Narges Ahmadi

Magnetic reconnection is a ubiquitous plasma phenomenon that plays a critical role in particle heating and energization. During reconnection, the topology of magnetic field rearranges, depositing energy into the surrounding plasma through bulk flow, thermal heating, or non‐thermal particle acceleration. While the pathways of this transformation from magnetic energy into kinetic have been studied extensively in recent years through theoretical or case‐by‐case observations, comprehensive statistical studies remain limited. In this paper, we present a statistical investigation using data from the Magnetospheric Multiscale (MMS) mission, and detail the particle energization mechanisms in magnetic structures found near reconnecting regions in turbulent Earth's magnetotail. We find that electrons with motion perpendicular to the magnetic field dominate dissipation. In contrast to the conventional picture of unidirectional energy transfer to particles by laminar two‐dimensional (2D) reconnection, we find that energy exchange within magnetic structures during turbulent reconnection tends to be bidirectional with only a small positive bias from electromagnetic fields to particles. Specific electron energization mechanisms are quantified, including those due to parallel electric field, Fermi energization from curvature drift, betatron heating from magnetic field inhomogeneity, and polarization drift.

Electric field instruments function by electrically coupling to the surrounding plasma, resulting in a response function that varies depending on local conditions. This variable coupling can complicate quantitative interpretation of wave measurements, yet is rarely considered. While this effect has recently been quantified for the Van Allen Probes, we present the first quantitative analysis for the Magnetospheric MultiScale (MMS) mission. First, MMS observations are evaluated against Faraday's Law, revealing that the angle between the measured whistler‐mode electric and magnetic fields directly depends on the wave propagation direction, a feature consistent with sheath impedance effects. A novel technique for determining electric field observations along each measurement direction is introduced, addressing limitations of previous works. This reveals that, for MMS in low‐density plasma, spin‐plane amplitudes are ∼60% of expected values, with small phase shifts, while spin‐axis measurements are accurate within 5%–10%, with phase shifts up to −20°. At intermediate densities, spin‐plane amplitudes match, or slightly exceed, expected values whereas spin‐axis observations can be overestimated by 70% and experience frequency‐dependent phase shifts. At high‐density, spin‐plane measurements generally agree with expected values, but spin‐axis observations are overestimated by ∼30% and experience ∼30°phase shifts. Accurate measurements are critical, with electric field fluctuations increasingly being used to infer wave‐particle energy exchange rates. If electric field observations can be under or over‐measured depending on the local plasma environment, this directly impacts these computations of energy exchange rates, potentially leading to a misinterpretation of the fundamental physical mechanisms that drive particle dynamics.

Cold protons play an important role in the Earth’s magnetosphere by modifying the dispersion relation of plasma waves. Their energy flux can be enhanced through a nonresonant response to electromagnetic ion cyclotron (EMIC) waves. This study combines Magnetospheric Multiscale (MMS) observations and a hybrid simulation to investigate cold proton dynamics during this nonresonant process. It is found that the energy flux of cold protons with kinetic energies below ~ 200 eV increases due to the bulk flow induced by EMIC waves, and this enhancement becomes stronger and extends to higher energies at higher magnetic latitudes. Despite the flux enhancement, the temperature and number density of cold protons remain constant. Moreover, we identify the formation mechanism of proton phase-bunching distribution in stationary gyrophase. During the nonresonant process, particles bunch in anti-phase with the wave magnetic fields, without exchanging energy with waves. In contrast, during the resonant process, particles bunch in anti-phase with the wave electric fields, facilitating energy transfer from particles to waves. We demonstrate that particle phase-bunching is a necessary but not sufficient condition for identifying resonant interactions with waves, and that the occurrence of energy transfer determines whether the response is nonresonant or resonant. Graphical Abstract

