Radar observations of auroral zone flows during a multiple-onset substorm

We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every \sim12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the preexisting extended current system driven by the large-scale flow. These filaments are \sim1 h MLT wide (\sim600 km), and initially expand poleward to a width of \sim300 km before contracting equatorward and coalescing with the preexisting current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s⁻¹ or less during the first few minutes, before surging equatorward at 0.5-1.0 km s⁻¹ during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed \sim100 mho, before decaying over a few minutes. This value compares with Hall conductivities of \sim20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.     

Meridian-scanning photometer, coherent HF radar, and magnetometer observations of the cusp: a case study

The dynamics of the cusp region and post-noon sector for an interval of predominantly IMF B_y, B_z < 0 nT are studied with the CUTLASS Finland coherent HF radar, a meridian-scanning photometer located at Ny Ålesund, Svalbard, and a meridional network of magnetometers. The scanning mode of the radar is such that one beam is sampled every 14 s, and a 30° azimuthal sweep is completed every 2 minutes, all at 15 km range resolution. Both the radar backscatter and red line (630 nm) optical observations are closely co-located, especially at their equatorward boundary. The optical and radar aurora reveal three different behaviours which can interchange on the scale of minutes, and which are believed to be related to the dynamic nature of energy and momentum transfer from the solar wind to the magnetosphere through transient dayside reconnection. Two interpretations of the observations are presented, based upon the assumed location of the open/closed field line boundary (OCFLB). In the first, the OCFLB is co-located with equatorward boundary of the optical and radar aurora, placing most of the observations on open field lines. In the second, the observed aurora are interpreted as the ionospheric footprint of the region 1 current system, and the OCFLB is placed near the poleward edge of the radar backscatter and visible aurora; in this interpretation, most of the observations are placed on closed field lines, though transient brightenings of the optical aurora occur on open field lines. The observations reveal several transient features, including poleward and equatorward steps in the observed boundaries, braiding of the backscatter power, and 2 minute quasi-periodic enhancements of the plasma drift and optical intensity, predominantly on closed field lines.     

CRRES/Ground-based multi-instrument observations of an interval of substorm activity

Observations are presented of data taken during a 3-h interval in which five clear substorm onsets/intensifications took place. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were recorded. In addition data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation, are available. The locations and movements of the substorm current system in latitude and longitude, determined from ground and spacecraft magnetic field data, have been correlated with the locations and propagation of increased particle precipitation in the E-region at EISCAT, increased particle fluxes measured by CRRES and DMSP, with auroral luminosity and with ionospheric convection velocities. The onsets and propagation of the injection of magnetospheric particle populations and auroral luminosity have been compared. CRRES was within or very close to the substorm expansion phase onset sector during the interval. The onset region was observed at low latitudes on the ground, and has been confirmed to map back to within L=7 in the magnetotail. The active region was then observed to propagate tailward and poleward. Delays between the magnetic signature of the substorm field aligned currents and field dipolarisation have been measured. The observations support a near-Earth plasma instability mechanism for substorm expansion phase onset.     

Distribution of heavy metals in the Severn Estuary

There has been considerable agitation in recent months about heavy metal pollution in the Bristol Channel and Severn Estuary and on land around Bristol. Concentrations of zinc, cadmium and lead in water, sediments, seaweeds and shore animals are recorded from a number of sites on the southern shore of the estuary. Contamination is detectable ninety miles downstream of Avonmouth where the highest concentrations of these heavy metals are found.     

A recent history of metal accumulation in the sediments of the Thames Estuary, United Kingdom

Sediment cores were collected from the Tilbury Tidal Basin in the Thames Estuary to determine the depositional history of metals in the estuary. Profiles of metals in sediments deposited in the lower Thames Estuary show a 30–50% decrease in concentration for Ag, Cd, Cu, Pb, and Zn, and a 70% decrease for Hg in recent decades. Historic depth soundings data showed the decreases in metal concentrations occurred between 1944 and 1966. The decline in sediment metal concentrations has been attributed to reduced inputs to the estuary, following updating of the major sewage treatment works in 1959 and 1963. This is indicated by the through-core distribution of Mn which implies that prior to 1960 the sediments were deposited in anoxic conditions, which subsequently improved. An increase in Mn concentrations observed in one of the cores has been attributed to increases in the dissolved oxygen of the estuarine waters resulting from the increased efficiency of the sewage treatment works.     

