Adv. Space Res. Vol. 11, No. 1, pp. (1)37—(1)40, 1991 0273—1177/91 $0.00 + .50Printedin GreatBritain. All rightsreserved. Copyright© 1991 COSPAR

THE SOLAR CYCLE VARIATION OF THERATES OF CMEs AND RELATED ACTIVITY

David F. Webb

Institute for SpaceResearch,Boston College, ChestnutHill, MA 02167,U.S.A.

ABSTRACT

Coronal Mass Ejections (CMEs) are an important aspect of the physics of the corona andheliosphere. We present results of a study of occurrence frequencies of CMEs and relatedactivity tracers over more than a complete solar activity cycle. To properly estimateoccurrence rates, observed CME rates must be corrected for instrument duty cycles,detection efficiencies away from the skyplane, mass detection thresholds, and geometricalconsiderations. We evaluate these corrections using CMEdata from 1976—1989 obtained withthe Skylab, SMMand SOLVIND coronagraphs and the HELIOS-2 photometers. The major resultsare: 1) The occurrence rate of CME5 tends to track the activity cycle in both amplitudeand phase; 2) The corrected rates from different instruments are reasonably consistent;and 3) Over the long term, no one class of solar activity tracer is better correlated withCME rate than any other (with the possible exception of type II bursts).

INTRODUCTION

With the large increase in CME observations over the last decade it has become clear thatCMEs are a fundamental part of coronal physics and a potentially significant contributorto the heliospheric mass flux /1,2/. However, we have only recently accumulated sufficientdata on CMEs to effectively study their long—term occurrence frequency and compare it tothat of related solar activity patterns. Previous studies have consistently shown a betterassociation between CMEs and certain classes of solar activity, such as eruptingprominences, than others, such as Ba flares /1,3,4/. Also, coronal shock waves (type IIbursts) and interplanetary shocks are well correlated with faster CHEs. Such studies leadus to expect different average long—term correlations between CHEs and other relatedclasses of solar activity.

The purpose of this study was to examine the long-term variation in the rate of CHEs overmore than a complete solar activity .cycle, from 1973—1989. I also studied the annualvariations of other activity related to CMES, calculating statistical correlations betweenthe rates of CHEs and these other tracers.

THE SOLAR CYCLE VARIATION OF THE RATE OF CMEs

CKEs are most directly detected and studied by white light emission arising fromThomson—scattering from free electrons. Large numbers of CMEs have been detected bycoronagraph experiments on Skylab in 1973-74, on SMM in 1980 and 1984—1989, and on P—78from 1979—1985. In addition, the HELlOS zodiacal light photometers have been used todetect CMEs in the interplanetary medium and to estimate their occurrence frequency from1976—1979, during a long gap in coronagraph coverage /2/. These observations form thebasic data base for this study.

To properly utilize CME rate data, the observed occurrence rates must be corrected forvarious instrumental—dependent effects. For a given coronagraph these effects include: theobservational duty cycle, the visibility function or detectability of CMEs away from theplane of the sky, and the detection threshold for events in the skyplane. In addition, ifone wants to compare the CME rates with in—ecliptic solar wind measurements, effects dueto the mass and geometrical distributions of CMEs must be taken into account. Onlyduty—cycle corrections have been published by the coronagraph groups /5,6,7/. These have

(1)38 D. F. Webb

functions, and including CME rates as determined with the HELIOS—2 photometers.

Metric type II bursts identified with Ha flares form a useful data base for evaluating thevisibility function of a coronagraph. Coronagraphs have their maximum sensitivity toevents in the skyplane, so they will miss or less efficiently detect CHEs with sourceregions further onto the solar disk. Since the longitude distribution of type II flares isrelatively flat, the distribution of such flares associated with and without CMEs can beused to estimate the visibility function of a coronagraph. We performed a comprehensiveanalysis following the procedure of Kahler et al. /9/ for the periods of theSOLVIND and SlIM observations. The results indicate that the detection efficiencies of bothcoronagraphs decreased from the limb inward. The normalized visibility functions suggestthat the SMM (SOLVIND) coronagraph missed about 40% (25%) of all CHEs.

The annualized CME rates, corrected for duty cycle and visibility functions as discussedabove, are shown In Figure 1. These data span a period of 17 years, or 1 1/2 solar cycles,from the start of the Skylab mission in June 1973 to the demise of SlIM in November 1989.The units of the rates are CME/day averaged over the observational period for eachinstrument. Two SOLWIND rates per year are given, one for all observed CHEs and a lowerone for only “major” (Y + Q) CMEs /6/. The HELIOS—2 data from 1976—1979 are from Webb andJackson /2/, and are corrected for spacecraft duty cycle and a crude visibility function.

