Solar X‐ray flares and ionospheric sudden phase anomalies relationship: A solar cycle phase dependence

Abstract

We investigate the relation between sudden phase anomaly (SPA) amplitudes (Δφ) and solar X‐ray flares importance and study if it has a solar activity cycle dependence. We find a very significant correlation between Δφ and the X‐ray fluences, FX (time‐integrated photon fluxes) in the range 0.5–2 Å. Compared with earlier works, the improvements of the Δφ versus FX relation allow us to study separately solar events as a function of their occurrence in the solar activity cycle, and we find that the Δφ versus FX relation is different depending on the epoch within the solar cycle. In particular, a minimum X‐ray fluence of 2.5 × 10−6 J m−2 is needed during solar activity minimum to trigger a SPA, while during solar maximum, 7.0 × 10−6 J m−2 is required. Similarly, a solar flare will produce a SPA during solar minimum that is greater by ∼2.6° M m−1 than would a flare of the same size during solar maximum. These results confirm recent findings about the dependence with the solar activity of the ionospheric undisturbed D region sensitivity. A possible consequence would be the monitoring of the long‐term solar irradiance, which maintains the D region, through measurements of VLF wave propagation properties.