X-Ray Emission from Jupiter, Saturn, and Earth: A Short Review

Jupiter, Saturn, and Earth - the three planets having dense atmosphere and a well developed magnetosphere - are known to emit X-rays. Recently, Chandra X-ray Observatory has observed X-rays from these planets, and XMM-Newton has observed them from Jupiter and Saturn. These observations have provided improved morphological, temporal, and spectral characteristics of X-rays from these planets. Both auroral and non-auroral (low-latitude) 'disk' X-ray emissions have been observed on Earth and Jupiter. X-rays have been detected from Saturn's disk, but no convincing evidence for X-ray aurora on Saturn has been observed. The non-auroral disk X-ray emissions from Jupiter, Saturn, and Earth, are mostly produced due to scattering of solar X-rays. X-ray aurora on Earth is mainly generated via bremsstrahlung from precipitating electrons and on Jupiter via charge exchange of highlyionized energetic heavy ions precipitating into the polar atmosphere. Recent unpublished work suggests that at higher (>2 keV) energies electron bremsstrahlung also plays a role in Jupiter's X-ray aurora. This paper summarizes the recent results of X-ray observations on Jupiter, Saturn, and Earth mainly in the soft energy (~0.1-2.0 keV) band and provides a comparative overview.


Introduction
Terrestrial X-rays were discovered in the 1950s. Launch of the first Xray satellite UHURU in 1970 marked the beginning of satellite-based Xray astronomy. After about two decades of search with balloon-, rocket-, and satellite-based experiments 1 , X-ray emission from Jupiter was discovered with the Einstein observatory 2 . During 1990s, Rontgensatellit (ROSAT) made important contributions to planetary X-rays by discovering emissions from Moon and comet and providing better observations on X-rays from Jupiter. With the advent of sophisticated Xray observatories, viz., Chandra and XMM-Newton, the field of planetary X-ray astronomy is advancing at a faster pace. Several new solar system objects are now known to shine in X-rays at energies generally below 2 keV [3][4][5] . These include Venus, Mars, Saturn, Galilean moons Io and Europa, Io plasma torus, rings of Saturn, and Earth and Martian exospheres. Higher spatial and spectral resolution information on planetary X-rays is improving our understanding on the physics of the X-ray production on the planetary bodies, which are much colder than traditional million-degree K or higher temperature plasmas in the solar corona and astrophysical objects 6 . In this paper we summarize the recent results of soft (~0.1-2.0 keV) Xray observations on Jupiter, Saturn and Earth: all the three planets having dense atmospheres and intrinsic magnetospheres, and known to emit Xrays. Table 1 provides some of the characteristic parameters of these planets. Reader are referred to other reviews for more details 1,3-5,7 .

Earth: Auroral Emissions
It is well known that the X-ray aurora on Earth is generated by energetic electron bremsstrahlung 8,9,10 , and the X-ray spectrum of the aurora has been very useful for studying the characteristics of energetic electron precipitation 9,[11][12][13][14][15] . The PIXIE X-ray imager on the Polar spacecraft measured X-rays in the range 2-60 keV 16 . The high apogee of the Polar satellite (~9 R E ) enabled PIXIE to image the entire auroral oval with a spatial resolution of ~700 km. PIXIE data showed that the substorm Xrays brighten up in the midnight sector and have a prolonged and delayed maximum in the morning sector due to the scattering of eastward-drifting electrons 13 . Statistically, the X-ray bremsstrahlung intensity is largest in the midnight substorm onset, is significant in the morning sector, and has a minimum in the early dusk sector 10 . During the onset/expansion phase of a typical substorm the electron energy deposition power is about 60-90 GW, which produces around 10-30 MW of bremsstrahlung X-rays 17 .  While harder X-ray emissions from electron bremsstrahlung are well known in the terrestrial aurora [9][10][11][12][13][14][15] , surprisingly, there were no searches for emissions at auroral latitudes at energies <2 keV until recently. Northern auroral regions of Earth were imaged using the High-Resolution Camera (HRC-I) of the Chandra X-ray Observatory at 10 epochs (each ~20 min duration) between mid-December 2003 and mid-April 2004 18 , to search for Earth's soft (<2 keV) X-ray aurora. The first Chandra soft X-ray observations of Earth's aurora showed that it is highly variable -sometimes intense arcs (Fig. 2), other times multiple arcs, or diffuse patches, and at times absent 18 . In at least one of the observations an isolated blob of emission is observed near the expected cusp location. A fortuitous overflight of DMSP satellite F13 provided SSJ/4 energetic particle measurements above a bright arc seen by Chandra on 24 January 2004, 20:01-20:22 UT. A model of the emissions expected strongly suggests that the observed soft X-ray signal is produced by electron bremsstrahlung 18 .

