Sohom Roy

The connection between turbulence and solar‐wind acceleration, long known in space physics, is further developed in this study by establishing a robust empirical law that relates the bulk‐flow speed to the magnetohydrodynamic‐scale fluctuation energy in the plasma. The model is based on analysis of 25 years of near‐Earth observations by NASA's Advanced Composition Explorer . It provides a simple way to estimate turbulence energy from low‐resolution speed data—a practical approach that may be of utility when high‐resolution measurements or advanced turbulence models are unavailable. Potential heliospheric applications include space‐weather forecasting operations, remote imaging data sets, and energetic‐particle transport models that require turbulence amplitudes to specify diffusion parameters.

In this project, we are exploring a few aspects of low frequency and wavenumber magnetic field energy spectra in the context of space physics, following observations of 1/f behavior (e.g. [1]-[4]). The origin of this phenomena is still debated; however studies have suggested that these processes could be generated from scale-invariant processes in the corona or further within the dynamo of the Sun. One of the paradigms that has been discussed ([5],[6]) for achieving scale invariant structure is the merger of two dimensional or quasi-two dimensional magnetic flux tubes or flux ropes. This may be particularly relevant in the corona. To further explore this connection, it becomes necessary to understand the distributions of size and magnetic flux content, as well as the morphology of magnetic structures/”islands” in two dimensional turbulence representations. These features of the magnetic field will be explored using methods described herein [7]. These will be implemented using magnetic fields obtained from synthetic construction and 2D simulation. [1]Burlaga, L. F., & Ness, N. F. 1998, JGR, 103, 29 719[2]Matthaeus, W. H., & Goldstein, M. L. 1986, PhRvL, 57, 495[3]Wang, J., Matthaeus, W. H., Chhiber, R., et al. 2024, SoPh, 299, 169[4]Pradata, R. A., Roy, S., Matthaeus, W. H., et al. 2025, ApJL, 984, L23[5]Matthaeus, W. H., & Goldstein, M. L. 1982, JGR, 87, 6011[6]Mullan, D.J.: 1990, Astron. Astrophys. 232, 520.[7]Servidio, S., Matthaeus, W., Shay, M., et al. 2010, Physics of Plasmas, 17

One of the key questions about magnetic reconnection is to understand how energy is partitioned between ions and electrons, especially inside the EDR and in the outflow regions. This requires studying the energy transport terms corresponding to kinetic, thermal and electromagnetic energies respectively, along with the energy conversion terms. Previous studies have shown that ion energy flux dominates close to the EDR in magnetopause reconnection, while the electron energy flux is dominant inside it. However, one must be careful while computing the energy transport terms using MMS data, since the results can be dominated by uncertainty. This is particularly true for magnetotail reconnection, where the plasma is tenuous. Here, we present a detailed analysis of the errors in these energy transport terms, and perform a comparative study between reconnection events observed in the magnetopause, magnetosheath and magnetotail regions.

Turbulent small‑scale dynamo action, magnetic reconnection, and kinetic instabilities in fully three‑dimensional magnetosheath turbulence must be investigated together to understand how energy is exchanged, redistributed, and dissipated in a collisionless plasma. Clarifying how these processes coexist and how they may sequence in time is essential for understanding turbulent energy transfer at sub‑ion scales. Using high‑resolution tetrahedral MMS observations in the magnetosheath, we compute a suite of diagnostics that characterize the dynamical role of velocity‑gradient structures, including field‑aligned stretching of the magnetic field, compressive motions, pressure–strain interactions, field–particle energy conversions, and pressure‑anisotropy instability measures. All quantities are derived directly from MMS time series. The measurements errors in the considered quantities are evaluated through Monte‑Carlo–based uncertainty analysis. As a working hypothesis, we examine whether regions with strong field‑aligned stretching or compression tend to coincide with magnetic‑field amplification associated with pressure‑anisotropy instabilities, conditions that may be favorable for turbulent dynamo‑like behavior. Conversely, we test whether intervals containing potentially reconnecting thin current sheets exhibit enhanced current density, elevated field particle and pressure-strain interactions and anisotropy relaxation. To explore the temporal relationships between these processes, we apply cross‑correlation analysis to the above diagnostic measures. This approach allows us to assess whether dynamo‑like amplification statistically precedes current‑sheet formation and dissipation, or whether these processes tend to overlap. Early results suggest that both ordered sequences and simultaneous occurrences are possible, reflecting the intermittent and multi‑scale nature of collisionless turbulence. The combined diagnostic and uncertainty‑quantification framework offers a possibility to evaluate the occurrence rates of magnetic‑field amplification, reconnection, and dissipation processes in collisionless space plasmas.

Turbulence in the terrestrial magnetosheath drives rapid energy exchanges between electromagnetic fields and flows through strong intermittent compressions, shear layers, and velocity gradient structures. These concurrent and competing processes can generate temperature anisotropies and drive plasma instabilities. Yet the dynamical pathways linking velocity-gradient processes to anisotropy evolution in compressible collisionless plasmas remain poorly understood. We combine high cadence multi-point MMS measurements to quantify the pressure–strain interaction Π: ∇u (decomposed into compressible and incompressible parts), the non-ideal work J·E′, and the electron heat flux q (and ∇·q, where the signal-to-noise ratio is sufficiently large) for selected turbulent magnetosheath intervals. Physically motivated thresholds (percentile-based and background relative) identify episodes of enhanced Π: ∇u, J·E′, and heat flux activity. Then, the electron temperature anisotropy Te⊥/Te, versus parallel electron plasma βe(“Brazil”) plots are obtained from the time series under investigation, with added theoretical thresholds corresponding to whistler and firehose instabilities. In this parameter space, the trajectories of the plasma, associated with the various enhanced energy conversion and transport terms, are visualized. Case studies and ensemble statistics reveal that a dominance of different channels occurs in overlapping but non-identical regions: Π: ∇u peaks are associated with rapid anisotropy excursions and compressive structures, J·E′, with localized current and electromagnetic activity, and heat flux events with directed heat-transport toward whistler and firehose thresholds. This approach offers a practical pathway to quantify how turbulence and localized structures push plasma toward or beyond linear instability thresholds, with implications for modeling dissipation and wave generation in collisionless plasmas.

