Influence of MHD Turbulence on Ion Kappa Distributions in the Earth's Plasma Sheet as a Function of Plasma β Parameter

Abstract

The possible influence of MHD turbulence on the energy distributions of ions in the Earth's plasma sheet was studied using data taken by the THEMIS satellites. Turbulence levels were traced using eddy diffusion coefficients (D), of which we measured one for each Geocentric Solar Magnetospheric (GSM) coordinates every 12 min. Ion fluxes between 1.75 and 210.5 keV during the same time windows that correspond to mainly suprathermal populations were fitted to Kappa distribution functions, which approximate a Maxwellian distribution when the κ-index (κ) is large. We found that the distribution of the eddy diffusion coefficients is bimodal, independently of both the eddy diffusion component and the plasma beta (β) parameter, which is defined as the ratio between plasma and magnetic pressures. The main peak corresponds to turbulent plasma flows with D > 103 km2 s−1. In such cases, the impact of turbulence on the κ index depends on the value of β and also on the direction of the turbulent transport. For eddy diffusion perpendicular to the neutral sheet, the values of κ decrease as Dzz increases for β < 2; while for higher values of β, κ increases with Dzz. For the other two directions, the values of κ decrease as D increases. This last tendency is stronger for β ~ 1 but almost null for β ~ 10. The secondary peak in the distribution of D values might represent quasi-laminar flows forming part of very large vortices, correct detection and description of which is beyond the scope of this study.

The Response of the Magnetosphere to Changes in the Solar Wind Dynamic Pressure: 2. Ion and Electron Kappa Distribution Functions

“…However, for P SW > 1 nPa the values of β in this local minimum start to be less than one, and—as shown experimentally by Eyelade, Espinoza, et al. (2021)—in this case for low β plasma the eddy diffusion might decrease the κ i . However, there is also strong evidence that at smaller scales the turbulence observed in the plasma sheet is dominated by electrostatic fluctuations (Ergun et al., 2018; Ovchinnikov et al., 2024).…”

“…At the same time, Eyelade, Espinoza, et al. (2021) showed that depending on the value of plasma β , the ion κ index may decrease (low β ) or increase (high β ) with the eddy diffusion. For diffusion perpendicular to the plasma sheet, this change takes place for β ≈ 1.…”

“…Recently, Stepanova and Antonova (2015) and Eyelade, Espinoza, et al. (2021) showed that turbulent transport may also affect the evolution of Kappa functions. In particular, Stepanova and Antonova (2015) analysed the parameters of the ion and electron kappa distributions obtained simultaneously at several THEMIS satellites along the plasma sheet and showed that for the majority of events the values of κ increase tailward.…”

“…Data from THEMIS‐B and THEMIS‐C were collected over a period of 3‐year (2008–2010), and data from THEMIS‐A, THEMIS‐D, and THEMIS‐E, span the 10‐year period 2008–2018. The measurements were averaged over 12 min intervals, which are long enough to obtain robust measurements of particle fluxes and at the same time short enough to ensure that the distributions do not vary significantly over this time interval (Espinoza et al., 2018; Eyelade, Stepanova, et al., 2021; Eyelade, Espinoza, et al., 2021; Stepanova & Antonova, 2015). We use magnetic field measurements obtained from the Flux Gate Magnetometer (FGM) onboard the THEMIS satellites (Auster et al, 2008), with a time resolution of 3 s which were used to calculate the magnetic fluctuation.…”

Ion Kappa Distribution Parameters in the Magnetosphere of the Earth at Geocentric Distances Smaller Than 20 RE During Quiet Geomagnetic Conditions

“…However, a decrease of can mean the action of turbulent acceleration leading to the increase of the rigidity of ion spectra. The plasma sheet of the Earth is a region of intense velocity and magnetic field fluctuations (see the recent reviews of Antonova & Stepanova, 2021; Borovsky et al., 2020 and results of Eyelade, Espinoza, et al., 2021). The level of such fluctuations increases toward the tail and it is low at the distances during quiet geomagnetic conditions (Pinto et al., 2011; Stepanova et al., 2009, 2011).…”