Cosmic Radio Noise Absorption in the Vicinity of Auroral Arcs

We have studied the relationship between cosmic radio noise absorption (CNA) in the ionosphere and auroral arcs. It is well known that CNA may occur in the same region as auroral particle precipitation. This is called as auroral absorption (AA) and it is caused by electron density enhancement in the D and lower E region due to energetic particle precipitation. However, in some cases CNA can occur outside of arcs, but in the vicinity of them. It is possible that in these cases CNA is produced by plasma waves in the ionosphere. To study under which conditions such events occur, we have looked for simultaneous optical, CNA and plasma parameter measurements. Optical measurements are made at Kilpisjarvi, Finland, (69.02ºN, 20.87ºE) by an all-sky camera or auroral television, CNA is measured by the IRIS imaging riometer at Kilpisjarvi and plasma parameters by the EISCAT incoherent scatter radar at Tromso, Norway (69.35ºN, 19.14ºE).