We present observations of an X1 flare (18 Jul. 1991) detected simultaneously with the Very Large Array (VLA), the PHOENIX Digital Radio Spectrometer and the Burst and Transient Source Experiment (BATSE) aboard the Gamma Ray Observatory (GRO). The VLA was used to produce snapshot maps of the impulsive acceleration in the higher corona several minutes before the onset of the hard x ray burst detected by BATSE. Comparisons with high spectral and temporal observations by PHOENIX reveal a variety of radio bursts at 20 cm, such as type 3 bursts, intermediate drift bursts, and quasi-periodic pulsations during different stages of the X1 flare. From the drift rates of these radio bursts we derive information on local density scale heights, the speed of radio exciters, and the local magnetic field. Radio emission at 90 cm shows a type 4 burst moving outward with a constant velocity of 240 km/s. The described X1 flare is unique in the sense that it appeared at the east limb (N06/E88), providing the most accurate information on the vertical structure of different flare tracers visible in radio wavelengths

Aschwanden, Markus J.

Benz, Arnold O.

Willson, Robert F.

English

NASA Technical Reports Server

N92-21945
VLA, PHOENIX and BATSE Observations of an X1 Flare
Robert F. Willson, Dept. of Physics and Astronomy, Tufts University, Medford, MA 02155
Markus J. Aschwanden, NASA/GSFC, Code 602.6, Greenbelt, MD 20771
Arnold O. Benz, Institute for Astronomy, ETH, CH-8082 Ziirich, Switzerland
ABSTRACT: We present observations of an Xl flare (July 18, 1991) detected
simultaneously with the Very Large Array (VLA), the PHOENIX Digital Radio Spec-
trometer and the Burst and Transient Source Experiment (BATSE) aboard the Gamma
Ray Observatory (GRO). The VLA was used to produce snapshot maps of the impulsive
burst emission on timescales of 1.7 sec at both 20 and 91 cm. Our results indicate electron
acceleration in the higher corona (h_ 0.4 - 0.5R®) several minutes before the onset of the
hard X-ray burst detected by BATSE. Comparisons with high spectral (Av---- 3 MHz) and
temporal (At = 40 ms) observations by PHOENIX reveal a variety of radio bursts at 20
cm, such as type III bursts, intermediate drift bursts, and quasi-periodic pulsations during
differnt stages of the X1 flare. From the drift rates of these radio bursts we derive infor-
mation on local density scale heights, the speed of radio exeiters, and the local magnetic
field. Radio emission at 90 cm shows a type IV burst moving outward with a constant
velocity of 240 km/s.
The described X1 flare is unique in the sense that it appeared at the east limb
(N06/E88), providing the most accurate information on the vertical structure of different
flare tracers visible in radio wavelengths.
1. GOES and BATSE/GRO data
We present some preliminary data from the solar largest flare simultaneously ob-
served by VLA, PHOENIX and GRO, which occurred on July 18, 1991. A GOES Xl-class
soft X-ray event was reported near the east limb (North 06 , East 88) in NOAA/USAF ac-
tive region 6734, starting at 1425 UT, with maximum at 1441 UT, and ending at 1519 UT.
An Ha flare with importance 1N was detected during this time, associated with eruptive
prominences and sprays.
BATSE recorded enhanced hard X-ray emission (> 25 keV) after 1425:34, which
was interrupted by GRO spacecraft night from 1430:10 until 1503:00 UT, and lasted until
1510:10 UT. The BATSE hard X-ray time profile (Fig. 1) shows two peaks in the early rise
of the impusive phase, and the end of the decay phase, while most of the main flare phase
was missed during spacecraft night. The major HXR peak at 1429-1430 UT is correlated
with a group of fast-drifting bursts at > 1.3 GHz.
2. VLA observations
The VLA observations were made at _,= 1441.6 MHz (20.8 cm) and 333 MHz (90
cm) with bandwidths of 3.25 MHz and a time resolution of 1.66 seconds. At the time
of these observations, the VLA was in the A configuration which provided synthesized
beamwidths of _ 1" at 20.8 cm and _5" at 90 cm.
515
https://ntrs.nasa.gov/search.jsp?R=19920012702 2020-03-17T13:14:11+00:00Z
Radio emissionat 90em indicates impulsive and relatively low-intensity bursts (10-
15 SFU) beginning at 14:19UT, or about three minutes after the first enhancements of
