Engineering Papers⌕ Search

Engineering topics

Hollweg, J. V.

Publications and source records attributed to Hollweg, J. V..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Non-WKB Alfven waves in the solar wind: Propagation and reflection of pulses

The non-WKB propagation of Alfven waves has been studied either for harmonic waves, or in terms of the evolution of power spectra. Here we present analytical and numerical solutions for the propagation of pulses, the goal being to understand how waves reflect in a smoothly varying medium. We here limit our discussion to a radial magnetic field. If we launch an outward-propagating delta function, it leaves behind an inward-propagaing signal which is roughly a square wave whose amplitude is proportional to the area under the initial pulse. The inward-propagating signal also reflects, producing an outward propagating pulse which is roughly triangular in shape and which grows with time. These signals also oscillate if v is less than v(A), but they grow if v is greater than v(A). The result reported by us earlier, that the 'ingoing Elsasser variable' can have outgoing phase, is now understood to be a consequence of interference. The inward-propagating signal depends to lowest order on the integral of the outgoing waves which have preceded it. Thus the ingoing signal can be expected to develop as a random walk. This will affect the radial evolution of cross-helicity in the solar wind.

Hollweg, J. V.↗
NASA NTRS1992

Status of solar wind modeling from the transition region outwards

In recent years, solar wind modeling has to some extent undergone a shift of emphasis, from attempts to produce high-speed streams far from the sun, to investigations of what conditions must exist in the solar corona and transition region in order to produce the observed conditions in both the solar wind and low corona. Thus there has been an increased awareness that the solar wind should really be treated as part of the solar atmosphere, and that the problems associated with heating and accelerating the solar wind should be treated in concert with the coronal (and perhaps chromospheric) heating problems. We will discuss several models which take this point of view but we will place particular emphasis on some outstanding problems, viz. the mass flux problem, some puzzling recent IPS data, our persistent difficulties with electron heat conduction, and observational uncertainties about the coronal and transition region boundary conditions which should be put into the models. We will conclude by suggesting some possible alternatives for future models.

Hollweg, J. V.↗
NASA NTRS1986

Viscosity and the Chew-Goldberger-Low equations in the solar corona

A general discussion of the dominant terms in the stress tensor in a magnetized plasma such as the solar corona is presented. The importance of dissipative terms such as electrical resistivity, heat conduction, and interspecies collisions is assessed. For average coronal conditions, the proton stress tensor is found to reduce to the dominant terms in the classical expression for the viscous stress. The classical expression can fail in the transition region, however. In the diffusion region of reconnection, classical viscosity will be appropriate if the resistivity is very large, so that the diffusion region is broad, but in that case the viscous heating is small compared to the resistive heating. On the other hand, the more general expression for the stress tensor is required if the diffusion region is thin; the stress tensor will be important in this case. The electron stress tensor is also considered, and it is shown how the classical expression for electron viscosity can fail in the transition region and lower corona.

Hollweg, J. V.↗
NASA NTRS1986

Transition region, corona, and solar wind in coronal holes

The formation of wind-driven solar model based on a new hypothesis for nonlinear wave dissipation is described. The equations and solutions used to derive the model are examined. The corona and solar wind are analyzed. The model is applied to observations and it is detected that it displays the proper steep temperature rise to a maximum coronal temperature in excess of 10 to the 6th K, a solar wind flux in excess of 3.5 x 10 to the 8th/sq cm per sec at 1 AU, and nonthermal velocities are observed at the bases of coronal holes; however, the model does not produce enough detail.

Hollweg, J. V.↗
NASA NTRS1985

Alfvenic pulses in the solar atmosphere

Some nonlinear aspects of Alfvenic pulses propagating in coronal loops and the underlying chromosphere are numerically investigated. Heat conduction and radiation are included. The Alfvenic pulses are modeled as axisymmetric twists on a vertical cylindrical flux tube. They nonlinearly couple into acoustic-gravity waves propagating along the flux tube. A single Alfvenic pulse is found to leave two acoustic-gravity pulses in its wake. These pulses can result in significant motions of the transition region and underlying chromosphere. These motions do not resemble spicules, but they may correspond to a variety of observations indicating that the solar atmosphere is in a continual dynamic state. It is suggested that a dynamic chromosphere and transition region may be the inevitable consequence of the coronal heating process itself.

