HR: 10:45h
AN: SA32A-02    [Abstracts]
TI: Solar Control of Earth's Ionosphere: Observations from Solar Cycle 23
AU: * Doe, R A
EM: doe@sri.com
AF: Center for Geospace Studies, SRI International, 333 Ravenswood Avenue, G-284, Menlo Park, CA 94025 United States
AU: Thayer, J P
AF: Department of Aerospace Engineering Sciences, University of Colorado, 429 UCB, Boulder, CO 80309 United States
AU: Solomon, S C
AF: High Altitude Observatory, National Center for Atmospheric Research, 3450 Mitchell Lane, Boulder, CO 80301 United States
AB: A nine year database of sunlit E-region electron density altitude profiles (N_e(z)) measured by the Sondrestrom ISR has been partitioned over a 30-bin parameter space of averaged 10.7 cm solar radio flux (F_10.7) and solar zenith angle (χ) to investigate long-term solar and thermospheric variability, and to validate contemporary EUV photoionization models. A two stage filter, based on rejection of N_e(z) profiles with large Hall to Pedersen ratio, is used to minimize auroral contamination. Resultant filtered mean N_e(z) compares favorably with subauroral N_e measured for the same F_10.7 and χ conditions at the Millstone Hill ISR. Mean N_e, as expected, increases with solar activity and decreases with large χ, and the variance around mean N_e is shown to be greatest at low F_10.7 (solar minimum). ISR-derived mean N_e is compared with two EUV models: (1) a simple model without photoelectrons and based on the 5 -- 105 nm EUVAC model solar flux [Richards et al., 1994] and (2) the GLOW model [Solomon et al., 1988; Solomon and Abreu, 1989] suitably modified for inclusion of XUV spectral components and photoelectron flux. Across parameter space and for all altitudes, Model 2 provides a closer match to ISR mean N_e and suggests that the photoelectron and XUV enhancements are essential to replicate measured plasma densities below 150 km. Simulated N_e variance envelopes, given by perturbing the Model 2 neutral atmosphere input by the measured extremum in A_p, F_10.7, and T_e, are much narrower than ISR-derived geophysical variance envelopes. We thus conclude that long-term variability of the EUV spectra dominates over thermospheric variability and that EUV spectral variability is greatest at solar minimum. ISR -- model comparison also provides evidence for the emergence of an H (Lyman β) N_e feature at solar maximum. Richards, P. G., J. A. Fennelly, and D. G. Torr, EUVAC: A solar EUV flux model for aeronomic calculations, J. Geophys. Res., 99, 8981, 1994. Solomon, S. C., P. B. Hays, and V. J. Abreu, The auroral 6300 Å emission: Observations and Modeling, J. Geophys. Res., 93, 9867, 1988. Solomon, S. C. and V. J. Abreu, The 630 nm dayglow, J. Geophys. Res., 94, 6817, 1989.
DE: 2407 Auroral ionosphere (2704)
DE: 2423 Ionization mechanisms
DE: 2479 Solar radiation and cosmic ray effects
SC: SPA-Aeronomy [SA]
MN: 2005 Joint Assembly