On the modulational instability of electrostatic ion cyclotron waves     

M.J. Giles 《Planetary and Space Science》1979,27(5):691-697

The instability of electrostatic ion cyclotron waves to low frequency density modulations is considered and nonlinear equations are derived which describe its development in terms of a coherent four wave interaction. A dispersion relation for the linear phase of the instability is obtained and threshold conditions for marginal stability determined. It is shown, using data from recent optical observations, that the conditions necessary for the instability to occur in the auroral plasma would probably be satisfied and that modulational frequencies in agreement with the observations are obtained for plausible wave amplitudes. The nonlinear development of the instability is then studied and it is shown that substantial modulation can occur. It is suggested therefore that this instability could lead to the development of a strongly turbulent state.  相似文献   

Optical detection of electrostatic ion cyclotron waves in the auroral plasma     

G. Martelli  P.N. Collis  M. Giles  P.J. Christiansen 《Planetary and Space Science》1977,25(7):643-650

Spectral analysis of recent photometric observations has revealed the existence of narrow frequency band pulsations in the brightness of some auroras. Peaks in the power spectra, between 25 and 32 Hz have been observed. We suggest that these observations are associated with electrostatic ion cyclotron waves, which are excited when the relative drift between ions and electrons due to field aligned (Birkeland) currents, exceeds a certain critical value.  相似文献   

Some comments on flares after many years of observation     

H. W. Dodson  E. R. Hedeman 《Solar physics》1976,47(1):267-275

Ground based observations of flares are reviewed to seek implications for a flare build-up on either a long or a short time scale. Plots of flare frequency and importance for certain individual centers of activity suggest a possible crescendo in flare occurrence days and hours before the development of large and significant flares. The X-ray records follow the same pattern of apparent build-up. A possible dependence between successive major flares, as phases one and two of a single complex flare event, suggests that the time scale in which the total flare event takes place may show extreme variation.Since all flares start as small features, there is a short term build-up in the optical records. The characteristics of this build up are not clear. The initial brightenings in a flare may or may not show a flash phase, and the rise to maximum may or may not be accompanied by filament activity. Flares rise to maximum H intensity at markedly different rates. Although most flares occur in centers of activity with well defined and often complex magnetic fields, certain large and relatively energetic flares have developed in centers of activity with apparently very simple circumstances.  相似文献   

Fossils explained 43: Anaspid fishes     

C. Giles Miller  Henning Blom  & Tiiu Märss 《Geology Today》2003,19(3):111-113

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Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues     

Anton T. Kearsley  Giles A. Graham  Mark J. Burchell  Michael J. Cole  Zu Rong Dai  Nicholas Teslich  John P. Bradley  Richard Chater  Penelope A. Wozniakiewicz  John Spratt  Gary Jones 《Meteoritics & planetary science》2007,42(2):191-210

Abstract— The known encounter velocity (6.1 kms^?1) and particle incidence angle (perpendicular) between the Stardust spacecraft and the dust emanating from the nucleus of comet Wild‐2 fall within a range that allows simulation in laboratory light‐gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre‐existing major‐ and trace‐element composition of the foil, geometrical issues for energy dispersive X‐ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.  相似文献   

甘肃西山坪遗址生物指标记录的中国最早的农业多样化   总被引：1，自引：0，他引：1  

李小强  周新郢  周杰  John Dodson  张宏宾  尚雪 《中国科学D辑》2007,37(7):934-940

通过花粉、农作物种子和植硅石等农业活动生物指标记录研究, 结合高精度AMS14C测年, 重建了甘肃西山坪遗址5250 ~ 4300 cal a BP时段的农作物类型和农业栽培特征. 5100 cal a BP以来农业活动增强, 5070 cal a BP出现中国西北最老的稻作农业遗存. 4600 cal a BP左右针叶林突然消失和栗树扩张是先民选择性砍伐针叶树, 保留并栽培栗树的结果. 西山坪遗址4650~4300 cal a BP期间种植有粟、黍、水稻、小麦、燕麦、青稞、大豆和荞麦等8种粮食作物, 囊括了东、西亚2个农业起源中心的主要作物类型. 不仅证实小麦和燕麦早在4650 cal a BP已传播到中国西北地区, 也揭示了中国最早的农业多样化可能出现在新石器时代的甘肃天水地区.  相似文献   

