Figure 1. — The solar background spectrum for May 12, 1979 (after Elgaröy et al. 1980). The dashed curve represents the Quiet Sun as given by Smerd (1964). Station names are those used by the WDCs.
K.M. BORKOWSKI1,
P. ZLOBEC2, C.A. ZANELLI2
1Inst.
of Astronomy, N. Copernicus Univ., Toruñ, Poland
20sservatorio
Astronomico di Trieste, Trieste, Italy
(Received 25 May 1980)
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SUMMARY Methods and problems of solar radio data recordings at different observing stations are given.Improvements and standardization of data are suggested. |
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RIASSUNTO Vengono presentati i metodi ed i problemi di registrazione dell'attivitá radio solare in vari Osservatori. Viene data un'indicazione per migliorare e rendere piú standardizzata tale attivitá. |
We did receive 16 replies and comments, some of which contain very interesting ideas. A review of the collected material that is presented in this paper seems to show the existence of a real need of further improvements in metric wavelength solar observations, regarding both data and instrumentation problems. Some valuable points remarked by J. van Nieuwkoop {1978) may be an outline for further efforts in this field.
The Tables are in general self-explanatory. The four-letter abbreviations of station names are those used by WDCs. The columns of Table I headed "gain" and "HPBW" give respectively the antenna gain and half-power beamwidth (in degrees): we are confident that these are the measured values, even in those replies in which this is not specified. In Table II, "dynamics" is the ratio between the highest and lowest sensitivity thresholds. The "noise" column gives the receiver noise figure, and that of "sensitivity" gives the least detectable signal in solar flux units (1 s.f.u. = 10–22 W·m–2·Hz–1). In Table III, the "speed" column contains recorder tape speed in cm·h–l or the sampling fre¹uency in Hz, "dynamics" represents the recorder ratio of greatest and lowest signals. The rightmost column gives an estimated accuracy figure in percent. If two figures in this column are separed by a comma, the first one regards the mean flux density measurement accuracy, the second one the distinctive event data accuracy.
Table I
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Antenna Systems
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Table II
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Radiometers
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Table III
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Recording and Observations
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Among problems of general interest it was pointed out that the construction of composite dynamic spectra, e.g. of type IV bursts, is difficult because of the small number of frequencies available, the different calibration systems, the different time constants, and the subjectiveness of choosing spectral contours, making a reference to wide-band (if low-dynamic) spectrograms necessary (Slottje).
The following stations take Cas A as reference source: Dwingeloo (every 3 months), Abastumani (once a year), Hoeven (every 2 months), Nançay (169 MHz interferometer), Toruñ (monthly) and Trieste (237 and 327 MHz polarimeter, 408 MHz interferometer, every 2 months).
Cyg A is used by: Nançay (169 MHz radioheliograph and interferometer), Harestua (comparing Cyg A region with a cold region at α = 9h40m and δ = 41°) and Trieste (408 MHz interferometer). Note that the Trieste 408 MHz polarimetric channel is calibrated on the interferometer.
At Culgoora the quiet fluxes at 80 and 160 MHz are periodically measured with interferometric techniques by comparison with sources of known flux (Vir A, Tau A, etc.) to determine the effective area of the radiometer aerial. The matching of the 80/160 MHz aerial feed is measured every day as a check on the stability of the effective area (Nelson).
In order to avoid difficulties connected with the absolute calibration and to reduce the inhomogeneity of data reported by various observatories it would be useful in the opinion of Kharadze and his colleagues to organize simultaneous observations of the Sun, at least for those stations where this is geographically possible. Through this cooperation more accurate results could be found, to be shared with other stations in order to overcome instrumental differences. In June 1979 a workshop was held in Nançay to discuss the results of such a joint observation day (May 12, 1979 was chosen) in which five observatories took part. The proceedings of this meeting are summarized in Elgaröy et al. (1980), from which Figure 1 is taken; considering that generally the data are given with an accuracy of about 10%, the fit must be accepted as rather good. In the same meeting it was proposed to repeat periodically this kind of joint observations; the observation of a "smooth" type IV burst would be useful as a test of the extrapolation to higher flux densities.
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Figure 1. — The solar background spectrum for May 12, 1979 (after Elgaröy et al. 1980). The dashed curve represents the Quiet Sun as given by Smerd (1964). Station names are those used by the WDCs. |
Suggestions were also made as to the agreement on
one absolute flux density scale. The researchers taking
part in the Nançay meeting have agreed to accept the
formula by Baars et al. (1977), slightly modified as
follows. The flux density of Cas A is expressed by:
S(f,t) = So exp[a(t – to)] fαo + b(t – to)
where to = 1980.0, a = –0.0097 yr–1, b = 0.00126 yr–1 and
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for 0.2 ≤ f < 30 GHz,
αo = –0.770, So
= 0.2720 sfu while for .02 < f < 0.2 GHz, αo = –0.681, So = 0.3140 sfu and S is in sfu, f in GHz. |
The flux density of Cyg A is given by:
log S(f) = 4.695 + 0.085·log f – 0.178·log2f
were S is in 10–26 W·m–2·Hz–1, f in MHz.
