Brightening of 630.0 nm equatorial spread‐F airglow depletions

Abstract

[1] Observations from the Boston University all-sky imaging system at Arecibo, Puerto Rico (18.3°N, 66.7°W, 28°N mag), show an unusual behavior of nighttime 630.0-nm airglow depletions. Associated with equatorial spread-F (ESF), these structures move eastward before reversing their motion and become airglow enhancements. Few other cases have been found, all during December solstices. For the case study presented here, data from the Arecibo incoherent scatter radar and the Republic of China Scientific Satellite (ROCSAT-1) provide supporting information. The radar shows that around local midnight the background zonal and meridional plasma motions reverse to westward and southward, respectively. ROCSAT-1 shows enhanced ion density, i.e., a low-latitude plasma blob, above the bright feature recorded by the all-sky imager, indicating a possible connection between both phenomena. Drifts parallel to the magnetic field are observed only in the region where the enhancement occurs. One possible interpretation of this change in the brightness of the depleted structure involves the influence of northward meridional winds and a reversal in the zonal drift motion, most likely caused by a zonal wind reversal.

Density enhancements associated with equatorial spread <I>F</I>

“…In a related example, we found that a meridional wind can act in such a way that a density enhancement (relative to the nearby background) occurs within an ESF plume and that this enhancement has ion-velocity profiles similar to those observed (Le et al, 2003;Yokoyama et al, 2007). In fact this same SAMI3/ESF study reproduced an observed ESF airglow phenomenon, ESF airglow enhancement, which is related to the above-mentioned ESF density enhancement Martinis et al, 2009). In more recent simulations, which we report below, we find that ESF density enhancements can occur in the absence of winds if the ESF plumes are growing relatively slowly.…”

“…This figure shows plots of log 10 n e , field-aligned O + velocity, vertical E × B velocity, and magnetic latitude versus longitude along a trajectory at fixed altitude 608 km. The top panel shows density increases similar to those seen in satellite data Le et al, 2003;Yokoyama et al, 2007;Park et al, 2008b;Martinis et al, 2009). We see that the E ×B drifts are everywhere upward and that the field aligned flows are everywhere poleward.…”

“…We have shown two circumstances in which the density on or above the ESF crests leads to a density enhancement, also called a "plasma blob," similar to those observed in situ Watanabe and Oya, 1986;Le et al, 2003;Yokoyama et al, 2007;Park et al, 2008b;Martinis et al, 2009). In one case, slowly growing ESF produces upward E × B drifts that move density upward faster than gravity moves it downward, with the result that densities on ESFcrest field lines, at or above the density crests, have densities that are enhanced relative that of the background ionosphere.…”

“…Krall (jonathan.krall@nrl.navy.mil) density gradients associated with ESF affect the propagation of electromagnetic signals, sometimes degrading global communication and navigation systems (de La Beaujardiere et al, 2004;Steenburgh et al, 2008), the studies have continued. One of the many ESF-related phenomena that have yet to be explained is the occurrence of ESF density enhancements, also called "plasma blobs" Watanabe and Oya, 1986;Le et al, 2003;Yokoyama et al, 2007;Park et al, 2008b;Martinis et al, 2009). These occur in regions of ESF activity, have been measured in situ by the Hinotori (altitude ∼650 km), ROCSAT (∼600 km), DMSP (∼840 km), STSAT (∼680 km), and CHAMP (∼350 km) satellites, and may be related to the phenomena of ESF airglow enhancements Martinis et al, 2009).…”

“…One of the many ESF-related phenomena that have yet to be explained is the occurrence of ESF density enhancements, also called "plasma blobs" Watanabe and Oya, 1986;Le et al, 2003;Yokoyama et al, 2007;Park et al, 2008b;Martinis et al, 2009). These occur in regions of ESF activity, have been measured in situ by the Hinotori (altitude ∼650 km), ROCSAT (∼600 km), DMSP (∼840 km), STSAT (∼680 km), and CHAMP (∼350 km) satellites, and may be related to the phenomena of ESF airglow enhancements Martinis et al, 2009). One aim of this paper is to provide an explanation for ESF density enhancements.…”