E-Layer Variations During X-Class Flares Inferred from Far Ultraviolet Dayglow Observations

A study is reported that addresses dayglow and ionospheric response to X-class flares that occurred on Oct 28 and Nov 4 2003. Data of interest are 1) the dayglow observations by GUVI and solar EUV/X-ray observations by SEE (both on NASA's TIMED satellite), 2) E-region measurements made by the EISCAT radar (on Oct 28) located at Tromsø, Norway, and 3) E-region/F-region IOX GPS occultation measurements (on Nov 4). The timing of the flares was fortunate in that TIMED was on the dayside portion of its orbit when activity peaked for both flares. During the Oct 28 flare, the EISCAT measurements were made near local noon under low geomagnetic activity conditions. Key results are the modeled and measured preflare and flare E-region electron densities. The basis of the modeled densities is QEUV, an integrated measure of solar EUV/XUV energy flux from 0 to 45 nm (that portion of the irradiance spectrum responsible for far ultraviolet dayglow). Use is made of spectra from SEE and the NRLEUV model along with GUVI nadir dayglow observations within its 135.6 and LBHS spectral channels to derive preflare and flare QEUV. The GUVI data are used to derive QEUV with the use of lookup tables, each distinguished by solar EUV/X-ray spectral shape, not by magnitude. Lookup tables have been produced using SEE and NRLEUV flare and preflare spectral shapes. The AURIC model is used to calculate the E-layer with the key input being a solar spectrum with a given shape (those discussed above) and magnitude (set by QEUV). The two sets of ionospheric measurements on their respective flare days show increases in NmE (E-layer peak density) by approximately a factor of three. The QEUV-based NmE agrees well with the measurements before and during these flares. NmE based on SEE spectra, on the other hand, exceeds the observed values, especially during the flares. The favorable agreement supports the derived GUVI QEUV values and argues for a significant reduction in SEE energy fluxes during periods of eruptive solar activity.