Coronal Mass Ejections'

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. A9, PAGES 8173-8191, SEPTEMBER 1, 1985

Coronal Mass Ejections' ^1979-1981

R. A. HOWARD, 1 N. R. SHEELEY, JR. 1 M J KOOMEN 2 AND D J MICHELS

In an examination of the Solwind coronagraph images obtained during the interval March 28, 1979, to December 31, 1981, we have identified 998 coronal mass ejections and recorded their structural classes, central latitudes, latitudinal spans, speeds, excess brightnesses, and relative importances. A statistical analysis revealed the following general results. (1) The properties of coronal mass ejections (CMEs) depended strongly on their structure. Curved front, halo, and complex CMEs were the most energetic, and single spike, streamer blowout, and diffuse fan CMEs were the least energetic. CMEs occurred over a wide range of position angles, broadly centered on the equator, and had an average angular span of 45 ø. The leading edge moved at an average of approximately 470 km/s, and the average

ejected mass and kinetic energy were 4.1 x 10 •5 g and 3.5 x 1030 erg, respectively. The average CME proton flux at the equator at 1 AU was 2.2 x 10 7 cm -2 s -• or approximately 5% of the measured in situ

During 1979-1981 the maximum in the 180-day average CME rate peaked in the second half of 1980, whereas the 180-day average sunspot number peaked during the first half of 1980. The 180-day average rate of fast CMEs (speeds of at least 800 km/s) had a monotonic increase that seemed to be more closely associated with the occurrence rate of large solar flares than with the variation of the sunspot

number.

1. INTRODUCTION

The purpose of this paper is to present an overview of

were obtained nearly continuously from the Solwind white light coronagraph on the P78-1 satellite and represent the longest continuous set of outer coronal observations ob-

(and stili growing) body of coronagraph data, we might expect to improve our knowledge of the statistical properties of coronal mass ejections beyond what has been learned previously from the relatively short-lived OSO-7, Skylab,

Earlier papers have summarized a number of properties of CMEs, both during the declining phase of the previous sunspot cycle and during sunspot maximum in the present cycle. For 77 CMEs that occurred during the Skylab mission in 1973-1974, Munro et al. [1979] identified several basic structures including loops, amorphous clouds, and "oth-

Hildner et al. [1976] reported that during 1973-1974, CMEs occurred at the rate of 0.34 CMEs/day and were correlated with the sunspot number, which itself indicated the presence

of an active longitude lasting for nine solar rotations. Hildner [1977], MacQueen [1980], and Rust et al. [1980] summarized the average properties of CMEs, including mass (6.2 x 10 •5 g) and total energy (11.9 x 10 30 erg). Gosling et al. [1976]

•E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washingon, D.C.

2 Sachs/Freeman Associates, Inc., Bowie, Maryland.

This paper is not subject to U.S. copyright, Published in 1985 by the American Geophysical Union.

Paper number 4A8222.

analyzed the speeds which ranged from less than 100 to 1200 km/s and had an average of 470 km/s.

Near sunspot maximum, Sheeley et al. [1980a, b] and Hundhausen et al. [1984] found that CMEs occurred at all position angles rather than just at position angles near the equator as had occurred during 1973-1974. Also, in a careful analysis of the CME occurrence rate, Hundhausen et al.

[1984] obtained 0.90 +- 15 CME/day for the 7-month interval during the Solar Maximum Mission in 1980 and 0.74 CME/

day for the 9-month Skylab interval 1973-1974. (The differ- ence between the Skylab rate obtained by Hildner [ 1977] and by Hundhausen et al. [1984] is due to different duty cycle corrections.) They concluded that the CME occurrence rate did not vary significantly during the sunspot cycle, in contrast with the earlier expectations based on Hildner's [1977] study. Finally, MacQueen and Fisher [1983] com- pared the speeds of eruptive prominence (EP) associated

CMEs and flare-associated CMEs and found that the EP- associated CMEs were initially slower but had large accel- erations, whereas the flare-associated CMEs were initially faster but had constant speeds.

Our initial studies of the Solwind data set revealed some

new types of mass ejections not reported prior to this era around sunspot maximum, such as halo CMEs [Howard et al., 1982], CMEs with high projected latitudes [Sheeley et

al., 1980c], and double spike and "streamer blowout" CMEs

Sheeley et al., 1984a; Kahler et al., 1984b; Cane and Stone, 1984], (4) type IV radio bursts [Munro et al., 1975], (5) interplanetary shocks [Gosling et al., 1974; Schwenn, 1983;

Sheeley et al., 1983b, 1984b; Woo et al., 1985], and (6)

8173

8174 HOWARD ET AL.' CORONAL MASS EJECTIONS

energetic proton events [Kahler et al., 1978, 1984a]. Finally, the massive statistical sample of Solwind CMEs observed during the years 1979-1981 might be expected to provide some insight into the question of whether the CME rate marches in step with the sunspot number, as Hildner et al [1976, 1977] found, or is essentially constant, as Hundhau- sen et al. [1984] concluded.

