Pulsating radio emissions from the solar corona

Three solar outbursts which show pulsating radio emissions at metric waves (239 MHz) are examined. The behaviour of the single frequency, high-time resolution records and the spectral diagrams seem to indicate that such phenomena are peculiar phases of type IV radiation, perhaps connected with absorptions in the solar corona. The spectral analysis of the low-frequency modulation of the emissions show a very definite spectral line with a period ranging from 1^s.7 to 3^s.1.     

Radioheliograph observations of a pulsating structure associated with a moving type IV burst

Observations of a pulsating structure with the Mark II Nançay Radioheliograph are reported. These fluctuations are found to occur early in the development of a moving type IV burst. It is confirmed that the source of these fluctuations is of small extent and that it is embedded in the moving type IV continuum, plausibly at the top of an expanding arch.The observations suggest that the pulsating structure consists of recurrent enhanced pulses (mean recurrency time 1.7 s) followed by trains of periodic pulses (mean periodicity 0.37 s). The intensity of the enhanced pulses has a damping time of about 5 s. It is shown that previous interpretation of the pulsating structure by Rosenberg (1970) cannot account for the present observations.     

The structure of the middle corona from observations at 80 and 160 MHz

Maps of the brightness distribution of the ‘quiet Sun’ at 80 and 160 MHz reveal the presence of features both brighter and darker than average. The ‘dark’ regions are well correlated with dark regions on UV maps; we deduce that they result from ‘coronal holes’. The ‘bright’ regions are associated with quiescent filaments and not plages or bright regions on microwave or UV maps; we deduce that they result from ‘coronal helmets’.

When coronal holes appear near the centre of the disk we can estimate the density and kinetic temperature in the holes from the radio observations. For a hole observed on 1972 July 20–21, we find T ≈ 0.8 × 10⁶ inside the hole and T ≈ 1.0 × 10⁶ in average regions outside the hole. Inside the hole the density is estimated to be about one-quarter of that in Newkirk's model of the spherically symmetric corona.

Variations in brightness at a fixed height above the limb are generally well correlated with scans at a similar height made with a K-coronameter. Occasional differences may result from streamers protruding beyond the limb from the back of the Sun. These can be seen by the K-coronameter but, because of refraction of the radio rays, not by the radio-heliograph.

     

Culgoora radio and skylab euv observations of emerging magnetic flux in the lower corona

Detailed comparisons of Culgoora 160 MHz radioheliograms of solar noise storms and Skylab EUV spectroheliograms of coronal loop structures are presented. It is concluded that: (1) there is a close association between changes in large-scale magnetic fields in the corona and the onset or cessation of noise storms; (2) these coronal changes result from the emergence of new magnetic flux at the photospheric level; (3) although new magnetic flux at the photospheric level is often accompanied by an increase in flare activity the latter is not directly responsible for noise storm activity; rather the new magnetic flux diffuses slowly outwards through the corona at rates 1–2 km s^–1 and produces noise storms at 160 MHz 1–2 days later; (4) the coronal density above or in large-scale EUV loop systems is sufficiently dense to account for noise storm emission at the fundamental plasma frequency; (5) the scatter in noise storm positions can be accounted for by the appearance and disappearance of individual loops in a system.     

Structure in the solar corona from radio scintillation measurements

Since the 1950s, a wide variety of radio observations based on scattering by electron density fluctuations in the solar wind has provided much of our information on density fluctuations and solar wind speed near the source region of the solar wind. This paper reviews recent progress in the understanding of the nature of these density fluctuations and their relationship to features on the Sun. The results include the first measurements of fine-scale structure within coronal streamers and evidence for structure in solar wind speed in the inner corona.     

Photometry of the outer solar corona from lunar-based observations

Two-dimensional isophotes of the extreme solar corona (r _max 45 R ) have been derived from integrated vidicon pictures taken from the Moon's surface by the unmanned probes Surveyors 6 and 7. These data were calibrated through use of previously published values for the coronal brightness gradient along the ecliptic. The resulting structure of the outer corona is compared to ground-based observations of the innermost corona 1.125 r/R 2.0 made by the High Altitude Observatory K-coronameter. The possible existence of a streamer seen by Surveyor 7 is analyzed over the region 15 r 22.5 R .     

