Electron Energization Research Articles

Enhanced radial transport and energization of radiation belt electrons due to drift orbit bifurcations.

[1]Relativistic electron intensities in Earth's outer radiation belt can vary by multiple orders of magnitude on the time scales ranging from minutes to days. One fundamental process contributing to dynamic variability of radiation belt intensities is the radial transport of relativistic electrons across their drift shells. In this paper we analyze the properties of three-dimensional radial transport in a global magnetic field model driven by variations in the solar wind dynamic pressure. We use a test particle approach which captures anomalous effects such as drift orbit bifurcations. We show that the bifurcations lead to an order of magnitude increase in radial transport rates and enhance the energization at large equatorial pitch angles. Even at quiet time fluctuations in dynamic pressure, radial transport at large pitch angles exhibits strong deviations from the diffusion approximation. The radial transport rates are much lower at small pitch angle values which results in a better agreement with the diffusion approximation.

Forward Acceleration of Ionospheric Electrons by NAU 40.75-kHz Whistler Waves Over Arecibo

We discuss a new mechanism to explain the forward acceleration of ionospheric electrons by whistler waves. We suggest this mechanism to be the key process responsible for our reported enhanced plasma lines detected by the Arecibo 430 MHz radar. These plasma lines are characterized by the frequency-downshifted spectra with a bandwidth of ~ 12 kHz. The backscatter radar operation indicates that electrons were accelerated upward along the Earth's magnetic field by the up-going 40.75 kHz whistler waves, which were launched from the NAU transmitter. They covered a broad range of altitudes ( ~ 300 km) and lasted for a period of a few minutes. This mechanism leads to the energization of electrons of ~ 13 eV inferred from Arecibo experiments.

Observation and Modeling of Geostationary Orbit Electron Energization Induced by Enhanced Dayside Whistler-Mode Waves

We provide correlated observations of enhanced dayside whistler-mode waves and energetic electron acceleration collected by the CLUSTER and GOES satellites during the 23∼24 September 2001 storm. Energetic (>0.6 MeV) electron fluxes are found to increase significantly during the recovery phase and the main phase, by a factor of ∼50 higher than the prestorm level. These high electron fluxes occur when strong dayside whistler-mode waves are present. Two-dimensional (2D) numerical simulations are carried out and the results demonstrate that the dayside whistler-mode wave can contribute to such enhancements in electron flux within 24 h, consistent with the observation.

A test electron model for the study of three dimensional magnetic reconnection effects

Understanding the mechanisms that enable the conversion of the explosive release of magnetic energy into the electron energization that is experimentally observed in space and laboratory plasmas represents a long-standing question in the study of magnetic reconnection.We present a test particle model able to follow the dynamics of the electron guiding centers in the presence of the electromagnetic fields characterizing a non-steady-state, three-dimensional magnetohydrodynamic (3D-MHD) description of magnetic reconnection. The resulting electron equations, based on a relativistic Hamiltonian formulation of the particle dynamics, have been implemented in a code that evolves the spatial position and the velocity of the electrons according to the fields obtained by the numerical solutions of a two-fluid reconnection model. By the calculation of the electron distribution function in the phase space, the electron kinetic moments can be obtained with a δf technique particularly suitable during the early stages of the reconnection process, which can be replaced by a full-f approach in the nonlinear reconnection phase. Thanks to its capability to reproduce the electron response in a quite realistic reconnection configuration, this numerical tool represents a new instrument for the investigation of the nonlinear electron dynamics in particularly complex reconnection regions, such as those characterized by magnetic field line stochasticity.

Bulk Energization of Electrons in Solar Flares by Alfvén Waves

Bulk energization of electrons to $10\,-\,20\,$keV in solar flares is attributed to dissipation of Alfv\'en waves that transport energy and potential downward to an acceleration region near the chromosphere. The acceleration involves the parallel electric field that develops in the limit of inertial Alfv\'en waves (IAWs). A two-potential model for IAWs is used to relate the parallel potential to the cross-field potential transported by the waves. We identify a maximum parallel potential in terms of a maximum current density that corresponds to the threshold for the onset of anomalous resistivity. This maximum is of order $10\,$kV when the threshold is that for the Buneman instability. We argue that this restricts the cross-field potential in an Alfv\'en wave to about $10\,$kV. Effective dissipation requires a large number of up- and down-current paths associated with multiple Alfv\'en waves. The electron acceleration occurs in localized, transient, anomalously-conducting regions (LTACRs) and is associated with the parallel electric field determined by Ohm's law with an anomalous resistivity. We introduce an idealized model in which the LTACRs are (upward-)current sheets, a few skin depths in thickness, separated by much-larger regions of weaker return current. We show that this model can account semi-quantitatively for bulk energization.

