G. A. Germany, P. G. Richards G. K. Parks, M. Brittnacher, and J. F. Spann
Presented at AIAA Plasmadynamics and Lasers Conference, June 25, 1997.
Images of the Earth's aurora, taken from space, can be used to exam plasma behavior throughout the magnetospheric regions surrounding the earth. The coupling of the magnetospheric plasmas through the ionosphere are discussed. A summary of past and current imaging technology is given and then specific examples of remote sensing are given using images from the Ultraviolet Imager aboard the POLAR satellite.
The plasmas in the near Earth space environment are of both ionospheric and solar wind origin and are spread across a volume of well over 10^16 cubic kilometers. The traditional approach of sending individual instruments for in situ monitoring of the plasma does not work well in the space about the Earth due to the large volume of space that must be sampled. The current modes of using multiplatform instrument campaigns with instruments positioned in key regions of space, e.g. the International Solar Terrestrial Physics (ISTP) program, is proving to be very successful. But even this approach is limited in its extent.
Fortunately the space plasma about the Earth is not random or chaotic in its distribution. Rather the plasma regions about the Earth are highly organized, highly coherent, and highly coupled [Siscoe, 1991b]. This organization is imposed by the Earth's magnetic field and its interactions with the solar plasma and the ionosphere. The interaction of the terrestrial magnetic field with the magnetized plasma streaming from the sun results in the formation of a complex magnetospheric cavity about the Earth, Figure 1. The principal features of the magnetosphere to note are the upstream bow shock, the magnetopause boundary, the long magnetic tail with its central plasma sheet, and the connection of the higher latitude magnetic field lines with all other features of the magnetosphere.
The magnetospheric regions are linked by extensive current systems, some of which flow through the Earth's ionosphere resulting in the auroral displays. The overall coherence of the magnetosphere enables us to examine a single observation of any one part of the magnetosphere in the context of the entire magnetospheric structure. One region of the coupled magnetospheric system, in particular, is uniquely suited for displaying the coherent nature of the magnetosphere. This region is the auroral zone found about the north and south magnetic poles of the Earth. These displays provide, in a relatively compact spatial location, compressed images of the plasma behavior throughout the extensive magnetospheric region. Imaging of the auroral regions from space thus allows estimation of the plasma processes and properties of magnetospheric structures throughout the region dominated by the Earth's magnetic field referred to collectively here as geospace. In this paper the use of global auroral imaging as diagnostics of the magnetosphere is reviewed with specific examples from the NASA Global Geospace (GGS) Ultraviolet Imager aboard the POLAR spacecraft [Torr et al., 1995].
The intense heat of the solar corona results in a constant mass loss in the form of the solar wind, a supersonic flow composed of protons with average energies near 1 kv and 100 ev electrons. The earth's magnetic field deflects the solar wind, forming a bullet shaped magnetospheric cavity with the dayside boundary about 10 Earth radii (Re) from the earth and a tail that stretches over 200 Re in a direction away from the sun.
Since the solar flow is magnetized, solar magnetic field lines can interact with terrestrial field lines, connecting across the magnetospheric boundary. This results in a configuration in which an electrical conductor (the solar plasma) moves in a magnetic field composed of the open field lines connected across the magnetospheric boundary. This constitutes a dynamo which, for the Earth, is capable of producing 20 - 150 kV of electric potential across the magnetosphere and can generate 10^6 MW of power.
The magnetic field lines act as conductors, tying together the various regions of geospace by magnetospheric current systems. The current systems close through the ionosphere generating auroral electrojets that carry currents on the order of 1 million amps. The ionosphere acts as a non- passive load on the dynamo circuits, changing in response to Joule heating and increased ionization. This changing load produces feedback throughout the magnetosphere. The aurora thus functions as a convenient screen displaying projections of behavior from throughout the extensive magnetospheric volume. To take advantage of this display proper instrumentation, along with a suitable vantage point, is needed. The best vantage point is from space.
