Quiet-time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja010220, 2004 Quiet-time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow D. J. Strickland, 1 R. R. Meier, 2 R. L. Walterscheid, 3 J. D. Craven, 4 A. B. Christensen, 3 L. J. Paxton, 5 D. Morrison, 5 and G. Crowley 6 Received 3 September 2003; revised 2 October 2003; accepted 30 October 2003; published 13 January [1] The TIMED/GUVI instrument is a far ultraviolet spectrograph that obtains images in five spectrally resolved wavelength channels. These images yield information on the dayside composition, temperature, solar EUV flux, large-scale wave structures, and auroral processes. In this paper we present an overview analysis of Earth-disk images for four seasons (March, July, and September 2002 and January 2003). Days were selected during geomagnetically quiet periods when the Sun was nearly in the orbital plane (noon orbits). Two of GUVI s five channels (designated as and LBH S and dominated by OI nm and short wavelength N 2 LBH band emission, respectively) are used when the instrument is in its imaging mode. These data are used to derive O/N 2 (column density ratio referenced to an N 2 column density of cm 2 ). The AURIC model is used to generate a lookup table that relates O/N 2 to the ratio of to LBH S for a given solar zenith angle. Global images of derived O/N 2 (designated as GUVI O/N 2 ) are presented for the 4 days. The initial validation of the retrieved composition ratio comes from comparison with the NRLMSIS model. Good overall qualitative agreement is obtained between GUVI and NRLMSIS. Both data and model exhibit similar latitudinal behaviors on the near-solstice days, namely a distinct gradient with O/N 2 decreasing from the winter to the summer hemisphere. Reductions in O/N 2 in the vicinity of magnetic poles are seen in both GUVI and NRLMSIS images. Globally, O/N 2 is smaller at the solstices and may be explained by the thermospheric spoon mechanism discussed by Fuller-Rowell [1998]. Alternatively, the greater overall values at the equinoxes may arise in part from global response to greater Joule heating at these times of the year. The sensitivity of O/N 2 to scalings of the N 2 LBH cross section and solar EUV below 20 nm is also addressed in response to recent papers on these topics. This initial look at the GUVI data demonstrates great promise of FUV remote sensing as a way to observe thermospheric composition changes over broad geographic scales. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 0355 Atmospheric Composition and Structure: Thermosphere composition and chemistry; 0358 Atmospheric Composition and Structure: Thermosphere energy deposition; 2427 Ionosphere: Ionosphere/atmosphere interactions (0335); KEYWORDS: thermosphere, far ultraviolet, seasonal behavior, remote sensing Citation: Strickland, D. J., R. R. Meier, R. L. Walterscheid, J. D. Craven, A. B. Christensen, L. J. Paxton, D. Morrison, and G. Crowley (2004), Quiet-time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow, J. Geophys. Res., 109,, doi: /2003ja Computational Physics, Inc., Springfield, Virginia, USA. 2 E.O. Hulbert Center for Space Research, Naval Research Laboratory, Washington D.C., USA. 3 Space Science Applications Laboratory, Aerospace Corporation, El Segundo, California, USA. 4 Geophysical Institute, Physics Department, University of Alaska Fairbanks, Fairbanks, Alaska, USA. 5 Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA. 6 Southwest Research Institute, San Antonio, Texas, USA. Copyright 2004 by the American Geophysical Union /04/2003JA Introduction [2] Satellite far ultraviolet (FUV) remote sensing is now playing a key role in space weather research on the global state of the thermosphere, solar extreme UV (EUV) variability, the ionosphere (especially the nightside equatorial ionosphere), and auroral processes (particle energy deposition, ionization, and composition changes). Important insights into thermospheric response to high-latitude forcing have already resulted from DE-1 FUV dayglow imaging data [Craven et al., 1994; Gladstone, 1994; Meier et al., 1995; Nicholas et al., 1997; Immel et al., 1997; Strickland et al., 1999a; Drob et al., 1999; Immel et al., 2001; Strickland et al., 2001a, 2001b]. Regions of O depletions and enhancements have been reported and related to high-latitude 1of10

2 heating, transport of disturbed air to lower latitudes, and changes to global circulation. Prölss and Craven [1998] observed negative ionospheric storm effects (reductions in NmF2, the peak electron density in the F2 layer) in regions of depressed dayglow associated with O depletions. Several more instances of this were reported by Strickland et al. [2001a, 2001b] in terms of O/N 2, the column density ratio referenced to a fixed depth in the column density of N 2. [3] Now, new opportunities exist through the routine measurements that are being made by the Global Ultraviolet Imager (GUVI) on board the Thermosphere, Ionosphere Mesosphere, Energetics, and Dynamics (TIMED) satellite [Christensen et al., 2003]. The TIMED satellite is in a 625 km circular orbit with an inclination of 74, allowing for dayglow (disk and limb), nightglow (again disk and limb), and aurora investigations. The present paper reports on a selected set of Earth-disk dayglow observations (i.e., viewing below the horizontal) by GUVI from which two parameters can be derived: O/N 2 and