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The extreme ultraviolet spectroscope for planetary science, EXCEED
Planetary and Space Science 85 (2013) 250–260
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The extreme ultraviolet spectroscope for planetary science, EXCEED K. Yoshioka a,n, G. Murakami a, A. Yamazaki a, F. Tsuchiya b, M. Kagitani b, T. Sakanoi b, T. Kimura a, K. Uemizu c, K. Uji d, I. Yoshikawa d a
Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan Tohoku University, Japan c National Astronomical Observatory of Japan, Japan d The University of Tokyo, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2013 Received in revised form 10 June 2013 Accepted 20 June 2013 Available online 29 June 2013
The extreme ultraviolet spectroscope EXtrem ultraviolet spetrosCope for ExosphEric Dynamics (EXCEED) on board the SPRINT-A mission will be launched in the summer of 2013 by the new Japanese solid propulsion rocket Epsilon as its first attempt, and it will orbit around the Earth with an orbital altitude of around 1000 km. EXCEED is dedicated to and optimized for observing the terrestrial planets Mercury, Venus and Mars, as well as Jupiter for several years. The instrument consists of an off axis parabolic entrance mirror, switchable slits with multiple filters and shapes, a toroidal grating, and a photon counting detector, together with a field of view guiding camera. The design goal is to achieve a large effective area but with high spatial and spectral resolution. In this paper, the performance of each optical component will be described as determined from the results of test evaluation of flight models. In addition, the results of the optical calibration of the overall instrument are also shown. As a result, the spectral resolution of EXCEED is found to be 0.3–0.5 nm Full Width at Half Maximum (FWHM) over the entire spectral band (52–148 nm) and the spatial resolution achieve was 10". The evaluated effective area is around 3 cm2. Based on these specifications, the possibility of EXCEED detecting atmospheric ions or atoms around Mercury, Venus, and Mars will be discussed. In addition, we estimate the spectra that might be detected from the Io plasma torus around Jupiter for various hypothetical plasma parameters. & 2013 Elsevier Ltd. All rights reserved.
Keywords: EUV observation EXCEED Io plasma torus Planetary science Spectroscope
1. Introduction The extreme ultraviolet (EUV) is an important spectral band for planetary science as it includes many interesting emissions from plasmas and atmospheres. However, it is not easy to observe there because to begin with the observing site must be outside of the Earth's atmosphere in order to avoid absorption. And because the emissions from many of the targets are very faint, the effective area of the detector should be as large as possible and long observation times are required. Beginning more than 30 years ago, a number of satellites and orbiters have made planetary observations in the EUV. The first attempt to observe planetary EUV emission was made by the spectrometer on board Mariner-10 (Broadfoot et al., 1977). The instrument took the spectra from the atmosphere of Mercury and Venus. Several years later, the EUV imager on board the Planet-B spacecraft took the first images of a part of the Earth's plasmasphere capturing the EUV emissions from the helium ions (30.4 nm) there (Nakamura et al., 2000; Yoshikawa et al., 2000).
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The Ultraviolet Imaging Spectrograph on board the Cassini spacecraft made both imaging and spectroscopic observations of Jupiter and Saturn (Esposito et al., 2004). The Erath orbiting EUVE satellite also made EUV observation of the Mars and Jupiter (Krasnopolsky et al., 1994; Herbert and Hall, 1998). In addition, the KAGUYA spacecraft observed the Earth's plasmasphere from the orbit of moon using EUV emissions (Yoshikawa et al., 2008; Murakami et al., 2013). Although these observations have delivered much valuable scientific information, it has limitations due to limited observation windows, narrow fields of view and spectral bands, and low spectral resolution; all of which create serious constraints on what we can say about of the respective planetary plasmas with respect to long period variations or phenomena with wide dynamic range. EXtrem ultraviolet spectrosCope for ExosphEric Dynamics (EXCEED) is an EUV spectroscope on board the Japanese small scientific satellite, “SPRINT-A” which is dedicated to planetary science. It will be launched in the summer in 2013 and carry out spectroscopic and imaging observation of EUV emissions from the plasmas and atmospheres around the solar system planets. The spectral range of EXCEED is wide enough to include the majority of planetary EUV emissions (52–148 nm) and its spectral resolution is high enough to separate those lines (Full Width at Half Maximum,
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FWHM: 0.3 nm). Since EXCEED is developed mainly for planetary science, its observation window can be dedicated to the solar system planets limited only by the allowable geometrical angles of the observing Sun–Spacecraft–target system. In this paper, the specifications and performance of EXCEED as measured during the flight model calibration is summarized, and possible observation scenarios will also be discussed.
2. Scientific targets EXCEED has two main scientific goals. One is to estimate the atmosphere escaping rate from the terrestrial planets. Although, heretofore there have been several in-situ observations by orbiters, the knowledge obtained from them has been limited (e.g., Shizgal and Arkos (1996)). VENUS EXPRESS and MARS EXPRESS measured physical parameters such as velocity and temperature along the orbits. They allowed good estimations of these flows based on their in-situ measurements (Lundin et al., 2008; Lammer et al., 2006). These orbiters measured local physical parameters such as velocity and temperature, but global aspects such as the total amount of outward-flow plasma are not straightforward to determine with a few exception such as Rosetta–Alice observation for Mars (Feldman et al., 2011). EXCEED will be able to deduce the density distribution of the planetary plasmas and determine the quantity of the escaping atmosphere. Its field of view is wide enough that we can get an image from the interaction region between solar wind and planetary plasmas all the way through to the tail at any one time. This will enhance our knowledge of the characteristics of outward-flowing plasmas, e.g., their composition, rate of flow, and dependence on solar activity. For Mercury, the first observation of its exosphere was made by the Mariner-10 spacecraft (Broadfoot et al., 1976). They found three atomic species there (H, He, and O). After later observations by ground-based telescopes, atomic sodium, potassium, and calcium have also been detected (Potter and Morgan, 1985, 1986; Bida et al., 2000). Next, the ultraviolet spectrometer on board NASA's MESSENGER detected atomic magnesium (McClintock et al., 2009). In addition, a distribution of magnesium, calcium, and sodium around the tail region of Mercury has been found (Vervack et al., 2010). The sodium imager with Fabry–Perot interferometer MSASI (Yoshikawa et al., 2010) and UV spectrometer PHEBUS (Chassefiere et al., 2010) on board the European Space Agency (ESA) and Japan Aerospace Exploration Agency's (JAXA) BepiColombo mission which will be launched in 2016 will improve our knowledge of the spatial variations of atmosphere around Mercury. Through EUV emissions, EXCEED will be able to map possible density distributions of other atomic species such as neon or sulfur which have bright emissions in the EUV. Such information will help us to understand the evolution of Mercury and the generation process of its exosphere. The other study objective for EXCEED is the plasma dynamics around the Jovian inner magnetosphere. This can be monitored by using the EUV emission surrounding the Io plasma torus which is located in the Jovian inner magnetosphere. The neutral volcanic ejecta from Io, one of the Galilean satellites is the main source of plasma for the torus, and drives the shape and dynamics of the rapidly rotating Jovian inner magnetosphere. Major ions such as sulfur and oxygen have many allowed transition lines in the EUV, and their radiation easily escapes to become observable from outside the plasma region (Delamere and Bagenal, 2003; Steffl et al., 2004a). EXCEED will carry out both spectroscopic measurements as well as making imaging observations, together they will enable us to deduce the radial distribution of the ion densities and surrounding electron temperatures through the spectral diagnosis (see Section 6.2).