Because of the importance of the auroral regions as a remote diagnostic of plasma processes and magnetospheric structure, spacebased instrumentation for imaging the auroral regions have been designed and operated for the last twenty-five years. These instruments were preceded by groundbased all-sky cameras that date back over forty years. The groundbased instruments are limited to visible wavelengths, of course, and can only view a limited section of the auroral region. Nevertheless, such cameras have proven very useful and are still in use today. For global viewing of the entire auroral zone, however, a spacebased viewing platform is needed.
A sampling of spacebased auroral imagers is given in Table 1. This table is not intended to be comprehensive. Rather it is intended to illustrate the general development of imager development to date. Spacebased imagers have been operational in some form since the early seventies. Initial optical observations used fixed direction photometers, typically with single filter bandpasses in the visible wavelengths. The first imaging capabilities came with the use of spin scan photometers that took advantage of spacecraft rotation combined with relative ground motion to build up an auroral image. The photometers also began devoting detector channels to specific spectral regions, including ultraviolet features. The imagers were typically limited to low orbital altitudes, however, which limited their ability to view the entire auroral region.
It was the imager on the DE-1 satellite [Frank et al., 1981] with an apogee near 3.65 Re (17,000 km altitude), however, that provided the first fully global auroral images on an extended basis. Much of our current understanding of auroral morphology and temporal evolution can be traced back to DE images. Later imagers have attempted to improve the spectral, spatial, and temporal resolution of the observed images to increase the science content retrieved from the data.
The ultraviolet imager on the Viking spacecraft [Anger et al., 1987] is an example of the increased spectral content of recent imagers with spectral channels devoted to atomic oxygen and N2 LBH emissions. As discussed below, this spectral selection allows quantification of the incident auroral energy flux. The latest generation of imagers, including those flown on the POLAR satellite, extends this quest for multispectral resolution by providing three separate imagers for the visible, ultraviolet, and, for the first time, X ray images of the aurora. The Visible Imaging System [Frank et al., 1995] is a descendent of the successful DE imager with improved filters, multiple scan mirrors, and the type of active targeting available with current CPU capabilities. The Ultraviolet Imager [Torr et al., 1995] employs advanced filter technology to spectrally resolve FUV features. Imagers continue to be important parts of planned future missions such as the Imager for Magnetopause to Aurora Global Exploration (IMAGE) Mission slated for launch near 2000.
Global auroral images have been most useful by providing information on the spatial and temporal morphology of the aurora. When used in conjunction with magnetospheric modeling and assumed mappings this provides information on the magnetospheric configuration and the solar wind behavior. Increasingly, auroral images are being used to provide quantitative estimates of energy and compositional changes during the auroral activity. Both of these funtions are discussed below.
A principal feature of using auroral images as remote diagnostics is the fact that distinct portions of the auroral regions can be mapped to specific magnetospheric regions, with varying degrees of success. This mapping can be exploited to allow remote monitoring of extended magnetospheric regions by observations of the localized auroral emissions. See Elphinstone et al. [1996] for a more detailed discussion of this idea of the aurora as a magnetospheric 'road map'. The principal mappings can be summarized in two rules. First, events closer to the magnetopause boundary region are seen in the aurora as events closer to local noon (dayside). Second, for a given local time, more equatorward events in the aurora correspond to events closer to the Earth. These simple mapping rules allow a great deal of quantitative probing of the magnetosphere using the auroral images.
For example, Elphinstone et al. [1994] used the mapping of dawn auroral fan arc structures to the dawn magnetopause and dayside flank regions of the magnetosphere to infer that the sources of the auroral structures were instabilities on the dayside of the magnetosphere. Similarly, Elphinstone et al. [1991] showed that the main UV auroral oval doesn't correspond to the open-closed field line boundary as previously expected but actually maps into the inner plasma sheet.
The auroral forms can be generally categorized as either dayside or nightside forms. The first of the mapping rules cited above tell us that nightside forms are indicators of the magnetotail behavior to which they are linked. The second rule supports the observations that the poleward auroral boundary is linked to the boundary plasma sheet while the more equatorward boundary is linked to the central plasma sheet. By monitoring the morphological changes of the observed aurora the (unseen) magnetospheric morphologies can be inferred. A classic example is the substorm model discussed below.
