Observations of flux rope – associated particle bursts with GEOTAIL in the distant tail

Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re < |X_GSM| < 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotro-pic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure.     

Observations of borehole-source amplitude reduction due to casing

Theoretical results show that the amplitude of a borehole source is reduced when the well in which it is operated is cased and cemented. This reduction is a strong function of the formation velocity and is more weakly dependent on the direction of propagation of the wave travelling from source to receiver and on the diameter of the borehole itself. We have tested these predictions with data gathered in a cross-hole seismic experiment conducted in two stages in 1990 and 1991. The source and receivers were located in the MIT/Stech 1-21A and MIT/Burch 1-20B wells at the Earth Resources Laboratory (ERL) test site in Michigan (USA). Though the source well (MIT/Stech 1-21A) was uncased in 1990, a steel casing and cement were added prior to the collection of data in 1991. Several receiver positions were reoccupied to compare data collected with the source in open and cased holes. Using a velocity model for this area and borehole diameter measurements from a calliper log, a compensation factor can be computed that will adjust the data collected in 1991 to have amplitudes comparable to that collected in the first stage of the experiment. The accuracy of the results demonstrates the validity of the theory, which can be very useful in better understanding seismic waveforms recorded in cross-hole experiments.     

Effect of Rigid PVC Bailer Contact on Detection of Pesticides in Water Samples

A field experiment was conducted to examine the effect of short-term (one minute) contact of pesticide-laden water with a polyvinyl chloride (PVC) bailer on quantitative laboratory measurements of seven pesticide concentrations in distilled water samples subsequently decanted from the bailer. Pesticides were tested at two initial concentrations (low. based on current FPA maximum contaminant levels, or MCL: and high, based on a multiple of approximately lour times the MCL). Pesticide species included bromoxynil, diclofop-m, dimethoate. MCPA, methyl parathion, propiconazole, and trifluralin. Dimethoate recoveries were poor for all treatments. For all other pesticides there was no systematic difference between pesticide concentrations measured before and after bailer contact. Effectiveness of bailer decontamination treatments consisting of distilled water rinse alone was related to water solubility (S) for each species. Distilled water samples decanted from a rigid PVC bailer following initial bailer contact with pesticide-spiked water, and after the bailer had been cleaned with a single distilled water rinse, had measured pesticide concentrations of less than 2 percent of the pesticide concentration in the initial pesticide-spiked water, regardless of S. A single distilled water rinse effectively removed all trace of contaminants having S> 500 mg/L. Multiple distilled water rinses, and multiple distilled water rinses followed by 15 bailings of a well, effectively removed all trace of contaminants having S> 50 mg/L. Below threshold S, cleaning effectiveness decreased as a power function of S.     

Relationship of Avalonian and Cadomian terranes to Grenville and Pan-African events

Suturing of the supercontinent Rodinia in the Grenville event (˜ 1000 Ma) was followed by rifting in the late Proterozoic (˜ 800-700 Ma), reorganization to Gondwana in the Pan-African (˜ 700-500 Ma) and further accretion to develop Pangea at the end of the Paleozoic. One of the Rodinian rifts followed part of the Grenville suture, it produced the margin of eastern North America and southern Baltica and the contrasting margin of west Gondwana in present South America. The Paleozoic accretionary wedge against the Grenville-age margin of North America and Baltica contains Avalonian/Cadomian terranes that exhibit Pan-African erogenic events ± sediments apparently developed while the terranes were in or near Gondwana. These terranes carry lower-Paleozoic fauna (Acado-Baltic) that are not indigenous to North America and Baltica.U---Pb zircon ages range from 1500-1000 Ma in Grenville terranes and from 800–500 Ma with minor inheritance in Avalonian terranes; they are generally much older in Cadomian terranes, implying very little resetting during Pan-African events. T_DM ages are generally 2000–1200 Ma in Grenville terranes, 1300–600 Ma in Avalonian terranes and 2000–1200 in Cadomian terranes. These summary data show that: (1) the Grenville orogenic event produced almost no juvenile crust; (2) the Avalonian terranes of North America contain crust that evolved primarily in the late Proterozoic, possibly as a mixture of juvenile Pan-African material and Grenville or slightly older material; (3) the Cadomian terranes of Europe consist of old (middle-Proterozoic to Archean) crust with minor juvenile Pan-African material. The Avalonian terranes apparently evolved near, and partly on, the Grenville-age crust now in South America during the intense orogeny associated with rotation of Gondwana away from North America. The Cadomian terranes of Europe, however, appear to be fragments of other parts of Gondwana, probably West Africa.     