I conclude the following from the CME rate data: 1) The CME rate tends to track that ofthe solar activity cycle, as usually traced by sunspot number, with minima in 1976 and1985 and maxima in 1980—1981 and 1989. 2) After corrections for duty cycles and visibilityfunctions (but not detection thresholds), the annual CME rates from different instrumentsare in reasonable agreement, except possibly in 1980. 3) The HELlOS and SOLVIND CME ratesin 1979 overlap. The near—equality of the HELlOS rate with that of “major” SOLVIND dIEsand the association of individual HELlOS CMEs with major SOLWIND dIEs /2/ suggest thatonly the larger, more energetic CMEs significantly influence the interplanetary medium.

3.5 I I I I I I I I I I0

YEAR

Fig. 1. Annual occurrence rates of dEs over solar cycle. Data are from coronagraphs onthe Skylab, SMM (o’s) and P-78 (X’s) satellites, and the HELIOS—2 photometers (squares).

THE SOLAR CYCLE VARIATION OF THE RATES OF ACTIVITY RELATED TO dIEs

Previous studies have consistently shown different degrees of association between CMEs andcertain classes of other solar activity. We wanted to examine the strength of theserelationships over solar—cycle time periods. For comparison with the CME rate data, wecollected monthly and annual counts of the following classes of activity during the

CMESolar Cycle Variations (1)39

interval 1972—1989: smoothed monthly sunspot numbers, Ha flares, metric type II bursts(coronal shocks), interplanetary shocks, long—duration 10.7 cm gradual rise and fallbursts (GRFs), and disappearing filaments (DFs). All of these classes have been consideredin previous CME association studies /3,4/. We made no attempt to derive absolute activityrates, but only to assure that each activity data set was internally consistent.

The data sets were obtained as follows. The sunspot numbers were taken from the SGD (SolarGeophysical Data Bulletins) tables of smoothed monthly sunspot numbers, a commonly usedtracer of solar activity. The flares were from the SGD tables of “Monthly Counts ofGrouped Solar Flares” and are dominated by subflares. The metric type II data included allbursts listed in the SGD tables of “Solar Radio Emission Spectral Observations”. Theinterplanetary shock wave data were from the Los Alamos plasma experiments on IMPs 6—8from 1972—1978 /10/, and from the Max Planck experiments on the HELlOS spacecraft from1975—1982. The GRF data consisted of events of > 4 hr. duration from the lists ofOutstanding Events of the Canadian Herzberg Institute of Astrophysics radio observatoriescollected by Kahler and Cliver /11/ and extended by me. These data are used as a proxy forX—ray LDEs. The data on DFs were from unpublished catalogs of DFs outside of activeregions collected by Joselyn from 1976—1986 and Wright from 1965—1980.

These data sets show similar trends, with minima in 1976—1977 and 1985—1986, and maximaabout 1980 and in 1989. As expected, the curves of sunspot numbers and flares were similar.The type II bursts also generally followed the activity cycle, but with a sharp rise in1978 and a rapid fall from maximum and a flattop distribution from 1978—1982. The rapidrise In 1978, only two years after minimum, also was reflected in the rates ofinterplanetary shocks, GRFs, and DF5; in addition, the GRF rate fell rapidly after 1982.We then compared the occurrence rates of CMEs with each activity class averaged only overthose months in a given year when CME observations were obtained, and computed the linearcorrelation functions. All of the graphical comparisons showed similar trends.

Table 1 summarizes these results. The activity classes are listed in descending order ofthe linear correlation coefficients, r, of their fits to the CME rates. Although allactivity classes are correlated with dEs at > 95% confidence level, the fits are best fortype II bursts and sunspot numbers. The amplitudes of the solar cycle variations of eachclass, given as the ratio of the annual activity rates at solar maximum to that atminimum, are given in column 2 for cycle 21 (1980/1976) and Column 3 for cycle 22(1989/1986). The dIE amplitudes are derived from the SMMand total SOLWIND rates in 1980.With the possible exception of the type II bursts, we attribute most of variations in theamplitudes and statistical fits to the varying quality of the data sets. Therefore, weconclude from these results that over the long term, no one class of activity tracer isbetter correlated with CIE rate than any other.

