Azimuth Ambiguity Resolution Workshop
Introduction
Bruce W. Lites
High Altitude Observatory
National Center for Atmospheric Research
CSAC/HMI Workshop, Boulder, 26-27 September 2005
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
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26 September 2005
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
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
•…………
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?)
SDO/HMI - CSAC Azimuth Ambiguity Resolution Workshop Agenda
NCAR CG-1 Room 2126
26-27 September 2005
Monday 26 September 2005
SDO/HMI - CSAC Azimuth Ambiguity Resolution Workshop Agenda
Tuesday 27 September 2005
8:30
Coffee, Continental Breakfast Items
8:30
Coffee, Continental Breakfast Items
9:00
Welcome: Michael Knoelker, Director HAO
9:00
Comparison of Ambiguity Resolution Methods: Application to Observations
9:05
Introduction to the Workshop: Bruce Lites
10:30 Break
SUMMARIES OF AMBIGUITY RESOLUTION METHODS
9:15
Allen Gary: Ambiguity Resolution at the MSFC Magnetograph
9:35
K.D. Leka: Hawaii Iterative Method
9:55
Tom Metcalf: Simulated Anealing
10:50 Discussion: Ambiguity Resolution Methods for Automatic Processing
(Discussion Leader, Jack Harvey)
Which extant method(s) now seem best for continuous large data sets from the
likes of SOLIS/VSM, Solar-B, HMI?
12:20 - 13:30 Lunch in CG-1 Cafeteria
10:15 Break
10:40 Bruce Lites: The AZAM Utility
11:00 Manolis Georgoulis: Structure Minimization and Nonpotential Magnetic Field
Calculation (NPFC)
11:30 Alexei Pevstov (presented by K.S. Balasubramaniam): New Iterative Method
of the Azimuth Ambiguity Resolution
11:50 Additional presentations by teleconference, if necessary
13:40 Discussion: Possible New Avenues for Ambiguity Resolution
*(Discussion Leader, K.S. Balasubramaniam)
15:00 Break
15:20 Tom Metcalf: Workshop Summary and Final Discussion
16:00 Adjourn
12:30 - 13:30 Lunch in CG-1 Cafeteria
13:40 Discussion: How Do We Compare Ambiguity Resolution Methods?
14:20 Comparison of Ambiguity Resolution Methods: Model Data
15:30 Break
15:45 Comparison of Ambiguity Resolution Methods: Model Data (continued).
17:30 Adjourn
19:30 Dinner at Selected Boulder Restaurant
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.