High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR) The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation. An Equal Opportunity/Affirmative Action Employer. Azimuth Ambiguity Resolution Workshop Introduction Bruce W. Lites High Altitude Observatory National Center for Atmospheric Research CSAC/HMI Workshop, Boulder, September September 2005

Azimuth Ambiguity Fundamentals Bruce Lites, CSAC/HMI Ambiguity Resolution Workshop, September 2005 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).

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 ………… Why We Concern Ourselves with the Azimuth Ambiguity

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, …..)

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?)

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?)

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

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.