Magnetic fields, bremsstrahlung, and synchrotron emission in solar flare of October 24, 196

Pruss, G.

Vorphal, J.

Zirin, H.

English

An impulsive flare October 24, 1969 produced two bursts with virtually identical time profiles of 8800 MHz emission and X-rays above 48 keV. The two spikes of hard X-rays correspond in time to the times of sharp brightening and expansion in the Hα flare. The first burst was not observed at frequencies below 3000 MHz. This cut off is ascribed to plasma cutoff above the low-lying flare. 



A model of the flare based on Hα observations at Big Bear shows that the density of electrons with energy above 10 keV is 5 × 10⁷ if the field density is 1011. The observed radio flux would be produced by this electron distribution with the observed field of 200 G. The Hα emission accompanying the hard electron acceleration is presumed due to excitation of the field atoms by the hard electrons

Pruss, Gail

Vorpahl, Joan

Caltech Authors - Main

Magnetic fields, bremsstrahlung and synchrotron emission in the flare of 24 October 1969

NASA Technical Reports Server

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MAGNETIC FIELDS, BREMSSTRAHLUNG AND
SYNCHROTRON EMISSION IN THE FLARE OF
24 OCTOBER 1969
H. Zirin & Gail Pruss
Hale Observatories
California Institute of Technology
Carnegie Institute of Washington
and
Joan Vorpahl
Space Sciences Laboratory
University of California, Berkeley
ABSTRACT
An impulsive flare October 24, 1969 produced two
bursts with virtually identical time profiles of 8800
MHz emission and x-rays above 48 KeV. The two spikes
of hard x-rays correspond in time to the times of sharp
brightening and expansion in the Ha flare. The first
burst was not observed at frequencies below 3000 MHz.
This cut off is ascribed to plasma cutoff above the
low-lying flare.
A model of the flare based on Ha observations at
Big Bear shows that the density of electrons with energy
above 10 KeV is 5 x 10 7 if the field density is 1011.
The observed radio flux would be produced by this electron
distribution with the observed field of 200 gauss. The
Ha emission accompanying the hard electron acceleration
is presumed due to excitation of the field atoms by the
hard electrons.
MAGNETIC FIELDS, BREMSSTRAHLUNG AND
SYNCHROTRON EMISSION IN THE FLARE OF
24 OCTOBER 1969
We have already published (Zirin, 1970) a description
of the eruptive flare of October 24, 1968, 2100 UT. We now
turn to the flare of October 24, 1969, 2112 UT. These
flares are actually different and the sunspot groups in
which they occurred were very different in magnetic con-
figuration. So the coincidence of position in the earth's
orbit is irrelevant, but amusing.
The October 24, 1969 flare is of interest in that
we have high resolution cinematograms from Big Bear Solar
Observatory, fine scan magnetograms from Mt. Wilson for
the following day, excellent hard x-ray data from UC
Berkeley (OGO-5) and radio observations from various
sources. These show a detailed correspondence between
Ha, radio and x-ray events. More important, the flare
produced two x-ray bursts of quite different spectra;
the contrast between these is most instructive for our
picture of emission by energetic electrons.
The flare occurred in a large, well-observed spot
group dominated by preceding spots. Figure 1 shows the
group in Ha on the morning of the 24th. Magnetic polarities
*2*
of larger scale features are marked, as deduced from Ha
structure and the fine scan magnetogram obtained by
Bhatnagar (1971, Figure 1B) on the 25th. There is an area
of bright plage north following the big spots: a filament
winds north from it. Sometime before the flare the fila-
ment separated from this region, which assumed bipolar form.
The two bright points making up the south part of the flare
are on opposite sides of the neutral line in that plage;
simultaneously two smaller points brightened under the
filament. Unfortunately all of this structure changed
after the flare, so the fine scan magnetogram of the 25th
is of limited utility and the field structure must be
deduced from the Ha morphology. The structure in the
flaring region is very complicated, with a number of
small spots; but it is definite that the two bright
kernels are on opposite sides of a neutral line. They
are not on the neutral line. The flare (Figure 2)
began between 12:11:40 and 21:12:00 (unfortunately the
interval between frames is 20 seconds) with two points
of opposite polarity which rapidly reached maximum
brilliance at 21:12:40, accompanied by brightening
along the filament neutral line. Growth in these slowed
around 21:13:30, followed by a new impulsive development.
*3*
beginning 21:13:58 and continuing to maximum around
21:16:00. This main phase of the flare was marked by
outward flow, in contrast to the first phase, which
was just brightening in place. The principal movement
was outward along the neutral line previously marked by
a filament. At the maximum the small bipolar plage as
well as the filament channel was all of flare brightness.
The flare was a relatively impulsive event as optical
flares go. Our observations ended just after the flare;
the next morning the entire structure was gone.
In Figure 3 we show the x-ray fluxes as measured
by the University of California Berkeley group with OGO-5,
compared to the radio data from Sagamore Hill. The higher
channels show two impulsive bursts, peaking at 21:12:30
and 21:14:00 as well as an extended burst peaking at
21:15:00 in the lowest channel. Because of pile-up of
softer photons, there is good reason to identify the
low energy maximum with the typical maximum phase soft
x-ray burst, such as we identified in the September 11,
1968 event (Vorpahl and Zirin, 1969). The soft x-ray burst
(2-12A) began 21:12 and peaked between 21:15 and 21:16.
The peaks of the two impulsive bursts agree perfectly in
time with the two fast brightness increases. Measurement
of areas also shows steep increases at the time of the
impulsive bursts. The spectra of these two bursts have
r
*4*
been calculated by Dr. Sharad Kane, who finds they
correspond to the following differential electron
energy spectra
Burst 1 (2113): dne = E -2.21 .
	
