Forecasting the Ionosphere Response to Solar Flares from Satellite Measurements of X-ray and EUV Flux

An X4.9-class solar flare occurred at approximately 01:00 hours UTC on 25 February 2014 causing widespread dayside blackouts of HF communication and radar, including SuperDARN. Nearly complete disruption persisted for about 10 minutes. The UHF Poker Flat Incoherent Scatter Radar (PFISR) measured significant plasma density enhancements in the D, E, and lower F regions during the flare event which occurred during PFISR Ion Neutral Observations of the Thermosphere (PINOT) campaign measurements.

In this case study, X-ray flux measurements from the Geostationary Operational Environmental Satellite (GOES-15) were used with the Self Consistent Ionosphere Model (SCIM) to calculate the ionosphere plasma density, temperature, and composition response over Poker Flat, Alaska. Under the assumption that the flare X-ray flux enhancement was confined to GOES-measured X-ray wavelengths between 0.05 and 0.8 nm, the calculated ionosphere plasma density perturbation above Poker Flat was restricted to the D region, contrary to the ISR measurements which in addition showed significant E- and lower F-region electron density enhancements. Incorporating measurements into the model from the GOES-15 and Solar Dynamics Observatory (SDO) extreme ultraviolet (EUV) sensors covering spectral bands between 5 and 36 nm improves the agreement between the modeled electron densities and its measurements but is not entirely sufficient, particularly in upper E and lower F regions.

Increasing the model soft X-ray energy flux in the 0.8 to 2.3 nm wavelength band by two orders of magnitude, consistent with TIMED-SEE measurement surveys of previous X-class flares, brings the calculated ionosphere response to good agreement with PFISR measurements. Finally, PFISR measurements made during the 7 January 2014 X1-class solar flare are shown that demonstrate some X-class flares produce only moderate electron density enhancements in the D and lower E regions, but not the upper E and lower F regions. This underscores the need for expanded continuous measurement of solar soft X-ray and EUV flux to adequately model the ionosphere response to X-class solar flares. No continuous measurements of this band are currently available.