K. Häusler

Effect of subauroral polarization streams on the thermosphere: A statistical study

Using 2 years of coordinated CHAMP and DMSP observations we have investigated for the first time the relationship between subauroral polarization streams (SAPS), ionospheric Hall current (electrojet), upper thermospheric zonal wind, and mass density at subauroral regions in the dusk and premidnight sectors, separately for both hemispheres. For comparison, we have also analyzed the same parameters as a function of magnetic latitude (30 degrees-80 degrees magnetic latitude) during non-SAPS periods. During periods of non-SAPS, the neutral wind exhibits similar features as during SAPS events in the dusk to premidnight sector, streaming westward in the same direction as the plasma drift. Both neutral and plasma velocities peak at the same latitude regardless of SAPS occurrence. For higher geomagnetic activity both velocities are faster and the peaks shift equatorward. During non-SAPS periods, the ratio between plasma and neutral wind velocity is on average 2.75 +/- 0.4 in both hemispheres irrespective of geomagnetic activity. The neutral wind during SAPS events gets enhanced by a factor of 1.5/1.2 for Kp = 4 in the Northern/Southern Hemisphere, respectively, as compared to non-SAPS time. The velocity difference between SAPS and neutral wind is also larger during SAPS period than during non-SAPS period, and the difference tends to increase with increasing geomagnetic activity. The peak latitude of the eastward auroral electrojet appears 1.5 degrees poleward of the plasma drift during SAPS events, confirming the formation of SAPS equatorward of the high-conductivity channel. These SAPS-induced large winds can heat the upper thermosphere. As a result we observe a 10% enhanced mass density at 400 km altitude with respect to periods without SAPS. In addition a density anomaly peak occurs collocated with the SAPS, displaced from the electrojet peak. We regard this as an indication for efficient thermospheric heating by ion neutral friction.

Global ionospheric and thermospheric response to the 5 April 2010 geomagnetic storm: An integrated data-model investigation

We present a case study of the 5 April 2010 geomagnetic storm using observations and numerical simulations. The event was driven by a fast-moving coronal mass ejection and despite being a moderate storm with a minimum Dst near −50 nT, the event exhibited elevated thermospheric density and surges of traveling atmospheric disturbances (TADs) more typically seen during major storms. The Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIMEGCM) was used to assess how these features were generated and developed during the storm. The model simulations gave rise to TADs that were highly nonuniform with strong latitude and longitude/local time dependence. The TAD phase speeds ranged from 640 m/s to 780 m/s at 400 km and were ~5% lower at 300 km and approximately 10–15% lower at 200 km. In the lower thermosphere around 100 km, the TAD signatures were nearly unrecognizable due to much stronger influence of upward propagating atmospheric tides. The thermosphere simulation results were compared to observations available from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE), CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites. Comparison with GOCE data shows that the TIMEGCM reproduced the cross-track winds over the polar region very well. The model-data comparison also revealed some differences, specifically, the simulations underestimated neutral mass density in the upper thermosphere above ~300 km and overestimated the storm recovery tome by 6 h. These discrepancies indicate that some heating or circulation dynamics and potentially cooling processes are not fully represented in the simulations, and also that updates to some parameterization schemes in the TIMEGCM are warranted.

Improved short-term variability in the thermosphere-ionosphere-mesosphere-electrodynamics general circulation model

We report on a new source of tidal variability in the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM). Lower boundary forcing of the TIME-GCM for a simulation of November–December 2009 based on 3-hourly Modern-Era Retrospective Analysis for Research and Application (MERRA) reanalysis data includes day-to-day variations in both diurnal and semidiurnal tides of tropospheric origin. Comparison with TIME-GCM results from a heretofore standard simulation that includes climatological tropospheric tides from the global-scale wave model reveal evidence of the impacts of MERRA forcing throughout the model domain, including measurable tidal variability in the TIME-GCM upper thermosphere. Additional comparisons with measurements made by the Gravity field and steady-state Ocean Circulation Explorer satellite show improved TIME-GCM capability to capture day-to-day variations in thermospheric density for the November–December 2009 period with the new MERRA lower boundary forcing.

Upper thermospheric responses to forcing from above and below during 1–10 April 2010: Results from an ensemble of numerical simulations

In this report we examine the spatial and temporal variability of the quiescent thermosphere leading up to and after the 5 April 2010 geomagnetic disturbance. We attribute the dominant driver of this variability to a combination of tides generated in situ and in the troposphere, stratosphere, and mesosphere. We identify nonmigrating tidal signatures attributable to the latter source that are ubiquitous, persistent, and significant at all thermospheric latitudes. Further, these perturbations underlie the upper atmospheric response to solar geomagnetic disturbances and are measurably altered, along with their migrating counterparts, during the storm. Our investigation is centered on a series of National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model simulations during 1–10 April 2010, including an optimal simulation with lower boundary forcing based on Modern-Era Retrospective Analysis for Research and Application reanalysis data and upper boundary forcing based on satellite and ground magnetometer measurements and the Assimilative Mapping of Ionospheric Electrodynamics procedure and three diagnostic simulations. The differences between the optimal and diagnostic simulations allow us to quantify thermospheric variability attributable to solar geomagnetic forcing and dynamical effects propagating into the thermosphere from below. We find that they can be comparable at high latitudes for some nonmigrating tidal components.

