Explaining Solar Flare‐Induced Ionospheric Ion Upflow at Millstone Hill (42.6°N)

• Published in: Journal of geophysical research. Space physics 2022-07, Vol.127 (7), p.n/a
• Main Authors: Liu, Xuanqing, Liu, Jing, Wang, Wenbin, Zhang, Shun‐Rong, Zhang, Kedeng, Lei, Jiuhou, Liu, Libo, Chen, Xuetao, Li, Shuhan, Zhang, Qing‐He, Xing, Zan‐Yang
• Format: Article
• Language: English
• Subjects: Ambipolar diffusion; Atmosphere; Continuity equation; Data comparison; Density gradients; Earth; Earth ionosphere; Electric fields; Electrodynamics; Electron density; F 2 region; General circulation models; Incoherent scatter radar; Ion motion; ion upflow; Ionosphere; Ionospheric plasma; Ionospheric plasma density; Photoionization; Plasma density; Plasmas (physics); Pressure gradients; Radiation; Solar EUV; solar flare; Solar flares; Thermosphere; TIEGCM; Ultraviolet radiation; Upper atmosphere
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2021JA030185

Previous studies have shown that solar flares can significantly affect Earth's ionosphere and induce ion upflow with a magnitude of ∼110 m/s in the topside ionosphere (∼570 km) at Millstone Hill (42.61°N, 71.48°W). We use simulations from the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) and observations from Incoherent Scatter Radar (ISR) at Millstone Hill to reveal the mechanism of ionospheric ion upflow near the X9.3 flare peak (07:16 LT) on 6 September 2017. The ISR observed ionospheric upflow was captured by the TIEGCM in both magnitude and morphology. The term analysis of the F‐region ion continuity equation during the solar flare shows that the ambipolar diffusion enhancement is the main driver for the upflow in the topside ionosphere, while ion drifts caused by electric fields and neutral winds play a secondary role. Further decomposition of the ambipolar diffusive velocity illustrates that flare‐induced changes in the vertical plasma density gradient is responsible for ion upflow. The changes in the vertical plasma density gradient are mainly due to solar extreme ultraviolet (EUV, 15.5–79.8 nm) induced electron density and temperature enhancements at the F2‐region ionosphere with a minor and indirectly contribution from X‐ray (0–15.5 nm) and ultraviolet (UV, 79.8–102.7 nm). Plain Language Summary Solar flares with a sudden enhancement in solar X‐ray and extreme ultraviolet (EUV) radiation heat the upper atmosphere and increase electron density dramatically, leading to an expansion of Earth's upper atmosphere. Previous observations have shown an upward ion motion of ∼110 m/s at middle latitudes during solar flares; however, the cause of it remains largely speculative. In this paper, we provide a quantitative analysis of the ionospheric ion upflow during the 6 September 2017 X9.3 flare via model‐data comparison. During solar flares, X‐ray and EUV radiation are intensified at different degrees, thus producing different magnitudes of enhancement in ionospheric plasma density and temperature at different altitudes. Enhanced electron density and temperature at ionospheric F‐region due to solar EUV photoionization build up an additional upward pressure gradient force, pushing the plasmas upward. Key Points Flare‐induced ionospheric ion upflow at midlatitudes was simulated by a 3‐D ionosphere‐thermosphere coupled model The midlatitude ion upflow was mainly due to changes in ambipolar diffusion during flares The solar extreme u