Extreme Ultraviolet Radiation Measurement for Planetary Atmospheres/Magnetospheres from the Earth-Orbiting Spacecraft (Extreme Ultraviolet Spectroscope for Exospheric Dynamics: EXCEED)
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The Sprint-A satellite with the EUV spectrometer (Extreme Ultraviolet Spectroscope for Exospheric Dynamics: EXCEED) was launched in September 2013 by the Epsilon rocket. Now it is orbiting around the Earth (954.05 km×1156.87 km orbit; the period is 104 minutes) and one has started a broad and varied observation program. With an effective area of more than 1 cm2 and well-calibrated sensitivity in space, the EUV spectrometer will produce spectral images (520–1480 Å) of the atmospheres/magnetospheres of several planets (Mercury, Venus, Mars, Jupiter, and Saturn) from the Earth’s orbit. At the first day of the observation, EUV emissions from the Io plasma torus (mainly sulfur ions) and aurora (H2 Lyman and Werner bands) of Jupiter have been identified. Continuous 3-month measurement for Io’s plasma torus and aurora is planned to witness the sporadic and sudden brightening events occurring on one or both regions. For Venus, the Fourth Positive (A1 Π-X1 Σ +) system of CO and some yet known emissions of the atmosphere were identified even though the exposure was short (8-min). Long-term exposure from April to June (for approximately 2 months) will visualize the Venusian ionosphere and tail in the EUV spectral range. Saturn and Mars are the next targets.
KeywordsEUV radiation Plasma emissions Jupiter Mars Venus EXCEED/SPRINT-A
1 Introduction: Mission Background
Japanese intensive exploration of space began with the launch of the first Earth-orbiting spacecraft, OHSUMI. It had a mass of only 24 kg, and today this spacecraft might be classified as a micro-satellite. After the success, the Institute of Space and Astronautical Science (ISAS) has built and launched more than 25 satellites for science and/or engineering purposes. Traditionally, many space researchers have carried out sounding-rocket experiments to have an access-to-space and to provide “hands-on” training ground for long-term missions. Sounding-rocket experiments were planned quickly and executed at low costs, and they provided frequent opportunities for space researches. However, in the 1990s, a solid propulsion rocket (M-V) developed by ISAS had been used as one of the powerful vehicles but at higher cost. As a result, the number of space programs decreased.
On the other hand, a series of small scientific satellites, which ISAS has started to develop, based on the concept ‘cheaper and faster realization of unique space experiments’ as a complementary program of mainstream scientific satellites. ISAS has released the plan to launch some small satellites over 5 years (Fukuda et al. 2008). The mass of the satellites is approximately 350 kg. This approach allows one to significantly reduce the time required to obtain science output. Moreover, the shorter development period will reduce overall cost. ISAS has built the standard bus architecture, where the bus and the payload are clearly separated in a modular manner. The series of the spacecraft employing standard bus architecture is named the Small scientific satellite Platform for Rapid Investigation and Test (Sprint series).
Fifteen working groups to propose small scientific satellites in this category were constituted under the Steering Committees of Space Science and Space Engineering in ISAS. Our EXCEED mission (EXtreme ultraviolet speCtroscope for ExosphEric Dynamics) was one of the proposals and was selected as the first program in 2007. It was based on our technological advances in the extreme ultraviolet (EUV) spectral range during the past two decades (e.g. Yoshikawa et al. 1997, 2001, 2005, 2010b; Murakami et al. 2013; Yoshioka et al. 2012; Chassefière et al. 2010). We have enabled quantitative spectroscopy to provide detailed information on neutral and ionized gases of atmospheric and astrophysical interest. There are several fundamental reasons why technological advances which have opened the EUV spectral range to investigate are important: (1) strong resonance transitions of the most abundant atmospheric and exospheric elements, e.g., hydrogen, helium, and oxygen have emission lines in the spectral range shortward of 1500 Å (Nakamura et al. 1999; Yoshikawa et al. 2008; Yamazaki et al. 2002), (2) for solar planets, interactions among solar wind, the magnetospheres, the exospheres, ionospheres, and atmospheres give rise to diagnostic EUV emission on a global scale (Yoshioka et al. 2011, 2013; Yamazaki et al. 2003), (3) many of the primary astrophysical interaction processes occur in the energy equivalent temperature from 104 to 106 Kelvin, producing transitions in the EUV (Yamazaki et al. 1996).
