Long-term aspect-sensitivity measurements of polar mesosphere summer echoes (PMSE) at Resolute Bay using a 51.5 MHz VHF radar

Abstract

Polar Mesosphere Summer Echoes (PMSE) are now a well recognized summer phenomenon in the polar regions. For more than a decade, the echoes have been continuously monitored at Resolute Bay (75.0°N, 95.0°W) using a 51.5 MHz VHF radar, which has a narrow vertically pointing beam as well as four tilted beams at 10.9° from zenith. The radar site is located inside the northern polar cap, and remains a true polar cap radar at any level of geomagnetic activity. PMSE at this location show some unique features compared with other high latitude radars operating at similar frequencies. In this paper, we investigate the aspect-sensitivity of the PMSE at Resolute Bay. This feature of PMSE, which led to debate among researchers regarding the scattering mechanisms of these echoes, has not been extensively studied, especially on a long-term basis. In this study, we use a decade-long data-set collected during 1998–2009 to investigate the aspect-sensitivity parameter by comparing the absolute backscattered powers in four off-zenith beams with the vertical beam. We concentrate on conditions when electron precipitation is weak or not present, in order to simplify comparison (precipitation is present less than 8% of the time during PMSE). It was found that the aspect-sensitivity of PMSE can be broadly categorized into three groups based on the PMSE signal strengths of strong, moderate and weak. The height profiles of the calculated aspect angle show a good agreement among all four off-zenith beams, especially in the cases of strong and moderate PMSE strengths. Our long-term investigation reveals that when PMSE are strong, they show a median aspect angle of 8° at around 84 km and a 11° at around 88 km, meaning lower PMSE layers are associated with more anisotropic scattering. These values are about 2–3° higher, when PMSE are only “moderate” strength at Resolute Bay. The height dependence of aspect-sensitivity remains consistent in the cases of strong and moderate PMSE strengths. The results are compared with the observations made by a similar radar at another polar cap location at Eureka (80.0°N, 86.0°W). The comparison shows agreement in terms of the shape of the aspect-sensitivity height profiles, but the values are about 2–3° smaller. Possible explanations are discussed.