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Construction of a particle climatology for the study of the effects of solar particle fluxes on the atmosphere
Pergamon www.elsevier.com/locate/asr
Adv. Space Res. Vol. 29, No. 10, pp. 1513-1522.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02 $22.00 + 0.00 PII: SO273-1177(02)00212-O
CONSTRUCTION OF A PARTICLE CLIMATOLOGY FOR THE STUDY OF THE EFFECTS OF SOLAR PARTICLE FLUXES ON THE ATMOSPHERE
J. R. Sharber’, J. D. Winningham*,
R. A. Frahm*, G. Crowley*, A. J. Ridley3, and R. Link2
‘NASA Headquarters, 300 E Street SW, Washington, DC 20546, U.S.A. ‘Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238, U.S.A. 3Universiv of Michigan, 2455 Hayward Street, Ann Arbor, Michigan 48109, U.S.A.
ABSTRACT In an investigation of the atmospheric effects of particle input, we are currently developing a particle climatology built using the UARS (Upper Atmosphere Research Satellite) particle database. UARS operates in a circular 585 km orbit and makes differential energy per unit charge measurements of electrons and positive ions in the range of a few eV to several MeV. The climatology is an empirical model that enables the user to obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. To further the goals of Working Group 1 of the International Solar Cycle Study (ISCS), we propose the construction of such a particle climatology. We suggest the UARS particle climatology as a starting point, to be augmented by additional iow-altitude particle data. In this paper we describe the development of the current climatology. show how it will be augmented to produce an improved ISCS climatology, and illustrate how this climatology will be used to carry out the objectives of the ISCS. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION A significant solar energy source in terms of its effect on planetary atmospheres is the outflow of particle energy from the Sun. Although we know that the energy and intensity of this outflow show spatial and temporal variations that depend on a variety of solar conditions, we lack an understanding of the nature of this dependence and of the mechanisms responsible for the particle production and energy variation. At the same time we are just now beginning to model globally the effects of this varying particle input on our atmosphere. One approach to improving our understanding of the production of particles on the Sun and their effects on the terrestrial atmosphere is to develop a model of particle input to that system. Ideally the model should provide spectral and total energy inputs as a function of position on the Earth as well as the integrated global or hemispheric power input. In principle, such a model would be related to the Sun by use of a suitable solar activity parameter to serve as a model sorting index. We propose to achieve this goal by building on a climatological model currently being developed using data from the Upper Atmosphere Research Satellite Particle Environment Monitor (UARS PEM). PEM operated continuously since the UARS launch in September 1991 until March 1995, after which time (until the present) it has operated on a duty cycle of -20%. The PEM particle detectors make differential spectral measurements over a wide energy range 5 eV to 5 MeV for electrons and 5 eV to 150 MeV for protons. (Details of the PEM instrumentation may be found in Winningham et al. (1993).) The result is a large body of particle observations beginning just after the peak of Solar Cycle 22, extending through its declining phase, and continuing into the ascending phase of Cycle 23. The PEM particle climatology was developed to exploit this data resource. It is being developed specifically to carry out one of the primary UARS/PEM goals, that of determining the magnetospheric and solar particle inputs to the terrestrial atmospheric system.
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UARS SPECTRAL MEASUREMENTS AND IONIZATION RATES We show some typical PEM electron spectra in Figures la and lb . These are selected from the larger body of spectra reported by Sharber eral. (1998) and were measured during the major magnetic storm of November 1993. They are measured at 585 km in the aurora1 precipitation region, one at 1102:(37-41) UT in the midnight sector and one at 1110:(40-44) UT from a region of softer precipitation later in the morning sector (4.7 MLT). Each spectrum is measured within the loss cone and consists of 63 energy levels which cover the full measurement range from 5 eV to 5 MeV. The intermediate to high energy portions of each spectrum have been fitted with kappa distributions (see Christon er al., (199 1)). The kappa distribution contains a near-thermal peak at characteristic energy E, which connects smoothly to a power law of slope -K at high energies. The kappa fitting parameters are shown on each plot. The ionization production rates computed from each spectrum are shown in Figure lc. They are computed assuming isotropic incidence using the CEPXS/ONELD multi-stream discrete ordinates code (Lorence, 1992), which solves the coupled electron-photon Boltzmann transport equations over the energy range 1 keV - 100 MeV. Included in the computations is the ionization of the bremsstrahlung X-rays produced by the incident electrons. This is seen as the secondary ledge of ionization beginning between 50 and 60 km and results in ionization down to stratospheric levels. The profile resulting from the most energetic spectrum shows an ionization rate peak of IO5cmm3s-’at an altitude of 96 km. As a reference marker, we include in the figure the rate of ionization production at 50” geographic latitude resulting from galactic cosmic rays (GCR) during solar minimum (top edge of curve) and solar maximum (lower edge of curve) conditions (Brasseur and Solomon, 1986). The ionization resulting from the spectrum of Figure la exceeds that due to cosmic rays at all altitudes above 33 km. Thus the ionizing effects of this spectrum reach the upper stratosphere. Additional electron spectra and a discussion of effects on the atmosphere may be found in Frahm et al. (1997). The UARS PEM CLIMATOLOGY
In the PEM climatology the fluxes at each energy level are binned into lo bins of invariant latitude (IL), 1 hour bins of magnetic local time (MLT), 1 hour bins of universal time (UT), and bins of activity levels of Dst, K,, and F10,7, and PC (the polar cap index). The latitudinal coverage is between 40” and 74” IL (Southern Hemisphere) and 70” IL (Northern Hemisphere), the maximum latitude being determined by the UARS 57’ inclination. Statistics are kept separately for the Northern and Southern Hemispheres. The PEM climatology is an empirical statistical model built using the PEM database from which the user may obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. A unique aspect of this model is the inclusion of differential measurements of high energy fluxes that extend to the -MeV range. The PEM climatology contains the ion (proton) component as well as electron component. Some of the attributes of the UARS climatology may be summarized as follows: Angular coverage and resolution are adequate to differentiate between precipitating and trapped particle fluxes. Local time and geographic longitude coverage are excellent with the exception of high magnetic latitudes (>73” IL in S. Hemi.; >68” in N. Hemi.) which are not sampled by UARS. As we discuss in a later section, this coverage will be increased by inclusion of high-latitude data from other spacecraft. The model is 3D in that the ionization production rates are functions of altitude. Spectral energy resolution is sharp enough to produce an accurate energy deposition (ionization rate) profile. The energy range is sufficiently high to investigate stratospheric effects. (X-ray production by the incident electrons is included.) The model is keyed to the D, I$, PC, and F10.7indices.