Storm‐induced energization of radiation belt electrons: Effect of wave obliquity

New Cluster statistics allow us to determine for the first time the variations of both the obliquity and intensity of lower‐band chorus waves as functions of latitude and geomagnetic activity near L∼5. The portion of wave power in very oblique waves decreases during highly disturbed periods, consistent with increased Landau damping by inward‐penetrating suprathermal electrons. Simple analytical considerations as well as full numerical calculations of quasi‐linear diffusion rates demonstrate that early‐time electron acceleration occurs in a regime of loss‐limited energization. In this regime, the average wave obliquity plays a critical role in mitigating lifetime reduction as wave intensity increases with geomagnetic activity, suggesting that much larger energization levels should be reached during the early recovery phase of storms than during quiet time or moderate disturbances, the latter corresponding to stronger losses. These new effects should be included in realistic radiation belt simulations.

ULF wave index as magnetospheric and space-weather parameters

Waves in the Ultra Low Frequency (ULF) band owe their existence to solar wind turbulence and transport momentum and energy from the solar wind to the magnetosphere and farther down. Therefore an index based on ULF wave power could better characterize solar wind–magnetosphere interaction than KP, Dst, AE, etc. indices which described mainly quasi-study state condition of the system. We have shown that the ULF wave index accurately characterize relativistic electron dynamics in the magnetosphere as these waves are closely associated with circulation, diffusion and energization of relativistic electrons in the magnetosphere. High speed solar wind streams also act as a significant driver of activity in the Earth’s magnetosphere co-rotating interaction region and are responsible for geomagnetic activities. In the present paper, we have analyzed various cases related with very weak (quiet) days, weak days, storm days and eclipse events and discussed the utility of the ULF wave index to explain the magnetospheric dynamics and associated properties. We have tried to explain that the ULF wave index can equally be useful as a space weather parameter like the other indices.

Generation and detection of whistler wave induced space plasma turbulence at Gakona, Alaska

We report on high-frequency wave injection experiments using the beat wave technique to study the generation of very-low-frequency (VLF) whistler waves in the ionosphere above Gakona, Alaska. This work is aimed at investigating whistler wave interactions with ionospheric plasmas and radiation belts. The beat wave technique involves injecting two X-mode waves at a difference frequency in the VLF range using the High-frequency Active Auroral Research Program (HAARP) heating facility. A sequence of beat wave-generated whistler waves at 2, 6.5, 7.5, 8.5, 9.5, 11.5, 15.5, 22.5, 28.5 and 40.5 kHz were detected in our 2011 experiments. We present Modular Ultra-high-frequency Ionospheric Radar (MUIR) (446 MHz) measurements of ion lines as the primary diagnosis of ionospheric plasma effects caused by beat wave-generated whistler waves. A magnetometer and digisonde were used to monitor the background ionospheric plasma conditions throughout the experiments. Our theoretical and data analyses show that VLF whistler waves can effectively interact with ionospheric plasmas via two different four-wave interaction processes leading to energization of electrons and ions. These preliminary results support our Arecibo experiments to study NAU-launched 40.75 kHz whistler wave interactions with space plasmas.

On sheath energization and Ohmic heating in sputtering magnetrons

In most models of sputtering magnetrons, the mechanism for energizing the electrons in the discharge is assumed to be sheath energization. In this process, secondary emitted electrons from the cathode surface are accelerated across the cathode sheath into the plasma, where they either ionize directly or transfer energy to the local lower energy electron population that subsequently ionizes the gas. In this work, we present new modeling results in support of an alternative electron energization mechanism. A model is experimentally constrained, by a fitting procedure, to match a set of experimental data taken over a large range in discharge powers in a high-power impulse magnetron sputtering (HiPIMS) device. When the model is matched to real data in this way, one finding is that the discharge can run with high power and large gas rarefaction without involving the mechanism of secondary electron emission by twice-ionized sputtered metal. The reason for this is that direct Ohmic heating of the plasma electrons is found to dominate over sheath energization by typically an order of magnitude. This holds from low power densities, as typical for dc magnetrons, to so high powers that the discharge is close to self-sputtering, i.e. dominated by the ionized vapor of the sputtered gas. The location of Ohmic heating is identified as an extended presheath with a potential drop of typically 100–150 V. Such a feature, here indirectly derived from modeling, is in agreement with probe measurements of the potential profiles in other HiPIMS experiments, as well as in conventional dc magnetrons.

