The longitudinal evolution of equatorial coronal holes

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Thursday, December 28, 2017

In 2011, three satellites—the Solar-Terrestrial RElations Observatory (STEREO) A & B, and the Solar Dynamics Observatory (SDO)—were in a unique spatial alignment that allowed a 360 degree view of the Sun. This alignment lasted until 2014, the peak of solar cycle 24.

total daily (black) and hemispheric sunspot numbers image
Panel A shows the evolution of the total daily (black) and hemispheric sunspot numbers over the past 22 years (northern hemisphere - red; southern hemisphere - blue) from the Solar Influences Data Center (SIDC) where the epoch studied herein is bracketed by vertical dashed black lines. The profound hemispheric asymmetry of solar cycle 24 is clearly visible. Panel B shows the derived CH latitude-time (or “butterfly) diagram, for the 2011 to 2015 epoch where we have global coverage of the EUV corona. The thick dashed red and blue lines indicate the time of sunspot maxima in the northern and southern hemispheres respectively. Panel B shows CH feature locations in heliographic latitude over the observation period. There are 190 CH features represented. Each is drawn as a straight line at the latitude at which it appeared, and for the duration for which it was detected. The associated lines are color-scaled to indicate the CH lifetimes, up to 250 days.

Using extreme ultraviolet images and Hovmoller 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 65 degree 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 55 degrees, and display velocities of ~55 m/s slower than the local differential rotation rate. A second, smaller population of CHs rotate prograde, with velocities between ~20-45 m/s. This population is also long-lived, but observed +/-10 degrees from the solar equator. A third population of CHs are short-lived (less than two solar rotations), appear over a wide range of latitudes (+/-65 degrees) and exhibit velocities between -140 to 80m/s. 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/STEREO/SDO archives is necessary to understand the contribution of CHs to the decadal-scale evolution of the Sun.

Publication Name: Astrophysical Journal Letters

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