First VIKING results: high frequency waves

The high frequency wave experiment onboard the Viking satellite measures electric and magnetic fields and the plasma frequency in the range 4 to 700 kHz. The magnetic field sensor is a loop antenna with its axis parallel to the satellite spin axis, the electric field sensor is one of the radial boom dipoles also used for DC and low frequencies. The sensitivity is 10⁻¹⁴ Tesla/√Hz for magnetic fields, 5 × 10⁻⁸V/m√Hz for electric fields. The plasma frequency is measured by wave activation and by impedance measurements of the plasma. The frequency range available corresponds to plasma densities from 1 to 3000 per cubic cm. Data from the initial operating period show frequent wave activity. In approximately half of the passes hiss and auroral kilometric radiation is observed in the auroral region in the evening side and sometimes also in the morning side. The radiation shows strong variations within a few seconds in spectral shape, and is often clearly limited by cutoffs in frequency. Very low plasma frequencies are often registered, corresponding to a few electrons per cubic cm at 1 to 2 Earth radii above the polar region.