A tentative model is proposed to account for some features of the microwave millisecond spike emission and its links with the physical processes of associated phenomena during the impulsive phase of large flares by comparing the optical, radio, and X-ray observations on May 16, 1981 to those on October 12, 1981

Fu, Qi-Jun

Jiang, Shuying

Li, Chunsheng

Li, Hongwei

English

NASA Technical Reports Server

N87-21805
MICROWAVE MILLISECOND SPIKE EMISSION AND ITS ASSOCIATED
PHENOMENA DURING THE IMPULSIVE PHASE OF LARGE FLARES
LI Chunsheng, JIANG Shuying, and LI Hongwei
Department of Astronomy
Nanjing University
Nanjing, China
FU Qijun
Beijing Observatory
Academia Sinina
Beijing, China
ABSTRACT
A tentative model is proposed to account for some features of the miczowave
millisecond spike emission and its links with the physical processes of associated
phenomena during the impulsive phase of large flares by conparing the optical,
radio and X-ray observations on May 16, 1981 to those on October 12, 1981.
I. INTRODUCTION
The emission of short duration (i-i00 ms), high brightness temperature
(7_.i0t5 K) spikes at microwave frequencies during the impulsive phase of some solar
flares is now well established [i, 2]. The occurrences of these spikes may give us a
clue to the physical process of microwave millisecond spike emission(MMSE) linked
with its associated phenomena.
On the basis of the observations described in [i], Melrose and Dulk (1982)
discussed the relation between the physical processes generating MMSE and hard X-ray
l ffr vjt.mw
193
https://ntrs.nasa.gov/search.jsp?R=19870012372 2020-03-20T11:44:27+00:00Z
bursts. They suggested that MM_E was caused by the loss-cone driven electron-cyclo-
tron maser in a flaring loop [3]. In this paper, %_ go further to find the physical
processes of the MMSE and its links with associated phenomena druing the impulsive
phase of some solar flares, but the MMSE is considered to be excited by the electron-
cyclorton instability associated with a hollow beam of electrons [4].
We have analysed two major flare-burst events of May 16 and October 12, 1981.
The following observational data during the impulsive phase of these two flares were
used for comparison.
(i) Radio observations made at 2.84 GHz with the time resolutions of 1 second
and 1 ms at the Beijing Cbservatory,
(2) H_ and photospheric magnetic field observations made at the Yunnan
Observatory,
(3)Hard X-ray burst observations made on Hinotori (by courtesy of Dr.K.Tanaka)
(4) Radio spectra for type III and type IVD_zM bursts published in Solar-
Geophysical Data.
All the data staed above are stmmarized in Table i.
II. A GENERAL DESCRIPTION OF THE T_D FLARE-BURST EVENTS
As shown by Table i, there existed some similar features between these two
events, such as the coincidence with H_ flares of inlDortance 3B, two-ribbon flares,
magnetic configurations of type _ and photospheric magnetic intensities _2500G.
However, we can see in the following their significant characteristics different
from each other.
i. There appeared strong MMSE's (Tb _ i0 I_" K) during the first event (1981 May
19 ) but appear during the second (1981 October 12 ).
2. A lot of intense decimetric bursts of type IIIs , type III_ and type IV_z M
occurred during the first event, but no decimetric burst and only weak metric bursts
occurred during the second.
3. Although only the decay phase of hard X-ray bursts was recorded on Hinotori
for the first event, the peak values (counts per second) of hard X-ray bursts for
the first event still far exceeded those of the second in the same energy range.
4. However, the peak flux density of radio burst at i0.6 an in the second
194
event is about six times larger than that in the first.
III. THE MDDEL AND KEY PROCESSES
It has been pointed out in [2,6] that after the main phase of the microwave
burst there still appeared MMSE's. This means that the fast electrons generating
the microwave bursts and those exciting MMSE's do not come from the same source.
Furthermore, the simultaneous observations of hard X-ray bursts by SMM and high
resolution microwave observations by ViA indicate that the sources of these two
bursts are mot ooincident with each other in _pace.
On the basis of these observations and a comparison between the two flare-burst
