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Vector Helium Magnetometer / Flux Gate Magnetometer

A fundamental feature of the heliosphere is the three dimensional structure of the interplanetary magnetic field. The simple, spherically symmetric and time-independent model of hydrodynamic expansion of the solar corona proposed originally by E.N. Parker in the early 1960's has provided a useful framework for describing interplanetary observations of the solar wind and of the interplanetary magnetic field. However, observations of the Sun and of the solar corona, as well as direct and indirect interplanetary measurements clearly indicate that there are very significant departures from both spherical symmetry and time independence.

The magnetic field investigation on Ulysses aims at determining the large scale features and gradients of the field, as well as the heliolatitude dependence of interplanetary phenomena so far only observed near the ecliptic plane.

The Ulysses magnetometer uses two sensors, one a Vector Helium Magnetometer (VHM), the other a Flux Gate Magnetometer (FGM) to measure the heliospheric magnetic field. The two magnetometers are located on the radial boom of the Ulysses spacecraft: the VHM sensor at the end of the 5m boom and the FGM sensor at 1.2m inboard from the VHM.

Both magnetometers are tri-axial sensors, but they use different physical principles to measure three orthogonal components of the magnetic field vector.

VHM - Vector Helium Magnetometer
The VHM was developed from instruments flown successfully on the Pioneers 10 and 11 missions and on the International Sun-Earth Explorer 3/International Cometary Explorer probe. The operating principle of the VHM is based on the effect an ambient magnetic field has on the efficiency with which a metastable population of He gas in the triplet ground state can be optically pumped.

Feedback currents provide highly linear measures of the three orthogonal components of the magnetic field vector in a reference frame defined with respect to the geometry of the sensor. The VHM has two operating ranges: +/-8.190 nT, equivalent to a sensitivity of 1.60 nT/V; and +/-65.52 nT, equivalent to 12.8 nT/V.

FGM - Flux Gate Magnetometer
The FGM sensor consists of three identical single axis ring-core fluxgate sensors, arranged in an orthogonal triad. The feedback signals to the three sensors represent in magnitude and sign the three components of the ambient magnetic field vector from the sensor.

Switchable feedback paths and output amplifier provide four ranges for the FGM measurements: +/-8 nT, +/-64 nT, +/-2.048 µT, and for pre-launch tests in the Earth's magnetic field, +/-44 µT. Range switching for the FGM is by ground command only.

Summary of Objectives

The prime scientific function of the instrument is to provide a time series of samples of the magnetic field vector along the trajectory of Ulysses. The types of questions which the Ulysses magnetic field observations will help answer are:

  • questions concerning the large scale structure, in particular to what extent the Parker model is modified by the effects of the mid-latitude and polar field structures during the descending phase of the solar cycle; this heading also includes the latitude gradients in the field
  • questions concerning dynamical features arising from different velocity solar wind streams interacting in interplanetary space
  • questions concerning waves, discontinuities and transient phenomena, in particular the latitudinal extent and propagation of shocks in the heliosphere

Finally, the Jupiter flyby needed for achieving the out of ecliptic orbit provided an opportunity to explore magnetic field features in the high latitude dusk regions of the Jovian magnetosphere.

In cooperation with other investigations, the magnetic field observations will also provide the framework for studies of energetic particle propagation in the high latitude heliosphere, in particular to address questions concerning the access and propagation of galactic cosmic rays.

Last Update: 08 December 2006

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