Elsevier

Advances in Space Research

Volume 47, Issue 3, 1 February 2011, Pages 411-418
Advances in Space Research

Review
Solar system planets observed with Suzaku

https://doi.org/10.1016/j.asr.2010.09.028Get rights and content

Abstract

Recent results of solar system planets observed with the Japanese X-ray astronomy satellite Suzaku are reviewed. Thanks to the low instrumental background and good energy resolution, X-ray CCDs onboard Suzaku are one of the best probes to study diffuse X-ray emission. An overview of the Suzaku data of Jupiter and Earth is presented, along with preliminary results of Mars. Firstly, diffuse hard X-ray emission is discovered in 1–5 keV at Jovian radiation belts. Its spectrum is represented by a power-law continuum with a photon index of ∼1.4. This emission could originate from inverse-Compton scattering of solar photons by tens MeV electrons. Secondly, variable diffuse soft X-rays are serendipitously found during observations in the directions of the north ecliptic pole and galactic ridge. Good time correlations with the solar wind and emission lines found in the X-ray spectra are firm evidences of a solar wind charge exchange emission with Earth’s exosphere. Thirdly, diffuse X-ray emission from Martian exosphere via the solar wind charge exchange is investigated for the first time at solar minimum. A stringent upper limit on the density of the Martian exosphere is placed from the Suzaku data.

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.

  • 1.

    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

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