Q EUV, the latter being defined as the solar EUV/soft X-ray flux below 45 nm [Strickland et al., 1995]. These parameters, derived from extended spatial and temporal FUV observations, provide valuable diagnostic insights into thermospheric behavior and solar EUV variability. D. J. Strickland et al. (Solar EUV irradiance variability derived from terrestrial far ultraviolet dayglow observations, submitted to Geophysical Research Letters, 2004, hereinafter referred to as Strickland et al., submitted manuscript, 2004) demonstrate the value of the observations for monitoring the latter including solar flare activity. Here, attention is directed to O/N 2 to show its seasonal behavior under geomagnetically quiet conditions and its uncertainty with respect to key algorithm inputs. The days selected are 17 March (day 76), 15 July (day 196), 21 September (day 264), all in 2002 and 16 January The algorithm inputs of interest are the N 2 Lyman-Birge- Hopfield (LBH) cross section and the solar EUV irradiance spectrum (represented by the Hinteregger et al. [1981] model) with specific attention given to its scaling below 20 nm. [4] GUVI has a spectrographic and an imaging mode as discussed by Christensen et al. [2003]. Limited data are recorded in the spectrographic mode for in-flight stellar calibrations and to monitor any background changes that may occur across the observed spectral region (115 to 180 nm). Imaging data are addressed here and were obtained at a wavelength resolution of 2.1 nm in two of GUVI s five spectral channels designated as and LBH S. These cover the wavelength intervals [ nm] and [ nm], respectively. The channel designations refer to the dominant emissions within these intervals, namely, atomic oxygen emission at nm and N 2 LBH molecular bands, respectively. The nearby bright OI nm triplet does not contribute to the channel except as part of a scattered light component that is removed in routine GUVI data processing. The LBH subscript identifies the LBH S channel as containing short wavelength bands and distinguishes it from a second lower sensitivity LBH channel at longer wavelengths not addressed in this work (for dayglow investigations of disk data, the only effect of adding the long channel is a slight improvement in signal-to-noise). [5] The next section discusses our approach to deriving O/N 2. Section 3 presents global dayside images of GUVI and NRLMSIS O/N 2. A more detailed examination of O/N 2 is given in section 4 for single traversals through the dayside region. A summary completes the paper in section Approach [6] The derivation of O/N 2 from disk and LBH dayglow observations was first addressed by Strickland et al. [1995]. A variation on the remote sensing technique was used to examine the global behavior of O/N 2 under disturbed conditions from DE-1 FUV observations as reported by Strickland et al. [1999a, 2001a, 2001b] and Drob et al. [1999]. The algorithm paper of D. J. Strickland (Dayglow and auroral remote sensing algorithms utilizing satellite far ultraviolet observations of the Earth s disk, manuscript in preparation, 2004, hereinafter referred to as Strickland, manuscript in preparation, 2004) applies the discussion in the 1995 paper to GUVI and LBH S channel observations. A lookup table is used to derive O/N 2 from a measured pair of and LBH S values. The table is produced from a series of AURIC runs [Strickland et al., 1999b] using a single Hinteregger solar EUV spectrum and an NRLMSIS atmosphere [Picone et al., 2002] with scalings of the O density profile to achieve a range of O/N 2 values. The outputs are nadir 135.6, nadir LBH S, and their ratio as functions of O/N 2 and solar zenith angle (SZA). Nadir values are sufficient given the nearly constant behavior of the ratio as a function of viewing angle across the disk (in the absence of auroral contamination and when either constrained in SZA or adjusted for its variation). This behavior of the ratio is seen in GUVI data and in our model calculations. Furthermore, any variations in SZA along a given line-of-sight for off-nadir disk viewing have an insignificant effect on the ratio. Using calculations of O/N 2 (referenced to an N 2 column density of cm -2 ) versus 135.6/LBH with 300 TIGCM atmospheres [Roble et al., 1987], Strickland et al. [1995] showed that a standard deviation of less than 1% exists in the functional relationship between the column density ratio and the intensity ratio (here, contains OI nm and LBH emission peaking at nm as in the present work). As discussed in the Strickland et al. [1995] paper, choosing the N 2 column density reference at cm -2 results in the smallest uncertainty for converting an intensity ratio to a column density ratio. [7] It is important to emphasize that O/N 2 is referenced to a fixed N 2 column density rather than to a fixed altitude. This is similar to the common practice of referencing atmospheric variables to fixed pressure levels. The 135.6/ LBH S column emission ratio is a measure of the relative abundance of O to N 2 (taking proper account of SZA) in the primary emission region where both features arise from photoelectron impact and thus compete for the energy loss of these nonthermal electrons. One must take into consideration that for any given atmosphere, the column density ratio changes with altitude or any other variable that specifies vertical location (e.g., the N 2 column density) since (volume) number density profile slopes are different for different species. 2of10

