Andrii Voshchepynets

Interaction faisceau-plasma dans un plasma aleatoirement non-homogene du vent solaire: Dans cette thèse nous avons présenté un modèle probabiliste auto cohérent décrivant la relaxation d'un faisceau d'électrons dans un vent solaire dont les fluctuations aléatoires de la densité ont les mêmes propriétés spectrales que celles mesurées à bord de satellites. On a supposé que, le système possédait différentes échelles caractéristiques en plus de l'échelle caractéristique des fluctuations de densité. Ceci nous a permis de décrire avec précision l'interaction onde-particule à des échelles inférieures à l'échelle caractéristique des fluctuations de densité en supposant que des paramètres d'onde sont connus: notamment, la phase, la fréquence et l'amplitude. Cependant, pour des échelles suffisamment plus grandes que l'échelle caractéristique des irrégularités de densité, l'interaction des ondes et des particules ne peut être caractérisée déterminé que par des quantités statistiques moyennes dans l'espace des vitesses à savoir: le taux de croissance/amortissement et le coefficient de diffusion des particules. En utilisant notre modèle, nous décrivons l'évolution de la fonction de distribution des électrons et d'énergie des ondes de Langmuir. Le schéma 1D suggérée est applicable pour des paramètres physiques de plasma du vent solaire à différentes distances du Soleil. Ainsi, nous pouvons utiliser nos calculs pour décrire des émissions solaires de Type III, ainsi que les interactions de faisceau avec le plasma, à des distances d'une Unité Astronomique du Soleil dans l'héliosphère et au voisinage des chocs planétaires.

The extended solar corona at 10–30 solar radii is essentially devoid of all waves below 100 kHz other than triggered ion acoustic waves (TIAWs), which consist of a low-frequency (LF) electromagnetic wave at a frequency of a few Hz coupled to one or more electrostatic waves at a few hundred Hz, such that the amplitudes of the high-frequency (HF) waves peak at a fixed phase of each LF wave period. All the waves in a TIAW event travel at the same phase speed, which is found to be 150 km s ⁻¹ (the ion acoustic speed was about 100 km s ⁻¹ ). It has not been possible to explain the TIAW as a resonant wave–wave interaction, so a nonresonant interaction has been considered in which the loss of energy by the LF wave is used to both heat the electrons and grow the HF waves. Evidence in support of this explanation is described, and a particle-in-cell simulation that discusses this process is summarized. This simulation demonstrates wave–particle interactions driven by the enhanced LF fluctuations, which subsequently modify the proton velocity distribution function and create conditions favorable for the growth of HF waves. This interplay between a pair of waves, mediated by modifications of plasma parameters and energy conversion, represents a significant nonlinear process in plasma physics, the study of which will deepen the understanding of energy transfer, wave generation, and plasma dynamics in diverse astrophysical environments.

On November 6, 2024, the Parker Solar Probe flew past Venus to make the first accurate electric field measurement in the nightside Venusian magnetosphere. To achieve this result, the electric field antennas were current biased in a way never before experienced by an electric field detector at Venus. This biasing requirement, that the positive bias current in the Venus shadow be about equal to the electron thermal current, is discussed and illustrated. About one minute of useful electric field data in the eight minute nightside magnetosphere crossing was obtained, during which the only feature observed was a few Hz signal. This result, along with the magnetic field measurements, showed that there were few if any electromagnetic waves, such as low frequency electromagnetic turbulence or whistlers, in the nightside crossing. Instead, a few Hertz, purely electrostatic signal was found. This suggests that the interaction of the solar wind with an unmagnetized body having an ionosphere may be different from that of previously studied magnetized bodies. In the sunlit flanks, many electromagnetic wave modes were observed.

Unmagnetized bodies with sufficiently dense ionospheres, such as Mars, form induced magnetospheres when interacting with the solar wind carrying the frozen-in interplanetary magnetic field (IMF). Mars Express equipped with the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) operating for 20 years over two solar cycles made fundamental contributions to our understanding of how the induced magnetosphere of Mars works. ASPERA-3 established the ion escape rate from 2 × 10²⁴ s⁻¹ to 4 × 10²⁴ s⁻¹ depending on the phase of the solar cycle. The measured empirical dependences of the escape rate on the solar wind dynamical pressure and UV fluxes allowed to determine the total atmospheric pressure lost over the past 4 billion years to be on the order of 10 mbars, i.e. small, contrary to long-standing expectations of a strong ion escape process. Comparing the measured escape rates from Mars with Venus and the Earth resulted in formulating the paradigm-shifting statement that the intrinsic magnetic field increases the escape rates and does not protect planetary atmospheres. Due to the long longevity of the mission, ASPERA-3 captured a number of extreme solar weather events and the unique encounter of Mars with the comet Siding-Spring. ASPERA-3 conducted the first-ever energetic neutral atom imaging of an induced magnetosphere, revealing the global periodic variability of the system, the significant precipitation of ENAs originating in the solar wind and magnetosheath, and the enhancement of ENA emissions from the Martian magnetic anomalies. ASPERA-3 conducted studies of the particles responsible for the discrete Martian aurora and characterized the precipitation of solar wind protons and alpha particles onto the atmosphere. The latter turned out to be a significant contribution to the helium balance on Mars. ASPERA-3 made several important findings outside its main science objectives among those are detection of the tentative signatures of backscattered ions from the Phobos surface, investigations for the first time of radar accelerated ions and electrons in non-magnetized environments, and measurements of heliospheric ENAs. Despite the significant progress following outstanding Mars Express results in the field of the Mars – solar wind interaction there is a broad spectrum of unsolved problems and unanswered questions to be addressed by future mission. The most fundamental one is the ionosphere – magnetosphere interactions.

