Like Earth, Jupiter forms spectacular auroras. More specifically, auroras are seen near and around Jupiter's poles, and occur in much the same fashion as they do on Earth. Aurora form an oval around each magnetic pole, where ions or electrons from the magnetosphere stream down into the ionosphere. These particles come from the Io orbital plasma cloud and beyond (Fig. 5.6). The aurora oval roughly marks the ring of the magnetic field lines that intersect Io's orbit and often has hotspots inside it [229].

Fig. 5.6. False color aurora. A false color composite of Galileo spacecraft images of Jupiter's northern aurora on the night side of the planet. The glow is caused by electrically charged particles impinging on the atmosphere from above. The particles travel along Jupiter's magnetic field lines, which are nearly vertical at th is latitude. The auroral arc marks the boundary between the "closed" field lines that are attached to the planet at both ends and the "open" field lines that extend out into planetary space. The colored background is light scattered from Jupiter's bright cresent. (Credit: NASA/JPL-Caltech).

The existence of Jovian auroral activity was first inferred from the particles and fields data collected by the Pioneer probes. The International Ultraviolet Explorer spacecraft, launched in 1978, made the first direct observations. The Galileo spacecraft later searched for auroral activity on the dark side of the planet, using its ultraviolet spectrometer [230]. Jupiter's auroras are best detected at far-ultraviolet wavelengths, where atoms and molecules of hydrogen radiate. These emissions can only be detected outside Earth's atmosphere. Consequently, the spacecraft missions of the 1970s and 1980s first detected the aurora in Jupiter's cloud tops, with the Earth orbiting observatory Copernicus being the first. Jupiter's aurora can also be detected from the Hubble Space Telescope (HST), as was first done in 1992 [231], and with modern infrared technology from ground based Earth instruments. Several research groups have made infrared images of Jupiter's aurora [232]. Earth orbiting spacecraft have also detected X-ray emissions coming from Jupiter's aurora regions [233]. The aurora can also be detected from Earth in the infrared g W at 3.5 ¡mm. In far ultraviolet and infrared, the auroral oval can be detected in both

© .£ the day and night side [234]. Auroral activity is concentrated in the oval ribbons,

C "]2 one around each magnetic pole [235]. It is thought that 'hazy clouds' are produced q 3 jg directly beneath auroral activity [236].

.¡5 2 Jupiter's aurora is the most powerful in the solar system. The main features

3 ^ being the main oval, or footprint, that generally co-rotates with the planet, and u (A a region of patchy, diffuse emission inside the oval on Jupiter's dusky side [237].

Jupiter's aurora is powered largely by energy extracted from planetary rotation, although there also seems to be a contribution from the solar wind. This contrasts with Earth's aurora, which is generated through the interaction of the solar wind with Earth's magnetosphere [238].

There is evidence that the auroral activity on Jupiter can flare and then subside rapidly. During a two hour period on September 21, 1999 the HST was used to make four imaging runs of Jupiter's northern far-ultraviolet aurora using HST's Imaging Spectrograph (STIS) in time-tagged mode. During a 4-min segment of the 2-h imaging run, a dramatic, rapidly intensifying, flare-like auroral emission was detected on Jupiter. The event began as a small 'pinpoint' emission near 167° System III longitude and 63° north latitude, which rose rapidly in intensity and became a structure several thousand kilometers across. The event apparently reached its maximum intensity in approximately 70 s, and then began to decrease in intensity and size. This entire sequence happened inside the four-minute segment. During this flare event, other auroral emissions remained virtually unchanged in both intensity and morphology. This flare event occurred within a region of fainter more diffuse emissions inside the main auroral oval [239].

The portion of the flare poleward of the main oval demonstrates that it is linked by magnetic field lines to a region of the magnetosphere lying at distances greater than 30 RJ on Jupiter's dayside. An analytical model indicates that the flare maps to a longitudinally extended region located at distances between 40 and 60 RJ in the morning sector. Therefore, it can be assumed that the flare was triggered by a disturbance in this region of the magnetosphere [240]. Calculations based upon measurements taken by the Advanced Composition Explorer spacecraft at the Earth L1 Langrarian position, indicates a series of sharp rises in the solar-wind dynamic pressure at the orbit of Jupiter near the time that the flare was observed [241].