soft X-ray emission and about six minutes before the start of the BATSE hard X-ray burst.
These metric bursts are located at different locations at heights of h = 0.4 - 0.5 Re) above
the limb (Fig. 2). They have angular sizes of _ 1.5' and peak brightness temperatures of
Tb =3.4-6.7x 10 7 K.
CxJ
4
CL_
E
L.")
c_
10-5
XRAYand OPTICALDATA BEI_IN: 18 JUL91
.6734' i_[.,734 _, ,_.6m ,
B
- A
GOES
i 1 I 1 I
18/1200 i300 I,,oo 16oo 1700 18/1800
BATSE
Fig. 1: Time protile of the GOES soft X-ray data (0.5-4.0 and 1.0-8.0 Angstrom) and
BATSE/GRO hard X-ray data (25-1000 keV) during the X1 class event on 1991 July 18,
1425-1510 UT. GRO observations are intermitted by spacecraft night from 1430 to 1503
UT.
516
Between14:40- 14:50UT, an intense 90 cm burst wasobservedto movesystem-
atically outward from the Sun with a velocity of _ 240 km s-1 (Fig. 3). This outward
movement is characteristic for a moving Type IV burst. 90 cm sourcesassociatedwith
slowly-varying emissionduring the decayphaseof the soft X-ray burst show no systematic
movement,but again, are located at heights of h_ OAR®abovethe limb.
The time profile of 20 cm emissionis depicted in Fig. 4 (top). Maps of the 20 cm
emissionduring the times of maximum brightness (14:35:19UT, 14:35:53UT) showthat
the burst consistsof at least two components (Tb = 2.8 x 107 K) which are located at
heights of h _ 0.2Ro (Fig. 2). The PHOENIX spectrometer data reveal a "broadband,
quasi-periodic pulsation event" during this period of time.
1441 MHz
©
19
t441 MHz
53
14 35 53
Fig. 2: VLA snapshot maps with 3.3 s time resolution o£ totM intensity at 1441 MHz
(top) and 333 MHz (bottom) at the times indicated. The contours mark levels of equal
brightness temperature, with an outermost contour and contour interval of Tb = 5.0 x
108 K at 1441 MHz and Tb = 8.4 x 10 e K at 333 MHz. The/]ducia/marks are drawn at
intervals ofl0" for the 1441 MHz maps, and 60" for the 333 MHz maps. The boxes drawn
on the 333 MHz maps denote the _eld of view of the 1441 MHz maps.
517
3. PHOENIX observations
The frequency-agile PHOENIX broadband spectrometer of ETH Zurich (Switzer-
land) was operated in two frequency bands around the VLA frequencies 333 MHz and
1441 MHz: 20 frequencies with 3 MHz resolution in the 300-360 MHz range, and 60 fre-
quencies with 10 MHz resolution in the 1150-1750 MHz band. The sweep spectrometer
was operated with a time resolution of 40 ms. Both circular polarizations Stokes I and
V are recorded, with a sensitivity of a few SFU at full time resolution. The PHOENIX
spectrometer provides unique complementary information to VLA observations, such as
high time resolution, broadband frequency spectra, and frequency-time drift rates.
The coregistered radio flux from VLA and PHOENIX is shown in Fig. 4 for the main
flare phase, at the same frequency of 1441 MHz and with similar bandwidth (_10 MHz) in
both instruments. The RCP correlated flux of the shortest VLA baseline is compared with
Stokes I of the full sun disk observation of PHOENIX. The broadband PHOENIX data
reveal a considerable variety of radio bursts: type III bursts in the metric 0.30-0.35 GHz
band; type IIIdm_ quasi-perlodic pulsations, and intermediate drift bursts in the
decimetric 1.2-1.7 GHz band. Enlarged dynamic spectra of two episodes are displayed in
Fig. 5.
0.5
0.4
0.2
OA ,
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14 35
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rw
v
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333 MHz Source
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oo
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/
V_240 km/sec
i i i l i i i , I I i _ i l i i i i I i
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40 45 5O 55 15
UNIVERSAL TIME
Fig. 3: A plot o# height versus time for the outwardly-moving 333 MHz source shown in
Figure 2. The solid line represents a constant-speed tit to the data.
518
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m. 3
#o
"0
2
= 2O.
E
0
C
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b_
0
14:20:00 14:25:00 14: 30:0i
1991 JULY 18, PHOENIX 1
1991 JULY 18, VLA 1441 MHZ, RCP, FILE=VLA18_20LCP
I
5:00 114:40:00 14:45:00 14:50:00
"OKES I, FILE=RAG729AI...HI
4OO
200
14:20:00 14:25:00 1 5:00 :00 14:45:00 14:50:00
lO00
1430:00 1436100 1437:00 1438:00
