The Longitudinal Evolution of Equatorial Coronal Holes

In 2011, three satellites - the Solar-Terrestrial RElations Observatory A & B, and the Solar Dynamics Observatory (SDO) - were in a unique spatial alignment that allowed a 360° view of the Sun. This alignment lasted until 2014, the peak of solar cycle 24. Using extreme ultraviolet images and Hovmöller diagrams, we studied the lifetimes and propagation characteristics of coronal holes (CHs) in longitude over several solar rotations. Our initial results show at least three distinct populations of "low-latitude" or "equatorial" CHs (below latitude). One population rotates in retrograde direction and coincides with a group of long-lived (over sixty days) CHs in each hemisphere. These are typically located between 30° and , and display velocities of ∼55 m s^-1 slower than the local differential rotation rate. A second, smaller population of CHs rotate prograde, with velocities between ∼20 and 45 m s^-1. This population is also long-lived, but observed ±10° from the solar equator. A third population of CHs are short-lived (less than two solar rotations), and they appear over a wide range of latitudes (±65°) and exhibit velocities between -140 and 80 m s^-1. The CH "butterfly diagram" we developed shows a systematic evolution of the longer-lived holes; however, the sample is too short in time to draw conclusions about possible connections to dynamo-related phenomena. An extension of the present work to the 22 years of the combined SOHO-SDO archives is necessary to understand the contribution of CHs to the decadal-scale evolution of the Sun.