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In addition, EXCEED will also observe the Jovian aurora. The radiation from the aurora comes from excitation by the precipitating electrons along the magnetic field line in the foot print of the middle (∼20 Jovian radii from the planet center) magnetosphere (Gerard et al., 1994; Bhattacharya and Thorne, 2001). Furthermore, it is thought that the electrons around the equatorial plane move toward inside through the process of injection (Mauk et al., 2002) or interchange instability (Russel et al., 2005). Therefore, since EXCEED can observe both the torus and aurora at same time, we will be able to discuss about the plasma transport process around the Jovian magnetosphere by comparing the spectra of the torus (inner magnetosphere) to that of the aurora (corresponding to the middle magnetosphere).
3. Launch date and orbit The EXCEED satellite will be launched in the summer of 2013 by the new Japanese solid propulsion rocket Epsilon (Morita, 2012). Epsilon has been developed to provide a more reliable and responsive space launch system with a lower life cycle cost for various small satellites. The rocket has 3 stages and is designed to provide flexible capabilities for various orbits and weights. The SPRINT-A is the first satellite that will be launched by Epsilon. The nominal mission life is 1-year but it could be extended to several years. As shown in the previous section, EXCEED will address various fundamental scientific questions pertaining to planetary plasma science through its capability for observations covering wide dynamic ranges and long time periods. Fig. 1 shows the Sun– spacecraft–planet angles from the beginning of 2013 to the end of 2015. Thanks to its entrance baffle, EXCEED can observe planets as long as the separation angle exceeds 20○. After the launch and inflight calibration, we will start to observe Venus for several weeks. Then the target will shift to Jupiter. After 2 months of observing Jupiter, Mars and Mercury will be selected. The orbital altitude has a 950 km perigee and 1150 km apogee. These come from optimizing the tradeoffs between contaminating emissions from the Geocoronal emission and Van Allen radiation belt (Yoshioka et al., 2010). The orbital period is 105 min and the inclination is 31○.
4. Design and performance of the EXCEED (efficiency) EXCEED consists of an entrance mirror, a slit plate with changeable filters, a grating, and a photon counting device together with an FOV guiding camera (Fig. 2). The incoming photons from the targets (planetary emissions) are incident onto
Fig. 1. The Sun–spacecraft–planet angle as a function of time. The angle must be larger than 20○ before targets can be observed (bold lines).
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Table 1 Major design parameters for EXCEED.
Fig. 2. The optical layout of EXCEED. The photons are incident on the entrance mirror through the baffle and collected onto the slit with its reflection angle of 5.4○. The FOV guiding camera monitors the reflected light from the slit plate. Light that passes through the slit and filter is diffracted by the grating. Finally an MCP detector converts the photons into electron events.
Wavelength range
52–148 nm
Entrance mirror
Shape: off-axis (5.4○) parabolic Effective diameter: 203 mm Reflecting surface: CVD-SiC Focal length: 1600 mm
Grating
Shape: toroidal (RH ¼ 400 mm, RV ¼393 mm) Effective diameter: 50 mm Surface: CVD-SiC Lines: laminar, 1800 mm−1
Slit widths
0.08 mm (10") 0.50 mm (60") 1.10 mm (140")a
Filters
Blank Indium thickness¼100 nm CaF2 thickness¼3 mm
Detector
MCP with CsI and RAE
Field of view
400" (10", 60", 140")
a
Dumbbell like shapes.
included in the telemetry is even finer (2"). Thus we can pair the attitude data with the spectral data to come up with time series integrated data with a pointing accuracy of 2". In the section below, detailed descriptions of each optical component are shown along with the results of the flight model ground calibration. 4.1. Entrance mirror
Fig. 3. A photo of the inner part of the spectrometer. Photons from the slit hit the grating and are diffracted to the MCP detectors. All inner walls are finished with a black light absorbing coating.
the 203 mm diameter entrance mirror and collected on the slit plate. The focal length is 1600 mm. There are three types of shapes for the slits and three types of filters behind the slits. We can choose the appropriate shape and filter to meet the observational requirements. Following the slit, the photons are diffracted by a toroidal grating and are focused onto a two-dimensional photon counting detector Micro Channel Plate (MCP). In order to minimize the stray light due to the scattering, inner wall of the spectrometer is black coated. The reflectivity of the wall for the Lyman-alpha with its incident angle of 45○ is lower than 0.01%. In addition, the light trap (also black coated) for the −1st order of the Lyman-alpha and 0th order is installed in front of the grating. Fig. 3 shows the inner part of the spectrometer (without the light trap). The specifications are summarized in Table 1. The satellite has a three-axis-stabilized attitude control system and the nominal pointing accuracy is 1.5'. In order to make the accuracy better, EXCEED includes a field of view (FOV) guiding camera in front of the slit plate. The camera monitors parts of the target images that are focused on the slit plate (but miss passing through the actual slit) it provides feedback for precise instrument pointing and attitude control. In this way, a pointing and attitude accuracy of 5" or less can be achieved (Tsuchiya et al., 2010). Moreover, the pointing angle data that is
The entrance mirror is an off axis parabolic and an effective diameter of 203 mm, and a focal length of 1600 mm. Since the required spectral and spatial resolutions are very high, the glancing angle is set very small (5.4○). However, it is a well-known fact that the reflectivity for EUV light on a conventional mirror is lower for smaller incident angles. Therefore, in order to enhance the reflectivity, the surface of the mirror is finished with Chemical Vapor Deposited Silicon Carbide (CVD-SiC), and the surface roughness is kept to better than 0.3 nm Root Mean Square (RMS). It is well-known fact that the multilayered mirror shows high reflectivity for some EUV lights (Yoshikawa et al., 2005; Murakami et al., 2006, 2011). But we did not choose it because our goal is to achieve high reflectivity for entire spectral band. Figs. 4 and 5 show a photo of the entrance mirror and the measured reflectivity as a function of wavelength. The results obtained show a very good match with the calculated expectation. After launching, we are planning to bake the mirror using a thinfilm type heater. By doing so, we can clear off possible contaminations and keep the surface of the mirror contamination free. 4.2. Filter and slit EXCEED has nine types of slits. By sliding the slit plate (Fig. 6) along the long axis of the slits, we can choose the most appropriate one for each observation. Slits (1)–(3) in Fig. 6 have no filters, and normally we will use them. When light contamination from Geocoronal emission is too high (especially Lyman alpha) we will use the slits with filters. Slits (4)–(6) have indium filters and (7)–(9) have CaF2 filters. The transmittances of the filters are shown in Fig. 7. Slits (4)–(6) are used when observing shorter wavelengths than Lyman alpha (121.6 nm). Conversely, slits (7)–(9) are used for wavelength lines longer than Lyman alpha. There are three types of shapes for the slits. In Fig. 6, slits (1), (4), and (7) are the narrowest (10") for high spectral resolution
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observation. Slits (2), (5), and (8) have wider (60") width slits. Although the spectral resolution is low, these slits can contain entire planet images. Slits (3), (6), and (9) are called “dumbbell” slits because of their shapes. These slits are specialized for Jupiter observations. As Fig. 8 shows, the center of the slits can be located to pass photons from the North/South Polar regions of Jupiter and as such they detect the auroral emission. At the same time, the wide end openings (140") pass the total emission from the Io
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plasma torus. Therefore, we can monitor the simultaneous relationship between the auroral activity and the Io plasma torus emissions. See Tsuchiya et al. (2010) for more detail. 4.3. Grating After the slit and filter, the photons impinge on a laminar type grating with incidence angle of 10.4○. For the grating too, the surface is coated with CVD-SiC. Since the grating is toroidal (vertical curvature giving spatial resolution and horizontal giving spectral resolution), we can get both spectral and spatially resolved images. Fig. 9 is a photo of the grating. The effective diameter is 50 mm and the line density is 1800 mm−1. The lines are ruled by an ion edging procedure. The measured diffraction efficiencies are shown in Fig. 10. The measured values are a little bit lower than expected. This is perhaps due to increased surface roughness of the ruled area caused by the ion bombardment during the line ruling process. On the backside of the grating we have attached a thin-film type heater in order to bake off possible contamination incurred during ground calibration. It would be energized for several weeks after the launch and before starting science observations. The door of the vacuum chamber (see the next subsection) will be opened after finishing the bake-out in order to avoid the possible damage on the detector by the contamination driven off the grating. 4.4. Photon detector
Fig. 4. A photo of the entrance mirror. The small mirror is installed for the alignment calibration.
