Introduction
SRBL is a novel spectrometer that can automatically detect and locate microwave bursts on the
sun. It uses a single 6-foot parabolic antenna with a planar, circularly-polarized feed and a stabilized 0.518 GHz frequency-agile superheterodyne reciever, together with interleaved Yagi antennas tuned to 245
and 410 MHz. The whole sun is held continuously in view while fluxes are measured at several dozen
selectable frequencies, and radio interference is dynamically avoided. Its log-spiral feed geometry
introduces spectral modulations which allow burst locations to be determined quickly, without need for
mechanical scanning or interferometry. When several stand-alone systems are deployed around the Earth,
the SRBL network will provide 24-hour, all-weather support for the United States Air Force's real-time
space-weather monitoring responsibilities, and create a unique data base of solar microwave activity useful
for scientific investigation.
This introduction will survey general factors that influence the design of SRBL, including brief
reviews of space weather, instrumentation, history, etc.. Prototype images and data will be presented for
orientation. Specific aspects of the instrument are covered in later chapters.
Background: Space Weather, Solar Radio Bursts, Instrumentation
The sun's atmosphere is a maelstrom of magnetic fields and intensly hot plasmas. Large coherent
configurations often form (sunspots, loops, tangled arcades, etc.), reaching out into the corona, in which
vast amounts of energy may be stored. These occasionally destabilize violently spewing tremendous
masses of particles. If directed toward us, blast waves of protons and electrons may buffet Earth within
hours of eruption. The resulting radiation and charging effects pose hazards to satellites, communications,
power grids, personnel in space, etc.
This activity takes many forms, so different instruments are used to monitor and study it. At the
highest energies, space-based sensors collect hard xray emissions as electrons crash into intact atoms near
the cooler solar surface. Optical telescopes on Earth image flares of sudden, superheated parcels of
hydrogen gas. Microwave detectors observe gyrosynchrotron radiation from electrons spiralling around
field lines in the lower corona. Long wavelength radio receivers follow plasma oscillations as shock waves
leave the sun. Particle and magnetic sensors on or near Earth record the resulting space-weather. Other
instruments extend and complement these. Several new ones are planned by the military and research
communities, promoting a more comprehensive view of each event.
Science is just beginning to understand some of the mechanisms involved. These insights are
combined with rough models of prevailing conditions and propagation effects to provide some warning of
impending geophysical "storm" activity.
In the radio, at least two broad regimes of burst emisson have been identified. These appear as
plasma temperatures and densities change while magnetic fields vary with coronal height. Typical
morphologies and causes include:
Spectra
Time scale
Polarization
Emisson kind
Source
Site
Types
below roughly 1/2 GHz
Structured, time varying
Many minutes to hours
Unpolarized
Plasma oscillations
Beams of electrons or shock waves
Mid to high corona
II (slow drift), III (fast drift), noise storms
above ~1/2 GHz
Generally smooth
Several seconds to a few minutes
Partial, circular, often fluctuating
Incoherent gyrosynchrotron
Hot plasmas or random fast electrons
Chromosphere or low corona
Impulsive or gradual
For the "quiet" sun, intensities generally increase smoothly with frequency. Integrated emissions
are unpolarized, and change slowly in time as sunspots come and go. Brightness can vary rapidly across
active regions, however, with sharper spectral features often appearing within sunspots.
Bursts at centimeter wavelengths (microwaves) imply solar electron energies of ~100 keV moving
within ~kGauss magnetic fields. These often "pulsate", but seldom on time-scales less than about 1 second.
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Sources can vary in size with frequency, but are usually a few arcseconds in diameter (geocentric), and
located around the same active region as concurrent optical flares. Conditions may vary over the source,
smearing burst spectra, which are typically quite broad and peak between 2 and 10 GHz. Overall intensity,
duration, and spectral "hardening" have been correlated with the ejection and arrival of protons at Earth.
Several radiometers, spectrometers, polarimeters, and interferometers have been used to study
microwave eruptions. Much has been learned, mostly from episodic bursts. The only continuous, 24-hour
monitor currently operating is the USAF's Radio Solar Telescope Network (RSTN), which samples
linearly-polarized fluxes every second simlutaneously at eight select frequencies.
Short History of SRBL
SRBL was conceived in the late 1980's by Dr. Gordon Hurford, a scientist then at the California
Institute of Technology (CIT, ACaltech@). He was investigating broad-band techniques at Caltech's Owens
Valley Radio Observatory (OVRO) using commercially available spiral feeds engineered to respond over
many GHz. He wanted to record real-time solar spectra by rapidly tuning to several frequencies. He found
that spectra for bursts viewed slightly off-center from the antenna boresite contained periodic dips. These
modulations were recognized as artifacts produced by detector geometry.
