Auroral NO mm emission observed from the Midcourse Space Experiment: Multiplatform observations of 9 February 1997

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006ja012120, 2007 Auroral NO mm emission observed from the Midcourse Space Experiment: Multiplatform observations of 9 February 1997 R. R. O Neil, 1 J. R. Winick, 1 R. H. Picard, 1 and M. Kendra 2,3 Received 12 October 2006; revised 26 January 2007; accepted 22 March 2007; published 29 June [1] The Spatial Infrared Imaging Telescope III (SPIRIT III) radiometer on the Midcourse Space Experiment satellite (MSX), observed enhanced 4.3 mm emission from a very well characterized aurora over the Barents Sea on 9 February 1997, in conjunction with observations by the POLAR and FAST satellites, the Loparskaya ground site, and ultraviolet and visible spectrometers aboard MSX. Measurements of the auroral location, form, spatial extent and dosing conditions were applied to specify the component of auroral 4.3 mm radiance due to the slowly produced and optically thick CO 2 n 3 ( ) transition. An analysis based on the Auroral Atmospheric Radiance Code (AARC) indicates: (1) the emission originates near and beyond the tangent point; (2) the optically thick CO 2 n 3 radiation is largely self absorbed by the intervening atmosphere; and (3) the auroral enhancement is predominantly due to NO + Dv = 1 vibrational state transitions. In addition, the analysis indicates that the previously reported laboratory result for the NO + v 1 vibrational yield from the reaction, N + +O 2, is insufficient to account for the observed 4.3 mm emission. In order to explain the current results, we propose that there is additional production from the reaction, N O, forming NO + in vibrational levels 0, 1, and 2 with relative populations of approximately 0.25, 0.5, and 0.25, respectively. The combined production processes yield an energetic electron induced efficiency of 0.56 ± 0.18 photons per auroral ion pair for NO + Dv = 1 emission at altitudes equal to or greater than 112 km. Citation: O Neil, R. R., J. R. Winick, R. H. Picard, and M. Kendra (2007), Auroral NO mm emission observed from the Midcourse Space Experiment: Multiplatform observations of 9 February 1997, J. Geophys. Res., 112,, doi: /2006ja Introduction [2] The Midcourse Space Experiment (MSX), a Missile Defense Agency (formerly Ballistic Missile Defense Organization) research program, was designed to support the development of advanced infrared space-based sensors. As part of an extensive series of measurements, MSX successfully completed 150 episodic measurements of infrared atmospheric radiance and spatial structure in Earth limb scenes at tangent heights ranging from 30 to 150 km. The satellite was launched on 24 April 1996 into a nearly Sun-synchronous orbit at an altitude of 900 km, and infrared measurements continued until the cryogenic supply of solid hydrogen was depleted on 25 February The MSX program objectives, sensors, and experiments are described by Mill et al. [1994] and O Neil et al. [1994]. Measurements of the terrestrial background included the initial space-based 1 Space Vehicles Directorate, Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA. 2 Atmospheric Environmental Research, Inc., Lexington, Massachusetts, USA. 3 Now at Space Vehicles Directorate, Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA. Copyright 2007 by the American Geophysical Union /07/2006JA midwavelength infrared (MWIR) observations of atmospheric gravity waves emanating from the CO mm n 3 band radiance layer near 40 km [Dewan et al., 1998; Picard et al., 1998]. [3] The characterization of auroral infrared emission was an important part of the MSX background measurements program. A number of Earth limb data collection events were scheduled to sample the polar region where auroral activity was shown to be more probable by the model of Hardy et al. [1985, 1991] based on long-term exoatmospheric energetic particle statistics. An analysis of aurorally enhanced nitric oxide emission, measured by the MSX SPIRIT III (Spatial Infrared Imaging Telescope III) mm radiometer band on 10 November 1996, is given by Sharma et al. [2001]. [4] The location and availability of other space-based sensors and the lunar phase and elevation angle for ground-based optical sites were also considered in scheduling the MSX auroral region data collection events. On 9 February 1997, a combination of space- and ground-based sensors observed the X-ray, optical, and infrared emissions and exoatmospheric electron energy and flux density associated with an aurora in the vicinity of the Kola Peninsula during a geomagnetic storm with a Kp index of 5+. The MSX measurement was scheduled in coordination with the POLAR and FAST (Fast Auroral Snapshot Explorer) satel- 1of22

2 Figure 1. The northern polar region and locations of the MSX satellite and ancillary sensors on 9 February The day night terminator at 1927 UT is indicated for altitudes of 0, 95 and 900 km and the orbital tracks of the MSX (red trace) and FAST (white) satellites are shown at selected times in the period from 1925 to 1948 UT. The end point of the LOS (yellow) of the MSX sensors is the location of the bore-sight tangent point at an altitude of 95 km. The nadir point of the POLAR spacecraft and the 75 half angle field of view of the O ( 1 D) all sky camera at Loparskaya, projected to 250 km, are also shown. lites and 2 days after a new moon to provide optimum viewing conditions for ground-based optical instruments in the region. An automated all-sky camera site at the Loparskaya ground site in Russia recorded a sequence of auroral O ( 1 D) 630-nm images in the region sampled by the space-based sensors. Enhanced auroral atomic and molecular emissions were observed by the MSX ultraviolet and visible imaging (UVISI) sensors and the MSX SPIRIT III MWIR radiometer bands, (band B1) and mm (band B2), at line-of-sight (LOS) tangent heights in the range from approximately 65 to 125 km. We have used results from the PIXIE (Polar Ionospheric X-ray Imaging Experiment) sensor on the POLAR satellite, the particle energy and flux density detectors on FAST, and the O( 1 D) 630-nm all-sky camera at the Loparskaya ground site to characterize the aurora and to estimate, with the help of an auroral