Fig. 5. The measured reflectivity of the entrance mirror as a function of wavelength. The incidence angle is set at 5.4○.
We chose to use MCPs with resistive anode encoders (RAE) for our photon-counting device. This type of detector has already been used in various former space missions such as UPI on KAGUYA spacecraft (Yoshikawa et al., 2008), and IMAP on international space
Fig. 7. Measured transmittance for the indium and CaF2 filters (FM).
Fig. 6. A photo of slit plate. There are 9 slits in all. (1)–(3) have no filter, (4)–(6) have an indium filter, and (7)–(9) have a CaF2 filter. (1), (4), and (7) are the narrowest with widths of 10". (2), (5), and (8) have 60" slits. (3), (6), and (9) have a rather special shaped “dumbbell” slits for observing Jupiter (see the details in the text). The slit lengths are 3.3 mm which corresponds to 400".
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Fig. 8. Schematic of the dumbbell slits observation setup. Aurora from the north or south pole passes through the narrow center slit whereas photons from the Io plasma torus pass through the wide ends.
Fig. 9. A photo of the grating. The surface is being illuminated with white light and here yellow light is being reflected to the camera. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
station (Yoshikawa et al., 2010). In order to increase the quantum detection efficiency (QDE), the surface of the top MCP is coated with CsI (cesium iodide) to form a photocathode with a depth of 300 nm except for the region where Lyman alpha (121.6 nm) would be incident. It is a well-known fact that CsI degrades on exposure to air (Yoshioka et al., 2012a). Therefore, the assembly must be kept under vacuum (lower than 10 Pa) during the entire time that it is on the ground. Unfortunately there are no materials available which are transparent to EUV. Therefore we must provide a vacuum chamber door, which can be opened after launch. Incoming photons interact with the CsI and initiate cascades of multiple electrons. The resulting electron cloud hits the surface of the RAE and the charge location can be measured by manipulating
Fig. 10. Theoretical and measured grating diffraction efficiency. The incident angle is set to 10.4○ and 1st order diffracted light is measured.
Fig. 11. A photo of the MCP detector in the SUS chamber. The door with the glass window will be opened after the launch. A part of the MCP surface is kept uncoated by CsI so as to prevent saturation from event overflow from intense Lyman alpha radiation.
the voltages measured at the four corners in order to produce the distances from each corner. Charge amplifiers are connected to each corner. A photo of the assembly is shown in Fig. 11. It is obvious that the more charge in the electron cloud, the better its location can be resolved. To increase the charge gain we stack five MCPs, and we apply inverse high voltages (HV) between the second and third MCPs to sharpen the pulse height ratio (Murakami et al., 2010). Fig. 12 shows the pulse height distributions of the Flight Model (FM) MCPs for several HV values, and the relationship between the HV value and the gain and pulse height ratio are shown in Fig. 13. Judging from this figure, we set our nominal HV value to be −3500 V, and to be possibly increased up
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Fig. 12. Pulse height distributions of the FM MCPs for EUV light at various HV values.
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Fig. 14. The QDEs of the FM MCP as a function of wavelength. For comparison the QDEs of Lab-MCP is also shown by the dotted line.
Fig. 13. A summary plot of the gain and PHR for the FM MCPs.
to −3700 V by the end of mission life to compensate for the degradation of the MCP channel gain. The absolute quantum detection efficiency of the MCP was measured by comparing it with a “calibrated” Lab-MCP. The LabMCP was calibrated with National Institute of Standard and Technology (NIST) certified photodiode using the same incident light source. The results for the MCP are shown in Fig. 14 as a function of wavelength together with the values of Lab-MCP (dotted line). As described in former study such as Tremsin and Siegmund (2000), and Yoshioka et al. (2012c), we see the remarkably high QDE of CsI-coated MCP which is kept under vacuum especially for longer wavelength (more than 100 times in wavelength longer than 140 nm). Then, the FM MCP shows efficiencies of 5–25% depending on the wavelength. Dark count noise in the MCP can be initiated by decay of radioactive elements which are included in the channel wall or the photocathode material. For the FM detector, those count rates and uniformity were evaluated at several HV values. As a result, a dark count rate at the nominal HV value (−3500 V) was found to be 0.43 cps/cm2. When we increased the HV value up to −3800 V, we saw that the dark count rate increased gradually. However, it was still less than 0.5 cps/cm2 (cps: counts per second). Fig. 15 shows the pulse height distribution of the dark signals and Table 2 summarizes the dark count rate. In addition to the MCP internal noise, we must also take into consideration contamination from Geocoronal emissions (airglow) and as well as due to secondary gamma rays caused by the high energy electrons traversing the satellite orbit. In order to reduce the effect of contamination from the former source, the orbital altitude should be made as high as possible. On the other hand, contamination from high-energy electrons also becomes more
Fig. 15. Dark image (at −3500 V) and pulse height distributions for various HV values.
serious at higher altitudes. Therefore, taking both contamination sources into account, we have chosen an orbital altitude to be from 950 km to 1150 km, and the detector itself is shielded by 8 mm of stainless steel. As a result of these choices, we estimate the noise counts from these contamination sources should be around 1.5 cps/cm2. The details are discussed in Yoshioka et al. (2010, 2012b). 4.5. Overall sensitivity The overall sensitivity of EXCEED can be calculated by multiplying the reflectance of the entrance mirror, transmittance of the filter (if used), diffraction efficiency of the grating, and the QDE of the MCP. Fig. 16 shows the result. The uncertainties are derived from the statistical errors (1−s) of measurements. In addition, we
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Table 2 The dark count rate of MCP. HV value Dark count rate
−3500 V (nominal) 0.43 cps/cm2
−3600 V 0.47 cps/cm2
−3700 V 0.47 cps/cm2
−3800 V 0.50 cps/cm2
Fig. 16. Overall sensitivity of EXCEED for 3 types of filters as a function of wavelength. The estimated counts per unit brightness accumulated over a half orbital period can also be seen on the right vertical axis.
should take the uncertainty of the NIST photodiode (around 10%) which was used to calibrate the QDE of MCP into account. The total sensitivity for some part of the wavelength will be also evaluated after the launch by observing the stars whose EUV flux is already known. The estimated counts for a unit brightness source after accumulating 50 min of a half orbit observation are also shown in the figure.
5. Design and performance of EXCEED (resolution) 5.1. Performance of the entrance mirror As shown in Section 4.1, the entrance mirror is a parabolic shape with an off-axis angle of 5.4○ and a focal length of 1600 mm. We can measure the resolution of the entrance mirror by seeing what happens to collimated visible light incident parallel to the light axis of the mirror. This test can also evaluate the preciseness of the geometrical setting of entrance mirror relative to the slit plate. The collimator is made from pinholes (2.5" and 10" in diameter), a diagonal mirror, and a parabolic concave mirror (203 mm in diameter). The collimated light is aligned to the alignment mirror that is installed near the entrance mirror (see Fig. 4). It is set parallel to the entrance mirror light axis. Fig. 17 shows an image of incident pinholes focused on the slit plate as taken by the FOV guiding camera. Since the slit plate is polished to reflect visible light, we can get very clear images on it. It has to be noted that the figure shows not the best focused image because the test was made in the ambient pressure. The optical path in the vacuum will be 0.3 mm shorter and the image will be sharper. Then, from the figure, we can conclude that the entrance mirror can focus the light from a point source onto the slit with a resolution of better than 5" which is well within the narrowest slit width (10"). 5.2. Spectral imaging The optical alignment inside the spectrometer (Fig. 3) was also evaluated. Light from the entrance mirror is incident on the slit. This time we used an EUV lamp as our light source. It uses high frequency excitation to generate emissions from gasses such as
Fig. 17. Spot diagram of the entrance slit plate taken by the FOV camera. Six focused images from three smaller pinholes (D ¼2.5") and three bigger pinholes (D ¼ 10") are captured.