Earth: Non-Auroral Disk Emissions
The non-auroral X-ray background above 2 keV from the Earth is almost completely negligible except for brief periods during major solar flares 10 .
However, at energies below 2 keV soft X-rays from the sunlit Earth's atmosphere have been observed even during quite (non-flaring) Sun conditions 19,20 . The two primary mechanisms for the production of X-rays from the sunlit atmosphere are: 1) the Thomson (coherent) scattering of solar Xrays from the electrons in the atomic and molecular constituents of the atmosphere, and 2) the absorption of incident solar X-rays followed by the emission of characteristic K-shell lines of Nitrogen, Oxygen, and Argon. Fig. 3 shows the PIXIE image of Earth demonstrating the X-rays (2.9-10 keV) production in the sunlit atmosphere during a solar flare of August 17, 1998. The X-ray brightness can be comparable to that of a moderate aurora. For two solar flare events during 1998 examined using the data from PIXIE, the shape of the measured X-ray spectra was in fairly good agreement with modeled spectra of solar X-rays scattered and fluoresced in the Earth's atmosphere 10 .

Jupiter: Auroral Emissions
Auroral X-rays from Jupiter were first detected by Einstein observatory in 1979 2 , and later studied by ROSAT observations 21,22 . The pre-Chandra understanding of Jovian auroral X-rays was that these emissions are mostly line emissions resulting from recombination and charge exchange transitions in high charged states of S and O ions precipitating from inner (~8-12 RJ) region of the magnetosphere 1,3,21-24 . The Chandra observations of Jupiter in December 2000 25 and February 2003 26 have revealed that: 1) most of Jupiter's northern auroral X-rays come from a "hot spot" that is fixed in latitude and longitude and located significantly poleward (>30 R J ) of the latitudes connected to the inner magnetosphere (Figs. 4 and 8), and 2) the auroral hot spot X-rays pulsate with periodicity that is regular (~45 min) at time 25 and chaotic at other times 26 (vary over the 20-70 min range, cf. Fig. 5). Chandra observations also found (Fig. 5) that X-rays from the north and south auroral regions are neither in phase nor in anti-phase, but that the peaks in the south are shifted from those in the north by about 120º (i.e. one-third of a planetary rotation) 26 . Periodic oscillations on time scale of 20-70 min are not observed in the XMM-Newton data 27,28 , perhaps due to lower spatial resolution of XMM-Newton relative to the Chandra.  XMM-Newton has provided spectral information on the X-rays from Jupiter, which is somewhat better than Chandra. The RGS on XMM-Newton clearly resolves the strongest lines in the spectra, while the EPIC camera has provided images of the planets in the strong OVII and OVIII lines present in the Jovian auroral emissions 28,29 .
The spectral interpretation of Chandra and XMM-Newton observations is consistent with a source due to energetic ion precipitation that undergoes acceleration to attain energies of >1 MeV/nucleon before impacting the Jovian upper atmosphere [26][27][28][29][30] . However, the source of precipitating ionswhether it's outer magnetospheric or solar wind origin, or a mixture of both, is currently not clear and arguments in favor of either of them have been presented [26][27][28][29][30] . Very recently, XMM-Newton and Chandra data 26 have suggested that there is a higher (>2 keV) energy component present in the spectrum of Jupiter's aurora. The observed spectrum and flux, at times, tentatively appears consistent with that produced by electron bremsstrahlung 1,32 at energies greater than 2 keV.

Jupiter: Low-Latitude Disk Emissions
X-ray emission from Jupiter's low latitudes was first reported using the ROSAT-HRI 33 . It was proposed that disk X-rays may be largely due to the precipitation of energetic sulfur or oxygen ions into the atmosphere from Jovian inner radiation belts. Later it was suggested 34 that elastic scattering of solar X-rays by atmospheric neutrals (H2) and fluorescent scattering of carbon K-shell X-rays from CH 4 molecules located below the Jovian homopause are also potential sources of disk X-rays. XMM-Newton's 69 hours of Jupiter observation, in Nov. 2003, demonstrated that day-to-day variation in disk X-rays of Jupiter are synchronized with variation in the solar X-ray flux (Fig. 7), including a solar flare that has a matching feature in the Jovian disk X-ray light curve 35 . The X-rays from the disk are quite uniformly distributed across the lowlatitudes (Fig. 8) -in contrast to the auroral X-rays. Auroral X-rays from the north (60-75º N latitude) are dominantly confined to ~150-190º longitude and those from the south (70-80º S latitude) spread almost half-way across the planet (~300-360º and 0-120º longitude), while the disk X-rays are quite uniformly distributed and are largely confined to <50º latitude in both hemispheres 36 . The spectrum of X-rays from the disk is also harder and extends to higher energies than the auroral spectrum (Fig. 9). No periodicity has been observed in disk X-ray lightcurve 26,36,37 .  Recent studies suggested that the X-ray emission from the Jovian disk is largely due to scattered solar X-rays and that processes occurring on the Sun control the X-rays from Jupiter's disk 35,36-38 .