We have organized our material as follows. In section 2 we introduce the reader to the instrument (section 2.1), to the method of identifying CMEs (section 2.2), to the concepts of CME structural classes (section 2.3), to importance catego- ries (section 2.4), and finally to the distribution of mass ejections among these structural classes and importance categories (section 2.5). In section 3 we present the proper- ties of CMEs, first, for all mass ejections regardless of their structural classes and importance categories (section 3.1), second, for each structural class (section 3.2), and third, for each importance category (section 3.3). In section 4, after a brief discussion of the instrumental duty cycle (section 4.1) we present the occurrence rate for all CMEs and for major

CMEs (section 4.2), for only the fast CMEs (section 4.3),

and then for only the equatorial CMEs (section 4.4). In section 5 we summarize the results and compare them to results obtained previously.

2. OBSERVATIONS 2.1. General

The data used in this study were obtained by the Solwind white light coronagraph on the Air Force Space Test Pro- gram satellite P78-1 [Michels et al., 1980]. Details of the instrument have been given elsewhere [Sheeley et al., 1980a, Michels et al., 1982]. Following the satellite launch on February 24, 1979, routine observations of the corona began on March 28, 1979, and continue to the present date. The observations show the outer corona between 2.5 and 10 Rs

with an angular subtense per pixel of 1.25 arc min. General-

daylight portion of the 96-min orbit. On some occasions this repetition period is decreased to 5 min, but on other occa-

occasionally foJ' weeks. In section 4 we will discuss these

With a few exceptions, we have reduced all of the data between the start of routine observations on March 28, 1979, and December 31, 1981. We have searched for coronal changes by forming the difference between two images separated by multiples of an orbital period. To begin, the first image taken after 0000 UT was used as the reference or base image and was digitally subtracted from an image in the following orbit. The resulting difference image was displayed on an imaging system and recorded on "3 x 5" Polaroid film.

The base image was then subtracted from the correspond- ing image in each subsequent orbit, and the process was repeated until evolutionary changes in the corona produced an obvious background that masked the fainter or more rapid coronal mass ejections. In practice, this occurred after about 12 hours, at which time a new base image was selected and subtracted from the images in the second half of the day. If sudden changes were detected from one orbit to the next, the base image was updated and subtracted from the remaining images in the relevant orbit to show the evolution of these fast changes. Evolutionary changes became noticeable as

faint enhancements and/or depletions on difference images 1.5-3 hours apart and became more enhanced as the differ- ence in time increased. These evolutionary changes seem to occur simultaneously at all radial positions.

In addition to the lateral motions and intensity variations of the streamers that constituted the gradual evolution of the coronal background intensity, we found outward proper motions of intensity on a variety of temporal and spatial

scales but we found no inward motions. We used the term

coronal mass ejection to refer to such outward proper motions of excess coronal intensity (and thus mass). The instrumental sensitivity (zX///) of approximately 0.022 corre-

sponds to an excess mass of about 1.0 x 10 -8 g cm -2 at a

Our objective was to identify and characterize all such CMEs during the initial 3 years of coronagraph operation

from March 28, 1979, to December 31, 1981. On occasion we

observed rapid brightness variations that we could not rigorously call CMEs, but which seemed to indicate some kind of coronal "activity." Although we recorded these occasions, we shall not discuss them further in this paper.

2.2. Method

During 1979-1981 we identified 998 CMEs and recorded their properties as follows. First, we indicated the radial position of the leading edge at the time that it was initially observed. Second, we measured the position angle of the center of emission and the angular span that the CME subtended, both at the time of first observation and at the time that the span was greatest. We have not attempted to convert these sky plane position angles into true latitude determinations by correcting for the unknown projection of the CME out of the plane of the sky. In the remainder of this paper we will use the term latitude to mean projected latitude which, of course, is an overestimate of the true solar latitude

for CMEs that are out of the sky plane. The angular span

included the boundaries of excess mass but not the stream- ers beyond those limits which were deflected away from the CME [cf. Gosling et al., 1975; Hi!dner et al., 1975; Sheeley et al., 1980a].

Third, when height-time diagrams of the leading edge could be constructed, we recorded the speed of the CME's leading edge together with an indication of the reliability of this measurement as good, fair, poor, and estimated. For most CMEs the sequence of measured points was not sufficiently precise to distinguish between linear and curved fits (i.e., between constant speeds and accelerations). How- ever, in some cases, the precision was sufficient, and of these, a few cases did show acceleration. For these cases we

entered the maximum speed.

Fourth, we estimated the CME intensity on a three-level scale as bright, average, or faint. Then to obtain the total mass, we multiplied the angular span by the mass per degree factor for that intensity. These mass per degree factors were derived from measurements of total mass and angular span of 15 CMEs in the manner described elsewhere [Poland et al., 1981; Howard et al., 1982]. Due to the low contrast nature of coronal observations, the mass per degree of the brightest class was about 6 times that of the faintest class.