Mariska, J. T.↗
NASA NTRS1985

Viscosity in a magnetized plasma - Physical interpretation

The viscosity of a fully ionized H plasma in a magnetic field is investigated theoretically, revising the derivation proposed by Braginskii (1965) for the viscous-stress-tensor viscosity coefficient eta(0). It is shown that eta(0) terms can be attributed to the tendency of the plasma to produce small thermal anisotropies during its evolution, and hence that they are fully incorporated in the gyrotropic diagonal pressure tensor, neglecting the off-diagonal terms. The role of collisions in preventing anisotropy production (and thus causing irreversible heating) is explored, and the implications of the eta(0) terms for the physics of 1000-10,000-km-scale structures in the solar corona are indicated.

Hollweg, J. V.↗
NASA NTRS1985

Alfvenic heating of the chromosphere and corona

The behavior in the chromosphere of Alfven waves propagating on closed magnetic field lines (e.g. coronal loops) is considered. It is found that the observed chromospheric nonthermal velocities are consistent with the predicted behavior of Alfven waves. If they are indeed Alfven waves, then the observed motions imply energy fluxes which are sufficient to heat the corona and chromosphere. It is further shown that the observed motions can reproduce the observed chromospheric heating, if the heating occurs via a Kolmogoroff cascade. But a definitive analysis will require a self-consistent treatment of nonlinear effects.

Hollweg, J. V.↗
NASA NTRS1984

Surface solitary waves and solitons

The solar atmosphere and solar wind are magnetically structured. The structuring can include tangential discontinuities, which can support surface waves. Such waves can be dispersive. This means that dispersion and nonlinearity can balance in such a way that solitary waves (or solitons) can result. This general point is illustrated by a two-dimensional nonlinear analysis which explicitly demonstrates the presence of long-wavelength solitary waves propagating on tangential discontinuities. If the waves are only weakly nonlinear, then they obey the Korteweg-de Vries equation and are true solitons.

Hollweg, J. V.↗
NASA NTRS1984

Alfvenic resonances on solar spicules

It is suggested that twisting and heating of solar spicules can be produced by Alfven waves which enter the spicule from below. The spicule is treated as a region of constant Alfven speed which is bounded above by a region of much higher Alfven speed (the corona) and below by a region of exponentially increasing Alfven speed (the photosphere and chromosphere). It is shown how the spicule can act as a resonant cavity. The transmission of the waves into the cavity is analytically determined to be enhanced at certain resonant frequencies. With reasonable spicule parameters, and assuming the spicule damping to be moderately large, it is found that twisting velocities of approximately 20-30 km/s can be induced on the spicule. It is suggested that the Alfven waves are dissipated via a turbulent cascade of their energy to higher wavenumbers. It is shown that the waves can thereby heat the spicules to the observed temperatures. It is further suggested that the continued input of energy can explain why H-alpha spicules fade, since the predicted heating rate is sufficient to heat the spicules to temperatures at which the hydrogen is fully ionized; thus H-alpha spicules may evolve into EUV spicules.

Sterling, A. C.↗
NASA NTRS1984

Resonant heating - An interpretation of coronal loop data

It is shown that the resonant heating theory of Hollweg can be used to organize the coronal loop data of Golub et al. (1980). When combined with a reasonable form for the input power spectrum, the resonant heating theory is fully compatible with the loop data.

Hollweg, J. V.↗
NASA NTRS1984

Alfvenic resonant cavities in the solar atmosphere - Simple aspects

It is noted in the present investigation of Alfven wave propagtion in a simple medium consisting of three uniform layers, each of which is characterized by a different Alfven speed value, that the central layer can, under general conditions, act as a resonant cavity. When this cavity is externally driven by an incident wave on one of the outer layers, resonant transmission peaks are generated which allow large energy fluxes to enter the cavity from outside. Two types of resonance are distinguished: one which occurs when the cavity has the largest or the smallest of the three Alfven speeds, as in coronal loops, and another which is generated when the cavity Alfven speed is intermediate between the two outer Alfven speed values, as may occur on solar spicules. It is also shown that if the energy lost to heat exceeds that lost through leakage out of the cavity, cavity heating can be independent of the damping rate.