The spatial boundaries of the auroral regions can also serve as useful proxies of auroral activity. For example, the equatorward boundary of the auroral oval can be correlated with the geomagnetic activity index Kp as a proxy for activity and cross-cap electric potential [Gussenhoven et al., 1983]. As another example, the oval size is a direct function of the power generated by the solar wind dynamo [Siscoe, 1991] and inferences about the solar wind plasma, particularly the IMF direction and solar wind speed, can be made from the oval size and location.
The temporal changes of the aurora are most evident in substorm activity which is strongly correlated with IMF configurations. Based on auroral observations the substorm sequence is now classified as being composed of three main phases: growth, expansion, and recovery. In the growth phase, typically during southward IMF, the solar wind energy is transferred into the magnetosphere, presumably by magnetic reconnection. The cross tail current increases during this phase and tailward stretching of the field lines in the near Earth tail is seen. This is manifested in the aurora as an equatorward expansion of the auroral form. In the expansion phase the cross tail current is partially shorted along magnetospheric field lines through the ionosphere creating the substorm current wedge and the expansive growth of the auroral form. Finally the magnetosphere returns to the prestorm configuration in the recovery phase.
The UVI image in Figure 3 illustrates an auroral substorm beginning at 09:45 UT over northern Alaska. Airglow is seen in the images in the bottom right hand portion of the circular field of view over Greenland and northern Europe. Some dayside auroral activity is also seen near the dusk terminator. The temporal development of the 09:45 UT substorm is shown in Figure 3 as a mosaic of six images from 09:44 to 10:34 UT. (The image times for each image, from top left to bottom right, are 09:44, 09:52, 10:06, 10:18, 10:24, and 10:34 UT.) The images have been registered in magnetic latitude and local time coordinates. Such a representation makes the magnetospheric mapping much easier to visualize. The substorm begins with an intensification near 22:00 magnetic local time. This is then followed by a classic expansion eastward and poleward until near 10:34 UT when the auroral is in recovery leaving a boundary arc on the poleward boundary of the expansion. The substorm advances westward in a series of discrete intensifications with each new brightening taking place westward of the previous one. The edge of the UVI field of view is seen near 06:00 LT
Many studies attempt to correlate the time and location of substorm onset with multispacecraft and groundbased instrumentation to understand the magnetospheric changes associated with the substorm activity. One of the first opportunities for this type of coordinated analysis, using UVI images, occurred during the substorm seen in Figure 3. This corresponded to a perigee pass of the WIND spacecraft which placed both the WIND and GEOTAIL spacecraft in the Earth's magnetotail. At the same time the IMP8 and INTERBALL-TAIL satellites were upstream of the bow shock sampling the solar wind and the interplanetary magnetic field. Added to these platforms were NOAA, DMSP, and GOES spacecraft as well as ground observations. Taken all together this has served as an excellent opportunity to characterize substorm events from multiple perspectives [Angelopoulos et al, 1996; Germany et al., 1996]. This type of multi- instrument coordinated analysis has been one of the principal modes of study since the POLAR launch with UVI images being used to provide a reference for multiple magnetospheric and groundbased measurements [e.g. Brittnacher et al., 1996; Elsen et al., 1996; Horwitz et al., 1996; Nagai et. al., 1996].
One principal advantage of having a continuously operating camera is the ability to observe unexpected or rapidly changing auroral morphologies. An example of this capability occurred during the preparation of this manuscript with the magnetospheric response of the solar coronal mass ejection events observed at Earth on January 10 and April 10, 1997. UVI images from the first period show extremely distorted ovals with multiple, distorted transpolar arcs evident. Several intense substorm events were triggered during this both events.
With increased spectral resolution and improved modeling capabilities auroral images are increasingly being used to provide more quantitative estimates of the incident auroral fluxes and their effects on neutral composition. Prior to the use of images, this information was typically provided on a statistical basis based on low-altitude polar orbiting satellites. Some of the most comprehensive statistical maps of energy parameters are based on DMSP overflights [Hardy et al., 1985; 1989]. The use of auroral images allows near instantaneous mappings to be produced of the entire auroral region.
