1995SoPh..160..277W
FLUX DISTRIBUTION OF SOLAR INTRANETWORK MAGNETIC
FIELDS
JINGXIU WANG
Beijing Astronomical Observatory, Chinese Academy
of
Sciences, Beijing 100080, China
and
HAIMIN WANG, FRANCES TANG, JEONGWOO
W.
LEE and HAROLD ZIRIN
Big Bear Solar Observatory, California Institute
of
Technology, Pasadena,
CA
91125, U.S.A.
(Received 5 December, 1994; in revised form 5 May, 1995)
Abstract.
Big Bear deep magnetograms
of
June
4,
1992 provide unprecedented observations for
direct measurements
of
solar intranetwork (IN) magnetic fields. More than 2500 individual IN
elements and 500 network elements are identified and their magnetic flux measured in a quiet region
of
300 x 235 arc sec. The analysis reveals the following results:
(1) IN element flux ranges from 10
16
Mx (detection limit)
to
2 x 10
18
Mx, with a peak flux
distribution
of
6 x 10
16
Mx.
(2) More than 20%
of
the total flux in this quiet region is in the form
of
IN elements at any given
time.
(3) Most IN elements appear as a cluster
of
mixed polarities from an emergence center (or centers)
somewhere within the network interior.
(4) The IN flux is smaller than the network flux by more than an order
of
magnitude. It has a
uniform spatial distribution with equal amount
of
both polarities.
It
is speculated that IN fields are intrinsically different from network fields and may be generated
from a different source as well.
1.
Introduction
The intranetwork (IN) magnetic fields are at the tail end of the spectrum
of
solar
magnetic fields in the photosphere. They are weakest in field strength and smallest
in flux. They appear as small elements of magnetic fields inside the network
(supergranular) cells. Like the network fields, their presence does not seem
to
be
dependent on the latitude on the globe. Unlike network fields, however, the IN
fields are not affected by decaying active region fields.
IN fields were first observed by Livingston and Harvey (1975) and Smithson
(1975). They were described
as
'discrete elements'
of
mixed polarities 'interior to
the network'. In recent years progress was made in the understanding
of
IN's mor-
phology, dynamics and some quantitative aspects from time sequences
of
deep
magnetograms obtained at the Big Bear and Huairou Solar Observatories (Livi,
Wang, and Martin, 1985; Martin, 1984, 1988, 1990; Shi
et
al., 1990; Wang
et
al.,
1985; Wang, Zirin, and Shi, 1989; Wang and Shi, 1988; Wang and Zirin, 1988;
Zirin, 1985, 1987, 1993).
The first Stokes
V line ratio measurement made by Keller
et
al.
(1994) has
placed an upper limit on the intrinsic strength of IN fields at 1000 G and with 68%
Solar Physics 160: 277-288, 1995.
© 1995 Kluwer Academic Publishers. Printed
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Academic Publishers • Provided by the NASA Astrophysics
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1995SoPh..160..277W
278
JINGXIU WANG
ET
AL.
probability at 500
G.
By using an infrared array technique, Lin (1994) was able
to directly measure the Zeeman splitting
of
the Fe I 15648.5 A and Fe I 15652.9 A
lines
of
the IN fields. His result revealed that the fields typically have field strengths
around 500
G.
The current knowledge about IN fields may be summarized as follows: (1) They
consist
of
'point-like' elements
of
mixed polarities within the network cells. (2) IN
flux elements move at a speed
of
0.3-1.0 km
s-
1
,
but they do not always move
radially toward the cell boundaries (Zirin, 1985). (3) They are intrinsically weak,
each estimated to have a total flux between 10
16
and10
17
Mx (Zirin, 1987). ( 4) They
interact with the network elements upon contact, either 'canceling' with opposite-
polarity elements or merging with same-polarity elements (Livi, Wang, and Martin,
1985). (5) Their lifetime has been estimated by several groups, but not statistically
determined. A wide range
of
lifetimes has been reported. High temporal resolution
magnetograms show that some
of
the IN elements are stable for at least several
hours.
While most
of
the network elements can be easily observed, most
of
the IN
fields are difficult
to
observe because
of
their smaller size and low flux density. The
June 4, 1992 data obtained at Big Bear provided the first set
of
observation with
which a statistical study
of
the IN and network fields may be carried out.
2.
Data
Analysis
2.1. DATA
The data on June
4,
1992 are the best quiet region observations ever obtained at
BBSO. The region is near the center
of
the Sun at SlO W3. The magnetograms
were acquired by integrating 4096 video frames, then recording in a 16-bit memory.
The calibration was made by the method described by Varsik (1994). The 7-hr
observation yielded 73 magnetograms, which were registered by crosscorrelation.
The sequence was then made into a movie from which the motion and evolution
of
the magnetic elements can be traced.
Throughout this paper, the term 'magnetic element' does not imply an ele-
mentary flux tube, which is beyond our present knowledge.
It
merely refers to an
observational entity: the intersection
of
a bundle
of
magnetic field lines with the
photosphere .
Individual IN elements were identified, and their flux measured. Figure 1 is a
magnetogram at 16:49 UT when the seeing was the best. The flux measurement is
based on this magnetogram. Solar south is at the top, east is to the right. The field
of
view is 617 x 473 pixels. Each pixel is 0.5 arc sec. Enhanced network can be
seen in most
of
the eastern part
of
the field
of
view, while quiet network can be
seen in parts
of
the western half.