During a special MMS Turbulence campaign, almost 80 min of continuous high resolution burst measurements were downlinked from a traversal across Earth's magnetosheath behind mixed quasi‐parallel/perpendicular bow shock conditions. Throughout the magnetosheath, we observed magnetic holes containing intense whistler waves known as “Lion Roars” (LRs). We compared the observed properties of LRs to determine why the whistler waves included higher order harmonics in some magnetic holes but not in others. We refer to the LR events with and without harmonics as “Higher Order Harmonic LRs (HOHLRs)” and “nonharmonic LRs”, respectively. From our observations of the LR events, we found that each wave train featured an electron beam moving at the electron Alfvén speed and parallel to the background magnetic field. We also observed that this electron beam had a stronger phase space density for the LRs near the bow shock. For the HOHLR events, we observed that they exhibited a strong antiparallel Poynting flux and high counts of solitary waves surrounding their corresponding magnetic holes, with some detected solitary waves at the edges of the magnetic hole. Additionally, the fundamental frequencies for the HOHLRs were observed around 0.18–0.23 of the electron cyclotron frequency, . For the nonharmonic LR events, we observed a strong presence of solitary waves within the confines of their magnetic hole, leading to a population of trapped electrons with a parallel/antiparallel temperature anisotropy, strong fluctuations, and Poynting flux flowing in an oblique direction with respect to the background magnetic field. Additionally, the fundamental frequencies of nonharmonic LRs were lower, ranging from 0.11 to 0.18 .

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.

Particle acceleration in magnetic reconnection is a critical area that demands investigation for a better understanding of energization processes in space plasma. NASA's Magnetospheric Multiscale Mission detected multiple separatrix crossings during the asymmetric magnetic reconnection at Earth's subsolar magnetopause. This unique observation revealed a highly field‐aligned beam of electrons and the coexistence of strong parallel electric fields greater than −23 mV/m along with bipolar electric field structures. For the first time, we report the observation of multiple plasma double layers (DLs) in asymmetric magnetic reconnection. Based on observations, we have identified that DLs are generated through the current‐driven Buneman instability along the reconnection separatrix layer, a finding well supported by existing theoretical and simulation studies. Such local electric fields can accelerate thermal electrons up to the keV range, which can undergo further energization downstream of the reconnection.

The Kelvin-Helmholtz (KH) instability plays an important role in the transfer of energy and mass from the solar wind into planetary magnetospheres. Reconnection outflows have been observed in the spine region of KH vortices, suggesting that these instabilities can enhance the dissipation rate of energy stored in the magnetic field. For 4.5 hours between 2020-06-17 and 2020-06-18, the four Magnetospheric Multiscale (MMS) spacecraft observed KH wave activity at the dawn flank of the magnetotail. In this study, we examine 15 individual current sheets with magnetic reconnection signatures generated by KH waves at average (x,y,z) _GSE coordinates of (-18.23, -21.34, 3.44) R _E . Direct evidence of magnetic reconnection on the leading edge of KH vortices at extended distances down-flank is presented for the first time. Furthermore, ion velocity distribution functions (VDFs) demonstrate the presence of ion beams in 8 of the 15 events. Signatures consistent with a KH vortex roll-up test confirms the advanced, non-linear stage of KHI development.

This study reports on a dayside magnetic reconnection event detected by the Magnetospheric Multiscale mission in the presence of a moderate guide field ( times the reconnecting magnetic field on the magnetosphere side) and assumed to be present in the whole reconnection region. The spacecraft traversed the compressed magnetospheric separatrix region, observing cold ions with densities up to 10 and a large magnetosheath density up to 150 . We provide a detailed analysis of current densities, generalized Ohm's law, and energy conversion processes in both the spacecraft and the fluid frames during the separatrix crossing. The normal electric field is directed away from the separatrix due to the cold ion drift on the magnetosphere side and to the magnetosheath ion drift in the presence of a guide field in the exhaust region. In the spacecraft frame, energy transfers from the plasma to the fields due to the convective field associated with the earthward motion of the magnetopause and the ion diamagnetic current associated with the large density gradient. In the fluid frame, energy conversion reverses due to the magnetic field‐aligned current density and electric field produced by the divergence of the electron pressure tensor . Additionally, we give insights into the local changes in electromagnetic, bulk flow, and thermal energies. We show that flow and thermal energy variations of the plasma are mostly driven by the compressible term of the electron pressure strain at the separatrix.

We investigate properties of lower hybrid drift waves (LHDWs) near and inside the electron diffusion regions in 17 magnetopause and 9 magnetotail reconnection events using Magnetospheric MultiScale (MMS) mission observations. Our analysis show that LHDW type depend on the electron beta, as electron beta increases LHDWs become more electromagnetic in nature. The energy transfer from electromagnetic fields to particles is higher in electrostatic LHDWs and it is largely in parallel direction with respect to the local magnetic field. Linear dispersion analysis shows that electrostatic LHDWs are perpendicular propagating while electromagnetic waves propagate in oblique directions and the normalized wavenumber of all LHDW types falls within 0.5–0.8 range. A simple estimate on the LHDW nonlinear saturation suggests a possibly important roles played by these waves in supporting the reconnection electric field.