Alfven Surface Research Articles

Dispersive effects for Alfven surface waves

Non-dispersive Alfven surface waves at a plasma interface became dispersive when non-uniformities in the equilibrium are introduced.

Dawn‐dusk electric field asymmetry of the Io plasma torus

We consider the effects of a combined convection and corotation electric field across the Io plasma torus. A dawn‐to‐dusk electric field Ec will modify the orbits of charged particles shifting them toward dawn. The radial drift imposed by the perturbed orbits implies a local time‐dependent modulation of low‐energy ion and electron temperatures with particles hotter at dusk than at dawn. With Ec ≃ 4 mV/m, the orbits near 6 RJ would be shifted by approximately 0.2 RJ. Then the electron temperature would be 20% higher at dusk than at dawn, an effect which could explain the local time asymmetry of EUV intensity found by the Voyager ultraviolet spectrometer. The source of the convection electric field is internal to the magnetosphere and is attributed to the tailward escape of Iogenic and Jovian plasma beyond the Alfven surface.

Comment on ?note on the stability of parallel magnetic fields?

It is shown that the conclusions arrived at regarding the instability of an incompressible fluid cylinder in the presence of the magnetic field and the streaming velocity in a recent communication easily follow from the study of propagation characteristics of Alfven surface waves along cylindrical plasma columns made earlier.

Compressibility effects on hydromagnetic surface waves

The dispersion equation for hydromagnetic surface waves along a plasma-plasma interface has been solved as a function of the compressibility factor c1/vA1, where c1 and vA1 are the acoustic and Alfven wave speed in one of the medium, for general wave propagation direction. Both slow and fast magnetosonic surface waves can exist. The nature and existence of these waves depends on the values of c1/vA1 and θ, the angle of wave propagation. For low-β plasmas only fast mode exists. The slow mode does not propagate below a critical value of c1. When c1 → ∞ the phase velocity of the slow wave tend to the Alfven surface wave velocity in the incompressible media and for large θ the phase velocity of the fast wave approaches this value. The phase velocity of the slow wave increases whereas for the fast wave it decreases with increase in the angle θ.

Two-Mode Structure of Alfvén Surface Waves

The Alfven surface waves propagating along a viscous conducting fluid-vacuum interface have been studied. It is found that besides the ordinary Alfven surface waves, modified by viscosity effects, the interface can support a second mode which is the over-damped solution of the dispersion equation. The possibility of observation of a two-mode structure of Alfven surface waves in the laboratory and in the solar coronal plasmas is discussed.

A note on the existence of Alfv�n surface waves

The Alfven surface waves can arise due to the discontinuity in the Alfven speed across the interface along which these waves propagate. This note studies the relationship between v A1 and v A2 which is required for the existence of Alfven surface waves in low-beta plasma.

Alfven surface waves along a cylindrical plasma column surrounded by neutral gas

The dispersion characteristics of Alfven surface waves along a cylindrical plasma column insulated by a neutral gas are discussed. There is no qualitative change in the characteristic curves below the critical magnetic field, given by vA approximately=s, as compared to the propagation of surface waves along the plasma-plasma interface. For magnetic fields above this critical value, there exists a cut-off wave number kc, which depends upon the azimuthal wave number, the radius of the cylinder, the strength of the magnetic field above the critical value and the gas pressure, such that surface waves do not exist for k<kc. The acoustic surface waves, which exist only in the presence of magnetic fields with a strength greater than the critical value, will therefore have restricted wavelength range 0< lambda < lambda c, when propagating along a cylindrical plasma column.

Implications of the high-state iron-line feature and soft X-ray emission of Hercules X-1

view Abstract Citations (12) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Implications of the high-state iron-line feature and soft X-ray emission of HER X-1. Bai, T. Abstract The interpretation that the soft X-ray emission of Her X-1 is due to reprocessing of the primary hard X-rays intercepted by a plasma layer in the Alfven surface imposes quite a stringent - but not an unreasonable - constraint on its parameters. By using the Alfven shell parameters constrained by this interpretation, the degree of ionization of the Alfven shell is calculated. Because the flux of hard X-ray from the neutron star is greatly attenuated inside the Alfven shell, except for a skin depth from its inner surface, the degree of ionization is determined by photoionization due to soft X-rays produced within the Alfven shell. On one hand, the large soft X-ray luminosity, which is about the same as the hard X-ray luminosity, indicates that a substantial fraction of primary hard X-rays is absorbed by the Alfven shell. On the other hand, the fact that the hard X-ray spectrum of Her X-1 observed during the high state shows neither a significant absorption feature down to about 3 keV nor an iron K-absorption dip tells us that the hard X-rays observed at the earth are mostly not reprocessed. This apparent conflict is resolved if the Alfven shell is clumpy (i.e., made of streams or blobs). The observed iron-line feature around 6.5 keV (E.W. about 300 keV) is explained by fluorescent iron K photons produced as a by-product of reprocessing of the hard X-rays by the Alfven shell; for this a large enhancement of the iron abundance with respect to the solar value is not required. Publication: The Astrophysical Journal Pub Date: August 1980 DOI: 10.1086/158188 Bibcode: 1980ApJ...239..999B Keywords: Binary Stars; Iron; Neutron Stars; Stellar Radiation; X Ray Sources; Abundance; Magnetohydrodynamic Waves; Metallic Stars; Photoionization; Stellar Envelopes; Stellar Luminosity; Astrophysics full text sources ADS | data products SIMBAD (1)