Since IN fields have low flux density (the magnitude
of
the vector B in
Maxwell's equations in a medium), noise in the magnetograms could be mis-
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1995SoPh..160..277W
FLUX DISTRIBUTION OF SOLAR INTRANETWORK MAGNETIC FIELDS 279
Fig.
1.
Magnetogram
of
a quiet region at 16:49
UT
on June 4, 1992. South is top, east is to the right.
The field
of
view is 308 x 237 arc sec centered at SlO W3.
construed as IN magnetic elements. To carefully determine the noise level,
we
select another magnetogram taken 5 min after the best seeing frame (Figure 1 ), and
subtract the two (avoiding the network boundary areas). Since the IN field does
not change appreciably in such a short time interval, the subtraction will yield the
noise
of
the magnetograms. The average r.m.s. value for the 20 network cells is
3.0
+ 0.2
G,
which places an upper limit
of
2.1 G to the real noise level in a single
magneto gram.
To match the real spatial resolution determined by the atmospheric seeing, a
2 x 2
pixel smooth average was made before the flux measurements were made. This
further increased the ratio
of
signal to noise. The final noise level after smoothing
is around
1 G for the best magnetograms with a spatial resolution
of
2 arc sec.
2.2.
IDENTIFICATION
OF
IN ELEMENTS
In most cases, it is not difficult to distinguish the IN magnetic elements from the
network elements. Three criteria are used to separate the two. (1) Flux density:
our empirical results from magneto grams with spatial resolution
of
1-
2 arc sec
indicate a maximum flux density higher than 40 G for network and lower for IN
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1995SoPh..160..277W
280
JINGXIU WANG ET AL.
fields. (2) Motion: network elements move at an average speed
of
0.06
km
s-
1
,
while IN elements move at a much higher average speed
of
0.4
km
s-
1
(Zirin,
1985). (3) Location: IN elements are within the supergranular network. The latter
has a lifetime
of
50 to 100 hours (Wang et al., 1989; Liu et al., 1994) and can be
easily identified most
of
the time.
For the study, a magnetic element is defined as a flux patch with a maximum
flux density higher than
2 G and an area larger than 1 (arc sec)
2
.
This results in a
detection limit for magnetic flux
of
10
16
Mx.
The flux measurements for IN and network magnetic elements were made with
an interactive data language (IDL) procedure. The elements are identified visually.
The boundary
of
the element is then traced with the mouse and its area, total flux
and maximum flux density are measured. Before flux measurements, all pixels
with flux density lower than 1 G were set
to
0 G (appears as gray in the image),
so that the periphery
of
an element is drawn at the 1 G level.
It
is assumed that
the flux distribution in an element is gaussian with a peak in flux density. Thus,
two closely connected elements are separated at the line
of
lowest flux densities
between them. The IN elements usually are well separated in the magnetogram, as
is shown in Figure
1. To separate the network elements which often clump together,
the dynamic range
of
the display is raised such that the elements shrink in size
and become distinct. For IN flux measurements, the network elements are masked
out first by use
of
the IDL. Likewise, for the network flux measurements, the IN
elements are masked out first.
3. Results
3.1.
THE
APPEARANCE AND DYNAMICS OF IN FIELDS
The IN magnetic elements emerge mainly in clusters
of
mixed polarities from a
localized area within a network cell.
We
call such an area a flux emergence center. In
Figure
2,
an example
of
a cluster emergence is shown. The circle drawn at 17:43
UT
indicates the site
of
an emergence center. By 19:50
UT,
most
of
the IN elements
in this group had emerged. Although the seeing deteriorated after
16:49
UT,
the
elements were still plainly visible. Note that this emergence center, unlike most
cases, is not located in the center
of
the network cell. In another such example,
an emergence center is right below a network magnetic element. IN elements are
seen to move continuously out from the boundary
of
the network element. Later
the network element itself is broken into two fragments as a result.
A typical sized IN element is indicated by an arrow at
16:49
UT
in Figure 2.
Even though it is visible on the first generation print, the readers may not be able
to make it out on theirs. This negative polarity flux measures
-6
x 10
16
Mx wiih
an area
of
5 (arc sec)
2
and a peak flux density
of
-4.8
G.
Other features
of
interest
in the figure are as follows: IN elements 'canceling' with network
of
opposite
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1995SoPh..160..277W
FLUX
DISTRIBUTION
OF
SOLAR INTRANETWORK MAGNETIC FIELDS 281
Fig.
2.
Time sequence
of
magnetograms showing the site
of
an
IN emergence center where a cluster
of
mixed polarity elements emerged (circle); an IN element with
flux
of
6 x
10
16
Mx (arrow at
16:49 UT); an IN element 'canceling' with network
of
the opposite polarity (a bracket); and a fast
moving IN element with an average speed
of
0.38 km
s-
1
(to the left
of
the arrow at 18:55 UT).
polarity (marked with a bracket), and a large positive IN element clearly visible
throughout the observation with an average speed
of
0.38 km
s-
1
.
The arrow in
the frame
of
18:55 UT marks the direction
of
the motion
of
the element, which
is to the left
of
the arrow. Later in the 22:13 UT frame, this IN element can be
seen merged with a network element
of
the same polarity. Occasionally a small
dipole appears in the cluster. Unlike other IN dipoles that travel in the same general
direction, this one grows as the two polarities move in opposite directions, much
like an ephemeral region (ER). At maximum development, the magnetic flux
of
this dipole is 2.5 x 10
18
Mx, typical for network magnetic elements. Parentheses in
Figure 2 indicate the evolution
of
the small ER. In the seven hours
of
observation, a
total
of
16 ephemeral regions are observed,
of
which 12 have magnetic flux similar
to the one shown.
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