bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quasi‐static magnetospheric MHD processes and the “ground state” of the magnetosphere
Authors: Voigt G.‐H. , Wolf R. A. 
Published: February 2008

From the viewpoint of magnetic equilibrium and convection theories, we discuss whether there exists a theoretically well‐defined ground state of the Earth's magnetosphere. As a basis for this discussion, we review the quasi‐static MHD theory and show in a two‐dimensional (2‐D) model that the convecting magnetosphere can (mathematically) reach a steady state. The steady state configuration has fairly stretched magnetotail field lines and is, therefore, likely to be unstable to the tearing mode instability which ultimately leads to substorm onset in magnetotail regions with stretched field lines. We present an interpretation of the substorm phenomenon in terms of sequences of quasi‐static MHD equilibrium states and argue that, under the influence of convection, magnetic substorms occur periodically in Earth's magnetosphere, thus being an integral part of the entire convection cycle. After explosively releasing plasma and magnetic field energies in a substorm, the magnetosphere starts the convection process again from a low‐energy state. In addition to “smooth” tail field configurations, there exist oscillatory equilibrium tail structures with lower potential energy.We discuss the concept of the ground state of the magnetosphere within the context of 2‐D quasi‐static MHD models for the near‐Earth magnetosphere. However, we are unable to find a definition of a single magnetospheric ground state that is conceptually simple and computationally feasible. The term “ground state” denotes the lowest energy state of a system, which for a magnetosphere in quasi‐static force equilibrium is the empty vacuum magnetic field configuration with zero thermal plasma pressure. The real magnetosphere never approaches that state. Therefore we propose the term “average magnetosphere” for defining a baseline configuration that corresponds to average solar wind conditions. Earth's magnetosphere is convectively driven and powered by the solar wind. Thus all potential energy states W = ∫ [P/(γ − 1) + B²/2µ0] dV that belong to a particular convection time sequence between substorm events are transient in time. Therefore the newly defined “average magnetosphere” has a potential energy 〈W − Wvacuum〉 representing the time average of equilibrium states over the substorm cycle. Thus a “quiet” magnetosphere would have a net potential energy lower than the average, whereas an “active” magnetosphere would have a higher energy. It is an unresolved theoretical problem, however, to determine the lowest and highest possible energy states for the magnetosphere that are compatible with the thermodynamic condition of quasi‐static convection.Three‐dimensional (3‐D) magnetospheric equilibria have not been computed yet, but we describe our experience with Stern's stretch transformations and a magnetofrictional method for calculating 3‐D magnetospheric MHD equilibria.
A Parametric Study of VLF Modes Below Anisotropic Ionospheres
Authors: Snyder Floyd P. , Pappert Richard A. 
Published: February 2008

Weakly attenuated components of the VLF mode spectrum associated with propagation below highly anisotropic ionospheres are presented as a function of frequency and azimuth. In particular, results for the phase velocity, attenuation, polarization mixing ratio, and excitation factor for vertical dipole excitation are presented in the frequency range of 10–30 kHz for midlatitude paths, whereas azimuthal dependencies are presented for a frequency of 19.8 kHz and dip angles of 0° and 60°. The ionospheres are described by exponential electron density and collision frequency profiles, and the primary region of wave‐plasma interaction falls in the highly anisotropic region of the ionosphere. It is shown that polarization mixing is much more pronounced for a westerly propagation path than for an easterly path at midlatitudes, and, as a result, principally TE modes can be expected to influence the mode sum for frequencies at least as low as 20 kHz for propagation to the west. On the other hand, as has been shown previously, contribution to the mode sum by principally TE modes for propagation to the east becomes significant only near 30 kHz.Azimuthal anomalies include drastic polarization changes in going from easterly to westerly paths. For example, in the case of transverse propagation at the magnetic equator, it is shown that modes that are pure TM for propagation to the east may be pure TE for propagation to the west. Tantamount to this is the statement that modes that have dominant excitation for propagation to the east may have vanishing excitation for propagation to the west. Azimuthal dependencies are shown to be very often characterized by rapid variation of the mode constants in the neighborhood of north‐south or south‐north propagation. These variations manifest themselves in marked differences of the mode sum for azimuthal changes at least as small as 10°. Unfortunately, the azimuthal dependencies of the mode constants do not appear to lend themselves to any simple analytical approximations.
Bow shock and its interaction with interplanetary shocks
Authors: Dryer Murray 
Published: February 2008

