Proceedings with Extended Abstracts (single PDF file) - Radio ...

Absorption of cosmic noise is caused by the increase of electron density in mesospheric heights(D-region). The enhancement of electron density is due primarily to the ionizing effects of highenergysolar protons down to even low altitudes of the mesosphere. This is called a Polar CapAbsorption (PCA) event, which spreads out all over the polar cap and can last for many hoursand days. We also observed the significant decrease of PMSE intensity during this PCA event,especially at the top of the PMSE layer above 86 km. Figure 1 shows the temporal variations ofnoise absorption, geomagnetic disturbance, PMSE signal power in the altitude of 89 km and HTIplot of PMSE. The drastic noise absorption, i.e. enhanced D-region electron density, occurred on14-16 July during the strong geomagnetic/ionospheric disturbance. Despite the increase inelectron density, the PMSE signal power around 86-90 km was depleted in comparison with themean diurnal variation of signal intensity deduced from data sets, recorded before (1-13 July) andafter this event (19-31 of July). The lower part of the PMSE layer below 85 km was not affectedat all in this event. This is in so far surprising, since we expect a decrease in PMSE signalstrength due to the underlying electron density, which causes an absorption of the radar signal aswell. When we consider this absorption and correct the measured signal strength of PMSE, wewould get an enhancement of the scatter cross section over the whole range. However, thereduction in the upper part still exists, although we cannot fully recover the significant reductionof PMSE signal in the heights above 86 km.Fig. 2 Height profiles of measured electron density Ne, heating rate dT/dt,neutral temperature T before and after the Joule heating (dashed lines), andthe limiting water vapor mixing ratio.We may assume that the reduction in PMSE signal power above 86 km during this event resultedfrom a neutral temperature increase by particle heating. To estimate particle heating effect (Banks,1979), we calculate the Joule dissipation rate, which is deduced from the electron density profileobtained with concurrent incoherent scatter observations with the EISCAT Svalbard Radar. Therequired profiles of mean background neutral temperature, neutral density, the collision- andgyro-frequencies were obtained from models. The ionospheric electric field measurements,needed to calculate the heating rate, were obtained from complementary incoherent scattermeasurements with the collocated EISCAT Svalbard Radar This deduction allowed us to determinethe temperature increase (heating rate) with time dT/dt.223