Starting with the first space explorations in the 60s, the space environment had always driven a high interest in the scientific community. From the study of planetary magnetosphere to the solar environment and the local interstellar medium, scientists have produced an enormous quantity of interesting research that gave us a clearer picture of Space and its mechanisms. Nevertheless, there are still open questions in the field even if our technology is developing faster than in the past. Thus, this Editor’s Challenge wants to assess the state of space physics, to guide researchers to address outstanding questions, and to open dialog about what might and might not be solved issues. The Specialty Chief Editor would like to invite the submissions of critical, ambitious, and courageous contributions that can provide new insights and stimulate a scientific constructive debate around the present and future status of Space Physics. The authors are challenged to address the following questions: •	Are there any solved problems in space physics? •	Are there problems that the community thinks are solved, but are not? •	What are the outstanding issues in space physics? •	In particular, what are the neglected outstanding issues in space physics? The authors should address these challenges in Perspective and Opinion articles, contributing viewpoints and interpretations about a specific area of investigation in Space Physics research. Reviews, Mini Reviews, Original Research and Brief Research Reports, are also welcome, focusing on ongoing questions in Space Physics. Please find details on the Article Type guidelines <a href="https://www.frontiersin.org/journals/astronomy-and-space-sciences/for-authors/article-types">here</a>. The Research Topic solicits contributions from the Editorial Board members of the Space Physics section, and invited contributors. The Specialty Chief Editors of Frontiers in Astronomy and Space Sciences launch a new series of Research Topics to highlight current challenges across the field of Astronomy and Space Sciences. Other titles in the series are: <a href="https://www.frontiersin.org/research-topics/33813/editors-challenge-in-astrostatistics-deep-learning-in-astrophysics---what-are-the-lessons">Editor's Challenge in Astrostatistics: Deep Learning in Astrophysics - What are the Lessons?</a> <a href="https://www.frontiersin.org/research-topics/34306/editors-challenge-in-astronomical-instrumentation-machine-learning-advances">Editor's Challenge in Astronomical Instrumentation: Machine Learning Advances</a> <a href="https://www.frontiersin.org/research-topics/36113/editors-challenge-in-exoplanets-next-generation-of-exoplanet-research">Editor's Challenge in Exoplanets: Next Generation of Exoplanet Research</a>

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Editor’s Challenge in Space Physics: Solved and Unsolved Problems in Space Physics

Editorial: Editor’s challenge in space physics: solved and unsolved problems in space physics

The solar wind forms the largest wind tunnel for plasma and magnetofluid turbulence that is accessible to Earth. It evolves from what is thought to be a turbulent source that continues to drive nonlinear turbulent dynamics as it expands outward via large-scale, energy-containing wind shear and shocks. In the outer heliosphere, once the gradients in the flow have coalesced and they no longer provide an adequate source for the turbulence, the excitation of wave energy by the injection of interstellar pickup ions becomes the dominant source of energy that continues to drive the turbulence. While there are established formalisms for the determination of the strength of the turbulence and the evolution of the turbulent spectra is well-established, the actual nonlinear dynamics that are responsible for its formation and evolution remain unresolved and the subject of considerable debate. We examine the evidence and attempt to illuminate the various theories while demonstrating what is needed to resolve the debates and bring the subject of plasma turbulence into a new level of understanding.

The solar wind forms the largest wind tunnel for plasma and magnetofluid turbulence that is accessible to Earth. It evolves from what is thought to be a turbulent source that continues to drive nonlinear turbulent dynamics as it expands outward via large-scale, energy-containing wind shear and shocks. In the outer heliosphere, once the gradients in the flow have coalesced and they no longer provide an adequate source for the turbulence, the excitation of wave energy by the injection of interstellar pickup ions becomes the dominant source of energy that continues to drive the turbulence. While there are established formalisms for the determination of the strength of the turbulence and the evolution of the turbulent spectra is well-established, the actual nonlinear dynamics that are responsible for its formation and evolution remain unresolved and the subject of considerable debate. We examine the evidence and attempt to illuminate the various theories while demonstrating what is needed to resolve the debates and bring the subject of plasma turbulence into a new level of understanding.

The unsolved problem of solar-wind turbulence

Quasilinear theories have been shown to well describe a range of transport phenomena in magnetospheric, space, astrophysical and laboratory plasma “weak turbulence” scenarios. It is well known that the resonant diffusion quasilinear theory for the case of a uniform background field may formally describe particle dynamics when the electromagnetic wave amplitude and growth rates are sufficiently “small”, and the bandwidth is sufficiently “large”. However, it is important to note that for a given wave spectrum that would be expected to give rise to quasilinear transport, the quasilinear theory may indeed apply for given range of resonant pitch-angles and energies, but may not apply for some smaller, or larger, values of resonant pitch-angle and energy. That is to say that the applicability of the quasilinear theory can be pitch-angle dependent, even in the case of a uniform background magnetic field. If indeed the quasilinear theory does apply, the motion of particles with different pitch-angles are still characterised by different timescales. Using a high-performance test-particle code, we present a detailed analysis of the applicability of quasilinear theory to a range of different wave spectra that would otherwise “appear quasilinear” if presented by e.g., satellite survey-mode data. We present these analyses as a function of wave amplitude, wave coherence and resonant particle velocities (energies and pitch-angles), and contextualise the results using theory of resonant overlap and small amplitude criteria. In doing so, we identify and classify five different transport regimes that are a function of particle pitch-angle. The results in our paper demonstrate that there can be a significant variety of particle responses (as a function of pitch-angle) for very similar looking survey-mode electromagnetic wave products, even if they appear to satisfy all appropriate quasilinear criteria. In recent years there have been a sequence of very interesting and important results in this domain, and we argue in favour of continuing efforts on: (i) the development of new transport theories to understand the importance of these, and other, diverse electron responses; (ii) which are informed by statistical analyses of the relationship between burst- and survey-mode spacecraft data.

