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Author name code: hollweg
ADS astronomy entries on 2022-09-14
author:"Hollweg, Joseph V." 

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Title: Radio Occultation Observations of the Solar Corona Over
1.60-1.86 R<SUB>⊙</SUB>: Faraday Rotation and Frequency Shift
    Analysis
Authors: Wexler, David. B.; Hollweg, Joseph V.; Efimov, Anatoli I.;
   Song, Paul; Jensen, Elizabeth A.; Lionello, Roberto; Vierinen, Juha;
   Coster, Anthea J.
2019JGRA..124.7761W    Altcode:
  The study of coronal energy transport, central to the solar wind
  acceleration problem, relies upon accurate representation of magnetic
  fields and plasma electron densities. This information is difficult
  to obtain in middle-to-lower coronal regions that may contain complex
  magnetic structures. Faraday rotation (FR) solar radio occultation
  observations, which reveal line-of-sight (LOS) integrated product of
  the coronal magnetic field and electron density, can help characterize
  the coronal environment and constrain magnetic field strengths. Global
  magnetohydrodynamic (MHD) models use specified synoptic solar surface
  magnetograms and may be used to facilitate FR interpretation by
  estimating detailed magnetic field properties along the radio LOS. We
  present a hybrid FR analysis incorporating magnetic field solutions from
  an MHD coronal model, and an electron density radial profile conforming
  to radio frequency shift observations. The FR modeled by the hybrid
  method is compared to MErcury Surface, Space ENvironment, GEochemistry
  and Ranging spacecraft radio FR observations through a coronal region
  of low heliolatitudes and radial distance 1.60-1.86 R<SUB>⊙</SUB>
  from the heliocenter, collected during a state of relative solar
  quiescence. The hybrid model reasonably reproduces the form, polarity,
  and magnitude of the observed FR. For this specific coronal region,
  the calculated radial profile of electron concentrations and varied
  magnetic field strengths indicate Alfvén wave speeds below 50 km/s
  close to the point of closest approach but near 400 km/s in adjacent
  regions along the sounding LOS. The new approach of combining MHD
  models with radio sounding observations supports study of MHD wave
  processes in the challenging middle-coronal magneto-ionic environment.

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Title: Spacecraft Radio Frequency Fluctuations in the Solar Corona:
    A MESSENGER-HELIOS Composite Study
Authors: Wexler, David B.; Hollweg, Joseph V.; Efimov, Anatoli I.;
   Lukanina, Liudmila A.; Coster, Anthea J.; Vierinen, Juha; Jensen,
   Elizabeth A.
2019ApJ...871..202W    Altcode:
  Fluctuations in plasma electron density may play a role in solar coronal
  energy transport and the dissipation of wave energy. Transcoronal
  spacecraft radio sounding observations reveal frequency fluctuations
  (FFs) that encode the electron number density disturbances, allowing an
  exploration of the coronal compressive wave and advected inhomogeneity
  models. Primary FF observations from MESSENGER 2009 and published FF
  residuals from HELIOS 1975-1976 superior conjunctions were combined to
  produce a composite view of equatorial region FF near solar minimum
  over solar offset range 1.4-25R <SUB>⊙</SUB>. Methods to estimate
  the electron number density fluctuation variance from the observed FF
  were developed. We created a simple stacked, magnetically structured
  slab model that incorporated both propagating slow density waves and
  advected spatial density variations to explain the observed FF. Slow
  density waves accounted for most of the FF at low solar offset,
  while spatial density inhomogeneities advected at solar wind speed
  dominated above the sonic point at 6R <SUB>⊙</SUB>. Corresponding
  spatial scales ranged 1-38 Mm, with scales above 10 Mm contributing
  most to FF variance. Magnetic structuring of the model introduced
  radial elongation anistropy at lower solar offsets, but geometric
  conditions for isotropy were achieved as the slab correlation scales
  increased further out in the corona. The model produced agreement with
  the FF observations up to 12R <SUB>⊙</SUB>. FF analysis provides
  information on electron density fluctuations in the solar corona,
  and should take into account the background compressive slow waves and
  solar wind-related advection of quasi-static spatial density variations.

