Energetic Electron Pitch Angle Distributions Research Articles

On the Formation of Trapped Electron Radiation Belts at Ganymede

AbstractThis study presents evidence of stably trapped electrons at Jupiter's moon Ganymede. We model energetic electron pitch angle distributions and compare them to observations from the Galileo Energetic Particle Detector to identify signatures of trapped particles during the G28 encounter. We trace electron trajectories to show that they enter Ganymede's mini‐magnetospheric environment, become trapped, and drift around the moon for up to 30 min, in some cases stably orbiting the moon multiple times. Conservation of the first adiabatic invariant partially contributes to energy changes throughout the electrons' orbits, with additional acceleration driven by local electric fields, before they return to Jupiter's magnetosphere or impact the surface. These trapped particles manifest as an electron population with an enhanced flux compared to elsewhere within the mini‐magnetosphere that are detectable by future spacecraft.

Constraining the Influence of Callisto's Perturbed Electromagnetic Environment on Energetic Particle Observations

AbstractThis study focuses on constraining the role that Callisto's perturbed electromagnetic environment had on energetic charged particle signatures observed during the Galileo mission. To do so, we compare data from the Energetic Particle Detector (EPD) obtained during four close encounters of the moon with a model framework that combines hybrid simulations for low‐energy plasma and test‐particle tracing simulations for high‐energy particles. By comparing model results for energetic particle dynamics in both uniform and perturbed electromagnetic fields, we systematically disentangle the role that geometric effects (i.e., absorption of particles by Callisto's solid surface) have on observed energetic particle signatures compared to those associated with Callisto's perturbed electromagnetic environment (generated by the moon's induced magnetic field and plasma interaction currents). We show that observed flux drop‐outs in the energetic ion pitch angle distributions (PADs) are largely driven by their absorption by Callisto's surface: their large gyroradii exceed the size of the moon, facilitating their impact onto the icy surface and preventing their detection by EPD. However, features observed in the energetic electron PADs can only be explained with an accurate representation of the moon's perturbed environment, since electrons closely follow the orientation of the electromagnetic fields. Our findings therefore illustrate the key role that the moon's induced field and magnetospheric plasma interaction have on the dynamics of energetic electrons, emphasizing the importance of accurately modeling Callisto's locally perturbed electromagnetic environment when attempting to interpret data from past and future encounters, including those anticipated from the upcoming JUICE mission.

Energetic Electron Pitch-angle Distributions in the Martian Space Environment: Pancake

We perform a statistical investigation of the occurrence rates of energetic electron (100–500 eV) pancake pitch-angle distributions (PADs) in the Martian space environment by utilizing 6 yr of MAVEN data. In the Martian ionosphere, we find the following: (1) at the same altitude in the terminator and night regions, the occurrences rates in the center of the southern magnetic anomaly regions are very low, but at the edges of strong magnetic fields, they increase significantly; (2) the occurrence rates of energetic electron perpendicular anisotropies on the Martian dayside increase with altitude; and (3) some closed magnetic lines in the 10°S–55°S, 30°W–125°W region at 400–800 km altitude gradually become open during the rotation of Mars from duskside to dawnside, while more closed magnetic lines are produced in the 40°S–65°S, 35°E–90°E region. In the Martian induced magnetosphere, we find the following: (1) the high-energy electron perpendicular anisotropy in the magnetosheath is the most significant; (2) the occurrence rates in the southern (Z MSO ≤−1 R M) magnetosheath are higher than those in the northern (Z MSO ≥ 1 R M) magnetosheath; (3) in the region of ∣Z MSO∣ < 0.5 R M, these high-energy electron pancake PADs are mainly concentrated in the magnetosheath region with Y MSO ∈ [−1.4RM, 2R M]; (4) the occurrence rates in the dawnside (Y MSO ≤−1 R M) magnetosheath are higher than those in the duskside (Y MSO ≥ 1 R M) magnetosheath; and (5) in the region of ∣Y MSO∣ < 0.5 R M, the occurrence rates throughout the magnetosheath are very high.

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

AbstractWe present a statistical study on the properties of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt, for the first time based on particle measurements from the entire Van Allen Probes mission. A detailed selection criteria is used to identify intervals when flux measurements at energies from 0.2 to 3.4 MeV are available across a wide range of pitch angles close to the geomagnetic equatorial plane. To better characterize the shape of each pitch angle distribution, the flux data is fitted to functions of the equatorial pitch angle based on Legendre polynomials. Using this technique, we show that the shape of the PADs strongly depends on the particle's energy, location and geomagnetic activity. These results are used to identify the dominant physical processes responsible for creating PADs with different shapes. The results presented here mainly focus on the occurrence statistics and properties of butterfly PADs. Significantly, we find clear evidence that butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the flux peaks at equatorial pitch angles ∼35° appear consistent with magnetopause shadowing, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude.