argon, neon, oxygen, and helium. We made the evaluations under vacuum conditions. By changing the slit plate position, the spectral resolution for the 10", 60", and dumbbell slits, were all evaluated. In addition, using the test pinhole on the slit plate (Fig. 6), the spatial resolution of the spectrometer could also be evaluated. The wavelength to pixel conversion formula was also deduced by measuring the famous landmark emission lines such as He 58.4 nm, Ne 736 nm, Ar 104.8 nm, O 130.4 nm, and so on. Fig. 18 shows the spectral images taken through the 3 slits and test pinhole without filters. Fig. 19 shows the spectrum of the pinhole with the line names, and the relation between the wavelength and pixel is shown in Fig. 20. By data fitting, we can deduce our fourth order pixel (x) to wavelength (λ) conversion formula as λ ½nm ¼ 1:5167 102 −8:4117 10−2 x−8:1793 10−5 x2 þ1:1077 10−7 x3 −4:8267 10−11 x4 7 0:3 The difference between the known wavelengths and the calculated ones from the formula are shown in the bottom panel of Fig. 20. They are smaller than 0.3 nm for entire spectral band.
5.2.1. Spatial resolution The spatial resolution of EXCEED can be measured by using the spectra taken through the pinhole whose diameter is 0.04 mm. For EXCEED, 0.04 mm on the slit corresponds to 5" in the sky. By convoluting the FWHM of the entrance mirror with that of the spectrometer, we can estimate the spatial resolution for a point source target. The results (Fig. 21) show that EXCEED has resolution around 10" for almost all wavelengths at the center of field of view. In this measurement, unit pixel on the detector along the spatial axis corresponds to 3".
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Fig. 18. Spectra taken through the 10" slit, 60" slit, dumbbell slit, and pinhole.
Fig. 19. Spectra taken by the EXCEED spectroscope through the pinhole slit.
Fig. 21. Spatial resolution as a function of wavelength.
Fig. 22. Spectral resolutions for 10" and 60" slits as a function of wavelength.
6. Possible observation scenarios
Fig. 20. Pixel to wavelength relation. The lower panel shows the delta from the approximation.
This section describes the possible observation scenarios for the planetary atmosphere or exosphere and Jovian magnetosphere by EXCEED. The results of the final calibrations that are shown in the former sections have been taken into account. 6.1. Detection of the planetary atmosphere
5.2.2. Spectral resolution The spectral resolution of EXCEED can also be measured by using the spectra taken through the 10" slit and the 60" slit. In both cases, the incident light is wider than the slit width. The FWHM of the lines for the spectral axis are measured. The results (Fig. 22) show that the spectral resolution for the 10" slit is 0.3– 0.5 nm FWHM which depends on the wavelength. Even for the 60" slit, the resolution is better than 1 nm FWHM over the entire spectral band.
As described above, one of the two main scientific goals is to estimate the amount of atmosphere around the various planets. In order to evaluate the detectability by EXCEED observations we assume some hypothetical column densities and brightnesses (in Rayleigh) as shown in Table 3. For Mercury, those values are set according to the model predictions by Leblanc et al. (2007) and Yoshioka et al. (2012c). The atom to photon conversion factor, the so called g-factor, is highly dependent on the solar activity and the
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distance from the Sun and planet (Killen et al., 2009). Although the observation window of EXCEED will be in the solar active phase, we calculate the g-factor using the solar flux at its minimum activity phase to be conservative in our estimation. The g-factors are calculated in the same manner for Venus and Mars (ionosphere). The column densities for those planets are taken from Fox and Sung (2001).
Table 3 Estimated brightness for planetary atmospheres. Wavelength (nm)
Column densitya (cm−2)
g-Factor (s−1)
Mercury Ne S+ S
63.0 90.9 142.5
6.0E+11 1.6E+11 4.1E+11
2.1E−6 1.4E−5 1.1E−5
0.05 0.05 0.11
Venus
O+ N+ C+
83.4 108.5 133.5
2.0E+13 1.0E+12 2.0E+12
2.8E−6 7.0E−6 1.9E−4
4.5 0.55 30
Mars
O+ N+ C+
83.4 108.5 133.5
4.6E+10 1.3E+09 1.3E+09
6.4E−7 1.6E−6 4.2E−5
Planet
Atom/ ion
Brightnessb (R)
0.027 0.002 0.51
a
Limb column density along the line of sight of EXCEED. The relative brightening area depends on the respective scale heights and planet's radius. b
Fig. 23. Lower detection limits of an EXCEED observation as a function of integration time.
We can consider the possibility of detecting those atmosphere components by comparing them with the measured sensitivity of EXCEED. Fig. 23 shows the possible detection limit (signal to noise ratio ¼ 1) as a function of integration time. In this figure we have also take into consideration the MCP internal noise and contamination from high energy electrons and stray light (see Yoshioka et al. (2010) for details). By comparing the model brightness with Fig. 23, we can imagine that some of the atmospheric species such as sulfur at Mercury, carbonic ion at Venus will be able to be detected by relatively short-term observations. On the other hand, we cannot be so optimistic about Martian atmospheres. It may require more than a month to detect even the brightest emissions. 6.2. Spectral diagnosis for the Jovian plasma torus The other scientific goal of EXCEED is to elucidate the plasma dynamics around the Jovian inner magnetosphere as obtained by observation of the Io plasma torus. Io is located in an orbit of 5.9 Jovian radii and has many active volcanoes. Volcanic plumes from Io are composed of oxygen, sulfur, sulfur dioxide, and so on. They accompany Io in its orbit around Jupiter until they are ionized through electron impact or charge exchange. Ionic sulfur, oxygen and their compounds such as So2+ have been observed together with background electrons in the plasma torus (Bagemal, 1985). In addition to those basic components, several in-situ observations have shown that a few percent of the electrons are energized upto an energy 100 times greater than the background electrons (Sittler and Strobel, 1987; Frank and Paterson, 1999, 2000). These hot electrons have a significant impact on the energy balance in the Jovian inner magnetosphere. However, their generation process has not yet been elucidated. One difficulty is that the available data all comes from in-situ observations, which cannot explore the temporal and spatial distributions explicitly. Therefore remote sensing, which could take a direct picture of the plasma dynamics is necessary. The EUV spectra from the Io torus can tell us the physical parameters of the plasma. The intensities of electron impact emissions from torus ions depend not only on the ion density but also on the ambient electron temperature and density (Shemansky, 1980; Steffl et al., 2004b). Moreover, the dependency of the emission intensity for each line is not uniform between the lines. Therefore, by comparing the brightness of each line, we can deduce relative composition of ions, ambient electron temperature, and its density as integrated along the line of sight. This method has been called Spectral Diagnosis (Osterbrock, 1989). By assuming the plasma parameters used in a previous study (Yoshioka et al., 2011), we can calculate the model spectrum that EXCEED might observe (Fig. 24). For comparison, a model
Fig. 24. Model spectra from the Io plasma torus as would be taken by EXCEED through its 10" slit. The model plasma parameters were taken from Yoshioka et al. (2011). The spectrum without a hot component is also shown on the same graph (offset by 150 counts). The lines in the gray hatched area are seen to be very sensitive to the hot component.
K. Yoshioka et al. / Planetary and Space Science 85 (2013) 250–260
spectrum for another temperature distribution (without a hot electron component) is also shown. It is clear that EXCEED will make possible taking useful spectra which can decisively deduce the plasma parameters at the Io torus as well as the system dynamics.