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Fig. 1. (a,b) Particle Environment Monitor electron spectra measured during the November 1993 magnetic storm (Sharber et al., 1998). Fit parameters for the kappa distributions are shown beside each plot. (c) Ionization production rates computed from the spectra. During the main phase of the storm, the ionization production rate by incident electrons and bremsstrahlung X-rays (solid curve) exceeded that of galactic cosmic rays (GCR) at all altitudes above 33 km.
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An example of the UARS climatology of electron number flux at an energy of 2.33 keV, incident on the Northern and Southern Hemispheres is shown in Figure 2a. Each polar plot is shown in invariant latitude (40” to 90“) and magnetic local time (O-24 MLT), and each represents a Kp value from 0 to 5. The energy input from the aurora1 oval is clear at all levels and shows a general increase in width and intensity with increasing Kp. The global distributions are consistent with the shape of the oval for various activity levels shown by previous studies (e.g., Hardy et al., 1985).
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In Figure 2b we show similar plots for precipitating
flux at much higher energy, 152 keV. The main band of precipita-
tion at intermediate latitudes is clearly seen in both hemispheres and shows much smaller variation with Kp. These electrons are associated with the outer radiation belt and cause ionization in the ionospheric D-region (Hartz, 1971; Whalen, 1982). The band of precipitation seen at low latitudes (40’ - 47’) in the Southern Hemisphere is flux from the South Atlantic Anomaly. It is not present in the Northern Hemisphere. The distributions of Figure 2 are for precipitating electrons only. This version of the UARS climatology contains 507 days of PEM spectral measurements made between Day 280 of 1991 and Day 341 of 1993. IMPROVING
THE CLIMATOLOGY
FOR ISCS
The UARS PEM data set is limited in two ways: (1) the latitudinal coverage and (2) the amount of data available. The restriction of latitude coverage results from the 57” inclination of the UARS orbit. This is not a severe problem during disturbed times when the oval is expanded; however, during quieter times, the orbits usually do not cross the auroral oval. To produce a fully global climatology the current model will need to be augmented to provide the required coverage. To be consistent with ISCS goals the database should be extended to cover a time interval equivalent to at least one solar cycle. An obvious candidate to extend the current UARS low-altitude database is the NOAA SEM (Space Environment Monitor) database. The SEM instruments have flown on NOAA or TIROS low-altitude satellites since 1978, and have thus produced an enormous amount of spectral data. The SEM high energy particle detectors measure both the precipitating and trapped components. Electrons are measured in three integral energy bands of E>30, E>lOO, and E>300 keV; protons are measured in 5 bands between 30 and 2500 keV (see Codrescu et al., 1997b). Low energy precipitating electrons and protons are measured with electrostatic analyzers over the range 0.3 to 20 keV. The NOAA inclination of -98” provides coverage to 90” invariant latitude. Data are archived at the National Geophysical Data Center. Another candidate is the large-volume database of the DMSP particle measurements also archived in the National Geophysical Data Center. This database contains differential measurements of precipitating electrons and positive ions over the range -30 eV to 30 keV and contains data as far back as 1976 (see Hardy et al., 1985; 1987). These two databases offer significant advantages. In both cases the data are obtained from low altitude polar satellites which have operated over two solar cycles. The particle data have been archived and are in the public domain. The archived data from both have already been used to develop auroral energy input models (i.e., the Air Force Research Laboratory models of Hardy et al. (1985; 1987; 1989; and 1991) and NOAA models of Fuller-Rowe11 and Evans (1987), Fuller-Rowe11 ef al. (1994), and Codrescu et al. (1997a; 1997b)). These models will provide benchmarks against which to validate our improved climatology, and because of their heritage, will provide insight into results expected from the application of our climatology. The resulting climatology will have the advantage of the extended, spectrally-resolved energy range of the UARS measurements as well as the greater global and temporal coverage provided by the instruments on NOAA and DMSP. The inclusion of additional particle databases will require both intercalibration of added data with UARS particle measurements and validation of the climatology as the data are added. These procedures will be accomplished using satellite conjunctions where possible and by comparison of the climatology spectral outputwith selected overpasses and images obtained from UARS, DMSP, or ISTP/Polar. When the current PEM climatology has been augmented and validated, it will be made available to the community at large, probably by release on a local website. This new climatology will not be static, but will instead be upgraded whenever it is deemed appropriate to add additional databases. It is particularly important that version control be maintained.
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Fig. 2a. Example of the UARS PEM climatology for electron number flux at 2.33 keV for the Northern and Southern Hemispheres. Each polar plot is shown in invariant latitude (40” to 90’) and magnetic local time (O-24 MLT) and each represents a K, value from 0 to 5.
