Upgrade of the Siberian Solar Radio Telescope

1. Re Im f1 f2 f3 f4 f5 Six antenna array example of the SSRT upgrading. Antenna array Fiber-optic line 3-9 GHz Mixing, delaying and digitizing Correlator The tuned LO set Out to computer Local oscillators Front-end receiver and mixer of antenna Am. R L S fm l/4 The tuned LO set A1 . . . . Am . . . . AN Im 0.1-10 MHz Polarization switching Local oscillators control Fringe stopping control Re 0.1-10 MHz Upgrade of the Siberian Solar Radio Telescope Lesovoi S.V., Zandanov V.G.,Smolkov G,Ya., Altyntsev A.T., Gubin A.V. Institute of Solar-Terrestrial Physics, P.O. Box 4026, Irkutsk, 664033, Russia, Goals and problems The variability of coronal magnetic fields is a key feature for an understanding of such problems of solar physics as flares and coronal mass ejections (CME).The measurement of the structure and dynamics of coronal magnetic fields is one of most attractive goals of the solar physics for last decades. There are difficulties of measuring these fields at other than radio wavelengths. On the other hand, spatially resolved (2D) radio observations of the polarization in a wide spectral range are most suitable method to measure the coronal magnetic fields strength. Unfortunately, there is not todaya radioheliograph operating in the wide frequency range with the relevant temporal and spatial resolutions. There are some problems with adaptation of the SSRT to these goals. The first problem is that the spatially resolved data are obtained at the only frequency.The second problem is the relative low temporal resolution of the SSRT (~2 min) is not enough to measure the flare-driven evolution of the coronal magnetic fields. Due to the above problems now it is difficult to use the SSRT in studying of the coronal magnetic field structure and dynamics. In order to adapt the SSRT to this task we plan to change the SSRT Earth rotation direct imaging to the Fourier synthesis imaging technique. Also, the frequency range would be extended to 4500–9000 MHz. Of course, it is not enough to cover the necessary frequency range, but it is determined by cost reason rather. On the other hand, it is very likely that this frequency range would be enough to determine second harmonic of gyroresonance emission (Gary, White). In turn, it would allow to measure the spot associated coronal magnetic fields. For flare events it would be fortunately to use simultaneously observations of the upgraded SSRT and NoRH (17,34 GHz). The SSRT would obtain positive slope of brightness temperature spectrum (optically thick) and NoRH would obtain negative one (optically thin). In order to improve the dynamical capability of the SSRT, the redundancy of the antenna array would be used. Up to five nested arrays would be formed to obtain two-dimensional images for five frequencies simultaneously. The reasonable first stage of the SSRT upgrade would be the 12-element T-shaped antenna array with the frequency agile receiver. Because the snapshot uv-planecoverage of 12-element array would be pure, it would be possible to measure slowly varying coronal magnetic fields onlywith this array. Instrumentation Antenna and front-end. We plane to use the existing SSRT antenna array. The best angular resolution would be reached at 9000 MHz (13). But the fundamental spacing at this frequency strongly depends on the observation time. Each antenna would be equipped with a wideband dual polarization feed, an attenuator, and an amplifier. Two feed types are considered now. For the crossed linear feeds, the hybrid sum has to be used. On the other hand, a multiplexer has to be used in the case of the wideband horn feed. The wideband signal would be transmitted by a single-mode optic-fiber cable to a working room. SSRT today The SSRT is a solar-dedicated radio telescope operating for two decades. The antenna configuration of the SSRT is a cross-shaped array. The maximum base length is 622 m; the operating frequency is 5730 MHz. The direct imaging is performed using the diurnal Earth rotation and the frequency scanning method (Grechnev et al. 2003). The temporal resolution due to this method is low, 0.5-3 minutes. The angular resolution of the SSRT is up to 21 arcsec. The today’s SSRT provides an opportunity to observe such solar phenomena as flares and CMEs (Uralov et al.) in 2D mode, and fast processes in 1D mode (Altyntsev et al. 1996, Lesovoi & Kardapolova 2003). Because the SSRT operates with both RCP and LCP, it is possible to measure the magnetic field structure and to estimate the magnetic field strength for optically thin bremsstrahlung (Grechnev et al. 2003, Ryabov et al. 2003). Fiber-optic cable and sampler. A single-mode optic-fiber cable would be used to transmit signals from antennas (4500-9000 MHz) to the working room. The maximum length of the cable is less then 400 m. One-bit sampling at the frequency of 20 MHz would be used. The accuracy of the time delays would be 0.25 ns in the range 1-2000 ns. Fine delay tuning (0.5-60 