Appendix E

A Frequency-Agile Solar Radio Telescope

WHAT IS FASR?

The Frequency-Agile Solar Radiotelescope (FASR) is a concept for a ground-based synthesis imaging radiotelescope designed specifically for observing the Sun. The solar-dedicated imaging spectrometer design takes advantage of recent advances in broadband microwave components, correlator technology, and computer technology to achieve radio-imaging performance far exceeding any currently available. Its combination of superb imaging and broadband spectral coverage will allow it to address a wide variety of fundamental solar science questions including direct measurement of coronal magnetic fields (a true coronal magnetograph), distribution and diagnostics of thermal and nonthermal electrons, and detection of coronal currents.

WHY FASR?

The Frequency-Agile Solar Radiotelescope will for the first time give high-quality, high-resolution images of the entire solar disk at a broad range of closely spaced microwave frequencies on timescales down to 1 s. The resulting spatially resolved, dual-polarization brightness temperature spectra can be inverted to quantitatively determine the important plasma parameters in most constituents of the solar atmosphere, such as active region loops, sunspots, filaments and prominences, coronal holes, bright points, supergranular network, CMEs and flaring loops. The relevant height range in the solar atmosphere is enormous, from 1000 km deep in the chromosphere to a solar radius or more into the corona. The radio emission mechanisms and techniques for extracting such information have long been understood, and there is a worldwide consensus among solar radiophysicists as to what kind of instrument is required to realize the full potential of radio emission. The technology exists to achieve the required performance at relatively low cost (from $10M to 20M, depending on whether existing antennas are utilized in the array).1 The FASR could be operational within 3 years of the beginning of funding.

MAIN SCIENCE OBJECTIVES
  1. A True Coronal Magnetograph: Radio emission due to both free-free and gyroemission is sensitive to the strength of the magnetic field. In the vicinity of active regions, where the magnetic field strength exceeds about 100 G, gyroresonance emission

NOTE: The material in this appendix is reprinted from the Frequency-Agile Solar Radiotelescope Fact Sheet (available online at <http://solar.njit.edu/fasr/>. The FASR science team includes D. Gary, New Jersey Institute of Technology; T. Bastian, National Radio Astronomy Observatory; S. White, University of Maryland; and G. Hurford, California Institute of Technology.

1  

Estimates for the cost of FASR have been published on line at <http://solar.njit.edu/fasr/>.



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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Appendix E A Frequency-Agile Solar Radio Telescope WHAT IS FASR? The Frequency-Agile Solar Radiotelescope (FASR) is a concept for a ground-based synthesis imaging radiotelescope designed specifically for observing the Sun. The solar-dedicated imaging spectrometer design takes advantage of recent advances in broadband microwave components, correlator technology, and computer technology to achieve radio-imaging performance far exceeding any currently available. Its combination of superb imaging and broadband spectral coverage will allow it to address a wide variety of fundamental solar science questions including direct measurement of coronal magnetic fields (a true coronal magnetograph), distribution and diagnostics of thermal and nonthermal electrons, and detection of coronal currents. WHY FASR? The Frequency-Agile Solar Radiotelescope will for the first time give high-quality, high-resolution images of the entire solar disk at a broad range of closely spaced microwave frequencies on timescales down to 1 s. The resulting spatially resolved, dual-polarization brightness temperature spectra can be inverted to quantitatively determine the important plasma parameters in most constituents of the solar atmosphere, such as active region loops, sunspots, filaments and prominences, coronal holes, bright points, supergranular network, CMEs and flaring loops. The relevant height range in the solar atmosphere is enormous, from 1000 km deep in the chromosphere to a solar radius or more into the corona. The radio emission mechanisms and techniques for extracting such information have long been understood, and there is a worldwide consensus among solar radiophysicists as to what kind of instrument is required to realize the full potential of radio emission. The technology exists to achieve the required performance at relatively low cost (from $10M to 20M, depending on whether existing antennas are utilized in the array).1 The FASR could be operational within 3 years of the beginning of funding. MAIN SCIENCE OBJECTIVES A True Coronal Magnetograph: Radio emission due to both free-free and gyroemission is sensitive to the strength of the magnetic field. In the vicinity of active regions, where the magnetic field strength exceeds about 100 G, gyroresonance emission NOTE: The material in this appendix is reprinted from the Frequency-Agile Solar Radiotelescope Fact Sheet (available online at <http://solar.njit.edu/fasr/>. The FASR science team includes D. Gary, New Jersey Institute of Technology; T. Bastian, National Radio Astronomy Observatory; S. White, University of Maryland; and G. Hurford, California Institute of Technology. 1   Estimates for the cost of FASR have been published on line at <http://solar.njit.edu/fasr/>.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE at harmonics of the cyclotron frequency gives a direct conversion between observing frequency and field strength. With high-resolution imaging at a large number of frequencies, the FASR will directly obtain iso-gauss contours at the corresponding field strength in every active region on the disk simultaneously. In lower-field regions such as quiet Sun network, the FASR will exploit the magnetic sensitivity of circularly polarized free-free emission to yield line-of-sight magnetic field strength. Both techniques measure the magnetic field well above the photospheric height accessible to optical magnetographs. True Coronal Temperatures: Radio emission allows direct measurement of the electron temperature, which is a true temperature that does not depend on abundances or atomic physics. Wherever the radio emission is optically thick (which occurs somewhere within the broad frequency range of FASR for virtually every solar feature), the radio brightness is strictly proportional to the electron temperature. For regions dominated by gyroresonance emission (B > 100 G), the temperature measurement refers to a very specific layer just 100 km wide which changes with the observing frequency. FASR will continuously map true coronal/chromospheric temperature over the entire solar disk and with exquisite height precision in active regions. Origins of Solar Activity: Radio emission is far more sensitive to non-thermal, high-energy electrons than is X-ray emission. The FASR will obtain diagnostics of electron energy distributions, with high spatial resolution, continuously over the entire visible hemisphere of the Sun, with activity ranging in size from tiny microflares to the largest solar events. The wide frequency range and high dynamic range will allow simultaneous, perfectly cospatial imaging of both the thermal and nonthermal counterparts of solar activity, including the initiation and propagation of CMEs, filament eruptions and flares. Comparison with pre-event coronal magnetograms and temperature maps from the FASR can help to reveal the root causes of the Sun's activity. KEY ARRAY CHARACTERISTICS Size of elements: 2 m Number of elements: ~ 40 Maximum baseline: 3 km Minimum baseline: 3 m Sky coverage: all-sky within 3 degrees of horizon Large elements for calibration: 1 to 3 large (25-meter-class) antennas allow determination of absolute flux and position using cosmic sources, to arc-sec accuracy KEY RECEIVER CHARACTERISTICS Time resolution: 1 s Frequency range: 300-26,000 MHz IF bandwidth: 500 MHz

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Frequency resolution: 32 MHz (> 800 frequency channels) Polarization: RCP, LCP Dynamic range: switchable attenuation to prevent saturation KEY IMAGING CHARACTERISTICS Field of view: Full Sun to 20 GHz Angular resolution: 1″ at 20 GHz Real-time images: I and V (or RCP and LCP) continuously Daily data products: Processed coronal magnetograms and coronal temperature maps several/day) OPERATION As a solar-dedicated, full-Sun instrument, the primary mode of operation of FASR would be continuous spectral imaging of the Sun in a standard observing sequence. Data would be non-proprietary, and would be immediately accessible to the scientific community. Real-time and daily processed data would be sent to world data centers. In addition, individual investigators could propose special observing sequences designed for specific projects (for example, increased time resolution at a smaller number of frequencies).

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