Unusual nighttime impulsivefoF2 enhancement below the southern anomaly crest under geomagnetically quiet conditions

Abstract

An unusual nighttime impulsive electron density enhancement was observed on 6 March 2010 over a wide region of South America, below the southern crest of the equatorial anomaly, under low solar activity and quiet geomagnetic conditions. The phenomenon was observed almost simultaneously by the F2 layer critical frequency (foF2) recorded at three ionospheric stations which are widely distributed in space, namely Cachoeira Paulista (22.4°S, 44.6°W, magnetic latitude 13.4°S), São José dos Campos (23.2°S, 45.9°W, magnetic latitude 14.1°S), Brazil, and Tucumán (26.9°S, 65.4°W, magnetic latitude 16.8°S), Argentina. Although in a more restricted region over Tucumán, the phenomenon was also observed by the total electron content (TEC) maps computed by using measurements from 12 GPS receivers. The investigated phenomenon is very particular because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. This compression was clearly visible both by the virtual height of the base of the F region (h′F) recorded at the aforementioned ionospheric stations, and by both the vertical electron density profiles and the slab thickness computed over Tucumán. Consequently, neither an enhanced fountain effect nor plasma diffusion from the plasmasphere can be considered as the single cause of this unusual event. A thorough analysis of isoheight and isofrequency ionosonde plots suggest that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could have likely played a significant role in causing the phenomenon.

Comparative analysis of spread‐F signature and GPS scintillation occurrences at Tucumán, Argentina

“…Using isoheight curves shown in Figure , it is possible to get an estimation of the GW period T equal to 45 min, a value comparable with that obtained from the FFT done on the v TEC perturbations and shown in Figure . The vertical phase velocity v z is calculated using the peak of two consecutive heights [see Pezzopane et al ., , Figure ], and a value of v z ≈ 34 m/s is found; the vertical wavelength λ z = v z T ≈ 92 km is then obtained. These values of T , v z , and λ z are consistent with the analysis performed by Klausner et al .…”

“…[] and by Pezzopane et al . [] who found nighttime MSTIDs with wavefronts elongated from northeast to southwest. Nevertheless, it is worth noting that Makela et al .…”

“…[] and Pezzopane et al . [] refer to geomagnetically quiet conditions, while here we focused on geomagnetically unstable periods. The point that MSTIDs represent an important triggering mechanism of irregularities is also confirmed by the fact that the v TEC wave‐like behavior is concomitant with the RSF appearance, as it is possible to observe in Figure c.…”

Ionosonde observations of daytime spread F at low latitudes

“…The gray bar indicates the occurred time of the daytime spread F at PUR. It indicates that there were traveling ionospheric disturbances [Pezzopane et al, 2011]. It indicates that there were traveling ionospheric disturbances [Pezzopane et al, 2011].…”

“…It indicates that there were traveling ionospheric disturbances [Pezzopane et al, 2011]. The directions of the phase and group velocity in the gravity waves are opposite [Pezzopane et al, 2011], which means that the phase velocity of gravity waves is downward and the group velocity is upward in the ionosphere. There were also traveling ionospheric disturbances occurred at about 10:00 LT over PUR.…”