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.

[1] We analyze in detail the zonal velocities of large-scale ionospheric plasma depletions over two conjugate stations inferred from OI 630 nm airglow all-sky images obtained during the Conjugate Point Equatorial Experiment (COPEX) campaign carried out in Brazil between October and November 2002. The conjugate stations were Boa Vista (BV) (geogr. 2.8N, 60.7W, dip angle 22.0°N) and Campo Grande (CG) (geogr. 20.5S, 54.7W, dip angle 22.32°S). Over Campo Grande, the zonal velocities were measured also by a system of spaced GPS scintillation receivers. The airglow zonal velocities at the conjugate sites were seen to agree very closely, except for a slightly increased velocity over CG which we attribute to the presence of the geomagnetic anomaly. The results show a high degree of alignment of the bubbles along the geomagnetic field lines during the bubble development phase and as the bubbles travel eastward, thereby suggesting that the neutral zonal wind effect in the zonal plasma motion is an integrated effect along the flux tube. The zonal velocities obtained from the GPS technique were always larger than those calculated by the airglow technique, which permitted observation of zonal plasma velocity shear between the altitudes of the airglow emitting layer and of the GPS scintillation. Theoretical ambient plasma zonal velocities calculated using the formulations by and Eccles (1998) are compared with the experimental results. Our results also reveal some degree of dependence of the zonal velocities on the solar flux (F10.7) and magnetic (Kp) indices during the COPEX period.

[1] The solar events that occurred at the end of October 2003 gave rise to very strong geomagnetic disturbances that peaked twice with Dst values reaching À345 nT around 0000 UT on 30 October and À400 nT around 2300 UT, on the same day. Disturbances in several ionospheric parameters were observed over Brazil. This work will focus on the ionospheric response to the initial westward prompt penetration electric field and on the strong intensification of the equatorial ionization anomaly that occurred because of the electric field polarity reversal that followed in the early morning hours of 29 October. The F layer peak height over the equator first decreased under the strong prompt penetration westward electric field, which was followed by significant height increase under eastward electric field. We have used Sheffield University Plasmasphere Ionosphere Model (SUPIM) with an intensified westward disturbed electric field in the presunrise hours, presumably due to prompt penetration from the magnetosphere, in order to study the effect of such a field in the ionosphere. The simulation results showed that prompt penetration of magnetospheric electric fields of westward polarity to the nightside equatorial region seems to be the most probable cause of the initial F layer height decreases. The intensification of the equatorial ionization anomaly and the unusual enhancement on F layer peak density, which was not modeled by the SUPIM, are explained as caused by the strong eastward electric field that followed the initial phase in combination with a highly variable disturbed meridional/transequatorial wind system as inferred from the F2 layer peak height variations. The highly dynamic wind pattern, with a short-term response (2-4 hours), is compatible with the predictions of some previous theoretical model calculations reported in the literature.

Using soft X‐ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high‐cadence Lyman‐α observations from the Large Yield Radiometer (LYRA) onboard the Project for On‐Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium‐size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub‐ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman‐α radiation excesses during the flares. Indeed, Lyman‐α excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman‐α emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman‐α variations along the solar cycle, they cannot be used to scale explosive Lyman‐α emission during flares.

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[1] We present recent observations of Sudden Phase Anomalies due to subionospheric propagation anomalies produced by solar X-ray flares. We use the new South America VLF Network (SAVNET) to study 471 ionospheric events produced by solar flares during the period May 2006 to July 2009 which corresponds to the present minimum of solar activity. For this activity level, we find that 100% of the solar flares with a X-ray peak flux above 5 × 10 −7 W/m 2 in the 0.1-0.8 nm wavelength range produce a significant ionospheric disturbance, while the minimum X-ray flux needed to do so is about 2.7 × 10 −7 W/m 2 . We find that this latter minimum threshold is dependent on the solar cycle, increasing when the Sun is more active, thus confirming that the low ionosphere is more sensitive during periods of low solar activity. Also, our findings are in agreement with the idea that the ionospheric D-region is formed and maintained by the solar Lyman-a radiation outside solar flare periods.

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Abstract. In this work, the F-region critical frequency (foF2) and peak height (hmF2) measured by digital ionosondes at two Brazilian low-latitude stations, namely Palmas The comparison at the latter station shows quite a reasonable agreement for both parameters. The former station exhibits a better agreement for hmF2 than for foF2. In general, the model generates good results, although some improvements are still necessary to implement in order to obtain better simulations for equatorial ionospheric regions.