arXiv subjects

Mats André

Publications and source records attributed to Mats André.

4 recordsLinked to original sources

arXiv2026

Plasma frequency waves in Earth's electron foreshock

At Earth's quasi-perpendicular bow shock, electrons can be reflected and accelerated to high velocities, forming beams. These beams excite Langmuir and beam-mode waves, which can then be converted to radio waves. We aim to understand the properties and evolution of Langmuir waves excited in the electron foreshock region using the Magnetospheric Multiscale (MMS) mission. We use fields and particle data from the four MMS spacecraft to investigate the properties of Langmuir/Z-mode waves in Earth's electron foreshock. MMS provides extended high-resolution snapshots of the three-dimensional electric field, enabling detailed analysis of wave properties. Probability distributions of the electric field are used to investigate the evolution of the waves and the role of density fluctuations. Distinct spectral peaks near the electron plasma frequency are often observed, suggestive of simultaneous observations of beam-mode and Langmuir waves, as well as nonlinear electrostatic decay of Langmuir waves or reflection off density gradients. In addition, the electric fields often have large perpendicular components, consistent with Z-mode waves. The statistical results show that the electric fields are largest near the electron foreshock boundary with the solar wind. Both the parallel and perpendicular components of the electric field exhibit close to log-normal probability distribution functions, consistent with predictions from Stochastic Growth Theory. These results suggest that small-scale density perturbations in the ambient plasma, in addition to nonlinear three-wave decay, are crucial to the evolution of Langmuir waves and the generation of radio waves. These results apply to Langmuir waves in the solar wind, such as in Type II and Type III solar radio burst source regions, where the same density fluctuations are expected and large-amplitude Langmuir waves with similar properties are observed.

physics.space-ph↗

arXiv2026

Electron Heat Flux and Whistler Instability in the Earth's Magnetosheath

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 MMS in situ 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.

physics.space-ph↗

arXiv2024

Whistler waves in the quasi-parallel and quasi-perpendicular magnetosheath

In the Earth's magnetosheath (MSH), several processes contribute to energy dissipation and plasma heating, one of which is wave-particle interactions between whistler waves and electrons. However, the overall impact of whistlers on electron dynamics in the MSH remains to be quantified. We analyze 18 hours of burst-mode measurements from the Magnetospheric Multiscale (MMS) mission, including data from the unbiased magnetosheath campaign during February-March 2023. We present a statistical study of 34,409 whistler waves found using automatic detection. We compare wave occurrence in the different MSH geometries and find three times higher occurrence in the quasi-perpendicular MSH compared to the quasi-parallel case. We also study the wave properties and find that the waves propagate quasi-parallel to the background magnetic field, have a median frequency of 0.2 times the electron cyclotron frequency, median amplitude of 0.03-0.06 nT (30-60 pT), and median duration of a few tens of wave periods. The whistler waves are preferentially observed in local magnetic dips and density peaks and are not associated with an increased temperature anisotropy. Also, almost no whistlers are observed in regions with parallel electron plasma beta lower than 0.1. Importantly, when estimating pitch-angle diffusion times we find that the whistler waves cause significant pitch-angle scattering of electrons in the MSH.

physics.space-ph↗

arXiv2018

Ion velocity and electron temperature inside and around the diamagnetic cavity of comet 67P

A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the inner-most part of the coma. Here, we combine Langmuir and Mutual Impedance Probe measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2-4 km/s, significantly above the expected neutral velocity $\lesssim$1~km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least w.r.t. the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials $\lesssim$-5~V throughout the cavity, showing that a population of warm ($\sim$5~eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold ($\lesssim0.1$~eV) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.

physics.space-ph↗