12:00 〜 12:15
[PEM10-11] Effects of CIR- and CME-driven magnetic storms on ion upflows in the low-altitude polar ionosphere
キーワード：イオン上昇流、共回転相互作用領域(CIR)、コロナ質量放出(CME)、磁気嵐、極域電離圏、欧州非干渉散乱(EISCAT)レーダー
Molecular ions (O2+/NO+/N2+) in the ring current of the terrestrial magnetosphere have been observed during the magnetic storms [e.g., Klecker et al., 1986; Seki et al., 2019]. These ions originate from the low-altitude ionosphere. In the ionosphere, upward ion transports (upflows) supply sources of the ions outflowing to the magnetosphere. Since the molecular ions usually exist in the low-altitude (< 300 km) ionosphere and can be affected by neutral winds, the generation mechanisms such as frictional heating and precipitations, and properties of ion upflows to transport molecular ions are different from those of O+[e.g., Ogawa et al., 2010; Yamazaki et al., 2017]. In particular, their dependence on solar activities is one of the important properties to understand formation mechanisms of the ion upflows. In a previous study by Ogawa et al. [2019], the characteristics of O+ion upflows in an altitude range of 400-500 km in the polar ionosphere were investigated during CIR- and CME-driven magnetic storms by using EISCAT radars. They reported that the upflows during CIR- and CME-driven storms have different dependence on magnetic local time: CME-driven storms have about 4 times larger upward ion flux in the nighttime than those under CIR-driven storms. They also showed that dayside ion upflows under small CIR-driven storms continue a few days longer than those under small CME-driven storms. Their study focused on the ion upflows in the altitude range between 400 and 500 km, where O+is the dominant species, and responses of the ion upflows to the different types of magnetic storms in the low-altitude ionosphere, where molecular ions exist, are far from understood. The purpose of this study is thus to understand effects of CIR- and CME-driven magnetic storms on ion upflows in the low-altitude ionosphere based on long-term observations of the EISCAT radars.
We used data from the EISCAT UHF radar at Tromsø (69º35'N, 19º14'E, Invariant Latitude: 66º12'N) and Svalbard radar at Longyearbyen (78º09'N, 16º03'E, Invariant Latitude: 75º10'N) from January 1996 to January 2016, and investigated statistical properties of ion upflows and ionospheric conditions during CIR- and CME-driven magnetic storms. We used 5-minute time resolution data when the radar was looking along the local magnetic field line. The ionospheric parameters such as electron density, ion velocity, and ion and electron temperatures were averaged over 250-350 km altitudes. We screened data to exclude unrealistic values with the following criteria: Absolute value of ion velocity was less than 1500 m/s, ion and electron temperatures were less than 10000 K, and electron density was more than 1010m-3and less than 1013m-3. We also selected reliable data based on the error values of ion and electron temperatures: The error value was less than 50% of each temperature. To understand the similarity and difference between low- and high- altitude upflows, we compared obtained results with those from the previous study [Ogawa et al., 2019]. The results show that low-altitude ion upflows were observed mainly in nightside and dawnside at Tromsø and Svalbard during magnetic storms. We also investigated the dependence of the low-altitude upflows on ion and electron temperatures to discuss the generation mechanisms. We assumed contribution from the frictional heating with ion temperature increase by >15% and the ratio of enhancement on electron temperature at ~110km, which indicates the enhancement of the electric field, were more than 1% compared to pre-storm value. On one hand, contribution of electron precipitations is assessed with electron temperature increase by >15%. The results indicate that the frictional heating mainly caused upflows during CME storms at both locations and possibly in dawnside during CIR storms at Svalbard, whereas precipitations mainly caused upflowsduring CIR storms at both locations and possibly in duskside during large CME storms at Tromsø.
We used data from the EISCAT UHF radar at Tromsø (69º35'N, 19º14'E, Invariant Latitude: 66º12'N) and Svalbard radar at Longyearbyen (78º09'N, 16º03'E, Invariant Latitude: 75º10'N) from January 1996 to January 2016, and investigated statistical properties of ion upflows and ionospheric conditions during CIR- and CME-driven magnetic storms. We used 5-minute time resolution data when the radar was looking along the local magnetic field line. The ionospheric parameters such as electron density, ion velocity, and ion and electron temperatures were averaged over 250-350 km altitudes. We screened data to exclude unrealistic values with the following criteria: Absolute value of ion velocity was less than 1500 m/s, ion and electron temperatures were less than 10000 K, and electron density was more than 1010m-3and less than 1013m-3. We also selected reliable data based on the error values of ion and electron temperatures: The error value was less than 50% of each temperature. To understand the similarity and difference between low- and high- altitude upflows, we compared obtained results with those from the previous study [Ogawa et al., 2019]. The results show that low-altitude ion upflows were observed mainly in nightside and dawnside at Tromsø and Svalbard during magnetic storms. We also investigated the dependence of the low-altitude upflows on ion and electron temperatures to discuss the generation mechanisms. We assumed contribution from the frictional heating with ion temperature increase by >15% and the ratio of enhancement on electron temperature at ~110km, which indicates the enhancement of the electric field, were more than 1% compared to pre-storm value. On one hand, contribution of electron precipitations is assessed with electron temperature increase by >15%. The results indicate that the frictional heating mainly caused upflows during CME storms at both locations and possibly in dawnside during CIR storms at Svalbard, whereas precipitations mainly caused upflowsduring CIR storms at both locations and possibly in duskside during large CME storms at Tromsø.