Magnetic clouds: Voyager observations between 2 and 4 AU

Magnetic clouds were observed in the solar wind between 2–4 AU by Voyagers 1 and 2, indicating that they are stable enough to persist without major changes out to such distances. The average size in radial extent of the clouds observed at these distances was 0.47 AU, compared to 0.25 for clouds observed at 1 AU. Assuming that these numbers are representative, we estimate that the clouds were expanding at a speed of the order of 45 km s^-1. This is consistent with the expansion speed derived from the difference of the speeds of the front and rear boundaries of the clouds, 33 km s^-1. The average Alfvén speed at the front and rear boundaries was 104 km s^-1, so our estimated expansion speed is nearly half of the Alfvén speed, consistent with an earlier estimate of the expansion speed of clouds between the Sun and 1 AU. The magnetic field configuration cannot be determined uniquely, but it is highly ordered and consistent with the passage of some kind of loop. The simple model of a magnetic tongue with magnetic field lines in planes, e.g., meridian planes, is not consistent with the data.