Magnetotail phenomena and auroral acceleration     

M.J. Giles 《Planetary and Space Science》1980,28(5):459-465

It is suggested that localised electrostatic potential wells could be generated in the plasma sheet by large amplitude electrostatic ion cyclotron waves. It is shown from a consideration of a simple one dimensional model that such wells could possess a double structure of oppositely directed fields elongated in longitude. The possibility that the waves could evolve from a turbulent ion wave cascade driven by Earthward streaming protons is discussed and the magnitude of the potentials that could be established in this way is estimated using results for condensed state turbulent equilibria.The projections of these wells along the highly conducting geomagnetic field lines form potential valleys across the field lines in the high latitude auroral plasma. It is shown that these valleys would be of the scale and depth needed to establish electrostatic shocks which would be of sufficient intensity to accelerate electrons to energies comparable to those observed in “inverted-V” events. Potential wells are formed predominantly in the midnight sector of the plasma sheet and propagate Earthwards. This implies a corresponding equatorwards motion of the valley which, typically, would have a velocity of a few hundred m s^?1.  相似文献   

Comments on the course of solar activity during the declining phase of solar cycle 20 (1970–74)     

H. W. Dodson  E. R. Hedeman 《Solar physics》1975,42(1):121-130

In the declining phase of solar cycle 20 (1970–74) three pulses of activity occurred and resulted in two well defined ‘stillstands’ in the smoothed means of sunspot, 2800 MHz, and calcium plage data. Marked diminutions in spot and 2800 MHz flux took place in 1970 and 1971, respectively, and were accompanied by concomitant decreases in flare-occurrence. Studies of the latitude distribution of spots and flares show the extent of the dominance of the northern hemisphere in cycle 20 and the marked phase shift between northern and southern hemispheres. In the years studied, the longitudes of centers of activity clustered in identifiable zones or hemispheres for relatively long intervals of time. From mid-1973 to mid-1974 the Sun had a relatively inactive hemisphere centered on ～0° longitude. The relationship of certain well defined ‘coronal holes’ to this inactive hemisphere of the chromosphere is noted. The first two spot groups of the new cycle formed in November 1974 and January 1975 in the longitude zone associated with relatively high levels of old cycle activity, a repetition of the pattern observed in 1963–64.  相似文献   

Non-linear resonant excitation of whistler mode waves in the earth''s ionosphere using two high frequency transmitters     

G. Martelli  M.J. Giles  P. Christiansen 《Planetary and Space Science》1973,21(12):2113-2121

We discuss the possibility of exciting whistler mode waves (WMWs) in the Earth's ionosphere, by using two high frequency beams of electromagnetic waves (f₁f₂) suitably orientated to the geomagnetic field H_o, so that a non-linear resonant interaction can take place in the natural ionospheric plasma, approximately at the altitude of the F2 maximum electron density. Within the limitations imposed by ionospheric inhomogeneities in the interaction region, it should be possible to excite a WMW which propagates along a predetermined direction, e.g. parallel to H_o.

If we assumef₁ andf₂ to be approx 30 MHz (i.e. well above the ionospheric plasma frequency), this method would make it possible to select and vary the frequency range of the excited WMW up to a few hundreds kHz without substantial alterations to the high frequency transmitting system.

Since the two beams should form an angle close to 90° to the direction of propagation of the WMW, this technique may prove particularly suitable for active wave experiments at low geomagnetic latitudes, where the geometry of the geomagnetic field limits the feasibility of direct wave injection experiments.

Using the results of theoretical calculations of the three wave coupling coefficients, it will be shown that the transmitters required to produce WMWs with field strengths comparable to that of naturally occurring strong whistlers are substantial, but feasible.  相似文献   

Trade in ‘virtual carbon’: Empirical results and implications for policy     

Giles Atkinson  Kirk HamiltonGiovanni Ruta  Dominique Van Der Mensbrugghe 《Global Environmental Change》2011,21(2):563-574

The fact that developing countries do not have carbon emission caps under the Kyoto Protocol has led to the current interest in high income countries in border taxes on the ‘virtual’ carbon content of imports. We use GTAP data and input-output analysis to estimate the flows of virtual carbon implicit in domestic production technologies and the pattern of international trade. The results present striking evidence on the wide variation in the carbon-intensiveness of trade across countries, with major developing countries being large net exporters of virtual carbon. Our analysis suggests that a tax on virtual carbon could lead to very substantial effective tariff rates on the exports of the most carbon-intensive developing nations. As an illustration, we find that average tariff rates of 10%, 8% and 12% would be faced by imports from China, India and South Africa if carbon is taxed at $50/ton CO₂. Moreover, there is wide variation in intensiveness across sectors within countries with implications for the disparate effective tariff burdens on particulars parts of the economies of these countries. Such empirical findings, we argue, are useful for framing on-going discussions about the principles and practice of border taxes on virtual carbon.  相似文献