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

Wave-induced loss of ultra-relativistic electrons in the Van Allen radiation belts.

The dipole configuration of the Earth's magnetic field allows for the trapping of highly energetic particles, which form the radiation belts. Although significant advances have been made in understanding the acceleration mechanisms in the radiation belts, the loss processes remain poorly understood. Unique observations on 17 January 2013 provide detailed information throughout the belts on the energy spectrum and pitch angle (angle between the velocity of a particle and the magnetic field) distribution of electrons up to ultra-relativistic energies. Here we show that although relativistic electrons are enhanced, ultra-relativistic electrons become depleted and distributions of particles show very clear telltale signatures of electromagnetic ion cyclotron wave-induced loss. Comparisons between observations and modelling of the evolution of the electron flux and pitch angle show that electromagnetic ion cyclotron waves provide the dominant loss mechanism at ultra-relativistic energies and produce a profound dropout of the ultra-relativistic radiation belt fluxes

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

A statistical study of the propagation characteristics of whistler waves observed by Cluster

International audience[1] VLF waves play a crucial role in the dynamics of radiation belts, and are responsible for the loss and the acceleration of energetic electrons. Modeling wave‐particle interactions requires the best possible knowledge for how wave energy and wave‐normal directions are distributed in L‐shells and for the magnetic latitudes of different magnetic activity conditions. In this work, we performed a statistical study for VLF emissions using a whistler frequency range for nine years (2001–2009) of Cluster measurements. We utilized data from the STAFF‐SA experiment, which spans the frequency range from 8.8 Hz to 3.56 kHz. We show that the wave energy distribution has two maxima around L ∼ 4.5 − 6 and L ∼ 2, and that wave‐normals are directed approximately along the magnetic field in the vicinity of the geomagnetic equator. The distribution changes with magnetic latitude, and so that at latitudes of ∼30°, wave‐normals become nearly perpendicular to the magnetic field. The observed angular distribution is significantly different from Gaussian and the width of the distribution increases with latitude. Since the resonance condition for wave‐particle interactions depends on the wave normal orientation, our results indicate that, due to the observed change in the wave‐normal direction with latitude, the most efficient particle diffusion due to wave‐particle interaction should occur in a limited region surrounding the geomagnetic equator

Correction to “A statistical study of the propagation characteristicsof whistler waves observed by Cluster”

International audience, 2011), the incorrect reference frame (i.e., ISR2) was used to analyze the data. In Figure 2, a statistical analysis of the distribution of wave-normal angles as a function of latitude was presented. The results were obtained using data from the STAFF-SA instrument from the Cluster Active Archive (CAA), by assuming that the data was in the ISR2 coordinate system as written in the data description [Cornilleau-Wehrlin et al., 1997; Cornilleau-Wehrlin and STAFF Team, 2011]. Unfortunately , recently it became evident that the true reference frame system of STAFF-SA data in the CAA is the SR2 (Spin Reference 2) coordinate system, which differs from the ISR2 (inverted SR2) by the inverted signs of the Y and Z axes. We have re-analyzed all of the data in the correct reference frame (i.e., SR2). The results of the new analysis are presented in Figure 1. In the present correction we describe the newly-obtained results and discuss how they impact the discussion in our original paper. [2] Identification of the error only recently became possible when the data of the STAFF-SC instrument in the burst mode, namely, waveforms with a sampling rate of 450 samples per second (that were obtained on January 26, 2001) became available in the CAA. Making use of this data, we performed cross-calibration tests between the STAFF-SC waveforms and the STAFF-SA spectrum matrices. For the comparison, we used observations from a whistler wave that had a frequency of 100 Hz (0.15 of the local f ce). For both STAFF-CA and STAFF-SA we determined that the wave had a circular polarization. The wave-normals determined from the STAFF-SA and STAFF-SC had different signs for the Y and Z components. The vector rotation with respect to the background magnetic field was right-handed from the STAFF-SC (as expected for whistler waves), but left-handed from the STAFF-SA. Since the coordinate system of the STAFF-SC has been verified using FGM data, the results unambiguously demonstrate that the true coordinate system of the STAFF-SA data in the CAA is SR2, not ISR2. The STAFF team, after verification , has confirmed that STAFF Spectral Matrix (SM) and Power Spectral Density (PSD) data at CAA are in SR2 reference frame and not in ISR2 as previously written by mistake (N. Cornilleau-Wehrlin, personal communication, 2012). The CAA documentation has now been modified accordingly by the STAFF team. [3] The corrected probability distribution functions (PDF) of the whistler wave-normal direction q (relative to the background

Statistical model of electron pitch angle diffusion in the outer radiation belt

International audience[1] We calculated the bounce averaged electron pitch angle diffusion coefficients 〈D a eq a eq 〉 using the statistical characteristics of lower band chorus activity collected by the Cluster mission from 2001–2009. Nine years of Cluster observations provide the distributions of the q angle between wave vectors and the background magnetic field; and the distributions of the wave total intensity B w 2 for relatively wide ranges of the magnetic latitude l, the magnetic local times, and the K p indices. According to Cluster observations, the probability of observing a larger B w 2 increases with l and depends upon the magnetic local time and K p. We compared the obtained results with the diffusion coefficients 〈D a eq a eq 〉 const that were calculated under an assumption of parallel whistler wave propagation with a constant intensity B w 2 = 10 4 pT 2. The last calculations substantially underestimated pitch angle diffusion for the small equatorial pitch angles, a eq , but likely overestimates 〈D a eq a eq 〉 const for a eq > 60. An important increase in 〈D a eq a eq 〉 for a eq 15. We took the probability density distribution of the wave mean amplitude into consideration instead of the averaged value. The obtained distribution of the diffusion coefficients indicated that approximately 20% of the most intense waves can provide the main portion of pitch angle diffusion for the dawn/day sector. For the dusk/night sector, wave intensity was significantly weaker and the relative importance of intense waves was not clearly pronounced