BRAZILIAN SOLAR SPECTROSCOPE (BSS) H. S. SAWANT, K. R. SUBRAMANIAN, C. FARIA, F. C. R. FERNANDES, J. H. A. SOBRAL, J. R. CECATTO, R. R. ROSA, H. O. VATS,J.A.C.F.NERI, E. M. B. ALONSO, F. P. V. MESQUITA, V. A. PORTEZANI and A. R. F. MARTINON Instituto Nacional de Pesquisas Espaciais - INPE, C.P. 515, 12201-970 São José dos Campos, SP, Brazil (Received 30 May 2000; accepted 15 January 2001) Abstract. Digital, decimetric (200 2500 MHz) Brazilian Solar Spectroscope (BSS) with high time (10 1000 ms) and frequency (1 10 MHz) resolution is in regular operation since April, 1998, at National Space Research Institute (INPE) at São José dos Campos, Brazil. BSS operates in conjunction with a 9-m diameter polar mounted antenna. It allows to select suitable observing frequency range, frequency and time resolutions and data can be digitized up to 100 channels. BSS has capabilities of quasi-real time display of the ongoing dynamic spectra of the solar activity that enables the observer to modify observational parameters so as to suit a specific type of activity such as spikes and improve the quality of data acquisition and storage. Minimum detectable flux density of the spectroscope, for different combinations of the observational parameters, is 3 s.f.u. Observations are carried out routinely from 11 UT to 19 UT. Necessary software for data acquisition and reductions has been developed in IDL 5.3 environment. Data are available in FITS and ASCII formats. Absolute timing accuracy of the station is less than 3 ms. Here, we present examples of the bursts which have been recorded by BSS and available display facilities. 1. Introduction SKYLAB observations of soft X-rays suggested that particle acceleration and/or heating is taking place in the region where electron densities are around 10 9 10 11 cm 3 (Moore et al., 1980, and references therein), corresponding to the decimetric frequency range. Realising the importance of high time and frequency resolution, Benz, Bernold, and Dennis (1983) initiated such observations for the first time in the frequency range of 100 1000 MHz and subsequently, the frequency range was increased to 4000 MHz (Messmer, Benz, and Monstein, 1999). Recently, Krüger and Voigt (1995) have reviewed solar spectroscopes in Europe and Bastian, Gary, and White (1999) have listed all telescopes operating for solar observations. These reviews indicate that there is a lack of high sensitivity and high resolution digital solar spectroscopes in the decimetre wavelengths in the western hemisphere. BSS fills this gap. Table I shows digital solar spectroscopes operating at present. BSS has the advantage of quasi-real time (delay of 1 5 min) display of the observed dynamic spectra, which enables to modify the parameters of observations On sabbatical leave from the Indian Institute of Astrophysics, Bangalore, India. On sabbatical leave from Physical Research Laboratory, Ahmedabahd, India. Solar Physics 200: 167 176, 2001. 2001 Kluwer Academic Publishers. Printed in the Netherlands.
168 H. S. SAWANT ET AL. TABLE I Solar digital spectroscopes in operation at present Station Frequency Time Reference range resolution (MHz) (ms) Zurich, CH 100 4000 0.5 1000 Benz et al., 1991 Ondřejov, CZ 800 2000 100 Jirička et al., 1993 Ondřejov, CZ 2000 4500 100 Tlamicha, 1990 INPE, BR 200 2500 10 1000 This work so as to suit planned observations of fine structures such as spikes, type III bursts, with improved quality of data acquisition. Normally, BSS operates with 50 ms time resolution. We believe this will enable one to investigate in detail the time evolution of the decimetric type III-like bursts and bursts observed with fine structures in frequency and time above 1000 MHz, since the duration of the type III-like bursts observed above 1000 MHz is < 300 ms (Meléndez et al., 1999). Signals sent by the GPS system are used to record timings of the events, with absolute accuracy 3 ms. The high absolute timing accuracy and the high time resolution observations will permit us to carry out simultaneous investigations of the short duration bursts observed in the other wavelengths more accurately. The analogue and digital system, including calibration, are discussed below. 2. Instrumentation The first phase of the development of INPE s Solar Spectrograph started in 1990, with the installation of the 9-m diameter antenna, with narrow band (1600 ± 100 MHz) observations (Sawant et al., 1993). That operated till 1994, and recorded about 350 groups of solar bursts (Sawant, Fernandes, and Neri, 1994). The second phase started in 1995, with the installation of a broadband log-periodic feed, to permit observations in the frequency range of (200 2500 MHz). This new instrument, including the digital data acquisition and GPS time code systems was put into operation in 1996 (Sawant et al., 1996). The new system was named as Brazilian Solar Spectroscope (BSS) (Figure 1), and is in regular operation for daily solar observations since April 1998. 2.1. ANALOGUE PART As shown in Figure 1, the signal is received by broad band crossed log-periodic feeder (Watkins and Johnson AR 274-10) kept at the focus of the 9-m diameter
BRAZILIAN SOLAR SPECTROSCOPE 169 Figure 1. Block diagram of the architecture of Brazilian Solar Spectroscope (BSS).
