Anisotropic Scattering of the Radio Emission of the Pulsar B0833–45 in the Vela Supernova Remnant

Very Long Baseline Interferometry (VLBI) observations of the pulsar B0833–45 have been carried out as part of the scientific program of the RadioAstron mission. Ground support was provided by the Long Baseline Array, which includes radio telescopes in Australia and other countries in the southern hemisphere. The VLBI observations of the pulsar are analyzed in order to derive the parameters characterizing the scattering of the pulsar radio emission: the angular size of the scattering disk, the spatial scale of the diffraction pattern, the drift velocity of this pattern relative to the observer, the pulse scattering time scale, and the characteristic scintillation time and frequency scales, as well as the index of the electrondensity fluctuation spectrum. Comparison of these values with the predictions of the theory of scattering on a thin screen enables the determination of the position of the effective screen along the line of sight. Estimates made using various methods give distances to the screen from the observer of 0.79 to 0.87 times the total distance to the pulsar. Although the position of the screen is beyond the boundary of the Vela supernova remnant, this object may play the dominant role in the scattering. The scattering disk is an ellipse with a 2:1 axis ratio and with the inferred position angle of the major axis being ≈ 50°, based on the changes in the visibility-function amplitude for various orientations of the projected baseline. This conclusion is supported by the shape of the visibility-function amplitude as a function of the delay.

     

The crustal structure of the western Arabian platform from the spectral analysis of long-period P-wave amplitude ratios

The crustal structure of the western Arabian platform is derived using the spectral analysis of long-period P-wave amplitude ratios. The ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations, crustal velocities, densities and the angle of emergence at the lower crust and upper mantle interface. Eleven well-defined earthquakes recorded at the long-period RYD station during the period from 1985 to 1994 were selected for analysis based on the following criteria: focal depths with a range between 7 and 89 km, body-wave magnitudes greater than 4.7, epicentral distances with a range from 8.8° to 26.5°, and back azimuthal coverage from 196° to 340°. Spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical P-wave motion obtained using the Thomson–Haskell matrix formulation for horizontally layered crustal models. The selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross-correlation coefficient with the observed transfer function ratio. By comparing the spectral peak positions of the observed and theoretical values, the thickness and velocity can be resolved within 3 km and 1 km/s, respectively, of the observed values. The spectral analysis of long-period P-waves can detect a thin layer near the surface of about 1.6 km thick and a velocity contrast of about 10% with that of the underlying layer. A strong velocity gradient of about 0.05 km/s per km was found in the upper crust and 0.02 km/s per km in the lower crust. The derived crustal model is not unique due to the theoretical assumptions (horizontal layering, constant densities and velocities in each layer), quality of the data and complexities of the crustal structure. The crustal model suggests that the crust consists of five distinct layers. The upper crustal layer has a P-wave velocity of about 5.6 km/s and is about 1.6 km thick. The second layer has a velocity of about 6.2 km/s and is 10.2 km thick. The third layer shows a velocity of 6.6 km/s and is 6.8 km thick. The fourth layer has a velocity of about 6.8 km/s and is 12.3 km thick. The lower crustal layer has a velocity of about 7.5 km/s and is 9.3 km thick. The Mohorovicic discontinuity beneath the western Arabian platform indicates a velocity of 8.2 km/s of the upper mantle and 42 km depth.     

Twenty-year-long monitoring of the H2O maser S269

Results of observations of the H₂O maser in S269 carried out from October 1980 to February 2001 on the 22-m telescope (RT-22) of the Pushchino Radio Astronomy Observatory are presented. During the monitoring of S269, variability of the integrated flux of the maser emission with a cyclic character and an average period of 5.7 years was observed. This may be connected with cyclic activity of the central star during its formation. Emission at radial velocities of 4–7 km/s was detected. Thus, the H₂O maser emission in S269 extends from 4 to 22 km/s, and is concentrated in three radial-velocity intervals: 4–7, 11–14, and 14–22 km/s. In some time intervals, the main group of emission features (14–22 km/s) had a triplet structure. The central velocity of the total spectrum is close to the velocity of the CO molecular cloud and HII region, differing from it by an amount that is within the probable dispersion of the turbulent gas velocities in the core of the CO molecular cloud. A radial-velocity drift of the component at V _LSR≈20 km/s with a period of ≈26 years has been detected. This drift is likely due to turbulent (vortical) motions of material.     

