Phys. Chem. Earth (C), Vol. 25, No. 5-6, pp. 499-503,200O 8 2000 Elsevier Science Ltd. All rights reserved 1464-1917/00/$ - see front matter
Modeling, of the Thermosphere-Ionosphere System S. C. Solomon Laboratory for Atmospheric and Space Physics, University of Colorado Received 31 October 1999; accepted 15 March 2000
star are its photon output in the far ultraviolet, extreme ultraviolet, and soft x-ray parts of the qectnnq its magnetic field, and its extended corona and solar wind. The most dramatic short term changes, particularly in the polar regions, are driven by magnetic field / solar wind processes that are manifested in the thermosphere-ionosphere as amoral energy deposition and high-latitude convection. Seasonal changes, influenced by the transformation of middle-ultraviolet and visible radiative energy through the middle and lower atmosphere, are also important But by far the largest effect on the density, extent, and charged particle component of the thermosphere is the variation in the solar radiation deposited directly in it, in the wavelength range -1 to -200 nm. Over the 11-year solar cycle, this variation drives a temperature-pressure change that alters the density by over a factor of 10 at altitudes used by low-Earth orbiting satellites, the STS “space shuttle”, and the International Space Station. Variation over a 27day solar rotation is usually not as dramatic, but can still be greater than a factor of e. Although there is considerable information on the behavior of the solar spectmm at these wavelengths, there is considerable uncertainty as well, and thus there is an ongoing need for thermosphereionosphere modelers to identify, parameter&, and incorporate solar measurements and model results. This is important for both theoretical and empirical modeling approaches. Although continuous observation of the solar farultraviolet spec&um by the UARS satellite has been ongoing for a decade, direct measurements of solar extremeultraviolet irradiance have only been available for limited periods in the last 30 years. Hence, proxy indices to which solar EUV observations have been correlated are often employed. The most widely accepted of these is the 10.7 cm radio flux, as it can be measured by ground-based observatories independent of weather, and has a 53-year history of stable continuity. Most thermosphereionosphere models, both empirical and theoretical, use this index, even though it is not a particularly good indi-
Abstract. Empirical, semi-empirical, and theoretical models of the thermosphere and ionosphere and of photochemical processes occuning therein are briefly described. An overview is provided of this section of the issue, containing papers on these topics and on the measurement of thermospheric and ionospheric properties from space. The importance to these endeavors of accurate measurement and speeiiication of the solar ultraviolet, extremeultraviolet, and sofl x-ray irradiance is discussed. Q 2000 Elsevier Science Ltd. All rights reserved
1. Introduction The terrestrial thermosphere from -90 to -600 km altitude is the most variable part of the Earth’s atmosphere. It is a region often considered to be “space”, and not atmosphere at all, because of its extremely low density, and because it is the region through which orbiting space vehicles travel. Nevertheless, what density it does possess is signiticant. Its charged component, the ionosphere, although less than a part in a thousand, is critical to its dynamics and important for communications and navigation. Its neutral component is equally variable, and affects the trajectories and degradation of orbiting spacecraft. Understanding, monitoring, and ultimately forecasting the changes of the thermosphere-ionosphere system is vital to the exploration and exploitation of near-Earth space. The reason for the extreme variability of the thermosphere is its rapid response to variable external forcing. This forcing occurs from above, by solar ultraviolet photons and by the energetic particles and electric fields imposed through the interaction between the solar wind, magnetosphere, and ionosphere, and from below; by atmospheric tides and gravity waves generated in the stratosphere and troposphere. All of these are ultimately driven by energy from the sun. The most variable aspects of our Correspondence
to: S. C.
S. C. Solomon: Thermosphere-Ionosphere System
cator of chromospheric activity. There are several other candidates for this role, the best of them probably the MgII 288 mn core-to-wing ratio, which is available from routine space-based measurements over last two decades. This index is an improved proxy for chromospheric fluxes, although F 10.7 is* still better for coronal emission lines. Combinations of these indices may prove a reasonable substitute, but for consistent and accurate modeling, actual solar EW measurements are needed. Fortunately, several new programs to measure the solar EW on a routine basis will soon.begin, as described in Section 1 of this issue. The following section of this issue addresses the current development of thermosphere-ionosphere models, the use of solar measurements in models, and methods for inferring thermosphere-ionosphere parameters from spacebased remote-sensing measurements. This paper serves as an introduction to the section and offers a brief overview of the field.
