The Response of Planetary Ionospheres to Solar Irradiance II
Presiding: S C Solomon, NCAR/High Altitude Observatory; M Galand, Center for Space Physics, Boston University
SA32A-01 10:30h
Integrated Solar Irradiance from 1-45 nm (Qeuv) as Determined from the GUVI instrument on TIMED
The N2 LBH emission produced by photoelectron impact is one of the core emissions used for remote sensing of the thermosphere. This emission forms the basis for a proxy of EUV solar flux (< 45 nm, Qeuv) when used with OI 135.6 nm signals to derive O/N2 column ratios in the thermosphere (Strickland et al. [1995]). The LBH cross section used comes from Ajello and Shemansky [1985]. Recently, experimenters have argued for an increase in this cross section by as much as a factor of 1.6 (Budzien et al. [1994] and Eastes [2000]). When the increased cross section is used the Qeuv estimated from GUVI data is considerably smaller than the solar irradiance measurements in this bandpass as measured by the SEE instrument, also flying on TIMED. This has led some to question the accuracy of the SEE values. We will show that using the established N2 LBH emission cross sections we can derive Qeuv measurements from the GUVI data that are very consistent with the SEE irradiance measurements, simply by relaxing an assumption that has normally been used to determine O/N2 ratios and Qeuv from the GUVI data.
SA32A-02 10:45h
Solar Control of Earth's Ionosphere: Observations from Solar Cycle 23
A nine year database of sunlit E-region electron density altitude profiles (Ne(z)) measured by the Sondrestrom ISR has been partitioned over a 30-bin parameter space of averaged 10.7 cm solar radio flux (F10.7) 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 Ne(z) profiles with large Hall to Pedersen ratio, is used to minimize auroral contamination. Resultant filtered mean Ne(z) compares favorably with subauroral Ne measured for the same F10.7 and χ conditions at the Millstone Hill ISR. Mean Ne, as expected, increases with solar activity and decreases with large χ, and the variance around mean Ne is shown to be greatest at low F10.7 (solar minimum). ISR-derived mean Ne 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 Ne and suggests that the photoelectron and XUV enhancements are essential to replicate measured plasma densities below 150 km. Simulated Ne variance envelopes, given by perturbing the Model 2 neutral atmosphere input by the measured extremum in Ap, F10.7, and Te, 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 ) Ne 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.
SA32A-03 INVITED 11:00h
The Effects of Variations in the Solar UV and Soft Xray Fluxes on Models of the Ionospheres/Thermospheres of the Terrestrial Planets
In the past few years, we have had a number of different solar UV flux models available, in addition to those of Hinteregger, which were derived from Atmosphere Explorer measurements in the 1970's, and which continue to be used in some models of planetary thermospheres/ionospheres. In the decades since Atmosphere Explorer, a number of satellites have measured the solar fluxes in different wavelength regions, including several which have measured solar fluxes in the UV and X-ray regions. Because Atmosphere Explorer did not measure the solar X-ray fluxes, these measurements have greatlty advanced our knowledge of the magnitude of the fluxes in this region of the spectrum and their variability. We discuss the effects on the ionospheres/thermospheres of the terrestrial planets of adopting different solar flux spectra, including the Solar 2000 (S2K) models of K. Tobiska, which have incorporated data from recent satellites. The S2K v1.24 models incorporate the solar soft X-ray fluxes as measured by the Student Nitric Oxide Explorer (SNOE) satellite, and the S2K v2.2x models incorporate measurements from the TIMED SEE experiment. Radio occultation measurements of the electron density profiles of both Mars (made by the Mars Global Surveyor) and Venus (made by from Pioneer Venus) sometimes show lower peaks that have been ascribed to the effects of the large and variable fluxes of solar soft xrays. This contrasts with models of the Earth's ion density profiles as modeled by Solomon et al. We discuss the reasons for these differences, and the wavelength regions which must be enhanced to cause the observed low altitude peaks to form on Mars and Venus.
SA32A-04 11:20h
Solar Soft X-ray Production of Thermospheric Odd-Nitrogen
Ionization by solar soft X-rays is a primary cause of non-auroral odd-nitrogen in the thermosphere, and the source of much of the variability of nitric oxide. Therefore, the long-standing problems of the reference level and amount of variability of solar soft X-ray irradiance are intertwined with the effort to obtain a consistent set of chemical rate coefficients that can describe thermospheric nitric oxide, and hence the thermal balance of the thermosphere through its radiation in the 5.3 micron band. Recent measurements of the solar EUV and X-ray irradiance, and of nitric oxide emissions, by the UARS, SNOE, TIMED, and SORCE satellites have yielded considerable new information that largely resolve earlier issues. These measurements are briefly reviewed, and a new method for introducing solar irradiance into thermospheric general circulation models is described. A chemical scheme for these models, the global distribution of nitric oxide and its cooling rates, and the effect on thermospheric temperature variation are presented.
SA32A-05 11:35h
MHD and kinetic modeling analysis of high-altitude photoelectron observations at Mars
The Electron Spectrometer (ELS) instrument of the ASPERA-3 package on the Mars Express satellite has observed photoelectron energy spectra up to apoapsis (~ 10,000 km). The characteristic photoelectron shape of the spectrum is sometimes seen well above the ionosphere in the evening sector across a wide range of near-equatorial latitudes. Two numerical models are used to analyze the characteristics of these high-altitude photoelectrons. The first is a global, multi-species MHD code that produces a 3-d representation of the magnetic field and bulk plasma values around Mars. It is used here to examine the possibility of magnetic connectivity between the high-altitude flanks and the subsolar ionosphere. It is found that some field lines in this region are draped IMF lines while others are open field lines, connected to both the IMF and the crustal magnetic field sources. The second model is a kinetic electron transport model that calculates the electron velocity space distribution along a non-uniform magnetic field line. It is used here to simulate the high-altitude ELS measurements. It is found that there are systematic differences in the electron distributions depending on the type of field line (open or draped).
SA32A-06 11:50h
Photoelectron Conic Distributions Observed by FAST
Observations from FAST of escaping photoelectrons at altitudes of 2000 to 3000 km and at solar zenith angles greater than ~100 degrees show conic distributions. I.e. distributions of upflowing photoelectrons with angular distributions peaked near the upward directed source cone created by the fall off in magnetic field intensity. We describe these observations and suggest how these distributions might be formed. We also discuss how these observations might be used to infer the density distribution above the ionosphere of Earth and other planets.