	45
neni V = 1.8 x 10
dE
Burst 2 (2114)
	 dne = E-3.01 ,	46
neni V	 1.6 x 10
dE
From these values and the observed area of the flares we
can derive the density of energetic electrons. For example,
the area A at the peak of the first burst is 2 x 10 18 cm2.
If we take a thickness 2 x 10 8 cm, we have V = 4 x 10 26 cm3.
For the second flare, which is four times greater in area,
and somewhat greater in height, we use V = 3 x 10 27 . Then
we have the parameters given in Table I,
TABLE I
FLARE PARAMETERS
BURST	 V	 n n	 V n  n N100 Nloo/BAe i
I	 4X1026	 1.8X1045 10 10 5x108 1.7x10 34 5x1012
10 11 5x107 1.7x10 33 5x1013
II	 3x1027	 1.6x1046 10 10 5x108 1.6x10 34 6x1011
lo ll 5x107 1.7x10 33 6x1012
where N100 is the total number of electrons with energy >100 Kev,
S
*5*
obtained by integrating eg (1) and N 100 /BA is the Holt-Ramaty
(1969) parameter, using B=200 gauss.
The radio burst as observed at Sagamore Hill is
shown in Figure 4. At the higher frequencies it agrees
with the x-ray burst in almost every detail. Since this
comparison showed slight delay in the radio data, we
checked with Dr. Castelli, who found that there was in
fact no delay; the radio data should be moved forward
10 seconds, which give perfect agreement. The first
radio pulse is cut off at frequencies below 3000 MHz,
a fact of great importance, which probably tells us
the height of the flare in the atmosphere.
There has been considerable discussion concerning
the height of flares in the atmosphere. Zirin (1964)
described a large explosive flare behind the limb
November 20, 1960 that produced a large fadeout with
little Ha visible; on the other hand Zirin et al (1969)
showed a perfect correlation between Ha and x-rays in
another behind-the-limb event December 2, 1967. Both
of these events radiated x-rays from high in the atmosphere;
the first was surely a hard event as well. But there is
considerable evidence for low-lying flares as well; the
hard x-ray burst of September 28, 1961 (Anderson & Winckler,
1962) showed a beautiful example of an impulsive burst at
9500 MHz, with no corresponding peak at 3000 MHz. It
*6*
appears that in flares without explosive motion the energetic
kernel remains low in the atmosphere, but in explosive events
the energetic particles cum magnetic field move outward so
that radiation comes from higher levels and no low-frequency
radio cutoff appears. If the burst comes from low in the
atmosphere, it might be cut off at lower frequencies by
overlying absorption (self-absorption is inadequate) or
by cutoff at the plasma frequency. If the burst is high
in the atmosphere, the cutoff would have to be produced
by the Razin effect.
For absorption by coronal material the opacity at
3000 MHz would be T-1.4 x 10 -30 ne 2 L; thus material at
coronal temperatures could produce no appreciable absorp-
tion. Plasma at 300000 would absorb 150 times more
effectively; then T-1 could be achieved with ne-l010
and L-100 Km. Alternatively, at ne -10 11 the plasma
frequency is 3000 MHz and all lower frequency emission
is cut off. Since we have no good model of an active
region we cannot choose between these, but neither a
temperature of 30000° nor a denzity of 10 11 will exist
above 3000 Kin 	 the active region; hence the Flare
kernel must be below 3000 Kin to explain the low frequency
cutoff. Since the cutoff is so sharp we believe it is
due to the plasma frequency. The fact that the second
pulse does not show a low-frequency cutoff and is connected
with an outward moving flare supports that view.
*7*
Holt and Ramaty (1969) have calculated the gyro-
synchrotron emission from a power law electron energy
distribution. The peak radio emission occurs typically
around 15 v B , where v  is the gyro-frequency, 2.8 MHz/gauss.
Thus for our case, where the peak frequency is 8800 MHz or
