Electron Pitch Angle Distributions Research Articles

Electron Pitch Angle Distributions in Compressional Pc5 Waves by THEMIS‐A Observations

AbstractAlthough compressional Pc5 waves are well known in the energy conversion and regulation of charged particles in the magnetosphere, the detailed features of associated electron pitch angle distributions (ePADs) are poorly understood. Based on Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations from 2010 to 2016, ePADs in the magnetic decreases (troughs) of compressional Pc5 waves are classified into three types: cigar, butterfly, and pancake. They are found in the electrons with respective energy ranges of 100–1,000 eV (larger than 10 keV), 1–10 keV (larger than 1 keV), and 6–30 keV (100 eV–10 keV) in (non)whistler‐type compressional Pc5 waves, that is, cases where whistler‐mode waves are present (absent) in the magnetic troughs. These statistical ePAD features observed in the whistler‐ and nonwhistler‐type waves are suggested to be associated with the Landau resonance of whistler‐mode waves and drift‐shell splitting effect.

Modulation of Whistler Mode Waves by Ultra‐Low Frequency Wave in a Macroscale Magnetic Hole: MMS Observations

AbstractWe report the quasi‐periodic whistler‐mode waves corresponding to electron temperature anisotropy T⊥/T// < 1 at the center of a macroscale magnetic hole (MH) near the Earth's magnetopause observed by the Magnetospheric Multiscale mission. A significant ultra‐low frequency (ULF) wave of background magnetic field was observed in this MH. The ULF wave dramatically exacerbates the evolution of MH and the formation of the donut‐shaped electron pitch angle distribution (PAD) in the macroscale MH. When the whistler‐mode waves were generated, they are consistent with troughs of the ULF wave and density increases, which is also related to the butterfly type PAD at those moments. The analysis of dispersion relation and cyclotron resonance condition indicate that whistler‐mode waves were mainly generated by the butterfly type PAD of electrons. Our results provides new insights into one possible excitation mechanism of whistler‐mode wave.

Characteristics of Suprathermal Electrons in Small-Scale Magnetic Flux Ropes and Their Implications on the Magnetic Connection to the Sun

Small-scale magnetic flux ropes (SMFRs) are observed more frequently than larger-scale magnetic flux ropes (e.g., magnetic clouds) in interplanetary space. We selected 235 SMFRs by applying cylindrical linear force-free fitting to 20-year observations of the Wind satellite, which meets the criteria of low beta, low temperature, an enhanced magnetic field, and a rotation feature. By examining the pitch angle distribution of suprathermal electrons for these events, we found that approximately 45.1% of the SMFRs were accompanied by unidirectional beams (strahl). A much smaller percentage of SMFRs (∼10.7%) were associated with bidirectional beams. We also found a small percentage (∼7.2%) of (sunward) conic distributions during SMFR events. Last, the remaining ∼37.0% of SMFRs were associated with complex electron distributions. The unidirectional beams and most of the conics (together corresponding to ∼50% of the total 235 SMFRs) imply open-field SMFRs with only one end connected to the Sun. For ∼37.7% of the unidirectional beam SMFRs, the local IMF field polarity was orthogonal or inverted (possibly due to interchange reconnection). Based on the solar wind conditions around the bidirectional beams, we suggest that more than half of the bidirectional beams were not necessarily closed-field-line SMFRs.

Formation of Pancake, Rolling Pin, and Cigar Distributions of Energetic Electrons at the Dipolarization Fronts (DFs) Driven by Magnetic Reconnection: A Two‐Dimensional Particle‐In‐Cell Simulation

AbstractA two‐dimensional particle‐in‐cell (PIC) simulation is performed to study the formation of the pitch angle distribution of energetic electrons at dipolarization fronts (DFs) driven by magnetic reconnection. The energetic electrons at DFs are originated from the lobe region, and experience a two‐step acceleration process at the reconnection x‐line and the DFs, respectively. Three kinds of pitch angle distributions commonly observed in the magnetotail, Pancake, Rolling pin, and Cigar distributions, are formed in sequence during the propagation of the DFs. In the early stage, Pancake distributions are formed through betatron acceleration. During this stage, the flux of energetic electrons with pitch angles around 0° and 180° is low because these electrons have no time to be reflected many times at the DFs to obtain sufficient Fermi acceleration. However, the electrons with pitch angles around 90° are difficult to be trapped around the DFs for a long time, and their flux saturates quickly; while the electrons with pitch angles around 0° and 180° can be trapped inside the closed field lines and they get continuous Fermi acceleration during the propagation of the DFs. Therefore, in the later stage, the flux of energetic electrons with pitch angles around 0° and 180° gradually increases and at last exceeds that of energetic electrons with pitch angles around 90°, forming Rolling pin and Cigar distributions in sequence.

