We investigate the dynamics of relativistic electrons in the Earth’s outer radiation belt by analyzing the interplay of several key physical processes: electron losses due to pitch angle scattering from electromagnetic ion cyclotron (EMIC) waves and chorus waves, and electron flux increases from chorus wave-driven acceleration of ~100-300 keV seed electrons injected from the plasma sheet. We examine a weak geomagnetic storm on April 17, 2021, using observations from various spacecraft, including GOES, Van Allen Probes, ERG/ARASE, MMS, ELFIN, and POES. Despite strong EMIC- and chorus wave-driven electron precipitation in the outer radiation belt, trapped 0.1-1.5 MeV electron fluxes actually increased. We use theoretical estimates of electron quasi-linear diffusion rates by chorus and EMIC waves, based on statistics of their wave power distribution, to examine the role of those waves in the observed relativistic electron flux variations. We find that a significant supply of 100-300 keV electrons by plasma sheet injections together with chorus wave-driven acceleration can overcome the rate of chorus and EMIC wave-driven electron losses through pitch angle scattering toward the loss cone, explaining the observed net increase in electron fluxes. Our study emphasizes the importance of simultaneously taking into account resonant wave-particle interactions and modeled local energy gradients of electron phase space density following injections, to accurately forecast the dynamical evolution of trapped electron fluxes.

Night-side chorus waves are often observed during plasma sheet injections, typically confined around the equator and thus potentially responsible for precipitation of $\lesssim 100 keV$ electrons. However, recent low-altitude observations have revealed the critical role of chorus waves in scattering relativistic electrons on the night-side. This study presents a night-side relativistic electron precipitation event induced by chorus waves at the strong diffusion regime, as observed by the ELFIN CubeSats. Through event-based modeling of wave propagation under ducted or unducted regimes, we show that a density duct is essential for guiding chorus waves to high latitudes with minimal damping, thus enabling the strong night-side relativistic electron precipitation. These findings underline both the existence and the important role of density ducts in facilitating night-side relativistic electron precipitation.

We present statistical analysis of 16,903 current sheets (CS) observed over 641 days aboard Ulysses spacecraft at 5 AU. We show that the magnetic field rotates across CSs through some shear angle, while only weakly varies in magnitude. The CSs are typically asymmetric with statistically different, though only by a few percent, magnetic field magnitudes at the CS boundaries. The dataset is classified into about 90.6\% non-bifurcated and 9.4\% bifurcated CSs. Most of the CSs are proton kinetic-scale structures with the half-thickness of non-bifurcated and bifurcated CSs within respectively 200–2,000 km and 500–5,000 km or 0.5–$5\lambda_{p}$ and 0.7–$15\lambda_{p}$ in units of local proton inertial length. The amplitude of the current density, mostly parallel to magnetic field, is typically within 0.05–0.5 nA/m$^{2}$ or 0.04–$0.4J_{A}$ in units of local Alfv\’{e}n current density. The CSs demonstrate approximate scale-invariance with the shear angle and current density amplitude scaling with the half-thickness, $\Delta \theta\approx 16.6^{\circ}\;(\lambda/\lambda_{p})^{0.34}$ and $J_0/J_{A}\approx 0.14\;(\lambda/\lambda_{p})^{-0.66}$. The matching of the magnetic field rotation and compressibility observed within the CSs against those in ambient solar wind indicate that the CSs are produced by turbulence, inheriting thereby its scale-invariance and compressibility. The estimated asymmetry in plasma beta between the CS boundaries is shown to be insufficient to suppress magnetic reconnection through the diamagnetic drift of X-line, but magnetic reconnection is probably suppressed by other processes. The presented results will be of value for future comparative analysis of CSs observed at different distances from the Sun.

