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  • 1
    Electronic Resource
    Electronic Resource
    Two New Plasma Instabilities in the Solar Wind (1970)
    [s.l.] : Nature Publishing Group
    Nature 225 (1970), S. 441-443 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Considering a hydrogen plasma, Stix's dispersion relation1 for circularly polarized electromagnetic waves propagating in the direction of the average magnetic field, B0, may be written in the form k*v2A + neOP^-e + OjUp = *(Qenp/Ve)^o(a+e) x (1) Our calculations are in the rest system of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/225441a0
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  • 2
    Electronic Resource
    Electronic Resource
    Solar Coronal Effects on Pulsar Signals (1968)
    [s.l.] : Nature Publishing Group
    Nature 220 (1968), S. 771-772 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] For the form of the average coronal electron concentration I shall adopt w0(p) = 1-55 x 108p-6 + 4-0 x 105p-2 cm-3 (1) where p is the distance from the solar centre in solar radii. Equation (1) is the Allen-Baumbach density (for p 〉 1-5), augmented by a term corresponding to the results ofParker2 ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/220771a0
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  • 3
    Electronic Resource
    Electronic Resource
    Resonant behaviour of MHD waves on magnetic flux tubes (1992)
    Springer
    Solar physics 138 (1992), S. 233-255 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The resonances that appear in the linear compressible MHD formulation of waves are studied for equilibrium states with flow. The conservation laws and the jump conditions across the resonance point are determined for 1D cylindrical plasmas. For equilibrium states with straight magnetic field lines and flow along the field lines the conserved quantity is the Eulerian perturbation of total pressure. Curvature of the magnetic field lines and/or velocity field lines leads to more complicated conservation laws. Rewritten in terms of the displacement components in the magnetic surfaces parallel and perpendicular to the magnetic field lines, the conservation laws simply state that the waves are dominated by the parallel motions for the modified slow resonance and by the perpendicular motions for the modified Alfvén resonance. The conservation laws and the jump conditions are then used for studying surface waves in cylindrical plasmas. These waves are characterized by resonances and have complex eigenfrequencies when the classic true discontinuity is replaced by a nonuniform layer. A thin non-uniform layer is considered here in an attempt to obtain analytical results. An important result related to earlier work by Hollweg et al. (1990) for incompressible planar plasmas is found for equilibrium states with straight magnetic field lines and straight velocity field lines. For these equilibrium states the incompressible and compressible surface waves have the same frequencies at least in the long wavelength limit and there is an exact correspondence with the planar case. As a consequence, the conclusions formulated by Hollweg et al. still hold for the straight cylindrical case. The effects of curvature are subsequently considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151914
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  • 4
    Electronic Resource
    Electronic Resource
    Resonant behaviour of MHD waves on magnetic flux tubes (1991)
    Springer
    Solar physics 133 (1991), S. 247-262 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The absorption of solar five-min oscillations by sunspots is interpreted as the resonant absorption of sound waves by a magnetic cylinder. The absorption coefficient is calculated both analytically under certain simplifying assumptions, and numerically under more general conditions. The observed magnitude of the absorption coefficient, which is up to 0.5 or even more, can be explained for suitable ranges of parameters. Limitations in the present model are also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00149889
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  • 5
    Electronic Resource
    Electronic Resource
    Resonant behaviour of MHD waves on magnetic flux tubes (1991)
    Springer
    Solar physics 133 (1991), S. 227-245 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A basic procedure is presented for dealing with the resonance problems that appear in MHD of which resonant absorption of waves at the Alfvén resonance point is the best known example in solar physics. The procedure avoids solving the full fourth-order differential equation of dissipative MHD by using connection formulae across the dissipation layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00149888
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  • 6
    Electronic Resource
    Electronic Resource
    Dissipative MHD solutions for resonant AlfvÉn waves in 1-dimensional magnetic flux tubes (1995)
    Springer
    Solar physics 157 (1995), S. 75-102 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The present paper extends the analysis by Sakurai, Goossens, and Hollweg (1991) on resonant Alfvén waves in nonuniform magnetic flux tubes. It proves that the fundamental conservation law for resonant Alfvén waves found in ideal MHD by Sakurai, Goossens, and Hollweg remains valid in dissipative MHD. This guarantees that the jump conditions of Sakurai, Goossens, and Hollweg, that connect the ideal MHD solutions forξ r , andP′ across the dissipative layer, are correct. In addition, the present paper replaces the complicated dissipative MHD solutions obtained by Sakurai, Goossens, and Hollweg forξ r , andP′ in terms of double integrals of Hankel functions of complex argument of order $$\frac{1}{3}$$ with compact analytical solutions that allow a straightforward mathematical and physical interpretation. Finally, it presents an analytical dissipative MHD solution for the component of the Lagrangian displacement in the magnetic surfaces perpendicular to the magnetic field linesξ⊥ which enables us to determine the dominant dynamics of resonant Alfvén waves in dissipative MHD.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00680610
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  • 7
    Electronic Resource
    Electronic Resource
    Alfvén waves in the solar atmosphere (1981)
    Springer
