Marian Lazar

Here, we discuss an extension of the modelling of the collisionless heat flux and the related parameter α , derived by Hollweg for a truncated Maxwellian to distributions with suprathermal tails. In the collisionless solar wind plasma, electrons and ions are described by a multitude of non-Maxwellian (nonequilibrium) velocity distributions, such as the standard, regularized κ , and Druyvesteyn distributions. We show that for these nonidealized distributions, the heat flux and the related parameter α can deviate significantly from the Maxwellian values derived by Hollweg. It is also shown that the heat flux is best described using a form that does not directly involve the truncation speed. Furthermore, we discuss the applicability of these distribution models for the nonrelativistic case, which constrains the κ values for the standard κ distribution to values above 4. In addition, we provide the possibility to easily obtain the heat flux parameter for a given solar wind environment.

Realistic modeling of type III solar radio bursts is essential for remotely diagnosing their exciters, the driving electron beams, and the ambient plasma. A new method is proposed to construct synthetic dynamic spectra of type III solar radio bursts, with frequency decreasing as a function of the distance or time traveled by their electron-beam plasma source in interplanetary space. This work introduces the first radio spectra computed from particle-in-cell (PIC) simulations within a dynamic model that is both quantitative and self-consistent, yet simplified by neglecting propagation effects (e.g., scattering and dissipation). Relevant to type III radio emissions, the input parameterization yields complex spectra that include the fundamental emission and the second and third harmonics, and thus determine a broadband consistent with that of the observations. These radio components are delimited and highlighted, enabling computations of their energy density on the basis of simulated first-principle mechanisms.

Context. Suprathermal electrons significantly contribute to the heat flux in the solar wind (SW), making their treatment essential for improved SW modelling. The still-significant presence of suprathermal electron populations (halo and strahl), recently revealed by the Parker Solar Probe in the young SW, makes their involvement in energy transfer between plasma populations highly plausible. Aims. We aim to quantify the impact of suprathermal electrons on steady-state SW solutions by incorporating their contribution to thermal conduction in a macroscopic magneto-hydrodynamic (MHD) framework. Methods. The Icarus heliosphere modelling tool implemented within the MPI-AMRVAC framework was used to simulate a steady-state SW. The effect of heat flux carried by free-streaming electrons was included through a source term in the energy equation. We adopted the collisionless heat flux prescription of Hollweg (1974, J. Geophys. Res., 79, 3845), ordered by a structural parameter ( α _H ) that has now been adjusted ( α _H → α _κ ) to incorporate the influence of kinetic processes that shape the electron velocity distribution. The new, still simplified version proposed here ( α _κ ) accounts for not only the skewness determined by the strahl but also the Kappa suprathermal tails of halo populations. The SW is driven with the semi-empirical Wang-Sheeley-Arge (WSA) coronal model. Our computations use the WSA maps as the inner boundary (IB) in Icarus, but also apply modified-IB conditions to reduce interactions between fast and slow wind streams and to adjust the IB pressure (and hence temperature) to the data recently reported by the Parker Solar Probe (PSP). Results. Including collisionless heat flux can considerably reduce the rate of temperature decrease with heliocentric distance, preventing cooling as rapidly as that dictated by adiabatic expansion. As expected, heat is deposited primarily in lower-temperature regions, tending to level out the structured temperature(-velocity) distribution. Furthermore, heat flux contributes to accelerating the SW, exerting a less significant yet concrete influence on radial velocity compared to temperature variations. These effects persist for both the original WSA and modified-IB conditions and can be markedly amplified by an enhanced heat flux associated with suprathermal Kappa-distributed electrons. Consistent with PSP observations, higher pressure at the IB further enhances SW acceleration, magnifying the impact of the heat flux. Conclusions. The implications of collisionless heat flux depend, to a large extent, on the IB conditions, particularly on the pressure and bulk speed of the young SW. For example, a lower thermal pressure (or temperature) at the IB can be compensated for by a heat flux input. The improved agreement between modelled and observed radial profiles in fast SWs is consistent with the enhanced electron strahl that is more specific to higher bulk speeds.

