Direct measurements of the Jovian magnetic field were first made by the Pioneers, and Voyager results generally confirm the initial findings. The strength of the Jovian field is about 4000 times greater than that of the Earth. The dipolar axis is not at the center of Jupiter, but offset by about 10 000 kilometers and tipped by 11 degrees from the axis of rotation. Each time the planet spins, the field wobbles up and down, carrying with it the trapped plasma of the radiation belts. The Voyager particles and fields instruments concentrated not on the planetary magnetic field but on the processes taking place in the magnetosphere. [Illustration: A probe of the Jovian atmosphere is obtained each time a spacecraft passes behind the planet as seen from the Earth. Passage through the ionosphere and atmosphere alters the phase of the radio telemetry signal, and subsequent computer analysis allows members of the Radio Science Team to reconstruct the profile of the atmosphere. Shown here is the atmospheric temperature as a function of pressure as derived from the Voyager 1 X-band occultation data, corresponding to a point at latitude 12°S, longitude 63°. The two curves represent extreme interpretations of the same data; the best fit lies somewhere between. Accuracy is high at greater depths but poor at levels above a pressure of about 0.03 bar. Clearly shown is the temperature minimum near 0.1 bar and the steady increase of temperature with depth as the radio beam probed toward the cloud level near 1.0 bar.] [Illustration: At a wavelength near 5 micrometers, the primary gases in the atmosphere of Jupiter are particularly transparent, and the infrared radiation from the planet comes from relatively great depths. At these depths, it is possible to see evidence of gases such as water vapor that condense at higher altitudes where the temperatures are lower. This IRIS spectrum in the 5-micrometer spectral region shows features identified with water (H₂O), germane (GeH₄), and deuterated methane (CH₃D), as well as the more easily detected ammonia (NH₃).] Wavelength (µm) H₂O CH₃D GeH₄ NH₃ Voyager 1 Voyager 2 Wave number (cm⁻¹) Voyager 1 brightness temperature (K) Voyager 2 brightness temperature (K) [Illustration: The structure of the Jovian atmosphere can be derived from infrared spectra as well as from the radio occultation data. This profile of temperature as a function of pressure covers the same range of altitudes as does the preceding figure and is in good agreement. Both profiles locate a temperature minimum of about 110 K near a pressure of 0.1 bar (1000 mb = 1 bar = 1 atm). The IRIS data also show variation of structure with position, including a cooler minimum temperature (about 100 K) over the Great Red Spot.]