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Title: Quasilinear Consequences of Turbulent Ion Heating by Magnetic
    Moment Breaking
Authors: Isenberg, Philip A.; Vasquez, Bernard J.; Hollweg, Joseph V.
2019ApJ...870..119I    Altcode: 2018arXiv181205021I
  The fast solar wind emerging from coronal holes is likely heated and
  accelerated by the dissipation of magnetohydrodynamic turbulence,
  but the specific kinetic mechanism resulting in the perpendicular
  ion heating required by observations is not understood. A promising
  mechanism has been proposed by Chandran et al., which in this paper we
  call “magnetic moment breaking” (MMB). As currently formulated,
  MMB dissipation operates only on the ion perpendicular motion, and
  does not influence their parallel temperature. Thus, the MMB mechanism
  acting by itself produces coronal hole proton distributions that
  are unstable to the ion-cyclotron (IC) anisotropy instability. This
  quasilinear instability is expected to operate faster than the
  nonlinear turbulent cascade, scattering ions into the parallel
  direction and generating quasi-parallel-propagating IC waves. To
  investigate the consequences of this instability on the MMB-heated
  protons, we construct a homogeneous model for protons with coronal
  hole properties. Using a simplified version of the resonant cyclotron
  interaction, we heat the protons by the MMB process and instantaneously
  scatter them to lower anisotropy while self-consistently generating
  parallel-propagating IC waves. We present several illustrative cases,
  finding that the extreme anisotropies implied by the MMB mechanism are
  limited to reasonable values, but the distinctive shape of the proton
  distribution derived by Klein &amp; Chandran is not maintained. We
  also find that these combined processes can result in somewhat higher
  particle energization than the MMB heating alone. These quasilinear
  consequences should follow from any kinetic mechanism that primarily
  increases the perpendicular ion temperature in a collisionless plasma.

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Title: Spacecraft Radio Frequency Fluctuations in the Corona:
    a Messenger-Helios Composite Study
Authors: Wexler, David B.; Hollweg, J. V.; Efimov, A. I.; Lukanina,
   L. A.; Coster, A. J.; Jensen, E. A.
2018shin.confE.249W    Altcode:
  Dissipation of locally generated slow compressive waves may play a role
  in Alfven wave heating and acceleration of the corona. Transcoronal
  spacecraft radio sounding observations reveal frequency fluctuations
  (FF) that encode the coronal electron number density disturbances,
  allowing exploration of coronal acoustic or slow magnetoacoustic wave
  models. FF observations from MESSENGER 2009 and HELIOS 1975-1976
  superior conjunctions were combined to produce a composite view of
  equatorial region FF near solar minimum over solar offset range 1.4-25
  Rs. We present a model of FF based on randomized compressive waves
  aligned with the coronal radial magnetic structure and traveling at
  the sonic speed. The model intrinsically includes anisotropic features
  at low solar offset on the basis of magnetically controlled flux tube
  dimensions. Agreement between the observations and the modeled FF over
  solar offset up to 12 Rs supports the possibility that magnetically
  guided slow compressive waves are ubiquitous in the inner coronal
  regions pertinent to slow solar wind formation and initial acceleration.

