F. I. Shimabukuro

On the temperature and emission measure of thermal radio bursts

Once the area of a thermal burst region has been determined, it is possible to obtain the temperature and emission measure of the burst by examination of the flux spectrum. Such determinations have been made for three events. Temperatures in the range 2 to 6 × 106K and emission measures in the area of 2.6 to 5.4 × 1049 cm−3 are consistent with radio observations. Electron densities are estimated to be of the order of 1 to 5 ]x 1010 electrons cm−3.

The observation of 3.3-mm bursts and their correlation with soft X-ray bursts

Observations of 3.3-mm bursts show that in most cases these bursts have slower rise times and are longer lived than the impulsive centimeter bursts. There is a good temporal correlation between the 3.3-mm and soft X-ray bursts, indicating that these bursts have their origin in the same thermal source mechanism, although these emissions may not arise from the same electrons.

Attenuation and emission of the atmosphere at 3.3 mm

Four empirical relationships are presented between the 3.3-mm attenuation determined from observations of the sun and 1) radiosonde measurements of the total amount of precipitable water in the atmosphere, 2) surface measurements of absolute humidity, 3) IR spectral hygrometer observations of the sun, and 4) observations of differential atmospheric emission at 3.3 mm. Fluctuations in atmospheric emission at 3.3 mm during good weather do not put any limit on the sensitivity of a dual-beam observing system using a receiver whose rms noise fluctuations in an output bandwidth of 0.25 Hz are \approx0.5\deg K.

Propagation through the atmosphere at a wavelength of 3.3mm

The general propagation characteristics of 3.3-mm waves through the atmosphere are determined. The measured absorption of the atmosphere is in fair agreement with the Van Vleck-Weisskopf theory. An approximation for the zenith attenuation in decibels through the atmosphere is 0.28+0.31w , where w is the precipitable water in centimeters. The effects of varying attenuation on the determination of the brightness temperature of extraterrestrial sources is considered. The usual radio refraction expression appears adequate for 3.3-mm waves for zenith angles less than 70 degrees.

On the source of the slowly varying component at centimeter and millimeter wavelenghts

The general features of the slowly varying component at centimeter and millimeter wavelengths are explained by magneto-ionic thermal emission. A model of an active region is constructed in which the electron temperature and density profile is based on recent EUV measurements, and the current-free magnetic field configuration is derived from a longitudinal magnetogram and scalar potential theory. In the model, the contributions of the reflected component of the inward extraordinary wave is important in determining the characteristic features of the radio flux and polarization. Emission by the mechanism of resonance absorption does not appear to be a significant factor in this model.

A comparison of 3.3-mm bursts and Hα emission during flares

Radio bursts observed at a wavelength of 3.3 mm are always associated with flares. The enhancement in the millimeter emission is strongly correlated with some optical event that is associated with the flare. Emissions in the bursts observed thus far, with the possible exception of those associated with proton flares, have not been large increases over quiescent readings and can be explained by a thermal mechanism.

3.3 Millimeter limb brightening measurements during the 30 June 1973 total solar eclipse

Solar limb brightening measurements at a wavelength of 3.3 mm were made during the 30 June 1973 total solar eclipse from a site at Lake Rudolf, Kenya. The results show that at this wavelength there is a limb brightening of about 20%, occurring within one half arc min of the limb.

The 3.3-mm brightness distribution of the quiet sun

Center-to-limb brightness distribution measurements of the quiet Sun at a wavelength of 3.3 mm show that there is a slight limb brightening at this wavelength. Within the measurement accuracy of the system used, the limb brightening function is only radially dependent. At 3.3 mm, the measurements are consistent with a solar brightness curve that is flat to about r = 0.8 with a rapid increase to a peak value of about 1.3 at the limb. The results show that most of the central disk 3.3-mm emission comes from a thin layer of relatively constant temperature about 1500–3500 km above the photosphere.

Design, development, and initial measurements of a 1.4-mm radiometric system

The development of a 1.4-mm heterodyne radiometric system and the initial measurements taken with it are described. The \Delta T of the system at the antenna terminals was approximately 18.5\deg K for a 0.25-Hz post-correlation noise bandwidth. Measurements show that the attenuation through the atmosphere at this wavelength is primarily due to water vapor, and an estimate of the zenith attenuation is given by A (dB)= 2.8w , where w is the precipitable water in centimeters. Measurements of the antenna half-power beamwidths using the sun as a source show that at 1.4 mm the atmospheric turbulence effects are not appreciable for observations through the atmosphere at zenith angles less than 45\deg with a 15-foot antenna.

Observations of the 1.4mm brightness distribution of the sun

Observations of the 1.4 mm center to limb solar brightness distribution show that there is little, if any, limb brightening at this wavelength. Knowledge of the antenna radiation pattern is vital in interpreting the radio measurements.