Stated in its most general terms, the main task in estimating the auroral energy characteristics is to find a spectral signature in the image that varies as a function of the incident energy. For example, the average energy of the incident auroral flux is typically identified by means of an altitude- dependent loss mechanism. Since the altitude of peak energy deposition is a function of energy, with lower altitudes corresponding to higher energies, the altitude-dependent loss mechanism translates into an energy-dependent loss mechanism.
For visible emissions the loss mechanism is typically chemical or collisional mechanisms that become prominent at lower altitudes. For example, the 630.0 nm 'red line' emission from atomic oxygen is produced by the decay of a long-lived atomic state that is collisionally deactivated at lower altitudes where collisions are more common. Therefore observed column intensities at this wavelength will decrease with increasing energy. By modeling the expected emission intensities the observed intensities can be used to estimate the incident energy of the auroral particles.
In the far ultraviolet (125.0 - 200.0 nm) the altitude- dependent loss mechanism is absorption by molecular oxygen. The O2 density decreases rapidly with increasing altitude and can serve as a significant loss mechanism only below about 150 km. The technique for using FUV emissions as remote diagnostics has been discussed elsewhere [Strickland et al., 1983; Germany et. al., 1994a,b; 1990; Lummerzheim et al., 1991] and will only be summarized here. The principal emissions in the FUV (125.0 - 200.0 nm) are atomic oxygen emissions and molecular N2 Lyman-Birge-Hopfield (LBH) emissions. Atomic nitrogen lines also appear. The LBH emissions are electric dipole forbidden and the only prominent excitation mechanism is electron impact excitation. Thus, in the absence of dayside photoelectrons, observed LBH intensities are direct diagnostics of the incident auroral flux. The O2 Schumann-Runge absorption continuum peaks within the UVI bandpass, decreasing with longer wavelength. FUV auroral emissions viewed from space in the spectral region of strong O2 absorption will exhibit losses, provided the incident auroral energy is high enough to reach the lower altitudes where O2 density is greatest. The observed emissions vary strongly (inversely) with increasing depth of penetration of the incident auroral electrons and hence with increasing energy, Figure 4.
The N2 LBH emissions can be divided into two regions: one at shorter wavelengths with significant losses due to O2 absorption (denoted LBHs and extending roughly from 140 to 160 nm) and longer wavelength emissions with less loss (LBHl, 160 to 180 nm). Modeling of expected emissions can be used to estimate incident energy flux from LBHl intensities and the average energy from the ratio of either OI 1356 or LBHs to LBHl. The 1356 emission is dependent on changes in the atomic oxygen density and can therefore exhibit significant variability. On the other hand since the emissions in the LBHs and LBHl filter passbands originate from the same species, their ratio is nearly independent of compositional changes with season or over a solar cycle [Germany et al., 1990]. The following discussion is therefore limited to the analysis of the LBHs/LBHl ratio, Figure 5.
In the ideal case, UVI energy analysis would be performed with two LBH bands - one at the wavelength of peak absorption and the other at a longer wavelength where O2 absorption is negligible. The longer wavelength emission would be essentially independent of average energy and solely dependent on energy flux. This is the case modeled previously by Germany et. al. [1994a,b; 1990]. In practice, the UVI LBH filters must necessarily include multiple LBH bands and therefore contain a range of loss factors. Thus the LBHl emission shows a weak dependence with average energy that is not indicated in the previous work by Germany et. al. [1994a,b; 1990]. This effect is small (<10% at 10 keV) and is not included in the analysis shown below. The result is a lookup table that can be used to estimate the energy parameters on a per pixel basis from the image. With this lookup table maps of energy parameters can be constructed.
