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Extreme ultraviolet spectra of Venusian airglow observed by EXCEED
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Extreme ultraviolet spectra of Venusian airglow observed by EXCEED Yusuke Nara a,∗, Ichiro Yoshikawa a, Kazuo Yoshioka a, Go Murakami b, Tomoki Kimura c, Atsushi Yamazaki b, Fuminori Tsuchiya d, Masaki Kuwabara b, Naomoto Iwagami e a
Department of Complexity Science and Engineering, University of Tokyo, Kashiwa, Chiba, 277-8561, Japan Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, Sagamihara, Kanagawa, 252-5210, Japan c RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama, 351-0198, Japan d Planetary Plasma and Atmospheric Research Center, Tohoku University, Sendai, Miyagi, 980-8577, Japan e Department of Earth and Planetary Science, University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan b
a r t i c l e
i n f o
Article history: Received 17 May 2016 Revised 16 September 2017 Accepted 23 October 2017 Available online xxx
a b s t r a c t Extreme ultraviolet (EUV) spectra of Venus in the wavelength range 520 − 1480 A˚ with 3 − 4 A˚ resolutions were obtained in March 2014 by an EUV imaging spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on the HISAKI spacecraft. Due to its high sensitivity and long exposure time, many new emission lines and bands were identified. Already known emissions such as the ˚ O I 989 A, ˚ H I Ly − β 1026 A, ˚ and the C I 1277 A˚ lines (Broadfoot et al., 1974; Bertaux et al., O II 834 A, 1980; Feldman et al., 20 0 0) are also detected in the EXCEED spectrum. In addition, N2 band systems such as the Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) (2, 0), (2, 1), (3, 1), (3, 2) and (5, 3) bands, the Birge Hopfield (b 1 u − X1 g+ ) (1, 3) band, and the Carroll-Yoshino (c4 1 u+ − X 1 g+ ) (0, 0) and (0, 1) bands together are identified for the first time in the Venusian airglow. We also identified the CO Hopfield-Birge (B 1 + − X 1 + ) (1, 0) band in addition to the already known (0, 0) band, and the CO Hopfield-Birge (C 1 + − X 1 + ) (0, 1), (0, 2) bands in addition to the already known (0, 0) band (Feldman et al., 20 0 0; Gérard et al., 2011). © 2017 Elsevier Inc. All rights reserved.
1. Introduction Extreme ultraviolet observations of the Venusian airglow play important roles in the study of the Venusian atmosphere. In 1970s, Mariner 10 (Broadfoot et al., 1974) and Venera 11/12 (Bertaux et al., 1980) observed the Venusian airglow using narrowband polychromators focusing on specific emission lines such as the He I 584 ˚ O II 834 A, ˚ H I 1216 A, ˚ and the O I 1304 A˚ although the interA, pretations of the data were ambiguous because of low signal to noise ratio due to short exposure time. In February 1990, Galileo flew by Venus on its way to Jupiter, and the Extreme Ultraviolet Spectrometer identified the H I Ly − 1026 A˚ and the O I 989 A˚ lines (Hord et al., 1991). In August 1994, an EUV telescope/spectrograph on a sounding rocket detected several emissions such as the N I 907 ˚ O I 1039 A, ˚ and the N II 1085 A, ˚ but the wavelength uncertainA, ties were as large as 9 A˚ (Stern et al., 1996). In March 1995, the Hopkins Ultraviolet Telescope (HUT) on the Astro-2 mission identified the CO Hopfield-Birge (B 1 + − X 1 + ) (0, 0) band, CO ˚ C I 1261 A, ˚ Hopfield-Birge (C 1 + − X 1 + ) (0, 0) band, N I 1134 A,
∗
Corresponding author. E-mail address: [email protected] (Y. Nara).
and the C I 1277 A˚ lines for the first time in the Venusian dayglow (Feldman et al., 20 0 0). The purpose of this paper is to present the spectra of the Venusian airglow features obtained by EXCEED on the HISAKI spacecraft (Yoshioka et al., 2013; Yoshikawa et al., 2014) and show their spectral identifications.
2. Analysis and database Since November 2013, EXCEED on the HISAKI spacecraft has measured the extreme ultraviolet spectra of several planets with exposure times significantly longer than any prior EUV spectroscopic measurements. The HISAKI spacecraft orbits around the earth between the altitudes of 1050 ± 100 km with a period of 106 min. The EXCEED spectro-imager consists of an entrance mirror, slits of different widths, a grating, and a microchannel plate detector. The narrow and the wide slits have widths of 10 and 60 , respectively, and the third one has a dumbbell-like shape. The spectrograph covers the wavelength region 520 − 1480 A˚ with a dispersion of around 1 A˚ per pixel. For the present observation, the 60 slit is used. Details of the instrument and the performance are given by Yoshioka et al. (2013) and Yoshikawa et al. (2014).
https://doi.org/10.1016/j.icarus.2017.10.028 0019-1035/© 2017 Elsevier Inc. All rights reserved.
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Fig. 2. The spectrum of the Venusian airglow after integration of as long as 21 days. Emissions extending vertically from -20 to 70 pixel such as at 1216 A˚ are geocoronal emissions. The Venusian airglow is seen at around y = 0. A narcissistic ghost of the H I Ly-α due to multiple reflection between the detector and the grating appears at ˚ 783 A.
Fig. 3. Spectra of 21-day integrations; Venus + geocorona (black curve) and geocorona alone (red curve).
Fig. 1. An example of raw spectral image with an exposure of 1 min. The x-axis represents wavelength and the y-axis represents east-west spatial position (west is upward), where 1 pixel corresponds to 4.2 . The strong feature at 1304 A˚ extending over 80 spatial pixels is the geocoronal O I emission. The Venus disk is seen at around y = 0 with a width of 7 pixels.
The Venusian airglow was observed with the 60 slit from March 9 through 29, 2014. During this observation of 21 days, the F10.7 index at 1 AU was 148 (http://www.spaceweather.gc.ca/) and the apparent diameter of Venus was at about 30 (7 pixels). The Venusian phase angle was kept at about 90 ° so that about 50% of Venus disk was illuminated. An example of a spectral image with an exposure of 1 min is shown in Fig. 1. A spectral image obtained by 21-day integration is shown in Fig. 2. Because the apparent diameter of Venus is smaller than the slit width, the spectral resolution may be different from the official value (Yoshioka et al., 2013). The object traces an elliptical path within the slit with a period of 106 min, due to the orbital motion of the HISAKI spacecraft (Yamazaki et al., 2014). This period is short enough compared to the 21-day integration time, so we consider that integrated spectrum is well averaged. Then, we estimate the spectral resolution by measuring widths of isolated lines. Because the integrated spectrum is well averaged, we assume the line shape as a gauss distribution and fitted it to isolated lines by using the nonlinear least square method (Marquardt, 1963). The es˚ timated spectral resolutions are between 3 and 4 A. There are three kinds of unwanted features. One is the geocoronal lines. Because the HISAKI’s orbit around the Earth is located between the altitudes of 1050 ± 100 km, the spectrum includes not
only the Venusian airglow but also contributions from the geocorona. The geocoronal emissions such as the H I Ly-α , Ly-β , Ly-γ , O, and the N emissions are also expected in the Venusian dayglow. We assume that the spectrum +210 (15 Rv ; 50 pixels) away from the center of Venus disk contains contributions only from the geocorona and that the geocorona is uniformly distributed in the slit’s field of view. Fig. 3 shows a comparison of two 21-day integrations. One includes contributions of both Venus and the geocorona (black curve), and the other includes those of the geocorona alone (red curve). The net intensity of Venus Iv (λ, yv ) is evaluated as
