Solar Flares, Type III Radio Bursts, CMEs, and
Energetic Particles
y
H. V. Cane
Laboratory for High Energy Astrophysics, NASA/GSFC, Greenbelt, MD USA
Bruny Island Radio Spectrometer, Tasmania, Australia
Abstract. Despite the fact that it has been well known since the earliest observations that solar energetic
particle events are well associated with solar ares it is often considered that the association is not physically
signicant. Instead, in large events, the particles are considered to be only accelerated at a shock driven by
the coronal mass ejection (CME) that is also always present. If particles are accelerated in the associated
are, it is claimed that such particles do not nd access to open eld lines and therefore do not escape from
the low corona. However recent work has established that long lasting type III radio bursts extending to low
frequencies are associated with all prompt solar particle events. Such bursts establish the presence of open eld
lines. Furthermore, tracing the radio bursts to the lowest frequencies, generated near the observer, shows that
the radio producing electrons gain access to a region of large angular extent. It is likely that the electrons
undergo cross eld transport and it seems reasonable that ions do also. Such observations indicate that particle
propagation in the inner heliosphere is not yet fully understood. They also imply that the contribution of are
particles in major particle events needs to be properly addressed.
INTRODUCTION
al. 5] indicating much lower temperatures argue
against a are contribution in major particle events
1]. However the Luhn et al. 5] study made observations at about 1 MeV/nuc. At these energies, large
particle events are dominated by shock acceleration
in the interplanetary medium. Thus the mean Fe
charge state of 15 that they found, (typical for a
plasma of Te 2 MK), does not represent material
accelerated at the Sun. More recent results from the
MAST experiment on SAMPEX, at much higher energy of 28-65 MeV/nuc., indicate Fe charge states
in the range 14-21 6]. The lower values were obtained for events with strong interplanetary shocks
(Cane 2002, in preparation). For other events, where
the measurements were dominated by particles accelerated at the Sun, the charge states are consistent
with \are{heated material".
Clearly the question of the contribution from
ares associated with major proton events needs
to be addressed. One problem in ongoing discussions is one of semantics. Some people use the term
are to describe localized brightenings in H . This
is clearly very restrictive. Cliver 7] discusses eruptive ares as being \formed by reconnection of eld
lines opened by a CME" but does not give a denition of the term. The denition provided by Hudson
et al. 8] \A are is a sudden energy release in the
solar atmosphere" is not unreasonable and will be
Solar energetic particle (SEP) events are often categorized into two classes based on various properties
1]. The most signicant dierence is the duration
of the particle emission. The less energetic events of
short duration, are called \impulsive". Such events
have heavy ion abundances about a factor of 10
above those determined for the corona and solar
wind. A signicant number of these events are associated with CMEs, but the CMEs tend to be rather
small in angular extent. In contrast, major, long
lasting solar energetic particle events, are associated
with large, fast CMEs 2]. As a consequence they
are also associated with strong interplanetary shocks
which can accelerate coronal/solar wind material.
Thus it is dicult to dierentiate and determine the
composition of the material accelerated at the Sun.
However data returned from the SIS instrument on
ACE 3] suggest that this material is also enhanced
in heavy ions 4].
Impulsive particle events have charge states that
indicate a source region that is heated to some 10
MK. Such temperatures are typical of the source
regions for soft Xray ares, so it is widely accepted that such particle events have their origins in
the magnetic reconnection regions associated with
ares. The charge state measurements of Luhn et
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
589
used in this study. Clearly are phenomena include
particle acceleration nonthermal particles produce
the classic H signatures. Another classic are signature is that of a type III radio burst caused by
streaming electrons. Since the emission is plasma
radiation, the frequency of emission is proportional
to the square root of the plasma density. Thus the
bursts drift rapidly to lower frequencies as the electron streams encounter decreasing densities further
from the Sun. Below about 20 MHz, the emission
must originate more than 1 solar radius above the
solar surface. Bursts that extend to lower frequencies indicate the escape of are particles along open
eld lines into the interplanetary medium. Thus the
presence or absence of type III bursts must indicate
whether or not are particles escape in a particular solar event. However, even if particles do escape
there is another question that needs to be addressed:
What regions of the inner heliosphere are accessible to these particles? Most researchers attempting to understand solar energetic particle increases
consider the interplanetary magnetic eld to generally follow a smooth Parker spiral. In such a picture,
are particles are released onto and remain conned
to a relatively small cone of eld lines. Thus are
particles are presumed to be detectable at Earth
only from events at about W60 on the Sun. However
the interplanetary medium is complex with numerous structures that can aect the ow of energetic
particles. For example, an Earth{based observer can
be \well{connected" to an eastern hemisphere are
if Earth is inside an interplanetary CME. Richardson et al. 9] showed how particles arrived from eld
lines to the east of the Earth{Sun line during the
ground{level enhancement of October 1981 associated with a are at E31 . Another important aspect
is the suggestion based on recent observations that
particles are transported across eld lines (e.g. McKibben et al. 10]). Thus if are particles can both
escape to the interplanetary medium in large CME
events, and can cross eld lines, they could make a
signicant contribution to major solar particle increases. This paper briey presents the results of
two papers which show that both these conditions
are fullled.