Our ultimate goal in this mission is to characterize the planetary atmospheres/magnetospheres. Planetary magnetospheres are characterized by the strength of the intrinsic dipole magnetic moment, planetary rotation period, and plasma sources. Compared with the Earth, Venus and Mars have a weak magnetic moment in the core and their atmospheres are forced to directly interact with the solar wind. The interaction results in loss of plasmas in the upper atmospheres to interplanetary space. The loss of atmospheric gases to space is thought to make a significant contribution to the evolution of the planetary atmosphere. On the other hand, Jupiter and Saturn have a strong magnetic moment, internal plasma sources (Io and Enceladus tori, respectively), and fast planetary rotations. Consequently, plasma convection in these magnetospheres is dominated by azimuthal flow around the planet, which is in contrast to the terrestrial magnetosphere, where the plasma flow is mainly driven by the solar wind.
Our science instrument (EXCEED) is solely boarded on the SPRINT-A spacecraft built by ISAS/JAXA. EXCEED on SPRINT-A is the world’s first observatory in space observing planets, Venus, Mars, Jupiter, and Saturn. Spectroscopic imaging in the EUV spectral range, which cannot be performed on the ground, allows us to collect information on the planetary atmospheres and magnetospheres. Here we present the instrument specifications with in-flight calibration and the initial results of the observations.
2 Observation Schedule
After the launch of the spacecraft (the details are shown later), we had to wait until the outgassing from the spacecraft dried up. On October 12th, we opened the door of the detector chamber by activating the paraffin actuator. We applied the high voltage onto the detector unit and started to increase the level of high voltage gradually until 17th November. It reached the high-voltage level by 3.55 kV on November 17th.
The first lights of Jupiter and Venus were obtained in November 18th. We started the continuous observation of Jupiter until the beginning of March. During the first and second weeks of 2014, joint observation with Hobble Space Telescope (HST) was successfully made. EXCEED changed the target to Venus in April and has taken a long time for the exposure, except the campaign period for the observation of Jupiter with X-ray telescopes (Suzaku, Chandra, and XMM). In this joint observation, we will observe the Jupiter intermittently for a couple of weeks.
From June 2014, we will face the challenge to observe Saturn looking small, Mars looking less bright, and with Mercury close to the Sun. The observation target was primarily chosen due to the elongation angle.
2.1 First Target: Jupiter
The interaction of planetary magnetosphere with solar wind generates a polar arc (aurora) on Earth, Jupiter, and Saturn. The Jovian magnetosphere with the strongest magnetic field in the solar system (10,000 times stronger than that of the Earth) and with planetary rotation at a high speed of ten hours per cycle is known to be dominated by the plasma flow co-rotating around the planet. Co-rotating regions are common in the magnetospheres of magnetized bodies (Mercury, Earth, Jupiter, and Saturn). The terrestrial magnetosphere has a co-rotating region named the plasmasphere and it is strongly affected by the solar wind. On the other hand, the strength of Jovian co-rotation induced electric field is orders of magnitude higher than that generated by the solar wind, therefore plasmas in the inner magnetosphere inside a radial distance of 10–20 R J (R J : Jovian radius=71,492 km) are considered to co-rotate with the planetary rotation. As a result, it has been thought that transports of plasma and energy across the azimuthal direction are difficult in the Jovian magnetosphere, unlike the Earth.