Statistical Characteristics of Energetic Electron Pitch Angle Distributions in the Van Allen Probe Era: 1. Butterfly Distributions with Flux Peaks at Preferred Pitch Angles

Statistical Characteristics of Energetic Electron Pitch Angle Distributions in the Van Allen Probe Era: 1. Butterfly Distributions with Flux Peaks at Preferred Pitch Angles

Statistical Characteristics of Energetic Electron Pitch-Angle Distributions in the Van Allen Probe Era

Statistical Characteristics of Energetic Electron Pitch-Angle Distributions in the Van Allen Probe Era

Cassini Observation of Relativistic Electron Butterfly Distributions in Saturn’s Inner Radiation Belts: Evidence for Acceleration by Local Processes

AbstractThe morphology of electron pitch angle distributions (PADs) helps to identify dynamic processes in Saturn’s magnetosphere. Previous studies demonstrated convective transport being important for relativistic electron acceleration at L > 4 in the inner magnetosphere, whereas closer to Saturn the situation is not as well established. We have investigated the PADs of relativistic electrons throughout radiation belts, using 13‐years measurements from Cassini spacecraft. Our results show that at 3.5 < L < 6, the PADs peak near 90°. However, at 2.5 < L < 3.5, the PADs transform to butterfly distributions, dominating over an energy range of one order of magnitude, from 800 keV to >7 MeV. These results suggest that convective transport continues to govern down to L = 3.5. For even smaller L‐shells, the independence of butterfly PADs from energy suggests that the acceleration by resonant interactions of electrons with waves likely serves as an additional key mechanism regulating the dynamics in the core region of radiation belts.

Numerical simulation of chorus-driving acceleration of relativistic electrons at extremely low L-shell during geomagnetic storms* *Project supported by the National Natural Science Foundation of China (Grant Nos. 41904149 and 12173038), Stable-Support Scientific Project of ChinaResearch Institute of Radiowave Propagation (Grant No. A132001W07), and the National Institute of Natural Hazards, Ministry of Emergency Management of China (Grant No. 2021-JBKY-11).

During 2018 major geomagnetic storm, relativistic electron enhancements in extremely low L-shell regions (reaching L∼3) have been reported based on observations of ZH-1 and Van Allen probes satellites, and the storm is highly likely to be accelerated by strong whistler-mode waves occurring near very low L-shell regions where the plasmapause was suppressed. It is very interesting to observe the intense chorus-accelerated electrons locating in such low L-shells and filling into the slot region. In this paper, we further perform numerical simulation by solving the two-dimensional Fokker–Planck equation based on the bounce-averaged diffusion rates. Numerical results demonstrate the evolution processes of the chorus-driven electron flux and confirm the flux enhancement in low pitch angle ranges (20°–50°) after the wave-particle interaction for tens of hours. The simulation result is consistent with the observation of potential butterfly pitch angle distributions of relativistic electrons from both ZH-1 and Van Allen probes.

Electron Trapping in Magnetic Mirror Structures at the Edge of Magnetopause Flux Ropes

AbstractFlux ropes are a proposed site for particle energization during magnetic reconnection, with several mechanisms proposed. Here, Magnetospheric Multiscale mission observations of magnetic mirror structures on the edge of two ion‐scale magnetopause flux ropes are presented. Donut‐shaped features in the electron pitch angle distributions provide evidence for electron trapping in the structures. Furthermore, both events show trapping with extended 3D structure along the body of the flux rope. Potential formation mechanisms, such as the magnetic mirror instability, are examined and the evolutionary states of the structures are compared. Pressure and force analysis suggest that such structures could provide an important electron acceleration mechanism for magnetopause flux ropes, and for magnetic reconnection more generally.

