Three-dimensional reconstruction techniques used to determine coronal density distributions from synoptic data are complicated and time consuming to employ. Current techniques also assume time invariant structures and thus mix both temporal and spatial variations present in the coronal data. The observed distribution of polarized brightness, pB, and brightness, B, of coronal features observed either at eclipses or with coronagraphs depends upon both the three-dimensional distribution of electron density within the structure and the location of the feature with respect to the plane-of-the-sky. By theoretically studying the signature of various coronal structures as they would appear during a limb transit, it is possible to recognize these patterns in real synoptic data as well as estimate temporal evolutionary effects

Fisher, R. R.

Munro, R. H.

English

NASA Technical Reports Server

INTERPRETATION OF CORONAL SYNOPTIC OBSERVATIONS
Richard H. Munro and Richard R. Fisher
High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO
I. INTRODUCTION
Three-dimensional reconstruction techniques used to determine coronal density distribu-
tions from synoptic data are complicated and time consuming to employ. Current techniques also
assume time invariant structures and thus mix both temporal and spatial variations present in the
coronal data. The observed distribution of polarized brightness, pB, and brightness, B, of coronal
features observed either at eclipses or with coronagraphs depends upon both the three-dimensional
distribution of electron density within the structure and the location of the feature with respect
to the plane-of-the-sky. By theoretically studying the signature of various coronal structures as
they would appear during a limb transit, it is possible to recognize these patterns in real synoptic
data as well as estimate temporal evolutionary effects.
II. TIME INDEPENDENT MODELS
Coronal streamers, large-scale white-light structures found in the solar corona, are the most
impressive and easily observed features of the extended solar atmosphere. In the summer of
1984, a period in which there were few structures associated with solar active regions, the corona
was dominated by streamers associated with the large-scale solar magnetic field. Rotating more
rigidly than the photospheric material, several streamers were observed near the solar limbs on
DOY 222 with both the spacecraft and ground based instruments. For this investigation, only the
distribution of polarized brightness (pB), at a reference height of 1.3 Ro, over latitude and longi-
tude has been used to estimate a model-dependent electron density at the base of these
streamers. Portions of the K-coronameter synoptic record are shown in Figure 1.
The investigation of the gross brightness properties of the solar corona does not require a
detailed model of the density structure. Thus we have chosen to represent coronal features using
simple "ad hoc" mathematical descriptions. Coronal streamers are assumed to be radial features
with circular cross sections that depend on radius. The bottom portion of "helmet-like" shape
(increasing then decreasing cross-sectional projection) was obtained from eclipse measurements.
The cross-sectional area is set to a constant above a given height ('2.5 Re). At 1.3 Ro, the
angular width of the streamer measured from Sun center is 15 degrees. For these calculations,
the density structure within the streamer has the identical radial logarithmic gradient as the
surrounding background, but with a constant enhancement over the background (corresponding
to an increased base pressure). Model streamers were placed in solar longitude and latitude at
the corresponding brightness maxima of the observations.
Note that during the period of these observations, the large-scale corona contained six
bright regions or streamers. On DOY 222, two of these were located in the southern hemisphere
on the east limb, two were located near the plane-of-the-sky on the west limb in the northern
hemisphere, and the remaining two streamers, somewhat fainter than the others, spanned the
equator near the center of the disk.
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An equatorial symmetric background corona was initially used to determine a multiplier to
the Alien (1963) Minimum Model to replicate the observed polarized brightness between the
streamers at 1.3 R o. The streamer enhancements required to match the observed maximum
polarized brightnesses for each structure averaged 3.1 times the equatorial background. This
enhancement in streamer density required is, of course, dependent upon the specific model
geometry given above, and is only a sufficient model. Taken at face value, a mean density of 5.6
x 107 electrons cm-3 in the Streamer is obtained at the reference height. The ratio of streamer pB to
the background corona is roughly the same as that found by Wilson (1977). On the other hand, the
absolute density for the entire corona is about a factor of 2 lower than the extrapolated value
obtained from Wilson's study. Estimates of the expected polarized brightness distribution (Fig. 2),
obtained by using the model density distribution, replicate the observed distribution of polarized
brightness with reasonable fidelity.
III. SIGNATURES OF STREAMERS IN K-CORONAMETER DATA
The major features found in the observed synoptic chart (Fig. 1) from the east and west
limb data are reproduced (Fig. 2), providing insight into the effects of the long integration path
through the corona and the variation of the apparent location of the streamer as a function of solar
rotation. Thus, it is clear that this relatively simple modeling technique allows the rapid interpreta-
