A static-state field-line reconnection model for the Earth's magnetosphere

A magnetostatic boundary value problem has been solved within the magnetospheric cavity and in the interplanetary space in order to incorporate the interplanetary magnetic field in a quantitative magnetospheric vacuum-B-field model. The boundary condition for the closed model, B_⊥ = 0 at the magnetopause, is replaced by the condition that the normal component of the total field penetrates the boundary continuously. Penetration of the B-field depends on a ‘merging-constant’ C₀, which is assumed to be independent of position on the magnetopause. C₀ = 0 indicates the closed model and C₀ = 1 describes, the complete interconnection between the magnetospheric and the interplanetary field.The model is valid for the field-line topology on the dayside including the polar cusp region. The position of the polar cusp's equatorial edge depends on the actual value of the merging constant C₀ and the orientation of the interplanetary magnetic field. Plasma-physical effects enter the model implicitly by the fact that a magnetopause shields the solar wind from the magnetospheric cavity. The static model neglects the hydrodynamic behaviour of the field-line merging process.     

Unintentional man-made modification effects in the magnetosphere

Evidence continues to accumulate that even very small input signals, such as power-line harmonic radiation (PLHR), can give rise to wave-particle interactions in the magnetosphere by stimulation of emission at VLF and contribute to the formation of the electron slot (2<L<3). Over North America and its geomagnetic conjugate, the satellite data indicate a permanent zone of VLF emission which appears to originate in the industrial areas of northeastern U.S.A. and southern Canada. The more intense stimulated emission in the summer (northern hemisphere) occurs poleward of the electron slot at 3<L<5. The resultant increased electron precipitation may explain the increase in thunderstorms at L>4 in the period 1935–1970 relative to 1900–1935. There is probably an intensity threshold at which PLHR becomes important. This is consistent with ground-based data from longitudinally separated stations near L = 4 (Siple, Halley, Yakutsk). The discovery that certain industrial plants (e.g. cement works) give rise to relatively much more intense PLHR requires further investigation. Also, geomagnetically induced currents in long power lines at high geomagnetic latitudes in Canada and Alaska can result in transformer saturation and possible damage, as well as greatly increased PLHR.Powerful VLF transmitters in the range 15–25 kHz can produce peaks in the spectra of energetic electrons observed on low altitude satellites. Morphological studies of the NAA wavefield intensity above the ionosphere show strong amplification immediately to the east of the South Atlantic Anomaly.     

The theory of radio windows in the ionosphere and magnetosphere

Radio waves in a stratified plasma can sometimes penetrate through a region where, according to a simple ray theory, they would be evanescent. They emerge on the far side in a different magnetoionic mode. This occurs when the incident wave normal is within a small cone of angles, called a radio window. The best known example is the Ellis window, used to explain the Z-trace in ionosonde records. Other phenomena where windows may be important have recently been studied. Simple approximate formulae are given for the transmission coefficient of a window and for its angular widths. These show the dependence on frequency, electron concentration gradient and direction of the ambient magnetic field. Comparison with more accurate calculations shows that these formulae are likely to be reliable in practical applications. The tracing of rays near a window is discussed, and the properties of a second kind of window are described.     

Global dynamics of the magnetosphere for northward IMF conditions

Ground-based and spacecraft observations of polar cap geophysical phenomena during periods of northward interplanetary magnetic field (IMF) show specific patterns of electric fields, field-aligned currents, aurora and particle precipitation. These are basically different from those when the IMF is southward. The total combination of observational data for northward IMF indicates rather a closed magnetosphere. This topology has led to the formation of a specific convection pattern in the distant plasma sheet. As different theoretical studies show, the connection of the IMF to geomagnetic flux tubes poleward of the cusp region may serve as the driving mechanism for plasma sheet convection and as the dynamo of current systems. Unfortunately, the direct observations of processes in the distant magnetosphere are too scarce either to accept or reject the concept of a closed magnetosphere. There are also some experimental data that are inconsistent with the closed magnetosphere topology. Definitive open or closed models must await future measurements.     

