**Statistical Properties of Radio-rich Coronal Mass Ejections**

N. GOPALSWAMY^1,2^, S. YASHIRO^1,2^ , G. MICHALEK^1,2^, M. L. KAISER^2^,
R. A. HOWARD^3^, and

J.-L. BOUGERET^4^

*^1^Center for Solar Physics and Space Weather, Department of Physics,
The Catholic University of America, Washington DC 20064, USA*

*^2^NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA*

*^3^Naval Research Laboratory, Washington DC 20375, USA*

*^4^Paris Observatory, Meudon, France*

**ABSTRACT**

Coronal mass ejections (CMEs) that produce type II radio bursts in the
near-Sun interplanetary medium are termed radio-rich owing to their
ability to drive MHD shocks. We summarize the statistical properties of
these CMEs in order to see if they constitute a special population
distinct from the general population. We found that these CMEs are
faster and wider than the regular CMEs and show significant deceleration
within the coronagraph field of view. Most of these CMEs were also found
to be proton accelerators. We conclude that these type II bursts may be
indicative of geoeffective CMEs and hence relevant to space weather.

**1. INTRODUCTION**

Coronal mass ejections (CMEs) that produce type II radio emission in the
Decameter-Hectometric (DH) wavelengths (1-14 MHz) are known as
radio-rich CMEs (Gopalswamy *et al.*, 2000a, 2001a). They drive shocks
in the outer corona (\> 2 Rs) that accelerate electrons, which in turn
produce type II radio bursts (Reiner and Kaiser, 1999).

The DH radio window became available through the WAVES experiment
(Bougeret *et al.*, 1995) on board the Wind spacecraft launched in 1994.
Since the DH type II bursts are indicative of shocks just departing near
the Sun, they may provide an excellent source of advanced warning of
geoeffective solar eruptions (Webb *et al.*, 2000). When the shocks are
near the Sun, they accelerate solar energetic particles (SEPs), which
arrive at 1 AU within tens of minutes (Tylka, 2001; Reames, 1999). The
IP shocks arriving at 1 AU signal the sudden commencement of geomagnetic
storms. They also bring along a population of locally accelerated SEPs
known as energetic storm particles (ESPs), which can be potentially
hazardous to astronauts and space-based technological systems.
Sometimes, the shock sheath can also cause geomagnetic storms if it
contains southward magnetic field component. When the eruption occurs
close to the Sun center, an ejecta usually follows the shock (Burlaga
*et al.*, 1981, Gopalswamy *et al.*, 2000b). Thus studying the
radio-rich CMEs may help us identify the population of geoeffective
CMEs.

**2. DATA**

In order to see what is special about the DH CMEs as compared to the
regular CMEs, we examined the speed, width and acceleration of the
radio-rich CMEs and also their association with major SEP events. The
Solar and Heliospheric Observatory (SOHO) mission's Large Angle and
Spectrometric Coronagraph (LASCO) detected the CMEs. The WAVES
experiment's high frequency radio detector RAD2 provided the dynamic
spectra of the type II bursts. From the C2 and C3 coronagraph images of
LASCO, we measured the height-time variation of CMEs. Linear and
quadratic fits to the height-time plot were used to derive the speed and
acceleration of the CMEs. We used the online Solar Geophysical Data to
obtain the solar sources of the CMEs. The solar sources were confirmed
by running EUV movies from SOHO's Extreme-ultraviolet Imaging Telescope.
Information on the SEPs was also obtained from the Solar Geophysical
Data. For the SEPs we simply checked the association with the DH type II
bursts. We have provided an illustrative example of the data sets in
Figure 1: the 2001 August 15 CME was a back-sided event from the
southwest quadrant. The CME was first seen above the occulting disk at
23:30 UT and moved with a speed of 1575 km/s. In radio, the event
started with an SA ("shock-associated") event at 2.5 MHz, followed by a
type II burst at 5 MHz. The type II burst from limb events is most
likely at twice the local plasma frequency, so the burst must have
originated from the corona where the local plasma frequency is 2.5 MHz
(density = 7.75x10^4^ cm^-3^). The density at the coronal base is much
larger (a few times 10^8^ cm^-3^) so the low starting frequency of the
type II burst must be due to occultation by the overlying corona.

