T. S. Bastian

A Search for Radio Emission from Extrasolar Planets

All magnetized planets in the solar system emit intense cyclotron maser radiation. Like Jupiter, extrasolar giant planets are probably magnetized. If, in addition, there is a source of energetic (keV) electrons in their magnetospheres, from auroral processes or as a result of magnetic coupling between the planet and a satellite, it is likely that extrasolar planets are cyclotron-maser emitters. Detection and follow-up observations of cyclotron maser radiation from an exoplanet would reveal the presence, strength, and complexity of the planetary magnetic field, the planets rotation rate, and possibly the presence of an Io-like moon within the planets magnetosphere. Magnetic fields may be necessary for life to exist on the surface of planets because they provide protection from the nefarious effects of energetic particles of stellar winds, stellar flares, and cosmic rays. We have conducted a search for radio emission from extrasolar planets and brown dwarfs at decimeter and meter wavelengths using the Very Large Array (VLA). We have observed seven extrasolar planets and two brown dwarfs at 333 and 1465 MHz, and one extrasolar planet and one brown dwarf at 74 MHz. Typical (1 σ) sensitivities were 0.02-0.07 mJy at 1465 MHz, 1-10 mJy at 333 MHz, and ~50 mJy at 74 MHz. To date, no detections have been made.

X-Ray Network Flares of the Quiet Sun

Temporal variations in the soft X-ray (SXR) emission and the radio emission above the solar magnetic network of the quiet corona are investigated using Yohkoh SXR images with deep exposure and VLA observations in the centimeter radio range. The SXR data show several brightenings, with an extrapolated occurrence probability of one brightening per 3 seconds on the total solar surface. During the roughly 10 minutes of enhanced flux, total radiative losses of the observed plasma are around 1025 ergs per event. These events are more than an order of magnitude smaller than previously reported X-ray bright points or active region transient brightenings. For all of the four SXR events with simultaneous radio observations, a corresponding radio source correlating in space and time can be found. There are several similarities between solar flares and the SXR/radio events presented in this paper. (1) Variations in temperature and emission measure during the SXR enhancements are consistent with evaporation of cooler material from the transition region and the chromosphere. (2) The ratio of the total energies radiated in SXR and radio frequencies is similar to that observed in flares. (3) At least one radio event shows a degree of polarization as high as 35%. (4) In three out of four substructures the centimeter radio emission peaks several tens of seconds earlier than in the SXR emission. (5) The associated radio emission tends to be more structured and to have faster rise times. These events thus appear to be flare-like and are called network flares.

Dynamic spectra of radio bursts from flare stars

The Arecibo 305 m telescope has been used to observe radio bursts from flare stars at 430 and 1415 MHz. Dynamic spectra of the emission with bandwidths of 10 MHz in the former case and 40 MHz in the latter are recorded. For AD Leo, the microwave burst emission was 100 percent right circularly polarized, achieved brightness temperatures near 10 to the 16th K, was generally broadband in character, but was superposed with finite structures in both frequency and time. Quasi-periodic pulsations were clearly present as well as a sudden reduction feature. For YZ CMi, the emission was 100 percent left circularly polarized and was relatively broadband with fine structures. Instabilities driven by anisotropies in the electron distribution, particularly the loss-cone distribution, are considered to account for the coherent radiation. 55 refs.

The Radio Spectrum of TVLM 513-46546: Constraints on the Coronal Properties of a Late M Dwarf

We explore the radio emission from the M9 dwarf TVLM 513-46546 at multiple radio frequencies, determining the flux spectrum of persistent radio emission, as well as constraining the levels of circular polarization. Detections at both 3.6 and 6 cm provide a spectral index measurement α (where Sν ∝ να) of -0.4 ± 0.1. A detection at 20 cm suggests that the spectral peak is between 1.4 and 5 GHz. The most stringent upper limits on circular polarization are at 3.6 and 6 cm, with V/I < 15%. These characteristics agree well with those of typical parameters for early- to mid-type M dwarfs, confirming that magnetic activity is present at levels comparable with those extrapolated from earlier M dwarfs. We apply analytic models to investigate the coronal properties under simple assumptions of dipole magnetic field geometry and radially varying nonthermal electron density distributions. Requiring the spectrum to be optically thin at frequencies higher than 5 GHz and reproducing the observed 3.6 cm fluxes constrains the magnetic field at the base to be less than about 500 G. There is no statistically significant periodicity in the 3.6 cm light curve, but it is consistent with low-level variability.

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 Subsystem Testbed (FST) and the Owens Valley Solar Array (OVSA) were used in parallel to observe the X1.5 flare on 2006 December 14. 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 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 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.

Broadband Quasi-periodic Radio and X-Ray Pulsations in a Solar Flare

We describe microwave and hard X-ray observations of strong quasi-periodic pulsations from the GOES X1.3 solar flare on 2003 June 15. The radio observations were made jointly by the Owens Valley Solar Array (OVSA), the Nobeyama Polarimeter (NoRP), and the Nobeyama Radioheliograph (NoRH). Hard X-ray observations were made by RHESSI. Using Fourier analysis, we study the frequency- and energy-dependent oscillation periods, differential phase, and modulation amplitudes of the radio and X-ray pulsations. Focusing on the more complete radio observations, we also examine the modulation of the degree of circular polarization and of the radio spectral index. The observed properties of the oscillations are compared with those derived from two simple models for the radio emission. In particular, we explicitly fit the observed modulation amplitude data to the two competing models. The first model considers the effects of MHD oscillations on the radio emission. The second model considers the quasi-periodic injection of fast electrons. We demonstrate that quasi-periodic acceleration and injection of fast electrons is the more likely cause of the quasi-periodic oscillations observed in the radio and hard X-ray emission, which has important implications for particle acceleration and transport in the flaring sources.

