Several solar hard X-ray events (greater than 100 keV) were observed simultaneously with identical instruments on the Venera 11, 12, 13, 14, and Prognoz spacecraft. High time resolution (= 2 ms) data were stored in memory when a trigger occurred. The observations of modulation are presented with a period of 1.6 s for the event on December 3, 1978. Evidence is also presented for fast time fluctuations from an event on November 6, 1979, observed from Venera 12 and another on September 6, 1981, observed from the Solar Maximum Mission. Power spectrum analysis, epoch folding, and Monte Carlo simulation were used to evaluate the statistical significance of persistent time delays between features. The results are discussed in light of the MHD model proposed by Zaitsev and Stepanov

Barat, C.

Desai, Upendra D.

Hurley, K.

Kouveliotou, Chryssa

Niel, M.

Talon, R.

Vedrenne, G.

English

NASA Technical Reports Server

NS÷- 21r90
PERSISTENT TIME INTERVALS BETWEEN FEATURES IN SOLAR FLARE
HARD X-RAY EMISSION
Upendra D. Desai and Chryssa Kouveliotou
Laboratory for Astronomy and Solar Physics
NASA Goddard Space Flight Center
C. Barat, K. Hurley, M. Niel, R. Talon, and G. Vedrenne
Centre d'Etudes Spatiale de Rayonnment
Toulouse, France
Abstract
Several solar hard X-ray events (> 100 keY) have been observed simul-
taneously with identical instruments on the Venera II, 12, 13, 14 and Prognoz
spacecraft. High time resolution (= 2 ms) data were stored in memory when
a trigger occurred. We present the observations of modulation with a
period of 1.6s for the event on 1978 December 3. We also present evidence
for fast time fluctuations from an event on November 6, 1979, observed from
Venera 12 and another on September 6, 1981, observed from the Solar Maximum
Mission. We have used power spectrum analysis, epoch folding, and Monte
Carlo simulation to evaluate the statistical significance of persistent
time delays between features. The results are discussed in light of the
MHD model proposed by Zaitsev and Stepanov (Soviet Astron. Letters, 1982,
8, 132, and Solar Physics, 1984, 92, 283).
I. Introduction
Light curves of a number of solar flare events observed in microwaves
and/or X-rays with high time resolution (- 0.1 s) show distinctive fea-
tures during the rise and/or decay phases. The time histories of hard
X-rays observations with time resolution of about a second on TD-IA
could be resolved into "elementary flare bursts" with full width at half
maximum of 4 to 25 s (van Beek 1974, de Jager and de Jonge 1978). The study
of a number of such elementary flare bursts in a single event or their dis-
tribution in time was not pursued in earlier papers. Recently, Loran
et al. (1985) have simulated the fast ripple structure by incorporating
in their model a variable repetition rate of these elementary bursts.
Sturrock et al. (1984) have tried to associate elementary event bursts
63
https://ntrs.nasa.gov/search.jsp?R=19870012357 2020-03-20T11:44:00+00:00Z
/ /'
with "elementary flux tubes", thus suggesting that the features are spatial
in origin. However, Ohki (1985) reports from Hinotorl imaging data
that the small size of the sources of impulsive flares and the lack of
motion indicate that spikes are temporal rather than spatial in origin,
i.e. that the same flux tube flares repeatedly.
Pioneering observations of solar hard X-rays (> 20 keV) by Parks
and Winckler (1969) with a balloon-borne detector revealed significant
power at a period of 16 s. They also reported similar simultaneous
behavior in microwaves at 15.4 GHz. Frost (1969) reported the existence
of modulation in OSO-5 data with a period of 35 s. Rosenberg (1970) ana-
lysing the solar radio event of February 15, 1969 (160-320 MHz), reported
modulations with a period of i s. He explained the modulations on the
basis of magnetohydrodynamic (MHD) oscillations in a coronal magnetic flux
tube. Janssens et al. (1973) also reported periodicities in radio data.
In two homologous microwave events on May 29, 1980, Urpo (1983) has reported
features with an average separation of 3.04 s. Recently Zodi et al. (1984)
have reported 1.5 s pulsations in both 22 GHz and 44 GHz high time resol-