radiance model, the fractional components of the auroral SPIRIT III B mm radiometer band radiance that are due to NO + (Dv = 1) and CO 2 n 3 emission. Although infrared spectra were not available, the simultaneous measurements of the same region by the distributed sensors provided a means to assess the relative contributions of NO + and CO 2 based on two other distinguishing characteristics, the long time constant for production and the optical opacity of CO 2 in contrast to the prompt and optically thin nature of NO + radiance in aurora. [5] The following sections present a description of the measurements from the MSX satellite (section 2) and from the POLAR and FAST satellites and the Loparskaya ground site (section 3). Section 3 continues with an estimate of the auroral form, location, dosing time history and dimensions along the LOS of the MSX sensors. After a discussion of auroral 4.3 mm emission processes in section 4, the loss processes for vibrationally excited NO + are described (section 5). The measured auroral MWIR radiance attributed to CO 2 and NO + based on an AARC (Auroral Atmospheric Radiance Code) model simulation is presented in section 6. Section 7 presents an estimate of the efficiency for the NO + MWIR radiance produced by energetic auroral electrons, expressed as a normalized value in terms of NO + Dv=1 photons per ion pair. An additional production mechanism for vibrationally excited NO +, N O, is proposed to account for the derived efficiency. Finally, the conclusions are summarized in section Auroral Event and MSX Measurements [6] The locations of the MSX, POLAR, and FAST spacecraft and the MSX bore sight tangent point in the northern polar region on 9 February 1997 are shown in Figure 1, along with the locations of the day-night terminator at 1927 UT for the Earth s surface and altitudes of 95 and 900 km (the nominal MSX altitude). At UT, the sunlit MSX satellite, operating in a constant tangent height push-broom scan mode, was oriented to look into the nightside atmosphere with a shadow height of approximately 900 km at the 95 km line-of-sight tangent point. The MSX and FAST orbital tracks, the POLAR nadir point 2of22

3 Figure 2. Earth limb images recorded by the SPIRIT III radiometer MWIR band (Figure 2a), the narrow FOV ultraviolet imager (2b), and the wide FOV ultraviolet and visible imagers (2c and 2d). The arrows below Figure 2a indicate the time corresponding to the images in Figures 2b, 2c, and 2d. Auroral emissions are evident from 1928 to approximately 1935 UT with local maxima in Figure 2a at 1929 and 1933 UT. A quiescent midlatitude night atmosphere is shown at 1944:06 UT in Figures 2b, 2c, and 2d. and the Loparskaya all-sky camera field of view (FOV) projected to an altitude of 250 km for a 75 half-angle are illustrated in Figure 1. The all-sky camera FOV includes the region observed from 1927 to 1930 UT along the MSX LOS near and beyond the tangent point. The MSX satellite, at an altitude of 900 km, initiated the push broom scan at 1927 UT positioned at 78 N, 190 E and completed measurements at 1948 UT at 9 N, 136 E. The coaligned MSX instrument suite was pointed to maintain the bore-sight at a constant LOS tangent point altitude of 95 km at an off-track angle of 120, resulting in initial and final tangent point locations of 74 N, 29 E and 25 N, 113 E. The locations of the MSX LOS to the 95-km tangent point are shown in Figure 1 with the time of primary interest in the present analysis, 1929 to 1930 UT, emphasized as a shaded region. The off-track angle was selected to position the constant tangent height scan through the auroral oval at magnetic local times and magnetic latitudes with higher probabilities for activity [Hardy et al., 1985, 1991] and, in this instance, also to coordinate the MSX Earth limb data with the FAST exoatmospheric particle measurements. [7] Figure 1 illustrates that, at 1929 UT, the FAST satellite at an altitude of 3777 km was slightly beyond the region of the MSX LOS 95 km tangent point and was near the northeasterly edge of the FOV of the Loparskaya all sky camera. The POLAR satellite was located at 81 N, 105 E at an altitude of 50,155 km at 1929 UT. This location changed little during the conjunction period because the satellite was near apogee and had an orbital period of approximately 18 hours. [8] SPIRIT III radiometer images for the MWIR bands from 1928:00 to 1936:24 UT on 9 February 1997 are shown in Figure 2, together with representative image frames from the coaligned visible and ultraviolet imagers. The SPIRIT III radiometer, the primary MSX sensor, contained six discrete wavelength bands including two MWIR bands, band B1 ( mm) and band B2 ( mm). The MWIR bands shared a single linear focal plane array, separated by a masked region and oriented vertically in this measurement, with the broader B2 band recording the radiance at the higher tangent altitudes. The MWIR focal plane contained two linear columns of 192 pixels, providing a one degree FOV, equivalent to an altitude range of 65 to 125 km and a spatial resolution of 0.3 km at the tangent point. During this data collection event, the MSX SPIRIT III radiometer was operating in the mirror scan mode at a data rate of 360 Hz with the mirror scanning ±1.5 in the horizontal dimension at 0.5 per second. The mirror scan mode allowed the radiometer to obtain multiple samples of the same tangent point region as illustrated in Figure 2 as the mirror swept the scene in alternating horizontal directions. The narrow vertical bars separating each MWIR image in Figure 2a are internal views of the cold optical system occurring each time the mirror reaches the scan limit and reverses direction. Each 3 by 1 radiometer image represents a frame time of 7.2 s, including mirror reversal, and tangent point scene dimensions of approximately km. [9] Figures 2b 2d show a series of images for the coaligned narrow field ( ) ultraviolet imager and wide field (10 13 ) ultraviolet and visible imagers, respectively. The arrows below the MWIR data in Figure 2a 3of22