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Title: Hybrid modeling of the lower corona using Faraday rotation
    observations and a MHD thermodynamic simulation
Authors: Wexler, David B.; Hollweg, Joseph V.; Jensen, Elizabeth;
   Lionello, Roberto; Macneice, Peter J.; Coster, Anthea J.
2017SPD....4830102W    Altcode:
  Study of coronal MHD wave energetics relies upon accurate representation
  of plasma particle number densities (n<SUB>e</SUB>) and magnetic
  field strengths. In the lower corona, these parameters are obtained
  indirectly, and typically presented as empirical equations as a function
  of heliocentric radial distance (solar offset, SO). The development
  of coronal global models using synoptic solar surface magnetogram
  inputs has provided refined characterization of the coronal plasma
  organization and magnetic field. We present a cross-analysis between a
  MHD thermodynamic simulation and Faraday rotation (FR) observations over
  SO 1.63-1.89 solar radii (R<SUB>s</SUB>) near solar minimum. MESSENGER
  spacecraft radio signals with a line of sight (LOS) passing through
  the lower corona were recorded in dual polarization using the Green
  Bank Telescope in November 2009. Polarization position angle changes
  were obtained from Stokes parameters. The magnetic field vector (B)
  and n<SUB>e</SUB> along the LOS were obtained from a MHD thermodynamic
  simulation provided by the Community Coordinated Modeling Center. The
  modeled FR was computed as the integrated product of n<SUB>e</SUB>
  and LOS-aligned B component. The observations over the given SO range
  yielded an FR change of 7 radians. The simulation reproduced this
  change when the modeled n<SUB>e</SUB> was scaled up by 2.8x, close
  to values obtained using the Allen-Baumbach equation. No scaling of
  B from the model was necessary. A refined fit to the observations was
  obtained when the observationally based total electron content (TEC)
  curves were introduced. Changes in LOS TEC were determined from radio
  frequency shifts as the signal passed to successively lower electron
  concentrations during egress. A good fit to the observations was
  achieved with an offset of 7e21 m<SUP>-2</SUP> added. Back-calculating
  n<SUB>e</SUB> along the LOS from the TEC curves, we found that the
  equivalent n<SUB>e</SUB> scaling compared to the model output was higher
  by a factor of 3. The combination of solar surface magnetogram-based MHD
  coronal simulations with transcoronal radio observations allows improved
  characterization of the lower corona. This hybrid approach potentially
  paves the way for more accurate use of Carrington rotation-specific
  coronal models.

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Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring
    the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence,
    and Radio Signatures of Solar Transients
Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell,
   J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa,
   M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess,
   D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen,
   C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.;
   Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.;
   Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson,
   E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury,
   T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper,
   J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh,
   V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.;
   Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas,
   D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.;
   Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.;
   Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin,
   S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.;
   Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle,
   A.; Werthimer, D.; Wygant, J. R.
2016SSRv..204...49B    Altcode: 2016SSRv..tmp...16B
  NASA's Solar Probe Plus (SPP) mission will make the first in situ
  measurements of the solar corona and the birthplace of the solar
  wind. The FIELDS instrument suite on SPP will make direct measurements
  of electric and magnetic fields, the properties of in situ plasma waves,
  electron density and temperature profiles, and interplanetary radio
  emissions, amongst other things. Here, we describe the scientific
  objectives targeted by the SPP/FIELDS instrument, the instrument
  design itself, and the instrument concept of operations and planned
  data products.

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Title: Deceleration of Alpha Particles in the Solar Wind by
Instabilities and the Rotational Force: Implications for Heating,
    Azimuthal Flow, and the Parker Spiral Magnetic Field
Authors: Verscharen, Daniel; Chandran, Benjamin D. G.; Bourouaine,
   Sofiane; Hollweg, Joseph V.
2015ApJ...806..157V    Altcode: 2014arXiv1411.4570V
  Protons and alpha particles in the fast solar wind are only weakly
  collisional and exhibit a number of non-equilibrium features,
  including relative drifts between particle species. Two non-collisional
  mechanisms have been proposed for limiting differential flow between
  alpha particles and protons: plasma instabilities and the rotational
  force. Both mechanisms decelerate the alpha particles. In this paper,
  we derive an analytic expression for the rate {Q}<SUB>{flow</SUB>} at
  which energy is released by alpha-particle deceleration, accounting
  for azimuthal flow and conservation of total momentum. We show
  that instabilities control the deceleration of alpha particles
  at r\lt {r}<SUB>{crit</SUB>}, and the rotational force controls
  the deceleration of alpha particles at r\gt {r}<SUB>{crit</SUB>},
  where {r}<SUB>{crit</SUB>}≃ 2.5 {AU} in the fast solar wind in
  the ecliptic plane. We find that {Q}<SUB>{flow</SUB>} is positive
  at r\lt {r}<SUB>{crit</SUB>} and {Q}<SUB>{flow</SUB>}=0 at r≥slant
  {r}<SUB>{crit</SUB>}, consistent with the previous finding that the
  rotational force does not lead to a release of energy. We compare
  the value of {Q}<SUB>{flow</SUB>} at r\lt {r}<SUB>{crit</SUB>}
  with empirical heating rates for protons and alpha particles, denoted
  {Q}<SUB>p</SUB> and {Q}<SUB>α </SUB>, deduced from in situ measurements
  of fast-wind streams from the Helios and Ulysses spacecraft. We find
  that {Q}<SUB>{flow</SUB>} exceeds {Q}<SUB>α </SUB> at r\lt 1 {AU},
  and that {Q}<SUB>{flow</SUB>}/{Q}<SUB>p</SUB> decreases with increasing
  distance from the Sun from a value of about one at r = 0.29-0.42 AU
  to about 1/4 at 1 AU. We conclude that the continuous energy input
  from alpha-particle deceleration at r\lt {r}<SUB>{crit</SUB>} makes an
  important contribution to the heating of the fast solar wind. We also
  discuss the implications of the alpha-particle drift for the azimuthal
  flow velocities of the ions and for the Parker spiral magnetic field.

