SA23B-01 INVITED
Magnetospheric forcing of the high-latitude thermosphere, as observed by CHAMP
It is known since quite a while that the magnetospheric input to the high-latitude thermosphere can surmount the solar energy deposition during disturbed time. There have been several model approaches to estimate the heating rate from ionospheric currents. Only during recent years CHAMP observations provide direct observations of the air density distribution at auroral latitudes. Opposed to earlier expectations these observations suggest particularly strong heating around the cusp region. This is present both during quiet and disturbed periods. A secondary enhancement in air density is observed in the pre-midnight substorm region. Interestingly, regions of low density are observed in the early morning sector, where the strongest electrojets occur. During geomagnetic storms major thermospheric heating occurs at auroral regions, in particular on the dayside. Density bulges formed at high latitudes propagate equatorward subsequently. Simultaneously with the density CHAMP is able to measure the cross-track wind. By combining many passes over the pole an average wind pattern has been deduced. Special features of that is the strong asymmetry between the dawn and dusk sides. While we observe strong zonal winds from day to night on the dawn side, an anti-cyclonic vortex develops on the dusk side. The "eye" of this cyclone forms at a latitude around 70° MLat and 18 MLT. The combined action of Coriolis and centrifugal force are the reason for the asymmetry. Only by considering the heating rates and the wind systems at the same time the observed thermospheric air distribution can be interpreted properly.
SA23B-02 INVITED
Thermospheric response to magnetospheric energy inputs
Analyzing DE-2 satellite data and fitting the data to analytic functions of the independent variables, a new quantitative empirical model of the high-latitude forcing of the thermosphere has been developed, including electric potential, magnetic potential and Poynting flux. Coupling this empirical model with NCAR-TIEGCM, the influence of the high-latitude energy inputs and energy distributions on the global thermospheric temperature, density, and composition has been investigated. First, the Joule heating calculated with the average electric field, which is called ¡°simple Joule heating", is compared with the Poynting flux from the empirical model to show the contribution of electric and magnetic field variability to the Joule heating. Secondly, an inter-comparison among three different methods to distribute the Poynting flux in altitude has been conducted. The difference of the thermosphere response suggests that not only the total amount of energy input, but the way to distribute the energy are significant for the impact of the magnetosphere on the thermosphere and ionosphere.
SA23B-03 INVITED
Sounding rocket observations of the small-scale neutral response in the E region to magnetospheric energy input at high latitudes
Sounding rocket measurements have provided some of the most detailed observations of the small-scale response of the neutral lower thermosphere to magnetospheric energy input in the auroral zone. In January and February 2007, a series of such launches were carried out at Poker Flat, Alaska, during substorm conditions. The experiments built on results from launches in 2003 and earlier that were also carried out at that location. The 2007 observations provided a particularly good specification of the local forcing due to the ground-based support from the Poker Flat Incoherent Scatter Radar (PFISR), which was installed there recently. The rocket measurements provided wind profiles and the north-south gradients in the winds, as well as detailed in situ measurements of the electric fields, electron densities, particle energies, and neutral densities, and temperatures. Theoretical support for the experiments has included model runs with several different numerical models of the thermosphere-ionosphere system. The results suggest that the contribution to the overall Joule heating from small-scale structure in the electric fields can be significant, that the response of the winds in the lower thermosphere to auroral forcing is stronger than expected, and that the response of the winds to comparable forcing in different events can be significantly different. The available data will be summarized and the characteristics of the small-scale structure in the neutral response to auroral forcing will be described.
SA23B-04 INVITED
Thermospheric Response to Energy and Momentum Inputs from the Magnetosphere
In this talk we review MI coupling via momentum and energy inputs, and the thermospheric response to these inputs. Our focus will be mainly at high latitudes. Using a global 3-D ionosphere-thermosphere model, we will review the various forcing terms and the balance of forces that leads to the thermospheric wind, temperature, density and compositional responses for various levels of geomagnetic activity. We will also review the energy balance at high latitudes. There is still much we do not understand regarding MI coupling, momentum and energy inputs and how they relate to the thermospheric response. We will discuss some of the outstanding research problems in this area and touch on some exciting opportunities for further research.
