A criterion of double-mode pulsation from coupling coefficients

A criterion for the occurrence of double-mode pulsations is derived from linear adiabatic coupling coefficients The criterion indicates that double-mode pulsations of classical Cepheids occur at a shorter period range than the observed one, and that the evolutionary mass models are preferable for the double-mode pulsations.     

Pulsating type IV solar radio bursts

Several models for pulsating type IV radio bursts are presented based on the assumption that the pulsations are the result of fluctuations in the synchrotron emission due to small variations in the magnetic field of the source. It is shown that a source that is optically thick at low frequencies due to synchrotron self-absorption exhibits pulsations that occur in two bands situated on either side of the spectral peak. The pulsations in the two bands are 180° out of phase and the band of pulsations at the higher frequencies is the more intense. In contrast, a synchrotron source that is optically thin at all frequencies and whose low frequency emission is suppressed due to the Razin effect develops only a single band of pulsations around the frequency of maximum emission. However, the flux density associated with the later model would be too small to explain the more intense pulsations that have been observed unless the source area is considerably larger than presently seems reasonable.     

Subsecond (0.1 s) Pulsations in the 11 April 2001 Radio Event

Radio pulsations observed during the 11 April 2001 event at six single frequencies (237, 327, 408, 610, 1420, and 2695 MHz) by the Trieste radio-polarimeter with a time resolution 10 ms are analyzed. A wavelet analysis method as well as time delay and polarization measurements are used. Both methods reveal pulsations with a period of about 0.1 s at all observed frequencies. Furthermore, the 0.1 s pulsations drift toward higher and lower frequencies, starting at about 1420 MHz. The polarization of pulsations increases with frequency and time. The remarkable fact that the detected 0.1 s period of pulsations does not depend on frequency in a very broad frequency range is discussed in terms of existing models of pulsations.     

A time profile of millisecond pulsations of solar microwave radio bursts

An hypothesis on the interference origin of millisecond pulsations of solar-burst microwave radio emissions based on the fact that the signal scintillation appears as a result of radio-wave propagation through an inhomogeneous turbulent corona is considered. It is shown that the time profile of pulsations depends on the phase difference of interfering waves and can either look like pulses of “emission” and “absorption” or it can have a sawtooth form with slow buildup and fast drop. The observed properties of pulsations were compared with predictions of this model; this comparison showed that the formation of pulsations and their observed properties are satisfactorily explained by multipath propagation, which takes place at traversal of the coronal plasma by radio waves.     

The role of the Hermanus Magnetic Observatory in geomagnetic field research

Geomagnetic field research carried out at the Hermanus Magnetic Observatory over the past decade is reviewed. An important aspect of this research has been the study of geomagnetic field variations, with particular emphasis on ULF geomagnetic pulsations. Features of geomagnetic pulsations which are unique to low latitude locations have been investigated, such as the cavity mode nature of low latitude Pi 2 pulsations and the role played by ionosphericO ⁺ ions in the field line resonances responsible for Pc 3 pulsations. A theoretical model has been developed which is able to account for the observed relationships between geomagnetic pulsations and oscillations in the frequency of HF radio waves traversing ionospheric paths. Other facets of the research have been geomagnetic field modelling, aimed at improving the accuracy and resolution of regional geomagnetic field models, and the development of improved geomagnetic activity indices.     

太阳微波低频段射电脉动现象的观测分析

综述云南天文台“四波段（１４２０ 、２０００ 、２８４０ 和４０００ ＭＨｚ） 太阳射电高时间分辨率同步观测系统”在１９８９ 年１２ 月—１９９４ 年１ 月期间所观测到的１２ 个射电脉动事件,发现在这些波段有多种不同特征的脉动现象,并在此基础上对脉动的形态、周期、带宽等观测特征作了分析和讨论     

Multisatellite observations of resonant hydromagnetic waves

Most measurements of long period ULF pulsations have come from ground based and single satellite observations. The observations have given strong support to the idea that these waves are resonant standing hydromagnetic waves on geomagnetic field lines. Simultaneous ground-satellite observations provide further details of the pulsation structure and are useful for examining the effect of the ionosphere on the transmission of the waves to the ground. Recently, multisatellite observations have been used to provide further insight into the nature of pulsations and we review the results obtained using this technique. Among the results presented are those from the ISEE 1 and 2 spacecraft which are closely spaced in identical orbits, making it possible to distinguish temporal from spatial structure in waves. The ISEE spacecraft have made measurements of resonant region widths and resonance harmonics. In addition, examples are shown of recent multisatellite observations of the global nature of some pulsations and the localization of Pi2 pulsations in space.     