Since auroral current systems close through the ionosphere, the ionospheric resistance as given by its conductance is a critically important parameter for thermospheric and magnetospheric modeling. Since global images can be used to estimate energy characteristics they can also be used as the basis for maps of global conductances. The derived energy characteristics depend on an assumed neutral composition. In the case of UVI we can determine the energy characteristics by dual methods and use the differences as diagnostics of neutral compositional changes. This offers the promise of extending the analysis to determine local compositional changes directly due to the auroral processes.
UVI images can be used to estimate the magnitude of the incident energy flux over the entire auroral zone, Figure 6. The inferred energy fluxes generally agree in magnitude and morphology with selected DMSP overflights [Germany et al., 1997]. The fluxes inferred from UVI images do not exhibit the same spatial or temporal resolution as in situ measurements but offer a global perspective unavailable from single satellite passes. The energy determinations have also been shown to agree with NOAA hemispherical power estimates [Brittnacher et al., 1997; Lummerzheim et al., 1997] and with radar estimates [Doe et al., 1997].
The near Earth space plasma environment is highly organized and coherent due to the structure imposed by the Earth's magnetosphere. Auroral emissions offer a convenient map of the plasma processes throughout the extensive magnetospheric cavity isolating the Earth from the solar wind. Imaging of the aurora from space offers monitors of the spatial and temporal morphology of the auroral processes. Increasingly images are being used to infer quantitative estimates of the processes and their affects on the earth's ionosphere. Since the ionosphere is intimately linked with the rest of the magnetosphere the auroral images provide insight to the plasma behavior throughout the vast magnetospheric region.
Angelopoulos, V., T. Phan, D. Larson, F. S. Mozer, R. P. Lin, M. Moyer, T. Mukai, K. Tsuruda, R. W. McEntire, D. J. Williams, G. Parks, R. Lepping, K. Yumoto, and E. Friis-Christensen, Correlative IMP8-WIND-GEOTAIL-POLAR measurements of magnetotail substorms, EOS Trans. AGU, 77(46), Fall Meet. Suppl., F609, 1996.
Anger, C. D., T. Fancott, J. McNally, and H. S. Kerr, ISIS-II scanning auroral photometer, Appl. Opt., 12, 1753, 1973.
Anger, C., D., et al., An ultraviolet auroral imager for the Viking spacecraft, Geophys. Res. Lett., 14, 387, 1987.
Brittnacher, M. J., G. K. Parks, L. J. Chen, R. Elsen, J. Spann, and G. Germany, Dipolarization of the plasma sheet observed without a classical substorm injection, EOS Trans. AGU, 77(46), Fall Meet. Suppl., F633, 1996.
Brittnacher, M., R. Elsen, G. Parks, L. Chen, G. Germany, and J. Spann, A dayside auroral energy deposition case study using the Polar Ultraviolet Imager, Geophys. Res. Lett., in press, 1997.
Doe, R. A., J. D. Kelly, D. Lummerzheim, G. K. Parks, M. J. Brittnacher, G. Germany, and J. Spann, Initial comparison of POLAR UVI and Sondrestrom IS radar estimates for auroral electron energy flux, Geophys. Rev. Lett., in press, 1997.
Elphinstone, R. D., J. S. Murphree, and L. L. Cogger, What is a global auroral substorm, Rev. Geophys., 34, 169, 1996.
Elphinstone, R. D., D. J. Hearn, and J. S. Murphree, Dayside aurora poleward of the main auroral distribution: Implications for convection and mapping, Physical Signatures of Magnetospheric Boundary Layer Processes, edited by J. A. Holtet and A. Egeland, pp. 189-200, Kluwer Acad., Norwell, Mass., 1994.
Elphinstone, R. D., D. J. Hearn, J. S. Murphree, and L. L. Cogger, Mapping using the Tsyganenko long mognetospheric model and its relationship to Viking auroral images, J. Geophys. Res., 96, 1467, 1991.
Elsen, R. K., G. K. Parks, M. J. Brittnacher, L. J. Chen, S. M. Petrinec, R. M. Skoug, R. M. Winglee, G. A. Germany, J. F. Spann, Mapping UVI images into the magnetosphere with empirical and global MHD magnetospheric models, EOS Trans. AGU, 77(46), Fall Meet. Suppl., F633, 1996.