Day‐to‐day variability of midlatitude ionospheric currents due to magnetospheric and lower atmospheric forcing

As known from previous studies on the solar quiet (Sq) variation of the geomagnetic field, the strength and pattern of ionospheric dynamo currents change significantly from day to day. The present study investigates the relative importance of two sources that contribute to the day-to-day variability of the ionospheric currents at middle and low latitudes. One is high-latitude electric fields that are caused by magnetospheric convection, and the other is atmospheric waves from the lower atmosphere. Global ionospheric current systems, commonly known as Sq current systems, are simulated using the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model. Simulations are run for 1–30 April 2010 with a constant solar energy input but with various combinations of high-latitude forcing and lower atmospheric forcing. The model well reproduces geomagnetic perturbations on the ground, when both forcings are taken into account. The contribution of high-latitude forcing to the total Sq current intensity (Jtotal) is generally smaller than the contribution of wave forcing from below 30 km, except during active periods (Kp≥4), when Jtotal is enhanced due to the leakage of high-latitude electric fields to lower latitudes. It is found that the penetration electric field drives ionospheric currents at middle and low latitudes not only on the dayside but also on the nightside, which has an appreciable effect on the Dst index. It is also found that quiet time day-to-day variability in Jtotal is dominated by symmetric-mode migrating diurnal and semidiurnal tidal winds at 45–60° latitude at ∼110 km.

The Geospace Response to Nonmigrating Tides

During the past five years, significant progress has been made in elucidating and delineating the geospace response to nonmigrating tides from the lower atmosphere. Satellite missions providing continuously and globally distributed measurements of the atmospheric parameters revealed astonishing findings not anticipated before. Special emphasis is put on the eastward propagating diurnal tide with zonal wavenumber 3 (DE3) which manifests itself not only in the neutral atmosphere but also in the ionosphere. The DE3 tide can be traced from its origin in the troposphere to its maximum in the mesosphere, lower thermosphere (MLT) region up to an altitude of 400 km. Thereby Hough Mode Extension (HME) modeling aids to bridge the data gap between satellite measurements performed in the MLT region and upper thermosphere.

Comparison of drift velocities of nighttime equatorial plasma depletions with ambient plasma drifts and thermospheric neutral winds

This is the first study to compare plasma depletion drifts with the ambient plasma drifts and neutral winds in the post sunset equatorial ionosphere using global-scale satellite observations. The local time and latitude variations of the drift velocities of O+ plasma depletions at 350–400?km altitude are derived from the observations of the far ultraviolet imager operated on the IMAGE satellite during 10 March to 7 June 2002. These depletion drift velocities are compared with the simultaneously measured ion drift velocities and neutral winds by the ROCSAT-1 and the CHAMP satellites for a similar time period. The analysis shows that the zonal drift velocity of plasma depletions is smaller than both the ambient ion zonal drift velocity and the neutral zonal wind at 18:00–20:00 magnetic local time, and after 21:00, the variations of these velocities are similar. The difference of the plasma depletion drift with the background is found to be smaller at lower latitudes. Furthermore, the zonal drift velocity of the depletion is found to have a large latitudinal gradient specifically at 12°–18° magnetic latitude, which again does not match the ambient ion drift and the neutral wind. This latitudinal difference has been reported by previous studies, but those studies use models and they only compare the depletion drifts with the modeled neutral winds. This study provides a measure of the difference that has never been studied before by any study using global observations. It has been suggested that polarization electric fields inside the plasma depletion structure drive the plasma to drift westward and thus the depletion structure moves to the east. The latitudinal gradient of the depletion drift velocity seen here in this study could also be explained by the polarization electric fields. For the C-shaped (reversed C) depletion, the polarization electric fields inside the depletion drive a westward drift of plasma and this drift velocity changes with increasing latitude. Consequently, the depletion drift has a latitudinal gradient becoming significant at higher latitudes.

Longitudinal Variations of the Thermospheric Zonal Wind Induced by Nonmigrating Tides as Observed by CHAMP

In July 2000 the very successful German mini-satellite mission CHAMP, an acronym for Challenging Minisatellite Payload, was launched. One of the scientific instruments on board is an accelerometer that allows us to derive the zonal wind at CHAMP’s altitude (~ 400 km). Previous to its launch, continuous and globally distributed measurements of the upper thermospheric wind have been rather sparse. With the launch of the CHAMP satellite we are now able to investigate the upper thermospheric zonal wind, and in particular its longitudinal variability, in a climatological sense. This capability has led to exciting and unanticipated findings such as the coupling from the troposphere to the thermosphere via nonmigrating tides. In this chapter we talk about the longitudinal variations of the CHAMP zonal wind at equatorial latitudes. Further, we present the nonmigrating tidal spectra embedded in the CHAMP zonal wind with special emphasis on the eastward propagating diurnal tide with zonal wavenumber 3 (DE3).