Ahmad Lalti

Collisionless shocks convert bulk flow energy into heat, electromagnetic fields, and non‐thermal particle populations. Recent studies suggest that downstream magnetic oscillations could play an important role in ion‐scale energy dissipation at low‐Mach‐number shocks; however, the specific shock and plasma parameters involved remain poorly understood. Interplanetary (IP) shocks, often characterized by low Mach numbers, provide an excellent opportunity for investigating these kinetic dissipation mechanisms. We demonstrate, using observations of an IP shock from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) missions, supported by test‐particle simulations, that gyrating protons generate the downstream magnetic oscillations. We found bursts of ion‐acoustic waves at the troughs and crests of the magnetic oscillations, suggesting their energy source is related to proton gyration. Comparing MMS and SolO observations, we conclude that the upstream flow speed to ion thermal speed ratio and magnetic compression ratio are key parameters controlling the ion kinetic behavior that produces downstream magnetic oscillations.

Earth is surrounded by two highly dynamic concentric belts of particle radiation. The outer radiation belt exhibits coherent variability at solar-cycle, seasonal, and 27-day (Carrington) time scales. While the solar-cycle and Carrington variations have been attributed to the recurrence of coronal hole solar wind, the seasonal variation has long been explained by local geometric effects that modulate the solar wind-magnetosphere coupling. Here, we challenge this paradigm by showing that the periodic recurrence of coronal hole solar wind likewise drives the seasonal variation. We further demonstrate that the Alfvénic nature of this solar wind is responsible for the observed electron flux enhancement in the outer radiation belt. These findings provide a unifying framework linking solar magnetic topology, solar wind properties, and magnetospheric dynamics across multiple time scales at Earth and beyond.

We report multipoint observations of a merger between a Short Large‐Amplitude Magnetic Structure (SLAMS) and 1‐Hz whistler wave packets upstream of Earth's quasi‐parallel bow shock. Using high‐resolution data from the Magnetospheric Multiscale (MMS) mission in a string‐of‐pearls configuration, we track the SLAMS as it evolves, exhibiting enhanced magnetic field strength, increasing spatial scale, variable ion reflection at the ramp, and increasingly complex morphology. The whistler wave packets show distinct propagation properties, propagating at a different velocity from the SLAMS and approaching it from upstream. Eventually, the SLAMS merges with the wave packets, altering its structure and forming a new ramp characterized by an intensified magnetic field and plasma density. These observations demonstrate how SLAMS and wave packets can coexist and interact, leading to changes in local plasma conditions and revealing the complexity and dynamic nature of bow shock transitions.

Not-yet-known not-yet-known not-yet-known unknown We investigate an interplanetary shock at two different heliocentric longitudes using data from the Magnetospheric Multiscale (MMS) and Solar Orbiter (SolO) spacecraft. We focus on the kinetic ion behavior to understand the wave environment around the shock at these distinct locations. Solar Orbiter observed the flank of the shock, while MMS crossed closer to its nose. The shock exhibited similar properties at both locations and comparable whistler precursors are observed by the two spacecraft. Despite these similarities, the ion behavior varied significantly between locations. At MMS, we observed magnetic fluctuations associated with protons gyrating as they crossed the ramp, reflected protons and alphas, and more intense ion-acoustic waves than at SolO. Notably, the ion-acoustic activity displayed a periodicity that corresponded with the low-frequency magnetic fluctuations. These findings indicate that solar wind parameters significantly influence the characteristics of interplanetary shocks.

Interplanetary (IP) shocks ripple through the heliosphere, altering the solar wind. These shocks are characterized by abrupt changes in the solar wind properties. Due to their size, different sections of an IP shock sweep through solar wind regions with different properties. Thus, the structure of an IP shock can differ across various longitudes. We investigate an IP shock at two different locations using the Solar Orbiter and Magnetospheric Multiscale (MMS) spacecraft. We found that the local shock profile is strongly influenced by the local solar wind conditions. Notably, downstream magnetic oscillations arise from gyrating protons in the region of the shock where faster incoming ions are present. Furthermore, we observe bursts of ion-acoustic waves occurring at the troughs and crests of the magnetic oscillations. This suggests a potential relationship between the ion-acoustic waves and the downstream magnetic oscillations, which are associated with the ion kinetic behavior.

Collisionless shocks can exhibit non‐stationary behavior even under steady upstream conditions, forming a complex transition region. Ion phase‐space holes, linked to shock self‐reformation and surface ripples, are a signature of this non‐stationarity. We statistically analyze their occurrence using 521 crossings of Earth's quasi‐perpendicular bow shock. Phase‐space holes appear in 65% of cases, though the actual rate may be higher as the holes may not be resolved during fast shock crossings. The occurrence rate peaks at 70% for shocks with Alfvén Mach numbers . These findings suggest that Earth's quasi‐perpendicular bow shock is predominantly non‐stationary.

We use the Magnetospheric Multiscale (MMS) mission to present a case study of electron acceleration at Earth's bow shock due to an interaction with a solar wind magnetic depression. The magnetic depression is identified as a reconnecting current sheet and is observed both at the bow shock, using MMS, and upstream of the shock at the Lagrange point 1 using the ACE, WIND, and DSCOVR spacecraft. The interaction with the current sheet and drop in magnetic field magnitude enables electrons to be accelerated from thermal energies (10–20 eV) up to suprathermal energies (1–5 keV) in a process producing a power‐law with a spectral index close to that predicted by first order Fermi acceleration . The observations are compared to a numerical model of Fermi acceleration considering two approaching magnetic mirrors and pitch angle scattering by whistler waves, and good agreement is shown. Thus, we add another piece to resolving the long‐standing electron injection problem.

The evolution of the properties of short‐scale electrostatic waves across collisionless shocks remains an open question. We use a method based on the interferometry of the electric field measured aboard the magnetospheric multiscale spacecraft to analyze the evolution of the properties of electrostatic waves across four quasi‐perpendicular shocks, with and . Most of the analyzed wave bursts across all four shocks have a frequency in the plasma frame lower than the ion plasma frequency and a wavelength on the order of 20 Debye lengths . Their direction of propagation is predominantly field‐aligned upstream and downstream of the bow shock, while it is highly oblique within the shock transition region, which might indicate a shift in their generation mechanism. The similarity in wave properties between the analyzed shocks, despite their different shock parameters, indicates the fundamental nature of electrostatic waves for the dynamics of collisionless shocks.

Ion‐acoustic waves are routinely observed at collisionless shocks and could be an important source of resistivity. The source of instability and the effects of the waves are not fully understood. We show, using Magnetospheric Multiscale mission observations and numerical modeling, that across low Mach number shocks a large relative drift between protons and alpha particles develops, which can be unstable to the proton‐alpha streaming instability. Linear analysis and a numerical simulation show that the resulting waves agree with the observed wave properties. The generated ion‐acoustic waves are predicted to become nonlinear and form ion holes, maintained by trapped protons and alphas. The instability reduces the relative drift between protons and alphas, and heats the ions, thus providing a source of resistivity at shocks.