Frank, L. A., J. D. Craven, K. L. Ackerson, M. R. English, R. H. Eather, and R. L. Carovillano, Global auroral imaging instrumentation for the Dynamics Explorer mission, Space Sci. Instrum., 5, 369, 1981.
Frank, L. A., J. B. Sigwarth, J. D. Craven, G. P. Cravens, J. S. Dolan, M. R. Dvorsky, P. K. Hardebeck, J. D. Harvey, and D. W. Muller, The Visible Imaging System (VIS) for the Polar spacecraft, Space Sci. Rev., 71, 297, 1995.
Germany, G. A., G. K. Parks, M. Brittnacher, J. Cumnock, D., Lummerzheim, J. F. Spann, L. Chen, P. G. Richards, and F. J. Rich, Remote determination of auroral energy characteristics during substorm activity, Geophys. Res. Lett., in press, 1997.
Germany, G. A., G. K. Parks, M. Brittnacher, L. Chen, R. Elsen, J. F. Spann, J. Cumnock, P. G. Richards, and F. Rich, Characterization of an auroral intensification using multiple spacecraft observations, EOS Trans. AGU, 77(46), Fall Meet. Suppl., F623, 1996.
Germany, G. A., M. R. Torr, D. G. Torr, P. G. Richards, Use of FUV auroral emissions as diagnostic indicators, J. Geophys. Res., 99, 383, 1994a.
Germany, G. A., D. G. Torr, P. G. Richards, M. R. Torr, and S. John, Determination of ionospheric conductivities from FUV auroral emissions, J. Geophys. Res., 99, 23297, 1994b.
Germany, G. A.., M. R. Torr, P. G. Richards, and D. G. Torr, The dependence of modeled OI 1356 and N2 LBH auroral emissions on the neutral atmosphere, J. Geophys. Res., 95, 7725, 1990.
Gussenhoven, M. S., D. A. Hardy, N. Heinemann, Systematics of the equatorward diffuse auroral boundary, J. Geophys. Res., 88, 5692, 1983.
Hardy, D. A., M. S. Gussenhoven, and E. Holeman, A statistical model of auroral electron precipitation, J. Geophys. Res., 90, 4229, 1985.
Hardy, D. A., M. S. Gussenhoven, and D. Brautigam, A statistical model of auroral ion precipitation, J. Geophys. Res., 94, 370, 1989.
Horwitz, J. L., G. Kunin, M. Hirahara, G. Germany, D. G. Brown, J. F. Spann, T. Nagai, and R. Lepping, Auroral and magnetospheric activity during intervals of steady southward interplanetary magnetic field from ISTP POLAR and WIND observations, EOS Trans. AGU, 77(46), Fall Meet. Suppl., F613, November 1996.
Imhof, W. L., K. A. Spear, J. W. Hamilton, B. R. Higgins, M. J. Murphy, J. G. Pronko, R. R. Vondrak, D. L. McKenzie, C. J. Rice, D. J. Gorney, D. A. Roux, R. L. Williams, J. A. Stein, J. Bjordal, J. Stadsnes, K. Njoten, T. J. Rosenberg, L. Lutz, and D. Detrick, The Polar Ionospheric X-ray Imaging Experiment (PIXIE), Space Sci. Rev., 71, 385, 1995.
Kaneda, E., Auroral TV observation by KYOKKO, in Proceedings of the International Workshop on Selected Topics of Magnetospheric Physics, p. 15, Japanese IMS Committee, Tokyo, 1979.
Lummerzheim, D., M. Brittnacher, D. Evans, G. A. Germany, G. K. Parks, M. H. Rees, and J. F. Spann, High time resolution study of the hemispheric energy flux carried by energetic electrons into the ionosphere during the May 19/20 auroral activity, Geophys. Rev. Lett., in press, 1997.
Meng, C.-I., and R. E. Huffman, Ultraviolet imaging from space of the aurora under full sunlight, Geophys. Res. Lett., 11, 315, 1984.
