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Title: ACTIVE-REGION TILT ANGLES: MAGNETIC VERSUS WHITE-LIGHT DETERMINATIONS OF JOY'S LAW

Abstract

The axes of solar active regions are inclined relative to the east-west direction, with the tilt angle tending to increase with latitude ({sup J}oy's law{sup )}. Observational determinations of Joy's law have been based either on white-light images of sunspot groups or on magnetograms, where the latter have the advantage of measuring directly the physically relevant quantity (the photospheric field), but the disadvantage of having been recorded routinely only since the mid-1960s. White-light studies employing the historical Mount Wilson (MW) database have yielded tilt angles that are smaller and that increase less steeply with latitude than those obtained from magnetic data. We confirm this effect by comparing sunspot-group tilt angles from the Debrecen Photoheliographic Database with measurements made by Li and Ulrich using MW magnetograms taken during cycles 21-23. Whether white-light or magnetic data are employed, the median tilt angles significantly exceed the mean values, and provide a better characterization of the observed distributions. The discrepancy between the white-light and magnetic results is found to have two main sources. First, a substantial fraction of the white-light ''tilt angles'' refer to sunspots of the same polarity. Of greater physical significance is that the magnetograph measurements include the contribution of plage areas,more » which are invisible in white-light images but tend to have greater axial inclinations than the adjacent sunspots. Given the large uncertainties inherent in both the white-light and the magnetic measurements, it remains unclear whether any systematic relationship exists between tilt angle and cycle amplitude during cycles 16-23.« less

Authors:
;  [1];  [2];  [3]
  1. Space Science Division, Naval Research Laboratory, Washington, DC 20375 (United States)
  2. Heliophysical Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 4010 Debrecen (Hungary)
  3. Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles, CA 90095 (United States)
Publication Date:
OSTI Identifier:
22364697
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 798; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; FACULAE; IMAGES; INCLINATION; MAGNETIC FIELDS; PHOTOSPHERE; PLAGES; SUN; SUNSPOTS; VISIBLE RADIATION

