Coronal observations have been processed for parts of each year during the interval 1979-1985. Around sunspot maximum, coronal mass ejections (CMEs) occurred at the rate of approximately 2 per day, and had a wide range of physical and morphological properties. During the recent years of relatively low sunspot number, CMEs occurred at the rate of only 0.2 per day, and were dominated by the class of so-called streamer blowout. These special CMEs maintained a nearly constant occurrence rate of roughly 0.1 per day during the entire interval

Howard, R. A.

Koomen, M. J.

Michels, D. J.

Sheeley, N. R., Jr.

English

NASA Technical Reports Server

SOLWlND OBSERVATIONS OF CORONAL MASS EJECTIONS DURING 1979-1985
(Invited)
N. R. Sheeley, Jr., R. A. Howard, M. J. Koomen,* and D. J. Michels
E.O. Hulburt Center for Space Research, Naval Research Laboratory
Washington, D.C. 20375-5000, U.S.A.
ABSTRACT
Coronal observations have been processed for parts of each year during the interval 1979-
1985. Around sunspot maximum, coronal mass ejections (CMEs) occurred at the rate of approxi-
mately 2 per day, and had a wide range of physical and morphological properties. During the
recent years of relatively low sunspot number, CMEs occurred at the rate of only 0.2 per day,
and were dominated by the class of so-called "streamer blowouts." These special CMEs
maintained a nearly constant occurrence rate of roughly 0.1 per day during the entire interval.
I. INTRODUCTION
All but a few months of the Solwind coronal observations have been processed for the
interval March 1979 to December 1981 around the maximum phase of sunspot cycle 21. During
this interval more than 1000 coronal mass ejections (CMEs) were identified. Their properties have
been measured and studied statistically by Howard et al. (1985), and summaries of this work
have been published elsewhere (cf., Howard et al., 1984; Michels et al., 1984). In addition,
several investigators have studied the associations between these CMEs and a variety of other solar
and interplanetary phenomena including soft X-ray events (Sheeley et al., 1983a), metric Type II
radio bursts (Sheeley et al., 1984; Kahler et al., 1984b), interplanetary shocks (Schwenn, 1983;
Sheeley et al., 1983b, 1985; Woo et al., 1985), energetic proton events (Kahler et al., 1984a),
and magnetic clouds (Burlaga et al., 1982).
The Solwind instrument has continued to obtain coronal images routinely since December
1981, and is still doing so at the time of this writing (May 1985). However, our processing of
these data was interrupted for about one year while we were converting to a new computer.
During the past few months, we have resumed the processing, and are rapidly reducing the obser-
vations during 1982-1985. The most striking characteristic of the recent observations is the
persistent streamer structure and the substantial reduction in the occurrence rate of coronal mass
ejections. The resulting correlation between the occurrence rate of CMEs and the level of sunspot
activity (the sunspot number) is similar to that deduced by Hildner et al. (1976) from their
study of mass ejections during the Skylab mission in 1973-1974. It is substantially different from
the nearly constant CME rate that Hundhausen et al. (1984) derived by combining their SMM
observations in 1980 with their reanalysis of the Skylab data. We suppose that the discrepancy
results from assumptions about the "dead time" of the Skylab and SMM coronagraphs.
Perhaps even more interesting is the fact that the recent CMEs have consisted primarily
of the so-called "streamer blowout" class (cf, Sheeley et al., 1982; Howard et al., 1985). During
1979-1981 we found that this class of CME constituted only 5% of all mass ejections and only
8% of all "major" mass ejections. During the four months of 1984-1985 for which we have
*Sachs/Freeman Associates, Bowie, MD 20715.
241
https://ntrs.nasa.gov/search.jsp?R=19860015164 2020-03-20T15:24:47+00:00Z
processed continuous data, these special CMEs have constituted 36% of all CMEs and 67% of all
major CMEs. Furthermore, it appears that the occurrence rate of "streamer blowout" CMEs
has been approximately constant during the interval 1979-1985 at a value of nearly 0.1 per day.
In this paper, we shall present a brief summary of these recent results, and refer the
reader to the published studies and review papers for more detailed properties of Solwind mass
ejections and their associations.
II. THE RESULTS
(a) The CME Occurrence Rate
From their study of 998 CMEs during 1979-1981, Howard et al. (1985) found that CMEs