Iv (λ, yv ) = I (λ, yv ) − I (λ, yv + 50 ), where I is the intensity observed by EXCEED, λ is wavelength, and yv is the position of the center of Venus disk. The center of Venus ˚ which is a is evaluated by using the emission of O I at 1356 A, relatively weak emission line in the Earth atmosphere at an altitude of 10 0 0 km and is strong in the Venus atmosphere (Fig. 3). We consider the center of Venus as the pixel value y which maximizes I(1356, y). The net intensities of Venusian spectrum are listed in Tables 1–3. Another unwanted feature is the narcissistic ghost (McCandliss et al., 1998). The narcissistic ghost of the H I 1216 A˚ appears at 783 A˚ as seen in Figs. 2 and 4. It is also found that of the O I 1304 A˚ appears at 705 A˚ as seen in Fig. 4. The other unwanted feature is the second order diffraction. There is the possibility that the emission at 1168 A˚ includes not only contributions from the N I at 1169 A˚ and the CO Hopfield˚ but also from the Birge (C 1 + − X 1 + ) (0, 3) band at 1169 A, ˚ which exhibits relatively strong second order of the He I at 584 A, emission among the spectra obtained by EXCEED. We identified the spectral features by the help of Meier (1991) for the geocorona, Herzberg (1950) for the di-
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Table 1 Identified atomic emission features. New emission features are indicated by bold letters. Wavelengths are obtained from the NIST database for the atomic lines except for the N I 853 A˚ which is suggested to exist in the Earth’s night glow by Chakrabarti (1984). Total blended intensity is recorded when the emission is blended with other species and is the sum of intensities which contribute to the emission. Individual intensity is the intensity which contains the contribution of only the transition. (a) For isolated emission lines, intensities are estimated by multiplying observed peak values by the instrumental width of 3.5 ± 0.5 A˚ and the errors are evaluated by propagation of errors. For the blended emissions, individual intensities are estimated by the spectrum from which the contributions of fitted spectrum are subtracted, and the errors are evaluated by propagation of errors of the EXCEED spectrum and the fitted spectrum. (b) The intensities are estimated by integrating spectrum of the emission and the errors are evaluated by calculating standard deviation divided by square root of the number data. HI Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh] (a)
Total blended intensity [Rayleigh] (b)
1s 2 S − 4 p 2 Po 1s 2 S − 3 p 2 Po 1s 2 S − 2 p 2 Po
972.5 1025.7 1215.7
24 ± 4 − −
− 189 ± 2 28, 100 ± 400
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1s2 1 S − 1s3 p 1 Po 1s2 1 S − 1s2 p 1 Po
537.0 584.3
9±2 220 ± 40
− −
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 2 p2 3 P − 2s2 2 p5d 3 Po 2s2 2 p2 3 P − 2s2 2 p4d 3 Do 2s2 2 p2 2 P − 2s2 2 p3d 3 Po 2s2 2 p2 3 P − 2s2 2 p3d 3 D0 2s2 2 p2 1 P − 2s2 2 p6d 1 Fo 2s2 2 p2 3 P − 2s2 p3 3 Po 2s2 2 p2 1 D − 2s2 2 p3d 1 Fo
1158.0 1193.2 1261.6 1277.6 1288.4 1329.1 1463.3
10 ± 2 − 16 ± 3 54 ± 8 4.2 ± 0.7 33 ± 5 −
− 29.8 ± 0.5 − − − − 224 ± 2
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 2 p 2 Po − 2s2 3d 2 D 2s2 p2 4 P − 2s2 p(3 Po )3s 4 Po 2s2 p2 2 D − 2s2 p(3 Po )3d 2 Do 2s2 p2 4 P − 2s2 p3 4 So 2s2 2 p 2 Po − 2s2 p2 2 D
687.3 806.8 809.9 1010.4 1335.7
0.7 ± 0.2 0.3 ± 0.2 0.4 ± 0.2 − −
− − − 3.4 ± 0.5 138.8 ± 0.8
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
N II + e− → N I + hν 2s2 2 p3 4 So − 2s2 2 p2 (3 P )5d 4 P 2s2 2 p3 4 So − 2s2 2 p2 (3 P )4d 4 P 2 p3 4 So − 2 p2 (3 P )3d 4 P 2 p3 4 So − 2 p2 (3 P )4s 4 P 2 p3 2 Do − 2 p2 (1 D )4s 2 D 2 p3 2 Do − 2 p2 (3 P )4d 2 D 2s2 2 p3 4 S − 2s2 p4 4 P 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )3d 2 D 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )3d 2 F 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )4s 2 P 2s2 2 p3 4 So − 2s2 2 p2 (3 P )3s 4 F
853 886.2 906.4 953.4 964.0 990.8 1097.2 1134.2 1163.9 1168.5 1177.7 1200.2
1.2 ± 0.3 1.9 ± 0.4 4.6 ± 0.7 − 4.7 ± 0.8 − − − − − 10 ± 2 100 ± 20
− − − 18.9 ± 0.3 − 100.6 ± 0.7 3.0 ± 0.2 26.0 ± 0.3 7.6 ± 0.7 10.5 ± 0.9 − −
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 p3 5 So − 2s2 p2 (4 P )3d 5 P 2s2 2 p2 3 P0 − 2s2 p3 3 So 2s2 2 p2 1 S − 2s2 2 p3s 1 Po 2s2 2 p2 3 P − 2s2 p3 3 Po 2s2 2 p2 3 P − 2s2 p3 3 Do
629.2 645.2 858.3 916.7 1085.7
0.7 ± 0.3 − − 11 ± 2 −
− 2.3 ± 0.2 3.5 ± 0.2 − 37.5 ± 0.3
He I
CI
C II
NI
N II
(continued on next page)
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Y. Nara et al. / Icarus 000 (2017) 1–9 Table 1 (continued) OI Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
771.1 792.2 859.3 879.0 931.5 938.0 950.1 988.8 999.5 1025.8 1039.2 1152.1 1304.4 1355.6
1.2 ± 0.3 2.1 ± 0.4 − 3.9 ± 0.7 6±1 8±2 − − 2.8 ± 0.5 − 15 ± 3 − − −
− − 3.5 ± 0.2 − − − 18.9 ± 0.3 100.6 ± 0.7 − 189 ± 2 − 108.6 ± 0.6 2100 ± 10 568 ± 2
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 2 p3 2 Do − 2s2 2 p2 (1 D )3s 2 D 2s2 2 p3 2 Po − 2s2 2 p2 (1 D )3s 2 D 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )3s 2 P 2s2 2 p3 2 Po − 2s2 p4 2 S 2s2 2 p3 2 Po − 2s2 2 p2 (3 P )3s 2 P 2s2 2 p3 2 Do − 2s2 p4 2 D 2s2 p4 4 P − 2s2 p3 (5 So )3s 4 So 2s2 2 p3 4 So − 2s2 p4 3 Po
555.1 600.6 617.1 644.2 673.8 718.5 739.9 834.5
2.2 ± 0.7 1.8 ± 0.5 12 ± 2 − 2.3 ± 0.4 5.4 ± 0.9 0.7 ± 0.2 130 ± 20
− − − 2.3 ± 0.2 − − − −
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
3s2 2 p6 1 S − 3s2 3 p5 (2 Po )3d2 3/2o 3 p6 1 S − 3 p5 (2 Po )4s 2 3/2o
866.8 1066.7
2.2 ± 0.4 1.7 ± 0.3
− −
2s2 2 p4 3 P − 2s2 2 p3 (2 Do )4d 3 D0 2s2 2 p4 3 P − 2s2 p5 3 Po 2s2 2 p4 1 D − 2s2 2 p3 (2 Do )5d 1 Fo 2s2 2 p4 3 P − 2s2 2 p3 (2 Po )3s 3 Po 2s2 2 p4 3 P − 2s2 2 p3 (4 So )7d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )6d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )5d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (2 Do )3s 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (2 Po )3s 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )3d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )4s 3 So 2s2 2 p4 1 D − 2s2 2 p3 (2 Do )3s 1 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )3s 3 So 2s2 2 p4 3 P − 2s2 2 p3 3s 5 So O II
Ar I
atomic molecules and the NIST database (http://www.nist.gov/ pml/data/asd.cfm) for atomic lines. 3. Discussion Because the 520 − 1480 A˚ region exhibits the highest quality in terms of signal to noise ratio among the spectral region reported so far, the present discussion focuses on the identification of new features in this region. Figs 4–6 show the net intensities of the Venusian airglow in each wavelength range with identifications of emissions. 3.1. General features of Venusian dayglow We found similarities in the EUV airglow of the terrestrial planets Venus, Earth and Mars. That is, the H I, He I, N I, and the O I emissions are dominant in neutral atomic emissions, and the N II and the O II emissions are dominant in ionic emissions in the airglow of these planets. No emissions due to divalent ions are found. That is, interactions between the solar wind and the planetary atmospheres are not observed even though Venus and Mars have no significant magnetic field. In molecular emissions, only CO and N2 emissions are found although CO2 is the main constituent of the Venusian and Martian atmosphere. It may be attributed to the difference in their dissociation potential energies. Those of CO (11.1 eV) and N2 (9.8 eV) are much higher than that of CO2 (5.5 eV) which is dissociated by sunlight. So, there are no CO2 band emissions, but CO band emissions exist in the spectra of Venus and Mars, and N2 bands also in the spectra of terrestrial planets (Gentieu et al., 1981; Stevens et al., 2015).
