Stormtime Evolution of the O + ${\mathrm{O}}^{+}$ Density: Magnetoseismic Analysis of Van Allen Probes Data

Measurements of singly ionized oxygen ($O^{+}$) ions in the inner magnetosphere during geomagnetic storms are important because the ions affect various magnetospheric processes. We apply a magnetoseismic technique to Van Allen Probes data to statistically determine the spatial and temporal development of the region of elevated $O^{+}$ number density $\left(n_{O^{+}}\right)$, referred to as the oxygen torus, during geomagnetic storms. This study is motivated by previous studies that reported magnetic local time (MLT) localization of the torus to the morning side. In our study, we first determine the frequencies of the fundamental through third harmonics of toroidal standing Alfvén waves to estimate the mass density $\left(\rho \right)$ and then use the electron density $\left(n_e\right)$ derived from plasma wave spectra to define the average ion mass $M_i$ (=$\rho /n_e$). We obtain $n_{O^{+}}$ from $M_i$ by making an assumption about the ${\text{He}}^{+}$ number density relative to the $H^{+}$ number density. All quantities are evaluated in a 15 min moving data window that moves in 5 min steps. By generating $L$-MLT maps of $M_i$ for different phases of 102 storms with SYMH minimum lower than $-50$ nT, we find that a region of high $M_i$ appears on the morning side during the storm main phase and lasts for a few days. $n_{O^{+}}$ is also high on the morning side during the main phase, but this MLT skewing disappears during the recovery phase. Therefore, the MLT asymmetry of the oxygen torus depends on what parameter is considered.