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Recent unpublished work suggests that at higher (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.
The X-ray emission from Saturn was unambiguously detected by XMMNewton
in October 200239 and by Chandra in April 200340. X-rays were
detected mainly from the low-latitude disk and no clear indication of
auroral X-rays was observed.
Chandra ACIS X-ray 0.24-2.0 keV images of Saturn on January 20, 26, 200441.
Each continuous observation lasted for one full Saturn rotation. The white scale bar in the
upper left of each panel represents 10?. The superposed graticule shows latitude and
longitude lines at intervals of 30?. The solid gray lines are the outlines of the planet and
rings, with the outer and inner edges of the ring system shown in white. The dotted white
line defines the region within which events were accepted as part of Saturn’s disk unless
obscured by the rings. The white oval around the south pole defines the polar cap region.
Recent Observation of Saturn (Fig. 10) by Chandra in January 2004
showed that X-rays from Saturn are highly variable – a factor of 2 to 4
variability in brightness in a week’s time41. 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 Sun41. 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 suggests41 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.
Fig. 11. Light curve of X-rays from Saturn and the Sun on 20 January 200441. All data
are binned in 30 minute increments, except for the TIMED/SEE data, which are 3 minute
observation-averaged fluxes obtained every orbit (~12 measurements per day). (a)
Background-subtracted low-latitude (non-auroral) Saturn disk X-rays (0.24–2.0 keV)
observed by Chandra ACIS, plotted in black (after shifting by -2.236 hr to account for the
light-travel time difference between Sun-Saturn-Earth and Sun-Earth). The solar 0.2–2.5
keV fluxes measured by TIMED/SEE are denoted by open green circles and are joined by
the green dashed line for visualization purpose. (b) Solar X-ray flux in the 1.6–12.4 and
3.1–24.8 keV bands measured by the Earth-orbiting GOES-12 satellite. A sharp peak in
the light curve of Saturn’s disk X-ray flux— an X-ray flare— is observed at about 7.5 hr,
which corresponds in time and magnitude with an X-ray solar flare. In addition, the
temporal variation in Saturn’s disk X-ray flux during the time period prior to the flare is
similar to that seen in the solar X-ray flux.
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
scattering34,38. 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-rays33.
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 satellites35,38,41.
The X-ray aurora on Earth is generated by energetic electron
bremsstrahlung8-10. 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 wind1,25-30. At higher energies
(>2.0 keV) the auroral X-rays at Jupiter31 could be produced by electron
bremsstrahlung process. However, at lower (~<2.0 keV) energies
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 aurora40,41. 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.
Recently, XMM-Newton has observed Saturn, for two planet rotations,
in April and November 2005; the data is being analyzed.
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, while in the Saturn system X-rays
have been detected from the rings of Saturn.