Space Physics Seminar spring-2014

The heliopause (HP)is the boundary that separates the plasma environment of the sun from that of the interstellar medium. I will discuss the recent Voyager spacecraft observations of this outer boundary and parallel theory and modeling efforts that suggest that the boundary is much more complex than a simple magnetic field rotation that separates two disparate plasma regions. Dropouts of energetic particles produced within the heliosphere and corresponding increases in the strength of the magnetic field measured by Voyager 1 in 2012 suggested that the spacecraft had crossed the HP. However, the absence of a corresponding rotation in the direction of the magnetic field convinced the Voyager science team that the spacecraft remained within the heliosphere. A parallel modeling effort of the global heliosphere and local processes at the HP suggested that Voyager 1 had crossed the HP. This conclusion was confirmed when wave measurements revealed that the plasma density at Voyager 1 had dramatically increased above heliospheric values. The present picture of the magnetic structure of the outer heliosphere and HP is that the large-scale magnetic field rotates gradually from its interstellar direction to that of the Parker spiral magnetic field at the HP. Locally at the HP magnetic reconnection has produced a complex set of nested magnetic islands leading to a porous HP in which galactic cosmic rays enter the heliosphere and heliospheric particles can exit into the local interstellar medium. However, not everyone is in agreement. There is a significant contingent who are convinced that Voyager 1 remains within the heliosphere. The reasons for the continued controversy will be discussed.