4 Figure 3. SPIRIT III radiometer MWIR limb profiles of enhanced auroral radiance (red) measured from 1929:10 to 1929:15 UT and an ambient midlatitude background (black) from 1944:03 to 1944:08 UT. The MWIR radiometer channel contained two filters separated by a mask, evident here as the region from 90 to 100 km. The upper and lower profiles correspond to the mm (B2) and the mm (B1) bands, respectively. indicate the times corresponding to coaligned ultraviolet and visible images. The ultraviolet and visible images in each column in Figures 2b 2d represent the same time period, recorded in sequence at intervals of 0.5 s; the first five columns are samples of auroral enhancements from 1929:14 to 1933:30 UT. The sixth column in Figures 2b 2d, at 1944:06 UT, illustrates a mid-latitude quiescent background, with the near infrared OH Meinel Band nightglow layer at 85 km evident in the nm wide FOV imager. [10] Local maxima in auroral MWIR radiance are apparent in Figure 2a at 1929 and 1933 UT. The greatest enhancement at 1929 UT is concentrated in the middle and upper portions of the wide field ultraviolet and visible imagers, Figures 2c and 2d, indicative of aurora near or beyond the tangent point of the sensors and partially masked by the terrestrial surface. This contrasts with the enhancement at 1933 UT which extends to lower portions of the wide FOV sensors, a feature characteristic of aurora located in the near field on the spacecraft side of the tangent point and observed against the solid earth. The relative intensities of the 4.3 mm radiometer and narrow FOV ultraviolet imager data show marked differences at 1929 and 1933 UT in Figure 2. The ultraviolet emission is slightly greater at the earlier time while the MWIR radiance is substantially greater at the later time. [11] MWIR 4.3 mm auroral enhancements are produced by radiation from vibrationally excited CO 2 and NO + [Whalen et al., 1985]. Winick et al. [1987b] review the production and loss processes for the CO 2 n 3 and NO + (Dv = 1) emission in auroras. They describe the complex and severe effects of self absorption on the 4.26 mm band of the 16 O 12 C 16 O (or 626, in HITRAN notation [Rothman et al., 2005]) main isotope along the LOS in earth limb views at tangent heights in the range from 90 to 110 km. The lower MWIR radiance observed at 1929 UT is attributed in part to self-absorption of the auroral CO 2 n 3 radiance emanating from more distant points along the LOS. The multiplatform measurements and the MSX ultraviolet and visible sensor data at 1929 UT provide a basis to characterize the auroral volume emission rate along the LOS of the MSX sensors, information essential to assess the relative contributions of NO + Dv = 1 transitions and the optically thick CO 2 n 3 band to auroral radiance at 4.3 mm. [12] Profiles of the MWIR limb radiance, observed during 5 s of a mirror scan frame, are shown in Figure 3 for an auroral observation from 1929:10 to 1929:15 UT and a midlatitude quiescent nightglow measurement from 1944:03 to 1944:08 UT. The data in Figure 3, recorded at 360 Hz, has been binned in 1 km altitude increments and averaged over 72 samples to provide 25 limb profiles in the 5 s mirror scan segment, representing a field of regard (FOR) of approximately 2.5 and a horizontal dimension of 150 km at the tangent point. The B1 ( mm) and B2 ( mm) SPIRIT III MWIR radiometer bands, represented by the lower and upper altitude profiles, respectively, in Figure 3, are separated by a masked region spanning about 8 km in tangent altitude that is centered at approximately 94 km. The higher-altitude auroral B2 band data is more than an order of magnitude greater than the nightglow background recorded at 1944 UT and is the focus of the current analysis. [13] The MSX data of the 9 February 1997 aurora included spectra recorded by the five UVISI image intensified spectrographic imagers (SPIMs) [Mill et al., 1994; Carbary et al., 1994] that provided spectral measurements from 110 to 900 nm. The emission rate of the N + 2 first negative (1N) (0-0) band at nm, a prominent feature of SPIM 4 auroral spectra, is proportional to the auroral ion pair production rate at a given altitude and serves as a convenient monitor of the energetic electron flux incident on the atmosphere [Dalgarno et al., 1965] and, in this instance, the integrated electron energy flux incident along the line of sight of the MSX limb viewing sensors. The measurement capabilities of the five SPIMs on the MSX satellite are described by Strickland et al. [2001] for a combined proton and electron aurora observed on 10 November In the latter experiment, the SPIMs were operated in the same manner as their operation in the 9 February 1997 event reported here. The SPIMs provided spectral images of 272 spectral pixels by 20 spatial pixels with a FOV of 1 (horizontal) by 0.05 (vertical). Auroral measurements from SPIMs 3 and 4, with spectral range (resolution) of (1.2) and (2.0) nm, respectively, were used in this analysis. The SPIM mirrors were adjusted in 0.05 increments in the vertical dimension to provide nearly synchronous staircase-like scans over the 1 FOVof the SPIRIT III radiometer. The limb profile of the integrated band intensity of the N + 2 1N (0-0) band at nm is shown in Figure 4 for two levels of auroral intensity as measured by the SPIM 4 pixel aligned with the 4of22

5 Figure 4. Limb profiles of auroral integrated N + 2 first negative (0-0) band emission at nm as measured by the image intensified spectrometer, SPIM 4, in 20 tangent height increments of 3 km over 25 s. The more intense layered profile, 100 to 750 kr, was generated as SPIM 4 scanned to higher altitudes from 1929:05 to 1929:30 UT and, as interpreted with the aid of the ancillary sensor data, consists of an auroral form with three segments near and beyond the tangent point. center position of the SPIRIT III scan mirror. The nm band profiles were generated by 20 incremental vertical steps of the SPIM slit position with a spatial resolution at the tangent point of 3 km in both the vertical and horizontal dimensions. The more intense profile in Figure 4 was recorded from 1929:05 to 1929:30 UT. During this time, the SPIRIT III scan mirror radiometer completed approximately 3.5 scans of the 3 horizontal FOR, Figure 2a. The 1 horizontal FOV of the SPIMs extended over the central one third of each radiometer frame and the opportunities for direct comparisons, in this event, represented 30% of the measurement time including scan mirror reversal. [14] SPIM 