The gain of a parabolic antenna (i.e. its "beam pattern") falls as the reciprocal of frequency and
with angular distance from its symmetry axis. But since spiral feeds do not form a series of concentric
circles, their effective electrical center (or center of greatest response) is periodically displaced as a
function of frequency. Therefore, off-axes sources are alternately viewed near to and far from the angles of
maximum reception, with varying phase and amplitude depending on the relative source position.
Traditionally, radio targets are pinpointed either by mechanically scanning the sky (with
resolutions depending upon beamwidth), or by operating several instruments together as an interferometer.
Although scanning is possible for stable sources, it is obviously not well suited for transient events like
solar flares. Interferometers are expensive, complicated, and usually operated at only one fixed frequency.
Now, after calibrating this newly recognized effect, modulations are found to differ sufficiently to locate
bright sources with useful accuracy. A single small dish can view the entire sun, take full spectra, and
locate bursts in real-time without diverting its motion. A breakthrough in speed, simplicity, and cost has
been made, well suited to microwave observations.
In the early 1990's, motivated by the prospect of a continuous spectrographic data-base of solar
activity, Caltech proposed SRBL to the USAF as an all-weather supplement to the flare-imaging Solar
Observing Optical Network (SOON), part of their Solar Electro-Optical Network (SEON). A period of
rapid development followed, borrowing heavily from existing hardware and software at OVRO. Two
identical research-grade (i.e. flimsy) "prototypes" were built. The first was installed at Palehua, Hawaii, to
be operated and maintained by USAF personnel, the other kept at OVRO. These could tune between 2 and
18 GHz, and performed with significant promise. Later, dual Yagi antennas were added and the receivers
were extended so that frequencies around 245, 410, 610 MHz, and from 1 to 2 GHz could be observed.
SRBL might therefore more fully replace the aging and costly RSTN systems.
Polarization
No. Freq's sampled per record
Frequencies tuned (MHz)
Single or Double SideBands
Sample period (per freq.)
Sample order
Stabilization
Update period
Burst Detection Threshold
Burst Location Threshold
Location Resolution
RSTN
linear
8
245, 410, 610, 1415, 2695,
4995, 8800, 15400
1 MHz SSB below 3GHz
30 MHz DSB above 3GHz
1 sec
simultaneous
na
6 sec
various
na
na
SRBL
linear (<1GHz) circular (1-18 GHz)
120
245, 410, and 610 (up to 5 samples
each), 1000-18000 (the rest of 120)
1 MHz SSB below 1 GHz
40 MHz DSB above 1 GHz
40 ms (30 ms of which is used)
sequential
noise diode (sampled concurrently)
9.6 sec
50 sfu
500 sfu
< 3 arcmin (for freq's > 2 GHz)
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Current Status
SRBL is in its adolescence. Despite the program's apparent age, budgetary contingencies have
resulted in an abbreviated R&D schedule. The required system features are in place, though some are more
rudementary than others. A number of possible enhancements have been identified which address realworld difficulties (e.g. RFI) and performance issues. Many of these will be outlined later.
The first prototype has been moved from Palehua to Indianapolis, where it supports engineering
changes being made by Raytheon Technical Systems Corporation. In 1997, RTSC was chosen by the
USAF to build more robust and servicable production instruments, improve the user interface, provide
event messaging in standardized formats, and prepare for installation at sites yet to be finalized. (These
added features will not be discussed in this review.)
Shown below are pictures of the OVRO system. This instrument is still in operation, recording
solar activity nearly every day, when not used as a testbed for development. Remote control is often taken
from Caltech. An automated website is maintained to display real-time data (http://srblserver.caltech.edu).
Several years worth of data have been accumulated, spanning all changes in the evolving hardware
and software. Many bursts were observed, but thorough scientific assessments are still wanting. The
recent performance of this system has been studied, however, and a report prepared. (See the website.)
The following pictures document an event that occured from 17:52 to 18:06 UTC on 28 August
1999. It proceeded as several pulses, peaking at 3200 SFU around 9 GHz at 17:59 UTC. SRBL located it
at 7 degrees West longitude by 25 degrees South latitude in heliographic coordinates. This is indicated by
the error circle superimposed on an H-alpha image taken simultaneously by Caltech's Big Bear Solar
Observatory. It agrees quite well with the flare position. Current active region numbers are shown for
reference.
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