On a difference of the chromospheric background intensity between the equatorial and polar regions of the sun

Small chromospheric emission centres (here defined as flocculi) were studied photometrically on K _{2, 3, 2} spectroheliograms taken at the Arcetri Observatory. The intensity ratio between I _fl, the flocculus brightness, and I _chr, the brightness of the chromospheric background, was determined. A difference was found between the polar and the equatorial limb values of the ratio I _fl/I _chr.     

Interplanetary sector structure and the helliomagnetic equator

The magnetohydrodynamics of solar-wind flow lead logically to the formation of one warped annular neutral surface that apparently extends from ≈ 2r _⊙ (solar radii) to the boundary of the heliosphere. The most likely asymptotic configuration for this neutral sheet intersects the heliomagnetic equatorial plane along four corotating arcs. The observer sees a reversal of magnetic polarity on each crossing of the neutral surface, and so interprets each reversal as the crossing of a sector boundary.     

Relationship between high-energy physical phenomena on the sun and in astrophysics

High energy phenomena on the surface of the Sun are manifestations of part of the solar dynamo cycle. Convection and magnetic field give rise to unstable, twisted flux loops that become solar flares when the resistive tearing mode proceeds to the nonlinear limit. If such twisted flux loops did not dissipate rapidly due to an enhanced resistivity, then the dynamo would not work. The act of dissipation leads to intense heating and acceleration leading to X-rays and accelerated particles. The particles in turn give rise to hard X-rays, gamma rays, neutrons, and solar cosmic rays. In high-energy astrophysics such phenomena occur in accretion disks around compact objects like black holes in quasars and SS433. The resulting acceleration may explain the observed extremely high-energy cosmic rays of up to 10²⁰ eV and the high-energy gamma rays of 10¹² to 10¹⁵ eV. These high energies are more readily explained by acceleration E parallel to B as opposed to stochastic shock acceleration. The anisotropy and localization of gamma rays from solar flares potentially may indicate which mechanism is prevalent.     

Absence of whistlers at the equator reaffirmed

Synoptic observations made on magnetic recording tape at Huancayo, Peru, at the magnetic dip equator, during the International Geophysical Year 1957–1958, were aurally reviewed at that time and no whistlers, hiss, or other emissions were heard. In view of the more recent observation of whistlers at geomagnetic latitudes as low as 12°, and in conjunction with a study of equatorial hiss observed in the topside ionosphere, these recordings have recently been reassessed by reducing them with modern real-time, digital spectrographic equipment. Although the observations were found to be of high quality, and to show the classical features of ground-wave and sky-wave propagation of sferics and VLF transmissions, again no evidence whatsoever of whistlers, hiss, or other emissions is found. Thus it is concluded that the whistlers observed at very low latitudes do not propagate subionospherically to the equator and it is confirmed that “hybrid” whistlers must be due to subionospheric propagation across the equator of the causative sferic rather than of the short whistler.     

On the difference in darkness between sunspots

The effects of the magnetic field as well as the velocity field on sunspot equilibrium are discussed. The gas pressure difference, P, between a spot and the environments in the same horizontal layer is primarily determined by the magnetic field. Using recent model atmospheres we find that P shows a maximum value, P _max, at a depth of 650 ± 150 km below the photosphere. The value of P _max suggests that the curvature of the field lines is important for the equilibrium.It appears that, at an optical depth of unity in the umbra, the density has a value close to that of the environment at the same geometric depth (see Figure 4). If such is the case the expression for the umbra temperature (Equation (15)) may be considerably simplified (Equations (17) and (18)) and compared with observations.     

Lightcurves and poles of seven asteroids

The Asteroids 5 Astraea, 19 Fortuna, 51 Nemausa, 68 Leto, 138 Tolosa, 196 Philomela and 409 Aspasia have been studied using Strömgren photometry observations made in 1997. Simultaneous lightcurves in the uvby Strömgren filters of synodic periods of 0.70004±0.00020, 0.31013±0.00003, 0.32400±0.00020, 0.61910±0.00100, 0.42087±0.00009, 0.34750±0.00020 and 0.37576±0.00060 days, and amplitudes, in the y filter, of 0.^m16±0.^m05, 0.^m30±0.^m02, 0.^m15±0.^m04, 0.^m29±0.^m02, 0.^m43±0.^m03, 0.^m30±0.^m04 and 0.^m15±0.^m06 have been found for 5 Astraea, 19 Fortuna, 51 Nemausa, 68 Leto, 138 Tolosa, 196 Philomela and 409 Aspasia, respectively. Additional observations of 138 Tolosa and 196 Philomela during February 2000, show lightcurve amplitudes equal or greater than 0.^m15 for 138 Tolosa and of 0.^m45 for 196 Philomela in 2000 opposition.Solutions for the sense of rotation, sidereal period, pole orientation and shape properties have been proposed for the first time for 138 Tolosa and improved solutions have been obtained for 5 Astraea, 19 Fortuna, 51 Nemausa, 196 Philomela and 409 Aspasia.     

