Solar Cycle Related Range Type Spread F Occurrence Characteristics over Equatorial and Low Latitude Stations in Brazil

摘要

Range type spread F occurrence rates from sunset to morning hours over Fortaleza and Cachoeira Paulista during the equinoctial month of October. Different occurrence rates were observed at the two stations especially in the early post sunset hours. Over Cachoeira Paulista the spread F occurrence rate at 20 and 21 LT increased by approximately a factor of 3 from low to high sunspot years, and starting from 22-23LT this rate of increase diminished and even showed an inverse dependence with sunspot number towards early morning hours. Fortaleza, the spread F occurrence rate in the early post sunset hours remained very high, 90-100%, during 20-21LT, with no variation from solar minimum to maximum epochs. The spread F occurrence rate is nearly independent of sunspot number remained high up to midnight and fell towards morning hours. The results are interpreted on the basis of the solar activity control on the statistical behavior of plasma bubble vertical rise velocity and F-region dynamo electric field.