Penetration of solar wind plasma elements into the magnetosphere

Assuming that the solar wind is unsteady and non-uniform, it is suggested that field aligned plasma elements dent the magnetopause surface. This indentation makes the magnetopause boundary convex and therefore locally unstable with respect to flute instabilities. The intruding element is slowed down and stopped within 1 or 2 Earth radii from the magnetopause. Its excess convection kinetic energy is dissipated in the lower polar cusp ionosphere after ~ 50–500 s, depending on the value of the integrated Pedersen conductivity. Once the plasma element has been engulfed, keeping its identity, the warm plasma content is dissipated by precipitation and by drifting. The magnetosheath particles with large pitch angles are mirrored, and feed the plasma mantle flow.Several consequences of this penetration mechanism are pointed out:Ionospheric heating beneath the polar cusp;Birkeland currents on the eastward and westward edges of the plasma element;Diamagnetic field fluctuations within 1–2 R_E from the magnetopause (multiple magnetopause crossings);Oscillation of the magnetopause surface after a new element has penetrated;Exit of energetic particles out of the magnetosphere, and entry of energetic solar wind particles into the magnetosphere along the magnetic field lines of the intruding element;Subtraction of magnetic flux from the dayside magnetosphere and its addition to the geomagnetic tail when the magnetic field of the element has a southward component.     

The suppressing effect of field-aligned currents on VLF emissions in the magnetosphere

The growth rate of whistler-mode waves is calculated analytically for a bi-Maxwellian plasma in the presence of a beam of cool electrons. This beam is moving in the same direction as the gyroresonant electrons and in the opposite direction to the waves which are considered to propagate parallel (or anti-parallel) to the imposed geomagnetic field. A somewhat surprising result is found. This is that even if the anisotropy is greater than a critical value, which is strongly frequency dependent, the beam reduces the growth of the waves near half the electron gyrofrequency. For a field-aligned current density ~ 1 μA m⁻², this mechanism can explain the lack of signals near 1.4 kHz on auroral (return current) flux tubes. It can also explain the observed absorption of signals at half the electron gyrofrequency, around 7 kHz on L = 4 flux tubes, near the equatorial plane and just outside the plasmapause.     

空中游:离开地球表面

正在天方地圆说盛行的年代,我们大多在平面的世界里欣赏山川河流,或登高望远,感万物之渺小,叹人生之须臾。后来,地球被证实是椭圆形,再后来,美国莱特兄弟发明了飞机。飞机的诞生使人们开始解除大地的束缚,离开地球表面,飞向梦寐以求的天空,探索打开世界的另一种方式,以神的角度俯瞰万物。     

Google Earth软件在考古探测中的应用评析

Google Earth软件因其独特的视角、先进的性能、广泛的用途、低廉的价格决定了其在考古工作中大有作为,在探测研究历史文化遗产上将起到不可替代的重要作用。探讨Google Earth软件在考古学中运用的优缺点,以便更好地开发考古专用软件。     

On modeling component processes in the Earth's global electric circuit

There are many component processes interacting simultaneously in the Earth's global electric circuit. The three main generators in the circuit are thunderstorms, the ionosphere wind dynamo, and the solar wind/magnetosphere dynamo. This paper reviews the progress made in recent years in modeling the various generators and their mutual interactions. The progress made in modeling various component processes that affect the distribution of electric fields and currents is also reviewed.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

Inversion of oblique ionograms including the Earth's magnetic field

A technique for determining ionospheric electron distribution from oblique ionograms is presented, based on the inversion method of Reilly and Kolesar (Radio Sci. 24, 575, 1989). It makes use of an equivalent operating frequency and an additional term to account for magnetoionic effects associated with the Earth's magnetic field. The technique is demonstrated by application to synthetic oblique ionograms, and to an experimentally obtained ionogram.     

