In the energy regime between the plasmasphere (a few eV) and the ring current (greater than 1 keV), there exists another magnetospheric particle population with energies from a few eV to a few keV, the origins of which are debated. Studies explore generation mechanisms for warm plasma energies in the inner magnetosphere through two observed phenomena: the warm plasma cloak and the oxygen torus. The relations between these two populations are unclear. Recent data reveal local heating of cold H+ and He+ ions to warm plasma energies by magnetosonic waves. In this study, we report first observations of thermal O+ heating by magnetosonic waves and link the heating to a possible formation mechanism for the warm plasma cloak. The O+ heating is observed by different plasmaspheric density profiles, including density channels. We observe that O+ heating always occurs with thermal H+ and He+ heating. We investigate the harmonic structure of the observed magnetosonic waves and find intense O+ heating is accompanied by discrete heavy ion gyroharmonics. We suggest that locally heated thermal ions to 100s eV by magnetosonic waves along the plasmapause could provide a possible mechanism for warm plasma cloak generation.

Abstract. For the first time, direct comparisons of the equatorial ion partial pressure and pitch angle anisotropy observed by TWINS and simulated by CIMI are presented. The TWINS ENA images are from a 4-day period, 7–10 September 2015. The simulations use both the empirical Weimer 2K and the self-consistent RCM electric potentials. There are two moderate storms in succession during this period. In most cases, we find that the general features of the ring current in the inner magnetosphere obtained from the observations and the simulations are similar. Nevertheless, we do also see consistent contrasts between the simulations and observations. The simulated partial pressure peaks are often inside the observed peaks and more toward dusk than the measured values. There are also cases in which the measured equatorial ion partial pressure shows multiple peaks that are not seen in the simulations. This occurs during a period of intense AE index. The CIMI simulations consistently show regions of parallel anisotropy spanning the night side between approximately 6 and 8 RE, whereas the parallel anisotropy is seen in the observations only during the main phase of the first storm. The evidence from the unique global view provided by the TWINS observations strongly suggests that there are features in the ring current partial pressure distributions that can be best explained by enhanced electric shielding and/or spatially localized, short-duration injections.

<p>We present the first coupled MHD-ring current simulation results that produce the global transpolar auroral arc phenomenon. We examine a unique observation of a midnight transpolar auroral arc that is produced during compressed magnetosphere conditions and persistent into an extended interval of southward IMF and substorm onset. The IMAGE satellite FUV-WIC camera observed the transpolar auroral arc in the southern hemisphere on 15 May 2005. The IMAGE observations show that the transpolar auroral arc originates at 24 MLT and stretches sunward across the polar cap to form a theta aurora with dusk-dawn motion that does not correspond with IMF By sign reversal or continuous magnitude decrease. Even though the theta aurora is typically a northern IMF phenomenon, the IMAGE observations show that the theta aurora persisted for almost an hour under disturbed geomagnetic conditions with peak AL below -1500 nT and Dst around -100 nT. We use the University of Michigan Space Weather Modeling Framework (SWMF) global geospace simulation to study the ionospheric conditions and magnetotail configuration throughout the observation period. Our SWMF simulation results show good agreement with the observed SYM-H and AL indices during the event interval. In the simulation, we identify peaks in Joule heating, precipitation, and anti-sunward flows in the region where the theta aurora is observed. We also demonstrate the temporal evolution of the open-closed field line boundary with respect to the observed theta aurora location, which suggests that the theta aurora is a closed field line phenomenon. We analyze the open-closed field line boundary mapping into the magnetotail and search for causes of precipitation within the simulation as well as analyze the hemispheric conjugacy of the event.</p>