Latitude Dependence of Low‐Altitude O+ Ion Upflow: Statistical Results From FAST Observations

We introduce a statistical model to explain the latitudinal dependence of the occurrence rate and energy flux of the ionospheric escaping ions, taking advantage of advances in the spatial coverage and accuracy of FAST observations. We use a weighted piecewise Gaussian function to fit the dependence, because two probability peaks are located in the dayside polar cusp source region and the nightside auroral oval zone source region. The statistical results show that: (1) the Gaussian Mixture Model (GMM) suitably describes the dayside polar cusp upflows, and the dayside and the nightside auroral oval zone upflows. (2) The magnetic latitudes of the ionospheric upflow source regions expand toward the magnetic equator as Kp increases, from 81° MLat (cusp upflows) and 63° MLat (auroral oval upflows) during quiet times to 76° MLat and 61° MLat, respectively. (3) The dayside polar cusp region provides only 3-5% O+ upflows among all the source regions, which include the dayside auroral oval zone, dayside polar cusp, nightside auroral oval zone and even the polar cap. However, observations show that more than 70% of upflows occur in the auroral oval zone and that the occurrence probability increases at the altitudes of 3500 - 4200 km, which is considered to be the lower altitude boundary of ion beams. This observed result suggests that soft electron precipitation and transverse wave heating are the most efficient ion energization/acceleration mechanisms at the altitudes of FAST orbit, and that the parallel acceleration caused by field-aligned potential drops become effective above that altitude.