LASCO Observations of a Streamer CME
on 13-14 Aug 1997
M. D. Andrews1,2, and R. A. Howard2
1
Raytheon ITSS, 4500 Forbes Blvd, Lanham, MD 20706
E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington DC 20375
2
Abstract. The LASCO instruments observed a low-velocity Streamer Blowout CME on 13-14 August 1997. Data
animations show the development typical of a streamer CME. The event begins with a slow brightening of the preexisting helmet streamer. This particular CME develops into two teardrop-shaped loops that may be the signature
of disconnected magnetic structures. Several features have been tracked through the C2 and C3 FOVs. All of the
structures show a constant acceleration of 1.6-2.7 m/s2. LASCO polarization data have been analyzed for this
event. The CME shows a peak polarization fraction of 0.91. This implies that the scattering electrons are close to
the plane of the sky. The total mass of the CME is measured to be 5.8 E +15 gr. The polarization data allow an
estimate to be made of the electron density in this CME. The largest density is 1.7 + 0.4 E+5 e-/cm3.
INTRODUCTION
1
The LASCO coronagraphs on SOHO have been
observing and measuring Coronal Mass Ejections
(CMEs) since early 1996. The initial results from
LASCO were reported by Howard et al. [1]. They
divided the LASCO CMEs into two classes: fast and
slow. This paper considers a single slow CME.
The slow events begin very slowly with the start of
the event often not clearly defined. These events
usually show significant acceleration in the C2 and/or
C3 fields-of-view (FOV). Sheeley et al. [2] examined a
particular CME observed on 27 May 1979 which
produced a splitting of a bright pre-existing coronal
streamer. They suggested that “Streamer Blowouts” are
relatively common. Howard et al. [3] presented some
general characteristics of Streamer Blowout CMEs.
They indicated that these events occur in two phases: a
pre-existing streamer brightens over a period of a few
hours to a few days followed by the ejection of material.
After the CME, the streamer was cleaned out and
appeared as a dark, depleted region.
1
LASCO is the Large Angle Spectroscopic Coronagraph which is one
of the instruments on the Solar and Heliospheric Observatory (SOHO)
which is a mission of international cooperation between NASA and
ESA. The LASCO experiment was developed by a consortium of
institutions from four countries: E. O. Hulburt Center for Space
Research, Naval Research Laboratory, Washington, DC; Laboratoire
d’Astronomie Spatiale, Marseille, France; Max-Planck-Institut fur
Aeronomie, Lindau, Germany; and Space Research Group, School of
Physics and Space Research, University of Birmingham, Birmingham,
UK
In this paper, we consider a single CME as observed
by LASCO on 13-14 August 1997. This CME
completely removed the pre-existing coronal helmet
streamer. The following section will present the
LASCO observations of this event. The observed
morphology,
time-height
profiles,
polarization
measurements, mass calculation, and estimated electron
density will be presented. The paper will conclude with
a discussion section in which we present a possible
physical interpretation of this CME and briefly consider
other observations of this event.
OBSERVATIONS
In this paper, we consider only the LASCO C2 and
C3 observations. The C2 telescope images the region
from approximately 2.0 to 6.0 R with a resolution of
~23" and a pixel size of 11.9". The C3 telescope
images the region of approximately 3.7 to 32 R with a
resolution of ~113" and a pixel size of 56". During
August 1997, the normal procedure was to observe only
the equatorial regions in C2 and C3. The data presented
here cover the region of –3.5 to +3.6 R (-17.7 to + 15.7
R) in the S-N direction for C2 (C3). Each of the
instruments has a number of optical filters. These data
were collected using the Orange filter (540-640 nm) for
C2 and the Clear filter (400-850 nm) for C3. Brueckner
et al. [4] present additional information on the design
and performance of the C2 and C3 coronagraphs.
Figure 1. Two LASCO C3 partial frame images of the 13-14 August 1997 CME as it propagates away from the sun. The points
labeled A and B indicate the rear edge of the first loop and the V-shaped “disconnection point.” The points labeled C, D, and E
indicate the rear and leading edge of the bright knot and trailing edge of the second loop. See text.
The morphology of the CME is illustrated in Figure
1 and the data animations, e.g. movies, on the CD-ROM
that accompanies this volume. This event is not a big,
bright CME. The structures being discussed in this
paper are not easily shown in a single, static image. The
structure of this CME changes dramatically as it moves
away from the Sun. These dynamics are much more
visible in the movies.