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Contents lists available at ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
The extreme ultraviolet spectroscope for planetary science, EXCEED K. Yoshioka a,n, G. Murakami a, A. Yamazaki a, F. Tsuchiya b, M. Kagitani b, T. Sakanoi b, T. Kimura a, K. Uemizu c, K. Uji d, I. Yoshikawa d a
Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan Tohoku University, Japan c National Astronomical Observatory of Japan, Japan d The University of Tokyo, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2013 Received in revised form 10 June 2013 Accepted 20 June 2013 Available online 29 June 2013
The extreme ultraviolet spectroscope EXtrem ultraviolet spetrosCope for ExosphEric Dynamics (EXCEED) on board the SPRINT-A mission will be launched in the summer of 2013 by the new Japanese solid propulsion rocket Epsilon as its first attempt, and it will orbit around the Earth with an orbital altitude of around 1000 km. EXCEED is dedicated to and optimized for observing the terrestrial planets Mercury, Venus and Mars, as well as Jupiter for several years. The instrument consists of an off axis parabolic entrance mirror, switchable slits with multiple filters and shapes, a toroidal grating, and a photon counting detector, together with a field of view guiding camera. The design goal is to achieve a large effective area but with high spatial and spectral resolution. In this paper, the performance of each optical component will be described as determined from the results of test evaluation of flight models. In addition, the results of the optical calibration of the overall instrument are also shown. As a result, the spectral resolution of EXCEED is found to be 0.3–0.5 nm Full Width at Half Maximum (FWHM) over the entire spectral band (52–148 nm) and the spatial resolution achieve was 10". The evaluated effective area is around 3 cm2. Based on these specifications, the possibility of EXCEED detecting atmospheric ions or atoms around Mercury, Venus, and Mars will be discussed. In addition, we estimate the spectra that might be detected from the Io plasma torus around Jupiter for various hypothetical plasma parameters. & 2013 Elsevier Ltd. All rights reserved.
Keywords: EUV observation EXCEED Io plasma torus Planetary science Spectroscope
1. Introduction The extreme ultraviolet (EUV) is an important spectral band for planetary science as it includes many interesting emissions from plasmas and atmospheres. However, it is not easy to observe there because to begin with the observing site must be outside of the Earth's atmosphere in order to avoid absorption. And because the emissions from many of the targets are very faint, the effective area of the detector should be as large as possible and long observation times are required. Beginning more than 30 years ago, a number of satellites and orbiters have made planetary observations in the EUV. The first attempt to observe planetary EUV emission was made by the spectrometer on board Mariner-10 (Broadfoot et al., 1977). The instrument took the spectra from the atmosphere of Mercury and Venus. Several years later, the EUV imager on board the Planet-B spacecraft took the first images of a part of the Earth's plasmasphere capturing the EUV emissions from the helium ions (30.4 nm) there (Nakamura et al., 2000; Yoshikawa et al., 2000).
n
Corresponding author. Tel.: +81 50 3362 7770; fax: +81 42 759 8218. E-mail address: [email protected] (K. Yoshioka).
0032-0633/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2013.06.021
The Ultraviolet Imaging Spectrograph on board the Cassini spacecraft made both imaging and spectroscopic observations of Jupiter and Saturn (Esposito et al., 2004). The Erath orbiting EUVE satellite also made EUV observation of the Mars and Jupiter (Krasnopolsky et al., 1994; Herbert and Hall, 1998). In addition, the KAGUYA spacecraft observed the Earth's plasmasphere from the orbit of moon using EUV emissions (Yoshikawa et al., 2008; Murakami et al., 2013). Although these observations have delivered much valuable scientific information, it has limitations due to limited observation windows, narrow fields of view and spectral bands, and low spectral resolution; all of which create serious constraints on what we can say about of the respective planetary plasmas with respect to long period variations or phenomena with wide dynamic range. EXtrem ultraviolet spectrosCope for ExosphEric Dynamics (EXCEED) is an EUV spectroscope on board the Japanese small scientific satellite, “SPRINT-A” which is dedicated to planetary science. It will be launched in the summer in 2013 and carry out spectroscopic and imaging observation of EUV emissions from the plasmas and atmospheres around the solar system planets. The spectral range of EXCEED is wide enough to include the majority of planetary EUV emissions (52–148 nm) and its spectral resolution is high enough to separate those lines (Full Width at Half Maximum,
K. Yoshioka et al. / Planetary and Space Science 85 (2013) 250–260
FWHM: 0.3 nm). Since EXCEED is developed mainly for planetary science, its observation window can be dedicated to the solar system planets limited only by the allowable geometrical angles of the observing Sun–Spacecraft–target system. In this paper, the specifications and performance of EXCEED as measured during the flight model calibration is summarized, and possible observation scenarios will also be discussed.
2. Scientific targets EXCEED has two main scientific goals. One is to estimate the atmosphere escaping rate from the terrestrial planets. Although, heretofore there have been several in-situ observations by orbiters, the knowledge obtained from them has been limited (e.g., Shizgal and Arkos (1996)). VENUS EXPRESS and MARS EXPRESS measured physical parameters such as velocity and temperature along the orbits. They allowed good estimations of these flows based on their in-situ measurements (Lundin et al., 2008; Lammer et al., 2006). These orbiters measured local physical parameters such as velocity and temperature, but global aspects such as the total amount of outward-flow plasma are not straightforward to determine with a few exception such as Rosetta–Alice observation for Mars (Feldman et al., 2011). EXCEED will be able to deduce the density distribution of the planetary plasmas and determine the quantity of the escaping atmosphere. Its field of view is wide enough that we can get an image from the interaction region between solar wind and planetary plasmas all the way through to the tail at any one time. This will enhance our knowledge of the characteristics of outward-flowing plasmas, e.g., their composition, rate of flow, and dependence on solar activity. For Mercury, the first observation of its exosphere was made by the Mariner-10 spacecraft (Broadfoot et al., 1976). They found three atomic species there (H, He, and O). After later observations by ground-based telescopes, atomic sodium, potassium, and calcium have also been detected (Potter and Morgan, 1985, 1986; Bida et al., 2000). Next, the ultraviolet spectrometer on board NASA's MESSENGER detected atomic magnesium (McClintock et al., 2009). In addition, a distribution of magnesium, calcium, and sodium around the tail region of Mercury has been found (Vervack et al., 2010). The sodium imager with Fabry–Perot interferometer MSASI (Yoshikawa et al., 2010) and UV spectrometer PHEBUS (Chassefiere et al., 2010) on board the European Space Agency (ESA) and Japan Aerospace Exploration Agency's (JAXA) BepiColombo mission which will be launched in 2016 will improve our knowledge of the spatial variations of atmosphere around Mercury. Through EUV emissions, EXCEED will be able to map possible density distributions of other atomic species such as neon or sulfur which have bright emissions in the EUV. Such information will help us to understand the evolution of Mercury and the generation process of its exosphere. The other study objective for EXCEED is the plasma dynamics around the Jovian inner magnetosphere. This can be monitored by using the EUV emission surrounding the Io plasma torus which is located in the Jovian inner magnetosphere. The neutral volcanic ejecta from Io, one of the Galilean satellites is the main source of plasma for the torus, and drives the shape and dynamics of the rapidly rotating Jovian inner magnetosphere. Major ions such as sulfur and oxygen have many allowed transition lines in the EUV, and their radiation easily escapes to become observable from outside the plasma region (Delamere and Bagenal, 2003; Steffl et al., 2004a). EXCEED will carry out both spectroscopic measurements as well as making imaging observations, together they will enable us to deduce the radial distribution of the ion densities and surrounding electron temperatures through the spectral diagnosis (see Section 6.2).