Long-term monitoring and interpretation of flares in the H<Subscript>2</Subscript>O maser emission of IRAS 16293–2422

The results of a study of the H₂O and OH maser emission from the cool IR source IRAS 16293?2422 are presented. The observations analyzed were obtained in H₂O lines with the 22-m telescope of the Pushchino Radio Astronomy Observatory during 1999–2015 and in OH lines with the Nanc¸ ay radio telescope (France). A large number of very strong flares of the H₂O maser were detected, reaching fluxes of tens of thousands of Jansky. Individual features can form organized structures resembling chains ～2 AU in length with a radial-velocity gradient along them. The observed drift of the H₂O emission (2003–2004) in space and velocity (from 4.3 to 5.3 km/s) is not due solely to proper motion of the features. The other origin of the drift is a drift of the emission maximum during a flare as the shock consecutively excites spatially separated features in the structure in the form of a chain. The OH-line observations at 18 cm show that the emission remains unpolarized and thermal, with a line width of 0.7 km/s, which corresponds to a cloud temperature of ～30 K.     

Upper mantle structure from the Trans‐Australia Seismic Survey (TASS) and other seismic refraction data

The recordings made during 1972 from large explosions at Kunanalling (W.A.), Mount Fitton (S.A.). and Bass Strait have added considerably to seismic refraction data measured over distances of 1000 km in continental Australia. Taken together with data from the 1956 Maralinga atomic bomb and 1970–71 Ord Dam explosions they show the existence of a refractor with apparent P‐wave velocity in the range 8.26–8.29 km/s, which is interpreted as the Moho under shield regions, at a depth of 34 km under Kalgoorlie and deepening eastward to 39 km under Maralinga. In northern South Australia and farther north and east this refractor is evident as a sub‐Moho refractor at a depth of about 60 km; the Moho refractor is also evident, with an apparent P velocity of 8.04 ± 0.04 km/s at a depth of 40 km. Two computer models (TASS‐1a and 2a) match the observed data. The subsequent arrivals recorded are consistent with the velocity of 8.53 km/s in a refractor at 165 km depth interpreted from the Ord Dam; there is little conclusive evidence for a low‐velocity zone above this depth.     

Monitoring of the H2O maser W31(2) in 1981–2003

A catalog of water-vapor maser spectra at 1.35 cm for the sourceW31(2), which is associated with an active star-forming region, is presented. The observations were carried out in 1981–2003 on the 22-m antenna of the Pushchino Radio Astronomy Observatory with a spectral resolution of 0.101 km/s. The mean interval between observations was about 1.5 months. The total velocity range in which emission was observed during the monitoring is from ?14 to +14 km/s. The spectrum is strongly variable and contains a large number of emission features. Two strong flares with an interval between their emission maxima (integrated flux) of about 12 years (1985–1986 and 1998–1999) were observed, as well as fast variations on a timescale of 0.5–2 years with amplitudes of up to 600 Jy km/s. No long-period component of the variations was found. A drift of the velocity centroid has been detected; it is well approximated by a third-power polynomial corresponding to a period of about 31–33 years. The two strong flares fall on different phases of this curve: the first (1985–1986) is located near the minimum, while the second (1998–1999) is at the maximum. The observed character of the variability of the emission is well explained by the existing model for the region of G10.6-0.4. The drift of the velocity centroid is probably associated with the nonstationary accretion of material onto an HII region formed by a cluster of OB stars.     

