Frequency Agile Solar Radiotelescope Research Articles

SPATIALLY AND SPECTRALLY RESOLVED OBSERVATIONS OF A ZEBRA PATTERN IN A SOLAR DECIMETRIC RADIO BURST

We present the first interferometric observation of a zebra-pattern radio burst with simultaneous high spectral (~ 1 MHz) and high time (20 ms) resolution. The Frequency-Agile Solar Radiotelescope (FASR) Subsystem Testbed (FST) and the Owens Valley Solar Array (OVSA) were used in parallel to observe the X1.5 flare on 14 December 2006. By using OVSA to calibrate the FST the source position of the zebra pattern can be located on the solar disk. With the help of multi-wavelength observations and a nonlinear force-free field (NLFFF) extrapolation, the zebra source is explored in relation to the magnetic field configuration. New constraints are placed on the source size and position as a function of frequency and time. We conclude that the zebra burst is consistent with a double-plasma resonance (DPR) model in which the radio emission occurs in resonance layers where the upper hybrid frequency is harmonically related to the electron cyclotron frequency in a coronal magnetic loop.

Statistical Hypothesis Testing and Variance Analysis for Radio Frequency Interference Identification in Solar Data

ABSTRACTThis work presents an effective algorithm for radio frequency interference (RFI) identification using dynamic power spectrum statistics in the frequency domain. Statistical signal processing techniques such as hypothesis testing and variance analysis are utilized to derive a test statistic for effective and efficient RFI identification. Starting from the generalized likelihood ratio test (GLRT), we formulate the problem systematically and propose a practical test statistic T(x; f), shown to be distributed, for RFI identification. A threshold approach working on this test statistic is developed to identify the presence of narrowband RFI in the power spectrum with additive Gaussian noise and/or solar flare background, corresponding to a desired constant false alarm rate (CFAR). Detailed analysis on detector performance and effect of RFI duty cycle are also provided. The proposed statistical test is applied to experimental solar data collected by our frequency-agile solar radio telescope (FASR) subsystem testbed (FST) to demonstrate the robustness and scalability of the algorithm, as well as its capability for real-time implementation.

Coronal Magnetism: Difficulties and Prospects

Prospects for advances in understanding the properties of the coronal magnetic field are discussed. A new generation of ground-based instrumentation presents possibilities of improved direct measurements of the field (the Advanced Technology Solar Telescope: ATST) and its inference from radio observations (the Frequency Agile Solar Radiotelescope: FASR). The latter in particular promises major advances in determining the structure of the strong magnetic fields present in active regions. Interpreting observations of coronal oscillations using MHD wave models to infer a magnetic field strength has become popular. While limb observations yield field strengths compatible with those obtained from infrared spectroscopy, disc observations yield values that seem on the low side, suggesting the need for a programme of forward modelling with realistic global magnetic fields. Global magnetic field models can now provide information on the field in the corona, and towards the Earth through the solar wind. Major challenges for such modelling are the incorporation of small-scale plasma effects.

The Stars Are Out in China

BEIJING— China is building a new set of ears tuned to our nearest star. Last month, the government of Inner Mongolia provided land to the National Astronomical Observatories of the Chinese Academy of Sciences for the Chinese Spectral Radioheliograph (CSRH), one of two major ground-based solar instruments that China's scientific community plans for the coming decade. Construction will begin later this month on the $7.3 million facility, which will listen in on radio bursts that could presage coronal mass ejections and solar flares. When directed at Earth, these ionic tidal waves can trigger geomagnetic storms that disable satellites and knock out power grids. Set to open in 2010, CSRH will consist of 40 radio dishes, each 4.5 meters wide. They will be clustered on the steppe in a zone devoid of earthly radio waves—apart from stray cell phone signals—260 kilometers northwest of Beijing. Meanwhile, there's work on a complementary facility, the Frequency-Agile Solar Radiotelescope (FASR). In June, the National Radio Astronomy Observatory (NRAO) and several university partners asked the U.S. National Science Foundation for $25 million to build FASR at Owens Valley Radio Observatory in California. If they receive the funds, the consortium wants to begin building a prototype array at Owens Valley next year, says NRAO's Tim Bastian.

