February 17, 2017, 1:00 pm - 2:00 pm
February 17, 2017, 1:00 pm - 2:00 pm, Heliophysics Director's Seminar
Coronal Holes Through Time and Across Missions
Michael S. Kirk (GSFC/Catholic University of America)
Coronal Holes offer a unique measurement of open magnetic flux on the sun. They are largely defined by their lack of emitting plasma in extreme ultraviolet solar images. Coronal holes are stable structures that exist at all latitudes throughout the solar cycle. Holes that cap the northern and southern solar poles are the longest-lived features observed on the Sun - persisting for nearly an entire solar cycle. Their longevity combined with this solar activity relationship makes polar coronal holes an ideal proxy for measuring the long-term evolution of the solar magnetic field. We use a perimeter tracking technique to measure the size and location of the polar coronal holes for 20 years starting in 1996. Utilizing images from SOHO EIT, STEREO A & B EUVI, PROBA2 SWAP, and SDO AIA, we present a comprehensive look at how polar coronal holes evolve and what they can tell us about our current and unusual solar cycle.
The Search for Ephemeral Coronal Holes
W. Dean Pesnell (NASA.GSFC), with Rachel O'Connor (Smith College), Michael S. Kirk (GSFC/CUA), & Nishu Karna (Harvard-Smithsonian CFA)
Ephemeral coronal holes are short-lived, volatile counterparts to equatorial coronal holes. The first exemplar of this phenomenon observed by NASA's Solar Dynamics Observatory (SDO) was on October 26, 2010. This led to a more complete survey of SDO 211 Angstrom images on a 12-hour cadence between June 2010 and June 2016. Each compact and isolated dim region we encountered was flagged as a potential ephemeral coronal hole for further analysis. This preliminary effort resulted in 149 candidate holes. For further analysis of their characteristics, we applied a strict definition criterion of an ephemeral coronal hole: the candidates had to be dark relative to the surrounding material, not influenced by a nearby eruption, not obviously connected to other coronal hole structures, and their lifetime had to occur completely within the disk crossing. This criterion was designed so that events could be completely analyzed, from beginning to end, to better understand the origins. Application of this criterion eliminated all candidates but 5 of the original 149. True ephemeral coronal holes are rare occurrences, appearing only six times in six years. We will offer some evidence that EphCHs are a direct link to the supergranules lying below them.
Estimating Total Open Heliospheric Magnetic Flux using Models & Observed Coronal Holes
C. N. Arge (NASA/GSFC) with Marios Pattichis (University of New Mexico), R. Hock (AFRL/Space Vehicles Directorate), & C. J. Henney (AFRL/Space Vehicles Directorate)
There are extended periods over the solar cycle where significant discrepancies exist between the observed total open heliospheric magnetic flux (i.e., those based on spacecraft observations) and that determined from coronal models. The reason(s) for these discrepancies remain unresolved, though several explanations have been proposed. To narrow down the possibilities, we compute total open magnetic flux using three different methods and compare these results with those obtained using in-situ interplanetary magnetic field observations. The first two methods make use of the Potential Field Source Surface (PFSS) model to calculate the total open magnetic flux using as its input: 1) traditional Carrington or diachronic maps and 2) Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model synchronic (i.e., instantaneous) maps. The third method makes use of observationally derived Helium and Extreme UltraViolet (EUV) coronal hole maps overlain on the above mentioned magnetic field maps to directly compute total open magnetic flux. Except near solar maximum, the total open magnetic flux estimates determined from the observed coronal holes and the PFSS model show surprisingly good, long-term agreement. While not providing a definitive explanation as to why the coronal (i.e., observed coronal holes and PFSS model) and in-situ open flux estimates differ so significantly, our results may suggest that time-dependent effects and/or the methodology for estimating open flux from in-situ observations are potential sources of the problem.
