Whistler waves in the inner heliosphere from 0.3 to 1 AU

The Helios mission (1974-1986), which aimed at probing the inner Heliosphere, provides an unique opportunity to study in-situ wave characteristics in solar wind as close as 0.3 AU. Using the search-coil magnetometer, which measures AC magnetic field spectra between 7 Hz and 680 Hz, we find that some of these spectra reveal bursty wave packets at a fraction of local electron cyclotron frequency, which we identify as whistler waves. For the first time we show the presence of narrow band whistler waves in inner heliosphere from 0.3 to 1 AU.

We have studied the radial evolution of observed whistlers for different physical parameters such as solar wind type and features of associated electron distribution functions. We find that most of the whistlers are observed in the slow (<500 km/s) solar wind almost irrespective of the radial distance from the Sun. It is worth noting that whistler wave activity is significantly weaker in the fast (>600 km/s) solar wind up to distances at about 0.8 AU, increasing at larger radial distances. This observed radial behavior seems to be related to the radial dependence of temperature anisotropy of halo, which determines the whistler temperature anisotropy instability (WTA) condition. We find evidence for WTA as a possible source of whistler generation in solar wind. These results allow us to anticipate what Parker Solar Probe will observe.