Radio Spectrograph Research Articles

ARTEMIS Mark-IV, THE NEW GREEK–FRENCH DIGITAL RADIO SPECTROGRAPH AT THERMOPYLES, GREECE

We present the new digital solar radio spectrograph located at the Thermopyles station, Greece, operated by the University of Athens. Observations cover the range from 110 to 600 MHz, using a 7-m parabolic antenna. The reception system uses two techniques in parallel: sweep frequency and multi-channel, the latter being based on the Acousto-Optical technique. The data acquisition system is based on two subsystems, a Sun Sparc-5 workstation and a front end based on a VME Motorola system. The two subsystems are connected through the Ethernet and are operated using the VxWorks real-time package. The daily operation is completely automated: pointing of the antenna to the sun, starting and stopping the observations at pre-set times, acquiring data, compressing data by silence suppression in real time, and archiving the data on a routine manner on DAT tapes. Apart from its usual function, this instrument will be used in conjunction with other instruments, including the Nancay decameter array and the low frequency radio receivers on the Wind spacecraft.

Solar radio burst (type III)

This article describes the observations of a type III radio burst observed at 103 MHz simultaneously by the two radio telescopes situated at Rajkot (22.3°N, 70.7°E) and Thaltej (23°N, 72.4°E). This event occurred on September 30, 1993 at about 0430 UT and lasted for only half a minute. The event consisted of several sharp spikes in a group. The rise and fall time of these are comparable, however the peaks of individual spikes varied by a factor of four. The comparison of these observations with the data of solar radio spectrograph HiRAS indicates that this was a metric radio burst giving highest emission at about 103 MHz.

Solar Radio Burst (Type III)

AbstractThis article describes the observations of a type III radio burst observed at 103 MHz simultaneously by the two radio telescopes situated at Rajkot (22.3°N, 70.7°E) and Thaltej (23°N, 72.4°E). This event occurred on September 30, 1993 at about 0430 UT and lasted for only half a minute. The event consisted of several sharp spikes in a group. The rise and fall time of these are comparable, however the peaks of individual spikes varied by a factor of four. The comparison of these observations with the data of solar radio spectrograph HiRAS indicates that this was a metric radio burst giving highest emission at about 103 MHz.

Solar radio spectrography in Europe

Recent technical progress of solar radio spectrography is concerned with digital data recording, the achievement of high time and frequency resolution, and with an improved coverage of the radio range at short and long wavelengths. A spatially-distributed network of radio spectrographs allows for complementary information about solar events and periods of activity selected for detailed investigation.

Fine structure of the auroral kilometric radiation: A Fermi acceleration process?

The fine structure of the auroral kilometric radiation is explained as a nonlinear interaction between the locally generated cyclotron emission created in an electron plasma cavity and the cavity boundary. Specifically, it is demonstrated that broadband cyclotron emission can form discrete tones after numerous interactions with an oscillating boundary that has quasi‐monochromatic wavelike motion. This boundary will Doppler shift most radio photons in a stochastic way, creating a diffuse background radio component. However, some radio photons will have regular, stable frequencies that form discrete tones that overlie on the diffuse background emission. The discrete tones are created in situations where the photon travel time in the cavity is an integer multiple of the boundary oscillation period, this effect establishing a resonance between the boundary oscillations and photon. A set of coupled nonlinear equations is used to describe the radio photon/cavity boundary interaction. Effects of wave growth in the local density cavity and boundary reflections are also included. As the character of the cavity slowly changes, the resonance condition also changes in an adiabatic way, shifting the resonance to different frequencies. The net result is the creation of drifting radio tones which are comparable to those actually observed on radio spectrograms of the auroral kilometric radiation.