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Fig. 2b. Same as Figure 2a except for electron number flux at 1.52 MeV. Note that, in addition to the radiation belt input, the contribution from a portion of the South Atlantic Anomaly is evident at the lowest latitudes (-40” 47’) in the Southern Hemisphere plots.
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APPLICATIONS OF THE ISCS CLIMATOLOGY Once a validated version of the climatology is released, it may be applied to the goals of the International Solar Cycle Study. The scientific focus of Working Group 1 Panel 2 of the International Solar Cycle Study may be stated as follows: To characterize quantitatively the short-to-long timescale variability of the solarphotonic (FUV, EW, andXUV) and particleflux so as to better understand the physical processes producing that solar variability and its effect on the terrestrial environment.
In the following subsections we suggest several ways in which the ISCS climatology may be applied to meet the particle aspect of the above objective. Effects on the Terrestrial Environment An important aspect of this climatology is the determination of ionization rates as functions of altitude at each location and activity level. These rates are available for any global scale study of atmospheric effects and can be incorporated as input parameters to global circulation and assimilative models as discussed below. Global Circulation Modeling. A primary use of the climatology is to provide an input for global circulation modeling. ‘Ihe NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) (Roble and Ridley, 1994) is one of the most advanced models for investigating effects on the atmosphere. It predicts winds, temperatures, major and minor composition, and electiodynamic quantities globally over an altitude range of -30 km to 500 km, which includes the upper stratosphere and mesosphere as well as the thermosphere and ionosphere. Inputs to the model are the particle and photonic energy inputs to the atmosphere, tidal inputs from the lower boundary, and the highaltitude electric fields. As an example of the use of the model to study storm temporal evolution and energy input, the TIME-GCM (with the ISCS climatology providing the particle inputs) may be run at selected intervals during the course of the storm. One likely focus will be on nitric oxide, as that constituent is expected to be enhanced during prolonged times of increased energy input. The storm-time response of the thermosphere-ionosphere system is reasonably well understood (Crowley et al., I898a, 1989b; Fuller-Rowe11et al., 1994,1996), and the ISCS climatology is not expected to change appreciably the model response above 110 km. On the other hand, below 100 km the atmospheric response is not well understood. Preliminary runs of the TIME-GCM using PEM average electron spectra measured during the November 1993 storm indicate increases in the mesosphem of NO and HOx by the order of 100% and 1O%,respectively, and corresponding teductions of ozone by a few percent. This run did not use the full PEM climatology and was not a long-term run of the TIME-GCM. However, it does indicate the significance of the global particle inputs to both the short and long-term atmospheric response during magnetically active intervals. In addition to studies of atmospheric effects during storm intervals, the climatology enables the study of solar particle inputs over much longer time intervals. The copious production of nitric oxide in the mesosphere produced by particle precipitation has been well documented (see Jackman et al., 1990). Because NO is long lived, transport is important. (e.g., Siskind et al. (1997), who have shown the importance of the transport of NO into the middle atmosphere). Application of the TIME-GCM with an accurate, global particle input will quantify the long-term production of NO (and other constituents), document the horizontal and vertical transport of NO, and quantify the effects of the transported NO on the mesosphere and stratosphere and at latitudes below the auroral precipitation region. In our own research we are currently porting the TIME-GCM to a metacomputing environment consisting of 40 workstations connected by an ATM fiber-optic network which provides communication speeds of 155 Mbps. This parallelized version of the TIME-GCM will run at speeds exceeding the performance of the present CRAY version, improving our efficiency for long-term applications of the model. Assimilative modeling. The assimilative mapping of ionospheric electrodynamics (AMIE) technique uses a method of data inversion to determine the high latitude (above 50’) ionospheric electric field pattern (Richmond and Kamide, 1988). The data inputs include ion velocities measured by radars, satellites, and ionosondes, ground magnetic perturba-
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tions, and magnetic perturbations measured by satellites. In addition, satellite measurements of precipitating particles and ultraviolet images of the aurora are used to determine the ionospheric conductances, which are needed to invert the ground magnetometer data. One of the important uses of the AMIE technique is the accurate specification of the electrodynamic state of the ionosphere which will be dependent on both the particle and photonic inputs. For example, in a magnetic storm study the ISCS climatology can be used as an input to AMIE to allow for a more accurate specification of the nightside condtctances and therefore, a more accurate specification of the electric and magnetic fields during active aurora1 conditions during the storm. Output from AMIE can also be used in the TIME-GCM, specifying the electric field upper boundary condition in a realistic fashion. The climatology described above can also be directly input into the TIME- GCM to specify the particle precipitation characteristics and electric fields which can significantly alter the chemistry in the model. By combiningthe output from both AMIE and the climatology as inputs into the TIME-GCM, a highly realistic thermosphere and mesosphere can be modeled. In summary, the ISCS climatology can be used in its own right to specify particle spectral inputs and ionization rates as functions of position in the aurora1 oval and to compute integrated global power inputs, or it can be used as inputs to atmospheric chemistry, global circulation, and assimilative models in order to study the effects of solar particle input and its variability on the atmosphere. Solar Particle Flux and its Variabilitv It is clear that the proposed climatology will make a contributbn toward understandingthe effects of solar particle input and its variability on the atmospheric system. The other part of the above