4 was operating in a gain state outside the range of the preflight calibration during the MSX auroral observation of 9 February 1997, and the N + 2 1N (0-0) band measurements, while valid indications of relative spectral intensity, were uncertain in absolute value. A calibration factor for the SPIM 4 measurement of N + 2 1N (0-0) band intensity was determined based on a comparison to selected transitions of the N 2 second positive (2P) system, as measured by the calibrated MSX SPIM 3 sensor, and reported values for the relative emission rate for N 2 2P and N + 2 1N transitions in aurora. This approach assumes the relative emission rates for N 2 2P and N + 2 1N transitions are essentially constant for auroras driven by energetic electrons with different energy distributions, ionization rates and altitude dosing profiles. The relative auroral intensities of selected N 2 2P and N + 2 1N bands have been studied extensively in ground-based [e.g. Gattinger et al., 1991] and space-based [e.g., Solomon, 1989; Hecht et al., 1995; Strickland et al., 2000] measurement programs and in model simulations of auroral electron energy loss processes based on comprehensive sets of excitation cross sections [Richards and Torr, 1990; Solomon, 1989]. In an analysis of Atmospheric Explorer C satellite data, Solomon [1989] concludes that the ratio of the N 2 2P (0-0) band to the N 2 + 1N (0 1) band at nm is constant and independent of the characteristic energy of the primary auroral electrons. Similarly, Gattinger et al. [1991] report no measurable variation of this ratio with auroral electron energy and set an upper limit for any change at 15%. [15] The ratio of the N 2 2P (0-0) nm band to the N 2 + 1N (0-0) nm band emission in aurora found in a number of studies, as summarized in Table 1, shows an average value of 0.30 ± The ratios reported by Hecht et al. [1995] and Richards and Torr [1990] are in good agreement with the earlier value given by Vallance Jones [1974] and are within the experimental uncertainty associated with the ground-based measurement reported by Gattinger et al. [1991]. [16] The N 2 2P band system measured by SPIM 3 at 1929:27 UT at a tangent height of approximately 112 km is shown in Figure 5. The summarized results for the ratios of the integrated band intensities for a number of isolated Table 1. Ratio of the N 2 Second Positive (0-0) Band to the N 2 + First Negative (0-0) Band in Aurora Reference Ratio Vallance Jones [1974] 0.31 Richards and Torr [1990] 0.30 Gattinger et al. [1991] 0.35 Hecht et al. [1995] 0.29 Strickland et al. [2000] 0.25 Average 0.30 ± of22

6 Figure 5. Auroral spectrum of the N 2 second positive band system measured by SPIM 3 at 1929:27 UT and a tangent height of approximately 112 km. The data represents a 1-s measurement by the SPIM spatial pixel corresponding to the SPIRIT III MWIR scan mirror center position. N 2 2P system transitions (v 0 + = 0 and 1) to those for the N 2 1N (0-0) band, as measured in this experiment over a range of tangent heights, are compared in Table 2 to ratios given by Vallance Jones [1974]. The relative rates for the electron induced auroral N 2 2P band system emissions [Vallance Jones, 1974] were calculated assuming the upper electronic state vibrational populations are proportional to the excitation Frank-Condon factors of Benesch et al. [1966] and using the transition probabilities of Shemansky and Broadfoot [1971]. The relative band intensities for the N 2 2P (v 0 = 0 and 1) progressions based on the excitation Frank-Condon factors and transition probabilities calculated by Gilmore et al. [1992] are in excellent agreement with the earlier values [Vallance Jones, 1974] presented in Table 2. On the basis of the results summarized in Table 2, a calibration factor of 2.04 was applied to the N + 2 1N (0-0) band profile presented in Figure 4 and the nm band intensities used in the analysis presented here. 3. POLAR, Loparskaya and FAST Measurements [17] The PIXIE sensor [Imhof et al., 1995] ( spasci.com/) on the POLAR spacecraft ( gsfc.nasa.gov/istp/polar/polar_inst.html) [Acuña et al., 1995], an X-ray multiple-pinhole camera designed to image bremsstrahlung X rays produced by the interaction of energetic electrons and the upper atmosphere, provides global images of precipitated energetic electron spectra, energy inputs, ionospheric electron densities and upper atmospheric conductivities. A PIXIE image of 2 12 kev X rays, recorded over the Northern Hemisphere from 1926:05 to 1931:04 UT on 9 February 1997 is shown in Table 2. Ratio of N 2 Second Positive (2P) Photon Emission Rates to the N 2 + First Negative (1N) (0-0) Band Rate SPIM Band System Wavelength, Ratio nm MSX AVJ a MSX/AVJ 4 N + 2 1N (0-0) N 2 2P (1-0) N 2 2P (0-0) N 2 2P (0 1) N 2 2P (1 3) N 2 2P average ratio 2.04 ± 0.37 a AVJ = Vallance Jones [1974]. 6of22

7 Figure 6. PIXIE image of 2 12 kev X-rays from 1926:05 to 1931:04 UT indicating the intense region of auroral energetic electron precipitation in the premidnight and postmidnight sector near and beyond the MSX LOS tangent point. Plot is in geomagnetic coordinates. The limits (red rectangle) and center (small beige circle with plus symbol) of the FOV of the PIXIE sensor are shown. The MSX locus of tangent points (white line), the day night terminator (dashed white line), and the POLAR subsatellite point (beige circle along the MSX locus of tangent points) are also shown. Figure 6 with the locations of the locus of tangent points in the midnight and postmidnight sectors. Figure 7, an enlargement of the PIXIE image, presents the MSX LOS and tangent point location and the Loparskaya all-sky camera FOVs projected to altitudes of 250 and 300 km. Figures 6 and 7 demonstrate that the major fraction of the auroral energy deposited along the MSX LOS is located beyond the tangent point and northeast of Loparskaya during this period. Bjordal et al. [2000] compare PIXIE images of auroral structures over the Kola Peninsula on 9 February 1997 from 1700 to 2100 UT with ground-based optical images recorded at the Loparskaya (68.25 N, E) and Lovozero (67.97 N, E) observatories, sites that are beyond the MSX tangent point at 1929 and 1930 UT (75.79 N, E and N, E, respectively) as indicated in Figure 1. The aurora in the period from 1920 to 