G. A. Dulk

Electron-cyclotron masers as the source of certain solar and stellar radio bursts

The theory of electron-cyclotron masers as they might apply in the astrophysical context is developed, and it is suggested that such masers provide an explanation for the very bright emission known to be associated with certain kinds of radio bursts observed on the Sun and other stars. Some of the observed properties of solar and stellar radio bursts that seem to require amplification of the radiation are summarized, including millisecond solar spikes, RS CVn binaries, and flares on M dwarf stars. Recent developments in the theory of electron-cyclotron masers are summarized and the current theory is applied to electrons with a loss cone anisotropy, estimating the growth rate and saturation levels. In the interpretation of solar microwave spikes and RS CVn binaries, the mechanisms of gyromagnetic absorption, maser at the second harmonic, polarization, and angular distribution are examined in the light of the theory.

Coronal magnetic fields

The observational evidence on the strength of the coronal magnetic field above active regions is reviewed. Recent advances in observations and plasma theory are used to determine which data are the more reliable and to revise some earlier estimates of field strength. The results from the different techniques are found to be in general agreement, and the relation 279-01, 1.02 ≲ R/R⊙ ≲ 10 is consistent with all the data to within a factor of about 3.

Tracing the Electron Density from the Corona to 1 au

We derive the electron density distribution in the ecliptic plane, from the corona to 1 AU, using observations from 13.8 MHz to a few kHz by the radio experiment WAVES aboard the spacecraft Wind. We concentrate on type III bursts whose trajectories intersect the spacecraft, as determined by the presence of burst-associated Langmuir waves, or by energetic electrons observed by the 3-D Plasma experiment. For these bursts we are able to determine the mode of emission, fundamental or harmonic, the electron density at 1 AU, the distance of emission regions along the spiral, and the time spent by the beams as they proceed from the low corona to 1 AU. For all of the bursts considered, the emission mode at burst onset was the fundamental; by contrast, in deriving many previous models, harmonic emission was assumed.By measuring the onset time of the burst at each frequency we are able to derive an electron density model all along the trajectory of the burst. Our density model, after normalizing the density at 1 AU to be ne(215 R0)=7.2 cm−3 (the average value at the minimum of solar activity when our measurements were made), is ne=3.3×105 r−2+4.1×106 r−4+8.0×107 r−6 cm−3, with r in units of R0. For other densities at 1 AU our result implies that the coefficients in the equation need to be multiplied by ne(1 AU)/7.2.We compare this with existing models and those derived from direct, in-situ measurements (normalized to the same density at 1 AU) and find that it agrees very well with in-situ measurements and poorly with ‘radio models’ based on apparent source positions or assumptions of the emission mode. One implication of our results is that isolated type III bursts do not usually propagate in dense regions of the corona and solar wind, as it is still sometimes assumed.

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.

Calibration of low-frequency radio telescopes using the galactic background radiation

We consider the calibration of flux densities of radio bursts from decametric to kilometric wavelengths using ground-based and space-based data. The method we derive is applicable to low-frequency radio telescopes where galactic background radiation is the principal contribution to system temperature. It can be particularly useful for telescopes of low angular resolution observing spectra of radio bursts from the Sun and the planets because absolute calibration of these telescopes is very difficult with conventional techniques. Here we apply the method to observations from about 7 to 47 MHz that were made on the ground with the Bruny Island Radio Spectrometer located in Tasmania, Australia, and those from about 20 kHz to 13.8 MHz were made with the radio experiment WAVES on the WIND spacecraft. The spectrum of the galactic background radiation from

Tracing shock waves from the corona to 1 AU: Type II radio emission and relationship with CMEs

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Radio flares from AE Aquarii - A low-power analog to Cygnus X-3?

MHz has been carefully measured with low-resolution telescopes, starting more than a decade ago. We use this known spectrum to calibrate both BIRS and WAVES on an absolute scale. The accuracy we achieve is about a factor of two, whereas the flux densities of solar and planetary radio sources vary by many orders of magnitude. Our method permits inter-calibration of ground-based and space-based observations, and allows corrections to be made for instrumental uncertainties on both radio experiments. In addition, on the ground, it allows the spectra to be corrected for ionospheric absorption and partial ground reflections. As an application we show the spectrum of a solar type III burst observed from 47 MHz to 20 kHz. Its flux density was largest,

Dynamic spectra of radio bursts from flare stars

S\approx 10^{-17}

Apparent changes in the rotation rate of Jupiter

W m -2 Hz -1 , at about 3 MHz, while at 60 kHz and at 47 MHz it was lower by a factor of about 300.

Electron beams and radio waves of solar Type III bursts

We report on 10 type II bursts observed with ground-based spectrographs in the meter-decameter range, and with the Radio and Plasma Wave Investigation on the Wind spacecraft from 13.8 to 0.01 MHz. We have selected events with contemporaneous observations of flares and of coronal mass ejections (CMEs) by Large-Angle and Spectrometric Coronagraph (LASCO) telescopes. We trace the history of each event from the time of the impulsive phase of the flare, the CME liftoff time, and the start time of the radio bursts. We derive the speed of the type II shock by using a coronal/solar wind density model, and the height-time progression is compared with that of the CME as observed in the plane of the sky and then converted into the radial direction. For most events a shock at 1 AU was observed in situ. The results show the following: (1) All type II bursts occurred within 2 or 3 min of the impulsive phase of a flare. (2) The speeds of the disturbances from the time of the flares to the time of the shocks at 1 AU were very similar to the speeds of the type II-emitting shocks: they were in the range of 600 to 1300 km s -1 . (3) When the type II burst was observed far out in the solar wind, the progression of the type II source had about the same speed in the solar wind as in the corona. (4) The CME liftoffs were before the flares and the type II bursts by 1-24 min for most of the selected events. As a consequence, in the corona, the type II bursts, being behind the fronts of the CMEs, are usually blast waves. (5) When a shock and CME material are observed at 1 AU, the time of arrival implies a deceleration of the CME in the solar wind, as is observed in the LASCO data. (6) Somewhere in the solar wind the shocks very likely become piston-driven. related to the CME.