Due to the Jupiter's strong magnetic field, the planet's magnetosphere fills a vast volume of space—1,000 times the volume of the Sun and a million times the volume of the Earth's magnetosphere. Material escaping from Io's atmosphere becomes ionized, supplying about a ton of plasma (sulfur and oxygen ions plus accompanying electrons) every second. This ionized material is trapped in the magnetic field and spun-up to rotate about the planet with Jupiter's 10-hour period, forming a giant torus (or donut-shaped cloud) that circles the planet. The Io plasma torus emits strong UV emission that the Alice instrument will observe as New Horizons approaches Jupiter. Alice will also observe the powerful auroral emissions produced when energetic charged particles from the magnetosphere bombard Jupiter's atmosphere.

Jupiter's Dynamic Torus and Aurora

Jupiter's aurora is about 103 times as powerful as Earth's, with emissions covering the electromagnetic spectrum (from x-rays to radio), and high spatial and temporal variability. Although there is an incredible amount of data available (from ground-based and space-based observatories, and deep-space missions), many outstanding questions remain. Since most observations have been made from Earth, one of the biggest mysteries is how the aurora behaves on the night side — Galileo's low data rate did not allow that mission to address this very well. Another key unanswered question is how the solar wind influences the high-latitude aurora (the main auroral oval is powered internally by Jupiter's rotation).

Another recent discovery is that near-infrared quadrupole emissions do not show the same morphology as the other auroral emissions — in particular, they seem to lack the northern "hot spot" seen at other wavelengths. No reason is known for this surprising behavior.

Most of the magnetosphere "real estate" New Horizons will cover is not new — the Voyagers, Galileo and Cassini have been there before. New Horizons does provide unique science opportunities, particularly deep in the magnetotail as well as for obtaining solar wind or outer magnetospheric plasma measurements (with primarily the SWAP instrument, but also PEPSSI) at the same time the Alice instrument observes emissions from the aurora and from Io's plasma torus. While Cassini attempted such observations, the lack of a scan platform meant that upstream solar wind measurements could not be made during remote sensing observations. This is not the case for the New Horizons — Alice and SWAP will take snapshots of auroral activity every few minutes from 38 days to 14 days before encounter. These data will be used to study the effect of the solar wind on Jupiter's aurora and how it changes with time.

As the New Horizons spacecraft moves away from Jupiter on the night side of the planet, LORRI will make night-side images of visible aurora. The visible camera on the Galileo orbiter got only a few tantalizing glimpses of these narrow bands of aurora at visible wavelengths. LORRI data will be used to study fine details of the aurora and how they change with altitude.

Interaction of Plasma with Io's Atmosphere

We've known for decades that Io plays a peculiar role in the magnetosphere of Jupiter — thanks to the 1964 discovery that Io triggers bursts of radio emissions — but details of the mechanism of this interaction have remained elusive. New Horizons will measure the atmospheric auroral emissions produced when the torus plasma bombards Io's atmosphere. Observable emissions include atomic oxygen (at wavelengths of 1,304 and 1,356 Angstroms) and atomic sulfur (at 1,251, 1,299, 1,389, 1,429, 1,479, and 1,667 Angstroms); visible wavelength emissions from sulfur dioxide (SO2) molecules (UV/blue continuum emission), sodium atoms (at 5,893 Angstroms), and oxygen (at 5,577 and 6,300 Angstroms); and sulfur monoxide (SO) emission at 1.7 microns.

These emissions are quite faint and observations are best made when Io is in Jupiter's shadow. During the Jupiter flyby, Alice will take spectra of Io, the LEISA component of Ralph will take images of the 1.7 micron emission on the night side (at a variety of sub-spacecraft and magnetic longitudes) and observations with LORRI, Alice and Ralph (both MVIC and LEISA) will be taken when Io is in Jupiter eclipse to investigate the response of the atmosphere to the eclipse and to image the visible-wavelength emissions at high spatial resolution.

UV Emission from the Icy Galilean Satellites

Europa and Ganymede have very tenuous oxygen atmospheres that can be detected through UV emissions of atomic oxygen (at 1,304 and 1,356 Angstroms) excited by bombarding magnetospheric electrons. Similar emissions have not yet been seen on Callisto. New Horizons will use the Alice UV spectrometer to obtain high quality disk-integrated spectra of these emissions to investigate the longitudinal distribution of the atmospheres, their response to changes in the Jovian magnetosphere, and to attempt to detect atmospheric emissions from Callisto for the first time.

Mysterious Airglow

Jupiter's airglow, excited by sunlight, is much fainter than auroral emissions but also presents a longstanding problem. The Voyager spacecraft both observed that the UV emission (specifically Lyman α wavelength emissions from atomic hydrogen) showed a strong longitudinal asymmetry, with one region having a much stronger emission, labeled the "Lyman α bulge." This feature was also monitored for many years by the Earth-orbiting International Ultraviolet Explorer (IUE) telescope. Is this mysterious "Lyman α bulge" similar to arcs of emission observed coming from tropical regions of Earth's upper atmosphere? Alice and Ralph/LEISA will each make maps of Jupiter's airglow five days before the encounter to study Jupiter's mysterious bulge.

Flying Down Jupiter's Magnetotail

Perhaps the most exciting magnetospheric observations will be made as New Horizons leaves the Jovian system. The path to Pluto happens to take the spacecraft down the "tail" of Jupiter's magnetosphere that is pulled back behind the planet. While the Voyager spacecraft encountered the magnetotail of Jupiter as it approached Saturn, no spacecraft has flown directly down the tail of any magnetosphere. After taking images of the planet and satellites, New Horizons will be spun up (to conserve fuel) and the cameras turned off.

But the particle instruments SWAP and PEPSSI will continue to make measurements of the plasma as it flows down Jupiter's long magnetotail. These observations will address whether the ton-per-second of material coming from Io is ejected from the magnetosphere in large, dense blobs (like coronal mass ejections from the Sun) or whether the flow is a steady "drizzle."