The CME occurs on the West limb of the sun in the
middle of a bright pre-existing helmet streamer. The
CME is first visible as a distinct structure in the C2
image recorded at 10:26UT on 13 August 1997. As the
CME slowly moves outward, the bright amorphous blob
evolves into two distinct loop-like structures both of
which are clearly seen in the C2 image at 16:23UT.
The trailing loop is visible in C2 until 23:50UT 13
August 1977.
Figure 1 shows the CME at two positions in the C3
FOV. The left panel displays one of the first images in
which both structures are distinctly visible. Only the
trailing portions of the leading loop can be seen. The
features labeled A and B are the rear edge of a dark
cavity within the loop and the V-shaped “disconnection
point.” The right panel shows the CME approximately
seven hours later. The leading loop has faded from
view. The trailing loop has a bright knot near the rear
of the loop. The features labeled C, D, and E are the
rear of the bright knot, the leading edge of the bright
knot, and the rear edge of the loop. This loop remains
visible throughout the C3 FOV until it disappears at
21:18UT on 14 August 1997.
The Height vs. Time plot of the five features labeled
in Figure 1 is shown in Figure 2. Each of the five
features has been individually tracked through the C2
and C3 FOVs. The C2 and C3 measurements are
labeled 2 and 3, respectively. The uncertainty in the
measurement is comparable to the size of the symbol.
The curves are a least squares fit of a second order
polynomial, e.g., constant acceleration. As Figure 2
shows, a constant acceleration is a very good match to
the data. All of the derived accelerations are between
1.6-2.7 m/s2. The accelerations of features C and D, 1.6
m/s2 and 2.7 m/s2, are labeled in Figure 2.
The features labeled A and B become too faint to
track at heights below 20 R, at which point the
velocities are approximately 275 km/s and 250 km/s
respectively. Features C, D, and E are visible to the
edge of the C3 FOV.
The final velocities are
approximately 260 km/s for features C and E and 320
km/s for feature D.
The observed velocities and calculated accelerations
for this CME are very small. The event is not bright
and the leading loop fades from view at low solar
heights. All of these factors could be explained if this
CME were located at a significant distance from the
plane-of-the-sky. The low velocities and accelerations,
the low brightness, and the fading could all be explained
as projection effects. As the following paragraphs will
acceleration are very close to the actual
radial values. The leading loop is not as
strongly polarized and this emission
probably originates at a larger angle with
respect to the plane-of-the-sky.
The coronal mass can be precisely
calculated from the observed white light
images if the distribution along the lineof-sight is known. For these data, we
have assumed that all of the electrons
were at an angle of 18 from the plane-of
the-sky and applied the method of Hildner
[6], to estimate the coronal mass.
Andrews, Wu, and Wang [7] discuss the
calculation of coronal mass based on
LASCO C3 images in more detail.
This calculation can have large
systematic errors. In particular, while the
use of 18 for the angle is justified for the
brightest parts of the CME, it probably
results in an underestimate of the mass for
the other areas of the corona. The zero
point of the mass calculation is completely
arbitrary and is based on the use of a
single image to remove the “background
mass.”
The results of the mass calculation are
shown in Figure 3 and the data animation
(mass_aug) on the accompanying CDROM. Figure 3 shows the mass of both
Figure 2. Height vs. Time plot of the five features labeled A-E in Figure
the East and West streamer belts, which in
1. The measurements from C2 and C3 are indicated by the symbols 2 and
this calculation was the region within
3. Each set of measurements has been fit to a second order polynomial,
approximately +6 R of the solar equator.
e.g., constant acceleration. The fit is excellent. The values of the
The mass on the West side slowly
acceleration are those determined for features C and D.
increases over a period of about 35 hours.
The calculated mass then decreases slowly
show, this is clearly not the case for the features labeled
as material moves out of the C3 FOV. The mass of the
C, D, and E.
Western steamer reaches a level significantly lower than
During this period, the LASCO C3 coronagraph was
prior to the CME. The mass of the CME is taken to be
recording polarization sequences every 12 hours. While
the difference between the largest and smallest mass
this is not a very rapid cadence, this CME was so slow
values: 5.8 E+15 gr. For this CME, the lowest value
that it was observed five times and quantitative analysis
occurs after the event.
is possible. In particular, the increase in brightness at 8
The mass of the East-side streamer belt shows much
R due to the bright knot is approximately 91%
smaller changes. The changes in the East are probably
polarized.
due to rotation.
