Abstract
Abstract. Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate ionospheric measurements of many magnetospheric processes (e.g. magnetic reconnection). This study compares the latitudes of Spectral Width Boundaries (SWBs), identified in the morning sector ionosphere using the Super Dual Auroral Radar Network (SuperDARN), with Particle Precipitation Boundaries (PPBs) determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the SWB represents a good proxy for the ionospheric projection 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 00:00-12:00 Magnetic Local Time (MLT) range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10min Universal Time (UT) window and within a ±1h MLT window. The results show that the SWB represents a good proxy for the OCB close to midnight (~00:00-02:00 MLT) and noon (~08:00-12:00 MLT), but is located some distance (~2°-4°) equatorward of the OCB across much of the morning sector ionosphere (~02:00-08:00 MLT). On the basis of this and other studies we deduce that the SWB is correlated with the poleward boundary of auroral emissions in the Lyman-Birge-Hopfield ``Long" (LBHL) UV emission range and hence, that spectral width is inversely correlated with the energy flux of precipitating electrons. We further conclude that the combination of two factors may explain the spatial distribution of spectral width values in the polar ionospheres. The small-scale structure of the convection electric field leads to an enhancement in spectral width in regions close to the OCB, whereas increases in ionospheric conductivity (relating to the level of incident electron energy flux) lead to a reduction in spectral width in regions just equatorward of the OCB.
Highlights
The boundary, or separatrix, between closed geomagnetic field lines with both foot points on the earth and open geomagnetic field lines with one end connected to the Interplanetary Magnetic Field (IMF) is a key diagnostic for the magnetospheric system
There are a wide range of proxies which are used to identify the open-closed field line boundary (OCB), including particle precipitation signatures measured by low-altitude spacecraft (Newell et al, 1991; Newell and Meng, 1992; Newell et al, 1996a), optical signatures of precipitation measured by all-sky cameras, spacecraft imagers, and meridian-scanning photometers (Blanchard et al, 1995, 1997; Sandholt et al, 1998; Brittnacher et al, 1999), E-region electron density signatures measured by incoherent scatter radar, the equatorward edge of HF radar backscatter (Milan et al, 1999; Milan and Lester, 2001), and the Doppler Spectral Width Boundary (SWB) measured by the Super Dual Auroral Radar Network (SuperDARN) (Baker et al, 1995, 1997; Chisham et al, 2001, 2002)
The major questions posed by the results are “what is the reason for the offset between the Spectral Width Boundaries (SWBs) and the OCB across much of the morning sector ionosphere” and “what does this tell us about the factors and mechanisms which determine the observed spectral width values?”
Summary
The boundary, or separatrix, between closed geomagnetic field lines with both foot points on the earth and open geomagnetic field lines with one end connected to the Interplanetary Magnetic Field (IMF) is a key diagnostic for the magnetospheric system. This boundary is typically termed the Open-Closed field line Boundary (OCB), its ionospheric projection is known as the polar cap boundary. Chisham et al.: Spectral width boundaries in the morning sector inonosphere
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