Geomagnetic control of polar mesosphere summer echoes

J Bremer,T L Hansen,P Hoffmann

doi:10.1007/s00585-000-0202-z

Abstract

Abstract. Using observations with the ALOMAR SOUSY radar near Andenes (69.3°N, 16.0°E) from 1994 until 1997 polar mesosphere summer echoes (PMSE) have been investigated in dependence on geomagnetic K indices derived at the Auroral Observatory Tromsø (69.66°N, 18.94°E). During night-time and morning hours a significant correlation between the signal-to-noise ratio (SNR) of the radar results and the geomagnetic K indices could be detected with a maximum correlation near midnight. The correlation becomes markedly smaller in the afternoon and early evening hours with a minimum near 17 UT. This diurnal variation is in reasonable agreement with riometer absorption at Ivalo (68.55°N, 27.28°E) and can be explained by the diurnal variation of ionization due to precipitating high energetic particles. Therefore, a part of the diurnal PMSE variation is caused by this particle precipitation. The variability of the solar EUV variation, however, has no significant influence on the PMSE during the observation period.Keywords: Ionosphere (auroral ionosphere) - Magnetospheric physics (energetic particles, precipitating) - Radio science (remote sensing)

Highlights

After the ®rst detection of polar mesosphere summer echoes (PMSE) at Poker Flat, Alaska, in 1979 by Ecklund and Balsley (1981) investigations of this phenomenon have been made with dierent radars at dierent places
The physical reason for this behaviour should be precipitation events of high energetic particles which are in one case positively connected with the geomagnetic activity and which in the other more often observed during nighttime and in the early morning hours than in the afternoon period
During the afternoon the correlation becomes smaller with a minimum near 17 UT. The reason of this diurnal variation is the variation of ionization due to precipitation of high energetic particles

Summary

Introduction

After the ®rst detection of polar mesosphere summer echoes (PMSE) at Poker Flat, Alaska, in 1979 by Ecklund and Balsley (1981) investigations of this phenomenon have been made with dierent radars at dierent places. Bremer altitudes between about 80±90 km, during summer months at polar latitudes, not all the details of this phenomenon are fully understood. Radar scatters from the mesosphere are caused by inhomogeneities in the electron density having spatial scales of about half the radar wavelength. Such small-scale electron densityuctuations are normally smoothed by diusion. The occurrence of large particles is markedly supported by the low mesospheric temperatures during polar summer months. A detailed review of existing PMSE observations and theories can be found in Cho and RoÈ ttger (1997)

Results

Discussion

Conclusion