[Illustration: The structure of the atmosphere can be inferred from IRIS spectra at many locations over the disk of Jupiter. Scientists are beginning to assemble this vast amount of information into maps that show the temperatures at a given pressure. The temperature contours are labeled in degrees Kelvin. The banded structure, with higher temperatures near the dark equatorial belt, is most clearly evident at the lower altitude. Surprisingly, the cool region associated with the Great Red Spot (latitude 23°S) is more apparent at high altitude.] [Illustration: The observed temperature at a depth near the cloud tops (0.8 bar).] [Illustration: The observed temperature at an altitude about 30 kilometers higher (0.15 bar).] The Magnetosphere Giant Jupiter has an enormous realm—from the size of its satellite system to its tremendous aurorae and superbolts of lightning, to the huge planet-sized cloud features that surround its atmosphere. The most gargantuan Jovian feature is its magnetosphere, which envelopes the satellites and constantly changes in size, pumping in and out at the whim of the solar wind. The Pioneer and Voyager spacecraft provided four cuts through this dynamic region, showing that its borders in the upwind solar direction lie between 50 R_J and 100 R_J from Jupiter. Downwind, away from the Sun, the magnetosphere extends much farther; some scientists postulate that a magnetotail may reach as far as the orbit of Saturn. Charged particles in the magnetosphere are subject to powerful forces. Tightly embedded in Jupiter, the magnetic field spins with a ten-hour period as the planet rotates. The particles are caught in the spinning field and accelerated to high speeds. The result is a co-rotating plasma in the magnetic equator of Jupiter, extending outward to at least 20 R_J. Beyond this distance, the flow breaks up and the magnetosphere is more unstable. Within the co-rotation region, the spinning plasma sets up a powerful electric current girdling the planet. Charged particles can be accelerated in the magnetosphere to high energies, corresponding to speeds tens of thousands of kilometers per second. Some of these particle streams escape from the inner parts of the magnetosphere and can penetrate the magnetopause and be ejected from the Jovian system. On Voyager 1, the low energy charged particle instrument began detecting these streams of “hot” plasma on 22 January, when Voyager 1 was still 600 R_J (almost 50 million kilometers) from the planet. Voyager 2 first detected Jovian particles at an even greater distance, 800 R_J. Hydrogen and helium ions (protons and alpha particles) dominate the magnetosphere at great distances from Jupiter, but increasing amounts of sulfur and oxygen appeared as the spacecraft crossed the magnetopause. The heavier ions presumably originate from Io.