17:15 〜 18:45
[PPS01-P14] ハレアカラ T60 望遠鏡と HISAKI 衛星の観測によるイオプラズマトーラスにおける太陽風応答の朝夕非対称性
キーワード：木星、イオ、電場、磁気圏
The purpose of this study is to clarify the characteristics of the electric field in the Jovian inner magnetosphere which responds to the solar wind. In this study, (1) we investigated the correlation between the temporal variation of the ribbon’s radial position in the io plasma torus (IPT) and the solar wind dynamic pressure, (2) ‘‘directly’’ estimated the dawn-to-dusk electric field with the ribbon’s radial position, and (3) revealed inhomogeneity of the dawn-to-dusk electric field in the Jovian inner magnetosphere associated with the solar wind response by using the data observed by the 60-cm telescope in the Haleakala observatory of Tohoku University (T60) & the Hisaki/EXtreme ultraviolet spectrosCope for ExosphEric Dynamics (EXCEED) data.
To get the ribbon’s radial position data, we used the data of the visible ([SII] 6716, 6731 Å) IPT image observed by the Visible Imager and SPectrograph with coronagraph (VISP) installed in the T60. The T60 has a spatial plate scale of 0.52"/pixel (corresponding to ~0.02 RJ at the Jupiter opposition), which is enough spatial scale to detect the ribbon’s radial shift of 0.15 RJ on average. We analyzed 1235 images, which had been observed from December 15, 2014, to May 18, 2017. We also analyzed the brightness of the sulfur SII, SIII, and SIV emission lines (640-770 Å) in IPT on the dawn and dusk side by the Hisaki/EXCEED. The analysis period is from December 15, 2014, to June 13, 2016.
(1) We found that the ribbon’s position moved dawnward when the solar wind dynamic pressure enhanced. We divided the position data into two groups, ‘‘quiet’’ and ‘‘disturbed’’, based on the solar wind dynamic pressure with a threshold of 0.1 nPa for each of the dawn and dusk sides. With the Man-Whitney U test, we compared distributions of the two groups whether the radial position changed according to the enhancement of solar wind dynamic pressure. The p-values are less than 0.01 in both sides (significant level of 99%). This means that difference between two groups is significant and shows that the radial position changed due to the solar wind fluctuation.
(2) From simultaneous observations of the T60 and Hisaki during a period from December 15, 2014, to June 13, 2016, the dawn-to-dusk electric field estimated by the T60 is 2.4 mV/m on average and 6.9 mV/m at the maximum and that estimated by Hisaki is 2.7 mV/m on average and 9.3 mV/m at the maximum. From a detailed event analysis for the data of February 19–23, 2016, the time responses of the electric field derived from the T60 and Hisaki to the solar wind are consistent with each other; i.e., an averaged intensity of electric field derived from the T60 is directly estimated in 3.9±0.8 mV/m, while 2.8±1.2 mV/m from the Hisaki observation.
(3) To confirm inhomogeneity of the dawn to dusk electric field due to the solar wind response, we checked the dawnward shift in each of the both sides and confirmed that the solar wind at first influence on the dawn side of IPT when the dynamic pressure enhanced, and the influence subsequently appears on the dusk side. Then, the radial position returns to the initial position in both sides simultaneously as the solar wind dynamic pressure decreases. From the radial variation of the ribbon’s position observed by the T60 on February 19–23, 2016, the amount of radial shift is 1.5 times larger in the dawn side than in the dusk side due to the solar wind enhancement. From the brightness variation observed by Hisaki on the same period, the amount of the brightness variation is about 1.3 times larger in the dawn side than in the dusk side. The results from the T60 and Hisaki are consistent with each other, which suggests that the dawn-to-dusk electric field has inhomogeneity in the inner magnetosphere. On the other hand, we could not conclude which was the more appropriate scenario for the generation of the electric field: the M-I coupling scenario or the tailward plasma flow scenario.
To get the ribbon’s radial position data, we used the data of the visible ([SII] 6716, 6731 Å) IPT image observed by the Visible Imager and SPectrograph with coronagraph (VISP) installed in the T60. The T60 has a spatial plate scale of 0.52"/pixel (corresponding to ~0.02 RJ at the Jupiter opposition), which is enough spatial scale to detect the ribbon’s radial shift of 0.15 RJ on average. We analyzed 1235 images, which had been observed from December 15, 2014, to May 18, 2017. We also analyzed the brightness of the sulfur SII, SIII, and SIV emission lines (640-770 Å) in IPT on the dawn and dusk side by the Hisaki/EXCEED. The analysis period is from December 15, 2014, to June 13, 2016.
(1) We found that the ribbon’s position moved dawnward when the solar wind dynamic pressure enhanced. We divided the position data into two groups, ‘‘quiet’’ and ‘‘disturbed’’, based on the solar wind dynamic pressure with a threshold of 0.1 nPa for each of the dawn and dusk sides. With the Man-Whitney U test, we compared distributions of the two groups whether the radial position changed according to the enhancement of solar wind dynamic pressure. The p-values are less than 0.01 in both sides (significant level of 99%). This means that difference between two groups is significant and shows that the radial position changed due to the solar wind fluctuation.
(2) From simultaneous observations of the T60 and Hisaki during a period from December 15, 2014, to June 13, 2016, the dawn-to-dusk electric field estimated by the T60 is 2.4 mV/m on average and 6.9 mV/m at the maximum and that estimated by Hisaki is 2.7 mV/m on average and 9.3 mV/m at the maximum. From a detailed event analysis for the data of February 19–23, 2016, the time responses of the electric field derived from the T60 and Hisaki to the solar wind are consistent with each other; i.e., an averaged intensity of electric field derived from the T60 is directly estimated in 3.9±0.8 mV/m, while 2.8±1.2 mV/m from the Hisaki observation.
(3) To confirm inhomogeneity of the dawn to dusk electric field due to the solar wind response, we checked the dawnward shift in each of the both sides and confirmed that the solar wind at first influence on the dawn side of IPT when the dynamic pressure enhanced, and the influence subsequently appears on the dusk side. Then, the radial position returns to the initial position in both sides simultaneously as the solar wind dynamic pressure decreases. From the radial variation of the ribbon’s position observed by the T60 on February 19–23, 2016, the amount of radial shift is 1.5 times larger in the dawn side than in the dusk side due to the solar wind enhancement. From the brightness variation observed by Hisaki on the same period, the amount of the brightness variation is about 1.3 times larger in the dawn side than in the dusk side. The results from the T60 and Hisaki are consistent with each other, which suggests that the dawn-to-dusk electric field has inhomogeneity in the inner magnetosphere. On the other hand, we could not conclude which was the more appropriate scenario for the generation of the electric field: the M-I coupling scenario or the tailward plasma flow scenario.