Pi(c) magnetic pulsations: their relation to ionospheric currents

The association of the phase of the H and D components of the Pi(c) pulsations with the phase of the broadscale H component magnetic bays confirms that Pi(c) pulsations result from auroral electron precipitation induced conductivity enhancements of existing current systems. Statistically determined relationships between the time delays and phases of the H and D components of magnetic Pi(c) pulsations with respect to the optical pulsations are used to infer a delay between the E-region Hall and Pedersen current fluctuations associated with the pulsating electron fluxes. Theoretical modelling of an auroral pulsation patch, as per Oguti and Hayashi, is used to show that the polarization of the Pi(c) pulsations is controlled principally by the delay between the Hall and Pedersen currents and the direction of the background E-region electric field.     

Day-time Pi pulsations at equatorial latitudes

Low latitude Pi2 pulsations are considered to be the best indicators of the onset of magnetospheric substornis (Rostoker and Olson, 1978; Saito, 1979) and are hitherto believed to be mainly night-time phenomena. It is seen from this study utilising the pulsation records from Choutuppal (geomagnetic: 7°.5, 149°.3 E)and Etaiyapuram (geomagnetic: –0°.6.147°.5 E)and the “Common Scale Magnetograms” from the Auroral Electrojet (AE) stations during January–April 1976, that Pi2s do appear even during day-time on many occasions at equatorial latitudes in simultaneity with the onset of magnetospheric substorms at AE stations located in the night hemisphere. It is also found that the day-time Pis, unlike the night-time Pi2s, show enhancement in their amplitudes of Hx component at Etaiyapuram, situated at the dipequalor as compared to those at Choutuppal, well away from it. The results thus not only show the appearance of Pi pulsations during daytime in the equatorial zone, but also bring out the possible influence of the equatorial electrojet on their amplitudes at the dip equator.     

Polarization characteristics and phase differences of Pi2 pulsations at conjugate stations

Spectral analysis was carried out on Pi2 pulsations occurring simultaneously at the conjugate stations St Anthony, Newfoundland and Halley Bay, Antarctica during the second half of 1976. For the strongest lines in each spectrum, the ellipticities and azimuths of the horizontal polarization ellipses at the two stations were approximately equal and opposite in sign generally consistent with the stations being conjugate, but there was noticeable scatter. When the differences in phase of the components between the stations were examined, they were found to be about 10° for the H-component and about 170° for the D (consistent with concept of an odd-mode oscillation of the field line) until about middle-late October. After this time, larger variations in the phase differences and decrease in coherence between the stations occurred and it is suggested that is a result of the presence of an ionosphere E-layer at Halley Bay throughout the night during Southern summer.     

Ionospheric mechanism for the two maxima in the spatial distribution of Pc3 geomagnetic pulsations

A modelling of the spatial distribution of Pc3 geomagnetic pulsations on the Earth's surface is carried out. We propose that the main contribution to the PC3 amplitude is due to ionospheric currents fluctuating because of conductivity variations associated with the modulation of electron precipitations which occurs in the field of compressional waves coming, probably, from the solar wind. A coincidence of the two dayside maxima in Pc3 geomagnetic pulsation amplitude (at latitudes ~ 70° and 55–60°) with two maxima in electron precipitations is in favour of such a proposition.     

Ionospheric propagation of ULF-waves

Features of mid-latitude ionospheric propagation of geomagnetic pulsations are studied. It is shown that the orientation of the horizontal component of the incident wave vector with reference to the geomagnetic meridian has a strong influence on the amplitude transformation and on the angle of rotation of the polarization plane. The effect of different conductivities of the Earth's crust and the contribution of Hall currents to the total pulsation field in the case of an inclined geomagnetic field are also considered.     

Electrodynamic response of the middle atmosphere to auroral pulsations

On the nights of 21 and 28 October 1987, two Nike Orion payloads (NASA 31.066 and 31.067) were launched from Andøya, Norway, as part of the MAC/EPSILON campaign, to study the effect of auroral energetics on the middle atmosphere. Each payload carried detectors to measure relativistic electrons from 0.1 to 1.0MeV in 12 differential energy channels, and bremsstrahlung X-rays from >5 to >80keV in 5 integral channels. In addition, instrumentation to measure bulk ion properties and electric fields was also carried by these and/or near simultaneous flights. Flight 31.066 was launched during the recovery phase of a moderate magnetic substorm, during relatively stable auroral conditions. Flight 31.067 was launched during highly active post-break-up conditions during which Pc 5 pulsations (> 150s period) were in progress. The energetic radiation of the first event was composed almost entirely of relativistic electrons below 200 keV with negligible contributions from bremsstrahlung X-rays, while the radiation of the second event was dominated by much softer electrons ( < 100 kcV), which produced high X-ray fluxes that exceeded the cosmic ray background as an ionizing source down to altitudes below 30 km. Simultaneous conductivity measurements during both events show consistency with the ionizing radiations, with the pulsation event producing free electrons down to 55 km. far below their expected altitude range during night-time. These comparisons are discussed to evaluate the impact of such events on the middle atmosphere.     

