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Type: Artigo de periódico
Title: On the detectability of solar X-ray flares using very low frequency sudden phase anomalies
Author: Raulin, JP
Pacini, AA
Kaufmann, P
Correia, E
Martinez, MAG
Abstract: We have studied ionization excesses produced by enhancements of X-ray emission during solar flares using the very low frequency (VLF) response of the lower edge of the ionospheric D region. We focus on whether or not the X-rays associated with a given solar flare were responsible for a sudden phase anomaly (SPA) event, independently of the characteristics of the SPA. Approximately 1300 and 200 solar events were found to cause an ionospheric event, during periods of high and low solar activity, respectively. The main results of the present work are: (i) definite spectral characteristics are required for a solar flare to produce a measurable SPA; (ii) the probability of SPA occurrence due to faint solar flares, of X-ray class C1-C2 or lower, is higher during solar minimum; (iii) the same probability for more intense solar flares (class C3 or higher) does not depend on the solar activity conditions. Our observations suggest that the low ionosphere has different sensitivities depending on the solar activity, being more sensitive when the Sun is less active. These results also constitute an observational confirmation of recent findings showing that the ionospheric reference height is lower (by about <= 1 km) during solar maximum. (c) 2005 Elsevier Ltd. All rights reserved.
Subject: solar flares
solar activity cycle
very low frequency propagation
Country: Inglaterra
Editor: Pergamon-elsevier Science Ltd
Citation: Journal Of Atmospheric And Solar-terrestrial Physics. Pergamon-elsevier Science Ltd, v. 68, n. 9, n. 1029, n. 1035, 2006.
Rights: fechado
Identifier DOI: 10.1016/j.jastp.2005.11.004
Date Issue: 2006
Appears in Collections:Unicamp - Artigos e Outros Documentos

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