Alfven surface waves along cylindrical plasma columns

The properties of Alfven surface waves along a cylindrical plasma column surrounded by vacuum or by another plasma medium are discussed. Both symmetric (m=0) and asymmetric (m=+or-1) modes are found to be dispersive in nature. The interfacial symmetric modes propagate in a certain frequency window ( omega A1, omega As), where omega As is the Alfven surface wave frequency along the interface of two semi-infinite media; when nu A1> nu A2 these modes propagate as backward waves and when nu A1 nu A2.

IRON LINES IN THE SPECTRA OF X‐RAY BINARIES

The paper presents a model explaining powerful iron line emission as gaseous shell fluorescence at the Alfven surface. Gas parameters in Alfven shells are analyzed, and thermal balance and ionization equilibrium are outlined. The equivalent width of an iron line is discussed, along with its profile. Consideration is given to soft X-ray emission and iron line emission in other X-ray binaries.

STELLAR MAGNETIC FIELDS: THE ROLE OF A MAGNETIC FIELD IN THE PECULIAR M GIANT, HD 4174.

ABSTRACT Principles of coronal heating via basic electrodynamic effects, viz., resonant absorption of Alfven surface waves (quiescent) and magnetic tearing instabilities (impulsive), are detailed to argue three principles which may have application to late-type evolved stars. First, if one observes that B-squared/8 pi is much greater than rho times v-squared in a stellar atmosphere, then the observed magnetic field must originate in an interior dynamo. Second, low mass-loss rates could imply the presence of closed magnetic flux loops within the outer atmosphere, which constrain hydrodynamic flows when the magnetic body forces exceed the driving forces. Third, given that such magnetic loops effect an enhancement of the local heating rate, a positive correlation is predicted between the existence of a corona and low mass-loss rates. Application of these principles is made in the case of the peculiar M giant star HD 4174, which is purported to have a kilogauss magnetic field. Several of its spectroscopic peculiarities are shown to be consistent with the above principles, and further observational checks are suggested. Possible application to dMe and RS CVn objects is sketched.

Resonant absorption of Alfvenic surface waves and the heating of solar coronal loops

A first-order mass- and energy-balance model is developed for steady-state EUV 'coronal rain' loops that are not associated with postflare events and are often seen over sunspot umbrae. The model disregards variations in a loop's thermodynamic and magnetic properties along magnetic-field lines and yields average properties of the resonant absorption sheath, the boundary layer, and the loop's interior. Both irreversible heating by Alfvenic surface waves and energy transport via induced boundary-layer convection are taken into account in the analysis. Results that include predictions of the temperatures of the sheath and the boundary-layer plasma, the temperature of the interior plasma, the radiative output of the loop, and the filling factor associated with this radiation are given in terms of the period and velocity amplitude of the relevant surface wave as well as various parameters that characterize a typical 'coronal rain' loop. These results are shown to be consistent with the observed minimum and maximum temperatures as well as the radiative output of typical 'coronal rain' loops.

Interaction of the accretion disk with the rotating stellar magnetic field - Plasma diffusion and angular momentum transfer across the Alfven surface

Starting with a general consideration for the force-balance condition between the accretion disk and the rotating stellar magnetic field, we investigate the plasma diffusion and angular momentum transfer across the Alfven surface. The accreting plasma penetrates into the boundary layer through the diffusion process caused by the anomalous electric resistivity, which in turn results from the magnetic field fluctuations created in the accretion disk. We thereby provide a formalism by which the position of the Alfven surface and the thickness of the boundary layer are explicitly determined. We apply those calculations to the accretion model of the X-ray pulsars and in particular compare the theoretical results on the spin-up rate with the observational data. In addition to reproducing the well-known agreement on the slow pulsars, we point to a way in which the spin-up characteristics of Her X-1 may be quantitatively explained.

Accretion by magnetic neutron stars. I - Magnetospheric structure and stability

view Abstract Citations (265) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Accretion by magnetic neutron stars. I. Magnetospheric structure and stability. Elsner, R. F. ; Lamb, F. K. Abstract Results are presented of a detailed study of the magnetic neutron star model, taking into account the particular case in which the accretion flow is approximately radial and the star is slowly rotating. An investigation is conducted of the physical processes which occur in the neighborhood of the magnetospheric boundary and, in particular, on the manner in which accreting plasma enters the magnetosphere. Attention is given to the formation of a magnetosphere, the scale and structure of static magnetospheres, equilibrium shapes, the stability of the magnetospheric boundary, the plasma flow into the magnetosphere, the scale and structure of the magnetosphere, and the Alfven surface. The presented results imply that, for a wide range of conditions relevant to compact X-ray sources, the Rayleigh-Taylor instability is the most important plasma entry process. Publication: The Astrophysical Journal Pub Date: August 1977 DOI: 10.1086/155427 Bibcode: 1977ApJ...215..897E Keywords: Magnetic Stars; Neutron Stars; Stellar Atmospheres; Stellar Magnetospheres; Stellar Mass Accretion; Atmospheric Models; Stellar Models; Stellar Rotation; X Ray Sources; Astrophysics full text sources ADS | data products SIMBAD (11)