focus statement deals with understandingthe processes producing that solar variability. This is certainly an importart as part of the Intemational Solar Cycle Study and will no doubt be undertaken vigorously within the solar community using the suite of observational and theoretical tools available. The ISCS climatology is a model whose primary use will be to study the variability of the solar particle flux as it effects the terrestrial atmosphere. However, it may also contribute to the study of more fundamental solar processes if a more direct link to the Sun can be established. This places prime importance on finding a solar parameter or index to which the climatology can be keyed. Recent studies using earth- and satellite-based(Yohkoh and SOHO) observations provide compelling evidence that coronal mass ejections are associated with very localized active regions (Hundhausen, 1997; Hudson and Webb, 1997). A recent study Bravo et al., (1998) found that intense geomagnetic storms (which will be readily modeled with the ISCS climatology) are each associated with a flare in one of the active regions adjacent to a coronal hole located near the Sun’s central meridian. As this kind of work continues it may soon be possible to obtain unambiguous signaturesof the locations on the Sun of sites producing geoeffective disturbances. The signature would thus become a solar proxy, which might ultimately provide a satisfactory key to the ISCS climatology enabling it to become not only reflective of fundamental solar processes, but a predictive model as well. CONCLUDING SUMMARY We have described the construction of a particle climatology for the study of the effects of solar particle fluxes on the atmosphere. The climatology is based on the UARS PEM particle climatology which will be improved by addition of more lowaltitude polar satellite data to improve both the size of the database and the global coverage, particularly at high latitudes. The resulting ISCS climatology is a statistical empirical model which will enable the user to obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. A unique aspect of this model is the inclusion of differential measurements of high energy fluxes that extend to the -MeV range. The climatology may be used as inputs to atmospheric chemistry, global circulation, and assimilative models in order to study a variety of effects of solar particle input and its variability on the terrestrial atmosphere. It may also contribute to the study of more fundamental solar processes if a proxy index to which the model may be keyed can be found.
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ACKNOWLEDGMENTS The work was supported by NASA grants NAG3-3 148 and NAG5685 and K. Butzke for helpful technical assistance.
1. We thank J. Mukherjee, E. Wilson,
REFERENCES Brasseur, G., and S. Solomon, Aeronomy of the Middle Atmosphere, D. Reidel, Dordrecht, The Netherlands (1986) Bravo, S., J. A. L. Cruz-Abeyro, and D. Rojas, The spatial relationshep between active regions and coronal holes and the Occurrence of intense geomagnetic storms throughout the solar activity cycle, Ann&es Geophysicue, 16,49-54 (1998). Codrescu, M. V., T. J. Fuller-Rowell, I. S. Kutiev, Modeling the F layer during specific geomagnetic storms, J.Geophys. Res., 102, 1431514329 (1997a). Codrescu, M.V., T.J. Fuller-Rowe& R.G. Roble, and D.S. Evans, Medium energy particle precipitation influences on the mesosphere and lower thermosphere, J. Geophys. Res., 102, 19,977 (1997b). Christon, S. P., D. J. Williams, D. G. Mitchell, C. Y. Huang, and L. A. Frank, Spectral characteristics of plasma sheet ion and electron populations during disturbed geomagnetic conditions, J. Geophys. Res., 96, 1-22 (1991). Crowley, G., B.A. Emery, R.G. Roble, H.C. Carlson and D.J. Knipp, Thermospheric dynamics during the September 18-19, 1984 storm: 1. Model Simulations, J. Geophys. Res. 94, 16925-16944, 1989a. Crowley, G., B.A. Emery, R.G. Roble, H.C. Carlson, J.E.Salah,V.B. Wickwar, K.L. Miller, W.L. Oliver,R.G. Bumside and F. A. Marcos, lhermospheric dynamics during the September 18-19,1984 storm 16959,1989b. Frahm, R. A., J. R. Sharber, R. Link, G. Crowley, J. D. Winningham, D. L. Chenette, and E. E. Gaines, B. J. Anderson, and T. A. Potemra, The diffuse aurora: a significant source of ionization in the middle atmosphere, J. Geophys. Res., 102,28,203-28,214 (1997). Fuller-Rowell, T. J. and D. S. Evans, Height-integrated Pedersen and Hall conductivity patterns inferred from the TIROS-NOAA satellite data, J. Geophys. Res., 92,7606-7618 (1987). Fuller-Rowell, T. J., M. V. Codrescu, R. J. Moffett, and S. Quegan, Response of the thermosphere and ionosphere to geomagnetic storms, J. Geophys. Res., 99,3893-3914, 1994. Fuller-Rowell, T.J., M.V Codrescu, H. Rishbeth, R.J. Moffett, and S. Quegan, On the seasonal response of the thermosphere and ionosphere to geomagnetic storms J. of Geophys. Res., 101,2343-2353, 1996. Hardy, D. A., M. S. Gussenhoven, and E. Holeman, A statistical model of aurora1 precipitation, J. Geophys. Res., 90, 42294248 (1985). Hardy, D.A., M.S. Gussenhoven, R. Raistrick, and W.J. McNeil, Statistical and functional representations of the pattern of aurora1 energy flux, number flux, and conductivity, J. Geophys. Res., 92, 12,275-l 2,294 (1987). Hardy, D. A., M. S. Gussenhoven, and D. Brautigam, A statistical model of aurora1 ion precipitation, J. Geophys. Res., 94,370 (1989). Hardy, D. A., W. McNeil, M. S. Gussenhoven, and D. Brautigam, A statitical model of aurora1 ion precipitation 2. Functional representation of average patterns, J. Geophys. Res., 96,5539 (1991). Hartz, T. R., Particle precipation patterns, in The Radiating Atmosphere (B. M. McCormac, ed.), 225-238, D. Reidel Publishing Company, Dordrecht, Holland (1971). Hudson, H.S., and D.F. Webb, Soft X-ray signatures of coronal ejections, Coronal Mass Ejections, Geophysical Monograph, 99, (N. Crooker, J.A. Joselyn, J. Feynman, eds.), 1-7, American Geophysical Union, Washington, DC (1997). Hundhausen, A.J., An introduction, Coronal Mass Ejections, Geophysical Monograph, 99, (N. Crooker, J.A. Joselyn, J. Feynman, eds.), 1-7, American Geophysical Union, Washington, DC, (1997). Jackman, C.H., A.R. Douglass, R B. Rood, R. D. McPeters, and P.E. Meade, Effect of solar proton events on the middle atmosphere during the past two solar cycles as computed using a two dimentional model, J. Geophys. Res., 95, 7417-7428 (1990). Lorence, L. J., Jr., CEPXS/ONELD version 2.0: A discrete ordinates code package for general one-dimensional coupled electron-photon transport, IEEE Trans. Nucl. Sci., 39, 103 1 (1992). Richmond, A.D., and Y. Kamide, Mapping electrodynamic features of the high-latitude ionosphere from Localized observations, J. Geophys. Res., 93,5741 (1988).
