Equatorial spread F statistics and empirical representation for IRI: A regional model for the Brazilian longitude sector

Abstract

The empirical representation of the equatorial spread F (ESF) statistics in the IRI scheme requires well established distribution statistics of ESF occurrence and intensity as a function of local time, season/month, latitude and solar and magnetic activity levels. We present here a regional model for the quiet-time spread F distribution in the Brazilian longitude sector. In view of the well known dependence of spread F occurrence on magnetic declination angle, and the fact that the declination angle varies rapidly from the west coast (Peruvian sector) to the east coast (Brazilian sector) of south America the present model can be said to be valid for the latter sector. 13 years of spread F data simultaneously collected (during 1978–1990) over the equatorial site Fortaleza and low latitude site Cachoeira Paulista are used in this model. Only spread F data that is believed to be related to plasma bubble developments is used in the model. The data were first grouped into solar flux range bins representing low, medium, and high solar activity levels, represented, respectively, by F10.7 ≤ 100; 100 < F10.7 < 180; and F10.7 ≥ 180. Spread F percentage occurrence as a function of (nocturnal) local time for each of the 12 months in each solar flux range was calculated, and form the database for the model. Cubic-B spline fits of the data in local time, latitude, season/month and solar flux nodes constitute the structure of the present model. The model confirms many characteristic features of the spread F statistics already known as well as brings out some new outstanding features for the Brazilian sector. Among the results to be highlighted are: The spread F onset and peak occurrence get delayed in local time with increasing distance from the equator, indicating the plasma bubble origin for the low latitude ionogram spread F traces; The plasma bubble occurrence as well as the vertical rise velocity increase with the increase in solar flux; They attain larger values in summer months (centered around December) than in equinoctial months (March and September). The latitudinal variation in spread F, though based only on two-station data sets in this study, looks compatible with the latitude variation of ion density fluctuations observed by the AE-E satellite. The model will be made available to interested users.