High Energy Solar Physics: Anticipating HESSI
ASP Conference Series, Vol. xxx, 2000
R. Ramaty and N. Mandzhavidze, eds.
Solar Radio Burst Locator
Brian L. Dougherty, William B. Freely, Harold Zirin
Solar Astronomy, Caltech, Pasadena, CA 91125
Dale E. Gary
Physics Deptartment, NJIT, University Heights, Newark, NJ 07102
Gordon J. Hurford
NASA/GSFC, Greenbelt, MD 20771
Abstract. The Solar Radio Burst Locator (SRBL) is a new ground- based
instrument used to record the spectra of microwave bursts and to locate their
positions on the solar disk. It was designed at Caltech and will be deployed at
several sites around the world in 1-2 years as part of the US Air Force's Solar
Electro-Optical Network (SEON). Each instrument is independently calibrated,
and employs a single, automated, 6 foot parabolic dish and a log-spiral receiving
element scanning through 100 selectable frequencies from 1 to 18 GHz every five
seconds. Additional data are taken around 245, 410, and 610 MHz. Solar burst
locations are determined from the amplitude and phase of modulations in the
observed microwave spectra. For bursts greater than 500 sfu in we have obtained
positions with an accuracy of less than 5 arcminutes rms, and hope to improve
this. The positional information is to be used in space weather forecasting, and the
spectral data will be a powerful resource for analysis of burst evolution and
electron energy distributions. Further insight should be gained through
comparisons with X-ray and particle observations. The database, with 24-hour allweather coverage, will be available on the Web. In this paper we describe the
instrument and look at data recorded in 1998 prototype archives.
1. Overview
After decades of solar research world-wide, many microwave bursts have been observed using
instruments with increasingly powerful spectral and imaging capabilities. Monitoring observations
have been limited to discrete frequencies. SRBL will provide the first full-spectrum measurements
with 24-hour coverage. The resulting complete record of centimeter-wavelength events should
enable more thorough studies of electron source kinematics and histories as well as comparisons
with observations in other frequency regimes.
SRBL utilizes a unique burst location technique based on spectral observations rather than
interferometry or mechanical scanning. Such information is especially useful in predicting
terrestrial consequences from solar activity.
The system was developed under a USAF contract, initially as a supplement to SEON optical
instruments for detecting and locating during inclement weather. The design borrowed heavily
from previous hardware and software operating at the Owens Valley Radio Observatory (OVRO)
(Hurford et al. 1984). The level of automation has been raised so that, under normal conditions, the
system operates unattended for several weeks at a time. Field testing began at OVRO and Hawaii
beginning in 1994 using antennas with equatorial mounts (Hurford et al. 1996). Two additional
prototypes with azimuth-elevation mounts and augmented frequency capability were then
developed to provide extended sky coverage and to expand SRBL's role to serve as a cost-effective
and higher-performance replacement for the aging SEON fixed-frequency radio telescope network
(RSTN).
Raytheon Systems Company in Indianapolis is now preparing to fabricate and field several
improved instruments. The choice of sites is not yet finalized. At this time, it appears that SRBL
will be co-located with RSTN systems at Palehua (Hawaii), Learmonth (Australia), San Vito
(Italy), and Holloman AFB (New Mexico). These instruments will be monitored from, and send
real-time data over dedicated communication circuits to a central facility operated by the Air Force
Weather Agency. Calibrated spectra and burst parameters for timely now-casting of solar activity
will be available on the Web. (For current data from the OVRO prototype, go to
http://goldilocks.caltech.edu.)
2. System Description
Full-sky coverage is obtained using an azimuth-elevation mount. Stepper motors and chain drives
steer the antenna without encoders in counted, 2 millidegree steps. A planar log-spiral feed at the
prime focus provides circularly polarized microwave reception from 610 MHz to 18 GHz. A dual,
interlaced Yagi extends from this feed to support linearly polarized measurements at 245 and 410
MHz. The frequency-agile superheterodyne receiver, utilizing 3 YIG local oscillators, is capable of
changing frequencies every 10 ms. A typical observing sequence consists of a series of 40 ms
samples, cycling through 120 frequencies every 4.8 seconds. The antenna and receiver are under
digital control from a desk station which includes a GPS clock and a backend for integrating and
conditioning the receiver IF output for digital readout. One dedicated PC maintains the time-critical
hardware tasking, passing data to and receiving commands from another PC which provides
automated scheduling, analysis, and archiving. Raw data rates are modest, at less than 3
Mbytes/day per instrument.
Daily operations include pre-dawn frequency and gain calibrations. Relative gain is then stabilized
throughout the day by following signals from regularly activated internal noise diode sources.
