Ordinary Meeting, 2006 May 31
Bright Lights on Giant Planets
Prof. Miller explained that his scientific background was in chemistry rather than astronomy, but that he had become involved with planetary science, and especially aurorae, through an interest in the chemical composition of planetary atmospheres. The various colours seen in aurorae were powerful probes of the chemical constituents of planetary atmospheres, and the speaker illustrated this with an image of the aurora borealis of our own planet. The deep red emission seen at the highest celestial altitudes could be attributed to atomic oxygen, and likewise the brighter green emission below it. Towards the lower edge of the aurora, closest to the horizon, reddy-pink emission stemmed from molecular nitrogen.
Not only were such aurorae revealing the chemical makeup of the Earth's atmosphere, but the dominance of different colours at different altitudes was also revealing its vertical structure. Above a certain point, the homopause, the atmosphere's various constituents ceased to be well mixed, being instead gravitationally stratified according to mass. Because molecules weighed more than atoms, the reddy-pink emission of molecular nitrogen was seen at lower altitudes than the emission of atomic oxygen.
Images taken from high-altitude aircraft presented compelling evidence that auroral emission arose high in the upper atmosphere, Prof. Miller explained – even from the highest-flying aircraft, one had to look upward to see it. Detailed study revealed it to emanate 70-200 km above the Earth's surface.
The speaker then turned to discuss the physical origin of aurorae. He began with a schematic of the Earth's magnetic field, which he compared to that of a bar magnet: field lines emanated from the Earth's surface at its north magnetic pole, wrapped longitudinally around the planet, and re-converged upon its south magnetic pole. However, they were permanently distorted from those of a bar magnet by their interplay with the solar wind – a continuous stream of high-energy charged particles, flowing outwards from the Sun through the solar system at around 400 km/s.
Michael Faraday had discovered in the 19th Century that when electric currents traversed circular paths, magnetic fields were generated – the principle behind the electromagnet. Conversely, he had also found that in the presence of magnetic fields, a force was exerted upon charge-carrying particles which caused them to follow circular paths around the field lines.
Thus, when solar wind particles came under the influence of the Earth's magnetic field, their paths were bent: they began to circle around the magnetic field lines. Broadly speaking, the Earth's magnetic field could be said to be an obstacle to their outward flow through the solar system.
This interaction also bent the Earth's magnetic field lines. On the sunward side, this distortion took the form of a compression, and at an altitude of around 70,000 km it exhibited an outer boundary called the magnetopause, outside of which the Sun's magnetic field dominated. It was upon impact with this boundary that solar wind particles came sharply into interaction with the Earth. More precisely, about 15,000 km upstream of it, compressed solar wind material piled up against the boundary to form a bow shock. In the anti-solar direction, the distortion had the opposite effect, stretching out the Earth's magnetic field into a long tail called the magnetotail, about 190,000 km in length.
Prof. Miller noted that the most profound consequence of this interplay between the Earth's magnetic field and the solar wind for the human species was that it shielded the Earth's surface from ionising solar wind particles: without such a shield, we could not survive. Aurorae were surely a secondary consequence. They arose when solar wind particles descended into the Earth's atmosphere and collided with one of the various atomic or molecular gas particles around them, dumping their energy into the gas, often leaving the particles ionised or in excited states. The visible light of the aurora arose when these gas particles subsequently de-excited via photon emission, but Prof. Miller added that the display of lights was not the only consequence of this process – it also effected a significant heating upon the atmosphere.
This descent of solar wind particles into the Earth's atmosphere was only possible in the Earth's polar regions, because these were the only places where magnetic field lines were directed up out of the surface of the Earth; these were the only places where the solar wind particles, spiralling around the field lines, could descend towards the Earth. More specifically, aurorae were actually most frequently observed slightly away from the pole, where the magnetic field lines were at a slight slant to the surface, in a circular region called the auroral oval.
