Solar wind‐driven variations of electron plasma sheet densities and temperatures beyond geostationary orbit during storm times

Abstract

The empirical models of the plasma sheet electron temperature and density on the nightside at distances between 6 and 11 RE are constructed based on Time History of Events and Macroscale Interactions During Substorms (THEMIS) particle measurements. The data set comprises ∼400 h of observations in the plasma sheet during geomagnetic storm periods. The equatorial distribution of the electron density reveals a strong earthward gradient and a moderate variation with magnetic local time symmetric with respect to the midnight meridian. The electron density dependence on the external driving is parameterized by the solar wind proton density averaged over 4 h and the southward component of interplanetary magnetic field (IMF BS) averaged over 6 h. The interval of the IMF integration is much longer than a typical substorm growth phase, and it rather corresponds to the geomagnetic storm main phase duration. The solar wind proton density is the main controlling parameter, but the IMF BS becomes of almost the same importance in the near‐Earth region. The root‐mean‐square deviation between the observed and predicted plasma sheet density values is 0.23 cm−3, and the correlation coefficient is 0.82. The equatorial distribution of the electron temperature has a maximum in the postmidnight to morning MLT sector, and it is highly asymmetric with respect to the local midnight. The electron temperature model is parameterized by solar wind velocity (averaged over 4 h), IMF BS (averaged over 45 min), and IMF BN (northward component of IMF, averaged over 2 h). The solar wind velocity is a major controlling parameter, and IMF BS and BN are comparable in importance. In contrast to the density model, the electron temperature shows higher correlation with the IMF BS averaged over ∼45 min (substorm growth phase time scale). The effect of BN manifests mostly in the outer part of the modeled region (r > 8RE). The influence of the IMF BS is maximal in the midnight to postmidnight MLT sector. The correlation coefficient between the observed and predicted plasma sheet electron temperature values is 0.76, and the root‐mean‐square deviation is 2.6 keV. Both models reveal better performance in the dawn MLT sector.

On the Accuracy of Reconstructing Plasma Sheet Electron Fluxes From Temperature and Density Models

“…The infinite value of corresponds to a Maxwellian distribution. As expected, the correlation coefficients are lower than 0.75 and 0.82 obtained in Dubyagin et al (2016) for the predictions of temperature and density themselves. To ease an interpretation of the MAE values in Figure 4b, which are computed for log 10 values of the fluxes, we convert them to the corresponding factors (or ratio of the measured and reconstructed fluxes) in the legend.…”

“…We use the same data set that was used by Dubyagin et al (2016) for the empirical model of the electron temperature and density construction. The fluxgate magnetometer measurements (Auster et al, 2008) were used to select the observations in the vicinity of the neutral sheet as described in Dubyagin et al (2016). We use the data of three THEMIS probes (A, D, and E) on the nightside in the radial distance range of 6-11R E .…”

“…The detailed data and event selection descriptions can be found in the original paper, and we will only briefly summarize them here. The fewer number of data records in comparison to the Dubyagin et al (2016) data set is due to the events with absent measurements for some energy channels inside one 1.6 min interval. These data were collected during all storms with min SYM-H < −50 nT that took place in the years 2007-2013.…”

“…We use the data of three THEMIS probes (A, D, and E) on the nightside in the radial distance range of 6-11R E . The plasma moments, which were used in Dubyagin et al (2016), could be computed for such events, but the spectra could not be averaged using simple methods for such intervals and were discarded. The fluxgate magnetometer measurements (Auster et al, 2008) were used to select the observations in the vicinity of the neutral sheet as described in Dubyagin et al (2016).…”

“…The electron temperature and density are estimated using Dubyagin et al (2016) model and also calculated from the observed flux spectra themselves. We intentionally concentrate on geomagnetic storm periods because they are the main targets of inner magnetosphere modeling.…”