Multipoint observations of low latitude ULF Pc3 waves in south-east Australia

Geomagnetic pulsations recorded on the ground are the signatures of the integrated signals from the magnetosphere. Pc3 geomagnetic pulsations are quasi-sinusoidal variations in the earth’s magnetic field in the period range 10–45 seconds. The magnitude of these pulsations ranges from fraction of a nT (nano Tesla) to several nT. These pulsations can be observed in a number of ways. However, the application of ground-based magnetometer arrays has proven to be one of the most successful methods of studying the spatial structure of hydromagnetic waves in the earth’s magnetosphere. The solar wind provides the energy for the earth’s magnetospheric processes. Pc3–5 geomagnetic pulsations can be generated either externally or internally with respect to the magnetosphere. The Pc3 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in Pc3 occurrence are of vital importance because they provide evidence which can be directly related to wave generation mechanisms both inside and external to the magnetosphere. At low latitudes (L < 3) wave energy predominates in the Pc3 band and the spatial characteristics of these pulsations have received little attention in the past. An array of four low latitude induction coil magnetometers were established in south-east Australia over a longitudinal range of 17 degrees at L = 1.8 to 2.7 for carrying out the study of the effect of the solar wind velocity on these pulsations. Digital dynamic spectra showing Pc3 pulsation activity over a period of about six months have been used to evaluate Pc3 pulsation occurrence. Pc3 occurrence probability at low latitudes has been found to be dominant for the solar wind velocity in the range 400–700 km/s. The results suggest that solar wind controls Pc3 occurrence through a mechanism in which Pc3 wave energy is convected through the magnetosheath and coupled to the standing oscillations of magnetospheric field lines.     

Transient ULF pulsation decay rates observed by ground based magnetometers: The contribution of spatial integration

Theoretical studies suggest that Joule dissipation in the ionosphere is the major source of damping for resonant ULF pulsations. The decay rates of transient pulsations (i.e. short-lived pulsations with latitude dependent periods) observed by ground based magnetometers are however generally larger than those predicted, and also larger than those observed in the magnetosphere. We have modelled the integration effects of ground based magnetometers on transient pulsations by considering empirical models of the associated ionospheric currents. The simulated ground magnetometer data show a smearing of the amplitude and period variations, which is more pronounced for smaller scale (specifically latitudinal) variations. The period increase with latitude is reduced, and may even be eliminated over appreciable latitude ranges. For all spatial scales the observed decay rates are typically 2–3 times larger than the true values, due to the additional decay resulting from spatial integration of the incoherent transient pulsations. Estimates of the ionospheric Pedersen conductance based on ground magnetometer observations of decay rates are correspondingly too small, and spurious gradients may be introduced. The present calculations reconcile observed decay rates on the ground with those predicted using the assumption that Joule dissipation is the dominant damping mechanism for toroidal mode resonant oscillations.     

Some features of Pc5 pulsations during a solar cycle

The morphological features of Pc5 pulsations during a solar cycle are studied using Fort Churchill data for the years 1962–1972. Some of the characteristics noted are as follows: (1) Increasing sunspot numbers show little influence on the diurnal variation of the occurrence, amplitude and the period except perhaps some noticeable change in the absolute magnitude of these parameters during different hours of the day. (2) The morning occurrence peak dominates during all phases of the solar cycle. (3) As noted earlier (Gupta 1973a), with increasing magnetic activity the day side region(s) of generation of Pc5 is found to shift closer to the subsolar point and in the midnight sector, the occurrence region (presumably the region of open and closed field lines) seemed to shift towards earlier hours with increasing magnetic activity and towards later hours with increasing sunspot numbers. (4) Despite the smaller number of data points for high magnetic activity levels the analysis indicates that the amplitude of Pc5 pulsations is directly related to all the levels of magnetic activity. (5) The periods of Pc5 pulsations show strong correlation with increasing sunspot numbers and the amplitude and occurrences are found to vary in accordance with the magnetic activity all through the cycle. (6) The annual and semi-annual variations of Pc5 parameters have been demonstrated especially for the pulsations occurring in the morning close to 8 ± 1 h LT and for those occurring near the midnight hours. (7) A suspected 27-day recurrence tendency has been clearly noticed for the occurrence, amplitude and period of Pc5 pulsations.     

