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Five minute solar oscillation power Five minute solar oscillation power within magnetic elementswithin magnetic elements
Rekha Jain & Andrew GascoyneSchool of Mathematics and Statistics (SoMaS)
University of Sheffield (UK)
Brad Hindman & Ben GreerJILA, University of Colorado at Boulder (USA)
Interaction with magnetic fieldsInteraction with magnetic fields
• Energy loss
• Energy redistribution
(i.e scattering)
Absorption
Damping
Power suppression
Phase shift
Mode mixing
Far field
Near-field (acoustic jacket modes)
Fast Fourier Transform (FFT) in time
Velocity Power maps
Dark region: suppressionBright region: enhancement
Jain and Haber (A&A, 2002)
Tracked &
RemappedMagnetogram
2.0-4.0 mHz
5.5-7.5 mHz
Sunspots Active Region
Doppler power images (integrating over diff. freq. ranges) Power suppressed in strong mag. field regions Power halos in strong mag. field but still suppressed in regions of strongest field
Doppler velocity dataLarge scale magnetic Large scale magnetic features & acoustic power features & acoustic power
Na line (589 nm)500 km
K line (770 nm)250 km
Ni line (677 nm)100 km
3 mHz 4 mHz 5 mHz 6 mHz 7 mHz 8 mHz
Moretti et al. (A&A, 2007)Three filters: different heights
• The suppression depends on height and frequency. Power halos are present in the limited range of frequencies that depend on the height
• The spatial extent of the region of power suppression grows as the height increases
Chitta et al. (ApJ, 2011) investigated the effect of magnetic fieldon photosphere/lower chromospheric intensity and velocity oscillations at the site of small scale magnetic features (|B| < 500 G) in quiet Sun close to the disk centre.
They chose quiet Sun with isolated small scale magnetic regions from different days with and without any visible large scale magnetic field regions such as sunspots and plages.
Small scale magnetic features and acoustic power Small scale magnetic features and acoustic power
Key findingsKey findings Both high resolution intensity observed in G band & velocity oscillations are influenced by the presence of magnetic field. Intensity oscillations are suppressed at all frequencies in strong magnetic regions
Key findingsKey findings Doppler velocity oscillations in magnetic elements are suppressed in the frequency range 2-5 mHz: compared to the surroundings (checked with separate data from MDI & HMI)
p-band
high-ν band
Diamond: reduction by a factor of 3
Key findingsKey findings Doppler velocity oscillations in magnetic elements are suppressed in the frequency range 2-5 mHz: there is 20-30% drop in power compared to the surroundings (checked with separate data from MDI & HMI)
p-band
high-ν band
The observed similarities between plage & small magnetic elements suggests that irrespective of the size of the magnetic regions, the physical mechanism that is responsible for the observed reduction of acoustic power is the same.
It is unlikely that the collective effect of tightly packed magnetic concentrations (as is typical of plage) is responsible??!!
Key findingsKey findings
However…However…
Sources of possible errors are not known.(so caution is needed & independent checks are needed.)
Simultaneous high resolution observations in different layers of solar atmosphere with co-temporal & co-spatial magnetic field information is needed.
The ModelThe Model
Isothermal
Polytrope
f and p mode solutionsf and p mode solutions
Isothermal
Polytrope
Sausage wave solutionsSausage wave solutions
Vertical displacement Vertical displacement of p modes (solid) of p modes (solid) & sausage waves& sausage waves
Re dashed
Im dotted
normalised by square root of density, as a function of dimensionless depth s. The vertical dotted line shows the position of the interface where the polytrope & isothermal regions are matched.
normalised by square root of density, as a function of dimensionless depth s. The vertical dotted line shows the position of the interface where the polytropeand isothermal regions are matched.
Vertical displacement of p modes (solid) & sausage wavesVertical displacement of p modes (solid) & sausage waves
Real part: dash
Imag. part: dot
Power ratioPower ratio
power of longitudinal (sausage) waves inside the tube to the (external) p mode power at a fixed height zR (from zphoto )
2
2
( )( )
( ),
v
nvz e
ws w
w
Dependence of power ratio on β for 3 mHz waves measured atthree different heights, zR in the isothermal region.
Assuming same amplitude
But observationally measured power maps have noBut observationally measured power maps have nowavenumber discrimination & the power measured inwavenumber discrimination & the power measured inany given pixel is the power in all modes at a given any given pixel is the power in all modes at a given ωω
where Pn is the power in the nth order mode.
( ) ( )n
n
P Pw w
( )magn
P w( )p
nP w
( )sn
P w sausage wave power
p-mode power
where
power in a magnetised pixel
f : filling factor
Power ratio
Since the fraction of p-mode power that is contained in any given order n is
( )( )
( )
pn
n pm
m
Pp
P
ww
w
obtained from helioseismic technique of ring-analysis as implemented in Greer et al. (2014)
1 1( ) [ ( )]n n
n
f r ps w
from theory
from HMI/SDO
POWER RATIO
f : black crossesP1: red asteriskP2: red asteriskP3: blue diamondsP4: yellow triangles
p5, p6, p7: turquoise
2
2
e
e
( )/ ( )
( )/ ( )
( )( , )
( )
HMI R n
HMI R m
z zHMIn
n R z zHMIm
m
pp z
p
a w
a w
ww
w
skin depth of the p-mode
solution in the isothermal atmosphere
Fractional power change as a Fractional power change as a function of height in the atmospherefunction of height in the atmosphere
Spectral lines are formed at different heights in magnetic and non-magnetic region - there will be some systematic error in the fractional power ratios.
We have investigated other data sets and our preliminary results suggest that the suppression in acoustic power at small scales, just like large-scale fields, is independent of the spectral line and instrument. However, caution is needed at this stage as observations at a very high spatial resolution can always open possibilities to deal with many of the questions we are trying to address in a much more effective way.
Suppression of intensity oscillations have been seen in Ca II K (which formin the lower to middle chromosphere). Since all Ca II K features have photospheric counterparts when observed in high-resolution G-band imaging, it remains to be seen if the intensity power suppression seen in the chromospheric Ca II K line is a result of already suppressed G-band intensity oscillations in the lower atmosphere.
centre to limb variation studies needed with different instruments & spectral lines