On Forecasting Thermospheric and Ionospheric Disturbances in Space Weather Events

Transcription

1 On Forecasting Thermospheric and Ionospheric Disturbances in Space Weather Events R. G. Roble High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado It is well known that solar EUV radiation and auroral heat and momentum sources have a significant effect on thermospheric and ionospheric structure and dynamics. Upper atmosphere general circulation models using these forcings have been reasonably successful in simulating the thermosphere and ionosphere responses for a number of geophysical event studies. These models can be used as forecast models of thermospheric and ionospheric structure and dynamics by using predicted inputs of solar EUV and UV radiation, auroral hemispheric power of precipitating particles, cross-polar cap potential drop and ion convection patterns. It is also necessary to have a satisfactory initial state to start the simulation. The NCAR TIE-GCM that simulates the thermosphere and ionosphere between 95 and 800 km altitude is used to show the sensitivity of the thermosphere and ionosphere to space weather events. 1. INTRODUCTION There have been several different upper atmosphere 3-D models that- have been developed over the years to study processes in the thermosphere and ionosphere and especially their dynamic response to solar and auroral variability [Mayr and Volland, 1966; Fuller-Row ell and Rees, 1980; Dickinson et ah, 1981; Mikkelsen and Larsen, 1993; Namgaladze et al, 1990]. These models have been used for thermospheric research and in the analysis of satellite and ground-based data for many years. In this brief paper, I will present results from one of these models the 'NCAR TIE-GCM' to illustrate the feasibility of using such a model in a time-dependent simulation of space weather using realistic solar and auroral forcings. Space Weather Geophysical Monograph 125 Copyright 2001 by the American Geophysical Union 2. TIE-GCM The National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) that extends between 95 and 800 km has been used by many investigators to simulate the time-dependent response of the thermosphere and ionosphere system to variations in solar EUV and UV radiation, auroral particle precipitation and ion convection during geomagnetic storms and substorms. It incorporates all of the features developed in previous general circulation models of the upper atmosphere, the TGCM [Dickinson et al, 1981, 1984], the TIGCM [Roble et al, 1988], and it has the main features of the TIE-GCM that have been described by [Richmond et al, 1992] but several changes to that model have been made as described in the numbered paragraphs below. The changes are to better represent aeronomical processes that in turn give a better comparison of model simulations with data. The T refers to thermosphere, I ionosphere, and E electrodynamics in each of the GCM characterizations described above. The model does not extend into the mesosphere and 369

2 370 THERMOSPHERE AND IONOSPHERE SPACE WEATHER PHOTOCHEMISTRY THERMOSPHERE STRUCTURE AND WINDS TIDES TIE-GCM SOLAR EUV, UV, AND AURORAL INPUTS IONOSPHERE 0+DIFFUSION WITH ION DRIFT -JxB MAGNETOSPHERIC ELECTRIC FIELD yexb DRIFT IONOSPHERIC DYNAMO Figure 1. Schematic of coupled physical and chemical processes in the TIE-GCM. thus must represent lower atmospheric variability by specifying boundary conditions at 95 km. Currently the model only includes a zonally averaged seasonal variation and Hough Mode tidal forcings for the diurnal (1,1) tide and the semi-diurnal (2-2 through 2-6) tidal components [Forbes et al, 1993]. The aeronomic processes in the model have been described by Roble et al. [1987] and Roble [1995]. The TIE-GCM includes self-consistent ionospheric electrodynamics, that is, a calculation of the electric fields and currents generated by the ionospheric dynamo, and consideration of their effects on the neutral and ionospheric dynamics. It is used for studies that focus on the thermosphere and its coupling with the ionosphere and magnetosphere. The thermosphere /ionosphere / electrodynamic interactions are shown schematically in Figure 1. These interactions are calculated at every grid point and time step in the model. The essential differences between the model that was described in those previous papers and the model used for these simulations include the following: 1. The neutral chemical reaction rates for the aeronomic scheme described by Roble [1995] have been updated to be consisted with the JPL-97 compilation [De- More et al, 1997]. The ion chemistry is consistent with Buonsanto et al. [1995]. 2. The CO2 infrared cooling parameterization has been updated to include the model of Fomichev et al. [1999] to account for a variable CO2 mixing ratio that is important for non-local thermodynamic equilibrium (non-lte) processes in the upper mesosphere and lower thermosphere. All calculations assume an O-CO2 vibrational relaxation rate of 3 x 10 cm" 3 s that seems to work reasonably well on all of the terrestrial planetary thermospheres [Bougher et al, 1998]. 