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Long-Term Measurements of Sunspot Magnetic Tilt Angles  

Jing Li   Submitted: 2012-10-06 18:05

Tilt angles of close to 30,600 sunspots are determined using Mount Wilson daily averaged magnetograms taken from 1974 to 2012, and MDI/SoHO magnetograms taken from 1996 to 2010. Within a cycle, more than 90% of sunspots have a normal polarity alignment along the east-west direction following Hale's law. The median tilts increase with increasing latitude (Joy's law) at a rate of ~0.5 degree per degree of latitude. Tilt angles appear largely invariant with time at a given latitude, but they decrease by ~0.9 degree per year on average, a trend which largely reflects Joy's law following the butterfly diagram. We find an asymmetry between the hemispheres in the mean tilt angles. On average, the tilts are greater in the southern than in the northern hemisphere for all latitudinal zones, and the differences increase with increasing latitude.

Authors: Jing Li & Roger Ulrich
Projects: None

Publication Status: ApJ. 758, 115 2012 October 20
Last Modified: 2012-10-08 19:21
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Active Longitudes Revealed by Large-scale and Long-lived Coronal Streamers  

Jing Li   Submitted: 2011-05-04 14:55

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Jing Li
Projects: STEREO

Publication Status: ApJ June 10, 2011, v734 - 1 issue
Last Modified: 2011-05-04 17:19
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Limitations of Absolute Current Densities Derived from the Semel & Skumanich Method  

Jing Li   Submitted: 2009-06-24 15:41

Semel and Skumanich proposed a method to obtain the absolute electric current density, |J_z|, without disambiguation of 180 degree in the transverse field directions. The advantage of the method is that the uncertainty in the determination of the ambiguity in the magnetic azimuth is removed. Here, we investigate the limits of the calculation when applied to a numerical MHD model. We found that the combination of changes in the magnetic azimuth with vanishing horizontal field component leads to errors, where electric current densities are often strong. Where errors occur, the calculation gives |J_z| too small by factors typically 1.2~2.0.

Authors: Jing Li, Yuhong Fan
Projects: None

Publication Status: October 2009 on Science in China Series G: Physics, Mechanics & Astronomy, special issue of ?International Workshop of 2008 Solar Total Eclipse?
Last Modified: 2009-06-25 13:25
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Vector Magnetic Fields and Electric Currents from the Imaging Vector Magnetograph  

Jing Li   Submitted: 2008-11-10 13:06

First, we describe a general procedure to produce high quality vector magnetograms using the Imaging Vector Magnetograph (IVM) at Mees Solar Observatory. Two IVM effects are newly discussed and taken into account: (1) the central wavelength of the Fabry-Perot is found to drift with time as a result of undiagnosed thermal or mechanical instabilities in the instrument; (2) the Stokes V-sign convention built into the IVM is found to be opposite to the conventional definition used in the study of radiative transfer of polarized radiation. At the spatial resolution 2x2, the Stokes Q,U,V uncertainty reaches ~0.001-0.0005 in time-averaged data over 1-hour in the quiet Sun. When vector magnetic fields are inferred from the time-averaged Stokes spectral images of FeI 6302.5, the resulting uncertainties are on the order of 10 G for the longitudinal fields, 40 G for the transverse field strength and 9 degree for the magnetic azimuth. The magnetic field inversion used in this work is the ``Triplet'' code, which was developed and implemented in the IVM software package by the late Barry J. LaBonte. The inversion code is described in detail in the Appendix. Second, we solve for the absolute value of the vertical electric current density, |J_z|, accounting for the above IVM problems, for two different active regions. One is a single sunspot region (NOAA 10001 observed on 20 June 2002) while the other is a more complex, quadrupolar region (NOAA10030 observed on 15 July 2002). We use a calculation that does not require disambiguation of 180n degree in the transverse field directions. The |J_z| uncertainty is on the order of ~7.0 mA m^-2. The vertical current density increases with increasing vertical magnetic field. The rate of increase is about 1-2 times as large in the quadrupolar NOAA 10030 region as in the simple NOAA 10001, and it is more spatially variable over NOAA 10030 than over NOAA 10001.

Authors: Li, J., van Ballegooijen, A.A., and Mickey, D.
Projects: None

Publication Status: ApJ February 2009 - 20 v692 issue
Last Modified: 2008-11-10 14:13
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Abstracts by Author
Long-Term Measurements of Sunspot Magnetic Tilt Angles
Active Longitudes Revealed by Large-scale and Long-lived Coronal Streamers
Limitations of Absolute Current Densities Derived from the Semel & Skumanich Method
Vector Magnetic Fields and Electric Currents from the Imaging Vector Magnetograph

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