Statistical Analysis of Low-Latitude Boundary of Polar-Type Medium-Scale Traveling Ionospheric Disturbances Observed by a 630-nm Airglow Imager at Nyrölä, Finland

Medium-scale traveling ionospheric disturbances (MSTIDs) are one of the ionospheric plasma density structures and are observable through 630-nm airglow images. Previous studies using airglow images at Tromso (69.6 degrees N, 19.2 degrees E; magnetic latitude: 66.7 degrees N), Norway, reported high-latitude MSTIDs (here we call them as polar-type MSTIDs) whose propagation direction changes associated with auroral brightening and magnetic field disturbances. However, there has been little statistical analysis on the connection of MSTIDs occurring at high and middle latitudes. In this study, we statistically analyzed the MSTIDs observed by an airglow imager at Nyrola (62.3 degrees N, 25.5 degrees E; magnetic latitude: 59.4 degrees N), Finland, which is located similar to 7 degrees south of Tromso, corresponding to subauroral latitudes. The period analyzed was from 23 January 2017, to 30 September 2021. We found 11 cases of MSTIDs during this period. Eight cases were found to be the polar-type MSTIDs whose motion changes associated with auroral brightening and magnetic field disturbances. We found that 9 cases of MSTID show the low-latitude boundary at 61 degrees +/- 2 degrees N for geographic latitude and 58 degrees +/- 2 degrees N for magnetic latitude, indicating disconnection between high- and mid-latitude MSTIDs. We also derived occurrence probability, velocity, wavelength, period, wave front direction, and propagation direction of these MSTIDs. The occurrence probability of MSTIDs at Nyrola is 1.9%, which is much lower than those at high (Tromsoe, more than 50%) and middle (Japan, similar to 30%) latitudes. We discuss these MSTID characteristics at subauroral latitudes based on possible difference of generation mechanisms of nighttime MSTIDs at high and middle latitudes. Medium-scale traveling ionospheric disturbances (MSTIDs) are one of the ionospheric plasma density structures and are observable through 630-nm airglow images. Previous studies using airglow images at Tromso (69.6 degrees N, 19.2 degrees E; magnetic latitude: 66.7 degrees N), Norway, reported high-latitude MSTIDs (here we call them as polar-type MSTIDs) whose propagation direction changes associated with auroral brightening and magnetic field disturbances. However, there has been little statistical analysis on the connection of MSTIDs occurring at high and middle latitudes. In this study, we analyzed 11 MSTID cases observed by an airglow imager at Nyrola (62.3 degrees N, 25.5 degrees E; magnetic latitude: 59.4 degrees N), Finland, which is located similar to 7 degrees south of Tromso, corresponding to subauroral latitudes. We found that 9 cases of MSTID show the low-latitude boundary at 61 degrees +/- 2 degrees N for geographic latitude and 58 degrees +/- 2 degrees N for magnetic latitude, indicating disconnection between high- and mid-latitude MSTIDs. We also derived occurrence probability, velocity, wavelength, period, wave front direction, and propagation direction of these MSTIDs. We statistically analyzed medium-scale traveling ionospheric disturbances (MSTIDs) observed at subauroral latitudes at Nyrola, Finland Most MSTIDs are the polar-type MSTIDs whose motion changes associated with auroral brightening and/or magnetic field disturbances The low-latitude boundary of these MSTIDs was at similar to 61 degrees N latitude, indicating disconnection between high- and mid-latitude MSTIDs