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Roble, R.G., and E.C. Ridley, A Thermosphere-Ionospher-Mesosphere-Electrodynamics General Circulation Model(Time-GCM): Equinox solar cycle minimum simulations (300-500 km), Geophys. Res. Letf., 21.,417-420 (1994). Sharber, J. R., R. A. Frahm, R. Link, G. Crowley, J. D. Winningham, E. E. Gaines, R. W. Nightingale, D. L. Chenette, B. J. Anderson, and C. A. Gurgiolo, UARS Particle Environment Monitor observations during the November 1993 storm: Aurora1 morphology, spectral characterization, and energy deposition, 1. Geophys. Res., 103,26,307-26,322 (1998). Siskind, David E., J.T. Bacmeister, M. E. Summers, and J. M. Russell III, Two-dimenssional model calculations of nitric oxide transport in the middle atmosphere and comparisons with Halogen Occultation Experiment data, J. Geophys. Res., (102), 3527-3545 (1997). Whalen, J. A., General characteristics of the aurora1 ionosphere in Physics of Space Plasmas, eds. T.S. Chang, B Coppi, J. R. Jasperse, pp 85-l 14, Scientific Publishers, Inc., Cambridge, MA (1982). Winningham et al., The UARS Particle Environment Monitor, J. Geophys. Res., 98, 10,649-10,666 (1993).
Adv. Space Res. Vol. 29, No. 10, pp. 1513-1522.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02 $22.00 + 0.00 PII: SO273-1177(02)00212-O
CONSTRUCTION OF A PARTICLE CLIMATOLOGY FOR THE STUDY OF THE EFFECTS OF SOLAR PARTICLE FLUXES ON THE ATMOSPHERE
J. R. Sharber’, J. D. Winningham*,
R. A. Frahm*, G. Crowley*, A. J. Ridley3, and R. Link2
‘NASA Headquarters, 300 E Street SW, Washington, DC 20546, U.S.A. ‘Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238, U.S.A. 3Universiv of Michigan, 2455 Hayward Street, Ann Arbor, Michigan 48109, U.S.A.
ABSTRACT In an investigation of the atmospheric effects of particle input, we are currently developing a particle climatology built using the UARS (Upper Atmosphere Research Satellite) particle database. UARS operates in a circular 585 km orbit and makes differential energy per unit charge measurements of electrons and positive ions in the range of a few eV to several MeV. The climatology is an empirical model that enables the user to obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. To further the goals of Working Group 1 of the International Solar Cycle Study (ISCS), we propose the construction of such a particle climatology. We suggest the UARS particle climatology as a starting point, to be augmented by additional iow-altitude particle data. In this paper we describe the development of the current climatology. show how it will be augmented to produce an improved ISCS climatology, and illustrate how this climatology will be used to carry out the objectives of the ISCS. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION A significant solar energy source in terms of its effect on planetary atmospheres is the outflow of particle energy from the Sun. Although we know that the energy and intensity of this outflow show spatial and temporal variations that depend on a variety of solar conditions, we lack an understanding of the nature of this dependence and of the mechanisms responsible for the particle production and energy variation. At the same time we are just now beginning to model globally the effects of this varying particle input on our atmosphere. One approach to improving our understanding of the production of particles on the Sun and their effects on the terrestrial atmosphere is to develop a model of particle input to that system. Ideally the model should provide spectral and total energy inputs as a function of position on the Earth as well as the integrated global or hemispheric power input. In principle, such a model would be related to the Sun by use of a suitable solar activity parameter to serve as a model sorting index. We propose to achieve this goal by building on a climatological model currently being developed using data from the Upper Atmosphere Research Satellite Particle Environment Monitor (UARS PEM). PEM operated continuously since the UARS launch in September 1991 until March 1995, after which time (until the present) it has operated on a duty cycle of -20%. The PEM particle detectors make differential spectral measurements over a wide energy range 5 eV to 5 MeV for electrons and 5 eV to 150 MeV for protons. (Details of the PEM instrumentation may be found in Winningham et al. (1993).) The result is a large body of particle observations beginning just after the peak of Solar Cycle 22, extending through its declining phase, and continuing into the ascending phase of Cycle 23. The PEM particle climatology was developed to exploit this data resource. It is being developed specifically to carry out one of the primary UARS/PEM goals, that of determining the magnetospheric and solar particle inputs to the terrestrial atmospheric system.