2. Empirical and Semi-Empirical Models Climatological models of thermospheric density and temperature were developed contemporaneously with early orbiting spacecmft to meet the need to estimate orbital decay due to atmospheric drag [e.g., Nicolet, 1961; Harris and Priester, 19621. Subsequent work [Jacehia, 1971, 1977; Bazhkov et al., 1984; Nazarenko, 19911resulted in improved operational models, but many standard satellite orbit propagation programs still use the Harris-Priester model. Gf the remainder, the Jacchia ‘71 model is probably the most widely employed. However, the research community has moved on, in particular with the mass spectrometer incoherent scatter models developed by A. E. Hedin using a variety of data sources, particularly from the Atmosphere Explorer program [Hedin, 1983, 1987, 19911. The MSIS series of models are more sophisticated in that they cover more species with greater fidelity for a broader variety of geophysical conditions, and MSIS densities are not identical to the Jacchia or Harris-Priester models for the”&me input conditions. MSIS is not widely used for satellite orbit propagation, even though there is some evidence that it is more accurate for density prediction than its predecessors [e.g., Chuo et al., 19971. The MSIS models arc best described as semi-empirical, as they employ the theory of diffusive equilibrium to extend analytic temperature profiles to density and composition. They are also in essence climatological models, and there will always be an intrinsic ~uncertaintyfor this type of model regardless of the accuracy of the inputs and the adequacy of the data sources employed. Current efforts for the new NRLMSIS [Picone et al., this issue] are focused on improvement of the solar and geomagnetic input specifications, and on the incorporation of new data sources. It is possible that these models will find increased acceptance in the operations community as well as
the research community; for example, MSIS90 will be employed for orbit prediction of the new TIMED spaceCdt. An extension of the MSIS approach to neutral atmosphere modeling to describe measurements of neutral winds, the I-IWM90model, was also developed by Hedin et al. . This has been used especially in theoretical models of the ionosphere (described below) as it enables calculations of &-layer height and ion density, which are strongly influenced by neutral winds, without the need for a fully-coupled thermosphere-ionosphere model. The ionosphere has greater short-term variability than the thermosphere in which it is embedded. In addition, ion composition is still not well-known, particularly in the E-region. Empirical and semi-empirical models of the ionosphere are therefore even more climatological than neutral atmosphere models. Early empirical models of ionospheric density include the Chiu [Chiu, 19751 and Bent [Llewellynand Bent, 19731models. Subsequent development in other locales (e.g., the reference ionospheric model [Chusovitinet al., 19871and the Chinese reference ionosphere [Wu et al., 19961) have to some extent been subsumed by efforts to develop the International Reference Ionosphere [Belitzu, 1990, and this issue] under sponsorship by COSPAR and URSI. Development of the IRI began with a mostly F-region focus, but has been extended downward to the E and D-regions, and contains composition estimates for the major ion species. Because, like MSIS, it is publicly available as source code, it is widely used, and has achieved significant acceptance as a reference standard. A more semi-empirical approach has been developed by D. N. Anderson and co-workers for the semi-emnirical, low-latitude ionospheric model (SLIM) and fully-analytic ionospheric model (FAIM) [Anderson et al., 1987, 19891. The latter of these was expressed using coefficients in analytic form so that users of the Chiu model would be able to use FAIM without having to change methodologies. FAIM is valid at mid-latitudes as well as low, and is implemented in a compact, efficient code. IRI, by comparison, employs large monthly coefficient files as inputs, but it is also reasonably fast.
3. Theoretical Models The most comprehensive theoretical models of the ther-
mosphere-ionosphere are global dynamical models that numerically solve the equations describing the neutral atmosphere distribution and motion simultaneously with those describing the ionosphere. These first-principles, fully-self-consistent models must nevertheless use parameterizations of forcings external to the region, either from theory, climatology, or observation. The best-known example of thus type of model is the series developed at the U.S. National Center for Atmospheric Research (NCAR) by R G. Roble and co-workers. Evolution of the thermo-
S. C. Solomon: Thermosphere-Ionosphere System
sphere general circulation model (TGCM), thermosphereionosphere general circulation model (TIGCM), thermosphere-ionosphere-e~cs general circulation and thermosphere-ionospheremodel (TIE-GCM), mesosphere-el~cs general circulation model (TIME-GCM) is described by Roble et al. [1982, 19881, Richmond et al. , and Roble and Ridley . Recent efforts have involved development of better coupling with the magnetosphere, either using the observation-based assimilative mapping of ionospheric electrodynamics (AMEX)method [Richmond and Kamide, 19881,or using linked models [e.g., Pepirat et al., 19981. An additional initiative is to couple the TIME-GCM with the NCAR community climate model (CCM) of the troposphere and lower stratosphere, and ultimately to construct an integrated ground-to-exosphere model. Other global dynamical models include the couple thermosphere-ionosphere model (C!TIh4),[Fuller-Rowe11 et al., 19871,and the global self-consistent model of the thermosphere, ionosphere, and protonosphere (GSM-TIP) [Namgaladze et al., 1988, and this issue]. Work on CTIM was begun at the University College London; development continues by T. J. Fuller-Rowe11and co-workers at the U.S. NOM Space Environment Center. GSM-TIP was constructed at the Kaliningrad observatory of IZMIRAN (now the West Department of IZM&AN) and work by A. A. Namgaladze and co-workers continues at the Polar Geophysical Institute in Murmansk and Murmansk State Technical University. There are several theoretical models that perform the ionospheric portion of this calculation, using semiempirical models to describe the neutral atmosphere and disturbance of the ionosphere by neutral winds. These include the Utah State University time-dependent ionosphere model (TDIM) [Schunk, 1988, Sojku, 19891, the U.S. Air Force Research Laboratory global theoretical ionospheric model (GTIM) [Anderson, 1973, Decker et al., 19941,and the University of Alabama field line interhemispheric plasma model (FLIP) [Richards and Torr, 1988, Torr et al., 19901. All three use MSIS-86 and HWM90 to specify the neutral atmosphere. Anderson et al. [ 19981compare these, the TIGCM, and CTIM to incoherent scatter radar measurements of the FZ region, and describe some of the specifics of their formulation. An additional category of model uses the results of empirical or theoretical models to describe the thermosphere and ionosphere, and then performs further detailed calculations of photochemical processes so as to describe minor species,composition, dissociation, excitation, and heating rates, and airglow or auroral emission intensities. The FLIP model performs these calculations in addition to the ionospheric ones described above. The atmospheric ultraviolet radiance integrated code (AURIC) model [Strickland et al., 19991 and global airglow (/glow) model [Solomon et al., 1988, Solomon anddbreu, 19891perform ionospheric calculations in the photochemical equilibrium region (below -200 km) and rely on other ionospheric
models at higher altitude. The /glow model is also capable of describing amoral electron transport processes and consequent emission rates, as are specialized implementations of the AURIC and FLIP models [Strickland et al., 1993, Germany et al., 19941. Additional photoelectron and auroral electron transport algorithms with linked photochemistry are described by Link  and by Lummerzhejm and Lilensten .