Thus the three-level brightness scale (B, A, F) corresponds to 2.1 x 10 TM, 1.05 X 10 TM, and 3.5 x 1013 g/deg.

Fifth, we recorded the apparent projected shape of the CME in one of 10 prototype classes and noted its apparent

HOWARD ET AL.' CORONAL MASS EJECTIONS 8175

6 SEP 80

LOOP

.

..

0456 UT

CURVED FRONT

.... . ...

.

. .

. .

. ..

- ..

... ::. ?. ... •. L.

., •!!:--': ... '-'*:-'--• y:'i-

•: ... •-•...:•?:-,•?: ...

:..:•y----:- . ... . .: ...

4 MAY 79 I 0355

HALO

... . ....

. .

... ....:

..;...

.: .

! 8 APR 8 t i 0227

SPIKE .! ...

.

.

DOUBLE SPIKE

... 1

.

.

..

.

..

.

.

MULTIPLE SPIKE

!

... .

... .

F •

21 MAR 81 2;551 29 JULY 79 0526 25 MAR 8 ! ' 2201

-W

STREAMER •BLOWOUT"

I

FAN COMPLEX

I

E w -w

29 MAY 80 1238 I 0 JUNE 79 1347 14 APR 80 I 080 I

S S

Fig. 1. Coronal difference images illustrating nine of the 10 structural classes of coronal mass ejections discussed in the text. The difference images are constructed by subtracting a preevent coronal image from the image taken at the date and time indicated below each image. The field of view extends from 2.5 to 8 Rs.

importance in one of three importance categories. We will discuss the structural classes and importance categories in the next two sections.

2.3. Structural Classes

As was noted above, we placed each CME in one of 10 structural classes. A representative image from nine of these classes is shown in Figure I. The tenth class was reserved for events whose shape could not be defined observational- ly. Included in this "other" class were events whose leading edge was beyond the outer limit of view (remnants). Also, if a CME had prominence material, it was noted as a subcate- gory rather than as a separate prominence material category.

Prominences were distinguished by their unpolarized inten- sity as well as their characteristic spatial structure. Finally,

we emphasize that some care must be used in comparing our CME classes with those used previously due to several factors including (1) structural changes that occur as a CME moves outward through the corona, (2) differences in the sensitivities and display techniques of individual corona- graphs, (3) the somewhat subjective nature of the classifica- tion technique itself.

Loops are CMEs which, in projection against the plane of the sky, are curvilinear structures with two legs and clearly

defined leading and trailing edges. Morphologically, a loop

CME is distinct from a so-called "curved front" CME which consists of a filled region of emission whose curved leading edge has no obvious trailing edge. Halo events involve material which surrounds the occulting disk [Howard et al., 1982] and represent solar material propagating approximate- ly toward (or away from) the observer.

8176 HOWARD ET AL.: CORONAL MASS EJECTIONS

Spikes are narrow jets of material which moved outward, sometimes but not always, along a streamer. Double spikes have straight legs that move radially outward at the same rate but are not bridged by perceptible emission. Curved fronts have curved leading edges bridging two frequently curved legs which expand laterally as they propagate out- ward. Multiple spikes are simply spikey CMEs with more than two spikes. One of our multiple spike CME events was described by Gergely et al. [1984] as an arcade of loops in the lower corona.

Streamer blowouts occur in two phases. In the first phase, a preexisting streamer brightens greatly and perhaps slightly broadens over a period of 6 hours to a few days. Difference images show material being expelled along the streamer. In the second phase, material is ejected along each side of the streamer and the streamer itselffades in a time of a few hours or less. If this sudden second phase had not occurred we would have counted the first phase as one or more especially bright jets along a streamer (spikes). In the example of May 29, 1980, shown in Figure 1, the second phase is in progress.

The difference image shows the preexisting streamer as a dark depletion region surrounded by excess material being pushed up from below. There is a net excess of material at

leading edge passed beyond 10 Rs or for which the last image was obtained when the leading edge was barely above the occulting disk). Generally, a major (Y) CME was bright and/

or large, a minor (N) CME was small and faint, and a Q CME was in between. In a later section we will discuss the

properties of CMEs as a function of their assigned impor- tance categories.

Figure 2 illustrates these three importance categories for five CME morphological classes. As was discussed earlier, the identification of structural form was based on a sequence of images. In Figure 2, several of the minor CMEs appear to have similar structure in these single frames, but their structural differences are clear in the full sequence of images (not shown here). The N importance and Q importance double spike CMEs bear a striking similarity to small loop CMEs and, in fact, several of our very small loop CMEs evolved into double-spike structures after the first or second frame [cf. Sheeley et al., 1982]. The Q importance, small loop CME on April 16, 1980, (Figure 2) originated as a clear loop structure in the lower corona and subsequently evolved into a double spike structure [cf. Wagner et al., 1983].

