Upgrade of the Siberian Solar Radio Telescope

1
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 today a 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
45009000 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-plane coverage of 12-element array would be
pure, it would be possible to measure slowly
varying coronal magnetic fields only with 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 todays 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. The flare 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