Period variation of Pc5 pulsations at high latitudes

Geomagnetic data for the year 1967 from seven Canadian observatories, spanning the subauroral, auroral and polar zones, have been analysed to investigate the characteristic variation of Pc5 period with several geophysical variables. Pulsations in the whole spectrum of Pc5 (period range 150–600 s) were found to occur at all of the observatories. Those with smaller periods occurred more frequently at lower latitudes while those with longer periods occurred more frequently at higher latitudes. Daily variation of the periods of Pc5 showed little change with seasons or with magnetic activity. Periods, in general, had two daily maxima which appeared at different local times in different zones. A predominant morning peak was noted at all stations except Baker Lake, where a mid-day maximum of the period was found. The Pc5 periods tended to increase with geomagnetic activity at lower latitude stations, and to decrease with activity at stations in the polar cap for low to moderateKp levels. At high activity levels these trends appeared to reverse, though results are less certain. In different seasons and for the whole year the periods increased almost linearly with latitude. However when similar analysis was done for individual hours of the day and for different magnetic activity groups, this linear relationship between period and geomagnetic latitude was not evident. Efforts to detect a 27-day recurrence tendency of Pc5 periods did not succeed.Contributions from the Earth Physics Branch No. 495.     

Source structure and dynamics of Pc 1 pulsations at low latitudes

Structured Pc 1 signals propagate in the ionospheric F2 region duct from their secondary sources at high latitudes to lower latitudes. Propagation directions to low latitude stations can be inferred from measurements of polarization parameters. The analysis of five events recorded at two low latitude stations (L = 1.9) are presented. Direction of arrival measurements are used to investigate the spatial and temporal structure of Pc 1 sources. Results show a close relationship between the structure of events identified in the frequency-time representation and direction of arrival measurement patterns. Multiple sources are sometimes indicated.     

Long-term prediction of Pc 1 geomagnetic pulsation occurrences

The monthly occurrences of Pc 1 geomagnetic pulsations at California stations over the 18.5 yr interval January 1955–June 1973, were spectral analyzed to obtain the characteristics of the major periodicities. There were eight lines in the spectrum with amplitudes significantly above the background noise and the periods, amplitudes and phases of the corresponding periodicities were determined from the spectral data. The periods of the eight lines, listed in order of their relative amplitudes, were ～14yr, 40.1 m, 13.8 m, 5.72 m, 3.08 m, 2.96 m, 10.5 m and 5.94 m. The eight periodicities were used to predict the trend of Pc 1 occurrences at middle latitudes over the remainder of the decade. It appears that the present moderately high rate of occurrence will remain approximately constant during 1974. After 1974 the rate should increase and reach a maximum level in 1977. From 1978 to 1980 a steady decrease in the rate of occurrence is indicated. There are no times when the level of Pc 1 occurrences drops nearly to zero.     

Generation mechanism of Pc3 magnetic pulsations at very low latitudes

Polarization properties of Pc3 magnetic pulsations at very low latitudes cannot be explained by existing theories which are based on the field line resonance model, because magnetic field lines at ¦Φ¦ < 22° are almost entirely in the ionosphere. In order to interpret Pc3 polarization characteristics observed at very low latitudes (¦Φ¦ < 20°), I would like to propose a possible, new qualitative model in which two superimposed ionospheric eddy currents, oscillating with slight differences in frequency in the Pc3 range and in azimuthal wave number, move azimuthally at very low latitudes. The equatorial ionospheric Pedersen eddy currents are believed to be predominantly caused by inductive electric fields of compressional Pc3 source waves which may possibly arrive in the equatorial ionosphere from the outer magnetosphere.     

Short-term prediction and a new method of classification of Pc 1 pulsation occurrences

Because of their known tendency to occur in the interval 2–7 days after the start of a geomagnetic storm, Pc 1 pulsations (0.2–5 Hz) are particularly well suited for a method of occurrence prediction based on the comparison of running means of a geomagnetic activity index. By comparing the running mean of a short interval (～ 2 days) of activity data with the mean of a longer interval (～ 5 days), it is possible to isolate the intervals of declining activity that contain a large proportion (if 66%) of Pc 1 pulsation occurrences. Assuming the real time availability of a daily activity index, predictions can be made for 3–10 days ahead of the probability of Pc 1 occurrences. The method of prediction generalizes the previous observations on the relation between Pc 1 pulsations and geomagnetic storms, and one of its important features is its ability to divide Pc 1 pulsation occurrences into a unified system of categories. It is probable that this system can be exploited to provide new information about the pulsations.     

CH stars at high Galactic latitudes

High-velocity outflows from supermassive black holes have been invoked to explain the recent identification of strong absorption features in the hard X-ray spectra of several quasars. Here, Monte Carlo radiative transfer calculations are performed to synthesize X-ray spectra from models of such flows. It is found that simple, parametric biconical outflow models with plausible choices for the wind parameters predict spectra that are in good qualitative agreement with observations in the 2–10 keV band. The influence on the spectrum of both the mass-loss rate and opening angle of the flow are considered: the latter is important since photon leakage plays a significant role in establishing an ionization gradient within the flow, a useful discriminant between spherical and conical outflow for this and other applications. Particular attention is given to the bright quasar PG 1211+143 for which constraints on the outflow geometry and mass-loss rate are discussed subject to the limitations of the currently available observational data.     