A statistical study of ion frictional heating observed by EISCAT

Results of a statistical survey of F-region ion frictional heating are presented, a survey which is based on over 4000 h of common programme observations taken by the European incoherent scatter (EISCAT) UHF radar facility. The criterion adopted in this study for the identification of ion frictional heating was that defined by McCrea et al., requiring an enhancement in the F-region field-parallel ion temperature exceeding 100 K over two consecutive integration periods, which was itself based on a selection criterion for frictional heating derived for the study of high-latitude F-region ion temperature observations from the Atmospheric Explorer-C satellite. In the present study, the diurnal distribution of ion frictional heating observed by EISCAT is established and, furthermore, its dependence on geomagnetic activity and the orientation of the interplanetary magnetic field (IMF) is investigated; results are interpreted with reference to corresponding distributions of enhanced ion velocity, again derived from the extended set of EISCAT UHF common programme observations. The radar, due to its location relative to the large-scale convection pattern, observes ion frictional heating principally during the night, although preferentially during the post-midnight hours where there is reduced coupling between the ion and neutral populations. There is an increased preponderance of frictional heating during intervals of high geomagnetic activity and for a southward z component of the IMF and, moreover, evidence of asymmetries introduced by the y component of the IMF.     

A crop-based rainfall chronology for pre-instrumental record southern California

Although measured climatic records begin in California about 1850, agricultural records from the earlier Spanish period (1769–1834) can be used to develop information about annual precipitation. This historical material should be used, however, only with a full understanding of the numerous cultural variables that influenced agricultural production. A study by Lynch in 1931, which is widely cited in the literature of Pacific Coast climate reconstruction, contains serious shortcomings because of incorrect assumptions about the Spanish agricultural system, and, more specifically, the amount of climatic information expressed in yearly harvest data. In this article, mission data are analyzed more rigorously to produce a revised annual rainfall index for southern California that encompasses most of the pre-instrumental record period.     

Calculation of equilibrium distributions of chemical species in aqueous solutions by means of monotone sequences

A numerical method for the calculation of equilibrium distributions of chemical species in aqueous solutions of electrolytes is presented. This method is constructed by transforming the problem of determining the set of unknown concentrations satisfying the mass balance and mass action equations into the equivalent problem of finding the limits of certain well-behaved mathematical sequences in a multivariable direct iteration scheme. The total (analytical) concentrations are taken as the starting estimates, and the recursive equations are constructed from the starting equations. It is shown that the sequences so constructed are both monotonic and bounded, hence convergent in an orderly fashion by a mathematical axiom. Each unknown free ion concentration (that is, each limit) is approached simultaneously from above and from below, being effectively sandwiched in an ever tightening manner. Strict error bounds therefore are easily constructed. The method has been found to be exceedingly efficient in practice, with the error bound (maximum fractional error in taking the upper estimate as the final answer) decreasing in an approximately exponential fashion with respect to iteration number, commonly 0.5–1.5 orders of magnitude per iteration. Correction for nonideality is presented, and the possibility of this giving rise to more than one solution is discussed in connection with three examples of natural waters of low-, medium-, and high-ionic strengths.     

Whistler duct structure and formation

Whistler components received by the VLF goniometer at Halley, Antarctica, have been scaled for nose frequency and direction of arrival. The data were taken from two separate one hour periods on either side of local midnight (0023–0133 UT and 0340–0450 UT, 5 June 1975). Because of the high whistler occurrence rate at this time and the rapid processing facilitated by the Sheffield semi-automated whistler analyser, it has been possible to scale 1850 whistler components and hence obtain high resolution mapping of the whistler duct structure. The number of observable ducts increased from 4 at 0030 UT to about 15 at 0130 UT and had reached about 31 by 0400 UT. This increasing number of ducts together with an observed clustering of ducts in the second one hour period, provides evidence for duct formation. The observations have been compared with the various duct generation mechanisms which have been proposed. The duct formation process may have been associated with a negative change in Dst which occurred at the same time, and this possibility is discussed.     

Sources of bias in weighted average inferences of environmental conditions

Weighted averaging is widely used for inferring environmental conditions from an observed species assemblage. However, weighted average inferences are known to be systematically biased, and linear corrections (i.e., deshrinking functions) are commonly applied to adjust for this bias. In this analysis, the magnitude of the biases in weighted average inferences (and therefore the values of the deshrinking coefficients) are shown to depend upon the range of conditions sampled in the calibration data set and the true optima and niche breadths of the species observed in the calibration data set. Since the range of conditions and the observed species can differ between the calibration data set and the new data set for which environmental conditions are inferred, the coefficients for the deshrinking function derived using the calibration data may not be applicable to inferences computed using a new data set. Thus, environmental inferences may still exhibit systematic errors even after application of the linear correction. The findings from the theoretical analysis are demonstrated using stream temperature and macroinvertebrate data collected from wadeable streams in the western United States.