3 Table 1. Scaling Factors Applied to LBH Cross Section [Ajello and Shemansky, 1985] and to Hinteregger Spectrum Below 20 nm for Generating the Lookup Tables Used in This Study Lookup Table LBH Scaling Factor Hinteregger Scaling Factor [8] In the part of the atmosphere where individual constituents are in static diffusion and distributed according to their own scale heights (diffusively separated), column density ratios may be calculated from just the temperature profile and species number densities at some lower altitude (boundary condition). Heavy species (smaller-scale heights) are more affected by temperature changes than light species (largerscale heights) and for fixed boundary conditions (fixed altitude) the effect is to reduce the column abundance ratio of a light species to a heavy species where warming occurs. Thus changes due to species transport become convolved with changes due to warming (by whatever mechanism) referenced to a fixed altitude. By contrast, the O/N 2 column density ratio we have defined has the special property that it is not sensitive to thermal changes combined with static diffusion, in spite of the fact that such changes alter scale heights and can produce significant volume density changes in the upper thermosphere. This can be shown with a static diffusion model such as that of Jacchia [1977], in which volume densities are fixed in the lower thermosphere. The change in either 135.6/LBH S or O/N 2 with exospheric temperature is insignificant. We have also confirmed this analytically with a simple static diffusion relation in an isothermal atmosphere. A model such as NRLMSIS, on the other hand, does exhibit changes in O/N 2 with changing thermal (e.g., solar activity) conditions because there are also changes in the O concentration relative to N 2 in the lower thermosphere caused by dynamical/photochemical effects. We conclude from this discussion that the O/N 2 column density ratio largely filters out thermal changes and consequently is a measure of dynamical and possibly photochemical changes. [9] A two-dimensional interpolation is performed to derive O/N 2 using a measured value of 135.6/LBH S and its corresponding SZA value (smoothing of the and LBH S data is applied by averaging nearest-neighbor values). Away from the limb, 135.6/LBH S is essentially constant with viewing angle at a fixed SZA, so that use of a lookup table containing nadir 135.6/LBHS is a fast, reliable means of conversion to geophysical parameters. For the global images of GUVI O/N 2 to follow, O/N 2 is derived using Level 1b disk data files spanning the look angle (referenced to nadir) range from 59.2 (sunward side of orbit) to (antisunward side). Angles greater than 65.5 refer more appropriately to limb viewing rather than disk viewing. Nevertheless, there is no noticeable change in O/N 2 at these large angles caused by the tablebased algorithm. Off-nadir effects are discussed by Strickland (manuscript in preparation, 2004) who notes that O/N 2 errors of the order of 10% occur at 60 for large O/N 2 values but are smaller at lower values. [10] In section 4, the sensitivity of O/N 2 to the N 2 LBH cross section and the Hinteregger solar flux below 20 nm is addressed. These quantities are singled out because of recent discussions in the literature that recommend changes to their magnitudes as commonly used in the modeling of photoelectron fluxes and associated excitation/ionization rates. The favored LBH cross section has come from the laboratory measurements of Ajello and Shemansky [1985] while, for the solar flux below about 20 nm, a scaling of the Hinteregger spectrum by a factor of about two has been used following the recommendation of Richards and Torr [1984] based on their fitting of calculated photoelectron fluxes to measured spectra. With regard to the Ajello and Shemansky cross section, Budzien et al. [1994] and Eastes [2000] have argued for an increase by as much as 1.6. J. Ajello (private communication, 2001) also suggests an increase may be necessary since published emission-based measurements to date have not captured possible radiative cascade contributions from the N 2 a 0 and w electronic states. Recent crossed-beam measurements by Campbell et al. [2001] also imply an increase. With regard to the solar EUV flux, an analysis of solar EUV measurements from the Student Nitric Oxide Experiment by Bailey et al. [2000] led to their recommendation of an increase of the Hinteregger et al. [1981] solar fluxes below 20 nm by as much as a factor of four. [11] Based on the above considerations, four alternative lookup tables have been constructed to examine the sensitivity of O/N 2 to the LBH cross section and solar EUV flux below 20 nm. The selected scaling factors for the lookup tables are listed in Table 1. The factor of 1.4 is in line with results coming from a study by two of us (RRM and DJS) comparing O/N 2 from GUVI limb and disk data. Unlike O/N 2 derived from disk data, limb-based O/N 2 is, in principle, insensitive to scalings of the OI nm and LBH cross sections and relies, instead, on the shapes of the emission profiles as functions of tangent altitude. The global behavior of O/N 2 presented in the next section is based on lookup table 2 that was produced with the 1.4 scaling factor. It should be noted that errors in O/N 2 arising from cross section errors pertain to the relative error between the OI nm and LBH cross sections [Strickland et al., 1995]. Thus while we focus on the LBH cross section, the effect produced by the scaling of 1.4 can also be achieved by reducing the OI nm cross section by a similar factor. In section 4, the sensitivity of the O/N 2 algorithm to these scalings and its consequences for the GUVI data analysis is discussed. 