Harbingers of significant magnetospheric motions consist of the interactions of interplanetary discontinuities with the standing bow shock. The most common discontinuity is the tangential discontinuity. Less frequent in occurrence, but of major significance to subsequent magnetospheric dynamics, is the flare‐generated interplanetary shock wave and its initiation of bow shock and magnetopause motion toward Earth. Early studies utilized the ordinary gas‐dynamic analogy of the well‐known Riemann splitting of an initial discontinuity (i.e., bow shock) into a reflected shock (the moving bow shock) and the transmitted shock (the inward‐moving interplanetary shock). It was shown that order‐of‐magnitude pressure increases at the subsolar point of the magnetopause are readily found for typical shock‐on‐shock studies. Phenomenological studies also demonstrated, in agreement with observations, that the magnetopause motion could be predicted on the basis of quasi‐static variation of the solar wind dynamic pressure. Recent hydromagnetic studies (for the simplified case of perpendicular shocks) have extended the theory to predict bow shock and magnetopause velocities which appear to be observed by spacecraft. Such one‐dimensional studies provide upper limits for all average plasma parameters within the region of the Sun‐Earth axis. The general case of the time‐dependent interaction in three dimensions has, as yet, not been done. Nevertheless, the general configurational and plasma details are, in principle, amenable to examination with the use of multiple spacecraft measurements during the interaction and the ensuing dynamic motions on a time scale of tens of min to the several hr required for the magnetopause to move to smaller geocentric distances. The physical processes of various energy transfers from kinetic to thermal and magnetic as predicted by the hydromagnetic theory could then be assessed by study of the plasma average velocity, direction of flow, magnetic field, temperature, and density.The magnetosheath plasma properties following such interactions could, on a more speculative note, provide new boundary conditions (associated with appropriate magnetosheath and magnetosphere magnetic field polarities) which could, in turn, initiate and drive the reconnection process within the magnetopause.
Magnetic results of Halley's expedition, 1698–1700
Authors: Bauer L. A. 
Published: February 2008

It will be recalled that the noted astronomer, Edmund Halley, in 1698, was placed by King George III in command of the sailing ship, the Paramour Pink, for “proceeding with her on an expedition to improve the knowledge of the Longitude and variations of the Cornpasse.” He was explicitly instructed that “in all the Course of your voyage you must be carefull to omit no opportunity of noting the variation of the Compasse, of which you are to keep a Register in your Journal.”1Halley set out on his first voyage from Deptford on October 20, 1698; returning to England towards the end of June, 1699, he sailed again from Deptford, September 16, 1699, this second voyage terminating at Long‐reach, September 7, 1700. The two voyages together extended over the Atlantic Ocean from 50°N. to about 52°S. Halley's expedition is memorable as being the first scientific one sent out by any country with the specific object of ascertaming the direction in which a compass sets itself at various places; it was the first ocean magnetic survey. The expedition fully justified itself, not only in the results immediately obtained, but by the impetus given to further investigation in magnetism and other subjects. It seems strange, therefore, that Halley, though a prominent, member of the Royal Society, published no report on his work. He appears to have been satisfied with the graphical representation of his magnetic results as shown on his famous chart—the first successful one of its kind—giving the lines of equal magnetic declination (variation of the compass) for the year 1700.
Measurements of the electrical conductivity in the free atmosphere up to 9,000 meters in height
Authors: Wigand Albert 
Published: February 2008