Hollweg, J. V.↗
NASA NTRS1984

Resonances of coronal loops

It is pointed out that any theoretical demonstration that the solar corona can be heated by waves requires a demonstration that the required energies can actually be carried from the convection zone to the corona by waves. In addition, it must be shown that the waves can dissipate their energy into heat in the corona. The present investigation is concerned specifically with the heating of closed magnetic structures in the corona, taking into account coronal loops or active region loops. Attention is given to the MHD Alfven wave. It is shown analytically that coronal active region loops can behave much like interference filters. The coronal part of the loop acts like a resonant cavity for Alfven waves. When the resonances are excited, large energy fluxes can be carried into the loop by Alfven waves which are generated in the solar convection zone. It is estimated that the energy fluxes can power the observed loops.

Hollweg, J. V.↗
NASA NTRS1983

Coronal heating by waves

Alfven waves or Alfvenic surface waves carry enough energy into the corona to provide the coronal energy requirements. Coronal loop resonances are an appealing means by which large energy fluxes enter active region loops. The wave dissipation mechanism still needs to be elucidated, but a Kolmogoroff turbulent cascade is fully consistent with the heating requirements in coronal holes and active region loops.

Hollweg, J. V.↗
NASA NTRS1983

On the preferential acceleration and heating of solar wind heavy ions

It is pointed out that the properties of solar wind ions heavier than H(+) have presented a puzzle to theorists for some time. The most recent theoretical efforts to explain the observations are considered, taking into account studies conducted by Dusenbery and Hollweg (1981) and Marsch et al. (1982). The first model of a wave-driven, three-fluid, supersonic solar wind is considered. Details of the resonant wave-particle interaction are discussed and the relevant equations for the model are derived. An investigation is conducted of the effects of resonant cyclotron acceleration and heating by a spectrum of nondispersive ion cyclotron waves on the model. It is found that there are serious difficulties with the hypothesis that the resonant cyclotron interaction is responsible for the preferential heating and acceleration of heavy ions in the solar wind. The model presented here is unable to produce the required energization by this mechanism. However, this model should be viewed as an approximate one, subject to possible correction by more advanced calculations.

Isenberg, P. A.↗
NASA NTRS1982

Surface waves on solar wind tangential discontinuities

It is demonstrated that (tangential) discontinuities in the magnetic field direction can support MHD surface waves. The surface waves are similar to the usual Alfven wave, but there are seven important differences. The first is that the surface waves exhibit a low-frequency cutoff; the second is that the velocity and magnetic field fluctuations are elliptically, and sometimes circularly, polarized. It is noted that they may account for the solar wind helicity spectrum. The third difference is that the surface waves are compressive, although there are special cases where they are noncompressive. The fourth is that the wave vector k, the local normals to the surfaces of constant phase, and the magnetic minimum variance direction do not all coincide. The fifth is that there is a tendency for the minimum variance direction to align itself with the mean magnetic field direction. The sixth difference is that the waves can be intrinsically nonplanar, and the seventh is that equipartition between magnetic and kinetic energies is not obeyed locally. These properties of the surface waves are interpreted to mean that surface waves may be common in the solar wind.

Hollweg, J. V.↗
NASA NTRS1982

Finite amplitude Alfven waves in a multi-ion plasma - Propagation, acceleration, and heating

An expression is derived for the wave action flux of finite-amplitude Alfven waves in a multi-ion plasma. The expression is valid in the presence of dissipative forces and permits an arbitrary angle between the average magnetic field and the wave vector. Applying the conservation of wave action and the first law of thermodynamics yields, for a multi-ion plasma, an expression for the spatial evolution of Alfven wave amplitude in the absence of dissipation. It also gives the relationship between the wave amplitude and the dissipative heating, as well as an expression for the acceleration of an ion species by finite-amplitude Alfven waves. It is pointed out that the acceleration comprises a nondissipative wave pressure that is identical to that derived previously under more restrictive conditions and a new term giving the acceleration that must accompany dissipative heating. The results are discussed in the context of the observations of heavy ions in the solar wind.

Isenberg, P. A.↗
NASA NTRS1982

On the preferential acceleration and heating of solar wind heavy ions

The feasibility of producing the observed velocities and temperatures of solar wind heavy ions by the resonant cyclotron interaction with left-polarized hydromagnetic waves was investigated. A "most favorable case" scenario in which the waves are parallel-propagating and dispersionless and the energy for the wave acceleration and heating is taken from saturated low-frequency Alfven waves via a cascade to higher frequencies, is incorporated into a numerical solar wind code and agreement with observation is tested. The resonant cyclotron interaction is shown to fail on at least three points, even in this most favorable case.

Isenberg, P. A.↗