Engineering topics

Melrose, D B

Publications and source records attributed to Melrose, D B.

11 records

DOE OSTI2012

Spin effect on parametric interactions of waves in magnetoplasmas

The parametric decay instability of upper hybrid wave into low-frequency electromagnetic Shear Alfven wave and Ordinary mode radiation (O-mode) has been investigated in an electron-ion plasma immersed in the uniform external magnetic field. Incorporating quantum effect due to electron spin, the fluid model has been used to investigate the linear and nonlinear response of the plasma species for three-wave coupling in a magnetoplasma. It is shown that the spin of electrons has considerable effect on the parametric decay of upper hybrid wave into Ordinary mode radiation (O-mode) and Shear Alfven wave even in classical regime.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2012

CHANGES IN POLARIZATION POSITION ANGLE ACROSS THE ECLIPSE IN THE DOUBLE PULSAR SYSTEM

We investigate the changes in polarization position angle in radiation from pulsar A around the eclipse in the Double Pulsar system PSR J0737-3039A/B at the 20 cm and 50 cm wavelengths using the Parkes 64 m telescope. The changes are {approx}2{sigma} during and shortly after the eclipse at 20 cm but less significant at 50 cm. We show that the changes in position angle during the eclipse can be modeled by differential synchrotron absorption in the eclipse regions. Position angle changes after the eclipse are interpreted as Faraday rotation in the magnetotail of pulsar B. Implied charge densities are consistent with the Goldreich-Julian density, suggesting that the particle energies in the magnetotail are mildly relativistic.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

GENERIC MODEL FOR MAGNETIC EXPLOSIONS APPLIED TO SOLAR FLARES

An accepted model for magnetospheric substorms is proposed as the basis for a generic model for magnetic explosions and is applied to solar flares. The model involves widely separated energy-release and particle-acceleration regions, with energy transported Alfvenically between them. On a global scale, these regions are coupled by a large-scale current that is set up during the explosion by redirection of pre-existing current associated with the stored magnetic energy. The explosion-related current is driven by an electromotive force (EMF) due to the changing magnetic flux enclosed by this current. The current path and the EMF are identified for an idealized quadrupolar model for a flare.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

MAGNETIC EXPLOSIONS: ROLE OF THE INDUCTIVE ELECTRIC FIELD

Inclusion of the inductive electric field, E{sub ind}, due to the temporally changing B, in magnetic explosions is discussed, with emphasis on solar flares. Several roles played by E{sub ind} are identified: on a global scale, E{sub ind} produces the electromotive force that drives the explosion; the associated E{sub ind} Multiplication-Sign B drift is identified with the inflow of magnetic field lines into a reconnection region; the polarization current, associated with {partial_derivative}E{sub ind}/{partial_derivative}t, implies a J Multiplication-Sign B force that accelerates this inflow; and the component of E{sub ind} parallel to B accelerates the energetic electrons that cause hard X-ray emission and type III radio bursts. Some simple models that describe these effects are presented. A resolution of the long-standing 'number problem' in solar flares is suggested.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

OBLIQUELY ROTATING PULSARS: SCREENING OF THE INDUCTIVE ELECTRIC FIELD

Pulsar electrodynamics has been built up by taking ingredients from two models, the vacuum-dipole model, which ignores the magnetosphere but includes the inductive electric field due to the obliquely rotating magnetic dipole, and the corotating-magnetosphere model, which neglects the vacuum inductive electric field and assumes a corotating magnetosphere. We argue that the inductive field can be neglected only if it is screened by a current, J{sub sc}, which we calculate for a rigidly rotating magnetosphere. Screening of the parallel component of the inductive field can be effective, but the perpendicular component cannot be screened in a pulsar magnetosphere. The incompletely screened inductive electric field has not been included in any model for a pulsar magnetosphere, and taking it into account has important implications. One effect is that it implies that the magnetosphere cannot be corotating, and we suggest that drift relative to corotation offers a natural explanation for the drifting of subpulses. A second effect is that this screening of the parallel inductive electric field must break down in the outer magnetosphere, and this offers a natural explanation for the acceleration of the electrons that produce pulsed gamma-ray emission.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

A resistive instability of lower hybrid-like waves in regions with parallel currents