Magnetic field rotation at high solar latitudes

Measurements of the rotation rate of polar magnetic features during 1974–76 lead to a significantly slower rotation rate than that found earlier for polar faculae in 1951–54. Similarly, the rotation rate of these features is slower than the Doppler-determined rate at polar latitudes or the rotation rate of polar filaments. It is suggested that the strong latitude rotation gradient in the subsurface magnetic flux tubes which is implied by these results may presage a very active solar maximum for cycle 21.     

Neutral hydrogen filaments at high galactic latitudes

Small angular scale twists in Hi filaments identified at high galactic latitudes are associated with the location of enhanced emission features (EEFs), structures that have traditionally been referred to as clouds. It is shown that in these directions the column density of gas is enhanced through geometrical effects because the line-of-sight intersects a greater pathlength when viewing along a segment of filament axis (flux tube) than when the tline-of-sight is more normal to the filament axis. By interpreting the EEFs as isolated entities (clouds) we derive an incorrect impression as regards the properties of interstellar Hi. For example, EEFs are typically a factor of five to ten times deeper than they are wide and, hence, the derived properties of Hi structures that have traditionally been taken to be as deep as they are wide are incorrect. This study leads to questions about the way observations of 21 cm, molecular, and 100 emission are currently being interpreted. It is concluded that much of what is observed to be cloud structure in the interstellar medium (except in regions directly associated with star formation) is telling us about geometry of filaments and not about the physics of clouds, The very notion of an interstellar cloud may have outlived its usefulness and previous work that has attempted to account for these structures in terms of gravitational stability or pressure equilibrium has to be reconsidered in the light of the existence of complex patterns of filamentary structure, not only in the Hi distribution but also of interstellar cirrus defined by 100 emission.     

Neutral hydrogen filaments at high galactic latitudes

Large-scale morphological waves with wavelengths of order 30° and amplitude=1/8 exist in several long Hi filaments in a 540 square degrees of sky aroundl=230°,b=+40°. The extent of the longest filament is greater than 72°, the limit set by the boundaries of the area surveyed. TheHi gas appears to be controlled by magnetic fields and the motion within flux tubes, determined by analysis of velocity along the filament axes, shows the presence of wave patterns with amplitude 5 to 6 km s^–1 on an angular scale similar to that seen in the spatial structure projected on the sky.     

Fine structures in solar radio bursts at a 21 cm wavelength and pulsation modulation

The solar burst of 13 July, 1986 at 21 cm wavelength was recorded with a time constant of 8 ms. In the course of the burst lasting for about 40 min there appeared distinct stages of the burst's evolution. They consisted of the pattern of energy release in flares which was proposed by Sturrock et al. (1984). There were pulsations with periodicities of 0.178 and 4 s superimposed on the flux density. The pulsations were quasiperiodical with features of almost unchanged mean periods. The relative amplitude of the pulsation modulation changed with the phase of the burst; in general, it reached 10–20% in the rising phase. The possible mechanisms of pulsations are discussed and some plasma parameters of oscillation sources are deduced.     

Filamentary structure of the ISM at high latitudes

Different mechanisms for filament formation are studied with a view to account for the results of Verschuur (1991a, b). It is shown that pinch phenomena can produce a filamentary structure of H I without any pressure contribution from the warm component of the interstellar medium (ISM). The mechanisms of filament formation involving the Parker instability and magnetoconvection are also discussed. It is shown that instabilities corresponding to all these mechanisms may result in a picture similar to that observed by Verschuur.     

Studies of blue objects at high galactic latitudes

Continuing the survey for faint blue objects at high galactic latitudes, a field of 25 square degrees centered at the Coma Cluster of Galaxies has been examined on two-color plates taken with the 90-67 cm Schmidt telescope of Asiago. 487 objects have been identified. A selected list of those with the largest negative color index is given in Table I. Identification charts, notes and comments follow.     

Reddenings of quasars VS PSA counts at high galactic latitudes

The correlation betweenE(B-V) reddenings of high latitude radio quasars (from Teerikorpi, 1981) and star/galaxy counts from the PSA prints was investigated. With the resolution down to 5×5, the reddenings and the counts show the relationship expected if it is assumed that most of the high latitude stars (B<21 mag) are situated above the dust clouds. The dust causing the reddenings is probably connected with the population of molecular clouds observed at high galactic latitudes. The present study underlines the importance of simple star/galaxy counts (all images counted) for the study of high latitude variable extinction. The existence of variable extinction shows that also the average extinction must be significant.     

