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Biomedical subjects

J F Drake

Publications and source records attributed to J F Drake.

18 recordsLinked to original sources
PubMed2006

Electron acceleration from contracting magnetic islands during reconnection.

A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields 'reconnect' and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1-3), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects. In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons. Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting 'magnetic islands' that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls--the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations.

Journal Article↗
PubMed2005

Catastrophe model for fast magnetic reconnection onset.

A catastrophe model for the onset of fast magnetic reconnection is presented that suggests why plasma systems with magnetic free energy remain apparently stable for long times and then suddenly release their energy. For a given set of plasma parameters there are generally two stable reconnection solutions: a slow (Sweet-Parker) solution and a fast (Alfvénic) Hall reconnection solution. Below a critical resistivity the slow solution disappears and fast reconnection dominates. Scaling arguments predicting the two solutions and the critical resistivity are confirmed with two-fluid simulations.

Journal Article↗
PubMed2005

Production of energetic electrons during magnetic reconnection.

The production of energetic electrons during magnetic reconnection is explored with full particle simulations and analytic analysis. Density cavities generated along separatrices bounding growing magnetic islands support parallel electric fields that act as plasma accelerators. Electrons because of their low mass are fast enough to make multiple passes through these acceleration cavities and are therefore capable of reaching relativistic energies.

Journal Article↗
PubMed2003

Impact of frustrated singularities on magnetic island evolution.

The growth of magnetic islands is explored using the magnetohydrodynamic model in a simple slab system in which the value of the tearing mode stability parameter Delta' can be varied continuously. Unless the system is close to marginal stability reconnection is controlled by Sweet-Parker current layers, whose formation is a consequence of the inherent singular structure of magnetic island equilibria.

Journal Article↗
PubMed2003

Formation of electron holes and particle energization during magnetic reconnection.

Three-dimensional particle simulations of magnetic reconnection reveal the development of turbulence driven by intense electron beams that form near the magnetic x-line and separatrices. The turbulence collapses into localized three-dimensional nonlinear structures in which the electron density is depleted. The predicted structure of these electron holes compares favorably with satellite observations at Earth's magnetopause. The birth and death of these electron holes and their associated intense electric fields lead to strong electron scattering and energization, whose understanding is critical to explaining why magnetic explosions in space release energy so quickly and produce such a large number of energetic electrons.

Journal Article↗
PubMed2001

Role of dispersive waves in collisionless magnetic reconnection.

Simulations of collisionless magnetic reconnection show a dramatic enhancement of the nonlinear reconnection rate due to the formation of an open outflow region. We link the formation of this open configuration to dispersive whistler and kinetic Alfvén wave dynamics at small scales. The roles of these two waves are controlled by two dimensionless parameters, which allow us to identify regions of fast and slow reconnection.

Journal Article↗
PubMed1997

Intraaortic balloon pumping for cardiac support: trends in practice and outcome, 1968 to 1995.

OBJECTIVES: A total of 4756 cases of intraaortic balloon pump support have been recorded at the Massachusetts General Hospital since the first clinical insertion for cardiogenic shock in 1968. This report describes the patterns of intraaortic balloon use and associated outcomes over this time period. METHODS: A retrospective record review was conducted. RESULTS: Balloon use has increased to more than 300 cases a year at present. The practice of balloon placement for control of ischemia (2453 cases, 11.9% mortality) has become more frequent, whereas support for hemodynamic decompensation (congestive heart failure, hypotension, cardiogenic shock) has been relatively constant (1760 cases, 38.2% mortality). Mean patient age has increased from 54 to 66 years, and mortality has fallen from 41% to 20%. Sixty-five percent (3097/4756) of the total patient population receiving balloon support underwent cardiac surgery. Placement before the operation (2038 patients) was associated with a lower mortality (13.6%) than intraoperative (771 patients, 35.7% mortality) or postoperative use (276 patients, 35.9% mortality). Independent predictors of death with balloon pump support were insertion in the operating room or intensive care unit, transthoracic insertion, age, procedure other than angioplasty or coronary artery bypass, and insertion for cardiogenic shock. Independent predictors of death with intraoperative balloon insertion were age, mitral valve replacement, prolonged cardiopulmonary bypass, urgent or emergency operation, preoperative renal dysfunction, complex ventricular ectopy, right ventricular failure, and emergency reinstitution of cardiopulmonary bypass. CONCLUSIONS: Balloons are being used more frequently for control of ischemia in more patients who are elderly with lower mortality. An institutional bias toward preoperative use of the balloon pump appears to be associated with improved outcomes.