Pulsating evershed flows and propagating waves in a sunspot

A comparative analysis of oscillatory spectra based on 66 time series for 14 active regions observed in 2001 shows that, although the chromospheric and photospheric oscillations in the Evershed flow zone possess many common features, there is no firm evidence that the direct and inverse flows have the same physical origin. The interactions between the various oscillation modes and stationary flows results in a complex pattern of wave motions in a sunspot. We studied the Doppler-velocity variations in the sunspot NOAA 0051 during its motion over the disk. The spatial-temporal distribution of the line-of-sight velocity in the chromospheric umbra displays a chevron structure, clearly indicating the presence of propagating waves. These waves move from the center of the umbra to outer regions with a phase speed of 45–60 km/s, a period of 2.8 min, and a measured Doppler speed of 2 km/s. The amplitude of these oscillations decreases abruptly at the boundary between the umbra and penumbra, and the observed waves are not directly related to propagating penumbral waves. Furthermore, the observed pattern of the photospheric velocities shows periodic motions (with a period of 5 min) directed from the inner boundary of the penumbra and superpenumbra toward the line of maximum Evershed velocity.     

The formation of radio-pulsar spectra

It is shown that scattering of electromagnetic waves by Langmuir waves taking into account the electrical drift motion of the particles is the most efficient nonlinear process contributing to a radio pulsar’s spectrum. If an inertial interval exists, stationary spectra with spectral indices of ?1.5 or ?1 can be formed, depending on the wave excitation mechanism. The obtained spectra are in satisfactory agreement with observational data.     

The crustal structure of the axis of the Great Valley,California, from seismic refraction measurements

In 1982 the U.S. Geological Survey collected six seismic refraction profiles in the Great Valley of California: three axial profiles with a maximum shot-to-receiver offset of 160 km, and three shorter profiles perpendicular to the valley axis. This paper presents the results of two-dimensional raytracing and synthetic seismogram modeling of the central axial profile. The crust of the central Great Valley is laterally heterogeneous along its axis, but generally consists of a sedimentary section overlying distinct upper, middle, and lower crustal units. The sedimentary rocks are 3–5 km thick along the profile, with velocities increasing with depth from 1.6 to 4.0 km/s. The basement (upper crust) consists of four units:

• 1.(1) a 1.0–1.5 km thick layer of velocity 5.4–5.8 km/s,
• 2.(2) a 3–4 km thick layer of velocity 6.0–6.3 km/s,
• 3.(3) a 1.5–3.0 km thick layer of velocity 6.5–6.6 km/s, and
• 4.(4) a laterally discontinuous, 1.5 km thick layer of velocity 6.8–7.0 km/s. The mid-crust lies at 11–14 km depth, is 5–8 km thick, and has a velocity of 6.6–6.7 km/s. On the northwest side of our profile the mid-crust is a low-velocity zone beneath the 6.8–7.0 km/s lid. The lower crust lies at 16–19 km depth, is 7–13 km thick, and has a velocity of 6.9–7.2 km/s. Crustal thickness increases from 26 to 29 km from NW to SE in the model.

Although an unequivocal determination of crustal composition is not possible from P-wave velocities alone, our model has several geological and tectonic implications. We interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment, since its thickness and velocity structure are consistent with that of oceanic crust. This fragment, which is not present 10–15 km to the west of the refraction profile, is probably at least partially responsible for the Great Valley gravity and magnetic anomalies, whose peaks lie about 10 km east of our profile. The middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating, or both. The crustal structure of the Great Valley is dissimilar to that of the adjacent Diablo Range, suggesting the existence of a fault or suture zone throughout the crust between these provinces.     

