Non-diffusive resonant acceleration of electrons in the radiation belts

International audienceWe describe a mechanism of resonant electron acceleration by oblique high-amplitude whistlerwaves under conditions typical for the Earth radiation belts. We use statistics of spacecraftobservations of whistlers in the Earth radiation belts to obtain the dependence of the angle hbetween the wave-normal and the background magnetic field on magnetic latitude k. According tothis statistics, the angle h already approaches the resonance cone at k 15 and remains close to itup to k 30–40 on the dayside. The parallel component of the electrostatic field of whistlerwaves often increases around k 15 up to one hundred of mV/m. We show that due to thisincrease of the electric field, the whistler waves can trap electrons into the potential well via waveparticle resonant interaction corresponding to Landau resonance. Trapped electrons then move withthe wave to higher latitudes where they escape from the resonance. Strong acceleration is favoredby adiabatic invariance along the increasing magnetic field, which continuously transfers theparallel energy gained to perpendicular energy, allowing resonance to be reached and maintained.The concomitant increase of the wave phase velocity allows for even stronger relative accelerationat low energy <50 keV. Each trapping-escape event of electrons of 10 keV to 100 keV results inan energy gain of up to 100 keV in the inhomogeneous magnetic field of the Earth dipole. Forelectrons with initial energy below 100 keV, such rapid acceleration should hasten their drop intothe loss-cone and their precipitation into the atmosphere. We discuss the role of the consideredmechanism in the eventual formation of a trapped distribution of relativistic electrons for initialenergies larger than 100 keV and in microbursts precipitations of lower energy particles

Statistics of whistler mode waves in the outer radiation belt: Cluster STAFF-SA measurements

International audience[1] ELF/VLF waves play a crucial role in the dynamics of the radiation belts and are partly responsible for the main losses and the acceleration of energetic electrons. Modeling wave-particle interactions requires detailed information of wave amplitudes and wave normal distribution over L-shells and over magnetic latitudes for different geomagnetic activity conditions. We performed a statistical study of ELF/VLF emissions using wave measurements in the whistler frequency range for 10 years (2001–2010) aboard Cluster spacecraft. We utilized data from the STAFF-SA experiment, which spans the frequency range from 8 Hz to 4 kHz. We present distributions of wave magnetic and electric field amplitudes and wave normal directions as functions of magnetic latitude, magnetic local time, L-shell, and geomagnetic activity. We show that wave normals are directed approximately along the background magnetic field (with the mean value of  — the angle between the wave normal and the background magnetic field, about 10 ı –15 ı) in the vicinity of the geomagnetic equator. The distribution changes with magnetic latitude: Plasmaspheric hiss normal angles increase with latitude to quasi-perpendicular direction at 35 ı –40 ı where hiss can be reflected; lower band chorus are observed as two wave populations: One population of wave normals tends toward the resonance cone and at latitudes of around 35 ı –45 ı wave normals become nearly perpendicular to the magnetic field; the other part remains quasi-parallel at latitudes up to 30 ı. The observed angular distribution is significantly different from Gaussian, and the width of the distribution increases with latitude. Due to the rapid increase of  , the wave mode becomes quasi-electrostatic, and the corresponding electric field increases with latitude and has a maximum near 30 ı. The magnetic field amplitude of the chorus in the day sector has a minimum at the magnetic equator but increases rapidly with latitude with a local maximum near 12 ı –15 ı. The wave magnetic field maximum is observed in the night sector at L > 7 during low geomagnetic activity (at L 5 for K p > 3). Our results confirm the strong dependence of wave amplitude on geomagnetic activity found in earlier studies. (2013), Statistics of whistler-mode waves in the outer radiation belt: Cluster STAFF-SA measurements

Ion kinetics of plasma interchange reconnection in the lower solar corona

The exploration of the inner heliosphere by Parker Solar Probe has revealed a highly structured solar wind with ubiquitous deflections from the Parker spiral, known as switchbacks. Interchange reconnection (IR) may play an important role in generating these switchbacks by forming unstable particle distributions that generate wave activity that in turn may evolve to such structures. IR occurs in very low beta plasmas and in the presence of strong guiding fields. Although IR is unlikely to release enough energy to provide an important contribution to the heating and acceleration of the solar wind, it affects the way the solar wind is connected to its sources, connecting open field lines to regions of closed fields. This "switching on" provides a mechanism by which plasma near coronal hole boundaries can mix with that trapped inside the closed loops. This mixing can lead to a new energy balance. It may significantly change the characteristics of the solar wind because this plasma is already pre-heated and can potentially have quite different density and particle distributions. It not only replenishes the solar wind, but also affects the electric field, which in turn affects the energy balance. This interpenetration is manifested by the formation of a bimodal ion distribution, with a core and a beam-like population. Such distributions are indeed frequently observed by the Parker Solar Probe. Here we provide a first step towards assessing the role of such processes in accelerating and heating the solar wind.Comment: Accepted in Parker Solar Probe Focus Issue (ApJ