Therefore it seems possible that double spikes may originate as loops close to the Sun (below the edge of our occulting this time. The blowout propagates outward at a very slow ' disk) and that the tops of these loops fade as they move pace (100-400 km/s) and leaves behind a cleaned-out corona,

usually without the initial streamer. However, on rare occa- sions, a leg may remain for several days as a new streamer.

Another streamer blowout on May 27, 1979, has been illustrated elsewhere [Sheeley et al., 1982; 1983b].

Fans have little or no internal structure, a poorly defined leading edge, and relatively straight lateral edges. Complex CMEs are events whose shapes could be defined, but not in terms of any of the other simple forms. Some of them had multiple phases, in which a "normal" event was followed within a short time by additional material. Others had multiple lobes or other unusual characteristics. For example, the complex event of April 14, 1980, shown in Figure 1, has an unusual curvature in its first image at 0801 UT. Subse- quent images show the outward motion characteristic of a mass ejection. Such CMEs with curvature in the trailing edge are rare in the Solwind images. (In a subsequent examination of a more complete sequence of SMM coronal images, Sime (private communication) noted that the April

14, 1980, CME had a looplike structure characteristic of many mass ejections.)

2.4. Importance Categories

At least two of us were present during the examination of every coronal difference image and the recording of the pertinent data. We had no trouble agreeing that large, bright CMEs were significant events. The question became wheth- er to include all faint or very narrow CMEs in our analysis.

Some projects might require the statistics of all CMEs regardless of their subjective importance, but other projects might require the statistics of only the significant events for example. Therefore to allow for a variety of subsequent analyses, three categories of importance were established:

yes (Y), questionable (Q), and no (N).

The yes or no events were those which we could agree were major or minor CMEs, respectively, from a visual inspection of the coronal difference images. The question- able events were those for which we could not agree or for which insufficient observations made it impossible to tell (i.e., cases for which the first image was obtained after the

outward into the corona. Alternatively, a significant leading edge may have been absent even lower in the corona as it was for example in the August 5, 1980, event observed at 1.2 Rs at Mauna Loa [cf. Fisher and Poland, 1981; Fisher et al.,

1981; Low et al., 1982].

2.5. Fractional Distribution

Table 1 shows how CMEs are distributed by structural class and importance category. We see that 20% of all CMEs were minor spikes and that no CMEs were major spikes. The next most frequent class was that of a major curved front which occurred 11% of the time. Q and N importance multiple-spike CMEs each occurred about 8% of the time.

Only 2% of all CMEs, 20 events were halos, but almost all of them were either in the Y or Q importance category. If we suppose that many of these halo CMEs are curved front CMEs seen moving along the line of sight [cf. Howard et al., 1982], then the combined class of curved fronts and halos would constitute 17% of all CMEs.

We have not assigned CMEs with prominence material to a separate structural class. However, only about 1.5% of our CMEs had such prominence material at the location of our first polarizer ring at approximately 4.0 Rs. We suppose that some CMEs, such as the April 16, 1980, CME described by Wagner et al. [1983] and shown in our Figure 2, were accompanied by prominence material that ionized before they reached 4.0 Rs.

In the outer corona, loops accounted for only 1% of all CMEs. As we have discussed above, many double spike CMEs may have originated as loops in the lower corona.

However, the combined set of loops and double spikes would still constitute only 13% of all CMEs in the outer

corona.

3. PROPERTIES OF CORONAL MASS EJECTIONS

3.1. Histograms for All CMEs

As was described in the previous section, 998 mass ejections have been identified and catalogued for the period between March 28, 1979, and December 31, 1981. Figure 3

HOWARD ET AL.' CORONAL MASS EJECTIONS 8177

.

.. F•-... ::•:!:•!i,i::.ii.??•::: .... • . o

.?..:::• .'.

• " ':•;:d•"'?•'•: ... :::.'""

ß .-::3 "' • ... .,,,.::'.:-: ... 0 0 ... ::::::::::::::::::::: ':::•:•'•:..'. %:, %':::":'" ::•":•

.

0 .o

..:::•"" . ..'::?:•%: •ø" • 0

::'?•'•:..i• •'• :'?--';d(: ... ".•: '.o

•.:"• :.:•'?•,. • ß ...

====================================== •::'" ß • • ... ,'"' ,..:"' o •::• ... .•.-..:..:::?•?... •

'": ß . "'•• •

....

.... •

.

:.-

. .

..

•...

