Absence of a Long Lasting Southward Displacement of the HCS Near the Minimum Preceding Solar Cycle 24

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Absence of a Long Lasting Southward Displacement
of the HCS Near the Minimum Preceding Solar Cycle
24

• X. P. Zhao, J. T. Hoeksema and P. H. Scherrer
• Stanford University
• D24 P110, July 15, 2008
• 37th COSPAR Scientific Assembly, Montreal
• July 13-20, 2008

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• Abstract
• Definitive observational evidence of a 10
  degres
• southward displacement of the heliospheric
  current sheet
• (HCS) from the heliographic equator was obtained
  from
• Ulysses observations near the minimum phase
  preceding
• Solar Cycle 23. The HCS southward displacement
  near the
• minimum phase has been suggested to be a
  persistent
• phenomenon. We found from the PFSS model
  prediction
• that there are 3-year lasting southward
  displacement of
• the HCS near the minimum preceding solar cycle
  22
• and 23 Zhao et al., 2005. This work shows,
  however, the
• absence of similar interval of HCS southward
• displacement between 2004 and 2008. Is the
  absence of
• the 3-year interval an abnormal phenomena?

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1. Introduction

• The heliospheric current sheet (HCS) separates
  the heliosphere into two magnetic hemispheres
  with opposite magnetic polarity.
• Definitive observational evidence of a 10 deg
  southward displacement of the HCS from the
  heliographic equator was obtained from Ulysses
  observations in the rapid transit from the south
  to north solar poles between September 1994 and
  May 1995 Simpson et al., 1996 Smith et al.,
  2000. A similar southward displacement in the
  green-line corona was observed during solar
  minima of cycles 21 and 22 statistical analysis
  of the IMF sector duration also shows a southward
  displacement of the average HCS around the
  minimum phase of cycles 20, 21 22.

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• The positive-negative and north-south HCS
  displacements computed using a potential field -
  source surface model applied to Wilcox Solar
  Observatory (WSO) observations
• between 1976 and 2001 revealed the existence
  of 3-year lasting HCS southward displacement
  near the minimum phase of solar cycles 21 and 22,
  consistent with the green-line corona
  observations and the statistical results of HMP
  polarity Zhao et al., 2005. We extend the study
  to April 2008 to see if the similar long lasting
  HCS southward displacement occurs between 2003
  and 2008 and discuss whether the absence is a
  normal or abnormal phenomenon.

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2. Prediction of the hemisphericasymmetry of the
heliomagnetosphere

• By defining the magnetic hemisphere with the same
  polarity as the dominant polarity in north
  (south) polar region as the north (south)
  heliomagnetic hemisphere (HH), and using the PFSS
  model, the effective displacement of the HCS from
  the Suns dipole equator, ?m, and from the
  heliographic equator, ?, can be calculated
• ?m arcsin (1 In dO / 2p )
  (1)
• ? ?m cos ?
  (2)
• Here In denotes the number of solid angle
  element (dO4p/2160 for WSO) in the north HH ?,
  tilt angle.

