Observation and identification of metre wave “blips”

Chin. Astron. Astrophys. (1994)18/T&222-229 A translation of Acta Astrophys. Sin. (1994)14/1,72-79 Copyright @ 1994 Else&r Science Ltd Printed in Great Britain. All rights reserved 0275-1062/94$24.00+.00

Pergamon

02751062(94)E0028-C

Observation and identification of metre wave “blips” t XIA Zhi-guo

MA Yuan

Yunnan Observatory,

Chinese

CHEN Jing-ying Academy

ZHENG Xiang-min

of Sciences,

Kunming

650011

Abstract High time-resolution (10 ms), high frequency-resolution (0.5 MHz) observations with the Yunnan Observatory spectrograph have led to the identification of narrow-band, short-duration, rapid frequency drift bursts in metre waves. It is a new type of metre wave bursts, different from the classical Type-III bursts, the spikes, or the Type-I bursts. Its features are similar to those of the “blips” in decimetre waves. Key words:

solar radio emission-metre

wave bursts-blips

1. INTRODUCTION Type-III

burst

is probably

the most thoroughly

researched

radio emission,

and there

have

been great efforts both in the theory and observations t,hat have yielded rather clear results. Electrons are accelerated to sub-relativistic velocities. (c/3) and pass through the corona along magnetic field lines. As the electron beams traverse the different heights in the solar atmosphere, Langmuir waves are excited at various frequencies, which subsequently change into electromagnetic waves. Here, not only is the acceleration process of the high-energy electrons in the corona an interesting topic for.study, we can also regard the electron beams as “probes” of the plasma physics of the solar atmosphere. Observation of fine structures in both the frequency and time domains in radio bursts can provide good determination of the parameters of the solar atmosphere plasma and the coronal magnetic field. Bursts that have a single fast frequency drift in Type-III burst has many subtypes. the range 30 kHz to 300 MHz are classical Type-III bursts. Benz et al.lll studied in detail decimetre wave quasi-Type-III bursts in the range 600 MHz to 1000 MHz and called them “blips”. Compared to the classical Type-III, blips have shorter lives and faster frequency drifts. They appear at higher frequencies and are better correlated with hard X-ray events. Benz summarized the characteristics of blips as follows: (1) Typical duration less than 200ms, typical bandwidth less than 100MHz. (2) Frequency range: 6000-1000 MHz. t Supported by National Natural Science Foundation Received 1992 February 10; revised version 1993 May 24

Blips in Metre Waves

(3) Average (4) Frequency -400 MHz/s.

bandwidth drifts:

Positive

Also, the spectra X-ray emission

20-30MHz, mostly

relative

negative

drifts have a similar of blips are mostly

223

bandwidth

drifts,

2.7%.

from -2OOMHz/s

to -lOOOMHz/s,

mean

spread. bell-shaped,

(40%), blips have weak polarizations

blips groups (lo%-30%),

often

coexist

with hard

etc..

Benz did not discover any blips below 400MHz. He thought that even if blips exist below 400 MHz, it will be difficult to distinguish them from Type-I bursts or Type-III bursts superimposed with spikes. In 1987, Stahli and Benz121 observed fast drifting microwave bursts between 3100MHz and 5205 MHz, with durations between 25 and 200 ms, drift rates as high as 8000 MHz/s, bandwidths greater than 150MHz. They are clearly different from spikes and bear some similarity to the decimetre blips. Now that blips are present in both microwave and decimetre wave, one naturally asks whether they are also in metre wave. For this, Yunnan Observatory improved the time resolution of their metre wave radio spectrograph to 10 ms and succeeded in observing what may be called “metre wave blips”.