Magnetic field-line curvature scattering (FLCS) of energetic particles in the equatorial magnetotail results in isotropization of pitch-angle distributions, loss-cone filling, and precipitation above a minimum energy at a given latitude. At a fixed energy, the lowest latitude of isotropization is the isotropy boundary (IB) for that energy. Nominally, the IB (latitude) exhibits a characteristic energy dependence due to the monotonic variation of the equatorial magnetic field intensity Beq with radial distance. Deviations from this nominal IB dispersion can occur if the radial Beq variation (spatial or temporal) is non-mononotic and/or if other precipitation mechanisms prevail. With its sensitive and detailed measurements of electron spectra up to relativistic energies, ELFIN’s recent observations reveal a variety of electron IBe patterns near magnetic midnight which are repeatable enough to warrant classification. This study aims to categorize the various IBe patterns observed by ELFIN’s high-fidelity but short lived dataset (a few months), compare them with simultaneous nearby POES observations, which are made with a limited energy coverage and resolution but last for decades, and discuss their possible interpretation. The general agreement between ELFIN and POES IB observations indicate a relatively large-scale nature of IBe patterns. Surprisingly, there exists a large number (up to 2/3 of all events) of non-monotonic-or steep/multiple-IB patterns. This suggest an abundance of non-trivial tail current sheet structures or a mixed contribution of two mechanisms in the vicinity of IBe in these cases.

Near-equatorial measurements of energetic electron fluxes, in combination with numerical simulation, are widely used for monitoring of the radiation belt dynamics. However, the long orbital periods of near-equatorial spacecraft constrain the cadence of observations to once per several hours or greater, i.e., much longer than the mesoscale injections and rapid local acceleration and losses of energetic electrons of interest. An alternative approach for radiation belt monitoring is to use measurements of low-altitude spacecraft, which cover, once per hour or faster, the latitudinal range of the entire radiation belt within a few minutes. Such an approach requires, however, a procedure for mapping the flux from low equatorial pitch angles (near the loss cone) as measured at low altitude, to high equatorial pitch angles (far from the loss cone), as necessitated by equatorial flux models. Here we do this using the high energy resolution ELFIN measurements of energetic electrons. Combining those with GPS measurements we develop a model for the electron anisotropy coefficient, n, that describes electron flux jtrap dependence on equatorial pitch-angle, αeq, jtrap ∼ sinnαeq. We then validate this model by comparing its equatorial predictions from ELFIN with in-situ near-equatorial measurements from Arase (ERG) in the outer radiation belt.

The two most important wave modes responsible for energetic electron scattering to the Earth’s ionosphere are electromagnetic ion cyclotron (EMIC) waves and whistler-mode waves. In this study, we report direct observations of energetic electron (from 50 keV to 2.5 MeV) scattering driven by the combined effect of whistler-mode and EMIC waves using ELFIN measurements. We analyze several events exhibiting such properties, and show that electron resonant interactions with whistler-mode waves may enhance relativistic electron precipitation by EMIC waves. During a prototypical event which benefits from conjugate THEMIS measurements, we demonstrate that below the minimum resonance energy (Emin) of EMIC waves, the whistler-mode wave may both scatter electrons into the loss-cone and also accelerate them to higher energy (1-3 MeV). These accelerated electrons above Emin resonate with EMIC waves that, in turn, quickly scatter those electrons into the loss-cone. This enhances relativistic electron precipitation beyond what EMIC waves alone could achieve.

Magnetotail earthward-propagating fast plasma flows provide important pathways for magnetosphere-ionosphere coupling. This study reexamines a flow-related red-line diffuse-like aurora event previously reported by Liang et al., (2011), utilizing THEMIS and ground-based auroral observations from Poker Flat. We find that time domain structures (TDSs) within the flow bursts efficiently drive electron precipitation below a few keV, aligning with predominantly red-line auroral intensifications in this non-substorm event. The diffuse-like auroras sometimes coexisted with or potentially evolved from discrete forms. We forward model red-line diffuse-like auroras due to TDS-driven precipitation, employing the time-dependent TREx-ATM auroral transport code. The good correlation (0.77) between our modeled and observed red line emissions underscores that TDSs are a primary driver of the red-line diffuse-like auroras, though whistler-mode wave contributions are needed to fully explain the most intense red-line emissions.