Volcanic gases (SO2, SO) released from Io are dissociated and ionized mainly by electron impacts. More than 1000 kg of neutral gases from Io’s atmosphere are ionized per second and produce a dense (approximately 2000 cm−3) torus of electrons, sulfur, and oxygen ions, trapped in Jupiter’s strong magnetic field. While in situ measurements of the Io plasma torus by the Voyager and Galileo spacecraft (Frank and Paterson 2000; Bagemal 1985) and remote-sensing observations from the ground and from space-based UV telescope have characterized the density, temperature and composition of the plasma as well as the basic structure (Broadfoot et al. 1981; Steffl et al. 2004a, 2004b; Thomas et al. 2003), the temporal variability of the torus remains poorly determined.
2.2 Second Target: Venus and Mars
Due to the thick atmosphere and the intrinsic magnetic moment, our Earth sustains the environment in which a variety of creatures have lived. In contrast, although Venus and Mars are Earth-like planets, with temperatures and atmospheric pressures of −40 °C and 750 Pa on Mars and 400 °C and 9000 kPa on Venus, these two planets have environments that are thoroughly exotic compared with that of the Earth. Since Mars and Venus have weak intrinsic magnetic dipole moments, their atmospheres are considered to have escaped or been escaping to space due to the solar wind interaction.
One of the most important studies in planetary science is the characterization of the formation of terrestrial planets’ environments and their divergent evolutions. The existence of CO2 and N2 in the past atmosphere indicates a mild climate in the planet. If we measure how much atmosphere is lost to space constantly, it will allow us to estimate the total loss that may have occurred during billions of years, possibly making Mars red and dry as it is today. EXCEED will witness the solar wind interaction with planetary atmospheres by the charge exchange process in the dayside (solar wind ions charge-exchanges atmosphere of planets). The distribution of escaping ionospheric ions to space, forming an ionospheric tail, is imaged by solar resonantly scattering emissions in the EUV spectral range (O+, C+, and N+, the necessary exposure period is discussed in Sect. 3.3). No instrument has ever been flown to measure the cumulated loss of atmosphere. This measurement is essential to understand why Earth-like planets could not become more exactly like Earth.
Collaboration with other space missions is helpful to understand the total escaping flux and the currently operating mechanism for it. In the past, many satellites investigated the plasma environment of a planet without measurement cumulated amount/loss of atmosphere. The MAVEN spacecraft with a magneto-sensor, plasma analyzer, and UV spectrometer, intended to study escape of Mars’s atmosphere, was launched in 2013 and will arrive at Mars in late 2014. It will be just after EXCEED starts the observation of Mars. In situ and local remote-sensing measurements on MAVEN will bring information on particle energy, velocity, density and their variation versus solar wind condition. Simultaneous measurements of cumulated amount/loss of atmosphere by EXCEED and particle characteristics by in situ (local remote-sensing) instrument on orbiter can fill the gap each other.
2.3 Third Target: Mercury and Saturn
Mercury, like the Moon, has no stable atmosphere. The gaseous environment is described as an exosphere. The mean free paths of the constituents are large and the total column density is less than 1012 cm−3. The existence of seven elements, O, H, He, Na, K, Ca, Si, and Mg has been identified, the first three elements by Mariner-10, the next three by ground-based telescopes, the last two by Messenger observations (Potter and Morgan 1985; Kameda et al. 2007; Bida et al. 2000; Vervack et al. 2010). It is highly probable that other elements, such as C, N, S, OH, Al, and Fe are also present. The exosphere of Mercury will be further investigated by BepiColombo mission (Chassefière et al. 2010; Yoshikawa et al. 2010a). EXCEED will search for predicted species, C and N have emission lines in EUV, not previously detected, as well as new species.
Saturn is the second largest planet in our solar system; it is surrounded by numerous rings and satellites. The Cassini spacecraft made observations in the magnetosphere after the arrival at Saturn on 30 June 2004. Saturn’s magnetic dipole moment is 0.21 G Rs3 (1 Rs=60,268 km), and the magnetic field intensity on the surface is similar to Earth’s one. The magnetic field axis is almost aligned with the rotation axis. Saturn’s magnetosphere shows both internal (e.g., Hill et al. 2005; Burch et al. 2007) and solar wind controls (e.g., Kurth et al. 2005; Mitchell et al. 2005).