Wave-Breaking Phenomena in a Relativistic Magnetized Plasma

We study the wave-breaking phenomenon of relativistic upper-hybrid (UH) oscillations in a cold magnetoplasma. For our purposes, we use the electron continuity and relativistic electron momentum equations, together with Maxwell's equations, as well as introduce Lagrangian coordinates to obtain an exact nonstationary solution of the governing nonlinear equations. It is found that bursts in the electron density appear in a finite time as a result of relativistic electron mass variations in the UH electric field, indicating a phase mixing or breaking of relativistic UH oscillations. We highlight the relevance of our investigation of the UH wave phase-mixing or UH wave-breaking process to electron energization and plasma particle heating.

PARTICLE-IN-CELL SIMULATIONS OF PARTICLE ENERGIZATION VIA SHOCK DRIFT ACCELERATION FROM LOW MACH NUMBER QUASI-PERPENDICULAR SHOCKS IN SOLAR FLARES

Low Mach number, high beta fast mode shocks can occur in the magnetic reconnection outflows of solar flares. These shocks, which occur above flare loop tops, may provide the electron energization responsible for some of the observed hard X-rays and contemporaneous radio emission. Here we present new 2D particle-in-cell simulations of low Mach number/high beta quasi-perpendicular shocks. The simulations show that electrons above a certain energy threshold experience shock-drift-acceleration. The transition energy between the thermal and non-thermal spectrum and the spectral index from the simulations are consistent with some of the X-ray spectra from RHESSI in the energy regime of $E\lesssim 40\sim 100$ keV. Plasma instabilities associated with the shock structure such as the modified-two-stream and the electron whistler instabilities are identified using numerical solutions of the kinetic dispersion relations. We also show that the results from PIC simulations with reduced ion/electron mass ratio can be scaled to those with the realistic mass ratio.

Spokes and charged particle transport in HiPIMS magnetrons

Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E × B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented.

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| < 5 nt) and mainly cigar type at |Bx| > 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.

Cluster observations of kinetic structures and electron acceleration within a dynamic plasma bubble

Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasma‐depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near‐Earth tail around X = −18 RE. The bubble is bounded by both sharp leading (∂bz/∂x < 0) and trailing (∂bz/∂x > 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field‐aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super‐Alfvénic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E × B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of Bz, or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field‐aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed.

Satellite observations of banded VLF emissions in conjunction with energy‐banded ions during very large geomagnetic storms

Frequency‐banded electromagnetic VLF waves up to 2000 Hz are observed concurrently with warm (10 s to 10,000 s of eV) energy‐banded ions in the low latitude auroral and sub‐auroral zones during every large geomagnetic storm encountered by the FAST and DEMETER satellites. The banded ions and waves persist for several FAST or DEMETER orbits, lasting up to 12 h, in both dawn and dusk sectors, in both the northern and southern hemispheres. If the waves are generated at harmonics of the proton gyrofrequency, the inferred source region would be ∼4000 km altitude. Previous investigations have shown that such waves can propagate from this source region to the locations of both spacecraft. An investigation into the growth of waves at harmonics of fci in the inferred source region suggests that these emissions could be generated by ion bands similar to those observed at the same time as the waves. Magnetospheric waves such as these play a role in energy transfer between distinct particle populations and may contribute to ion heating and ion outflow as well as electron energization. All of these phenomena occur during the strongest magnetic storms. The appearance of the banded ions and associated wave activity suggests that there may be distinct changes in the geospace system that characterize large magnetic storms.

Alfvén's critical ionization velocity observed in high power impulse magnetron sputtering discharges

Azimuthally rotating dense plasma structures, spokes, have recently been detected in several high power impulse magnetron sputtering (HiPIMS) devices used for thin film deposition and surface treatment, and are thought to be important for plasma buildup, energizing of electrons, as well as cross-B transport of charged particles. In this work, the drift velocities of these spokes are shown to be strongly correlated with the critical ionization velocity, CIV, proposed by Alfvén. It is proposed as the most promising approach in combining the CIV and HiPIMS research fields is to focus on the role of spokes in the process of electron energization.

Electron energization during magnetic island coalescence

Radio emission from colliding coronal mass ejection flux ropes in the interplanetary medium suggested the local generation of superthermal electrons. Inspired by those observations, a fully kinetic particle-in-cell simulation of magnetic island coalescence models the magnetic reconnection between islands as a source of energetic electrons. When the islands merge, stored magnetic energy is converted into electron kinetic energy. The simulation demonstrates that a mechanism for electron energization originally applied to open field line reconnection geometries also operates near the reconnection site of merging magnetic islands. The electron heating is highly anisotropic, and it results mainly from an electric field surrounding the reconnection site that accelerates electrons parallel to the magnetic field. A detailed theory predicts the maximum electron energies and how they depend on the plasma parameters. In addition, the global motion of the magnetic islands launches low-frequency waves in the surrounding plasma, which induce large-amplitude, anisotropic fluctuations in the electron temperature.