Multiple radio echoes in the solar corona

Echo-type solar radio bursts are associated with preceding short-lived bursts in double events. The peculiar and rather rare decameter echoes are observed with a UTR-2 antenna. The initial narrow-band burst is followed, some 5 to 10 s later, by an echo-like burst at the same frequency. The observational data obtained for decameter echo evens are, on the whole, consistent with the model of a pulsed source emitting radio signals at the plasma-frequency harmonic, which is placed in a non-uniform corona and rotates together with the Sun.Intensity-time profiles of 25 MHz echo bursts are of an unusual shape, featuring an extended leading edge and an abrupt decay at the trailing edge and also showing some fine structures in the form of an additional, weakly pronounced maximum or a step at the final stage of the burst. Time parameters characterizing the profiles are evaluated. The step is delayed with respect to the main pulse at about two times longer than the principal echo maximum. At the same time, the time delays depend essentially on the heliolongitude of the active region and achieve their maximum values at the meridian. The step height does not exceed 0.5 of the echo maximum. At this level, the echo-decay time almost coincides with the initial burst duration but is about 1.7 times less than the echo-rise time. The feature at the echo tail can be interpreted as a result of a repeated reflection of the burst from the source region. The causes and conditions for the formation of multiple echoes are discussed. The extended leading edge of the echo permits us to assume a quasi-radial fibrous structure of the corona, capable of back-scattering the incident radiation.     

On the observation of scattered radio emission from sources in the solar corona

The observations of a brief flaring region between two plages on the eastern limb of the Sun and the subsequent coronal transient are reported for June 16, 1972 by Koomen et al. (1974). Both of these events have unambiguous and closely timed associations with the solar noise bursts observed at 2800 and 2700 MHz and are also accompanied with good X-ray and SID effects but faint subflare (Solar-Geophysical Data). The two frequencies are those monitored at widely separated stations operated by the Astrophysics Branch of the National Research Council at Lake Traverse, Ontario and at Penticton, B.C.     

Gyro-resonance absorption of plasma waves in the corona and the fine structure of solar radio bursts

We consider the important part played by plasma wave absorption at the higher harmonics of the electron gyrofrequency (under conditions of the so-called double plasma resonance) in the solar corona. An explanation is proposed for the fine structure of Type III bursts on the basis of this effect.     

Recent observations of the decimetric radio emission from Jupiter

The position angle of linear polarization and the degree of circular polarization of the decimetric emission show no marked variations with time, although there may be an indication in the position angle data of a secular decrease in the inclination of the magnetic field structure. High resolution observations of the radiation belts show that the asymmetry in the emission found in synthesis maps originates from an extended region. The presence of this features accounts well for the observed variation in the position of the centroid of the emission with rotation. The radio observations suggest that any equatorial displacement of the main magnetic field structure from the center of the planet is less than 0.05 R_J.     

Cyclotron wave instability in the corona and origin of solar radio emission with fine structure

The longitudinal waves (Bernstein modes and plasma waves near the hybrid frequency) in a mixture of equilibrium coronal plasma and a small group of energetic electrons are investigated. The energetic electrons have a nonequilibrium momentum distribution inherent in trapped particles. The frequency dependence of the cyclotron instability increments is studied. Attention is paid to a significant role of the relativistic effects for the cyclotron instability of longitudinal waves. For sufficiently large velocity of nonequilibrium electrons the increments are shown to increase when the hybrid frequency coincides with one of the gyrofrequency harmonics (double plasma resonance). The results obtained are used in Parts II and III to explain tadpoles and zebra-pattern in solar radio bursts.     

The importance of source positions during radio fine structure observations

The measurement of positions and sizes of radio sources in the observations of the fine structure of solar radio bursts is a determining factor for the selection of the radio emission mechanism. The identical parameters describing the radio sources for zebra structures（ZSs） and fiber bursts confirm there is a common mechanism for both structures. It is very important to measure the size of the source in the corona to determine if it is distributed along the height or if it is point-like. In both models of ZSs（the double plasma resonance（DPR） and the whistler model） the source must be distributed along the height, but by contrast to the stationary source in the DPR model, in the whistler model the source should be moving. Moreover, the direction of the space drift of the radio source must correlate with the frequency drift of stripes in the dynamic spectrum. Some models of ZSs require a local source, for example,the models based on the Bernstein modes, or on explosive instability. The selection of the radio emission mechanism for fast broadband pulsations with millisecond duration also depends on the parameters of their radio sources.     

Type III radio bursts in the outer corona

Type III solar radio bursts observed from 3.0 to 0.45 MHz with the ATS-II satellite over the period April–October 1967 have been analyzed to derive two alternative models of active region streamers in the outer solar corona. Assuming that the bursts correspond to radiation near the electron plasma frequency, pressure equilibrium arguments lead to streamer Model I in which the streamer electron temperature derived from collision damping time falls off much more rapidly than in the average corona and the electron density is as much as 25 times the average coronal density at heights of 10 to 50 solar radii (R ). In Model II the streamer electron temperature is assumed to equal the average coronal temperature, giving a density enhancement which decreases from a factor of 10 close to the Sun to less than a factor of two at large distances (> 1/4 AU). When the burst frequency drift is interpreted as resulting from the outward motion of a disturbance that stimulates the radio emission, Model I gives a constant velocity of about 0.35c for the exciting disturbance as it moves to large distances, while with Model II, there is a decrease in the velocity to less than 0.2c beyond 10 R .     