2. OBSERVING

EQUIPMENT

AND

DATA

OBTAINED

The digital acousto-optical spectrograph of Yunnan Observatory uses a Reticon to transform frequency information into data streams, then uses a fast A/D transformer to collect the data. The clock frequency of the Reticon can be adjusted within a wide range and can work stably below 4MHz. The A/D data acquisition rate can also be set by computer software and can be made as fast as 1~s. The highest frequency resolution of the spectrograph is determined by the acousto-optical parts to be 250 kHz. According to the requirement of the job on hand, the frequency resolution can be set below the maximum in order to reduce the volume of data to be collected. With the use of Reticon, the background is conveniently subtracted. Instrumental noise and sky background should, in general, be subtracted. According to the requirement of the observation, emission from the quiet sun and slow-varying emission from the active region can also be removed. In this way, fine structures in the radio emission can be brought out and investigated. The observational data used in this paper had a time resolution of lOms, and the band 230-300MHz was divided into 140 channels, giving a frequency resolution of 0.5MHz. When used with 512 channels, the sprctrograph had a time resolution as high as 1 ms. Even when used with a frequency 0.5MHz and a time resolution 10 ms, there is still 12.8 Kbytes of data every second, and each 5-inch diskette only covers 6.8 seconds. It would be difficult to make routine observation at this data rate. So we adopted a threshold such that only when the flux exceeds the preset threshold are the data stored on the hard disk. At the end of each day, the data is copied onto soft disks for long-term storage. The radio spectrograph is connected to a 10-m parabolic antenna, which has good sensitivity and resistance to interference. Between 1990 July and 1991 December, a total of 2100 soft disks of burst data were obtained. Good correspondence was found with the 245MHz bursts, metre wave Type-III bursts and soft X-ray events published in P. R. F. and S. G. D.

224

XIA Zhi-guo et al.

and Ha flare lists.

Our data contained

Type-III

bursts,

spike groups

and blips groups.

Posi-

tive and negative frequency drifts of different speeds, double peak structures, superpositions of spikes and Type-III bursts-a host of phenomena were observed. If blips are present in metre waves, they should be narrow-band. According to Benz’ observed decimetre data, the lower the starting frequency the narrower is the bandwidth, for frequencies 6~0-700, 800-900, 9O~lOOOMHz, the bandwidth are 19, 28, 32MHz, respectively. If we extrapolate this to the 200-300 MHz range, we expect a bandwidth of only a few MHz. And there is confusion with spikes and noise bursts, so we need high-r~olution equipment to distinguish them, If we use usual analogue type spectrographs the only difference will be a slight difference in the shape of the images on spectrograms. Rusdjak131 pointed out that often different authors obtained different results in regard to the statistical relation between Type-I bursts and Ha flares. This is because Type-I bursts are confused with blips and the latter have a better correlation with Ho flares. Fig. 1 is a dynamic spectrum of a mixture of spikes, blips and Type-III burst. The event occurred on 1990 August 23, at 02:53:12UT. According to reports from LEAR, Australia, and PALE, Hawaii, 02:12UT was precisely the time of maximum of an Ha subflare, which lasted only 7 minutes. Yunnan Observatory also recorded the course of this flare, giving 02:53:17UT as the time of maximum. Referring to Fig. 1, the energy of the radio burst at 02:53:12UT is concentrated in the range 250-275MHe, where a, b, c, d, e are blips with bandwidths lo-25 MHz, durations about 300 ms, half-power durations about 150ms. The three blips a, b, c occurred at neighbouring frequencies, d and e, at slightly lower frequencies. These 5 blips have these in common: rich spikes are superposed at the lower frequencies; as they move to higher frequencies the duration lengthens to a maximum of about 3OOms at around 268MHz, then shortens and finally disappears at about 280MHz. The variation of the duration with frequency is bell-shaped, altogether different from Type-III bursts. As the negative drift bursts move to lower frequencies, the duration lengthens, the spectrum is bean-sprout shaped and can be expressed in the form141 td = ~OV-‘.~. Recall that at about 300 MHz, the mean duration of a Type-III burst is 1.1 &*I. This is clearly greater than that of the blips. In the range 253-260 MHz, typical spike groups are superposed on the blips. The duration of the spikes was 20-30 ma-spikes and the blips are clearly phenomena belonging to different time scales. The spikes have a bandwidth of 7MHz, or a relative bandwidth of 2.7%. Fig. 2 is another typical blips event. It occurred on 1991 May 7 at 08:35:39WT and 08:56:30 UT, where there was an eruptive prominence on the surface of the sun. Fig. 2(a) is a typical fast-drift, narrow-band drift event. The bandwidth was 10 MHz, and it occurred in the range 2~0~25OMHz. The half-flux width is 1lOms and the peak frequency drift, +65 MHz/s (drifting from the outer corona inwards). Fig. 2(b) shows 5 blips, with durations between 200 and 500 ms. The blip C had a bandwidth of 9 MHz, a duration of about 200 rns and a frequency drift of 82MHz/s. On May 13, at 01:35UT, the chromosphere telescope of Yunnan Observatory recorded the course of eruption of a loop prominence. At 01:16:59 UT and 01:21:59UT, our radio spectrograph recorded the accompanying spikes, blips and Type111 burst, providing a good

