Azimuth Ambiguity Resolution Workshop Introduction

1. Azimuth Ambiguity Resolution WorkshopIntroduction Bruce W. Lites High Altitude Observatory National Center for Atmospheric Research CSAC/HMI Workshop, Boulder, 26-27 September 2005 26 September 2005

2. Azimuth Ambiguity Fundamentals Zeeman Effect produces linear polarization for a transverse magnetic field: Note the degeneracy of the polarization wrt the sense of the transverse field (up or down in this case). Bruce Lites, CSAC/HMI Ambiguity Resolution Workshop, September 2005

3. Why We Concern Ourselves with the Azimuth Ambiguity • The “local solar frame” (one axis normal to the solar surface) is the preferred reference frame for physical interpretation • For fields measured away from disk center, the ambiguity of the azimuth in the observer’s frame affects both the local frame azimuth and inclination to the vertical • The azimuth ambiguity must be resolved for many problems that depend on vector field measurements: • Extrapolation of fields to higher layers • Flux emergence and decay • Active region evolution • …………

4. Ambiguity resolution necessitates adding more physical information to that provided by the Zeeman Effect alone. • Additional information on the physical system: • Field connectivity (H, X-ray, EUV, ….) • Solenoidal condition B = 0 • Scattering polarization (in chromospheric lines) • ….Others (add to the list at this workshop!) • Ad-hoc physical assumptions: • Closest approach to potential field • Force-Free field assumption (currents parallel to B) • Minimize vertical currents (spatial continuity) • Assumptions on physical structure of field topology (e.g., plage fields usually close to vertical, orientation gradients at location of strong field gradients, …..)

5. Ambiguity Resolution – “There Is No Magic Bullet” • As of yet, none of the class of additional physical information provides us with a reliable ambiguity resolution • Some ad-hoc physical assumptions do better, but still fail in particular circumstances • Ad-hoc assumptions can be patently wrong (e.g., the assumption of a force-free field in photosphere is often incorrect) • Data quality and analysis techniques strongly affect the local frame azimuth, hence the ambiguity resolution • Manual (human interaction) adjustment is sometimes required for any extant method (what are the assumptions involved?)

6. Workshop Goals (Personal View) • Further our collective understanding of the methods that are available • Examine the strengths and weaknesses of the methods • Compare, to the extent possible, the accuracy and usefulness of available methods • Assess the utility of various methods for bulk, automatic processing of data from HMI, Solar-B, SOLIS, ….. • Discuss possibilities for future advances • Recommend a course for further study (Another workshop? Working groups?)

7. Note: Agenda purposefully left open to allow ample time for discussions and comparisons.

9. Some Logistical Information: • Proceedings: We will post all presentations and summaries on the web. Please give your presentations to Steve Tomczyk. • Money: Those of you being reimbursed from NCAR for travel expenses, please see Louise Beierle at this morning’s coffee break. She will have travel vouchers, etc. for you to sign. • Dinner tonight: We will break into smaller groups for dinner at various restaurants. We will meet in front of the Residence Inn at 19:00 to arrange transportation and select restaurants.