On spectral breaks in the power spectra of magnetic fluctuations in fast solar wind between 0.3 and 0.9 AU

Abstract

We analyze the radial variation of the power spectra of the magnetic field from 0.3 to about 0.9AU, using Helios 2 spacecraft measurements in the fast solar wind. The time resolution of the magnetic field data allows us to study the power spectra up to 2Hz. Generally, the corresponding spectral break frequency fb and the Doppler-shifted frequencies, which are related to the proton gyroradius and inertial scales, are close to a frequency f of about 0.5Hz at a distance of 1AU from the Sun. However, studying the radial evolution of the power spectra offers us the possibility to distinguish between those scales. Recent Ulysses observations show that, while the proton scales vary, fb stays nearly constant with the heliocentric distance R. In our study we confirm that fb varies within a small interval of [0.2, 0.4]Hz only, as R varies from 0.3 to 0.9AU. Moreover, if we assume parallel propagating fluctuations (with respect to the solar wind flow or background magnetic field), we can show that none of the proton scales are coincident with the break scale. If, however, we take into account the two-dimensional nature of the turbulent fluctuations, then we can show that the spatial scale corresponding to fb (R) does follow the proton inertial scale, λp(R), but not the proton gyroradius scale, ρp(R), as a function of heliocentric distance. These observations indicate that the spectral break at the proton inertial scale might be related to the Hall effect, or be controlled by the ion-cyclotron damping of obliquely propagating fluctuations or the formation of current sheets scaling like λp, which could be responsible for ion heating through magnetic reconnection. © 2012 The American Astronomical Society. All rights reserved.