Properties of whistler mode waves between 0.3 and 1.0 AU from Helios observations

Abstract

A study of magnetic fluctuations in the solar wind in the distance range from 0.3 to 1 AU and the frequency range 4.7 Hz‐2.2 kHz by the Technical University of Braunschweig search‐coil magnetometer experiment onboard Helios 1 and 2 during the time intervals from December 1974 to April 1975 for Helios 1 and January 1976 to May 1976 for Helios 2 yields the following results: (1) Near 1 AU electromagnetic wave modes are restricted to frequencies less than 220 Hz for 99% of the time. Inside 0.5 AU the waves are restricted to less than 470 Hz in 94% of all cases. (2) For a spectral density [P(f)]1/2; ∝ fα with the quantity √P measured in nT/√Hz the spectral index α varies between −1.4 and −2.0 with a most probable value of −1.7 and a slight tendency toward steeper spectra towards the sun. (3) No clear correlation between simple plasma parameters and magnetic field magnitude F on one side and spectral densities on the other side is found. The proportionality between spectral density and F is simply attributed to the radial variation. (4) High wave intensities occur in the vicinity of stream interfaces with a decrease in wave intensity with increasing distance from the stream interaction region. (5) Magnetic field energy density above 4.7 Hz varies between 5 × 10−18 erg/cm³ and 10−14 erg/cm³ with a maximum of occurrence of 5 × 10−17 erg/cm³ near 1 AU and 1.6 × 10−15 erg/cm³ near 0.3 AU. The values are therefore orders of magnitude below the total magnetic field energy density. (6) The fluctuations are tentatively interpreted as locally generated whistler waves.