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Abstract 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).
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Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2 f ce Roar Radio Emissions
Abstract Competing theories exist for the generation mechanism of auroral medium‐frequency burst (MFB). In an effort to constrain MFB source heights, this study analyzes 33 events in which MFB and auroral 2fceroar co‐occurred at Sondrestrom, Greenland. Using measurements from an array of receiving antennas, direction‐of‐arrival calculations indicate that in a given co‐occurrence, the elevation angle of MFB typically is higher than that of roar. Ray tracing is used to determine source heights of the MFB signals. Density profiles are obtained from the International Reference Ionosphere (IRI) and shifted in magnitude until each event's roar signals originate at heights where the frequency‐matching condition for 2fceroar generation is satisfied. This shifting method is validated using density measurements from the Sondrestrom incoherent scatter radar (ISR) facility for the two events with available ISR data. After shifting, ray tracing demonstrates that in 25 of the 33 events, burst originates at a height of about 200 km, lower than the typical altitude of peak electron density. However, ISR measurements show that the density profile is enhanced at low altitudes while MFB is observed, peaking in theEregion rather than theFregion. This finding implies that the MFB sources at 200 km are on the topside of the density peak, in a region of downward pointing density gradient, in qualitative agreement with the mechanism of MFB generation by Langmuir waves in the topside ionosphere. These results also suggest a new method of estimating density in the polar cap using roar signals to calibrate IRI profiles.
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- Award ID(s):
- 1915058
- PAR ID:
- 10453639
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 125
- Issue:
- 10
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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