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Fig. 1 Evolution of Suns tilt angle ?
calculated using WSO data from June 1976 to April
2008. The positive (negative) magnetic polarity
is in the north polar region when ? 0 (180).
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Fig 2. The difference between positive and
negative solid angles corresponding to positive
and negative heliomagnetic hemisphere. In 1980s,
south positive area is less than north negative
area in 1990s, north positive area is greater
than south negative area. Both show that the
north hemisphere is more strongly developed and
greater than the south hemisphere.
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Fig. 3 Calculated ?m (dotted line) and ? (solid
line). There are two 3-year intervals of
southward displacement of the HCS (between
198303 198607 and between199204 199505)
preceding Cycle 22 and 23. There is no such
3-year interval preceding Cycle 24 !?
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Fig. 4 Predicted IMF radial component Br in the
positive (away) and negative (toward)
heliomagnetic hemisphere. Between 8303
8607, south positive Br is greater than north
negative Br between 9204 9505, south
negative Br is greater than north positive Br,
consistent with the HCS southward displacement.
There is no such systematicly N-S asymmetry
between 2005 and 2008.
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3. Photospheric field in north and south polar
regions
The southward displacement of the HCS near the
minimum has been attributed to the north-south
asymmetry in the Suns polar magnetic field. The
HMF is composed mainly of lower multipole
components the dipole, the quadrupole, and the
hexapole. Around sunspot minimum these
lower-order multipoles are orientated basically
parallel to the Suns rotation axis and can be
approximately represented by the zonal harmonic
coefficients g10, g20 and g40. The polar field
represented by g10 has opposite polarity in the
north and south polar regions, but the field
described by g20 g40 has the same polarity in
both polar regions, giving rise to an asymmetry
in the field strength. The absence of the 3-year
southward displacement of the HCS preceding the
cycle 24 implies the absence of the asymmetry in
the field strength between the south and north
polar regions.
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Fig. 5 The difference in WSO mean field amplitude
between the north and south polar caps above 55
degrees of north and south heliographic latitude.
Each point denotes the rotation-averaged value of
the north polar field subtracted by the
rotation-averaged value of the south polar field.
There are 3-year interval when south polar field
is stringer than north polar field. But there is
no such asymmetry in the polar field preceding
cycle 24.
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Fig 6. The evolution of the amplitude and sign of
the zonal lower-order multipole components, g10,
g20 and g40 between June 1976 and April 2008. The
opposite sign between g10 and g20 lasts near
3-year in the earlier two intervals, but g20
shows small fluctuations around zero preceding
the cycle 24 , and g10 is significantly less than
what in the earlier two intervals.
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4. Duration of two IMF sector structures
The southward displacement of the HCS should
affect the duration of two sector structures
observed near the Earth, but the effect may be
contaminated by the ICME, large-amplitude
Alfvenic waves, and various stream-stream
interaction. As shown in Figure 3, the southward
displacement of the HCS is mostly less than 7.2
degrees, its effect may be shown in the in situ
observations made in Fall and Spring seasons when
the Earth reaches 7.2 degrees of solar latitude.
Mursula and Hiltula 2003 have calculated the
Toward sector occurrence fractions T/(TA ) using
hourly IMF data between 1967 2001 (here T A
denote the total number of Toward Away sector
hours for each 3-month season of SpringFeb--Apr
and FallAugOct). They found that the IMF sector
in the northern hemisphere is systematically more
strongly developed, suggesting the southward
displacement of the HCS. We extend the analysis
to April, 2008 using IMF daily polarity of OMNI
and Leif Svalgaard ( http//omniweb.gsfc.nasa.gov/
and http//www.leif.org/research/).
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Figure 7. The same as Figure 1 of Mursula and
Hiltula 2003, but extending to April 2008 from
2001 and using daily IMF polarity. The evolution
of the toward sector occurrence fraction before
2002 is basically the same as what Mursula and
Hiltula obtained, showing that the IMF sector in
the northern hemisphere is systematically more
strongly developed, but there is no such
asymmetry after 2003.
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Figure 8. The same as Figure 7 but using the IMF
daily polarity inferred by Dr. Leif Svalgaard.
The evolution of the toward sector occurrence
Fraction here is basically the same as Figure 7,
implying the validation of IMF polarity data
inferred by Dr. Leif Svalgaard in analyzing
latitudinal Variation of dominate IMF polarity.
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Figure 9. The same as Figure 8, but starting from
the year of 1926, increasing 4 more solar cycles.
The evolution of the toward sector occurrence
fraction before the year of 1940 is the same as
after 2003, showing the absence of the asymmetry
in the development of IMF sector structures
between the north and south hemisphere, i.e.,
the absence of the long lasting southward
displacement of the HCS near the minimum phase in
solar cycles 16, 17, and 23.
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Figure 10. The time variation of the sunspot
number between Janual1926 and April 2008, showing
the normal evolution of the solar activity in
solar cycle 23.
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6. Summary Discussion

• 6.1 Using the algorithm developed for
  quantitative estimate of the north-south
  displacement of the HCS Zhao et al., 2005 and
  the WSO data observed until April 2008, we find
  that, different from Solar cycles 21 and 22,
  there is no the 3-year lasting HCS southward
  displacement near the minimum phase in solar
  cycle 23. In addition, there is no N-S asymmetry
  of the polar field amplitude between the two
  hemispheres between 2003 and 2008.

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6.2 The occurrence fraction of the toward sector
calculated using the daily IMF polarity both
observed and inferred shows the same evolution,
i.e. the absence of the asymmetry of IMF sector
development between northern and southern
hemisphere after the year of 2003. It support our
PFSS model prediction of the absence of the long
lasting southward displacement of the HCS. By
using the daily IMF polarity inferred by Dr. Leif
Svalgaard, it is found that there is also no such
asymmetry near the minimum phase in solar cycles
16 and 17, suggesting that the long lasting HCS
southward displacement near the minimum phase is
not a persistent phenomenon and thus, the absence
of the long lasting southward displacement of the
HCS near the minimum phase of solar cycle 23 is
not an abnormal phenomenon.