The Response of Planetary Ionospheres to Solar Irradiance I
Presiding: S C Solomon, NCAR/High Altitude Observatory; M Galand, Center for Space Physics, Boston University
SA31A-01 INVITED 08:30h
Solar EUV models and photoelectron transport at Earth: challenges
This paper presents an overview of models of solar EUV and soft X-ray fluxes that are responsible for producing the ions and photoelectrons in the Earth's ionosphere. In particular, solar fluxes from models based on earlier measurements from the Atmosphere Explorer satellites are compared with recent measurements of the solar EUV and X-ray fluxes from the TIMED satellite. Model photoelectron fluxes are also compared with measurements from the earlier Atmosphere Explorer and recent FAST satellites. This paper also discusses the impact on the models from uncertainties in our knowledge of the photoionization and electron impact cross sections that are required for accurate modeling of the density and temperature of the ionosphere.
SA31A-02 INVITED 08:50h
Recent Measurements of the Solar Ultraviolet Irradiance Variability and Application for Planetary Atmospheric Research
The recent measurements of the solar ultraviolet (UV) irradiance from the NASA Thermosphere-Ionosphere-Mesosphere-Energetics-Dynamics (TIMED) mission and the NASA SOlar Radiation and Climate Experiment (SORCE) mission will be presented. The solar far ultraviolet (FUV: 120-200 nm) irradiance is being measured by the SORCE SOLar STellar Irradiance Comparison Experiment (SOLSTICE) with 0.1 nm spectral resolution and 5% accuracy. The solar extreme ultraviolet (EUV: 30-120 nm) irradiance is being measured by the TIMED Solar EUV Experiment (SEE) with 0.4 nm spectral resolution and 15% accuracy. The solar soft x-ray (XUV: 0.1-30 nm) irradiance is being measured by the XUV Photometer System (XPS), which is aboard both the TIMED and SORCE satellites, with 10 nm resolution and 20% accuracy. The TIMED satellite was launched in December 2001 near solar cycle maximum, and the SORCE satellite was launched in January 2003 during moderate solar cycle activity. The solar activity has now declined to low-moderate activity. The solar UV irradiance varies on all time scales, seconds to years, and this variation is very dependent on wavelength. In addition to the new results on the solar variations related to the 11-year solar cycle and the 27-day solar rotation, the TIMED and SORCE solar instruments have observed several hundred flares concentrated primarily during the solar storm periods in June 2003, October 2003, July 2004, November 2004, and January 2005. The coronal emissions, such as the Fe XVI 33.5 nm emission and x-rays, vary the most, with variations of a factor of 30 for the large flares, a factor of 2 for solar rotation, and a factor of 5 during the TIMED mission (3 years). The transition region emissions, such as the H I 121.6 nm and He II 30.4 nm emissions, vary less with variations of a factor of 1.5 for the large flares, a factor of 1.2 for solar rotation, and a factor of 1.6 during the TIMED mission. The chromospheric and photospheric emissions vary even less. The variations of the solar UV irradiance shortward of 190 nm will be discussed in the context of planetary atmospheric research.
http://lasp.colorado.edu/see/
SA31A-03 09:10h
Ionization frequencies calculated using TIMED/SEE observations
The recent measurements of the solar extreme ultraviolet (EUV) irradiance by the TIMED Solar EUV Experiment (SEE) have allowed us to re-examine the ionization frequencies for part of decreasing phase of the solar cycle 23. In the past, ionization frequencies for various species (CO2, O, O2, N2) have been calculated based on limited observations from satellites and rockets and their solar cycle dependence was parameterized from discrete sets of measurements. We compare the ionization frequencies obtained from TIMED/SEE to those presently in use in the community. We also examine the time evolution of these frequencies on several time scales - solar cycle and solar rotational variability as well as short-term flare activity. The relationship of these ionization frequencies to different solar indices will also be discussed.
SA31A-04 INVITED 09:25h
Solar Forcing at Titan Within the Context of Cassini Observations
A brief review will be given of the response of planetary ionospheres to the input of energy from the Sun and from external sources such as the solar wind. Particular emphasis will be given to the role that superthermal electrons play in planetary ionospheres. Superthermal electron populations in planetary ionospheres include photoelectrons associated with the photoionization of atmospheric neutrals by solar EUV and soft x-ray radiation and external fast electrons from magnetospheres or from the solar wind. The ionosphere of Titan will be emphasized in this talk. The results of ionospheric model calculations of densities and superthermal electron fluxes will be compared with measurements made by a number of instruments onboard the Cassini Orbiter.
SA31A-05 09:45h
Photoelectron transport at Titan, Saturn, and Earth: comparative approach
Solar irradiance in the extreme ultra-violet and X-ray spectral range is a major source of variability in planetary upper atmospheres through photoionization and heating processes. The induced photoelectrons have enough energy to also interact with the atmospheric population. Their transport is important for ionospheric densities and temperatures. As support of the on-going Cassini mission we recently upgraded the version of Titan's multistream electron transport model and adopted it to the atmosphere of Saturn. The goals of such calculations, unprecedented for Saturn, are to compute the electron/ion production rates induced by energetic electrons and the heating rate associated with the energy transfer between suprathermal electrons and ionospheric electrons. We will discuss how sensitive these quantities are with solar activity and compare solar-induced to photoelectron-induced electron productions. Saturn and its largest moon, Titan, do not share the same atmospheric neutral species, whereas Earth and Titan have similar atmospheric constituents at different distances from the Sun. Therefore comparing the transport of photoelectrons between these three bodies, primary goal of this study, is of particular interest.
http://www.bu.edu/csp/uv/cp-aeronomy/aeronomy-sol-sys.html