III radio bursts. The two exceptions were weaker
events for which particles were not measured until
several hours after the are. The radio bursts occurred in conjunction with H ares for all events
determined to originate on the front side of the Sun.
The ares occurred at the times of large, fast CMEs.
For the large proton events (i.e. excluding the >20
MeV proton events that would be described as `impulsive') the radio bursts were not normal type III
bursts. Such bursts occur at the onset of ares, typically last about 5{10 minutes, and commence at frequencies above 200 MHz. Instead the radio bursts
associated with major proton events were longer{
lasting, started at frequencies below about 100 MHz
and were much more fragmented than a group of
normal type IIIs. At frequencies observable from the
ground (typically above 15 MHz) they were not particularly intense and usually superimposed on slow
drifting features. Slow drift (type II) bursts are attributed to shocks but these bursts are generally
considered to occur behind CMEs (e.g. Leblanc et
al. 12]). Independent of the nature of these type II
bursts, the type III electron streams appear to originate lower in the corona and therefore are unlikely
to be shock accelerated. Below 1 MHz, as observed
from the WAVES experiment on Wind, the type III
events associated with major proton events are very
intense and generally much more prominent that any
type II emission. Cane et al. 11] called these events
\type III-l". Examples (above 18 MHz) may be
seen on the solar/Culgoora/historical data/SEP related events, section of the following website: http:
//www.ips.gov.au. The low frequency counterparts
may be seen at http://lep694.gsfc.nasa.gov/
waves/waves.html. Cane et al. 11] show composite spectra covering the entire frequency range
from 2 GHz to 20 kHz. The coverage is completed
by including data from the Bruny Island Radio
Spectrometer ( http://fourier.phys.utas.edu.
au/birs/) which bridges the gap between the Culgoora and WAVES frequency ranges.
Cane et al. 11] also performed a counter study
looking for type III-l bursts that were not associated with SEP events. This was undertaken in two
ways. First a random study was made of long{lasting
radio bursts seen in the WAVES data. Most were
found to be either type III-l bursts with associated
SEP events or long lasting groups of type III bursts,
as reported by ground based observers. Second, a
study was performed to look at the radio bursts associated with all large (angular extent >140 ) CMEs
in 1997-2000 which were frontside events as determined by the SOHO observers. Again, most type III-l
bursts found were already identied as being associated with an SEP event. It was also found that large
THE OBSERVATIONS
Cane et al. 11] have determined the solar associations for 123 events comprising essentially all >20
MeV proton events detected by the Goddard experiment on IMP 8 for the period 1997 to mid{2001.
They found that all but two were preceded by type
590
10
10
WAVES TNR May 6, 1998 0700-1300 UT
FREQUENCY (kHz)
FREQUENCY (kHz)
WAVES TNR October 29, 2000 0100-0700 UT
3
2
FIGURE 1. A type III-l burst associated with a solar
10
10
3
2
FIGURE 2. A type III-l burst associated with a solar
event from E35 . Note that the drift rate of the burst
becomes almost zero and the event does not extend below
50 kHz implying that the associated electron beam did
not intercept the spacecraft. The associated slow particle
increase is shown in Figure 3.
event from W65 . Note that the burst drifts very rapidly.
The associated particle increase rose very rapidly as may
be seen in Figure 3.
2
PROTONS (cm -sec-ster-MeV)
CMEs without associated radio bursts were in general slow. It was found that type III-l bursts without
associated SEP events were from the eastern hemisphere of the Sun. In these cases the radio emission
did not extend to the lowest frequencies observed
by WAVES indicating that the associated electrons
did not reach the vicinity of the spacecraft consistent with the absence of an SEP event. A number of
type III-l bursts that were associated with energetic
particle increases also did not extend to the lowest frequencies and all but one these came from the
eastern hemisphere. An example is shown in Figure
1 (the solar event occurred on October 29, 2000)
which shows data from the TNR receiver on the
WAVES experiment. In this case, and others like
it, the particle event had a very slow rise. For the
event illustrated the near{Earth 25 MeV intensity did not rise above background until about 10
hours after the solar event (see Figure 3). However
at Ulysses, located at 3 AU, at extreme southern
latitudes, and east of the Earth{Sun line the event
began about 6 hours earlier. Consistent with the
more rapid particle increase, the type III-l burst drifted more rapidly to low frequencies in the Ulysses
radio data. (The radio and particle data are shown
in Cane and Erickson 13]).
