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Full-Text Articles in Physics
Intrinsic Rotation Of Toroidally Confined Magnetohydrodynamics, Jorge A. Morales, Wouter J. T. T. Bos, Kai Schneider, David C. Montgomery
Intrinsic Rotation Of Toroidally Confined Magnetohydrodynamics, Jorge A. Morales, Wouter J. T. T. Bos, Kai Schneider, David C. Montgomery
Dartmouth Scholarship
The spatiotemporal self-organization of viscoresistive magnetohydrodynamics in a toroidal geometry is studied. Curl-free toroidal magnetic and electric fields are imposed. It is observed in our simulations that a flow is generated, which evolves from dominantly poloidal to toroidal when the Lundquist numbers are increased. It is shown that this toroidal organization of the flow is consistent with the tendency of the velocity field to align with the magnetic field. Up-down asymmetry of the geometry causes the generation of a nonzero toroidal angular momentum.
Magnetic Field Amplification In Electron Phase-Space Holes And Related Effects, R. A. Treumann, W. Baumjohann
Magnetic Field Amplification In Electron Phase-Space Holes And Related Effects, R. A. Treumann, W. Baumjohann
Dartmouth Scholarship
No abstract provided.
A Model Of So-Called "Zebra" Emissions In Solar Flare Radio Burst Continua, R. A. Treumann, R. Nakamura, W. Baumjohann
A Model Of So-Called "Zebra" Emissions In Solar Flare Radio Burst Continua, R. A. Treumann, R. Nakamura, W. Baumjohann
Dartmouth Scholarship
A simple mechanism for the generation of elec- tromagnetic “Zebra” pattern emissions is proposed. “Zebra” bursts are regularly spaced narrow-band radio emissions on the otherwise broadband radio continuum emitted by the ac- tive solar corona. The mechanism is based on the generation of an ion-ring distribution in a magnetic mirror geometry in the presence of a properly directed field-aligned electric po- tential field. Such ion-rings or ion-conics are well known from magnetospheric observations. Under coronal condi- tions they may become weakly relativistic. In this case the ion-cyclotron maser generates a number of electromagnetic ion-cyclotron harmonics which modulate the electron maser …
Resistive Mhd Reconstruction Of Two-Dimensional Coherent Structures In Space, W L. Teh, B U. Sonnerup, J Birn, R E. Denton
Resistive Mhd Reconstruction Of Two-Dimensional Coherent Structures In Space, W L. Teh, B U. Sonnerup, J Birn, R E. Denton
Dartmouth Scholarship
We present a reconstruction technique to solve the steady resistive MHD equations in two dimensions with initial inputs of field and plasma data from a single space- craft as it passes through a coherent structure in space. At least two components of directly measured electric fields (the spacecraft spin-plane components) are required for the reconstruction, to produce two-dimensional (2-D) field and plasma maps of the cross section of the structure. For con- venience, the resistivity tensor η is assumed diagonal in the reconstruction coordinates, which allows its values to be es- timated from Ohm’s law, E+v×B=η·j. In the present paper, …
On Arc-Polarized Structures In The Solar Wind, B U. Sonnerup, S E. Haaland, G Paschmann
On Arc-Polarized Structures In The Solar Wind, B U. Sonnerup, S E. Haaland, G Paschmann
Dartmouth Scholarship
A theoretical model is proposed to account for some of the behavior of arc-polarized magnetic structures seen in the solar wind. To this end, an exact analytical solu- tion is developed that describes infinite plane wave trains of arbitrary amplitude in a plasma governed by ideal Hall MHD. The main focus is on intermediate-mode wave trains, which display double-branched magnetic hodogram signatures sim- ilar to those seen in the solar wind. The theoretically derived hodograms have field rotation in the ion-polarized sense at a slightly depressed field magnitude on one branch and an electron-polarized rotation at a slightly enhanced field …
Hydrodynamic And Magnetohydrodynamic Computations Inside A Rotating Sphere, P. D. Mininni, D. C. Montgomery, L. Turner
Hydrodynamic And Magnetohydrodynamic Computations Inside A Rotating Sphere, P. D. Mininni, D. C. Montgomery, L. Turner
Dartmouth Scholarship
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations are reported for the interior of a rotating, perfectly-conducting, rigid spherical shell that is insulator-coated on the inside. A previously-reported spectral method is used which relies on a Galerkin expansion in Chandrasekhar–Kendall vector eigenfunctions of the curl. The new ingredient in this set of computations is the rigid rotation of the sphere. After a few purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial waves), attention is focused on selective decay and the MHD dynamo problem. In dynamo runs, prescribed mechanical forcing excites a persistent velocity field, usually turbulent at modest …
Structured Waves Near The Plasma Frequency Observed In Three Auroral Rocket Flights, M Samara, J Labelle
Structured Waves Near The Plasma Frequency Observed In Three Auroral Rocket Flights, M Samara, J Labelle
Dartmouth Scholarship
Abstract. We present observations of waves at and just above the plasma frequency (fpe) from three high frequency electric field experiments on three recent rockets launched to altitudes of 300–900 km in active aurora. The predominant observed HF waves just above fpe are narrowband, short- lived emissions with amplitudes ranging from <1mV/m to 20 mV/m, often associated with structured electron den- sity. The nature of these HF waves, as determined from frequency-time spectrograms, is highly variable: in some cases, the frequency decreases monotonically with time as in the “HF-chirps” previously reported (McAdams and La- Belle, 1999), but in other cases rising frequencies are ob- served, or features which alternately rise and fall in fre- quency. They exhibit two timescales of amplitude variation: a short timescale, typically 50–100 ms, associated with in- dividual discrete features, and a longer timescale associated with the general decrease in the amplitudes of the emissions as the rocket moves away from where the condition f ∼fpe holds. The latter timescale ranges from 0.6 to 6.0 s, corre- sponding to distances of 2–7 km, assuming the phenomenon to be stationary and using the rocket velocity to convert time to distance.
