Abstract
Abstract. In this study, we investigate two global climatological data sets; the occurrence rate of Equatorial Spread-F (ESF), associated with equatorial plasma irregularities, at ~400 km altitude obtained from CHAMP observations, and the evening equatorial vertical plasma drift, vz, 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 between 84% and 93% in the seasons December solstice, equinox and June solstice. The highest correlation is found for the solstice seasons when vz is integrated over local time around the prereversal enhancement (PRE) and displaced 6° towards east. The integrated vz is a suitable estimate of the ionospheric height at the time just after the PRE and the 6° 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 vz 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. 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.
Highlights
Over the last several decades the name Equatorial Spread-F (ESF) has been used to designate equatorial plasma density irregularities over a large range of scale sizes which occur regularly in the low-latitude ionosphere after sunset
This study investigated independent climatological data sets of the ESF occurrence rate at ∼400 km altitude obtained from CHAMP magnetic field observations and vertical plasma drift velocities obtained from ROCSAT-1 measurements
The longitudinal variation of these prereversal enhancement (PRE) quantities were both found to correlate with the ESF occurrence rate to about 90% in each of the investigated seasons, December solstice, equinox, and June solstice
Summary
Over the last several decades the name Equatorial Spread-F (ESF) has been used to designate equatorial plasma density irregularities over a large range of scale sizes which occur regularly in the low-latitude ionosphere after sunset. Su et al (2008) provided the global evidence of the relation between these two parameters These authors compared the mean equatorial vertical plasma drift velocity between 18:00 LT and 19:00 LT with plasma depletion observations, both derived from ROSCAT-1 satellite measurements in different longitude sectors and found correlations coefficients up to 0.91 in December solstice. The occurrence rate was calculated as the number of orbital segments with ESF encounters over the sum of segments with or without ESF encounters, e.g. in a longitudinal bin The climatological features such as seasonal/longitudinal and local time occurrence rate distributions derived from the multi-years magnetic survey (Stolle et al, 2006) compare very well with the global distribution of ESFs derived from plasma depletion observations At few locations no PRE peak occurs like at −75◦ E during June solstice (i.e. Fig. 1)
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