... ,...:. •

I.nt ..l t. nl

'• Z ¸

Z •

, ,...•

8178 HOWARD ET AL.: CORONAL MASS EJECTIONS

TABLE 1. Fractional Distribution of Coronal Mass Ejections Importance Category

Structural Class Y Q N All

Spike 0.0 2.2 19.8 22.0

Double Spike 1.1 4.3 6.5 11.9

Multiple Spike 3.4 7.8 7.6 18.8

Curved Front 10.5 3.5 1.2 15.2

Loop 0.5 0.3 0.3 1.1

Halo 0.8 0.9 0.3 2.0

Complex 3.2 1.4 0.5 5.1

Streamer Blowout 2.8 1.7 0.6 5.1

Fan 0.4 3.8 5.8 10.0

Other 1.3 6.0 1.7 9.0

All 24.0 31.9 44.3 100.0

presents histograms of speed, angular span, central latitude, mass, and kinetic energy for CMEs and a histogram of latitudes for daily ejected mass. In each panel the plot is normalized to 100% of the given maximum number. In the histogram of daily ejected mass the maximum value is in units of grams per degree per day, whereas in the other five panels the maximum is the number of events. To compute the kinetic energy we used one half the speed of the CME's leading edge.

The CME speeds range from under 50 to 1680 km/s and have an average value of approximately 470 km/s and a median value of approximately 200 km/s. The distribution has a peak near 300 km/s and falls slowly with higher speed.

The angular spans range from 2 ø to 360 ø (although values greater than 120 ø are lumped into the bin at 120 ø) and have a peak at 10 ø. The average span is 45 ø and the median value is 30 ø. Thus most CMEs tend to be relatively narrow.

The central latitudes range from -90 ø to +90 ø , showing that CMEs occurred at all latitudes during 1979-1981. The

distribution has a very broad peak centered at the solar equator and falling to 25-30% of this value at the poles.

Recall that central latitudes and angular spans are projected latitudes, having been measured in the plane of the sky.

The mass ranges from 2 x 10 TM to 4 x 1016 g, with an average value of 4.1 x 10 ]5 g. On this logarithmic scale the distribution is symmetric about the value 2 x 1015 g. The kinetic energy ranges from less than 1 x 10 29 to 6 x 10 TM erg and has an average value of 3.5 x 10 30 erg. The distribution is peaked at 1 x 10 30 erg. The latitude histogram of daily

3.2. Properties by Structural Class

In this section we will illustrate the way in which the properties vary from one structural class to another. We begin with histograms for the three most populous structural classes, spikes, multiple spikes, and curved fronts, which together account for 56% of all CMEs. Then we tabulate some average properties for all the structural classes.

Figures 4a, 4b, and 4c show the corresponding histograms for curved fronts, multiple spikes, and single spikes in the same format as for all CMEs in Figure 3. A comparison of

ioo • ioo

50 L• MAX=59CME

0 500 I000 1500 2.000

SPEED (KM/SEC)

IOO IOO

o t i-- •1

0 30 60 90 120

ANGULARSPAN(DEG) IOO

,,

MAX:125CME

•90 -45 0 45 90

CENTRALLATITUDE(DEG)

50

= iC,

i ME

MASS (1014gins)

1CME 50

01 ... ! ... I ,

I I 0 I O0 1000

K,E, ( I Oeeergs)

IOO _so

MAX•5.5(•3)•M/•E•/•A!

I ,I I I

0-90 -45 0 45 90

LATITUDE (DEG)

Fig. 3. Properties of all CMEs. The distributions of speed, spread, central latitude, mass, and kinetic energy are plotted as histograms. Each of the plots is normalized to 100% of the maximum number of CMEs in a single bin. The maximum value used in the normalization is indicated in the plot. The sixth histogram distribution gives the mass ejected into each degree of latitude. The normalization is to the maximum value of 5.5 x 1013 g deg -• day -•. To derive the daily ejected mass distribution, the correction for instrumental duty cycle has been applied. Note that all angular measurements are made projected in the plane of the sky.

IOO

MAX-18CME 50

0 I [-r• [-"q" ]

0 500 I000 1500 2000

SPEED (KM/SEC)

I00 [--]--1__ MAX=29CME

0 r I I

,-4

ANGULAR SPAN (DEG)

MAX=I 9CME

0 I I , J

-90 -45 0 45 90

CENTRAL LATITUDE (DEG) IOO

9CME 50

0 , , ,,,,,•'"L,i,

...

i , , i, ...

i

MASS (10 •4 gins)

I00

5O

MAX:4CME

,F, ... , ... ,

I I o I GO I ooo

K.E. (I 02eergs)

IOO

5o

DAY

0 l • • i

-90 -45 0 45 90

LATITUDE (DEG)

I00

50 •_ MAX=llCME 0 i L-•l•

SPEED (KM/SEC)

I00

5O

i 2000

-• MAX=35CME

0 30 60 90 120

ANGULAR SPAN (DEG)

I00

5o •

MAX=27C.ME

0 90 45 0 45 90

CENTRAL LATITUDE (DEG)

I00 1 MAX=I 2CME I

0 ... •

I I o I GO I ooo

MASS ( I 0 •4 grns)

IOO

MAX:2CME 50

0 ... i ... l ... I

I0 I00 I000

K.E. ( I 02•ergs)

IOO •

MAX:9.6 ( 12 ) GM/DEG/DAY

i i i

0.. _ 0 45 90

LATITUDE (DEG)

I O0 I O0

MAX:I 1CME MAX=I 8CME

50 50

0

i[• •-• i

I

I 0 [ ...

