09:30 〜 09:45
[PEM12-03] Electron density depletion and electric field variation of plasma bubble observed by the Arase satellite during a geomagnetic storm in April 2023
キーワード：磁気嵐、全球測位衛星システム、あらせ衛星、プラズマバブル、電場
After sunset, a plasma density depletion (plasma bubble) often occurs at the bottom of the F region ionosphere over the dip equator. Plasma bubbles are sometimes extended from the equator to mid-latitudes during geomagnetic storms. An eastward electric field is one of the most important factors for generating plasma bubbles through the Rayleigh-Taylor instability. Plasma bubbles also move upward due to an eastward polarization electric field generated inside them with the northward geomagnetic field. The large-sale evolution of plasma bubbles was extensively studied using ground and satellite observations, whereas there have been few direct observations of the electric field inside plasma bubbles during geomagnetic storms. In this study, we clarify a possible generation mechanism of equatorial plasma bubbles extending to the mid-latitudes during a geomagnetic storm in April 2023 by analyzing global navigation satellite system-total electron content (TEC), rate of TEC index (ROTI) and Arase satellite data.
During the main phase of the geomagnetic storm, equatorial plasma bubbles were generated in the European sector and extended to mid-latitudes of ~40° N in geomagnetic latitude. The plasma bubbles indicated by the ROTI enhancement extended northward and they tilted westward. The Arase satellite passed through the plasma bubbles and observed electron density depletions in the dusk sector (20–21.5 h MLT) around 15° N in geomagnetic latitude at an altitude of 450–700 km. Some of the density depletions correspond to the enhanced ROTI regions related to the plasma bubbles. One of the electron density variations showed a sharp depletion from 9.84×1011 m-3 to 1.68×109 m-3. An electric field enhancement was also observed by the Arase satellite when it passed through one of the density depletion regions. This is the first time that Arase has identified the eastward electric field enhancement inside storm-time plasma bubbles. The observed electric field shows a perturbation in the northward, upward, and eastward directions in solar magnetic coordinates. The amplitude of the eastward (polarization) electric field was ~25 mV/m, which corresponds to the upward velocity of ~480 m/s. On the other hand, some previous studies have reported observations of the upward velocity (eastward electric field) related to plasma bubbles using satellite or radar data and shown that the amplitude of the E×B velocity was a few hundred m/s. This suggests that the upward velocity inside the plasma bubbles was more intensified during the geomagnetic storm as compared with that during geomagnetically quiet times. This is because the penetration electric field in the equatorial region is further enhanced during geomagnetic storms. In the equatorial region, the charge separation in the electron density perturbation region can be enhanced by the enhanced eastward Pedersen current driven by the eastward electric field, generating a larger polarization electric field to hold the current continuity. Our analysis results would contribute to the understanding of the development mechanism of plasma bubbles extending to the mid-latitudes during geomagnetic storms.
During the main phase of the geomagnetic storm, equatorial plasma bubbles were generated in the European sector and extended to mid-latitudes of ~40° N in geomagnetic latitude. The plasma bubbles indicated by the ROTI enhancement extended northward and they tilted westward. The Arase satellite passed through the plasma bubbles and observed electron density depletions in the dusk sector (20–21.5 h MLT) around 15° N in geomagnetic latitude at an altitude of 450–700 km. Some of the density depletions correspond to the enhanced ROTI regions related to the plasma bubbles. One of the electron density variations showed a sharp depletion from 9.84×1011 m-3 to 1.68×109 m-3. An electric field enhancement was also observed by the Arase satellite when it passed through one of the density depletion regions. This is the first time that Arase has identified the eastward electric field enhancement inside storm-time plasma bubbles. The observed electric field shows a perturbation in the northward, upward, and eastward directions in solar magnetic coordinates. The amplitude of the eastward (polarization) electric field was ~25 mV/m, which corresponds to the upward velocity of ~480 m/s. On the other hand, some previous studies have reported observations of the upward velocity (eastward electric field) related to plasma bubbles using satellite or radar data and shown that the amplitude of the E×B velocity was a few hundred m/s. This suggests that the upward velocity inside the plasma bubbles was more intensified during the geomagnetic storm as compared with that during geomagnetically quiet times. This is because the penetration electric field in the equatorial region is further enhanced during geomagnetic storms. In the equatorial region, the charge separation in the electron density perturbation region can be enhanced by the enhanced eastward Pedersen current driven by the eastward electric field, generating a larger polarization electric field to hold the current continuity. Our analysis results would contribute to the understanding of the development mechanism of plasma bubbles extending to the mid-latitudes during geomagnetic storms.