Local vortex-structured auroral spiral and a large-scale transpolar arc (TPA) were contemporaneously observed by the Polar ultraviolet imager (UVI), when a substorm almost recovered. The TPA grew along the dawnside auroral oval from the nightside to the dayside (oval-aligned TPA), and a chain of multiple auroral spots and spiral were located azimuthally near the poleward edge of the nightside auroral oval. Contemporaneous appearances of the TPA and the auroral spiral can be seen after the spiral appeared alone. Polar also detected the oval-aligned TPA and another dawnside TPA with the nightside end distorted toward the premidnight sector (J-shaped TPA) before and after the spiral’s formation, respectively. To examine these associated magnetotail structures, we performed global magnetohydrodynamic (MHD) simulations, based on two different types of code, BAT-S-RUS and improved REPPU, and examined how the field-aligned current (FAC) profiles varied in association with changes of the auroral form to TPA and/or auroral spiral. Global MHD simulations with the two different types of code can reproduce the TPAs and the associated FAC structures in the magnetotail. The auroral spiral and its nightside FAC profile, however, were not formed in both simulations, suggesting that its formation process cannot be treated within an MHD framework but is closely related to some kinetic process. When the J-shaped TPA and the auroral spiral contemporaneously appeared, the two MHD simulations could not reproduce the TPA, spiral and their associated magnetotail FAC structures, also advocating that a kinetic effect related to the spiral formation might prevent the TPA occurrence.

The ultraviolet imager (UVI) of the Polar spacecraft and an all-sky camera at Longyearbyen contemporaneously detected an auroral vortex structure (so-called “auroral spiral”) on 10 January 1997. From space, the auroral spiral was observed as a “small spot” (one of an azimuthally-aligned chain of similar spots) in the poleward region of the main auroral oval from 18 h to 24 h magnetic local time. These auroral spots were formed while the substorm-associated auroral bulge was subsiding and several poleward-elongated auroral streak-like structures appeared during the late substorm recovery phase. During the spiral interval, the geomagnetically north-south and east-west components of the geomagnetic field, which were observed at several ground magnetic stations around Svalbard island, showed significant negative and positive bays caused by the field-aligned currents related with the aurora spiral appearance. The negative bays were reflected in the variations of local geomagnetic activity index (SML) which was provided from the SuperMAG magnetometer network at high latitudes. To pursue the spiral source region in the magnetotail, we trace each UVI image along field lines to the magnetic equatorial plane of the nightside magnetosphere using an empirical magnetic field model. Interestingly, the magnetotail region corresponding to the auroral spiral covered a broad region from Xgsm ~ -40 to -70 RE at Ygsm ~ 8 to 12 RE. The appearance of this auroral spiral suggests that extensive areas of the magnetotail (but local regions in the ionosphere) remain active even when the substorm almost ceases, and geomagnetic conditions are almost stable.

The near-Earth plasma sheet becomes cold and dense under northward interplanetary magnetic field (IMF) condition, which suggests efficient solar wind plasma entry into the magnetosphere across the magnetopause for northward IMF and a possible contribution of ionospheric oxygen ion outflow. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are the interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the solar wind plasma entry across the magnetopause and resultant formation of the cold-dense plasma sheet (CDPS) in the tail flank regions. However, the transport path of the cold-dense plasma inside the magnetotail has not been understood yet. Here we present a case study of the CDPS in the dusk magnetotail by Magnetospheric Multiscale (MMS) spacecraft under strongly northward IMF and high-density solar wind conditions. The ion distribution function consists of high- and low-energy components, and the low-energy one intermittently shows energy dispersion in the directions parallel and anti-parallel to the local magnetic field. The time-of-flight analysis of the energy-dispersed low-energy ions suggests that these ions originate in the region farther down the tail, move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch-angle dispersion analysis gives consistent results on the traveling time and path length of the energy-dispersed ions. Based on these observations, we discuss possible generation mechanisms of the energy-dispersed structure of the low-energy ions during the northward IMF.

Nightside magnetospheric processes (dynamics) directly reflect to auroral morphology and type. By investing type of auroras and the auroral morphological changes, we can expect to understand what physical processes would take place in the magnetotail. Under northward Interplanetary Magnetic Field (IMF) conditions, transpolar arcs (TPAs) and aurora spiral can be observed. A source of TPA is considered as field-aligned currents induced by the plasma flow shear (including the plasma flow vortices) between the fast plasma flows generated by magnetotail magnetic reconnection and slower background magnetospheric plasma flows. On the other hand, it is well-known that aurora spiral is also likely to be formed by the field-aligned current induced by the flow shear in the magnetotail, such as the Kelvin-Helmholtz instabilities. Based on the contemporaneous observations of TPA and aurora spiral, we try to investigate (diagnose) how the plasma and its energy are transported in the nightside magnetosphere toward ionosphere under northward IMF conditions. On January 10th, 1997, transpolar arc (TPA) and aurora spiral contemporaneously occurred for about 5.5 hours between 17:58 UT and 22:23 UT even when Interplanetary Magnetic Field (IMF) orientation changed from weakly southward to northward at ~21:00 UT. Because no in-situ magnetotail observations were unfortunately found in this day, we performed global MHD simulations based on the Open Geospace General Circulation Model (Open GGCM) distributed in the Community Coordinated Modeling Center (CCMC), and discussed the physical relation between two different auroral appearances and nightside magnetospheric processes. In this simulation, after the IMF-Bz orientation turned from weakly southward to northward, clear flow shear between fast earthward plasma flows triggered by magnetotail reconnection and slower tailward background magnetospheric flows was seen around Xgsm ~ -40 Re in the dawn sector, being consistent with the TPA and aurora spiral brightening. These flow shears may be a “source” of field-aligned currents to form the TPA. Furthermore, they bifurcated toward dawn and dusk, and showed stronger vortices in the dusk region than those in the dawnward sector. These vortex(-like) structures, bifurcated duskward, and associated field-aligned currents would be linked to the formation of the aurora spiral. In this presentation, we will discuss further the relation between the variations of these flow shear (vortex) structures, TPA and aurora spiral formations under northward IMF conditions, followed by weak southward IMF intervals.

The RBSP and the Arase satellites have different inclinations and sometimes they fly both near the equator and off the equator on the same magnetic field line, simultaneously. Such conjunction events give us opportunities to compare the electron density at different latitudes. In this study, we analyzed the plasma waves observed by Arase and RBSP-A or B during the three conjunction events during and after the 7 Sep 2017 storm event. The electron number density at the satellite positions were estimated from frequencies of the UHR emissions obtained by the HFA/PWE onboard the Arase and the Waves instrument onboard RBSP, respectively. During the three conjunction events, the satellites passed through the plume, inner trough (the narrow region with low electron density between main body of the plasmasphere and the plume), plasmatrough with variable electron density, and partially-refilled plasmasphere. The power-law index m for the inner trough and plume was inferred to be 6~8 and ~0, respectively. This is interpreted to mean that the trough was close to collisionless and the plume was near diffusive equilibrium. In the plasmatrough with the varying density, both the high-density and low-density regions had m~0. The low-density portion of this region may have a different origin from the inner trough, because of the different m-indices. For the partially-refilled plasmasphere in the storm recovery phase, the power-law index m showed negative values, meaning that the density in the equatorial plane was higher than at higher latitudes.