The intensity variation of the atomic oxygen red line during morning and evening twilight on 9–10 April 1969

During the evening of 9 April and the morning of 10 April 1969, the twilight zenith intensity of the atomic oxygen red line OI(³P-¹D) at 6300 Å was measured at the Blue Hill Observatory (42°N, 17°W). At the same time incoherent scatter radar data were being obtained at the Millstone Hill radar site 50 km distant. We have used a diurnal model of the mid-latitude F-region to calculate the ionospheric structure over Millstone Hill conditions similar to 9–10 April 1969. The measured electron temperature, ion temperature, and electron density at 800 km are used as boundary conditions for the model calculations. The diurnal variation of neutral composition and temperature were obtained from the OGO-6 empirical model and the neutral winds were derived from a semiempirical three-dimensional dynamic model of the neutral thermosphere. The solar EUV flux was adjusted to yield reasonable agreement between the calculated and observed ionospheric properties.This paper presents the results of these model computations and calculations of the red line intensity. The 6300 Å emission includes contributions from photoelectron excitation, dissociative recombination, Schumann-Runge photodissociation and thermal electron impact. The variations of these four components for morning and evening twilight between 90–120° solar zenith angles, and their relative contributions to the total 6300 Å emission line intensity, are presented and the total is compared to the observations. For this particular day the Schumann-Runge photodissociation component, calculated using the solar fluxes tabulated by Ackermann (1970), is the dominant component of the morning twilight 6300 Å emission. During evening twilight it is necessary to utilize a lower O₂ density than for the morning twilight in order to bring the calculated and observed 6300 Å emission rates into agreement. The implication that there may be a diurnal variation in the O₂ density at the base of the thermosphere is discussed in the light of available experimental data and current theoretical ideas.     

Relationships between radio aurora,visual aurora and ionospheric currents during a sequence of magnetospheric substorms

In an earlier paper the latitudinal and longitudinal structure of ionospheric current flow during a sequence of magnetospheric substorms was presented (McDiarmid and Harris, 1976). In the present paper the relationships between the electrojets, the radio aurora observed at 48 MHz and the all-sky camera-recorded visual aurora are presented for the same substorm sequence. The previously described morphology of radio aurora during substorms is confirmed and the observed relationships can be explained.     

Über ein halbautomatisches Irisblendenphotometer

A method automatically giving the equality-information in iristype photometers is described in this paper. It bases on digital techniques. Its resolution is 0,001 mag for brighter stars and 0,004 mag for fainter ones. For a single measurement of a star image the mean error ist ±0,004 mag and ±0,010 mag, respectively, including errors in centering, equilization, and reading-out. The range of a reduction curve extends over more than 10 magnitudes. The rate of measurements ist 200 to 360 stars per hour depending on the density of stars on the plate and the measurer's practice.     

Further VLA observations of hydrogen deficient stars

VLA observations at 6 cm have been obtained for three hydrogen-deficient objects υ Sgr, V 348 Sgr, and A bell 58. A bell 58 was also observed at 2 cm. Only upper limits to the flux density could be set for these sources. A new radio source at 6 cm was found in the field of υ Sgr. The upper limit for 6 cm flux density of V348 Sgr sets an upper limit to its reddening asE(B–V) ≤ 0.65. The hydrogen deficient planetary nebula A 58 shows much lower radio flux than expected from the infrared-radio flux density relationship of planetary nebulae. National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National Science Foundation, USA.     

Thermosolutal-convective instability in a stellar atmosphere

The thermosolutal-convective instability of a stellar atomsphere is studied in the presence of suspended particles. The criteria for monotonic instability are derived and are found to hold good also in the presence of uniform rotation and uniform magnetic field on the thermosolutal-convective instability. The thermosolutal-convective instability of a stellar atmosphere is also studied in the presence of suspended particles and radiative transfer effects and the criteria for monotonic instability are obtained in terms of source function.     

Solar flare neutron production and the angular dependence of the capture gamma-ray emission

The depth dependence of the production of neutrons and capture gamma-ray line emission are calculated by Monte Carlo simulation of the nuclear processes taking place when flare-accelerated ions interact with the solar atmosphere. The calculations also give the heliocentric-angular dependence of the 2.223 MeV neutron capture line emission as a function of accelerated-ion energy spectrum and angular distribution. These results are compared with observations to determine the energy spectrum shape and total ion number for various flares.     

The spatial structure of a solar flare in soft X-rays and centimetric wavelengths

High-resolution images of the decay phase of a soft X-ray flare observed by the S-054 experiment on Skylab are compared with interferometric scans of the radio burst obtained simultaneously at 2.8 cm (Felli et al., 1975). The spatial resulution of the radio instrument in one direction, although lower than the X-ray telescope resolution, is high enough for a detailed comparison. The comparison clarifies the relationship between the sources of soft X-ray and thermal radio emission in solar flares. The X-ray emission is localized in a loop-like structure which appears spatially coincident with the rapidly varying component of the radio burst. The more stable components of the radio source, which do not appear to contribute substantially to X-ray emission, are found to be spatially associated with the extremes of the X-ray loop. A model of plasma-filled loops is suggested which accounts for the emissions in both spectral ranges and for their spatial location and temporal development.On leave from Osservatorio Astrofisico di Arcetri, Florence, Italy.     

Ariel 1118-61 — A very close binary system or a slowly rotating neutron star?

The transient X-ray source Ariel 1118-61 has a period of 6.75 min. We review possible models for the X-ray source and in particular we consider orbital and rotational origins for the periodicity. Finally we discuss the possible identification of Ariel 1181-61 with the Mira-type variable RS Cen.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Interplanetary scattering of fast solar electrons deduced from type III bursts observed at low frequencies

Observations of low frequency solar type III radio bursts and the associated fast solar electrons show that the total path length traveled by the particles between the Sun and the Earth is significantly greater than the length of the smooth Archimedean spiral trajectory followed by the centroid of the type III exciter (Alvarez et al., 1975). Here we assume that the ratio of electron path length and the spiral length increases approximately as r ⁿ, where r is heliocentric distance, and then compute the radio bursts arrival time at 1 AU for different values of n. A comparison with the radio observations indicates that the best fit occurs for n = 1.5 ± 1.0. We interpret these results in terms of the variation of electron scattering with heliocentric distance.     

On the galactic plane discontinuity nearl=140°

Star counts, from measurements made by the GALAXY measuring machine, in unit B magnitude intervals were made in nine regions, each of 1.75 square degrees distributed about (l, b)= (140°, 0°). The mean density function and the mean variation of interstellar absorption with distance for the various areas were determined using a combination of theoretical and observed cumulative Wolf diagrams. For the same regions multi-colour (UBV) photographic photometry was used to identify early type stars. The distribution of these stars shows three main peaks which correspond well in distance with the Perseus, - and -spiral arms found by Verschuur from radio measurements of neutral hydrogen. The pattern made by the stellar arms in the (l, r) plane shows a discontinuity nearl=140°. From a consideration of both the star counts and the distribution of the reddened early type stars it is suggested that the cause of the discontinuity is a cloud with 0 _. ^m 7 total obscuration situated much nearer to the Sun than the Perseus arm and terminating on one side atl=140°.