3. Global Behavior of O/N 2 [12] The global behavior of GUVI O/N 2 for the four seasons can be seen in Figure 1 along with NRLMSIS O/N 2 for each of these days. Fourteen contiguous satellite orbits or revolutions of data (hereafter designated as revs similar to Table 2. Geomagnetic and Solar Input Parameters to NRLMSIS for Days of Interest Date DOY A p Daily F 10, F 10, Previous Day F 10, 81 Day Average 17 March July September January of10

4 the designation used for data files) are used for the GUVI images with the first beginning near 0 hrs UT. We note again that TIMED is in a circular 625 km orbit at a 74 inclination. The UT scales above the color bars emphasize the fact that results contained within each image cover approximately one day. The times marked as dots on the UT scales correspond to the location of TIMED at the equator, also indicated by dots in the images. The local time for each day is approximately noon when TIMED crossed the equator on the dayside. The saw-tooth patterns at the tops and bottoms reflect scan cutoffs due to either orbit angle or SZA constraints (O/N 2 is not derived beyond SZA = 90 o ). NRLMSIS O/N 2 is calculated using appropriate input parameters at each location. The geomagnetic and solar inputs for the four days are given in Table 2. [13] The thin gaps seen in Figure 1 near the equator are caused by lack of overlap in the disk scans. At higher latitudes, overlap does occur but the O/N 2 in these regions corresponds to the values derived in the latest revs (i.e., no attempt has been made to average values where overlap occurs). Red x symbols that identify the magnetic poles are included because high-latitude structure is often ordered in geomagnetic coordinates. Figure 2 gives solar (F 10.7 ) and geomagnetic (K p ) conditions for each day of interest and its previous 9 days. Solar activity is moderate while reasonably quiet conditions prevail for geomagnetic activity on the days of interest. Given the low level of geomagnetic activity, the images have been extended to about 70 N and S (where the SZA constraint permits the extension). An examination of the auroral ovals (north and south) recorded by GUVI confirms that auroral contamination on these quiet days has little impact on the derived O/N 2 near the tops and bottoms of the GUVI images. Another potential source of dayglow contamination, this time at low latitudes, is OI nm recombination emission (O + + e ) from the Appleton Anomaly region that has been observed by GUVI at its larger tangent altitudes where dayglow emission is weak (unpublished). Recombination emission within the channel for disk viewing is minor and leads to no discernable false enhancements in O/N 2 (bands to either side of the magnetic equator) presented here or derived by us from datasets at other times (over numerous days) Day 76 (3/17/02) [14] The equinox noontime behavior seen in GUVI O/N 2 (Figure 1a) is similar to that on other quiet days near the selected date. Both GUVI and NRLMSIS show O/N 2 decreases near the magnetic poles that are consistent with atmospheric upwelling caused by high-latitude heating. Outside of these regions, GUVI O/N 2 exhibits a broad, weak minimum at low latitudes with an offset to the south of the equator. The general behavior of smaller values in the vicinity of the subsolar point is consistent with greater heating and upwelling in this region. Toward higher latitudes, larger values occur in the northern hemisphere (30 to 60 ) compared to corresponding southern latitudes. NRLMSIS exceeds GUVI by as much as 25% at low to middle latitudes with an approximately constant latitudinal variation up to 30 to 40 on either side of the equator followed by weak decreases thereafter (with stronger decreases near the poles). The variations being described are much better seen in Figure 6 to be introduced in section 4. [15] The gaps that occur on the right and left of the images arise from the use of data from 14 revs, which constitute slightly less than 24 hours of observations. The first rev of the day occurs just to the left of the gap (on the right side of the panel). Each of the 14 swaths is associated with a single rev because the start of each rev (when TIMED crosses the equator moving north) occurs on the nightside. Looking ahead to the remaining images in Figure 1, similar gaps are seen in Figure 1b where again, the start of each rev on day 196 is on the nightside. By contrast, the gaps in Figures 1c and 1d have a different appearance where the revs on days 264 and 16 begin on the dayside, and consequently portions of two revs are needed to produce a full dayside swath Day 196 (7/15/02) [16] The images in Figure 1b under solstice conditions are much different from those in Figure 1a. Again, there is good qualitative agreement between GUVI and NRLMSIS, both in overall magnitude of O/N 2 and in latitudinal gradients. Globally, GUVI shows a reduction in O/N 2 on day 196 compared to day 76. This is consistent with the well-known semiannual