Geoscience Reference
In-Depth Information
X-RAY EMISSION FROM JUPITER, SATURN, AND
EARTH: A SHORT REVIEW
ANIL BHARDWAJ
Space Physics Laboratory, Vikram Sarabhai Space Center
Trivandrum 695022, India
anil bhardwaj@vssc.org
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 emis-
sions 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 highly ionized energetic heavy ions precip-
itating 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 Recent unpublished work suggests that at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.
1. Introduction
Terrestrial X-rays were discovered in the 1950s. Launch of the first X-ray
satellite UHURU in 1970 marked the beginning of satellite-based X-ray
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, Rontgensatellite (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 X-ray 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
-
5
These include Venus, Mars,
Saturn, Galilean moons Io and Europa, Io plasma torus, rings of Saturn,
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