The solar radio observation system at
Hiraiso was renovated in 1993. The frequency range of the initial
spectrograph (70-500 MHz) was expanded about five times to
25-2500 MHz, making it sufficient to monitor
the coronal radio emissions associated with flares
[e.g., Kundu, 1965].
observations of the sun using the new Hiraiso radio spectrograph (HiRAS
: Hiraiso radio spectrograph) began late in May 1993. Daily
operation is fully automated, i.e., the antennas automatically track the
sun from sunrise to sunset, and the data acquired by the
spectrograph are processed by a workstation to produce a composite dynamic
spectrogram. At the frequencies that HiRAS monitors, radio signals
from the sun are always contaminated by the artificial signals such as
radio and TV broadcasts, especially at
frequencies lower than 1 GHz . This contamination is thought to be
worsening year by year as social activities progress.
In order to make meaningful observations,
we have developed software that effectively removes
interference from the raw spectrum data. This data
processing method is being successfully applied to actual observation
data, and the solar-burst spectrogram
quality has been significantly improved in terms of
This improvement has
considerably increased the HiRAS ability to monitor solar radio
2. Hiraiso Radio Spectrograph (HiRAS)
A schematic block diagram of the new radio spectrograph at Hiraiso is
shown in Figure 1
[see Kondo et al, 1995 for details].
The HiRAS consists of
three antennas, HiRAS-1, HiRAS-2, and HiRAS-3, of which
the receiving frequency
ranges are 25-70 MHz, 70-500 MHz and 500-2500 MHz.
Both right- and left-handed circular
polarization signals received by each antenna
are amplified with low-noise pre-amplifiers and fed to spectrum
Day-to-day observation is fully automated. In the daytime
data obtained by the spectrum analyzers are
acquired by a system control computer (HP9000/R362) via the GPIB
transferred to a workstation (HP9000/710) through the Ethernet.
The information is then stored on a hard-disk mass storage system.
The time resolution of observations is limited by the data-acquisition
capability of the system control computer.
When the computer is devoted exclusively to data acquisition,
it can gather data from the six analyzers every two seconds.
In an actual operation, however, the period varies from 2 to
5 seconds because the same computer also controls the antenna
to track the sun, and this task sometimes interrupts data
The variation in
sampling period is corrected during data processing that combines data
from the three
form a spectrogram.
3. Data processing
Raw spectrum data observed at Hiraiso are always contaminated by strong
interference signals from local radio and television stations and
other artificial sources. This interference degrades the quality
of solar-burst spectrograms in terms of signal-to-noise ratio,
resulting in less sensitive in the detection of the solar radio
events. To improve the quality of spectrogram a new
data-processing algorithm was developed.
The bandwidth of interference is usually narrower
than that of solar-burst signals. Therefore it is possible to get
only the solar burst signal alone by reading the data points
immediately next to the
interference signals (spikes). This re-sampling may degrade
the frequency resolution,
however this is thought to be of negligible importance
in solar radio observations, especially for the purpose of the
To implement this idea
we have developed software that reduces the number of samples in the
frequency domain by about of a factor three, from 1803 points to
501 points, by
re-sampling the data to select the minimum value in the frequency
range corresponding to each new sample point. By adopting
this re-sampling, i.e., minimum-data selection, most of the interference
has been effectively removed.
In addition to this processing, the solar-emission background level
at the observation is subtracted from the spectrum data to increase
the signal-to-noise ratio and thus to help separate burst events
from the quiet-sun baseline.
The background level is determined every three hours using the actual
observation data for this period. To avoid the fluctuation of the
received level due to a below horizon antenna image,
the background measurement is limited to sun elevation angles
higher than 10 degrees.
This processing also serves to
absorb differences in received signal level among the antennas.
Two gaps will appear at 83 MHz and 280 MHz in a spectrogram due to
notch filters inserted in the receiver to avoid saturation of the
post amplifier by strong local FM-radio and telecommunicating signal.
Arc-like structures seen both at sunrise and sunset are interference fringes
due to an image of the sun reflected from the sea (at sunrise) and the ground
4. Concluding Remarks
Conditions of radio observations of weak natural signals becomes worse
year by year due to
the increase of artificial noise, especially at the one meter
and longer wavelength bands. Under these negative conditions
we have successfully improved the quality of observation by adopting
a minimum-data selection technique for data processing.
This improvement makes it easy to detect solar bursts and to
classify them into five major types according to their appearance
on the spectrograms [e.g., Gary, 1990].
The authors would like to acknowledge the many people of
Nihon Tsusinki Company and Toyo corporation who constructed the
new Hiraiso radio spectrograph.
Gary, D.E., Radio Observations During Max'91 Campaign 1,
MAX'91 Workshop \#3: MAX'91/SMM Solar Flares: Observations and Theory,
Colorado, pp.1-10, 1990.
Kondo,T., T.Isobe, S.Igi, S.Watari, and M.Tokumaru, The Hiraiso Radio
Spectrograph (HiRAS) for Monitoring Solar Radio Bursts,
J. Commun. Res. Lab, Vol.42, No.1, pp.111-119, 1995.
Kundu, M.R., Solar Radio Astronomy, New York:
P.S. The authors would like to thank Robert S. Fritzius for the correction
of this article. (Feb.3,1997)