Appendix A1 (VHM/FGM/HED Experiment)

PI Comments/Explanations

The magnetic field parameters on the CDF tape consist of the X, Y, and Z components of the field in the spacecraft coordinate system. In this system, the Z direction is aligned with the spacecraft spin axis and is positive towards the Earth; the X direction is perpendicular to Z lying in the plane containing the Z direction and the spacecraft-sub line, positive towards the sun; and the Y direction then completes the right handed system.

On the plots distributed from JPL, the field magnitude, elevation angle, and azimuth angle are plotted with respect to the RTN coordinate system. The R direction is defined by the sun-spacecraft line, positive radially away from the sun. The N direction is perpendicular to R and lies in the plane defined by R and the solar rotation axis, positive northwards. The T direction completes the right handed system. The elevation angle (theta) is the angle between the magnetic field vector and its projection onto the R-T plane. A positive theta direction is northward with respect to the R-T plane. A negative theta direction is southward. The azimuth angle (phi) is the angle in the R-T plane between the projection of the field vector onto this plane and the R direction. The phi = 0 deg direction is away from the sun. The phi = +/180 deg direction is towards the sun. The azimuth angle increases in a right handed sense.

The field magnitude can be calculated from the components in the usual way. Conversion between spacecraft and RTN coordinates requires trajectory information from the SEDR tape. 

PI Caveats

A preliminary value for the magnetic field Z direction offset is used in processing the data input to the CDF tape. At very low field magnitudes, less than approximately 0.3nT, small errors (rarely more than 0.05nT) in the Z offset value can lead to relatively large errors in magnitudes and directions calculated from the field components.

Information supplied by:

Dr. R. J. Forsyth, Blackett Laboratory, Imperial College, London SW7 2BZ (UK)

Fax: +44 71 823 8250

E-mail: 19773::RJF

17 April 1993

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