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In addition, EXCEED will also observe the Jovian aurora. The radiation from the aurora comes from excitation by the precipitating electrons along the magnetic field line in the foot print of the middle (∼20 Jovian radii from the planet center) magnetosphere (Gerard et al., 1994; Bhattacharya and Thorne, 2001). Furthermore, it is thought that the electrons around the equatorial plane move toward inside through the process of injection (Mauk et al., 2002) or interchange instability (Russel et al., 2005). Therefore, since EXCEED can observe both the torus and aurora at same time, we will be able to discuss about the plasma transport process around the Jovian magnetosphere by comparing the spectra of the torus (inner magnetosphere) to that of the aurora (corresponding to the middle magnetosphere).
3. Launch date and orbit The EXCEED satellite will be launched in the summer of 2013 by the new Japanese solid propulsion rocket Epsilon (Morita, 2012). Epsilon has been developed to provide a more reliable and responsive space launch system with a lower life cycle cost for various small satellites. The rocket has 3 stages and is designed to provide flexible capabilities for various orbits and weights. The SPRINT-A is the first satellite that will be launched by Epsilon. The nominal mission life is 1-year but it could be extended to several years. As shown in the previous section, EXCEED will address various fundamental scientific questions pertaining to planetary plasma science through its capability for observations covering wide dynamic ranges and long time periods. Fig. 1 shows the Sun– spacecraft–planet angles from the beginning of 2013 to the end of 2015. Thanks to its entrance baffle, EXCEED can observe planets as long as the separation angle exceeds 20○. After the launch and inflight calibration, we will start to observe Venus for several weeks. Then the target will shift to Jupiter. After 2 months of observing Jupiter, Mars and Mercury will be selected. The orbital altitude has a 950 km perigee and 1150 km apogee. These come from optimizing the tradeoffs between contaminating emissions from the Geocoronal emission and Van Allen radiation belt (Yoshioka et al., 2010). The orbital period is 105 min and the inclination is 31○.
4. Design and performance of the EXCEED (efficiency) EXCEED consists of an entrance mirror, a slit plate with changeable filters, a grating, and a photon counting device together with an FOV guiding camera (Fig. 2). The incoming photons from the targets (planetary emissions) are incident onto
Fig. 1. The Sun–spacecraft–planet angle as a function of time. The angle must be larger than 20○ before targets can be observed (bold lines).
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Table 1 Major design parameters for EXCEED.
Fig. 2. The optical layout of EXCEED. The photons are incident on the entrance mirror through the baffle and collected onto the slit with its reflection angle of 5.4○. The FOV guiding camera monitors the reflected light from the slit plate. Light that passes through the slit and filter is diffracted by the grating. Finally an MCP detector converts the photons into electron events.
Wavelength range
52–148 nm
Entrance mirror
Shape: off-axis (5.4○) parabolic Effective diameter: 203 mm Reflecting surface: CVD-SiC Focal length: 1600 mm
Grating
Shape: toroidal (RH ¼ 400 mm, RV ¼393 mm) Effective diameter: 50 mm Surface: CVD-SiC Lines: laminar, 1800 mm−1
Slit widths
0.08 mm (10") 0.50 mm (60") 1.10 mm (140")a
Filters
Blank Indium thickness¼100 nm CaF2 thickness¼3 mm
Detector
MCP with CsI and RAE
Field of view
400" (10", 60", 140")
a
Dumbbell like shapes.
included in the telemetry is even finer (2"). Thus we can pair the attitude data with the spectral data to come up with time series integrated data with a pointing accuracy of 2". In the section below, detailed descriptions of each optical component are shown along with the results of the flight model ground calibration. 4.1. Entrance mirror
Fig. 3. A photo of the inner part of the spectrometer. Photons from the slit hit the grating and are diffracted to the MCP detectors. All inner walls are finished with a black light absorbing coating.
the 203 mm diameter entrance mirror and collected on the slit plate. The focal length is 1600 mm. There are three types of shapes for the slits and three types of filters behind the slits. We can choose the appropriate shape and filter to meet the observational requirements. Following the slit, the photons are diffracted by a toroidal grating and are focused onto a two-dimensional photon counting detector Micro Channel Plate (MCP). In order to minimize the stray light due to the scattering, inner wall of the spectrometer is black coated. The reflectivity of the wall for the Lyman-alpha with its incident angle of 45○ is lower than 0.01%. In addition, the light trap (also black coated) for the −1st order of the Lyman-alpha and 0th order is installed in front of the grating. Fig. 3 shows the inner part of the spectrometer (without the light trap). The specifications are summarized in Table 1. The satellite has a three-axis-stabilized attitude control system and the nominal pointing accuracy is 1.5'. In order to make the accuracy better, EXCEED includes a field of view (FOV) guiding camera in front of the slit plate. The camera monitors parts of the target images that are focused on the slit plate (but miss passing through the actual slit) it provides feedback for precise instrument pointing and attitude control. In this way, a pointing and attitude accuracy of 5" or less can be achieved (Tsuchiya et al., 2010). Moreover, the pointing angle data that is
The entrance mirror is an off axis parabolic and an effective diameter of 203 mm, and a focal length of 1600 mm. Since the required spectral and spatial resolutions are very high, the glancing angle is set very small (5.4○). However, it is a well-known fact that the reflectivity for EUV light on a conventional mirror is lower for smaller incident angles. Therefore, in order to enhance the reflectivity, the surface of the mirror is finished with Chemical Vapor Deposited Silicon Carbide (CVD-SiC), and the surface roughness is kept to better than 0.3 nm Root Mean Square (RMS). It is well-known fact that the multilayered mirror shows high reflectivity for some EUV lights (Yoshikawa et al., 2005; Murakami et al., 2006, 2011). But we did not choose it because our goal is to achieve high reflectivity for entire spectral band. Figs. 4 and 5 show a photo of the entrance mirror and the measured reflectivity as a function of wavelength. The results obtained show a very good match with the calculated expectation. After launching, we are planning to bake the mirror using a thinfilm type heater. By doing so, we can clear off possible contaminations and keep the surface of the mirror contamination free. 4.2. Filter and slit EXCEED has nine types of slits. By sliding the slit plate (Fig. 6) along the long axis of the slits, we can choose the most appropriate one for each observation. Slits (1)–(3) in Fig. 6 have no filters, and normally we will use them. When light contamination from Geocoronal emission is too high (especially Lyman alpha) we will use the slits with filters. Slits (4)–(6) have indium filters and (7)–(9) have CaF2 filters. The transmittances of the filters are shown in Fig. 7. Slits (4)–(6) are used when observing shorter wavelengths than Lyman alpha (121.6 nm). Conversely, slits (7)–(9) are used for wavelength lines longer than Lyman alpha. There are three types of shapes for the slits. In Fig. 6, slits (1), (4), and (7) are the narrowest (10") for high spectral resolution
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observation. Slits (2), (5), and (8) have wider (60") width slits. Although the spectral resolution is low, these slits can contain entire planet images. Slits (3), (6), and (9) are called “dumbbell” slits because of their shapes. These slits are specialized for Jupiter observations. As Fig. 8 shows, the center of the slits can be located to pass photons from the North/South Polar regions of Jupiter and as such they detect the auroral emission. At the same time, the wide end openings (140") pass the total emission from the Io
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plasma torus. Therefore, we can monitor the simultaneous relationship between the auroral activity and the Io plasma torus emissions. See Tsuchiya et al. (2010) for more detail. 4.3. Grating After the slit and filter, the photons impinge on a laminar type grating with incidence angle of 10.4○. For the grating too, the surface is coated with CVD-SiC. Since the grating is toroidal (vertical curvature giving spatial resolution and horizontal giving spectral resolution), we can get both spectral and spatially resolved images. Fig. 9 is a photo of the grating. The effective diameter is 50 mm and the line density is 1800 mm−1. The lines are ruled by an ion edging procedure. The measured diffraction efficiencies are shown in Fig. 10. The measured values are a little bit lower than expected. This is perhaps due to increased surface roughness of the ruled area caused by the ion bombardment during the line ruling process. On the backside of the grating we have attached a thin-film type heater in order to bake off possible contamination incurred during ground calibration. It would be energized for several weeks after the launch and before starting science observations. The door of the vacuum chamber (see the next subsection) will be opened after finishing the bake-out in order to avoid the possible damage on the detector by the contamination driven off the grating. 4.4. Photon detector
Fig. 4. A photo of the entrance mirror. The small mirror is installed for the alignment calibration.
