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.
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.
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 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.
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 (IAGA Resolutions, 2013; IAGA Resolution, 2021). 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.
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.