Pulsating radio emission at decametre wavelengths from the sun

Observations on the pulsation pattern in the time profile of short duration solar radio bursts at decametre wavelengths are presented. The pulsations are found to be present predominantly in the saturation phase of the burst. A tentative physical model based on the non-linear development of the waves interacting in a turbulent medium is invoked to explain the origin of the pulsations.     

Correlations between ground observations of Pi 2 geomagnetic pulsations and satellite plasma density observations

Ground observations of Pi 2 geomagnetic pulsations are correlated with satellite measurements of plasma density for three time intervals. The pulsations were recorded using the IGS network of magnetometer stations and the plasma density measurements were made on board GEOS-1 and ISEE-1. Using the technique of complex demodulation, the amplitude, phase and polarisation characteristics of the Pi 2 pulsations are observed along two meridional profiles; one from Eidar, Iceland (L = 6.7) to Cambridge, U.K. (L = 2.5) and the other from Tromso, Norway (tL = 6.2) to Nurmijarvi, Finland (L = 3.3). The observed characteristics of the Pi 2 pulsations are then compared with the plasma density measurements. Close relationships between the plasmapause position and the position of an ellipticity reversal and a variation in H component phase are observed. A small, secondary amplitude maximum is observed on the U.K./Iceland meridian well inside the position of the projection of the equatorial plasmapause. The primary maxima on the two meridians, in general occur close to the estimated position of the equatorward edge of a westward electrojet. Using the plasma density measurements, the periods of surface waves at the plasmapause for two intervals are estimated and found to be in good agreement with the dominant spectral peaks observed at the ground stations near the plasmapause latitude and within the plasmasphere. The polarisation reversal, together with phase characteristics, spectral evidence and the agreement between the theoretical and observed periods leads to the suggestion that on occasions a surface wave is excited on the plasmapause as an intermediate stage in the propagation of Pi 2 pulsations from the auroral zone to lower latitudes.     

Photometric and spectroscopic investigation of the oscillating Algol type binary: EW Boo

We obtained the physical and geometrical parameters of the EW Boo system, which exhibits short period and small amplitude pulsations as well as brightness variations due to orbital motion of components. Towards this end we carried out photometric observations at Ankara University Kreiken Observatory (AUKO) as well as spectroscopic observations at TUBITAK National Observatory (TNO). The light and radial velocity curves obtained from these observations have been simultaneously analyzed with PHOEBE and the absolute parameters of the system along with the geometric parameters of the components have been determined. Using model light curves of EW Boo, light curve regions in which the pulsations are active have been determined and as a result of analyses performed in the frequency region, characteristic parameters of pulsations have been obtained. We find that the results are compatible with current parameters of similar systems in the literature. The evolutionary status of the components is propounded and discussed.     

High resolution method of direct measurement of the magnetic field lines'' eigen frequencies

The simultaneous observations of Pc4 geomagnetic pulsations at the two temporary stations, located along the geomagnetic meridian 50 km to the North and South from the observatory Borok (L = 2.8), have been used for the investigation of amplitude gradients of both H- and D-components of these pulsations. It has been discovered that the direction of a meridional component of the gradient H (grad_MH) depends on the frequency ƒ of a spectral component of pulsations. The grad_MD is directed more or less permanently northward independently from the frequency ƒ These results are the consequence of a local amplification of geomagnetic pulsations due to Alfvén waves resonance along the magnetic field lines. It has been demonstrated that the frequencies ƒ_R for which the northward direction of grad_MH is replaced by the southward one (with increasing ƒ) can be interpreted as the eigen frequencies of the field line which intersects the meridian in the middle between two temporary stations, i.e. in Borok.

The possible applications of a gradient method of measurement of the magnetic field lines' eigen frequencies are discussed.     

The position of ET And in the Hertzsprung-Russell diagram

The interstellar lines of Call (K) and NaI (D) are the main means used to estimate the distance of the Ap star ET And to derive its position in the HRD, because the classical methods for the determination of M are affected by some restrictions. By these means the star lies distinctly above the main sequence. The post-main sequence evolutionary stage suggests to assume that pulsations are the reason for the observed short-time spectroscopic and photemetric variations. Possibly, the 1.61-day period is also caused by pulsations.     