ution microwave data. Wiehl and Matzler (1980) studied both hard X-ray
and microwave events and reported the existence of quasi-periodic modu-
lations with periods as short as five seconds. In their study a change of
slope during the rise or fall was accepted as a modulation feature.
In the studies mentioned above emphasis has been given on results
that demonstrate periodicities or quasl-periodicities. We present solar
events which show the prevalence of persistent time-delays between suc-
cessive or alternate features indicative of the existence of a synchro-
nized series of features. We examine this in the light of the proposed
MHD model of Zaitsev and Stepanov (1982, 1984). Various models have
been proposed to explain pulsations both in hard X-rays and in radio
data (Rosenberg 1970, McClean et al. 1975, Chiu 1970).
Power spectrum analysis of transient events to find out periods
which are fractions of the total duration of the transients are not
adequate. We have also pursued auto correlation, the epoch-folding tech-
nique, and Monte-Carlo stimulation to statistically evaluate the persist-
ence of delay times between features. In highly dynamic turbulent phenomena
like solar flares, strict periodicities may not prevail but parameter
dependent characteristic times - rise, decay, and delay times - could
prevail.
2. Observations
Observations reported here were made on the Venera 12 deep space
probe (VI2) and the Earth orbiting Solar Maximum Mission (SMM) satellite.
Detailed dlscriptlons of the instrumentation have appeared in Barat et al.
(1981), Chambon et al. (1979), and Orwig et al. (1980). The basic detec-
tors on VI2 are 4.5-cm radius, 3.7-cm thick NaI(T£) scintillators with a
64
plastic anticolncldence shield; the Hard X-Ray Burst Spectrometer (HXRBS)
on SMMconsists of a Csl(Na) scintillator surrounded by a Csl(Na) shield
with an opening angle of - 40° FWHM.In both detectors, an increase in
counting rate above a certain threshold initiates data storage in memory.
The finest time resolution available is 2 ms for VI2 and 1 ms for S_IM.
The average photon energy threshold is - 100 keV for VI2 and - 30 keV
for SMM. For the present study data with 109 ms and 15 ms integration
are used for VI2 and 128 ms for SMMdata.
The Event on 1978 December 3
On 1978 December 3 at 20h30 m UT, an Ha flare of importance SB was
observed from McMath region 15694 located at $22E65. Figure 1 shows the
time history of the hard X-rays observed from VI2 with 109 ms resolution.
VI2 was closer to the Sun than Earth-orbltlng spacecraft and consequently
had better statistics. The total duration of the event was 16 s with
the maximum counting rate occurring 9 s from the start. Significant tem-
poral variations are seen during the rising phase. The filtered data are
shown at the bottom of Figure 1 with the times of various statistically
significant peaks indicated. Two series of peaks with delay times of
about 1.6 s are indicated in the figure. Figure 5 shows the histograms
of the delay times between alternate features. The amplitude of the
pulses of the first series appears to be constant while the amplitude
of the second series of pulses is growing. The average pulse profile
after proper phase folding for 6 cycles is also indicated in Figure 4a.
The shape of the first series of pulses is symmetrical with rise and
decay times of about 300 ms. The time-varying second series of pulses
is poorly defined statistically. During the decay phase after about II
s, the pulsations are significantly attenuated.
Bogovalov et al. (1983) have analysed several solar events for
quasi-periodicltles and have reported for this same 1978 December 3
event pulsation frequencies of 1.22 ± 0.03 Hz and 0.50 ± 0.03 Hz with
99% confidence limits. We have also carried out the power spectrum
analysis and agree with their results, but to emphasise the existence of
two synchronised series of features, we have also used the epoch-foldin_
technique and evaluated the X2 for various periods. We found that X_
peaks at a period of 1.64 s.
The light curve of this event shows a succession of ten prominent
peaks. The times of occurrence of all ten peaks can be described as a
periodic series of seven peaks with an "inter-pulse" which occasionally
