DISCRETE CHANGES IN SOLAR CORONAL HOLE BOUNDARIES

Solar coronal holes are large-scale regions of open magnetic fields that are easily observed in soft X-ray images. Previous studies by Nolte et al. using Skylab X-ray images established that large-scale (≥9 × 104 km) changes in coronal hole boundaries were due to coronal processes, i.e., magnetic reconnection, and not to photospheric diffusive motions. However, the low time resolution (1 day) of their data did not allow them to deduce the size and time scales of discrete boundary changes. We have used sequences of Skylab X-ray images near central meridian passages of a large coronal hole to examine hole boundary changes with a time resolution of 90 minutes. We find that X-ray bright points play a key role in both the expansion and contraction of the coronal hole. The size scale of the boundary changes is ∼2 × 104 km, and the time scale is ≤3 hr. We suggest a scenario in which a large-scale closed magnetic field interacts with a bright point to effect a change in the hole boundary.