HF doppler observations on the occurrence of equatorial spread-F

HF doppler observations of the vertical drift velocity and group height of the 5.5 MHz plasma frequency level of the post-sunset bottomside F-region obtained on a fewESF (equatorial spread-F) and non-ESF days at Trivandrum are presented. The results show that on the non-ESF days, the maximum group height attained is about 400 km and the maximum velocity is less than 30 m/sec. On theESF days, however, the corresponding values are found to be in the range of 400–650 km and 30–50 m/sec. TheESF onset is found to be significantly delayed relative to the velocity peak indicating that it is more closely linked to the layer height than to the drift velocity.     

Crustal structure under the Sydney Basin and Lachlan Fold Belt,determined from explosion seismic studies

The Lachlan Fold Belt has the velocity‐depth structure of continental crust, with a thickness exceeding 50 km under the region of highest topography in Australia, and in the range 41–44 km under the central Fold Belt and Sydney Basin. There is no evidence of high upper crustal velocities normally associated with marginal or back‐arc basin crustal rocks. The velocities in the lower crust are consistent with an overall increase in metamorphic grade and/or mafic mineral content with depth. Continuing tectonic development throughout the region and the negligible seismicity at depths greater than 30 km indicate that the lower crust is undergoing ductile deformation.

The upper crustal velocities below the Sydney Basin are in the range 5.75–5.9 km/s to about 8 km, increasing to 6.35–6.5 km/s at about 15–17 km depth, where there is a high‐velocity (7.0 km/s) zone for about 9 km evident in results from one direction. The lower crust is characterised by a velocity gradient from about 6.7 km/s at 25 km, to 7.7 km/s at 40–42 km, and a transition to an upper mantle velocity of 8.03–8.12 km/s at 41.5–43.5 km depth.

Across the central Lachlan Fold Belt, velocities generally increase from 5.6 km/s at the surface to 6.0 km/s at 14.5 km depth, with a higher‐velocity zone (5.95 km/s) in the depth range 2.5–7.0 km. In the lower crust, velocities increase from 6.3 km/s at 16 km depth to 7.2 km/s at 40 km depth, then increase to 7.95 km/s at 43 km. A steeper gradient is evident at 26.5–28 km depth, where the velocity is about 6.6—6.8 km/s. Under part of the area an upper mantle low‐velocity zone in the depth range 50–64 km is interpreted from strong events recorded at distances greater than 320 km.

There is no substantial difference in the Moho depth across the boundary between the Sydney Basin and the Lachlan Fold Belt, consistent with the Basin overlying part of the Fold Belt. Pre‐Ordovician rocks within the crust suggest fragmented continental‐type crust existed E of the Precambrian craton and that these contribute to the thick crustal section in SE Australia.     

Shear-wave velocity structure of the western part of the Mediterranean Sea from Rayleigh-wave analysis

The lithospheric structure of the western part of the Mediterranean Sea is shown by means of S-velocity maps, for depths ranging from 0 to 35 km, determined from Rayleigh-wave analysis. The traces of 55 earthquakes, which occurred from 2001 to 2003 in and around the study area have been used to obtain Rayleigh-wave dispersion. These earthquakes were registered by 10 broadband stations located on Iberia and the Balearic Islands. The dispersion curves were obtained for periods between 1 and 45 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source zones to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and after inverted according to the generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks with a size of 1° × 1°. The shear velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several depths. These maps show the existence of lateral and vertical heterogeneity. In these maps is possible to distinguish several types of crust with an average S-wave velocity ranging from 2.6 to 3.9 km/s. The South Balearic Basin (SBB) is more characteristic of oceanic crust than the rest of the western Mediterranean region, as it is demonstrated by the crustal thickness. We also find a similar S-wave velocity (ranging from 2.6 km/s at the surface to 3.2 km/s at 10 km depth) for the Iberian Peninsula coast to Ibiza Island, the North Balearic Basin (NBB) and Mallorca Island. In the lower crust, the shear velocity reaches a value of 3.9 km/s. The base of the Moho is estimated from 15 to 20 km under Iberian Peninsula coast to Ibiza Island, continues towards NBB and increases to 20–25 km beneath Mallorca Island. While, the SBB is characterized by a thinner crust that ranges from 10 to 15 km, and a faster velocity. A gradual increase in velocity from the north to the south (especially in the upper 25 km) is obtained for the western part of the Mediterranean Sea. The base of the crust has a shear-wave velocity value around of 3.9 km/s for the western Mediterranean Sea area. This area is characterized by a thin crust in comparison with the crustal thickness of the eastern Mediterranean Sea area. This thin crust is related with the distensive tectonics that exists in this area. The low S-wave velocities obtained in the upper mantle might be an indication of a serpentinized mantle. The obtained results agree well with the geology and other geophysical results previously obtained. The shear velocity generally increases with depth for all paths analyzed in the study area.     

