Maria E. Usanova

Wave‐particle interactions are a major mechanism responsible for dissipating energy and redistributing particles within geospace. This study investigates the effects of electromagnetic ion cyclotron waves on ring current ions. We present a case study of the effects of EMIC waves on the ring current during the 15 August 2015 geomagnetic storm. We use a kinetic model to simulate the ring current, including the effects of pitch‐angle scattering and local acceleration due to interactions with EMIC waves. We find that EMIC waves have energy‐ and species‐dependent effects on the ring current trapped and precipitating ion populations; for example, while EMIC wave‐driven local acceleration had minimal effects on ring current O ⁺ , the effects on H ⁺ were significant. Moreover, both mechanisms have distinct, and often opposing, effects on the ring current ions. These results show that both EMIC wave‐induced mechanisms play an important role in the dynamics of the ring current.

We investigate the magnetic ( B ) and electric ( E ) field spectra in the dissipation range of strong turbulence of a collisionless plasma. This investigation, which is relevant to turbulence studies in many astrophysical settings, is enabled by high-resolution measurements from the four-spacecraft Magnetospheric Multiscale (MMS) mission in the Earth’s magnetotail. B and E spectra are derived as a function of the product of the wave number and electron skin depth ( k d e ) using a novel technique that employs time-delay analysis on multiple intervals of B and E . Using the MMS tetrahedral formation with close (several d _e ) spacing, velocities of B and E signals can be derived so that native frequency-based spectra can be accurately translated to k spectra. The most important finding is a mathematically significant break in the B spectral index that appears at k d e ≈ 1 . In the subion range, which spans from the ion inertial length ( d _ι ) to d _e , the B spectral index is −2.35, then steepens to − 3.13 at sub- d _e scales. As expected from previously derived frequency spectra, E has a particularly shallow spectral index (−0.67) in the subion range. At scales smaller than d _e and/or the electron thermal gyroradius ( ρ _e ), the E spectral index steepens to −2.73. Spectral breaks in both B and E in the dissipation range indicate a change in the physical dissipation processes from ion to electron domination at k d e ≈ 1 . We also confirm that at k ρ e > ~ 2 , the energy density of B and E approaches equipartition, suggesting that energy transfer is near complete.

On 15 February 2018 a co‐rotating interaction region (CIR) from an equatorial coronal hole reached the Earth. The CIR initiated a moderate and slowly intensifying geomagnetic storm, which began with a large and strong substorm injection. The substorm injection was exceptionally well‐observed by an array of spacecraft including LANL‐GEO satellites, Van Allen Probes (RBSP), Arase (ERG), and MetOp/POES, as well as ground‐based instruments. These observations enable the unambiguous identification of several important features that have been impossible to measure directly in other events. The substorm injection extended well inside the geosynchronous orbit. A fortuitous conjunction of RBSP‐A (moving inbound) and Arase (simultaneously moving outbound at the same magnetic local time) allows us to establish, very precisely, the location of the inner edge of the injection region at L = 3.8−3.9. In supporting observations, North American riometers saw precipitation extending down to L ≈ 4 but not lower. Arase and RBSP‐A also observed whistler‐mode hiss waves inside the plasmasphere. Analysis of the resonance conditions shows, conclusively, and for the first time, that they were produced by the drifting injected electrons. RBSP‐A observations also show the injection (or transport) of electrons into or through the slot region within hours of the substorm injection onset. Previous studies were not able to clearly connect or separate substorm injections and slot‐filling processes. These new observations clearly identify slot‐filling as a spatially and temporally separate process that is not a direct result of substorm injection.

Electromagnetic ion cyclotron (EMIC) waves are plasma waves found in Earth’s magnetosphere and ionosphere, as well as in the solar wind and planetary magnetospheres. They drive losses of energetic electrons and protons that populate the radiation belts and ring current and can also heat lower-energy electrons and ions. Here, we analyze a Van Allen Probes event on 4 November 2015 during a period of dayside magnetospheric compression. Rising- and falling-tone EMIC waves were detected along with electromagnetic harmonics and proton heating in the few to ~tens of eV range. Quasi-linear diffusion modeling shows that high-frequency harmonics in the H⁺ band can transfer energy efficiently to low-energy protons via cyclotron resonance when the wave frequency nears the local proton cyclotron frequency. Our analysis suggests that intervals of increased solar wind dynamic pressure may lead to cold plasma heating through EMIC interactions on time scales of 2-3 minutes.

The Magnetospheric Multiscale mission has identified regions of magnetic reconnection surrounded by strong turbulence associated with particle acceleration in the Earth’s magnetotail. In these turbulent reconnection regions, ion and electron distributions have substantially elevated temperatures and display nonthermal tails indicating acceleration to over ∼100 times their initial thermal energies. We investigate electron energization and acceleration using 3D test-particle simulations based on the measured magnetic and electric fields in a representative magnetic reconnection geometry. We explore stochastic, Fermi, betatron, Speiser, and turbulence-boosted Speiser-like energization and acceleration. We find that stochastic energization and Speiser-like motion (Speiser orbits and turbulence-boosted Speiser-like orbits) dominate energization and acceleration at high turbulence levels and make the largest contribution to the nonthermal tail. Electron energization comes mostly from second-order impulses from electric fields with frequencies above the ion cyclotron frequency. Perpendicular electric fields contribute to the majority of the energization (∼75%). However, the exiting electron fluxes show the reverse; most (∼62%) of the electron energy flux exits as parallel energy. Trapping in the low magnetic field near the magnetic reconnection current sheet plays an important role by increasing dwell time in the turbulence, which enhances stochastic and Speiser-like processes. One of our primary conclusions is that magnetic reconnection and turbulence appear to combine as an efficient accelerator; the turbulent acceleration near a magnetic reconnection current sheet favors high-energy electrons, generating the energetic tail. Given its widespread presence in space and astrophysical plasmas, turbulent magnetic reconnection should be considered an important source of accelerated electrons.

