Quasi-periodic solar radio pulsations at decimetric wavelengths

Observations are reported of radio-pulsations at dm-wavelengths. The pulsations are quasiperiodic with a period of 0.5 s, they have a bandwidth > 300 MHz and show up to a 50% enhancement of the underlying type IV continuum.     

The solar brightness temperature at millimeter wavelengths

The authors have previously discussed an improved method for obtaining the absolute solar brightness temperature using the new Moon as a calibration source. New measurements of the Sun-to-new Moon ratio at three frequencies near 36 GHz ( = 8 mm) and also at two frequencies near 93 GHz ( = 3 mm) are reported. The slopes of the solar brightness temperature spectrum based on these ratios are then discussed. The absolute solar brightness spectrum derived from all current available measurements is also presented and discussed.     

Improved solar wavelengths between 7780 and 7925 Å

By utilizing the known rotational constants for the 7820 and 7883 Å CO₂ bands, we are able to derive substantially improved wavelengths for solar lines in the 7780–7925 Å region.     

Interferometric studies of spectral lines in the solar corona

A photographic Fabry-Perot interferometer was used to measure the breadth and wavelength of the Fexiv spectral line at 5303 Å in the solar corona, during the eclipse of 7 March, 1970, in Mexico. The observations were consistent with a possible large-scale vortex structure in a bright streamer, in which cooler gases revolved about a hotter core.Work supported by the United States National Science Foundation and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant AF-AFOSR-68-1496.     

Interferometric phase velocity measurements in the auroral electrojet

A double-probe electric field detector and two spatially separated fixed-bias Langmuir probes were flown on a Taurus-Tomahawk sounding rocket launched from Poker Flat Research Range in March 1982. Interesting wave data have been obtained from about 10s of the downleg portion of the flight during which the rocket passed through the auroral electrojet. Here the electric field receiver and both density fluctuation (δn/n) receivers responded to a broad band of turbulence centered at 105 km altitude and at frequencies generally below 4 kHz. Closer examination of the two (δn/n) turbulent waveforms reveals that they are correlated, and from the phase difference between the two signals, the phase velocity of the waves in the rocket reference frame is inferred. The magnitude and direction of the observed phase velocity are consistent either with waves which travel at the ion sound speed (C_s) or with waves which travel at the electron drift velocity. The observed phase velocity varies by about 50% over a 5 km altitude range—an effect which probably results from shear in the zonal neutral wind, although unfortunately no simultaneous neutral wind measurements exist to confirm this.     

Positions and motions of solar bursts at decameter wavelengths

The positions and motions of solar bursts in the range 20 to 60 MHz have been measured by the means of a sweep-frequency grating interferometer with angular resolution of 5 arc at 60 MHz decreasing to 15 arc at 20 MHz. The positional characteristics of the decameter wavelength bursts are discussed in terms of the commonly accepted theories of the origin of radio bursts from plasma and synchrotron radiations.     

Relative timing of solar flares observed at different wavelengths

The timing of 503 solar flares observed simultaneously in hard X-rays, soft X-rays and H is analyzed. We investigated the start and the peak time differences in different wavelengths, as well as the differences between the end of the hard X-ray emission and the maximum of the soft X-ray and H emission. In more than 90% of the analyzed events, a thermal pre-heating seen in soft X-rays is present prior to the impulsive flare phase. On average, the soft X-ray emission starts 3 min before the hard X-ray and the H emission. No correlation between the duration of the pre-heating phase and the importance of the subsequent flare is found. Furthermore, the duration of the pre-heating phase does not differ for impulsive and gradual flares. For at least half of the events, the end of the non-thermal emission coincides well with the maximum of the thermal emission, consistent with the beam-driven evaporation model. On the other hand, for 25% of the events there is strong evidence for prolonged evaporation beyond the end of the hard X-rays. For these events, the presence of an additional energy transport mechanism, most probably thermal conduction, seems to play an important role.     