TABLE 1 Comparison of the Long—Term Occurrence Ratesof CIEs with Related Solar Activity

Linear Correlation Cycle 21 Cycle 22 1Activity Class r % Confidence Amplitude Amplitude

dIEs: SMM ——— ——— 14; 342 >14SOLWIND ——— ——— 3; 22

Type II Bursts 0.88 >99 (l9)~ 13 >14Sunspot Number 0.875 >99 (19) 12 >11All Ha Flares 0.82 >99 (19) 16.5 >10.510.7 cm GRF Bursts 0.80 >99 (19) 11.5 >9Dis. Filaments 0.76 99 (15) 2.5IP Shock Waves 0.74 98 (10) 7 - 8

1: The Cycle 22 ratios are given as lower limits since sunspot maximum may have -occurredafter 1989. 2: The CME ratios are in the order 1980/1985; 1980/1976. 3: ( ) indicate thenumber of degrees of freedom.

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CONCLUSIONS

The main conclusions of this study are: 1) The frequency of occurrence of CMEs tends totrack the solar activity cycle in both amplitude and phase; 2) The CME rates measured bydifferent instruments, when corrected for both duty cycles and visibility functions, arereasonably consistent; 3) The 1979 data suggest that only the larger, more energetic dEssignificantly influence the interplanetary medium; 4) Considering only long—term averages,no one class of tracer of solar activity is better correlated with CME rate than any other(with the possible exception of type II bursts).

These results provide new information about CMEs which in some cases differ from previousconcepts about CMEs and their relation to other solar activity. Previous studies involvingthe origins of CME5 have generally shown better associations between CMEs and certainclasses of activity than others. Our results do not support such a differentiation byactivity class over the long term. One idea was that since dIEs often do notoriginate in active regions and are not always associated with flares, the phase of theiroccurrence rate might differ from that of the sunspot activity cycle. With the long CMEdata base, it is clear that the overall average CME rate does indeed track the sunspotcycle suggesting that dEs, like most forms of solar activity, are also controlled by theemergence and evolution of the global solar magnetic field. However, close examination ofother details of this relationship, such as spatial distributions as a function of time ofdifferent classes of activity, are needed to better understand the causes of dEs.Finally, when corrected for visibility functions, we find consistent results for the CMErate from different coronagraphs during equivalent observation periods. Therefore, ourresults differ from some who have found discrepancies between such data sets /1,5/. On theother hand, the upwardly revised SMM CIE rate In 1980 still indicates a substantialdiscrepancy between the SlIM and SOLWIND rates at solar maximum; note that the SlIM rates atthe maxima of the two solar cycles agree with each other.

I thank the following for providing data: A. Hundhausen, R.A. Howard, J. Joselyn, S.Kahler, and R. Schwenn. Work on this study was supported by NASA contract NAS5—28727, andAir Force Geophysics Lab contracts AF19628—87-K—0O33 and AF19628—9O—K—0006.

REFERENCES

1. S.K. Kahler, Rev, of Geophys., 25, 663 (1987).

2. D.F. Webb and B.V. Jackson, J. Geophys. Res., in press (1990).

3. D.J. Munro, J.T. Gosling, E. Hildner, R.M. MacQueen, A.I. Poland, and C.L. Ross,

Solar Phys., 61, 201 (1979).

4. D.F. Webb and A.J. Hundhausen, Solar Phys., 108, 383 (1987).

5. A.J. Hundhausen, C.B. Sawyer, L. House, R.M.E. Illing, and W.J. Wagner,J. Geophys. Res., 89, 2639 (1984).

6. R.A. Howard, N.R. Sheeley, Jr., M.J. Koomen, and D.J. Michels,J. Geophys. Res., 90, 8173 (1985).

7. R.A. Howard, N.R. Sheeley, Jr., D.J. Michels, and M.J. Koomen, in: The Sun and theHeliosphere in Three Dimensions, ed. R.G. Marsden, D. Reidel, Dordrecht, 1986, p. 107.

8. D.F. Webb, in: Solar—Terrestrial Physics, Proceedingsof SecondIndo—US Workshopon Solar—Terrestrial Physics, ed. M.R. Kundu, et al., Insdoc, New Delhi, 1986, p. 283.

9. S. Kahler, N.R. Sheeley, Jr., R.A. Howard, N.J. Koomen, and D.J. Michels,Solar Phys., 93, 133 (1984).

10. G. Borrini, J.T. Gosling, S.J. Bame, and W.C. Feldman, J. Geophys. Res., 87,4365 (1982).

11. K. Kahler and E.W. diver, Solar Phys., 115, 385 (1988).