ns) would be provided by a dedicated time-delay chip, with a coarse delay realized by a shift register. Correlator. We plan to use usual one-bit complex correlators (Nakajima et al. 1994) to calculate the complex visibilities. It is more convenient to use the programmable logic device technology, but the use of a dedicated correlator chipset is considered too. Because the number of visibilities is big (18336), to lower cost of the correlator it would be used the combined method to calculate the visibilities – one correlator unit would be shared by 16, for example, visibilities. Data flow.The whole number of complex correlations of the 192-element array is 18336. The sampling frequency is 20 MHz. The size of each sample is 24 bits. So, the data flow is about 20 MB/sec with an integration time of 0.02 sec. The routine-mode temporal resolution (integration time) is 1 sec, and the flare-mode temporal resolution is 0.02 sec. Because the observational time is in a range of 6-10 hours, the maximal data flow is about 14 GB per day excluding the flare event data. First stage – 12-element array. We plan to develop the 12-element antenna array with the frequency agile receiver as first stage of the SSRT upgrade. At this stage we hope to solve some technical problems (for example, now it is considered two alternative types of wideband feeds) also we hope to find out the real cost of the whole project. On the other hand, this array could be used for some observational goals also. Although the instantaneous uv coverage for this array would be rather pure, we would use Earth rotate aperture synthesis. Thus, it would be possible to use this array to studying slow variable coronal magnetic fields. The figure below shows the uv-plane coverage for the array 3∙(1d,2d,16d,64d) for declinations of 23º (left pane) and -23º (right pane) obtained during day by the Earth rotation synthesis. It would be possible to obtain spatially resolved spectrum in range 4500..9000 MHz every day with this array. Current parameters of the SSRT • Angular resolution: up to 21 arcsec in 2D mode, up to 15 arcsec in 1D mode • Temporal resolution: 2..3 min in 2D mode, 14 ms in 1D mode • Operating frequency: 5730 MHz • Frequency range: 120 MHz (≈2%) • Polarization: Stokes I,V (RCP and LCP) • Data flow: about 0.3 GB per day in routine mode, about 0.1 MB per second in fast 1D mode Upgrading ways To improve the temporal resolution of the SSRT, its direct imaging (by the Earth rotation & frequency scanning) has to be changed to an indirect one (synthesis technique). We plan to use only the T-shaped array of the cross-shaped array of the SSRT. This array would consist of 192 antennas. The whole number of complex correlations of this array is 18336. 8192 correlations would be used to obtain a 2D image, remainder correlations would be used to reduce the instrumental magnitude and phase errors. The wide frequency range would be reached using the wideband fiber-optic cable and a tuned local oscillator set. The SSRT would operate in two modes. The routine mode is a consecutive imaging the whole Sun by the whole array at different frequencies. Theflare mode is a simultaneous imaging a flare region by nested arrays at five frequencies. Expected parameters of the upgraded SSRT • Frequency range: 4500..9000 MHz (up to five frequencies simultaneously) • Temporal resolution: 0.02..1 s • Angular resolution: 26..13 arcsec (4500..9000 MHz) • Spectral resolution: about 100 MHz (≈2%) in routine mode, ≈20% in flare mode • Polarization: Stokes I,V (RCP and LCP) • Data flow: about 14 GB per day in routine mode, about 20 MB/s in flare mode Goals of the upgraded SSRT • Measurement of coronal magnetic fields • Studying of dynamics and 3D structures of CME • Physics of flares • Studying of sub-second processes in solar flares References: Altyntsev A.T., Grechnev V.V., Konovalov S.K., Lesovoi S.V., Lisysian E.G., Treskov T.A., Rosenrauch Yu.M., Magun A. 1996, ApJ, 469, 976 Gary D.E. “Radio spectral diagnostics” in “Solar and space weather radiophysics”, 2004, In press (http://www.ovsa.njit.edu/fasr/author_final.html), pp. 71..87 Grechnev V.V., Lesovoi S.V., Smolkov G.Ya., Krissinel B.B., Zandanov V.G., Altyntsev A.T., Kardapolova N.N., Sergeev R.Y., Uralov A.M., Maksimov V.P., Lubyshev B.I., 2003, Solar Phys. Lesovoi S.V., Kardapolova N.N., 2003, Solar Phys. Nakajima H., Nishio M., Enome S., Shibasaki K., Takano T., Hanaoka Y., Torii C., Sekiguchi H., Bushimata T., Kawashima S., Shinhara N., Irimajiri Y., Koshiishi H., Kosugi T., Shiomi Y., Sawa M., Kai K., 1994, Nobeyama Radio Observatory Report 357, pp. 705..713 Ryabov B. I., Maksimov V.P., Lesovoi S.V., Shibasaki K., Nindos A., Pevtsov A.A., 2004, Solar Phys., submitted Uralov A.M., Lesovoi S.V., Zandanov V.G., Grechnev V.V., 2002, Solar Phys., 208, 69 White S.M. “Coronal magnetic field measurements through gyroresonance emission” in “Solar and space weather radiophysics”, 2004, In press (http://www.ovsa.njit.edu/fasr/author_final.html), pp. 89..113