[P-EM11] Coupling Processes in the Atmosphere-Ionosphere System

[PEM11-06] Generation mechanisms of plasma density irregularity from equatorial to midlatitude ionosphere during a geomagnetic storm on 21 and 22 December 2014

*惣宇利 卓弥¹、新堀 淳樹¹、大塚 雄一¹、津川 卓也²、西岡 未知²、吉川 顕正³ (1.名古屋大学　宇宙地球環境研究所、2.情報通信研究機構、3.九州大学)

キーワード：磁気嵐、プラズマバブル、rate of TEC index (ROTI)、過遮蔽電場、非干渉散乱レーダー、赤道ジェット電流

Plasma bubbles having a significantly sharp depletion of plasma density are one of the ionospheric irregularities in the low-latitude and equatorial regions. Within the plasma bubbles, plasma density irregularities with different spatial scales exist. The plasma bubbles are generated in the bottomside F region of the nighttime equatorial ionosphere after sunset by Rayleigh-Taylor instability (RTI) mechanism [Farley et al., 1970]. The growth rate of RTI increases with increasing eastward electric fields and increasing ionospheric altitude. During geomagnetic storms, three kinds of electric field originating from interplanetary magnetic field (IMF), region-2 field-aligned currents (FAC) and disturbance dynamo are imposed in the ionosphere from the midlatitude to equator. Especially, an over-shielding electric field generated by the enhanced region-2 FACs is dominant for 1-2 hours after northward turning of the IMF and/or the onset of substorms. However, how the over-shielding electric field contributes to generation of plasma bubbles remains unknown. In this study, we investigate temporal and spatial variations of plasma bubbles and electric fields during a geomagnetic storm on 21 and 22 December 2014 by using global navigation satellite system-rate of total electron content index (ROTI), incoherent scatter (IS) radars and magnetometers data, and discuss the origin of plasma bubble generation during the geomagnetic storm. The ROTI enhancements related to plasma bubbles were observed three times in the American sector from the low latitude to equator at 00:50, 03:10 and 05:30 UT on 22 December, respectively. Before the second ROTI enhancement appeared, the AL index showed a sudden decrease and reached the peak value of -805 nT at 02:14 UT. The equatorial electrojet (EEJ) estimated from the difference of the X component of geomagnetic field at Yap and Guam decreased from 33 nT at 02:13 UT to -28 nT at 02:50 UT. Plasma drift velocity of E×B (upward and northward perpendicular to the geomagnetic field) direction observed by the Arecibo IS radar increased from -9.0 m/s at 02:23 UT to 17.6 m/s at 02:37 UT. The E×B drift velocity over Arecibo turned into upward around 02:30 UT. Vertical drift velocity obtained from the Jicamarca IS radar also increased from -22.0 m/s at 02:22 UT to 28.8 m/s at 02:52 UT. The vertical drift velocity over Jicamarca turned into upward direction around 02:42 UT. The north-south component of the IMF showed a constant value of -15 nT during 01:56–02:58 UT. The equivalent currents derived from global magnetometer data were directed westward at 40–55° in geomagnetic latitude (GMLAT) and eastward at 60–70° in GMLAT in the dusk sector while those in the dawn sector were directed eastward at 40–50° in GMLAT and westward at 50–75° in GMLAT around 02:30 UT. The EEJ variation at the dayside equator suggests that the eastward ionospheric electric field turned into westward after the AL decrease associated with a substorm. The plasma drift velocities over Arecibo and Jicamarca on the nightside also turned into upward 20–30 min after the AL index decreased. From these observational facts, it can be inferred that the reversed convection electric field penetrates to the low-latitude and equatorial regions in both the day and night. The second ROTI enhancement appeared 30–40 min after the plasma drift velocities were directed upward. Further, a distribution of global equivalent currents suggests that the region-2 FACs were dominant after the AL decrease. These results suggest that the second plasma bubble was generated by the enhanced eastward over-shielding electric field driven by the substorm in the pre-midnight sector even though the IMF was directed southward. Therefore, it can be concluded that an over-shielding electric field originating from the enhanced region-2 FAC system during geomagnetic storms and substorms can be also an important factor contributing to generation of plasma bubbles in the nighttime.