Type III radio bursts are signatures of the fluxes of near-relativistic electrons ejected during solar flares. These bursts are frequently observed by spacecraft such as the Parker Solar Probe. It has been traditionally believed that these electron beams generate Langmuir waves through the two-stream instability, which are then converted into electromagnetic waves. In this study, we revise that model, by examining how the electron distribution becomes truncated due to the “time-of-flight” effect, as the beam travels through a randomly inhomogeneous and gently varying solar wind plasma. Rather than the two-stream instability, this truncation destabilizes the distribution and leads to the generation of Langmuir waves via a linear instability; we confine our analysis to this linear regime and do not take into account the backreaction of the generated Langmuir waves on the electron distribution, which is nonlinear. The instability grows until slower electrons arrive and dampen the waves. Our qualitative analysis shows that the resulting wave intensity growth and decay closely match the intensity–time profile of observed type III radio bursts at the fundamental frequency, supporting this modified theory.

On 2024 March 30, the Parker Solar Probe crossed the heliospheric current sheet at 13 solar radii while encountering two distinctly different regions at sharp boundaries on several occasions. The two regions had very different plasma densities, electric-field spectra, and magnetic-field geometries. In one region the strahl flowed only along the direction from the Sun, and there were relatively few particles at pitch angles near 90°, while in the other region the strahl flowed both toward and away from the Sun, and there were relatively many particles at pitch angles near 90°. These different properties are interpreted as being due to the spacecraft being in the heliospheric current sheet on long open magnetic field lines in the case of unidirectional strahl flow and the spacecraft being in coronal loops having much shorter closed magnetic field lines in the case with bidirectional strahl flow. The two regions intermingled on time scales less than 100 ms to create a complex magnetic field geometry. Broadband waves were observed in the open field-line regions, while narrowband electrostatic harmonic waves were observed in both regions. These harmonic frequencies correlated with the proton plasma frequency, f _pp, with the lowest frequency at ∼0.1f _pp. This result, plus the field-aligned electric field waves and plasma density fluctuations, requires that the observed electrostatic mode and associated harmonics were ion acoustic waves.

Spacecraft discharge time constants are calculated from measurements of electron differential flux before and during operation of an ionospheric sounding radar. Determining these time constants provides insight into how the operation of a sounding radar affects the surrounding plasma's interaction with the spacecraft. The analysis is enabled by the fixed‐frequency operation mode of a sounding radar which enhances resonant interaction with the ambient plasma. This mode's effect on measured energy spectra of ion and electron fluxes is described. Measurements of electron fluxes disturbed by radar operation serve as input to a model of spacecraft discharge for calculating capacitive discharge time constants. A case study using electron fluxes measured at Mars yields discharge time constants in the range 0.6–0.8 ms and reveals that a residual potential around V remains on the spacecraft long after radar operation ceases. The minimum spacecraft potential cannot be determined with these data and model due to the narrow energy range of electrons in the ambient plasma.

Solar type III radio bursts are amongst the most intense emissions found within Solar System. The bursts are generated by the relativistic electron beams ejected from the Sun as they propagate through the corona and solar wind. One the key parameters that can control beam plasma interactions is level of the density fluctuations. The density fluctuations can change the local phase velocity of the Langmuir waves generated by the beam instability, resulting in changes of the resonant conditions of wave-particle interaction. Changes in the wave phase velocity during the wave propagation can be described in terms of probability distribution function determined by distribution of the density fluctuations. This enables an approach that describes beam-plasma interaction with a system of equations, similar to well known quasi-linear approximation, but with the conventional velocity diffusion coefficient and the wave growth rate are replaced by the averaged in the velocity space. This approach, known as probabilistic model, allows to describe generation of the Langmuir waves in randomly inhomogeneous solar wind in self-consistent manner. Although previous version of the probabilistic model could explain some of the observational features of the emission, it had significant limitation, as it did not include time of flight effects. Here we present results of the numerical simulation based on an updated set of equations that can describe generation of the Langmuir waves by the electron beam as it propagates from the source region up to 10 Solar radii.

The self-consistent generation of type III radio emissions and their morphological features has been an open problem since the 1950s. Using data from the Parker Solar Probe, namely the radio frequency spectrometer (RFS) to analyze type III fine structures (striae) and link them to the statistical properties of density fluctuations from 13 to 60 solar radii obtained using the floating potential technique. We find that the average level of density fluctuations decreases with distance from the Sun, while the intermittency is high at the characteristic scales of the striae. Our findings demonstrate that the probabilistic model of beam-plasma interaction describes, step by step, the self-consistent generation of type III radio bursts, from the excitation of Langmuir waves to electromagnetic wave emission.