Observations of daytime F2‐layer stratification under the southern crest of the equatorial ionization anomaly region

Abstract

Ionospheric vertical sounding observations are being carried out at Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), Brazil, under the southern crest of the equatorial ionization anomaly (EIA) since August 2000. In this paper, we present and discuss the observations of daytime F2‐layer stratification near the crest of EIA, for the first time, under magnetically quiet high solar activity conditions. Three examples and a year of statistics are presented. The F2‐layer stratification and F3‐layer were observed between 10:40 and 11:45 UT on 31 December 2000, between 13:30 and 14:30 UT on 1 January 2001, and between 13:15 and 15:15 UT on 11 February 2001. The statistics during September 2000 to August 2001 shows that the F3‐layer occurs only for 66 days (18% occurrence), and it occurs only during September–February (spring–summer), with maximum occurrence in September–October and longest duration in February. The F2‐layer stratification seems to be associated with gravity waves (GWs), which have periods of about 30–60 min, downward phase velocities of about 60–140 m/s, and vertical wavelengths of about 200–500 km. The presence of powerful gravity waves in a vertically extended F‐layer seems to stratify the F2‐layer and produce the F3‐layer. Because the stratifications are observed during geomagnetically quiet periods, the source of the gravity waves are most likely to be associated with local tropospheric disturbances and not with high‐latitude disturbances.

Stratification of the Low-Latitude and Near-EquatorialF2Layer, Topside Ionization Ledge, andF3Layer: What We Know about This? A Review

“…Their model result shows that an upward E × B drift with ordinary neutral wind cannot produce the humps in the latitudinal variation of TEC as in the observation. The mechanism suggested by Thampi et al [54] resembles that of Fagundes et al [26] for the EIA region. However, they have not calculated the wave parameters through the ionosonde data to rule out possibility of the ordinary F3 layer which have the following signature: (a) it remains at the same virtual height and does not propagate downward to lower frequencies as a gravity wave would be expected to do; (b) no periodicity is observed in the intensity of the cusp.…”

“…Vasiliev [25] suggested that with increase of distance from equator, the role of internal gravity waves in the formation of the equatorial F2 layer stratifications increases. This hypothesis is used at present to explain the formation of the stratifications at the latitudes of the EIA crests in quiet geomagnetic conditions [26].…”

“…Fagundes et al [26] reported daytime F2 layer stratification over an equatorial anomaly crest location at Sao Jose dos Campos (23.2…”

“…They have shown that (I) the frequency of occurrence during high solar activity (HSA) is 11 times greater than during low solar activity (LSA), (II) during HSA there is a maximum occurrence of F3 layer during summer time and a minimum during winter time; while during LSA there is no seasonal variation in the F3 layer occurrence, and (III) during the maximum occurrence season in HSA F3 layer was generated almost every day and lasted for several hours, while during the month with minimum occurrence, the F3 layer lasted for about one hour or even less time. The seasonal and solar activity dependence of the F3 layer occurrence suggests that the vertical extension of the F2 layer is an important factor that is favorable for the formation of the F3 layer if AGWs mechanism works, that is the F layer vertical extension must be larger than the gravity wave wavelength to create favorable conditions for stratification and formation of the F3 layer [26]. During the HSA and summer season, large daytime upward E × B drift and/or large equatorward wind can cause unusual vertical extension of the F2 layer near the EIA crest.…”

Features of theF3layer in the low-latitude ionosphere at sunset

“…These observations suggest that the distortion in the equatorial electron density profile, associated with a movement toward the base of the F 2 layer as magnetic field lines, descended with increasing latitude. Fagundes et al [2007] reported stratification of the F 2 layer over an equatorial anomaly crest location in Brazil, which was mainly considered as the result of the propagation of atmospheric gravity waves (AGWs) often observed in the middle latitudes [Heisler, 1962].…”