Frontiers in Physics | Space Physics section | New and Recent Articles
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RSS Feed for Space Physics section in the Frontiers in Physics journal | New and Recent Articlesen-usFrontiers Feed Generator,version:12019-09-22T08:50:22.4432872+00:0060https://www.frontiersin.org/articles/10.3389/fphy.2019.00108
https://www.frontiersin.org/articles/10.3389/fphy.2019.00108
Sign Singularity of the Local Energy Transfer in Space Plasma Turbulence2019-08-20T00:00:00ZLuca Sorriso-ValvoGaetano De VitaFederico FraternaleAlexandre GurchumeliaSilvia PerriGiuseppina NigroFilomena CatapanoAlessandro RetinòChristopher H. K. ChenEmiliya YordanovaOreste PezziKhatuna ChargaziaOleg KharshiladzeDiana KvaratskheliaChristian L. VásconezRaffaele MarinoOlivier Le ContelBarbara GilesThomas E. MooreRoy B. TorbertJames L. BurchIn weakly collisional space plasmas, the turbulent cascade provides most of the energy that is dissipated at small scales by various kinetic processes. Understanding the characteristics of such dissipative mechanisms requires the accurate knowledge of the fluctuations that make energy available for conversion at small scales, as different dissipation processes are triggered by fluctuations of a different nature. The scaling properties of different energy channels are estimated here using a proxy of the local energy transfer, based on the third-order moment scaling law for magnetohydrodynamic turbulence. In particular, the sign-singularity analysis was used to explore the scaling properties of the alternating positive-negative energy fluxes, thus providing information on the structure and topology of such fluxes for each of the different type of fluctuations. The results show the highly complex geometrical nature of the flux, and that the local contributions associated with energy and cross-helicity non-linear transfer have similar scaling properties. Consequently, the fractal properties of current and vorticity structures are similar to those of the Alfvénic fluctuations.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00114
https://www.frontiersin.org/articles/10.3389/fphy.2019.00114
Kinematic Collisionless Relaxation of Ions in Supercritical Shocks2019-08-14T00:00:00ZMichael GedalinWe show that kinematic collisionless relaxation in the macroscopic electric and magnetic fields plays the main role in the formation of the downstream ion distributions in a super-critical shock with ion reflection present. It is done by comparison of a theoretically predicted magnetic profile with the magnetic profile of a shocks observed by MMS mission. It is shown that pressure balance remains the main constraint for the shock stability.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00063
https://www.frontiersin.org/articles/10.3389/fphy.2019.00063
A First Assessment of a Regression-Based Interpretation of Langmuir Probe Measurements2019-05-03T00:00:00ZJonathan ChalaturnykRichard MarchandA new approach is presented for interpreting low level Langmuir probe measurements in terms of physical plasma parameters such as density or temperature. Instead of relying on analytic expressions as in most analyses, the method uses regressions combined with a suitably prepared solution library consisting of precomputed probe characteristics for selected plasma parameters. In machine learning language, this amounts to generating a training data set, constructing and training a model, and validating it over a domain of physical parameters of interest. This study aims at establishing the feasibility and limits of the method by using synthetic data sets that can be generated quickly from analytic approximations. The ultimate goal is to use this approach with model training on data sets constructed with detailed kinetic simulations capable of accounting for more physical processes, and more realistic geometry, than are possible with analytic models.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00008
https://www.frontiersin.org/articles/10.3389/fphy.2019.00008
A Note on Capon's Minimum Variance Projection for Multi-Spacecraft Data Analysis2019-02-01T00:00:00ZYasuhito NaritaCapon's minimum variance projection for the multi-point measurements is revisited using the method of likelihood function to derive the minimum variance projection and a simplified error estimate analytically. Theoretical construction of the minimum variance projection assumes a Gaussian form of the likelihood function and also regards the data covariance as a proxy of the noise covariance. The minimum variance projection is extended to the problem of two-spacecraft mode decomposition in the Mercury magnetosphere in which the magnetic field is a superposition of the constant field from the current sheet and the dipolar field from the planet. The extension of the Capon estimator (the data-variance projection) can identify the signal amplitudes of the different fields with a sufficient accuracy when the statistical averaging is properly done. The Capon estimator serves as a powerful analysis tool when the spatial resolution is limited to only a few points.]]>https://www.frontiersin.org/articles/10.3389/fphy.2018.00144