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UARS SPECTRAL MEASUREMENTS AND IONIZATION RATES We show some typical PEM electron spectra in Figures la and lb . These are selected from the larger body of spectra reported by Sharber eral. (1998) and were measured during the major magnetic storm of November 1993. They are measured at 585 km in the aurora1 precipitation region, one at 1102:(37-41) UT in the midnight sector and one at 1110:(40-44) UT from a region of softer precipitation later in the morning sector (4.7 MLT). Each spectrum is measured within the loss cone and consists of 63 energy levels which cover the full measurement range from 5 eV to 5 MeV. The intermediate to high energy portions of each spectrum have been fitted with kappa distributions (see Christon er al., (199 1)). The kappa distribution contains a near-thermal peak at characteristic energy E, which connects smoothly to a power law of slope -K at high energies. The kappa fitting parameters are shown on each plot. The ionization production rates computed from each spectrum are shown in Figure lc. They are computed assuming isotropic incidence using the CEPXS/ONELD multi-stream discrete ordinates code (Lorence, 1992), which solves the coupled electron-photon Boltzmann transport equations over the energy range 1 keV - 100 MeV. Included in the computations is the ionization of the bremsstrahlung X-rays produced by the incident electrons. This is seen as the secondary ledge of ionization beginning between 50 and 60 km and results in ionization down to stratospheric levels. The profile resulting from the most energetic spectrum shows an ionization rate peak of IO5cmm3s-’at an altitude of 96 km. As a reference marker, we include in the figure the rate of ionization production at 50” geographic latitude resulting from galactic cosmic rays (GCR) during solar minimum (top edge of curve) and solar maximum (lower edge of curve) conditions (Brasseur and Solomon, 1986). The ionization resulting from the spectrum of Figure la exceeds that due to cosmic rays at all altitudes above 33 km. Thus the ionizing effects of this spectrum reach the upper stratosphere. Additional electron spectra and a discussion of effects on the atmosphere may be found in Frahm et al. (1997). The UARS PEM CLIMATOLOGY
In the PEM climatology the fluxes at each energy level are binned into lo bins of invariant latitude (IL), 1 hour bins of magnetic local time (MLT), 1 hour bins of universal time (UT), and bins of activity levels of Dst, K,, and F10,7, and PC (the polar cap index). The latitudinal coverage is between 40” and 74” IL (Southern Hemisphere) and 70” IL (Northern Hemisphere), the maximum latitude being determined by the UARS 57’ inclination. Statistics are kept separately for the Northern and Southern Hemispheres. The PEM climatology is an empirical statistical model built using the PEM database from which the user may obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. A unique aspect of this model is the inclusion of differential measurements of high energy fluxes that extend to the -MeV range. The PEM climatology contains the ion (proton) component as well as electron component. Some of the attributes of the UARS climatology may be summarized as follows: Angular coverage and resolution are adequate to differentiate between precipitating and trapped particle fluxes. Local time and geographic longitude coverage are excellent with the exception of high magnetic latitudes (>73” IL in S. Hemi.; >68” in N. Hemi.) which are not sampled by UARS. As we discuss in a later section, this coverage will be increased by inclusion of high-latitude data from other spacecraft. The model is 3D in that the ionization production rates are functions of altitude. Spectral energy resolution is sharp enough to produce an accurate energy deposition (ionization rate) profile. The energy range is sufficiently high to investigate stratospheric effects. (X-ray production by the incident electrons is included.) The model is keyed to the D, I$, PC, and F10.7indices.
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Fig. 1. (a,b) Particle Environment Monitor electron spectra measured during the November 1993 magnetic storm (Sharber et al., 1998). Fit parameters for the kappa distributions are shown beside each plot. (c) Ionization production rates computed from the spectra. During the main phase of the storm, the ionization production rate by incident electrons and bremsstrahlung X-rays (solid curve) exceeded that of galactic cosmic rays (GCR) at all altitudes above 33 km.
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An example of the UARS climatology of electron number flux at an energy of 2.33 keV, incident on the Northern and Southern Hemispheres is shown in Figure 2a. Each polar plot is shown in invariant latitude (40” to 90“) and magnetic local time (O-24 MLT), and each represents a Kp value from 0 to 5. The energy input from the aurora1 oval is clear at all levels and shows a general increase in width and intensity with increasing Kp. The global distributions are consistent with the shape of the oval for various activity levels shown by previous studies (e.g., Hardy et al., 1985).
