solar-cycle variation of oscillation frequencies and...
TRANSCRIPT
Solar-cycle variation of oscillation
frequencies and surface magnetic
field
Shao Min Tan
Carleton College
Mentors: Michael Thompson, Rebecca Centeno-Elliot (HAO)
Pulsating Stars
• Cepheid variables
• Standard candle
Image credit: European Science Agency, http://sci.esa.int/science-e-
media/img/20/cepheid-variables.jpg
Closer to Home…
-2000 2000
Velocity (m/s)
-2000 2000
Velocity (m/s)
Image credit: Christensen-Dalsgaard, 2002 [1]
Instruments
• Michelson Doppler Interferometer (MDI) on
SOHO spacecraft
• Global Oscillation Network Group (GONG)
Image credit: GONG, http://gong.nso.edu/
Solar Oscillations
• Oscillation period ~ 5 minutes
• Data averaged over 72 days (SOHO) or 36 days
(GONG) to find frequencies(GONG) to find frequencies
• p-modes and g-modes
Motivation
Me
asu
re o
f su
nsp
ot
nu
mb
er
Me
asu
re o
f su
nsp
ot
nu
mb
er
Image credit: Broomhall et al., 2009 [2]
Oscillation Modes
Radial direction:
• Radial order, n
Surface:
• Degree, l
• Azimuthal order, m
•
Image credit: GONG, http://gong.nso.edu/
Spherical Harmonics
l = 1, m = 0 l = 1, m = 1l = 1, m = 0 l = 1, m = 1
l = 2, m = 0 l = 2, m = 1 l = 2, m = 2
Image credit: Christensen-Dalsgaard, 2003 [3]
Surface Magnetic Field
Sin
e la
titu
de L
atitu
de
Longitude
Sin
e la
titu
de L
atitu
de
Image credit: Solar Oscillations Investigation,
http://soi.stanford.edu/magnetic/index6.html
Surface Magnetic Field
Latitu
de
Sin
e l
ati
tud
e
Magnetic flux (G)
Longitude
Latitu
de
Sin
e l
ati
tud
e
Legendre Decomposition of B-field
Scaled Pk
for:
k = 0
k = 1
k = 2
k = 3k = 3
k = 4Sum (40
components)
= cos θ
Bk
Coefficients over the Solar Cycle
B2
B4
Image credit: David Hathaway, http://solarscience.msfc.nasa.gov
Rising phase Declining phase
R = -.995
R = -.990
R = -.998
R = -.984
ak
vs. Bk
over the Solar Cycle
SOHO
GONG
Absolute slope of the linear fit
My data
Ab
solu
te s
lop
e (
nH
z/G
)
GONG data
Antia et al.
(2001) [4]
Index number, k
Ab
solu
te s
lop
e (
Conclusion
• Linear correlation between ak
and Bk
–
corroboration of Antia’s result
• Correlation strength is similar for rising and
declining phases of solar cycle – what does this declining phases of solar cycle – what does this
mean for subsurface effects?
– Further work: separating modes with different
penetration depths
• Slope varies nonmonotonically with k, regardless
of handling of decomposition at poles