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.     

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.     

The relationship between the strength of the IMF and the frequency of magnetic pulsations on the ground and in the solar wind

It has been established for some time that there is a correlation between the frequency of Pc 3–4 geomagnetic pulsations observed on the ground and the strength of the interplanetary magnetic field (IMF). The recent discovery of an apparently similar relationship between pulsations in the same frequency band in the solar wind and the strength of the IMF led to the suggestion that some magnetospheric Pc 3–4 pulsations have an exogenic source.In this paper we offer a statistical reappraisal of some of the earlier results, and an analysis of newly available ground and solar wind pulsation data sets, which suggest that on the basis of a frequency-field strength relationship alone, the case for an exogenic source is still unproven.We do, however, find support for the frequency-field strength relationship (for ground pulsations), which was the original basis for the Borok B index for prediction of the strength of the IMF. We also confirm that pulsation frequency is, at best, an imprecise predictor and show that any derived relationship is strongly dependent on the data sets used.     

Derivation of the amplitude equations of acoustic modes of an unstable semi-infinite polytrope

The coupling of non-radial pulsations and convection is studied on a simple example: an unstable semi-infinite polytrope. An expansion is proposed and the equations that should describe correctly the pulsations are isolated. The additional complications of the real solar case are discussed at the end.     

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.     

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.     

Hard x-ray Pulsations in the Initial Phase of Flares

When analyzing light curves of hard X-ray bursts recorded by the Hard X-Ray Spectrometer on board the MTI satellite, we have found three events (all associated with major solar flares, two of them in the same active region) which show pulsations in the very initial phase of the burst. Periods of the pulsations range from 25 to 48 s. We compare them with other observations of pulsations of radio waves and in X-rays and conclude that pulsations of this kind have not been observed before. We mention several possible causes and prefer interactions between current-carrying loops as the most likely interpretation of the observed variations.     

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.     

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.     

Pi2 pulsations in the magnetosphere

Several substorms were observed at Explorer 45 in November and December 1971, and January and February 1972, while the satellite was in the evening quadrant near L = 5. These same substorms were identified in ground level magnetograms from auroral zone and low latitude stations. The satellite vector magnetic field records and rapid run ground magnetograms were examined for evidence of simultaneous occurrence of Pi2 magnetic pulsations. Pulsations which began abruptly were observed at the satellite during 7 of the 13 substorms studied and the pulsations occurred near the estimated time of substorm onset. These 7 pulsation events were also observed on the ground and 6 were identified in station comments as Pi2. All of the events observed were principally compressional waves, that is, pulsations in field magnitude. There were also transverse components elliptically polarized counter-clockwise looking along the field line. Periods observed ranged from 40 to 200 sec with 80 sec often the dominant period.     

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.     

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.     

Power Fluctuations in Meteor Head Echoes Observed with the EISCAT VHF Radar

We present observations and preliminary results from a meteor experiment carried out with the 224 MHz EISCAT VHF radar in Tromsø, Norway, which was run for 6 h on November 26, 2003. The data set contains echoes with peculiar pulsations in received power in the frequency range 20–200 Hz, limited by instrumental parameters. The process causing the echo power pulsations has not yet been identified. Plasma effects are the most likely cause, a possible mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate.     

The behaviour of the geomagnetic pulsations near the boundary of the plasmasphere

Based on the observational data obtained at eleven stations along a geomagnetic meridian (Φm = 45–63°), the characteristics of pc 3, 4 pulsations are investigated. It has been shown that pc 3, 4 pulsations possess two amplitude maxima: one in the high latitudes and the other in middle latitudes. Consequently, the amplitude minimum between the two maxima is observed in subauroral latitudes (Φm ≈ 60°). Examining the peculiarities of the polarization behaviour of pc 3, 4 pulsations along the meridian array, two different regions, where the pulsations are generated, are noticed. One is situated in the middle latitudes of about 55–60°, and the other in the auroral area of about 65–70° in geomagnetic latitude. The former region corresponds to a projection of an area inside the plasmapause and the latter of an area of the outer radiation belt in the magnetosphere. The dependence of the pc 3, 4 periods on the position of the plasmapause is clarified. It is also shown that both the position of the pc 3 amplitude maximum in the middle latitudes and the position of pc 4 minimum in the subauroral area shift according to the variation in the magnetic activity and the position of plasmapause.The dynamic spectra of the simultaneous wave-packets of Pc-pulsations are investigated along the meridional profile. The maximum time delay of the Pc-signals is found at a latitude of about 57°, corresponding to the region of low values of Alfvén velocity inside the plasmasphere. On the other hand, a sharp decrease in the time delay is observed at a latitude of about 60°, the region of the rapid increase of Alfvén velocity at the plasmaspheric boundary in the magnetosphere.     

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     

Observational evidence for solar coronal decimetric radio pulsations with very short periods

Using the decimetric (700–1500 MHz) radio spectrometer and the synchronous observational system with high temporal resolution at four frequencies (1420, 2130, 2840 and 4260 MHz) of Yunnan Observatory, two rare events were observed on 2001 June 24 and 1990 July 30. The former was a small radio burst exhibiting pulsations with short periods (about 29, 40 and 100 ms) in the impulsive phase. The latter was a large radio burst, which at 2840 MHz produced radio pulsations with period of about 30 ms. This paper focuses on pulsations with very short periods in the range of 29–40 ms. The mechanism of generation of such pulsations may be modulation of radio radiation by the periodic trains of whistler packets originating in unstable regions of the corona. Alternatively, these pulsations can be attributed to wave-wave non-linear interactions of electrostatic upper hybrid waves driven by beams of precipitating electrons in flaring loops.     

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.     

Observation of quasi-periodic pulsations in the solar flare SF 900610

A quasi-periodic component was found at the maximum of the X-ray light curve for the June 10, 1990 solar flare detected by the Granat observatory. The pulsation period was 143.2±0.8 s. The intensity of the pulsing component is not constant; the maximum amplitude of the pulsations is ～5% of the total flare intensity. An analysis of the data showed the characteristic size of the magnetic loop responsible for these pulsations to be ～(1–3)×10¹⁰ cm.     

Doubling period effect in the Ez(air) component of the geomagnetic pulsation field

The effect of doubling of the period of Pc3–4 type of geomagnetic pulsations in the spectrum of the vertical electric component measured in the air is found.