Fig. 4: Comparison of VLA (top) and PHOENIX (middle) observations at 1441 MHz.
The two enlargements (bottom) show broadband dynamic spectra of the "intermediate
drif_ burs_s" (bottom left) and of the "quasi-periodic pulsation" event (bottom right). The
time resolution is 3.3 s (VLA, top), 0.04 s (PHOENIX, middle), and 0.16 s (PHOENIX,
bottom). No_e that the brightest radio emission during the/]are is related to the quasi-
periodic pulsation event.
519
1300-
1400
O'
1500
1600-
1700-
1435:00.0 1435:30.0 1436:00.0
Time [UT]
1700-
1431:40.0 1431:50.0 1432:00.0 1432:10.0
Time [UT]
Fig. 5: PHOENIX broadband observations at 1165-1725 MHz during the impulsive
phase of the X1 flare. The white color in the grayseale indicates enhanced emission. The
episode of "broadband quasi-periodic pulsations" at 1435-1437 UT is shown on top. Weak
"intermediate drift bursts" are visible during 1431-1432 UT (bottom).
52O
ORIGINAL PA _-,_
BLACK AND WHITE PHOTOGRAP14
3.1 Broadband, decimetric, quasi-periodic pulsations
The brightest radio emission at 20 cm occurs during 1435:00-1436:30 UT. While the
VLA does not resolve many individual pulses, a total of about 80 pulses is identified in
the PHOENIX record, with individual pulse durations of 1-2 s and frequency bandwidth
of > 500 MHz (Fig. 5). The frequency-time drift rate of these pulses has been measured
with an automatic pattern recognition code, showing a symmetric distribution for positive
and negative drift rates, where 80% of the pulses have a drift rate of ldu/dtl > 1000 MHz/s.
Monte-Carlo simulations of frequency-time drift rates with similar distributions have shown
that the exciters of such radio emission originate in coronal structures with scale heights
of 2.103 - 2 • 104 km (Aschwanden and Benz, 1986). This is in good agreement with
the spatial scale measured by the VLA, which is _ 7 • 103 km (10") for the half width of
the 20 cm emission (Fig. 2, top) during the brightest pulses. This spatial scale probably
corresponds to the transverse cross-section of large loops extending to heights of > 0.2R®.
Assuming plasma emission, the location of the 20 cm source corresponds to a density of
> 10 l° cm -3. The time interval of the radio pulsation coincides roughly with the time
of the steepest derivative in the soft X-ray time profile (Fig. 1), indicating the maximum
heating rate during the flare, usually synchronized with the peak of hard X-ray emission.
The characteristics of the pulsating radio time profile indicate the nonlinear dynamics of
the underlying energy release mechanism (see review by Aschwanden, 1987); they cannot
be explained by MHD eigen-modes which have much stricter periodicity.
3.2 Intermediate drift bursts
During the rise time of the impulsive flare phase we detected intermediate drift
bursts with drift rates of _ -80 MHz/s (Fig 4,5). These bursts are detected for the first
time in this high frequency range at 1.5-1.7 GHz. The normalized drift rate (du/dt)/u
0.05 is consistent with intermediate drift bursts found in the metric and decimetric range,
also called "fibre" bursts. They represent a frequent fine structure of type IV bursts and
can be used for diagnostics of the magnetic field. In current models they are interpreted in
terms of Alfven wave solitons (Treumann et al. 1990) or whistler wave trains (Mann et al.,
1987). 3D reconstruction of the magnetic field (using the Sakurai code) and the electron
density (using the diagnostics from plasma emission of simultaneously localized type III
bursts) should allow us for the first time to decide between the two theoretical models and
to exploit their diagnostic potential.
Acknowledgements Solar radio and X-ray studies of the Sun at Tufts University are
supported by NASA grant NAGW-2383 and by grant AFOSR-89-0147 with the Air Force
Office of Scientific Research. The work at NASA/GSFC was performed under grant NAS5-
30442 at USRA. The VLA is operated by Associated Universities, Inc., under contract with
the National Science Foundation.
References
Aschwanden,M.J. 1987, Sol.Phys., 111,113.
Aschwanden,M.J. and Benz, 1986, A&A, 158, 102.
Mann,G., Karlicky, M., and Motschmann,U. 1987, Sol.Phys., 110, 381.
Treumann,R.A., Giidel,M. and Benz,A.O. 1990, A&A, 236,242.
521


VLA, PHOENIX, and BATSE observations of an X1 flare

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