could be conveniently classified by morphological structure and importance. They identified nine
more or less distinct classes, examples of which are shown in Figure 1. The importance of a CME
is a somewhat subjective quantity, but in effect it is a measure of the CME's projected size and
brightness. In practice, the classification was performed by two observers, and the assignment was
either major (Y) or minor (N) when both observers agreed on this assignment. The assignment
was questionable (Q) when the observers could not agree or when data gaps made the assignment
indeterminate. Figure 2 illustrates the importance categories for five different morphological
classes of mass ejection.
Howard et al. (1985) found that during the entire interval 1979-1981 the occurrence
rate for all CMEs (regardless of structural class or importance category) was 1.8 per day. This
determination was based on their choice of 4.5 hrs as the minimum "dead time" required to
constitute a data gap. This gave a 66.5% duty cycle. When the minor (N) CMEs were excluded,
the occurrence rate was 0.9 per day, which is essentially the same as Hundhausen et al. (1984)
obtained from their analysis of CMEs observed by the SMM satellite during 1980.
Figures 3, 4, and 5 illustrate the variation of the occurrence rate on a variety of time
scales during 1979-1981. Although there was considerable variation on the shorter scale of 7
days, this variation does not seem to be strongly or consistently related to fluctuations in the
sunspot number. However, when we began to process observations during 1984-1985, we found
intervals of several weeks at a time when there were no CMEs. We never found such intervals
during 1979-1981. After processing completely the observations during June 1984, October and
November 1984, and March 1985, we found only 22 CMEs (of all classes and importance cate-
gorys), for an average rate of only 0.2 per day. If one corrects this rate using the 4.5-hr dead
time correction that was used for the CMEs during sunspot maximum, then the rate increases to
0.4 per day. However, we think this correction is not necessary during sunspot minimum because
our difference images show long intervals of 12 hrs or more during which there is no appreciable
change at all. Thus, if a CME had occurred in a 12-hr data gap during this phase of the sunspot
cycle, we would have seen indications of it.
Figure 6 summarizes our measurements to date. The recent observations have been
assigned to the date 1984, and for consistency both the uncorrected value of 0.2 per day and
the "overcorrected" value of 0.4 per day have been plotted. For reference, the dashed line
indicates the rate that Hildner et al. (1976) deduced from their analysis of the Skylab observa-
tions. Although it is probably premature to emphasize the detailed agreement between the
Solwind and Skylab results, the comparable trend with sunspot number is both obvious and
significant.
242
If we adopt the value of 0.2 per day for CMEs (of all classes and importances) during
1984-1985, then we find a value of only 0.1 per day when the minor CMEs are excluded. Of
these, 67% or 0.07 per day consisted of streamer blowouts. This is comparable to the rate of
0.09 per day for streamer blowouts during 1979-81 when this class constituted only 8% of all
major CMEs. Figure 7 shows the occurrence rate for these special CMEs during the interval
1979-1985. Although the statistics are poor, especially for the recent time, the resulting variation
is in sharp contrast to that obtained for all classes of CME shown in Figure 6.
(b) The Properties
Figure 8 shows the evolution of a typical "curved front" CME on 18,19 November 1981.
Mass ejections like this one constituted at least 15% of the CMEs that occurred in the years
around sunspot maximum. Howard et al. (1985) found that such CMEs had average speeds of
584 km]s, spans of 62 degrees in the sky plane, mass of 8.4 x 101 s gm, and kinetic energy of
6.4 x 1030 ergs. Characteristically, they were associated with long-duration X-ray events (Sheeley
et al., 1983a), interplanetary shocks (Sheeley et al., 1985), and energetic protons (Kahler et al.,
1984a).
Notice in Figure 8 that a coronal streamer moved southward as the CME transited the
field of view. Figure 9 shows the subsequent eruption or "blowout" of this streamer during a
19 hr interval on 19_20 November !981. Howard et al. (1985) found that the average properties
of such streamer blowouts included a relatively low speed of 200 km/s, a span of 44 degrees, a