greater, the field would be around 200 gauss, exactly what
we observe on the magnetogram. At higher frequencies the
emission falls off as v(1-K)/2 where K is the exponent in
the differential electron energy spectrum; in our case
the first burst will fall off as v -^. At the lower
frequencies the burst is cut off by self-absorption, or
if v B<v P , by the Razin effect. The condition for a sharp
cutoff by the Razin effect is
ay = 3 yBy	 =	 314 BY	 < 1	 y =	 1$ vP	
a	 1- vz
c2
In our case, T = 200 gauss, so a is 0.2 for n  = 10 11 and
0.6 for n  = 10 10 . The value of y of course varies with
the electron energies responsible for any given frequency;
this is taken account of in the Holt and Ramaty calcula-
tions, but we can use y-3 as a rough value. Thus for the
ob:;:-- ed magnetic field the Razin cutoff takes place for
densities similar to those required for plasma cutoff;
again the source must lie low in the atmosphere. We
*6*
cannot use lower values of B to reduce a^because then
the peak emission would occur at lower. frequencies.
It is instructive to examine the applicable curves
of Holt & Ramaty (their Figure 1) using the values for
Nloo/BA (which they call N/BA) from Table 1. The value
for n  = 10 11 (extrapolating to a = 0.2) gives a peak at
5600 MHz of 100 sfu; with n  = 10 10 we find a higher peak
around 9000 MHz with 3000 sfu, about 15 times too much
flux. The latter case also gives too slow a fallout to
lower frequencies. What if the burst is higher in the
atmosphere, where density is lower? Then a is lar ge and
there is no cutoff; there also is too much radio emission
because N becomes large. If we decrease B, then the maxi-
mum wavelength of the radio emission drops lower, Clearly
the best solution is reac:zed with n i = 10 11 . But in that
case plasma cutoff cuts out the radiation below 3000 MHz
and we don't need the Razin effect.
The Holt-Ramaty spectra, even if they give the right
flux, are too steeply peaked to correspond to what is
observed in most bursts (the reader will convince himself
of this by simply glancing at recorded bursts in Solar -
Geophysical data or other publications; microwave bursts
often show flat curves between 5000 at 30000 MHz, with
*9*
peak values usually at the higher frequencies for
energetic events). Since the Holt-Ramatv calculations
are presumably correct, 	 we can only assume that the
magnetic field strength and particle spectrum are not
uniform across the region. Although the magnetogrant
does not reveal_ such small field changes in the bright
points that make up the flare, it cannot be ruled out,
in view of the inhomoceneity of the Ha emission, and is
in fact likely.
O'Dell and Sartori (1970) have recently called
attention to a low-frequency cutoff due to the pitch
angle distribution of the electrons. The low frequency
slope of this cutoff is vK . However our burst falls
from 100 sfu to nothing within a factor 2 of frequency;
again, plasma cutoff seems to be the answer.
The second radio spike shows no cutoff at low
frequencies. Since it appears to fly out from the region,
it must be supposed that it reaches higher elevations,
where plasma cutoff is no longer effective; the magnetic
field, hence a, is smaller here and surely B decreases
faster than ne . A Razin cutoff, if effective in the
first spike, would be even stronger i:-^ the second.
Plasma cutoff seems a better answer.
*In*
We have explained the characteristics of our flare
as those of a low lying energy release at the surface.
But what of those flares that have been proven, since
they come from spots behind the limb, to come from
20,000 or 30,000 km high? Good examples are November 20,
1960 (Zirin, 1964) and March 30, 1970 (Kane and Donnelly,
1970). The latter even showed a peak at at least 8800 MHz,