1930 UT, described as a double breakup, is highly dynamic over both sites with correlated enhancements in the ground based optical images and the PIXIE X-ray images of these regions. [18] Figure 8 shows an all-sky camera image of O ( 1 D) nm emission recorded from the ground site in Loparskaya, Russia at 1929:13 UT. The auroral emission profile of the metastable O ( 1 D) state is characterized by a broad layer resulting from auroral-electron-induced ionization, dissociation and excitation and peaking at an altitude of approximately 250 km due to collisional deactivation at lower altitudes. The energetic electrons are constrained by the steeply inclined, 83 inclination, geomagnetic field above Loparskaya as they deposit their energy in the denser atmosphere. Hence the O ( 1 D) image provides an indication of the auroral morphology and dosing conditions at lower altitudes. The arcs near the edge of the image in Figure 8 appear brighter in part because the path length through a concentric atmospheric shell increases with increasing zenith angle of the camera LOS. The projection of the MSX LOS in Figure 8 indicates that two or more arcs were in the MSX FOV during this time. This is consistent with the MSX narrow FOV ultraviolet image at 1929:14 UT in Figure 2b and the nm profile from SPIM 4 in Figure 4 as recorded from 1929:05 to 1929:30 UT. 7of22

8 Figure 7. A more detailed view of the PIXIE image, 1926: :04 UT, with the FOV of the Loparskaya all sky camera projected to altitudes of 250 and 300 km (large white circles) and the MSX LOS and tangent point (black circle) indicated. Figure 8. The O ( 1 D) nm all sky image recorded by the ground based camera at Loparskaya at 1929:12.56 UT. The multiple auroral arcs, segments 1, 2, and 3, have been indicated to correspond to the emission layers identified in the N nm profile in Figure 4. 8of22

9 Figure 9. Exoatmospheric electron energy and flux density measured by the FAST particle detectors on 9 February At 1929 to 1930 UT, FAST was located above the tangent point region observed by the MSX sensors. [19] The FAST satellite, at an altitude of 3777 km, recorded increased auroral electron and ion energies and flux densities from approximately 1923 to 1934 UT (Figure 9). At 1923 UT, FAST was located at 66 N, 38 E, beyond the MSX tangent point approaching the auroral region from a southerly direction on a path nearly collinear with the MSX LOS (Figure 1). In the interval from 1928 to 1930 UT, FAST measured electron energies in excess of 10 kev and energy fluxes in the range from 20 to 45 erg cm 2 s 1. During this time, the FAST orbit traversed the northeast section of the FOVof the ground site at Loparskaya and the higher-altitude region beyond and near the MSX tangent point (Figure 1). The electron energy and flux density measured by the FAST particle detectors (Figure 9) decreased precipitously at 1930 UT when the FAST satellite, moving in a track toward MSX above and along the MSX LOS reached a point directly above the MSX tangent point. [20] The PIXIE (Figures 6 and 7), Loparskaya (Figure 8), and FAST data (Figure 9) and the MSX nm SPIM profile (Figure 4) have been used to characterize the form, location, and altitude of the enhanced auroral ultraviolet, visible, and infrared emissions observed by the MSX satellite in the period from 1929 to 1930 UT on 9 February The auroral form, estimated as three segments at and beyond the MSX LOS tangent point, is shown schematically in Figure 10 with a conceptual indication of the sensor locations and the LOS of the SPIRIT III radiometer. D. Strickland (private communication, 2001) estimated the Figure 10. Schematic representation of the MSX, POLAR, and FAST satellites and the multisegmented aurora observed at UT on 9 February The tangent point altitudes of the bore sight and limiting SPIRIT III radiometer 4.3-mm MWIR band (band B) pixels are shown in relation to the auroral segments and the FOV of the Loparskaya all sky camera. 9of22

10 Table 3. Estimated Auroral Dosing along the MSX Line of Sight at 1929:15 UT Segment Distance Beyond LOS Tangent Point, km Electron Energy, kev Energy Width, kev Energy Flux Density, erg cm 2 s path length, electron energy, and electron flux density in the three auroral segments as indicated in Table 3, based in part on the layered emission of the nm band profile presented in Figure 4. Segments 2 and 3 are beyond the tangent point (Figure 10) and thus their emissions appear at lower tangent height altitudes in Figure 4. For example, auroral emission at an altitude of approximately 112 km that is 650 km beyond the tangent point contributes to the radiance peak at 80 km in Figure 4 that is attributed to segment 3. [21] The auroral dosing conditions, derived from the multiplatform measurements of 9 February 1996 and presented in Table 3, are considered to be estimates rather than definitive values. However, the location and spatial extent of this aurora, difficult to establish from a single space-based platform, are unusually well characterized and are used with the dosing conditions as inputs to an updated version of the original AARC auroral model [Winick et al., 1987b] to evaluate the relative contribution of CO 2 and NO + to the MWIR auroral enhancement measured from the MSX satellite from 1929:00 to 1929:30 UT. This updated version calculates the efficiencies using the currently supplied dosing and model atmosphere conditions in contrast to the 1987 model which used precomputed stored efficiencies. The AARC simulations of this event that are presented in the following sections all use the updated AARC model. 