Ulysses radio observations of the heliospheric activity in early June 1991

The radio receiver of the URAP (Unified Radio and Plasma Wave) experiment on Ulysses has recorded a heliospheric activity particularly intense between late May and early June 1991. Many solar radio emissions of types III and II were observed together with interplanetary (IP) shocks. In the same time, the radio spectrograph ARTEMIS at Nancay (France) observed several intense type II bursts. We investigate the association and/or interaction of these radio emissions, which are remotely observed, with some IP shocks detected in situ, in the context of a Coronal Mass Ejection (CME) induced scenario.

Efficient information access in an on-line image archive

The rapid development of world-wide networking during the last two years, due partly to the simple concept of addressing with uniform resource locators (URLs), has drastically simplified information exchange. This expanding communication media has created new possibilities in the access of large databases, and has therefore found a particular interest in astronomy. The most used means of information transfer is text. If large sets of multi-dimensional data, commonly images, have to be accessed, longer transfer times compared with text urge the data manager to find more efficient solutions. Modern tools are not as efficient at browsing among images through a network compared with browsing text. Either the images being browsed are too large to be transferred quickly, or the image size is reduced so much that the interesting information is lost during the data reduction. In this article, an alternative to the transfer of plain images is investigated, based on the fact that only a part of an image's information content is needed for browsing. Images may therefore be reduced to icons which only symbolically represent the information content. Such image icons are used to browse solar radio spectrograms in the Zurich Radio Astronomy Group archive.

Radio Type IV Burst Fine Structures and the Dynamics of the Flare Process

AbstractWe present signatures of quasi-periodic release of nonthermal particles in different flare phases. In the decimetric range a characteristic pattern is identified with a recurrence period of 1...3 min accompanied by a slow drift toward lower frequencies. We explain these spectral features as evidence for repeated coronal energy release in a slowly (0.1vA) uprising particle accelerator. The analysis allows quite a new prospect to identify the source of the metric type II burst and its timing in the type IV radio spectrogram.

Measurements of Solar Magnetic Fields Using Radio Observations with the RATAN-600

AbstractThree ways of measuring magnetic fields in the corona and chromosphere have been developed based on spectral polarization observations with high spatial resolution using the radio telescope RATAN-600. The methods rely on effects from the theory of the generation and propagation of microwaves in the solar atmosphere: thermal bremsstrahlung, thermal cyclotron emission, and inversion of the polarization sign in quasi-transverse field regions. The new radio spectrograph (PAS) at the RATAN-600 resulted in higher accuracy measurements of the strength and structure of coronal magnetic fields. Using observations made with the PAS on 1992 January 10, we show that the magnetic field strength in the lower corona above all large sunspots can be measured with an accuracy ≈ 3%. The correlation with photospheric magnetic fields is good, ‘radio’ magnetic fields being weaker by ≈ 20%.

Spectral Characteristics of a Coronal Oscillation with a Period of the Order of 10 MS

AbstractA coronal oscillation with a changing period was observed with the acousto-optic radio spectrometer of Yunnan Observatory. The bandwidth of the oscillation was about 10 MHz. It accompanied radiation which lasted for about 500 ms. A corresponding burst was found on a radio spectrogram obtained at Weissenau, Germany. A 245 MHz burst was reported at the same time and we found a bright spot on the Hα film records of Yunnan Observatory. In this paper, we present a series of spectra of the oscillation at 10 ms intervals. In the spectra the oscillation appears in the form of a 3.5 MHz drift of radio flux with frequency.

Evolution of the spectral fine structure of Jupiter's decametric S-storms

High-resolution spectra of Jupiter's decametric S-storms are studied with an acousto-optical radio spectrograph operating over the frequency range 20–30 MHz. In 1985–1989 20 S-storms were recorded in the Io-B region. There is only a slight average zoning effect of certain types of fine structure in the Io-B region, with sporadic S-bursts occurring most often in the early CML values, and S-trains in the late values. Emissions of type N and its variants occur at lower values of the Io phase than S-emissions and their variants. There is no exact storm-to-storm correspondence, nor any Io-B-centered zones in which the various types of fine structure could be accurately placed. Every storm is different and has a ‘signature’ of its own. An important exception is formed by the wide-range quasi-periodic FDS-S storms that occur at the edge of the Io-B region with Io phase values greater than 80 ‡. These are outstanding storms in which the individual bursts may extend across the full spectral width of 20–30 MHz and be repeated in rapid succession at quasi-periodic rates of 20–40 s−1. It is suggested that these be referred to as type Q storms. It is estimated that only 10% of the S-burst types are recorded so far.