' ' ' "L]-rn'"

...

'

SPEED (KM/SEC) MASS (: I0 • gins)

I O0 I O0 i

NAX=l 11CNE MAX=2CME

lil

! I I 0 ... i , ,, ...

O0 30 60 90 I 20 I I 0 I O0 I 000

ANGULAR SPAN (DEG) K.E. (102e ergS)

Ioo J I J _{l_i-h} Ioo

MAX:25CME •DAY

o i i I i o

-90 -45 0 45 90 -90 -45 0 45 90

CENTRAL LATITUDE (DEG) LATITUDE (DEG)

Fig. 4. Properties of CMEs for three structural classes. (a) Curved front CMEs. (b) Multiple spike CMEs. (c) Single

8180 HOWARD ET AL.' CORONAL MASS EJECTIONS

these figures shows that curved fronts are faster than multi-

ple spikes, which in turn are much faster than the relatively slow single spikes. Similarly, the angular spans decrease as one progresses from curved fronts to multiple spikes to single spikes. As expected, single spikes are unique in their

Curved front CMEs seem to have a bimodal distribution of

central latitude with a pair of peaks in the solar activity belts superimposed on a relatively flat background. For multiple spike CMEs the high-latitude contribution is greatly reduced (especially in the southern hemisphere), and the equatorial contribution is relatively greater. For single spike CMEs the high-latitude contribution is less reduced (but still has the north-south asymmetry), and the equatorial contribution is

For the ejected mass rate the latitude distribution has an

even broader dependence for curved front CMEs than it had for all CMEs in Figure 3. Also, the activity belt peaks that

have been washed out in this latitude of ejected mass,

especially in the southern hemisphere. The peaks in the

shows a pronounced structure with pairs of peaks at low

latitude peaks are nearly blended into a single equatorial peak.) The pair of high-latitude peaks is not exactly symmet- ric about the solar equator, much as the corresponding

prominence-filament zones that characterize this phase of the sunspot cycle [Lockyer, 1931' d'Azambuja and d'Azam-

significant fraction of all spike CMEs occur along (possibly

For multiple spikes the logarithmic mass distribution is

symmetric about a peak at 2 x 1015 g. However, for curved fronts the peak occurs at 7 x 1015 g and has a tail to lower values, whereas for single spikes the peak occurs at 5 x 10 TM

values as one goes from curved fronts to single spikes.

Table 2 summarizes several CME properties by structural

class. An inspection of this table reveals the principal characteristics as follows. Together, spikes, double spikes,

CMEs. Streamer blowouts had the lowest average speed (200 km/s), and halos, curved fronts, and complex CMEs

Curved front and complex CMEs had angular spans in

span for all CMEs. Also, their masses and kinetic energies

for all CMEs. As was expected from their definitions, single spikes were narrow (15ø), and halos were broad (309ø).

(Some halo CMEs did not completely surround the occulting

outs, and diffuse fans were the least energetic CMEs with kinetic energies (0.5 x 10 30 erg) of only one seventh the average kinetic energy for all CMEs (3.5 x 10 30 erg). Thus from a kinetic energy standpoint, the large masses of the streamer blowouts compensates somewhat for their very low

The extraordinarily large mass (21 x 10 • g) and kinetic energy (18 x 1030 erg) of the halo CMEs may be due to the following reasons. First, the occulting disk may be selective-

distribution. Thus if these CMEs were curved fronts, then one might expect to obtain masses and kinetic energies on the order of 1016 g and 10 31 erg, (compare Figure 4a), consistent with the values in Table 2 for halos. Second, the

perspective. Thus if mass continues to flow out, the larger volume associated with the halo perspective may include

more mass than does the smaller volume of the broadside

perspective. This comparison illustrates a fundamental prob- lem in the measurement of ejected coronal mass and sug- gests that CME mass determinations may be udderes-

timates.

3.3. Properties by Importance Category

In section 2.4 we defined the three categories of CME importance of major (Y), questionable (Q), and minor (N). In Figures 5a, 5b, and 5c the histograms provide a quantitative characterization of these three importance categories. Fur- ther, they provide a basis for understanding the effects of

Referring to Figures 5a, 5b, and 5c we see that even though speed was not explicitly used in the definition of N

importance CMEs, there are relatively fewer high-speed CMEs in this category than in the Y importance category.

However, some minor CMEs are fast. N importance CMEs are relatively narrow, reflecting the subjective evaluation that small CMEs should be less important than big ones. The bimodal distribution of central latitude for Y importance CMEs changes to an equatorially peaked distribution with few high-latitude events for N importance CMEs. The latitude distribution of the daily ejected mass shows a similar behavior. As was expected from our definition, the mass distribution shifts to smaller values as one goes from Y to N.