Parker Solar Probe observations in the young solar wind reveal new properties of both plasma particle velocity distributions (VDs) and associated electromagnetic (EM) wave fluctuations. The quasilinear (QL) kinetic theory of plasma wave instabilities has recently shown that new hammerhead (HH) proton distributions can be generated by the relaxation of proton beams through the instabilities of right-handed (RH) polarized waves. Such RH waves have indeed been reported in association with HH distributions. In this paper, new results from hybrid simulations of proton beam–plasma systems with properties typical of those observed to excite EM-RH wave instabilities are presented. From the long-term evolution of these systems, it is found that beam relaxation is driven by instabilities and growing wave fluctuations, leading to HH-type features in the VDs. The production of these features, as well as their prominence, depends on the magnetic power of the waves generated by the instabilities and, therefore, implicitly on the available free energy, quantified by the plasma beta parameter and the relative beam drift. The simulation results capture the self-consistent evolution of the instabilities and their nonlinear development. Linear theory, together with simulations, helps identify the nature of the unstable modes and the plasma conditions under which they arise. The good agreement with QL theory further indicates that it can serve as a computationally efficient complementary framework for interpreting the associated wave–particle interactions.

Context. Despite the fact that electrons observed in situ in space plasmas have three major components – the quasi-thermal core, suprathermal halo, and strahl – the analysis of instabilities triggered by kinetic, velocity-space anisotropies (such as relative drifts and temperature anisotropy) generally considers only two of these components. Aims. We aim to demonstrate that a realistic modeling with all three components is achievable in the present analysis focusing on heat-flux instabilities. In the absence of particle–particle collisions, these instabilities are responsible for the regulation of the heat flux carried mainly by suprathermal electrons. Methods. The velocity distributions of the electron populations were modeled according to in situ observations, with a Maxwellian core and Kappa-distributed halo and strahl components. We exploited new advanced numerical codes capable of solving the linear dispersion and stability properties of any plasma system with Maxwellian- and Kappa-distributed populations. Results. The unstable solutions differ significantly from those obtained with simplified models with only two components (such as core-strahl or core-beam models). The growth rates predict the systematic excitation and interplay of two unstable modes, whistler heat-flux and/or fire-hose heat-flux instabilities. The numerical solver “ALPS” was successfully applied to systems with regularized Kappa distributions, for which the analytical derivation of dispersion relations is not straightforward. Conclusions. The two instabilities are triggered by the two relative drifts, core-strahl and halo-strahl, and may have new consequences concerning the regulation of the heat flux. Particularly important are the cases when the core-strahl instability known from previous studies is in competition with the new instability driven by the halo-strahl drift, as well as when the two instabilities have the same nature and can accumulate. Future studies are thus motivated to confirm these predictions in quasilinear theories and numerical simulations.

The fluid behavior of the solar wind is affected by the heat flux carried by the suprathermal electron populations, especially the electron strahl (or beam) that propagates along the magnetic field. In turn, the electron strahl cannot be stable, and in the absence of collisions, its properties are regulated mainly by self-generated instabilities.This paper approaches the description of these heat-flux instabilities in a novel manner using regularized Kappa distributions (RKDs) to characterize the electron strahl. RKDs conform to the velocity distributions with suprathermal tails observed in-situ, and at the same time allow for consistent macromodeling, based on their singularity-free moments.In contrast, the complexity of RKD models makes the analytical kinetic formalism complicated and still inaccessible, and therefore, here heat-flux instabilities are resolved using the advanced solver ALPS. Two primary types of instabilities emerge depending on plasma conditions: the whistler and firehose heat-flux instabilities.The solver is successfully tested for the first time for such instabilities by comparison with previous results for standard distributions, such as Maxwellian and Kappa.Moreover, the new RKD results show that idealized Maxwellian models can overrate or underestimate the effects of these instabilities, and also show differences from those obtained for the standard Kappa, which, for instance, underestimate the firehose heat-flux growth rates.