During an auroral display, the solar wind might dump energy into the Earth's atmosphere with a power of around 100 GW, raising the temperature of the upper atmosphere by around 100 K. This effect was very significant, though somewhat less dramatic than it sounded for the fact that the absorption of solar UV radiation already heated this part of the atmosphere to around 1000 K. The change made by aurora was thus a significant but not overwhelming 10%.
The speaker then turned to discuss the aurorae of other planets, and first of all, of Jupiter. Infrared images from the United Kingdom Infrared Telescope (UKIRT), on the summit of Mauna Kea, Hawaii, revealed compelling evidence for a bright auroral oval, not dissimilar to our own, on Jupiter. Prof. Miller noted in passing that this was one of very few areas of work where ground-based telescopes could usefully be employed in planetary science; in the visible, the resolution of the Hubble Space Telescope (HST) ruled supreme. In addition to UKIRT, another ground-based infrared telescope, NASA's Infrared Telescope Facility (IRTF), also on the summit of Mauna Kea, provided a complementary facility to take high-resolution spectra of Jovian aurorae.
The emission seen in these images was arising high in Jupiter's atmosphere, at an altitude of 450-2000 km above the surface. At this altitude, the atmosphere was thin, having a particle number density less than 1018 particles per cubic metre, but also apparently incredibly hot, ranging between 900 and 1100 K. This compared with 400 K for Saturn's upper atmosphere and 500-750 K for that of Uranus. The speaker would return to the puzzle of how the Jovian upper atmosphere came to be so hot later.
The primary constituent of the Jovian atmosphere was hydrogen gas, and so narrow-band images centred upon the Lyman transition lines of atomic hydrogen produced detailed maps of the excited gas. These lines lay in the ultraviolet part of the spectrum, unobservable from the ground because of absorption from the Earth's atmosphere, but could be imaged by the HST. Such images contained a wealth of information, both about the auroral oval and its neighbourhood; for the present talk, the speaker concentrated upon the former, specifically upon the question of how it compared with the Earth's auroral oval. Were the aurorae of Jupiter similarly controlled by the solar wind?
To answer this question, Prof. Miller started by outlining what was known about the Jovian magnetosphere. It was a huge structure. Its magnetopause and bow shock lay a colossal 1-2 million km above Jupiter's surface, and if these were visible structures, their projection on the night sky would appear 2½ times the size of a Full Moon from the Earth. The magnetotail was larger still, stretching 750 million km in the anti-solar direction – so far that it stretched beyond Saturn's orbit; Saturn could indeed pass through it. The Jovian magnetosphere was arguably the second largest 'structure' in the solar system after the Sun.
Apart from its sheer size, it differed from the Earth's magnetosphere in one additional respect, which arose from Jupiter's interaction with its nearest moon, Io. Orbiting at a mere 350,000 km above Jupiter's surface – closer than the Earth-Moon distance – Io experienced extreme tidal gravitational forces, stirring up its internal structure. The resultant strain rendered Io the most volcanic body in the solar system, as had compellingly been seen in many images returned by the Voyager probes. Volcanic plumes spewed around a tonne of ionised material into the neighbourhood of Jupiter each second. This material spread out to form a thin circular sheet termed a plasma torus, extending out to a distance of a million km from Jupiter. When initially spewed from Io, this material would share its orbital period of 42 hours. However, because of its electrical charge, it interacted with the magnetic field of Jupiter, bringing about a rapid change in its rotation speed. For such a large planet, Jupiter was remarkably fast-spinning: in fact, it was not only the largest planet in the solar system, but also that with the shortest rotation period – a mere 9 hours. As Jupiter spun, it carried its magnetic field around with it, and the effect of the interaction between this rapidly-rotating magnetic field and the plasma torus was to spin up the ionised material, draining rotational energy from Jupiter at a rate of 10 TW – sufficient to completely halt Jupiter's rotation within 60 times the current age of the Universe.