4. Auroral 4.3 mm Infrared Emission Processes [22] The 4.3-mm emissions from CO 2 and NO + in the lower thermosphere are not in local thermodynamic equilibrium (LTE), and as a consequence, to estimate the auroral emission, one must evaluate all the production and loss processes for the radiating states [López-Puertas and Taylor, 2001]. The production and loss processes for MWIR auroral radiance, described by Kumer [1977] and reviewed by Winick et al. [1987a, 1987b, 1988, 2004], are briefly summarized here. The vibrational yields in the chemiluminescent ion-molecule reaction: N þ þ O 2 ) NO þ ðx; vþþo ð1þ have been studied in a laboratory experiment by Smith et al. [1983], who measured a bimodal NO + vibrational population with v 14 and the major fraction of the population in v 8. O Keefe et al. [1986] used a kinetic energy ion cyclotron resonance spectroscopy technique to measure the kinetic energy of the reaction products of reaction (1) and to infer their relative internal energy distribution. The reaction product channels, exothermicity and maximum allowed vibrational state resulting from (1) are [Winick et al., 1987b]: N þ þ O 2 ) NO þ ðþþo v 1 S ; DE ¼ 2:43 ev; vmax ¼ 8 ð1aþ N þ þ O 2 ) NO þ ðþþ v O 1 D ; DE ¼ 4:66 ev; vmax ¼ 18 ð1bþ N þ þ O 2 ) NO þ ðvþþo 3 P ; DE ¼ 6:63 ev; vmax ¼ 28: ð1cþ [23] Given the NO + vibrational population measured by Smith et al. [1983] and their own measurement of the kinetic energy (0.83 to 1.58 ev) of NO + resulting from (1), O Keefe et al. [1986] conclude: reaction (1a) is not a significant source of NO + ; reaction (1b) and (1c) produce NO + (v = 0 8) and (v = 8 16), respectively, assuming little or no rotational excitation; and reaction (1b) accounts for 80% of total NO + produced from (1). Using a flowing afterglow and visible chemiluminescence technique, Langford et al. [1986] measured branching ratios of 0.1% and 70 ± 30% for reactions (1a) and (1b), respectively, in agreement with the results of O Keefe et al. [1986]. [24] It is energetically possible for the reaction: N þ 2 þ O ) NOþ ðvþþn 2 D ; DE ¼ 0:69 ev; vmax ¼ 2 ð2þ to produce NO + (v) with v 2 but no definitive vibrationalyield measurements have been reported. This process has been included in the present analysis as a potential source of auroral NO + emission. [25] Other exothermic ion-molecule reactions that may produce NO + vibration include the charge exchange reactions: O þ 2 þ NO ) NOþ ðþþo v 2 X 3 S g ; DE ¼ 2:83 ev; v max ¼ 9 ð3aþ O þ 2 þ NO ) NOþ ðvþþo 2 a 1 D g ; DE ¼ 1:85 ev; vmax ¼ 6 ð3bþ O þ 2 þ NO ) NOþ ðþþo v 2 b 1 S þ g ; DE ¼ 1:20 ev; v max ¼ 4: ð3cþ [26] However, the vibrational state populations of the NO + (v) product ions of these charge exchange reactions 10 of 22

11 have not been determined and since dissociative recombination competes with charge exchange as an auroral O 2 + loss process, auroral NO + production by this mechanism is dependent on the auroral dosing conditions and electron density. While an accurate assessment of the auroral O 2 + relative loss rates requires knowledge of the electron and NO densities along the LOS of a space based sensor, it is estimated that charge exchange with NO accounts for approximately 10 percent or less of the O 2 + loss rate for the MSX IBC (International Brightness Coefficient) Class III aurora observed at 112 km on 9 February 1997 where electron densities of cm 3 or more were present. Charge transfer from O 2 + is a more significant source of NO + under conditions of enriched NO concentration and lower electron densities such as encountered during the Auroral Dynamics rocket probe observation of an IBC Class II aurora reported by Lee et al. [1989]. However, definitive experimental evidence is required to support reactions (3a), (3b), and (3c) as an auroral source of vibrationally excited NO +. [27] Caledonia et al. [1995] propose the reaction: N 2 P þ O ) NO þ ðþþe; v DE ¼ 0:79 ev; v max ¼ 2 as a potential source of NO + (v) with v 2 in aurora but no direct experimental evidence is available to confirm this chemi-ionization mechanism as a source of NO + MWIR emission. [28] Another possible source of NO + (v = 1) in an intense and sustained aurora is V-V transfer from N 2 (v = 1) in the reaction, N 2 ðv ¼ 1ÞþNO þ ðv ¼ 0Þ ) NO þ ðv ¼ 1ÞþN 2 ðv ¼ 0Þ: ð5þ [29] The significance of this source was evaluated using the AARC model to compare the steady state vibrational temperature of NO + resulting from the sum of the chemiexcitation reactions (1) and (2) with the N 2 vibrational temperature obtained for several representative dosing times. For the current event at 110 km, the vibrational temperature of NO + from the chemi-excitation reactions is 763 K and the N 2 vibrational temperature is 403, 467, and 496 K after 1, 3, and 5 min of continuous dosing, respectively. Thus at these altitudes and in this event V-V transfer with N 2 is a sink of vibrationally excited NO + and not a source. [30] Auroral NO + Dv = 1 photon emission rates are presented here based on AARC simulations assuming production solely from reaction (1) [Winick et al., 1987a] and from both reactions (1) and (2) [Winick et al., 1987b] using assumed NO + v 2 population distributions for reaction (2). [31] CO 2 n 3 rovibrational emission at 4.3 mm results from vibrational energy transfer (V-V) from excited N 2 to CO 2. The excited N 2 is produced in aurora by excitation from low-energy secondary electrons: e þ N 2 ) N 2 ðþþe v CO 2 v 0 1 ; v0 2 ; v0 3 þ N2 () CO 2 v 0 1 ; v0 2 ; v0 3 1 þ N2 ðv ¼ 1Þ: ð7þ ð4þ ð6þ [32] N 2 vibration is also produced by chemical reaction of N( 4 S) with NO and by electronic energy transfer (E-V) from O ( 1 D). CO 2 n 3 production and loss also result from radiative transfer of photons, which may include solar photons, LTE photons from the lower atmosphere (earthshine) and/or photons emitted or absorbed elsewhere in the non-lte region of the atmosphere, CO 2 v 0 1 ; v0 2 ; v0 3 () CO2 v 0 1 ; v0 2 ; v00 3 þ hv: ð8þ [33] CO 2 n 3 production and loss also result from collisional transfer (V-T) of energy from/to another molecule M (M = N 2,O 2,orO), CO 2 v 0 1 ; v0 2 ; v0 3 þ M () CO2 v 0 1 ; v0 2 ; v00 3 þ M: ð9þ [34] Prompt excitation of the CO 2 (001) state could result from direct collisions of CO 2 with low-energy secondary electrons, e þ CO 2 ð000þ ) CO 2 ð001þþe: ð10þ [35] In order to compare the efficiency of this process with that of indirect excitation via N 2 vibration, reactions (6) and (7), the efficiency was determined using the secondary electron flux calculated as in AARC and the electron excitation cross sections of Porter and Mayr [1979] for the dosing conditions representing the aurora observed on 9 February 1997 at 1929:12 UT. The AARC computation provided excitation rates of and CO 2 (001) states per ion pair at 110 and 120 km, respectively. The fraction of the production rate contributing to the LOS radiance is subject to the effects of radiation transport and losses due to energy transfer