Dawnward shift of the dayside O~+ outflow distribution: The importance of field line history in O~+ escape from the ionosphere

摘要

Observations and modeling of thermal upwelling O~+ in the topside ionosphere are used to demonstrate that the well-known dawnward shift of energetic escaping fluxes at higher altitudes is the result of the expansion and contraction of low energy plasma on magnetic field lines and noon focused energization mechanisms. Recent research has shown that magnetospheric impacts of ion outflow are dependent on the local time source location. However, most modelers assume that the peak escaping O~+ flux on the dayside is coincident with energy inputs from the magnetosphere associated with the cusp. The dawnward offset has been observed by multiple spacecraft and most recently in a new DMSP database of low altitude upwelling O~+. The documented control of the cusp orientation with the Y component of the interplanetary magnetic field is generally a smaller effect than the observed dawnward bias. The dawnward offset is largest for thermal O~+ during geomagnetically quiet intervals and smallest for energetic escaping O~+ during active intervals. The present study investigates the efficacy of precipitating electrons, solar irradiance and neutral winds on dayside upwelling O ~+. An ionosphere model is used to show that solar irradiance causes significant upwelling O~+ on dawn flux tubes and when combined with noon focused energization mechanisms, establishes the observed dayside escaping energetic O~+ flux distribution. Our analysis also reveals that, during geomagnetically quiet intervals, high latitude neutral winds may be as important as electron precipitation in establishing the dayside distribution of upflowing thermal O~+.