Blips in Metre Waves

225

F.

Fig. 1 A spectrum of a solar metric radio burst with fast frequency drift and narrow band (a) A outline of the burst (b), (c) A fine structure of the burst

XIA Zhi-guo et al.

226

n

I__

___.

4

I

Fig. 2

(a) A spectrum of a typical blip (b) The spectra of 5 blips

opportunity for their intercomparison. II burst from 0122 to 01:42. Fig.3(a)

Immediately following the two bursts shows the burst events at 01:16:59.

was a TypeEvent d is a

typical Type-III burst, with a frequency drift of 94MHz/s, and a duration of 1.38ms at 245 MHz. Events a, b, c are biips superposed on burst flux, Event e is a single spike which, at finer spectral resolution (0.5MHz between the time curves), showed maturation both in the frequency and the time domains. Event f marks a spike group. In Fig.3(b), a, b, c are blips; their low frequency portion fell outside the working frequency range, so an accurate bandwidth cannot be determined. d is a single spike and e is a spike group. Unlike Fig. l(b) w h ere large numbers of spikes are superposed on the blips, here, in Fig. 3(b) the spike groups are located in the 252~267MHz range, and the blips, in 237-246 MIIz range, with a comparatively calm, burst-free interval in between. Between 1990 July and 1991 December, a large number of blips were recorded at the times of Ha flares, eruptive prominences and soft X-ray events. Preliminary statistics on the parameters of several hundred blips were obtained, showing that the durations and bandwidths are narrowly distributed about their typical values while there is a large spread in the drift rate. This is because there are many factors affecting the drift rate, for example, the direction of motion of the high-energy particle beams has an important effect on the drift rate.

227

Blips in Metre Waves

F

a? 253.

m

k52

249

247

24%

242

233

Fig, 3 The spectra of the blips duringf a eruptive prominence (1991 05 13) (a) The blips overlapped by spikes and type III bursts; fb) There is a quiet intervat of 2-3 MHz (na significant burst) between the spike groups and the bfips

3. DIFFERENCES

TYPE-I 3.1 fluration

BETWEEN AND

METRIC

CLASSICAL

WAVE BLIPS AND SPIKES,

TYPE-III

BURSTS

of Single Bursts For classical TypeIII bursts, the duration is 1.1s; for LOO-3~Orns~ for the spikes, 20-1OOms. Most of the spikes have durations between 20 and 5Oms.

Type-f bursts, Is; for the blips, observed at Yimnan Observatory

3.2 Evolution of the Duration of a Single Burst in the Frequency Type-III bursts, as evofution proceeds in the frequency domain, the duration giving a bean-sprout profile. For the metric wave blips, the duration profile first increases then decreases. This is an important distinction between “contraction”

Domain For often increases, is the two. The

228

XIA Zhi-guo et al.