Quasilinear theories have been shown to well describe a range of transport phenomena in magnetospheric, space, astrophysical and laboratory plasma &#x201c;weak turbulence&#x201d; scenarios. It is well known that the resonant diffusion quasilinear theory for the case of a uniform background field may formally describe particle dynamics when the electromagnetic wave amplitude and growth rates are sufficiently &#x201c;small&#x201d;, and the bandwidth is sufficiently &#x201c;large&#x201d;. However, it is important to note that for a given wave spectrum that would be expected to give rise to quasilinear transport, the quasilinear theory may indeed apply for given range of resonant pitch-angles and energies, but may not apply for some smaller, or larger, values of resonant pitch-angle and energy. That is to say that the applicability of the quasilinear theory can be pitch-angle dependent, even in the case of a uniform background magnetic field. If indeed the quasilinear theory does apply, the motion of particles with different pitch-angles are still characterised by different timescales. Using a high-performance test-particle code, we present a detailed analysis of the applicability of quasilinear theory to a range of different wave spectra that would otherwise &#x201c;appear quasilinear&#x201d; if presented by e.g., satellite survey-mode data. We present these analyses as a function of wave amplitude, wave coherence and resonant particle velocities (energies and pitch-angles), and contextualise the results using theory of resonant overlap and small amplitude criteria. In doing so, we identify and classify five different transport regimes that are a function of particle pitch-angle. The results in our paper demonstrate that there can be a significant variety of particle responses (as a function of pitch-angle) for very similar looking survey-mode electromagnetic wave products, even if they appear to satisfy all appropriate quasilinear criteria. In recent years there have been a sequence of very interesting and important results in this domain, and we argue in favour of continuing efforts on: (i) the development of new transport theories to understand the importance of these, and other, diverse electron responses; (ii) which are informed by statistical analyses of the relationship between burst- and survey-mode spacecraft data.

The challenge to understand the zoo of particle transport regimes during resonant wave-particle interactions for given survey-mode wave spectra

The influence of the Sun on the Earth’s atmosphere and climate has been a matter of hot debate for more than two centuries. In spite of the correlations found between the sunspot numbers and various atmospheric parameters, the mechanisms for such influences are not quite clear yet. Though great progress has been recently made, a major problem remains: the correlations are not stable, they may strengthen, weaken, disappear, and even change sign depending on the time period. None of the proposed so far mechanisms explains this temporal variability. The basis of all solar activity is the solar magnetic field which cyclically oscillates between its two components—poloidal and toroidal. We first briefly describe the operation of the solar dynamo transforming the poloidal field into toroidal and back, the evaluated relative variations of these two components, and their geoeffective manifestations. We pay special attention to the reconstruction of the solar irradiance as the key natural driver of climate. We point at some problems in reconstructing the long-term irradiance variations and the implications of the different irradiance composite series on the estimation of the role of the Sun in climate change. We also comment on the recent recalibration of the sunspot number as the only instrumentally measured parameter before 1874, and therefore of crucial importance for reconstructing the solar irradiance variations and their role in climate change. We summarize the main proposed mechanisms of solar influences on the atmosphere, and list some of the modelling and experimental results either confirming or questioning them. Two irradiance-driven mechanisms have been proposed. The “bottom-up” mechanism is based on the enhanced absorption of solar irradiance by the oceans in relatively cloud-free equatorial and subtropical regions, amplified by changes in the temperature gradients, circulation, and cloudiness. The “top-down” mechanism involves absorption by the stratospheric ozone of solar UV radiation whose variability is much greater than that of the visible one, and changes of large-scale circulation patterns like the stratospheric polar vortex and the tropospheric North Atlantic Oscillation. The positive phase of the tropospheric North Atlantic Oscillation indicative of a strong vortex is found to lag by a couple of years the enhanced UV in Smax. It was however shown that this positive response is not due to lagged UV effects but instead to precipitating energetic particles which also peak a couple of years after Smax. The solar wind and its transients modulate the flux of galactic cosmic rays which are the main source of ionization of the Earth’s atmosphere below ∼50 km. This modulation leads to modulation of the production of aerosols which are cloud condensation nuclei, and to modulation of cloudiness. Increased cloudiness decreases the solar irradiance reaching the low atmosphere and the Earth’s surface. Variations of the galactic cosmic rays also lead to variations of the electric currents and the ionospheric potential in the polar caps which may intensify microphysical processes in clouds and thus also cause cloudiness variations. Solar energetic particles are produced during eruptive events at the Sun. They produce reactive odd hydrogen HOx and nitrogen NOx which catalytically destroy ozone in the mesosphere and upper stratosphere—“direct effect.” NOx which are long-lived in the lack of photoionization during the polar night, can descend to lower altitudes and destroy ozone there producing a delayed “indirect effect.” In the absence of sunlight ozone absorbs longwave outgoing radiation emitted by the Earth and atmosphere. Ozone depletion associated with ionization increases leads to cooling of the polar middle atmosphere, enhancing the temperature contrast between polar and midlatitudes and, thus, the strength of the stratospheric polar vortex. Solar energetic particles are powerful but sporadic and rare events. An additional source of energetic particles are the electrons trapped in the Earth’s magnetosphere which during geomagnetic disturbances are accelerated and precipitate into the atmosphere. They are less energetic but are always present. Their effects are the same as that of the solar energetic particles: additional production of reactive HOx and NOx which destroy ozone resulting in a stronger vortex and a positive phase of the North Atlantic Oscillation. It has been shown that the reversals of the correlations between solar activity and atmospheric parameters have a periodicity of ∼60 years and are related to the evolution of the main forms of large-scale atmospheric circulation whose occurrence has a similar periodicity. The large-scale circulation forms are in turn influenced by the state of the polar vortex which can affect the troposphere-stratosphere interaction via the propagation of planetary waves. Two solar activity agents are supposed to affect the stratospheric polar vortex: spectral solar irradiance through the “top-down” mechanism, and energetic particles. Increased UV irradiance was found to lead to a negative phase of the North Atlantic Oscillation, while increased energetic particles result in a positive phase. Solar irradiance, like sunspots, is related to the solar toroidal field, and energetic particle precipitation is related to the solar poloidal field. In the course of the solar cycle the irradiance is maximum in sunspot maximum, and particle precipitation peaks strongly in the cycle’s declining phase. The solar poloidal and toroidal fields are the two faces of the solar large-scale magnetic field. They are closely connected, but because they are generated in different domains and because of the randomness involved in the generation of the poloidal field from the toroidal field, on longer time-scales their variations differ. As a result, in some periods poloidal field-related solar drivers prevail, in other periods toroidal field-related drivers prevail. These periods vary cyclically. When the poloidal field-related drivers prevail, the stratospheric polar vortex is stronger, and the correlation between solar activity and atmospheric parameters is positive. When toroidal field-related drivers prevail, the vortex is weaker and the correlations are negative.

