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.

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.

Waves which couple to energetic electrons are particularly important in space weather, as they drive rapid changes in the topology and intensity of Earth’s outer radiation belt during geomagnetic storms. This includes Ultra Low Frequency (ULF) waves that interact with electrons via radial diffusion which can lead to electron dropouts and rapid acceleration and inward transport of electrons during. In radiation belt simulations, the strength of this interaction is specified by ULF wave radial diffusion coefficients. In this paper we detail the development of new models of electric and magnetic radial diffusion coefficients derived from in-situ observations of the azimuthal electric field and compressional magnetic field. The new models use L* as it accounts for adiabatic changes due to the dynamic magnetic field coupled with an optimized set of four components of solar wind and geomagnetic activity, Bz, V, Pdyn and Sym-H, as independent variables (inputs). These independent variables are known drivers of ULF waves and offer the ability to calculate diffusion coefficients at a higher cadence then existing models based on Kp. We investigate the performance of the new models by characterizing the model residuals as a function of each independent variable and by comparing to existing radial diffusion models during a quiet geomagnetic period and through a geomagnetic storm. We find that the models developed here perform well under varying levels of activity and have a larger slope or steeper gradient as a function of L* as compared to existing models (higher radial diffusion at higher L* values).

Energetic electron precipitation (EEP) from the radiation belts into Earth’s atmosphere leads to several profound effects (e.g., enhancement of ionospheric conductivity, possible acceleration of ozone destruction processes). An accurate quantification of the energy input and ionization due to EEP is still lacking due to instrument limitations of low-Earth-orbit satellites capable of detecting EEP. The deployment of the ELFIN (Electron Losses and Fields InvestigatioN) CubeSats marks a new era of observations of EEP with an improved pitch-angle (0°–180°) and energy (50 keV–6 MeV) resolution. Here, we focus on the EEP recorded by ELFIN coincident with electromagnetic ion cyclotron (EMIC) waves, which play a major role in radiation belt electron losses. The EMIC-driven EEP (~200 keV – ~2 MeV) exhibits a pitch-angle distribution (PAD) that flattens with increasing energy, indicating more efficient high-energy precipitation. Leveraging the combination of unique electron measurements from ELFIN and a comprehensive ionization model known as Boulder Electron Radiation to Ionization (BERI), we quantify the energy input of EMIC-driven precipitation (on average, ~3.3x10-2 erg/cm2/s), identify its location (any longitude, 50°–70° latitude), and provide the expected range of ion-electron production rate (on average, 100–200 pairs/cm3/s), peaking in the mesosphere – a region often overlooked. Our findings are crucial for improving our understanding of the magnetosphere-ionosphere-atmosphere system as they accurately specify the contribution of EMIC-driven EEP, which serves as a crucial input to state-of-the-art atmospheric models (e.g., WACCM) to quantify the accurate impact of EMIC waves on both the atmospheric chemistry and dynamics.

AbstractElectromagnetic ion cyclotron waves in the Earth's outer radiation belt drive rapid electron losses through wave-particle interactions. The precipitating electron flux can be high in the 100s keV energy range, well below the typical minimum resonance energy. One of the proposed explanations relies on nonresonant scattering, which causes pitch-angle diffusion away from the fundamental cyclotron resonance. Here we propose the fractional sub-cyclotron resonance, a second-order nonlinear effect that scatters particles at resonance order $n = 1/2$, as an alternate explanation. Using test-particle simulations, we evaluate the precipitation ratios of sub-MeV electrons for wave packets with various shapes, amplitudes, and wave normal angles. We show that the nonlinear sub-cyclotron scattering produces larger ratios than the nonresonant scattering when the wave amplitude reaches sufficiently large values. The ELFIN CubeSats detected several events with precipitation ratio patterns matching our simulation, demonstrating the importance of sub-cyclotron resonances during intense precipitation events.

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.

