Emergence of Small-Scale
Magnetic Loops in the Quiet
Sun Internetwork
R. Centeno, H Socas-Navarro, B. Lites, M. Kubo
High Altitude Observatory (NCAR), Boulder CO 80301, USA
Z. Frank, R. Shine, T. Tarbell, A. Title
Lockheed Martin Space and Astrophysics Laboratory, Palo Alto, CA, USA
K. Ichimoto, S. Tsuneta, Y. Katsukawa, Y. Suematsu
National Astronomical Observatory of Japan, Tokyo, Japan
T. Shimizu
Japan Aerospace Exploration Agency, Tokyo, Japan
S. Nagata
Kwasan and Hida Observatories, Kyoto University, Japan
The Astrophysical Journal, Volume 666, Issue 2, pp. L137-L140.
Presented by Angelo P. Verdoni
Center for Solar-Terrestrial Research
Fall 2007
FALL 2007
CSTR Journal Club
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:
a
1. Strong kG field strengths associated with small filling factors
b
2. Predominance of weak magnetic fields (~300 – 500 G)
c
• Lites , 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
d
Telescope (SOT) on board Hinode meets all of these requirements.
FALL 2007
CSTR Journal Club
09/27/07
Observations: Hinode SP/SOT
Figures taken from: http://solarb.msfc.nasa.gov/documents/Tarbell_SolarB.pdf
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
Observations
• 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,
• Figure shows 4 consecutive snapshotsV)( @
4’’every
X 4’’ position
) of the
data set Δt = 125 sec
• 0.16’’ step size resulting in 4’’
wide maps with a spatial
• Background shows integrated continuum
intensity
resolution
of 0.32’’
revealing photospheric granulation.
• Spectral region contains Fe I
lines @ λ =
6301.5 Å and 6302.5
• Contours show the non-negligible polarization
signals.
Å with 21.5 mÅ sampling
• Red: positive circular polarization Green: negative
• Noise level in continuum
2
circular polarization Orange: net linear polarization
(Q -3
polarization ~ 1.2 X 10 Ic
2 1/2
+U )
FALL 2007
CSTR Journal Club
09/27/07
Magnetic Flux Density and Field
Topology
• To quantify the magnetic flux density and its topology, full Stokes LTE
e
inversions ( using LILIA ) 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
f
densities were computed from the integrated polarization signals and the LTE
e
inversion was used to determine the field topology (which remained
consistently independent of parameter variation).
FALL 2007
CSTR Journal Club
09/27/07
Magnetic Flux Density and Field
Topology
• 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.
FALL 2007
CSTR Journal Club
09/27/07
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.
FALL 2007
CSTR Journal Club
09/27/07
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.
FALL 2007
CSTR Journal Club
09/27/07
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.
17
• This event brings ~ 10 Mx of apparent longitudinal magnetic flux and
does not seem to have any major influence on the shape of the underlying
g
granulation pattern. In agreement with simulations where small scale
18
magnetic loop structures with less than 10 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
FALL 2007
CSTR Journal Club
09/27/07