appears at a stable phase. The probability of the peaks happening with
this arrangement can be calculated by a Monte Carlo simulation. Specif-
ically, we ask what is the probability that ten peaks can be randomly
scattered and yield ten "hits" to a periodic peak with interpulse. In
one thousand Monte Carlo trials none demonstrated periodicity. In fact,
on average, only four out of ten peaks could be fitted into a periodic
pattern.
65
160
140
120
(J
loo
O_
0
8O
I--
Z
.-_
o
o 6O
4O
2O
0
40
_ 2o
o
ffl
Z
_o
,_ -20
,,>,
-40
1.09 SEC
December 3, 1978
Venera 12-M1
98-544 keY
To=20h 31m 43.501 _c
t t t f
n
%
I [ I I I I I
0 1 2 3 4 5 6
t _ X = extrapolated data points
l [ I I I I [ I I
7 8 9 10 11 12 13 14 15
TIME IN SECONDS AFTER To
Figure i. Time history of the December 3, 1978 event with the filtered
variational part shown by the lower curve. Arrows indicate
the 2 synchronized time series mentioned in the text.
66
The Event on 1969 November 6
A second event occurred on 1969 November 6 at 08h38 m UT. Figure 2
shows the light curve with 109 ms resolution. The total duration of
the event is about I0 s. There are three major features and three
secondary features. The delay times between both series are indicated in
Figure 2. The folded light curve of the pulse profiles is presented in
Figure 4b with a time resolution of about 15 ms. Time variations down
to 30 ms are seen in the average pulse profile. The initial rise occurs
within 150 ms and the decay takes - 360 ms. The distribution of inter-
val times between features is shown in Figure 5.
400
300
d
n-
2oo
u I
I I
0 1 2
November6, 1979
Venera 12-M1
127-705 keY
To=8h 38m 13.115 sec
J
t
t t
t t
I I I I
3 4 5 6
1.09 uc
{
L
I l I
7 8 9
TIME IN SECS AFTER TO
Figure 2. Time history of the November 6, 1979 event.
67
The Event on 1981 September 6
Another short event which shows well defined multiple pulse struc-
ture occurred on 1981 September 6 at 23h53 m UT. Figure 3a shows the
total event. A time-expanded view of both the initial low level and
later high intensity multiple structures are shown in Figure 3b. There
is a total of nine peaks with a characteristic time delay of - 2.5 s.
The light curve could also be interpreted as repetitive groups of three
peaks with a time delay of about 6 s. The successive light curves of
these groups are shown in Figure 4c. This pattern is revealed at energies
up to 300 keV. The distribution of interval times between features is
shown in Figure 5.
2000
15OO
_- 10OO
z
O
L_
500
2000
1500
_- 1OO0
z
O
L)
O_ " ' i .
2353 qO- 2353 20
//
/
/
/
///
/
/
/
/
/
/
/
/
September 6, 1981
SMM-HXRBS
30.5-286 keV
A
ll _ .A . L JL
2353:40 "'2354:00 2354:20
l
l l
L
L "--...J
B
B
0.25H6 sec
0
2353:10 2353:15 2353:20 2353:25 2353:30 2353:35 2353:40 UT 2353:45 2353:50
Figure 3. Time history of the September 6, 1981 event.
68
In order to evaluate the characteristic parameters of the repetitive
features we have epoch-folded appropriate intervals of the light curves
of events I and 2 and the results are shown in Figure 4a and 4b, respect-
ively. The similarities of the features of the third event are revealed
by the aligned profiles of Figure 4c.
1 20(J
I.U
0
"2. 100
o
F-
Z
o 80
0
6O
epoch folded pulie _ 3, 1978
cycles 1-6 2000
b = 550 rm_c
_ 120(1
8_
I I
i i a
i_-.. 1.64 se¢ .---_ 0
September 6, 1981,
30.5-286.1 keY,
c 20 msec resolution
2353 :42 2353:44 2353:46 2353: 48
i Nov. 6, 1979 !
epoch 1folded
pulse
cycles 1-3
; 530 ms ; b
1750
1400
u_
1050
I--
z
:3
o
u 700
5o
350
4o
8 0
_ 2225
20
1780
10 o=
1335
z
8 890
445
B
, , I , , i I i i i I i i i i i
2353: 36 2353: 38 2353:40 2353: 42
i
2353 28 2353 30 2353 32 UT 2353:34
C
Figure 4. Common temporal aspects of the pulsed features of events
I, 2 and 3.
69
All of these events are short with total durations not exceeding
40 s. Significant features with widths of about a second are revealed
as distinct peaks with persistant delays between them. The characteristic
time delay for three events is 1.6 s, 2.3 s, and 6 s, respectively.
Figure 5 shows the frequency distribution of these intervals for all 3
events.
.J
<
>
cr
tu
b-
_z
M.
o
rr
t_
n_
D
Z
! I I I
4 6
November 6, 1979
6
September 6, 1981
0 ! I
2 3 4 5 6 7 secs
TIME BETWEEN ALTERNATE FEATURES
