The Radial Evolution of Magnetic Clouds From Helios to Ulysses

Abstract

The evolution of a magnetic cloud (MC) from the inner heliosphere to the outer heliosphere has been investigated for decades. Although many studies have reported on the evolution of MCs, there is no relevant statistical study about the continuous parametric evolution of the flux rope model of the Gold–Hoyle solution for MCs from near the Sun to 5.4 au. Based on the velocity-modified uniform-twist force-free flux rope model, in this study we explore the evolution with heliodistance for some parameters from 139 MCs observed by the Helios, Wind, and Ulysses spacecraft. We find a negative/positive correlation between the central axial field strength/the radius of the cross section and the heliodistance. The angle between the axis of the MC and the Sun–spacecraft line (Θ), the expansion velocity (v e ), and the poloidal velocity (v p ) did not show any evident tendency to increase or decrease with the heliodistance. In addition, the number of turns of the magnetic field lines per unit length winding around the magnetic flux rope, τ, shows a weak decrease with heliodistance. Also, there is an evident negative correlation between τ and the radius of the flux rope, R. The axial magnetic flux (F z ) and the magnetic helicity (H m ) show a tendency to decrease within 1 au, after which they remain almost unchanged until 5.5 au. Furthermore, we do not find any evident difference in the parametric properties of MCs on and outside the ecliptic.