Fig. 5. The measured reflectivity of the entrance mirror as a function of wavelength. The incidence angle is set at 5.4○.
We chose to use MCPs with resistive anode encoders (RAE) for our photon-counting device. This type of detector has already been used in various former space missions such as UPI on KAGUYA spacecraft (Yoshikawa et al., 2008), and IMAP on international space
Fig. 7. Measured transmittance for the indium and CaF2 filters (FM).
Fig. 6. A photo of slit plate. There are 9 slits in all. (1)–(3) have no filter, (4)–(6) have an indium filter, and (7)–(9) have a CaF2 filter. (1), (4), and (7) are the narrowest with widths of 10". (2), (5), and (8) have 60" slits. (3), (6), and (9) have a rather special shaped “dumbbell” slits for observing Jupiter (see the details in the text). The slit lengths are 3.3 mm which corresponds to 400".
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Fig. 8. Schematic of the dumbbell slits observation setup. Aurora from the north or south pole passes through the narrow center slit whereas photons from the Io plasma torus pass through the wide ends.
Fig. 9. A photo of the grating. The surface is being illuminated with white light and here yellow light is being reflected to the camera. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
station (Yoshikawa et al., 2010). In order to increase the quantum detection efficiency (QDE), the surface of the top MCP is coated with CsI (cesium iodide) to form a photocathode with a depth of 300 nm except for the region where Lyman alpha (121.6 nm) would be incident. It is a well-known fact that CsI degrades on exposure to air (Yoshioka et al., 2012a). Therefore, the assembly must be kept under vacuum (lower than 10 Pa) during the entire time that it is on the ground. Unfortunately there are no materials available which are transparent to EUV. Therefore we must provide a vacuum chamber door, which can be opened after launch. Incoming photons interact with the CsI and initiate cascades of multiple electrons. The resulting electron cloud hits the surface of the RAE and the charge location can be measured by manipulating
Fig. 10. Theoretical and measured grating diffraction efficiency. The incident angle is set to 10.4○ and 1st order diffracted light is measured.
Fig. 11. A photo of the MCP detector in the SUS chamber. The door with the glass window will be opened after the launch. A part of the MCP surface is kept uncoated by CsI so as to prevent saturation from event overflow from intense Lyman alpha radiation.
the voltages measured at the four corners in order to produce the distances from each corner. Charge amplifiers are connected to each corner. A photo of the assembly is shown in Fig. 11. It is obvious that the more charge in the electron cloud, the better its location can be resolved. To increase the charge gain we stack five MCPs, and we apply inverse high voltages (HV) between the second and third MCPs to sharpen the pulse height ratio (Murakami et al., 2010). Fig. 12 shows the pulse height distributions of the Flight Model (FM) MCPs for several HV values, and the relationship between the HV value and the gain and pulse height ratio are shown in Fig. 13. Judging from this figure, we set our nominal HV value to be −3500 V, and to be possibly increased up
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Fig. 12. Pulse height distributions of the FM MCPs for EUV light at various HV values.
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Fig. 14. The QDEs of the FM MCP as a function of wavelength. For comparison the QDEs of Lab-MCP is also shown by the dotted line.
Fig. 13. A summary plot of the gain and PHR for the FM MCPs.
to −3700 V by the end of mission life to compensate for the degradation of the MCP channel gain. The absolute quantum detection efficiency of the MCP was measured by comparing it with a “calibrated” Lab-MCP. The LabMCP was calibrated with National Institute of Standard and Technology (NIST) certified photodiode using the same incident light source. The results for the MCP are shown in Fig. 14 as a function of wavelength together with the values of Lab-MCP (dotted line). As described in former study such as Tremsin and Siegmund (2000), and Yoshioka et al. (2012c), we see the remarkably high QDE of CsI-coated MCP which is kept under vacuum especially for longer wavelength (more than 100 times in wavelength longer than 140 nm). Then, the FM MCP shows efficiencies of 5–25% depending on the wavelength. Dark count noise in the MCP can be initiated by decay of radioactive elements which are included in the channel wall or the photocathode material. For the FM detector, those count rates and uniformity were evaluated at several HV values. As a result, a dark count rate at the nominal HV value (−3500 V) was found to be 0.43 cps/cm2. When we increased the HV value up to −3800 V, we saw that the dark count rate increased gradually. However, it was still less than 0.5 cps/cm2 (cps: counts per second). Fig. 15 shows the pulse height distribution of the dark signals and Table 2 summarizes the dark count rate. In addition to the MCP internal noise, we must also take into consideration contamination from Geocoronal emissions (airglow) and as well as due to secondary gamma rays caused by the high energy electrons traversing the satellite orbit. In order to reduce the effect of contamination from the former source, the orbital altitude should be made as high as possible. On the other hand, contamination from high-energy electrons also becomes more
Fig. 15. Dark image (at −3500 V) and pulse height distributions for various HV values.
serious at higher altitudes. Therefore, taking both contamination sources into account, we have chosen an orbital altitude to be from 950 km to 1150 km, and the detector itself is shielded by 8 mm of stainless steel. As a result of these choices, we estimate the noise counts from these contamination sources should be around 1.5 cps/cm2. The details are discussed in Yoshioka et al. (2010, 2012b). 4.5. Overall sensitivity The overall sensitivity of EXCEED can be calculated by multiplying the reflectance of the entrance mirror, transmittance of the filter (if used), diffraction efficiency of the grating, and the QDE of the MCP. Fig. 16 shows the result. The uncertainties are derived from the statistical errors (1−s) of measurements. In addition, we
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Table 2 The dark count rate of MCP. HV value Dark count rate
−3500 V (nominal) 0.43 cps/cm2
−3600 V 0.47 cps/cm2
−3700 V 0.47 cps/cm2
−3800 V 0.50 cps/cm2
Fig. 16. Overall sensitivity of EXCEED for 3 types of filters as a function of wavelength. The estimated counts per unit brightness accumulated over a half orbital period can also be seen on the right vertical axis.
should take the uncertainty of the NIST photodiode (around 10%) which was used to calibrate the QDE of MCP into account. The total sensitivity for some part of the wavelength will be also evaluated after the launch by observing the stars whose EUV flux is already known. The estimated counts for a unit brightness source after accumulating 50 min of a half orbit observation are also shown in the figure.