Kp
In Figure 2b we show similar plots for precipitating
flux at much higher energy, 152 keV. The main band of precipita-
tion at intermediate latitudes is clearly seen in both hemispheres and shows much smaller variation with Kp. These electrons are associated with the outer radiation belt and cause ionization in the ionospheric D-region (Hartz, 1971; Whalen, 1982). The band of precipitation seen at low latitudes (40’ - 47’) in the Southern Hemisphere is flux from the South Atlantic Anomaly. It is not present in the Northern Hemisphere. The distributions of Figure 2 are for precipitating electrons only. This version of the UARS climatology contains 507 days of PEM spectral measurements made between Day 280 of 1991 and Day 341 of 1993. IMPROVING
THE CLIMATOLOGY
FOR ISCS
The UARS PEM data set is limited in two ways: (1) the latitudinal coverage and (2) the amount of data available. The restriction of latitude coverage results from the 57” inclination of the UARS orbit. This is not a severe problem during disturbed times when the oval is expanded; however, during quieter times, the orbits usually do not cross the auroral oval. To produce a fully global climatology the current model will need to be augmented to provide the required coverage. To be consistent with ISCS goals the database should be extended to cover a time interval equivalent to at least one solar cycle. An obvious candidate to extend the current UARS low-altitude database is the NOAA SEM (Space Environment Monitor) database. The SEM instruments have flown on NOAA or TIROS low-altitude satellites since 1978, and have thus produced an enormous amount of spectral data. The SEM high energy particle detectors measure both the precipitating and trapped components. Electrons are measured in three integral energy bands of E>30, E>lOO, and E>300 keV; protons are measured in 5 bands between 30 and 2500 keV (see Codrescu et al., 1997b). Low energy precipitating electrons and protons are measured with electrostatic analyzers over the range 0.3 to 20 keV. The NOAA inclination of -98” provides coverage to 90” invariant latitude. Data are archived at the National Geophysical Data Center. Another candidate is the large-volume database of the DMSP particle measurements also archived in the National Geophysical Data Center. This database contains differential measurements of precipitating electrons and positive ions over the range -30 eV to 30 keV and contains data as far back as 1976 (see Hardy et al., 1985; 1987). These two databases offer significant advantages. In both cases the data are obtained from low altitude polar satellites which have operated over two solar cycles. The particle data have been archived and are in the public domain. The archived data from both have already been used to develop auroral energy input models (i.e., the Air Force Research Laboratory models of Hardy et al. (1985; 1987; 1989; and 1991) and NOAA models of Fuller-Rowe11 and Evans (1987), Fuller-Rowe11 ef al. (1994), and Codrescu et al. (1997a; 1997b)). These models will provide benchmarks against which to validate our improved climatology, and because of their heritage, will provide insight into results expected from the application of our climatology. The resulting climatology will have the advantage of the extended, spectrally-resolved energy range of the UARS measurements as well as the greater global and temporal coverage provided by the instruments on NOAA and DMSP. The inclusion of additional particle databases will require both intercalibration of added data with UARS particle measurements and validation of the climatology as the data are added. These procedures will be accomplished using satellite conjunctions where possible and by comparison of the climatology spectral outputwith selected overpasses and images obtained from UARS, DMSP, or ISTP/Polar. When the current PEM climatology has been augmented and validated, it will be made available to the community at large, probably by release on a local website. This new climatology will not be static, but will instead be upgraded whenever it is deemed appropriate to add additional databases. It is particularly important that version control be maintained.
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Fig. 2a. Example of the UARS PEM climatology for electron number flux at 2.33 keV for the Northern and Southern Hemispheres. Each polar plot is shown in invariant latitude (40” to 90’) and magnetic local time (O-24 MLT) and each represents a K, value from 0 to 5.
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Fig. 2b. Same as Figure 2a except for electron number flux at 1.52 MeV. Note that, in addition to the radiation belt input, the contribution from a portion of the South Atlantic Anomaly is evident at the lowest latitudes (-40” 47’) in the Southern Hemisphere plots.
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APPLICATIONS OF THE ISCS CLIMATOLOGY Once a validated version of the climatology is released, it may be applied to the goals of the International Solar Cycle Study. The scientific focus of Working Group 1 Panel 2 of the International Solar Cycle Study may be stated as follows: To characterize quantitatively the short-to-long timescale variability of the solarphotonic (FUV, EW, andXUV) and particleflux so as to better understand the physical processes producing that solar variability and its effect on the terrestrial environment.
In the following subsections we suggest several ways in which the ISCS climatology may be applied to meet the particle aspect of the above objective. Effects on the Terrestrial Environment An important aspect of this climatology is the determination of ionization rates as functions of altitude at each location and activity level. These rates are available for any global scale study of atmospheric effects and can be incorporated as input parameters to global circulation and assimilative models as discussed below. Global Circulation Modeling. A primary use of the climatology is to provide an input for global circulation modeling. ‘Ihe NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) (Roble and Ridley, 1994) is one of the most advanced models for investigating effects on the atmosphere. It predicts winds, temperatures, major and minor composition, and electiodynamic quantities globally over an altitude range of -30 km to 500 km, which includes the upper stratosphere and mesosphere as well as the thermosphere and ionosphere. Inputs to the model are the particle and photonic energy inputs to the atmosphere, tidal inputs from the lower boundary, and the highaltitude electric fields. As an example of the use of the model to study storm temporal evolution and energy input, the TIME-GCM (with the ISCS climatology providing the particle inputs) may be run at selected intervals during the course of the storm. One likely focus will be on nitric oxide, as that constituent is expected to be enhanced during prolonged times of increased energy input. The storm-time response of the thermosphere-ionosphere system is reasonably well understood (Crowley et al., I898a, 1989b; Fuller-Rowe11et al., 1994,1996), and the ISCS climatology is not expected to change appreciably the model response above 110 km. On the other hand, below 100 km the atmospheric response is not well understood. Preliminary runs of the TIME-GCM using PEM average electron spectra measured during the November 1993 storm indicate increases in the mesosphem of NO and HOx by the order of 100% and 1O%,respectively, and corresponding teductions of ozone by a few percent. This run did not use the full PEM climatology and was not