Ranking of sudden ionospheric disturbances by means of the duration of Vlf perturbed signal in agreement with satellite X-ray flux classification

Abstract

Ionosphere undergoes permanently solar flares that quickly change its properties inducing sometime unwanted effects. These changes, or events, are known as Sudden Ionospheric Disturbances (SIDs) and the knowledge of their magnitude may be of great interest to anticipate probable damages. Currently, there does not exist any classification of these ionospheric changes based on their amplitude due to the wide variability of its responses. The only way to surmise their importance is to study them indirectly, throughout the classification of the X-ray flux intensity recorded by satellites. An attempt of classification based on their duration was proposed by the American Association of Variable Star Observers (AAVSO) but it is not very accurate because SID's duration is measured directly from the raw signal of the Very Low Frequency (VLF) signal and/or the Low Frequency (LF) signal. The aim of this work is to investigate, through a set of simple mathematical techniques applied to VLF/LF signals recorded by ground based receivers, the best method to estimate SIDs durations and then propose a new classification based on these durations.