[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,

Using ground‐based measurements we investigate the occurrence of dark band structures in the OI 630 nm nightglow emission all‐sky images in the Brazilian low latitude region during the periods from January 1990 to December 1990 (high solar activity period‐HSA), May 1995 to July 1996 (low solar activity period‐LSA) and from January 1997 to December 1999 (ascending solar activity period‐ASA). Unfortunately we do not have all‐sky images for the descending phase of solar activity period. The OI 630 nm images obtained during these periods show thermospheric Dark Band Structures (DBS) in the low latitudes region propagating from southeast to northwest. These dark patches moved with average speed of about 50–200 m/s at an altitude of 220–300 km, which is the typical altitude range of the OI 630.0 nm airglow emission. Only during low solar activity period (LSA) and ascending solar activity period (ASA), the DBS occurrences were observed in the OI 630 nm nightglow emission all‐sky images. It should be pointed out that these thermospheric/ionospheric events are not related to geomagnetic disturbed conditions. In this paper, we present and discuss this phenomena in the Brazilian sector under different solar activity conditions. A possible mechanism for generation of these dark band structures is presented.

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An atypical nighttime spread‐F structure is observed on ionograms at or above the F2 trace, near the crest of the ionospheric equatorial ionization anomaly (EIA) region. This ionospheric atypical spread‐F phenomenon was observed using two closed spaced (∼115 km) ionospheric soundings stations located in Sao Jose dos Campos (23.21°S, 45.97°W) and Cachoeira Paulista (22.70°S, 45.01°W), Brazil, in a low‐latitude station (near the southern crest of the EIA region), during nighttime, low solar activity, and quiet geomagnetic conditions. This structure, in the initial phase, appears in the ionogram as a faint spread‐F trace above or at the F2‐layer peak height. After a few minutes, it develops into a strong spread‐F trace, and afterwards, it moves to altitudes below the F2‐layer peak heights. Finally, the atypical nighttime F‐layer trace structure may remain for a while between the F‐layer bottom side and peak height or can move to an altitude above the F‐layer peak height, and then it disappears. In order to have a comprehensive view of the ionospheric environment characterizing the phenomenon under study, complementary data from six GPS station were used to investigate the ionosphere environment conditions, during both events. The six GPS stations used in this study are distributed from near the equatorial region to low latitudes and provide evidence that the atypical nighttime spread‐F structures are not related with large scale equatorial irregularities (plasma bubbles).

An interesting case of plasma blob event was observed on August 27-28, 1987 over Cachoeira Paulista (22.7ºS, 45.0ºW; magnetic latitude 13.25ºS, declination 20ºW) showing localized plasma density enhanced by a factor of, approximately, 2 above the background level. The F-region parameters were obtained from a Digisonde 256, which provide a good idea of the ionospheric behavior during the event. On this night, geomagnetic activity was moderately disturbed with Dst > -70 nT. Complementary data was obtained from an all-sky imager operating on a routine basis at the same site. All-sky images were used to map the spatial extension and temporal location of plasma blob that showed, typically, east-west and north-south extensions of 320-350km and of 360-380km, respectively. In this paper, important features from this observation is presented and discussed.