Two strong radio bursts at high and medium Galactic latitude

The Nasu Observatory, which is composed of eight 20 m elements, was constructed for observing radio transients over a wide field at 1400 MHz. We report on two radio transients detected in consecutive drift scanning observations at declination 32° over a period of about two months. One of the two transients, WJN J1039+3200, appeared at =10^h39^m40^s±10^s, δ=32°±0.4° on March 4, 2005, and the other one, WJN J0645+3200, appeared at =06^h45^m25^s±10^s, δ=32°±0.4° on March 24, 2005. Both exhibited flux densities in excess of 1 Jy, and the burst durations were up to two days. Since there are few examples of radio transients outside the Galactic plane, these are very important observations. We have previously reported on four radio transients with features that look like the two transients detected this time. Of these six WJN transients in total, five had a duration of up to two days, and one up to three days. Four of the transients were detected at high Galactic latitude of b > 30°. Counterparts of the six WJN transients included X-ray sources in four events and had a consistency of 66%. The consistency of γ-ray, PGC Galaxy, NVSS, and FIRST sources was concentrated at about 50%. We were not able to find any special features in the counterparts. The distribution was verified by making a log N–log S plot using data for the four previously detected transients and the new ones. As a result, the distribution of the radio transients that we observed might have an isotropic distribution not dependent on Galactic longitude and Galactic latitude. The detection probability was calculated based on the assumption of an isotropic distribution. The 2σ upper probability limit for detection of transients of 1000 mJy or more is 0.0049 [deg⁻² yr⁻¹]. We cannot yet identify these two radio transients, because their features are different from any radio bursts observed in the past.     

Convection at high latitudes when the interplanetary magnetic field is northward

It is shown that the pattern of convection at high latitudes when the IMF is northward is the sum of two types of convection: (a) four-vortex convection completely closed in high latitudes and (b) homogeneous sunward convection across all high-latitude regions including the auroral zone. The decay of the three-dimensional current system which existed before the turning of IMF northwards may maintain the homogeneous sunward convection.     

The north–south asymmetry of solar activity at high latitudes

Solar long-term activity runs at high latitudes in three ways: (i) in phase with solar long-term activity at low latitudes; (ii) in antiphase with solar long-term activity at low latitudes and (iii) does not follow either (i) or (ii), and mainly occurs around the times of maxima of (i) and (ii). In the present study, we investigate the north–south asymmetry of solar activity at high latitudes and found the following. In Case (i), high-latitude filament activity, for example, is inferred to have the same dominant hemisphere as low-latitude activity in a cycle. In Case (ii), the north–south asymmetry of high-latitude activity, represented by both the polar faculae and the Sun's polar field strength, is usually different from that of low-latitude activity in a sunspot cycle, and even in a cycle of high-latitude activity (polar faculae and the Sun's polar field strength), suggesting that the north–south asymmetry of solar activity at high latitudes should have little or no connection with that of low latitudes. In Case (iii), the north–south asymmetry of solar activity at high latitudes (polar flares) should have little connection with that at low latitudes as well. The observed magnetic field at high latitudes is inferred to consist of two components: one comes from the emergence of the magnetic field from the Sun's interior and the other comes from the drift of the magnetic activity at low latitudes.     

Pc3-4 geomagnetic pulsations at very low latitude in Brazil

In order to investigate Pc3-4 geomagnetic pulsations at very low and equatorial latitudes, L=1.0 to 1.2, we analyzed simultaneous geomagnetic data from Brazilian stations for 26 days during October-November 1994. The multitaper spectral method based on Fourier transform and singular value decomposition was used to obtain pulsation power spectra, polarization parameters and phase. Eighty-one (81) simultaneous highly polarized Pc3-4 events occurring mainly during daytime were selected for the study. The diurnal events showed enhancement in the polarized power density of about 3.2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of pulsation observed at stations near the magnetic equator showed a delay of 48-62° in relation to the most distant one. The peculiarities shown by these Pc3-4 pulsations close to the dip equator are attributed to the increase of the ionospheric conductivity and the intensification of the equatorial electrojet during daytime that regulates the propagation of compressional waves generated in the foreshock region and transmitted to the magnetosphere and ionosphere at low latitudes. The source mechanism of these compressional Pc3-4 modes may be the compressional global mode or the trapped fast mode in the plasmasphere driving forced field line oscillations at very low and equatorial latitudes.     

Interstellar neutral hydrogen filaments at high galactic latitudes and the bennett pinch

Observed properties of interstellar neutral hydrogen filaments suggest the presence of the Bennett pinch as described by the Carlqvist relationship with rotation around the filament axes included. A brief summary is first given of three ways in which a filament model for interstellar cloud structure was tested. Preliminary results from highresolution HI mapping of gas and dust in an apparent HI cloud indicate that the neutral gas and dust within and around its boundary is itself highly filamentary. An attempt to detect magnetic fields in this and similar features using the Zeeman effect technique at the 21-cm wavelength of interstellar neutral hydrogen set upper limits of a fewµG. In contrast, the strength of the toroidal magnetic field expected from the examination of the Carlqvist relationship is of order 5µG, which would be produced by a current of 1.4 · 10¹³ A. Zeeman effect technology is at present not able to detect toroidal magnetic fields of this order at the edge of barely resolved HI filaments. Nevertheless, currently available high-resolution HI data suggest that interstellar filament physics should take into account the role of currents and pinches for creating and stabilizing the structures.