(Aerophysikalischer Forschungsfonds Halle, Abhandlung 11.)For the measurement of the electrical conductivity of the air in manned free balloons, Gerdien has designed a serviceable piece of apparatus, and has accomplished with it successful measurements. He finds that the conductivity shows considerable variations in the lower 1,000 meters, but it increases considerably at greater heights, at 5,760 meters reaching the value 13.65×10−4 E. S. U., and at 6,000 meters, the value 20.6×10−4 E. S. U.; that is, 22 times the amount found for the Potsdam mean value for the normal day (λ = λ+ + λ− − 0.95×10−4 E. S. U.) at a height of 85 meters. At Davos, altitude 1,600 meters, the conductivity was found 3 times as great as that at Potsdam (2.8×10−4).
Chapters in the history of terrestrial magnetism
Authors: Mitchell A. Crichton 
Published: February 2008

Chapter III—The discovery of the magnetic inclination1—It has been shown in Chapter II that the literature of the fourteenth century contains references to the fact that the direction of a suspended or pivoted magnetic needle is not, always and everywhere, that of the geographical meridian at the place of observation; that for nearly 200 years the existence of this variation was either denied, or doubted, or ascribed to irregularity in the magnetization of the needle or to faulty methods of observation; but that finally, about the middle of the fifteenth century, it was recognized as a general or world‐wide phenomenon. Thus was reached the conception of magnetic declination.
Tables on sunspot‐frequency for 1749–1938
Authors: Brunner W. 
Published: February 2008

In view of the numerous requests received for the whole series of Zürich sunspot‐numbers and the depletion of the supply of reprints of the table of observed relative sunspot‐numbers for 1749–1933 published in this JOURNAL (September, 1934), Table 1 is reprinted here with additions for the years 1934–1938. The new Table 2 is added for the so‐called “smoothed monthly means” of sunspot‐numbers for the same period and Table 3 for the epochs of maxima and minima of sunspots from 1610 to 1938.Table 1 gives the monthly means of the observed daily relative sunspot‐numbers compiled by Rudolf Wolf, using old observations of the Sun from 1749 to 1849 and published yearly since 1849 by the astronomers of the Zürich Observatory (Wolf, Wolfer, and Brunner in the Astronomische Mitteilungen of this Observatory). Since 1849 these means have been based upon careful daily observations (280 to 310 days a year) at our Observatory in which the same instrument has always been used. As is seen from the Astronomische Mitteilungen these observations are supplemented by observations made elsewhere with different instruments and by different observers. These observations are rendered comparable with our own by means of empirically determined reduction‐factors. Our daily relative spot‐numbers form therefore a completely unified system.
Ionospheric effects associated with magnetic disturbances
Authors: Berkner L. V. , Wells H. W. , Seaton S. L. 
Published: February 2008

Consideration of ionospheric effects coinciding with magnetic disturbances must, as yet, be qualitative. Discussion in this paper is directed toward elucidating the nature of ionospheric changes coincident with magnetic activity in the temperate and equatorial zones as interpreted from automatic multifrequency ionospheric records. Observations at Kensington (Maryland) during summer are described and illustrated. During summer night, following commencement of a storm, ion‐density of the F‐ or F2‐regions decreases and height increases. The region loses its smooth horizontal distribution and appears to break up into “clouds” of non‐uniform ion‐density. There is evidence of turbulence in the F‐region during the disturbance. When a storm begins at night during summer, the F2‐region is observed to separate from the F1‐region immediately at daybreak, or for more severe disturbances to appear first at great heights above the F1‐region with greatly reduced ion‐density. Winter‐night and summer‐night conditions are similar. For moderate magnetic disturbances during the day in winter little ionospheric disturbance is noticed, the ionosphere recovering rapidly to nearly normal conditions after daylight. Penetration‐frequencies remain slightly disturbed during winter day, ranging from a little above normal to below normal depending upon intensity of the disturbance, and the region appears to be thicker than usual.The severe magnetic disturbance of April 16, 1938, is selected for study in connection with equatorial‐zone ionospheric disturbances. It is shown that this ionospheric disturbance can be divided into three main phases. With the advent of the storm the maximum electron‐density of the F‐region fell by some three orders of magnitude (essentially to zero) in the first hour. This was followed by appearance of an intensely ionized absorbing region at about 85 km from which echoes were weak. After about 45 minutes this region disappeared as suddenly as it had appeared. Virtual heights of the F‐region then started to rise, first with increasing and then with decreasing ion‐density, until the maximum ion‐density again fell to substantially zero, with the few remaining electrons (about 5000 per cc) being at enormous heights. The surprising thing about these facts is that the ion‐density should fall essentially to zero throughout the stratosphere. The appearance of an absorbing region at low levels at different latitudes is considered in connection with sporadic E‐region ionization. Dependence of observed changes of radio transmission upon the observed ionospheric effects are considered.
The electrical characterization of days at Colaba (Bombay, India) during 1930–38
Authors: Mukherji S. M. , Pillai A. R. 
Published: February 2008

