A new kind of burst emission component was discovered, exhibiting fast and distinct pulses (approx. 60 ms durations), with spectral peak emission at f approx. 100 GHz, and onset time coincident to hard X-rays to within approx. 128 ms. These features pose serious constraints for the interpretation using current models. One suggestion assumes the f approx. 100 GHz pulses emission by synchrotron mechanism of electrons accelerated to ultrarelativistic energies. The hard X-rays originate from inverse Compton scattering of the electrons on the synchrotron photons. Several crucial observational tests are needed for the understanding of the phenomenon, requiring high sensitivity and high time resolution (approx. 1 ms) simultaneous to high spatial resolution (0.1 arcsec) at f approx. 110 GHz and hard X-rays

Correia, E.

Costa, J. E. R.

Kaufmann, P.

Vaz, A. M. Z.

English

NASA Technical Reports Server

SHORT-LIVED SOLAR BURST SPECTRAL COMPONENT AT f > 100 GHz
P. Kaufmann, E. Correia, J. E. R. Costa, and A. M. Zodi Vaz
INPE: Instituto de Pesquisas Espaciais, CNPq, C.P. 515, 12.200 S_o Jose
dos Campos, SP, Brazil
ABSTRACT
A new kind of burst emission component was discovered, exhibiting fast and distinct
pulses (_60 ms durations), with spectral peak emission at f _> 100 GHz, and onset time coin-
cident to hard X-rays to within _<128 ms. These features pose serious constraints for the inter-
pretation using current models. One suggestion assumes the f _> 100 GHz pulse emission by
synchrotron mechanism of electrons accelerated to ultrarelativistic energies. The hard X-rays
originate from inverse-Compton scattering of the electrons on the synchrotron photons. Several
crucial observational tests are needed for the understanding of the phenomenon, requiring
high sensitivity and high-time resolution ("1 ms) simultaneous to high-spatial resolution (<0.1
are s) at f > 100 GHz and hard X-rays.
DISCUSSION
An extraordinary solar burst was observed on 21 May 1984, 1326:20 UT, with high
sensitivity and time resolution using the Itapetinga 14-m antenna, at 30 GHz and 90 GHz, and
at hard X-rays (>25 keV) by the HXRBS experiment onboard SMM (Kaufmann et al., 1985a).
The event is shown and described in Figure 1. The spectrum derived for the Structure A of
this event is shown in Figure 2, clearly suggesting an emission component towards millimeter
(or sub-millimeter) waves only. The existence of such a burst emission component was, in fact,
already suggested in the past, although the data were obtained with very poor time resolution.
In Figure 3, spectra of various bursts obtained in the past, by different authors, do suggest
either a "flattening" or a rise towards higher frequencies. Therefore, the occurrence of an
emission component in the mm to sub-ram range of frequencies might be common to many
solar flares.
The basic diagnostics of the 21 May 1984 burst can be summarized as follows:
- Maximum emission frequency > 1011 Hz
- Timescale "-30 ms (60 ms durations)
- Several packets of distinct pulses. Large relative amplitude (AF/F) -_ 60%. Much
larger than the usual "ripple" ('_1%) found at f < 100 GHz.
- Hard X-rays (>25 keV) onset coincident to <100 ms.
- "Delays" or lack of time correlation at lower frequencies (<100 GHz) with respect
to hard X-rays, do not exist at >100 GHz.
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The 21 May 1984 burst-observed emission (for Structure A, Figure 1) in the radio and
hard X-ray ranges is shown as thick solid lines in Figure 4, which also includes the typical
fluxes reported for white-light flares (Neidig and Cliver, 1983). Several tentative spectral fits,
based on different energy loss mechanisms, are indicated in the figure and described in its
caption.
All the loss processes, spectra are shown in Figure 4, imply timescales considerably
larger than the observed ones (60 ms). If hard X-rays are produced by bremsstrahlung, the
short timescales observed require ambient plasma densities of the order of 10x5 cm "3, which
are quite too high. Therefore, another loss process might be acting in order to account for
the short lifetime of the pulses observed. One possibility is to assume the effectiveness of
inverse-Compton (I.C.) scattering, reducing the electron energies in short timescales, and
originating the observed hard X-rays. This possibility was already considered in the past
(Shklovsky, 1964). One interpretation was recently suggested to explain the features of the
21 May 1984 event (Kaufmann et al., 1985b). The pulses are attributed to primary accel-
erators located somewhere in an unstable loop, producing "instantaneously" a population of
ultrarelativistic electrons which constitute a self-absorbed synchrotron source inside which the
I.C. quenching on the source photons originate the hard X-ray photons. Equating the syn-
chrotron and I.C. conditions, the model predicts the observed fluxes at radio and at hard
X-rays, and accounts for the small timescales observed, which are shown in Figure 4. The
predicted Synchrotron 1 spectrum peaks in the infrared ("1013 Hz), and implies electrons
accelerated to >100 MeV energies, in magnetic fields ranging from 100 to 1000 G. A similar
mechanism spectrum has been proposed by Stein and Ney (1963) to explain white-light flare
