Observations of Cassiopeia A at 20 and 25 MHz with the uran-1 interferometer

The variation in elemental depletions seen in diffuse clouds in the interstellar medium is shown to be consistent with the exposure of grains to weak shocks. Shocks remove a weakly bound layer of amorphous oxide material from refractory cores. Depletions are shown to be the result of the incorporation of elements in refractory cores formed during equilibrium condensation in high temperature sources together with accretion under low (T1000K) temperature conditions. Evidence is presented for the high temperature condensation of CaAlTi grains. In addition to refractory CaAlTi particels, a refractory condensate containing 20% of available Mg, 60% of Si and 90% of Fe is inferred from data on depletions. An amorphous layer with a composition (2MgO) (SiO) condenses on these cores.     

Radio Interferometric Observations of the Sun Using Commercial Dish TV Antennas

This memoir is a summary of my early childhood, education, and research career at the Naval Research Laboratory (NRL) in Washington, DC. I describe my early interest in astronomy and how I wound up working in the fields of solar physics and X-ray-UV spectroscopy of high temperature plasmas. I describe some of my home life and other interests, my education at the University of Pittsburgh, and the various projects and management activities that I have been fortunate to work on at NRL. I have been blessed with being able to work at a first-class research laboratory populated by outstanding scientists. I am particularly blessed to have worked with my many friends and colleagues in the NRL Space Science Division. Perhaps I am most blessed by having had wonderful parents that gave me the interests I have in life and the passion to pursue them, and an outstanding wife that has been my partner through good and bad times for over 50 years. I am now retired but for three years I was a participant in the NRL Voluntary Emeritus Program (VEP). However, this memoir is a personal account, and not work done as a VEP.

     

Chromospheric Internetwork Oscillations at Various Locations of the Quiet Sun

We analyze oscillation behaviours in chromospheric internetwork regions using spectral observations of the CⅡ1334 A line obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) aboard Solar and Heliospheric Observatory (SOHO). Three areas, 26 ×120arcsec2 each, at the various latitudes from the disk center to the north polar coronal hole, were rastered with a cadence of about 40-60 s in the solar minimum year. We obtained the time evolution of two-dimensional (2D) line intensity, continuum and line core shift. The continuum and the line shift show -3 min chromospheric oscillations in the internetwork regions underlying the coronal hole as well as at the disk center. We find that the CⅡ1334A line shift oscillates with an average speed of -1.7 km s-1, independent of the latitude, while its coherent scale decreases with latitude. On the other hand, the oscillation amplitude of the continuum around the 1334 A and the phase delay between the Doppler shift and continuum sli     

Radio emission from the Sun at 843 MHz

The Molonglo Observatory Synthesis Telescope (MOST) has recently been modified to permit observations of the Sun. With a collecting area of 18000 m², MOST makes high-sensitivity measurements in right-hand circular polarisation over a 3 MHz bandwidth at 843 MHz. The maximum baseline of the multi-element interferometer is 1600 m, so that one-dimensional spatial resolution as fine as 32 arc sec is available. A resistor array produces simultaneously a set of 64 beams separated by 22 arc sec, which may be offset electronically to cover the entire Sun in a few seconds. Observations may be made with a beam shape corresponding to either a multiplying or an adding interferometer. By exploiting the technique of Earth-rotation synthesis the telescope may be used to make two-dimensional maps of the Sun at the time of the austral solstice with a synthesized beamwidth of 43 × 110 arc sec. This paper describes the instrument and the procedures used to make various types of solar observations, and exhibits some of the first data collected.     