variation. A proposed mechanism consistent with this observation is the thermospheric spoon mechanism discussed by Fuller-Rowell [1998]. He argued that asymmetrical heating under solstice conditions produces meridional circulation enhanced over that during equinox, thereby stirring the thermosphere. Such an effect leads to reductions in the O/N 2 number density ratio on constant pressure surfaces. This will also lead to reductions in the column density ratio. Another phenomenon that may play a role in the contrast between the observed equinox and solstice O/N 2 is the semiannual variation of Joule heating with the maxima in Joule heating occurring at the equinoxes [Walterscheid, 1982]. At the equinoxes this heating (which is a maximum at high latitudes) drives an equatorward circulation that opposes the poleward circulation driven by solar heating (which is a maximum at low latitudes). This should lead to some reduction in the net globally averaged upwelling and in turn greater overall O/N 2 than would otherwise be the case. As discussed in section 2, thermal changes alone have little effect on O/N 2 and here this applies to globally averaged temperature differences between solstice and equinox. [17] Both GUVI and NRLMSIS exhibit hemispherical asymmetries with smaller O/N 2 values in the Southern (summer) Hemisphere where stronger upwelling associated with solar EUV heating occurs [e.g., Carignan, 1975]. This latitudinal gradient in O/N 2 is also consistent with the wellknown seasonal or winter anomaly in the dayside ionosphere. Lower peak F-region densities (NmF2) at middle latitude occur in the summer compared with the winter hemisphere in spite of smaller SZAs [see, e.g., Rishbeth and Garriot, 1969]. Reduced O/N 2 is associated with upwelling of air from the lower thermosphere, which results in enhanced molecular concentrations at F-region heights, that in turn lead to increased electron-ion recombination and reductions in NmF2 [e.g., Rishbeth and Edwards, 1989, 1990; Burns et al., 1989]. We retrieved fof2 (plasma frequency corresponding to NmF2) from 4of10

5 Figure 1. GUVI and NRLMSIS O/N 2 on days 76 (3/17/02), 196 (7/15/02), 264 (9/21/02), and 16 (1/16/ 03). Quiet geomagnetic conditions prevail on these days for which daily K p is less than two. The time scales illustrate the fact that 24 hours of data were used to produce the GUVI images. Local times throughout the images at low latitudes are near noon. The selected dates serve to illustrate O/N 2 behavior under near-equinox and near-solstice conditions. 5of10

6 Figure 2. Solar EUV index F 10.7 and geomagnetic index K p on the days of interest and the 9 days preceding these dates. Labeling in the upper left corners of the panels (e.g., a-1 and a-2) correspond to labeling of the quadrants in Figure 1 (quadrant a for the example). selected ground-based ionosonde measurements (accessed through NOAA s NGDC website) for day 196 and found the expected behavior: lower midday fof2 values at, e.g., Wallops Island, VA (37.9 N) of 7 MHz and higher values at Grahamstown, S. Africa (33.3 S) of 9 MHz. NmF2 at Wallops is 0.6 that of Grahamstown. The corresponding O/N 2 values at these locations are 0.6 and 0.9, respectively. [18] The above effect leading to depressed NmF2 is also common to another well-known phenomenon, namely substorm related decreases in NmF2 (negative ionospheric storm effects). Strickland et al. [2001a, 2001b] observed significantly depressed NmF2 in high to middle latitude regions following various substorms, where reductions in O/N 2 (observed in DE-1 far ultraviolet imaging data) were present. Daniell and Strickland [2001] modeled NmF2 in one of these storm regions; they concluded molecular enhancements are required to match reduced ionosonde NmF2 values. Examples of other recent investigations supporting such behavior are Mikhailov and Foster [1997] (based on incoherent scatter radar data) and Richards and Wilkinson [1998] (based on ionosonde data). While these various studies focused on negative ionospheric storms, reduced O/N 2 and associated F-region molecular enhancements are common themes of their conclusions Day 264 (9/21/02) [19] The second example of global behavior of O/N 2 under quiet-time equinox conditions is given in Figure 1c. The overall behavior is similar to that on day 76 with a weak minimum in GUVI O/N 2 at low latitudes in the central region of the image. However, the reduced O/N 2 across the image at high northern latitudes is not present on day 76. NRLMSIS replicates qualitatively the lower O/N 2 reductions at the high northern (and southern) latitudes. In an attempt to isolate the differences between the two equinox days, we produced an NRLMSIS image for day 264 using the solar-geophysical conditions of day 76; the resulting image from day 264 is in close agreement with that of day 76 (although smaller values are still present across the top of 6of10