Pitch-angle distribution of accelerated electrons in 3D current sheets with magnetic islands

Aims. This research aims to explore variations of electron pitch-angle distributions (PADs) during spacecraft crossing of reconnecting current sheets (RCSs) with magnetic islands. Our results can benchmark the sampled characteristic features with realistic PADs derived from in situ observations. Methods. Particle motion is simulated in 2.5D Harris-type RCSs using the particle-in-cell method and considering the plasma feedback to electromagnetic fields induced by accelerated particles. We evaluate particle energy gains and PADs in different locations with virtual spacecraft passing the current sheet while moving in the different directions. The RCS parameters are comparable to heliosphere and solar wind conditions. Results. The energy gains and the PADs of particles would change depending on the specific topology of the magnetic fields. In addition, the observed PADs also depend on the crossing paths of the spacecraft. When the guiding field is weak, the bi-directional electron beams (strahls) are mainly present inside the islands and are located just above or below the X-nullpoints in the inflow regions. The magnetic field relaxation near the X-nullpoint alters the PADs towards 90°. As the guiding field becomes larger, the regions with bi-directional strahls are compressed towards small areas in the exhausts of RCSs. Mono-directional strahls are quasi-parallel to the magnetic field lines near the X-nullpoint due to the dominant Fermi-type magnetic curvature-drift acceleration. Meanwhile, the high-energy electrons confined inside magnetic islands create PADs of around 90°. Conclusions. Our results link the electron PADs to local magnetic structures and the directions of spacecraft crossings. This can help to explain a variety of the PAD features reported in recent observations in the solar wind and the Earth’s magnetosphere.

Electron Pitch Angle Distributions Around Dipolarization Fronts at the Off Magnetic Equator

AbstractThe pitch angle distributions of energetic electrons (10 s–200 keV) around the dipolarization front (DF) have been studied extensively. However, the pitch angle distributions of energetic electrons around the DF at the off magnetic equator have been unclear so far. In this study, we present the observations of the DF at the off magnetic equator in the near‐Earth tail. Some properties of the DF at the off magnetic equator are presented. Importantly, the triple‐peaked pitch angle distribution of energetic electrons around the DF is found. Such a new type of distribution is a peak at around 22.5°, 90°, and 157.5°, which is a combinational consequence of betatron acceleration, Fermi acceleration, and magnetic mirror effect. Using an analytical model, we quantitatively reproduce the processes of betatron acceleration, Fermi acceleration, and magnetic mirror effect. These results will further enhance our understanding of the electron dynamics around DFs.

Evolution of Pitch Angle Distributions of Relativistic Electrons During Geomagnetic Storms: Van Allen Probes Observations

AbstractWe present a study analyzing relativistic and ultra relativistic electron energization and the evolution of pitch angle distributions using data from the Van Allen Probes. We study the connection between energization and isotropization to determine if there is a coherence across storms and across energies. Pitch angle distributions are fit with a J0sinnθ function, and the variable “n” is characterized as the pitch angle index and tracked over time. Our results show that consistently across all storms with ultra relativistic electron energization, electron distributions are most anisotropic within around a day of Dstmin and become more isotropic in the following week. Also, each consecutively higher energy channel is associated with higher anisotropy after storm main phase. Changes in the pitch angle index are reflected in each energy channel; when 1.8 MeV electron pitch angle distributions increase (or decrease) in pitch angle index, so do the other energy channels. We show that the peak anisotropies differ between coronal mass ejection (CME)‐ and corotating interaction region (CIR)‐driven storms and measure the relaxation rate as the anisotropy falls after the storm. The isotropization rate in pitch angle index for CME‐driven storms is −0.15 ± 0.02 day−1 at 1.8 MeV, −0.30 ± 0.01 day−1 at 3.4 MeV, and −0.39 ± 0.02 day−1 at 5.2 MeV. For CIR‐driven storms, the isotropization rates are −0.10 ± 0.01 day−1 for 1.8 MeV, −0.13 ± 0.02 day−1 for 3.4 MeV, and −0.11 ± 0.02 day−1 for 5.2 MeV. This study shows that there is a global coherence across energies and that storm type may play a role in the evolution of electron pitch angle distributions.

Non-storm erosion of MeV electron outer radiation belt down to <em>L</em>* < 4.0 associated with successive enhancements of solar wind density

We report an unusual non-storm erosion event of outer zone MeV electron distribution during three successive solar wind number density enhancements (SWDEs) on November 27−30, 2015. Loss of MeV electrons and energy-dependent narrowing of electron pitch angle distributions (PAD) first developed at <italic>L</italic>* = 5.5 and then moved down to <italic>L</italic>* &lt; 4. According to the evolution of the electron phase space density (PSD) profile, losses of electrons with small pitch angles at <italic>L</italic>* &gt; 4 during SWDE1 are mainly due to outward radial diffusion. However during SWDE2&amp;3, scattering loss due to EMIC waves is dominant at 4 &lt; <italic>L</italic>* &lt; 5. As for electrons with large pitch angles, outward radial diffusion is the primary loss mechanism throughout all SWDEs which is consistent with the incursion of the Last Closed Drift Shell (LCDS). The inner edge of EMIC wave activity moved from <italic>L</italic>* ~5 to <italic>L</italic>* ~4 and from <italic>L</italic> ~6.4 to <italic>L</italic> ~4.2 from SWDE1 to SWDE2&amp;3, respectively, observed by Van Allen Probes and by ground stations. This is consistent with the inward penetration of anisotropic energetic protons from <italic>L</italic>* = 4.5 to <italic>L</italic>* = 3.5, suggesting that the inward extension of EMIC waves may be driven by the inward injection of anisotropic energetic protons from the dense plasma sheet.