The interaction was not quite strong enough, however, to completely bring the plasma torus into co-rotation with Jupiter, and the difference between the two rotation speeds was especially great at large radii. The resulting sheer in electric field produced a break in Jupiter's magnetosphere through which solar wind particles could break.
Prof. Miller noted in passing that the plasma torus also seemed to have another effect: in recent HST images, a clear 'footprint' of auroral activity could be seen beneath Io, suggesting that its volcanic activity produced a secondary source of ions, in addition to the solar wind, which produced their own aurorae. Rather curiously, Ganymede and Europa also had visible auroral footprints, despite not being appreciably volcanic; this remained unexplained.
Returning to the question of why Jupiter's upper atmosphere was so hot, the speaker discussed whether energy input from aurorae could be the answer. He explained that the effect of solar wind electrons upon the Jovian atmosphere was primarily to ionise hydrogen through the reactions:
H2 + e- → H2+ + 2e- (1)
H2+ + H2 → H3+ + H (2)
Emission lines, resulting from the rotational excitation and de-excitation of the H3+ ions produced on the right-hand side of Reaction (2), were responsible for producing the infrared emission seen in the UKIRT images discussed earlier. By contrast, it was the second product on the right-hand side of Reaction (2), the atomic hydrogen, which was responsible for the ultraviolet emission seen in the narrow-band Lyman-line images returned by the HST.
Across the whole planet, the energy input from the solar wind through these reactions could be calculated to be about 1014 W – more than two orders of magnitude in excess of the power absorbed by Jupiter from sunlight. Aurorae did thus seem a plausible mechanism for heating, although the situation was actually rather more complicated than suggested by this simple evaluation of power input alone. The bright infrared emission seen from H3+ ions demonstrated that they were very efficient at re-radiating absorbed energy, and thus much of the energy absorbed from solar wind particles seemed not to be retained by the planet's atmosphere.
Turning next to Saturn, the speaker explained that images of aurorae in its polar regions had been returned by the Cassini probe, but that they appeared to be rather modest as compared to those on Jupiter. It also appeared that those on Saturn were controlled exclusively by the solar wind, rather than any volcanism on its moons. Uranus showed signs of aurorae as well, perhaps contributing up to 20% of its total emission, but they were rather difficult to image on account of being spread very widely and thinly across the planet's entire disk. The lack of an auroral oval on Uranus was not well understood, but could perhaps be attributed to the planet's unusual axial tilt, at 98° to its orbital plane.
The speaker closed with a brief discussion of the possible effects of aurorae on extrasolar planets. He remarked that over 100 planets had now been discovered around stars other than our own, and that many of them seemed to be gas giants not unlike Jupiter, but in very close orbits around their parent stars – perhaps as close as 1/20th of an astronomical unit. Such discoveries raised many questions about how these planets came to be found so close to their parent stars. One especial problem was that models of planetary atmospheres predicted that planets in such hot environments would completely boil away within a timescale of 105-106 years. In astronomical terms, these were very short timescales, and so we would not expect to observe such planets so close to stars.
Prof. Miller argued that aurorae on these planets might have a rôle to play in extending their lifespans: the production of H3+ ions on their sunward sides could lead to very intense infrared emission in the H3+ rotational transitions. The cooling effect of this emission might have a thermostatic effect, preventing the atmosphere from boiling away. The speaker added that if this idea was correct, then the first direct detection of emission from exoplanets might well be in the form of H3+ emission lines in the spectra of their parent stars.
Following the applause, the President thanked Prof. Miller for his thorough account and invited questions. A member asked at what wavelength the UKIRT images of Jovian aurorae had been taken. The speaker replied that they were taken in the atmospheric window around the photometric L-band, specifically, at 3.42 and 3.53 μm.
The President then adjourned the meeting until the occasion of the Exhibition Meeting, to be held in Cambridge on June 24.
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Dominic Ford
© 2006 Dominic Ford / The British Astronomical Association.