Relation between the occurrence rate of ESF and the equatorial vertical plasma drift velocity at sunset derived from global observations

C. Stolle, H. Lühr, B. G. Fejer
2008 Annales Geophysicae  
<p><strong>Abstract.</strong> In this study, we investigate two global climatological data sets; the occurrence rate of Equatorial Spread-<I>F</I> (ESF), associated with equatorial plasma irregularities, at ~400 km altitude obtained from CHAMP observations, and the evening equatorial vertical plasma drift, <I>v<sub>z</sub></I>, from ROCSAT-1 measurements. First, as retrieved for a solar flux level of F10.7=150, the longitudinal variation of the two independently derived quantities correlates
more » ... ities correlates between 84% and 93% in the seasons December solstice, equinox and June solstice. The highest correlation is found for the solstice seasons when <I>v<sub>z</sub></I> is integrated over local time around the prereversal enhancement (PRE) and displaced 6&amp;deg; towards east. The integrated <I>v<sub>z</sub></I> is a suitable estimate of the ionospheric height at the time just after the PRE and the 6&amp;deg; displacement is consistent with ESF eastward drift during 2 h which is assumed between creation and detection at satellite altitudes. Second, our analyses reveal a global threshold <I>v<sub>z</sub></I> which is required to observe ESF at satellite altitudes. This threshold depends linearly on solar flux with correlations of 97%. Both results bring global evidence on the linear relations between ESF and the vertical plasma drift which have been proven only by local observations so far. <br><br> This paper includes the first global map of the seasonal/longitudinal variation of the ESF occurrence rate over local time being valid for high solar flux years 2001–2004. The map reveals, e.g. a longitudinal dependence of the persistence of the plasma irregularities indicating that longitude dependent mechanisms other than the PRE determine the ESF lifetime.</p>
doi:10.5194/angeo-26-3979-2008 fatcat:3bxd6e2grzhr3ezduwrmuk5stq