On the vertical electric component of the geomagnetic pulsation field

The existence of a close correlation between the atmospheric electric field and geomagnetic pulsations in the range of Pc 2–4 is demonstrated experimentally and theoretically (from the horizontal propagation of pulsations). The need to take this into account in any geological-geophysical interpretation of experimental data is stressed.     

日冕分米波射电甚短周期脉动的观测证据

利用国家天文台云南天文台“分米波(700—1500MHz)射电频谱仪”和“四波段太阳射电高时间分辨率同步观测系统”分别于2001年6月24日和1990年7月30日观测到了两个稀少事件，前者是一个小射电爆发，其上升相伴随有短周期(约29、40和100毫秒)的脉动，后者是一个射电大爆发，在2840MHz上产生了周期约30毫秒的射电脉动，还着重讨论其甚短周期(如29—40毫秒)的脉动现象，甚短周期脉动可能是归因于起源在日冕深处不稳定区域的哨声波束周期链对射电辐射的调制，或沉降电子束驱动的静电高混杂波，经由波-波非线性相互作用导致甚短周期的射电脉动。     

The effect of damping on geomagnetic pulsation amplitude and phase at ground observatories

We present some results from a model of forced oscillations of the magnetosphere. The purpose of this work is to examine the effects and consequences of damping on geomagnetic pulsations as observed on the ground. The aim of the current work is to quantify the amount of damping applicable to geomagnetic pulsation waveforms. Ionospheric conductivities vary with latitude and time of day and this variation will effect the damping of geomagnetic pulsations. The variations in ionospheric conductivities are taken into account to predict the changes in amplitude and phase of geomagnetic pulsations over an extended latitudinal array of ground observatories. Three situations are modelled where the damping factor γ/ω_n, which is related to the amplitude loss per cycle, is different: (i) γ/ω_n approximately equal to 0.01, this corresponds to the ionospheric Joule damping of Newton et al. (1978); (ii) λ/ω_n equal to 0.1, this value is consistent with the empirically determined day-time damping factors from the observed latitude-dependent transient decays of the pulsation single effect events discussed by Siebert (1964). The value of 0.1 as the damping factor is taken as typical of day-time conditions and its effect on amplitude and phase for continuous pulsations is considered; and (iii) λ/ω_n is latitude-dependent; three different levels of damping are used appropriate for the night-time conditions associated with the auroral electrojet, plasmatrough and plasmasphere.The results from the model suggest that observationally determined damping factors are greater than those computed from ionospheric Joule damping alone. The model also illustrates the broadening of the latitudinal resonance width with increasing damping and the reducing of the phase change across resonance to less than 180°. The model also successfully reproduces features of pulsation single effect events and Pi2 pulsations.     

Pi 2 pulsations and the auroral electrojet

Measurements of the properties of Pi 2 pulsations along a magnetic meridian at high latitudes during a number of substorms have been analyzed for their relationship to the auroral electrojet. It is found that the maximum Pi 2 pulsation amplitudes are closely associated with the instantaneous position of the electrojet. That is, the average pulsation amplitude in the Pi 2 band as well as the amplitudes of pulsations at specific frequencies in the band have maximum amplitudes at latitudes close to the instantaneous electrojet location. Stations equatorward of the electrojet tend to observe a classical Pi 2 waveform concurrent with the onset of the substorm electrojet. Stations near the electrojet observe a broad spectrum of pulsations indicating a multiplicity of sources. Stations poleward of the initial electrojet position see little pulsation activity until the electrojet moves overhead. The appearance of large amplitude Pi 2 pulsations at a station which was poleward of the electrojet at the onset of a substorm appears to be coincident with the arrival of the poleward border of the electrojet.     

Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: Location of source regions with respect to the auroral electrojets

Substorm onsets and intensifications are accompanied on a one-to-one basis by a Pi 2 magnetic pulsation burst. The source region for these pulsations is generally thought to lie in the region of substorm disturbance in the auroral oval. In this paper we outline the characteristics of Pi 2 pulsations in regions near the substorm enhanced electrojet but removed from the locale of the westward travelling surge. We show that a resonance region for the pulsations lies at the equatorwad edge of the westward electrojet, which in the evening sector marks the locus of the Harang discontinuity. Finally we show examples where the maximum amplitude of the Pi 2 is located at or equatorward of the southern border of the eastward electrojet or at the southern border of the westward electrojet. This is clear evidence for the coupling of wave energy into the L-shells far distant from the source of the energy. Mechanisms for Pi 2 generation are discussed in the context of the results presented in this paper.