Electron Energy Spectra and e–e Bremsstrahlung from Anisotropic Electron Distributions in Extreme Solar Flares

We study the electron energy spectra of two powerful solar flares SOL2003-10-28T 11:06:16 (GOES class X17.2) and SOL2002-07-23T00:18:16 (X4.8) on the basis of HXR and gamma-ray spectra obtained from RHESSI data. Electron-electron (e-e) bremsstrahlung makes a significant contribution to the X-ray flux at energies above 500 keV. In X-ray flux calculations different electron pitch-angle distributions were considered: quasi-transverse and quasi-longitudinal. It was shown that breaks in hard X-ray spectra occur on energies $\epsilon\approx100$ keV with photon spectral-index difference $\Delta\gamma\ge 1$ in the case of quasi-longitudinal anisotropy and hard power-law electron energy spectra. We deduce the parameters of non-thermal electrons energy spectra from RHESSI data for the extreme solar flares of 2003 October 28 and 2002 July 23. After including e-e bremsstrahlung, the non-thermal electrons energy spectra obtained in the thick-target model can be fitted using power-law with spectral index $\delta\approx 5$. For the 2002 July 23 flare, there is a pronounced asymmetry towards the southern footpoint, starting from energies above 100 keV. The separation of HXR sources at energies above 100 keV may be related to a feature of relativistic electron transport in flaring loops.

Magnetospheric Multiscale Observations of the Off-equatorial Dipolarization Front Dynamics in the Terrestrial Magnetotail

Abstract We report the Magnetospheric Multiscale observations of dynamics at an off-equatorial dipolarization front (DF) in the Terrestrial Magnetotail. Three different plasma waves, namely electromagnetic ion cyclotron (EMIC) waves, lower hybrid drift waves, and electrostatic solitary waves (ESWs), associated with different electron pitch angle distributions were detected at different subregions of a single DF. It is interesting to note that the EMIC wave was linearly polarized, associating with a parallel current as a result of the antiparallel drift of electrons in the energy range of about 0.3–2 keV. These suggest that the wave was most likely to be locally generated. This generation could be explained by the current-driven kink-like instability due to the electron drift. The current-driven instabilities may dissipate the energy of the field-aligned current at the DF and thus play important roles in the magnetosphere–ionosphere coupling. On the other hand, the detected ESWs are interpreted as multidimensional electromagnetic electron holes (EHs) which are manifestations of several distinguishing features in electric and magnetic field perturbation. The EHs with a strong positive central potential suggested the likelihood of nonlinear behavior.

Internal structures of the ion-scale flux rope associated with dayside magnetopause reconnection

Ion-scale magnetic flux ropes have been observed in the reconnection diffusion region. In this paper, we show a typical ion-scale flux rope observed by the Magnetospheric Multiscale spacecraft at the subsolar magnetopause. The most intriguing feature of this flux rope is that the rapid variations of electron density are correlated with the changes of electron temperature and pitch angle distribution inside the flux rope. In the region of low electron density, the perpendicular temperature is higher than parallel temperature. Furthermore, the electrons within higher energies (>70 eV) are mainly trapped near pitch angle $\sim 90^{\circ}$, but electrons with lower energies (<70 eV) are missing. However, in the region of high electron density, perpendicular temperature is lower than the parallel temperature. In addition, the pitch angle of electrons with higher energies is essentially field-aligned, and the fluxes of electrons with lower energies rapidly increase. It implies that the coexistence of different magnetic field line topologies inside the ion-scale flux rope is responsible for the observed electron pitch angle distributions within the flux rope. These results can be useful for understanding the reconnection process in three-dimension and the generation mechanism of the ion-scale flux rope.

The Heliospheric Current Sheet and Plasma Sheet during Parker Solar Probe’s First Orbit

Abstract We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe’s (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP’s first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer.

