[1] Measurements of atmospheric density near 410 km from the STAR accelerometer on the CHAMP satellite are used to illustrate the spatial-temporal dependence of the thermospheric response to the severe solar storms occurring during 29 October to 1 November 2003. This interval includes periods of elevated magnetic activity with K P values of 5-9, as well as undisturbed intervals that serve to define quiet time baseline densities. Measurements are available from À87°to +87°latitude during both day and night at local times near 1300 and 0100 hours, respectively. During times of maximum geomagnetic activity for this study, density measurements exhibit enhancements of 200-300%. Northern Hemisphere daytime responses are much larger than in the Southern Hemisphere; the origins of this effect are unknown. Nighttime density disturbances more readily propagate to equatorial latitudes, possibly facilitated by the predominant equatorward flow in both hemispheres due to the diurnal tides driven by in situ EUV heating. The CHAMP density measurements are compared with density predictions from the NRL-MSISe00 empirical density model and demonstrate some model shortcomings. Measurements of cross-track accelerations provide the opportunity to estimate zonal winds from the equator to about ±60°latitude, transitioning to a measure of purely meridional winds at the turning point of the orbit near ±87°latitude. A periodic variation in cross-track winds with an apparent period of 24 hours appears at high latitudes and exhibits similar amplitudes and temporallatitudinal structures to the empirical HWM-93 wind model when projected into the cross-track direction. This periodicity is due to the displacement of geomagnetic and geographic coordinates. At low latitudes, CHAMP and HWM-93 both yield westward winds of order 100 ms À1 during midday under quiet magnetic conditions; however, during severely disturbed periods the HWM-93 winds generally show a greater westward intensification (to 250 ms À1 ) than the CHAMP measurements. At night, CHAMP winds are near zero under quiet conditions whereas HWM-93 indicates eastward winds of order 50-100 ms À1 . Under disturbed conditions the CHAMP winds shift to westward values of order 200 to 250 ms À1 , while HMW-93 values do not exceed about 50 ms À1 in the westward direction. The physical origins of the observed effects are difficult to isolate, and unequivocal interpretation will require sophisticated numerical modeling taking into account self-consistent interactions between the neutral winds, drifts, and ionization densities.

We report discovery of a solar‐terrestrial connection between rotating solar coronal holes and density variations in Earth's thermosphere. Specifically, during 2005, a 9‐day recurrence of fast streams in the solar wind exists due to solar coronal holes distributed roughly 120 degrees apart in longitude; this periodicity is transmitted to the geospace environment where it modulates geomagnetic activity and thermospheric densities derived from accelerometer measurements on the CHAMP satellite. Our discovery demonstrates a solar‐terrestrial connection that has not been appreciated before, and by its nature is characterized by an element of predictability. Its potential predictability has practical relevance for collision avoidance and other applications affected by density variability in the terrestrial space environment.

[1] We report on periodic oscillations in thermosphere density, measured by the accelerometer on the CHAMP satellite during 2006, and relate these periodicities to oscillations observed in solar wind speed and Kp index. Common periodic oscillations at 4-5, 6-7, and 9-11 day periods are observed in the neutral density at 400 km in the 2006 data set, with the 7 day period being the predominant oscillation. Spectral analysis reveals that similar periodicities are present in both the solar wind and the planetary magnetic index Kp but not in the EUV solar flux proxy F 10.7 . We suggest that the periodic oscillations observed in thermosphere density are a direct response to recurrent geomagnetic activity and associated high-speed streams in the solar wind. The lack of response in F 10.7 at the 7 day period enables storm effects on the thermosphere density to be isolated from solar flux effects. The Kp index for these events correspond to moderate levels of geomagnetic activity, and the resultant perturbations in thermosphere density are ±20-30% of background levels. Although these levels of perturbation are small compared to major magnetic storms, their much higher occurrence frequency and characteristic long recovery time may lead to a cumulative effect on the state of the thermosphere and ionosphere.Citation: Thayer, J. P., J. Lei, J. M. Forbes, E. K. Sutton, and R. S. Nerem (2008), Thermospheric density oscillations due to periodic solar wind high-speed streams,

Atmospheric mass density estimates derived from accelerometers onboard satellites such as CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) are crucial in gaining insight into open science questions about the dynamic coupling between space weather events and the upper atmosphere. Recent advances in physics‐based satellite drag coefficient modeling allow derivation of new density data sets. This paper uses physics‐based satellite drag coefficient models for CHAMP and GRACE to derive new estimates for the neutral atmospheric density. Results show an average difference of 14–18% for CHAMP and 10–24% for GRACE between the new and existing data sets depending on the space weather conditions (i.e., solar and geomagnetic activity levels). The newly derived densities are also compared with existing models, and results are presented. These densities are expected to be useful to the wider scientific community for validating the development of physics‐based models and helping to answer open scientific questions regarding our understanding of upper atmosphere dynamics such as the sensitivity of temporal and global density variations to solar and geomagnetic forcing.

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