https://www.frontiersin.org/articles/10.3389/fphy.2018.00144
History of Los Alamos Participation in Active Experiments in Space2018-12-18T00:00:00ZMorris B. PongratzLos Alamos has a long history of participation in active experiments in space beginning with the Teak nuclear test in 1958. Above-ground nuclear testing stopped in 1962 because of the Partial Test Ban Treaty, and a program of non-nuclear chemical release experiments began in 1968. Los Alamos has participated in nearly 100 non-nuclear experiments in space, the last being the NASA-sponsored strontium and europium doped barium thermite releases in the Arecibo beam in July of 1992. The rationale for these experiments ranged from studying basic plasma processes such as gradient- driven structuring and velocity-space instabilities to illuminating the convection of plasmas in the ionosphere and polar cap to ionospheric depletion experiments to the B.E.A.R. 1-MeV neutral particle beam (NPB) test in 1989. This report reviews the objectives, techniques and diagnostics of Los Alamos participation in active experiments in space.]]>https://www.frontiersin.org/articles/10.3389/fphy.2018.00050
https://www.frontiersin.org/articles/10.3389/fphy.2018.00050
Determination of Polar Cap Boundary for the Substorm Event of 8 March 20082018-05-28T00:00:00ZChi WangJiangyan WangRamon LopezHui LiJiaojiao ZhangBinbin TangThe polar cap boundary (PCB) is a fundamental indicator of magnetospheric activities especially during a substorm cycle. Taking a period on 8 March 2008 as an example, we investigate the location of PCB and its dynamics during a substorm event. The PCB location is determined from the Piecewise Parabolic Method with a Lagrangian Remap (PPMLR) -Magnetohydrodynamic (MHD) simulation data and Defense Meteorological Satellite Program (DMSP) observations, respectively. Model-observation comparison indicates that the PPMLR-MHD model gives a reliable estimate of PCB location during a complex substorm sequence. We further analyze the evolution of PCB in that period. The polar cap expands under southward interplanetary magnetic field (IMF), since the low-latitude dayside reconnection produces new open magnetic flux. Meanwhile, more solar wind energy enters and stores in the magnetosphere with the decreasing SML (SuperMAG Auroral Lower) index. After the substorm expansion onset, the polar cap contracts for a while due to the explosive increase of nightside reconnection. When the IMF direction turns northward, the polar cap contracts continuously, since the dayside reconnection ceases and no more open magnetic flux are supplied, and the storage energy in the magnetosphere releases with the increasing SML index. The model results are in good accord with the features from observations.]]>https://www.frontiersin.org/articles/10.3389/fphy.2017.00008
https://www.frontiersin.org/articles/10.3389/fphy.2017.00008
Ion-Scale Sideband Waves and Filament Formation: Alfvénic Impact on Heliospheric Plasma Turbulence2017-02-23T03:46:41ZYasuhito NaritaUwe MotschmannA mini-review is given about two recent discoveries in the solar wind turbulence research on ion-kinetic scales: the existence of sideband waves (or breakdown of the linear mode wave picture) and the wavevector anisotropy leading to a persistent filament formation in a wide range of plasma beta.]]>https://www.frontiersin.org/articles/10.3389/fphy.2016.00029
https://www.frontiersin.org/articles/10.3389/fphy.2016.00029
On Electron Adiabaticity in Collisionless Shocks2016-07-22T00:00:00ZHoria Comişelhttps://www.frontiersin.org/articles/10.3389/fphy.2015.00040
https://www.frontiersin.org/articles/10.3389/fphy.2015.00040
Broad current sheets, current bifurcation, and collisionless reconnection—An Opinion on “Onset of fast magnetic reconnection via subcritical bifurcation” by Z. Guo and X. Wang2015-06-08T00:00:00ZRudolf A. TreumannWolfgang Baumjohannhttps://www.frontiersin.org/articles/10.3389/fphy.2015.00034
https://www.frontiersin.org/articles/10.3389/fphy.2015.00034
Information kinetics—an extension2015-05-19T00:00:00ZRudolf A. TreumannWolfgang BaumjohannWe present an operator formalism for the recently developed kinetic information theory, construct Poisson brackets between the Liouville and information operators in μ space, proposing its quantum version. Making use of the universal quantum of time, the Planck time τ_{p}, a pseudo-energy-time uncertainty relation is constructed. It suggests that tiny amounts of information production may cause large variations in energy. The Hubble time τ_{H} sets an upper bound on information in the universe.]]>https://www.frontiersin.org/articles/10.3389/fphy.2015.00022