Fine structure in solar microwave bursts

We have designed and constructed a new multi-channel radio spectrograph for the study of short-lived structures in solar microwave bursts. It measured the integrated flux over the whole solar disc in two circular polarizations at 36 frequencies between 4 and 8 GHz, with a time constant of 0.5 ms. We have analyzed all 119 recorded bursts observed in 1981 and 1983. We focused our attention on events with a lifetime of less than 1 s. Fine structure occurs in about 30% of the observed bursts, and can be as rich in detail as in bursts observed at lower frequencies. We found at least four different classes of events. In one event neither bandwidth nor time resolution of the receiver appear to be sufficient to resolve the fine structure. The bulk of the drifts is found to be towards higher frequencies. Periodic flux variations were found in two cases.

Spike observations in flares in China

Variations on short time-scales have been found in solar flares at different wavelengths. Millisecond scale radio spikes are a quickly developing area of solar radio astronomy. The solar radio astronomy group of Beijing Astronomical Observatory (BAO) has found fine structures of microwave bursts with millisecond time-scale at 2840 MHz. In this paper, we briefly summarize the observations. A joint-observation network for observing solar radio bursts with high time resolution has also been established. The equipment in the network covers a frequency domain of more than 10:1, including 1.3, 2.0, 6, 10, 15, 20 cm, and meter wavelengths. In particular, a multi-channel polarimeter with super-fast sampling (10 μs) at 2600 MHz, an intensity interferometer with 1 ms sampling rate at 6 cm wavelength, and an auto-correlation radio spectrograph with 8 ms time constant at 21 cm wavelength are being established. We pay close attention to research on the spike emission features over wide bands, and their relationship to special characteristics in other spectral ranges.

The magnetoionic modes and propagation properties of auroral radio emissions

The different magnetoionic wave modes which accompany the aurora are identified using DE 1 not only by their appearance on satellite radio spectrograms, but also by concurrent measurements of their wave polarization and arrival directions, and by ray‐tracing models of their expected propagation behavior. Of the four possible propagation modes, designated O, X, W, and Z for the ordinary, extraordinary, whistler, and Z modes, respectively, all four are found to occur in the auroral zone, as follows: The most intense, of course, is the well‐known auroral kilometric radiation (AKR), which originates primarily in the X mode near the electron cyclotron frequency, but which is frequently also accompanied by a weaker O‐mode component from the same location. The next most prominent auroral emission is the W‐mode auroral hiss originating from altitudes always well below the DE 1 satellite at frequencies below the local cyclotron frequency. For a particular case which was studied in detail, both of these components were found to originate from approximately the same source field lines near the poleward edge of the auroral plasma cavity, with the latter exhibiting a funnel shape on the radio spectrograms, centered on the source field lines but having different shapes inside and outside the cavity because of the differing plasma densities of these two regions. Equatorward of the cavity and at frequencies above the minimum plasma frequency within the cavity, this upcoming W mode exhibits a shadow zone which is attributed to the blockage of W‐mode propagation at the plasma frequency. The previously reported Z‐mode auroral radiation was also detected, but from sources also below the satellite and at the poleward edge of the cavity, and not from the expected AKR source at the cyclotron frequency. A weaker O‐mode component seems to accompany these emissions also, both within the polar cap poleward of the source and inside the cavity, the latter seemingly being guided upward by the cavity's lower plasma densities. Finally, exactly on the source field lines at the poleward edge of the cavity, there also occasionally seems to be localized Z‐mode emissions extending from the Z‐mode cutoff at quite low frequencies up to and above the plasma frequency.

