Se

E "]2 Next to our Sun, Jupiter is the strongest and most interesting X-ray source within ig 3 ¡^ our solar system. Observations with the Chandra X-ray Observatory (CXO) and

2 the XMM-Newton Observatory revealed that Jupiter's environment is a rich source nvurup of X-rays and that the structure is very complicated. There appear to be four ui Ifl n distinct sources of X-ray emission (1) the high-latitude auroral zones, or polar auroral zones on Jupiter; (2) the disk of Jupiter; (3) the Io plasma torus; (4) and the Galilean moons [287].

Elsner et al., explain the production of X-rays as: "A number of interactions among electrons, protons, ions, neutral atoms, and electromagnetic fields lead to the production of X-rays. One of the simplest is the interaction between an electron and proton or ion leading to the emission of a photon. The electron begins and ends unbound to the heavy positively charged particle. This process is called bremsstrahlung and leads to broadband continuum emission. For sufficiently energetic electrons, the spectrum can peak in the X-ray band or at higher frequencies" [288].

X-ray emission in Jupiter's auroral regions is attributed to charge-exchange between energetic ions and neutral atoms high in the polar atmosphere. X-ray emission from Jupiter's moons may result from the energetic ions incident on their surfaces ionizing and exciting neutral surface atoms leading to fluorescent K-shell line emission [289]. High spatial resolution observations of < 1 arcsec of Jupiter with the CXO in 2000 found that most of the auroral X-rays were located in small high-latitude regions, and in the north were confined to longitudes between 160° and 180° System 3, and in latitude between 60° and 70°. This correlated strongly with the Jovian magnetic field and mapped along magnetic field lines to distances greater than 30 Jupiter radii (RJ) from the planet. CXO observations in 2003 of the southern hemisphere found auroral emissions to be more extended in longitude than in the north, but still tied to magnetic field lines. Astronomers conclude that the X-ray auroral emission regions reside well within the ultraviolet (UV) auroral oval (discussed earlier). Typically the auroral regions emit -0.5-1.0 GW in soft X-rays [290].

Gladstone et al., call this concentration of auroral X-rays a "pulsating auroral X-ray hot spot." Comparison of Chandra X-ray emission mapping with simultaneous (December 18, 2000) far-ultraviolet images obtained by the HST imaging spectrograph revealed that the northern auroral X-rays are concentrated in a "hot spot" within the main ultraviolet auroral oval at high magnetic latitudes. Gladstone et al., agree with Elsner et al., that the hot spot is located at -60-70° north latitude and -160-190° system III longitude. No southern latitude hot spot has been detected, but that may be due simply to the poor viewing geometry of the southern polar cap during these observations [291].

Astronomers have also concluded that the emission from the auroral region is variable. Short, seconds to minutes, UV flares have been observed to be accompanied by an X-ray flare, with the location of the X-ray flare slightly offset from the location of the UV flare. An unexpected observation of a -40-min periodic oscillation in the X-ray emission from Jupiter's northern auroral zone was detected by the CXO in December 2000. Surprisingly, the February 2003 CXO observations did not find a 40-min oscillation. However, CXO did detect variability on a timescale from 10 to 100 min. In February 2002, the Ulysses spacecraft observed 40-minute oscillations that were correlated with periodic radio bursts from Jupiter. Although at the time of the 2003 CXO observations, the Ulysses radio observations did not detect periodic 40-min oscillations, but did detect variations on timescales similar j- ®

to that in the X-ray emission [292]. Astronomers do not yet fully understand the c -n relationship of X-ray emissions to other forms of emission. j| j-

Astronomers have also established beyond a doubt that the bright X-ray emission O 3 ¡5

from the Jovian polar-regions is line emission, not a continuum, and is likely 2

caused by charge exchange between energetic highly-stripped heavy ions and neutral ¡5

atoms in Jupiter's upper atmosphere [293]. According to Elsner et al., the CXO X-ray ^ Ifl n spectrum for the northern region of Jupiter indicates strong evidence that one of the major constituents of the incoming ion stream is highly ionized oxygen. There must be at least one other major constituent to account for the line emission. The two strongest candidates are highly ionized sulfur and highly ionized carbon. Sulfur would favor a magnetospheric origin while carbon would favor a solar origin. The CXO data seem to favor sulfur. However, XMM-Newton spacecraft data from April 2003 seems to favor carbon. Astronomers have not been able to arrive at a conclusion [294].

The planetary disk of Jupiter seems to be awash in X-ray emission. CXO data indicates Jupiter's X-ray emission appears to be a featureless disk illuminated by solar X-rays [295], or from a combination of reflected and fluoresced solar X-rays [296]. This seems to be a rather simple picture, but astronomers continue to investigate other possible causes of X-ray emission from Jupiter's disk.

The Io Plasma Torus is also an emitter of X-rays. According to Elsner et al., CXO observations in November 1999 and December 2000 detected a faint diffuse source of soft X-rays from the region of the Io plasma torus. Apparently, knowledge of the spatial structure in X-rays and X-ray spectra of the Io plasma torus are limited by its faintness. So far, Na, Cl, S, and O ions and possibly protons, have been detected in the Io plasma torus X-ray spectrum [297].

X-ray emissions have also been observed from the Galilean moons, and how these emissions occur is of great interest to astronomers. In 1999 and 2000, the CXO detected X-ray emission from the Galilean moons, specifically X-ray emissions from Io and Europa. The estimated power of the emission from Io was 2 MW, and 3 MW for Europa. X-ray emission from Callisto is suspected but was not detected by CXO, apparently occurring below the spacecraft's sensitivity level during the observation. According to Elsner et al., "the X-ray emissions from Europa are best explained by energetic H, S, and O ion bombardment of the icy surface with subsequent fluorescent emission from the deposition of energetic particle energy in the top 10 |im of the surface" [298].

Thus, the environment of Jupiter is a rich, diverse source of X-ray emission. While our knowledge of Jovian X-ray emission is limited, perhaps future spacecraft missions in orbit around Jupiter making in situ observations will eventually yield the data astronomers long for, as advocated by Elsner et al. [299].

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