Dr Louis Richard

Abstract:

Magnetospheric-ionospheric coupling studies often rely on multi-spacecraft conjunctions, which require accurate magnetic field mapping tools. For example, linking measurements from the magnetotail with those in the ionosphere involves determining when the orbital magnetic footpoint of THEMIS or Magnetospheric Multiscale (MMS) intersects with the footpoint of Swarm. The Tsyganenko models are commonly used for tracing magnetic field lines. In this study, we aim to analyze how the footpoint locations are impacted by the input parameters of these models, including solar wind conditions, geomagnetic activity, and the location in the magnetotail. A dataset of 2394 bursty bulk flows (BBFs) detected by MMS was mapped to Earth’s ionosphere with six different Tsyganenko models. Approximately 90% of the ionospheric footpoints are concentrated within 70° ± 5° magnetic latitude (MLAT) and ±3 h of magnetic local time (MLT) around midnight, with a pronounced peak in the pre-midnight sector. The MLT position showed a difference of approximately ±1 h MLT across the models. Footpoint locations were linked to the dawn-dusk position of the BBFs, with differences between models associated with variations in the interplanetary magnetic field clock angle. The MLAT values exhibited similar differences of approximately ±4° around the mean value, with a systematic shift toward lower latitudes in the T89 model. This position is also influenced by the input parameters of the model representing the dynamics of Earth’s magnetosphere, where stronger magnetospheric activity typically corresponds to lower latitudes. The uncertainty on the BBF footpoint location impacts the number of conjunctions with Swarm. Generally, Swarm B exhibited more conjunctions than Swarm A or C in the Northern Hemisphere. However, when considering only Swarm-BBF conjunctions where the distance between footpoints computed with T89 and TA15n is smaller than the size of the BBF footprint, the number of conjunctions is reduced to less than half of the total.

Abstract:

We investigate turbulence in magnetic reconnection jets in the Earth's magnetotail using data from the Magnetospheric Multiscale spacecraft. We show that signatures of a limited inertial range are observed in many reconnection jets. The observed turbulence develops on the timescale of a few ion gyroperiods, resulting in intermittent multifractal energy cascade from the characteristic scale of the jet down to the ion scales. We show that at sub-ion scales, the fluctuations are close to monofractal and predominantly kinetic Alfvén waves. The observed energy transfer rate across the inertial range is ∼10^{8}  J kg^{-1} s^{-1}, which is the largest reported for space plasmas so far.

Abstract:

We present a statistical analysis of ion distributions in magnetic reconnection jets using data from the Magnetospheric Multiscale spacecraft. Compared with the quiet plasma in which the jet propagates, we often find anisotropic and non-Maxwellian ion distributions in the plasma jets. We observe magnetic field fluctuations associated with unstable ion distributions, but the wave amplitudes are not large enough to scatter ions during the observed travel time of the jet. We estimate that the phase-space diffusion due to chaotic and quasiadiabatic ion motion in the current sheet is sufficiently fast to be the primary process leading to isotropization.

Abstract:

AbstractPlasma jets are ubiquitous in the Earth's magnetotail. Plasma jet fronts (JFs) are a seat of particle acceleration and energy conversion. JFs are commonly associated with dipolarization fronts (DFs), which are often characterized by solitary sharp large‐amplitude increases in the northward component of the magnetic field Bz. However, MHD and kinetic instabilities can develop at JFs and disturb the front structure which questions on the occurrence of solitary DFs at the JFs. We investigate the structure of JFs using 5 years (2017–2021) of the Magnetospheric Multiscale observations in the central plasma sheet (CPS) in the Earth's magnetotail. We compiled a database of 2394 CPS jets. We find that 42% of the JFs are associated with large‐amplitude changes in Bz. However, solitary DFs constitute a quarter of these large‐amplitude events, while the rest are associated with more complex structures.

Abstract:

AbstractWe investigate two flow bursts in a series of Earthward bursty bulk flows (BBFs) observed by the Magnetospheric Multiscale spacecraft in Earth's magnetotail at (−24, 7, 4) RE in Geocentric Solar Magnetospheric coordinates. At the leading edges of the BBFs, we observe complex magnetic field structures. In particular, we focus on one BBF which contains large‐amplitude magnetic field fluctuations on the time scale of the proton gyroperiod, and another with a large scale dipolarization. For both events, the magnetic field structures are associated with flux increases of supra‐thermal ions with energies ≳100 keV. We observe that helium ions dominate the ion flux at energies ≳150 keV. We investigate the ion acceleration mechanism and its dependence on the mass and charge state of H+ and He2+ ions. We show that for both events, the ions with gyroradii smaller than the dawn‐dusk scale of the structure are accelerated by the ion bulk flow. For ions with larger gyroradii, the acceleration is likely due to a localized spatially limited electric field for the event with a large‐scale dipolarization. For the event with fluctuating magnetic field, the acceleration of ions with gyroradii comparable with the scale of the magnetic fluctuations can be explained by resonance acceleration.