(Absolute calibrations are obtained from occasional nighttime observations of the moon or CygnusA, or from comparisons against RSTN measurements of quiet-sun output.) Radio frequency
interference is rejected in real time by the analysis software. Observing sequences are reset daily
based on the environmental RFI noted previously among the 596 total, logarithmically-spaced
candidate frequencies. Thresholds for burst detection and location are presently set at 20 and 100
sfu respectively, ahead of our design goal of 50 and 500 sfu.
For each frequency, the pointing offsets (radius and angle) of maximum reception deviate in a
uniquely corresponding manner from the boresite axis, following the log-spiral geometry of our
feed. Thus, off-center bursts are effectively viewed from several continuously varying offsets, and
spectra are modulated in log(frequency) with a cyclic artifact. The depth of modulation increases
with frequency and distance off-center. To calibrate this effect, a separate run is performed (every
few months, or after dish/feed maintenance) in which the quiet sun is viewed from many
commanded offsets in both azimuth and elevation. Peak-response positions at each frequency are
then found and later used to invert the phase and amplitude of modulations in burst spectra, fitting
for the angle and radius of event locations (Hurford et al. 1986).
3. 1998 data
In 1998, 102 microwave bursts were recorded: 57 with peak above 100 sfu, 17 above 200 sfu, and
7 above 500 sfu. The last are tabulated in chronological order below. (We show a spectrum from
the largest event in Figure 1.) Locations are given in geocentric (az,el) coordinates, with respect to
sun center. The average rms location uncertainty is 5.0 arcminutes.
___date
03 May
13 Aug
19 Aug
24 Aug
05 Nov
06 Nov
18 Dec
start
(UTC)
21:16:46
23:11:59
21:39:24
21:57:28
19:41:05
15:10:25
17:17:35
peak
(UTC)
21:17:01
23:23:11
21:45:57
22:03:14
19:43:48
15:10:54
17:18:33
peak
end
(UTC)
21:36:56
23:45:16
21:51:14
22:36:40
20:07:57
15:12:59
17:53:26
peak location
freq.
flux
(az,el) ± rms
(MHz)
(sfu)
(arcmin)
2260
3322 ( -1.0,-8.2) ± 6.8
9900
694 (not computed)
18000
5250 (-17.6, 1.6) ± 2.0
4240
4277 ( -2.5, 5.4) ± 2.0
7960
1148 ( 7.2, 2.2) ± 6.6
10200
780 ( 4.8, 3.0) ± 5.2
3760
2040 (-11.5, 4.0) ± 7.5
Optical were seen during each of these events. The mean SRBL location error with respect to the
corresponding image is 0.9 times our rms uncertainty.
Most events lasted less than 5 minutes and had spectra that peaked between 2 and 10 GHz. Roughly
a dozen bursts, however, had rising spectra that peaked offscale (> 18 GHz) and which tended to
last longer (more than 10 minutes for most). Bursts that both SRBL and RSTN saw were relatively
well correlated in timing, and peak frequency, as expected. For events that both SRBL and GOES
or BATSE observed, however, there was little correspondence between and more study is needed
to address the timing relationships.
We intend to investigate more fully the spectral shapes and evolution variations. Plans are also
being made to study associations between microwave bursts and sunspot configurations, and to
look for event precursors.
Figure 1. One of many spectra recorded for the 19 August 1998 event. The modulations seen in the
lower curve placed this burst on the solar limb, coincident with active region 8307.
Acknowledgments. We gratefully acknowledge the important contributions of the late Dr. R. B.
Read in hardware design and development; of K. Nelin at OVRO in assembly, testing, and
operations; of M. Fyffe in drafting and troubleshooting; and of USAF personnel whose enthusiasm
and support for the project has contributed in no small part to its success. This work was supported
by the USAF under contract number F19628-93-C-0013.
References
Hurford, G. J., Read, R. B., and Zirin, H. A., Frequency-Agile Interferometer for Solar Microwave
Spectroscopy, Solar Physics, 94, 413-426, 1984
Hurford, G. J., Solar Microwave Spectroscopy, Proceedings of the Second Indo-US Workshop on
Solar-Terrestrial Physics, ed. M. R. Kundu, B. Biswas, B. M. Reddy, and S. Radadurai,
259-268, National Physics Laboratory, New Delhi, India, 1986
Hurford, G. J., Zirin, H., Freely, W. B., and Gary, D. E., Solar Radio Burst Locator, Proceedings of
the Solar-Terrestrial Predictions Workshop, Hitachi, Japan, 1996