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Title: Magnetohydrodynamic Slow Mode with Drifting He<SUP>++</SUP>:
    Implications for Coronal Seismology and the Solar Wind
Authors: Hollweg, Joseph V.; Verscharen, Daniel; Chandran, Benjamin
   D. G.
2014ApJ...788...35H    Altcode: 2014arXiv1404.4625H
  The MHD slow mode wave has application to coronal seismology,
  MHD turbulence, and the solar wind where it can be produced by
  parametric instabilities. We consider analytically how a drifting ion
  species (e.g. He<SUP>++</SUP>) affects the linear slow mode wave in
  a mainly electron-proton plasma, with potential consequences for the
  aforementioned applications. Our main conclusions are as follows. 1. For
  wavevectors highly oblique to the magnetic field, we find solutions that
  are characterized by very small perturbations of total pressure. Thus,
  our results may help to distinguish the MHD slow mode from kinetic
  Alfvén waves and non-propagating pressure-balanced structures, which
  can also have very small total pressure perturbations. 2. For small ion
  concentrations, there are solutions that are similar to the usual slow
  mode in an electron-proton plasma, and solutions that are dominated by
  the drifting ions, but for small drifts the wave modes cannot be simply
  characterized. 3. Even with zero ion drift, the standard dispersion
  relation for the highly oblique slow mode cannot be used with the
  Alfvén speed computed using the summed proton and ion densities, and
  with the sound speed computed from the summed pressures and densities
  of all species. 4. The ions can drive a non-resonant instability
  under certain circumstances. For low plasma beta, the threshold drift
  can be less than that required to destabilize electromagnetic modes,
  but damping from the Landau resonance can eliminate this instability
  altogether, unless T<SUB>e</SUB> /T<SUB>p</SUB> Gt 1.

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Title: Velocity-shear-induced Mode Coupling in the Solar Atmosphere
and Solar Wind: Implications for Plasma Heating and MHD Turbulence
Authors: Hollweg, Joseph V.; Kaghashvili, Edisher Kh.; Chandran,
   Benjamin D. G.
2013ApJ...769..142H    Altcode:
  We analytically consider how velocity shear in the corona and solar
  wind can cause an initial Alfvén wave to drive up other propagating
  signals. The process is similar to the familiar coupling into other
  modes induced by non-WKB refraction in an inhomogeneous plasma, except
  here the refraction is a consequence of velocity shear. We limit our
  discussion to a low-beta plasma, and ignore couplings into signals
  resembling the slow mode. If the initial Alfvén wave is propagating
  nearly parallel to the background magnetic field, then the induced
  signals are mainly a forward-going (i.e., propagating in the same
  sense as the original Alfvén wave) fast mode, and a driven signal
  propagating like a forward-going Alfvén wave but polarized like the
  fast mode; both signals are compressive and subject to damping by the
  Landau resonance. For an initial Alfvén wave propagating obliquely
  with respect to the magnetic field, the induced signals are mainly
  forward- and backward-going fast modes, and a driven signal propagating
  like a forward-going Alfvén wave but polarized like the fast mode;
  these signals are all compressive and subject to damping by the Landau
  resonance. A backward-going Alfvén wave, thought to be important in
  the development of MHD turbulence, is also produced, but it is very
  weak. However, we suggest that for oblique propagation of the initial
  Alfvén wave the induced fast-polarized signal propagating like a
  forward-going Alfvén wave may interact coherently with the initial
  Alfvén wave and distort it at a strong-turbulence-like rate.