In the Venusian airglow spectrum, many emission features are attributed to not one but several emission lines, especially for ˚ such as shown in Fig. 6. To resolve wavelengths above 1200 A, such blends, we consider measured and theoretical emission ratios of the N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands and the CO fourth positive (A 1 − X 1 + ) bands. We generated synthetic spectra of the N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands with a 4 A˚ resolution referring to the theoretical estimations by Benesch et al. (1966) and Conway (1982). Benesch et al. (1966) calculated the Franck-Condon factors for transitions in N2 theoretically and Conway (1982) calculated the transition probabilities of the N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands by using the Franck-Condon factors. Durrance (1981) calculated the transition probabilities of the CO fourth positive (A 1 − X 1 + ) bands by using the observations of International Ultraviolet Explorer. Using these information and the nonlinear least square method (Marquardt, 1963), we investigated blended emission features. Specifically, we have conducted following 3 steps. First, a Gaussian function is fitted to the molecular bands which are not blended with other species. Second, intensities of the other bands but for (ν , 0) bands, which cause self-absorption, are estimated. Third, the fitted spectrum is subtracted from the EXCEED spectrum. The same approach is done for the residual spectrum, repeatedly. In the estimation of relative intensities, we adjusted the wavelength manually because of the wavelength inaccuracy of EX˚ while CEED spectrum. We adopted the spectral resolution as 4 A, ˚ That does not affect our conit is estimated between 3 and 4 A. clusion because we focus on the integrated values. Fig. 7 shows the result. We fitted the CO A-X (14, 3), (9, 2), (6, 1), the N2 a-X (3, 1), and (2, 0) band features to the residual spectrum. Because Conway (1982) found the self-absorption of the N2 a-X (2, 0) is
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˚ by the 21-day integration. Many of them are identified for the first time. Vertical lines represent peaks of Fig. 4. The net intensity of the Venusian airglow (520–900 A) the observed spectrum. We refer to the NIST database for the wavelength used in the labels for each feature. The differences between the observed and reference (NIST) ˚ wavelengths are usually less than 1 A.
˚ The CO Hopfield-Birge (B 1 + − X 1 + ) (1, 0) bands, another Hopfield-Birge (C 1 + − X 1 + ) (0, 1) and Fig. 5. The net intensity of the Venusian airglow (90 0 –120 0 A). (0, 2) bands, the N2 Birge-Hopfield (b 1 u − X1 g+ ) (1, 3) band, and the Carroll-Yoshino (c4 1 u+ − X 1 g+ ) (0, 0) and (0, 1) bands are also newly identified. Vertical lines represent peaks of the observed spectrum. We refer to the NIST database for the atomic and ionic emissions, and to Herzberg (1950) for the molecular emissions for the ˚ wavelength used in the labels for each feature. The differences between the observed and reference (NIST and Herzberg (1950)) wavelengths are usually less than 2 A.
negligible on Earth, we remove the contribution of the N2 a-X (2, 0) and (2, 1) from the residual spectrum (Fig. 7e). Fig. 8 shows the residual spectrum and synthetic spectrum of CO A-X (5, ν ) bands fitted to CO A-X (5, 0) band. The intensity of the (5, 1) band is derived by assuming no self-absorption. The candidates for species which contribute to the emission at 1435 A˚ are N2 a-X (2, 1), CO ˚ The contribution from N2 a-X (2, 1) is A-X (5, 1), and C I 1432.5 A. already subtracted from the spectrum in Fig. 8. The intensity of the synthetic (5, 1) band is 78 ± 2 Rayleighs while the observed intensity is 82 ± 3 Rayleighs. So, we conclude that the self-absorption of
the CO A-X (5, 0) band is negligible and there is no contribution of C I 1432.5 A˚ to this emission. 3.2. Atomic lines While emissions brighter than 10 Rayleighs have been reported by the previous observations, EXCEED could detect emissions as ˚ many new emission lines were weak as 1 Rayleigh. Below 10 0 0 A, ˚ the O II 600.6, 673.8, 739.9 detected such as the O I 771.1, 792.2 A, ˚ the N II 629.2, 858.3 A˚ (or the O II 589.3 A), ˚ the C II 687.3, 806.8, A,
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˚ The N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) (3, 1) band is newly identified, and the CO fourth positive Fig. 6. The net intensity of the Venusian airglow (1200–1480 A). (A 1 − X 1 + ) (14, 1), (14, 3), (6, 0) and (6, 1) bands, which were previously identified, are also identified. Vertical lines represent peaks of the spectrum. The differences ˚ between the observed and the reference (NIST and Herzberg (1950)) wavelengths are usually less than 3 A.
Fig. 7. The results of the fitting to EXCEED spectrum. The red curves represent the synthetic spectra fitted to the CO A-X (14, 3) band for (a), the CO A-X (9, 2) band for (b), the CO A-X (6, 1) band for (c), the N2 a-X (3, 1) band for (d) and the N2 a-X (2, 0) band for (e). These features are marked with red. For each panel, we estimate the relative intensities of these bands by referring to Benesch et al. (1966) and Conway (1982) for the N2 a-X bands, and to Durrance (1981) for the CO A-X bands. The black curve of (a) represents the Venusian spectrum observed by EXCEED, and those of the others represent the Venusian spectrum from which each fitted component is subtracted.
˚ which and 809.9 A˚ lines. The Ar I emissions at 866.8 and 1066.7 A, had not been detected in the Venus atmosphere, are observed. Although, the Ar I emission at 1048.2 A˚ is not observed. There is ˚ possibility that this emission is contaminated by the O I 1039.2 A. The observed intensities are listed in Table 1.