Figure 5. Frequency distribution of the alternate time intervals between
significant features of the three events.
7O
The ratio of the amplitude of the pulse AJ to the average intensity
J is - 50%. The pulses occur at energies up to 360 keV. The amplitude
of the successive peaks of the December 3 event are nearly constant giving
a high value of the quality Q of the assumed generating oscillator, where
Q = _t/P, t is the e-folding time of the decrease in the pulse ampli-
tude, and P is the pulse period. In Figure 6a,b, and c, we show the
simultaneous behavior at various energies for all events.
Oecember 3, 1978 To=20" 31" 43.501'
100_j
0
_ 250
_ 2_
5O;
0 ;_ ,o " ;, .... 2'o.... _;s.... _,' ',;_
TIME AFTER TO
September6, 1981
_°. ft.
o _160-138 key
lOOO_ |_ 31-60keV
, , , . . , , , , .... , .... •
2353:10 2353:15 2353:20 2353:25 2353:30 2353:35 2353:4OUT 2353:45 2353:50
lo November 6, 1979
1070-1540 keV
20 705-1070 keY
o
111(I ._l _ 345-705 keV
0 O"
200
0 1 2345678910
SEC
Figure 6. Time profiles at various energies for all 3 events.
?]
We have studied the evolution of the X-ray spectrum for the event
of September 6, 1981 with a time resolution of 256 ms. Figure 7 shows
the spectral index A 2 of the best fitted power law along with the inten-
sity profile of the event. At least three of the peaks are associated
with lower power law index values indicating a harder spectrum than in
the valleys. This type of behavior of spectral variation is also reported
by Kosugi (1985) at this meeting.
<
1 500
{J
UJ
_/)
o3
_- 1000
z
0
500
f September 6, 1981
4 FIL
2
0.256s
H
A
0
B
A I , , , , I , , , i I
2353:30 2353:40 UT
Figure 7. Temporal variation of the power law index A2 and the HXRBS
count rate for the event of September 6 in the energy range
from 30 to 290 keV.
72
In the MHD model reported by Zaitsev and Stepanov (1982, 1984),
compact (_ i0 I0 cm) magnetic loops with plasma density and electron
temperature one or two orders of magnitude higher than the ambient values
are considered as the resonators for fast mode MHD waves. The period P
of fast magnetoacoustic oscillations in a loop with a radius r that is
much smaller than the length is given by the following expression:
P = r(c2 + c2)-I/2s
where cA is the Alfven speed and cs is the sound speed. With r between
10 8 and 109cm and (c_ - 2,1/2t Cs) = 108cm s-I, one gets P in the range
of 1-10s, in good agreement with our results. Values of the period P,
the degree of modulation AJ/J, and the quality of the oscillator Q obtain-
ed from the observations enable estimations to be made of the plasma
density, the plasma temperature, and the magnetic field strength in
the loop (Zaitsev and Stepanov, 1982).
This type of analysis for the three events presented here indicates
temperatures of " 107 K- magnetic fields of - I00 Gauss and plasma
densities between I0 I0 and I012cm -3. These are in general agreement
with values of these parameters derived from other models.
We want to thank B.R. Dennis for providing SMM-HXRBS data, for
critically reviewing the text, and for suggesting valuable improvements
for this final presentation.
References
Barat, C., Chambon, G., Hurley, K., Niel, M., Vedrenne, G., Estulin, I.V.,
Kuznetsov, A.V. Zenchenko, V.M., 1981, Sp. Sci. Inst., 5, 229.
Bogovalov, S.V., lyudin, A.F., Kotov, Yu D., Dolidze, V.I., Estulin, I.V.,
Vedrenne, G., Niel, M., Barat, C., Chambon G. and Talon, M. 1983,
Sov. Astron. Lett., 9, 3.
Chambon, G., Hurley, K., Niel, M., Vedrenne, G., Zenchenko, V.M.,
Kuznetsov, A.V., and Estulin, I.G., 1979, Sp. Sci. Instr., 5, 340
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Kosugi, T., 1985, these proceedings.
Loran, J.M., Brown, J.C., Correia, Emilia and Kaufmann, P., 1985, Solar
Phys., 97, 363.
McLean D.I., Sheridan, K.V., Stewart R.T. and Wild, I.P. 1971, Nature,
234, 140.
Ohki, K., 1985, these proceedings
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Parks and Winckler, 1969, Ap.J., 155, LII7.
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74


Persistent time intervals between features in solar flare hard X-ray emission

Persistent time intervals between features in solar flare hard X-ray emission

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