Saturn’s inner magnetosphere is dominated by water group neutrals (H2O, OH, and O) originating from Enceladus’ water plume (e.g., Shemansky et al. 1993; Esposito et al. 2005; Delamere et al. 2007; Melin et al. 2009; Fleshman et al. 2010; Cassidy and Johnson 2010; Tadokoro et al. 2012). Enceladus supplies ∼150–350 kg/s of neutral H2O gas to the magnetosphere (e.g., Hansen et al. 2006). The neutral cloud is an important loss process of plasmas in the inner magnetosphere (e.g., Paranicas et al. 2007, 2008). Cassini observations suggest that ions with several tens of keV are significantly lost by charge exchange (e.g., Krimigis et al. 2007; Paranicas et al. 2008). An understanding of the neutral cloud is required to understand plasma dynamics in the magnetosphere. The observations by the Hubble Space Telescope (HST) reveal numerous OH (Shemansky et al. 1993). The observations by the Cassini spacecraft clarify H2O plume with temporal variation (e.g., Porco et al. 2006; Spencer et al. 2006; Hansen et al. 2008) and O with temporal and spatial variations (e.g., Esposito et al. 2005; Melin et al. 2009). Although the O distributions observed by Cassini show asymmetries in the local time distribution, it is difficult to explain the asymmetric distribution, since the orbital period of Enceladus (∼1.4 days) is shorter than the period of observations (∼order of weeks). EXCEED will observe the emission of O I at 1304 Å in the inner magnetosphere within 10 Rs. It is expected to produce a mosaic per 1 day. EXCEED will produce the image at O I to clarify the local time structure with a temporal variation. The atomic oxygen observation by EXCEED is a key parameter for understanding the neutral and plasma dynamics in Saturn’s magnetosphere.
3 Instrument and Calibrations
3.1 EXCEED: Extreme Ultraviolet Spectroscope for Exospheric Dynamics
Calcium fluoride (CaF2) bandpass filters are placed on the slits for aversion to our mission full abort. CaF2 has a sharp cutoff of transmittance at the shortward of 1300 Å. If we had encountered with high irradiation of stray light (for example, Lyman-α line at 1216 Å), we must have used the slit with the CaF2 filter at the expense of sensitivity and shortward spectral measurements. Indium (In) has a relatively wide bandpass in 700–1100 Å. We may use the In-filter for the observation of the bright EUV stars to avoid the signal saturation at the detector electronics. Transmittances of the materials are shown in Fig. 5.
Behind the slit, a laminar-type grating with a toroidal curvature surface manufactured by Shimazu Co. is placed (the substrate was manufactured by Nikon Co.). It was optically fabricated by a holographic method. The line density is 1800 lines/mm. The calibrated efficiency at first order is presented in Fig. 5 together with a theoretical curve.
The spectrograph covering 520–1480 Å in first order with a dispersion of approximately 1 Å per pixel was built with the above optical system. The data processor has a temporal memory stores 1300 photon events (position and time). Position data consists of four outputs from the channels (A, B, C, D), each is digitized by 16-bit, each channel data is stored with 16-bit. We calculate the position of each photon event by X=(A+B)/(A+B+C+D), Y=(A+C)/(A+B+C+D) and produce the spectral image on the ground. This method has a significant benefit. We can obtain not only a spectral image (X–Y matrix) but also a pulse-height distribution (PHD) of signals, i.e., histogram of amplitudes of A+B+C+D. It realizes the detector’s health check even during the observation.
