Behavior of the equatorial ionosphere during the prolonged solar cycle 23-24 minimum is one of the interesting phenomena. Many authors concluded that the annual means of the critical ionospheric frequency foF2 and the global TEC (total electron content) were reduced due to EUV variations (extreme ultraviolet), while other investigations found no essential variations compared to the previous solar minimum. To resolve these doubts we have examined the factors which can change the annual ionospheric variations of the critical frequency foF2 at the equatorial ionization anomaly stations Vanimo during the two last solar minima. With that end in view the annual variations of foF2 at Vanimo station have been calculated and compared with variations of Dst-index and of the solar wind parameters. We show that in addition to low level of the EUV during the last solar minima, the geomagnetic variations effect, associated with solar coronal holes, has to be included as the influencing factor on the equatorial ionosphere.
Sun and Geosphere
- Pub Date:
- March 2014
- Equatorial ionosphere; Dst-index; Solar minimum; Annual variation