TY - JOUR

T1 - A new method for resolving the 180° ambiguity in solar vector magnetograms

AU - Moon, Y. J.

AU - Wang, Haimin

AU - Spirock, Thomas J.

AU - Goode, P. R.

AU - Park, Y. D.

N1 - Funding Information:
We greatly appreciate the referee’s constructive comments. We are very thankful to the BBSO observing staff for obtaining the data. We thank Dr J. Qiu and Dr T. Met-calf for providing us with data analysis programs. This work has been supported by NASA grants NAG5-10894, NAG5-7837, and NAG 5-12782, by NSF-ATM-0086999, by a MURI grant of AFOSR, by the US-Korea Cooperative Science Program (NSF INT-98-16267), and by a NRL grant M10104000059-01J000002500 of the Korean government.

PY - 2003/10

Y1 - 2003/10

N2 - We present a new method to resolve the 180° ambiguity for solar vector magnetogram measurements. The basic assumption is that the magnetic shear angle (Δθ), which is defined as the difference between the azimuth components of observed and potential fields, approximately follows a normal distribution. The new method is composed of three steps. First, we apply the potential field method to determine the azimuthal components of the observed magnetic fields. Second, we resolve the ambiguity with a new criterion: -90° + Δθmp ≤ Δθ ≤ 90° + Δθmp, where Δθmp is the most probable value of magnetic shear angle from its number distribution. Finally, to remove some localized field discontinuities, we use the criterion Bt · Bmt ≥ 0, where Bt and Bmt are an observed transverse field and its mean value for a small surrounding region, respectively. For an illustration, we have applied the new ambiguity removal method (Uniform Shear Method) to a vector magnetogram which covers a highly sheared region near the polarity inversion line of NOAA AR 0039. As a result, we have found that the new ambiguity solution was successful and removed spatial discontinuities in the transverse vector fields produced in the magnetogram by the potential field method. It is also found that our solution to the ambiguity gives nearly the same results, for highly sheared vector magnetograms and vertical current density distributions, of NOAA AR 5747 and AR 6233 as those of other methods. The validity of the basic assumption for an approximate normal distribution is demonstrated by the number distributions of magnetic shear angle for the three active regions under consideration.

AB - We present a new method to resolve the 180° ambiguity for solar vector magnetogram measurements. The basic assumption is that the magnetic shear angle (Δθ), which is defined as the difference between the azimuth components of observed and potential fields, approximately follows a normal distribution. The new method is composed of three steps. First, we apply the potential field method to determine the azimuthal components of the observed magnetic fields. Second, we resolve the ambiguity with a new criterion: -90° + Δθmp ≤ Δθ ≤ 90° + Δθmp, where Δθmp is the most probable value of magnetic shear angle from its number distribution. Finally, to remove some localized field discontinuities, we use the criterion Bt · Bmt ≥ 0, where Bt and Bmt are an observed transverse field and its mean value for a small surrounding region, respectively. For an illustration, we have applied the new ambiguity removal method (Uniform Shear Method) to a vector magnetogram which covers a highly sheared region near the polarity inversion line of NOAA AR 0039. As a result, we have found that the new ambiguity solution was successful and removed spatial discontinuities in the transverse vector fields produced in the magnetogram by the potential field method. It is also found that our solution to the ambiguity gives nearly the same results, for highly sheared vector magnetograms and vertical current density distributions, of NOAA AR 5747 and AR 6233 as those of other methods. The validity of the basic assumption for an approximate normal distribution is demonstrated by the number distributions of magnetic shear angle for the three active regions under consideration.

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U2 - 10.1023/A:1027365413021

DO - 10.1023/A:1027365413021

M3 - Article

AN - SCOPUS:1842529705

SN - 0038-0938

VL - 217

SP - 79

EP - 94

JO - Solar Physics

JF - Solar Physics

IS - 1

ER -