Indication of anomalous conductivity at the nigerian dip equator

The geomagnetic daily variations at the Nigerian dip equator have been analyzed with the methodology introduced in a previous paper. It has been found that the height integrated current presents a notoriously higher amplification in Nigeria than in Peru. It has also been found that there exists a strong and inhomogeneous anomaly in the Earth's conductivity in Nigeria. And contrary to what is usually accepted, it is shown that its latitudinal distribution can not be precisely determined until the distribution and magnitude of the ionospheric currents at F-region heights is more accurately known.     

The difference between the nearby galaxies and the Virgo cluster

The sample of galaxies within 7 Mpc has been compared with that of the Virgo cluster in order to find new and confirm already known differences which may be caused by the environment. The well-known morphology-density relation is found to be valid not only for luminous galaxies but also for faint objects. The differences in the global luminosity functions are mainly caused by the different composition of the samples, whereas the luminosity functions of narrow morphological subgroups, with the exception of the irregulars, probably are not dependent on the density or on other quantities. The already observed HI-deficiency of the interstellar medium of the Virgo cluster spirals has been confirmed. There is a steep drop of this quantity with increasing distance from the dominant galaxies M 87 and M 49. The interstellar dust masses of Virgo cluster spirals and irregulars have been estimated from far-infrared fluxes. These data have been combined with the HI masses in order to obtain the gas-to-dust ratios, which come out to be smaller by a factor of 4 for the Virgo cluster spiral galaxies than for the nearby systems, on the average. The gas-to-dust ratio is strongly correlated with the HI-deficiency. These results support the concept of the ram pressure of the intracluster medium on the spiral galaxies in the core of the Virgo cluster.     

The correspondence between x-ray bright points and evolving magnetic features in the quiet sun

Coronal bright points, first identified as X-ray Bright Points (XBPs), are compact, short-lived and associated with small-scale, opposite polarity magnetic flux features. Previous studies have yielded contradictory results suggesting that XBPs are either primarily a signature of emerging flux in the quiet Sun, or of the disappearance of pre-existing flux. With the goal of improving our understanding of the evolution of the quiet Sun magnetic field, we present results of a study of more recent data on XBPs and small-scale evolving magnetic structures. The coordinated data set consists of X-ray images obtained during rocket flights on 15 August and 11 December, 1987, full-disk magnetograms obtained at the National Solar Observatory - Kitt Peak, and time-lapse magnetograms of multiple fields obtained at Big Bear Solar Observatory. We find that XBPs were more frequently associated with pre-existing magnetic features of opposite polarity which appeared to be cancelling than with emerging or new flux regions. Most young, emerging regions were not associated with XBPs. However, some XBPs were associated with older ephemeral regions, some of which were cancelling with existing network or intranetwork poles. Nearly all of the XBPs corresponded to opposite polarity magnetic features which wereconverging towards each other; some of these had not yet begun cancelling. We suggest that most XBPs form when converging flow brings oppositely directed field lines together, leading to reconnection and heating of the newly-formed loops in the low corona.     

Photometry and spectroscopy of the solar granulation along the polar axis and equator

From high-resolution CCD images of granulation obtained at the Swedish Solar Tower of the Observatorio del Roque de los Muchachos at different positions on the solar disk, mean bi-dimensional spatial power spectra have been obtained at each position and restored following the method proposed by Rodríguez Hidalgoet al. (1990). The variation with latitude of the power spectrum, of the granular brightness contrast and of the mean wave number of the power spectrum are presented. The analysis is completed with spectroscopic observations obtained at the Gregory Coudé Telescope of the Observatorio del Teide. The centre-to-limb and latitudinal variation of the bisectors, residual intensities and equivalent widths are shown for several photometric lines.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Extension of a polar ionospheric current to the nightside equator

A detailed analysis of rapid-run magnetograms from Guam (geomagnetic latitude = 4.2°) revealed that there are two kinds of geomagnetic sudden commencement (SC) observed in nighttime. One is the ordinary SC consisting of a main impulse only which has a smooth rise of the H-component. The other is a superposition by a small positive impulse on the very beginning part of the smooth rise of the main impulse and consequently the SC starts with a small stepwise increase of the H-component. The latter type of SC occurs between 20 and 08 h L.T. and its occurrence rate takes the maximum value of about 50% around 03 h L.T. Corresponding magnetograms from a dayside equatorial station (Huancayo, geomagnetic latitude = ?0.7°) were examined and a good correlation was found between the stepwise SC at the nightside (Guam) and SC¹ with a preliminary reverse impulse (PRI) at the dayside (Huancayo). Since PRI observed at the dayside equator may be interpreted as an extension of an ionospheric current due to an dusk-to-dawn electric field impressed on the polar ionosphere, our results show that a polar originating ionospheric current can extend to the nightside equator and produce a small but observable magnetic effect in spite of much reduced nighttime ionospheric conductivity.     