The source location and beaming of broadband bursty radio emissions from Uranus

Among the radio emissions detected with Voyager 2 at Uranus was a so‐called broadband bursty or “b‐bursty” component at frequencies of 300‐700 kHz, which was first observed✓ just before closest approach and then during subsequent planetary rotations about 1‐6 hours before the more prominent smooth component. Although not obvious for individual rotations because of the bursty nature of these signals, a systematic occurrence pattern for these emissions has been detected in a new composite radio spectrogram where the bursts from different rotations were superimposed. Consisting of symmetrical emissions flanking a quiet region, with their peak frequencies occurring about an hour and a half on either side, this pattern is attributed to Voyager intercepting a hollow emission cone from the nightside of Uranus which tipped past its direction and back during successive planetary rotations. On the assumption that this cone was circular and aligned with the magnetic field at the cyclotron frequency, its source location and beam angles were determined by minimizing the apparent cone‐angle deviations at the highest frequencies. The source was thus found to occur at a southern Uranian latitude of 45°, west longitude 235°, and at a height of 1.43 RU, in excellent agreement with previous studies. The beam angle at the highest frequencies, with respect to the source magnetic field, was found to be about 85°, and that for the entire emission pattern about 80°‐87°.

Short-lived Jovian decametric events observed with an acousto-optical radio spectrograph

S-bursts and other short-lived events in Jupiter's decametric radio spectra are studied with an acousto-optical radio spectrograph (AORS) and a charge-coupled device (CCD) readout. The lifetimes of regular S-bursts are of the order of tens of milliseconds. Lifetimes of S-bursts that occur in quasi-periodic trains are mostly below 20 ms. Short-lived bursts that do not seem to drift in frequency are also observed. Such bursts (nondrift bursts) may occur in S-trains and at the vertex of tilted-V patterns. The vertex bursts have lifetimes comparable to those of the S-train bursts. Preliminary measurements of the risetimes of bursts indicate that a significant proportion of the vertex bursts exhibit risetimes of less than 2 ms.

Beaming of Jupiter's decametric emission

Dynamic spectra of a Jovian non-Io-A storm recorded simultaneously by the Voyager 1 spacecraft and by the Kiiminki radio spectrograph are compared. It seems that the emission beam of the storm co-rotates with the planet and has a sloped leading edge, in accordance with the result of Maeda and Carr (1984).

Radio spectrographs for observing the planet Jupiter

A variety of techniques and instruments have been developed at the University of Florida to study the decametric radio emissions from the planet Jupiter. Several types of radio spectrograph are being utilized, each offering a unique advantage in speed, resolution or frequency coverage. The individual instruments, as well as their integration into a multiresolution system, are discussed. Representative data are presented.

Affirmation of triggered Jovian radio emissions and their attribution to corotating radio lasers

The diverse criticisms of jovian radio triggering, by Desch and Kaiser, are considered invalid primarily because they stemmed from ignoring the original event criteria. Moreover, attributing the observed triggerings to accidental encounters with a constant rotating beam is unjustified, as should have been obvious from the original analysis, since they occurred at the wrong jovian phases. On the other hand, there is nothing wrong with a rotating source also being triggerable, and it is proposed that the triggered emissions originate from just such sources.

A Catalogue of Major Metre-Wavelength Solar Events Recorded by the Dapto and Culgoora Solar Radio Observatories (1961-1981)

We present a catalogue of 668 major metre-wavelength radio events. The data was collected from dynamic spectra (taken at Dapto, N.S.W. until 1966 and at Culgoora, N.S.W. thereafter) by means of a radio spectrograph, which records the intensity of radio emission as a function of both frequency and time. A description of the major types of metre-wavelength phenomena can be found in Wild et al. (1963), Kundu (1965), Wild and Smerd (1972) and Kruger (1979), and details of the radio spectrograph in Sheridan (1963, 1967) and Labrum (1972).