Magnetic signatures of equatorial spread F as observed by the CHAMP satellite

摘要

Magnetic observations on board the CHAMP satellite are used for the first comprehensive study of magnetic signatures of the postsunset equatorial spread F (ESF) events. This is derived from a continuous database covering the years 2001–2004. On the basis of an extended survey, the global distribution of magnetic signatures is derived. We find a distinct seasonal/longitudinal variation of the occurrence rate of magnetic signatures that is consistent with that obtained from previous satellite observations of plasma depletions. The latitudinal distribution of the ESF magnetic signatures from CHAMP is symmetrical about the dip equator. It can be approximated by two Gaussian curves peaking at ±9.5° magnetic latitude, both exhibiting an 1σ-width of 4.5°. We further find a close relation between the occurrence frequency and the solar EUV flux. The global average of the occurrence rate is linearly proportional to solar activity attaining ～0.1% times the F10.7 value. The response of the ESF magnetic signatures to geomagnetic activity is also investigated. However, only a weak relation between the signature occurrence rate and the Kp index is found. Using high-resolution magnetic field measurements of the ESF structures, we are able to identify very small spatial scales of spread F of only few tens of meters. The vector magnetic field observations provide experimental evidence of the electromagnetic characteristics of ESF, valuable for testing model predictions. Finally, we discuss the effect of the ESF phenomenon on magnetic field modeling efforts based on satellite data.