On the analogy between the zebra patterns in radio emission from the sun and the crab pulsar

We investigate the close analogy between the solar radio emission with a quasi-harmonic spectrum structure and one of the microwave emission components of the Crab pulsar in the form of the so-called zebra pattern. The radio emission mechanism of this component can be provided by instability at double plasma resonance and can be realized in extraordinary (for a radio pulsar) conditions, namely in a nonrelativistic plasma with a relatively weak magnetic field. We point out possible models of the emission source in the form of a magnetic trap or a neutral current sheet with a transverse magnetic field localized in the corotating region of the pulsar magnetosphere far from the neutron star surface.     

Equinox and equator determinations from hypothetical minor planet observations

Hypothetical observations of minor planets 1, 2, 3, 4, 6, 7, 8, 9, and 15 were computed for a ten year observing period commencing 2 Jan. 1980. The mean errors of the equinox and equator corrections were determined versus: the distribution of clear nights during the year; the length of the observing program; the number of minor planets included in the program; and the elongation from the Sun. It is concluded that the equinox and equator are best determined by: choice of a site with a minimum of 60% clear nights well distributed throughout the year; an observing program that lasts a minimum of four years, but that little is gained by extension of the program to more than ten years; inclusion of a minimum of four or five minor planets in the program, but that more than nine is not necessary; that the best four minor planets to use are numbers 1, 4, 7, and 15; and that observations should be made as far from opposition as possible, but that it is not essential to observe beyond quadrature.Presented at the Symposium Star Catalogues, Positional Astronomy and Celestial Mechanics, held in honor of Paul Herget at the U.S. Naval Observatory, Washington, November 30, 1978.     

Origin of the stepped topography of the Martian poles

The circumpolar stepped topography observed within the Martian polar regions can have originated from one of a limited number of processes, including (i) erosion of resistant layers, (ii) erosion rates inversely proportional to slope gradient, (iii) basal sapping, and (iv) bistable rates of erosion and deposition. The last mechanism appears most likely to operate on the polar escarpments, driven by ablation of volatiles on the dark scarps and deposition on the icy flats. Decreasing albedo and a corresponding increase in radiation input caused by dust accumulations on the ablating layered deposits on steeper slopes provides a metastable erosion rate model sufficient to produce a stepped topography. Wind erosion is presuured later to remove the loose excess residual dust which accumulated during ablation of the scarps. The ablation of the scarps contemporaneously with ice accumulation on the flats implies the layered deposits exposed on the scarps have formed beneath overlying flats, and the observed unconformities within these deposits can due to the exposure of deposits laid down under more than one flat with different gradients. The linearity and mutual parallelism of the scarps is a result of scarp retreat on a regional slope or with a prefered direction of scarf retreat. The spiral arrangement of the scarps is probably due to more rapid retreat of scarps facing slightly west of the equatorward meridian, that is, in the direction of greatest solar and atmospheric warming. The model suggest, but does not prove, that the layered deposits are mostly water ice, with small amounts of codeposited silicate dust and volcanic ash.     

YO molecule in the sun

Vibrational transition probabilities, namely Franck—Condon factors and -centroids have been evaluated by an approximate analytical method for the (A–X), (A–X), and (A–X) system of YO molecule. Morse potential energy curves forX ²⁺,A ²₂,A²₂, andA²₂, states of YO have been constructed using the latest spectroscopic data. The value of -centroids for the band have been found to decrease linearly with the corresponding wavelengths. We show results for two new transitions of (A–X) and (A–X) and five new bands of (A–X) of YO in the umbral spectrum of the Sun.     

On convection in the sun

Convective motions driven by a superadiabatic temperature gradient in a viscous thermally conductive medium are considered. Approximate linearized equations governing the perturbation are derived under the following conditions: (i) The ratio of the excess temperature gradient over the adiabatic gradient is small compared with the gradient itself, (ii) The perturbation is of low-frequency type, (iii) The rotation is slow. Only the convective mode is described by these equations (as in the Boussinesq approximation), and the equations are valid for compressible configurations with any ratio between the scale heights of the equilibrium and perturbed quantities. Results of a numerical calculation of unstable perturbations for configurations with a large density stratification are given. They show that under conditions appropriate for the solar convection zone an extremely strong instability is expected to occur if the mixing length is assumed to be equal to 1.5 times the pressure scale height. The horizontal scale of the instability is intermediate between those of granulation and supergranulation. The larger the mixing length, the smaller the growth rate of the instability, and the larger its horizontal scale. Therefore it seems possible to adjust the mixing length to obtain the characteristics corresponding to those of the solar supergranulation. The possible origin of the granulation as an instability in a subsurface zone, where a local increase in the density scale height takes place, is also discussed. To achieve agreement with observations, it seems necessary to assume that the ratio of the mixing length to pressure scale height is an increasing function of the pressure.     

Lunar tidal oscillations in the horizontal ionospheric drift at the equator

The effect of lunar tides on the apparent ionospheric drift velocity (V) for an equatorial station Thumba (0.6°S dip) is computed by using nearly six years of data at fixed solar hours. Significant tides are observed in the E-region drifts, particularly around 12.00 hr and in the F-region drifts around 15.00 hr.A good correlation in the phases of the lunar tides in H and V is found to exist, suggesting a strong electrojet control of the horizontal ionospheric drift around these hours.