POSSIBLE EVIDENCE OF ALFVEN-CYCLOTRON WAVES IN THE ANGLE DISTRIBUTION OF MAGNETIC HELICITY OF SOLAR WIND TURBULENCE

The fluctuating magnetic helicity is considered an important parameter in diagnosing the characteristic modes of solar wind turbulence. Among them is the Alfven-cyclotron wave, which is probably responsible for the solar wind plasma heating, but has not yet been identified from the magnetic helicity of solar wind turbulence. Here, we present the possible signatures of Alfven-cyclotron waves in the distribution of magnetic helicity as a function of theta(VB), which is the angle between the solar wind velocity and local mean magnetic field. We use magnetic field data from the STEREO spacecraft to calculate the theta(VB) distribution of the normalized reduced fluctuating magnetic helicity sigma(m). We find a dominant negative sigma(m) for 1 s < p < 4 s (p is time period) and for theta(VB) < 30 degrees in the solar wind outward magnetic sector, and a dominant positive sigma(m) for 0.4 s < p < 4 s and for theta(VB) > 150 degrees in the solar wind inward magnetic sector. These features of sigma(m) appearing around the Doppler-shifted ion-cyclotron frequencies may be consistent with the existence of Alfven-cyclotron waves among the outward propagating fluctuations. Moreover, right-handed polarized waves at larger propagation angles, which might be kinetic Alfven-waves or whistler waves, have also been identified on the basis of the sigma(m) features in the angular range 40 degrees < theta(VB) < 140 degrees. Our findings suggest that Alfven-cyclotron waves (together with other wave modes) play a prominent role in turbulence cascading and plasma heating of the solar wind.