The influence of the Sun on the Earth’s atmosphere and climate has been a matter of hot debate for more than two centuries. In spite of the correlations found between the sunspot numbers and various atmospheric parameters, the mechanisms for such influences are not quite clear yet. Though great progress has been recently made, a major problem remains: the correlations are not stable, they may strengthen, weaken, disappear, and even change sign depending on the time period. None of the proposed so far mechanisms explains this temporal variability. The basis of all solar activity is the solar magnetic field which cyclically oscillates between its two components—poloidal and toroidal. We first briefly describe the operation of the solar dynamo transforming the poloidal field into toroidal and back, the evaluated relative variations of these two components, and their geoeffective manifestations. We pay special attention to the reconstruction of the solar irradiance as the key natural driver of climate. We point at some problems in reconstructing the long-term irradiance variations and the implications of the different irradiance composite series on the estimation of the role of the Sun in climate change. We also comment on the recent recalibration of the sunspot number as the only instrumentally measured parameter before 1874, and therefore of crucial importance for reconstructing the solar irradiance variations and their role in climate change. We summarize the main proposed mechanisms of solar influences on the atmosphere, and list some of the modelling and experimental results either confirming or questioning them. Two irradiance-driven mechanisms have been proposed. The “bottom-up” mechanism is based on the enhanced absorption of solar irradiance by the oceans in relatively cloud-free equatorial and subtropical regions, amplified by changes in the temperature gradients, circulation, and cloudiness. The “top-down” mechanism involves absorption by the stratospheric ozone of solar UV radiation whose variability is much greater than that of the visible one, and changes of large-scale circulation patterns like the stratospheric polar vortex and the tropospheric North Atlantic Oscillation. The positive phase of the tropospheric North Atlantic Oscillation indicative of a strong vortex is found to lag by a couple of years the enhanced UV in Smax. It was however shown that this positive response is not due to lagged UV effects but instead to precipitating energetic particles which also peak a couple of years after Smax. The solar wind and its transients modulate the flux of galactic cosmic rays which are the main source of ionization of the Earth’s atmosphere below ∼50 km. This modulation leads to modulation of the production of aerosols which are cloud condensation nuclei, and to modulation of cloudiness. Increased cloudiness decreases the solar irradiance reaching the low atmosphere and the Earth’s surface. Variations of the galactic cosmic rays also lead to variations of the electric currents and the ionospheric potential in the polar caps which may intensify microphysical processes in clouds and thus also cause cloudiness variations. Solar energetic particles are produced during eruptive events at the Sun. They produce reactive odd hydrogen HOx and nitrogen NOx which catalytically destroy ozone in the mesosphere and upper stratosphere—“direct effect.” NOx which are long-lived in the lack of photoionization during the polar night, can descend to lower altitudes and destroy ozone there producing a delayed “indirect effect.” In the absence of sunlight ozone absorbs longwave outgoing radiation emitted by the Earth and atmosphere. Ozone depletion associated with ionization increases leads to cooling of the polar middle atmosphere, enhancing the temperature contrast between polar and midlatitudes and, thus, the strength of the stratospheric polar vortex. Solar energetic particles are powerful but sporadic and rare events. An additional source of energetic particles are the electrons trapped in the Earth’s magnetosphere which during geomagnetic disturbances are accelerated and precipitate into the atmosphere. They are less energetic but are always present. Their effects are the same as that of the solar energetic particles: additional production of reactive HOx and NOx which destroy ozone resulting in a stronger vortex and a positive phase of the North Atlantic Oscillation. It has been shown that the reversals of the correlations between solar activity and atmospheric parameters have a periodicity of ∼60 years and are related to the evolution of the main forms of large-scale atmospheric circulation whose occurrence has a similar periodicity. The large-scale circulation forms are in turn influenced by the state of the polar vortex which can affect the troposphere-stratosphere interaction via the propagation of planetary waves. Two solar activity agents are supposed to affect the stratospheric polar vortex: spectral solar irradiance through the “top-down” mechanism, and energetic particles. Increased UV irradiance was found to lead to a negative phase of the North Atlantic Oscillation, while increased energetic particles result in a positive phase. Solar irradiance, like sunspots, is related to the solar toroidal field, and energetic particle precipitation is related to the solar poloidal field. In the course of the solar cycle the irradiance is maximum in sunspot maximum, and particle precipitation peaks strongly in the cycle’s declining phase. The solar poloidal and toroidal fields are the two faces of the solar large-scale magnetic field. They are closely connected, but because they are generated in different domains and because of the randomness involved in the generation of the poloidal field from the toroidal field, on longer time-scales their variations differ. As a result, in some periods poloidal field-related solar drivers prevail, in other periods toroidal field-related drivers prevail. These periods vary cyclically. When the poloidal field-related drivers prevail, the stratospheric polar vortex is stronger, and the correlation between solar activity and atmospheric parameters is positive. When toroidal field-related drivers prevail, the vortex is weaker and the correlations are negative.

Solar influences on the Earth’s atmosphere: solved and unsolved questions

Recently it has been identified that our Moon had an extensive magnetosphere for several hundred million years soon after it was formed when the Moon was within 20 Earth Radii (RE) from the Earth. Some aspects of the interaction between the early Earth-Moon magnetospheres are investigated by mapping the interconnected field lines between the Earth and the Moon and investigating how the early lunar magnetosphere affects the magnetospheric dynamics within the coupled magnetospheres over time. So long as the magnetosphere of the Moon remains strong as it moves away from the Earth in the antialigned dipole configuration, the extent of the Earth’s open field lines decreases. As a result, at times it significantly changes the structure of the field-aligned current system, pushing the polar cusp significantly northward, and forcing magnetotail reconnection sites into the deeper tail region. In addition, the combined magnetospheres of the Earth and the Moon greatly extend the number of closed field lines enabling a much larger plasmasphere to exist and connecting the lunar polar cap with closed field lines to the Earth. That configuration supports the transfer of plasma between the Earth and the Moon potentially creating a time capsule of the evolution of volatiles with depth. This paper only touches on the evolution of the early Earth and Moon magnetospheres, which has been a largely neglected space physics problem and has great potential for complex follow-on studies using more advanced tools and due to the expected new lunar data coming in the next decade through the Artemis Program.

