Searching%20for%20the%20Magnetic%20Fields%20at%20the%20Base%20of%20the%20Convection%20Zone%20%20Dean-Yi%20Chou

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Searching for the Magnetic Fieldsat the Base of
the Convection ZoneDean-Yi Chou

• Institute of Astronomy Department of Physics
• Tsing Hua University, Taiwan
• (March 2003, Seoul)

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Probing Subsurface Magnetic Fields
0.96 R lt r lt R Mode Frequency (c,
v) Time-Distance (c, v) Acoustic Imaging (c)
0.9 R lt r lt R Time-Distance (v)
r ? 0.7 R Time-Distance (c)
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MOTIVATION

• The change in frequency by magnetic fields near
  the surface depends on frequency only.
• The change in frequency by magnetic fields deep
  in the convection zone depends on horizontal
  phase velocity.
• To study magnetic fields at the base of the
  convection zone, we have to separate these two
  dependences.

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Modeling

1. Simulating effects of magnetic fields by
   perturbing sound speed in a standard solar model.
2. Two perturbed regions One is near the surface
   another is near the base of the convection zone.
3. The mode frequencies are computed from the
   perturbed solar model.
4. The change in frequency due to the perturbed
   sound speed is studied.

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Procedure

1. The dependence of mode mass is removed by
   multiplying frequency change by mode mass.
2. Using the modes with low horizontal phase
   velocity as a reference, the frequency dependence
   of frequency change is removed.
3. The frequency change of modes in a range of
   2.5-3.5 mHz is plotted vs. horizontal phase
   velocity.
4. To see small variation in frequency change as a
   function of horizontal phase velocity, modes in
   certain range of horizontal velocity are
   averaged.

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Frequency change
Frequency change mode mass
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What we learn from modeling.

• The frequency change remains constant for modes
  which can not penetrate into the base of the
  convection zone.
• The frequency change increase rapidly for modes
  whose lower turning points are located near the
  base of the convection zone.
• For modes which can penetrate into the radiative
  zone, the frequency change drops a little.

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MDI Central Frequency

• MDI central frequencies are used in this study.
• The frequency change relative to the solar
  minimum in different periods along the solar
  cycle is computed for each mode.
• The frequency change is analyzed as in modeling.

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• N is the number of bounces which takes the wave
  packet travel around the Sun.
• N is anti-correlated with the horizontal phase
  velocity.

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Conclusion from Studies of Central Frequency

• No anomaly in frequency change is found at
  horizontal phase velocity corresponding to the
  base of the convection zone.
• A upper limit of change in sound speed,
• about one millionth, can be given from this
  study.
• This limit is smaller than previous studies.

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ml modes

1. Since the power of mode ml is concentrated near
   the equator, the probability to detect the effect
   of the magnetic fields at the base of the
   convection is higher.
2. The frequency change is analyzed as the central
   frequency.
3. We use MDI and GONG frequencies.

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Conclusion

• An anomaly near N8 , which corresponds to the
  base of the convection zone, is seen in both MDI
  and GONG frequencies.
• It is unlikely due to the measurement artifacts.
• It is not clear what causes this anomaly.