Propagation and evolution of a magnetic cloud from ACE to Ulysses

Abstract

The chance of a magnetic cloud (MC) being observed by widely separated spacecraft is rare. However, such an event provides us a good opportunity to study the propagation and evolution of magnetic cloud in the interplanetary space. A magnetic cloud event observed by ACE at Earth on 4–6 March 1998 is tracked from the location of ACE to Ulysses. Using a one‐dimensional (1‐D) MHD solar wind model, we propagate the ACE data to the location of Ulysses and confirm that the two magnetic clouds observed by both spacecraft have the same solar origin. The Grad‐Shafranov (GS) reconstruction technique is then employed to recover the 2‐D cross sections of the magnetic clouds at locations of ACE and Ulysses, respectively. The magnetic clouds observed at ACE and Ulysses both show magnetic flux rope configurations of the same chirality and unidirectional axial magnetic field along approximately the same direction. It is found that the magnetic cloud is expanding while propagating outward. Their relevant toroidal (axial) and poloidal magnetic flux contained within each flux rope are different but approximately of the same order of magnitude. However, the relative magnetic helicity contained in each flux rope differs significantly. We discuss the causes of such differences, taking into account the underestimate of the area of the flux rope at Ulysses and the effect of magnetic cloud evolution that may not be fully addressed by 2‐D models.