The resistive instabilities and dispersion of obliquely propagating waves near the lower hybrid (LH) frequency are studied in plasma carrying a current parallel to the magnetic field. Possible applications of these instabilities include magnetic reconnection regions where LH-like waves may accelerate and heat both ions and electrons. A resistive instability is found in one of the four LH-like wave modes for electron drift speeds several times greater than the electron thermal speed for representative parameters. Numerical fully electromagnetic kinetic calculations of the solutions to the linear dispersion equation are compared with more approximate analytic calculations and show good agreement. The analytic results indicate that ion magnetization effects play a critical role in the resistive instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2011

Reactive instabilities of lower hybrid-like waves in regions with parallel currents

The dispersion and reactive instabilities of obliquely propagating waves near the lower hybrid (LH) frequency are studied in plasma carrying a current parallel to the magnetic field. Possible applications of these instabilities include magnetic reconnection regions, where LH-like waves may accelerate and heat both ions and electrons. In plasmas with a bulk drift of electrons relative to the ions at speed v{sub d} along the magnetic field, the forward and backward propagating LH modes are shown to be replaced by four LH-like modes. Reactive instabilities are discovered here for a forward propagating mode with Re({omega}){approx_equal}k{sub ||}v{sub d}/2 and a backward propagating mode with Re({omega})<{approx}5{Omega}{sub i}. Numerical warm, fully electromagnetic, kinetic calculations are compared with cold plasma calculations and agree well, confirming that the discovered instabilities are reactive. In the cold plasma limit, the forward and backward propagating instabilities occur for v{sub d} below and above some thresholds, respectively.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2011

Classical relativistic model for spin dependence in a magnetized electron gas

The response of a cold electron gas is generalized to include the spin of the electron described by the relativistically correct quasiclassical Bargmann-Michel-Telegdi (BMT) equation. The magnetization of the electron gas is assumed to be along the background magnetic field B and the spin-dependent contribution to the response tensor is proportional to the magnitude of the magnetization. The dispersion equation is shown to be quadratic in the refractive index squared, and dispersion curves for the two wave modes are plotted for cases where the magnetic field associated with magnetization is comparable with B. Two intrinsically spin-dependent wave modes are identified: one bounded by two resonances and the other by two cutoffs. The counterpart of the z mode can escape without encountering a resonance or a cutoff.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2011

Wave dispersion in a counterstreaming, relativistic thermal, magnetized, electron-positron plasma

The dispersion equation is analyzed for waves in a strongly magnetized, electron-positron plasma in which counterstreaming electrons and positrons have a relativistic thermal distribution in their respective rest frames, for propagation parallel to the magnetic field. We derive the response tensor for the medium, demonstrate the dispersion curves for different temperatures, and discuss the differences from the cold-plasma case. Application to the case of pulsar magnetospheres is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2010

FARADAY ROTATION: EFFECT OF MAGNETIC FIELD REVERSALS

The standard formula for the rotation measure (RM), which determines the position angle, {psi} = RM{lambda}{sup 2}, due to Faraday rotation, includes contributions only from the portions of the ray path where the natural modes of the plasma are circularly polarized. In small regions of the ray path where the projection of the magnetic field on the ray path reverses sign (called QT regions) the modes are nearly linearly polarized. The neglect of QT regions in estimating RM is not well justified at frequencies below a transition frequency where mode coupling changes from strong to weak. By integrating the polarization transfer equation across a QT region in the latter limit, I estimate the additional contribution {Delta}{psi} needed to correct this omission. In contrast with a result proposed by Broderick and Blandford, {Delta}{psi} is small and probably unobservable. I identify a new source of circular polarization, due to mode coupling in an asymmetric QT region. I also identify a new circular-polarization-dependent correction to the dispersion measure at low frequencies.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

Plasma dispersion function for a Fermi-Dirac distribution

A plasma dispersion function (PDF) is defined for a nonrelativistic Fermi-Dirac distribution and its properties are explored. The degree of degeneracy is described by a parameter {xi}=e{sup {mu}}{sub e}{sup /T}{sub e}, for electrons, with {mu}{sub e}/T{sub e} large and negative in the nondegenerate limit, and large and positive in the completely degenerate limit. The PDF is denoted Z(y,{xi}), where the variable y={omega}/{radical}(2)kV{sub e}, is the argument of the conventional PDF, Z(y)=Z(y,0), for a Maxwellian distribution. In the completely degenerate limit, Z(y,{xi}) approaches a logarithmic function that depends on the Fermi temperature and is independent of T{sub e}. Analytic approximations to Z(y,{xi}) are derived in terms of polylogarithmic functions for y{sup 2}>>1 and for y{sup 2}<<1.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