Distribution of the relative plasma turbulence level in the galaxy

The statistical dependence of τ/(DM)² (the ratio of the broadening of a pulsar pulse due to scattering in the interstellar medium to the square of the pulsar’s dispersion measure) on the pulsar’s dispersionmeasure, Galactic coordinates, age, and the angular distance to the nearest supernova remnant are studied. This parameter describes the relative level of electron density fluctuations in the turbulent interstellar plasma. It is shown that the interstellar plasma turbulence level is three orders of magnitude higher in the spiral arms of the Galaxy than outside the arms. The plasma turbulence level is approximately an order of magnitude higher in the Galactic arms, in regions within ?0.3° of supernova remnants, than outside these regions. We conclude that the source of energy for the turbulence in the Galactic arms is supernova explosions in the denser medium there.     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.     

Seismic Hazard Analysis for the Bangalore Region

Indian peninsular shield, which was once considered to be seismically stable, is experiencing many earthquakes recently. As part of the national level microzonation programme, Department of Science and Technology, Govt. of India has initiated microzonation of greater Bangalore region. The seismic hazard analysis of Bangalore region is carried out as part of this project. The paper presents the determination of maximum credible earthquake (MCE) and generation of synthetic acceleration time history plot for the Bangalore region. MCE has been determined by considering the regional seismotectonic activity in about 350 km radius around Bangalore city. The seismotectonic map has been prepared by considering the faults, lineaments, shear zones in the area and historic earthquake events of more than 150 events. Shortest distance from the Bangalore to the different sources is measured and then peak ground acceleration (PGA) is calculated for the different source and moment magnitude. Maximum credible earthquake found in terms of moment magnitude is 5.1 with PGA value of 0.146 g at city centre with assuming the hypo central distance of 15.88 km from the focal point. Also, correlations for the fault length with historic earthquake in terms of moment magnitude, yields (taking the rupture fault length as 5% of the total fault length) a PGA value of 0.159 g. Acceleration time history (ground motion) and a response acceleration spectrum for the corresponding magnitude has been generated using synthetic earthquake model considering the regional seismotectonic parameters. The maximum spectral acceleration obtained is 0.332 g for predominant period of 0.06 s. The PGA value and synthetic earthquake ground motion data from the identified vulnerable source using seismotectonic map will be useful for the PGA mapping and microzonation of the area.     

Chain-type structure in the H<Subscript>2</Subscript>O maser NGC 7538N

An analysis of the H₂O maser emission toward the source NGC 7538N, which is associated with an active star-forming region, is reported. The analysis is based on 24 years of monitoring in the 1.35-cm line using the the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 1981–2005 with a spectral resolution of 0.101 km/s. Individual spectral components have been isolated, and temporal drifts in their radial velocities found. From time to time, the drifts were accompanied by velocity jumps. This can be explained if there are chains consisting of clumps of material that are elongated in the radial direction toward the star and have a radial-velocity gradient. In 1982–2005, two maser activity cycles were observed, during which the chains were activated. We propose that shocks consecutively cross the chain elements and excite maser emission in them. The longest chain, at a radial velocity of ?58 km/s, has not fewer than 15 links. For a shock velocity of 15 km/s, the chain step is estimated to be ≤1.5 AU. The chains could be located in a circumstellar disk with a width of ≤10¹⁵ cm. A structure in the form of a rotating nonuniform vortex with the rotation period of about 1.6 years has also been detected. The translational motion of the vortex may be a consequence of its orbital motion within the protoplanetary disk.     