Quiet Sun and slowly varying component at meter and decameter wavelengths

Comparison of maps of the Sun obtained over the period June 29 to July 8, 1982 at 169 MHz with the Nançay Radioheliograph and at 73.8, 50, and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly varying component at meter and decameter wavelengths is not always thermal emission. During the period under study weak noise storm continua were the most frequent sources of slowly varying component at 169 and 73.8 MHz. Most filaments show no radio counterpart on the disk. A streamer has been detected on the disk from 169 to 30.9 MHz with an optimum observability at 50 MHz. The brightest source of the slowly varying component from 73.8 to 30.9 MHz for most of the period was located above an extended coronal hole in a region where a depression was observed at 169 MHz. In favorable cases, electron densities can be derived from the positions of noise storms and radio streamers; these are in agreement with previous K-corona eclipse observations.     

Coronal X-ray holes and the quiet radio Sun at 2800 MHz

Radio cool regions observed on strip scans of the Sun made at 2800 MHz with a 1.5 min arc fan beam are associated with X-ray coronal holes and are used to derive lower envelopes which are similar to spotless Sun drift curves. Fluxes are evaluated from the Ottawa-ARO solar patrol;. e.g. that of Coronal Hole 1 observed during the Skylab Mission with central meridian passage on July 25, 1973 is 66.5 s.f.u. ± 0.6%. This level is identified as that observed during sunspot minimum by comparison with the flux of 67.2 observed in July, 1964, and with the low daily values of 67.5 and 67.1 observed in April, 1975 and January, 1976. The enhancement of the quiet Sun of 3.0 s.f.u. for the optically inactive hemisphere of May 20, 1974 suggests that the radio quiet Sun may vary during the sunspot cycle.     

Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations

Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun’s atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun.     

Orientation of X-Ray Bright Points in the Quiet Sun

Thanks to the high-resolution images from the X-ray telescope (XRT) aboard the Hinode satellite, X-ray bright points (XBPs) in the quiet region of the Sun are resolved and can be seen to have complex loop-like structures. We measure the orientation of such loop structures for 488 XBPs picked up in 26 snapshot X-ray images near the disk center. The distribution of the orientation is slightly but clearly biased to the east – west direction: the random distribution is rejected with a significance level of 1% by the χ ²-test. The distribution is similar to the orientation distribution for the bipolar magnetic fields. The XBP orientation is, however, much more random than that of the bipolar magnetic fields with similar size. 24% of the XBPs are due to emerging bipoles, while the remaining 76% are due to chance encounters of opposite polarities.     

Observations of the quiet Sun at meter and decameter wavelengths

The new TeePee Tee array of the Clark Lake Radio Observatory has been used to observe the quiet Sun at 121.5, 73.8 and 26.3 MHz. The equatorial brightness distributions at all three frequencies, and the polar brightness distributions at the two higher ones have been measured. From the observed total fluxes and half-power diameters we have derived the peak brightness temperatures of the solar disk as well as of some sources of the slowly varying component.On leave of absence from Instituto Argentino de Radioastronomía, Argentina.     

Some features of the pulsed radiation from the pulsar 1919+21 at 16.7, 20 and 25 MHz

An intense interpulse radio frequency radiation of PSR 1919+21 has been detected in the range of 16.7, 20 and 25 MHz. An integrated waveform of this radiation was investigated with time resolutions ofT/16 andT/64, and several characteristic regions of the intensity maxima and minima have been revealed. As has been shown, the maximum number of the best pronounced interpulses is about four, their location being symmetrical relative to the centre of the main pulse. The fine frequency structure of the integrated radiation was also investigated. It has been found that the shape of the signal can differ considerably with a frequency diversity of 10 to 100 kHz and only slightly with a separation of 5 to 10 MHz, the difference diminishing with an increase of the observation bandwidth and of the averaging time. Pulse broadening was studied at 16.7 and 25 MHz and has been found to agree with the interstellar scattering mechanism. The mean intensity of the main pulse and the interpulses has been evaluated and an essential difference of their spectral indices established.     

