Radial evolution of the power spectral density of the magnetic field from 0.3 to 0.9 AU

The power spectra of the magnetic field from 0.29 to about 0.9 AU using Helios 2 spacecraft measurements is analyzed. It is known that, at distance of 1 AU from the Sun, the scale corresponding to the break frequency f_b and the Doppler- shifted proton scales are all close to f ∼ 0.5Hz. However, the radial evolution of the power spectra studied here gives the possibility to distinguish between those scales. In this study we show that f_b varies in a tiny interval [0.2,0.4] along the distance R. Also, when we assume parallel propagating (with respect to the solar wind flow or to the background magnetic field) fluctuations we find that none of the proton scales is following the spectral break scale. However, taking into account the 2-dimensional nature of turbulent fluctuations, we show that scale corresponding to f_b(R) follows λ _p(R) and not the f_cp(R) or ρ _p(R) along the heliocentric distance. These observations may indicate that the spectral break at the high frequency range might be controlled by the Hall effect or by the onset of the ion dissipation due to 2D magnetic reconnection near the ion skin depth scale.