moderately high mass of 5.4 x 101 s g, but a relatively low kinetic energy of 0.56 x 103° ergs.
One of these streamer eruptions (cf., Figure 10a) was associated with a large shock (cf., Figure
10b) at the Helios 1 spacecraft (Sheeley et al., 1983b, 1985). However, during 1979-1981 most
of these streamers and their eruptions occurred at relatively high latitudes and thus did not affect
the near ecliptic spacecraft conditions. However, like the streamers with which they originate,
the streamer blowout CMEs have tended to occur at relatively lower latitudes during 1984-1985
than they did during 1979-1981. Figure 11 shows a sample of these more recent events, and
Figure 12 shows the evolution of a particularly well observed one.
It is interesting to compare some average properties of CMEs observed in the two intervals
1979-1981 and 1984-1985. Whereas the average projected latitude was 45 degrees near sunspot
maximum, it was 22 degrees toward minimum. Similarly, the average span in position angle
dropped from 44 degrees to 22 degrees. The average CME speed dropped from 472 km/s to
roughly 130 km/s, which is consistent with the fact that most of the recent CMEs were the
characteristically slow streamer blowouts.
III. DISCUSSION
In view of the preliminary state of our recent results, we can only speculate on their
significance. The order-of-magnitude drop in the CME rate between sunspot maximum and
the present time is surely significant. It points to a strong connection between the occurrence
of CMEs and the level of solar magnetic activity. This seems consistent with the idea that CMEs
are tracers of the ejection of flux from the Sun. It is also consistent with the fact that a large
fraction of fast CMEs are associated with major X-ray flares, metric Type II bursts, interplanetary
shocks, and energetic protons.
On the other hand, the nearly constant occurrence rate of the streamer blowout CMEs
points to their possible identification as a normal state of the evolution of coronal streamers.
243
Such structures are present at all phases of the sunspot cycle. Only their heliographic location
evolves with time. Thus, one might speculate that the streamer blowouts are simply phases in the
evolution of streamers just as disappearing filaments (eruptive prominences) are phases in the
evolution of filaments (prominences). Indeed, it is tempting to suppose that streamer blowouts
and eruptive prominences are two aspects of the same process. However, further study is
certainly required before we can take this speculation seriously.
Finally, we note the similarity between the helmet-like magnetic structure of coronal
streamers and the teardrop structure of planetary magnetotails and comet tails. Thus, it is
possible that an understanding of the physics of streamer blowouts will have a broad application
to other astrophysical objects.
Acknowledgments. We are grateful to D. Roberts, F. Harlow, and W. Funk for their con-
tinuing data processing and other technical support. The Department of Defense Space Test
Program was responsible for the P78-1 vehicle and for integration, launch, and operational
support. The Air Force Satellite Control Facility has provided the orbital operations support.
The NASA Office of Heliospheric Physics provided some financial support through DPR W 14,
429.
244
REFERENCES
Burlaga, L. F., Klein, L., Sheeley, N. R., Jr., Michels, D. J., Howard, R. A., Koomen, M. J.,
Schwenn, R., and Rosenbauer, H., 1982, Geophys. Res. Lett., 9, 1317.
Hildner, E., Gosling, J. T., MacQueen, R. M., Munro, R. H., Poland, A. I., and Ross, C. L., 1976,
Solar Phys., 48, 127.
Howard, R. A., Sheeley, N. R., Jr., Koomen, M. J., and Michels, D. J., 1984, Adv. Space Res., 4,
307.
Howard, R. A., Sheeley, N. R., Jr., Koomen, M. J., and Michels, D. J., 1985, J. Geophys. Res.
(submitted).
Hundhausen, A. J., Sawyer, C. B., House, L., Illing, R. M. E., and Wagner, W. J., 1984, J. Geophys.
Res., 89, 2639.
Kahler, S. W., Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Michels, D. J., McGuire, R. E.,
von Rosenvinge, T. T., and Reames, D. V., 1984a, J. Geophys. Res., 89, 9683.
Kahler, S. W., Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., and Michels, D. J., 1984b,
Solar Phys., 93, 133.
Michels, D. J., Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Schwenn, R., Muhlhauser,
K. H., and Rosenbauer, H., 1984, Adv. SpaceRes., 4, 311.
Schwenn, R., 1983, Space Sci. Rev., 34, 85.
Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Michels, D. J., Harvey, K., and Harvey, J.,