SATURN
The X-ray emission from Saturn was unambiguously detected by XMM-Newton in October 2002 39 and by Chandra in April 2003 40 . X-rays were detected mainly from the low-latitude disk and no clear indication of auroral X-rays was observed. showed that X-rays from Saturn are highly variable -a factor of 2 to 4 variability in brightness in a week's time 41 . In these observations an Xray flare has been detected from the non-auroral disk of Saturn, which is seen in direct response to an M6-class flare emanating from a sunspot that was clearly visible from both Saturn and Earth (Fig. 11). This is the first direct evidence suggesting that Saturn's disk X-ray emission is principally controlled by processes happening on the Sun 41 . Also a good correlation has been observed between Saturn X-rays and F10.7 solar activity index. The spectrum of X-rays from Saturn disk is very similar to that from the disk of Jupiter (Fig. 12). The Chandra observations in January 2004 also revealed X-rays from Saturn's south polar cap on Jan. 20 (see Fig. 10, left panel). However, the analysis suggests 41 that X-ray emissions from the south polar cap region on Saturn are unlikely to be auroral in nature; they might instead be an extension of its disk X-ray emission.  Table 2 presents a summary of the main characteristics of X-rays from the three planets. X-rays from the low-latitude (non-auroral) disk of all the three planets are mostly produced by scattering of solar X-rays by atmospheric species. On Jupiter and Saturn the scattering is dominantly resonant scattering with minor (~<10%) contribution from fluorescent scattering 34,38 . However, not all the incident solar X-rays in the ~0.2-2.0 keV are scattered back. The energy-average geometric X-ray albedo of Jupiter and Saturn over this energy range is ~5?10 -4 [ref. 35,41]. At Jupiter precipitation of radiation belt ions can also make some contribution to the disk X-rays 33 . It has been suggested that the upper atmospheres of the giant planets Saturn and Jupiter act as "diffuse mirrors" that backscatter solar X-rays. Thus, these planets might be used as potential remote-sensing tools to monitor X-ray flaring on portions of the hemisphere of the Sun facing away from near-Earth space weather satellites 35,38,41 . The X-ray aurora on Earth is generated by energetic electron bremsstrahlung [8][9][10] . The auroral X-rays from Jupiter are produced by charge-exchange of highly-ionized energetic heavy ions precipitating from the outer magnetosphere and/or solar wind 1,25-30 . At higher energies (>2.0 keV) the auroral X-rays at Jupiter 31 could be produced by electron bremsstrahlung process. However, at lower (~<2.0 keV) energies Comparison of Chandra ACIS-S X-ray Spectrum of Satun and Jupiter Disk electron bremsstrahlung falls short by orders of magnitude in explaining the Jupiter auroral X-ray flux. Also the spectrum shape at lower energies is inconsistent with the bremsstrahlung shape (see Figs. 6 and 9) 26,37 . At Saturn there is no clear indication of an X-ray aurora 40,41 . X-ray aurora produced by electron bremsstrahlung is expected at Saturn, but it will probably be weak and could escape detection by present-day instruments, because Saturn aurora is relatively weaker than that on Jupiter (see Table  1), and Saturn does not have copious heavy ion source, like Io on Jupiter.

Discussion
Recently, XMM-Newton has observed Saturn, for two planet rotations, in April and November 2005; the data is being analyzed. The values quoted are "typical" values at the time of observation. X-rays from all bodies are expected to vary with time. For comparison the total X-ray luminosity from the Sun is 10 20 W.
In addition to X-rays from the planet itself, in the Jupiter system the Xray emission has been observed from the Io plasma torus and from Galilean satellites Io and Europa 42 , while in the Saturn system X-rays have been detected from the rings of Saturn 43 .