A document by Muhammad Fraz Bashir. Click on the document to view its contents.

A document by Muhammad Shahid. Click on the document to view its contents.

Energetic electron losses by pitch-angle scattering and precipitation to the atmosphere from the radiation belts are controlled, to a great extent, by resonant wave particle interactions with whistler-mode waves. The efficacy of such precipitation is primarily controlled by wave intensity, although its relative importance, compared to other wave and plasma parameters, remains unclear. Precipitation spectra from the low-altitude, polar-orbiting ELFIN mission have previously been demonstrated to be consistent with energetic precipitation modeling derived from empirical models of field-aligned wave power across a wide-swath of local-time sectors. However, such modeling could not explain the intense, relativistic electron precipitation observed on the nightside. Therefore, this study aims to additionally consider the contributions of three modifications – wave obliquity, frequency spectrum, and local plasma density – to explain this discrepancy on the nightside. By incorporating these effects into both test particle simulations and quasi-linear diffusion modeling, we find that realistic implementations of each individual modification result in only slight changes to the electron precipitation spectrum. However, these modifications, when combined, enable more accurate modeling of ELFIN-observed spectra. In particular, a significant reduction in plasma density enables lower frequency waves, oblique, or even quasi-field aligned waves to resonate with near $\sim1$ MeV electrons closer to the equator. We demonstrate that the levels of modification required to accurately reproduce the nightside spectra of whistler-mode wave-driven relativistic electron precipitation match empirical expectations, and should therefore be included in future radiation belt modeling.

Earth’s magnetotail is filled with solar wind and ionospheric electrons, whose initial energies are significantly lower than the typical energies (temperatures) of plasmasheet electrons. One of the most common mechanisms responsible for heating of solar wind and ionospheric electrons in Earth’s magnetotail is adiabatic heating caused by earthward convection of these electrons from the deep tail (i.e., from the region of a weak magnetic field) towards the region of stronger magnetic fields closer to Earth. This heating is moderated by electron losses into the ionosphere due to local wave scattering. In this study, we compare electron spectra from simultaneous observations of The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at different radial distances with spectra obtained from a simple model that includes adiabatic heating and losses. Our comparison shows that the model heating significantly overestimates the increase in energetic (>1 keV) electron fluxes, indicating that losses are essential for accurate modelling of the observed spectra. The required electron losses are similar to or even greater than the losses in the strong diffusion limit (when the loss cone is full). The latter can be interpreted as loss cone widening by field-aligned electron acceleration.

Energetic electron precipitation to the Earth’s atmosphere is a key process controlling radiation belt dynamics and magnetosphere-ionosphere coupling. One of the main drivers of precipitation is electron resonant scattering by whistler-mode waves. Low-altitude observations of such precipitation often reveal quasi-periodicity in the ultra-low-frequency (ULF) range associated with whistler-mode waves, causally linked to ULF-modulated equatorial electron flux and its anisotropy. Conjunctions between ground-based instruments and equatorial spacecraft show that low-altitude precipitation concurrent with equatorial whistler-mode waves also exhibits a spatial periodicity as a function of latitude over a large spatial region. Whether this spatial periodicity might also be due to magnetospheric ULF waves spatially modulating electron fluxes and whistler-mode chorus has not been previously addressed due to a lack of conjunctions between equatorial spacecraft, LEO spacecraft, and ground-based instruments. To examine this question, we combine ground-based and equatorial observations magnetically conjugate to observations of precipitation at the low-altitude, polar-orbiting CubeSats ELFIN-A and -B. As they sequentially cross the outer radiation belt with a temporal separation of minutes to tens of minutes, they can easily reveal the spatial quasi-periodicity of electron precipitation. Our combined datasets confirm that ULF waves may modulate whistler-mode wave generation within a large MLT and $L$-shell domain in the equatorial magnetosphere, and thus lead to significant aggregate energetic electron precipitation exhibiting both temporal and spatial periodicity. Our results suggest that the coupling between ULF and whistler-mode waves is important for outer radiation belt dynamics.