The Kelvin–Helmholtz instability (KHI) is a shear-driven phenomenon that generates a chain of vortices, located along the shear layer. As these vortices grow, they interact and fragment, eventually leading to turbulence and the dissipation of kinetic energy. The exact pathway through which KHI moves and converts energy across scales still remains elusive. Using Magnetospheric Multiscale (MMS) spacecraft data, we explore energy conversion pathways during KHI events at Earth's magnetopause.We quantify, for the first time, KHI energy conversion channels via pressure–strain and J·E′ diagnostics. Enhanced energy conversion between flow and thermal energy is observed inside vortices, associated with both local non-thermal features and perpendicular temperature anisotropies. Conversely, at the boundaries, enhanced magnetic fluctuations are associated with peaks in the ion agyrotropy. Finally, we investigate how reconnecting current sheets, observed at the vortex boundaries, affect energy conversion terms.

Close to the Sun, Parker Solar Probe (PSP) traverses the sub-Alfvénic solar wind, a magnetically controlled plasma environment. Farther from the Sun, the magnetic field amplitude and plasma density weaken enough to establish a super-Alfvénic environment where kinetic energy and turbulence become more prominent. Switchbacks (i.e. large directional deviations from the mean field) are shown to occur almost exclusively in the super-Alfvénic regime. In this study, we analyse magnetic fluctuations from PSP encounters 8 through 19 to study their relationship to Alfvén Mach number ($M_\mathrm{ A}$) and switchback parameter (Z). We find that the fluctuation of the magnetic field magnitude normalized to the mean magnetic field and of the radial velocity normalized to local Alfvén speed increase with both Alfvén Mach number and switchback parameter. However, there is distinct saturation in the increments of the normalized radial velocity fluctuations in the Alfvén Mach number range of $\sim$4–6. Results are interpreted in terms of the Chandrasekhar criterion for Kelvin–Helmholtz activity. Overall, these findings are in agreement with earlier studies suggesting switchback generation through non-linear shear flow dynamics.

Close to Earth, the solar wind is usually super-Alfvénic, i.e., the speed of the solar wind is much larger than the Alfvén speed. However, in the lower coronal regions, the solar wind is mostly sub-Alfvénic. With the Parker Solar Probe (PSP) crossing the boundary between the sub- and super-Alfvénic flow, R. Bandyopadhyay et al. performed a turbulence characterization of the sub-Alfvénic solar wind with initial data from encounters 8 and 9. In this study, we reexamine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency, and switchback strength using PSP data from encounters 8–19. The later orbits probe lower altitudes and experience sub-Alfvénic conditions more frequently, providing a greater statistical coverage to contrast sub- and super-Alfvénic solar wind. These later orbits also extend the observations from near solar minimum at launch to near solar maximum conditions. Also, by isolating the intervals where the solar wind speed is approximately equal to the Alfvén speed, we explore the transition in more detail. We show that the amplitude of the normalized magnetic field fluctuation is smaller for the sub-Alfvénic samples. While solar wind turbulence in general is shown to be anisotropic, the sub-Alfvénic samples are more anisotropic than the super-Alfvénic samples, in general. Further, we show that the sub- and super-Alfvénic samples do not show much distinction in terms of intermittency strength. Finally, consistent with prior results, we find no evidence for polarity reversing >90° switchbacks in the sub-Alfvénic solar wind.

We study the statistics of dynamical quantities associated with magnetic reconnection events embedded in a sea of strong background magnetohydrodynamic turbulence using direct numerical simulations. We focus on the relationship of the reconnection properties to the statistics of global turbulent fields. We show that the distribution in turbulence of reconnection rates (determined by upstream fields) is strongly correlated with the magnitude of the global turbulent magnetic field at the correlation scale. The average reconnection rates, and associated dissipation rates, during turbulence are thus much larger than predicted by using turbulent magnetic field fluctuation amplitudes at the dissipation or kinetic scales. Magnetic reconnection may, therefore, be playing a major role in energy dissipation in astrophysical and heliospheric turbulence.

One of the central problems in collisionless plasma turbulence is to understand the nature of the cross-scale energy transfer and dissipation occurring at kinetic scales. Magnetic reconnection plays an important role in dissipating energy and driving energy transfer in these systems. However, for reconnection, a detailed picture of the fluxes of energy in its several forms, and the conversion among them, until now remains unclear. In this study, a scale filtering method is adopted to explore how energy is converted between different forms and transferred across scales in turbulent magnetic reconnection using a combination of Magnetospheric Multiscale observations and particle-in-cell simulations. Two novel findings of this paper are that both ion and electron dissipation start to occur at sub-ion scale, at nearly same scale, and that most of the energy cascade due to reconnection is carried by the electron fluid.