Citation Formats

Wang, Y.-M., Colaninno, R. C., Baranyi, T., and Li, J., E-mail: yi.wang@nrl.navy.mil, E-mail: robin.colaninno@nrl.navy.mil, E-mail: baranyi@tigris.unideb.hu, E-mail: jli@igpp.ucla.edu. ACTIVE-REGION TILT ANGLES: MAGNETIC VERSUS WHITE-LIGHT DETERMINATIONS OF JOY'S LAW. United States: N. p., 2015. Web. doi:10.1088/0004-637X/798/1/50.
Wang, Y.-M., Colaninno, R. C., Baranyi, T., & Li, J., E-mail: yi.wang@nrl.navy.mil, E-mail: robin.colaninno@nrl.navy.mil, E-mail: baranyi@tigris.unideb.hu, E-mail: jli@igpp.ucla.edu. ACTIVE-REGION TILT ANGLES: MAGNETIC VERSUS WHITE-LIGHT DETERMINATIONS OF JOY'S LAW. United States. doi:10.1088/0004-637X/798/1/50.
Wang, Y.-M., Colaninno, R. C., Baranyi, T., and Li, J., E-mail: yi.wang@nrl.navy.mil, E-mail: robin.colaninno@nrl.navy.mil, E-mail: baranyi@tigris.unideb.hu, E-mail: jli@igpp.ucla.edu. Thu . "ACTIVE-REGION TILT ANGLES: MAGNETIC VERSUS WHITE-LIGHT DETERMINATIONS OF JOY'S LAW". United States. doi:10.1088/0004-637X/798/1/50.
@article{osti_22364697,
title = {ACTIVE-REGION TILT ANGLES: MAGNETIC VERSUS WHITE-LIGHT DETERMINATIONS OF JOY'S LAW},
author = {Wang, Y.-M. and Colaninno, R. C. and Baranyi, T. and Li, J., E-mail: yi.wang@nrl.navy.mil, E-mail: robin.colaninno@nrl.navy.mil, E-mail: baranyi@tigris.unideb.hu, E-mail: jli@igpp.ucla.edu},
abstractNote = {The axes of solar active regions are inclined relative to the east-west direction, with the tilt angle tending to increase with latitude ({sup J}oy's law{sup )}. Observational determinations of Joy's law have been based either on white-light images of sunspot groups or on magnetograms, where the latter have the advantage of measuring directly the physically relevant quantity (the photospheric field), but the disadvantage of having been recorded routinely only since the mid-1960s. White-light studies employing the historical Mount Wilson (MW) database have yielded tilt angles that are smaller and that increase less steeply with latitude than those obtained from magnetic data. We confirm this effect by comparing sunspot-group tilt angles from the Debrecen Photoheliographic Database with measurements made by Li and Ulrich using MW magnetograms taken during cycles 21-23. Whether white-light or magnetic data are employed, the median tilt angles significantly exceed the mean values, and provide a better characterization of the observed distributions. The discrepancy between the white-light and magnetic results is found to have two main sources. First, a substantial fraction of the white-light ''tilt angles'' refer to sunspots of the same polarity. Of greater physical significance is that the magnetograph measurements include the contribution of plage areas, which are invisible in white-light images but tend to have greater axial inclinations than the adjacent sunspots. Given the large uncertainties inherent in both the white-light and the magnetic measurements, it remains unclear whether any systematic relationship exists between tilt angle and cycle amplitude during cycles 16-23.},
doi = {10.1088/0004-637X/798/1/50},
journal = {Astrophysical Journal},
number = 1,
volume = 798,
place = {United States},
year = {Thu Jan 01 00:00:00 EST 2015},
month = {Thu Jan 01 00:00:00 EST 2015}
}
  • In this paper, we study the magnetic energy (ME) structure contained in the solar corona over the active region NOAA 11158. The time period is chosen as from 0:00 to 06:00 UT on 2011 February 15, during which an X-class flare occurred. The nonlinear force-free field (NLFFF) and the potential field extrapolation are carried out to model the coronal magnetic field over this active region, using high-quality photospheric vector magnetograms observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory as boundary conditions. We find that the volume distribution for the density of the ME (B {supmore » 2}/8{pi}) and the ohmic dissipation power (ODP, j {sup 2}/{sigma}), in which j is the electric current density (c/4{pi}{nabla} Multiplication-Sign B) and {sigma} is the conductivity in the corona, can be readily fitted by a broken-down double-power law. The turn-over density for the spectrum of the ME and ODP is found to be fixed at {approx}1.0 Multiplication-Sign 10{sup 4} erg cm{sup -3} and {approx}2.0 Multiplication-Sign 10{sup -15} W cm{sup -3} (assuming {sigma} = 10{sup 5} {Omega}{sup -1} m{sup -1}), respectively. Compared with their first power-law spectra (fitted below the corresponding turn-over value) which remain unchanged, the second power-law spectra (fitted above the corresponding turn-over value) for the NLFFF's ME and ODP show flare-associated changes. The potential field remains steady. These results indicate that a magnetic field with energy density larger than the turn-over energy density plays a dominant role in powering the flare.« less
  • Tilt angles of close to 30,600 sunspots are determined using Mount Wilson daily averaged magnetograms taken from 1974 to 2012, and SOHO/MDI 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 {approx}0.{sup 0}5 per degree of latitude. Tilt angles of spots appear largely invariant with respect to time at a given latitude, but they decrease by {approx}0.{sup 0}9 per year on average, a trend that largely reflects Joy's law followingmore » 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 latitude zones, and the differences increase with increasing latitude.« less
  • The tilt angles of sunspot groups represent the poloidal field source in Babcock-Leighton-type models of the solar dynamo and are crucial for the build-up and reversals of the polar fields in surface flux transport (SFT) simulations. The evolution of the polar field is a consequence of Hale's polarity rules, together with the tilt angle distribution which has a systematic component (Joy's law) and a random component (tilt-angle scatter). We determine the scatter using the observed tilt angle data and study the effects of this scatter on the evolution of the solar surface field using SFT simulations with flux input basedmore » upon the recorded sunspot groups. The tilt angle scatter is described in our simulations by a random component according to the observed distributions for different ranges of sunspot group size (total umbral area). By performing simulations with a number of different realizations of the scatter we study the effect of the tilt angle scatter on the global magnetic field, especially on the evolution of the axial dipole moment. The average axial dipole moment at the end of cycle 17 (a medium-amplitude cycle) from our simulations was 2.73 G. The tilt angle scatter leads to an uncertainty of 0.78 G (standard deviation). We also considered cycle 14 (a weak cycle) and cycle 19 (a strong cycle) and show that the standard deviation of the axial dipole moment is similar for all three cycles. The uncertainty mainly results from the big sunspot groups which emerge near the equator. In the framework of Babcock-Leighton dynamo models, the tilt angle scatter therefore constitutes a significant random factor in the cycle-to-cycle amplitude variability, which strongly limits the predictability of solar activity.« less
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