![](media/image1.png){width="6.166666666666667in"
height="6.166666666666667in"}

Fig.1 SOHO/LASCO images (top) at three instances (00:06, 00:42, and
01:42 UT) during the DH type II burst (middle) and the CME height-time
plot (bottom) for the 2001 August 15 event. The Type II burst and the
"shock associated" events are marked as II and SA, respectively. By
01:42 UT, the radio emission has moved to frequencies below 1 MHz. The
CME had an average speed of 1575 km/s. The vertical lines mark the times
of the three LASCO images. The 01:42 UT LASCO image indicates the
arrival of SEPs at the L1 point. The dark disc in LASCO images is the
occulting disk and the white circle represents the optical Sun. The
rectangular features after 01:00 UT represent calibration interval.

Figure 1 also shows three LASCO images corresponding to various
instances of the type II burst. The outer edge of the CME can be taken
as the approximate coronal height from which the type II emission is
generated. The type II burst continued to lower frequencies beyond the
RAD2 spectral range (not shown). The height-time plot of the CME shown
in Figure 1 indicates where the type II burst might originate at later
times. The LASCO image at 01:42 is severely streaked with SEPs hitting
the SOHO detector. This is image corresponds roughly to 2 hours after
the CME onset.

**3. SUMMARY OF RESULTS**

A detailed analysis of the statistical properties of CMEs associated
with DH type II bursts was recently published by Gopalswamy *et al.*
(2001a). Here we provide a brief summary of the results extended to a
larger set of events during the study period January 1996 to June 2001.
In all there were 132 DH type II bursts that had overlapping
observations from SOHO. The statistical properties of the CMEs
associated with the DH type II bursts are shown in Figure 2. We can
infer the following: (1) the average speed of the DH CMEs (1030 km/s) is
more than two times larger than that of regular CMEs (\~450 km/s). Only
16% of the CMEs had speed around the average value of regular CMEs. We
have not corrected for the projection effects so the speeds in Figure 1
represent a lower limit (Gopalswamy et al., 2000a). (2) The typical
width of regular CMEs is \~ 60^o^. This is in sharp contrast to the
width distribution of DH CMEs shown in Figure 2 (middle panel). Only 7%
of the CMEs have width in the average range. Another important result is
that a majority of the DH CMEs are halo events. It is difficult to know
the width of the halo CMEs, but it is significant that 88/132 CMEs were
halo events. In other words, 93% of the DH CMEs are either halo events
or have above-average widths. (3) Both positive and negative
accelerations are found, but there is a clear bias towards events with
deceleration. About 24% of the events show very little acceleration.
Events with deceleration constitute the largest fraction (45%). The
remaining 31% show acceleration. The measured acceleration of the CMEs
within the coronagraph field of view depends on the relative
contribution from the propelling and retarding forces. We infer that the
coronal drag may play a significant role in deciding the propagation of
CMEs in the IP medium.

![](media/image2.png){width="6.990277777777778in"
height="2.2506944444444446in"}

Fig. 2: Histograms of speed (left), width (middle) and acceleration
(right) of the 132 radio-rich CMEs considered in this study. The average
speed of the distribution is marked. The last bin in the width histogram
includes all CMEs with a measured width greater than 185^o^. This bin is
shown broken because it was out of scale with 88 CMEs.

We also examined the association between the major SEP events of this
solar cycle until October 2001. There were 44 proton events that could
be classified as major SEP events. All but three of the SEP events had
DH type II bursts. In the three events without DH type II bursts, there
were clear "shock associated" type III events starting at frequencies
below 2 MHz. These events usually accompany type II bursts. We also did
not examine the possibility of type II bursts starting at frequencies
below 1 MHz. The preliminary investigation suggests that the electron
accelerators are also good proton accelerators. When we examined the
fast CMEs without associated DH type II bursts, we found them to be of
average width. We need to examine additional factors such as ambient
Alfven speed before making a firm conclusion as to why these fast CMEs
were not associated with DH type II bursts.