https://www.frontiersin.org/articles/10.3389/fphy.2015.00022
Ideal MHD turbulence: the inertial range spectrum with collisionless dissipation2015-04-09T00:00:00ZRudolf A. TreumannWolfgang BaumjohannYasuhito NaritaThe inertial range spectrum of ideal (collisionless/dissipationless) MHD turbulence is analyzed in view of the transition from the large-scale Iroshnikov-Kraichnan-like (IK) to the meso-scale Kolmogorov (K) range under the assumption that the ultimate dissipation which terminates the Kolmogorov range is provided by collisionless reconnection in thin turbulence-generated current sheets. Kolmogorov's dissipation scale is identified with the electron inertial scale, as suggested by collisionless particle-in-cell simulations of reconnection. Transition between the IK- and K-ranges occurs at the ion inertial length allowing determination of the IK-coefficient. With the electron inertial scale the K-dissipation scale, stationarity of the spectrum implies a relation between the energy injection and dissipation rates. Application to solar wind is critically discussed.]]>https://www.frontiersin.org/articles/10.3389/fphy.2015.00019
https://www.frontiersin.org/articles/10.3389/fphy.2015.00019
Kinetic theory of information—the dynamics of information2015-03-30T00:00:00ZRudolf A. TreumannWolfgang BaumjohannA kinetic approach to the notion of information is proposed, based on Liouville kinetic theory. The general kinetic equation for the evolution of the N-particle information _{N} in a Hamiltonian system of large particle number N ≫ 1 is obtained. It is shown that the N-particle information is strictly conserved. Defining reduced particle number information densities in phase space should be possible to obtain a kinetic equation for the ordinary one-particle information _{1} ≡ following the Bogoliubov prescription. The kinetic equation for is a kind of generalized Boltzmann equation with interaction term depending on the hierarchy of reduced informations. This term in its general form is the most general expression for the Kolmogorov entropy rate of evolution of the information.]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00076
https://www.frontiersin.org/articles/10.3389/fphy.2014.00076
Lessons on collisionless reconnection from quantum fluids2014-12-15T00:00:00ZYasuhito NaritaWolfgang BaumjohannMagnetic reconnection in space plasmas remains a challenge in physics in that the phenomenon is associated with the breakdown of frozen-in magnetic field in a collisionless medium. Such a topology change can also be found in superfluidity, known as the quantum vortex reconnection. We give a plasma physicists' view of superfluidity to obtain insights on essential processes in collisionless reconnection, including discussion of the kinetic and fluid pictures, wave dynamics, and time reversal asymmetry. The most important lesson from the quantum fluid is the scenario that reconnection is controlled by the physics of topological defects on the microscopic scale, and by the physics of turbulence on the macroscopic scale. Quantum vortex reconnection is accompanied by wave emission in the form of Kelvin waves and sound waves, which imprints the time reversal asymmetry.]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00059
https://www.frontiersin.org/articles/10.3389/fphy.2014.00059
The strongest magnetic fields in the universe: how strong can they become?2014-10-16T00:00:00ZRudolf A. TreumannWolfgang BaumjohannAndré BaloghMagnetic fields in the universe are in general weak, of the order of μGauss only. However, in compact objects they assume extraordinarily large values. These are produced by gravitational collapse of massive magnetized objects. Clearly, fields in the massive progenitor are energetically limited by the available energy which can be fed into the generation of currents and magnetic fields. However, when collapsing down to small scales magnetic fields become superstrong exceeding any limits which can be reached in the laboratory. A brief review and discussion is given on the absolute limitation to the magnetic field strengths which can be obtained during such collapses.]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00049
https://www.frontiersin.org/articles/10.3389/fphy.2014.00049
