No abstract

Dark brown cyclonic spots ('barges') at 14øN were studied by using Voyager 1 and 2 images of Jupiter. Movie sequences were made to study the spots' behavior over intervals of 50 days and longer. These movies revealed that the length and width vary by +9% with a period of about 15 days, while the area remains approximately constant. The horizontal velocity field was investigated for an interval of about 1 day. Flow around the largest barge (feature 6) occurs as a ring current. The vorticity inferred is about 2« times that of the ambient cyclonic zonal circulation, and about one-haft the value of the local planetary vorticity. Length and width variations appear to be associated with a nonzero horizontal divergence field. If the oscillations are a natural mode of the system, the 15-day period will provide an important datum for testing models of stable closed vortices.

The flow of salt glaciers is investigated. On the basis of data extracted from published sources, order of magnitude estimates (and probable upper and lower limits) for the stress and strain rate in a salt glacier in Iran are determined. The estimate of strain rate is much larger (by a factor of ∼108) than strain rates predicted by previously investigated flow mechanisms for halite (i.e., Nabarro‐Herring creep and dislocation climb) at the stress estimated. The observed stress and strain rate can better be explained as creep due to the diffusion of ions through an interstitial salt‐water solution.

Accurate measurements of ambient planetary and interplanetary magnetic fields using spacecraft magnetometers typically require accounting for interfering magnetic fields generated by the flight system (FS). The most common method for removing FS-generated time-variable magnetic fields is narrow-band and low-pass filtering of magnetic field data in the frequency domain. However, if fluctuations in the ambient field contain frequencies overlapping those in the FS field, it can be difficult to construct a filter that will not affect both signals. Here we present an alternate method for removing FS time-variable signatures from magnetic field measurements. For spacecraft that make use of a magnetic gradiometer (i.e. with two or more instruments on a boom at different distances from the center of the spacecraft), the dominant frequencies in the FS field can be identified using spectra of the differenced field components. The amplitudes of the FS field at those frequencies can then be suppressed without removing spectral peaks present in the ambient field. We demonstrate the successful application of this method, referred to as gradiometry peak suppression, both to modeled data sets and to 128 Hz Venus Express magnetometer data.

The objective of the Psyche Magnetometry Investigation is to test the hypothesis that asteroid (16) Psyche formed from the core of a differentiated planetesimal. To address this, the Psyche Magnetometer will measure the magnetic field around the asteroid to search for evidence of remanent magnetization. Paleomagnetic measurements of meteorites and dynamo theory indicate that a diversity of planetesimals once generated dynamo magnetic fields in their metallic cores. Likewise, the detection of a strong magnetic moment ($>2\times10^{14}~\text{Am}^{2}$ > 2 × 10 14 Am 2 ) at Psyche would likely indicate that the body once generated a core dynamo, implying that it formed by igneous differentiation. The Psyche Magnetometer consists of two three-axis fluxgate Sensor Units (SUs) mounted 0.7 m apart along a 2.15-m long boom and connected to two Electronics Units (EUs) located within the spacecraft bus. The Magnetometer samples at up to 50 Hz, has a range of $\pm80{,}000~\text{nT}$ ± 80 , 000 nT , and an instrument noise of $39~\text{pT}\,\text{axis}^{-1}\,3\sigma $ 39 pT axis − 1 3 σ integrated over 0.1 to 1 Hz. The two pairs of SUs and EUs provide redundancy and enable gradiometry measurements to suppress noise from flight system magnetic fields. The Magnetometer will be powered on soon after launch and acquire data for the full duration of the mission. The ground data system processes the Magnetometer measurements to obtain an estimate of Psyche’s dipole moment.