Atmospheric gravity waves due to the Tohoku-Oki tsunami observed in the thermosphere by GOCE

International audienceOceanic tsunami waves couple with atmospheric gravity waves, as previously observedthrough ionospheric and airglow perturbations. Aerodynamic velocities and density variations are computed from Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) accelerometer and thruster data during Tohoku-Oki tsunami propagation. High-frequency perturbations of these parameters are observed during three expected crossings of the tsunami-generated gravity waves by the GOCE satellite. From theoretical relations between air density and vertical and horizontal velocities inside the gravity wave, we demonstrate that the measured perturbations are consistent with a gravity wave generated by the tsunami and provide a way to estimate the propagation azimuth of the gravity wave. Moreover, because GOCE measurements can constrain the wave polarization, a marker (noted C 3 ) of any gravity wave crossing by the GOCE satellite is constructed from correlation coefficients between the observed atmospheric state parameters. These observations validate a new observation tool of thermospheric gravity waves generated by tsunamis above the open ocean

Medium-scale gravity wave activity in the thermosphere inferred from GOCE data

This study is focused on the effect of solar flux conditions on the dynamics of gravity waves (GWs) in the thermosphere. Air density and crosswind in situ estimates from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) accelerometers are analyzed for the whole mission duration. The analysis is performed in the Fourier spectral domain averaging spectral results over periods of 2 months close to solstices. A new GW marker (called C3f) is introduced here to characterize GWs activity under low, medium, and high solar flux conditions, showing a clear solar damping effect on GW activity. Most GW signal is found in a spectral range above 8 mHz in GOCE data, meaning a maximum horizontal wavelength of around 1000 km. The level of GW activity at GOCE altitude is strongly decreasing with increasing solar flux. Furthermore, a shift in the dominant frequency with solar flux conditions has been noted, leading to larger horizontal wavelengths (from 200 to 500 km) during high solar flux conditions. The correlation between air density variability and GW marker allows to identify most of the large-amplitude perturbations below 67∘ latitudes as due to GWs. The influence of correlated error sources, between air density and crosswinds, is discussed. Consistency of the spectral domain results is verified in the time domain with a global mapping of high-frequency air density perturbations along the GOCE orbit. This analysis shows a clear dependence with geomagnetic latitude with strong perturbations at magnetic poles and an extension to lower latitudes favored by low solar activity conditions. These results are consistent with previous Challenging Minisatellite Payload (CHAMP) data analysis and with general circulation models

Seasonal dependence of northern high‐latitude upper thermospheric winds: A quiet time climatological study based on ground‐based and space‐based measurements

This paper investigates the large‐scale seasonal dependence of geomagnetically quiet time, northern high‐latitude F region thermospheric winds by combining extensive observations from eight ground‐based (optical remote sensing) and three space‐based (optical remote sensing and in situ) instruments. To provide a comprehensive picture of the wind morphology, data are assimilated into a seasonal empirical vector wind model as a function of season, latitude, and local time in magnetic coordinates. The model accurately represents the behavior of the constituent data sets. There is good general agreement among the various data sets, but there are some major offsets between GOCE and the other data sets, especially on the duskside. The assimilated wind patterns exhibit a strong and large duskside anticyclonic circulation cell, sharp latitudinal gradients in the duskside auroral zone, strong antisunward winds in the polar cap, and a weaker tendency toward a dawnside cyclonic circulation cell. The high‐latitude wind system shows a progressive intensification of wind patterns from winter to equinox to summer. The latitudinal extent of the duskside circulation cell does not depend strongly on season. Zonal winds show a mainly diurnal variation (two extrema) around polar and middle latitudes and semidiurnal variation (four extrema) at auroral latitudes; meridional winds are primarily diurnal at all high latitudes. The strength of zonal winds channeling through the auroral zone on the duskside is strongest in the summer season. The vorticity of the wind pattern increases from winter to summer, whereas divergence is maximum in equinox. In all three seasons, divergence is weaker than vorticity.Key PointsFirst ever investigation of the large‐scale seasonal dependence of northern high‐latitude upper thermospheric winds in magnetic coordinatesResults show progressive intensification of wind circulation from winter to equinox to summerThe vorticity increases from winter to summer. In all the seasons, the strongest divergences occur primarily in and above auroral latitudesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136373/1/jgra53329.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136373/2/jgra53329_am.pd

Swarm accelerometer data processing from raw accelerations to thermospheric neutral densities

