Correlation of solar X-ray flux and SID modified signal strength

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Sunday, 4 January 2015
Shannon N. Kranich, Air Force Institute of Technology, Wright-Patterson AFB, OH; and W. F. Bailey, R. D. Loper, and K. S. Balasubramaniam

This paper presents a quantitative comparison of the X-ray flux during solar flares, as measured by the GOES 15 satellite, and the associated effects on the ionization levels in the lower ionosphere as measured by Sudden Ionospheric Disturbance (SID) monitors around the globe. In operation since March 2010, GOES15 has recorded 490 solar flares of M-class or greater. The Solar Center at Stanford University has designed receiver that can be used to monitor SIDs. These monitors detect signals from a variety of different transmitting stations, each sending a unique Very Low Frequency (VLF) radio wave signal ranging from 16.4 to 45.9 kHz. Global signal propagation distances are achieved via the Earth-ionosphere waveguide propagation mode. During a solar flare, the X-ray flux increases dramatically producing an enhanced ionization response in the upper atmosphere. The rapid increase of ionization causes a SID in the lower ionosphere and signal propagation is altered. The monitored VLF signal strength increases as a result of the decrease in height of the ionosphere boundary as ionization penetrates deeper into the neutral atmosphere. Using the numerical data gathered by GOES15 and data from a variety of SID monitors, X-ray flux and SID modified VLF signal strength were analyzed for the period beginning in March 2010 to present. Ionospheric incubation times, VLF signal rise time and the duration of signal enhancement are related to flare strength via the x-ray flux enhancement.