Beyond Gibbs-Boltzmann-Shannon: general entropies—the Gibbs-Lorentzian example2014-08-14T00:00:00ZRudolf A. TreumannWolfgang BaumjohannWe propose a generalization of Gibbs' statistical mechanics into the domain of non-negligible phase space correlations. Derived are the probability distribution and entropy as a generalized ensemble average, replacing Gibbs-Boltzmann-Shannon's entropy definition enabling construction of new forms of statistical mechanics. The general entropy may also be of importance in information theory and data analysis. Application to generalized Lorentzian phase space elements yields the Gibbs-Lorentzian power law probability distribution and statistical mechanics. The corresponding Boltzmann, Fermi and Bose-Einstein distributions are found. They apply only to finite temperature states including correlations. As a by-product any negative absolute temperatures are categorically excluded, supporting a recent “no-negative T” claim.]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00047
https://www.frontiersin.org/articles/10.3389/fphy.2014.00047
Alfvén wave characteristics of equatorial plasma irregularities in the ionosphere derived from CHAMP observations2014-08-11T00:00:00ZHermann LührJaeheung ParkChao XiongJan RaubergWe report magnetic field observations of the components transverse to the main field in the frequency range 1–25 Hz from times of equatorial plasma irregularity crossings. These field variations are interpreted as Alfvénic signatures accompanying intermediate-scale (150 m–4 km) plasma density depletions. Data utilized are the high-resolution CHAMP magnetic field measurements sampled at 50 Hz along the north-south satellite track. The recorded signals do not reflect the temporal variation but the spatial distribution of Alfvénic signatures. This is the first comprehensive study of Alfvénic signatures related to equatorial plasma bubbles that covers the whole solar cycle from 2000 to 2010. A detailed picture of the wave characteristics can be drawn due to the large number (almost 9000) of events considered. Some important findings are: Alfvénic features are a common feature of intermediate-scale plasma structures. The zonal and meridional magnetic components are generally well correlated suggesting skewed current sheets. The sheets have an orientation that is on average deflect by about 32° away from magnetic east toward upward or downward depending on the hemisphere. We have estimated the Poynting flux flowing into the E region. Typical values are distributed over the range 10^{−8}–10^{−6} W/m^{2}. Large Poynting fluxes are related to steep spectra of the Alfvénic signal, which imply passages through regularly varying electron density structures. No dependence of the Poynting flux level on solar activity has been found. But below a certain solar flux value (F10.7 < 100 sfu) practically no events are detected. There is a clear tendency that large Poynting flux events occur preferably at early hours after sunset (e.g., 20:00 local time). Toward later times the occurrence peak shifts successively toward lower energy levels. Finally we compare our observations with the recently published results of the high-resolution 3-D model simulations by Dao et al. [1].]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00029
https://www.frontiersin.org/articles/10.3389/fphy.2014.00029
Fractional Laplace transforms—a perspective2014-06-10T00:00:00ZRudolf A. TreumannWolfgang BaumjohannA new form of the Laplace transform is reviewed as a paradigm for an entire class of fractional functional transforms. Various of its properties are discussed. Such transformations should be useful in application to differential/integral equations or problems in non-extensive statistical mechanics.]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00015
https://www.frontiersin.org/articles/10.3389/fphy.2014.00015
Systematic study of intermediate-scale structures of equatorial plasma irregularities in the ionosphere based on CHAMP observations2014-03-19T00:00:00ZHermann LührChao XiongJaeheung ParkJan RaubergEquatorial spread-F ionospheric plasma irregularities on the night-side, commonly called equatorial plasma bubbles (EPB), include electron density variations over a wide range of spatial scales. Here we focus on intermediate-scale structures ranging from 100 m to 10 km, which play an important role in the evolution of EPBs. High-resolution CHAMP magnetic field measurements sampled along north–south track at 50 Hz are interpreted in terms of diamagnetic effect for illustrating the details of electron density variations. We provide the first comprehensive study on intermediate-scale density structures associated with EPBs, covering a whole solar cycle from 2000 to 2010. The large number of detected events, almost 9000, allows us to draw a detailed picture of the plasma fine structure. The occurrence of intermediate-scale events is strongly favored by high solar flux. During times of F10.7 <100 sfu practically no events were observed. The longitudinal distribution of our events with respect to season or local