Solar flare detection sensitivity using the South America VLF Network (SAVNET)

Abstract

[1] We present recent observations of Sudden Phase Anomalies due to subionospheric propagation anomalies produced by solar X-ray flares. We use the new South America VLF Network (SAVNET) to study 471 ionospheric events produced by solar flares during the period May 2006 to July 2009 which corresponds to the present minimum of solar activity. For this activity level, we find that 100% of the solar flares with a X-ray peak flux above 5 × 10 −7 W/m 2 in the 0.1-0.8 nm wavelength range produce a significant ionospheric disturbance, while the minimum X-ray flux needed to do so is about 2.7 × 10 −7 W/m 2 . We find that this latter minimum threshold is dependent on the solar cycle, increasing when the Sun is more active, thus confirming that the low ionosphere is more sensitive during periods of low solar activity. Also, our findings are in agreement with the idea that the ionospheric D-region is formed and maintained by the solar Lyman-a radiation outside solar flare periods.

Study of Antarctic-South America Connectivity From Ionospheric Radio Soundings

“…The analyses of SPAs have shown that the D-region is affected by the 11-year variations of the solar ionizing radiation, and that it is more sensitive during periods of low solar activity, when the ionospheric reference height is about 1km higher ( degrees. Figure adapted from Raulin et al (2010) Long term data analysis of the daytime VLF amplitude has been considered to study the solar forcing in the D-region. Considering the averaged level of the daytime VLF amplitude during the summer season (December-January), when the solar illumination is maximum in the NPM-EACF path, the VLF amplitude from 2004 through 2009 shows a slow decrease of about -0.63dB/year, which accompanies the decrease of the solar Lymanalpha radiation intensity (Figure 3, Correia et al 2010a) during the decay of the 23rd solar cycle.…”

“…This region is maintained by the solar Lymanalpha radiation (Nicolet & Aikin 1960) during quiet periods, but can be highly disturbed by enhancements of X-ray emission during solar flares (McRae & Thomson 2004, Raulin et al 2010, as well by electron precipitation events from the radiation belts (helliwell et al 1973, Inan et al 1982, Fernandez et al 2003, Peter & Inan 2004, Correia et al 2007, Correia et al 2010b. These disturbances produce changes in the electron density of the D-region that affects the conductivity gradient and/ or the reference height, which govern the refractive index (Wait & Spies 1964).…”

Monitoring of atmospheric changes related to sun-earth interactions

“…Solar flare probability detection P, as a function of the soft X-ray peak flux Px for the long NPM -ATI VLF propagation path, and for solar zenith angle greater (dashed line) or lower (full line) than 40 degrees. Figure adapted from Raulin et al (2010).…”

Analysis of daytime ionosphere behavior between 2004 and 2008 in Antarctica

“…The reference height of quiescent ionosphere was found to be about 1 km lower at solar maximum compared to that at solar minimum (McRae and Thomson, 2004;Raulin et al, 2006). VLF studies also had shown that the lower ionosphere is more sensitive during the periods the solar activity is at lower levels, when even weaker solar XͲray events produce significant phase advances (McRae and Thomson, 2000;Pacini and Raulin, 2006;Raulin et al, 2010).…”