3.2 Field-Of-View Guidance Camera (FOVGC)
The SPRINT-A satellite has a three-axis-stabilized attitude control system, but the original absolute pointing accuracy was poor (±1.5 arc-minutes). Therefore, the Field-Of-View Guidance Camera (FOVGC) is boarded to improve the pointing accuracy. If the target (planet) image produced by the entrance mirror of EXCEED does not fall onto the slit, the image reflected at the outer slit frame is captured by FOVGC. It has an FOV by 240″×240″. A mission by data processor (MDP) identifies the centroid of the reflected image and calculates the gap and direction between the slit and target every 5 s. The gap and direction are informed the spacecraft attitude control system (Common Bus system in Sprint series). The satellite bus system leads the spacecraft attitude system so that the line-of-sight (LOS) of EXCEED should direct the target.
3.3 In-Orbit Calibration: Operational HV Level, Plate-Scale, Spatial/Spectral Resolutions, and Overall Sensitivity
As cited in Sect. 3.1, we can obtain the pulse-height distribution (PHD) of 5-stage MCP assembly from the raw data of EXCEED. The upper panel of Fig. 9 shows the PHDs with high voltage applied in space (red line) and on the ground-based calibrations (black lines). On the ground, the average gain (peak of the distribution) became higher, growing from 1 pC to 4.5 pC, while the applied level of high voltage increased from −3.4 kV to −3.7 kV. However, increment of high voltage in space might induce the discharge and potentially result in a mission failure. In order to avoid the worst case, we have increased the applied voltage step by step for approximately 3 days. As previously stated, we can obtain a pulse-height distribution and spectral image simultaneously. The lower panels of Fig. 9 show the PHDs available at 584 Å (labeled as (a)), 834 Å as (b), 1216 Å as (c), and 1304 Å as (d) for the first light of the geocorona. Their features of PHDs were similar; however, you can see the peak of the gain is lower, while the wavelength is longer (energy is lower). The wavelength dependence is by the nature of the detector due to the interaction between a pore of MCP and incident photons. We found that the minimum level of HV was −3.5 kV and that it realized fairly good spatial resolution of the detector at the longer wavelength. The slight difference in the peak of PHD between the ground and in-flight calibrations at −3.5 kV might be due to the degradation of MCP gain after the launch.
Using the opportunity of star observation, we have determined the spatial resolution (point-spread function) by 17 arc-seconds (FWHM), the plate scale by 4.2 arc-second/pixel along the spatial direction, and the FOV in spatial direction by 360 arc-seconds. The spectral resolution is determined by the point-spread function as 3 Å (FWHM). All the dimensions are shown in the lower panel of Fig. 6.
We have re-considered the necessary exposure periods for visualizing very dark targets (the escaping ions) based on the results from in-flight calibration (effective area versus wavelength, detector noise due to high-energy particle bombardment (2.0 cps/cm2)). The details are given in the Appendix. We should integrate the spectra for approximately 30 passes (<3 days) for identification of C II (1335 Å) at ionospheric tail and for approximately 120 passes (<12 days) for identification of N II (1085 Å). We have scheduled the observation period of Venus from March to May in 2014.
4 Launch and Orbit
The epsilon rocket was used to launch a three-axis stabilized satellite (Sprint-A) with EXCEED, making its first flight. The four-stage Epsilon rocket was launched from the Uchinoura Space Center at 05:00 UTC on 14 September 2013. Following its successful insertion into the orbit and deployment of its solar arrays, Sprint-A was renamed Hisaki. The perigee and apogee are 954.05 km and 1156.87 km, respectively, as planned. The orbital inclination is 29.704°. The orbital period is 104 minutes.
4.1 Foreground Emissions: Geocorona Measurement
Although contaminations from stray lights and high-energy particles were negligible, a narcissistic ghost line appeared near 800 Å (McCandliss et al. 1998). Bright emission at Lyman-α (1216 Å) was incident on the face of the detector (MCP), back onto the detector by the grating in zero order. We always have to always be careful about narcissistic ghosts for the identifications of weak emissions. The level of contamination is 1/1000. We will be able to re-examine this feature by the EUV star observations in the near future.