Recently it has been identified that our Moon had an extensive magnetosphere for several hundred million years soon after it was formed when the Moon was within 20 Earth Radii (RE) from the Earth. Some aspects of the interaction between the early Earth-Moon magnetospheres are investigated by mapping the interconnected field lines between the Earth and the Moon and investigating how the early lunar magnetosphere affects the magnetospheric dynamics within the coupled magnetospheres over time. So long as the magnetosphere of the Moon remains strong as it moves away from the Earth in the antialigned dipole configuration, the extent of the Earth’s open field lines decreases. As a result, at times it significantly changes the structure of the field-aligned current system, pushing the polar cusp significantly northward, and forcing magnetotail reconnection sites into the deeper tail region. In addition, the combined magnetospheres of the Earth and the Moon greatly extend the number of closed field lines enabling a much larger plasmasphere to exist and connecting the lunar polar cap with closed field lines to the Earth. That configuration supports the transfer of plasma between the Earth and the Moon potentially creating a time capsule of the evolution of volatiles with depth. This paper only touches on the evolution of the early Earth and Moon magnetospheres, which has been a largely neglected space physics problem and has great potential for complex follow-on studies using more advanced tools and due to the expected new lunar data coming in the next decade through the Artemis Program.

Effects of the evolving early Moon and Earth magnetospheres

Understanding the harmful effects of galactic cosmic rays (GCRs) on space exploration requires a substantial amount of nuclear data. Specifically, the interaction of energetic GCR charged particles with spacecraft materials generates secondary radiations that, through energy deposition, can harm astronauts and electronic systems. By identifying the gaps in our knowledge of the relevant nuclear data—such as interaction cross sections—and identifying ways to fill those gaps—with measurements, compilations, evaluations, dissemination, reaction modeling, sensitivity studies, and uncertainty quantification—the safety and viability of space exploration can be improved. This work surveys the state of the art in this interdisciplinary field and identifies promising collaborative research topics that have significant potential to advance our understanding of the effects of the space radiation environment on space exploration.

Understanding the harmful effects of galactic cosmic rays (GCRs) on space exploration requires a substantial amount of nuclear data. Specifically, the interaction of energetic GCR charged particles with spacecraft materials generates secondary radiations that, through energy deposition, can harm astronauts and electronic systems. By identifying the gaps in our knowledge of the relevant nuclear data&#x2014;such as interaction cross sections&#x2014;and identifying ways to fill those gaps&#x2014;with measurements, compilations, evaluations, dissemination, reaction modeling, sensitivity studies, and uncertainty quantification&#x2014;the safety and viability of space exploration can be improved. This work surveys the state of the art in this interdisciplinary field and identifies promising collaborative research topics that have significant potential to advance our understanding of the effects of the space radiation environment on space exploration.

Nuclear data for space exploration

For community use, a new composite whole-Earth index E(1) and its matching composite solar wind driving function S(1) are derived. A system science methodology is used based on a time-dependent magnetospheric state vector and a solar wind state vector, with canonical correlation analysis (CCA) used to reduce the two state vectors to the two time-dependent scalars E(1)(t) and S(1)(t). The whole-Earth index E(1) is based on a diversity of measures via six diverse geomagnetic indices that will be readily available in the future: SML, SMU, Ap60, SYMH, ASYM, and PCC. The CCA-derived composite index has several advantages: 1) the new “canonical” geomagnetic index E(1) will provide a more powerful description of magnetospheric activity, a description of the collective behavior of the magnetosphere–ionosphere system. 2) The new index E(1) is much more accurately predictable from upstream solar wind measurements on Earth. 3) Indications are that the new canonical geomagnetic index E(1) will be accurately predictable even when as-yet-unseen extreme solar wind conditions occur. The composite solar wind driver S(1) can also be used as a universal driver function for individual geomagnetic indices or for magnetospheric particle populations. To familiarize the use of the new index E(1), its behavior is examined in different phases of the solar cycle, in different types of solar wind plasma, during high-speed stream-driven storms, during CME sheath-driven storms, and during superstorms. It is suggested that the definition of storms are the times when E(1) >1.

For community use, a new composite whole-Earth index E(1) and its matching composite solar wind driving function S(1) are derived. A system science methodology is used based on a time-dependent magnetospheric state vector and a solar wind state vector, with canonical correlation analysis (CCA) used to reduce the two state vectors to the two time-dependent scalars E(1)(t) and S(1)(t). The whole-Earth index E(1) is based on a diversity of measures via six diverse geomagnetic indices that will be readily available in the future: SML, SMU, Ap60, SYMH, ASYM, and PCC. The CCA-derived composite index has several advantages: 1) the new “canonical” geomagnetic index E(1) will provide a more powerful description of magnetospheric activity, a description of the collective behavior of the magnetosphere–ionosphere system. 2) The new index E(1) is much more accurately predictable from upstream solar wind measurements on Earth. 3) Indications are that the new canonical geomagnetic index E(1) will be accurately predictable even when as-yet-unseen extreme solar wind conditions occur. The composite solar wind driver S(1) can also be used as a universal driver function for individual geomagnetic indices or for magnetospheric particle populations. To familiarize the use of the new index E(1), its behavior is examined in different phases of the solar cycle, in different types of solar wind plasma, during high-speed stream-driven storms, during CME sheath-driven storms, and during superstorms. It is suggested that the definition of storms are the times when E(1) &gt;1.

A system science methodology develops a new composite highly predictable index of magnetospheric activity for the community: the whole-Earth index E(1)

Auroral radio emissions are of intrinsic interest as part of the Earth’s environment but also provide remote sensing of ionospheric conditions and processes and a laboratory for emission processes applicable to a wide range of space and astrophysical plasmas. At VLF and above, four broad classes of radio emissions occur. All have been observed with ground-based and, in some cases to a lesser degree, with space-based instruments. Related to each type of radio emission, many experimental and theoretical challenges remain, for example: explanations of frequency and time structure, relations to auroral substorms or current systems, and application to remote sensing of the auroral ionosphere. In some cases, basic parameters such as source heights or generation mechanisms are uncertain. Emerging technological advances such as cubesat fleets, ultra-large capacity disk drives, and software defined radio show promise for developing better understanding of auroral radio emissions.

Auroral radio emissions are of intrinsic interest as part of the Earth&#x2019;s environment but also provide remote sensing of ionospheric conditions and processes and a laboratory for emission processes applicable to a wide range of space and astrophysical plasmas. At VLF and above, four broad classes of radio emissions occur. All have been observed with ground-based and, in some cases to a lesser degree, with space-based instruments. Related to each type of radio emission, many experimental and theoretical challenges remain, for example: explanations of frequency and time structure, relations to auroral substorms or current systems, and application to remote sensing of the auroral ionosphere. In some cases, basic parameters such as source heights or generation mechanisms are uncertain. Emerging technological advances such as cubesat fleets, ultra-large capacity disk drives, and software defined radio show promise for developing better understanding of auroral radio emissions.

