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Solar Wind Observations Over the Poles of the Sun

The solar wind plasma experiment on Ulysses accurately characterizes the bulk flow and internal state conditions of the interplanetary plasma in three dimensions as a function of solar latitude.

Solar wind electrons and ions are measured simultaneously with two completely separate instruments. Both instruments make use of a curved-plate electrostatic analyzer equipped with multiple Channel Electron Multipliers (CEMs). The CEMs are arranged to detect particles at chosen polar angles from the spacecraft spin axis; resolution in spacecraft azimuth is obtained by timing measurements with the spacecraft Sun clock as the spacecraft spins.

Electron Analyzer Experiment
Electrons with central energies extending from 0.86 eV to 814 eV are detected at seven polar angles and various combinations of azimuth angle to cover the unit sphere comprehensively, so as to enable computation of the pertinent electron velocity distribution parameters. As the average electron flux level changes with heliocentric distance, command control of the CEM counting intervals is used to extend the dynamic range.

Ion Analyzer Experiment
Ions are detected between 255 eV/q and 34.4 keV/q using appropriate subsets of 16 CEMs at spin angles designed to provide matrices of counts as a function of energy per charge, azimuth angle, and polar angle centred on the average direction of solar-wind flow. Data matrices are obtained every 4 minutes when the spacecraft is actively transmitting and every 8 minutes during data store periods. These matrices contain sufficient energy and angle resolution to permit a detailed characterization of the ion velocity distributions, from which ion bulk parameters are derived.

As the average ion flux intensity changes with heliocentric distance, the entrance aperture size is periodically optimized by command selection from a set of seven apertures on a disk driven by a stepping motor. Changes in the average solar wind flow direction relative to the Earth-pointing spacecraft spin axis are accommodated by command selection of the proper measurement matrix from a set of 11 matrices. In a separate mode of operation and under favourable conditions, heavy ions of oxygen, silicon, and iron at various charge levels are resolved.

Summary of Objectives

The primary objective of the Ulysses solar wind plasma investigation is to investigate and establish bulk flow parameters and internal state conditions of the solar wind as a function of solar latitude. Further important goals include studies of radial gradients of solar wind properties between Earth and Jupiter and investigations of the solar wind interaction with the Jovian magnetosphere. The important goals of SWOOPS are:

  • Determine systematic variations in the solar wind bulk flow with solar latitude
  • Examine physical processes important for driving the coronal expansion
  • Investigate variations in the evolution of high speed streams with latitude
  • Investigate changes in the nature of transient disturbances with latitude
  • Determine latitudinal variations in the relative abundance and charge state composition of solar wind minor ions
  • Search for and identify plasma mechanisms regulating the electron heat flux ion beam relative flow velocities, and the He++ to H+ ion temperature ratio
  • Survey latitudinal variations in Alfvén wave amplitudes and the relative numbers of tangential and rotational discontinuities
  • Determine coronal hole temperatures at which high speed stream solar wind Fe ionization states are established
  • Estimate the electrostatic potential and electron collision length dependence on latitude
  • Identify local heating mechanisms of solar wind ions
  • Study the interaction of interstellar neutrals with the solar wind
  • Study large amplitude hydromagnetic waves
  • Evaluate radial and meridional gradients in solar wind electron temperature and anisotropy

Last Update: 08 December 2006

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