1982, Space Sci. Rev., 33, 219.
Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Michels, D. J., 1983a, Astrophys. J., 272, 349.
Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Michels, D. J., Schwenn, R., Muhlhauser, K.
H., and Rosenbauer, H., 1983b, in M. Neugebauer (ed.), Solar Wind Five, NASA Confer-
ence Publication 2280, Washington, D.C., p. 693.
Sheeley, N. R., Jr., Steward, R. T., Robinson, R. D., Howard, R. A., Koomen, M. J., and Michels,
D. J., 1984, Astrophys. J., 279, 839.
Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Michels, D. J., Schwenn, R., Muhlhauser,
K. W., Rosenbauer, H., 1985, J. Geophys. Res., 90, 163.
Woo, R., Armstrong, J. W., Sheeley, N. R., Jr., Howard, R. A., Koomen, M. J., Michels, D. J., and
Schwenn, R., 1985, J. Geophys. Res., 90, 154.
245
1268 iO .Ji,;_',,_E79 _,547
Figure 1. A sample of Solwind coronal mass ejections during 1979-1981 illustrating the variety
of morphological classes as defined by Howard et el. (1985). The small white disk
indicates the size of the solar disk.
246
Figure 2. A sample of Solwind CMEs during 1979-1981 illustrating major (Y), questionable (Q), and minor (N)
-,a importance categories according to Howard et al. (1985).
OCCURRENCERATE OF ALL CME8
FREQUENCY OF CMEs (7 DAY SUM)
30-
r_
z 00
1979 1980 198 I 1982
OBSERVATIONALDUTYCYCLE
0 50-
W
13..
0
1979 1980 198 I 1982
CORRECTEDDAILY CME RATE
)-- 3-
" A°Z I - I0
979 1980 198 I 1982-
AVERAGE SUNSPOT NUMBER
300 -
200 -
N
r_
I00 -
0 I I i
979 1980 198 I 1982_
DAY OF YEAR
Figure 3. The occurrence rate of all CMEs during 1979-1981 averaged over 7-day intervals.
248
OCCURRENCERATE OF ALL CMEs
FREQUENCY OF CMEs (2_7 DAY SUM)
60 -
m 40 - [-
FD 20 -
= LF 40 1 I
1979 1980 1981 1982
OBSERVATIONAL DUTYCYCLE
I00 -
(.) 50-
n,"
UJ
13_
0 l
1979 1980 198 I 1982
CORRECTED DAILY CME RATE
<
(:3,_ 2- _ i__r-
z I -
0 , i
979 1980 1981 1982
AVERAGE SUNSPOT NUMBER
300 -
N 200-I00 -
0 l j j
979 1980 198 I 198?-
DAY OF YEAR
Figure 4. The occurrence rate of all CMEs during 1979-1981 averaged over 27-day intervals.
249
OCCURRENCERATE OF ALL CMEs
FREQUENCY OF CME8 (180 DAY SUM)
220 -
123 IIO-
:E) 55-
Z
0 I I I
979 1980 198 I 1982_
OBSERVATIONAL DUTYCYCLEI00-
I--
Z
l.l.I
o 5o- I , i L__
n,,'
I.d
Q..
0 I I ]
9"79 1980 198 I 1982
CORRECTED DAILY CME RATE
3-
,,_ 2- I I I
,--, ! L_
Z I-
0 J I I
979 1980 198 I 1982
AVERAGE SUNSPOT NUMBER
300 -
200 -N
13E I , ,----
I00 -
0 I I
9"79 1980 198 I 1982
DAY OF YEAR
Figure 5. The occurrence rate of all CMEs during 1979-1981 averaged over 180-day intervals.
250
! 984 1 Current work
.......... Hildner et sl (1976)
0 i I I f
20 40 60 BO 100 120 140 160
AVERAGE SUNSPOT NUMBER
Figure 6. The Solwind CME occurrence rate plotted as a function of the average sunspot number
during 1979-1984. The solid line is a least-squares fit to the data with the two points
in 1984 treated equally•
0.16
T I I I I
0.14 _
0.12
0.1
0.08 _
1979
)_ 1984 uncorrected 198Z
t_ 0.06
Q)
_. 0.04
0.02
0 . I J i I I I
20 40 60 B0 I00 120 140 160
AVERAGE SUNSPOT NUMBER
Figure7. The Solwindoccurrencerateforstreamerblowoutmass ejectionsillustrating
thcnearlyconstantratcduring1979-1984.
251
iiii_iiii
Figure 8. A typical "curved front" mass ejection according to Howard et al. (1985). Such CMEs have become rare in 1984-1985.
Note the streamer that was deflected as the CME transited the coronagraph's field of view on 18 November 1981 (cf., Fig. 9).
Figure 9. The eruption of the 18 November streamer during a 19-hr interval on 19-20 November 1981 (cf., Fig. 8). This
illustrates a typical high-latitude "streamer blowout" CME near sunspot maximum.
Figure 10a. A streamer blowout on 27 May 1979, associated with the Helios 1 shock
on 28 May (cf., Fig. 10b).
shock
/
Vp 600 -- -]r
[kms_l ] _ _j *--_ A_.,_.,_
300 ..............
 -jz%1
Np .__i'_ _,[cm-3] 10
Tp lOS _ "i_'_---/I_-
['K] r.i
104 , I , , 1 ,
1200 2LO0 1200
May 28[UT] May 29
Figure 10b. The plasma measurements of the interplanetary shock at Helios 1
located at 0.43 AU, W90 degrees on 28 May 1979.
254
Figure 11. A sample of recently observed streamer blowout CMEs during October-November 1984. Note the
relatively low latitudes of the events on 21 October and 14 November.
-W
i ¸¸
....PRE-EVENT
Figure 12. The evolution of a well-observed streamer blowout during 29 June 1984. The moon is visible on the difference
images at 1105, 1241, and 1416 UT. The white disks also indicate the size of the solar disk.


Solwind observations of coronal mass ejections during 1979-1985

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server