Electromagnetic ion cyclotron (EMIC) waves effectively scatter relativistic electrons in the Earthâ\euro™s radiation belts and energetic ions in the ring current. Empirical models parameterizing EMIC wave characteristics are important elements of inner magnetosphere simulations. Two main EMIC wave populations included in such simulations are the population generated by plasma sheet injections and another population generated by magnetospheric compression due to solar wind. In this study, we investigate a third class of EMIC waves, generated by hot plasma sheet ions modulated by compressional ultra-low-frequency (ULF) waves. Such ULF-modulated EMIC waves are mostly observed on the dayside, between magnetopause and the outer radiation belt edge. We show that ULF-modulated EMIC waves are weakly oblique (with wave normal angle $\approx 20^\circ\pm10^\circ$) and narrow banded (with spectral width of $\sim 1/3$ of the mean frequency). We further construct an empirical model of EMIC wave characteristics as a function of $L$-shell and MLT. The low ratio of plasma frequency to electron gyrofrequency ($f_{pe}/f_{ce}\sim 5-10$) around the EMIC wave generation region does not allow these waves to scatter energetic electrons. However, these waves provide very effective (comparable to strong diffusion) quasi-periodic precipitation of plasma sheet protons.

An interplanetary shock can abruptly compress the magnetosphere, excite magnetospheric waves and field-aligned currents, and cause a ground magnetic response known as a sudden commencement (SC). However, the transient (<~1 min) response of the ionosphere-thermosphere system during an SC has been little studied due to limited temporal resolution in previous investigations. Here, we report observations of a global reversal of ionospheric vertical plasma motion during an SC on 24 October 2011 using ~6 s resolution SuperDARN ground scatter data. The dayside ionosphere suddenly moved downward during the magnetospheric compression due to the SC, lasting for only ~1 min before moving upward. By contrast, the post-midnight ionosphere briefly moved upward then moved downward during the SC. Simulations with a coupled geospace model suggest that the reversed E X B vertical drift is caused by a global reversal of ionospheric zonal electric field induced by magnetospheric compression during the SC.

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.

Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are frequently observed in Earth’s foreshock, which can significantly disturb the bow shock and therefore the magnetosphere-ionosphere system and can accelerate particles. Previous statistical studies have identified the solar wind conditions (high solar wind speed and high Mach number, etc.) that favor their generation. However, backstreaming foreshock ions are expected to most directly control how HFAs and FBs form, whereas the solar wind may partake in the formation process indirectly by determining foreshock ion properties. Using Magnetospheric Multiscale mission and Time History of Events and Macroscale Interactions during Substorms mission, we perform a statistical study of foreshock ion properties around 275 HFAs and FBs. We show that foreshock ions with a high foreshock-to-solar wind density ratio (>~3%), high kinetic energy (>~600eV), large ratio of kinetic energy to thermal energy (>~0.1), and large perpendicular temperature anisotropy (>~1.4) favor HFA and FB formation. We also examine how these properties are related to solar wind conditions: higher solar wind speed and larger (angle between the interplanetary magnetic field and the bow shock normal) favor higher kinetic energy of foreshock ions; foreshock ions are less diffuse at larger ; small , high Mach number, and closeness to the bow shock favor a high foreshock-to-solar wind density ratio. Our results provide further understanding of HFA and FB formation.

We report observational evidence for a class of coherent magnetic dipolarization structures that are long lived and radially extensive. The reported dipolarization structures, a subset of general dipolarizations, typically remain coherent over 20-30 min in real time, 3-6 hours in MLT, and 10-20 Re in radial distance. Arrays of more than three spacecraft in non-collinear geometry are used to determine the propagation vector, including both the normal speed and direction, of such dipolarizations in the equatorial plane. The determined azimuthal propagation is ~3 deg/min, which corresponds to ~50 km/s at 6.6 Re. This speed is consistent with those obtained from two azimuthally separated spacecraft in previous works. Further analysis suggests that these azimuthally propagating dipolarizations (APDs) are often finger-like in shape, ranging from 5 to 20 Re in length and several Re in width. The reported APD may accompany the earthward flow channel and dipolarizing flux bundle (DFB).