collisions. However, as subsequently shown, even this total CO 2 (001) excitation rate is a small fraction of the photons per ion pair measured in this experiment. Consequently, direct excitation of CO 2 (001) is not a significant source of MWIR radiance in this measurement. [36] The V-V transfer rate for reaction (7) is slow, with time constants of approximately 1 min at 90 km and 30 min at 110 km, and thus CO 2 auroral emission from this process is strongly dependent on the dosing history. The vibrational to translational (V-T) energy exchange process with atomic oxygen: N 2 ðv ¼ 1ÞþO! N 2 ðv ¼ 0ÞþO ð11þ is also a loss mechanism for the N 2 (v = 1) state as reported by Taylor [1974]. A significant fraction of CO 2 (v 0 3) quanta may be thermalized by the V-V process in reaction (7) followed by the V-T mechanism in reaction (11). The loss rate for N 2 (v = 1) in reaction (11) exceeds the rate for V-V transfer to CO 2, reaction (7), at an altitude of approximately 110 km and is the dominant loss process for N 2 (v = 1) at altitudes greater than 110 km. The lifetime for the N 2 (v = 1) state is approximately 30 min at altitudes from 110 to 150 km as controlled by reaction (11) [Winick et al., 1987b]. Thus the production time constant for CO 2 (001) emission is approximately 30 min at altitudes above 110 km and this slow response to auroral dosing has been utilized as a 11 of 22

12 distinguishing characteristic to estimate the CO 2 component of the radiometric MWIR measurement. [37] In reactions (8) and (9), usually we have v 00 3 =v3 0 ±1 and these processes correspond to the absorption/emission of 4.3-mm photons or exchange of a comparable amount of energy with the thermal bath. In the daytime, the reverse of reaction (8) also should include fluorescence excited by shorter-wavelength solar photons, associated with other changes of vibrational quantum numbers. There is a strong diurnal dependence for excitation by reaction (8), with a daytime peak resulting from solar excitation. At nighttime the weak bands have prominent ambient emissions due to pumping from the warmer lower atmosphere. The main-band emission from the 626 isotope becomes optically thick in the limb below 115 km, while the minor isotopes and hot bands are less opaque. [38] In auroral limb measurements such as the current MSX measurement, the long optical paths involved result in large optical depths for the main band of CO 2, while NO + emission remains optically thin. This should serve to enhance the relative contribution of NO + above levels observed in vertically viewing sounding rocket probes such as the rocket-borne field-widened interferometer (FWI) experiment [Picard et al., 1987; Espy et al., 1988; Winick et al., 1988] and the Auroral Dynamics radiometric measurement [Lee et al., 1989]. Since the FWI possessed moderate spectral resolution, it was possible to identify NO + unambiguously as an auroral radiator for the first time from FWI spectra. However, due to the moderate dosing level (IBC II + ) and near-vertical path, the enhanced auroral NO + radiance was relatively small while the CO 2, being much less opaque than in the limb views, dominated. The Auroral Dynamics radiometer observed enhanced auroral 4.3 mm emission from a stable IBC Class II event at altitudes greater than payload apogee, 121 km. At these higher altitudes CO 2 emission from N 2 V-V transfer is a less efficient auroral process as N 2 (v) is primarily lost in deactivating collisions with atomic oxygen and the enhanced MWIR radiance observed in the Auroral Dynamics measurement is attributed to NO + [Lee et al., 1989]. 5. Auroral NO + (X, v > 0) Loss Processes [39] Loss processes for vibrationally excited NO + represented in the AARC model used in this analysis include radiative decay, NO þ ðv 0 Þ ) NO þ ðv 0 nþþ hv n ¼ 1 2; ð12þ dissociative recombination, NO þ ðv 0 Þþe ) N þ O; and collisional deactivation by N 2, ð13þ NO þ ðv 0 ÞþN 2 ) NO þ ðv 0 nþþn 2 n 1: ð14þ [40] Both experimental and theoretical studies have reported Einstein coefficients and N 2 collisional deactivation rate coefficients for selected vibrational states of NO + (X). These results are reviewed in Appendix A. [41] The Einstein coefficients calculated by Werner and Rosmus [1982] for the NO + (X) Dv = 1 and 2 transitions, used in the AARC model for the current analysis, are reproduced in Table A1 in Appendix A. The transition probability given by Werner and Rosmus [1982] for NO + (v = 1) is 10.9 s 1 with increasingly larger values for the higher vibrational states. A rate coefficient of (T e /300 K) 0.85 cm 3 s 1, where T e is the electron temperature, was used in AARC to represent dissociative recombination of NO +, reaction (13), for the ground state as well as the higher vibrational states considered here. This assumption provides an upper limit for this loss process in view of recent experimental [Mostefaoui et al., 1999] and theoretical [Motapon et al., 2006] studies that indicate vibrationally excited NO + ions recombine more slowly than the ground state ion. The auroral ion composition and electron density are calculated in AARC based on electron impact ionization cross sections and ion chemistry, charge exchange, and dissociative recombination rate coefficients. The AARC model simulation of this auroral event provides a maximum steady state electron density of cm 3 at a tangent height of 109 km and lesser amounts at higher altitudes and when combined with the rate coefficient for reaction (13), the electron density yields an upper limit rate of 0.4 s 1 for the dissociative recombination of NO + v>0 at 109 km. The MSIS model atmosphere kinetic temperature was used for the electron temperature, T e, in reaction (13) in this calculation. However, this assumption is not critical to the current analysis given the relatively slight temperature dependence of reaction (13) and the magnitude of this loss process. Since the upper limit rate for dissociative recombination is approximately 4 percent of the rate of radiative decay for the NO + v = 1 state and less for the higher vibrational levels, dissociative recombination is not a significant loss process for vibrationally excited NO + in the auroral measurements reported here. [42] The temperature dependent rate coefficient for collisional