A Subsystem Test Bed for the Frequency‐Agile Solar Radiotelescope

ABSTRACTThis paper describes the design and operation of the Frequency‐Agile Solar Radiotelescope (FASR) Subsystem Testbed (FST) and the first observational results. Three antennas of the Owens Valley Solar Array have been modified so that each sends a 1–9 GHz band of radio frequency to a central location using a broadband analog optical fiber link. A dynamically selected 500 MHz subset of this frequency range is digitized at 1 Gsps (gigasample per second) and recorded to disk. The full‐resolution time‐domain data thus recorded are then correlated through offline software to provide interferometric phase and amplitude spectra on three baselines. An important feature of this approach is that the data can be reanalyzed multiple times with different digital signal‐processing techniques (e.g., different bit‐sampling, windowing, and radio frequency interference [RFI] excision methods) to test the effect of different designs. As a prototype of the FASR system, FST provides the opportunity to study the design, calibration, and interference‐avoidance requirements of FASR. In addition, FST provides, for the first time, the ability to perform broadband spectroscopy of the Sun with spectrograph‐like spectral and temporal resolution, while providing locating ability for simple sources. Initial observations of local RFI, geostationary satellite signals, global positioning system (GPS) satellite signals, and the Sun are presented to illustrate the system performance.

Joint Discussion 3 Solar active regions and 3D magnetic structure

AbstractKeeping in view of the modern powerful observing tools, among othersHinode(formerlySOLAR-B),STEREOand Frequency-Agile Solar Radiotelescope, and sophisticated modelling techniques, Joint Discussion 3 during the IAU General Assembly 2006 focused on the properties of magnetic field of solar active regions starting in deep interior of the Sun, from where they buoyantly rise to the coronal heights where the site of most explosive events are located. Intimately related with the active regions, the origin and evolution of the magnetic field of quiet Sun, the large scale chromospheric structures were also the focal point of the Joint Discussion. The theoretical modelling of the generation and dynamics of magnetic field in solar convective zone show that the interaction of the magnetic field with the Coriolis force and helical turbulent convection results in the tilts and twists in the emerging flux. In the photosphere, some of these fluxes appear in sunspots with field strengths up to about 6100 G. Spectro-polarimetric measurements reveal that the line of sight velocities and magnetic field of these locations are found to be uncombed and depend on depth in the atmosphere and exhibit gradients or discontinuities. The inclined magnetic fields beyond penumbra appear as moving magnetic features that do not rise above upper photospheric heights. As the flux rises, the solar chromosphere is the most immediate and intermediary layer where competitive magnetic forces begin to dominate their thermodynamic counterparts. The magnetic field at these heights is now measured using several diagnostic lines such as CaII854.2 nm, HI656.3 nm, and HeI1083.0 nm. The radio observations show that the coronal magnetic field of post flare loops are of the order of 30 G, which might represent the force-free magnetic state of active region in the corona. The temperatures at these coronal heights, derived from the line widths, are in the range from 2.4 to 3.7 million degree. The same line profile measurements indicate the existence of asymmetric flows in the corona. The theoretical extrapolation of photospheric field into coronal heights and their comparison with the observations show that there exists a complex topology with separatrices associated to coronal null points. The interaction of these structures often lead to flares and coronal mass ejections. The current MHD modelling of active region field shows that for coronal mass ejection both local active region magnetic field and global magnetic field due to the surrounding magnetic flux are important. Here, we present an extended summary of the papers presented in Joint Discussion 03 and open questions related to the solar magnetic field that are likely to be the prime issue with the modern observing facilities such asHinodeandSTEREOmissions.