Settler geology: Earth's deep history and the governance of in situ oil spills in Alberta

Alberta's bitumen industry is frequently identified as a key site of environmental politics in the Anthropocene owing to the scale of its fossil fuel extraction operations. While popular images of surface mining activities often focus these discussions, approximately 80% of the bitumen reserves in the Canadian province lie too deep for surface mining and are extracted through in situ technologies, including processes that inject high-temperature, high-pressure steam to mobilize geologic formations of the tar-like fossil fuel. This article examines how in situ extraction was governed in response to four flow-to-surface (FTS) events in which bitumen unexpectedly migrated to Earth's surface as the result of in situ operations. The governance response to these events is of particular interest because it counters the assertion that existing governance institutions operate on time scales that are incommensurate with those relevant to the Anthropocene. The Alberta case shows the opposite owing to how Earth's deep history was used to provide temporal syntax for a geotechnical debate that ensued over what caused the FTS events. By detailing the controversy over what caused the FTS events, and the search for “enabling conditions” that would link causal explanations to the spatial distribution of the four bitumen seeps, Earth's deep history was also made commensurate with the political geography of settler colonialism in Alberta. The article introduces and develops the notion of ‘settler geology’ in order to capture the naturalization of geologic forms of reasoning about Earth's deep history, the geologic force of anthropogenic in situ operations, and the temporal framework of settler colonial governance in Alberta.     

Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere,ionosphere and atmosphere

The physical mechanism of a cyclotron resonance interaction between trapped energetic electrons and whistler-mode waves in the magnetosphere is discussed. Not only do the electrons have their pitch angles reduced in this interaction, so that they may be precipitated into the upper atmosphere, but also the waves can be amplified. Such a flux of precipitating electrons can, either by direct ionisation or via bremsstrahlung radiation, cause a pimple to be produced on the bottom of the ionosphere. That can significantly modify the amplitude and/or phase of very low frequency radio signals propagating in the Earth-ionosphere waveguide. Various experimental observations that demonstrate the reality of such effects are reviewed. The conditions necessary for a positive feedback situation are discussed, and some evidence for its existence assessed.     

The Water Balance and Groundwater Resources in Connection with the Vertical Water Exchange in the Upper Zone of the Earth's Crust

For research on regional estimates of the water balance and future water-resource planning, it is important to determine the amount of water leakage downward across the base level, i.e. from the upper layers of the earth's crust drained by streams to the lower layers. The author presents a methodology based on water-balance equations that shows such leakage to be negligible in terms of the margin of error of precipitation and evapotranspiration records. It can therefore be assumed that the groundwater discharge into streams is practically identical with the recharge of groundwater reserves through percolation.     

Energetic electron precipitation at the South Atlantic Magnetic Anomaly: a review

This paper reviews the status of knowledge concerning energetic electron precipitation at the South Atlantic Magnetic Anomaly (SAMA). The main purpose is to place recent results in the context of the long-standing problems about energetic electron precipitation at the SAMA region. A synopsis of results achieved in the last two decades, in relation to the various physical mechanisms responsible for precipitating energetic electrons, are also presented. The major uncertainties in the understanding of the energetic electron precipitation at the SAMA include: (1) temporal and spatial precipitation changes from magnetically quiet to disturbed periods; (2) the role of wave-induced precipitation processes.     

Predicting the Fate of Artefacts in Energetic, Shallow Marine Environments: an Approach to Site Management

A poor understanding of the physical environment often hinders management of marine artefacts. A study was conducted of an early-18th-century shipwreck to test whether the wreckage could have settled through ∼3.5 m of substrate. Results indicate that the wreck could have settled via episodic scour processes driven by storms and tidal inlet migration. A numerical model, modified to include characteristics of the underlying geology, predicts continued scour under moderate waves. Scour processes appear to have been interrupted by an erosion-resistant underlying layer, so that the wreck now remains exposed, subject to degradation. A generalized approach to predict burial or exposure of other shallow-water artefacts is developed.
© 2006 The Authors     

Modelling the Neolithic transition in a heterogeneous environment

The Neolithic transition, involving the change of subsistence from foraging to agriculture, can fruitfully be modelled mathematically, as, e.g., in the three-population model of Aoki et al. (1996). Here that model is modified to include some features of population dynamics in a realistic, two-dimensional environment, and including population pressure, competition for resources between farmers and foragers, and the dependence of the population carrying capacities and diffusivities on the environmental conditions. This modified model allows for the survival of foragers in regions where environmental conditions do not favour farming. The model is tentatively applied to the Indian subcontinent, which is a complicated example of this transition involving multiple domestication centres. The results are briefly compared with published archaeological data.