arXiv subjects

Ahmad Lalti

Publications and source records attributed to Ahmad Lalti.

11 recordsLinked to original sources

arXiv2026

Solar Magnetic Configuration Control over Radiation Belt Electrons

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) timescales. 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 Alfv\'enic 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 timescales at Earth and beyond.

physics.space-ph↗

arXiv2025

Occurrence of Non-Stationarity at Earth's Quasi-Perpendicular Bow Shock

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 $M_A>7$. These findings suggest that Earth's quasi-perpendicular bow shock is predominantly non-stationary.

physics.space-ph↗

arXiv2025

Debye-scale electrostatic waves across quasi-perpendicular shocks

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 $1.4 \leq M_A \leq 4.2$ and $66^\circ \leq θ_{Bn} \leq 87^\circ$. Most of the analyzed wave bursts across all four shocks have a frequency in the plasma frame $f_{pl}$ lower than the ion plasma frequency $f_{pi}$ and a wavelength on the order of 20 Debye lengths $λ_D$. 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.

physics.space-ph↗

arXiv2025

Revealing an unexpectedly low electron injection threshold via reinforced shock acceleration

Collisionless shock waves, found in supernova remnants, interstellar, stellar, and planetary environments, and laboratories, are one of nature's most powerful particle accelerators. This study combines in situ satellite measurements with recent theoretical developments to establish a reinforced shock acceleration model for relativistic electrons. Our model incorporates transient structures, wave-particle interactions, and variable stellar wind conditions, operating collectively in a multiscale set of processes. We show that the electron injection threshold is on the order of suprathermal range, obtainable through multiple different phenomena abundant in various plasma environments. Our analysis demonstrates that a typical shock can consistently accelerate electrons into very high (relativistic) energy ranges, refining our comprehension of shock acceleration while providing insight on the origin of electron cosmic rays.

astro-ph.HE↗

arXiv2025

Ion-Acoustic Waves and the Proton-Alpha Streaming Instability at 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 (MMS) 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. The results from 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.

physics.space-ph↗

arXiv2024

Multi-Mission Observations of Relativistic Electrons and High-Speed Jets Linked to Shock Generated Transients

Shock-generated transients, such as hot flow anomalies (HFAs), upstream of planetary bow shocks, play a critical role in electron acceleration. Using multi-mission data from NASA's Magnetospheric Multiscale (MMS) and ESA's Cluster missions, we demonstrate the transmission of HFAs through Earth's quasi-parallel bow shock, associated with acceleration of electrons up to relativistic energies. Energetic electrons, initially accelerated upstream, are shown to remain broadly confined within the transmitted transient structures downstream, where betatron acceleration further boosts their energy due to elevated compression levels. Additionally, high-speed jets form at the compressive edges of HFAs, exhibiting a significant increase in dynamic pressure and potentially contributing to driving further localized compression. Our findings emphasize the efficiency of quasi-parallel shocks in driving particle acceleration far beyond the immediate shock transition region, expanding the acceleration region to a larger spatial domain. Finally, this study underscores the importance of multi-scale observational approach in understanding the convoluted processes behind collisionless shock physics and their broader implications.

astro-ph.HE↗

arXiv2024

Electron heating at quasi-perpendicular collisionless shocks

Adiabatic and non-adiabatic electron dynamics have been proposed to explain electron heating across collisionless shocks. We analyze the evolution of the suprathermal electrons across 310 quasi-perpendicular shocks with $1.7<M_A<48$ using in-situ measurements. We show that the electron heating mechanism shifts from predominantly adiabatic to non-adiabatic for the Alfvénic Mach number in the de Hoffman-Teller $\gtrsim 30$ with the latter constituting 48\% of the analyzed shocks. The observed non-adiabatic heating is consistent with the stochastic shock drift acceleration mechanism.

physics.space-ph↗

arXiv2023

Backstreaming ions at a high Mach number interplanetary shock: Solar Orbiter measurements during the nominal mission phase