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

Position of the Energetic Electron Trapping Boundary Relative to Auroral Oval Boundaries during the Magnetic Storm on December 19–22, 2015, Based on Data from the Meteor-M2 Satellite

This paper studies the position of the trapping boundary of electrons with energies of >100 keV relative to the equatorial boundary of the auroral oval during a large magnetic storm on December 19–22, 2015, with a minimum Dst of –170 nT as measured by the Meteor-M2–1 satellite. Energetic electrons with energies from 0.1 to 13 MeV and fluxes of low-energy electrons with energies from 0.13 to 16.64 keV have been measured. It is taken into account that the pitch-angle distribution of energetic electrons near the trapping boundary is almost isotropic. It is shown that the energetic electron trapping boundary during the considered storm is detected inside the auroral oval or near its polar boundary. The distance along the geomagnetic latitude between the energetic electron trapping boundary and the equatorial boundary of the auroral oval is determined. The dependence of this distance on time for crossings of the oval before and after midnight is analyzed. It is shown that the distance between the trapping boundary and the equatorial boundary of the oval during the storm decreases after midnight and increases before midnight. These values are almost equal near minimum Dst. The significance of the results obtained for a description of changes in the magnetospheric topology during magnetic storms is discussed.

On the Contribution of EMIC Waves to the Reconfiguration of the Relativistic Electron Butterfly Pitch Angle Distribution Shape on 2014 September 12—A Case Study*

Abstract Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron–Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L &lt; 5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.

Energetic Electron Pitch Angle Distributions During the Cassini Final Orbits

AbstractPitch angle distributions (PADs) of very energetic electrons (110–365 keV) are examined during the ring‐grazing and proximal orbits of the Cassini spacecraft, from day 320 2016 (15 November) to day 257 2017 (14 September). These repeating orbits allowed a statistical evaluation of the PADs within the magnetopause on the nightside of Saturn. Along L‐shells (i.e., equatorial crossing distances of magnetic field lines) near and outside that of Titan and north of the equator, the electron fluxes were unidirectionally field‐aligned going away from Saturn. Along L‐shells inside Titan's and south of the equator, the electrons had bidirectional or pancake (trapping) PADs. This behavior suggests that the field lines within Titan's L‐shell are generally closed, while those outside of that L‐shell are generally open. This result strictly applies only to the nightside local times sampled during the final Cassini orbits, but one may infer a similar behavior at other times.

Survey of radiation belt energetic electron pitch angle distributions based on the Van Allen Probes MagEIS measurements

AbstractA statistical survey of electron pitch angle distributions (PADs) is performed based on the pitch angle‐resolved flux observations from the Magnetic Electron Ion Spectrometer (MagEIS) instrument on board the Van Allen Probes during the period from 1 October 2012 to 1 May 2015. By fitting the measured PADs to a sinnα form, where α is the local pitch angle and n is the power law index, we investigate the dependence of PADs on electron kinetic energy, magnetic local time (MLT), the geomagnetic Kp index, and L shell. The difference in electron PADs between the inner and outer belt is distinct. In the outer belt, the common averaged n values are less than 1.5, except for large values of the Kp index and high electron energies. The averaged n values vary considerably with MLT, with a peak in the afternoon sector and an increase with increasing L shell. In the inner belt, the averaged n values are much larger, with a common value greater than 2. The PADs show a slight dependence on MLT, with a weak maximum at noon. A distinct region with steep PADs lies in the outer edge of the inner belt where the electron flux is relatively low. The distance between the inner and outer belt and the intensity of the geomagnetic activity together determine the variation of PADs in the inner belt. Besides being dependent on electron energy, magnetic activity, and L shell, the results show a clear dependence on MLT, with higher n values on the dayside.

