Long-Term Characteristics of Midlatitude Mesosphere Summer Echoes (MSE) Observed with a VHF Radar at Wakkanai, Japan (45.4ºN)

Abstract Midlatitude mesosphere summer echoes (MSE) at the VHF band (VHF-MSE) were observed for 13 years (2000-2002 and 2009-2018) with a 46.5 MHz radar at Wakkanai, Japan (45.4ºN, 141.8ºE). VHF-MSE are active during June-July and appear only in the daytime mainly at altitudes of 80-88 km with a maximum occurrence at 85 km and altitude extents of 1-4 km for a duration of about half an hour or more. The VHF-MSE occurrences are positively correlated with solar activity, but not with geomagnetic activity except for very high activity. Such long-term characteristics are mostly consistent with past VHF-MSE observations at higher midlatitudes in Europe. No VHF-MSE were observed in 2002, 2014 and 2018, possible reasons for which are discussed. It is shown that cold ice particles in the upper mesosphere inducing MSE are advected from high latitudes to midlatitudes with equatorward wind. Thus, the MSE occurrences over Wakkanai are fundamentally controlled by both the solar activity and equatorward ice particle advection. One example of MSE at the HF band (HF-MSE) is presented to discuss spatial and temporal relationship between VHF-MSE and HF-MSE.