**4. DISCUSSION AND CONCLUSIONS**

The primary result of this study is that DH type II bursts are
indicative of faster and wider CMEs. Though the solar sources of these
DH CMEs were from all longitudes, most of them were halo or partial halo
CMEs. This may prove to be a useful way to identify geoeffective CMEs,
even if white light observations are not available. The CME width also
seems to be a critical parameter in deciding whether a fast CME is
radio-rich or not. The large apparent width of the DH CMEs suggest the
possibility that most of these CMEs may expand rapidly in all
directions, in addition to the rapid outward motion. Gopalswamy *et al.*
(2001b) pointed out such a possibility from independent considerations.
The observed deceleration of DH CMEs within 30 solar radii from the Sun
may indicate that the coronal drag is an important factor that
determines the propagation characteristics of fast CMEs in the
interplanetary medium. The connection between SEPs and DH type II bursts
is being investigated, but the preliminary results indicate that the
west-limb DH type II bursts are also good indicators of SEP
accelerators.

**ACKNOWLEDGMENT**

This work was supported by NASA, Air Force Office of Scientific
Research, and NSF.

**REFERENCES**

Bougeret, J.-L., M. L. Kaiser, P. J. Kellogg, R. Manning, K. Goetz, S.
J. Monson, N. Monge, L. Friel, C. A.

> Meetre, C. Perche, L. Sitruk, and S. Hoang, Waves: The Radio and
> Plasma Wave Investigation on the Wind Spacecraft, *Space Sci. Rev.,
> 71*, 231, 1995.

Burlaga, L., Sittler, E., Mariani, F., and Schwenn, R., Magnetic loop
behind an interplanetary shock -- Voyager,

> Helios, and IMP 8 observations, *J. Geophys. Res., 86,* 6673, 1981.

Gopalswamy, N., Y. Hanaoka, T. Kosugi, R. P. Lepping, J. T. Steinberg,
S. Plunkett, R. A. Howard, B. J.

> Thompson, J. Gurman, G. Ho, N. Nitta, and H. S. Hudson, On the
> relationship between coronal mass ejections and magnetic clouds,
> *Geophys. Res. Lett., 25,* 2485, 1998.

Gopalswamy, N., M. L. Kaiser, B. J. Thompson, L. F. Burlaga, A. Szabo,
A. Vourlidas, A. Lara, S. Yashiro, and

> J. L. Bougeret, Radio-rich Solar Eruptive Events, *Geophys. Res.
> Lett., 27,* 1427, 2000a.

Gopalswamy, N., A. Lara, R. P. Lepping, M. L. Kaiser, D. Berdichevsky,
and O. C. St. Cyr, Interplanetary

> acceleration of coronal mass ejections, *Geophys. Res. Lett., 27,*
> 145, 2000b.

Gopalswamy, N., S. Yashiro, M. L. Kaiser, R. A. Howard, and J.-L.
Bougeret, Characteristics of coronal mass

> ejections associated with long-wavelength type II radio bursts , *J.
> Geophys. Res.,* 106 , 29,219, 2001a.

Gopalswamy, N., A. Lara, S. Yashiro, M. L. Kaiser, and R. A. Howard,
Predicting the 1-AU arrival times of

> coronal mass ejections , *J. Geophys. Res.,*106, 29,207, 2001b.

Reames, D. V., Particle acceleration at the Sun and in the heliosphere,
*Space Sci. Rev.,* 90, 413, 1999.

Reiner, M. J. and M. L. Kaiser, High-frequency type II radio emissions
associated with shocks driven by coronal

> mass ejections, *J. Geophys. Res,*104, 16,979, 1999.

Tylka, A., New insights on solar energetic particles from Wind and ACE,
*J. Geophys. Res.,* 106, 25,233, 2001.

Webb, D. F., E. W. Cliver, N. U. Crooker, O. C. St. Cyr, and B. J.
Thompson, *J. Geophys. Res., 105,* 7491, 2000.