Roble and Emery (1983) The Thermosphere’s primary heat sources are solar EUV absorption, Joule heating, and auroral particle heating

electron and Ion energy equation ionosphere with O + diffusion: O 2 + N 2 + NO + N + in PCE* major species diffusion O 2 O and N 2 minor species diffusion N( 2 D) N( 4 S) NO neutral gas energy equation Solar UV & EUV heatingIon/neutralchemistry heatingOrecombination photoelectron heating photoionization O 2 + h  O + O heating electron/ion/neutral collisions heating neutral-neutral chemistry N 2 + h  N + N nnnnnn nininini    n    n n TnTnTnTn nnnnnnnn * photochemical equilibrium

Polar Energy Input to Thermospheric Heating Electromagnetic Energy Flux Kinetic Energy Flux Neutral Momentum Neutral Heating  Chemical Reactions Cascading e – Excitation Ionization TeTe TiTi   h

Simulations of Joule Heating and Thermosphere Density The global mean density variation computed from TIMEGCM for about 18 days in November 2004 (black line), together with the Joule heating derived from TIMEGCM (red line). A major storm on days , 2004 produced large density enhancements. Courtesy G. Crowley

SATELLITE DRAG AND THE EVOLUTION OF MODEL ATMOSPHERES The aerodynamic force is the force created by a spacecraft’s movement through a neutral density atmosphere. The force results from momentum exchange between the atmosphere and the spacecraft and can be decomposed into components of lift, drag, and side slip. The drag force is considered the most dominant force on low-earth orbiting spacecraft and serves to change the energy of the spacecraft through the work done by the drag force. This alters the period and semimajor axis of a spacecraft over time.

Through Kepler's laws, one can derive the rate of change of orbital period ( T ) in terms of the atmospheric density: where B = B-factor (ballistic coefficient) =  P =density at perigee  =density s =satellite path From radar tracking, one can derive the atmospheric density (the more accurate the tracking, the shorter time required to determine density). Typical resolution is about 1 day below 200 km and 5 days at 500 km. (Much better with laser beacon, etc.) Atmospheric Density determined by tracking the change of the orbital period

The above procedure requires some knowledge about the variation of density with height. In practical applications of this method, if a reasonable initial first guess of the vertical structure is provided, an iterative procedure leads to an accurate determination that is independent of the initial guess. Given the equations defining the hydrostatic law, the chemical composition at some height, and an expression for the temperature profile shape (example: Bates Temperature Model), a density profile can be retrieved from the satellite observations. A set of "exospheric temperatures” emerges from this process.

Analyses of many satellite orbits led to the so-called "static diffusion models" developed by Jacchia and which form the basis of many operational drag models. These are based on a parameteric dependence of density on exospheric temperature. The derived temperature is more of a 'virtual' temperature than 'real' (kinetic) temperature Rocket measurements of O, O 2 at the lower boundary are difficult to interpret (i.e., O recombines into O 2 against walls of measuring device, meaning that O can be underestimated and O 2 overestimated). Wind-induced diffusion is also important, i.e., for [O]; The 'static diffusion' or 'hydrostatic' restriction is not amenable to addressing vertical transport (i.e., upwelling) or horizontal transport. DESPITE THE IMMENSE SUCCESS OF THESE MODELS AT THE TIME, THEY SUFFER FROM SOME FUNDAMENTAL LIMITATIONS: The derived exospheric temperatures (and the densities) reveal many of the variations typical of the thermosphere: annual, semiannual, solar activity, magnetic activity, diurnal, etc. (see following figures).

IN THE 1970'S, TWO DATA SETS BECAME AVAILABLE THAT ADDRESS THE PREVIOUSLY-MENTIONED LIMITATIONS FOR DRAG- BASED MODELS: However, the MSIS models are not optimized with respect to satellite drag, and so have not been widely adopted for ephemeris computations in lieu of the Jacchia models. In principle, though, getting closer to the correct physics should lead to improved orbital predictions. Mass Spectrometer Incoherent Scatter (MSIS) models of A. Hedin (NASA/GSFC). HENCE LEADING TO THE: determinations of T ex from incoherent scatter radar measurements. satellite mass spectrometer measurements of O, O 2, N 2, He, H, etc., and also measurements of T ex. (satellites like OGO-6, AE, and many others).

ASEN 5335 Aerospace Environments -- Upper Atmospheres10 Some of the other models developed during the past 30 years NCAR Thermosphere-Ionosphere GCM TGCMTIGCMTIE-GCMTIME-GCM University College London Thermosphere-Ionosphere Model (now Coupled Thermosphere-Ionosphere-Plasmasphere Model (CTIP) at CU/CIRES) NUMERICAL/THEORETICAL MODELS U.S. Standard 1962 Jacchia 1964 U.S. Standard Supplements, 1966 CIRA-1961, 1965 MSIS86, MSIS90, MSISE90 Jacchia-1971, 1977 CIRA-1986 NRLMSISE-00 Jacchia-Bowman 2006 EMPIRICAL

Correlation of density and temperature with long- term changes in solar activity.

Thermosphere density variability on various time scales

ASEN 5335 Aerospace Environments -- Drag & Re-entry13 J. M. Forbes 1, E.K. Sutton 1, S. Bruinsma 2, R. S. Nerem 1 1 Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA 2 Department of Terrestrial and Planetary Geodesy, CNES,Toulouse, France Solar-Terrestrial Coupling Effects in the Thermosphere: New Perspectives from CHAMP and GRACE Accelerometer Measurements of Winds And Densities

The physical parameters of the CHAMP satellite are: Total Mass 522 kg Height m Length (with m Boom) m Width m Area to Mass Ratio m 2 kg -1 The CHAMP satellite was launched in July 2000 at 450 km altitude in a near-circular orbit with an inclination of 87.3°

Non-gravitational forces acting on the CHAMP and GRACE satellites are measured in the in-track, cross-track and radial directions by the STAR accelerometer Separation of accelerations due to mass density (in- track) or winds (cross-track and radial) require accurate knowledge of à spacecraft attitude à 3-dimensional modeling of the spacecraft surface (shape, drag coefficient, reflectivity, etc.) à accelerations due to thrusting à solar radiation pressure à Earth albedo radiation pressure STAR accelerometer by Onera

CHAMP and GRACE offer new perspectives on thermosphere density response characterization: latitude, longitude, temporal and local time sampling ≈ 45 minutes Local time precession rate of CHAMP is about 24 hours/133 days Only cross -track winds can be inferred DMSP November 20, 2003

Liu et al., JGR, 2005: “Global Distribution of the Thermospheric Mass Density Derived from CHAMP” Percent Differences from MSIS90 for K p = 0-2 during 2002 > 50 o Latitude Co-location with field-aligned currents Cusp region enhancement Pre-midnight/ midnight enhancement North-South asymmetry

18 Thermosphere Density Response to the October Storms from CHAMP Accelerometer Measurements (Sutton et al., JGR, 2005) EUV Flare ~50% ~ % ~200%

Time Delay and Traveling Atmospheric Disturbances

November Storm Density Structures at Scales > 1200 km: “Large-Scale” Waves (150-sec (~1200 km) running means applied to raw data) ……. before _____ after

~800 ms -1 % density difference from orbit average April 2002 storm period