3. The tidal forcing at the lower boundary near 95 km, that represents diurnal and semi-diurnal components excited in the lower atmosphere, has been specified using results from the Global Scale Wave Model (GSWM) [Hagan, 1996]. The amplitudes and phases of the propagating diurnal (1,1) and semi-diurnal tide (2-2 through 2-6) were obtained from the GSWM at 95 km altitude for various seasons and used as lower boundary amplitudes and phases in the TIE-GCM. They, however, have been adjusted to be consistent with the UARS measurements of winds by McLandress et al. [1996]. 4. Solar Ionization rates are calculated using the EUVAC solar flux model and absorption cross-sections from Richards et al [1994]. Solar photodissociation rates for thermospheric processes have been determined using, in part, the parameterizations given in Brasseur and Solomon [1986]. The output fields calculated by the TIE-GCM are given in Appendix A. 3. TIE-GCM CALCULATED STRUCTURE FOR EQUINOX CONDITIONS The TIE-GCM has been used to calculate the global circulation, temperature and compositional structure for equinox and solstice conditions. A diurnally reproducible solution for equinox conditions during solar cycle maximum is shown in Figure 2. The upper panel shows the global temperature variation as well as vectors of neutral wind velocity at an altitude near 350 km. There is a diurnal temperature variation of about 500K and the winds have high velocity in the polar region driven by ion convection. At low latitudes the winds are greater at night when the ion drag is relatively small. The lower panel shows the ionospheric dynamo calculation of electric potential and vectors of ion drift. At high latitudes, the ions follow the two cell pattern of ionospheric convection with drift speed on the order of m/s. At low latitudes the ion drifts are small during the daytime when the E-region is electrically conducting but drift with velocities approaching the neutral wind velocities at low latitudes at night. These drifts are driven by the F-region dynamo that operates after the E-region recombines at night. These and other electrodynamic couplings between the thermosphere/ionosphere have been discussed in detail by Richmond and Roble [1997].

3 ROBLE TIE-GCM 350 km, 0 UT, Equinox, Solar Maximum NEUTRAL TEMPERATURE AND WIND LONGITUDE (DEG) ;.'O O 4 LOCAL TIME (HRS) Figure 2. (a) Calculated neutral gas temperature and vectors of neutral wind at 350 km altitude for equinox solar cycle maximum conditions and (b) calculated electric potential (volts) and vectors of ion drift. used in the simulation is shown in Figure 4. The parameterizations based on Kp are derived from many space weather studies using satellite data and the results of AMIE simulations. The main driving parameters include the cross-polar cap potential drop, the configuration of the convection pattern and its expansion with increased geomagnetic activity. It also includes an analytic auroral oval as described by Roble and Ridley [1987] with characteristic auroral particle mean energy and energy flux related to variations in Kp derived from satellite particle precipitation studies. Similar to the procedure used in space weather event studies, the TIE-GCM is run for a geomagnetic quiet period with constant solar and auroral forcings until a diurnally reproducible steady state solution is obtained. From the steady state solution the TIE-GCM is run then forward in time using the time-dependent geophysical indicies to drive the solar spectral irradiance and auroral models. The TIE-GCM time step is 3 minutes and the solar spectral irradiance is linearly interpolated between daily values and the Kp parameterizations are linearly varied in response to the 3 hr Kp values. The simulation described below saves histories hourly through the period. The results, however, are displayed as daily zonal averages as a satellite in a sun synchronous orbit at 12 Local Time would see the thermosphere and ionosphere properties. This is just F10.7 CM DAILY FLUX ( 76 DAYS FROM TO 98140) 4. SIMULATING SPACE WEATHER FOR MARCH 15 - MAY 15, 1998 The TIE-GCM is used to calculate the thermosphere and ionosphere response to solar and auroral forcing during the period March 15 through May 15, The solar F10.7 variation during the period is shown in Figure 3 and it clearly illustrates a 27 day solar rotation period on a background characteristic of near solar minimum conditions. The solar F10.7 variation is used to drive the EUVAC solar spectral irradiance model of Richards et al. [1994] that covers the spectral range from 0.1 to 200. nm. This model is used to calculate the solar ionization, photodissociation and heating rates as described by Roble et al. [1987] and Roble [1995]. The auroral model used in the TIE-GCM has been described by Roble and Ridley [1987]. It includes an ion convection model and aurora particle precipitation model and both are driven by parameterizations related to the 3 hr geomagnetic index Kp. The variation of Kp Figure 3. Solar F10.7 variation during days in 1998.

