Localized High-Latitude Ionosphere-Thermosphere Ionization Events during the High Speed Stream Interval of 29 April – 5 May 2011

Abstract:

We analyze localized ionospheric – thermospheric (IT) events in response to external driving by a high-speed stream (HSS) event during the ascending phase of the solar cycle 24. The HSS event occurred from 29 April to 5 May, 2011. The HSS (and not the associated co-rotating interaction region) caused a moderate geomagnetic storm with peak SYM-H = -55 nT and prolonged auroral activity. We analyze TIMED/SABER measurements of nitric oxide (NO) cooling emission during the interval as a measure of thermospheric response to auroral heating. We identify several local cooling emission (LCE) events in high- to sub-auroral latitudes that are presumed to be in response to external driving. Individual cooling emission profiles during these LCE events are enhanced at ~100 to 150 km altitude (ionospheric E layer). For the first time, we present electron density profiles in the vicinity of the LCE events using COSMIC radio-occultation measurements. Measurements at local nighttime show the formation of an enhanced E-layer (about 2.5 times increase over the undisturbed value) at the same approximate altitudes as the LCE peaks. Daytime electron density profiles show relatively smaller enhancements in the E-layer. We suggest that the IT response is due to additional ionization caused by medium energy electron (>20 keV) precipitation into the sub-auroral to high-latitude atmosphere during the HSS event. POES/MEPED electron precipitation data are presented to support this hypothesis. Consequences for space weather forecasting are discussed.