3.3. Carbon monoxide bands In Fig. 6, the CO fourth positive (A 1 − X 1 + ) bands are identified. Bands of the (14, ν ) progression appear because they are accidently pumped by the solar H I Ly-α at the (14, 0) band (Durrance et al., 1980). Bands of the (9, ν ) progression also ap-
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Table 2 Same as Table 1 but for CO emissions. Wavelengths are obtained from the NIST database for the atomic lines and Herzberg (1950) for the molecular bands. c) For the blended emissions, individual intensities are estimated by the spectrum from which the contributions of fitted spectrum are subtracted, and the errors are evaluated by propagation of errors of EXCEED spectrum and fitted spectrum. The CO Hopfield-Birge (E 1 − X 1 + ) bands Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
E 1 − X 1 + ( 2, 0 ) E 1 − X 1 + ( 1, 0 ) E 1 − X 1 + ( 0, 0 )
1029.0 1052.1 1076.2
− 1.1 ± 0.2 2.2 ± 0.4
189 ± 2 − −
The CO Hopfield-Birge (C 1 + − X 1 + ) bands Transition C 1 + − X 1 g+ C 1 + − X 1 + g C 1 + − X 1 + g 1 + 1 + C − X g
( 0, ( 0, ( 0, ( 0,
0) 1) 2) 3)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1088.0 1113.9 1140.8 1168.6
− 7±1 4.9 ± 0.8 −
37.5 ± 0.3 − − 10.5 ± 0.9
The CO Hopfield-Birge (B 1 + − X 1 + ) bands Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
B 1 + − X 1 + g ( 2, 0 ) B 1 + − X 1 + g ( 1, 0 ) B 1 + − X 1 g+ (0, 0 )
1098.0 1123.7 1150.7
− 0.7 ± 0.2 −
3.0 ± 0.2 − 108.6 ± 0.6
The CO Hopfield-Birge (b 1 u − X 1 g+ ) bands Transition b 3 + − X 1 g+ b 3 + − X 1 g+ b 3 + − X 1 g+ b 3 + − X 1 g+
( 3, ( 2, ( 1, ( 0,
0) 0) 0) 0)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1106.0 1133.5 1162.5 1193.0
2.2 ± 0.4 − − −
− 26.0 ± 0.3 7.6 ± 0.7 29.8 ± 0.5
The CO fourth positive (A 1 − X 1 + ) bands Transition A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 +
(14, 0) (14, 1) (14, 2) (9, 0) (14, 3) (9, 1) (14, 4) ( 6, 0 ) (9, 2) (14, 5) ( 5, 0 ) ( 6, 1 ) (9, 3) ( 4, 0 ) ( 5, 1 ) (3, 0) ( 6, 2 ) ( 4, 1 )
Wavelength ˚ [A]
Individual intensity [Rayleigh] (c)
Total blended intensity [Rayleigh]
1214.4 1246.9 1280.7 1301.5 1315.9 1338.8 1352.6 1367.7 1377.9 1390.9 1392.7 1409.0 1418.8 1419.2 1435.5 1447.5 1452.3 1464.7
− 31 ± 5 56 ± 9 − 110 ± 20 29 ± 1 210 ± 3 11 ± 2 53 ± 8 101 ± 2 46.0 ± 0.8 60 ± 10 37 ± 2 − 78 ± 2 − 28 ± 1 −
28, 100 ± 400 − − 2100 ± 10 − 138.8 ± 0.8 568 ± 2 − − 150.1 ± 0.9 150.1 ± 0.9 − 106.1 ± 0.8 106.1 ± 0.8 242 ± 2 153 ± 1 153 ± 1 224 ± 2
pear accidently pumped by the O I 1304 A˚ at the (9, 0) band (Durrance, 1981). In the CO fourth positive (A 1 − X 1 + ) system, (5, 1), (6, 0), (6, 1), (9, 2), and (14, 3) bands are clearly seen without blend with other transitions. In Fig. 5, the CO Hopfield-Birge (B 1 + − X 1 + ) (1, 0) bands and another Hopfield-Birge (C 1 + − X 1 + ) (0, 1) and (0, 2) bands are newly identified whereas the CO (B 1 + − X 1 + ) (0, 0), (2, 0), and (C 1 + − X 1 + ) (0, 0) bands are blended with atomic lines. Only the (0, 0) band of these systems has been detected by HUT (Feldman et al., 20 0 0) and by Cassini (Gérard et al., 2011). Feldman et al. (20 0 0) suggested that the main excitation source of
these bands is photoelectron impact. The observed intensities are also listed in Table 2. 3.4. Molecular nitrogen bands We find that the EUV dayglow of Venus is rich in N2 and N emissions as well as CO and C emissions. Among the N2 LymanBirge-Hopfield (a 1 g − X 1 g+ ) bands, only the (2, 0), (3, 1), and (5, 3) bands are observed without blending whereas the (0, 0), (1, 0), (1, 1), (2, 1), (3, 0), and (3, 2) bands are blended with other bands or lines as seen in Fig. 6. By fitting the spectrum mentioned in Section 3.1, the contributions of N2 a-X (2, 1) and (3, 2) bands have been confirmed. Because N2 a-X is electric dipole forbidden
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Y. Nara et al. / Icarus 000 (2017) 1–9 Table 3 Same as Table 1 but for N2 emissions.
The N2 Carroll-Yoshino (c4 1 u+ − X 1 g+ ) bands Transition 1 + c4 1 + u − X g ( 0 , 0 ) 1 + c4 1 + u − X g ( 0 , 1 )
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
958 980
5.0 ± 0.8 10 ± 2
− −
The N2 Birge-Hopfield (b 1 u − X 1 g+ ) bands Transition b1 u b1 u b1 u b1 u b1 u
− X 1 + g − X 1 + g 1 + − X g 1 + − X g − X 1 + g
( 1, ( 1, ( 1, ( 1, ( 1,
0) 1) 2) 3) 4)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
985.6 1008.8 1032.0 1057.6 1083.3
− − − 0.9 ± 0.2 −
100 ± 1 3.4 ± 0.5 189 ± 2 − 37.5 ± 0.3
The N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands Transition a1 g − X1 + g a1 g − X1 + g 1 1 + a g − X g 1 1 + a g − X g a1 g − X1 + g a1 g − X1 + g a1 g − X1 + g 1 1 + a g − X g 1 1 + a g − X g a1 g − X1 + g a1 g − X1 + g
( 5, ( 5, ( 3, ( 2, ( 3, ( 1, ( 5, ( 2, ( 3, ( 0, ( 1,
0) 1) 0) 0) 1) 0) 3) 1) 2) 0) 1)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1298.1 1338.6 1353.6 1383.8 1397.7 1416.0 1425.9 1429.9 1444.4 1450.2 1464.3
− − − 37 ± 6 18 ± 3 − 25 ± 4 160 ± 3 13 ± 2 − −
2100 ± 10 138.8 ± 0.8 568 ± 2 − − 106.1 ± 0.8 − 242 ± 2 153 ± 1 153 ± 1 224 ± 2
This can be applied for the Venusian atmosphere. The observed intensities are also listed in Table 3. 4. Conclusion We have obtained EUV spectra of Venusian airglow in the range 520 − 1480 A˚ with 3 − 4 A˚ resolutions. The observations of Venus by EXCEED provide the highest quality spectra than any other observations so far. Emissions of the N2 Lyman-BirgeHopfield (a 1 g − X 1 g+ ) (2, 0), (2, 1), (3, 1), (3, 2), and (5,
3) bands, the Carroll-Yoshino (c4 1 u+ − X 1 g+ ) bands and BirgeHopfield (b 1 u − X1 g+ ) (1, 3) band in the EUV wavelength range are found for the first time in the Venusian airglow. Acknowledgments Fig. 8. The residual spectrum after fitting to the EXCEED spectrum and the synthetic spectrum of CO A-X (5, v ) bands fitted to the CO A-X (5,0) band which is marked with red by assuming no self-absorption (red curve).
transition, this band feature is excited by photoelectron impact. Considering these features, we investigated blended emissions. In addition, the N2 Birge-Hopfield (b 1 u − X1 g+ ) (1, 3) band at 1058 A˚ and the Carroll-Yoshino (c 1 u+ − X 1 g+ ) (0, 0) at 960 A˚ and 4
(0, 1) at 981 A˚ bands are identified. These band systems were detected in the geocorona previously (Gentieu et al., 1981), but are identified for the first time in the Venusian airglow. In Fig. 3 of Gérard et al. (2011), the N2 c4 − X (0, 1) band appears while it
is blended with O I emission. The identification of N2 c4 − X (0, 1) and (0, 0) bands together by EXCEED confirms the existence of this band system. In the Earth’s atmosphere, although the transitions of these band features are dipole allowed, their main excitation source is photoelectron impact due to weak sunlight at the wavelengths where the absorption occurs (Chakrabarti et al., 1983).
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Meier, R.R., 1991. Ultraviolet spectroscopy and remote sensing of the upper atmosphere. Space Sci. Rev. 58, 1–185. doi:10.10 07/BF012060 0 0. Stevens, M.H., Evans, J.S., Schneider, N.M., Stewart, A.I.F., Deighan, J., Jain, S.K., Crismani, M., Stiepen, A., Chaffin, M.S., McClintock, W.E., Holsclaw, G.M., Lefèvre, F., Lo, D.Y., Clarke, J.T., Montmessin, F., Bougher, S.W., Jakosky, B.M., 2015. New observation of molecular nitrogen in the Martian upper atmosphere by IUVS on MAVEN. Geophys. Res. Lett. 42, 9050–9056. doi:10.1002/ 2015GL065319. Stern, S.A., Slater, D.C., Gladstone, G.R., Wilkenson, E., Cash, W.C., Green, J.C., Hunten, D.M., Owen, T.C., Paxton, L., 1996. The 825–1110A˚ EUV spectrum of Venus. Icarus 122, 200–204. doi:10.1006/icar.1996.0119. Yamazaki, A., Tsuchiya, F., Sakanoi, T., Uemizu, K., Yoshioka, K., Murakami, G., Kagitani, M., Kasaba, Y., Yoshikawa, I., Terada, N., Kimura, T., Sakai, S., Nakaya, K., Fukuda, S., Sawai, S., 2014. Field-of-view guiding camera on the HISAKI (SPRINTA) satellite. Space Sci. Rev. 184, 259–274. doi:10.1007/s11214-014-0106-y. Yoshioka, K., Murakami, G., Yamazaki, A., Tsuchiya, F., Kagitani, M., Sakanoi, T., Kimura, T., Uemizu, K., Uji, K., Yoshikawa, I., 2013. The extreme ultraviolet spectroscope for planetary science, EXCEED. Planet. Space Sci. 85, 250–260. doi:10.1016/j.pss.2013.06.021. Yoshikawa, I., Yoshioka, K, Murakami, G., Yamazaki, A., Tsuchiya, F., Kagitani, M., Sakanoi, T., Terada, N., Kimura, T., Kuwabara, M., Fujiwara, K., Hamaguchi, T., Tadokoro, H., 2014. Extreme ultraviolet radiation measurement for planetary atmospheres/magnetospheres from the Earth-orbiting spacecraft (Extreme ultraviolet spectroscope for exospheric dynamics: EXCEED). Space Sci. Rev. 184, 237– 258. doi:10.1007/s11214-014-0077-z.
