Application of Empirical Mode Decomposition to the detection of Sudden Ionospheric Disturbances by monitoring the signal of a distant Very Low Frequency transmitter by

Sudden Ionospheric Disturbances (SIDs) result from an abnormal increase of the ionization density of the D region of the ionosphere originating from x-ray solar flares. They can affect radio communications and possibly lead to a radio blackout. A classic means to detect the most powerful solar flares and resulting SIDs is to monitor the amplitude and phase of distant Very Low Frequency (VLF) transmitters. The Hilbert-Huang transform (HHT) and the Empirical Mode Decomposition (EMD) have offered a breakthrough in the analysis of nonlinear and non-stationary data. This signal processing technique has been used with great success in many applications where traditional methods do not apply. This document describes how the Empirical Mode Decomposition (EMD) technique helps in the detection of SIDs from recordings of the signal levels of VLF stations. The author has written software to apply EMD to actual data. By comparing with GOES spacecraft measurements of solar x-ray flux, the study confirms that a careful choice of an Intrinsic Mode Function (IMF) allows extracting the uncommon changes of the signal strength resulting from x-ray solar flares. With respect to the analysis of raw measurements, this study describes how a more precise timing of the disturbance is obtained. In addition, the extraction of faint changes from the noise is improved. The detection threshold appears reduced from the usual C1 to approximately B5. Introduction The propagation of Very Low Frequency (VLF) radio communications is affected by the ionospheric disturbances. They are triggered by the sudden release of highenergy radiation from x-ray solar flares. Measurement of the amplitude and/or phase of the signal of a distant transmitter is a means to detect from the ground the most powerful solar flares and to characterize their effects in the Earth vicinity. Firstly, this paper presents background information on x-ray solar flares and the specificities of VLF signal propagation. It shortly describes how the ionosphere can be affected by solar flares and create Sudden Ionospheric Disturbances (SIDs). The author's monitoring station, used to record the signal strength data, is briefly presented. Then, the basic principles of the Hilbert-Huang Transform (HHT) and its key part, the Empirical Mode Decomposition (EMD), are explained. The algorithm is detailed and the software written by the author to analyze the data is described. Finally, the EMD has been applied to actual data representative of typical recordings. Several examples, from a quiet day with no event to a very active day with many disturbances, are presented. The objective is to illustrate how the various IMFs resulting from the EMD are affected by SIDs of various importance. This paper describes how the components that correctly represent the propagation anomalies are chosen. By comparing with GOES spacecraft measurements of x-ray flux, the validity and consistency of the analysis is demonstrated. When compared with the analysis of raw measurements, this method lessens the ambiguities in the detection of abnormal conditions. Also, the timing of the disturbance is more precisely obtained, especially for the end time that is usually difficult to determine with confidence, the exponential decay of the event leading to an unclear transition between the disturbed and normal propagation conditions. In addition, the results suggest that, with respect to the traditional data reduction method, the detection of fainter events is achievable. Very small disturbances, barely visible on original recordings, have been successfully extracted. Flares as small as B5—instead of the usual limit around C1—seem to produce, under some conditions, small VLF propagation disturbances. 1 Contact: http://sidstation.lionelloudet. homedns.org/email-en.xhtml