TABLE 2. Average Properties of Coronal Mass Ejections 1979-

1981 Percent

of Speed, Span, Mass, Kinetic Energy, Form Total km/s deg 10 • 5 g 10 3o ergs

Spike 22 297 15 0.93 0.44

Double Spike 12 425 30 2.6 2.4

Multiple Spike 19 425 45 3.5 1.9

Curved Front 15 584 62 8.4 6.4

Loop 1 530 44 4.7 3.7

Halo 2 630 309 21. 18.

Complex 5 592 65 7.2 5.7

Streamer Blowout 5 200 44 5.4 0.56

Diffuse Fan 10 377 33 1.7 0.60

Other 9 483 59 4.4 3.4

All 100 472 45 4.1 3.5

I O0

•

MAX=24CME

0 • F-• •

0 500 I000 1500 2.000

SPEED (KM/SEC)

o i i

o 30 60 90

ANGULAR SPAN (DEG) 120

•oo

MAX:26CME

-90 -45 0 45 90

CENTRAL LATITUDE (DEG)

IOO

21CME õ0

0 ... • ... • ' ' •'

1 I 0 100 1000

MASS ( I 014 gins)

I00 •

MAX=7CME

I I o I oo I ooo

K.E. ( I 02eergs)

5O

MAX:2,8(1 3 ) GM/DEG/DAY

0 • •

-90 -45 0 45 90

LATITUDE (DEG)

IOO

50 -L_,-LL

0 500 1000 1500 2000

SPEED (KM/SEC)

IOO

MAX=18CME

5O

_]-[

o 30 60 90 120

ANGULAR SPAN (DEG)

,oo 50

•I.•

•'•[ MAX:46CME

CENTRAL LATITUDE (DEG)

I I 0 I O0 1000

MASS (1014 gms)

I00

MAX=4CME 5O

I I0 I00 I000

K.E. (I OZeergs)

I00

MAX=I ,6(1 3)GM/DEG/DAY

I I i

0-90 -45 0 45 90

LATITUDE (DEG)

IOO

MAX=20CME

50 [

0 IL• • I I

SPEED (KM/SEC) I00

MAX=176CME

0 . •

0 50 60 9O 120

ANGULAR SPAN (DEG) I00

MAX=58CME

0 • • I I

-9O -45 0 45 9O

CENTRAL LATITUDE (DEG)

I00

• MAX=37CME

50 . L

I I 0 100 1000

MASS ( I 014 gms)

I00

MAX=3CME 5O

0 ,I , , ... i ... I

I I 0 I O0 1000

K.E. (1029 ergs)

I00

MAX:8,4 ( 1 2 ) GM/DE G/DAY

0 i i i

-90 -45 0 45 90

LATITUDE (DEG)

Fig. 5. Properties of CMEs for the three importance categories. (a) Y importance CMEs. (b) Q Importance CMEs. (c) N importance CMEs. The format for the histogram plots is the same as in Figure 3.

8182 HOWARD ET AL.' CORONAL MASS EJECTIONS

A ;50

15

0

FREQUENCY Of CMEs (7 DAY SUM)

1980 1981 1982

I- z Ld

Ld IOO

50

o 1979

OBSERVATIONAL DUTY CYCLE

1980 1981 1982

>-

<[

z

CORRECTED DAILY CME RATE

4

3

0 •

1979 1980 1981 1982.

N

AVERAGE SUNSPOT NUMBER

[

200

f

DAY OF YEAR Fig. 6a

Fig. 6. Occurrence rate of all coronal mass ejections. (a) Seven day sums. (b) Twenty-seven day sums. (c) One- hundred eighty day sums. The first panel in each figure is the total number of CMEs observed in the sum interval. The second panel gives the observational duty cycle for the same interval allowing a 4.5-hour gap between observations.

The third panel gives the occurrence rate corrected for the duty cycle. The last panel displays the average Zurich sunspot number for the same interval. The average corrected occurrence rate is 1.8 CME/day.

4. OCCURRENCE RATE OF CORONAL MASS EJECTIONS 4.1. Duty Cycle

In this section we will be discussing the occurrence rate of CMEs during 1979-1981. The instrument was not operated at the same rate throughout this interval. Thus in order to compare the rates at different times, a correction for the duty

cycle needs to be applied to the observed number of CMEs.

Even with "complete" coverage (an image every 5-10 min throughout the 60-min daylight portion of every orbit), the orbital night gives rise to a 36-rain gap every 96 min. In actuality, data gaps from single orbits to days, and occasion- ally weeks, occurred throughout this period.

A problem is to determine how long a data gap can be before the occurrence rate is significantly affected. If the leading edge of a CME is moving at the average speed of 470 km/s, then it will traverse the 7.5 Rs coronagraph field of view in 3 hours. In order to have two views of the average CME's leading edge, a gap of less than 3 hours would be required. However, a number of CMEs were identified even though the leading edge of the event had already left the field, since material continues to flow outward for up to several hours behind the leading edge. The maximum gap that can be tolerated and still detect the occurrence of the average CME is about 4-5 hours.