We have already discussed the effect of the solar wind on the shape and size of the Jovian magnetosphere. We have also discussed the effect of the solar wind on changes in the intensity of auroral emissions. It is speculated that a sharp jump in the solar-wind dynamic pressure at Jupiter's dayside magnetopause produced the disturbance that manifested itself in the polar-cap flare. Solar-wind conditions at the time of the flare were not unusual, suggesting that such flares, if triggered by changes in the solar-wind pressure, may not be uncommon. The response of Earth's aurora to solar-wind dynamic pressure pulses are evidenced by rapid global brightenings associated with the passing of interplanetary shocks, and as smaller scale transient auroral events. Similar events may be at work at Jupiter [242].

During the Cassini spacecraft flyby of Jupiter, as Cassini was enroute to Saturn, scientists took advantage of the encounter to take data on Jupiter's auroral activity. In order to correlate changes in the intensity and morphology of Jupiter's aurora with the state of the solar wind, Cassini carried out a coordinated observation campaign with the HST and the Galileo spacecraft in orbit around the planet. Inbound, Cassini monitored the state of the solar wind, Galileo observed magnetosphere properties from within the magnetosphere, and HST observed Jupiter's aurora. Outbound, Cassini observed the night side aurora and traversed the edge of the magnetosheath, monitoring fluctuations in the shape and width j- ®

of Jupiter's magnetosphere, while Galileo monitored the solar wind and HST ®

monitored the dayside aurora [243]. The Cassini spacecraft observed three inter- j| j-

planetary shocks due to activity in the solar wind as it approached Jupiter from O 3 ¡5

Earth. A distinct brightening in Jupiter's aurora followed each of these shocks. 2

This proved that the interaction of the solar wind would in fact result in such ¡5

brightenings [244, 245]. UHA

Jupiter's aurora has been observed to vary on short (minutes to hours) and long (days to weeks) timescales. This variability is thought to be due to the combined influences of internal magnetosphere processes and external solar wind driven changes. Similar to the processes driving the Earth's Aurora, a direct relationship between injection of electrons and a transient auroral feature was observed. Unlike the solar wind driven aurora at Earth, Jupiter's auroral morphology shows dependence on both the solar wind and Jupiter's rotation [246].

We know that energetic electrons and ions that are trapped in Earth's magnetosphere can suddenly be accelerated towards the planet. Some of the dynamic features of Earth's aurora (the northern and southern lights) are created by the fraction of these injected particles that travel along magnetic field lines and strike the upper atmosphere. The aurora of Jupiter resemble those of Earth in some respects; for example, both appear as large ovals encircling the poles and both show transient events. But the magnetosphere of Earth and Jupiter are so different in the way they are powered, that it was not known whether the magnetospheric drivers of Earth's aurora also caused them on Jupiter. Mauk et al., were able to demonstrate a direct relationship between Earth-like injections of electrons in Jupiter's

Fig. 5.7. Night side Jovian aurora. Galileo spacecraft image. The upper bright arc is auroral emission seen "edge on" above the planetary limb with the darkness of space as a background. The lower bright arc is seen against the dark clouds of Jupiter. The aurora is easier to see on the night side of Jupiter because it is fainter than the clouds when they are illuminated by sunlight. North is at top of image. (Credit: NASA/JPL-Caltech).

Fig. 5.7. Night side Jovian aurora. Galileo spacecraft image. The upper bright arc is auroral emission seen "edge on" above the planetary limb with the darkness of space as a background. The lower bright arc is seen against the dark clouds of Jupiter. The aurora is easier to see on the night side of Jupiter because it is fainter than the clouds when they are illuminated by sunlight. North is at top of image. (Credit: NASA/JPL-Caltech).

magnetosphere and a transient auroral feature in Jupiter's polar region. The discovery of Earth-like charged particle injections within Jupiter's magnetosphere g W is surprising because Jupiter's magnetosphere is powered mostly from the inside

© .£ by the rapid but steady planetary rotation rather than from the outside by the vari-

C "]2 able solar wind. Mauk et al., took advantage of the Cassini flyby in late 2000 and q 3 jg early 2001, and the joint Cassini, Galileo, HST observation campaign and discov-

.¡5 2 ered the previously unknown role of injection in generating auroral emissions at

3 Jupiter [247]. According to Scott Bolton, a member of the Galileo spacecraft plasma u (A spectrometer team, the auroral arc on Jupiter is thin and patchy, contrary to those on Earth. Jupiter's auroral arc is estimated to occur between 300 and 600 km above the 1 bar reference level [248].

Three of Jupiter's moons, Io, Ganymede, and Europa, each produce a very distinct auroral "footprint" at Jupiter (Fig. 5.7) . The footprint of Io is by far the brightest, since its volcanic activity puts heavy ions into the Jovian magnetosphere [249, 250]. These footprints will be more fully discussed in the sections on The Io Flux Tube and Magnetic Footprints on Jupiter.

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