deactivation of NO + (X, v 0) by N 2, given by Morris et al. [1988] as 2.4 ± (300/T) 2.3 cm 3 s 1, has been used to calculate the efficiency for NO + Dv=1 auroral emission in the present analysis with three different quenching models designated SSQ1, SQQV, and MQQV. For the SQQ1 case, quenching proceeds in single quantum steps and the Morris et al. [1988] rate is used for all vibrational levels, v = 1 14, relevant to the current analysis. In the SQQV model, quenching is also by single quantum steps but the rate coefficient for vibrational level, v, is directly proportional to v in accord with the Landau-Teller scaling law. The measurements of Bien [1978] support the SQQ1 interpretation while the V-V transfer calculations of Sharma [2006] are more consistent with the SQQV quenching model. Finally, for the multiquantum quenching model (MQQV), the quenching rate is also proportional to v but quenching for v 2 proceeds in a single multiple quantum step (sudden death) directly to the ground vibrational state (NO + X, v = 0). 6. CO 2 and NO + Components of the Measured MWIR Auroral Enhancement [43] The AARC model [Winick et al., 1987b] was used to simulate the MWIR auroral radiance measured by the 12 of 22

13 Figure 11. A comparison of the radiance profile measured in the SPIRIT III B2 radiometer band at 1929:12 UT and an AARC simulation using the segment dimensions and dosing conditions specified in Table 3 and a dosing duration of 5 min. The calculated auroral radiance enhancement from the NO + and CO 2, combined, is shown for each segment. The total profile includes the auroral radiance enhancement in each segment and the ambient airglow contribution from CO 2 along the SPIRIT III LOS. In Figure 11a, the NO + auroral production is from reaction (1); in Figure 11b, NO + is produced by reactions (1) and (2). SPIRIT III radiometer at 1929:12 UT assuming the auroral excitation conditions specified in Table 3 had continuously dosed the observed region for 1, 3, and 5 min prior to the time of the measurement. The simulation described in this section was performed to compare the measured radiance profile with the model and to provide an estimate of the relative contributions of vibrationally excited CO 2 and NO + to the aurorally enhanced MWIR radiance profile. [44] The AARC model calculation of auroral NO mm radiance assumes steady-state, a reasonable assumption in most events given that the auroral NO + production time constants for (1) and (2) are approximately 0.1 s or less at altitudes below 120 km based on the rate coefficients for N + and N 2 + loss processes [Jones and Rees, 1973] and the MSIS model atmosphere for this event. However, since the time constant for N 2 to CO 2 V-V energy transfer can be greater than the dosing duration in many auroras, a time dependent approach with the specification of the dosing time history is used in AARC to estimate the CO 2 n 3 radiance [Winick et al., 1987b, 1988]. [45] The dosing levels given in Table 3 represent an intense event, approximately equivalent to IBC III auroral conditions for a ground-based observer, and as observed from space, extended over a path length of 800 km along the MSX LOS, the radiance is equivalent to IBC IV intensity, as indicated in Figure 4. As discussed in section 3, the time history and dynamics of the intense aurora of 9 February 1997 were monitored by the PIXIE pinhole X-ray camera on the POLAR satellite. Bjordal et al. [2000] in their analysis of the PIXIE images of auroral structures over the Kola Peninsula on 9 February 1997 from 1700 to 2100 UT and ground-based optical images recorded at Loparskaya and Lovozero describe several dynamic auroral formations with rapid motions and activation periods of 20 to 100 s. The period from 1920 to 1930 UT is described as a double breakup with auroral arcs formed both north and south of the Lovozero observatory. At approximately 1927 UT a bright arc appears and persists for several minutes at 71 N, north of the Lovozero site and south and beyond the MSX tangent point at 75 N. The Loparskaya ground site images, similar to Figure 8, show variations in the form and location of multiple auroral arcs along the MSX LOS in the three minutes prior to the SPIRIT III measurement at 1929 UT. The AARC computation, based on the assumption of continuous electron dosing for 1, 3, and 5 min prior to the MSX observation, is a simplification of the dynamic conditions in the MSX tangent point region evident in the PIXIE and Loparskaya data. Nevertheless, the simplified dosing conditions used in the AARC simulation are a reasonable approximation to the actual conditions and should serve to delineate the components of auroral 4.3 mm radiance due to CO 2 and NO +. The assumption of continuous dosing for 5 min prior to the MSX observation provides an upper limit estimate for the dosing duration prior to the MSX measurement. [46] The results of the AARC simulation of the MSX mm band auroral enhancement are compared in Figure 11 with the MSX measurement at 1929:12 (the maximum profile in Figure 3). The AARC simulation in Figure 11 is based on 5 min of dosing as specified in Table 3, includes the NO + and CO 2 components of the auroral enhancement in each segment, and includes the CO 2 ambient airglow background (not significant for NO + ) in the total radiance profile. Figure 11a illustrates NO + production from reaction (1) and Figure 11b, production from reactions (1) and (2). Several characteristics of the AARC results are common to Figures 11a and 11b. The contribution from segment 1, which is closest to the tangent point, reaches a maximum at a tangent altitude of 105 km and drops off dramatically at tangent heights below 100 km. The contribution from segment 2 reaches a maximum at a tangent height altitude of 100 km and continues to dominate down to 90 km (recall from Figure 3 that the MSX B mm band data is limited to tangent heights from 100 to 120 km). The contribution from segment 3 is small and less than 3 percent of the total radiance at altitudes above 100 km. [47] The addition of reaction (2) in the AARC computation increases the NO + radiance, as filtered by the system spectral response of the MSX B2 radiometer band, slightly more than a factor of 2 in Figure 11b, and the simulation 13 of 22