Radio emissions of auroral origin observable at ground level: outstanding problems

Solar wind (SW) quantities, referred to as coupling parameters (CPs), are often used in statistical studies devoted to the analysis of SW–magnetosphere–ionosphere couplings. Here, the CPs and their limitations in describing the magnetospheric response are reviewed. We argue that a better understanding of SW magnetospheric interactions could be achieved through estimations of the energy budget in the magnetosheath (MS), which is the interface region between the SW and magnetosphere. The energy budget involves the energy transfer between scales, energy transport between locations, and energy conversions between electromagnetic, kinetic, and thermal energy channels. To achieve consistency with the known multi-scale complexity in the MS, the energy terms have to be complemented with kinetic measures describing some aspects of ion–electron scale physics.

Solar wind (SW) quantities, referred to as coupling parameters (CPs), are often used in statistical studies devoted to the analysis of SW&#x2013;magnetosphere&#x2013;ionosphere couplings. Here, the CPs and their limitations in describing the magnetospheric response are reviewed. We argue that a better understanding of SW magnetospheric interactions could be achieved through estimations of the energy budget in the magnetosheath (MS), which is the interface region between the SW and magnetosphere. The energy budget involves the energy transfer between scales, energy transport between locations, and energy conversions between electromagnetic, kinetic, and thermal energy channels. To achieve consistency with the known multi-scale complexity in the MS, the energy terms have to be complemented with kinetic measures describing some aspects of ion&#x2013;electron scale physics.

How to improve our understanding of solar wind-magnetosphere interactions on the basis of the statistical evaluation of the energy budget in the magnetosheath?

Dynamic mesoscale flow structures move across the open field line regions of the polar caps and then enter the nightside plasma sheet where they can cause important space weather disturbances, such as streamers, substorms, and omega bands. The polar cap structures have long durations (apparently at least ∼1½ to 2 h), but their connections to disturbances have received little attention. Hence, it will be important to uncover what causes these flow enhancement channels, how they map to the magnetospheric and magnetosheath structures, and what controls their propagation across the polar cap and their dynamic effects after reaching the nightside auroral oval. The examples presented here use 630-nm auroral and radar observations and indicate that the motion of flow channels could be critical for determining when and where a particular disturbance within the nightside auroral oval will be triggered, and this could be included for full understanding of flow channel connections to disturbances. Also, it is important to determine how polar cap flow channels lead to flow channels within the auroral oval, i.e., the plasma sheet, and determine the conditions along nightside oval/plasma sheet field lines that interact with an incoming polar cap flow channel to cause a particular disturbance. It will also be interesting to consider the generality of geomagnetic disturbances being related to connections with incoming polar cap flow channels, including the location, time, and type of disturbances, and whether the duration and expansion of disturbances are related to flow channel duration and to multiple flow channels.

Dynamic mesoscale flow structures move across the open field line regions of the polar caps and then enter the nightside plasma sheet where they can cause important space weather disturbances, such as streamers, substorms, and omega bands. The polar cap structures have long durations (apparently at least &#x223c;1&#xbd; to 2&#xa0;h), but their connections to disturbances have received little attention. Hence, it will be important to uncover what causes these flow enhancement channels, how they map to the magnetospheric and magnetosheath structures, and what controls their propagation across the polar cap and their dynamic effects after reaching the nightside auroral oval. The examples presented here use 630-nm auroral and radar observations and indicate that the motion of flow channels could be critical for determining when and where a particular disturbance within the nightside auroral oval will be triggered, and this could be included for full understanding of flow channel connections to disturbances. Also, it is important to determine how polar cap flow channels lead to flow channels within the auroral oval, i.e., the plasma sheet, and determine the conditions along nightside oval/plasma sheet field lines that interact with an incoming polar cap flow channel to cause a particular disturbance. It will also be interesting to consider the generality of geomagnetic disturbances being related to connections with incoming polar cap flow channels, including the location, time, and type of disturbances, and whether the duration and expansion of disturbances are related to flow channel duration and to multiple flow channels.

Unsolved problems: Mesoscale polar cap flow channels’ structure, propagation, and effects on space weather disturbances

An assessment of the status quo of fast subauroral flows—subauroral ion drifts (SAID) and subauroral polarization streams (SAPS), is presented. For a few decades, their development has been interpreted in terms of the voltage and current magnetospheric generators based largely on the drift motion of test particles. Recent multispacecraft observations revealed serious flaws in the generator paradigm and called for a new generation mechanism of fast-time subauroral flows and ring current (RC) injections. A novel model includes them in the overarching problem of the penetration of magnetotail plasma flow bursts (MPFs) into the plasmasphere and the substorm current wedge (SCW) development. SAID are created near the plasmapause, where inbound MPFs are short-circuited by the cold plasma. This stops the MPF’s electrons and forms the “dispersionless” plasma sheet (PS) boundary. The SAID electric field—the inherent part of the short-circuiting loop—stops the inward-moving MPF’s ions. In turn, SAPS are an integral part of the two-loop SCW system, or SCW2L, where the downward (R2) current emerges in response to the upward (R1) current in the SCW’s “head.” The meridional Pedersen current, which connects the R1 and R2 currents, leads to SAPS that ultimately drive the fast-time RC injections on the duskside.

An assessment of the status quo of fast subauroral flows—subauroral ion drifts (SAID) and subauroral polarization streams (SAPS), is presented. For a few decades, their development has been interpreted in terms of the voltage and current magnetospheric generators based largely on the drift motion of test particles. Recent multispacecraft observations revealed serious flaws in the generator paradigm and called for a new generation mechanism of fast-time subauroral flows and ring current (RC) injections. A novel model includes them in the overarching problem of the penetration of magnetotail plasma flow bursts (MPFs) into the plasmasphere and the substorm current wedge (SCW) development. SAID are created near the plasmapause, where inbound MPFs are short-circuited by the cold plasma. This stops the MPF’s electrons and forms the “dispersionless” plasma sheet (PS) boundary. The SAID electric field—the inherent part of the short-circuiting loop—stops the inward-moving MPF’s ions. In turn, SAPS are an integral part of the two-loop SCW system, or SCW2L, where the downward (R2) current emerges in response to the upward (R1) current in the SCW’s “head.” The meridional Pedersen current, which connects the R1 and R2 currents, leads to SAPS that ultimately drive the fast-time RC injections on the duskside.