曹丕文学史料三则

曹丕是我国历史上第一个留下大量文学作品的皇帝作家.在曹丕研究中,有许多值得讨论的问题,本文就其<典论>、<柳赋>的创作年代提出有别于前辈学者的看法,对"南皮之游"的时间亦有新的解释.     

皮锡瑞《经学历史》的编纂特点

《经学历史》对于经学通史的编纂有许多开创性。该书不仅从时间的跨度和内容的广度继承和发展了传统经学史的编纂特点 ,且吸收了西方史学的章节体体例 ,将经学在传统社会的发展史用一主题思想贯穿起来 ,从而成为第一部完整意义的经学史并对近代的经学史研究产生了一定的影响。但由于作者门户之见的限制 ,使得书中在探寻经学发展的原因时 ,在对经学史上各流派、重要的经学人物的评价上又表现出很大的局限性     

Zu RV 7,88,6c und 5b (avrká-)

Ohne Zusammenfassung     

Theory of generation of ULF pulsations by ionospheric modification experiments

A model characterized by three regions is considered. A thin dynamo region is separated by a region of free space from a perfectly conducting earth below. Above, the dynamo region is bounded by a magnetosphere in which hydromagnetic waves in the fast and in the Alfvén mode can propagate. Two limited extensions of a previous theory that is valid for frequencies well below 1 Hz and that neglects the effect of currents flowing into the magnetosphere, are considered. First, the frequency dependence of the ionospheric current system is determined, neglecting currents flowing into the magnetosphere. Secondly, the effects of currents flowing into the magnetosphere are evaluated in the low frequency limit and the Alfvén wave fields excited in the magnetosphere in the equatorial plane are calculated.     

A study of pearl pulsations in the equatorial region of India

Pearl pulsations in the equatorial region, despite their relative paucity, are known to exhibit definite features in their diurnal, seasonal and annual occurrence patterns. In the present study, attempts are made to examine and bring out in detail some more aspects of these pulsations recorded at Choutuppal (Geomagnetic latitude: 7°28′N) over a period of nine years during 1967–1975. The mid-period (t) of pearls is found to bear a linear relation to the pearl repetition period (T) and is of the form T = 70t + 11.2. The periods of pearls are seen to be influenced by the average magnetic activity level prevailing during the preceding few days rather than that existing on the day of pearl occurrence. There is an increase in the mid-frequency of pearls that occurred succeeding a magnetic disturbance particularly when the disturbance persisted over some days. The amplitude ratio (H_x/H_y) averaged for each two-hourly interval is seen to attain a value of unity around local midnight and decrease towards the dawn and morning hours. From these results, namely the dependence of mid-frequency of pearls on the average magnetic activity (represented by (Σ Kp >) during the preceding few days and the occurrence of shorter period pearls even after a few days following a magnetic storm, it is suggested that the plasmapause takes much longer time to re-establish to the quiet time position. In the alternative, it may be visualized that possible presence of localized regions of high plasma density gradients inside the plasmasphere after the cessation of a storm might provide favourable conditions for the generation of shorter period pearls.     

Ionospheric control of polarization of low-latitude geomagnetic micropulsations at sunrise

Continuous observations of low-latitude Pc3 and Pc4 geomagnetic micropulsations were carried out at ASO (22.0°N, 198.0° geomagnetic coordinates) from November 1979 to July 1980 to confirm the ionospheric control of polarization characteristics of low-latitude pulsations presented by Saka etal. (1980). The present study confirms the previous result that D-component amplitude starts to increase with sunrise. From the present study, the following results are obtained : (1) the D-component amplitude, which is much smaller than the H-component amplitude before sunrise, increases as much as that of the H-component after sunrise, and this brings about the tilting of the major axis of the polarization ellipse from north to northwest; (2) the onset-time of the D-component increment (or tilting of the major axis) coincides with the appearance of the E-layer in the ionosphere within an hour, and the time of the coincidence shifts from season to season, in parallel with the change of sunrise ; and (3) the ellipticity of the polarization in the horizontal plane is not affected appreciably by sunrise.It is suggested that the Hall conductivity increment associated with the E-layer sunrise enhancement affects the characteristics of the D-component on the ground.     

The phase relationships of the ionospheric signatures of Pc1 geomagnetic pulsations

In this study we consider the phase relationships between the oscillations of various ionospheric signatures associated with Pcl geomagnetic pulsations. Investigations using a simple analytical method and a numerical model, which has proved successful when applied to longer period pulsations, both suggest that Doppler velocity oscillations should be predominantly in anti-phase with oscillations of the rates of change of group range and echo amplitude. However, observations indicate that the Doppler velocity oscillations are in quadrature with the other two types of oscillations. Possible causes for this discrepancy are suggested.     

The Construction of the Khufu I Vessel (c.2566 BC): a Re-Evaluation

A re-evaluation of the published reports on the Khufu I vessel suggests: first, Egyptian boatwrights used as much of a plank's original width and length as possible to minimize waste, resulting in unpredictable stealer and scarf placement; second, crafting tenons to be narrower than corresponding mortises may have been one of many standardized techniques of Egyptian boatbuilding; third, temporary and permanent edge-to-edge transverse lashing was an important structural feature. Finally, woven straps instead of ropes were probably used to lash this vessel, and seams were probably sealed with wadding and battens, while the frames appear to have been recycled.
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