events stated above, _ propose a tentative model to account for the links of the
physical processes of MMSE's with their associated phencmena as follows (Fig. i. ).
i. During the impulsive phase of large solar flares, there probably appear two
acceleration regions, One (region A) of them formed in the current sheet by a tear-
ing mode instability is located over the top of the flaring loop, the other (region
B) is established just at the top of the flaring loop by turbulence acceleration.
Regions A and B are also the energy release regions.
2. A stream of fast electrons escaping outward from region A along open field
lines is able to excite type III bursts with a negative frequency drift under
certain conditions, while the other stream injected downward with a certain incident
angle into the flaring loop is capable of establishing an anisotropic pitch angle
distribution of "hollow beam" and stimulate an electron-cyclotron maser to radiate
MMSE's or generate type IVp¢_M bursts with positive frequency drift. As soon as the
fast electrons radiate away energy in the direction perpendicular to the magnetic
field, they immediately precipitate into the transition region or the chromosphere,
collide with the surrounding plasma and emit hard X-ray bursts (thick target model).
3. The background radiation, i.e. microwave radio bursts superimposed by
MMSE's, is generally accepted as a gyrosynchrotron radiation emitted by nonthermal
electrons, gyrating about field lines, with an isotopic pitch angle distribution
and power law energy spectrun. The microwave burst source is located in region B at
the top of the flaring loop.
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! ' I i I ' I ' I ' I ' I i I '8-
' fLOG F 1/_1 >7--_ 7 MICROWAVE BURS1rj /AT k= 10.6cm
6 -_ 1981 MAY 16
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Fig. i. The large microwave burst at _ = 10.6 an
and hard X-ray bursts recorded by Prognoz 8
satellite during the large flare-burst event
of 1981 May 16 [5].
I
IV. EXPLANATION AND DISCUSSION
I. According to a quasilinear theory, the general formula for the growth rate
at s-th harmonic for wave in the magneto-ionic mode is [3, Appendix B]
197
TYPE III
O BURST
o
ACCELERATION
REGION
TRANSITIOI_
REGION
X-RAY
N S
Fig. 2 A model of MMSE and associated phenomena
68
where p = (p_ + p._ )_ : rmv is the electron's momentum, 6_)8=-_-_ is the electron-
cyclotron frequency, and _ and I/ denote components of wave number and velovity to
the direction of the magnetic field B, p, = pcos_, p_ = psin_ , and_ is the pitch
angle, r is the Lorentz factor.
of
It can be shown [8] that the sign of the integrand is determined by the sign
suO_ ix)
Positive contributions to Df favour the growth and negative contributions favour
the damping of the waves.
198
Suppose that the distrioution function of nonthermal electrons with an isotro-
pic pitch angle and power law energy spectrum for gyro-synchrotron radiation can be
written as in [9, i0]
here G is a constant and g is the spectral index, gin0. one obtains _<o , so that
_(r)< 0. Therefore, the distribution function of fast electrons in expression (3)
is capable of producing gyrosynchrotron radiation of microwave bursts [9, i0] but
it can not amplify the s-th harmonic waves and generate MM_E's.
Just on the contrary, the "hollow beam' distribution of fast electrons favours
the growth of waves and leads to the generation of MMSE's [4].
2. The distribution of the pitch angles of the electrons injected from Region
A to Region B on top of the magnetic arch is determined by the distance D between A
and B. If D is sufficiently large, then the distribution is isotropic; otherwise,
it will be anisotropic. This is because, the large the D, the greater will be the
diffusion of the electrons; the smaller the D, the more restricted will be the angle
of injection of the electron beams.
Table 1 shows that, for the event of May 16, the Type III bursts began in the
decimetric wave range (aroung 60 cm, corresponding to a plasma frequency of 500 MHz,
and a height of 23 xl0_km above the photosphere, see below). For the Oct. 12 event,
they began in the meter wave bands (e.g., 2 m, corresponding plasma frequency
150 MHz and height 2.3 x 105 km above photosphere). Since both events have the same