SA23B-05
Modeling Suspersonic Flow in the High-Latitude Thermosphere
Supersonic flow has been observed in the high-latitude thermosphere via DE-2 satellite measurements. The supersonic flow events were observed to occur most frequently in the dawn sector at high magnetic latitudes and in the altitude range from about 300-600 km. To gain theoretical insight into these supersonic flow events, we used a high-resolution model of the global thermosphere to study the geophysical conditions that give rise to supersonic flow. In particular, we considered the dependence of the neutral wind on IMF orientation, magnetic activity level, convection pattern shape, solar activity level, season, and universal time. We also studied the extent to which narrow plasma convection channels can induce supersonic neutral winds. We found that the predominant factor controlling the thermospheric wind speed at high latitudes is the magnitude of the cross- polar-cap potential, indicating that interaction with the convecting ionosphere can drive the thermosphere to supersonic speeds. The region of supersonic flow predicted by the model corresponds closely to the region where supersonic flow events were most frequently observed by the DE satellite.
SA23B-06
The Response of the Thermosphere and Ionosphere to Magnetospheric Inputs as Determined from LEO UV Remote Sensing Measurements - Model/Data Comparisons
UV remote sensing from low Earth orbit provides the opportunity to investigate the response of the coupled ionosphere/thermosphere system to magnetospheric inputs. Low Earth orbit (LEO) is particularly advantageous because a simple, low-cost, low-mass instrument such as GUVI can make limb and disk measurements that characterize the ionosphere and thermosphere. In this paper we will show the results of some of the comparisons we have carried out between the GITM model driven by typical AMIE inputs and when the UV data are included. The UV data are used to provide the hemispheric power due to particle precipitation based on the Zhang and Paxton [2007] model of the auroral energy distribution. The UV data are also used to provide global scale maps of the conductances. These real measurements show important departures from the conductance pattern inferred from AMIE. We also find that the model output can, indeed, be meaningfully challenged by the UV data. The UV data embodies information on the O. N2, O2 and electron density profiles on a global basis as well as the lower thermospheric O/N2 data. Note that the column O/N2 ratio observed by instruments like GUVI provides a unique insight into the thermospheric composition at about 135 km. We will discuss these differences for superstorms as well as moderate and quiet conditions because we find that there are significant differences between the models, climatologies such as MSIS and the data.
SA23B-07
Effect of the Interplanetary Magnetic Field on the Thermospheric Density at High Latitude
The high-latitude thermospheric total mass density near 400 km altitude, derived from the high-accuracy accelerometer on board the Challenging Minisatellite Payload (CHAMP) spacecraft in November 2001 through February 2002, is statistically analyzed as a function of the direction and strength of the interplanetary magnetic field (IMF) for southern hemisphere. The difference densities, which are obtained by subtracting values for zero IMF from these for nonzero IMF, show the IMF dependence as follow: 1) Difference density for negative By shows increase in the early morning and dawn sectors, but decrease in the premidnight and dusk sectors. 2) For positive By it is opposite in the signs for negative By. 3) Those for negative Bz show significant increase in the cusp region and premidnight sector, but decrease in the dawn sector. High-latitude thermospheric total mass densities and variations with IMF simulated by the National Center for Atmospheric Research Thermosphere- Ionosphere Electrodynamics General Circulation Model (NCAR/TIEGCM), coupled with a new quantitative empirical model of the high-latitude forcing on the thermosphere, show reasonable agreement with CHAMP observations for the dependence of density on the IMF direction. Using the numerical simulation, we can gain insight into sources responsible for the thermospheric density variations. It is suggested that high-latitude thermospheric density variations with different IMF conditions can be strongly determined by thermospheric winds, which vary strongly with respect to the direction of IMF. In addition, we find that the density variations are also influenced by the local heating associated with ionospheric current or auroral particle precipitation, or by the local cooling, which varies with IMF conditions.