Ionosphere response to recurrent geomagnetic activity in 1974

The 9 day variation of Earth's thermosphere neutral density in 2005 and 2006 is known to have been influenced by rotating solar coronal holes and the quasiperiodic solar wind high‐speed streams (HSSs) and by the concomitant recurrent geomagnetic activity that they induce. The corresponding responses in ion temperature, ionospheric electron density, and total electron content have also been reported during the same period. In 1974, a significant 13.5 day periodicity in geomagnetic activity was created by the quasiperiodic HSS associated with two major solar coronal hole regions separated by about 180°. In the present paper, ionospheric F‐ region peak plasma density ( N m F 2 ) and height ( h m F 2 ) in the daytime and nighttime from 12 ionosonde stations are analyzed to delineate the responses to this epoch of HSS forcing of the geospace system. Results show that the ionospheric responses to this 13.5 day periodic forcing are similar in some ways to responses to the 9 day periodicity. For instance, in middle and high latitudes, daytime and nighttime N m F 2 is mostly out of phase with the fluctuations in the daily mean K p index ( K p ¯ ), while the daytime h m F 2 are in phase with theK p ¯ fluctuations. Empirical model results confirm the important role of thermal expansion in connecting thermospheric and ionospheric changes driven by high‐speed streams and recurrent geomagnetic activity. At low latitudes, the 13.5 day signatures are not as straightforward as those at middle and high latitudes, and significant spectral energy thought to be connected with planetary waves and perhaps other lower‐atmosphere influences exists at periods of less than 13.5 days.