tion of K-coronameter and coronagraph results, at least in the case of the largest features found
in the solar corona. However, some of the characteristic signatures displayed in the model are
absent in the data. The dashed lines on the observed synoptic chart mark the theoretical track of
the two streamers closest to the solar poles. While the southern streamer matches the model cal-
culations, a glaring discrepancy exists for the northern-most high-latitude streamer. In this case,
the data do not display a symmetric feature, but rather a strange trailing extension from the maxi-
mum polarization brightness region (time runs right to left in coronal synoptic maps). To gain
insight into this problem, synoptic observations from the three HAO K-coronameters covering the
years 1965-1984 were used to classify the signature of streamers as they appeared during limb
passage. The shape categories included round, trailing extensions, leading extensions, symmetric
extensions, and other complex features. The results are summarized in Table 1. Although it is
possible to construct static models of coronal density distributions that produce the observed
shape of the extensions, round coronal signatures cannot be produced by a static structure.
Emerging from an examination of a long-term synoptic record is the perception that while
the underlying organization of the large-scale photospheric magnetic field tends to be quite per-
sistent, at least one-third of the streamers identified in K-coronameter synoptic charts must evolve
with a timescale approximately equal to the length of time such a structure is visible using coro-
nagraphic techniques. Taking this argument further, the overwhelming number of trailing exten-
sions over leading extensions implies that the temporal scales for injecting material into a streamer
are shorter than for removing thematerial.
IV. TIME DEPENDENT MODELS
To investigate how time-dependent density enhancements and reductions might appear in
synoptic observations, the model streamers described above were subjected to linear rates of
formation and destruction. Figure 3 depicts sample signatures for streamers at a latitude of 45
degrees North where the maximum density for the streamers occurs one day after, at, and one day
258
before limb passage for the top, middle and bottom panels, respectively. With each panel, the
model streamers form in one day, but decay in one, two, three, and four days. The signatures for
slow formation and rapid decay are mirror images of these forms about the central time of the
individual panels (or 90 degrees longitude).
These calculations show that:
(1) The absence of an extension implies rapid density variations and "round" signatures
require both rapid injection and removal of material.
(2) The location of the observed maximum brightness depends on the time when the
streamer attained its maximum density, and only weakly on the spatial location of the streamer.
The physical location of the streamer remained constant in the models displayed in Figure 3;
only the time of maximum density was allowed to vary. As a rule of thumb, the observed maxi-
mum brightness signals the time of maximum density within 8 hours as long as the peak density
was attained within 2 days of the streamer's limb passage. Thus, caution must be used when
associating the physical location of streamers as determined by synoptic observations with under-
lying photospheric features.
(3) The shape of the brightness distribution of an observed streamer can provide clues as
to the nature of the temporal evolution of streamer material. This variation can be seen by select-
ing a particular injection and removal rate (e.g., 1:4, the lower right of each panel), and compar-
ing the signatures as the time of peak density varies from after (top) to before (bottom) limb
passage.
REFERENCES
Allen, C. W., 1963, in Astrophysical Quantities, University of London, London, p. 176.
Wilson, D. C., 1977, The Three Dimensional Solar Corona - A Coronal Streamer, NCAR Coopera-
tive Thesis No. 40, National Center for Atmospheric Research, Boulder, Colorado.
259
/I Z26 2?.4 2ZZ 220 2re I 228 2z6 22.4 22?_ 220 218 x I
t DAY OF OBSERVATION,1984 t
I != EAST LIMB DATA =1 =- WEST LIMB DATA -"
Figure 1. Portions of the K-coronameter synoptic record, from both the east and west limbs,
centered on DOY 222 1984. Amplitude of the polarized brightness of the corona is plotted
as a function of solar latitude and day of observation. Dashed lines mark the theoretical
track of the northern-most and southern-most streamers.
90 I I I
-45
-9o I I
-90 0 90 180 270
Figure 2. The estimated polarized brightness that would be observed from the earth if material
were distributed over the volume of the corona in accordance with a model that has (1) a quiet
background contribution, the axis of symmetry which is tilted with respect to the solar rotation
axis,.and (2) six embedded streamers with the geometrical properties given in the model (see text).
The streamer densities have been adjusted so as to achieve the observed peak polarized brightness.
260
-so
I:1 1:2
I::l 1:4
z [I.gQ I:1 i: 2
"1:3 1:4
-Io
I:l 1:2it-to
[ 1:3 1:4
_TIM[
Figure 3. Synoptic observational signatures of time-dependent model streamers with varying
rates of material removal and time of maximum density enhancement.
(See text for additional information.)
TABLE 1. OBSERVED STREAMER SIGNATURES IN THE HAO
K-CORONAMETERS 1965-1984
Obvious Temporal Evolution
Stationary Sum Other, Total
Round Trailing Leading Complex
Extension Extension Features
Number 77 250 412 57 796 1131 1927
Fraction 4% 13% 21% 3% 41% 59% 100%
Fraction 10% 31% 52% 7% 100%
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Interpretation of coronal synoptic observations

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