Alexandros Chasapis

Turbulence properties in the solar wind change with increasing heliospheric distance. Observations from different missions have shown how spectral indices converge toward an f ^−5/3 spectrum in the inertial range as the solar wind reaches 1 au and how the cascade rate decays as the plasma moves away from the Sun. We evaluate whether these properties display similar trends in a smaller astrophysical system, that of a planetary magnetosheath. Using data from the 2023 Magnetospheric Multiscale (MMS) unbiased magnetosheath campaign, we determine the evolution of the spectral indices and the cascade rate along 15 inbound magnetosheath crossings. WIND observations were permitted to monitor the state of the solar wind during the campaign and allowed to study the impact of the upstream plasma conditions on the evolution of the cascade rate downstream from the bow shock. Our results show an increase of the cascade rate between 2 and 3 orders of magnitude after the plasma crosses the bow shock into the magnetosheath. Once in the magnetosheath, turbulence develops with increasing distance to the bow shock in a way similar to solar wind observations, despite the smaller size of the system. Incompressible Hall magnetohydrodynamics (MHD) scaling-law estimates of the cascade rate show a clear power-law decay of the cascade rate in the magnetosheath, particularly for the cases with upstream slow wind. This suggests that the way energy is injected into the system plays a role in the decay of turbulence as the plasma moves away from the bow shock.

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.

Collisions are nearly negligible in many space and astrophysical plasmas, allowing charged-particle velocity distribution functions (VDFs) to depart from local thermodynamic equilibrium (LTE). How collisionless plasmas relax these non-LTE distributions and convert turbulent energy into particle heating remains an open question. We investigate deviations from LTE in ion velocity distribution functions (iVDFs) within collisionless plasma turbulence using high-resolution measurements from the Magnetospheric Multiscale (MMS) mission. We find that the iVDFs' non-bi-Maxwellian features are widespread and can be significant. Their complexity increases with ion plasma beta and turbulence intensity, with pronounced high-order non-LTE features emerging during intervals of large-amplitude magnetic field fluctuations. In addition, we show that turbulence-induced magnetic curvature plays a significant role in ion scattering and contributes to the isotropization of the iVDF. These results highlight the complex interaction between turbulence and the velocity distribution of charged particles, providing new insight into the kinetic processes responsible for energy conversion in collisionless plasmas.

We present a first record of significantly enhanced occurrences of magnetic reconnection exhausts as measured by the Wind satellite across a stream interaction region (SIR) at 1 AU from 10:11:20 UT on 4 Jan 2019 to 09:58:00 UT on 5 Jan 2019. The activity is clustered in a slow wind compression regime ahead of the SIR interface with a deflected, compressed fast wind. The 43 exhausts of this 1-day SIR dominate a distribution of 71 exhausts as obtained by a multi-window sliding technique application to the 8-day period on 1-9 Jan 2019. Active current sheets inside the SIR are associated with normal directions mostly near the ecliptic plane and a more azimuthal-than-Parker magnetic field direction at 1 AU. We find that exhausts wider than 500 ion inertial lengths are typically present just upstream and inside this SIR rather than within unperturbed slow and fast winds beyond a shocked solar wind. The observations suggest that plasma and field compressions may be crucial elements in driving a break-up of large-scale current sheets embedded in SIRs into smaller, multi-layered current sheet segments through magnetic reconnection.

In the Earth’s magnetosheath, several processes contribute to energy dissipation and plasma heating, one of which is wave-particle interactions between whistler waves and electrons. The whistler-heat flux instability is known to scatter the strahl electrons in the solar wind. However, the heat flux properties and evolution across the Earth’s magnetosheath region have not yet been explored.We present Magnetospheric Multiscale (MMS) observations from 18 hours of burst-mode measurements using the unbiased magnetosheath campaign. We quantify the electron heat flux in the magnetosheath and examine the role of whistler instabilities in regulating it. Our results show that the heat flux follows the magnetosheath magnetic field as it drapes around the magnetosphere. We find that the heat flux is constrained by whistler instability thresholds and is aligned with the propagation of low-frequency whistler waves. We also present a case study investigating direct evidence for the whistler-heat flux instability in the magnetosheath.

Plasma Observatory (PO) is one of the three candidate ESA M7-class missions currently in Phase A. Its primary goal is to investigate the fundamental multi-scale processes that govern plasma energization and energy transport within Earth's magnetospheric system. To address these objectives, PO will deploy a constellation of seven spacecraft in a double-nested tetrahedral configuration with a common vertex, enabling simultaneous measurements at both fluid and ion scales and, crucially, their coupling.Compared to previous multi-spacecraft missions such as Cluster and MMS, PO's expanded constellation introduces unprecedented opportunities to resolve multi-scale dynamics in space plasmas. However, these opportunities come with significant challenges. Realizing PO's full scientific potential requires the development and application of novel multi-point and advanced data analysis methodologies capable of exploiting measurements from more than four spacecraft.The Multi-Point and Advanced Data Analysis Methods Working Group has been established to support the mission's Science Study Team (SST) in evaluating how PO's science goals can be achieved through its unique configuration. The Working Group brings together expertise in multi-spacecraft diagnostics and the analysis of in situ plasma observations. We present the composition and ongoing activities of the Working Group, highlight the represented analysis methods (both established and under active development), and outline ongoing efforts to assess and enhance the scientific capabilities of the PO mission.