A first interpretation of crustal structure in the Adelaide Geosyncline in South Australia using quarry blasts

Explosions at two large open cut mines (Leigh Creek and Iron Baron) were used as sources of seismic energy to record along two linear profiles, parallel and approximately transverse to the axis of the Adelaide geosyncline in South Australia. Records at approximately 120 sites were obtained out to distances of the order of 350 km with Kinemetrics PS‐1A portable seismographs, using smoked paper and a recording speed of 4 mm/s. Times of blasting were determined from records at some of the permanent stations of the University of Adelaide seismograph network. Station spacing was normally 5 km, but at large distances from the source this increased to the order of 10 km.

The simplest model of the crust consistent with the observed travel times comprises two essentially homogeneous layers overlying the mantle. The average P wave velocities in the upper and lower crustal layers are 5.94 km/s and 6.46 km/s, with the boundary between the layers at approximately 18 km and possibly 8 km below Eyre Peninsula. Although such a division has been found in other parts of Australia, none of the earlier studies in S.A. found evidence for such a discontinuity or velocity gradient. The P wave velocity in the upper mantle is 7.97 km/s and the mean thickness of the crust is 39 km. Both the intermediate and Moho “discontinuities” may vary by up to 5 km from their mean depths. Shear waves have velocities of 3.43 and 4.45 km/s in the upper crustal layer and the upper mantle, respectively.     

Seismogenic Structure around the Epicenter of the May 12, 2008 Wenchuan Earthquake from Micro‐seismic Tomography

Abstract: A three-dimensional local-scale P-velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event-to-receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network. Checkerboard tests show that our tomographic model has lateral and vertical resolution of ~2 km. The high-resolution P-velocity model revealed interesting structures in the seismogenic layer: (1) The Guanxian-Anxian fault, Yingxiu-Beichuan fault and Wenchuan-Maoxian fault of the Longmen Shan fault zone are well delineated by sharp upper crustal velocity changes; (2) The Pengguan massif has generally higher velocity than its surrounding areas, and may extend down to at least ~10 km from the surface; (3) A sharp lateral velocity variation beneath the Wenchuan-Maoxian fault may indicate that the Pengguan massif’s western boundary and/or the Wenchuan-Maoxian fault is vertical, and the hypocenter of the Wenchuan earthquake possibly located at the conjunction point of the NW dipping Yingxiu-Beichuan and Guanxian-Anxian faults, and vertical Wenchuan-Maoxian fault; (4) Vicinity along the Yingxiu-Beichuan fault is characterized by very low velocity and low seismicity at shallow depths, possibly due to high content of porosity and fractures; (5) Two blocks of low-velocity anomaly are respectively imaged in the hanging wall and foot wall of the Guanxian-Anxian fault with a ~7 km offset with ~5 km vertical component.     

Seismic evidence for thick and underplated late Archaean crust of eastern Dharwar craton

The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.     

Interferometric observations of 4C 21.53 and PSR 1937+214 at decameter wavelengths

Preliminary resuts of interferometric observations of 4C 21.53 and PSR 1937+214 at 25 and 20 MHz are presented. The observations were obtained using the URAN-1 and URAN-2 interferometers, with baselines of 42.4 and 152.3 km. In addition to the pulsar radiation, which provides about 70% of the total flux of the object, radio emission from extended components with dimensions of several tens arcseconds has been detected for the first time. The angular size of the pulsar is 3″ at 25 MHz and 4″.8 at 20 MHz. The pulsar’s low-frequency spectrum deviates appreciably from the power law derived at higher frequencies.