Developing a Nowcasting Capability for X‐Class Solar Flares Using VLF Radiowave Propagation Changes.

Abstract

A technique for analyzing very low frequency (VLF) radiowave signals is investigated in order to achieve rapid, real-time detection of large solar flares, through the monitoring of changes in VLF radio signal propagation conditions. The reliability of the use of VLF phase and amplitude perturbations to determine the X-ray fluxes involved during 10 large solar flare events (>X1) is examined. Linear regression analysis of signals from the NPM transmitter in Hawaii, received at Arrival Heights, Scott Base, Antarctica, over the years 2011-2015 shows that VLF phase perturbations during large solar flares have a 1.5-3 times lower mean square error when modeling the long wavelength X-ray fluxes than the equivalent short wavelength fluxes. The use of VLF amplitude observations to determine long or short wavelength X-ray flux levels have a 4-10 times higher mean square error than when using VLF phase. Normalized linear regression analysis identifies VLF phase as the most important parameter in the regression, followed by solar zenith angle at the midpoint of the propagation path, then the initial solar X-ray flux level (from 5 min before the impact of the solar flare), with F10.7 cm flux from the day beforehand providing the least important contribution. Transmitter phase measurements are more difficult to undertake than amplitude. However, networks of VLF receivers already exist which include the high quality phase capability required for such a nowcasting product. Such narrowband VLF data can be a redundant source of flare monitoring if satellite data is not available.

Electron Precipitation From the Outer Radiation Belt During the St. Patrick's Day Storm 2015: Observations, Modeling, and Validation

“…This allows relative phase variations to be determined for a period of 29 days in a row—something that is not normally possible because of instability in either transmitter phase or receiver phase‐lock. The transmitter amplitude was also logged at Reykjavik; however, the amplitude levels during geomagnetic storms were highly variable and less understandable as a monitor of long‐lasting perturbations, consistent with the findings of George et al () for solar flare analysis. A second receiver location is also shown in Figure by a red diamond close to the transmitter, at Eskdalemuir geomagnetic observatory.…”

“…The superimposed diamonds indicate the phase calculated by LWPC for the GVT‐Reykjavik path on 16 March, using D‐region ionospheric electron number density Wait‐based profiles for solar zenith angle‐defined beta (sharpness) and H ′ (reference height) values determined by McRae and Thomson () and midlatitude nighttime beta and H ′ values from Thomson and McRae (). Several features of note can be observed, including the sudden phase change effects of a M2 solar flare just prior to midday (see George et al, for a discussion of large solar flares and their VLF responses) and a sunrise shoulder, relative to the daytime phase levels, which is caused by ozone layer absorption of solar UV during high solar zenith angle conditions (Macotela et al, ). Although these two features are not captured by the LWPC modeling, the close fit between the rest of the observed phase variations, the QDC, and LWPC modeling results indicate a high‐quality knowledge of the background, undisturbed ionospheric conditions prior to the geomagnetic storm on 17 March.…”

Analysis of Global Ionospheric Response to Solar Flares Based on Total Electron Content and Very Low Frequency Signals

“…Ionospheric response to solar flares based on VLF radio signals has also achieved numerous breakthrough results [18][19][20]. For example, George et al [21] studied the long or short wave X-ray flux level by using the VLF phase and amplitude of the NPM Station. They found that the mean squared error of using VLF amplitude to determine long or short waves X-ray flux level is 4-10 times higher than that of using VLF phase.…”

The space weather station at the University of Alcala

“…in H-alpha observations. Nevertheless, flares can also be monitored by considering the propagation of radio waves of distant VLF transmitters (Stokes, 1989;Raulin et al, 2010;Wenzel et al, 2016;George et al, 2019), which not only gives information about the flare but also about the actual impact on the Earth. When the electromagnetic energy released during solar flares reaches the Earth, it is absorbed in the upper terrestrial atmosphere and greatly enhance ionization of neutral species, including oxygen and nitrogen (Mitra, 1974).…”