A search for cyclotron maser radiation from substellar and planet-like companions of nearby stars

Several stars within 5 pc have variations in their rectilinear or Keplerian motion that suggest that they have substellar or planetlike companions. However, direct, unambiguous evidence of such companions is difficult to obtain. It is argued that such objects should commonly emit cyclotron maser radiation and that this radiation should be observable with modern radio telescopes. Its detection would confirm the presence of the companions and permit many of the properties of the stars and companions to be derived. A search was made for the cyclotron maser emission from six nearby stars with suspected companions using the VLA at frequencies of 0.33 and 1.4 GHz. None was detected. Typical observing times were 3.5 hr, and typical upper limits were 30 mJy at 0.33 GHz and 0.3 mJy at 1.4 GHz.

Radio spectral evolution of an X-ray-poor impulsive solar flare : Implications for plasma heating and electron acceleration

We present radio and X-ray observations of an impulsive solar flare that was moderately intense in microwaves, yet showed very meager EUV and X-ray emission. The flare occurred on 2001 October 24 and was well observed at radio wavelengths by the Nobeyama Radioheliograph (NoRH), the Nobeyama Radio Polarimeters (NoRP), and the Owens Valley Solar Array (OVSA). It was also observed in EUV and X-ray wavelength bands by the TRACE, GOES, and Yohkoh satellites. We find that the impulsive onset of the radio emission is progressively delayed with increasing frequency relative to the onset of hard X-ray emission. In contrast, the time of flux density maximum is progressively delayed with decreasing frequency. The decay phase is independent of radio frequency. The simple source morphology and the excellent spectral coverage at radio wavelengths allowed us to employ a nonlinear χ2-minimization scheme to fit the time series of radio spectra to a source model that accounts for the observed radio emission in terms of gyrosynchrotron radiation from MeV-energy electrons in a relatively dense thermal plasma. We discuss plasma heating and electron acceleration in view of the parametric trends implied by the model fitting. We suggest that stochastic acceleration likely plays a role in accelerating the radio-emitting electrons.

Acceleration of electrons at type II shock fronts and production of shock-accelerated type III bursts

We present evidence of electron acceleration by type II-burst-emitting shocks in the corona. Some of the electrons travel outward along open magnetic field lines and produce “shock-accelerated type III bursts” (or SA type III bursts) along their paths. The SA type III bursts are evident in dynamic spectra that cover part or all of the range from metric to kilometric wavelengths. The unique feature of our observations is the complete or near-complete frequency coverage from about 2 GHz to < 0.1 MHz, that is, ≲ 1.01 Ro to 1 AU. A sample of eight events is presented. All would be classified as “shock accelerated events” at hectometric wavelengths, as first defined by Cane et al. [1981]. Our complete spectra frequently show several to many type III-like bursts emanating from near the type II burst toward low frequencies, with no trace of emission at frequencies higher than that of the type II burst. The drift rates of these SA type III bursts are similar to those of normal type III bursts, and the exciting electrons have speeds of order 0.1c to 0.2c, or energies of 3–10 keV and higher. Their intensity at hectometer wavelengths is similar to that of normal type III bursts. They often persist to the lowest frequencies observable, near the local plasma frequency at 1 AU. In most of the events examined, there were no microwave bursts from the low corona whose intensity profiles were similar to the hectometric profiles. We therefore conclude that these SA type III bursts originate in type II shocks and are caused by energetic electrons accelerated at the shocks. Of the eight events analyzed, three contain only SA type III bursts. For the remainder, normal type III bursts predominate initially, followed by SA type III bursts later in the event. We emphasize the need for spectra with near-continuous coverage, especially from decametric to kilometric wavelengths, to identify SA type III bursts unambiguously and to distinguish between the contributions of normal and SA type III bursts.

Decimetric solar type U bursts - VLA and Phoenix observations

Observations of type U bursts, simultaneously detected by the VLA at 1.446 GHz and by the broadband spectrometer Phoenix in the 1.1-1.7 GHz frequency band on August 13, 1989 are reported. Extrapolations of the coronal magnetic field, assuming a potential configuration, indicate that the VLA 20 cm source demarcates an isodensity level. The source covers a wide angle of diverging magnetic field lines whose footpoints originate close to a magnetic intrusion of negative polarity into the main sunspot group of the active region with dominant positive polarity. The centroid of the 20-cm U-burst emission, which corresponds to the turnover frequency of the type U bursts and remains stationary during all U bursts, coincides with the apex of extrapolated potential field lines at a height of about 130,000 km. It is demonstrated that the combination of radio imaging and broadband dynamic spectra, combined with the magnetic field reconstruction from magnetograms, can constrain all physical parameters of a magnetic loop system.