1.Introduction

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]. Routine 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 signal-to-noise ratio. This improvement has considerably increased the HiRAS ability to monitor solar radio bursts.

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 analyzers. 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 and immediately 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 acquisition. The variation in sampling period is corrected during data processing that combines data from the three antennas to 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 emission-type classification. 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 (at sunset).

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].

Acknowledgements

The authors would like to acknowledge the many people of Nihon Tsusinki Company and Toyo corporation who constructed the new Hiraiso radio spectrograph.

References

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: Interscience Publ., 1965.


P.S. The authors would like to thank Robert S. Fritzius for the correction of this article. (Feb.3,1997)