Observations of GW/TID oscillations in the F2 layer at low latitude during high and low solar activity, geomagnetic quiet and disturbed periods

Abstract

Ionospheric vertical sounding observations, using a digital ionosonde, are being carried out on a routine basis at Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S, hereafter referred to as SJC), Brazil, located under the southern crest of the equatorial ionization anomaly (EIA), since August 2000. In this paper, we present and discuss the seasonal variation of gravity wave (GW) and traveling ionospheric disturbance (TID) oscillations in the ionospheric F2 layer during high solar activity (HSA, September 2000 to August 2001) and low solar activity (LSA, January 2006 to December 2006) observed at SJC during different levels of geomagnetic activity. The GW/TID signatures in the F2 layer can be seen in the isofrequency lines of virtual height daily variations for six fixed frequencies (3, 4, 5, 6, 7, and 8 MHz) which show quasiperiodic oscillations (crests and valleys). The crests and valleys when seen in close frequencies present a phase difference (i.e., first it is observed at higher frequency then at lower frequency), indicating a downward phase velocity. These quasiperiodic oscillations induced in the virtual heights are divided into three groups as small amplitude (lower than 40 km), medium amplitude (between 40 km and 60 km), and large amplitude (greater than 60 km). The observations show that GWs/TIDs are much more pronounced at F layer heights during HSA than LSA and the large‐amplitude GWs/TIDs are present normally only during HSA.

Satellite traces, range spread-F occurrence, and gravity wave propagation at the southern anomaly crest

“…3a and d), and the corresponding parameters (T , V z , and λ z ) are in agreement with those found by Klausner et al (2009). The UT of such signatures suggests that for these cases GWs are excited by the solar terminator passage (Somsikov and Ganguly, 1995;Galushko et al, 1998).…”

“…However, the most important thing to note in Table 1 is that all the RSF signatures appear well after the local sunset, which in this month occurs on average at 19:00 local time (LT=UT−4), and also well after the time an EPB, that was formed at sunset at the dip equator, takes to drift to the EA, ∼95 min as illustrated in a very comprehensive way by Whalen (2000). This suggests the two following considerations: 1) the observed RSF do not seem to be signature of EPBs generated around sunset at the magnetic equator; instead they may be considered signatures of plasma instabilities that were locally generated; 2) the PSSR, that can extend under the southern crest of the EA as was shown by Klausner et al (2009) and by Abdu et al (2009), cannot be considered itself as the only cause of RSF appearance on the ionogram, and a triggering process associated with PSSR is needed to cause instabilities responsible for RSF development. In support of this argument, Fig.…”

“…With regard to GW propagation features, Klausner et al (2009) presented a very good work illustrating its seasonal variation at the southern crest of the equatorial anomaly.…”

Low-latitude equinoctial spread-F occurrence at different longitude sectors under low solar activity

“…6 and 9). In fact, in these cases STs are due to corrugation in the isodensity surfaces caused by GW propagation which, in turn, can be considered to be the mechanism that, amplifying the post sunset seeding sources of irregularities (Fejer et al, 1999;Hysell and Burcham, 2002;Klausner et al, 2009), gives rise to the instabilities responsible for the RSF signatures occurring on the ionogram. On the other hand, under the southern crest of the EIA and for medium-low solar activity, also Fagundes et al (2009) andCandido et al (2011) have recently found evidence that spread-F events can be caused by ionospheric disturbances that are unrelated to equatorial processes, suggesting that traveling ionospheric disturbances and TPWIDs might be one of the main sources of these non-equatorial phenomena.…”

Ionospheric response to the 2009 sudden stratospheric warming over the equatorial, low, and middle latitudes in the South American sector

“…These waves are strongly connected with the ionospheric day-to-day variability in the equatorial and low latitudes and are important topics of investigation. Considerable research has gone into understanding the physics behind the observed high variability, particularly in the F2 layer [Fagundes et al, 2005Klausner et al, 2009;MacDougall et al, 2009;Rishbeth et al, 2009]. For many years, it was thought that during geomagnetic quiet conditions, the equatorial and low-latitude F2 layer was mainly perturbed by waves or electric fields generated by the equatorial electroject (EEJ), via tidal winds, which are transmitted along the magnetic field lines to equatorial F-region altitudes .…”