170 H. S. SAWANT ET AL. TABLE II Parameters of BSS Parameters Specifications Frequency range 200 2500 MHz Frequency resolution 1 10 MHz Number of digital channels 25/50/100 Time resolution 10/20/50/100/1000 ms Observing time 11 19 UT Absolute timing accuracy 3ms Antenna 9-m diameter parabolic, mesh-type, polar mount parabolic antenna, in two linear polarisations (horizontal and vertical). The signal of each polarisation is amplified by the broad band MITEQ (Serial no. 208593) low-noise amplifiers kept at the base of the feeder having gain 25 db. These two signals are combined in phase by a broad band combiner and sent to the observatory 30 m away, via a helical cable. Signal is directly fed to a spectrum analyser (HP 8559A) operating in the frequency range of 200 2500 MHz, which enables to choose the desired observing parameters such as observing frequency range, frequency and time resolutions. Table II lists the parameters of the BSS (Sawant et al., 1996). 2.2. MINIMUM DETECTABLE FLUX DENSITY Solar background fluxes at various frequencies are estimated by using the calibration method suggested by Guidice and Castelli (1971) taking the dimension of the Sun into account. The following equations are used to determine the flux, S(f), at desired frequency, f: [ ] S(fa ) (f ) S(f) = S(f a ), (1) S(f b ) with (f ) = log(f/f a) log(f a /f b ), (2) where f is the observing central frequency in between f a and f b and corresponding flux densities at these frequencies are S(f a ) and S(f b ), respectively. The values of solar flux are obtained via Internet from Solar Geophysical Data (http:/www.ngdc. noaa.gov/stp) or from Cracow Radio Observatory (http://www.nac.oa.uj.edu.pl/sol) homepages.
BRAZILIAN SOLAR SPECTROSCOPE 171 Calibration is made in db above the solar background. Hence, direct comparison of the burst intensity with this calibration gives the flux of the solar bursts. Occasionally, at the input of the pre-amplifiers we use a noise generator HP (346B), which enables us to estimate the antenna temperatures. Minimum detectable flux density, S min, is estimated by the following equation (Kraus, 1966, p. 103): S min = 2k(T sy + T bg ), (3) A eff ( f T ) 1/2 where k is the Boltzmann s constant, T sy is the system temperature ( 300 K), T bg is the background temperature, including the temperature of quiet Sun ( 1.3 10 4 K), A eff ( 40 m 2 ) is the effective area of the antenna for an efficiency of 60%, f is the IF bandwidth and T is the integration time, given by T = t/n ch,where t is the time resolution, in ms, and N ch is the number of channels. Since IF bandwidth is variable from 1 to 3 MHz, depending upon the observed frequency range and the number of channels in use, frequency resolution will vary from 1 to 10 MHz. Minimum detectable flux density of BSS, obtained for different combinations of the observing frequency range, time resolution and number of channels is of the order of 3 s.f.u. Efforts are made to choose such combinations so that sensitivity will remain almost the same for different combinations. 2.3. DIGITAL PART, DATA ACQUISITION AND SOFTWARE As shown in Figure 1, vertical output of the spectrum analyser HP 8559A is fed to an A/D 12 bit digital board along with the synchronisation and blanking pulses. The digitized signal is recorded in the IBM-PC(1) files. From the beginning of the sweep time, t = t 0, digitisation starts and digitised signal averaged over the 1 th time interval is stored locally into the RAM memory 100 of an IBM-PC(1). Depending upon the combination of time and frequency range one can digitise 25, 50, or 100 channels. The duration of each file can be chosen from 1 to 20 min. Each file is sent to another computer IBM-PC(2) for quasireal time visualisation and preliminary analysis, which enables to choose proper parameters of the observations such as time/frequency resolutions and frequency range, if needed (Sawant et al., 2000). In order to have better absolute timing accuracy, the signal of GPS station Clock Model 8820, situated 300 meters away, is used. The GPS clock sends the signal into IRIG-B format of the date, hour, minute, second, millisecond. The signal modulated at 1 khz is received via telephone line from the above GPS system at Observatory and it is demodulated and decoded by a TRAK-839537 decoder. The decoded output is also recorded into RAM memory along with each file. Necessary software is developed to use RS232 as an interface between decoder and IBM- PC(1).