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Title: Observational Constraints on the Role of Cyclotron Damping
    and Kinetic Alfvén Waves in the Solar Wind
Authors: Smith, Charles W.; Vasquez, Bernard J.; Hollweg, Joseph V.
2012ApJ...745....8S    Altcode:
  Certain few intervals with high-β plasma (ratio of gas pressure to
  magnetic pressure) have been interpreted as containing turbulent
  fluctuations with wave vectors that are confined to very oblique
  angles with respect to the mean magnetic field. The fluctuations are
  theorized to be Kinetic Alfvén Waves (KAWs) engaged in an energy
  cascade that dissipates primarily at electron scales. Dissipation by
  ions, and by cyclotron damping in particular, is argued to be minimal to
  non-existent. This interpretation is not supported, generally, by the
  analysis of larger data sets using other data analysis methods. These
  prior studies, however, were not conducted for specific β ranges. In
  this study, we reconsider the analysis for a moderately large set of
  high-β intervals. The analysis includes magnetic variance, the Bieber
  ratio test, the cross-helicity versus magnetic helicity correlation, and
  the implied break frequency versus angle relationship. In our analysis,
  the results do not support the exclusive KAW interpretation as applied
  generally to solar wind intervals of high-β while the results do
  support the presence of cyclotron damping at a significant level.

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Title: Alfvén Waves in Shear Flows Revisited
Authors: Hollweg, Joseph V.; Kaghashvili, Edisher Kh.
2012ApJ...744..114H    Altcode:
  We revisit our earlier study of the evolution of an initial propagating
  Alfvén wave in a magnetic-field-aligned flow with a cross-field
  velocity shear. Our goal is to show how the Alfvén wave drives up
  plasma density fluctuations which might be observed and serve as a
  signature of the presence of Alfvén waves in regions such as the
  solar corona which are inaccessible to direct observations. Here,
  we introduce a new initial condition which takes into account the
  initial distortion of the streamlines by the Alfvén wave, and we
  present new analytical results for the driven waves. We find that the
  density fluctuations of a properly placed linearly polarized Alfvén
  wave in a shear flow are much smaller than we originally estimated.

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Title: Observational Constraints on the Role of Kinetic Alfven Waves
    in the Solar Wind
Authors: Smith, C. W.; Vasquez, B. J.; Hollweg, J. V.
2011AGUFMSH43C1969S    Altcode:
  Several recent publications have proposed an extreme view of
  ion inertial and dissipation range dynamics that can be readily
  tested. Specifically, a view has been proposed by Alexandrova et
  al. (2009) that suggests a second turbulent inertial range may exist at
  scales smaller than the ion inertial scale where electron MHD dynamics
  may provide energy transport without dissipation. This leads to a
  predictable and reproducible spectrum at spacecraft frame frequencies
  greater than the proton cyclotron frequency and less than the electron
  cyclotron frequency. However, other very radical interpretations of
  the observations have suggested that highly perpendicular Kinetic
  Alfven Waves (KAW) can be used to describe the ion inertial scales
  through these high frequencies and extending to the electron cyclotron
  frequency such that the high frequency measurements are just Doppler
  shifted extensions of this highly perpendicular inertial range dynamics
  (Sahraoui et al. 2009, 2010). We take exception to these interpretations
  and provide a series of observational tests that clearly refute the
  arguments.

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Title: The Argument Against Kinetic Alfven Waves Forming the Whole
    of the Dissipation Range
Authors: Smith, Charles William; Vasquez, Bernard J.; Hollweg,
   Joseph V.
2011shin.confE..96S    Altcode:
  For over a decade, carefully performed analyses of the magnetic
  spectrum at scales surrounding those at which dissipation occurs in
  association with ion dynamics have resulted in a clear and consistent
  view of this part of the spectrum. That view involves the complimentary
  dynamics of cascade and dissipation. Most recently, a more radical
  interpretation of a very small number observations have suggested
  that highly perpendicular Kinetic Alfven Waves (KAW) describe the
  ion inertial scales through the highest frequencies extending to the
  electron cyclotron frequency such that the high frequency measurements
  are just Doppler shifted extensions of the highly perpendicular inertial
  range dynamics (Sahraoui et al. 2009, 2010). This interpretation leads
  to simple tests that can be performed to confirm or refute the assumed
  underlying radical 2D geometry of the spectrum. We describe a series
  of observational tests that clearly demonstrate that this radical
  interpretation does not apply, in general, to the interplanetary
  spectrum.