四颗南天脉冲星在1560 MHz上的偏振观测

本文给出88年6月和7月期间在澳大利亚Parkes进行的一次南天脉冲星偏振观测的部分有意义的结果。频率为1560MHz。采用了Jodrell Bank天文台新研制成的2×16×5MHz滤波系统,使这次观测具有很强的消色散能力和较高的灵敏度。这4颗南天脉冲星具有较强的线偏振,为首次获得的高频偏振资料。     

Quiet Sun Contribution to Variations in the Total Solar Irradiance

An analysis of spatially-resolved measurements of the intensity of the photospheric continuum by the Michelson Doppler Imager (MDI) on the SOHO spacecraft indicates that these data can be used to study variations of the Total Solar Irradiance (TSI). Since the techniques employed depend upon ratios of intensities measured by MDI, they are independent of the absolute photometric calibration of the instrument. The results suggest that, while it is possible to account for short-term (weeks to months) variation in TSI by variations in the irradiance contributions of regions with enhanced magnetic fields (larger than ten G as measured by MDI), the longer-term variations are influenced significantly by variations in the brightness of the quiet Sun, defined here as regions with magnetic field magnitudes smaller than ten G. The latter regions cover a substantial fraction of the solar surface, ranging from approximately 90% of the Sun near solar minimum to 70% near solar maximum. The results provide evidence that a substantial fraction, 50% or more, of the longer term (≥one year) variation in TSI is due to changes in the brightness of the quiet Sun.     

Comparison of Nonlinear Force-Free Field and Potential Field in the Quiet Sun

In this paper, a potential field extrapolation and three nonlinear force-free (NLFF) field extrapolations (optimization, direct boundary integral (DBIE), and approximate vertical integration (AVI) methods) are used to study the spatial configuration of magnetic field in the quiet Sun. It is found that differences in the computed field strengths among the three NLFF and potential fields exist in the low layers. However, they tend to disappear as the height increases, and the differences are of the order of 0.1 gauss when the height exceeds ≈ 2000 km above the photosphere. The difference in azimuth angles between each NLFF field model and the potential field is as follows: for the optimization field, it decreases evidently as the height increases; for the DBIE field, it almost stays constant and shows no significant change as the height increases; for the AVI field, it increases slowly as the height increases. Our analysis shows that the reconstructed NLFF fields deviate significantly from the potential field in the quiet Sun.     

Stochastic Seismic Emission from Acoustic Glories and the Quiet Sun

Helioseismic images of multipolar active regions show enhanced seismic emission in 5-mHz oscillations in a halo surrounding the active region called the `acoustic glory'. The acoustic glories contain elements that sustain an average seismic emission 50% greater than similar elements in the quiet Sun. The most intense seismic emitters tend to form strings in non-magnetic regions, sometimes marking the borders of weak magnetic regions and the separation between weak magnetic regions of opposite polarity. This study compares the temporal character of seismic emission from acoustic glories with that from the quiet Sun. The power distribution of quiet-Sun seismic emission far from solar activity is exponential, as for random Gaussian noise, and therefore not perceivably episodic. The distribution of seismic power emanating from the most intense elements that comprise the acoustic glories is exponential out to approximately 4 times the average power emitted by the quiet Sun. Above this threshold the latter distribution shows significant saturation, suggesting the operation of a hydromechanical non-linearity that sets limits on the acoustic power generated by the convection zone. This could give us considerable insight into the physical mechanism of seismic emission from the near subphotosphere.     

The Number of Magnetic Null Points in the Quiet Sun Corona

The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points. These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO’s MDI. The majority of null points occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null point, with altitude greater than 1.5 Mm, above every 322 Mm² patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete elements with mean flux 〈|φ|〉=1.0×10¹⁹ Mx distributed randomly with density n=0.007 Mm⁻².     

Observations of Jupiter at 26.3 MHz using a large array

A 640 element phase-steerable dipole array has been used to make highly sensitive observations of the planet Jupiter during the 1973 apparition. The satellite Io is found to have very little influence at the low flux levels, whereas the definition of sources A and B appears to be relatively flux independent. A two-dimensional analysis of the data in the Jupiter-Io plane has revealed considerable source B activity at low intensities which is not influenced by Io.