Systematic recording of atmospheric electric potential‐gradient at Colaba commenced from the middle of June, 1930, with a photographic electrograph of the Cambridge Scientific Instrument Co. The instrument was first installed in the old Milne Seismograph room of the Observatory but in February 1931 was transferred to a specially constructed hut in the open space near the sea‐coast in the northeast corner of the Observatory compound.A radium‐spiral screwed to the outer end of an insulated metal rod which projects out horizontally through a hole in the western wall of the hut, is used as the collector. The spiral is placed 171 cm above the ground with its middle point 40 cm away from the outer surface of the wall during normal days. The distance of the collector from the wall is reduced suitably during disturbed weather.
Geomagnetic three‐hour‐range indices for the years 1938 and 1939
Authors: Bartels J. , Heck N. H. , Johnston H. F. 
Published: February 2008

The paper gives three‐hour‐range indices K for seven observatories for July 1, 1938, to December 31, 1939 (Table 1), world‐wide three‐hour‐range indices Kw for 1938 and 1939 (Table 2, which will be used most often), and, based on Kw, daily indices B (Table 3) and monthly frequencies and averages (Table 4). Section (1) contains all the information necessary for using these Tables in geophysical or ionospheric work.Sections (2) to (5) show how the indices K are standardized by means of keys for transforming K into reduced indices Kr. The world‐wide Kw is introduced as an average of the reduced indices Kr, in which the Kr from polar stations enter with higher weight than the Kr from equatorial stations. Kw is proposed as a measure of the intensity P of solar corpuscular radiation.Note—Throughout this paper, a superior cross (×) denotes a half unit, thus, 0× =0.5, 1× = 1.5, etc.
Watheroo Magnetic Observatory January to March, 1946
Authors: Parkinson W. C. 
Published: February 2008

(Latitude 30° 19′.1 S., longitude 115° 52′.6 or 7h 43m.5 E. of Gr.)January 3–4— A magnetic disturbance of moderate severity began with a “sudden commencement” at 08h 10m GMT, January 3. At this time H increased very rapidly by 32 gammas, the westerly declination decreased by 4′.5, and Z decreased by 32 gammas. Rapid and irregular fluctuations began immediately afterwards, two periods of major disturbance being worthy of mention. Between 11h 00m and 11h 30m, January 3, H decreased by 137 gammas; between 11h 13m and 11h 43m Z decreased by 122 gammas, and between 11h 17m and 11h 42m westerly declination increased by 18′. The second period of activity was between 15h 10m and 16h 10m, January 3, when a peak appeared in the H‐trace, the maximum point of which was 161 gammas above the trace before and after. Corresponding movements appeared in D and Z. Small to moderate movements in all three elements continued until 20h, January 4, after which the traces were normal, with the H still slightly below its predisturbance value. Ranges: D, 26′.7; H, 226 gammas; Z, 168 gammas.
Pre‐history of the Earth's magnetic field
Authors: Johnson E. A. , Murphy Thomas , Torreson O. W. 
Published: February 2008

