Eric K. Sutton

Density and Winds in the Thermosphere Deduced from Accelerometer Data

With the emergence and increased use of highly accurate accelerometers for geodetic satellite missions, a new opportunity has arisen to study nonconservative forces acting on a number of satellites with high temporal resolution. As the number of these satellite missions increases, so does our ability to determine the spatial characteristics and time response of total density and winds in the thermosphere. This paper focuses on the derivation and methodology of inferring density and winds from along-track and cross-track accelerometer measurements, with themain goal of determining the feasibility of this data set. The principal sources of error such as solar radiation pressure, the unknown coefficients of drag and lift, instrument precision and biases, and unaccounted-for winds are discussed in the context of both density and winds. In the context of our treatment of errors, density errors are generally less than 15%, whereas wind-speed errors are more substantial. Finally, comparisons of results to existing empirical models (i.e., horizontal wind model 93) and to self-consistent numerical models (i.e., thermosphere–ionosphere electrodynamic general circulation model) are provided. Comparisons of results to ion drift velocities (as measured by Defense Meteorological Satellite Program) are also provided.

Annual and semiannual variations of thermospheric density: EOF analysis of CHAMP and GRACE data

[1] In this paper, observations from CHAMP and GRACE during 2002–2010 are used to study the seasonal variations of thermospheric density by characterizing the dominant modes of thermospheric density variability as empirical orthogonal functions (EOFs). Our results showed that the first three EOFs captured most of the density variability, which can be as large as 98% of total density variability. Subsequently, the obtained mean field, first three EOFs and the corresponding amplitudes of three EOFs are applied to construct a thermospheric density model at 400 km to study seasonal variations of thermospheric density under geomagnetically quiet conditions. Thermospheric density shows strong latitudinal dependence in seasonal variation, although it usually has maxima near the equinoxes and minimum in the local winter at middle and high latitudes. Semiannual variations imbedded in the annual variations are seen at all latitudes; annual variations however become dominant in the southern hemisphere. Specifically, the observations show that the annual amplitude can reach as large as 40–50% of the annual mean at high latitudes in the southern hemisphere and it decreases gradually from the southern to northern hemisphere. The semiannual component to the annual mean is about 15–20% without significant latitudinal dependence. Additionally, the relative amplitudes of annual and semiannual variations in the MSISE00 density agree fairly well with the observations, albeit the MSISE00 gives an opposite solar activity dependence for the annual and semiannual variations compared with the positive F107 dependence seen in the observations.

Energy coupling during the August 2011 magnetic storm

Abstract : We present results from an analysis of high-latitude ionosphere-thermosphere (IT) coupling to the solar wind during a moderate magnetic storm which occurred on 5 6 August 2011. During the storm, a multipoint set of observations of the ionosphere and thermosphere was available. We make use of ionospheric measurements of electromagnetic and particle energy made by the Defense Meteorological Satellite Program and neutral densities measured by the Gravity Recovery and Climate Experiment satellite to infer (1) the energy budget and (2) timing of the energy transfer process during the storm. We conclude that the primary location for energy input to the IT system may be the extremely high latitude region. We suggest that the total energy available to the IT system is not completely captured either by observation or empirical models.

Interannual and latitudinal variability of the thermosphere density annual harmonics

[1] In this paper we investigate the intra-annual variation in thermosphere neutral density near 400 km using 4 years (2002–2005) of CHAMP measurements. The intra-annual variation, commonly referred to as the ‘‘semiannual variation,’’ is characterized by significant latitude structure, hemispheric asymmetries, and interannual variability. The magnitude of the maximum yearly difference, from the yearly minimum to the yearly maximum, varies by as much as 60% from year to year, and the phases of the minima and maxima also change by 20–40 days from year to year. Each annual harmonic of the intraannual variation, namely, annual, semiannual, terannual and quatra-annual, exhibits a decreasing trend from 2002 through 2005 that is correlated with the decline in solar activity. In addition, some variations in these harmonics are correlated with geomagnetic activity, as represented by the daily mean value of Kp. Recent empirical models of the thermosphere are found to be deficient in capturing most of the latitude dependencies discovered in our data. In addition, the solar flux and geomagnetic activity proxies that we have employed do not capture some latitude and interannual variations detected in our data. It is possible that these variations are partly due to other effects, such as seasonallatitudinal variations in turbopause altitude (and hence O/N2 composition) and ionosphere coupling processes that remain to be discovered in the context of influencing the intraannual variations depicted here. Our results provide a new data set to challenge and validate thermosphere-ionosphere general circulation models that seek to delineate the thermosphere intra-annual variation and to understand the various competing mechanisms that may contribute to its existence and variability. We furthermore suggest that the term ‘‘intra-annual’’ variation be adopted to describe the variability in thermosphere and ionosphere parameters that is well-captured through a superposition of annual, semiannual, terannual, and quatra-annual harmonic terms, and that ‘‘semiannual’’ be used strictly in reference to a pure 6-monthly sinusoidal variation. Moreover, we propose the term ‘‘intraseasonal’’ to refer to those shorter-term variations that arise as residuals from the above Fourier representation.