The Swarm satellites were launched on November 22, 2013, and carry accelerometers and GPS receivers as part of their scientific payload. The GPS receivers do not only provide the position and time for the magnetic field measurements, but are also used for determining non-gravitational forces like drag and radiation pressure acting on the spacecraft. The accelerometers measure these forces directly, at much finer resolution than the GPS receivers, from which thermospheric neutral densities can be derived. Unfortunately, the acceleration measurements suffer from a variety of disturbances, the most prominent being slow temperature-induced bias variations and sudden bias changes. In this paper, we describe the new, improved four-stage processing that is applied for transforming the disturbed acceleration measurements into scientifically valuable thermospheric neutral densities. In the first stage, the sudden bias changes in the acceleration measurements are manually removed using a dedicated software tool. The second stage is the calibration of the accelerometer measurements against the non-gravitational accelerations derived from the GPS receiver, which includes the correction for the slow temperature-induced bias variations. The identification of validity periods for calibration and correction parameters is part of the second stage. In the third stage, the calibrated and corrected accelerations are merged with the non-gravitational accelerations derived from the observations of the GPS receiver by a weighted average in the spectral domain, where the weights depend on the frequency. The fourth stage consists of transforming the corrected and calibrated accelerations into thermospheric neutral densities. We present the first results of the processing of Swarm C acceleration measurements from June 2014 to May 2015. We started with Swarm C because its acceleration measurements contain much less disturbances than those of Swarm A and have a higher signal-to-noise ratio than those of Swarm B. The latter is caused by the higher altitude of Swarm B as well as larger noise in the acceleration measurements of Swarm B. We show the results of each processing stage, highlight the difficulties encountered, and comment on the quality of the thermospheric neutral density data set.Astrodynamics & Space Mission

How to reach a few percent level in determining the Lense-Thirring effect?

In this paper we discuss and compare a node-only LAGEOS-LAGEOS II combination and a node-only LAGEOS-LAGEOS II-Ajisai-Jason1 combination for the determination of the Lense-Thirring effect. The new combined EIGEN-CG01C Earth gravity model has been adopted. The second combination cancels the first three even zonal harmonics along with their secular variations but introduces the non-gravitational perturbations of Jason1. The first combination is less sensitive to the non-conservative forces but is sensitive to the secular variations of the uncancelled even zonal harmonics of low degree J4 and J6 whose impact grows linearly in time.Comment: Latex2e, 22 pag. 1 table, 2 figures, 45 references. Changes in the Abstract, Introduction and Conclusions. Discussion on the non-gravitational perturbations on Ajisai and on the impact of the secular rates of the even zonal harmonics added. EIGEN-CG01C CHAMP+GRACE+terrestrial gravimetry/altimetry Earth gravity model used. Reference adde

HL‐TWiM Empirical Model of High‐Latitude Upper Thermospheric Winds

We present an empirical model of thermospheric winds (High‐latitude Thermospheric Wind Model [HL‐TWiM]) that specifies F region high‐latitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HL‐TWiM represents the large‐scale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical high‐latitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 ground‐based stations (Fabry‐Perot Interferometers and Scanning Doppler Imaging Fabry‐Perot Interferometers) located at various northern and southern high latitudes and two space‐based instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HL‐TWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HL‐TWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HL‐TWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Model‐data intra‐annual comparisons in this study show semiannual oscillation‐like behavior of GOCE winds, rarely observed before in wind data.Key PointsWe developed a comprehensive empirical model of high‐latitude F region thermospheric winds (HL‐TWiM)Universal Time variations in high‐latitude winds are stronger in the Southern than Northern HemisphereHL‐TWiM provides a necessary benchmark for validating new high‐latitude wind observations and tuning first principal modelsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/1/jgra55363_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/2/jgra55363-sup-0001-Figure_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/3/jgra55363.pd

Plasma-neutral interactions in the lower thermosphere-ionosphere : The need for in situ measurements to address focused questions

The lower thermosphere-ionosphere (LTI) is a key transition region between Earth's atmosphere and space. Interactions between ions and neutrals maximize within the LTI and in particular at altitudes from 100 to 200 km, which is the least visited region of the near-Earth environment. The lack of in situ co-temporal and co-spatial measurements of all relevant parameters and their elusiveness to most remote-sensing methods means that the complex interactions between its neutral and charged constituents remain poorly characterized to this date. This lack of measurements, together with the ambiguity in the quantification of key processes in the 100-200 km altitude range affect current modeling efforts to expand atmospheric models upward to include the LTI and limit current space weather prediction capabilities. We present focused questions in the LTI that are related to the complex interactions between its neutral and charged constituents. These questions concern core physical processes that govern the energetics, dynamics, and chemistry of the LTI and need to be addressed as fundamental and long-standing questions in this critically unexplored boundary region. We also outline the range of in situ measurements that are needed to unambiguously quantify key LTI processes within this region, and present elements of an in situ concept based on past proposed mission concepts.Peer reviewe

Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

The lower-thermosphere-ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere, the solar wind, as well as the wind dynamo. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either in situ or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.Peer reviewe