Geomagnetic lunar and solar daily variations during the last 100 years

This paper describes long-term changes in the geomagnetic lunar (L) and solar (S) daily variations. We analyze the eastward component of the geomagnetic field observed at eight midlatitude stations during 1903–2012. The amplitude and phase for the semidiurnal component of the L and S variations are examined. Both L and S amplitudes correlate with the solar activity index F10.7, revealing a prominent 11 year solar cycle. In both cases, the correlation is slightly better with inline image than F10.7. The sensitivity of the L variation to solar activity is comparable with that of the S variation. The solar cycle effect is also found in the phase of the S variation but not apparent in the phase of the L variation. The ratio in the amplitude of the L to S variation shows a long-term decrease (approximately 10% per century), which may be due to a reduction in lunar tidal waves from the lower atmosphere to the upper atmosphere in association with climate change

Evidence for stratospheric sudden warming effects on the upper thermosphere derived from satellite orbital decay data during 1967–2013

We investigate possible impact of stratospheric sudden warmings (SSWs) on the thermosphere by using long-term data of the global average thermospheric total mass density derived from satellite orbital drag during 1967–2013. Residuals are analyzed between the data and empirical Global Average Mass Density Model (GAMDM) that takes into account density variability due to solar activity, season, geomagnetic activity, and long-term trend. A superposed epoch analysis of 37 SSW events reveals a density reduction of 3–7% at 250–575 km around the time of maximum polar vortex weakening. The relative density perturbation is found to be greater at higher altitudes. The temperature perturbation is estimated to be −7.0 K at 400 km. We show that the density reduction can arise from enhanced wave forcing from the lower atmosphere

Linkages between the cold summer mesopause and thermospheric zonal mean circulation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95392/1/grl28827.pd

A Year‐Long Comparison of GPS TEC and Global Ionosphere‐Thermosphere Models

The prevalence of GPS total electron content (TEC) observations has provided an opportunity for extensive global ionosphere‐thermosphere model validation efforts. This study presents a year‐long data‐model comparison using the Global Ionosphere‐Thermosphere Model (GITM) and the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM). For the entire year of 2010, each model was run and compared to GPS TEC observations. The results were binned according to season, latitude, local time, and magnetic local time. GITM was found to overestimate the TEC everywhere, except on the midlatitude nightside, due to high O/N2 ratios. TIE‐GCM produced much less TEC and had lower O/N2 ratios and neutral wind speeds. Seasonal and regional biases in the models are discussed along with ideas for model improvements and further validation efforts.Key PointsTIE‐GCM generally underpredicted GPS TEC, while GITM overpredicted itTIE‐GCM GPS TEC predictions were most accurate near the poles; GITM compared best in the Northern HemisphereBoth TIE‐GCM and GITM have model descriptions which may be improved for better TEC comparisonsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142881/1/jgra54053.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142881/2/jgra54053_am.pd

Oscillation of the Ionosphere at Planetary-Wave Periods

F‐region ionospheric oscillations at planetary‐wave (PW) periods (2–20 days) are investigated, with primary focus on those oscillations transmitted to the ionosphere by PW modulation of the vertically propagating tidal spectrum. Tidal effects are isolated by specifically designed numerical experiments performed with the National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model for October 2009, when familiar PW and tides are present in the model. Longitude versus day‐of‐month perturbations in topside F‐region electron density (Ne) of order ±30–50% at PW periods occur as a result of PW‐modulated tides. At a given height, these oscillations are mainly due to vertical oscillations in the F layer of order ±15–40 km. These vertical movements are diagnosed in terms of changes in the F2‐layer peak height, ΔhmF2, which are driven by the vertical projections of E × B drifts and field‐aligned in situ neutral winds. E × B drifts dominate at the magnetic equator, while the two sources play more equal roles between 20° and 40° magnetic latitudes in each hemisphere. The in situ neutral wind effect arises from vertical propagation of PW‐modulated tides, whereas the E × B drifts originate from dynamo‐generated electric fields produced by the E‐region component of the same wind field; the former represents a new coupling mechanism for production of ionospheric oscillations at PW periods. Roughly half the above‐mentioned variability in Ne and hmF2 is associated with zonally symmetric (S0) oscillations, which contribute at about half the level of low‐level magnetic activity during October 2009. The thermosphere‐ionosphere‐electrodynamics general circulation model simulates the S0 oscillations in Ne observed from the CHAMP satellite well during this period and reveals that S0 oscillations in E × B play a significant role in driving S0 oscillations in ΔhmF2, in addition to neutral winds

Energy transport in the thermosphere during the solar storms of April 2002

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94816/1/jgra17969.pd