A statistical comparison of SuperDARN spectral width boundaries and DMSP particle precipitation boundaries in the afternoon sector ionosphere
The open-closed magnetic field line boundary (OCB) is best measured at the foot points of the boundary in the Earth's ionosphere where continuous and extensive spatiotemporal measurements can be made. The ability to make routine observations of this type is crucial if accurate global measurements of energy transfer processes occurring at the boundary, such as magnetic reconnection, are to become a reality. The spectral width boundary (SWB) measured by the Super Dual Auroral Radar Network (SuperDARN) has been shown to be a reliable ionospheric proxy for the OCB at certain magnetic local times (MLTs). However, the reliability of the SWB proxy in the afternoon sector ionosphere (12:00-18:00 MLT) has been questionable. In this paper we undertake a statistical comparison of the latitudinal locations of SWBs measured by SuperDARN and particle precipitation boundaries (PPBs) measured by the Defense Meteorological Satellite Program (DMSP) spacecraft, concentrating on the PPB which best approximates the location of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 12:00-18:00 MLT range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10 min universal time (UT) window and within a ±1 h MLT window. The results show that when the SWB is identified at higher geomagnetic latitudes (poleward of ~74), it is a good proxy for the OCB, with 76% of SWBs lying within 3 of the OCB. At lower geomagnetic latitudes (equatorward of ~74), the correlation is poor and the results suggest that most of the SWBs being identified represent ionospheric variations unassociated with the OCB, with only 32% of SWBs lying within 3 of the OCB. We propose that the low level of precipitating electron energy flux, typical of latitudes well equatorward of the OCB in the afternoon sector, may be a factor in enhancing spectral width values at these lower latitudes. A consequence of this would be low latitude SWBs unrelated to the OCB.