Relativistic electron beams accelerated by an interplanetary shock

Collisionless shock waves have long been considered amongst the most prolific particle accelerators in the universe. Shocks alter the plasma they propagate through and often exhibit complex evolution across multiple scales. Interplanetary (IP) traveling shocks have been recorded in-situ for over half a century and act as a natural laboratory for experimentally verifying various aspects of large-scale collisionless shocks. A fundamentally interesting problem in both helio and astrophysics is the acceleration of electrons to relativistic energies (more than 300 keV) by traveling shocks. This letter presents first observations of field-aligned beams of relativistic electrons upstream of an IP shock observed thanks to the instrumental capabilities of Solar Orbiter. This study aims to present the characteristics of the electron beams close to the source and contribute towards understanding their acceleration mechanism. On 25 July 2022, Solar Orbiter encountered an IP shock at 0.98 AU. The shock was associated with an energetic storm particle event which also featured upstream field-aligned relativistic electron beams observed 14 minutes prior to the actual shock crossing. The distance of the beam's origin was investigated using a velocity dispersion analysis (VDA). Peak-intensity energy spectra were anaylzed and compared with those obtained from a semi-analytical fast-Fermi acceleration model. By leveraging Solar Orbiter's high-time resolution Energetic Particle Detector (EPD), we have successfully showcased an IP shock's ability to accelerate relativistic electron beams. Our proposed acceleration mechanism offers an explanation for the observed electron beam and its characteristics, while we also explore the potential contributions of more complex mechanisms.Comment: Main text: 6 pages, 2 figures. Supplementary material: 6 pages, 7 figure

Dynamics of Strong Collision-Free Shock Waves and Radiation Generation

Available from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio

Dynamical shock: Theory and observations

International audienceCollisionless shock waves in plasmas are usually considered as stationary nonlinear waves that cause irreversible changes of plasma state. However, in the very beginning of the collisionless shock physics it was observed experimentally (Morse et al., 1971) and in computer simulations (Biscamp and Welter, 1972) that quasiperpendicular shock behavior can be nonstationary. Later it was hypothesized that nonstationary dynamics is typical for high-Mach-number shocks. Now it is clear that there exist several types of nonstationary effects. Both computer simulation and experimental observations have shown different manifestations of shock front variability, which differ by dimensionality and strength. In general, temporal variations result in spatially inhomogeneous multidimensional shock front structure. Relatively weak effects will result in a "vibrating" shock front structure resembling that of a stationary shock with relatively small variations of the number of reflected ions and wave activity upstream of the shock, while strong effects may cause the shock front disruption and overturning. In this last case the shock front "disappears" and a "new" one is formed in the vicinity of the "old" front, this phenomenon is usually called a reformation. Another feature of quasiperpendicular shocks observed experimentally and in simulations is a phenomenon of front rippling, which is essentially multidimensional. The question whether rippling is always related to the shock front nonstationarity or it can exist in quasistationary shocks is still open. Several spacecraft programs, including multi-spacecraft missions like ISEE, AMPTE and Cluster provided an opportunity to measure spatial scales and characteristic times more carefully. This allowed observing unambiguous manifestations of shock front reformation and rippling. We discuss also the observational features of particle distributions associated with the shock front reformation

Electron beam-plasma instability in the randomly inhomogeneous solar wind

International audienceWe propose a new model that describes effects of plasma density fluctuations in the solar wind on the relaxation of the electron beams ejected from the Sun during the solar flares. The density fluctuations are supposed to be responsible for the changes in the local phase velocity of the Langmuir waves generated by the beam instability. We use the property that for the wave with a given frequency the probability distribution of density fluctuations uniquely determines the probability distribution of phase velocity of wave. We replace the continuous spatial interval by a discrete one, consisting of small equal spatial subintervals with linear density profile. This approach allows us to describe the changes in the wave phase velocity during the wave propagation in terms of probability distribution function. Using this probability distribution, we describe resonant wave particle interactions by a system of equations, similar to a well-known quasi-linear approximation, where the conventional velocity diffusion coefficient and the wave growth rate are replaced by the averaged in the velocity space. The averaged diffusion coefficient and wave growth rate depend on a form of the probability distribution function for the density fluctuations. This last distribution is obtained from the spectrum of the density fluctuations measured aboard ISEE satellites when they were in the solar wind. It was shown that the process of relaxation of electron beam is accompanied by transformation of significant part of the beam kinetic energy to energy of the accelerated particles via generation and absorption of the Langmuir waves. We discovered that for the very rapid beams with beam velocity vb>15vt, where vt is a thermal velocity of background plasma, the relaxation process consists of two well-separated steps. On first step the major relaxation process occurs and the wave growth rate almost everywhere in the velocity space becomes close to zero or negative. At the second stage the system remains close to the state of marginal stability long enough to explain how the beam may be preserved traveling distances over 1 AU while still being able to generate the Langmuir waves