Traveling ionospheric disturbances and ionospheric perturbations associated with solar flares in September 2017

Abstract

Solar flares provide strong impulsive radiation and energy injection to the sunlit upper atmosphere. The impact on the ionosphere is immense in spatial scale, and therefore, it is not immediately evident if dramatically elevated neutral heating can lead to excitation of acoustic gravity waves. Using primary observations from Global Navigation Satellite System differential TEC (total electron content) over the continental United States, this paper presents postflare ionospheric observations associated with three X-class flares on 6, 7, and 10 September 2017. Postflare ionospheric changes had two significant morphological characteristics: (1) A few minutes after the X9.3 flare peak on 6 September, clear traveling ionospheric disturbance (TID) fronts emanated near the sunrise terminator with alignment parallel to its direction—TIDs propagated predominantly eastward into the dayside with a 150 m/s phase speed and a ∼30-min period; (2) synchronized differential TEC oscillations over continental United States with ∼60-min periodicity and damping amplitude over time, following all three X-class flares. Postflare ionospheric oscillation spectra exhibited significantly enhanced amplitudes and changes of periodicities (including the appearance of the 60-min oscillations). The Millstone Hill incoherent scatter radar observed large ionospheric up-welling occurring nearly simultaneously as detected TIDs at the X9.3 flare peak, with up to 80 m/s enhancements in vertical drift at 500 km lasting for ∼30 min. Results suggest that significant solar flare heating and associated dynamical effects may be an important factor in TID/acoustic gravity wave excitation.