16:00 〜 16:15
[PEM09-22] Seasonal and longitudinal dependence of the Sub-Auroral Polarization Streams occurrence observed by the SuperDARN radars
キーワード：SuperDARN、Sub-Auroral Polarization Streams (SAPS)、longitudinal dependence
We conduct a statistical study on occurrence characteristics of Sub-Auroral Polarization Streams (SAPS), using the Super Dual Auroral Radar Network (SuperDARN) data. We use six years of mid-latitude SuperDARN data and study the seasonal dependence of SAPS type flows. We identify the equatorward auroral oval boundary using the precipitating energy flux data of the National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellites (POES) satellites. We use only ionospheric echoes sampled in the region equatorward of this boundary. We set the following criteria to identify SAPS events: 1. the flow speed should exceed 150 m/s [Nagano et al., 2015]. 2. the flow should be directed westward (-45 deg < Δθ < +45 deg).
Some past studies suggested the seasonal dependence of SAPS, e.g., SAPS-like high velocity flows occur over a larger magnetic local time (MLT) extent during winter months [Koustov et al., 2006], lower velocity flows are observed more often in the summer Northern Hemisphere than in the winter Southern Hemisphere [Kunduri et al., 2012]. These results are in slight disagreement with another interhemispheric study [Parkinson et al., 2005], which showed that summer Southern Hemisphere velocities are slightly higher than in the winter Northern Hemisphere. Nevertheless, there have been few comprehensive studies on the statistics of seasonal dependence.
One of the remarkable SAPS characteristics found in the present study is that the occurrence rate in spring and autumn are generally larger than other seasons for each radar, which can be interpreted in terms of the Russel-McPherron effect (Russell & McPherron, 1973). On the other hand, we see no consistent tendency among the radars regarding the summer-winter difference of the SAPS occurrence, but the peak MLT of SAPS occurrence in summer and winter depends on the radar's location. These results are interpreted in terms of the differences in the dipole tilt angle (DTA). To the best of our knowledge, this is the first result of the SAPS occurrence rate discussed as a function of the dipole tilt angle in addition to the solar zenith angle.
References
Koustov, A. V., et al., Ann. Geophys., https://doi.org/10.5194/angeo-24-1591-2006, 2006.
Kunduri, B. S. R., et al., J. Geophys. Res., https://doi.org/10.1029/2012JA017784, 2012.
Nagano, H., et al., Earth, Planets and Space, https://doi.org/10.1186/s40623-015-0299-7, 2015.
Parkinson, M. L., et al., Ann. Geophys., https://doi.org/10.5194/angeo-23-1371-2005, 2005.
Russell, C. T., & McPherron, R. L., J. Geophys. Res., https://doi.org/10.1029/ja078i001p00092, 1973.
Some past studies suggested the seasonal dependence of SAPS, e.g., SAPS-like high velocity flows occur over a larger magnetic local time (MLT) extent during winter months [Koustov et al., 2006], lower velocity flows are observed more often in the summer Northern Hemisphere than in the winter Southern Hemisphere [Kunduri et al., 2012]. These results are in slight disagreement with another interhemispheric study [Parkinson et al., 2005], which showed that summer Southern Hemisphere velocities are slightly higher than in the winter Northern Hemisphere. Nevertheless, there have been few comprehensive studies on the statistics of seasonal dependence.
One of the remarkable SAPS characteristics found in the present study is that the occurrence rate in spring and autumn are generally larger than other seasons for each radar, which can be interpreted in terms of the Russel-McPherron effect (Russell & McPherron, 1973). On the other hand, we see no consistent tendency among the radars regarding the summer-winter difference of the SAPS occurrence, but the peak MLT of SAPS occurrence in summer and winter depends on the radar's location. These results are interpreted in terms of the differences in the dipole tilt angle (DTA). To the best of our knowledge, this is the first result of the SAPS occurrence rate discussed as a function of the dipole tilt angle in addition to the solar zenith angle.
References
Koustov, A. V., et al., Ann. Geophys., https://doi.org/10.5194/angeo-24-1591-2006, 2006.
Kunduri, B. S. R., et al., J. Geophys. Res., https://doi.org/10.1029/2012JA017784, 2012.
Nagano, H., et al., Earth, Planets and Space, https://doi.org/10.1186/s40623-015-0299-7, 2015.
Parkinson, M. L., et al., Ann. Geophys., https://doi.org/10.5194/angeo-23-1371-2005, 2005.
Russell, C. T., & McPherron, R. L., J. Geophys. Res., https://doi.org/10.1029/ja078i001p00092, 1973.