(WLF) continuum emission.
According to this interpretation (Kaufmann et al., 1985b), the pulse sources are very
short-lived ("60 ms), very small (<107 cm), and exhibiting high apparent brightness tem-
peratures (>10 '° K). As these sources vanish, the electrons decay into lower energy levels
and might still be able to produce the better,known longer-lasting emissions at microwaves
and X-rays.
Alternative interpretations might be considered by placing the primary accelerator
sources deep into the chromosphere, in a very high ambient plasma atmosphere. This new
physical situation needs further investigations. One attempt is currently in progress reconciling
it with the burst impulsive phase description recently proposed by de Jager (1985).
Acknowledgments. This research was partially supported by Brazilian research agency
FINEP. One of the authors (PK) is a Guest Investigator on SMM.
209
REFERENCES
Akabane, K., Nakajima, H., Ohki, K., Moriyama, F., and Miyaji, T., 1973, Solar Phys., 33, 431.
Cogdell, J. R., 1972, Solar Phys., 22, 147.
Croom, D. L., 1970, Solar Phys., 15, 414.
de Jager, C., 1985, these proceedings.
Kaufmann, P., Correia, E., Costa, J. E. R., Zodi Vaz, A. M., and Dennis, B. R., 1985a, Nature,
313, 380.
Kaufmann, P., Correia, E., Costa, J. E. R., and Zodi Vaz, A. M., 1985b, Astron. Astrophys.
(submitted).
Neidig, D. F. and Cliver, E. W., 1983, "A Catalog of Solar White:Light Flares (1859-1982), Includ-
ing Their Statistical Properties and Associated Emissions," Rept. AFGL-TR-83-0257.
Ohki, K. and Hudson, H. S., 1975, Solar Phys., 43, 405.
Shimabukuro, F. I., 1970, Solar Phys., 15, 424.
Shimabukuro, F. I., 1972, Solar Phys., 23, 169.
Shklovsky, J., 1964, Nature (Letter to Editor), 202, 275.
Stein, W. A. and Ney, E. P., 1963, J. Geophys. Res., 68, 65.
Zirin, H. and Tanaka, K., 1973, Solar Phys., 32, 173.
210
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132:30 :31 :32 :33 :34 U.T.
Figure l. The solar burst of 21 May 1984 is shown in compressed timescale (A), with two
hard X-ray energy ranges at the top, and 90 GHz, 30 GHz, and 7 GHz at the bottom. All
hard X-ray burst structures correlate in time very well with the 90-GHz emission, but not
well for lower frequencies emissions. The major Structure A is time expanded in (B),
with hard X-ray (24 to 219 keV) data at the top, restricted to 128-ms time resolution,
and 90-GHz data at the bottom, displayed with 10-ms time constant. The structure
consists in a packet of distinct pulses, repeating at about 7 Hz. The e-folding rise time
of a pulse is about 30 ms. The hard X-ray onset is coincident to the 90-GHz onset
by less than 128 ms.
211
3
I0 - J J-" J] _ _ I _1 J i , j_
21 MAY1984, 1326 UT -
- STRUCTUREA -"
_- ITAPETIN_
- • AFGL -
02 OTTAWA_- 1 0
Z -
x I
m f _
< /
o I0 _ _ l
-- % I
- -
10° _ -._ i I i [ .... __t..... L_.J_J..J_ I ] _.L_L
10-I 100 10 102
FREQUENCY(GHz)
Figure 2. The observed spectrum of Structure A of 21 May 1984 solar burst (Figure 1).
The existence of a burst emission component peaking somewhere at f _ 100 GHz
is clearly evidenced.
212
104 J ---r
SPECTRA OF /"_"_---_-o
OTHERBURSTS /
103 /_
/ /
x 102
.-.I
i,
r,,--
0
lO1
..v?100 _J ._........ 1...............
lO8 109 109 iOII
FREQUENCY(Hz)
:_ CROOM(1970)
CI COGDELL(1972)
o SHIMABUKURO(1972)
I SHIMABUKURO(1970)
AKABANEET AL. (1973)
ZIRIN ANDTANAKA(1973)
Figure 3. Spectra of various solar bursts obtained in the past by various authors, with symbols
and names quoted at the bottom. They confirm that the occurrence of a burst emission
component in the mm to sub-mm range of frequencies is common to many flares. One
of these spectra (Shimabukuro, 1970) is possibly very similar to the spectrum of
the event studied here (Figure 2).
213
I l I e I I I I l .... -I- ! I I .
j,°.°°
• °°
-10 o. QUIET SUN
t
oe
,, _ WLF (REPORTED)
/ .......... :_'_'_ ...... THERMAL1
-14 / \/
m \ SYNCHROTRON1
'_- -I 8 -
SYNCHROTRON2 HARDX-RAYS
,-Y %
.- K--..............K----N F------*
"-' -22 SUB-MMx VISIBLEANDIR
h / •
"-" , ", OBSERVED
"-" -26t.L
_.1
SPECTRALFITS TO ,,
-30 21 MAY1984 BURST "
OBSERVATION \
-34 , , , , ----_.... -F------_--_----F- ....---I _ , _
8 I0 12 14 16 18 20 22
LOG,4 (FREQUENCYIN Hz)
Figure 4. A radio-hard X-ray range spectral display, showing the observed emissions of 21 May
1984 solar burst (Structure A, Figure 1), and the typical fluxes reported for white-fight flares
(Neidig and Cliver, 1983). The tentative spectral fits (points' lines) are: Thermal 1: a thermal
component with turnover frequency somewhere in the sub-mm or far infrared range of fre-
quencies, fitting white-light flare reported fluxes (Ohki and Hudson, 1975); Thermal 2: a
thermal component with turnover frequency at about 100 GHz (Shimabukuro, 1970);
and Synchrotron 2: losses by electrons with relativistic energies, with emission spectrum
peaking at about 100 GHz (implying in electrons energies 7 MeV in a 500 Gauss magnetic
field). The Synchrotron 1 plot in dashed line, peaking in the infrared, is a model prediction
for a synchrotron/inverse-Compton mechanism (implying electrons' energies ~100 MeV in a
500 Gauss field) (Kaufmann et al., 1985b). The quiet-Sun blackbody spectrum is shown
at the top.
214


Short-lived solar burst spectral component at f approximately 100 GHz

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