Meng, C.-I., R. E. Huffman, F. Del Greco, and R. Eastes, UVI images of the dayside auroral oval, abstract published in Eos Trans. AGU, 68, 396, 1987.
Murphree, J. S., R. A. King, T. Payne, K. Smith, D. Reid, J. Adema, B. Gordon, and R. Wlochowicz, The Freja ultraviolet imager, Space Sci. Rev., 70, 421, 1994.
Nagai, T., Y. Saito, T. Yamamoto, T. Mukai, A. Nishida, S. Kokubun, M. Hirahara, G. Germany, J. L. Horwitz, J. Spann, and M. Brittnacher, GEOTAIL-POLAR simultaneous observations of substorm onsets, EOS Trans. AGU, 77(46), Fall Meet. Suppl., F631, 1996.
Oguti, T., E. Kaneda, M. Ejiri, S. Sasaki, T. Yamamoto, K. Hayashi, R. Fujii, and K. Makita, Studies of auroral dynamics by aurora-TV on the Akebono (EXOS-D) satellite, J. Geomagn. Geoelectr., 42, 555, 1990.
Rogers, E. H., D. F. Nelson, and R. C. Savage, Auroral photography from a satellite, Science, 183, 951, 1974.
Shepherd, G. G., T. Fancott, J. McNally, and H. S. Kerr, ISIS-II atomic oxygen red line photometer, Appl. Opt., 12, 1767, 1973.
Siscoe, G. L., What determines the size of the auroral oval?, in Auroral Physics, edited by D.-I. Meng, M. J. Rycroft, and L. A. Frank, p. 159, Campbridge 1991a.
Siscoe, G., The magnetosphere: A union of interdependent parts, EOS, Transactions of the American Geophysical Union, November 5, 1991b.
Strickland, D. J., J. R. Jasperse, and J. A. Whalen, Dependence of auroral FUV emissions on the incident electron spectrum and neutral atmosphere, J. Geophys. Res., 88, 8051, 1983.
Torr, M. R., D. G. Torr, M. Zukic, R. B. Johnson, J. Ajello, P. Banks, K. Clark, K. Cole, C. Keffer, G. Parks, B. Tsurutani, J. Spann, A far ultraviolet imager for the International Solar-Terrestrial physics mission, Space Sci. Rev., 71, 329, 1995.
Figure 1.The Earth's magnetosphere formed by the interaction of the terrestrial magnetic field and the magnetized solar wind.
Figure 2.An auroral substorm on March 27, 1996 as seen by UVI. Airglow is seen in the image in the bottom right hand portion of the image over Greeland and nothern Europe.
Figure 3.Temporal development of substorm on March 27, 1986 as a mosaic of six images from 09:44 to 10:34 UT. The image times for each image, from top left to bottom right, are 09:44, 09:52, 10:06, 10:18, 10:24, and 10:34 UT.
Figure 4.LBH volume emission rates for Maxwellian incident energy flux, from Strickland et al., 1983.
Figure 5. Ratio of LBH long and short wavelength intensities as a function of auroral energy.
Figure 6. Map of incident energy flux and average energy derived from UVI LBHI and LBHs images . The images are from May 19, 1996.
Imager/Platform Era Wavelength (nm) Apogee (km) Notes
ISIS 2 1973 391.4, 555.7, 630.0 1,400 three imaging photometers, spin scan, 15 min DMSP 1974 400-113 800 Air Force, scanning radiometer KYOKKO 1979 first UV 4,000 2 min/frame DE 1 1981 115 - 180, 120 - 180, 17,000 three imaging photometers, 135 - 180 spin scan 12 min HILAT 1983 1,000 Air Force, line scan Viking 1987 124.0 - 150.0, 13,500 2 cameras, spin scan 133.0 - 195.0 Polar BEAR 1986 1,000 Air Force Akebono 1989 10,500 Freja 1992 130 - 160, 130.4 1,763 dual channel CCD PIXIE 1995 Xray 57,000 VIS 1995 multi-spectral visible 57,000 UVI 1995 multi-spectral far UV 57,000