ELF and VLF wave generation by HF heating: A comparison of AM and CW techniques

Antenna field 2 of the Tromsø Heating facility consists of six rows of six horizontal crossed full wave dipoles aligned with the rows running geographic east-west. In previous experiments on ELF/VLF wave generation it has been the practice to feed the rows in parallel, with HF radiation amplitude modulated at the ELF/VLF frequency it was desired to radiate (AM configuration). Here we describe how the antenna array was also configured so that it could be fed with a continuous wave (CW) input power but still carry information at an ELF/VLF frequency. To effect this the three southern most rows of the antenna array were driven with a CW signal at 4.04 MHz and the three northernmost rows with a CW signal at frequency greater than 4.04 MHz by the ELF/VLF frequency it was desired to radiate (CW configuration). Experiments were performed with modulation/difference frequencies of 565 and 2005 Hz and the signals were received at the Lycksele Geophysical Observatory, 500 km south of the heating facility. The signals were typically 11 dB greater in the AM than the CW configuration, despite the fact that the average power delivered to the ionosphere in the CW configuration was four times that in the AM configuration. Significant harmonic radiation (both odd and even) was produced in the AM configuration but no harmonic radiation was detected in the CW configuration. A simple theory has been developed to model the fields produced by HF heating using the two techniques (AM,CW). A good agreement has been obtained between the experimental observations and model computations. The model has been extended to show how the relative efficiency of generation, AM/CW, varies with the frequency of the ELF/VLF radiation and HF antenna element spacing. Radiation patterns for the ‘ionospheric ELF/VLF antenna’ have also been derived for the two generation techniques.     

Simulation studies of processes associated with stimulated electromagnetic emission (SEE) in the ionosphere

Results of numerical simulation studies of processes associated with Stimulated Electromagnetic Emission (SEE) produced during ionospheric heating experiments are presented. A one-dimensional magnetized electrostatic Particle-In-Cell (PIC) simulation model with uniform plasma density is used to investigate electrostatic wave generation in the region where the pump frequency ω₀ approximately equals the upper hybrid frequency ω_uh. In particular, the simulation plasma is driven with a uniform oscillating electric field to represent the long wavelength pump wave and power spectra of the electrostatic waves produced are taken. The pump wave frequency and amplitude are varied to consider the effects on the simulation power spectrum. The upper hybrid frequency in the model is varied through harmonics of the electron cyclotron frequency Ω_ce to consider the effects of stepping the pump frequency through cyclotron harmonics. The power spectrum from the simulation plasma is richly structured. The resulting power spectra show sidebands upshifted and downshifted from the pump frequency by multiples of the lower hybrid frequency ω_lh. The structure of the spectrum is highly sensitive to the proximity of the upper hybrid frequency to the cyclotron harmonic frequencies.     

Effect of scallop shells and sediment grain size on phytoplankton flux to the bed

A flume study was made of bed skin friction and phytoplankton (Thalassiosira weissf ogii) deposition about a sea scallop (Placopecten magellanicus) mimic on a coarse (mean grain dia. =1200 μm), medium (615 μm) and fine (170 μm) quarry sand. Bed skin friction immediately upstream, and at one shell diameter downstream of the mimic was 1.2–2.4 times higher than ambient values (8.1 × 10⁻² Pa). Directly downstream of the mimic there was a region of near-zero skin friction. Bed diatom density was correlated with changes in skin friction; after 21 h, cell densities were 36–87% greater in the regions of high skin friction upstream and downstream of scallop than in control experiments. The pattern of bed diatom density about the scallop was similar in the coarse and medium sands, but deposition to the fine bed was not affected by the mimic. Diatom density was significantly correlated with grain diameter; deposition in the coarse bed was 2.3 times higher than in the medium sand, and 7.4 times higher than in the fine sand. A field experiment confirmed that the coarse sand was a greater sink of phytoplankton pigment than the fine sand. Previous studies and scaling arguments suggest that the differences in bed diatom density were controlled by the magnitude of interfacial solute fluxes. Regions of high skin friction about the mimic increased the porewater exchange, resulting in greater concentrations of diatoms retained within the bed. Similarly, the more permeable coarse sand would have a greater rate of porewater exchange than the fine sand, explaining the higher bed diatom density. Differences in the predicted volume of interstitial void space as a function of grain size closely matched the observed differences in bed diatom density. Results suggest that alteration of boundary layer flows by centimetre scale topography such as scallops, increases the flux of particulate organic matter toward the bed, but whether it is retained within the bed, and thus made available to the benthos is dependent on the sediment granularity.