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Extreme ultraviolet spectra of Venusian airglow observed by EXCEED Yusuke Nara a,∗, Ichiro Yoshikawa a, Kazuo Yoshioka a, Go Murakami b, Tomoki Kimura c, Atsushi Yamazaki b, Fuminori Tsuchiya d, Masaki Kuwabara b, Naomoto Iwagami e a
Department of Complexity Science and Engineering, University of Tokyo, Kashiwa, Chiba, 277-8561, Japan Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, Sagamihara, Kanagawa, 252-5210, Japan c RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama, 351-0198, Japan d Planetary Plasma and Atmospheric Research Center, Tohoku University, Sendai, Miyagi, 980-8577, Japan e Department of Earth and Planetary Science, University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan b
a r t i c l e
i n f o
Article history: Received 17 May 2016 Revised 16 September 2017 Accepted 23 October 2017 Available online xxx
a b s t r a c t Extreme ultraviolet (EUV) spectra of Venus in the wavelength range 520 − 1480 A˚ with 3 − 4 A˚ resolutions were obtained in March 2014 by an EUV imaging spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on the HISAKI spacecraft. Due to its high sensitivity and long exposure time, many new emission lines and bands were identified. Already known emissions such as the ˚ O I 989 A, ˚ H I Ly − β 1026 A, ˚ and the C I 1277 A˚ lines (Broadfoot et al., 1974; Bertaux et al., O II 834 A, 1980; Feldman et al., 20 0 0) are also detected in the EXCEED spectrum. In addition, N2 band systems such as the Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) (2, 0), (2, 1), (3, 1), (3, 2) and (5, 3) bands, the Birge Hopfield (b 1 u − X1 g+ ) (1, 3) band, and the Carroll-Yoshino (c4 1 u+ − X 1 g+ ) (0, 0) and (0, 1) bands together are identified for the first time in the Venusian airglow. We also identified the CO Hopfield-Birge (B 1 + − X 1 + ) (1, 0) band in addition to the already known (0, 0) band, and the CO Hopfield-Birge (C 1 + − X 1 + ) (0, 1), (0, 2) bands in addition to the already known (0, 0) band (Feldman et al., 20 0 0; Gérard et al., 2011). © 2017 Elsevier Inc. All rights reserved.
1. Introduction Extreme ultraviolet observations of the Venusian airglow play important roles in the study of the Venusian atmosphere. In 1970s, Mariner 10 (Broadfoot et al., 1974) and Venera 11/12 (Bertaux et al., 1980) observed the Venusian airglow using narrowband polychromators focusing on specific emission lines such as the He I 584 ˚ O II 834 A, ˚ H I 1216 A, ˚ and the O I 1304 A˚ although the interA, pretations of the data were ambiguous because of low signal to noise ratio due to short exposure time. In February 1990, Galileo flew by Venus on its way to Jupiter, and the Extreme Ultraviolet Spectrometer identified the H I Ly − 1026 A˚ and the O I 989 A˚ lines (Hord et al., 1991). In August 1994, an EUV telescope/spectrograph on a sounding rocket detected several emissions such as the N I 907 ˚ O I 1039 A, ˚ and the N II 1085 A, ˚ but the wavelength uncertainA, ties were as large as 9 A˚ (Stern et al., 1996). In March 1995, the Hopkins Ultraviolet Telescope (HUT) on the Astro-2 mission identified the CO Hopfield-Birge (B 1 + − X 1 + ) (0, 0) band, CO ˚ C I 1261 A, ˚ Hopfield-Birge (C 1 + − X 1 + ) (0, 0) band, N I 1134 A,
∗
Corresponding author. E-mail address: [email protected] (Y. Nara).
and the C I 1277 A˚ lines for the first time in the Venusian dayglow (Feldman et al., 20 0 0). The purpose of this paper is to present the spectra of the Venusian airglow features obtained by EXCEED on the HISAKI spacecraft (Yoshioka et al., 2013; Yoshikawa et al., 2014) and show their spectral identifications.
2. Analysis and database Since November 2013, EXCEED on the HISAKI spacecraft has measured the extreme ultraviolet spectra of several planets with exposure times significantly longer than any prior EUV spectroscopic measurements. The HISAKI spacecraft orbits around the earth between the altitudes of 1050 ± 100 km with a period of 106 min. The EXCEED spectro-imager consists of an entrance mirror, slits of different widths, a grating, and a microchannel plate detector. The narrow and the wide slits have widths of 10 and 60 , respectively, and the third one has a dumbbell-like shape. The spectrograph covers the wavelength region 520 − 1480 A˚ with a dispersion of around 1 A˚ per pixel. For the present observation, the 60 slit is used. Details of the instrument and the performance are given by Yoshioka et al. (2013) and Yoshikawa et al. (2014).
https://doi.org/10.1016/j.icarus.2017.10.028 0019-1035/© 2017 Elsevier Inc. All rights reserved.
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Fig. 2. The spectrum of the Venusian airglow after integration of as long as 21 days. Emissions extending vertically from -20 to 70 pixel such as at 1216 A˚ are geocoronal emissions. The Venusian airglow is seen at around y = 0. A narcissistic ghost of the H I Ly-α due to multiple reflection between the detector and the grating appears at ˚ 783 A.
Fig. 3. Spectra of 21-day integrations; Venus + geocorona (black curve) and geocorona alone (red curve).
Fig. 1. An example of raw spectral image with an exposure of 1 min. The x-axis represents wavelength and the y-axis represents east-west spatial position (west is upward), where 1 pixel corresponds to 4.2 . The strong feature at 1304 A˚ extending over 80 spatial pixels is the geocoronal O I emission. The Venus disk is seen at around y = 0 with a width of 7 pixels.
