This project aims to characterize the variability of the thermosphere over a wide range of spatial and temporal scales. It will accomplish this by utilizing satellite observations from the CHAMP and GRACE satellites to derive total mass densities and cross-track winds from 2002 through 2008. These spacecraft data are particularly useful since observations are obtained almost continuously with better than 80-km resolution along the satellite track which extends from pole to pole. The database will be provided to the research community via the web, yielding a comprehensive view of space weather from solar cycle maximum through solar minimum. These datasets will then be used in a variety of other investigations. One of these will delineate and interpret the global magnetic storm response of the thermospheric densities, winds, and exospheric temperatures as a function of longitude, local time, and season. A similar study will be performed to elucidate the local time response of the density, winds, and exospheric temperatures for different levels of solar activity; this study includes an investigation of the relative contributions of the tides originating in the lower atmosphere relative to the tidal components forced in situ by solar EUV absorption. The dataset will also be used to investigate a variety of regional phenomena such as the equatorial ionization anomaly, the midnight temperature maximum, and wave 4 longitude structures recently observed in the ionosphere; these features can be examined for the global view of their seasonal, solar cycle, and magnetic activity dependences. The CHAMP satellite is scheduled to re-enter the atmosphere in 2009 and it may be possible to use data obtained during this period to derive vertical winds. The project will provide a cross-disciplinary training opportunity for graduate students in aerospace engineering and in space weather science and will help meet national needs in terms of training future engineers and scientists while addressing problems relevant to society such as orbital drag prediction and ionospheric effects on communication. It will also enable the research team to participate in the ESA mission GOCE which will further international collaborations. The space weather benefits include development of improved empirical density and wind models, validation and testing of first-principles models, conducting ion-neutral coupling studies using coincident orbital plane data from the DMSP satellites; and coordinated studies with data from the CNOFS satellite.
The main thrust of the research under this grant was to delineate the variability of the thermosphere and ionosphere imposed by the Sun as well as meteorological effects from below. The altitude region of interest is roughly 300-500 km. Understanding the nature of this variability and its origins represents an important step towards predicting satellite orbits, avoiding collisions in space, and predicting conditions in space that affect navigation and communications systems. Attaining our research goal relied heavily on analyzing data from the CHAMP (Challenging Minisatellite Payload) and GRACE (Gravity Recovery and Climate Experiment) satellites. CHAMP was launched on 15 July 2000 and reentered the Earth’s atmosphere on 19 September 2010. CHAMP had a circular near-polar orbit with a 87.3-degree inclination. Over the years its orbital altitude gradually decreased from about 460 km to about 300 km. GRACE was launched on 17 March 2002 and consisted of two identical spacecraft flying about 220 km apart in a 89-degree inclination orbit. CHAMP’S primary mission was to measure Earth’s gravity and magnetic fields, but it also carried accelerometers that measured the drag forces on the satellite, and a Langmuir probe that measured the density of charged particles in the ionosphere. From the accelerometer data, we derived neutral densities and winds to perform our scientific analyses. The GRACE satellites also carried accelerometers from which densities could be inferred, but because of its higher altitude neutral winds could not be inferred. GRACE is still providing accelerometer measurements, but the data quality has significantly degraded since about 2010. An unprecedented view of "thermosphere-ionosphere weather" extending from solar maximum to solar minimum was attained within our research due to the exceptional spatial and temporal sampling offered by the CHAMP and GRACE satellites. Some highlights are provided below. Neutral Atmosphere Waves CHAMP and GRACE data enabled characterization of various-scale waves all over the globe. These included density waves at scales of order 600-2400 km in terms of local time, latitude, solar cycle and geomagnetic activity dependences. We showed that waves generated in polar-auroral regions in connection with solar disturbed conditions can propagate to the equator and even penetrate into the opposite hemisphere. We also discovered a solar terminator wave in neutral thermosphere densities, and demonstrated that a general circulation model could replicate the salient features of the wave structure that moves with the terminator. Concerning waves that originate in the lower atmosphere and propagate to satellite altitudes, we demonstrated that longitude variability is imposed on upper atmosphere densities by thermal tides with 24-hour and 12-hour periodicities that are excited in the troposphere and strongly influenced by the global land-sea distribution; and examined the global structure of the lunar atmospheric gravitational tide, and constructed multi-year average depictions of its effects, and also demonstrated significant intra-seasonal variability that contributes to the "space weather" of the thermosphere. Ionospheric Effects We extended our utilization of measurements from the CHAMP satellite to study the space weather of the neutral thermosphere, to studies of complementary effects on the ionosphere. In this connection we quantified the response of the ionosphere to recurrent geomagnetic activity; the intra-annual variability of ionospheric longitude structures due to upward-propagating diurnal tides excited in the troposphere; and the disruption and recovery of these longitude structures during the evolution of a magnetic storm. We demonstrated for the first time that longitudinal structures exist in the global ionospheric "solar quiet, or Sq" current system, consistent with the presence of nonmigrating tidal winds in the dynamo region. Although we had difficulties in deriving the effects of solar flares on the ionosphere from the CHAMP electron density data, we were successful in delineating solar flare ionospheric responses using instruments on the ground called digisondes. Global Responses Associated with Geomagnetic Disturbances When charged particles and magnetic fields from the Sun’’s "solar wind" interact with Earth’s geospace environment, injections of solar wind energy into the thermosphere and ionosphere occur that generate significant modifcations to neutral and charged-particle densities. Some of the scientific contributions that we made in this area include: (1) Demonstration that the time delay between enhanced geomagnetic activity and neutral density response is a function of latitude, and furthermore, much shorter than the 6-hour delay typically assumed in empirical models. (2) Characterized the response of high-latitude neutral winds and densities in terms of features in the charged-particle convection pattern and direction of the interplanetary magnetic field (IMF) embedded in the solar wind. (3) Quantified how the direction as well as magnitude of the IMF influence thermospheric densities, especially during equinoctial seasons. (4) Discovered periodicities in neutral thermosphere density variability at sub-harmonics of the 27-day rotation rate of the Sun, and ascribed them to rotating coronal holes on the sun, their manifestation in periodic high solar wind episodes, and the recurrent geomagnetic activity that results from the interaction of these high-speed streams with the geospace environment.