Figure 2 shows data for a radio event on May 6,
1998 that drifted very rapidly to the lowest frequencies consistent with the implied good magnetic connection to the are region the associated are was
located at W63 . Figure 3 shows the proton prole
for the associated particle event and also for the increase associated with the event of Figure 1. The
particle increases have rise times consistent with the
low frequency drift rates of the type III-l bursts.
101
May 6, 1998
100
10-1
10-2
10-3
October 29, 2000
10-4
4
10
16
22
HOURS AFTER SOLAR EVENT
FIGURE 3. Proton intensities in the range 24-29
MeV associated with the two radio events illustrated in
Figure 1.
Similarly from a statistical study of the type IIIl bursts associated with >20 MeV proton events it
can be shown 13] that the drift rates of the radio
emission organize the proton data. Figure 4 shows
how the \radio delay" is related to the magnetic
connection of the observer. The radio delay is the
time interval between the start of the burst at the
Sun and when its leading edge reached the local
plasma frequency. The local plasma frequency is
determined from noise in the antenna which appears
as a dark intensication across the bottom of the
TNR data, indicated by a bar along the left edges
of Figures 1 and 2. The magnetic connection is
the dierence between the are longitude and the
presumed footpoint of the eld line of the observer
591
RADIO DELAY (hours)
CONCLUSIONS
1. Type III radio bursts show that there are open
eld lines from aring regions accompanying all
solar particle events.
2. The lowest frequency emissions of the radio
bursts show that the responsible electrons can
gain access to a large region of the inner heliosphere. The radio data organize proton onsets suggesting that ares could contribute to
all solar particle events.
3. Interplanetary transport of energetic particles,
in particular transport across the mean eld, is
not yet fully understood.
4
3
2
1
0
-100
0
100
ACKNOWLEDGMENTS
CONNECTION ANGLE (degrees)
FIGURE 4. Radio burst delays as a function of mag-
The use of the data made available via the NSSDC
CDAWeb is acknowledged. This work was partially
funded by a NASA contract with USRA.
netic connection to the are longitude.
taking into consideration the solar wind speed.
Note that some of the radio bursts take several
hours to drift to the local plasma frequency whereas
others take less than an hour. This does not mean
that the responsible electrons are faster in some
events than in others - the radio emission is generated by those electrons which form a positive slope
in the parallel distribution function which at 1 AU
is typically in the 2-20 keV range. Rather the delays
reect the time it takes the responsible electrons to
propagate to 1 AU. Delays in the arrival of solar energetic particles as a function of connection angle of
the associated are have been known for years 14]
but attributed to delays occurring at the Sun. In
recent years delays were attributed to the time required for CME{driven shocks to deposit particles
on the observer's eld line. If this were the case for
the radio generating electrons, one would then expect a separate signature at the Sun of their escape,
in some cases several hours after the are. This is
not what is observed. Instead the radio producing
electrons actually may take up to several hours to
propagate from the Sun to near Earth. Since some
events originate at locations well removed from the
region of good magnetic connection, there must be
cross eld transport for the radio generating electrons. Since the bursts also organize the proton characteristics it seems reasonable that the ions undergo
cross eld transport as well.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
592
Reames, D. V., Space Sci. Rev. 90, 413{(1999).
Cane, H. V., Proc. 27th Internat Cosmic Ray
Conf. (Hamburg) 8, 3231 (2001).
Stone, E. C., et al., Space Sci. Rev. 86, 357
(1998).
von Rosenvinge, T. T., et al., in Solar
and Galactic Composition, AIP Conference
Proceedings 598, New York: American
Institute of Physics, 2001, pp. 343{348.
Luhn, A., et al., Proc. 19th Internat. Cosmic
Ray Conf. (La Jolla) 4, 241{244 (1985).
Leske, R. A., et al., in Solar and Galactic
Composition, AIP Conference Proceedings
598, New York: American Institute of Physics,
2001, pp. 171{176.
Cliver, E. W., Solar Physics 157, 285 (1995).
Hudson, H., J. Geophys. Res. 100, 3473
(1995).
Richardson et al., J. Geophys. Res. 96, 7853
(1991).
McKibben, R. B. et al. Proc. 27th Internat
Cosmic Ray Conf. (Hamburg) 8, 3281{3284
(2001).
Cane, H. V., et al. J. Geophys. Res. 107, 1315
(2002).
Leblanc et al. J. Geophys. Res. 105, 18,215
(2000).
Cane, H. V. and W. C. Erickson, J. Geophys.
Res. in press 2003.
Reinhard, R. and Wibberenz, G., Solar Phys.
36, 473 (1974).