The Thomson scattering of solar radiation by
The movie (mass_aug on the CD-ROM) shows
electrons is described by Billings [5]. The polarization
coronal images processed into units of mass/pixel. The
of the scattered radiation is a function of the three
movie clearly shows that after the CME, the Western
dimensional structure of the coronal plasma.
A
equatorial region mass is reduced to the background
polarization of 91% implies that the CME must be close
coronal level. This CME does clean out the streamer
to the plane-of-the-sky. With reasonable assumptions
and the term Streamer Blowout is quite appropriate.
for the structure of the CME, the emission is found to be
We have estimated the electron density for the bright
at an angle of approximately 18 from the plane-of-theknot at solar elevations near 8 R. This it not usually
sky. This implies that the features labeled C, D, and E
possible since the determination of the density requires
are not at large angles and the observed velocity and
that the distribution along the line-of-sight be known.
small. Second, this CME formed two
loops: one was “hollow” while the other
had a bright core. At this time, we have no
explanation for either of these unusual
features.
This CME was observed at a height of
8 R by UVCS as reported by Strahan
[10]. They report outflow velocities and
densities consistent with the LASCO
results.
On
19-20
August
1997,
an
interplanetary CME was observed by the
Near Earth Asteroid Rendezvous (NEAR)
magnetometer
(D.
Rust,
personal
communication). NEAR was located at an
angle of approximately 30 forward from
the
plane-of-the-sky. The possibility that
Figure 3. The calculated change in coronal mass for the East- and West-limb
the
leading
loop of the LASCO CME was
streamer belts during the 13-14 August 1997 CME. The Eastern (Western)
observed at NEAR is being investigated.
data is indicated by  (). The mass of the CME is approximately 5.8 E +15
For the two observations to be of the same
gr.
event, the average propagation velocity
For this particular CME, the large observed polarization
must be approximately 480 km/s. While the features A
implies that the scattering electrons are close to the
and B in the LASCO CME may correspond to the
plane-of-the-sky and that the depth along the line-ofNEAR observations, this would require additional
sight is not large. The bright knot has the projected size
acceleration since the velocities observed by LASCO
of approximately 3 R. With the assumption that the
are too small to reproduce the observed arrival time.
knot is 3 R deep, the electron density is obtained by
dividing the mass/pixel by the appropriate volume. The
largest estimated density of the bright knot is 1.7 + E+5
REFERENCES
e-/cm3. Typical values are 0.5 to 1.0 E+5 e-/cm3.
1. Howard, R. A., et al., “Observations of CMEs from
DISCUSSION
The Streamer Blowout CME of 13-14 August 1997
shows several features that are typical of this type of
event. The event begins with the slow swelling and
brightening of a pre-existing helmet streamer. The
initial velocities are small with significant, in this case
constant, acceleration throughout the C2 and C3 FOVs.
As the CME develops, the shape evolves into two
teardrop shaped loops. With the notable exception of
the twin loops, all of these features are typical of a
Streamer Blowout CME.
Simnett et al. [8] have presented LASCO
observations of two streamer CMEs: 28 July and 5
November 1996. They interpret the loop-like structures
and the trailing V shape as the signature of disconnected
plasmoids.
Wu, Guo, and Andrews [9] presented a MHD model
of the 28 July 1996 CME. They show that the
emergence of an evacuated flux rope will trigger the
CME. The observed teardrop shaped loop is the
deformed flux tube.
The 13-14 August 1997 CME is unusual in two
ways. First, the velocity and acceleration are unusually
SOHO/LASCO,” in Coronal Mass Ejections, edited by
N. Crooker, J. Joselyn, and J. Feynman, AGU
Monograph 99, 1997, pp.17-26.
2. Sheeley, Jr., N. R., et al., Space Sci. Rev., 33, 213 (1982).
3. Howard, R. A., et al. JGR, 90, 8173 (1985).
4. Brueckner, G. E., et al., Solar Phys., 162, 291 (1997).
5. Billings, D. E., A Guide to the Solar Corona, Academic
Press, San Diego, 1966, pp. 65.
6. Hildner, e., Gosling, J. T., MacQueen, R. M., Oland, A.
I., and Ross, C. L., Solar Phys., 42, 163 (1975).
7. Andrews, M. D., Wang, A.-H., and Wu, S. T., Solar
Phys., submitted (1999).
8. Simnett, G. M., Solar Phys., 175, 685 (1997).
9. Wu, S. T., Guo, W. P., Andrews, M. A., et al., Solar
Phys., 175, 719 (1997).
10. Strachan, L., et al., this proceedings (1999).