A. E. Hedin

The new NRLMSISE‐00 empirical atmospheric model extends from the ground to the exobase and is a major upgrade of the MSISE‐90 model in the thermosphere. The new model and the associated NRLMSIS database now include the following data: (1) total mass density from satellite accelerometers and from orbit determination (including the Jacchia and Barlier data sets), (2) temperature from incoherent scatter radar covering 1981–1997, and (3) molecular oxygen number density, [O 2 ], from solar ultraviolet occultation aboard the Solar Maximum Mission. A new component, “anomalous oxygen,” allows for appreciable O + and hot atomic oxygen contributions to the total mass density at high altitudes and applies primarily to drag estimation above 500 km. Extensive tables compare our entire database to the NRLMSISE‐00, MSISE‐90, and Jacchia‐70 models for different altitude bands and levels of geomagnetic activity. We also explore scientific issues related to the new data sets in the NRLMSIS database. Especially noteworthy is the solar activity dependence of the Jacchia data, with which we study a large O + contribution to the total mass density under the combination of summer, low solar activity, high latitude, and high altitude. Under these conditions, except at very low solar activity, the Jacchia data and the Jacchia‐70 model indeed show a significantly higher total mass density than does MSISE‐90. However, under the corresponding winter conditions, the MSIS‐class models represent a noticeable improvement relative to Jacchia‐70 over a wide range of F 10.7 . Considering the two regimes together, NRLMSISE‐00 achieves an improvement over both MSISE‐90 and Jacchia‐70 by incorporating advantages of each.

The MSIS‐86 empirical model has been revised in the lower thermosphere and extended into the mesosphere and lower atmosphere to provide a single analytic model for calculating temperature and density profiles representative of the climatological average for various geophysical conditions. Tabulations from the Handbook for MAP 16 are the primary guide for the lower atmosphere and are supplemented by historical rocket and incoherent scatter data in the upper mesosphere and lower thermosphere. Low‐order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, and simplified local time and longitude variations. While month to month details cannot be completely represented, lower atmosphere temperature data are fit to an overall standard deviation of 3 K and pressure to 2%. Comparison with rocket and other data indicates that the model represents current knowledge of the climatological average reasonably well, although there is some conflict as to details near the mesopause.

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground‐based incoherent scatter radar and Fabry‐Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground‐based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately 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 are found to extend to 300 km at some local times and pose complications for interpretation of Fabry‐Perot observations.

The MSIS‐83 empirical model of temperature, density and composition is a revision of the original MSIS model (including longitude/UT effects) and is based on temperature, density, and composition data from a comprehensive summary of rocket flights, seven satellites, and five incoherent scatter radars, including data from high solar activity. The model extends the previous description of neutral parameters to the base of the thermosphere in a continuous manner while maintaining the basic structure of the MSIS model at higher altitudes. As the altitude decreases, composition approaches lower atmosphere values, while yearly, and to a lesser extent daily, variations in temperature and density are in reasonable agreement with previous results for the lower thermosphere. An alternate description of magnetic storm variations is provided based on the three hour ap indices and an 8‐ to 10‐hour exponential decay in thermospheric density and temperature response after a heating event. Additional coefficients are included for the time independent and magnetic activity terms, including a longitudinally dependent seasonal magnetic activity effect. The molecular oxygen description is based on mass spectrometer and EUV absorption measurements rather than ion chemistry.