Abstract
Abstract. 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.
Highlights
Understanding the processes which control the transfer of mass, momentum and energy from the solar wind into the Earth’s magnetosphere is a major goal in magnetospheric science
By determining the probability distribution of the spectral width boundary (SWB) with latitude and magnetic local times (MLTs), Chisham and Freeman (2004) showed how the preferred latitudinal location of SWBs in data measured by the Halley SuperDARN radar became less clearly defined in the afternoon sector ionosphere
The doc boundary distribution has moved so that its peak is located ∼4◦−6◦ poleward of the SWB, the distributions showing a similar amount of overlap with the SWB as the deq and dds boundaries. These results suggest that the SWBs being identified equatorward of 74◦ are unrelated to the open-closed field line boundary (OCB) and that they correspond to another latitudinal transition that exists in the boundary plasma sheet (BPS)/low-latitude boundary layer (LLBL) precipitation region
Summary
Understanding the processes which control the transfer of mass, momentum and energy from the solar wind into the Earth’s magnetosphere is a major goal in magnetospheric science Most of this energy transfer occurs through the process of magnetic reconnection, in which geomagnetic flux is transferred between closed field line regions (with both ends connected to the Earth’s ionosphere), and open field line regions (with one end connected to the interplanetary magnetic field (IMF)). By identifying and tracking the open-closed field line boundary (OCB), the addition and removal of open magnetic flux can be measured, and the net global reconnection rate can be determined (Siscoe and Huang, 1985; Cowley and Lockwood, 1992; Milan et al, 2003, 2004). SuperDARN presently comprises 9 Northern Hemisphere, and 7 Southern Hemisphere, HF radars whose
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