Ionospheric electron density response to solar flares as viewed by Digisondes

Solar flares are explosive events on the Sun that release energetic particles, X‐rays, EUV, and radio emissions that have an almost immediate impact on Earth's ionosphere‐thermosphere (IT) system and/or on operational systems that are affected by IT conditions. To assess such impacts, it is a key that we know how the ionosphere is modified. An objective of this paper is to evaluate how digisondes might serve in this role. Toward this end we utilize data from the Millstone Hill digisonde to reveal the height versus time bottomside F region responses to three X‐class flares (X28, X8.3, and X1.7) at a middle latitude site. In terms of percent increase with respect to a preflare hourly mean, the long‐lived (> 15–30 min) responses to these flares maximize between about 150 and 250 km and measurably last ~0.75–1.5 h after flare maximum. The relative magnitudes of these responses are complicated by flare position on the solar disk, which determines how much of the EUV solar emissions are attenuated by the solar atmosphere. At Millstone Hill there was little measurable response to these flares near the F 2 layer peak; however, at the magnetic equator location of Jicamarca, the F 2 peak electron density increased by ~15–40%. Herein, all of these flare response characteristics are interpreted in terms of available modeling results. We propose that such digisonde data, in combination with first‐principles models and high‐resolution measurements of solar EUV flux emissions (e.g., from Solar Dynamics Observatory/EUV Variability Experiment), can lead us to a deeper understanding of the ionospheric photochemistry and dynamics that underlies a predictive capability.