[1] In this paper, we report a long-term study of medium-scale traveling ionospheric disturbance (MSTID) occurrence using all-sky images of O I 630 nm airglow emission. Our study is based on a 10.5 year data series in which 5.5 years compound a new data set. The images were obtained by an all-sky imager installed at Cachoeira Paulista (22.7°S, 45°W, 15°S magnetic latitude). Our results show the greatest occurrence of MSTIDs during solar minimum and a minor occurrence rate during descending and ascending solar activity periods. During solar maximum, we have not detected MSTID signatures on all-sky images. All detected events have occurred during geomagnetically quiet conditions. Simultaneous measurements using both all-sky images and ionograms obtained at the same site show the occurrence of spread F and a sharp rise in the ionospheric F layer at the same time as dark bands are optically registered over the zenith. It is possible that for certain weaker events, the ionosonde may resolve bands that are not sufficiently raised to cause darkening of the all-sky images. In order to investigate this possibility, we have conducted a study using only ionograms for a 1 year period (March 2000-February 2001 during solar maximum, when we have not seen any MSTID events in the all-sky images. As we conjectured, MSTIDs also occur during solar maximum, and the risings of the ionospheric F layer are not able to disturb the airglow layer during such events.Citation: Amorim, D. C. M., A. A. Pimenta, J. A. Bittencourt, and P. R. Fagundes (2011), Long-term study of medium-scale traveling ionospheric disturbances using O I 630 nm all-sky imaging and ionosonde over Brazilian low latitudes,

A digital ionosonde of the type known as the Canadian Advanced Digital Ionosonde (CADI) is operational at São José dos Campos (23.2°S, 45.9°W), Brazil. This ionospheric sounding station is located under the southern crest of the ionospheric equatorial anomaly. The F layer electron density profile presents considerable day‐to‐day variability, even during undisturbed conditions, and this variability is still one of the less understood aspects of the physics of the ionosphere. The propagation of waves into the ionosphere may play an important role in this day‐to‐day ionospheric variability. In this paper we present a new technique that uses multifrequency virtual height variations, from ionospheric sounding observations at this low‐latitude station, to investigate how the F layer is modulated by planetary wave type oscillations. We have also considered the possible influence of oscillations due to solar origin (solar rotation variation). In the present study, observations during the months of June to September 2003 (season of low‐range type equatorial spread F occurrence at this location) have been used. The present study indicates the presence of 2‐day (the observed 3‐day periods are possibly associated with the quasi 2‐day oscillations), 5‐day, 10‐day, and 16‐day periods related to planetary wave type oscillations due to tropospheric sources during all the day. Also, it appears that during the day and dusk times, the longer periods 24–30 days are related to solar rotation (27 days), whereas the shorter periods 9–17 days have possibly some influence from half solar rotation (13.5 days). During the nighttime, possibly the short period oscillations 3–9 days may have some influence from the geomagnetic disturbances.

[1] Using two ground-based all-sky imaging systems, measurements of moving dark band structures in the OI 630.0 nm nightglow emission were detected in the low-latitude region of Brazil. On the nights of 30-31 August 1995, 18-19 July 1998, and 13-14 July 1999, the all-sky imaging observations of the OI 630 nm emission, carried out at Cachoeira Paulista (22.7°S, 45°W, magnetic declination 20°W), Brazil, showed dark band structures that stretched across the entire image and propagated from southeast to northwest. The observed band structures are thus most likely formed in the midlatitude region and move into the field of view. Digisonde observations, available for two of the events, registered abrupt increases in both the F-layer peak height (h m F 2 ) and minimum virtual height (h'F) on 19 July 1998 and 13 July 1999, when the low-intensity band passed over Cachoeira Paulista. It should be pointed out that these thermospheric events are not related to geomagnetic disturbed conditions, as the nights for which data are presented are geomagnetically quiet (Kp < 2). In this paper we present and discuss the first observations of these thermospheric dark band structures in the Brazilian sector and their effects on the nighttime ionosphere. A possible mechanism for generation of these dark band structures is presented.

Abstract.A very intense geomagnetic storm (superstorm) began with storm sudden commencement (SSC) at 08:03 UT on 20 November 2003, as a result of the coronal mass ejection (CME) by sunspot 484 hurled into space on 18 November 2003. The geomagnetic storm attained |D st | max =472 nT at 20:00 UT (20 November). In this paper we present the simultaneous ionospheric sounding observations, using the Canadian Advanced Digital Ionosondes (CADIs), carried out from Palmas (PAL; 10.2 • S, 48.2 • W; dip latitude 5.5 • S; a near equatorial station) and São José dos Campos (SJC; 23.2 • S, 45.9 • W; dip latitude 17.6 • S; station located under the crest of equatorial ionospheric anomaly), Brazil. In addition, total electron content (TEC) measurements from several GPS receiving stations in the Brazilian sector during this storm are presented. The simultaneous ionospheric sounding observations carried out at SJC and PAL, and TEC observations on 3 consecutive days viz., 19 November (quiet), 20 November (disturbed) and 21 November (recovery phase) are presented. Salient features from the ionospheric observations in the Brazilian sector during the superstorm are discussed. The difference in the observed ionospheric response at the two stations (PAL and SJC) is considerable. This is not surprising given that PAL is close to the magnetic equator and SJC is near the crest of the equatorial ionospheric anomaly (EIA). It should be pointed out that soon after the SSC (about 4 h later), the F-region critical frequency (foF2), the F-region peak height (hpF2), and variations of virtual heights at different frequencies (iso-frequency plots) all show Correspondence to: F. Becker-Guedes (fabio@univap.br) wavelike oscillations of the F-region during daytime at both the ionospheric sounding stations. Unusual rapid uplifting of F-region at PAL was observed during both the main and recovery phases of the storm.

Abstract. The 28 October 2003 solar flare (X-ray Class X17.2) was one of the most intense solar flares observed in the recent past. In the present investigation we show the unusual ionospheric effects observed in the Brazilian sector during this solar flare, using both the ionospheric sounding observations obtained at the UNIVAP stations: