ReviewSolar system planets observed with Suzaku
Introduction
In recent years, most of solar system planets have been recognized to show X-ray emission. The detected objects are Venus (Dennerl et al., 2002a), Earth (Winckler et al., 1958, Grader et al., 1968, Snowden et al., 1994, Snowden et al., 2004, Wargelin et al., 2004), Mars (Dennerl, 2002b, Dennerl et al., 2006, Metzger et al., 1983, Gladstone et al., 2002, Elsner et al., 2002, Branduardi-Raymont et al., 2004, Branduardi-Raymont et al., 2007), and Saturn (Ness et al., 2004a, Ness et al., 2004b). Hence, remote sensing X-ray astronomy satellites are becoming a novel probe to investigate planetary environments (see Bhardwaj et al. (2007) for review).
Planetary atmospheres are at most 1000 K and far cooler than astrophysical plasmas in stars, galaxies and cluster of galaxies, whose temperatures reach 106−8 K. X-ray emission mechanisms of the planets can be roughly divided into three classes. The first class is due to scattering of solar X-rays from neutrals. This needs a large neutral column density of >1019 cm−2 that is easy to obtain in a planetary upper and lower atmosphere but difficult in a more tenuous exosphere. This type is often called disk emission. The second class is a charge exchange of solar wind ions (keV/amu) or energetic ions (MeV/amu) accelerated in planetary magnetospheres with neutrals. Due to its large cross sections at solar wind ion energies, typically a few times 10−15 cm2, this type of emission can be seen in the planetary exosphere and is sometimes called halo emission. The third class is continuum and line emission by high energy electrons accelerated in planetary magnetic fields. This is seen in Earth’s and Jovian aurorae.
Fig. 1 summarizes typical X-ray fluxes of planets and other solar system objects. Almost all the objects show the X-ray to optical flux ratio of 10−9 ∼ 10−11, while comets emit X-rays far more efficiently (10−4 ∼ 10−5) because their tenuous widely spread atmosphere is very efficient for the charge exchange process. The Sun, a prototypical low-mass main-sequence star (G2V), emits X-rays from its corona with the X-ray to optical flux ratio of ∼10−7. Hence, in terms of the X-ray to optical flux ratio, the planets are in general fainter than the low-mass stars.
In the last decade, our knowledge on the X-ray emission from planets has rapidly advanced by two X-ray astronomy satellites, Chandra and XMM-Newton, with their superb angular resolution and large effective area. The Japanese fifth astronomy satellite Suzaku is of great benefit in advancing the understanding of the planetary X-rays and discovering a new type of emission. It was launched in 2005 (Mitsuda et al., 2007), 6 years after the launches of Chandra and XMM-Newton. Due to the low earth orbit (and thus low particle background) and large effective area, X-ray CCDs onboard Suzaku have one of the highest sensitivity to diffuse X-ray emission in 0.2–10 keV accompanied with a good energy resolution. The telescope vignetting over the XIS field of view is well reproduced by ray-tracing codes within ∼10%, while the flux of the standard calibration source, the Crab Nebula, is consistent with the past observations within ∼2% (Serlemitsos et al., 2007). Furthermore, the instrumental background has small uncertainty of ≲5% from the model based on observations (Tawa et al., 2008). In this paper, we review recent studies on diffuse X-ray emission associated with Jupiter’s radiation belts, and Earth’s and Martian exospheres using the Suzaku data.
Section snippets
Jupiter’s radiation belts
Jupiter is the most luminous planet in the solar system at the X-ray wavelength range. Chandra obtained high angular resolution X-ray images (Gladstone et al., 2002) and revealed the presence of auroral and disk emission. Subsequent observations with XMM-Newton (Branduardi-Raymont et al., 2004, Branduardi-Raymont et al., 2007) obtained high resolution energy spectra. The spectral shape of the auroral emission is consistent with the bremsstrahlung continuum by electrons precipitating from the
Earth’s exosphere
In addition to Jupiter, Suzaku has made significant advances in the investigation of solar wind charge exchange with the Earth’s exosphere. When an ion in the solar wind interacts with a neutral atom in Earth’s exosphere, it strips an electron(s) from the atom, and then X-ray or ultra-violet photon(s) are released as the electron relaxes into the ground state. Thus, the solar wind charge exchange emission is characterized by emission lines such as OVII, OVIII, and CVI Kα.
A first sign of the
Martian exosphere
The last topic is the X-ray emission from Mars. It was first detected with Chandra in July 2001 by Dennerl (2002b). The emission consists of an almost fully illuminated disk and faint halo component surrounding Mars. The former showed an X-ray luminosity in 0.5–1.2 keV of ∼4 MW that is consistent with the scattering of solar X-rays in the upper Mars atmosphere. The X-ray emission was dominated by a single emission line which is most likely fluorescence emission of neutral oxygen. The latter is
Conclusion
In this paper, we have reviewed Suzaku observations of diffuse X-ray emission associated with Jupiter’s radiation belts, and Earth’s and Martian exospheres. We have obtained the following results.
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The diffuse hard X-ray emission spatially associated with Jupiter’s radiation belts is detected, exhibiting a 1–5 keV luminosity of 330 MW. It is distributed over ∼16 × 8 Jovian radii. It showed a power-law spectrum with a photon index of ∼1.4. We suggested an inverse-Compton scattering off solar photons
References (31)
- et al.
X-rays from solar system objects
Planetary and Space Science
(2007) - et al.
Ultra-relativistic electrons in Jupiter’s radiation belts
Nature
(2002) - et al.
First observation of Jupiter by XMM-Newton
Astron. Astrophys.
(2004) - et al.
A study of Jupiter’s aurorae with XMM-Newton
Astron. Astrophys.
(2007) - et al.
Identifying XMM-Newton observations affected by solar wind charge exchange. Part I
Astron. Astrophys.
(2008) Discovery of X-rays from Mars with Chandra
Astron. Astrophys.
(2002)- et al.
X-ray emissions from comets detected in the Röntgen X-ray satellite all-sky survey
Science
(1996) - et al.
Discovery of X-rays from Venus with Chandra
Astron. Astrophys.
(2002) - et al.
First observation of Mars with XMM-Newton: high resolution X-ray spectroscopy with RGS
Astron. Astrophys.
(2006) - et al.
X-ray emissions from comets detected in the Röntgen X-ray satellite all-sky survey
Science
(1997)
Discovery of soft X-ray emission from Io Europa, and the Io plasma torus
Astrophys. J.
Discovery of diffuse hard X-ray emission around Jupiter with Suzaku
Astrophys. J.
Time variability of the geocoronal solar-wind charge exchange in the direction of the celestial equator
Publ. Astron. Soc. Jpn.
Evidence for solar-wind charge-exchange X-ray emission from the Earth’s magnetosheath
Publ. Astron. Soc. Jpn.
A pulsating auroral X-ray hot spot on Jupiter
Nature
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