Electron fluxes and pitch‐angle distributions at dipolarization fronts: THEMIS multipoint observations

AbstractTaking advantage of multipoint observations from a Cluster‐like Time History of Events and Macroscale Interactions during Substorms (THEMIS) probe configuration repeated in three events, we study pitch‐angle distributions (PAD) of lower energy (0.2–keV) electrons and omnidirectional energy‐time spectrograms of higher energy (30–500 keV) electrons observed at and near dipolarization fronts in the plasma sheet. Recent observations have shown that dipolarization fronts in the plasma sheet provide an impulsive electric field suggested to cause electron energization and dispersionless injections. Increase and decrease in energetic electron flux are equally probable at the fronts, however. Our case studies demonstrate increased energetic electron flux in the front's central region but decreased flux on its dusk side, where diverted plasma flow forms a vortex. An electric field associated with this vortex causes the electron flux decrease. We also find that shorter‐term energetic flux decreases, often observed before injections, coincide with a dip in the northward magnetic field ahead of the front. We attribute these decreases to particle energy loss via the inverse betatron effect. Our case studies reveal that pancake‐type (maximum at 90° pitch angle) and cigar‐type (maxima at 0 and 180°) PADs coexist at the same front. Our data analysis suggests that energetic electron PADs are mainly pancake type near the neutral sheet (|Bx| &lt; 5 nt) and mainly cigar type at |Bx| &gt; 10 nt. These results, to be confirmed in statistical studies, provide important constraints for further modeling of electron energization and transport toward the inner magnetosphere.

Storm-Time Evolution of Energetic Electron Pitch Angle Distributions by Wave-Particle Interaction

The quasi-pure pitch-angle scattering of energetic electrons driven by field-aligned propagating whistler mode waves during the 9∼15 October 1990 magnetic storm at L ≈ 3 ∼ 4 is studied, and numerical calculations for energetic electrons in gyroresonance with a band of frequency of whistler mode waves distributed over a standard Gaussian spectrum is performed. It is found that the whistler mode waves can efficiently drive energetic electrons from the larger pitch-angles into the loss cone, and lead to a flat-top distribution during the main phase of geomagnetic storms. This result perhaps presents a feasible interpretation for observation of time evolution of the quasi-isotropic pitch-angle distribution by Combined Release and Radiation Effects Satellite (CRRES) spacecraft at L ≈ 3 ∼ 4.

Substorm associated magnetotail energetic electrons pitch angle evolutions and flow reversals: Cluster observation

The Cluster satellites observed a distinct pattern in the evolution of energetic electron pitch angle distributions on November 13, 2003, in the mid‐tail (radius &gt;10RE) associated with intensifications of these electron fluxes that accompanied an observed local dipolarization of the magnetic field. The intensity of electrons (20–200 keV) were first observed to increase for perpendicular pitch angles (pancake distribution), then evolve into isotropic distributions, then further evolve into mixed distributions ‐ a combination of perpendicularly peaked distributions and beamlike distributions (cigar distribution), and at the end of each of three episodes always evolved into a beamlike or field‐aligned distribution. This evolution pattern seen at a relative fixed observing (satellite) location usually developed within the locally observed dipolarization time scale. We have interpreted this pattern to indicate that electrons were coming from different regions along the magnetotail and have gotten accelerated differently. This has led us to suggest and present a model in which Fermi acceleration dominates for electrons that are accelerated from the reconnection region, while betatron acceleration dominates for electrons that are accelerated from the mid‐tail region that is very close to the satellite location.

Evolution of energetic electron pitch angle distributions during storm time electron acceleration to megaelectronvolt energies

We investigate the pitch angle distributions of 0.15–1.58 MeV electrons observed during the 9–15 October 1990 storm measured by the Combined Release and Radiation Effects Satellite (CRRES) spacecraft. This storm period is characterized by an enhancement in the electron flux at L ≈ 4 by more than an order of magnitude over the prestorm level. The overall change in flux at L ≈ 6.6 is small in comparison. Previous work shows that radial diffusion underestimates the flux enhancement by up to a factor of 5 for L ≤ 4.5 [Brautigam and Albert, 2000], indicating the need for an additional acceleration process. The pitch angle distributions presented here are examined for evidence of the acceleration mechanism. The distributions at L ≈ 2 are rounded and are dominated by Coulomb collisions. They show little variation during the storm. The distributions at L ≈ 3 are pancake‐shaped before the storm, characteristic of pitch angle scattering by plasmaspheric hiss. During the main phase, they become broad and flat, and they evolve back into pancake distributions during the recovery phase. At L ≈ 4–6, the pitch angle distributions are characterized as butterfly distributions at storm onset, and they become broad flat top distributions during the recovery phase. The flat top distributions persist throughout the ∼3‐day recovery phase and are observed in the region of highest flux enhancement. The flat top distributions are energy dependent and are broader at lower energies (30°–150°) than at higher energies (50°–130°). The higher energies exhibit a much faster fall off toward the loss cone than at lower energies. Inward radial diffusion should result in anisotropic distributions peaked near 90° and does not explain the observed energy dependence. Furthermore, the direction of diffusion is outward at higher energies. Model calculations of the pitch angles resonant with whistler mode waves show that flat top distributions are consistent with pitch angle and energy scattering in regions where fpe/fce ∼ 1. Although radial diffusion may be very important for particle energization, the observed pitch angle distributions provide strong evidence that wave particle interactions play an important role in the energization process.