7 Figure 3. Nadir/near-nadir and LBHS data, their ratio, SZA, and longitude for revs 1478 and 3255 on days 76 and 196, respectively. A given data point is an average value obtained by a two dimensional sum over GUVI s 14 along-track spatial pixels and over cross-track nadir angles from 4.8 to +4.8 (13 pixels). the day 264 image, reflecting an apparent seasonal effect in NRLMSIS). We conclude that the differences in O/N 2 on the 2 days are mostly due to differences in geomagnetic forcings (From Figure 2, K p approaches 3 on day 264 and is less than 1 on day 76; relative activity is also higher on the preceeding day 263 compared to day 75.) 3.4. Day 16 (1/16/03) [20] Figure 1d shows O/N 2 on day 16, near winter solstice. The GUVI image exhibits approximately the opposite hemispherical behavior to that on day 196. The NRLMSIS image closely matches its hemispherical inverse on day 196. Although the agreement is good for GUVI O/N 2 in the summer hemispheres on these two days, a somewhat higher O/N 2 is present in the winter hemisphere on day 16 (as well as at the equator) than on day 196. Contrasting levels of solar activity over the several days leading up to the image days (see Figure 2) may play a role in explaining these differences. 4. Further Examination of O/N 2 [21] The above discussion was based on the use of lookup table 2 for deriving O/N 2. In this section, we look at the dependence of the derived O/N 2 on the assumed LBH cross section and solar EUV below 20 nm by presenting results using all four lookup tables identified in Table 1. For illustration, we use GUVI and LBH S measurements and their ratios from single dayside swaths near 0 o longitude on the four days addressed in the previous section GUVI Data [22] Observations taken between 50 S and 50 N on days 76 and 196 are displayed in Figure 3. Similar results from days 264 and 16 appear in Figure 4. The top panels give column emission rates for the and LBH S channels. Their ratios are plotted in each of the middle panels. The bottom panels give SZA and longitude for the observations. Because the revs on days 76 and 196 start at the nightside equator, the data are identified by single rev numbers. However, on days 264 and 16, the revs begin at the dayside equator so that portions of two revs are needed to identify the data in Figure 4. Each data point is an average obtained by a two-dimensional sum over GUVI s 14 along-track spatial pixels and over 13 cross-track nadir angles from 4.8 to +4.8 (a total of 182 pixels spanning along- and cross-track distances of 105 km and 71 km). The averaging reduces statistical uncertainty, while having no significant impact on spatial characterization. Error bars are not shown but are on the order of 10 15% for either channel as indicated by the point-to-point scatter. [23] Both compositional and SZA effects are present in the data. The intensities decrease with increasing SZA whereas their ratios increase. The ratio increase is due to the increase in altitude of solar EUV energy deposition with SZA; there, more O is encountered. Superimposed on this SZA behavior are compositional changes that are evident in 7of10

8 Figure 4. Similar to Figure 3 except for selected revs on days 264 (2002) and 16 (2003). the data from season to season. Under equinox conditions, 135.6/LBH S equals or exceeds a value of 1.2 (days 76 and 264) at all latitudes, whereas under near-solstice conditions (days 196 and 16), its value is 1.0 over extended latitudinal regions in the summer hemisphere. This leads to the smaller O/N 2 values discussed in section 3 and below Using Lookup Tables 1 4 to Derive O/N 2 [24] The lookup tables identified in Table 1 are used in derivations of O/N 2 from the latitudinal profiles of 135.6/ LBH S and SZA on days 76 and 264 in Figure 3. Latitudinal profiles of O/N 2 corresponding to the four lookup-tables are shown in the upper panel in Figure 5 for day 76. Corresponding profiles on day 196 appear in the lower panel. O/N 2 derived by using lookup table 2 is 20% greater than obtained using table 1. This corresponds to a scaling of the LBH cross section by 1.4 that in turn produces increases both the computed and LBH S channel intensities. The increase in arises from a minor LBH component within this channel and is a function of O/N 2 and SZA (e.g., smaller increases at large values of these variables due to increasing dominance by OI nm). The increase in the LBH S channel is essentially independent of these variables since all emission arises from N 2. [25] Increasing the modeled solar EUV flux below 20 nm by a factor of two over that in used in tables 1 and 2 (i.e., a factor of four over the original Hinteregger et al. flux) causes an increase in the derived O/N 2 by 8%. The larger solar EUV produces more FUV excitation at lower altitudes, where the N 2 density is larger than O, and therefore the predicted intensity ratio is lower. As a result, the inferred O/N 2 is larger for a given observation of the intensity ratio (Strickland, manuscript in preparation, 2004) GUVI and NRLMSIS O/N 2 [26] Figure 6 compares GUVI and NRLMSIS O/N 2 for all data in Figures 3 and 4 using lookup table 2. The upper panel addresses days 76 and 196 while the lower panel does the same for days 264 and 16. The differences between the various days and between GUVI and NRLMSIS have already been discussed in section 3. There is nearly a factor of two difference in O/N 2 between equinox and solstice (at 50 N in the upper panel and 50 S in the lower panel). GUVI O/N 2 for the two equinoxes is similar. The same is true for the two solstices in their respective summer and winter hemispheres. [27] GUVI and NRLMSIS latitudinal gradients agree at the solstices, although the overall magnitude of GUVI O/N 2 is 10% lower. There is a greater disagreement at the equinoxes, where the most striking difference is the broad low-latitude minimum in the GUVI O/N 2 compared to the more nearly flat behavior for NRLMSIS. GUVI O/N 2 at low latitudes is 25% less than NRLMSIS. We have also examined GUVI O/N 2 on other near-equinox quiet days and observe similar behavior, so the results in Figure 6 are representative of the seasonal behavior. As noted in section 3, these equinox minima coincide with maximum upwelling (versus latitude) associated with solar heating. It should be kept in mind that any errors in the excitation cross sections or solar EUV do not affect 8of10

