FALL 2007 CSTR Journal Club

Emergence of Small-Scale Magnetic Loops in the Quiet Sun Internetwork

R. Centeno, H Socas-Navarro, B. Lites, M. KuboHigh Altitude Observatory (NCAR), Boulder CO 80301, USA

K. Ichimoto, S. Tsuneta, Y. Katsukawa, Y. SuematsuNational Astronomical Observatory of Japan, Tokyo, Japan

T. ShimizuJapan Aerospace Exploration Agency, Tokyo, Japan

S. NagataKwasan and Hida Observatories, Kyoto University, Japan

Presented by Angelo P. VerdoniCenter for Solar-Terrestrial

Research Fall 2007

The Astrophysical Journal, Volume 666, Issue 2, pp. L137-L140.

Z. Frank, R. Shine, T. Tarbell, A. TitleLockheed Martin Space and Astrophysics Laboratory, Palo Alto, CA, USA

FALL 2007 CSTR Journal Club 09/27/07

Introduction• Presented in this paper is clear evidence of the emergence and temporal evolution of a small-scale InterNetwork (IN) magnetic loop in the quiet Sun photosphere.

• The nature of InterNetwork (IN) magnetic fields is currently a hot topic of debate:

1. Strong kG field strengths associated with small filling factorsa

2. Predominance of weak magnetic fields (~300 – 500 G)b

• Litesc , using the Advanced Stokes Polarimeter (ASP), reports Horizontal Internetwork Fields (HIFs) with typical sizes of 1” and lifetimes of ~ 5 minutes, suggesting small magnetic loops are being advected towards the surface by the upward motion of the plasma inside the granule.

• Measurement of the full topology of a magnetic loop requires accurate 2-D spectropolarimetric maps of the four Stokes parameters, with high S/N ratio (~ 10-3 continuum intensity), high spatial resolution and good consistent seeing conditions. The Spectro-Polarimetr (SP) of the Solar Optical Telescope (SOT) on board Hinoded meets all of these requirements.

FALL 2007 CSTR Journal Club 09/27/07

Figures taken from: http://solarb.msfc.nasa.gov/documents/Tarbell_SolarB.pdf

Observations: Hinode SP/SOT

Figure taken from: Shimizu, T. SolarB Solar Optical Telescope (SOT), The Solar-B Mission and the Forefront of Solar Physics , Astronomical Society of the Pacific Conference Series, 2004, Vol. 325

FALL 2007 CSTR Journal Club 09/27/07

• March 10, 2007

• 5-hour long time series of 4’’ X 82’’ spectropolarimetric maps • Cadence of 2 minutes

• 25 positions on spectrograph slit @ 4.8 s per position. (I, Q, U, V) @ every position

• 0.16’’ step size resulting in 4’’ wide maps with a spatial resolution of 0.32’’

• Spectral region contains Fe I lines @ λ = 6301.5 Å and 6302.5 Å with 21.5 mÅ sampling

• Noise level in continuum polarization ~ 1.2 X 10-3 Ic

• Figure shows 4 consecutive snapshots ( 4’’ X 4’’ ) of the data set Δt = 125 sec

• Background shows integrated continuum intensity revealing photospheric granulation.

• Contours show the non-negligible polarization signals.

• Red: positive circular polarization Green: negative circular polarization Orange: net linear polarization (Q2 + U2)1/2

Observations

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Magnetic Flux Density and Field Topology

• To quantify the magnetic flux density and its topology, full Stokes LTE inversions ( using LILIAe ) of pixels with non-negligible linear or circular polarization signals.

• LTE inversions should give reliable magnetic flux density values. However, some of the signals are marginally above noise level.

• By adjusting various parameters ( one example, keeping field height constant or allowing linear variation in height ) different values of the flux density were calculated. So, the apparent transverse and longitudinal flux densities were computed from the integrated polarization signalsf and the LTE inversione was used to determine the field topology (which remained consistently independent of parameter variation).

FALL 2007 CSTR Journal Club 09/27/07

• Figure shows ( for the 4’’ X 4’’ region ) the time sequence of the longitudinal and transverse flux density ( 1st and 2nd row respectively ). The bottom row shows the field orientation with color-coded pixels representing inclination values and arrows representing the direction of positive polarity.

Magnetic Flux Density and Field Topology

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Magnetic Flux Density and Field Topology

• t = 0, barely any magnetic signal present in the granular region centered at approximately (1’’,2’’)

• t = 2 min, new concentration of mostly horizontal ( transverse ) flux density appears. The field is parallel to the surface and azimuth makes angle ~ 60 degrees with E-W direction

• t = 4 min, magnetic feature has “stretched” in the linear direction. Magnetic poles now apparent.

• t = 6 min, transverse flux is not detectable with vertical dipoles visibly drifting towards granule boundary.

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Magnetic Flux Density and Field Topology

• Due to the azimuth ambiguity there are two possible topology configurations for the magnetic loop seen at t = 6 min.

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Conclusions• Observational evidence is presented of an emergent magnetic loop structure at quiet sun disk center. The flux emerges within granular region showing strong horizontal magnetic signal flanked by traces of two vertical opposite polarities.

• This event brings ~ 1017 Mx of apparent longitudinal magnetic flux and does not seem to have any major influence on the shape of the underlying granulation pattern. In agreement with simulationsg where small scale magnetic loop structures with less than 1018 Mx of longitudinal flux are not sufficiently buoyant to rise coherently against the granulation, and produce no visible disturbances.

• The convective motions carry the vertical magnetic flux towards the intergranular lanes, where it stays confined for longer times. This could explain why transverse magnetic flux (observed at disk center) is in general co-spatial with granules while longitudinal flux tends to be concentrated in the intergranular lanes.

FALL 2007 CSTR Journal Club 09/27/07

References

a. Sanchez Almeida, J., Lites, B.W., ApJ, 532, 1215

b. Lin, H, 1995, ApJ, 446, 421 Lin, H., Rimmele, T., 1999, ApJ, 514, 448

c. Lites, B.W., Leka, K.D., Skumanich, A., Martinez Pillet, V., Shimizu, T., 1996, ApJ, 460, 1019

d. Kosugi, T. et al, 2007, Solar Physics, submitted

e. Socas-Navarro, H., 2001, in Advanced Solar Polarimetry-Theory, Observation and Instrumentation, edited by M. Sigwarth, 236, 487

f. Lites, B.W. et al, 2007, ApJ, submitted

g. Cheung, M.C.M., et al, 2007, A&A, 467, 703