Trapped electrons in Ganymede's magnetic field

The NASA Galileo satellite encountered the Jovian moon Ganymede on 7 May 1997. The Energetic Particles Detector (EPD) measured energetic electron pitch angle distributions characteristic of closed magnetic field lines. Significantly different from distributions observed during other encounters with Ganymede, they displayed loss cone features near both 0° and 180° pitch angles and an additional minimum near 90° pitch angle. These double loss cone, butterfly distributions are characteristic signatures of particles drifting in a distorted magnetic field configuration. Distortions due to the flow of Jovian plasma past Ganymede qualitatively could account for the observed distributions. Electron injection could then occur either through the equatorial downstream hemisphere or at low Ganymede altitudes on high‐latitude Ganymede‐Jovian field lines adjacent to the closed field line region. The data indicate maximum injection efficiences of 1–10%.

Characteristics of the magnetospheric boundary layer and magnetopause layer as observed by Imp 6

Imp 6 observations of the low‐latitude magnetospheric boundary layer indicate that the plasma within it is supplied primarily by direct entry of magnetosheath plasma across the magnetopause layer. We define the magnetopause layer as the current layer (separating the magnetosheath from the boundary layer) through which the magnetic field shifts in direction. High temporal resolution (3‐s average) data reveal that in a majority of Imp 6 magnetopause crossings, no distinct changes in electron density or energy spectra are observed at the magnetopause layer. In all Imp 6 crossings, some magnetosheathlike plasma is observed earthward of the magnetopause layer, implying the existence of a boundary layer. Boundary layer electron energy spectra are often virtually indistinguishable from the adjacent magnetosheath spectra. Low‐latitude boundary layer bulk plasma flow as observed by Imp 6 almost always has an antisunward component and often has a significant cross‐field component. The boundary layer thickness is highly variable and is generally much larger than the magnetopause layer thickness. Energetic electron pitch angle distributions indicate that the low‐latitude boundary layer is normally on closed field lines. We conclude that diffusive as well as nondiffusive processes probably contribute to the entry of magnetosheath plasma into the boundary layer.

Observations of ionospheric ion flow and related convective electric fields in and near an auroral arc

Measurements of the ionospheric ion distribution function and energetic electron and ion precipitation during an auroral substorm near local midnight are presented. Thermal ion measurements are used to determine the ion bulk flow velocity, temperature, and density. It is shown that the ion flow velocity is highly correlated with energetic electron precipitation. The convective component (perpendicular to the local magnetic field B) was found to be large (equivalent electric field E⊥ ∼ 100–200 mV/m) poleward of a series of poleward expanding arcs. Inside the arc the convective flow dropped to a low value (E⊥ ∼ 10–30 mV/m). High-velocity (∼2 km/s) ion flows parallel to B were also observed and found to be correlated with electron precipitation. Poleward of the arc the flow was directed away from the ionosphere, whereas inside the arc the flow switched to earthward. Parallel electric fields of ∼0.1 mV/m required to produce these vertical flows are shown to be consistent with energetic electron pitch angle distributions. Convective electric field measurements reported here are compared with previous observations.

Temporal behavior of energetic particle precipitation during an auroral substorm

Results from two rocket experiments launched from Fort Churchill Research Range into the breakup and post-breakup phases of an auroral substorm are described. Unusual characteristics were observed in energy spectra and pitch-angle distributions of energetic electrons (E> 3 kev) and protons ( >28 kev) simultaneously with the arrival of the northern edge of a series of northward-propagating auroral arcs. When the northern edge passed the rocket, electron intensities increased and the spectrum hardened appreciably, both changes occurring with time constants less than about 2 seconds. Dispersion in arrival times of electrons was observed with the low-energy particles arriving first. After the initial injection the particle fluxes were observed to decay at rates consistent with the maximum decay rates for these latitudes. Decay times of the order of a few minutes for electrons and a few hours for protons were observed. Electron angular distribution characteristics were found to be energy dependent. The recovery-phase of the substorm was found to be characterized by soft isotropic electron precipitation.