Studies of the PMWE : Polar Mesosphere Winter Echoes

Abstract: This Master thesis examines a phenomenon that occurs in the upper polar atmosphere, namely, Polar Mesospheric Winter Echoes, or PMWEs. PMWEs are radar echoes observed by Very High Frequency Mesosphere Stratosphere Troposphere (VHF MST) radar, from altitudes of 60 to 76 km at 7 to 15 UT, in the winter months from the middle of September to the beginning of May. The aforementioned specifications are the partial results of this thesis.PMWEs are generally understood to be caused by turbulence; however, radar data indicate some rare cases where PMWEs can be created with velocities exceeding the speed of sound, which is not possible with current turbulence theory. Kirkwood et al., 2006 and Belova, Kirkwood, and Sergienko, 2013 hypothesised that infrasound could generate the necessary conditions for PMWEs with velocities equal to or exceeding the speed of sound. Observations of PMWEs presented in this thesis have been carried out by the MST radar ESRAD, located at Esrange (67 56’N, 21 04’E) near Kiruna in northern Sweden. The radar operates at 52 MHz and has been performing continuous radar observations since Dec 1996. Observations of the infrasounds presented in this thesis were carried out by a microbarometer located close to Rymdcampus in Kiruna. Access to the data is restricted, but through university administration, this master’s thesis has been granted permission to use the data for the study of PMWEs. The instrument performed continuous infrasound measurements from the 24th of May 2016 and forward. This thesis will perform a full analysis of all radar data from the 17th of Dec 1996 until the 31st of Jan 2021 to assess the altitude interval, diurnal interval and yearly interval. The data given in the first section are a result of this work. Additionally, space weather parameters relation with the occurrence of PMWEs is analysed. Space weather parameters are very important for the chemistry and conditions present in the mesosphere. It is shown thatt here is a strong relation between solar wind and PMWE occurrence, decent relation with Kp index and no to weak relation with solar particle event (SPE). Correlating space weather and PMWEs in greater detail could be the subject of other studies. Last but most interesting, microbarometer data will be analysed for days where high-speed PMWEs are detected. Because of the rarity of these high-speed PMWEs, only seven total cases were found from 24 May 2016 onward, making the analysis of the infrasound measurements very limited, and no connection was found. However, it was found that days with high-speed PMWEs had an abnormally low amount of infrasound detections, further making analysis difficult but raising questions of why. This could mean that infrasound signals might be hindered from reaching the ground on days where we have high-speed PMWEs. No further conclusions can be made, as this indicates relation but not causation. Airborne infrasound instruments could be used to detect weaker infrasound signals due to being unaffected by wind disturbance at the ground.