Ionosphere response to recurrent geomagnetic activity: Local time dependency

Abstract

Observations of global positioning system total electron content (TEC) and in situ electron densities at altitudes of ∼350–370 km from the CHAMP satellite are used to illustrate the local time and latitude dependence of 9 day periodicities in the ionosphere due to recurring high‐speed solar wind streams and geomagnetic activity during 2005. A local time dependence is found, with nighttime TEC oscillations concentrated at high latitudes and close to ±40% of background levels. The largest oscillations in daytime TEC occur at midlatitudes and are ±25% of background levels. Furthermore, the daytime response is generally symmetric about the geomagnetic equator with anticorrelation between high and low latitudes, whereas at night the high‐latitude Northern Hemisphere is generally in‐phase with low latitudes and anticorrelated with the high‐latitude Southern Hemisphere. A combination of enhanced equatorward neutral winds and changes in neutral composition are thought to be the primary mechanisms responsible for the observed ionospheric response. Although similar mechanisms are driving the response, the local time dependency arises because of the presence (lack) of photoionization during the daytime (nighttime). Similar trends are observed in CHAMP in situ electron densities; however, the oscillations at a near‐constant altitude are ∼10–15% larger than the TEC oscillations. Additionally, the CHAMP observations reveal possible variations in the strength of the equatorial ionization anomaly, indicating that disturbance dynamo electric fields may also contribute to the ionospheric response to recurrent geomagnetic activity. The results presented are the first to reveal the significant differences between the daytime and nighttime response of the ionosphere to periodic forcing from solar wind high‐speed streams.