The Venusian airglow was observed with the 60 slit from March 9 through 29, 2014. During this observation of 21 days, the F10.7 index at 1 AU was 148 (http://www.spaceweather.gc.ca/) and the apparent diameter of Venus was at about 30 (7 pixels). The Venusian phase angle was kept at about 90 ° so that about 50% of Venus disk was illuminated. An example of a spectral image with an exposure of 1 min is shown in Fig. 1. A spectral image obtained by 21-day integration is shown in Fig. 2. Because the apparent diameter of Venus is smaller than the slit width, the spectral resolution may be different from the official value (Yoshioka et al., 2013). The object traces an elliptical path within the slit with a period of 106 min, due to the orbital motion of the HISAKI spacecraft (Yamazaki et al., 2014). This period is short enough compared to the 21-day integration time, so we consider that integrated spectrum is well averaged. Then, we estimate the spectral resolution by measuring widths of isolated lines. Because the integrated spectrum is well averaged, we assume the line shape as a gauss distribution and fitted it to isolated lines by using the nonlinear least square method (Marquardt, 1963). The es˚ timated spectral resolutions are between 3 and 4 A. There are three kinds of unwanted features. One is the geocoronal lines. Because the HISAKI’s orbit around the Earth is located between the altitudes of 1050 ± 100 km, the spectrum includes not
only the Venusian airglow but also contributions from the geocorona. The geocoronal emissions such as the H I Ly-α , Ly-β , Ly-γ , O, and the N emissions are also expected in the Venusian dayglow. We assume that the spectrum +210 (15 Rv ; 50 pixels) away from the center of Venus disk contains contributions only from the geocorona and that the geocorona is uniformly distributed in the slit’s field of view. Fig. 3 shows a comparison of two 21-day integrations. One includes contributions of both Venus and the geocorona (black curve), and the other includes those of the geocorona alone (red curve). The net intensity of Venus Iv (λ, yv ) is evaluated as
Iv (λ, yv ) = I (λ, yv ) − I (λ, yv + 50 ), where I is the intensity observed by EXCEED, λ is wavelength, and yv is the position of the center of Venus disk. The center of Venus ˚ which is a is evaluated by using the emission of O I at 1356 A, relatively weak emission line in the Earth atmosphere at an altitude of 10 0 0 km and is strong in the Venus atmosphere (Fig. 3). We consider the center of Venus as the pixel value y which maximizes I(1356, y). The net intensities of Venusian spectrum are listed in Tables 1–3. Another unwanted feature is the narcissistic ghost (McCandliss et al., 1998). The narcissistic ghost of the H I 1216 A˚ appears at 783 A˚ as seen in Figs. 2 and 4. It is also found that of the O I 1304 A˚ appears at 705 A˚ as seen in Fig. 4. The other unwanted feature is the second order diffraction. There is the possibility that the emission at 1168 A˚ includes not only contributions from the N I at 1169 A˚ and the CO Hopfield˚ but also from the Birge (C 1 + − X 1 + ) (0, 3) band at 1169 A, ˚ which exhibits relatively strong second order of the He I at 584 A, emission among the spectra obtained by EXCEED. We identified the spectral features by the help of Meier (1991) for the geocorona, Herzberg (1950) for the di-
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Table 1 Identified atomic emission features. New emission features are indicated by bold letters. Wavelengths are obtained from the NIST database for the atomic lines except for the N I 853 A˚ which is suggested to exist in the Earth’s night glow by Chakrabarti (1984). Total blended intensity is recorded when the emission is blended with other species and is the sum of intensities which contribute to the emission. Individual intensity is the intensity which contains the contribution of only the transition. (a) For isolated emission lines, intensities are estimated by multiplying observed peak values by the instrumental width of 3.5 ± 0.5 A˚ and the errors are evaluated by propagation of errors. For the blended emissions, individual intensities are estimated by the spectrum from which the contributions of fitted spectrum are subtracted, and the errors are evaluated by propagation of errors of the EXCEED spectrum and the fitted spectrum. (b) The intensities are estimated by integrating spectrum of the emission and the errors are evaluated by calculating standard deviation divided by square root of the number data. HI Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh] (a)
Total blended intensity [Rayleigh] (b)
1s 2 S − 4 p 2 Po 1s 2 S − 3 p 2 Po 1s 2 S − 2 p 2 Po
972.5 1025.7 1215.7
24 ± 4 − −
− 189 ± 2 28, 100 ± 400
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1s2 1 S − 1s3 p 1 Po 1s2 1 S − 1s2 p 1 Po
537.0 584.3
9±2 220 ± 40
− −
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 2 p2 3 P − 2s2 2 p5d 3 Po 2s2 2 p2 3 P − 2s2 2 p4d 3 Do 2s2 2 p2 2 P − 2s2 2 p3d 3 Po 2s2 2 p2 3 P − 2s2 2 p3d 3 D0 2s2 2 p2 1 P − 2s2 2 p6d 1 Fo 2s2 2 p2 3 P − 2s2 p3 3 Po 2s2 2 p2 1 D − 2s2 2 p3d 1 Fo
1158.0 1193.2 1261.6 1277.6 1288.4 1329.1 1463.3
10 ± 2 − 16 ± 3 54 ± 8 4.2 ± 0.7 33 ± 5 −
− 29.8 ± 0.5 − − − − 224 ± 2
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 2 p 2 Po − 2s2 3d 2 D 2s2 p2 4 P − 2s2 p(3 Po )3s 4 Po 2s2 p2 2 D − 2s2 p(3 Po )3d 2 Do 2s2 p2 4 P − 2s2 p3 4 So 2s2 2 p 2 Po − 2s2 p2 2 D
687.3 806.8 809.9 1010.4 1335.7
0.7 ± 0.2 0.3 ± 0.2 0.4 ± 0.2 − −
− − − 3.4 ± 0.5 138.8 ± 0.8
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
N II + e− → N I + hν 2s2 2 p3 4 So − 2s2 2 p2 (3 P )5d 4 P 2s2 2 p3 4 So − 2s2 2 p2 (3 P )4d 4 P 2 p3 4 So − 2 p2 (3 P )3d 4 P 2 p3 4 So − 2 p2 (3 P )4s 4 P 2 p3 2 Do − 2 p2 (1 D )4s 2 D 2 p3 2 Do − 2 p2 (3 P )4d 2 D 2s2 2 p3 4 S − 2s2 p4 4 P 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )3d 2 D 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )3d 2 F 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )4s 2 P 2s2 2 p3 4 So − 2s2 2 p2 (3 P )3s 4 F
853 886.2 906.4 953.4 964.0 990.8 1097.2 1134.2 1163.9 1168.5 1177.7 1200.2
1.2 ± 0.3 1.9 ± 0.4 4.6 ± 0.7 − 4.7 ± 0.8 − − − − − 10 ± 2 100 ± 20
− − − 18.9 ± 0.3 − 100.6 ± 0.7 3.0 ± 0.2 26.0 ± 0.3 7.6 ± 0.7 10.5 ± 0.9 − −
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 p3 5 So − 2s2 p2 (4 P )3d 5 P 2s2 2 p2 3 P0 − 2s2 p3 3 So 2s2 2 p2 1 S − 2s2 2 p3s 1 Po 2s2 2 p2 3 P − 2s2 p3 3 Po 2s2 2 p2 3 P − 2s2 p3 3 Do
629.2 645.2 858.3 916.7 1085.7
0.7 ± 0.3 − − 11 ± 2 −
− 2.3 ± 0.2 3.5 ± 0.2 − 37.5 ± 0.3
He I
CI
C II
NI
N II
(continued on next page)
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Y. Nara et al. / Icarus 000 (2017) 1–9 Table 1 (continued) OI Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
771.1 792.2 859.3 879.0 931.5 938.0 950.1 988.8 999.5 1025.8 1039.2 1152.1 1304.4 1355.6
1.2 ± 0.3 2.1 ± 0.4 − 3.9 ± 0.7 6±1 8±2 − − 2.8 ± 0.5 − 15 ± 3 − − −
− − 3.5 ± 0.2 − − − 18.9 ± 0.3 100.6 ± 0.7 − 189 ± 2 − 108.6 ± 0.6 2100 ± 10 568 ± 2
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
2s2 2 p3 2 Do − 2s2 2 p2 (1 D )3s 2 D 2s2 2 p3 2 Po − 2s2 2 p2 (1 D )3s 2 D 2s2 2 p3 2 Do − 2s2 2 p2 (3 P )3s 2 P 2s2 2 p3 2 Po − 2s2 p4 2 S 2s2 2 p3 2 Po − 2s2 2 p2 (3 P )3s 2 P 2s2 2 p3 2 Do − 2s2 p4 2 D 2s2 p4 4 P − 2s2 p3 (5 So )3s 4 So 2s2 2 p3 4 So − 2s2 p4 3 Po
555.1 600.6 617.1 644.2 673.8 718.5 739.9 834.5
2.2 ± 0.7 1.8 ± 0.5 12 ± 2 − 2.3 ± 0.4 5.4 ± 0.9 0.7 ± 0.2 130 ± 20
− − − 2.3 ± 0.2 − − − −
Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
3s2 2 p6 1 S − 3s2 3 p5 (2 Po )3d2 3/2o 3 p6 1 S − 3 p5 (2 Po )4s 2 3/2o
866.8 1066.7
2.2 ± 0.4 1.7 ± 0.3
− −
2s2 2 p4 3 P − 2s2 2 p3 (2 Do )4d 3 D0 2s2 2 p4 3 P − 2s2 p5 3 Po 2s2 2 p4 1 D − 2s2 2 p3 (2 Do )5d 1 Fo 2s2 2 p4 3 P − 2s2 2 p3 (2 Po )3s 3 Po 2s2 2 p4 3 P − 2s2 2 p3 (4 So )7d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )6d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )5d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (2 Do )3s 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (2 Po )3s 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )3d 3 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )4s 3 So 2s2 2 p4 1 D − 2s2 2 p3 (2 Do )3s 1 Do 2s2 2 p4 3 P − 2s2 2 p3 (4 So )3s 3 So 2s2 2 p4 3 P − 2s2 2 p3 3s 5 So O II
Ar I
atomic molecules and the NIST database (http://www.nist.gov/ pml/data/asd.cfm) for atomic lines. 3. Discussion Because the 520 − 1480 A˚ region exhibits the highest quality in terms of signal to noise ratio among the spectral region reported so far, the present discussion focuses on the identification of new features in this region. Figs 4–6 show the net intensities of the Venusian airglow in each wavelength range with identifications of emissions. 3.1. General features of Venusian dayglow We found similarities in the EUV airglow of the terrestrial planets Venus, Earth and Mars. That is, the H I, He I, N I, and the O I emissions are dominant in neutral atomic emissions, and the N II and the O II emissions are dominant in ionic emissions in the airglow of these planets. No emissions due to divalent ions are found. That is, interactions between the solar wind and the planetary atmospheres are not observed even though Venus and Mars have no significant magnetic field. In molecular emissions, only CO and N2 emissions are found although CO2 is the main constituent of the Venusian and Martian atmosphere. It may be attributed to the difference in their dissociation potential energies. Those of CO (11.1 eV) and N2 (9.8 eV) are much higher than that of CO2 (5.5 eV) which is dissociated by sunlight. So, there are no CO2 band emissions, but CO band emissions exist in the spectra of Venus and Mars, and N2 bands also in the spectra of terrestrial planets (Gentieu et al., 1981; Stevens et al., 2015).