The effect of periodic variations of thermospheric density on CHAMP and GRACE orbits

[1] In this paper thermosphere densities observed by the CHAMP and GRACE satellites and their orbital parameters are used to investigate the effect of periodic oscillations in thermospheric densities (7–27 days) caused by solar rotation and periodic magnetic activity on satellite orbits during 2003–2005. Two new results are obtained in this study. First, the response of the mean radius of the satellite orbit per revolution (MRPR) to the oscillations in the mean atmospheric density per revolution (MDPR) increased linearly with oscillation periods. Therefore, MRPR had a strong oscillation near the 27 day period. However, it had no obvious 7, 9, and 13.5 day oscillations, although there were strong oscillations at the same periods in MDPR. Second, there was a phase difference of � =2 between the oscillations of MRPR and MDPR. The phases of the oscillations in MRPR led the phases of the variations in MDPR. The correlation coefficient between the 27 day oscillations in MRPR and those in MDPR was 0.83 with a phase difference of −6.8 days for CHAMP; the correlation for GRACE was 0.67 with a phase difference of −6.4 days. The amplitudes of the oscillations in MRPR of CHAMP were larger than those of GRACE because GRACE had a higher orbit than CHAMP. These features are in good agreement with our theoretical analysis.

Ionospheric tomography using the residual correction method

Ionospheric tomography systems provide data that can be used to reconstruct images of ionospheric electron density. Since ionospheric tomography systems have fundamentally poor vertical resolution, a priori information on the vertical distribution of ionospheric electron density must be used in the reconstruction algorithm. This can be accomplished using an existing technique called orthogonal decomposition. However, the poor vertical resolution of the imaging system makes the reconstruction problem numerically ill conditioned. This paper presents a new ionospheric reconstruction algorithm called the residual correction method (RCM). The RCM is a fast, efficient, and numerically stable ionospheric tomography algorithm. This paper will present results demonstrating the performance of the RCM algorithm using a realistic example.

Altitude variations in the thermosphere mass density response to geomagnetic activity during the recent solar minimum

Accelerometer data from coplanar orbits of Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites were used to study the complex altitude and latitude variations of the thermosphere mass density response to geomagnetic activity during 1–10 December 2008 near 09 LT. Helium number densities near 500 km altitude were extracted from the CHAMP and GRACE measurements and clearly show the presence of a winter hemisphere helium bulge. This recent extreme solar minimum indicates that wintertime helium concentrations exceed NRLMSISE-00 model estimates by 30%–70% during quiet geomagnetic activity after adjusting F10.7 input into MSIS. The perturbation in mass density from quiet to active conditions is found to be less enhanced in the winter hemisphere at the higher GRACE altitudes (25%) than at the lower CHAMP altitudes (60%) and is attributed to dynamic behavior in the helium/oxygen transition. The investigation revealed the maximum storm time density perturbation to occur near the He/O transition region with a much weaker maximum near the O/N2 transition region. The altitude of maximum density perturbation occurs where the perturbation in the weighted pressure scale height is equal and opposite to the perturbation in the weighted mean molecular weight scale height. The altitude structure of density scale height perturbation is significantly influenced by the changes in the molecular weight scale height and can account for 50% of the change in mass density scale height in a region correspondingly close to the He/O transition during the 2008 solar minimum period.

A self-consistent model of helium in the thermosphere

We have found that consideration of neutral helium as a major species leads to a more complete physics-based modeling description of the Earths upper thermosphere. An augmented version of the composition equation employed by the Thermosphere-Ionosphere-Electrodynamic General Circulation Model (TIE-GCM) is presented, enabling the inclusion of helium as the fourth major neutral constituent. Exospheric transport acting above the upper boundary of the model is considered, further improving the local time and latitudinal distributions of helium. The new model successfully simulates a previously observed phenomenon known as the “winter helium bulge,” yielding behavior very similar to that of an empirical model based on mass spectrometer observations. This inclusion has direct consequence on the study of atmospheric drag for low-Earth-orbiting satellites, as well as potential implications on exospheric and topside ionospheric research.

New density estimates derived using accelerometers on board the CHAMP and GRACE satellites

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.

Intercalibration of neutral density measurements for mapping the thermosphere

This paper describes a technique for mapping exospheric temperatures, derived from neutral density measurements from the Challenging Mini-satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites. The Naval Reasearch Laboratory Mass Spectrometer, Incoherent Scatter Radar Extended Model (NRLMSISE-00) thermosphere model is used for the conversion. Adjustments for each satellite were needed in order for the time-averaged densities to agree with the model. It was necessary to correct for inexact modeling of the annual and semiannual oscillations in the density, as well as the declining densities during the solar minimum. It was found that a time-varying perturbation in the atomic oxygen in the model could produce a good agreement at both altitudes. The time series of this oxygen variation was found to have a very high correlation with independent measurements of CO2 emissions measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. The temperature data are averaged on a spherical grid having equal areas in each pixel, avoiding functional fits that would blur finer details. The use of solar magnetic rather than geographic coordinates enhances the auroral ovals. There are strong elevations in the exospheric temperatures in the polar regions, particularly near the dayside cusp. Spatial filtering with spherical wavelets is used to remove statistical fluctuations, although some details are lost. The exospheric temperature maps are well ordered by the nitric oxide emission measurements from SABER. The technique that is described here could be applied to future improvements of empirical density models, having an accuracy and spatial resolution that is not presently available.