Extension of the solar limb at sub-millimeter and millimeter wavelengths

Solar limb scanning at 5 wavelengths from 0.35 to 2 mm on the JCMT has revealed significant limb extension which increases rapidly with wavelength, in agreement with other measurements. This appears to be related to the increasing opacity of overlapping spicules which appear to become optically thick at about 1mm.     

New observations of solar noise storm radiation at decameter wavelengths

We report multifrequency observations of storm continuum and other radio bursts. Based on their positional study and their correlation with other coronal and photospheric features, we deduce that the storm source is located in the magnetic field lines lying above a single bipolar active region. Energetic electrons trapped in the magnetic structures above the spots must be responsible for the storm radiation. We show that spontaneous emission of Langmuir waves by anisotropic distributions can explain both storm continuum and bursts self-consistently. Whenever the collisional damping ( _c ) is more than the growth (- _A ), there is a steady emission responsible for the continuum, and whenever _c = - _A (which may be satisfied randomly) there is a sudden jump in T _b giving rise to bursts. The number density of energetic particles required to explain the storm continuum at 73.8, 50, and 30.9 MHz frequencies is estimated to lie in the limits n _b /n _e 10^–10–10^–9 in the context of the present observations. The brightness spectrum of the storm continuum is computed and compared with observations.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

A new high-speed solar spectrograph for meter and decameter wavelengths

A new high-speed digital solar radio spectrograph has been designed and is being operated at the Clark Lake Radio Observatory in California. The spectrograph design attempts to optimize sensitivity, dynamic range, and frequency-time resolution while utilizing modern high-speed computer data-handling techniques. The system is described and initial data observations are presented.     

Dual pulsations in solar radio bursts at short centimeter wavelengths

It is well known that the oscillating MHD waves drive periodic variations in the magnetic field. But how the MHD waves can be triggered in the flaring loops is not yet well known. It seems to us that this problem should be connected with the physical processes occurring in the flare loop during a solar flare. A peculiar solar flare event at 04:00–04:51 UT on May 23, 1990 was observed simultaneously with time resolutions 1 s and 10 ms by Nanjing University Observatory and Beijing Normal University Observatory, which are about 1000 km apart, at 3.2 cm and 2 cm wavelengths, respectively. Two kinds of pulsations with quasi-periods 1.5 s and 40 s were found in radio bursts at the two short centimeter waves; however, the shorter quasi-periodic pulsations were superimposed upon the longer ones. From the data analysis of the above-mentioned quasi-periodic pulsations and associated phenomena in radio and soft X-ray emissions during this flare event published in Solar Geophysical Data (SGD), the authors suggest that the sudden increase in plasma pressure inside (or underlying) the flare kernel due to the upward moving chromospheric evaporated gas, which is caused by the explosive collision heating of strong non-thermal electrons injected downwards from the microwave burst source, plays the important role of triggering agents for MHD oscillations (fast magneto-acoustic mode and Alfvén mode) of the flare loop. These physical processes occurring in the flare loop during the impulsive phase of the solar flare may be used to account for the origin and observational characteristics of quasi-periodic pulsations in solar radio bursts at the two short centimeter wavelengths during the flare event of 1990 May 23. In addition, the average physical parameters N, T, B inside or underlying the flare kernel can be also evaluated.     

A model for solar oscillations at cm and mm wavelengths

It is suggested that large scale oscillations of the Sun give rise to the periodic fluctuations observed in the quiet Sun at radio wavelengths. These fluctuations would be most readily observed from regions where there is a large gradient of electron temperature with electron density, and are thus more readily observed at cm wavelengths than at millimetre wavelengths.     

Decay time of type III solar bursts observed at kilometric wavelengths

Type III bursts were observed between 3.5 MHz and 50 kHz by the University of Michigan radio astronomy experiment aboard the OGO-5 satellite.Decay times were measured and then combined with published data ranging up to about 200 MHz. The observed decay times increase with decreasing frequency but at a rate considerably slower than that expected from electron-proton Coulomb collisions. At 50 kHz values differ by about a factor of 100. Using Hartle and Sturrock's solar wind model, Coulomb collisional frequencies were computed and compared with the apparent collisional frequencies deduced from the observations. It was found that the ratio of observed to computed values varies with heliocentric distance according to an inverse 0.71 power. This is similar to an ad hoc function used by Wolff, Brandt, and Southwick to increase the electron-proton collisional energy exchange and make the solar wind theory agree with the measurements of electron and proton temperature near the Earth. These results may provide a clue about the nature of the non-collisional plasma wave damping process responsible for the short duration of type III bursts.     