4. Section Overview
Recent progress in semi-empirical models of the thermosphere and ionosphere, respectively, is described in papers by Picone et al. and by Bilitza. Continued work on the MSIS series of models has been taken on by the U.S. Naval Research Laboratory. Picone et al. are reevaluating historical measurements, incorporating recent ones, and adapting improved solar proxies for use with the new edition. Meanwhile, work proceeds on the International Reference Ionosphere. Bilitza delineates the importance of accurate indices of solar EW for this endeavor. The effect of solar ultraviolet irradiance on a global dynamical model is illustrated through studies with the GSM-TIP by Bessarab et al., and results from this model for geomagnetically quiet and disturbed conditions are presented by Namgaladze et al. The capability of this model should receive wider dissemination. Figure 1 of Namgaladze et al. well illustrates the difference between empirical/climatological and theoretical models, and compares with results from tomographic reconstruction of the ionosphere using Global Positioning System signals. This is a powerful new technique that exploits serendipitous active remote sensing being made continuously by GPS tl-iltlSmitterS.
Ionospheric short-term effects from solar flares and long-term effects from solar EW variation are examined by Kudryashev and Avakyan, and by Mikhailov. Although flares have a small effect on ambient electron densities, they can have a significant effect on photoelectrons, causing consequent increases in airglow emission rates. Conversely, Mikhailov finds that the small E-region electron density increases during flares is evidence that solar sofl x-rays provide a minority of E-region ionization, and hence that suggestions to drastically increase the x-ray portion of standard solar spectrum models am not warranted. Theoretical work pertaining to the interpretation of space-based measurements is described in papers by Lund and Mobius, and by Eastes. The latter paper shows calculations of the vibrational distribution of the N2 LBH band system as influenced by cascade from the a’ and w states into the a state. This eBect may have important consequences for the effective total cross section for electron impact excitation of Nz LBH bands, and hence for impending remote sensing observations of thermospheric
S. C. Solomon: Thermosphere-Ionosphere System
composition. New laboratorymeasurementsof Nz LBH electronimpactexcitationcrosssectionsareindicated.
5. Discussion Improvedmodeling of the thermosphereand ionosphere by any of the methodsmentionedhereindependscritically on improvedspecificationof the solar ultravioletand soft x-ray spectrum. Whetherthis specificationis done using single or multiple proxy indices, using model spectra drivenby proxy indices or by broad-bandmeasurements, or using actual measurements,more accurateresultswill follow from moreaccuratesolar inputs. The challengeto modelersis to develop methodsto use the new measurements and proxies without disruptingthe heritageof recent and currentpractice. Use of existing indices (e.g., Ji0.7) has become quite ingrained, and for good reason. Modelersand model users prefer well-established,wellcalibrated,and continuous input variables,and it is not always simple to convert to a more operationalmode basedon space-basedmeasurementsfrom diversesources. In some cases, it may proveexpedientfor analyststo convert experimentalresults to an indexed form for use in existing models, such as a pseudo-valuefor the daily 10.7 cm flux. For fully theoreticalmodels,solarmeasurements arebest used as directinputs,but everymodelhas its own requirementsfor resolutionand efficiency, so it is important to accommodatetransitionbetween various spectral resolutions.%Fiually,observational results are seldom adoptedas model inputs if controversyremains with regardto calibration. The 21” centmy will see a multitude of new solar measurementsfrom different nations and differentagencies,and using differentmethods. It is crucially important that these measurements be wellcalibrated,inter-calibrated,and inter-compared,and that any discrepanciesbe resolvedthrough a physical understandingof instrumentcharacteristicsand changes. It is also vital to continue the comparisonof measurements fromlong-term orbiting instrumentswith regular suborbitalcalibrationflights.
This work was supportedby NASA grantNAG54027 to the Universityof Colorado.
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