a long-term run of the TIME-GCM. However, it does indicate the significance of the global particle inputs to both the short and long-term atmospheric response during magnetically active intervals. In addition to studies of atmospheric effects during storm intervals, the climatology enables the study of solar particle inputs over much longer time intervals. The copious production of nitric oxide in the mesosphere produced by particle precipitation has been well documented (see Jackman et al., 1990). Because NO is long lived, transport is important. (e.g., Siskind et al. (1997), who have shown the importance of the transport of NO into the middle atmosphere). Application of the TIME-GCM with an accurate, global particle input will quantify the long-term production of NO (and other constituents), document the horizontal and vertical transport of NO, and quantify the effects of the transported NO on the mesosphere and stratosphere and at latitudes below the auroral precipitation region. In our own research we are currently porting the TIME-GCM to a metacomputing environment consisting of 40 workstations connected by an ATM fiber-optic network which provides communication speeds of 155 Mbps. This parallelized version of the TIME-GCM will run at speeds exceeding the performance of the present CRAY version, improving our efficiency for long-term applications of the model. Assimilative modeling. The assimilative mapping of ionospheric electrodynamics (AMIE) technique uses a method of data inversion to determine the high latitude (above 50’) ionospheric electric field pattern (Richmond and Kamide, 1988). The data inputs include ion velocities measured by radars, satellites, and ionosondes, ground magnetic perturba-
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tions, and magnetic perturbations measured by satellites. In addition, satellite measurements of precipitating particles and ultraviolet images of the aurora are used to determine the ionospheric conductances, which are needed to invert the ground magnetometer data. One of the important uses of the AMIE technique is the accurate specification of the electrodynamic state of the ionosphere which will be dependent on both the particle and photonic inputs. For example, in a magnetic storm study the ISCS climatology can be used as an input to AMIE to allow for a more accurate specification of the nightside condtctances and therefore, a more accurate specification of the electric and magnetic fields during active aurora1 conditions during the storm. Output from AMIE can also be used in the TIME-GCM, specifying the electric field upper boundary condition in a realistic fashion. The climatology described above can also be directly input into the TIME- GCM to specify the particle precipitation characteristics and electric fields which can significantly alter the chemistry in the model. By combiningthe output from both AMIE and the climatology as inputs into the TIME-GCM, a highly realistic thermosphere and mesosphere can be modeled. In summary, the ISCS climatology can be used in its own right to specify particle spectral inputs and ionization rates as functions of position in the aurora1 oval and to compute integrated global power inputs, or it can be used as inputs to atmospheric chemistry, global circulation, and assimilative models in order to study the effects of solar particle input and its variability on the atmosphere. Solar Particle Flux and its Variabilitv It is clear that the proposed climatology will make a contributbn toward understandingthe effects of solar particle input and its variability on the atmospheric system. The other part of the above focus statement deals with understandingthe processes producing that solar variability. This is certainly an importart as part of the Intemational Solar Cycle Study and will no doubt be undertaken vigorously within the solar community using the suite of observational and theoretical tools available. The ISCS climatology is a model whose primary use will be to study the variability of the solar particle flux as it effects the terrestrial atmosphere. However, it may also contribute to the study of more fundamental solar processes if a more direct link to the Sun can be established. This places prime importance on finding a solar parameter or index to which the climatology can be keyed. Recent studies using earth- and satellite-based(Yohkoh and SOHO) observations provide compelling evidence that coronal mass ejections are associated with very localized active regions (Hundhausen, 1997; Hudson and Webb, 1997). A recent study Bravo et al., (1998) found that intense geomagnetic storms (which will be readily modeled with the ISCS climatology) are each associated with a flare in one of the active regions adjacent to a coronal hole located near the Sun’s central meridian. As this kind of work continues it may soon be possible to obtain unambiguous signaturesof the locations on the Sun of sites producing geoeffective disturbances. The signature would thus become a solar proxy, which might ultimately provide a satisfactory key to the ISCS climatology enabling it to become not only reflective of fundamental solar processes, but a predictive model as well. CONCLUDING SUMMARY We have described the construction of a particle climatology for the study of the effects of solar particle fluxes on the atmosphere. The climatology is based on the UARS PEM particle climatology which will be improved by addition of more lowaltitude polar satellite data to improve both the size of the database and the global coverage, particularly at high latitudes. The resulting ISCS climatology is a statistical empirical model which will enable the user to obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. A unique aspect of this model is the inclusion of differential measurements of high energy fluxes that extend to the -MeV range. The climatology may be used as inputs to atmospheric chemistry, global circulation, and assimilative models in order to study a variety of effects of solar particle input and its variability on the terrestrial atmosphere. It may also contribute to the study of more fundamental solar processes if a proxy index to which the model may be keyed can be found.
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ACKNOWLEDGMENTS The work was supported by NASA grants NAG3-3 148 and NAG5685 and K. Butzke for helpful technical assistance.
1. We thank J. Mukherjee, E. Wilson,
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Roble, R.G., and E.C. Ridley, A Thermosphere-Ionospher-Mesosphere-Electrodynamics General Circulation Model(Time-GCM): Equinox solar cycle minimum simulations (300-500 km), Geophys. Res. Letf., 21.,417-420 (1994). Sharber, J. R., R. A. Frahm, R. Link, G. Crowley, J. D. Winningham, E. E. Gaines, R. W. Nightingale, D. L. Chenette, B. J. Anderson, and C. A. Gurgiolo, UARS Particle Environment Monitor observations during the November 1993 storm: Aurora1 morphology, spectral characterization, and energy deposition, 1. Geophys. Res., 103,26,307-26,322 (1998). Siskind, David E., J.T. Bacmeister, M. E. Summers, and J. M. Russell III, Two-dimenssional model calculations of nitric oxide transport in the middle atmosphere and comparisons with Halogen Occultation Experiment data, J. Geophys. Res., (102), 3527-3545 (1997). Whalen, J. A., General characteristics of the aurora1 ionosphere in Physics of Space Plasmas, eds. T.S. Chang, B Coppi, J. R. Jasperse, pp 85-l 14, Scientific Publishers, Inc., Cambridge, MA (1982). Winningham et al., The UARS Particle Environment Monitor, J. Geophys. Res., 98, 10,649-10,666 (1993).