SA24A-01
Ionospheric Longitude Storm Dependence Upon the Magnitude of the Earth's Magnetic Field
The Earth's magnetic field in the ionosphere is understood to be non-dipolar with significant deviations in magnitude and orientation across the globe. This study models the mid-latitude ionospheric response to a geomagnetic storm for different idealizations of the Earth's magnetic field strength. In so doing the study addresses the question whether or not a longitude dependence in ionospheric storm responses could exist due to the longitude dependence of the magnetic field [ Huang et al., 2005], and if so, how significant is the effect? The mechanism by which the magnetic field magnitude has a first order effect is through the E x B plasma drift that has a vertical components, i.e., usually described as a meridional plasma drift caused by the zonal electric field. This vertical drift is inversely proportional to the magnitude of the magnetic field. A vertical drift raises or lowers the F-region into regions of lesser or greater recombination rates respectively, hence, directly affecting the plasma density. The Utah State University (USU) Time Dependent Ionospheric Model (TDIM) uses a tilted dipole magnetic field model to represent the Earth's field. The magnitude of magnetic field is specified by the dipole moment, in fact, the magnetic field strength on the surface of the Earth at the magnetic equator. Changing this one parameter enables studies to be made under identical storm conditions of the effect of different magnetic field magnitudes. For this study the normal 0.31 Gauss surface magnetic field is replaced by 0.24 Gauss and 0.41 Gauss. These two numbers represent the magnitude of the minimum and maximum observed field strength around the Earth equatorial region. The TDIM results are shown for a storm simulation that occurred on 5-6 November 2001. For otherwise identical model conditions and drivers, the difference in magnetic field strength results in a factor of 2 difference in TEC, NmF2, etc. Since the magnetic field magnitude is weakest in the Atlantic (South Atlantic specifically) and largest over the central Asian continent, these simulations predict that the Atlantic storm densities would be many 10's of percent larger than those in Asia for identical electric fields. The simulated mechanism will contribute to a longitude dependence that produces larger ionospheric densities over the Atlantic sector provided an eastward electric field is present. This is very likely to be the case during major geomagnetic storms as the high-latitude convection pattern extends to mid- and low-latitudes. Huang, C.-S., J. C. Foster, L. P. Goncharenko, P. J. Erickson, W. Rideout, and A. J. Coster, (2005), A strong positive phase of ionospheric storms observed by the Millstone Hill incoherent scatter radar and global GPS network, J. Geophys. Res., 110, A06303, doi:10.1029/2004JA010865.
SA24A-02
Long-term Temperature Trends in the Thermosphere Based on Incoherent Scatter Radar Data
We have developed a series of empirical models of Earth's ionosphere and thermosphere based on data from most of the world's incoherent scatter radars (ISRs). These models depend on solar and geomagnetic activity, but until recently have not included any long-term trend independent of those due to differences in geophysical indices from solar cycle to solar cycle. Greenhouse gases such as CO2 and CH4 are well known to be increasing in the lower atmosphere. The effect of this on the upper atmosphere, in particular, the ionosphere, has become an active topic of research since the publication of a theoretical modeling study by Roble and Dickinson suggesting a major greenhouse cooling in the thermosphere in response to increases in CO2 and CH4 concentration at 60 km. This cooling effect leads to a global reduction in neutral densities including O, N2 and total neutral mass density as the neutral temperature Tn decreases. We have addressed this using almost three solar cycles of Millstone Hill measurements of the ion temperature Ti, which is closely coupled to Tn. There is a statistically highly-significant long-term Ti trend ranging from -4 K/year to -8 K/year depending on time-of-day and altitude. Using the ion energy equation we find a similar trend for Tn.
SA24A-03
A Comparative Planetary Study: Exosphere Temperature Responses of Earth and Mars to Long-Term Solar Change
Using densities derived from precise orbit determination of the Mars Global Surveyor (MGS) spacecraft from 1999 to mid-2005, the response of Mars' exosphere to long-term solar change is established and compared to that of Earth. At Mars, exosphere temperatures change only 36-50 percent as much as those at Earth as solar activity increases from solar minimum to solar maximum. General circulation models suggest that this difference may be strongly influenced by adiabatic cooling associated with the thermosphere general circulations. However, other processes such as differences in CO2 cooling rates may also be playing a role.