Fiber bursts as 3D coronal magnetic field probe in postflare loops

Fiber bursts appear in some complex solar radio bursts as a continuum fine structure in the frequency range of 1503000 MHz. We present and test a new method to use fiber bursts as a probe of the magnetic field strength and the 3D field structure in postflare loops. Thereby we assume that fiber bursts are driven by whistler waves ascending in the postflare loops which act as magnetic traps for nonthermal flare electrons. For a selected event (1997 April 07) we derive from dynamic radio spectra (Potsdam) and Nançay Radio Heliograph imaging data of fiber bursts the coronal magnetic field strength within the fiber burst source. We compare the fiber burst source positions and field strength estimates with the extrapolated potential magnetic field above the flaring active region NOAA 8027 using SOHO-MDI photospheric field data. The field strength from fiber bursts are within a factor of 0.6 to 1.4 of the field strength of the selected subset of potential field lines and give preference to a 3.5 times Newkirk (1961, ApJ, 133, 983) coronal density model within the evolving postflare loops. We find independent proof of the physical significance of considering selected potential field lines as postflare loop field information regarding topology and field strength over a time interval of one hour after the impulsive flare phase. We conclude that radio decimeter and meter wave spectra and radio imaging at two representative frequencies are sufficient for a reliable estimate of the (otherwise not measurable) coronal magnetic field strength in postflare loops. This can be an important field sounding method using the forthcoming FASR (Frequency Agile Solar Radiotelescope) instrument.

Low-frequency solar radiophysics with LOFAR and FASR

Low-frequency radio observations offer unique diagnostics of the solar corona and solar wind. After a prolongued hiatus, there is renewed interest in this important frequency regime. Two new ground-based instruments will provide critical new low-frequency observations: the low-frequency array (LOFAR) and the frequency agile solar radiotelescope (FASR). This brief topical review summarizes low-frequency radio phenomena that will be accessible to detailed study by LOFAR and FASR in the coming decade. Energy release, drivers of space weather, and studies of the solar wind are emphasized. Both instruments are expected to play important roles in both basic research problems and national and international space weather capabilities. While FASR is a solar-dedicated instrument, LOFAR is not. Solar observing requirements for LOFAR are briefly discussed.

THE FREQUENCY AGILE SOLAR RADIOTELESCOPE

Solar radio astronomy is about to undergo a revolution with the advent of a new radio synthesis array, the Frequency Agile Solar Radiotelescope (FASR). The array will consist of more than 100 antennas (5000 baselines), and will be designed to meet the special challenges of solar imaging. It will produce high-quality images at hundreds of frequencies in the range 20 MHz-24 GHz. We briefly describe the plans for the instrument, and then concentrate on the range of science that is expected to be addressed, using current state-of-the-art solar radio observations and modeling to illustrate FASR performance. We end with an assessment of the current status of the instrument, and plans for future.

The Frequency Agile Solar Radiotelescope

Abstract The Frequency Agile Solar Radiotelescope (FASR), a telescope concept currently under study, will be a ground based solar-dedicated radio telescope designed and optimized to produce high resolution, high-fidelity, and high-dynamic-range images over a broad frequency range (∼0.1–24 GHz). As such, FASR will address an extremely broad science program, including the nature and evolution of coronal magnetic fields, the physics of flares, drivers of space weather, and the quiet Sun. An important goal is to mainstream solar radio observations by providing a number of standard data products for use by the wider solar physics community. The instrument specifications and the key science elements that FASR will address are briefly discussed, as well as several operational issues.

The Frequency Agile Solar Radiotelescope

The Frequency Agile Solar Radiotelescope (FASR), a telescope concept currently under study, will be a ground based solar-dedicated radio telescope designed and optimized to produce high resolution, high-fidelity, and high-dynamic-range images over a broad frequency range (∼0.1–24 GHz). As such, FASR will address an extremely broad science program, including the nature and evolution of coronal magnetic fields, the physics of flares, drivers of space weather, and the quiet Sun. An important goal is to mainstream solar radio observations by providing a number of standard data products for use by the wider solar physics community. The instrument specifications and the key science elements that FASR will address are briefly discussed, as well as several operational issues.