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Title: ASYMMETRIC SUNSPOT ACTIVITY AND THE SOUTHWARD DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET

Abstract

Observations of the interplanetary magnetic field (IMF) have suggested a statistical tendency for the heliospheric current sheet (HCS) to be shifted a few degrees southward of the heliographic equator during the period 1965-2010, particularly in the years near sunspot minimum. Using potential-field source-surface extrapolations and photospheric flux-transport simulations, we demonstrate that this southward displacement follows from Joy's law and the observed hemispheric asymmetry in the sunspot numbers, with activity being stronger in the southern (northern) hemisphere during the declining (rising) phase of cycles 20-23. The hemispheric asymmetry gives rise to an axisymmetric quadrupole field, whose equatorial zone has the sign of the leading-polarity flux in the dominant hemisphere; during the last four cycles, the polarity of the IMF around the equator thus tended to match that of the north polar field both before and after polar field reversal. However, large fluctuations are introduced by the nonaxisymmetric field components, which depend on the longitudinal distribution of sunspot activity in either hemisphere. Consistent with this model, the HCS showed an average northward displacement during cycle 19, when the 'usual' alternation was reversed and the northern hemisphere became far more active than the southern hemisphere during the declining phase of the cycle. Wemore » propose a new method for determining the north-south displacement of the HCS from coronal streamer observations.« less

Authors:
 [1];  [2]
  1. Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352 (United States)
  2. Royal Observatory of Belgium, 1180 Brussels (Belgium)
Publication Date:
OSTI Identifier:
21578205
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 736; Journal Issue: 2; Other Information: DOI: 10.1088/0004-637X/736/2/136; Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASYMMETRY; FLUCTUATIONS; HELIOSPHERE; INTERPLANETARY MAGNETIC FIELDS; MAGNETISM; SIMULATION; SUN; SUNSPOTS; ATMOSPHERES; MAGNETIC FIELDS; MAIN SEQUENCE STARS; SOLAR ACTIVITY; SOLAR ATMOSPHERE; STARS; STARSPOTS; STELLAR ACTIVITY; STELLAR ATMOSPHERES; VARIATIONS

Citation Formats

Wang, Y.-M., and Robbrecht, E., E-mail: yi.wang@nrl.navy.mil, E-mail: eva.robbrecht@oma.be. ASYMMETRIC SUNSPOT ACTIVITY AND THE SOUTHWARD DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET. United States: N. p., 2011. Web. doi:10.1088/0004-637X/736/2/136.
Wang, Y.-M., & Robbrecht, E., E-mail: yi.wang@nrl.navy.mil, E-mail: eva.robbrecht@oma.be. ASYMMETRIC SUNSPOT ACTIVITY AND THE SOUTHWARD DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET. United States. doi:10.1088/0004-637X/736/2/136.
Wang, Y.-M., and Robbrecht, E., E-mail: yi.wang@nrl.navy.mil, E-mail: eva.robbrecht@oma.be. Mon . "ASYMMETRIC SUNSPOT ACTIVITY AND THE SOUTHWARD DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET". United States. doi:10.1088/0004-637X/736/2/136.
@article{osti_21578205,
title = {ASYMMETRIC SUNSPOT ACTIVITY AND THE SOUTHWARD DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET},
author = {Wang, Y.-M. and Robbrecht, E., E-mail: yi.wang@nrl.navy.mil, E-mail: eva.robbrecht@oma.be},
abstractNote = {Observations of the interplanetary magnetic field (IMF) have suggested a statistical tendency for the heliospheric current sheet (HCS) to be shifted a few degrees southward of the heliographic equator during the period 1965-2010, particularly in the years near sunspot minimum. Using potential-field source-surface extrapolations and photospheric flux-transport simulations, we demonstrate that this southward displacement follows from Joy's law and the observed hemispheric asymmetry in the sunspot numbers, with activity being stronger in the southern (northern) hemisphere during the declining (rising) phase of cycles 20-23. The hemispheric asymmetry gives rise to an axisymmetric quadrupole field, whose equatorial zone has the sign of the leading-polarity flux in the dominant hemisphere; during the last four cycles, the polarity of the IMF around the equator thus tended to match that of the north polar field both before and after polar field reversal. However, large fluctuations are introduced by the nonaxisymmetric field components, which depend on the longitudinal distribution of sunspot activity in either hemisphere. Consistent with this model, the HCS showed an average northward displacement during cycle 19, when the 'usual' alternation was reversed and the northern hemisphere became far more active than the southern hemisphere during the declining phase of the cycle. We propose a new method for determining the north-south displacement of the HCS from coronal streamer observations.},
doi = {10.1088/0004-637X/736/2/136},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 736,
place = {United States},
year = {2011},
month = {8}
}