4 372 THERMOSPHERE AND IONOSPHERE SPACE WEATHER Kp (every 3 hours) ( 76 DAYS FROM TO 98140) 9 i ' 1 ' 8 - the otherwise straight lines that would be generated in the absence of any solar or auroral variability. These figures illustrate the space weather effects on thermospheric and ionospheric variability. 7h 5. DEVELOPMENT OF SPACE WEATHER MODELS FOR OPERATION PURPOSES The above simulation illustrates the variability of various thermospheric and ionospheric properties associated with space weather. The existing space weather models can be used in either the nowcast or forecast NEUTRAL TEMPERATURE (OEG K) HEIGHT = LOCAL TIME = DAY OF 1998 Figure 4. Geomagnetic index Kp variation during days in for presentation of results in this paper, but the global data comprising the figures are available on an hourly basis for other types of space weather uses and displays. The calculated neutral gas temperatures at two altitudes (300 km and 150 km) are shown in Figure 5. The solar F10.7 peaks near days 80, 100 and 125 and there are corresponding increases in thermospheric temperature on those days. The Kp index peaks near day 125 and the are major changes to temperature between days 122 and 126. The variability in these figures are caused by variations in the solar EUV ionizing radiation and the Joule heating and particle precipitation in the aurora. The atomic oxygen number density and the O/O2 ratio variations during the period are shown in Figures 6 and 7 respectively. There is a slow seasonal variation as well as enhancements during the peaks in solar F10.7 and Kp, similar to the daily temperature variations shown in Figure 5. The O/O2 ratio is important for predicting density variations detected by remote UV sensing instruments. The calculated electron density variations at the same two heights are shown in Figure 8. Since the presentation is along a constant height and constant solar local time (12 hr), the variability shows up as variations to UT (DAYS) (MTIMES TO ) NEUTRAL TEMPERATURE (DEG K) I» ' " 1 '» i/iv 1 ii «J * ' 1 I 1 ili Iai 1 k I II an,/hf V, f I i H it im UT (DAYS) (MTIMES TO ) Figure 5. Calculated zonal average temperature (K) as sampled daily by a sun synchronous satellite at 12LT at two altitudes (a) 300 km and (b) 150 km.