Age Distribution↗
PubMed1977

Growth kinetics of Colpoda steinii on Escherichia coli.

Colpoda steinii was grown in two-stage continuous cultures with Escherichia coli as prey species. The concentration of prey and the ciliate mean cell volume, dry weight, and number per milliliter were determined at known growth rates. Steady states were reached in the second-stage continuous cultures at all growth rates. Although changes occurred in mean cell size of the ciliates and in the number per milliliter at various growth rates, the yield of protozoan biomass per unit of prey consumed was constant at all growth rates. The data were compared with several equations proposed to describe the kinetics of protozoan growth as a function of prey density.

Animals↗
PubMed1976

Predation of Escherichia coli by Colpoda steinii.

A study of single-stage chemostat cultures of Colpoda steinii. Escherichia coli, and glucose is reported here. Two levels of glucose were fed as the limiting nutrient to the chemostat cultures. The cultures were studied at three holding times. Oscillations developed at short holding time and damped oscillations developed a long-residence times that approached steady-state conditions of populations of C. steinii and E. coli and concentrations of glucose. The experimental data are fitted to and compared with Jost's model.

Animals↗
PubMed1973

Differential counting in mixed cultures with coulter counters.

A critical comparison of Coulter, viable, and microscope counts for several mixed cultures of microorganisms has been made. This investigation shows that Coulter counting can provide reliable estimates of microbial numbers in mixed cultures. Precautions and limitations of Coulter counting in mixed cultures are discussed.

Analysis of Variance↗
PubMed1973

Interactions of Tetrahymena pyriformis, Escherichia coli, Azotobacter vinelandii, and glucose in a minimal medium.

A study was made of the food web formed from a protozoon, two bacteria, and a glucose minimal medium in chemostat culture. The system was also divided into simpler parts, first by omitting the protozoon to obtain a competition system, and then by omitting one or the other of the bacteria to obtain two food chains. In the competition studies, one bacterium was displaced by the other at all holding times used. In the food chain studies, sustained oscillations of the population densities of predator and prey developed at short holding times, and then changed to damped oscillations at longer holding times. In addition, the level of residual glucose remained high at long holding times. A new model of microbial growth is necessary to explain these results. In the food web studies, predation of the protozoon on the two bacteria stabilized the competition between the latter and allowed their coexistence in the same habitat. Thus, Gause's principle was circumvented.

Antibiosis↗
PubMed1972

Predator-prey interactions of Dictyostelium discoideum and Escherichia coli in continuous culture.

Dictyostelium discoideum and Escherichia coli were aerobically propagated in mixed continuous culture in a predator-prey relationship, and the effects of temperature and holding times were examined. Oscillations developed in the concentration of glucose, the limiting substrate for E. coli, and in the densities of the two populations, but eventually steady-state populations were reached. The experimental data were analyzed according to the Lotka-Volterra model for prey-predator relationships and by the Monod model for saturation kinetics. A comparison of the adequacy of the two models in describing predation is given.

Aerobiosis↗
PubMed1970

Automated counting of microbial colonies.

The time required for direct counting of colonies on agar plates for estimating population density of viable microorganisms has precluded studies requiring measurement of such a parameter. Bowman, Blume, and Vurek have described a photo-electrical capillary tube scanner which automates the counting. Results obtained with an instrument similar to that of Bowman et al. have been intensively analyzed with respect to precision and accuracy. The sources of "errors" have been ascertained, and the instrument's potentialities and limitations are discussed.

Automation↗