MMS observations of reconnection separatrix region in the magnetotail at different distances from the active neutral X-line

<p>The region surrounding the reconnection separatrix consists of the multitude of particle and wave transient features (electron, cold and hot ion beams, Hall E&B fields, kinetic Alfven and LH waves, e-holes etc) whose pattern and intensities may vary depending on the stage of reconnection process as well as on the distance from the active neutral line (XNL), whose characterization from observations is not a trivial task. We explore quick MMS entries into the plasma sheet boundary layer from the lobe in 2017 and 2018 tail seasons which potentially could be the crossings of the active separatrix as suggested by energy dispersed beams and polar rain gap features. By combining  the observations of beam dispersion with the measured plasma convection and PSBL motion (obtained using the timing method) we attempt to separate  temporal and spatial (velocity filter) contributions  to the observed beam energy dispersion and evaluate the MMS distance from the XNL. In this report we discuss similarities and differences of separatrix manifestations  observed far from the XNL (at distances exceeding several tens Re) and those found close to it (where the outermost electron beam directed toward the XNL is seen).  One of surprizes was that we were often able to identify the intense Hall-like E&B field structures at very large distances from the XNL.  </p>

The Polar Wind Modulated by the Spatial Inhomogeneity of the Strength of the Earth’s Magnetic Field

<p>When the geomagnetic field is weak, the small mirror force allows precipitating charged particles to deposit energy in the ionosphere. This leads to an increase in ionospheric outflow from the Earth’s polar cap region, but such an effect has not been previously observed because the energies of the ions of the polar ionospheric outflow are too low, making it difficult to detect the low-energy ions with a positively charged spacecraft. In this study, we found anti-correlation between ionospheric outflow and the strength of the Earth’s magnetic field. Our results suggest that the electron precipitation through the polar rain can be a main energy source of the polar wind during periods of high levels of solar activity. The decreased magnetic field due to spatial inhomogeneity of the Earth’s magnetic field and its effect on outflow can be used to study the outflow in history when the magnetic field was at similar levels.</p>

DMSP/SSUSI observations of the high-latitude dayside aurora (HiLDA

<p>High-latitude dayside aurora (HiLDA) are large-scale discrete arcs or spot-like aurora poleward of the cusp, observed previously in the northern hemisphere by the Viking UV imager [Murphree et al., 1990] and by the IMAGE FUV [Frey et al., 2003]. The particular interest on HiLDA is to understand its formation related to the dayside reconnection and the resulted field-aligned currents (FACs) configuration in the polar cap (open field line region). In addition, the occurrence of HiLDA in the southern hemisphere is not well known.</p><p>In this study, we investigate the properties of HiLDA using DMSP/SSUSI images from the satellites F16, F17, F18, and F19. The combined data with auroral images from DMSP/SSUSI, ion drift velocity from SSIES, magnetic field perturbations from SSM, and energetic particle spectrum from SSJ make it possible to study the electrodynamics in the vicinity of the HiLDA and its connection the dayside cusp. HiLDA is formed due to monoenergetic electron precipitation (inverted-V structures) with the absence of ion precipitation. The field-aligned potential drop can be up to tens of keV. Applying the current-voltage relation, we suggest accelerated polar rain as the source of HiLDA, indirectly controlled by the solar wind/magnetosheath plasma population. The upward field-aligned current associated with the potential drop is a part of the cusp current system, produced by the dayside reconnection. Both lobe reconnection and reconnection on the duskside flanks play a role in the formation of HiLDA.</p><p>The occurrence of HiLDA is highly associated with the sunlit hemisphere and IMF By dominated conditions. Our results agree with previous observations, which show that HiLDA occurs during positive By dominated conditions in the northern summer hemisphere. We also confirmed that HiLDA occurs during negative By dominated conditions in the southern hemisphere. In addition, the fine structures of HiLDA are studied.</p><p>References</p><p><span>Murphree, J. S.</span>, <span>Elphinstone, R. D.</span>, <span>Hearn, D.</span>, and <span>Cogger, L. L.</span> ( <span>1990</span>), <span>Large‐scale high‐latitude dayside auroral emissions</span>, <em>J. Geophys. Res.</em>, <span>95</span>( <span>A3</span>), <span>2345</span>– <span>2354</span>, doi:.</p><p><span>Frey, H. U.</span>, <span>Immel, T. J.</span>, <span>Lu, G.</span>, <span>Bonnell, J.</span>, <span>Fuselier, S. A.</span>, <span>Mende, S. B.</span>, <span>Hubert, B.</span>, <span>Østgaard, N.</span>, and <span>Le, G.</span> ( <span>2003</span>), <span>Properties of localized, high latitude, dayside aurora</span>, <em>J. Geophys. Res.</em>, <span>108</span>, 8008, doi:, <span>A4</span>.</p>

The nightside magnetic field line open–closed boundary and polar rain electron energy-latitude dispersion

The polar rain electrons near the open–closed field line boundary on the nightside often exhibit energy-latitude dispersion, in which the energy decreases with decreasing latitude. The solar wind electrons from the last open-field line would E × B drift equatorward as they move toward the ionosphere, resulting in the observed dispersion. This process is modeled successfully by an open-field line particle precipitation model. The existing method for determining the magnetotail X line distance from the electron dispersion underestimates the electron path length from the X line to the ionosphere by at least 33%. The best estimate of the path length comes from using the two highest energy electrons in the dispersion region. The magnetic field line open–closed boundary is located poleward of the highest energy electrons in the dispersion region, which in turn is located poleward of Defense Meteorological Satellite Program (DMSP) b6, b5e, and b5i boundaries. In the four events examined, b6 is located at least 0.7–1.5° equatorward of the magnetic field line open–closed boundary. The energy-latitude dispersion seen in the electron overhang may result from the plasma sheet electron curvature and gradient drifts into the newly closed field line.