Observations of the ionospheric impact of M‐class solar flares on local and hemispheric scales

Abstract

The ionospheric impact of a series of M‐class solar flares that occurred on 12 March 2015 is described. A combination of data sets from GPS receivers and two different colocated radio telescopes, the very large array (VLA) and the long wavelength array (LWA), was used to detect and characterize flare‐induced irregularities. The data demonstrate that each flare causes a rapid step‐like increase in total electron content (TEC) of about 0.2 total electron content units (TECU), 1 TECU = 1016 el m−2. The rise times of these steps are on the order of 1–3 min. The GPS data show signs of traveling ionospheric disturbances likely associated with gravity waves whose magnitudes were temporarily enhanced by the impact of two of the flares. Increased activity within the horizontal TEC gradients observed with the VLA was apparent for several minutes following the flare X‐ray peak. The properties of these VLA‐detected disturbances are strongly indicative of field‐aligned irregularities within the plasmasphere. They initially form at two altitudes, approximately 2600 and 5600 km. Those that form higher quickly disappear, whereas those that form lower appear to descend over a period of ∼7 min to an altitude of roughly 1450 km before disappearing. LWA observations of ionospheric reflections of the HF radio station WWV show a significant and brief (∼few minutes) Doppler frequency disturbance near the onset of TEC enhancement, followed by a ∼5 min drop in received power, indicating increased ionization in the D region.