In the Venusian airglow spectrum, many emission features are attributed to not one but several emission lines, especially for ˚ such as shown in Fig. 6. To resolve wavelengths above 1200 A, such blends, we consider measured and theoretical emission ratios of the N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands and the CO fourth positive (A 1 − X 1 + ) bands. We generated synthetic spectra of the N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands with a 4 A˚ resolution referring to the theoretical estimations by Benesch et al. (1966) and Conway (1982). Benesch et al. (1966) calculated the Franck-Condon factors for transitions in N2 theoretically and Conway (1982) calculated the transition probabilities of the N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands by using the Franck-Condon factors. Durrance (1981) calculated the transition probabilities of the CO fourth positive (A 1 − X 1 + ) bands by using the observations of International Ultraviolet Explorer. Using these information and the nonlinear least square method (Marquardt, 1963), we investigated blended emission features. Specifically, we have conducted following 3 steps. First, a Gaussian function is fitted to the molecular bands which are not blended with other species. Second, intensities of the other bands but for (ν , 0) bands, which cause self-absorption, are estimated. Third, the fitted spectrum is subtracted from the EXCEED spectrum. The same approach is done for the residual spectrum, repeatedly. In the estimation of relative intensities, we adjusted the wavelength manually because of the wavelength inaccuracy of EX˚ while CEED spectrum. We adopted the spectral resolution as 4 A, ˚ That does not affect our conit is estimated between 3 and 4 A. clusion because we focus on the integrated values. Fig. 7 shows the result. We fitted the CO A-X (14, 3), (9, 2), (6, 1), the N2 a-X (3, 1), and (2, 0) band features to the residual spectrum. Because Conway (1982) found the self-absorption of the N2 a-X (2, 0) is
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˚ by the 21-day integration. Many of them are identified for the first time. Vertical lines represent peaks of Fig. 4. The net intensity of the Venusian airglow (520–900 A) the observed spectrum. We refer to the NIST database for the wavelength used in the labels for each feature. The differences between the observed and reference (NIST) ˚ wavelengths are usually less than 1 A.
˚ The CO Hopfield-Birge (B 1 + − X 1 + ) (1, 0) bands, another Hopfield-Birge (C 1 + − X 1 + ) (0, 1) and Fig. 5. The net intensity of the Venusian airglow (90 0 –120 0 A). (0, 2) bands, the N2 Birge-Hopfield (b 1 u − X1 g+ ) (1, 3) band, and the Carroll-Yoshino (c4 1 u+ − X 1 g+ ) (0, 0) and (0, 1) bands are also newly identified. Vertical lines represent peaks of the observed spectrum. We refer to the NIST database for the atomic and ionic emissions, and to Herzberg (1950) for the molecular emissions for the ˚ wavelength used in the labels for each feature. The differences between the observed and reference (NIST and Herzberg (1950)) wavelengths are usually less than 2 A.
negligible on Earth, we remove the contribution of the N2 a-X (2, 0) and (2, 1) from the residual spectrum (Fig. 7e). Fig. 8 shows the residual spectrum and synthetic spectrum of CO A-X (5, ν ) bands fitted to CO A-X (5, 0) band. The intensity of the (5, 1) band is derived by assuming no self-absorption. The candidates for species which contribute to the emission at 1435 A˚ are N2 a-X (2, 1), CO ˚ The contribution from N2 a-X (2, 1) is A-X (5, 1), and C I 1432.5 A. already subtracted from the spectrum in Fig. 8. The intensity of the synthetic (5, 1) band is 78 ± 2 Rayleighs while the observed intensity is 82 ± 3 Rayleighs. So, we conclude that the self-absorption of
the CO A-X (5, 0) band is negligible and there is no contribution of C I 1432.5 A˚ to this emission. 3.2. Atomic lines While emissions brighter than 10 Rayleighs have been reported by the previous observations, EXCEED could detect emissions as ˚ many new emission lines were weak as 1 Rayleigh. Below 10 0 0 A, ˚ the O II 600.6, 673.8, 739.9 detected such as the O I 771.1, 792.2 A, ˚ the N II 629.2, 858.3 A˚ (or the O II 589.3 A), ˚ the C II 687.3, 806.8, A,
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˚ The N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) (3, 1) band is newly identified, and the CO fourth positive Fig. 6. The net intensity of the Venusian airglow (1200–1480 A). (A 1 − X 1 + ) (14, 1), (14, 3), (6, 0) and (6, 1) bands, which were previously identified, are also identified. Vertical lines represent peaks of the spectrum. The differences ˚ between the observed and the reference (NIST and Herzberg (1950)) wavelengths are usually less than 3 A.
Fig. 7. The results of the fitting to EXCEED spectrum. The red curves represent the synthetic spectra fitted to the CO A-X (14, 3) band for (a), the CO A-X (9, 2) band for (b), the CO A-X (6, 1) band for (c), the N2 a-X (3, 1) band for (d) and the N2 a-X (2, 0) band for (e). These features are marked with red. For each panel, we estimate the relative intensities of these bands by referring to Benesch et al. (1966) and Conway (1982) for the N2 a-X bands, and to Durrance (1981) for the CO A-X bands. The black curve of (a) represents the Venusian spectrum observed by EXCEED, and those of the others represent the Venusian spectrum from which each fitted component is subtracted.
˚ which and 809.9 A˚ lines. The Ar I emissions at 866.8 and 1066.7 A, had not been detected in the Venus atmosphere, are observed. Although, the Ar I emission at 1048.2 A˚ is not observed. There is ˚ possibility that this emission is contaminated by the O I 1039.2 A. The observed intensities are listed in Table 1.