172 H. S. SAWANT ET AL. The visualisation of the data is done by BSSView software. BSSView is written in IDL version 5.3, in which a number of procedures are available for visualisation and preliminary analysis of the solar bursts recorded digitally with high time and frequency resolutions. Graphic interface to call specific functions and procedures is available for users. Presently, the system is working with Windows 98, while another version using UNIX system is being tested. Main characteristics of the BSSView are the following: (1) 2D Visualisation of the spectra in quasi-real time, with colour table and facility to change colour table. (2) Procedure for preliminary analysis in quasi-real time. (3) Reading and writing of the data in the formats FITS, ASCII, and internal format ESP. (4) Visualisation of time profiles of various channels independently and zooming of them, as shown in Figure 2. (5) Availability of 2D dynamic spectrum, 3D surface and contours, and multiple plots of selected data set. (6) Availability of various procedures and functions such as calibration, integration in time and frequency, subtraction of the background, derivative filtering, and smoothing. Examples are shown in Figure 3. 3. Initial Results Since April 1998, more than 70 flares were observed using BSS. Examples of the observed bursts are shown in Figures 2 5. A catalogue of bursts observed will be published elsewhere (Fernandes et al., 2000). 4. Conclusions Since 1990, efforts have been made to develop a digital spectroscope operating over a wide band with high resolution. Around 1995, the spectroscope was operating over narrow band. Since 1998, with a modified antenna tracking system, and with a broad band feeder, the spectroscope is operating over wide band with high resolution. Digitized data are transferred every 1 to 5 min and displayed on another PC, thus permitting quasi-real time visualisation. This enables us to change the desired observing parameters such as time resolution, observing bandwidth, frequency range and number of frequency channels. Data are in FITS format and efforts are made to link it with Solar SoftWare (SSW). Absolute timing accuracy and high resolutions permit comparative studies with other wavelengths as well as one can carry out investigations independently. BSS is the only digital solar spectroscope in regular operation in the western hemisphere giving good coverage to VLA and OVRO observations. Efforts are made to calibrate it using radio sources.
BRAZILIAN SOLAR SPECTROSCOPE 173 Figure 2. Time profiles and zoom of solar bursts observed by BSS on 29 April 1998 (16:25:45 UT), in the frequency range of 1200 1700 MHz. Frequency increases upward. These bursts are associated with a M6.8 X-ray flare that peaked at 16:37 UT (SEC-NOAA, 1999). The bursts show fine structures in time ( 100 ms) and frequency ( 5 MHz), to the limit of BSS limit resolution.
174 H. S. SAWANT ET AL. Figure 3. Dynamic spectra of solar bursts observed on 9 November 1999 ( 12:52:25 UT), in the frequency range of 1200 1400 MHz. These events are associated with a C5.6 X-ray flare that peaked at 12:58 UT. Emissions in radio wavelengths from 245 MHz up to 8800 MHz were also reported (SEC-NOAA, 1999). The three different graphics represent (top) original data, (middle) background subtracted data and (bottom) derivative filtered data.
