Steven J. Schwartz

In Earth's collisionless magnetosheath, turbulence evolution and energy transformation across the system, especially at kinetic scales, remain poorly quantified without full‐coverage of high‐resolution observations. Using multi‐traversal observations from the 2023 and 2024 Magnetospheric Multiscale (MMS) Unbiased Magnetosheath Campaign, we construct cross‐sheath profiles of the electric fields () and find that the parallel electric field () peaks on the bow‐shock side and decreases rapidly toward the magnetopause. Enhanced defines an extended “turbulence evolution region” occupying ∼20% of the sheath, corresponding to several tens to order of 100 ion gyro‐radii (). This region is significantly larger than the reflected‐ion shock transition region (∼several to order of 10 ). The signals are primarily ion‐acoustic‐like wave packets, with occasional double layers and phase‐space holes. These observations suggest a region where enhanced corresponds to intensified sub‐kinetic activity and the early stage of magnetosheath turbulence evolution.

Space weather predictive modeling currently utilizes real‐time data from the spacecraft ACE and DSCOVR, with SOLAR‐1 and IMAP recently online. This study assesses whether DSCOVR solar wind plasma and interplanetary magnetic field (IMF) data exhibit large‐scale discrepancies from the corresponding ACE data, and if so, whether these discrepancies can be accounted for in predictive models. We chose several month‐long intervals of ACE and DSCOVR when they were maximally separated in Y location and separately in X location. In general, DSCOVR measurements of solar wind density, velocity and magnetic field magnitude correlate quite well with ACE. However, although the measured IMF magnitude is essentially identical between the two platforms, the various IMF components show significant differences, particularly in B _ZGSE . The data frequently display a significant (10–20 min) time offset between the two sets of measurements, consistent with the plasma structures being aligned with the Parker spiral. Comparison with MMS data taken in the solar wind confirm this variation from ballistic timing. Thus, space weather predicted by models relying on L1 monitors may be significantly impacted, representing a fundamental uncertainty in the prediction accuracy. Our results suggest that for the most accurate estimate of the IMF reaching the magnetopause, modelers should choose the spacecraft which is closest to the Sun/Earth line (−20 < Y _GSE < 10 R _E ) and nearest to Earth in X _GSE . However, for predictions with the longest lead time, as for time sensitive space weather alerts, the realtime monitor used should be the one with the largest + Y _GSE and X _GSE location.

Despite heat flux's role in regulating energy conversion in collisionless plasmas, its properties and evolution in the magnetosheath downstream of the Earth's bow shock are scarcely explored. We use Magnetospheric Multiscale measurements to quantify and characterize the electron heat flux in the magnetosheath. We find that the heat flux is shaped by the magnetosheath magnetic field as it drapes around the magnetosphere. While it is affected by solar wind upstream conditions and increases with magnetic field strength, it is not substantially changed by local magnetosheath processes. Also, the heat flux is limited by whistler instability thresholds.

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.

Particles are energized—heated and accelerated to nonthermal energies—in laboratory, space, solar, and astrophysical plasmas. In collisionless plasmas, ion and electron temperatures are often unequal and cannot be fully understood within the framework of magnetohydrodynamics (MHD). In this context, a relation, Δ ε _s = q _s VBL _s , for each species can be useful, where Δ ε _s is the energy gain for species s, measured in the plasma rest frame, relative to the upstream region of shocks and magnetic reconnection; q _s is the charge; V is the plasma bulk flow speed; B is the magnetic field strength; and L _s is a characteristic length scale of energization. From this relation, we recently derived semiempirical scalings for ion and electron temperature increases across shocks and magnetic reconnection in Earth’s plasma environment. However, it remains unclear how broadly these scalings apply. Here we show that the same scalings explain temperature increases in other plasma environments such as laboratory experiments, planetary magnetospheres, solar flares, and supernova remnant shocks. Combined with another recent report that the maximum energy of particles in various plasma environments follows the same relation when L _s is taken as the system size, our results indicate that Δ ε _s = q _s VBL _s provides a novel framework that universally captures particle energization—both heating and acceleration to nonthermal energies. Additionally, the scaling captures the essential MHD trends while revealing systematic deviations that point to kinetic effects beyond fluid models, highlighting promising directions for theoretical and simulation studies.

Shocks in collisionless astrophysical plasmas redistribute some of the incident flow energy into both thermal and non‐thermal energy. Quantifying the partition of that energy amongst various particle species or their sub‐populations, and electromagnetic energy, represents a fundamental goal of shock physics. It embodies the role of the equation of state for the system. Here we apply a framework to assess all the incident and downstream energy fluxes at a crossing of Earth's bow shock for which the upstream magnetic field was roughly aligned with the shock normal direction. Such quasi‐parallel shocks are known to be non‐steady and to produce significant populations of suprathermal particles. We quantify the evolution of all the important carriers of energy flux through the shock region. We sub‐divide the proton population into thermal, suprathermal, and energetic components in order to investigate the shock's efficiency in energizing the nonthermal particles. While the largest energy fluxes are found in the incident proton ram energy and downstream proton thermal enthalpy fluxes, a significant suprathermal population pervades the regions both up‐ and downstream. We also evaluate the energy fluxes attributable to fluctuations in the fluid and field parameters.

Shocks in collisionless astrophysical plasmas redistribute some of the incident flow energy into both thermal and non-thermal energy. Quantifying the partition of that energy amongst various particle species or their sub-populations, and electromagnetic energy, represents a fundamental goal of shock physics. It embodies the role of the equation of state for the system. Here we apply a framework to assess all the incident and downstream energy fluxes at a crossing of Earth's bow shock for which the upstream magnetic field was roughly aligned with the shock normal direction. Such quasi-parallel shocks are known to be non-steady and to produce significant populations of suprathermal particles. We quantify the evolution of all the important carriers of energy flux through the shock region. We sub-divide the proton population into thermal, suprathermal, and energetic components in order to investigate the shock's efficiency in energizing the nonthermal particles. While the largest energy fluxes are found in the incident proton ram energy and downstream proton thermal enthalpy fluxes, a significant suprathermal population pervades the regions both up- and downstream. We also evaluate the energy fluxes attributable to fluctuations in the fluid and field parameters.

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.

We report a rare regime of Earth's magnetosphere interaction with sub‐Alfvénic solar wind in which the windsock‐like magnetosphere transforms into one with Alfvén wings. In the magnetic cloud of a Coronal Mass Ejection (CME) on 24 April 2023, NASA's Magnetospheric Multiscale mission distinguishes the following features: (a) unshocked and accelerated low‐beta CME plasma coming directly against Earth's dayside magnetosphere; (b) dynamical wing filaments representing new channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope; (c) cold CME ions observed with energized counter‐streaming electrons, evidence of CME plasma captured due to by reconnection between magnetic‐cloud and Alfvén‐wing field lines. The reported measurements advance our knowledge of CME interaction with planetary magnetospheres, and open new opportunities to understand how sub‐Alfvénic plasma flows impact astrophysical bodies such as Mercury, moons of Jupiter, and exoplanets close to their host stars.

Whitepaper #432 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: fundamental space plasma theory; inner heliosphere; space-based missions/projects.