In the opinion of Yamashita the above mentioned method cannot be regarded as decisive. A similar idea is expressed also by Slottje who thinks that interferometry is needed when the Quiet Sun is to be measured (of course the problem is less critical when the bursts are concerned as they appear above the background). Presently in Torun interferometric technique is used routinely and so this is not a real problem; this technique is even more developed for the Nançay instruments (the 169 MHz interferometer having a resolving power of 3.5', the 169 MHz radioheliograph 1.15').
In Toruñ, broad beam antennas are used and this leads to a continuous influence of ground-reflected radiation on observations. The effect is present all the year as a function of the altitude of the Sun. It is hoped that after a few years of observations the required factors may be established to correct the previously derived data.
Further, it is not difficult to find out whether a phenomenon is of solar origin if high-time resolution recordings are used: on these, an interference is clearly visible because of its sudden beginning and end. Also checking the sky in a neighboring re.gion may be useful. Raffaelli (1975) presented a system for 408 MHz observations at San Miguel (not included in the Tables) consisting of two antennas one of which is directional and the other omnidirectional. An input of the radiometer is switched alternatively from one aerial to the other with a frequency of 1 kHz. A solar event, unlike interference, is observed by both antennas with the same shape.
It is also possible to distinguish between solar emission and interference by using a noise discriminator — an electronic circuit or simply human ears. Ears are a sensitive detector of interference, once trained, but are not suitable for routine observations. An electronic noise discriminator is a combination of two RC circuits of different time constants. When connected to the same detector these two circuits show different responses and their ratio is constant for random noise while it varies in other cases (Yamashita, 1974).
In Bordeaux they use an automatic divider of the output signal by 4, 16 or 64 (Poumeyrol). In the 200 MHz receiving system of Hiraiso the input signal is divided in two levels, namely the Quiet Sun (low) and the Active Sun (high) levels. The high level has a dynamic range of about 30 dB, while the low level is limited to 20 dB or less for reasons of linearity. This method, however, has some deficiencies (smooth connection between the channels). A quite new design has been put into operation for 500 MHz observations. It is a receiver that keeps the input level of the linear detector and its driving stage rather constant, with some definite clearance, with the aid of attenuators inserted automatically at the input when large signals are present; at the output a microprocessor is employed. Logarithmically compressed flux (10 to 100,000 sfu) is recorded beside linear-scale flux (0 to 400 sfu) and poiarization degree (–1 to +1) (Yamashita).
At the Trieste Observatory, paper strip recorders (with a speed of 20 cm·h–l) are automatically attenuated by a factor of 2 when the recorded signal reaches end-of-scale level. There is the possibility of attenuating the signal up to 16 times. When the output signal exceeds a given threshold, an attenuation of the antenna signal (in steps of 10 dB) is automatically triggered.
In Ondøejov two paper strip recordes per frequency are used; the ratio of sensitivity between them is 1:10 (Tlamicha).
Finally one of us (KMB) pays attention to the lack of consistent criteria for variability index evaluation. He suggests a slight modification of the Manual by Tanaka (1975) that would take into account the dependence of noise storm burst height on the integrating time constant of a system (Borkowski, 1977).
From the above reported material and suggestions, a fair proposal to obtain a much-needed standardization of data recording is the following. For mean flux density measurements (hourly data) a paper strip recorder having a speed of 20 cm·h–1 and a time constant of 1 sec would look sufficient. When distinctive events are considered, as 0.1 sec (or less) lasting burst structures do exist, a time constant of 0.01 sec is needed and the recorder speed must be proportionately higher, while logarithmic amplifiers should be employed. Such a "high rate" recording system should of course be used only during active periods, a magnetic tape support being preferable to a paper strip recorder.
The most unpleasant difficulty concerns the absolute calibration; though it should not be expected that this problem will be solved quickly and easily, the main factors responsible for this may be singled out and the best actions to overcome them could be decided. One of these is the fixing of an absolute flux scale for the overall metric band, to be respected by every observatory. This, as well as other problems outlined in the preceding paragraphs, may be relatively easily solved by international cooperation. Something very promising in this direction has been done during the above mentioned Nançay meeting, though among few stations and without the support of a major international agency; all these problems would in fact require an official organization to be effectively solved, such as a working group formed by URSI, CESRA or IAU participants, in which experienced scientists would lead the action.
Borkowski, K .M.: 1977, Physica Solariterr. 4, 13.
Elgaröy, Ö., Slottje, C., Tlamicha, A., Urbarz, H., Zanelli, C., Zlobec, P., Bougeret, J.-L., Kerdraon, A. and de la Noë, J.: 1980, Astron. Astroph. Suppl. Ser., (in press) [44 (1981), 165].
van Nieuwkoop, J.: 1978, private communication.
Raffaelli, J.C.: 1975, private communication.
Smerd, S.F.: 1964, Ann. of the IGY 34, 331.
Tanaka, H.: 1975, Instruction Manual for Monthly Report, Toyokawa.
Yamashita, F.: 1974, private communication.