Magnetospheric solitary structure maintained by 3000 km/s ions as a cause of westward moving auroral bulge at 19 MLT

Abstract

Abstract. In the evening equatorial magnetosphere at about 4 R E geocentric distance and 19 MLT, the four Cluster spacecraft observed a solitary structure with a width of about 1000∼2000 km in the propagation direction. The solitary structure propagates sunward with about 5∼10 km/s carrying sunward electric field (in the propagation direction) of up to about 10 mV/m (total potential drop of about 5∼10 kV), depletion of magnetic field of about 25%, and a duskward E × B convection up to 50 km/s of He + rich cold plasma without O + . At the same time, auroral images from the IM-AGE satellite together with ground based geomagnetic field data showed a westward (sunward at this location) propagating auroral bulge at the magnetically conjugate ionosphere with the solitary structure. The solitary structure is maintained by flux enhancement of selectively 3000 km/s ions (about 50 keV for H + , 200 keV for He + , and 750 keV for O + ). These ions are the main carrier of the diamagnetic current causing the magnetic depletion, whereas the polarization is maintained by different behavior of energetic ions and electrons. Corresponding to aurora, field-aligned accelerated ionospheric plasma of several keV appeared at Cluster from both hemispheres simultaneously. Together with good correspondence in location and propagation velocity between the auroral bulge and the solitary structure, this indicates that the sunward moving auroral bulge is caused by the sunward Correspondence to: M. Yamauchi (m.yamauchi@irf.se) propagation of the solitary structure which is maintained by energetic ions. The solitary structure might also be the cause of Pi2-like magnetic variation that started simultaneously at Cluster location.

Cluster observation of few-hour-scale evolution of structured plasma in the inner magnetosphere

“…We made the same plot for local noon where the asymmetric cases are peaked (not shown here), but only a minor association with AL dependence is obtained with a large uncertainty. Thus, a substantial percentage of the low-energy bursts is generated without substorms, while some low-energy bursts are generated during substorms (Yamauchi et al, 2009a). At the moment, we cannot make a general conclusion on the relation between the low-energy burst and substorm activity.…”

“…Short bursts of low-energy ions that are isolated from the above structures: They have a peak energy flux at less than 100 eV, but at higher energy than plasmaspheric ions (Yamauchi et al, 2009a). The majority of the events are flowing in nearly field-aligned directions (Yamauchi et al, 1996(Yamauchi et al, , 2009a.…”

Storm time impulsive enhancements of energetic oxygen due to adiabatic acceleration of preexisting warm oxygen in the inner magnetosphere

“…Cluster observations reported by Yamauchi et al . [] indicated a solitary structure composed of polarization electric field (up to 10 mV/m) propagating at a speed of ~5–10 km/s at 4.4 R E [ Yamauchi et al ., ]. Dipolarizations have been observed even in the deep inner magnetosphere [ Sergeev et al ., ; Fu et al ., ; Apatenkov et al ., ; Ohtani et al ., ; Nosé et al ., ; Ohtani et al ., ; Gkioulidou et al ., ; Turner et al ., ].…”

“…Such a precondition enables spatially localized electric fields to more easily generate a large number of ring current oxygen ions (>100 keV). Although the Van Allen Probes spacecraft were positioned slightly away from the core region of the injection region during the examined event, strong (up to >1 mV/m), highly variable electric fields have been reported in the inner magnetosphere [e.g., Maynard et al, 1983;Rowland and Wygant, 1998;Wygant et al, 1998;Nishimura et al, 2006;Yamauchi et al, 2009]. Such electric fields can be inductive due to spatially Journal of Geophysical Research: Space Physics 10.1002/2016JA022384 localized and/or temporally impulsive magnetic field reconfiguration (dipolarization) [e.g., Ohtani et al, 2010] or potential fields caused by fast, small-scale convective flow (bursty bulk flow (BBF)) [e.g., Runov et al, 2015], as discussed below.…”

Energy conversion through mass loading of escaping ionospheric ions for different Kp values

“…Cluster observation found a travelling compressional structure in the inner magnetosphere, which actually caused evening sector aurora, i.e. strong FACs (Yamauchi et al, 2009). Yamauchi (1994) made a MHD simulation of FACs by such a longitudinal charge separation in a compressional structure, and showed that its intensity is sufficient in explaining the cusp current system with flowing directions consistent with the observations.…”