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

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Journal of Geophysical Research: Space Physics
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Pacini A.A.
Raulin J.-P.
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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 producer 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. Copyright 2006 by the American Geophysical Union.
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