The spectrum and position of solar noise storms at decameter wavelengths

Decametric storm radiation during the period July–August 1970 has been observed simultaneously with a high sensitivity spectrograph at Arecibo Observatory and with the log-periodic, swept-frequency array of the Clark Lake Radio Observatory. The observations complement each other; different types of fine structure emissions can be easily identified on the spectrograph records and their position can be determined from the swept-frequency recordings. We study the relative positions of the different emissions which have been observed during the storms. Four distinct sources appeared to be present. The continuum emission, the type I bursts and the flare-related type III's were all emitted at different locations. The storm type III bursts, type IIIb's and drift pairs overlapped in position, but appeared at different locations than the previously mentioned sources.On leave of absence from Instituto Argentino de Radioastronomia, Argentina.     

The investigation of the solar wind transsonic region at meter wavelengths

Radio astronomy experiments designed to probe the interplanetary plasma close to the Sun have been carried out at the wavelength = 2.92 m using the occultation method. The experiments are based on a modification of the occultation method by which the sources, in this case quasars, are used to measure the radial dependence of the apparent source size, the scattering angle (R). The radial dependence of this source size (R) reveals that an anomalous enhancement in the scattering appears at radial distances from about 16 to 30 R , which is associated with the solar wind transsonic region. It is shown by analysis of the theoretical equations that the radial profiles of both the source size (R) and the scintillation index m(R) are consistent in the near solar interplanetary medium (R 40 R ). Combining these two independent applications of the occultation method extends the range of the investigation and provides a powerful new diagnostic of the solar wind transsonic region.     

Spectrum of slowly varying component of solar radio emission on millimeter wavelengths

The spectrum of the S-component of solar radio emission has been investigated at 4 GHz, 17 GHz, 35 GHz, 70 GHz, and 94 GHz. The spectrum for a spot group which appeared late in March 1966 (McMath plage No. 8223), seems to be flat at the frequencies above about 35 GHz. This implies that the emission is due to pure free-free emission at the frequencies above 35 GHz.     

Analysis of the impulsive phase of a solar flare at submillimeter wavelengths

We present a report on the strong X5.3 solar flare which occurred on 25 August 2001, producing high-level γ-ray activity, nuclear lines and a dramatic long-duration white-light continuum. The bulk of millimeter radio fluxes reached a peak of ∼100 000 solar flux units at 89.4 GHz, and a few thousands of solar flux units were detected in the submillimeter range during the impulsive phase. In this paper we focus on and discuss (i) the implications inferred from high frequency radio observations during the impulsive phase; (ii) the dynamics of the low corona active region during the impulsive phase. In particular we found that 4–5 × 10³⁶ accelerated (>20 keV) electrons s⁻¹ radiating in a 1000–1100 G region, are needed to explain the millimeter to submillimeter-wave emissions. We present evidence that the magnetic field in the active region was very dynamic, and that strong non-thermal processes were triggered by the appearance of new, compact, low-lying (few thousand kilometers) loop systems, suggesting the acceleration site(s) were also located in the low solar atmosphere.     

Radio interferometric observations of solar bursts at 3.7 and 11.1 cm wavelengths

Several solar active regions were observed during the 1976 vernal equinox with the 3-element interferometer of the National Radio Astronomy Observatory. The element spacings for these observations were 600, 2100, and 2700 m, resulting in maximum angular resolutions of about 3 arc sec at 3.7 cm and 8.5 arc sec at 11.1 cm. We fitted the fringe visibility for each baseline pair as a function of projected baseline with a single gaussian component, and calculated the extrapolated flux at zero baseline, F ₀, the FWHM source size, a, and the peak brightness temperature T _b. We present physical parameters derived for the observed bursts.