db/dt = 0.01v’.s4. According to this formula, for 260MHz. we have a rate of 280MHz/s. Benz gave, for decimetre wave blips, a mean value of -400 MHz/s, and there were also positive drifts. For the metric wave blips we found, the-rate varied between 60 and 200 MHz, with both signs; this is related to the fast moving (c/3) exciting source, as in the case of Type-III bursts. Type-I bursts have slow drifts, with rates less than 1 MHz/s (McLean, 1973), some two orders of magnitude slower than the blips. According to the data of Nansay radio heliometer, more than half of Type-I sources are stable, the rest are slow moving (corresponding to velocities of a few km/s). 3.5 Grouping When Type-III bursts occur in groups, the number in each group does not exceed 10 and the group covers a time interval of a few minutes. For decimetre wave blips, there is usually a few tens in a group, typical values reported by Benz being 4lOOindividuals per group and the group covers a frequency interval between several tens and several hundreds MI%, each group having a life time of around 1 minute. For metre wave blips, a group often contains a dozen or so individuals and each group lasts a few tens of seconds. For spikes, there are often thousands and thousands in a group, Benz’ statistics is 8000 N 13000 per group, spread over time intervals of about 20s and frequency intervals of a few MHz. Type-I bursts may have durations of several hours or even several days; they are concentrated in a narrow frequency range and mostly appear around 120MHz, rarely above 250 MHz. 3.6 Size of Source Rogion Benz estimated that the source region for spikes have sizes less than 200km~51. XIA Guo-zhi et al.lel measured the size distribution to be: maximum size 830 km, modal size 150 km. Benz using the formula Ah = ~AAcIv/v, calculated the size of the source region of decimetre wave blips to be 6.6~10~ km. Metre wave and decimetre wave blips have comparable bandwidths but their frequencies differ by a factor of 2-3, hence the relative bandwidth is 2-3 times larger in metre than in decimetre waves and the same formula then gives Ah c4~ 1.6x lo4 km. In a Type-III burst, electron beams go through the corona and beyond into interplanetary space, so the spatial scale involved is very large. 3.7 Correlations Spikes, blips Type-III bursts and X-ray events often occur correlated in time, showing that they are all related to fast electron beams in the solar atmosphere. In the frequency domain we sometimes see spikes evolving into blips and then incipient form of Type-III bursts, a typical example of this can be seen in Fig. l(b). Also, these events often occur when there are Ho flares or eruptive prominences. On the other hand, Type-I bursts are caused by weaker perturbations at the time of increasing solar activity.

4. RESULTS

AND DISCUSSION

(1) Narrow-band, fast-drift bursts or blips, exist both in decimeter wave and microwave. The fast drift rate shows the exciting source to be fast electron beams. In metric wave, the presence of many more Type-I bursts made identification of blips difficult. Spectrographic observation with fine time and frequency resolution at Yunnan Observatory has established the presence of blips in metre wave.

229

Blips in Metre Waves

(2) The main characteristics of metre wave blips are: bandwidth lo-25MHz, duration 100-300 ms, drift rate f60 MHz-f200 MHz. (3) The excitation of narrow-band radio bursts by high-energy electrons moving at about c/3 may be due to the following possible causes: (a) Rapid deceleration of the electron beam in passing through the corona, losing most of the energy over a short distance. (b) While the ccelerated electron beam executes long range motion in the solar atmosphere, only the plasma in a limited region is excited, undergoes oscillation, hence generates radio emission. Over the rest of the journey the physical parameters of the plasma are not suitable for triggering off oscillation. (c) The electron beam is confined in a small region by mirror reflection. (4) The duration of metre wave blips reaches a maximum at a certain frequency, then gradually “contracts”. This phenomenon suggests that the cause for the narrow band is inhomogeneity of the solar plasma. Prom the relative bandwidth of 4%-lo%, we infer that the size of inhomogeneity is about 1.5~ lo4 km. ACKNOWLEDGEMENT

We thank Professor ZHANG Bo-rong for valuable suggestions.

References

[l]

Benz A. O., ApJ, 1983, 271,355

[ 2]

Stahli M., A&A, 1987,175,271

[ 31

Ruzdjak V., Proceedings of the 4th Workshop on Solar Noise Storms, Trieste, 1982

[ 41

Elgr+‘y, A&A, 1972,16,1

[ 51

Benz A. d., Solar Physics, 1986,104,102,107

[ 6]

Xia Guo-zhi, Ma Yuan, Chen Jing-ying & Zheng Xiang-min, AApS, 1992,12,

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