The evolving paradigm of the subauroral geospace

It has become well-established that strong outer radiation belt enhancements are due to wave-driven electron energization by whistler-mode chorus waves. However, in this study, we examine strong MeV electron injections on 10 July 2019 and find substantial evidence that such injections may be a crucial contributor to outer radiation belt enhancement events. For such an examination, it is essential to precisely separate temporal flux changes from spatial variations observed as Van Allen Probes move along their orbits. Employing a new “hourly snapshot” analysis approach, we discover unprecedented details of electron flux evolutions that suggest that for this event, the outer belt enhancement was not continuous but instead intermittent, mostly composed of 4 large discrete injection-driven flux increases. The injections appear as sharp flux increases when observed near apogee. Otherwise, by comparing hourly snapshots for different times, we infer injections and infer temporally stable fluxes between injections, despite strong and continuous chorus emission. The fast and intermittent electron flux growth successively extending earthwards implies cumulative outer belt enhancement via a series of repetitive inward transport associated with injection-induced electric fields.

It has become well-established that strong outer radiation belt enhancements are due to wave-driven electron energization by whistler-mode chorus waves. However, in this study, we examine strong MeV electron injections on 10 July 2019 and find substantial evidence that such injections may be a crucial contributor to outer radiation belt enhancement events. For such an examination, it is essential to precisely separate temporal flux changes from spatial variations observed as Van Allen Probes move along their orbits. Employing a new “hourly snapshot” analysis approach, we discover unprecedented details of electron flux evolutions that suggest that for this event, the outer belt enhancement was not continuous but instead intermittent, mostly composed of 4 large discrete injection-driven flux increases. The injections appear as sharp flux increases when observed near apogee. Otherwise, by comparing hourly snapshots for different times, we infer injections and infer temporally stable fluxes between injections, despite strong and continuous chorus emission. The fast and intermittent electron flux growth successively extending earthwards implies cumulative outer belt enhancement via a series of repetitive inward transport associated with injection-induced electric fields.

Can strong substorm-associated MeV electron injections be an important cause of large radiation belt enhancements?

The Earth’s atmosphere near both the geographic and magnetic equators and at altitudes between 120 and 200 km is called the low-latitude valley region (LLVR) and is among the least understood regions of the ionosphere/thermosphere due to its complex interplay of neutral dynamics, electrodynamics, and photochemistry. Radar studies of the region have revealed puzzling daytime echoes scattered from between 130 and 170 km in altitude. The echoes are quasi-periodic and are observed in solar-zenith-angle dependent layers. Populations with two distinct types of spectral features are observed. A number of radars have shown scattering cross-sections with different seasonal and probing-frequency dependencies. The sources and configurations of the so-called 150-km echoes and the related irregularities have been long-standing riddles for which some solutions are finally starting to emerge as will be described in this review paper. Although the 150-km echoes were discovered in the early 1960s, their practical significance and implications were not broadly recognized until the early 1990s, and no compelling explanations of their generation mechanisms and observed features emerged until about a decade ago. Now, more rapid progress is being made thanks to a multi-disciplinary team effort described here and recent developments in kinetic simulations and theory: 18 of 27 riddles to be described in this paper stand solved (and a few more partially solved) at this point in time. The source of the irregularities is no longer a puzzle as compelling evidence has emerged from simulations and theory, presented since 2016 that they are being caused by photoelectrons driving an upper hybrid plasma instability process. Another resolved riddle concerns the persistent gaps observed between the 150-km scattering layers—we now understand that they are likely to be the result of enhanced thermal Landau damping of the upper hybrid instability process at upper hybrid frequencies matching the harmonics of the electron gyrofrequency. The remaining unsolved riddles, e.g., minute-scale variability, multi-frequency dependence, to name a few, are still being explored observationally and theoretically—they are most likely unidentified consequences of interplay between plasma physics, photochemistry, and lower atmospheric dynamic processes governing the LLVR.

The Earth&#x2019;s atmosphere near both the geographic and magnetic equators and at altitudes between 120 and 200&#xa0;km is called the low-latitude valley region (LLVR) and is among the least understood regions of the ionosphere/thermosphere due to its complex interplay of neutral dynamics, electrodynamics, and photochemistry. Radar studies of the region have revealed puzzling daytime echoes scattered from between 130 and 170&#xa0;km in altitude. The echoes are quasi-periodic and are observed in solar-zenith-angle dependent layers. Populations with two distinct types of spectral features are observed. A number of radars have shown scattering cross-sections with different seasonal and probing-frequency dependencies. The sources and configurations of the so-called 150-km echoes and the related irregularities have been long-standing riddles for which some solutions are finally starting to emerge as will be described in this review paper. Although the 150-km echoes were discovered in the early 1960s, their practical significance and implications were not broadly recognized until the early 1990s, and no compelling explanations of their generation mechanisms and observed features emerged until about a decade ago. Now, more rapid progress is being made thanks to a multi-disciplinary team effort described here and recent developments in kinetic simulations and theory: 18 of 27 riddles to be described in this paper stand solved (and a few more partially solved) at this point in time. The source of the irregularities is no longer a puzzle as compelling evidence has emerged from simulations and theory, presented since 2016 that they are being caused by photoelectrons driving an upper hybrid plasma instability process. Another resolved riddle concerns the persistent gaps observed between the 150-km scattering layers&#x2014;we now understand that they are likely to be the result of enhanced thermal Landau damping of the upper hybrid instability process at upper hybrid frequencies matching the harmonics of the electron gyrofrequency. The remaining unsolved riddles, e.g., minute-scale variability, multi-frequency dependence, to name a few, are still being explored observationally and theoretically&#x2014;they are most likely unidentified consequences of interplay between plasma physics, photochemistry, and lower atmospheric dynamic processes governing the LLVR.

Solved and unsolved riddles about low-latitude daytime valley region plasma waves and 150-km echoes

