Highlights of Solar Radio Physics 1/2007:
Are there Radio-quiet Solar Flares?
Solar flares accelerate particles, and their distribution is usually
non-thermal. Non-thermal electrons are prone to velocity space
instabilities driving various plasma waves which, in turn, couple into
observable radio waves. Such coherent emissions are the result of the
combined action of many electrons organized by kinetic plasma waves
and can therefore be extremely efficient. Small coherent radio bursts
at the limit of present routine observations emit an energy of some
1015 erg. Even if the conversion of particle energy into radio
emission may occur at a ratio as low as 10-6, the energy in the
electron population that excites coherent radio emission is a tiny
fraction of the flare energy. Thus a slight deviation from an isotropic
Maxwellian electron distribution in the course of acceleration,
as a result of particle propagation or magnetic trapping, may be
sufficient to cause coherent radio emission.
Radio-quiet flares should be infrequent!
It is not surprising that many radio bursts are not accompanied
with X-ray emission as observed with present sensitivity.
More astonishing are reports of radio-quiet flares. Simnett and Benz
(A&A 165, 1986) found no coherent radio emission between 100 and 1000 MHz in 15%
of the events having count rate >1000 cts/s at photon energies >25 keV
observed by the Hard X-Ray Burst Spectrometer
(HXRBS) on the Solar Maximum Mission.
The detection ratio did not improve in statistics based on more sensitive
radio observations and larger frequency range, extending up to 4000 MHz:
In 17% of the X-ray events observed by the Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI)
, no coherent radio emission was associated in
observations between 100 and 4000 MHz (Benz et al.,
Solar Physics 226, 2005).
The enhanced sensitivity and the larger search range of frequencies must
have increased the association rate. On the other hand, the extension
to smaller events in X-ray magnitude from HXRBS to RHESSI may have
decreased the association rate. The two opposing effects have
apparently cancelled out. The existence of such 'radio-quiet' flares
puts severe constraints on the acceleration and containment of
energetic flare electrons.
Some correlate well! A more typical case of correlation between X-rays
and coherent radio emission in the 100 to 4000 MHz range, observed on
Phoenix-2 in Zurich and
Analysis of the radio-quiet flares
So, what about these radio-quiet flares? We have analyzed them in
detail and find the following properties:
- Out of a total number of 201 coincident flares
(RHESSI – Phoenix-2) having a GOES class >C5, 29 flares were found
initially radio-quiet in the 100–4000 MHz range. A closer
inspection with increased sensitivity by integration yielded radio
emission in only one case. Thus radio-quiet is a yes/no property.
- A surprising number of 22 (76%) out of the 29 radio-quiet events
occurred at a radial distance of more than 800" from disk center,
indicating that radio waves associated with a limb flare may be
- The remaining 7 radio-quiet flares that occurred within 800" had
a property in common: All of them were accompanied with metric type III
radio emission below 100 MHz, as reported by the
- These 7 radio-quiet flares occurring within 800" had another
startling property: All of them had no RHESSI counts above 25 keV. Thus
they had weak non-thermal emission in general or were soft in their
non-thermal X-ray spectrum. In fact, there is evidence for both:
the radio-quiet flares were all below C7.0 class except for one event.
There were two events with a large count rate at 12–25 keV
(> 300 cts/s). Thus they must have been very soft to vanish at 25 keV.
So how does a flare become radio-quiet?
What does a flare have to do to remain undetected by current radio
instruments? The best idea is to stay small.
Only one flare out of 11 with
class larger than M1 was radio-quiet. A good idea is furthermore
to occur at the limb.
Assuming a random distribution of flare positions, 33.4 flares out of
the total number of 201 are expected to occur at the limb (>800").
The fact that 22 of the radio-quiet flares were indeed found at the limb,
indicates that 66% of the limb flares larger than C5 are radio-quiet.
A third idea is to be soft in non-thermal
electron distribution. The duration, impulsive or extended, does not
seem to help, however. Of course, the gyrosynchrotron emission in
centimeter waves and thermal emission in millimeter waves cannot escape
detection given a large enough instrument, but these radiations are not
the issue here.
The cloak of invisibility of the flare process against radio detection
in the 100–4000 MHz range — small, on the limb and soft —
may be interpreted by reduced emission and/or absorption.
- In limb events, the propagation path is initially nearly horizontal.
The dwelling time of the radio waves in a region having a plasma
frequency close to the radiation frequency is much longer near the limb.
Thus the absorption is higher, as well as the chances to meet an
occulting region having a plasma frequency above the radiation
frequency. Limb events are evidently more likely to be absorbed or
- Soft electron energy distributions are less likely to become
loss-cone unstable and need a longer beam propagation time to
develop a bump-on-tail instability. Thus hard X-ray events originating
from a soft energy distribution are less likely to have decimetric
loss-cone or beam-propagation emissions.
- The most difficult characteristic to
interpret seems to be the smallness of radio-quiet flares. Remember
that the radio emission of the smallest detectable X-ray flare
constitutes a minute fraction of its energy. Thus, it may not be the
lack of energy in small flares that reduces the association rate, but
the smaller number of elements yielding fewer chances for radio
emission to escape.
- There are no really radio-quiet flares >C5 if the
full meter and decimeter wave ranges are surveyed and the source is
observed from above (i.e. not from the limb).
- There is no evidence for purely thermal flares >C5 without
super-thermal electrons. Given enough propagation path length, all
these flares produce coherent radio emission at least by electron
- A comparison of radio and X-ray emissions from the flare electron
accelerator must focus on events near the center.
- The results motivate to compare radio and X-ray emissions not
only in association, but in position. Future multi-frequency radio
capabilities necessary to accomplish this, combined with soft and
hard X-ray imaging, could make a break-through in the understanding
Based on a paper recently submitted to
by Arnold O. Benz, Roman Brajša, and Jasmina Magdalenić
January 4, 2007
Arnold O. Benz (),
with thanks to S. Pohjolainen
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