Macro-modeling of the solar corona and solar wind seeks to describe the physical mechanisms that accelerate plasma particles to supersonic speeds and to explain their properties at various heliographic coordinates, increasingly sampled by space missions. Recent observational evidence provided by Parker Solar Probe on suprathermal electrons with Kappa-type velocity distributions at the origins of the solar wind has revived interest in deciphering kinetic effects and their consequences. We present new results from two complementary approaches of the solar wind, namely the kinetic-exospheric models and the HD/MHD fluid models with kinetic components. In this case, both approaches capture and quantify the effects of suprathermal electrons modeled with Kappa distributions, standard and regularized Kappa models. The latter have been introduced more recently for a physically and statistically consistent characterization of suprathermal populations and their consequences. Overestimations of the energy transferred directly or indirectly (e.g., through the ambipolar field created in the acceleration regions) to the solar wind by suprathermal electrons are thus prevented. One can consider for the first time those more energetic electrons, with strong suprathermal tails and associated with the source of energetic solar outflows, such as coronal mass ejections and bursts. Furthermore, such conditions cannot be ruled out as being conducive to the shaping of stellar winds in the astrospheres of stars with coronas much hotter than those of the solar corona.

Context. In recent years, significant progress has been made in the velocity-moment-based quasi-linear (QL) theory of waves and instabilities in plasmas with nonequilibrium velocity distributions (VDs) of the Kappa (or κ ) type. However, the temporal variation of the parameter κ , which quantifies the presence of suprathermal particles, is not fully captured by such a QL analysis, and typically κ remains constant during plasma dynamics. Aims. We propose a new QL modeling that goes beyond the limits of a previous approach, realistically assuming that the quasithermal core cannot evolve independently of energetic suprathermals. Methods. The case study is done on the electron-cyclotron (EMEC) instability generated by anisotropic bi-Kappa electrons with A = T _⊥ / T _∥ > 1 (∥, ⊥ denoting directions with respect to the background magnetic field). The parameter κ self-consistently varies through the QL equation of kurtosis (fourth-order moment) coupled with temporal variations of the temperature components, relaxing the constraint on the independence of the low-energy (core) electrons and suprathermal high-energy tails of VDs. Results. The results refine and extend previous approaches. A clear distinction is made between regimes that lead to a decrease or an increase in the κ parameter with saturation of the instability. What predominates is a decrease in κ , i.e., an excess of suprathermalization, which energizes suprathermal electrons due to self-generated wave fluctuations. Additionally, we found that VDs can evolve toward a quasi-Maxwellian shape (as κ increases) primarily in regimes with low beta and initial kappa values greater than five. Conclusions. Instability-driven relaxation only partially resolves temperature anisotropy in bi-Kappa electron VDs, as wave fluctuations generally act to further energize suprathermal electrons. The present results show a preliminary agreement with in situ observations in the solar wind, suggesting that the new QL model could provide a sufficiently explanatory theoretical basis for the kinetic instabilities in natural plasmas with Kappa-like distributions.

Recent evidence from Parker Solar Probe on the suprathermal electrons with Kappa‐type velocity distributions in the outer corona has revived interest in the kinetic‐based macro‐modeling of the solar (SW), aiming to explain its properties. Invoked in kinetic modeling of nonequilibrium plasmas, standard Kappa distributions (SKDs) have been adjusted to the regularized Kappa distributions (RKDs) to fix the inconsistencies of SKD and develop consistent fluid modeling of space plasmas. We propose a new analysis of these properties at large heliocentric distances based on the existence of RKD electrons at the exobase. This new semi‐analytic formalism is inspired by the methodology proposed initially by Meyer‐Vernet and Issautier (1998), https://doi.org/10.1029/98ja02853 . Compared to SKDs, the results for RKDs have extended applicability, since all moments can be defined and calculated consistently for all values of the parameter, even lower than the critical ones (e.g., imposed to the second‐order moment) of SKDs. However, the excess energy of the more energetic suprathermal electrons associated with low values of , is regulated by the RKD‐specific cutoff parameter . The estimates for, for example, the temperature and bulk velocity of the SW, remain at realistic values even for small , which would otherwise exceed specific observations. One can thus model a higher abundance of suprathermal electrons at the exobase (e.g., ), which is plausible for the sources of energetic events (flares and coronal mass ejections), and also in the astrospheres of stars with coronas hotter than the Sun's.