We study the dependencies of Earth’s magnetosphere on Universal Time, UT. These are introduced because Earth’s magnetic axis is not aligned with the rotational axis and complicated because it is eccentric, which makes the offset of the magnetic and rotation poles considerably greater in the Southern hemisphere and the longitudinal separation of the magnetic poles less than 180°: hence consequent UT variations in the two hemispheres are not in equal in amplitude nor in exact antiphase and do not cancel, as they would for a geocentric dipole. We use long series of a variety of geomagnetic data to demonstrate the inductive effect of motions of the polar caps in a “geocentric-solar” frame, which is phase-locked to the Russell-McPherron (R-M) effect on solar-wind magnetosphere coupling. This makes the response of the magnetosphere-ionosphere system different for the two polarities of the Y-component of the Interplanetary Magnetic Field in the GSEQ reference frame, explaining the difference in response to the March and September equinox peaks in solar wind forcing. The sunward/antisunward pole-motion effect is detected directly in satellite transpolar voltage data and is shown to have a greater effect on the geomagnetic data than the full dipole tilt effect which generates the equinoctial pattern, the potential origins of which are discussed in terms of the dipole tilt effect on ionospheric conductivities and the stability of the near-Earth tail. Persistent UT variations in Region-1 and Region-2 field-aligned currents and in partial ring current indices are presented: their explanation is an important challenge for numerical modelling of the magnetosphere-ionosphere-thermosphere system which we need to quantify the relative contributions of the various mechanisms and to give understanding of the effect of arrival time on the response of the system to large, geoeffective disturbances in interplanetary space.Plain language summary: The effect on terrestrial space weather of Earth’s magnetic axis not being aligned with the rotational axis is investigated. It is complex because not only do these two axes not align in direction (the “dipole tilt”), the magnetic axis does not pass through the centre of the Earth, which sets a requirement for an “eccentric” model of the field and not the commonly-used “geocentric” one. For many years, it has been known that the dipole tilt gives a peak in geomagnetic activity at the equinoxes (the semi-annual variation) through the “Russell-McPherron” (R-M) effect. However, although the variation with Universal Time is consistent with the R-M effect for the September equinox, it is not for the March equinox. We here solve this long-standing puzzle by investigating the effects of the motions of the two poles in a frame fixed with respect to both the Earth and the Sun for an eccentric dipole model. But solving one puzzle generates many others. We present observations of the Universal Time variations that these mechanisms combine to generate, which set an important challenge to the numerical modelling of the near-Earth space environment.

We study the dependencies of Earth’s magnetosphere on Universal Time, UT. These are introduced because Earth’s magnetic axis is not aligned with the rotational axis and complicated because it is eccentric, which makes the offset of the magnetic and rotation poles considerably greater in the Southern hemisphere and the longitudinal separation of the magnetic poles less than 180°: hence consequent UT variations in the two hemispheres are not in equal in amplitude nor in exact antiphase and do not cancel, as they would for a geocentric dipole. We use long series of a variety of geomagnetic data to demonstrate the inductive effect of motions of the polar caps in a “geocentric-solar” frame, which is phase-locked to the Russell-McPherron (R-M) effect on solar-wind magnetosphere coupling. This makes the response of the magnetosphere-ionosphere system different for the two polarities of the Y-component of the Interplanetary Magnetic Field in the GSEQ reference frame, explaining the difference in response to the March and September equinox peaks in solar wind forcing. The sunward/antisunward pole-motion effect is detected directly in satellite transpolar voltage data and is shown to have a greater effect on the geomagnetic data than the full dipole tilt effect which generates the equinoctial pattern, the potential origins of which are discussed in terms of the dipole tilt effect on ionospheric conductivities and the stability of the near-Earth tail. Persistent UT variations in Region-1 and Region-2 field-aligned currents and in partial ring current indices are presented: their explanation is an important challenge for numerical modelling of the magnetosphere-ionosphere-thermosphere system which we need to quantify the relative contributions of the various mechanisms and to give understanding of the effect of arrival time on the response of the system to large, geoeffective disturbances in interplanetary space.Plain language summary: The effect on terrestrial space weather of Earth’s magnetic axis not being aligned with the rotational axis is investigated. It is complex because not only do these two axes not align in direction (the “dipole tilt”), the magnetic axis does not pass through the centre of the Earth, which sets a requirement for an “eccentric” model of the field and not the commonly-used “geocentric” one. For many years, it has been known that the dipole tilt gives a peak in geomagnetic activity at the equinoxes (the semi-annual variation) through the “Russell-McPherron” (R-M) effect. However, although the variation with Universal Time is consistent with the R-M effect for the September equinox, it is not for the March equinox. We here solve this long-standing puzzle by investigating the effects of the motions of the two poles in a frame fixed with respect to both the Earth and the Sun for an eccentric dipole model. But solving one puzzle generates many others. We present observations of the Universal Time variations that these mechanisms combine to generate, which set an important challenge to the numerical modelling of the near-Earth space environment.

Universal Time variations in the magnetosphere

An overview of recent advances made in understanding the phenomenon of equatorial spread F (ESF) is presented and a discussion of unresolved issues that need to be addressed. The focus is on research that has occurred in the last decade. The topics include satellite observations, theory, and modeling. The suggested areas that require further exploration are a unified theory of turbulence extending from 100 s m to 10 s cm, the impact of geomagnetic storms on the development of equatorial spread F, the need for accurate thermospheric wind measurements and models, and identifying the underlying physics of ESF in the post-midnight sector during solar minimum.

An overview of recent advances made in understanding the phenomenon of equatorial spread F (ESF) is presented and a discussion of unresolved issues that need to be addressed. The focus is on research that has occurred in the last decade. The topics include satellite observations, theory, and modeling. The suggested areas that require further exploration are a unified theory of turbulence extending from 100&nbsp;s&nbsp;m to 10&nbsp;s&nbsp;cm, the impact of geomagnetic storms on the development of equatorial spread F, the need for accurate thermospheric wind measurements and models, and identifying the underlying physics of ESF in the post-midnight sector during solar minimum.

Resolution of the equatorial spread F problem: Revisited

The polar cap magnetic activity PC index was approved by IAGA as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling (<xref ref-type="bibr" rid="B41">IAGA Resolutions, 2013</xref>; <xref ref-type="bibr" rid="B40">IAGA Resolution, 2021</xref>). The paper summarizes experimental results attesting the validity of this PC index essence. The following issues are examined: the PC index derivation method, making allowance for regular and irregular variations of ionospheric conductivity; relationships between the PC index and solar wind electric field (EKL) and factors controlling response of PC index to the EKL field alterations; relation of PC index to the magnetospheric field-aligned currents (FAC) and to solar wind dynamic pressure pulses (Pdyn). The PC index alterations in course of 23/24 solar activity cycles have been analyzed in relation to various solar wind parameters and the conclusion was made that linear link between the PC index and solar wind electric field EKL remained valid irrespective of solar activity cycle. New ideas concerning (1) the nature of occasional differences between the PC indices in summer and winter polar caps and (2) two simultaneously acting mechanisms of the solar wind influence on the magnetosphere (Dungey and Tverskoy concepts) are discussed, as a result of the PC index application. It is emphasized that the ground-based PC index ensures a permanent on-line information on geoefficiency of the solar wind impact on the magnetosphere and, correspondingly, on the varying geophysical situation.

