|Table of Contents||ECSS||Model Page|
|Background Information||Background Information|
|Atmosphere and ionosphere models|
The MSIS models use a Bates-Walker temperature profile as a function of geopotential height for the upper thermosphere and an inverse polynomial in geopotential height for the lower thermosphere. Exopsheric temperature and other atmospheric quantities are expressed as functions of geographical and solar/magnetic parameters. The temperature profiles allow for exact integration of the hydrostatic equation for a constant mass to determine the density profile based on a density specified at 120 km as a function of geographic and solar/magnetic parameters.
The model version implemented in SPENVIS is NRLMSISE-00. The MSIS software can be obtained at the Naval Research Lab or at NSSDC.
The essence of the MET-V 2.0 model is the calculation of atmospheric density in two major regions: the lower thermosphere (altitudes between 90 km and 105 km) and the upper thermosphere (above 105 km). Between the base of the thermosphere (assumed to be at 90 km) and 105 km, turbulent mixing is assumed to predominate, and diffusion dominates at higher altitudes. The density for all points on the globe at 90 km altitude is assumed constant and mixing of atmospheric constituents prevails to 105 km. Between these two altitudes, the mean molecular mass varies as a result of dissociation of O2 to O. An empirical process is employed in the determination of the mean molecular mass distribution between 90 and 105 km, such that the ratio of O to O2 is 1.5 at 120 km.
The input parameters required by the model are altitude, latitude, longitude, date (month, day, and year), time (hour and minute), 3 hourly geomagnetic index (linear or logarithmic), and the daily 10.7 cm solar radio flux and its average over six solar rotations referenced to the midpoint.
The model version implemented in SPENVIS is MET-V 2.0. The MET-V 2.0 computer program is available upon request to Jerry K. Owens at Jerry.K.Owens@nasa.gov.
Since satellite drag data lead to total density in the vicinity of the perigee, some assumptions must be introduced to obtain concentrations for each atmospheric constituent. The construction of DTM is based on the fact that molecular nitrogen, atomic oxygen and helium are successively the major atmospheric constituents above 120 km as a consequence of diffusion separation in the Earth's gravitational field. Barlier et al.  used an iterative procedure to represent the three major constituents N2, O and He in terms of spherical harmonics at 120 km altitude. Using a thermopause temperature model and an analytical temperature profile it then is possible to compute concentrations for the major atmospheric constituents at a given altitude as a function of local solar time, latitude, day number in the year, solar decimetric flux F10.7 and geomagnetic index Kp. The detailed formalism is given by Barlier et al. .
Solar cycle variations are included (since HWM90), but they are found to be small and not always very clearly delineated by the current data. Variations with magnetic activity index (Ap) are included. Mid- and low-latitude data are reproduced quite well by the model. The polar vortices are present, but not to full detail. The model describes the transition from predominantly diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind extend up to 300 km at some local times.
The model software provides zonal and meridional winds for specified latitude, longitude, time, and Ap index. A comparison of the HWM values with winds derived from IRI parameters and from ionosonde measurements have shown in general good agreement [Miller et al., 1990]. The model version implemented in SPENVIS is HWM93.
The HWM93 software can be obtained at NSSDC.
IRI is updated yearly during special IRI Workshops (e.g., during COSPAR general assembly). Several extensions are planned, including models for the ion drift, description of the auroral and polar ionosphere, and consideration of magnetic storm effects. The model version implemented in SPENVIS is IRI2001 [Bilitza, 2001].
The IRI software can be obtained at NSSDC.
Following the increasing amount of available data, the model formulation has been updated in order to improve its capabilities to provide representations of the ionosphere at global scales. In particular, major changes have been introduced in the model topside and bottomside formulation. In addition, specific revisions have been applied to the computer package associated to NeQuick in order to improve its computational efficiency. As a result, a new version of the model, NeQuick v2.0, has been made available [Nava, Coïsson & Radicella, 2008].
The model version implemented in SPENVIS is NeQuick v2.0. The original NeQuick source code can be obtained at the International Telecommunication Union, Radiocommunication Sector ( ITU-R).
Barlier, F., C. Berger, J. L. Falin, G. Kockarts, and G. Thuillier, J. Atmosph. Terr. Phys., 41, 527-541, 1979.
Hedin, A. E., A Revised Thermospheric Model Based on Mass Spectrometer and Incoherent Scatter Data: MSIS-83, J. Geophys. Res., 88, 10170, 1983.
Hedin, A. E., MSIS-86 Thermospheric Model, J. Geophys. Res., 92, 4649, 1987.
Hedin, A. E., Extension of the MSIS Thermosphere Model into the Middle and Lower Atmosphere, J. Geophys. Res., 96, 1159, 1991.
Hedin, A. E., N. W. Spencer, and T. L. Killeen, Empirical Global Model of Upper Thermosphere Winds Based on Atmosphere and Dynamics Explorer Satellite Data, J. Geophys. Res., 93, 9959-9978, 1988.
Hedin, A. E., et al., Revised Global Model of Thermosphere Winds Using Satellite and Ground-Based Observations, J. Geophys. Res., 96, 7657-7688, 1991.
Miller, K. L., A. E. Hedin, P. J. Wilkinson, D. G. Torr, and P. G. Richards, Neutral Winds Derived from IRI Parameters and from the HWM87 Wind Model for the SUNDIAL Campaign of September, 1986, Adv. Space Res., 10, 99, 1990.
Nava, B., Coïsson, P. & Radicella, S. M., A new version of the NeQuick ionosphere electron density model, Journal of Atmospheric and Solar-Terrestrial Physics, 70, 1856-1862, 2008.
Owens, J. K., NASA Marshall Engineering Thermosphere Model-Version 2.0, NASA/TM—2002-211786, 2002.