Interplanetary magnetic field dependence of the suprathermal energetic neutral atoms originated in subsolar magnetopause

Using energetic neutral atom (ENA) emission observations of the subsolar magnetopause measured by the Interstellar Boundary Explorer (IBEX), we study the correlation between the upstream interplanetary magnetic field (IMF) conditions and the spectral index of the source ion population. Our ENA data set includes hour-averaged ENA measurements at energies between 0.5 and 6 keV obtained by the IBEX High Energy ENA imager from January 2009 to May 2011. Under the condition of quiet geomagnetic activity (SYM-H index >-20 nT), we find that the shallower spectra in the suprathermal tail of the ion population of the subsolar magnetopause is weakly correlated (correlation coefficient of -0.30) with the shock angle of the Earth's bow shock, but not correlated with parameters related to magnetic reconnection (i.e., elevation and clock angle of the interplanetary magnetic field orientation). The observed correlation suggests suprathermal ion energization from diffusive shock acceleration and thus that the suprathermal ions in the subsolar magnetopause are of shocked solar wind origin. We also argue that the roles of magnetospheric ion leakage or ion acceleration by magnetic reconnection are reduced in the magnetopause emissions compared to shock acceleration processes.