9 Figure 5. O/N 2 corresponding to the data in Figure 3 using lookup tables 1 4. A scaling of the LBH cross section by 1.4 produces a change of 20% in O/N 2. Similarly, a scaling change of solar EUV below 20 nm by a factor of two (from the factor two to four within the tables) produces a change of 8% in O/N (9/21/02), and 16 (1/16/03), all at local times near noon. The key features at or near equinox (days 76 and 264) are weak gradients from low to middle latitudes with an overall magnitude in O/N 2 of approximately 0.9. GUVI O/N 2 exhibits weak noontime minima at low latitudes not exhibited by NRLMSIS and coincides with maximum upwelling associated with solar heating at these times of the year. Both GUVI and NRLMSIS exhibit reductions in O/N 2 in the vicinity of the south magnetic pole that can be attributed to heating and associated upwelling within auroral regions. Both also show a more subtle reduction in the vicinity of the north magnetic pole on day 76. By contrast, there are noticeable reductions on day 264 by both in this region as well as elsewhere above 50 N. The slightly higher geomagnetic activity on day 264 probably accounts for these differences. [30] O/N 2 near solstice (days 196 and 16) is in sharp contrast to the equinox observations. Distinctive latitudinal gradients are present with less O/N 2 in the summer hemisphere. Such behavior is expected due to the greater solar EUV heating in the summer hemisphere that leads to more upwelling and reductions in O/N 2. Thermal effects on density profiles (higher temperatures in the summer hemisphere) can be ruled out since, as discussed in section 2, O/N 2 is not sensitive to such effects. Here there is overall good agreement between GUVI and NRLMSIS except that GUVI O/N 2 is larger at high latitudes in the winter hemispheres. O/N 2 is lower compared to days 76 and 264, possibly as a result of the thermospheric spoon mecha- latitudinal profile shapes or the relative differences observed between the various days. 5. Summary [28] In this paper, we have presented the initial overview of GUVI Earth-disk observations of the O/N 2 column density ratio. This quantity is easily derivable from the ratio of atomic oxygen to molecular nitrogen intensity ratios, with the appropriate algorithm. The ratio is a remarkably sensitive diagnostic of thermospheric changes due to dynamical effects associated with (especially) geomagnetic or solar forcings. To establish a baseline of understanding for future work, we looked at data from four seasons under geomagnetically quiet conditions when the Sun was nearly in the orbital plane. The comparison for this preliminary validation of the GUVI results was with the NRLMSIS model [Picone et al., 2002]. Overall, we find good agreement between GUVI and NRLMSIS, although differences are indeed present. The latest available version (8) of the data was selected for the reported analysis. Key findings are summarized below Global Behavior of O/N 2 [29] Global images of GUVI and NRLMSIS O/N 2 were presented in Figure 1 on days 76 (3/17/02), 196 (7/15/02), Figure 6. Comparisons of GUVI to NRLMSIS O/N 2 for the revs addressed in Figures 3 and 4. 9of10

10 nism discussed by Fuller-Rowell [1998]. Changes in global circulation arising from globally enhanced Joule heating at the equinoxes [Walterscheid, 1982] may also play a role Sensitivity of O/N 2 to Scalings of the LBH Cross Section and Solar EUV [31] If the N 2 LBH electron impact excitation cross section were increased by 40%, the derived O/N 2 would increase by 20% with little impact from SZA variations. Changes arising from scaling the solar EUV are less. An increase of the derived O/N 2 by 8% arises from doubling the scaling below 20 nm from an original factor of two. Our preferred lookup table for deriving both O/N 2 and Q EUV is table 2 based on (1) the work of Budzien et al. [1994], Eastes [2000], and Campbell et al. [2001], (2) ongoing studies of spatially coincident GUVI disk and limb dayglow data, and (3) the paper of Strickland et al. (submitted manuscript, 2004) that reports Q EUV derived from GUVI data. Further insights into cross sections (OI nm and LBH) and solar EUV inputs for constructing lookup tables is expected from future studies of GUVI disk and limb dayglow data. [32] Acknowledgments. Support was provided through the Aerospace Corporation under NASA grant NAG Software support was provided by Jerome Alfred, Harold Knight, and Wade Woo of CPI. Partial support for RRM also came from the Office of Naval Research. [33] Arthur Richmond thanks Thomas J. Immel and another reviewer for their assistance in evaluating this paper. References Ajello, J. M., and D. E. Shemansky (1985), A re-examination of important N 2 cross sections by electron impact with application to the dayglow: The Lyman-Birge-Hopfield band system and NI ( nm), J. Geophys. Res., 90, Bailey, S. M., T. N. Woods, C. A. Barth, S. C. Solomon, L. R. Canfield, and R. Korde (2000), Measurements of the solar soft X-ray irradiance by the Student Nitric Oxide Explorer: First analysis and underflight calibrations, J. Geophys. 