In a first‐of‐its‐kind study utilizing the four Magnetospheric Multiscale satellites with accurate 3D electric field measurements, we characterize large‐amplitude (≥25 mV m ⁻¹ ) parallel electric fields ( E _|| ) in Earth's magnetotail during periods of magnetic reconnection‐driven strong turbulence. Twelve turbulent periods covering roughly 3 hours are examined. The results of this study reveal that the largest‐amplitude E _|| structures reside near the plasma sheet boundary layer and display a wide variety of nonlinear structures. Many of the phase‐coherent E _|| structures appear in roughly one second wave‐like packets over a broad range of frequencies. One of the most intriguing findings is that, despite the broad range of frequencies, the wavelengths of the E _|| structures are narrowly confined from ∼1 to ∼4 electron skin depths, which suggests that they are participating in the kinetic range of turbulent dissipation. Many of these wave‐like packets display a particular nonlinearity; the signals have a positive‐to‐negative asymmetry in which one polarity has sharp high‐amplitude peaks, whereas the opposite polarity displays broad, lower‐amplitude peaks. Another impactful finding is that a sub‐class of E _|| structures can carry net potentials greater than 10,000 V, which exceeds the electron temperature in eV. This study indicates a rich variety of behavior in E _|| and its potential role in the evolution and dissipation of turbulence.

Electromagnetic ion cyclotron (EMIC) waves are discrete electromagnetic emissions separated by multiple ion gyrofrequencies. Harmonic EMIC waves are defined as waves with a strong electric or magnetic field (or both) at the harmonics of the fundamental EMIC mode. In this paper, for the first time, we present a statistical study on harmonic EMIC waves by the Van Allen Probes. The EMIC waves are categorized into three types based on their harmonics: (a) Fundamental mode only (without higher harmonics), (b) electrostatic (ES) harmonics, and (c) electromagnetic (EM) harmonics. We find that most harmonic EMIC wave events are observed on the dayside, outside the plasmasphere and are associated with a low , a high proton , and a strong fundamental EMIC mode. ES harmonic EMIC waves tend to exhibit a larger ratio compared to EM harmonic EMIC waves. The results advance our understanding of harmonic EMIC waves and their generation mechanisms.

Cold plasma originating from the ionosphere is a highly influential component of the magnetosphere, dominating its mass. It is typically treated as a passive background represented as an empirical density in models of the ring current, outer radiation belt, and wave propagation. While it is well established that cold plasma density plays a critical role in controlling wave properties - such as amplitude and wave vector - increasing theoretical and observational evidence suggests that cold plasma populations may actively couple with waves, exchanging energy through processes that are not yet fully understood. Importantly, these processes can energize cold plasma, gradually transforming it into warm and eventually hot plasma. This mini-review explores the role of wave-particle interactions in cold and warm plasma heating, highlighting their significance in facilitating coupling between different plasma populations in Earth’s magnetosphere.

Electromagnetic ion cyclotron (EMIC) waves are believed to play an important role in the dynamics of the inner magnetosphere, including the ring current, the radiation belts and potentially, the cold plasma. In this work, we investigate their occurrence in the magnetosphere and the geomagnetic and solar wind conditions which lead to their excitation. We use an automated detection algorithm of EMIC waves observed by Van Allen Probes over the entire mission duration between 2012 and 2019. Consistent with earlier studies, we find that the H⁺ band occurrence maximizes in the dayside magnetosphere during enhancements of solar wind dynamic pressure. Both the H⁺ and He⁺ band are also generated along the duskside magnetosphere during disturbed geomagnetic conditions. In addition, to H⁺ and He⁺ bands commonly surveyed, we investigate the occurrence of H⁺ waves above and below 0.5 H⁺ gyrofrequency, as well as wave occurrence in the N⁺ and O⁺⁺ bands. Most H⁺ waves are observed in the band below 0.5f_H+. We find several events in the N⁺ band, indicative of their very low occurrence. The O⁺⁺ band is observed during disturbed geomagnetic conditions and high solar wind dynamic pressure at low L‐shells. Its radial localization coincides with the O⁺⁺ torus. This study provides a comprehensive picture of EMIC wave distribution and insight into ion composition in the inner magnetosphere during variable geomagnetic conditions.

Magnetic reconnection and plasma turbulence are ubiquitous and key processes in the Universe. These two processes are suggested to be intrinsically related: magnetic reconnection can develop turbulence, and, in turn, turbulence can influence or excite magnetic reconnection. In this study, we report a rare and unique electron diffusion region (EDR) observed by the Magnetospheric Multiscale mission in the Earth’s magnetotail with significantly enhanced energetic particle fluxes. The EDR is in a region of strong turbulence within which the plasma density is dramatically depleted. We present three salient features. (1) Despite the turbulence, the EDR behaves nearly the same as that in 2D quasi-planar reconnection; the observations suggest that magnetic reconnection continues for several minutes. (2) The observed reconnection electric field and inferred energy transport are exceptionally large. However, the aspect ratio of the EDR (one definition of reconnection rate) is fairly typical. Instead, extraordinarily large-amplitude Hall electric fields appear to enable the strong energy transport. (3) We hypothesize that the high-energy transport rate, density depletion, and the strong particle acceleration are related to a near-runaway effect, which is due to the combination of low-plasma-density inflow (from lobes) and possible positive feedback between turbulence and reconnection. The detailed study on this EDR gives insight into the interplay between reconnection and turbulence, and the possible near-runaway effect, which may play an important role in other particle acceleration in astrophysical plasma.