type photospheric magnetic field with strengths _ 2500 G, for both then, we may
regard the microwave (10.6 cm) burst source in Region B at the top of the magnetic
arch during the impulsive phase of the flare to have about the same size (or i0// ).
It then follows that the angle of the cone subtended by the electron beams issuing
from Region A (the height corresponding to the starting frequency of the Type III
burst) must greater than 24020 ' for the earlier event in order to achieve isotropic
distribution, and need only 1°58 ' for the later event. Clearly it is easy to a beam
ejected at A to be diffused through the collision with the background particls into
a cone as small as 1° 58' when reaching B. Thus, for the later event, of October 12,
the electron beams injected from A into B had an isotropic distribution of pitch
199
angle and so could not generate any spike emission or intense x-ray bursts. But
such isotropic beams would add to the isotropic electrons in Region B and greatly
increase the gyro-synchrotron radiation, hence the n_Dre intense at 10.6 cm.
For the earlier event of May 16, the Type III burst was strong, its starting
frequency was high, the generating electrons bad high energies, the downward
injected bear,%ras energetic and difficult to disperse, making the incident cone far
below the value of 20°20 ' required for isotropization. The bea_, then, was a
"hollow-beam" and so generated intense spike endssion and also led to strong bursts
in hard X-ray. According to the radio dynamic spectrum measured at Dwingloo 5,
during the inpulsive phase, this event (May 16), at about 0814.5 UT, a Type III
burst with a negative frequency drift appeared in 300-380 MHz, and at sa_e time,
one with a positive drift appeared in 509-666 MHz. This observed fact shows
certainly that Region A in the neutral current sheet can simultaneously eject both
one upward (towards the outer corona) and an inward (toward the coronal base)
electron bean_. It was estimated that the electron density in this acceleration
region was about 3xl0_km above the photospere.
3. Type III_ bursts are the type III bursts (_ i0) appearing in groups. In
the event of 1981 May 16, the M_E's recorded at wavelength 10.6 cm often happended
in a group-like type III_ burst. Since the radiating electrons of type III_ bursts
and _E's escape from the same acceleration process. Probably, the duration of
each group of _4SE's corresponds to that of each subburst in type III burst. The
switch-off structure of MMSE's in [2] corresponds to the interval between t_
subbursts and the switch-on structure manifests the start of a group of F_4SE's due
to the injected electrons with a "hollow beam" distribution.
As can be seen in Table i, an intense type-III burst appears from 0810 to
0816.2UT. It might be in correspondence with the significant switch-off and switch-
on structures of _4SE's at about 0815UT during the rising phase in micro-
wave burst (at 10.6 cm) of May 16, 1981.
About 0814.5UT in the band 300-480 F_z, type III bursts with negative fre-
quency drifts of about 100MHz/sec were observed with Dwingloo radio-spectrograph,
while at the higher frequencies 509-666 _ positive drifting bursts about
+ 50 MHz/sec occurred [5]. This is a strong evidence indicating the sin_itaneous
acceleration of electrons upwards and downwards. The electron density in the
acceleration region was estimated to be 3 x i0 9 cTn.3 and the corresponding
2OO
height about 2.3 x 104 Km.
5. Becausethe radiating electrons of hard X-ray bursts and type III bursts
comefrom the sameacceleration region A, the key problem is whether these electrons
possess enough energy to radiate _E's and hard X-ray bursts. We should answer this
question by evaluating the energy of these nonthermal electrons which may produce
type III bursts.
Type III bursts present a rapid drift from high to low frequencies at a rate
described in [II].
Generally speaking, Type III emission is ascribed to the scattering of
Cerenkov plasma waves produced by fast electron streams. It is believed that most
type III bursts are observed at the second harmonic of the local plasma frequency
cD
where R(t) represents the position of type III burst source at instant t. N(R) is
the local electron density in cm -3 .