The relaxation of many physical systems is constrained by collisions. However, most space and astrophysical plasmas are nearly collisionless, leaving open questions about the pathways of energy transfer and dissipation. In many turbulent plasmas, the electric field takes on the role of energy transfer leading to dissipation. Using measurements from the Magnetospheric Multiscale Mission, we study the statistical properties of the electric field spectrum in the kinetic range of strong turbulence generated by magnetic reconnection in the Earth's magnetotail. From the inertial to the kinetic range (often called the dissipation range) of scales, we find that turbulent fluctuations develop increasingly non-Gaussian features. The kinetic range contains two regimes with distinct behaviors in the power spectrum and measures of non-Gaussianity. In the subelectron kinetic regime (smaller than the electron gyroradius), the turbulence becomes isotropic and exhibits energy equipartition between the electric field and magnetic field. Our analyses indicate (1) a growing presence of intermittent structures that are expected to lead to enhanced energy dissipation, (2) changes in the electric field dynamics at the transitions between turbulence regimes, and (3) an asymptotic relaxation to a state of energy equipartition in the electromagnetic field in the subelectron kinetic range, where the energy transfer between the magnetic and electric fields appears to be near complete. These results reveal the importance of the electric field in mediating turbulence dissipation and relaxation in collisionless plasmas.

In space plasmas, the rarity of collisions leads to complex structures in the velocity space where a turbulent cascade of the velocity distribution function (VDF) fluctuations is thought to occur. Previous studies have explored this phenomenon using the Hermite decomposition of the ion VDF in both magnetosheath data and numerical simulations. In this work, we investigate the Hermite spectrum of the ion VDFs measured by the Parker Solar Probe in the inner heliosphere. We analyze a super-Alfvénic stream at a radial distances of R ≈ 28 R _⊙ and a sub-Alfvénic stream at R ≈ 11 R _⊙ , the former characterized by a prevalence of VDFs with suprathermal beams (also known as hammerhead distributions). The Hermite analysis is also compared with various proxies of energization and dissipation, in order to establish a connection between turbulent cascades in real space and those in the velocity space. A qualitative agreement between the energization proxies and the Hermite analysis is observed. The results are suggestive of the presence of a dual cascade in real and velocity space.

In recent years, pressure–strain interaction, which describes a pathway of internal energy evolution due to a spatially varying bulk flow, has received significant attention in collisionless magnetic reconnection. However, the influence of an out-of-plane (guide) magnetic field on the structure and amplitude of the pressure–strain interactions during reconnection has yet to be systematically explored. In this study, we use 2.5-dimensional kinetic particle-in-cell simulations of antiparallel and guide field reconnection and explore the changes due to the guide field of the electron pressure–strain interaction and its decompositions close to the X-line. We find that the structure and amplitude of the electron pressure–strain interaction for larger guide field cases differ strongly from the antiparallel case; it is non-zero over a larger region of space, and its amplitude is considerably greater when there is a guide field. It is mostly dominated by Pi−D, especially the contribution due to velocity shear. Physically, this is because in guide field reconnection, the electrons form thin current sheets along one branch of the separatrices in the exhaust region, leading to strong velocity shear. We perform a scaling analysis near the electron diffusion region in the large guide field limit to estimate the value of electron pressure–strain interaction as a function of upstream parameters for symmetric guide field reconnection and confirm the result with the simulations. We anticipate that this scaling and structure will be useful for spacecraft observations of a wide range of guide field reconnection in Earth's magnetosphere and the solar wind.

We present a first record of significantly enhanced occurrences of magnetic reconnection exhausts as measured by the Wind satellite across a stream interaction region (SIR) at 1 au from 2019 January 4 10:11:20 UT to 2019 January 5 09:58:00 UT. The activity is clustered in a slow wind compression regime ahead of the SIR interface with a deflected, compressed fast wind. The 43 exhausts of this 1 day SIR dominate a distribution of 71 exhausts as obtained by a multiwindow sliding technique application to the 8 day period on 2019 January 1–9. Active current sheets inside the SIR are associated with normal directions mostly near the ecliptic plane and a more azimuthal-than-Parker magnetic field direction at 1 au. We find that exhausts wider than 500 ion inertial lengths are typically present just upstream and inside this SIR rather than within unperturbed slow and fast winds beyond a shocked solar wind. The observations suggest that plasma and field compressions may be crucial elements in driving a breakup of large-scale current sheets embedded in SIRs into smaller, multilayered current sheet segments through magnetic reconnection.