implying that it carried a field of 200 gauss up to this
high level in the atmosphere. Clearly these are huge
events which do not show low frequency cutoffs. The
typical flare with low frequency cutoff does not show
such huge eruptions.
There are, in summary, two interesting features in
this flare: the beautiful correlation between Ha, x-ray,
and radio emission; and the juxtaposition of two hard
bursts in the same flare, one with a low frequency cutoff
and one without. From the first we may conclude that the
same electrons are responsible for the radio and x-ray
emission, and either the same electrons or the accelerating
process are responsible for the Ha emission and the
energetic electrons. Of course it has been pointed out
many times that Ha is typically radiated at ten or twenty
thousand degrees, whereas the energetic electrons are of
course at far greater "temperatures." But as we saw, the
energetic electrons make up less than 1% of the total
*11*
clectrons, hence 99% of the material may be at a lower
temperature. Therefore, it is perfectly reasonable that
the Ha emission is produced by excitation of atoms of
neutral hydrogen in the flare. Indeed, we are most
fortunate that we have such a fine tracer of the mor-
phology of the x-ray flare. From the second point we
conclude that the low frequency cutoff of some bursts is
due to plasma frequency absorption.
It remains to explain why the hard electrons are so
well correlated with the extremely bright kernels of Ha
emission in flares (Vorpahl and Zirin, 1969). We can only
guess that the great brightness results from high density
or compression, and only in these cases is the field strong
enough to produce the harder electrons. Since the Ha
emission from the bright kernels is only 10 24 ergs/sec,
the energetic electrons along with those below 10 KeV
could provide the excitation of Ha, as well as enough
excess thermal energy for the subsequent emission after
the electrons themselves are lost or decelerated.
We wish to thank Dr. Kane for the use of his x-ray
data and several valuable discussions; Drs. Castelli,
van Allen, and Bhatnagar for their data, and Dr. Alan Moffet
for several discussions on the question of gyro-synchrotron
emission. The Caltech group was supported by NASA under
/^(4 L OS 	 o3+
grant	 8d4 and NSF under grant CA 24015; the
Berkeley group by NASA, under contract NAS 5-9094 and
Oil
grant NSL 05 00''0+7.
REFERENCES
Anderson, K. and Winckler, J. (1962) J Geophys Res. 67, 4103
Bhatnagar, A. 197.1: Solar Phys, in press.
Holt, Stephen S. and Ramaty, Reuven 1969: Solar Phys 9,
119.
Kane, S.R. and Donnelly, R.F. 1970: submitted to Ap J.
O'Dell, S.L. and Sartori, L. 1970: Ap J 62, 137.
Vorpahl, J. and Zirin, H.: Solar Phys Z7, 285.
Zirin, H. 1970: ESSA Report UAG-8, 30.
Zirin, H., Ingham, W., Hudson, H., and McKenzie, D. 1969:
Solar Phys 9, 269.
Zirin, H. 1964: Ap J Z40, 1216.
*12*
FIGURE CAPTIONS
Figure 1 - Ha Big Bear filtergram of the spot group 1838 UT
October 24. At the top center we see the small
bipolar satellite plage where the flare occurred;
the polarity change is marked by a band of trans-
verse dark fibrils in its center. Polarities marked
p and f.
Figure 2 - Development of the flare.
Figure 3 - OGO-V X-ray flux compared to radio flux. We
are mostly concerned with the hard burst in the
higher energy channels. The great rounded peak
at lower energies is mostly due to pileup from
lower energies.
Figure 4 - Radio emission from Sagamore Hill. Times shown
should be reduced by 10 seconds.
*13*
Fig. I
BIG BEAR SOLAR OBSERVATORY
Fig. 2
OCTOBER 24, 1969
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240
1


Magnetic fields, bremsstrahlung and synchrotron emission in the flare of 24 October 1969

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