time agrees well with that of the EPBs, qualifying the fine structure as a common feature, but the occurrence rates are smaller by a factor of 4 during the period 2000–2005. Largest amplitude electron density variations appear at the poleward boundaries of plasma bubbles. Above the dip-equator recorded amplitudes are small and fall commonly below our resolution. Events can generally be found at local times between 19 and 24 LT, with a peak lasting from 20 to 22 LT. The signal spectrum can be approximated by a power law. Over the frequency range 1–25 Hz we observe spectral indices between −1.4 and −2.6 with peak occurrence rates around −1.9. There is a weak dependence observed of the spectral index on local time. Toward later hours the spectrum becomes shallower. Similarly for the latitude dependence, there is a preference of shallower spectra for latitudes poleward of the ionization anomaly crest. Our data suggest that the generation of small plasma structures is part of the early stage EPB development, during times when the vertical plasma drift surpasses a certain threshold.]]>https://www.frontiersin.org/articles/10.3389/fphy.2014.00013
https://www.frontiersin.org/articles/10.3389/fphy.2014.00013
Spatial structure of ion-scale plasma turbulence2014-03-05T00:00:00ZYasuhito NaritaHoria ComiselUwe MotschmannSpatial structure of small-scale plasma turbulence is studied under different conditions of plasma parameter beta directly in the three-dimensional wave vector domain. Two independent approaches are taken: observations of turbulent magnetic field fluctuations in the solar wind measured by four Cluster spacecraft, and direct numerical simulations of plasma turbulence using the hybrid code AIKEF, both resolving turbulence on the ion kinetic scales. The two methods provide independently evidence of wave vector anisotropy as a function of beta. Wave vector anisotropy is characterized primarily by an extension of the energy spectrum in the direction perpendicular to the large-scale magnetic field. The spectrum is strongly anisotropic at lower values of beta, and is more isotropic at higher values of beta. Cluster magnetic field data analysis also provides evidence of axial asymmetry of the spectrum in the directions around the large-scale field. Anisotropy is interpreted as filament formation as plasma evolves into turbulence. Axial asymmetry is interpreted as the effect of radial expansion of the solar wind from the corona.]]>https://www.frontiersin.org/articles/10.3389/fphy.2013.00031
https://www.frontiersin.org/articles/10.3389/fphy.2013.00031
Collisionless magnetic reconnection in space plasmas2013-12-31T00:00:00ZRudolf A. TreumannWolfgang BaumjohannMagnetic reconnection, the merging of oppositely directed magnetic fields that leads to field reconfiguration, plasma heating, jetting and acceleration, is one of the most celebrated processes in collisionless plasmas. It requires the violation of the frozen-in condition which ties gyrating charged particles to the magnetic field inhibiting diffusion. Ongoing reconnection has been identified in near-Earth space as being responsible for the excitation of substorms, magnetic storms, generation of field aligned currents and their consequences, the wealth of auroral phenomena. Its theoretical understanding is now on the verge of being completed. Reconnection takes place in thin current sheets. Analytical concepts proceeded gradually down to the microscopic scale, the scale of the electron skin depth or inertial length, recognizing that current layers that thin do preferentially undergo spontaneous reconnection. Thick current layers start reconnecting when being forced by plasma inflow to thin. For almost half a century the physical mechanism of reconnection has remained a mystery. Spacecraft in situ observations in combination with sophisticated numerical simulations in two and three dimensions recently clarified the mist, finding that reconnection produces a specific structure of the current layer inside the electron inertial (also called electron diffusion) region around the reconnection site, the X line. Onset of reconnection is attributed to pseudo-viscous contributions of the electron pressure tensor aided by electron inertia and drag, creating a complicated structured electron current sheet, electric fields, and an electron exhaust extended along the current layer. We review the general background theory and recent developments in numerical simulation on collisionless reconnection. It is impossible to cover the entire field of reconnection in a short space-limited review. The presentation necessarily remains cursory, determined by our taste, preferences, and knowledge. Only a small amount of observations is included in order to support the few selected numerical simulations.]]>