PARTICLE ACCELERATION AT A FLARE TERMINATION SHOCK: EFFECT OF LARGE-SCALE MAGNETIC TURBULENCE

We investigate the acceleration of charged particles (both electrons and protons) at collisionless shocks predicted to exist in the vicinity of solar flares. The existence of standing termination shocks has been examined by flare models and numerical simulations e.g., Shibata,Forbes. We study electron energization by numerically integrating the equations of motion of a large number of test-particle electrons in the time-dependent two-dimensional electric and magnetic fields generated from hybrid simulations (kinetic ions and fluid electron) using parameters typical of the solar flare plasma environment. The shock is produced by injecting plasma flow toward a rigid piston. Large-scale magnetic fluctuations -- known to exist in plasmas and known to have important effects on the nonthermal electron acceleration at shocks -- are also included in our simulations. For the parameters characteristic of the flaring region, our calculations suggest that the termination shock formed in the reconnection outflow region (above post-flare loops) could accelerate electrons to a kinetic energy of a few MeV within 100 ion cyclotron periods, which is of the order of a millisecond. Given a sufficient turbulence amplitude level ($\delta B^2/B_0^2 \sim 0.3$), about 10% of thermal test-particle electrons are accelerated to more than 15 keV. We find that protons are also accelerated, but not to as high energy in the available time and the energy spectra are considerably steeper than that of the electrons for the parameters used in our simulations. Our results are qualitatively consistent with the observed hard X-ray emissions in solar flares.

Comparison of energetic electron flux and phase space density in the magnetosheath and in the magnetosphere

Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into the magnetosphere and further energized through radial diffusion is significant in understanding the physical mechanisms for producing the radiation belt electrons (>100s of keV) in the magnetosphere. In this study, we analyze more than two hundred magnetopause crossing events using the energetic electron and magnetic field measurements from Geotail and compare the flux and phase space density (PSD) of the energetic electrons on both sides of the magnetopause. It is found that for most of the events (>70%), the fluxes and PSDs of energetic electrons in the magnetosheath are less than those in the magnetosphere, suggesting that the energetic electrons in the magnetosheath cannot be a direct source sufficient for the energetic electrons inside the magnetosphere. In fact, our analysis suggests a possible leakage of the energetic electrons from inside to outside the magnetopause. By investigating the average energetic electron flux distribution in the magnetosheath, we find that the energetic electron fluxes are higher near the bow shock and the magnetopause than in between. The high energetic electron flux near the bow shock can be understood as due to energization of electrons when they go through the bow shock. The relatively low flux of the energetic electrons in between indicates that it is difficult for the energetic electrons to travel from the bow shock to the magnetopause and vice versa, possibly because the energetic electrons near the bow shock and the magnetopause are all on open magnetic field lines and these two relatively intense energetic electron populations in the magnetosheath rarely get mixed.

ULF wave derived radiation belt radial diffusion coefficients

Waves in the ultra‐low‐frequency (ULF) band have frequencies which can be drift resonant with electrons in the outer radiation belt, suggesting the potential for strong interactions and enhanced radial diffusion. Previous radial diffusion coefficient models such as those presented by Brautigam and Albert (2000) have typically used semiempirical representations for both the ULF wave's electric and magnetic field power spectral densities (PSD) in space in the magnetic equatorial plane. In contrast, here we use ground‐ and space‐based observations of ULF wave power to characterize the electric and magnetic diffusion coefficients. Expressions for the electric field power spectral densities are derived from ground‐based magnetometer measurements of the magnetic field PSD, and in situ AMPTE and GOES spacecraft measurements are used to derive expressions for the compressional magnetic field PSD as functions of Kp, solar wind speed, and L‐shell. Magnetic PSD results measured on the ground are mapped along the field line to give the electric field PSD in the equatorial plane assuming a guided Alfvén wave solution and a thin sheet ionosphere. The ULF wave PSDs are then used to derive a set of new ULF‐wave driven diffusion coefficients. These new diffusion coefficients are compared to estimates of the electric and magnetic field diffusion coefficients made by Brautigam and Albert (2000) and Brautigam et al. (2005). Significantly, our results, derived explicitly from ULF wave observations, indicate that electric field diffusion is much more important than magnetic field diffusion in the transport and energization of the radiation belt electrons.