5 ROBLE 373 ATOMIC OXYGEN (CM3) HEIGHT = LOCAL TIME = ATOMIC OXYGEN (CM3) UT (DAYS) (MTIMES TO ) UT (DAYS) (MTIMES TO ) Figure 6. Same caption as for Figure 5 except for units of 10 8, (b) particles/cm 3 in units of atomic oxygen number density, (a) Particles/cm 3 in modes as well as for previous event studies once the appropriate solar and auroral inputs can be specified. Nowcasting uses previous and instantaneous solar and geophysical indices or AMIE data to drive the TIE- GCM. Model solar EUV and UV spectral irradiance inputs are specified by instantaneous solar indicies and model ion convection and aurora particle precipitation parameterizations are driven by specified instantaneous geomagnetic index or solar wind properties. RATIO 0/02 () HEIGHT = LOCAL TIME Assimilative Mapping of Ionospheric Electrodynamics (AMIE) can be used with ground-based and satellite data to derive instantaneous timedependent auroral inputs for the TIE-GCM. Forecasting uses predicted solar and auroral indicies to integrate the model forward in time for future forecasts and they can be updated as often as desired. With solar and auroral indicies predicted for various days in advance the TIE-GCM can be run to give, say, 1, 3, 5, or 10 day forecasts of space RATIO 0/02 () UT (DAYS) (MTIMES TO ) Li (i rrttty I. ^, V T ft,,a, /, I,,V, I,,, VJ, /, {\\,111 MV., I,UH I UT (DAYS) (MTIMES TO r00) Figure 7. Same caption as for Figure 5 except for ratio. atomic oxygen to molecular oxygen number density

6 374 THERMOSPHERE AND IONOSPHERE SPACE WEATHER ELECTRON DENSITY (CM3) HEIGHT = LOCAL TIME = ELECTRON DENSITY (CM3) UT (DAYS) (MTIMES TO ) UT (DAYS) (MTIMES TO ) 135 Figure 8. Same caption as for Figure 5 except for electron number density, (a) Particles/cm 3 of 10, (b) particles/cm 3 in units of in units weather, much as the weather forecasters do for predicting the future troposphere and stratosphere weather. The TIE-GCM is numerically fast so the predictions can be updated daily or as needed. 6. FUTURE SPACE WEATHER MODEL DEVELOPMENT A major problem that has not yet been addressed for space weather studies is data assimilation for numerical initial conditions. There is a strong need to develop such techniques so that the model can be run forward smoothly without introducing transients. There is also a need for continued aeronomical research and data analysis to upgrade many aspects of the aeronomy in the TIE-GCM, such as the rate coefficients for CO2 and NO vibrational exchange with O. There is a need to improve boundary conditions to better represent magnetosphere/ionosphere energy and plasma exchange. There is a need to improve the lower boundary of the TIE-GCM to represent coupling with the lower atmosphere. The recent development of the TIME-GCM (M indicating Mesosphere) that in- Table Al. TIE-GCM Output Fields Field Neutral gas temperature Neutral winds (zonal, meridional and vertical) Neutral composition (major species) Neutral composition (minor species) Neutral density Electron density Ion Composition Electron and Ion temperature Electric Fields and Currents Electrical conductivity Airglow emissions Symbol Tn Un, Vn, Wn N 2, 0 2, O NO, N( 4 S), N( 2 D), 0( 1 D), 0( 1 S), H e, and Ar Rho (gm/cm 3 ) Ne NO+, O+, 0 + ( 4 S), 0+( 2 D), 0 + ( 2 P), N+, H+ Te, Ti E, J local and height integrated Pedersen, Hall and Parallel conductivity a large number of UV and visible airglows

7 ROBLE 375 eludes the mesosphere and lower thermosphere down to 10 mb or 30 km is a first step in that direction [Roble and Ridley, 1994]. Ultimately, a model of the whole atmosphere is needed to adequately represent coupling with the lower atmosphere [Roble, 2000]. It is important to continually validate model performance by comparisons with observations. There is also a need to develop skill tests to provide an objective assessment of the various models' ability to specify space weather events accurately. Satellite Observations - There are many satellite projects that have a need for a comprehensive model that predict dynamics, temperature, compositional and airglow structure. These include the military and commercial satellites, NASA satellites such as TIMED, ISTP, UARS, as well as various ground-based programs such as CEDAR, GEM, and Space Weather. APPENDIX A: TIE-GCM OUTPUT FIELDS The TIE-GCM output includes a global description of the following fields at every time step. REFERENCES Brasseur, G., and S. Solomon, Aeronomy of the Middle Atmosphere, 452 pp., D. Reidel Publishing Co., second edition, Dordrecht, Holland, Buonsanto, M. J., P. G. Richards, W. K. Tobiska, S. C. Solomon, Y.