From equation (7), the drift rate in frequency can be expressed as
 o4: .e (6)
./N
where _ is the gradient of coronal electron density and _-v is the velocity
vector of the fast electron stream. If e is the angle between_ and v, and 6 the
angle between the line of sight and the direction of the electron stream, then for
a relativistic electron stream and because of Doppler effect and _< i, we can
l
obtain approximately fv =-/S/d_. From Equation (6), the rat io_=_ can be
l-
given by
a.
201
and
For simplicity, we assume that the electron density of the corona is in
spherically symmetric distribution and the fast electron streams move outward along
the radial direction.
When 8=_ = O, _ have
The energy of the fast electrons producing type III bursts is given by
Moreover, _e use the model of N(R) for solar maximum activity fiven by Table 2 of
referenoe [12]. Finally, the kinetic energy of fast electrons producing type III
bursts for different frequencies is shown in Table 2.
If _ and _ are not equal to zero, the value of_(_,_) evaluated from equa-
tion(7) n_st be larger than that from equation (9) with the same frequency. Hence
the energy of fast electrons increases. Therefore, as long as the fast electrons
injected downward from acceleratiom region A possess the same energy as those
exciting type III bursts, they are still abl_ to produce hard X-ray bursts as those
recorded on "Hinotori" or on Prognoz 8 satellite even after losing some energy
about tens of kev due to MMSE's.
Table 2. Energy of fast electrons producing type
III bursts at different frequencies
i _ i =,
f(_z ) 600 300 200
_ (KEY) 11.24 69 34
V. CONCLUDING WORDS
Based on our model and the mechanism of electron-cyclotron instability
202
associated with a hollow beam of electrons, we have explained why there occurred
strong MMSE's during the event of 1981 Fay 16 and why there appeared no MMSE but
more intene microwave bursts at 10.6 am during the event of 1981 October 12.
Furthermore, we have shown that there exist some intimate links of MMSE's with
their associated phenomena in the physical processes of generation and evolution
during the impulsive phase of large flares. Obviously, the discovery of these links
with ere another is /mportant for clarifying the mechanism of large flares.
The authors are indebted to Dr. K. Tanaka for sending us the data of hard
X-ray bursts recorded on "Hinotori" satellite and to the Yunnan Observatory for
the data of photospheric magnetic fields.
REF_RmgCES
[i] Slottje, C., Fast Fine Structure in Solar Microwave Flares, Nature,
Vol. 157(1978), p. 20.
[2] ZHAO Renyan and JIN Shengzhen, Analysis of Ultrafast Fine Structures of Micro-
wave Bursts, Scientia Sinica (Series A), Vol 25(1982), p. 422.
[3 ] Melrose D.B. and Duld C.A., Electron-Cyclotron Masers as the SOurce of Certain
Solar and Stellar Tadio Bursts, Ap. J., Vol. 259(1982), p. 844
[4] LI Hongwei LI Chunsheng and FU Qijun, Electron-Cyclotron Masers and Solar Micro-
wave Millisecond Spike Emission, This Workshop.
[5] Farnik F. et al., X-ray, H and Radio Cbservation of the Two-ribbon Flare of
May 16, 1981, Dwingeloo Reprint.
[6] FU Qijun, LI Chunsheng and JIN Shengzhen, Spike Groups Event in Solar Micro-
wave Bursts, submitted to Acta Astronomica Sinica
[7] Marsh K.A. et al., Properties of Solar Flare Electrons, Deduced from Hard X-ray
and Spatially Resolved Microwave Observation, Ap. J. Vol. 251(1881), p. 797.
203
[8] Hewitt, R.G.et al., A Cyclotron Tneory for the Beaming Pattern of Jupiter's
Decimetric Radio Emission, Proc. ASA, Vol. 4(1981), p. 221.
[9] Takakura T. and Scalise, E.J., Gyro-synchrotron Emission in A Magnetic Dipole
Field for the Application to the Center-To-Limb Variation of Microwave Impul-
sive Bursts, Solar Phys. Vol. 11(1970), p. 434.
[i0] LI Chunsheng et al., The Mechanism of the U-shaped Spectrt_n of Type IV Solar
Radio Bursts, Chinese Astronomy, Vol. 4(1980), No. 2, p. 155.
[II] Alvarez, H. and Haddock, F.T., Solar Wind Density Model from Km-Wave Type III
Bursts, Solar Phys., Vol. 29( 1973 ), p. 197.
[12] Zheleznyakov, V.V., Radio Emission from the Sun and Planets, Per Press,
Oxford, 1970.
2O4


Microwave millisecond spike emission and its associated phenomena during the impulsive phase of large flares

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