The Structure Of Flux Transfer Events Recovered From Cluster Data, H Hasegawa, B U. Ö Sonnerup, C J. Owen, B Klecker, G Paschmann, A Balogh, H Re`Me
The Structure Of Flux Transfer Events Recovered From Cluster Data, H Hasegawa, B U. Ö Sonnerup, C J. Owen, B Klecker, G Paschmann, A Balogh, H Re`Me
Dartmouth Scholarship
The structure and formation mechanism of a to- tal of five Flux Transfer Events (FTEs), encountered on the equatorward side of the northern cusp by the Cluster space- craft, with separation of ∼5000 km, are studied by apply- ing the Grad-Shafranov (GS) reconstruction technique to the events. The technique generates a magnetic field/plasma map of the FTE cross section, using combined magnetic field and plasma data from all four spacecraft, under the assump- tion that the structure is two-dimensional (2-D) and time- independent. The reconstructed FTEs consist of one or more magnetic flux ropes embedded in the magnetopause, suggest- ing …
Low Magnetic Prandtl Number Dynamos With Helical Forcing, Pablo D. Mininni, David C. Montgomery
Low Magnetic Prandtl Number Dynamos With Helical Forcing, Pablo D. Mininni, David C. Montgomery
Dartmouth Scholarship
We present direct numerical simulations of dynamo action in a forced Roberts flow. The behavior of the dynamo is followed as the mechanical Reynolds number is increased, starting from the laminar case until a turbulent regime is reached. The critical magnetic Reynolds for dynamo action is found, and in the turbulent flow it is observed to be nearly independent on the magnetic Prandtl number in the range from ∼0.3 to ∼0.1. Also the dependence of this threshold with the amount of mechanical helicity in the flow is studied. For the different regimes found, the configuration of the magnetic and velocity …
Optimal Reconstruction Of Magnetopause Structures From Cluster Data, H Hasegawa, B U. Ö Sonnerup, B Klecker, G Paschmann
Optimal Reconstruction Of Magnetopause Structures From Cluster Data, H Hasegawa, B U. Ö Sonnerup, B Klecker, G Paschmann
Dartmouth Scholarship
The Grad-Shafranov (GS) reconstruction tech- nique, a single-spacecraft based data analysis method for recovering approximately two-dimensional (2-D) magneto- hydrostatic plasma/field structures in space, is improved to become a multi-spacecraft technique that produces a single field map by ingesting data from all four Cluster spacecraft into the calculation. The plasma pressure, required for the technique, is measured in high time resolution by only two of the spacecraft, C1 and C3, but, with the help of spacecraft po- tential measurements available from all four spacecraft, the pressure can be estimated at the other spacecraft as well via a relationship, established from C1 …
High-Latitude Propagation Studies Using A Meridional Chain Of Lf/Mf/Hf Receivers, J Labelle
High-Latitude Propagation Studies Using A Meridional Chain Of Lf/Mf/Hf Receivers, J Labelle
Dartmouth Scholarship
For over a decade, Dartmouth College has oper- ated programmable radio receivers at multiple high-latitude sites covering the frequency range 100–5000 kHz with about a 1-s resolution. Besides detecting radio emissions of auro- ral origin, these receivers record characteristics of the iono- spheric propagation of natural and man-made signals, docu- menting well-known effects, such as the diurnal variation in the propagation characteristics of short and long waves, and also revealing more subtle effects. For example, at auroral zone sites in equinoctial conditions, the amplitudes of dis- tant transmissions on MF/HF frequencies are often enhanced by a few dB just before …
A Study Of Pc-5 Ulf Oscillations, M K. Hudson, R E. Denton, M R. Lessard, E G. Miftakhova, R R. Anderson
A Study Of Pc-5 Ulf Oscillations, M K. Hudson, R E. Denton, M R. Lessard, E G. Miftakhova, R R. Anderson
Dartmouth Scholarship
A study of Pc-5 magnetic pulsations using data from the Combined Release and Radiation Effects Satellite (CRRES) was carried out. Three-component dynamic mag- netic field spectrograms have been used to survey ULF pul- sation activity for the approximate fourteen month lifetime of CRRES. Two-hour panels of dynamic spectra were exam- ined to find events which fall into two basic categories: 1) toroidal modes (fundamental and harmonic resonances) and 2) poloidal modes, which include compressional oscillations. The occurence rates were determined as a function of L value and local time. The main result is a comparable probabil- ity of occurence of …
Velocity Field Distributions Due To Ideal Line Vortices, Thomas D. Levi, David C. Montgomery
Velocity Field Distributions Due To Ideal Line Vortices, Thomas D. Levi, David C. Montgomery
Dartmouth Scholarship
We evaluate numerically the velocity field distributions produced by a bounded, two-dimensional fluid model consisting of a collection of parallel ideal line vortices. We sample at many spatial points inside a rigid circular boundary. We focus on “nearest-neighbor” contributions that result from vortices that fall (randomly) very close to the spatial points where the velocity is being sampled. We confirm that these events lead to a non-Gaussian high-velocity “tail” on an otherwise Gaussian distribution function for the Eulerian velocity field. We also investigate the behavior of distributions that do not have equilibrium mean-field probability distributions that are uniform inside the …