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

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, v z , 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 v z is integrated over local time around the prereversal enhancement (PRE) and displaced 6 • towards east. The integrated v z 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 v z 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][2002][2003][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.

Features of highly structured equatorial plasma irregularities deduced from CHAMP observations

“…They also stated that the R-T growth rate maximizes at the time of the peak height rather than at the time of peak velocity. Stolle et al (2008) and Su et al (2008) have confirmed that the EPB occurrence rate has a higher correlation with the integrated vertical plasma drift velocity than with the peak value of the vertical plasma drift velocity during the pre-reversal enhancement (PRE).…”

“…In order to prove this suggestion we have also investigate the vertical plasma drift, V Z , data of ROCSAT-1. Su et al (2008) and Stolle et al (2008) have shown that EPB occurrence rates show a good correlation with the peak vertical plasma drift during the pre-reversal enhancement, V P Z . But the correlation between EPB occurrence rate and the integrated vertical velocity, S PRE , is even higher.…”

“…The close relationship between EPB occurrence rate and the amplitude of the vertical plasma drift, or the layer height, was shown empirically in several studies (Farley et al, 1970;Fejer et al, 1999;Stolle et al, 2008). Fejer et al (2008) used ROCSAT-1 data to develop a model for the vertical plasma drift, which was used by Stolle et al (2008) for their analysis.…”

“…Fejer et al (2008) used ROCSAT-1 data to develop a model for the vertical plasma drift, which was used by Stolle et al (2008) for their analysis. Jayachandran et al (1993) investigated HF Doppler radar and ionosonde measurements in India, and stated that the height of the F-layer, determined by the evening vertical plasma drift, is the deciding factor for an EPB onset.…”

Climatology of GPS signal loss observed by Swarm satellites

“…As the GPS signal loss at low latitudes is caused by the EPIs, similar seasonal dependences are expected from them, e.g., higher occurrence around equinoxes and December solstice months from a global view (Burke et al, 2004;Su et al, 2006;Stolle et al, 2008;Xiong et al, 2010). However, the seasonal dependence of polar patches is still an open issue: some studies reported that the polar patches are mainly a local winter phenomenon in both hemispheres (e.g., Coley and Heelis, 1998;Kivanç and Heelis, 1998;Carlson, 2012;Spicher et al, 2017), while there are also studies found that polar patches have higher occurrence during December solstice months in both hemispheres (e.g., Noja et al, 2013;Chartier et al, 2018).…”