5. Design and performance of EXCEED (resolution) 5.1. Performance of the entrance mirror As shown in Section 4.1, the entrance mirror is a parabolic shape with an off-axis angle of 5.4○ and a focal length of 1600 mm. We can measure the resolution of the entrance mirror by seeing what happens to collimated visible light incident parallel to the light axis of the mirror. This test can also evaluate the preciseness of the geometrical setting of entrance mirror relative to the slit plate. The collimator is made from pinholes (2.5" and 10" in diameter), a diagonal mirror, and a parabolic concave mirror (203 mm in diameter). The collimated light is aligned to the alignment mirror that is installed near the entrance mirror (see Fig. 4). It is set parallel to the entrance mirror light axis. Fig. 17 shows an image of incident pinholes focused on the slit plate as taken by the FOV guiding camera. Since the slit plate is polished to reflect visible light, we can get very clear images on it. It has to be noted that the figure shows not the best focused image because the test was made in the ambient pressure. The optical path in the vacuum will be 0.3 mm shorter and the image will be sharper. Then, from the figure, we can conclude that the entrance mirror can focus the light from a point source onto the slit with a resolution of better than 5" which is well within the narrowest slit width (10"). 5.2. Spectral imaging The optical alignment inside the spectrometer (Fig. 3) was also evaluated. Light from the entrance mirror is incident on the slit. This time we used an EUV lamp as our light source. It uses high frequency excitation to generate emissions from gasses such as
Fig. 17. Spot diagram of the entrance slit plate taken by the FOV camera. Six focused images from three smaller pinholes (D ¼2.5") and three bigger pinholes (D ¼ 10") are captured.
argon, neon, oxygen, and helium. We made the evaluations under vacuum conditions. By changing the slit plate position, the spectral resolution for the 10", 60", and dumbbell slits, were all evaluated. In addition, using the test pinhole on the slit plate (Fig. 6), the spatial resolution of the spectrometer could also be evaluated. The wavelength to pixel conversion formula was also deduced by measuring the famous landmark emission lines such as He 58.4 nm, Ne 736 nm, Ar 104.8 nm, O 130.4 nm, and so on. Fig. 18 shows the spectral images taken through the 3 slits and test pinhole without filters. Fig. 19 shows the spectrum of the pinhole with the line names, and the relation between the wavelength and pixel is shown in Fig. 20. By data fitting, we can deduce our fourth order pixel (x) to wavelength (λ) conversion formula as λ ½nm ¼ 1:5167 102 −8:4117 10−2 x−8:1793 10−5 x2 þ1:1077 10−7 x3 −4:8267 10−11 x4 7 0:3 The difference between the known wavelengths and the calculated ones from the formula are shown in the bottom panel of Fig. 20. They are smaller than 0.3 nm for entire spectral band.
5.2.1. Spatial resolution The spatial resolution of EXCEED can be measured by using the spectra taken through the pinhole whose diameter is 0.04 mm. For EXCEED, 0.04 mm on the slit corresponds to 5" in the sky. By convoluting the FWHM of the entrance mirror with that of the spectrometer, we can estimate the spatial resolution for a point source target. The results (Fig. 21) show that EXCEED has resolution around 10" for almost all wavelengths at the center of field of view. In this measurement, unit pixel on the detector along the spatial axis corresponds to 3".
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Fig. 18. Spectra taken through the 10" slit, 60" slit, dumbbell slit, and pinhole.
Fig. 19. Spectra taken by the EXCEED spectroscope through the pinhole slit.
Fig. 21. Spatial resolution as a function of wavelength.
Fig. 22. Spectral resolutions for 10" and 60" slits as a function of wavelength.
6. Possible observation scenarios
Fig. 20. Pixel to wavelength relation. The lower panel shows the delta from the approximation.
This section describes the possible observation scenarios for the planetary atmosphere or exosphere and Jovian magnetosphere by EXCEED. The results of the final calibrations that are shown in the former sections have been taken into account. 6.1. Detection of the planetary atmosphere
5.2.2. Spectral resolution The spectral resolution of EXCEED can also be measured by using the spectra taken through the 10" slit and the 60" slit. In both cases, the incident light is wider than the slit width. The FWHM of the lines for the spectral axis are measured. The results (Fig. 22) show that the spectral resolution for the 10" slit is 0.3– 0.5 nm FWHM which depends on the wavelength. Even for the 60" slit, the resolution is better than 1 nm FWHM over the entire spectral band.
As described above, one of the two main scientific goals is to estimate the amount of atmosphere around the various planets. In order to evaluate the detectability by EXCEED observations we assume some hypothetical column densities and brightnesses (in Rayleigh) as shown in Table 3. For Mercury, those values are set according to the model predictions by Leblanc et al. (2007) and Yoshioka et al. (2012c). The atom to photon conversion factor, the so called g-factor, is highly dependent on the solar activity and the
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distance from the Sun and planet (Killen et al., 2009). Although the observation window of EXCEED will be in the solar active phase, we calculate the g-factor using the solar flux at its minimum activity phase to be conservative in our estimation. The g-factors are calculated in the same manner for Venus and Mars (ionosphere). The column densities for those planets are taken from Fox and Sung (2001).
Table 3 Estimated brightness for planetary atmospheres. Wavelength (nm)
Column densitya (cm−2)
g-Factor (s−1)
Mercury Ne S+ S
63.0 90.9 142.5
6.0E+11 1.6E+11 4.1E+11
2.1E−6 1.4E−5 1.1E−5
0.05 0.05 0.11
Venus
O+ N+ C+
83.4 108.5 133.5
2.0E+13 1.0E+12 2.0E+12
2.8E−6 7.0E−6 1.9E−4
4.5 0.55 30
Mars
O+ N+ C+
83.4 108.5 133.5
4.6E+10 1.3E+09 1.3E+09
6.4E−7 1.6E−6 4.2E−5
Planet
Atom/ ion
Brightnessb (R)
0.027 0.002 0.51
a
Limb column density along the line of sight of EXCEED. The relative brightening area depends on the respective scale heights and planet's radius. b
Fig. 23. Lower detection limits of an EXCEED observation as a function of integration time.
We can consider the possibility of detecting those atmosphere components by comparing them with the measured sensitivity of EXCEED. Fig. 23 shows the possible detection limit (signal to noise ratio ¼ 1) as a function of integration time. In this figure we have also take into consideration the MCP internal noise and contamination from high energy electrons and stray light (see Yoshioka et al. (2010) for details). By comparing the model brightness with Fig. 23, we can imagine that some of the atmospheric species such as sulfur at Mercury, carbonic ion at Venus will be able to be detected by relatively short-term observations. On the other hand, we cannot be so optimistic about Martian atmospheres. It may require more than a month to detect even the brightest emissions. 6.2. Spectral diagnosis for the Jovian plasma torus The other scientific goal of EXCEED is to elucidate the plasma dynamics around the Jovian inner magnetosphere as obtained by observation of the Io plasma torus. Io is located in an orbit of 5.9 Jovian radii and has many active volcanoes. Volcanic plumes from Io are composed of oxygen, sulfur, sulfur dioxide, and so on. They accompany Io in its orbit around Jupiter until they are ionized through electron impact or charge exchange. Ionic sulfur, oxygen and their compounds such as So2+ have been observed together with background electrons in the plasma torus (Bagemal, 1985). In addition to those basic components, several in-situ observations have shown that a few percent of the electrons are energized upto an energy 100 times greater than the background electrons (Sittler and Strobel, 1987; Frank and Paterson, 1999, 2000). These hot electrons have a significant impact on the energy balance in the Jovian inner magnetosphere. However, their generation process has not yet been elucidated. One difficulty is that the available data all comes from in-situ observations, which cannot explore the temporal and spatial distributions explicitly. Therefore remote sensing, which could take a direct picture of the plasma dynamics is necessary. The EUV spectra from the Io torus can tell us the physical parameters of the plasma. The intensities of electron impact emissions from torus ions depend not only on the ion density but also on the ambient electron temperature and density (Shemansky, 1980; Steffl et al., 2004b). Moreover, the dependency of the emission intensity for each line is not uniform between the lines. Therefore, by comparing the brightness of each line, we can deduce relative composition of ions, ambient electron temperature, and its density as integrated along the line of sight. This method has been called Spectral Diagnosis (Osterbrock, 1989). By assuming the plasma parameters used in a previous study (Yoshioka et al., 2011), we can calculate the model spectrum that EXCEED might observe (Fig. 24). For comparison, a model
Fig. 24. Model spectra from the Io plasma torus as would be taken by EXCEED through its 10" slit. The model plasma parameters were taken from Yoshioka et al. (2011). The spectrum without a hot component is also shown on the same graph (offset by 150 counts). The lines in the gray hatched area are seen to be very sensitive to the hot component.
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spectrum for another temperature distribution (without a hot electron component) is also shown. It is clear that EXCEED will make possible taking useful spectra which can decisively deduce the plasma parameters at the Io torus as well as the system dynamics.
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