SummaryIn order to determine the origin and nature of the Earth's magnetic field and to test the various hypotheses which have been advanced to explain the field, it is desirable to determine the history of this field throughout geologic time and to investigate more carefully its spatial variations, both inside and outside the Earth's surface. This research is concerned with the determination of the history of the Earth's field as it can be deduced from the present polarization of crustal material. Unconsolidated fresh‐ and salt‐water sediments have been investigated. These sediments are in the form of clays and offer one of the simplest types of polarization, since the clays can be redeposited under laboratory conditions. A particularly lengthy investigation of the polarization of glacial varves has been made, together with measurements on core samples of sediments from the Pacific. From a study of anomalous deposits in the glacial clays, the geologic stability of the polarization of these clays has been established over geologic time. From the measurements of the glacial clays, it is concluded that the Earth's field has not changed substantially in direction or intensity during the last 15,000 years. From measurements of the Pacific cores, it is tentatively concluded that the direction and intensity of the Earth's magnetic field has probably remained substantially constant during the last million years. A much more complete investigation is necessary to verify these tentative conclusions. It would be desirable to extend the measurements to periods of the order of one billion years. These results are consistent with the “fundamental” theory proposed by Schuster, Babcock, and Blackett, but do not provide positive evidence to support this theory.
Hysteresis During Infiltration and Redistribution in a Soil Column at Different Initial Water Contents
Authors: Vachaud Georges , Thony Jean‐Louis 
Published: February 2008

Experimental results dealing with flow processes involving hysteresis effects in the suction‐water content relationship were obtained on a vertical column of soil submitted to the following successive controlled flow conditions: (1) redistribution of water following a constant head infiltration in an initially air‐dried column. (2) constant head infiltration followed by redistribution, and (3) constant flux infiltration followed by controlled evaporation and redistribution.Water contents were measured by using gamma ray attenuation, and water suctions were obtained simultaneously from 10 pressure transducers distributed along the soil column, each one being connected to a tensiometer. A data acquisition system permitted fully automated measuring sequences. All the data were punched on tape and treated with a computer.Examination of the changes of water content and water suction at a given depth permitted us to obtain precise information on the hysteretic behavior of Ψ (θ). This analysis showed that the scanning curves for a single reversal can be defined uniquely with reference to the transition water content θ*, that the independent domain theory appears to be inadequate for describing soil water hysteresis completely, and that k(θ) can be considered practically unique.
Daily Precipitation Modeling by Discrete Autoregressive Moving Average Processes
Authors: Chang Tiao J. , Kavvas M. L. , Delleur J. W. 
Published: February 2008

To account for the statistical dependence among the daily precipitation magnitudes and among the daily precipitation occurrence times, various models from the discrete autoregressive moving average (DARMA) family are constructed for and applied to the daily precipitation data in Indiana. The structure of the DARMA family is specified by the marginal distribution of independent identically distributed (i.i.d.) discrete random variables and by their random linear combination. A fundamental property of the DARMA family is that its correlation structure can be specified independently of its marginal density. The DARMA correlation structure remains invariant under transformation of the marginal density. This property is satisfied by the Indiana daily precipitation data, and it facilitates their modeling by the DARMA family of processes. It is shown that the Markov chain, a popular model for the daily precipitation sequences, is a specified model within DARMA family. The Indiana daily precipitation data is transformed into a multistate discrete precipitation sequence by discretizing the daily precipitation amount into a number of magnitude states. The stochastic process of the daily precipitation sequences is analyzed in terms of the locally stationary seasons defined within the paper. In each season the candidate DARMA models are identified in terms of the covariance function within the season. The parameter estimation of these candidate models is done by fitting their theoretical autocorrelation functions to their empirical counterparts, and the state probabilities are obtained through the multistate mean run lengths. The theoretical run‐length distributions of the candidate models are derived for each of the precipitation magnitude states, including the dry state. The goodness‐of‐fit tests of the candidate models and the best model selection are based on the theoretical multistate run‐length distributions of the daily precipitation data.Supplement (data, tables, calculations, etc.) is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document W84‐001; $2.50. Payment must accompany order.