Peculiar Moving Magnetic Features

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Peculiar Moving Magnetic Features Observed With
The Flare Genesis Experiment
Pietro N. Bernasconi1, David M. Rust1, Manolis K.
Georgoulis1 , Barry J. LaBonte2, Brigitte
Schmieder3
1JHU/Applied Physics Laboratory, 11100 Johns
Hopkins Rd., Laurel, MD 20723, United
States 2Institute for Astronomy, 2680 Woodlawn
Dr., Honolulu, HI 96822, United
States 3Observatory of Paris Meudon, DASOP P1
Janssen, Meudon PPL Cdx 92195, France
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Overview
In January 2000 the Flare Genesis Experiment, an
80 cm solar telescope, flew for 17 days suspended
from a balloon in the stratosphere, 35 km above
Antarctica, observing the Sun at high spatial
resolution and without interruptions. FGE was
equipped with a vector polarimeter and a
lithium-niobate Fabry-Perot narrow-band filter.
It recorded time series of filtergrams, vector
magnetograms and dopplergrams at the CaI 6122.2 Å
line, as well as H? filtergrams with a cadence
between 2.5 and 7 minutes. The image resolution
was 0.5" without seeing induced distortion. On
January 25, 2000 FGE observed for several hours
the active region NOAA 8844. It was a new,
rapidly growing, flux emergence that appeared at
the solar surface only two days before. In H? -1
Å off-band filtergrams we observed several small
brightenings lasting for 7 to 20 minutes (so
called Ellerman Bombs). Most of these
brightenings appeared near magnetic neutral
lines, over regions where new flux was emerging
and just above peculiar moving magnetic features
(MMFs). Most of the MMFs observed were moving
towards the trailing sunspot of the AR and their
part closer to the spot had polarity opposite to
the one of the spot itself. The analysis of the
FGE data suggests that the observed MMFs are
U-shaped folds instead of small ?-loops. Web
site http//sd-www.jhuapl.edu/FlareGenesis/
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Instrument description

• Balloon borne solar telescope. Fly over
  Antarctica at 35 km altitude (125,000 ft).
• No seeing induced image distortions.
• Uninterrupted observations during entire flight
  (17 days).
• Main telescope
• Cassegrain, 80 cm Ø, F/1.5
• Spatial resolution theoretical 0.2" in
  flight 0.5"
• Gondola pointing accuracy 10"

• Image Motion Compensation system (IMC) fast
  tip-tilt mirror that stabilizes the image at the
  CCD focal plane to about 1".
• Imaging Vector Magnetograph is the main
  scientific instrument
• Polarization analyzer 2 liquid-crystal
  polarization modulators 1 linear polarizer.
• 3 thin-film 1.25 Å prefilters for wavelength
  selection.
• 0.16 Å lithium-niobate Fabry-Perot etalon filter
  coated for 6000 - 6600 Å operation.
• 1024 x 1024-pixel Kodak Megaplus CCD camera - 20
  MByte/s read rate.
• Field of view 100" x 100"
• Data products Time series of high spatial
  resolution vector magnetograms, vector velocity
  maps, and intensity filtergrams in the CaI 6122.2
  Å (g 1.75) and H? lines.

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Data reduction

• Preliminary reduction
• Dark current and flat field correction

• Compute Stokes parameters

• Compute LOS vector fields Use V, Q, and U
  polarized filtergrams in the red wing of the CaI
  6122.2 Å line.
• Infer Longitudinal and Transversal Field
  components
• p , n calibration parameters. Determined by
  comparing FGE data with Hawaii IVM vector
  magnetograms acquired simultaneously.
• Calculate vector field direction
• Transformation from LOS to Local Coordinate
  Frame
• ? to local Zenith Angle 0 ? vertical UP
  90 ? horizontal 180 ?
  vertical DOWN
• ? to local Azimuth from local W counted
  positive
  counterclockwise
• Solution of the 180 ambiguity Solved in part by
  considering a standard potential field geometry
  of the observed active region, and in part
  empirically, by just following simple logical
  continuity and geometrical considerations.

Btrans
g
c
Q
Blong
LOS

• Compute Doppler velocity maps Use filtergrams
  recorded on both wings of the CaI 6122.2 Å line

c calibration parameter. Determined by
comparing FGE and Hawaii IVM dopplergrams of the
same region and recorded simultaneously.
Positive values denote Up flowfs.
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Active Region NOAA 8844

• The active region NOAA 8844 emerged at the solar
  surface on January 22-23, 2000.
• Until the 24th apparently quiescent, with only
  two, small, stable sunspots.
• Started a rapid growth on the 24th or early on
  the 25th. The two main flux concentrations of
  opposite polarity moved rapidly away from each
  other and new magnetic flux was constantly
  emerging from its center.
• Continued growing for only about 24 hours.
• FGE started the observations of NOAA 8844 on the
  25th at 1519 UT and ended at 1913 UT.
• At the time of the observations the active
  region was located at approximately 5N 30W.

