On the Rotation of the Magnetic Field Across the Heliopause

Based on the difference between the orientation of the interstellar and the solar magnetic fields, there was an expectation by the community that the magnetic field direction will rotate dramatically across the heliopause (HP). Recently, the Voyager team concluded that Voyager 1 ( V 1) crossed into interstellar space last year. The question is then why there was no significant rotation in the direction of the magnetic field across the HP. Here we present simulations that reveal that strong rotations in the direction of the magnetic field at the HP at the location of V 1 (and Voyager 2) are not expected. The solar magnetic field strongly affects the drapping of the interstellar magnetic field ( B ISM ) around the HP. B ISM twists as it approaches the HP and acquires a strong T component (East–West). The strong increase in the T component occurs where the interstellar flow stagnates in front of the HP. At this same location the N component B N is significantly reduced. Above and below, the neighboring B ISM lines also twist into the T direction. This behavior occurs for a wide range of orientations of B ISM . The angle δ = a sin ( B N / B ) is small (around 10°–20°), as seen in the observations. Only after some significant distance outside the HP is the direction of the interstellar field distinguishably different from that of the Parker spiral.