The polar cap magnetic activity PC index was approved by IAGA as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling (<a href="#B41">IAGA Resolutions, 2013</a>; <a href="#B40">IAGA Resolution, 2021</a>). The paper summarizes experimental results attesting the validity of this PC index essence. The following issues are examined: the PC index derivation method, making allowance for regular and irregular variations of ionospheric conductivity; relationships between the PC index and solar wind electric field (EKL) and factors controlling response of PC index to the EKL field alterations; relation of PC index to the magnetospheric field-aligned currents (FAC) and to solar wind dynamic pressure pulses (Pdyn). The PC index alterations in course of 23/24 solar activity cycles have been analyzed in relation to various solar wind parameters and the conclusion was made that linear link between the PC index and solar wind electric field EKL remained valid irrespective of solar activity cycle. New ideas concerning (1) the nature of occasional differences between the PC indices in summer and winter polar caps and (2) two simultaneously acting mechanisms of the solar wind influence on the magnetosphere (Dungey and Tverskoy concepts) are discussed, as a result of the PC index application. It is emphasized that the ground-based PC index ensures a permanent on-line information on geoefficiency of the solar wind impact on the magnetosphere and, correspondingly, on the varying geophysical situation.

PC index as a ground-based indicator of the solar wind energy incoming into the magnetosphere: (1) relation of PC index to the solar wind electric field EKL

Auroral kilometric radiation (AKR) is the paradigm of intense radio emission from planetary magnetospheres. Being close to the electron gyro frequency and/or its lower harmonics, its observation indicates the non-thermal state of the source plasma. Emission is produced when the plasma enters a state of energetic excitation which results in deformation of the electron distribution function. Under certain conditions this leads to “quasi-coherent” emission. It is believed that the weakly-relativistic electron-cyclotron-maser instability is responsible for this kind of radiation. Since energetically radio radiation normally is not of primary importance in the large-scale magnetospheric phenomena, AKR as such has, for the purposes of large-scale magnetospheric physics, become considered a marginal problem. Here this notion is questioned. AKR while applying to the auroral region mainly during magnetospherically disturbed times carries just a fraction of the total substorm energy. It is, however, of diagnostic power in the physics of the upper auroral ionosphere and Space Weather research. As a fundamental physical problem of generation of radiation in non-thermal plasmas it remains not resolved yet. Many questions have been left open even when dealing only with the electron-cyclotron-maser. These can advantageously be studied in the magnetosphere proper both by observation and theory, the only continuously accessible place in space. The most important are listet here with hint on how they should be attacked. Its value is to be sought in the role it should play in application to the other magnetized planets, extra-solar planets, and to strongly magnetized astronomical objects as an important tool to diagnose the matter state responsible for radiation in the radio frequency range beyond thermal, shock or synchrotron radiation.

Auroral kilometric radiation (AKR) is the paradigm of intense radio emission from planetary magnetospheres. Being close to the electron gyro frequency and/or its lower harmonics, its observation indicates the non-thermal state of the source plasma. Emission is produced when the plasma enters a state of energetic excitation which results in deformation of the electron distribution function. Under certain conditions this leads to &#x201c;quasi-coherent&#x201d; emission. It is believed that the weakly-relativistic electron-cyclotron-maser instability is responsible for this kind of radiation. Since energetically radio radiation normally is not of primary importance in the large-scale magnetospheric phenomena, AKR as such has, for the purposes of large-scale magnetospheric physics, become considered a marginal problem. Here this notion is questioned. AKR while applying to the auroral region mainly during magnetospherically disturbed times carries just a fraction of the total substorm energy. It is, however, of diagnostic power in the physics of the upper auroral ionosphere and Space Weather research. As a fundamental physical problem of generation of radiation in non-thermal plasmas it remains not resolved yet. Many questions have been left open even when dealing only with the electron-cyclotron-maser. These can advantageously be studied in the magnetosphere proper both by observation and theory, the only continuously accessible place in space. The most important are listet here with hint on how they should be attacked. Its value is to be sought in the role it should play in application to the other magnetized planets, extra-solar planets, and to strongly magnetized astronomical objects as an important tool to diagnose the matter state responsible for radiation in the radio frequency range beyond thermal, shock or synchrotron radiation.

Auroral kilometric radiation—The electron cyclotron maser paradigm

In this Perspective article discussing solved and unsolved problems in space physics, the focus is on the unsolved problem of the spatial-temporal variability of the magnetospheric plasma waves that produce the spatial-temporal atmospheric luminosity of the pulsating aurora. In particular the outstanding issue of what causes the spatial-temporal variations of the chorus-wave intensities is highlighted: Two great unknowns are (1) how does it work and (2) what are the controlling factors. The point is made that the whistler-mode chorus waves that produce the pulsating aurora are the same chorus waves that energize the Earth’s electron radiation belt. Hence, beyond not understanding the cause of pulsating aurora there is (1) a lack of understanding of the magnetosphere-ionosphere system behavior and (2) a lack of understanding of how the electron radiation belt is energized. It is noted that the pulsating aurora is perhaps the most-obvious example of an “emergent phenomena” in the magnetosphere-ionosphere-thermosphere system, and so perhaps the clearest indication that the magnetosphere-ionosphere-thermosphere system is a truly “complex system”, not just a complicated system. Future needs for solving this unsolved problem are discussed: the most-critical need is argued to be gaining an ability to measure cold-electron structuring in the equatorial magnetosphere.

In this Perspective article discussing solved and unsolved problems in space physics, the focus is on the unsolved problem of the spatial-temporal variability of the magnetospheric plasma waves that produce the spatial-temporal atmospheric luminosity of the pulsating aurora. In particular the outstanding issue of what causes the spatial-temporal variations of the chorus-wave intensities is highlighted: Two great unknowns are (1) how does it work and (2) what are the controlling factors. The point is made that the whistler-mode chorus waves that produce the pulsating aurora are the same chorus waves that energize the Earth&#x2019;s electron radiation belt. Hence, beyond not understanding the cause of pulsating aurora there is (1) a lack of understanding of the magnetosphere-ionosphere system behavior and (2) a lack of understanding of how the electron radiation belt is energized. It is noted that the pulsating aurora is perhaps the most-obvious example of an &#x201c;emergent phenomena&#x201d; in the magnetosphere-ionosphere-thermosphere system, and so perhaps the clearest indication that the magnetosphere-ionosphere-thermosphere system is a truly &#x201c;complex system&#x201d;, not just a complicated system. Future needs for solving this unsolved problem are discussed: the most-critical need is argued to be gaining an ability to measure cold-electron structuring in the equatorial magnetosphere.

What produces and what controls the spatial-temporal structuring of the magnetospheric chorus waves that create the pulsating aurora: An unsolved problem in need of new measurements

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