    Solar physics 70 (1981), S. 25-66 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The linearized propagation of axisymmetric twists on axisymmetric vertical flux tubes is considered. Models corresponding to both open (coronal hole) and closed (active region loops) flux tubes are examined. Principal conclusions are: Open flux tubes: (1) With some reservations, the model can account for long-period (T ≈ 1 hr) energy fluxes which are sufficient to drive solar wind streams. (2) The waves are predicted to exert ponderomotive forces on the chromosphere which are large enough to alter hydrostatic equilibrium or to drive upward flows. Spicules may be a consequence of these forces. (3) Higher frequency waves (10 s ≲ T ≲ few min) are predicted to carry energy fluxes which are adequate to heat the chromosphere and corona. Nonlinear mechanisms may provide the damping. Closed flux tubes: (1) Long-period (T ≈ 1 hr) twists do not appear to be energetically capable of providing the required heating of active regions. (2) ‘Loop resonances’ are found to occur as a result of waves being stored in the corona via reflections at the transition zones. The loop resonances act much in the manner of antireflectance coatings on camera lenses, and allow large energy fluxes to enter the coronal loops. The resonances may also be able to account for the observed fact that longer coronal loops require smaller energy flux densities entering them from below. (3) The waves exert large upward and downward forces on the chromosphere and corona.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00154391
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  • 8
    Electronic Resource
    Electronic Resource
    Alfvén waves in the solar atmosphere (1982)
    Springer
    Solar physics 75 (1982), S. 35-61 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The nonlinear propagation of Alfvén waves on open solar magnetic flux tubes is considered. The flux tubes are taken to be vertical and axisymmetric, and they are initially untwisted. The Alfvén waves are time-dependent axisymmetric twists. Their propagation into the chromosphere and corona is investigated by solving numerically a set of nonlinear time-dependent equations, which couple the Alfvén waves into motions parallel to the initial magnetic field (motion in the third coordinate direction is artificially suppressed). The principal conclusions are: (1) Alfvén waves can steepen into fast shocks in the chromosphere. These shocks can pass through the transition region into the corona, and heat the corona. (2) As the fast shocks pass through the transition region, they produce large-velocity pulses in the direction transverse to B o. The pulses typically have amplitudes of 60 km s−1 or so and durations of a few tens of seconds. Such features may have been observed, suggesting that the corona is in fact heated by fast shocks. (3) Alfvén waves exhibit a strong tendency to drive upward flows, with many of the properties of spicules. Spicules, and the observed corrugated nature of the transition region, may therefore be by-products of magnetic heating of the corona. (4) It is qualitatively suggested that Alfvén waves may heat the upper chromosphere indirectly by exerting time-dependent forces on the plasma, rather than by directly depositing heat into the plasma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00153458
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  • 9
    Electronic Resource
    Electronic Resource
    A Kinetic Model of Coronal Heating and Acceleration by Ion-Cyclotron Waves: Preliminary Results (2000)
    Springer
    Solar physics 193 (2000), S. 247-257 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present a kinetic model of the heating and acceleration of coronal protons by outward-propagating ion-cyclotron waves on open, radial magnetic flux tubes. In contrast to fluid models which typically insist on bi-Maxwellian distributions and which spread the wave energy and momentum over the entire proton population, this model follows the kinetic evolution of the collisionless proton distribution function in response to the combination of the resonant wave-particle interaction and external forces. The approximation is made that pitch-angle scattering by the waves is faster than all other processes, resulting in proton distributions which are uniform over the resonant surfaces in velocity space. We further assume, in this preliminary version, that the waves are dispersionless so these resonant surfaces are portions of spheres centered on the radial sum of the Alfvén speed and the proton bulk speed. We incorporate the fact that only those protons with radial speeds less than the bulk speed will be resonant with outward-propagating waves, so this rapid interaction acts only on the sunward half of the distribution. Despite this limitation, we find that the strong perpendicular heating of the resonant particles, coupled with the mirror force, results in substantial outward acceleration of the entire distribution. The proton distribution evolves towards an incomplete shell in velocity space, and appears vastly different from the distributions assumed in fluid models. Evidence of these distinctive distributions should be observable by instruments on Solar Probe.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005253706166
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  • 10
    Electronic Resource
    Electronic Resource
    A new resonance in the solar atmosphere (1979)
    Springer
    Solar physics 62 (1979), S. 227-240 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We consider a horizontally stratified isothermal model of the solar atmosphere, with vertical and uniform B 0, and v A 2 ≫v s 2 . The equations of motion are linearized about a background which is in hydrostatic equilibrium. A homogeneous wave equation results for the motions perpendicular to B 0; this wave equation is similar to the equation for the MHD fast mode. On the other hand, the equation for the parallel motions is inhomogeneous, containing ‘driving terms’ which arise from the presence of the fast mode; the homogeneous form of this equation is identical to the equation describing vertically-propagating gravity-modified acoustic waves. We demonstrate that a resonance can exist between the (driving) fast wave and the (driven) gravity-modified acoustic wave, in such a way that very large parallel velocities can be driven by small perpendicular velocities. Applications of this resonance to solar spicules, ‘jets’, and other phenomena are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155355
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