-K. Tung, and J. A. Fennelly, Ionospheric electron densities calculated using different EUV flux models and cross-sections: Comparison with radar data, J. Geophys. Res., 100, , DeMore, W. B., et al., Chemical kinetics and photochemical data for use in stratospheric modeling, Evaluation Number 10, JPL-Publication 94-1, NASA-JPL, Pasadena, CA, Dickinson, R. E., E. C. Ridley, and R. G. Roble, A threedimensional general circulation model of the thermosphere, J. Geophys. Res., 86, , Dickinson, R. E., E. C. Ridley, and R. G. Roble, Thermospheric general circulation with coupled dynamics and composition, J. Atmos. Sci., 1±1, , Flemming, E. L., S. Chandra, M. R. Schoeberll, and J. J. Barnett, Monthly mean global climatology of temperature, wind, geopotential height, and pressure for km, NASA Tech. Memo TM , 85 pp., Goddard Space Flight Center, Greenbelt, Maryland, Fomichev, V. I., J.-P. Blanchet, and D. S. Turner, Matrix parameterization of the 15/zm CO2 band cooling in the middle and upper atmosphere for variable CO2 concentration, J. Geophys. Res., in press, Forbes, J. M., R. G. Roble, and C. G. Fesen, Acceleration, heating and compositional mixing of the thermosphere due to upward-propagating tides, J. Geophys. Res., 98, , Fuller-Rowell, T. J., and D. Rees, A three-dimensional timedependent global model of the thermosphere, J. Atmos. Sci., 37, , Hagan, M. E., Comparative effects of migrating solar sources on tidal signatures in the middle and upper atmosphere, J. Geophys. Res., 101, 21,213-21,222, McLandress, C, G. G. Shepherd, B. H. Solheim, M. D. Burrage, P. B. Hays, and W. R. Skinner, Combined mesosphere/thermosphere winds using WINDII and HRDI data from the Upper Atmosphere Research Satellite, J. Geophys. Res., 101, , Mikkelsen, I. S., and M. F. Larsen, Comparisons of spectral thermospheric general circulation model simulations and E and F region chemical release wind observations, J. Geophys. Res., 98, , Namgaladze, A. A., Yu. N. Koren'kov, V. V. Klimenko, I. V. Karpov, F. S. Bessarb, V. A. Surotkin, T. A. Glushcenko, and N. M. Naumova, A global numerical model of the thermosphere, ionosphere, and protonosphere, Geomagnetism and Aeronomy, 30, , Richards, P. G., J. A. Fennelly, and D. G. Torr, EUVAC: A solar EUV flux model for aeronomical calculations, J. Geophys. Res., 99, , Richmond, A. D., E. C. Ridley, and R. G. Roble, A thermosphere/ionosphere general circulation model with coupled electrodynamics, Geophys. Res. Lett., 19, , Richmond, A. D., and R. G. Roble, Electrodynamic coupling effects in the thermosphere/ionosphere system, Adv. Space Res., 20, , Roble, R. G., and E. C. Ridley, An auroral model for the NCAR thermosphere general circulation model (TGCM), Annales. Geophysicae, 5A, (6), , Roble, R. G., E. C. Ridley, A. D. Richmond, and R. E. Dickinson, A coupled thermosphere/ionosphere general circulation model, Geophys. Res. Lett, 15, , Roble, R. G., and E. C. Ridley, A thermosphere-ionospheremesosphere-electrodynamics general circulation model (TIME-GCM): Equinox solar cycle minimum simulations ( km), Geophys. Res. Lett, 21, , Roble, R. G., E. C. Ridley, and R. E. Dickinson, On the global mean structure of the thermosphere, J. Geophys. Res., 92, , Roble, R. G., Energetics of the mesosphere and thermosphere, in The Upper Mesosphere and Lower Thermosphere: A Review of Experiment and Theory, Geophys. Mono., 87, 1-21, Roble, R. G., On the feasibility of developing a global atmospheric model extending from the ground to the exosphere, in Coupling of Processes Across the Stratopause, Geophys. Mono., 92, in press, R. G. Roble, High Altitude Observatory, National Center for Atmospheric Research, 3450 Mitchell Lane, Boulder, CO ( roble@ucar.edu)