3.3. Carbon monoxide bands In Fig. 6, the CO fourth positive (A 1 − X 1 + ) bands are identified. Bands of the (14, ν ) progression appear because they are accidently pumped by the solar H I Ly-α at the (14, 0) band (Durrance et al., 1980). Bands of the (9, ν ) progression also ap-
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Table 2 Same as Table 1 but for CO emissions. Wavelengths are obtained from the NIST database for the atomic lines and Herzberg (1950) for the molecular bands. c) For the blended emissions, individual intensities are estimated by the spectrum from which the contributions of fitted spectrum are subtracted, and the errors are evaluated by propagation of errors of EXCEED spectrum and fitted spectrum. The CO Hopfield-Birge (E 1 − X 1 + ) bands Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
E 1 − X 1 + ( 2, 0 ) E 1 − X 1 + ( 1, 0 ) E 1 − X 1 + ( 0, 0 )
1029.0 1052.1 1076.2
− 1.1 ± 0.2 2.2 ± 0.4
189 ± 2 − −
The CO Hopfield-Birge (C 1 + − X 1 + ) bands Transition C 1 + − X 1 g+ C 1 + − X 1 + g C 1 + − X 1 + g 1 + 1 + C − X g
( 0, ( 0, ( 0, ( 0,
0) 1) 2) 3)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1088.0 1113.9 1140.8 1168.6
− 7±1 4.9 ± 0.8 −
37.5 ± 0.3 − − 10.5 ± 0.9
The CO Hopfield-Birge (B 1 + − X 1 + ) bands Transition
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
B 1 + − X 1 + g ( 2, 0 ) B 1 + − X 1 + g ( 1, 0 ) B 1 + − X 1 g+ (0, 0 )
1098.0 1123.7 1150.7
− 0.7 ± 0.2 −
3.0 ± 0.2 − 108.6 ± 0.6
The CO Hopfield-Birge (b 1 u − X 1 g+ ) bands Transition b 3 + − X 1 g+ b 3 + − X 1 g+ b 3 + − X 1 g+ b 3 + − X 1 g+
( 3, ( 2, ( 1, ( 0,
0) 0) 0) 0)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1106.0 1133.5 1162.5 1193.0
2.2 ± 0.4 − − −
− 26.0 ± 0.3 7.6 ± 0.7 29.8 ± 0.5
The CO fourth positive (A 1 − X 1 + ) bands Transition A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 + A 1 + − X 1 +
(14, 0) (14, 1) (14, 2) (9, 0) (14, 3) (9, 1) (14, 4) ( 6, 0 ) (9, 2) (14, 5) ( 5, 0 ) ( 6, 1 ) (9, 3) ( 4, 0 ) ( 5, 1 ) (3, 0) ( 6, 2 ) ( 4, 1 )
Wavelength ˚ [A]
Individual intensity [Rayleigh] (c)
Total blended intensity [Rayleigh]
1214.4 1246.9 1280.7 1301.5 1315.9 1338.8 1352.6 1367.7 1377.9 1390.9 1392.7 1409.0 1418.8 1419.2 1435.5 1447.5 1452.3 1464.7
− 31 ± 5 56 ± 9 − 110 ± 20 29 ± 1 210 ± 3 11 ± 2 53 ± 8 101 ± 2 46.0 ± 0.8 60 ± 10 37 ± 2 − 78 ± 2 − 28 ± 1 −
28, 100 ± 400 − − 2100 ± 10 − 138.8 ± 0.8 568 ± 2 − − 150.1 ± 0.9 150.1 ± 0.9 − 106.1 ± 0.8 106.1 ± 0.8 242 ± 2 153 ± 1 153 ± 1 224 ± 2
pear accidently pumped by the O I 1304 A˚ at the (9, 0) band (Durrance, 1981). In the CO fourth positive (A 1 − X 1 + ) system, (5, 1), (6, 0), (6, 1), (9, 2), and (14, 3) bands are clearly seen without blend with other transitions. In Fig. 5, the CO Hopfield-Birge (B 1 + − X 1 + ) (1, 0) bands and another Hopfield-Birge (C 1 + − X 1 + ) (0, 1) and (0, 2) bands are newly identified whereas the CO (B 1 + − X 1 + ) (0, 0), (2, 0), and (C 1 + − X 1 + ) (0, 0) bands are blended with atomic lines. Only the (0, 0) band of these systems has been detected by HUT (Feldman et al., 20 0 0) and by Cassini (Gérard et al., 2011). Feldman et al. (20 0 0) suggested that the main excitation source of
these bands is photoelectron impact. The observed intensities are also listed in Table 2. 3.4. Molecular nitrogen bands We find that the EUV dayglow of Venus is rich in N2 and N emissions as well as CO and C emissions. Among the N2 LymanBirge-Hopfield (a 1 g − X 1 g+ ) bands, only the (2, 0), (3, 1), and (5, 3) bands are observed without blending whereas the (0, 0), (1, 0), (1, 1), (2, 1), (3, 0), and (3, 2) bands are blended with other bands or lines as seen in Fig. 6. By fitting the spectrum mentioned in Section 3.1, the contributions of N2 a-X (2, 1) and (3, 2) bands have been confirmed. Because N2 a-X is electric dipole forbidden
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Y. Nara et al. / Icarus 000 (2017) 1–9 Table 3 Same as Table 1 but for N2 emissions.
The N2 Carroll-Yoshino (c4 1 u+ − X 1 g+ ) bands Transition 1 + c4 1 + u − X g ( 0 , 0 ) 1 + c4 1 + u − X g ( 0 , 1 )
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
958 980
5.0 ± 0.8 10 ± 2
− −
The N2 Birge-Hopfield (b 1 u − X 1 g+ ) bands Transition b1 u b1 u b1 u b1 u b1 u
− X 1 + g − X 1 + g 1 + − X g 1 + − X g − X 1 + g
( 1, ( 1, ( 1, ( 1, ( 1,
0) 1) 2) 3) 4)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
985.6 1008.8 1032.0 1057.6 1083.3
− − − 0.9 ± 0.2 −
100 ± 1 3.4 ± 0.5 189 ± 2 − 37.5 ± 0.3
The N2 Lyman-Birge-Hopfield (a 1 g − X 1 g+ ) bands Transition a1 g − X1 + g a1 g − X1 + g 1 1 + a g − X g 1 1 + a g − X g a1 g − X1 + g a1 g − X1 + g a1 g − X1 + g 1 1 + a g − X g 1 1 + a g − X g a1 g − X1 + g a1 g − X1 + g
( 5, ( 5, ( 3, ( 2, ( 3, ( 1, ( 5, ( 2, ( 3, ( 0, ( 1,
0) 1) 0) 0) 1) 0) 3) 1) 2) 0) 1)
Wavelength ˚ [A]
Individual intensity [Rayleigh]
Total blended intensity [Rayleigh]
1298.1 1338.6 1353.6 1383.8 1397.7 1416.0 1425.9 1429.9 1444.4 1450.2 1464.3
− − − 37 ± 6 18 ± 3 − 25 ± 4 160 ± 3 13 ± 2 − −
2100 ± 10 138.8 ± 0.8 568 ± 2 − − 106.1 ± 0.8 − 242 ± 2 153 ± 1 153 ± 1 224 ± 2
This can be applied for the Venusian atmosphere. The observed intensities are also listed in Table 3. 4. Conclusion We have obtained EUV spectra of Venusian airglow in the range 520 − 1480 A˚ with 3 − 4 A˚ resolutions. The observations of Venus by EXCEED provide the highest quality spectra than any other observations so far. Emissions of the N2 Lyman-BirgeHopfield (a 1 g − X 1 g+ ) (2, 0), (2, 1), (3, 1), (3, 2), and (5,
3) bands, the Carroll-Yoshino (c4 1 u+ − X 1 g+ ) bands and BirgeHopfield (b 1 u − X1 g+ ) (1, 3) band in the EUV wavelength range are found for the first time in the Venusian airglow. Acknowledgments Fig. 8. The residual spectrum after fitting to the EXCEED spectrum and the synthetic spectrum of CO A-X (5, v ) bands fitted to the CO A-X (5,0) band which is marked with red by assuming no self-absorption (red curve).
transition, this band feature is excited by photoelectron impact. Considering these features, we investigated blended emissions. In addition, the N2 Birge-Hopfield (b 1 u − X1 g+ ) (1, 3) band at 1058 A˚ and the Carroll-Yoshino (c 1 u+ − X 1 g+ ) (0, 0) at 960 A˚ and 4
(0, 1) at 981 A˚ bands are identified. These band systems were detected in the geocorona previously (Gentieu et al., 1981), but are identified for the first time in the Venusian airglow. In Fig. 3 of Gérard et al. (2011), the N2 c4 − X (0, 1) band appears while it
is blended with O I emission. The identification of N2 c4 − X (0, 1) and (0, 0) bands together by EXCEED confirms the existence of this band system. In the Earth’s atmosphere, although the transitions of these band features are dipole allowed, their main excitation source is photoelectron impact due to weak sunlight at the wavelengths where the absorption occurs (Chakrabarti et al., 1983).
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Please cite this article as: Y. Nara et al., Extreme ultraviolet spectra of Venusian airglow observed by EXCEED, Icarus (2017), https://doi.org/10.1016/j.icarus.2017.10.028
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