Electron dynamics near diamagnetic regions of comet 67P/Churyumov- Gerasimenko

The Rosetta spacecraft detected transient and sporadic diamagnetic regions around comet 67P/Churyumov-Gerasimenko. In this paper we present a statistical analysis of bulk and suprathermal electron dynamics, as well as a case study of suprathermal electron pitch angle distributions (PADs) near a diamagnetic region. Bulk electron densities are correlated with the local neutral density and we find a distinct enhancement in electron densities measured over the southern latitudes of the comet. Flux of suprathermal electrons with energies between tens of eV to a couple of hundred eV decreases each time the spacecraft enters a diamagnetic region. We propose a mechanism in which this reduction can be explained by solar wind electrons that are tied to the magnetic field and after having been transported adiabatically in a decaying magnetic field environment, have limited access to the diamagnetic regions. Our analysis shows that suprathermal electron PADs evolve from an almost isotropic outside the diamagnetic cavity to a field-aligned distribution near the boundary. Electron transport becomes chaotic and non-adiabatic when electron gyroradius becomes comparable to the size of the magnetic field line curvature, which determines the upper energy limit of the flux variation. This study is based on Rosetta observations at around 200 ​km cometocentric distance when the comet was at 1.24 AU from the Sun and during the southern summer cometary season.

Characterisation of suprathermal electron pitch-angle distributions

Context. Suprathermal electron pitch-angle distributions (PADs) contain substantial information about the magnetic topology of the solar wind. Their characterisation and quantification allow us to automatically identify periods showing certain characteristics. Aims. This work presents a robust automatic method for the identification and statistical study of two different types of PADs: bidirectional suprathermal electrons (BDE, often associated with closed magnetic structures) and isotropic (likely corresponding to solar-detached magnetic field lines or highly scattered electrons). Methods. Spherical harmonics were fitted to the observed suprathermal PADs of the 119–193 eV energy channel of STEREO/SWEA from March 2007 to July 2014, and they were characterised using signal processing analysis in order to identify periods of isotropic and bidirectional PADs. The characterisation has been validated by comparing the results obtained here with those of previous studies. Results. Interplanetary coronal mass ejections (ICMEs) present longer BDE periods inside the magnetic obstacles. A significant amount of BDE remain after the end of the ICME. Isotropic PADs are found in the sheath of the ICMEs, and at the post-ICME region likely due to the erosion of the magnetic field lines. Both isotropy and BDE are solar-cycle dependent. The isotropy observed by STEREO shows a nearly annual periodicity, which requires further investigation. There is also a correspondence between the number of ICMEs observed and the percentage of time showing BDE. Conclusions. A method to characterise PADs has been presented and applied to the automatic identification of two relevant distributions that are commonly observed in the solar wind, such as BDE and isotropy. Four catalogues (STEREO-A and STEREO-B for isotropic and BDE periods of at least 10 min) based on this identification are provided for future applications.

On the loss mechanisms of radiation belt electron dropouts during the 12 September 2014 geomagnetic storm

Radiation belt electron dropouts indicate electron flux decay to the background level during geomagnetic storms, which is commonly attributed to the effects of wave-induced pitch angle scattering and magnetopause shadowing. To investigate the loss mechanisms of radiation belt electron dropouts triggered by a solar wind dynamic pressure pulse event on 12 September 2014, we comprehensively analyzed the particle and wave measurements from Van Allen Probes. The dropout event was divided into three periods: before the storm, the initial phase of the storm, and the main phase of the storm. The electron pitch angle distributions (PADs) and electron flux dropouts during the initial and main phases of this storm were investigated, and the evolution of the radial profile of electron phase space density (PSD) and the (<italic>μ</italic>, <italic>K</italic>) dependence of electron PSD dropouts (where <italic>μ</italic>, <italic>K</italic>, and <italic>L</italic>* are the three adiabatic invariants) were analyzed. The energy-independent decay of electrons at <italic>L</italic> &gt; 4.5 was accompanied by butterfly PADs, suggesting that the magnetopause shadowing process may be the major loss mechanism during the initial phase of the storm at <italic>L</italic> &gt; 4.5. The features of electron dropouts and 90°-peaked PADs were observed only for &gt;1 MeV electrons at <italic>L</italic> &lt; 4, indicating that the wave-induced scattering effect may dominate the electron loss processes at the lower <italic>L</italic>-shell during the main phase of the storm. Evaluations of the (<italic>μ</italic>, <italic>K</italic>) dependence of electron PSD drops and calculations of the minimum electron resonant energies of H⁺-band electromagnetic ion cyclotron (EMIC) waves support the scenario that the observed PSD drop peaks around <italic>L</italic>* = 3.9 may be caused mainly by the scattering of EMIC waves, whereas the drop peaks around <italic>L</italic>* = 4.6 may result from a combination of EMIC wave scattering and outward radial diffusion.