High spatial resolution microwave observations of the Sun

Over the past decade two large arrays — the Westerbork Synthesis Radio Telescope (WSRT) and the Very Large Array (VLA) built primarily for sidereal radio astronomy have been used for solar radio astronomical studies with spatial resolution of a few seconds of arc. In this review, we discuss some results obtained at Maryland using these instruments.The quiet Sun observations made with the WSRT have premitted us to produce synthesized maps of supergranulation network at 6 cm wavelength. The brightness temperatures of typical network elements and cells are respectively 2.5 × 10⁴ K and 1.5 × 10⁴ K; thus the contrast is 1.71 which compares with 1.31 for Ca ⁺ K and 20 for L networks. Limb profiles in both equatorial and polar directions have been obtained; limb brightening is observed at both west and south limbs, peak limb temperature being about 40% higher than disk temperature. We have produced synthesized maps of disk filaments which correspond well to H disk filaments and regions of reduced emission in He i 10 830 Å spectroheliograms. Using the WSRT synthesized maps at 6 cm, we have compared the structure of a sunspot associated source with model computations. Using a new method of analysis we have been able to map the vertical as well as the horizontal component of the sunspot magnetic field at specific locations in the low corona. Using the VLA, we have mapped coronal loops at 20 cm; the radio emission is attributed to bremsstrahlung near the loop footpoints whereas gyroresonance process dominates near the loop top. Using the VLA, we have carried out simultaneous observations of a microwave burst at 2 and 6 cm. The 6 cm burst source is apparently located near the top of a flaring loop, while the 2 cm emission originates from the loop footpoints. The 6 cm emission is attributed to gyrosynchrotron radiation of thermal electons in the bulk heated plasma at 4 × 10⁷ K, while the 2 cm emission is due to nonthermal particles released and accelerated during the flare process. A DC electric field flare model appears to explain the observed delay between the peaks at the two wavelengths. From the delay, the strength of the electric field in the flaring region is estimated.     

Multiple moving magnetic structures in the solar corona

We report the study of moving magnetic structures inferred from the observations of a moving type IV event with multiple sources. The ejection contains at least two moving radio emitting loops with different relative inclinations. The radio loops are located above multiple H flare loops in an active region near the limb. We investigate the relationship between the two systems of loops. The spatial, temporal and geometrical associations between the radio emission and near surface activities suggest a scenario similar to coronal mass ejection (CME) events, although no CME observations exist for the present event. From the observed characteristics, we find that the radio emission can be interpreted as Razin suppressed optically thin gyrosynchrotron emission from nonthermal particles of energy 100, keV and density 10²–10⁵ cm^–3 in a magnetic field 2 G.     

Geology,geochemistry and geochronology of the Archaean Peninsular Gneiss around Gorur,Hassan District,Karnataka, India

The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking gneissosity parallel to the axial planes of a set of isoclinal folds (DhF₁). These have been over printed by near-coaxial open folding (DhF₁₂) and non-coaxial upright folding on almost N-S trend (DhF₂). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically with DhF₁ deformation. The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated REE patterns (Ce_N/Yb_N < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7). A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30 Ma with Sr _i of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr _i = 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal residence history of less than a 100 Ma.     

High resolution observations of solar bursts at 3.7 and 11.1 cm wavelengths

Four bursts were observed on August 9, 1973 with the NRAO 3-element interferometer at 3.7 and 11.1 cm. By using a simple source model we have calculated the temperature, flux, size and position of the small scale components of the bursts as a function of time. We obtained peak temperatures around 10⁷ K for the components with size of about 10 at 11.1 cm and 3 at 3.7cm. The peak flux of these components lies between 4 and 40 % of the total peak flux of the bursts. Two of the bursts were found to be right circularly polarized. There is evidence that the location of these two bursts does not coincide with the location of the brightest point of the associated active regions. The existence of burst structures with temperatures of the order of 10⁷K indicates that at least part of the radiation in these bursts is generated by a non-thermal mechanism.NAS-NRC Research Associate at Goddard Space Flight Center on Sabbatical Leave from the University of Maryland.

---

     

A new microwave/X-ray diagnostic for the thermal phase of solar flares

We have observed 10 solar bursts during the thermal phase using the Haystack radio telescope at 22 GHz. We show that these high frequency flux observations, when compared with soft X-ray band fluxes, give useful information about the temperature profile in the flare loops. The microwave and X-ray band fluxes provide determinations of the maximum loop temperature, the total emission measure, and the index of the differential emission measure (q(T)/T = cT^–1). The special case of an isothermal loop ( = ) has been considered previously by Thomas et al. (1985), and we confirm their diagnostic calculations for the GOES X-ray bands, but find that the flare loops we observed departed significantly from the isothermal regime. Our results ( = 1–3.5) imply that, during the late phases of flares, condensation cooling ( 3.5) competes with radiative cooling ( 1.5). Further, our results appear to be in good agreement with previous deductions from XUV rocket spectra ( 2–3).     

Wave forces on a seawater intake caisson

The wave force on a seawater intake structure consisting of a perforated square caisson of 400 mm×400 mm size encircling a vertical suction pipe of 160-mm diameter is investigated using physical model studies. The porosity of caisson was varied from 1.6 to 16.9%. Regular and random waves of wide range of heights and periods were used. It is found that the force ratio (ratio of the force on perforated caisson to the force on caisson with zero percent porosity) reduces to an extent of up to 60% with increase in porosity of the caisson from 1.6 to 16.9%. The force ratio was found to increase with increase in relative wave height and reduces with increase in relative width. Multiple regression analysis of the measured data points was carried out and predictive equations for wave force ratios are obtained both for regular and random waves. The results of this investigation can be used in the hydrodynamic design of perforated caissons, which are widely used as seawater intake structures.     

Meterwave observations of a coronal hole

We present meterwave maps showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in He i l10830 spectroheliograms.Using the model of coronal holes by Dulk et al. (1977) we derive the electron density from the radio observations of the brightness temperature. The discrepancy between the density value derived from the Skylab EUV data and that computed from our radio data is even larger than in Dulk et al. 's comparison at similar and higher frequencies.     

The spatial distribution of 6 centimeter gyroresonance emission from a flaring X-ray loop

We compare simultaneous high resolution soft X-ray and 6 cm images of the decay phase of an M3 X-ray flare in Hale Region 16413. The photographic X-ray images were obtained on an AS & E sounding rocket flown 7 November, 1979, and the 6 cm observations were made with the VLA. The X-ray images were converted to arrays of line-of-sight emission integrals and average temperature throughout the region. The X-ray flare structure consisted of a large loop system of length 1.3 arc min and average temperature 8 × 10⁶ K. The peak 6 cm emission appeared to come from a region below the X-ray loop. The predicted 6 cm flux due to thermal bremsstrahlung calculated on the basis of the X-ray parameters along the loop was about an order of magnitude less than observed. We model the loop geometry to examine the expected gyroresonance absorption along the loop. We find that thermal gyroresonance emission requiring rather large azimuthal or radial field components, or nonthermal gyrosynchrotron emission involving continual acceleration of electrons can explain the observations. However, we cannot choose between these possibilities because of our poor knowledge of the loop magnetic field.     

Soil erosion estimation of Bhandara region of Maharashtra,India, by integrated use of RUSLE,remote sensing,and GIS

Natural Hazards - The agricultural land of the whole world is deteriorating due to the loss of top fertile soil reducing agricultural productivity and groundwater availability. Mainly, natural...