BRAZILIAN SOLAR SPECTROSCOPE 175 Figure 4. Dynamic spectra of spikes observed by BSS on 24 June 1999 (16:54:43 UT), in the frequency range of 1200 1600 MHz. Emissions in 245 MHz were also reported (SEC-NOAA, 1999). Figure 5. Dynamic spectra of narrow-band bursts observed by BSS on 27 November 1999 (12:08:47 UT) in the frequency range of 1000 1400 MHz. These events are associated with a X1.4 X-ray flare started at 12:05 UT. Emissions in radio wavelengths up to 15 400 MHz were also reported (SEC-NOAA, 1999). BSS will be operated throughout the ongoing solar cycle. Data are available on request for collaborative programmes. Acknowledgements This project is supported by INPE, FAPESP, CNPq, and Capes. F.C.R. Fernandes and H.O. Vats would like to acknowledge the scholarships from CNPq (381647/97-8) and FAPESP (99/10529-0) and from CNPq (301049/98-0), respectively.
176 H. S. SAWANT ET AL. References Bastian, T. S., Gary, D. E., and White, S. M.: 1999, Report to the Astronomy and Astrophysics Survey Committee, http://solar.njit.edu/fasrt/panel99r.html. Benz, A. O., Bernold, T. E. X., and Dennis, B. R.: 1983, Astron. J. 271, 355. Benz, A. O., Güdel, M., Isliker, H., Miszkowicz, S., and Stehling, W.: 1991, Solar Phys. 133, 385. Fernandes, F. C. R., Sawant, H. S., Vats, H. O., Cecatto, J. R., Portezani, V. A., Martinon, A. R. F.: 2000, Astrophys. J. Suppl. Ser., in press. Guidice, D. A. and Castelli, J. P.: 1971, IEEE Trans. Aerosp. Electronic System 7, 226. Jiřička, K., Karlický, M., Kepka, O., and Tlamicha, A.: 1993, Solar Phys. 147, 203. Kraus, J. D.: 1966, Radio Astronomy, McGraw-Hill, New York. Krüger, A. and Voight, W.: 1995, Solar Phys. 161, 393. Meléndez, J. L., Sawant, H. S., Fernandes, F. C. R., and Benz, A. O.: 1999, Solar Phys. 187, 77. Messmer, P., Benz, A. O., and Monstein, C.: 1999, Solar Phys. 187, 335. Moore, R. et al.: 1980, in P. A. Sturrock (ed.) Solar Flares: a Monograph from Skylab Solar Workshop II, Colorado University Press, Boulder, Cap. 8, pp. 341 409. Sawant, H. S., Fernandes, F. C. R., and Neri, J. A. C. F.: 1994, Astrophys. J. Suppl. Ser. 90, 689. Sawant,H.S.,Sobral,J.H.A.,Neri,J.A.C.F.,Fernandes,F.C.R.,Rosa,R.R.,andCecatto,J.R.: 1993, Adv. Space Res. 13, 199. Sawant,H.S.,Sobral,J.H.A.,Fernandes,F.C.R.,Cecatto,J.R.,Day,W.R.G.,Neri,J.A.C.F., Alonso, E. M. B., and Moraes, A.: 1996, Adv. Space Res. 17, 385. Sawant, H. S., Subramanian, K. R., Faria, C., Stephany, S., Fernandes, F. C. R., Cecatto, J. R., Rosa, R. R., Portezani, V. A., Mesquita, F. P. V., and Alonso, E. M. B.: 2000, ASP Conf. Series 206, 347. Space Environment Center / National Oceanic and Atmospheric Administration - SEC NOAA, http://www.sec.noaa.gov, 1999. Tlamicha, A: 1990, Spectral Observations of Solar Radio Bursts in the Range of 2.0 4.5 GHz, Astronomical Institute of the Czechoslovak Academy of Science.