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Strickland (2001), Modeling negative ionospheric storm effects caused by thermospheric disturbances observed in satellite UV images, J. Geophys. Res., 106, 30,307. Drob, D. P., R. R. Meier, J. M. Picone, D. J. Strickland, R. J. Cox, and A. C. Nicholas (1999), Atomic oxygen in the thermosphere during the July 13, 1982 solar proton event deduced from far ultraviolet images, J. Geophys. Res., 104, Eastes, R. W. (2000), Modeling the N 2 Lyman-Birge-Hopfield bands in the dayglow: Including radiative and collisional cascading between the singlet states, J. Geophys. Res., 105, 18,557. Fuller-Rowell, T. J. (1998), The thermospheric spoon : A mechanism for the semiannual density variation, J. Geophys. Res., 103, Gladstone, G. R. (1994), Simulations of DE 1 UV airglow images, J. Geophys. Res., 99, 11,441. Hinteregger, H. E., K. Fukui, and B. R. Gibson (1981), Observational, reference and model data on solar EUV from measurements of AE-E, Geophys. Res. Lett., 8, Immel, T. J., J. D. Craven, and L. A. Frank (1997), Influence of IMF By on large-scale decreases of O column density at middle latitudes, J. Atmos. Terr. Phys., 59, 725. Immel, T. J., G. Crowley, J. D. Craven, and R. G. Roble (2001), Dayside enhancements of thermospheric O/N 2 following magnetic storm onset, J. Geophys. Res., 106, 15,471. Jacchia, L. G. (1977), Thermospheric temperature, density, and composition: New models, Rep. 375, Smithson. Astrophys. Observ., Cambridge, Mass. Meier, R. R., R. J. Cox, D. J. Strickland, J. D. Craven, and L. A. Frank (1995), Interpretation of Dynamics Explorer far UV images of the quiet time thermosphere, J. Geophys. Res., 100, Mikhailov, A. V., and J. C. Foster (1997), Daytime thermosphere above Millstone Hill during severe geomagnetic storms, J. Geophys. Res., 102, 17,275. Nicholas, A. C., J. D. Craven, and L. A. Frank (1997), A survey of largescale variations in thermospheric oxygen column density with magnetic activity as inferred from observations of the FUV dayglow, J. Geophys. Res., 102, Picone, J. M., A. E. Hedin, D. P. Drob, and A. C. Aikin (2002), NRLMSISE-00 empirical model of the atmosphere: Statistical comparisons and scientific issues, J. Geophys. Res., 107(A12), 1468, doi: /2002ja Prölss, G. W., and J. D. Craven (1998), Perturbations of the FUV dayglow and ionospheric storm effects, Adv. Space Sci., 22, 129. Richards, P. G., and D. G. Torr (1984), An investigation of the consistency of the ionspheric measurements of the photoelectron flux and solar EUV flux, J. Geophys. Res., 89, Richards, P. G., and P. J. Wilkinson (1998), The ionosphere and thermosphere at southern midlatitudes during the November 1993 ionospheric storm: A comparison of measurement and modeling, J. Geophys. Res., 103, Rishbeth, H., and R. Edwards (1989), The isobaric F2-layer, J. Atmos. Terr. Phys., 51, 321. Rishbeth, H., and R. Edwards (1990), Modeling the F2 layer peak height in terms of atmospheric pressure, Radio Sci., 25, 757. Rishbeth, H., and O. K. Garriot (1969), Introduction to Ionospheric Physics, pp , Academic, San Diego, Calif. Roble, R. G., J. M. Forbes, and F. A. Marcos (1987), Thermospheric dynamics during the March 22, 1979, magnetic storm: 1. Model simulations, J. Geophys. Res., 92, Strickland, D. J., J. S. Evans, and L. J. Paxton (1995), Satellite remote sensing of thermospheric O/N 2 and solar EUV: 1. Theory, J. Geophys. Res., 100, 12,217. Strickland, D. J., R. J. Cox, R. R. Meier, and D. P. Drob (1999a), Global O/N 2 derived from DE-1 FUV imaging dayglow data: Technique and examples from two storm periods, J. Geophys. Res., 104, Strickland, D. J., J. E. Bishop, J. S. Evans, T. Majeed, P. M. Shen, R. J. Cox, R. Link, and R. E. Huffman (1999b), Atmospheric ultraviolet radiance integrated code (AURIC): Theory, software architecture, inputs, and selected results, J. Quant. Spectrosc. Radiat. Transfer, 62, 689. Strickland, D. J., R. E. Daniell, and J. D. Craven (2001a), Negative ionospheric storm coincident with DE-1 observed thermospheric disturbance on October 14, 1981, J. Geophys. Res., 106, 21,049. Strickland, D. J., J. D. Craven, and R. E. Daniell (2001b), Six days if thermospheric/ionospheric weather over the Northern Hemisphere in late September 1981, J. Geophys. Res., 106, 30,291. Walterscheid, R. L. (1982), The semiannual oscillation in the thermosphere as a conduction mode, J. Geophys. Res., 87, 10,527. A. B. Christensen and R. L. Walterscheid, Space Science Applications Laboratory, Aerospace Corporation, El Segundo, CA 90245, USA. (andrew.b.christensen@aero.org; richard.walterscheid@aero.org) J. D. Craven, Geophysical Institute, Physics Department, University of Alaska Fairbanks, Fairbanks, AK 99775, USA. (craven@gi.alaska.edu) G. Crowley, Southwest Research Institute, San Antonio, TX 78238, USA. (crowley@picard.space.swri.edu) R. R. Meier, E. O. Hulbert Center for Space Research, Naval Research Laboratory, Washington, D.C., 20375, USA. (meier@uap2.nrl.navy.mil) D. Morrison and L. J. Paxton, Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA. (danny.morrison@jhuapl. edu; larry.paxton@jhuapl.edu) D. J. Strickland, Computational Physics, Inc., Springfield, VA 22151, USA. (dsrick@cpi.com) 10 of 10