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Data set for AR NOAA 8844, derived from FGE
observations
Longitudinal magnetogram
All images here are nearly simultaneous. Distance
between tick marks is 1".
Filtergram at 6122.43 Å, blue wing of CaI 6122.5 Å
Filtergram at 6562 Å (H? - 1Å)
Vector- magnetogram
The time sequence started at 1515 UT and ended
at 1913 UT. 55 CaI Filtergrams and Vector
Magnetograms. 1 frame every 3.5 min. 28 H?-1Å
Filtergrams and Dopplergrams. 1 frame every 7 min.
Dopplergram
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Direction of the transversal magnetic field
component
The same data as in the figure on the left, but
with the transverse field direction indicated by
lines superimposed to the 6122.428 Å filtergram.
The orientation of the transverse component is
indicated by the pixel color, as in the color
wheel at the lower left.
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Peculiar Moving Magnetic Features
Emergence cell
1
2

• MMFs are usually small magnetic dipoles that
  move together, as a unique entity.
• Many of the MMFs observed in AR 8844 on Jan.
  25, 2000 first appear at the center of a cell
  with emerging flux. Then they migrate towards the
  edges of the cell. Most of them towards the
  trailing sunspot of negative polarity, where they
  merge with it.
• The polarity of the part closer to the negative
  spot is positive.
• Their life time is highly variable from tens of
  minutes to hours, depending on their distance
  from the edges of the cell and their speed.
• Average MMF speed 350 m/sec
• Average MMF size 1.5" ? 4"
• Average MMF peak longitudinal field strength
  200 G

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5
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Magnetic structure of MMFs
1605 UT
1744 UT
Longitudinal magnetograms on the left indicate
location and direction (arrows) of the 5 selected
MMFs. Tick marks separation is 1".
View in perspective of the filed lines geometry
for the 5 MMFs
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MMFs , Ellerman Bombs, and Vertical Flows
Overlay
Longitudinal Magnetogram
Dopplergram (bright up flow)
MMFs
Downdrafts
Ellerman Bombs
Contours of the magnetic signature (positive
red, negative blue) and the Ellerman Bombs
(green) plotted over the dopplergram.

• MMFs are coincident with downdrafts in the
  photosphere.
• Brightenings observed in the wings of the H?
  line (Ellerman bombs) appear frequently at or
  near the neutral lines of MMFs.
• Ellerman Bombs are energy releases in the lower
  chromosphere. Probably triggered by magnetic
  reconnection.

Filtergram at H? 1 Å
Distance between tick marks in images is 1".
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MMFs Average Properties
This diagram shows some of the properties of the
five MMFs highlighted in slide 10. The positions
are taken from a line cutting the MMFs
longitudinally, across both polarities, and the
zero is positioned on the MMF neutral line. The
five selected MMFs show very similar properties.
The peak longitudinal magnetic field is about 200
G on both polarities. The white polarity seems to
be more elongated then the black polarity (b),
and has shallower field lines. The maximum
inclination to the local horizontal is about 20
in the white polarity versus an angle of up to
60 in the black polarity (c). In the vicinity of
the MMFs peaks is clearly visible a down-flow
relative to the surrounding plasma flow (d).

• (a) Absolute value of the total magnetic field
  strength
• (b) Field strength of the component parallel to
  the local vertical.
• (c) Zenith Angle in the local reference system
  0 is vertical UP, 90 is horizontal, 180 is
  vertical DOWN.
• (d) Plasma velocity parallel to the line of
  sight direction. Positive is towards the observer
  (UP-flow).

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MMFs in motion
Use red and blue stereo glasses (with red on the
left eye) to view these images.
The arrows represent the orientation of the
magnetic field lines in 3D. Background images are
the longitudinal magnetograms. The MMFs move
from left to right, towards the trailing sunspot
of negative polarity (on the upper right corner).
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Interpretation and Conclusions

• The filtergrams, vector magnetograms and
  dopplergrams acquired by FGE provided
  unprecedented insight in the structure and
  evolution of the magnetic fields in a new flux
  emergence region.
• Our observations indicate that magnetic fields
  emerged as folded flux ropes. The folds appeared
  as Moving Magnetic Features in longitudinal
  magnetograms even though the the fields were
  mostly horizontal.
• The vector magnetograms show that the observed
  MMF field structure is inconsistent with ?-shaped
  loops. The filed lines in MMFs are U-shaped.
• Our interpretation
• The magnetic filed emerges at the photosphere as
  horizontal, folded flux ropes.
• The lower part of the folds remains anchored
  below the photosphere, while the upper part
  balloons in the chromosphere enhancing the folds
  and pushig them towards the foot points of the
  arch filament system, i.e. the main sunspots.
• Material transported in the lower chromosphere
  by the emerging flux rope drains downward
  following both sides of the U-shaped filed lines.
  This produces the downdraft signature in
  dopplergrams.
• In the lower chromosphere, above the MMFs,
  field lines of opposite polarity in the folds
  occasionally get into contact. They reconnect and
  the energy radiated is seen as brightenings in
  the wings of H? ? Ellerman Bombs.