X-ray spectra of Hercules X-1. I - Iron line fluorescence from a subrelativistic shell

The X-ray spectrum of Hercules X-1 was observed in the energy range 2-24 keV from August 29 to September 3, 1975. A broad iron line feature is observed in the normal high state spectrum. The line equivalent width is given along with its full-width-half-maximum energy. Iron line fluorescence from an opaque, cool shell of material at the Alfven surface provides the necessary luminosity in this feature. The line energy width can be due to Doppler broadening if the shell is forced to corotate with the pulsar at a radius 800 million cm. Implications of this model regarding physical conditions near Her X-1 are discussed.

Soft X-rays from Hercules X-1 during the 'off' state

A significant 0.18--0.28 keV flux has been detected from Her X-1 during the off portion of the 35 day cycle. The intensity observed in a rocket observation 6 days before turn-on is 4 percent of that previously reported for the on state. Another observation, made 3 days before turn-on from an Apollo spacecraft, failed to detect any soft X-ray emission above 2 percent of the on state flux.The soft X-ray flux may be attributed to an opaque shell near the Alfven surface of the neutron star, with T=85. x 10/sup 5/ K and R=1.5 x 10/sup 8/ cm. The varying intensities observed are interpreted in terms of obscuration by the accretion disk. (AIP)

The existence of an ultrarelativistic plasma beyond the Alfven cylinder of a pulsar

Previous studies of the steady-state magnetohydrodynamic models of pulsar magnetospheres have indicated that at the Alfvenic critical surface of the flow in such magnetospheres there exists a shock discontinuity. In this paper, the relativistic theory of magnetohydrodynamic shocks is employed to determine the state of the plasma across this discontinuity. It is found that an appreciable fraction of the magnetic energy of the plasma which crosses the shock is thermalized, and that to within a first-order approximation, the mean particle energy in the shocked plasma is given by E'approx. =3.7 x 10/sup 161/X/subv//suby/'B/sub 6//sup 2/n/sub 6//sup -1/eV, where ..gamma..' is the Lorentz factor based on the flow velocity of the plasma beyond the Alfven cylinder, v which depends on the thickness of the shock is a factor of the order of unity, and B/sub 6/ and n/sub 6/ are the poloidal magnetic field and the particle number density behind the Alfven cylinder in units of 10/sup 6/ gauss and 10/sup 6/ cm/sup -3/, respectively. For the estimated values of the parameters at the Alfven cylinder: which in the limiting case of a massless magnetosphere is coincident with the light cylinder: this result agrees both with the brightness temperature of themore » radio emission from most pulsars and with the upper limit so far established by observations of the cosmic ray energies in the Crab nebula. (AIP)« less

A Model for Compact X-Ray Sources: Accretion by Rotating Magnetic Stars

The physics of accretion onto compact stars is considered, taking into account both the effects of stellar rotation and a stellar magnetic field. We show that far from the star the stellar magnetic field is screened by currents flowing in the accreting plasma, while close to the star the stellar field forces matter to corotate with the star. The location of the Alfven surface, where the transition between the two regimes occurs, depends on the flow pattern of the accreting matter beyond the Alfvea surface and the rotation period of the compact star, as well as the mass accretion rate and the strength of the stellar magnetic field. Three types of flow pattern are considered: radial inflow toward the compact object, orbital motion about it, and streaming motion past it. For the accretion rates of interest the radius of the Alfven surface r/sub A/, is found to be ~10/sup 7/-10/sup 8/ cm for a typical neutron star and ~ 10/sup 8/ -10/sup 10/cm for a typical magnetic degenerate dwarf: For neutron stars it is shown that inside the Alfvea surface there is comparatively little flow of matter across field lines, so that accreting matter is channeled toward the magnetic polesmore » of the star where it forms hot spots. The resulting radiation is shown to emerge from the neighborhood of the stellar surface in a strongly anisotropic angular pattern with a spectrum which depends on the details of the accretion process; for an oblique rotator, one has a natural mechanism for the production of pulsed radiation, which is not generally expected to he blackbody in character, will be in excess of 6 keV. Comparison of the above model with the observations suggests strongly that the X-ray stars in the pulsating binary X-ray sources Cen X-3 and Her X-1 are accreting neutron stars with their magnetic axes inclined at substantial angles to their axes of rotation. The implications of this interpretation for the observed X-ray spectra and pulse wave-forms are discussed. The change in the X-ray pulsation period due to accretion of matter is calculated; the calculated time scales for the spin-up which occurs when there is orbital inflow toward a star rotating ia the same sense as the orbital motion, is in excellent agreement with these observed in Her X-1 and Gn X-3. The possibility of significant optical pulsations and the likely existence of a minimum pulsation period for accreting X-ray stars are discussed. (auth)« less