Estimating statistics of sky brightness using radio interferometric observations

Radio interferometers are used to construct high resolution images of the sky at radio frequencies and are the key instruments for accessing the statistical properties of the evolution of neutral hydrogen over cosmic time. Here we use simulated observations of the model sky to assess the efficacy of different estimators of the large-scale structure and power spectrum of the sky brightness distribution. We find that while the large-scale distribution can be reasonably estimated using the reconstructed image from interferometric data, estimates of the power spectrum of the intensity fluctuations calculated from the image are generally biased. This bias is found to be more pronounced for diffuse emission. The visibility based power spectrum estimator, however, gives an unbiased estimate of the true power spectrum. This work demonstrates that for an observation with diffuse emission the reconstructed image can be used to estimate the large-scale distribution of the intensity, while to estimate the power spectrum, visibility based methods should be preferred.With the upcoming experiments aimed at measuring the evolution of the power spectrum of the neutral hydrogen distribution, this is a very important result.     

Results of polarization observations of the outer corona from a jet aircraft

White-light photographs of the solar corona were taken during the March 1970 eclipse from an Air Force NC-135 jet aircraft at an altitude of 36 380 ft. Four photographs each were made for several exposure times varying from 0.1 to 10.0 s. Three each were made with plane polaroid filters whose orientation was varied at 60° intervals. Stabilization of the camera was approximately 10 even during the longest exposures. The corona was recorded to distances beyond 12 R .A preliminary study of per cent polarization as a function of position with respect to the solar disk for a set of 1.0 s exposures shows an inversion in per cent polarization in the region 6 to 8 R -polarization decreasing outward to the region and increasing again beyond it. This inversion is most apparent along the major streamers.Intensities traced outward from the Sun in both polar and equatorial directions are compared with previous observations.Work done under the auspices of the U.S. Atomic Energy Commission.     

The origin of interplanetary sectors from radio observations

The interplanetary sectors have been correlated to observations of solar coronal active centers and condensations in the metric wavelengths. We have found that (1) a sector boundary is always located to the west of a coronal condensation, and (2) the effect of active centers is to displace systematically the boundary toward the east, thus enlarging the sector. A physical interpretation is discussed.     

High resolution observations of the radio emission from luminous YSOs

New, high-resolution 1.66 GHz MERLIN maps of Cep A2 and LkH101 are presented and discussed.     

Prominence–corona transition region plasma diagnostics from SOHO observations

New results concerning prominence observations and in particular the prominence–corona transition region (PCTR) are presented. In order to cover a temperature range from 2 × 10⁴ to 7 × 10⁵ K, several emission lines in many different ionization states were observed with SUMER and CDS on board SOHO. EM and DEM were measured through the whole PCTR. We compared the prominence DEM with the DEM from other solar structures (active region, coronal hole and the chromosphere–corona transition region (CCTR)). We notice a displacement of the prominence DEM minimum towards lower temperatures with respect to the minimum of the other structures. Electron density and pressure diagnostics have been made from the observed C III lines. Local electron density and pressure for T ∼ 7 × 10⁴ K are respectively log N _e = 9.30_−0.34 ^+0.30 and 0.0405_−0.014 ^+0.012. Extrapolations over the entire PCTR temperature range are in good agreement with previous SOHO results (Madjarska et al., 1999). We also provide values of electron density and pressure in two different regions of the prominence (center and edge). The Doppler velocity in the PCTR shows a trend to increase with temperature (at least up to 30 km s -1 at T ∼ 7 × 10⁴ K), an indication of important mass flows. A simple morphological model is proposed from density and motion diagnostics. If the prominence is taken as a magnetic flux tube, one can derive an opening of the field lines with increasing temperature. If the prominence is represented as a collection of threads, their number increases with temperature from 20 to 800. Derived filling factors can reach values as low as 10⁻³ for a layer thickness of the order of 5000 km. The variation of non-thermal velocities is determined for the first time, in the temperature range from 2 × 10⁴ to 7 × 10⁵ K. The quite clear similarity with the CCTR non-thermal velocities would indicate that heating mechanisms in the PCTR could be the same as in the CCTR (wave propagation, turbulence MHD).     

High-frequency Faraday rotation observations of the solar corona

This thesis, presented on January 31, 2007 under the supervision of Professor Christopher T. Russell, discusses the solar coronal magnetic field observations that can be obtained using the phenomenon of Faraday rotation. It was defended in the Department of Earth and Space Sciences at the University of California, Los Angeles (595 Charles E. Young, Dr. East, Los Angeles, CA 90095). A resume can be found at http://acs-consulting.com/.