If we adopt a value of 4.5 hours for this gap, then we can calculate the fraction of each day not interrupted for times exceeding this value. In Figures 6-9 we have plotted this observational duty cycle for the 3-year interval 1979-1981.

Its 3-year average value is 66.5%. Similar 3-year average values are shown in the following table for other choices of the maximum tolerable gap.

Allowable Gap Duty Cycle, %

3.0 54.9

4.0 57.8

4.5 66.5

5.0 67.6

6.0 76.0

As can be seen from this table, a variation of 1.5 hours about the 4.5-hour gap length causes only a 15% change in the average duty cycle and thus in the corrected CME occur-

rence rate.

4.2. Rate for all CMEs and for Major CMEs

Figures 6a, 6b, and 6c show the occurrence rate of all

mass ejections during the 3-year interval 197921981. Each of

HOWARD ET AL.' CORONAL MASS EJECTIONS 8183

),.. o z b.I •) C• b.I

la. la.i _.1 o o

-

O

o co la.i

- O

I I I I (• I I (• I I (• I I I (• , 0 • 0 • O-- 0 0 0 I'0 • -- O-- 0 0 0 0 • • -- • 0 • 0 0 0

• •18•nN .LN•IO•Id A•/(i/N Z• bJ

•) J

bJ

b_ _

0

I I I 0 0 0 0 0 0 0 ,q- I'0 • -- 0 bJ I--- 0 13. Z •)

-

i •- (• I I I

o o o o o o o

• W3B•nN .LN•IOW•Id A•O/N ZW

8184 HOWARD ET AL.' CORONAL MASS EJECTIONS

2O

I0

0

FREQUENCY OF CMEs (7 DAY SUM)

1980 1981 1982

I- z i,i

i,i IOO

5o

OBSERVATIONAL DUTY CYCLE

1979 1980 1981 1982

CORRECTED DAILY "ME RATE

3

0 I

1979 1980 _{1981 1982}

N 300

200

I00

1979

AVERAGE SUNSPOT NUMBER

I I i

1980 198 I 1982

DAY OF YEAR Fig. 7a

Fig. 7. Occurrence rate of Y plus Q importance CMEs. (a) Seven day sums. (b) Twenty-seven day sums. (c) One- hundred eighty day sums. The format is the same as in Figure 6. The average corrected occurrence rate is 0.9 CME/day.

of a plot of the observed CME rate (top panel), a plot of the observational duty cycle (second panel) as discussed above, a plot of the corrected CME rate (third panel), and a plot of the corresponding average sunspot number (bottom panel).

The occurrence rate has not been corrected for intervals when the duty cycle was less than 25%.

In Figure 6a the 7-day averages reveal a high variability of the CME rate on a relatively short time scale. In particular,

less than 1 to more than 3 CMEs/day during intervals of only 7-14 days. In general, these variations are not accompanied by corresponding variations of the sunspot number (correla- tion coefficient -- 0.22), even allowing for possible phase shifts for 7 days in either direction. (Similar comparisons using rates for only east limb or only west limb CMEs do not show an improved correspondence with the sunspot num-

when a correlation does seem to be present (correlation coefficient = 0.55), there are many other intervals when it is

In Figure 6b the 27-day averages show that there is also

changes of 1-3 CMEs/day during only three to four solar

to be accompanied by a similar variation in the 27-day

Thus for example, although the CME and sunspot rates decreased together in mid- 1981, 3 months later the CME rate dropped while the sunspot rate increased.

Figure 6c shows the 180-day averages. Here the effects of the duty cycle have flattened out because of the long-term averaging. Thus the corrected CME rate more accurately reflects the long-term variation. There is a gradual rise to a peak of 2.5 CMEs/day in late 1980 and a slight fall off to approximately 1.5 CME/day during 1981. In contrast, the 180-day sunspot number was much less variable, peaking in late 1979 to early 1980, and then declining only slightly during 1980 and 1981. Thus if there is a long-term relation between CME rate and sunspot number, then the CME rate peaked one half year after the sunspot number reached maximum. Averaged over the entire 3-year interval near sunspot maximum, the CME rate was 1.8 CMEs/day for all

CMEs.

It is of some interest to examine the occurrence rate when

minor CMEs are excluded. Thus Figures 7a, 7b, and 7c show

the corresponding 7-, 27-, and 180-day average occurrence

rates for only CMEs in the combined Y plus Q importance

categories, respectively. Again, one finds a high variability

on the short time scales without obvious persistent corre-

sponding variations of the sunspot number (correlation coef-

ficient = 0.20). For these relatively major CMEs the occur-

rence rate fluctuated from 0.0 to 2.2 CMEs/day during

HOWARD ET AL.' CORONAL MASS EJECTIONS 8185

0 0 0 0 0 0 • -- 0 0 0 0 0

• • 0 tO 0 0 0

= •1381AIFIN IN3083d A•/(I/N Z•I