Solar Orbiter, a mission developed by the European Space Agency, explores in situ plasma across the inner heliosphere while providing remote-sensing observations of the Sun. Our study examines particle observations for the 30 October 2021 shock. The particles provide clear evidence of ion reflection up to several minutes upstream of the shock. Additionally, the magnetic and electric field observations contain complex electromagnetic structures near the shock, and we aim to investigate how they are connected to ion dynamics. The main goal of this study is to advance our understanding of the complex coupling between particles and the shock structure in high Mach number regimes of interplanetary shocks. We used observations of magnetic and electric fields, probe-spacecraft potential, and thermal and energetic particles to characterize the structure of the shock front and particle dynamics. Furthermore, ion velocity distribution functions were used to study reflected ions and their coupling to the shock. To determine shock parameters and study waves, we used several methods, including cold plasma theory, singular-value decomposition, minimum variance analysis, and shock Rankine-Hugoniot relations. To support the analysis and interpretation of the experimental data, test-particle analysis, and hybrid particle in-cell simulations were used. The ion velocity distribution functions show clear evidence of particle reflection in the form of backstreaming ions several minutes upstream. The shock structure has complex features at the ramp and whistler precursors. The backstreaming ions may be modulated by the complex shock structure, and the whistler waves are likely driven by gyrating ions in the foot. Supra-thermal ions up to 20 keV were observed, but shock-accelerated particles with energies above this were not.

physics.space-ph↗

arXiv2022

Short wavelength electrostatic wave measurement using MMS spacecraft

Determination of the wave mode of short-wavelength electrostatic waves along with their generation mechanism requires reliable measurement of the wave electric field. We investigate the reliability of the electric field measurement for short-wavelength waves observed by MMS. We develop a method, based on spin-plane interferometry, to reliably determine the full 3D wave vector of the observed waves. We test the method on synthetic data and then apply it to ion acoustic wave bursts measured in situ in the solar wind. By studying the statistical properties of ion acoustic waves in the solar wind we retrieve the known results that the wave propagation is predominantly field-aligned. We also determine the wavelength of the waves. We find that the distribution peaks at around 100 m, which when normalized to the Debye length corresponds to scales between 10 and 20 Debye lengths.

physics.space-ph↗

arXiv2021

Whistler Waves in the foot of Quasi-Perpendicular Super-Critical Shocks

Whistler waves are thought to play an essential role in the dynamics of collisionless shocks. We use the magnetospheric multiscale (MMS) spacecraft to study whistler waves around the lower hybrid frequency, upstream of 11 quasi-perpendicular super-critical shocks. We apply the 4-spacecraft timing method to unambiguously determine the wave vector $\mathbf{k}$ of whistler waves. We find that the waves are oblique to the background magnetic field with a wave-normal angle between $20^{\circ}$ and $42^{\circ}$, a wavelength around 100 km which is close to the ion inertial length. We also find that $\mathbf{k}$ is predominantly in the same plane as the magnetic field and the normal to the shock. By combining this precise knowledge of $\mathbf{k}$ with high-resolution measurements of the 3D ion velocity distribution we show that a reflected ion beam is in resonance with the waves, opening up the possibility for wave-particle interaction between the reflected ions and the observed whistlers. The linear stability analysis of a system mimicking the observed distribution, suggests that such a system can produce the observed waves.

physics.space-ph↗

arXiv2021

Automated classification of plasma regions using 3D particle energy distributions

We investigate the properties of the ion sky maps produced by the Dual Ion Spectrometers (DIS) from the Fast Plasma Investigation (FPI). We have trained a convolutional neural network classifier to predict four regions crossed by the MMS on the dayside magnetosphere: solar wind, ion foreshock, magnetosheath, and magnetopause using solely DIS spectrograms. The accuracy of the classifier is >98%. We use the classifier to detect mixed plasma regions, in particular to find the bow shock regions. A similar approach can be used to identify the magnetopause crossings and reveal regions prone to magnetic reconnection. Data processing through the trained classifier is fast and efficient and thus can be used for classification for the whole MMS database.

physics.space-ph↗