Observation of the multifractal spectrum in the heliosphere and the heliosheath by Voyager 1 and 2

摘要

We present first results of the multifractal scaling of the fluctuations of the interplanetary magnetic field strength as measured onboard Voyager 2 in the very distant heliosphere and even in the heliosheath. More specifically, the spectra observed by Voyager 2 in a wide range of heliospheric distances from 6 to 90astronomical units (AU) are compared with those of Voyager 1 already analyzed between 7 and 107AU. We focus on the singularity multifractal spectrum before and after crossing the termination heliospheric shock by Voyager 1 at 94 AU and Voyager 2 at 84AU from the Sun. It is worth noting that the spectrum is prevalently right-skewed inside the whole heliosphere. Moreover, we have observed a change of the asymmetry of the spectrum at the termination shock, where the spectrum changes from (left-) right-skewed in the very distant heliosphere to the (right-) left-skewed or possibly symmetric spectrum in the heliosheath. We confirm that the degree of multifractality falls steadily with the distance from the Sun. In addition, the multifractal structure is apparently modulated by the solar activity, with a time shift of several years, corresponding to a distance of about 10AU, resulting from the evolution of the whole heliosphere. Hence this basic result also brings significant additional support to some earlier claims suggesting that the solar wind termination shock is asymmetric.