4.2 First Images of Jupiter and Venus
The EXCEED observations of Jupiter and Venus represent the highest quality spectra, in terms of spectral/spatial resolutions and instrument sensitivity. The present discussion will focus on the identification of spectral features and comparison with previous observations. Future works will concern the modeling of these spectra in terms of current atmospheric models based on in situ measurements (GALILEO, Venus Express, etc.).
Both figures show the existence of sulfur ion emissions, e.g. at 680 Å (S III), 765 Å (S II). (For reference, major transitions are indicated at the top.) This spectral image involves at least 11 individual spectral transitions (S II, S III, S IV and O II transitions). We can identify aurorally excited H2 Lyman and Werner bands extending toward shorter wavelength from Lyman-α. The S III line at 680 Å is by far the most energetic feature in the EUV spectrum of the torus. The general shape of the profile observed by EXCEED is fairly similar to the UVIS profile by the Cassini spacecraft (Steffl et al. 2004a, 2004b); however, our spectral image seems thinly. The weak emissions, e.g. S II (642 Å), need a longer exposure period to appear in the spectrum.
The spectral image includes the desired disk emissions at Venus as well as contributions from the terrestrial foreground emissions, even though the exposure was limited by 8-minute. Geocoronal emissions illuminated the whole of the slit as shown in the upper panel of Fig. 15, however, the brightness at O II (834 Å) and O I (1304 Å) around Venus surpassed the geocorona brightness alone. The lower panel of Fig. 15 is the spectrum at the Venus disk. (Cross-sectional view at the Venus disk position of the spectral image.) A large number of individual bands of the CO Fourth Positive (A1 Π-X1 Σ +) system and Hopfield–Birge bands (B1 Π-X1 Σ +) were seen, however, some were blended with another line. The 4P bands at (14,3)–(14,6) are due to the optical pumping up by solar Lyman-α line producing the CO 4P (14,v″) band system through resonance with the (14,0) band (Derrance 1981). Cascading 4P products at (10,0) and (12,2) were identified on the disk spectra in addition to the principal C II multiplets (1335 Å) and O I (989 Å), whose existences were previously reported by Feldman et al. (2000) and Hubert et al. (2012). The intensities of C II (1335 Å) and of O I (989 Å) were approximately 100 R, which are consistent with the past measurements.
The emission at 867 Å is probably ascribed to Ar I, that was as previously reported by Stern et al. (1996). However, they did not regard it as Ar I emission, because their measured intensity was approximately two orders of magnitude brighter than expected for Ar I emission. We have measured the intensity at 867 Å by less than 10 R, while it was reported as more than 100 R (Stern et al. 1996). Noble gases are highly useful tool for studies of the origin and evolution of planetary atmospheres. We will clarify the existence of Ar I emission (867 Å) and its intensity with higher S/N ratio after a longer exposure period is taken.
As shown in Sect. 2.2, the feature of escaping ions can be visualized after the accumulation of many spectra available for several days.
4.3 Data Downlink, Processing, and Distribution of EXCEED
An operation program is uploaded every week. The raw data produced by EXCEED during the observation consist of time-tag photon events, i.e., four 16-bit digitized channel outputs with time code for each photon. The data table is produced every minute. Science, engineering, and housekeeping data are stored on board the spacecraft and downlinked to JAXA ground network. Raw data is converted to spectral images (matrix: 1024×1024: LEVEL-1 data). Some corrections are made and additional information is attached to convert it to LEVEL-2 data.
The LEVEL-2 data will be available to the space science community and general public after the examination by the EXCEED team. The LEVEL-2 data are accessible on the Web for display and download from the Center for Science satellite Operation and Data Archive (C-SODA) in ISAS/JAXA. Documentation and analysis software will also be downloaded.
The authors thank the operation and project members of Sprint-A for their support. The major mechanical components were fabricated by Sumitomo Heavy Industries, Ltd. The microchannel plates were provided by Hamamatsu Photonics, the metallic filter was provided by Luxel Corporation, the entrance mirror was provided by Nikon Corporation and the diffractive grating was provided by Shimazu Corporation.
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