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Title: Observational Constraints on the Role of Kinetic Alfven Waves
    in the Solar Wind
Authors: Smith, Charles William; Vasquez, Bernard J.; Hollweg,
   Joseph V.
2011shin.confE..97S    Altcode:
  Several recent publications have proposed an extreme view of
  ion inertial and dissipation range dynamics that can be readily
  tested. Specifically, a view has been proposed by Alexandrova et
  al. (2009) that suggests a second turbulent inertial range may exist at
  scales smaller than the ion inertial scale where electron MHD dynamics
  may provide energy transport without dissipation. This leads to a
  predictable and reproducible spectrum at spacecraft frame frequencies
  greater than the proton cyclotron frequency and less than the electron
  cyclotron frequency. However, other very radical interpretations of
  the observations have suggested that highly perpendicular Kinetic
  Alfven Waves (KAW) can be used to describe the ion inertial scales
  through these high frequencies and extending to the electron cyclotron
  frequency such that the high frequency measurements are just Doppler
  shifted extensions of this highly perpendicular inertial range dynamics
  (Sahraoui et al. 2009, 2010). We take exception to these interpretations
  and provide a series of observational tests that clearly refute the
  arguments.

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Title: Recent Successes of Wave/Turbulence Driven Models of Solar
    Wind Acceleration
Authors: Cranmer, S. R.; Hollweg, J. V.; Chandran, B. D.; van
   Ballegooijen, A. A.
2010AGUFMSH41B1786C    Altcode:
  A key obstacle in the way of producing realistic simulations of the
  Sun-heliosphere system is the lack of a first-principles understanding
  of coronal heating. Also, it is still unknown whether the solar wind
  is "fed" through flux tubes that remain open (and are energized by
  footpoint-driven wavelike fluctuations) or if mass and energy are
  input intermittently from closed loops into the open-field regions. In
  this presentation, we discuss self-consistent models that assume the
  energy comes from solar Alfven waves that are partially reflected,
  and then dissipated, by magnetohydrodynamic turbulence. These models
  have been found to reproduce many of the observed features of the fast
  and slow solar wind without the need for artificial "coronal heating
  functions" used by earlier models. For example, the models predict
  a variation with wind speed in commonly measured ratios of charge
  states and elemental abundances that agrees with observed trends. This
  contradicts a commonly held assertion that these ratios can only be
  produced by the injection of plasma from closed-field regions on the
  Sun. This presentation also reviews two recent comparisons between the
  models and empirical measurements: (1) The models successfully predict
  the amplitude and radial dependence of Faraday rotation fluctuations
  (FRFs) measured by the Helios probes for heliocentric distances between
  2 and 15 solar radii. The FRFs are a particularly sensitive test of
  turbulence models because they depend not only on the plasma density
  and Alfven wave amplitude in the corona, but also on the turbulent
  correlation length. (2) The models predict the correct sense and
  magnitude of changes seen in the polar high-speed solar wind by Ulysses
  from the previous solar minimum (1996-1997) to the more recent peculiar
  minimum (2008-2009). By changing only the magnetic field along the polar
  magnetic flux tube, consistent with solar and heliospheric observations
  at the two epochs, the model correctly predicts that the wind speed
  remains relatively unchanged, but the in-situ density and temperature
  decrease by approximately 20 percent and 10 percent, respectively.

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Title: Ultraviolet Coronagraph Spectroscopy: A Key Capability for
    Understanding the Physics of Solar Wind Acceleration
Authors: Cranmer, S. R.; Kohl, J. L.; Alexander, D.; Bhattacharjee,
   A.; Breech, B. A.; Brickhouse, N. S.; Chandran, B. D. G.; Dupree,
   A. K.; Esser, R.; Gary, S. P.; Hollweg, J. V.; Isenberg, P. A.; Kahler,
   S. W.; Ko, Y. -K.; Laming, J. M.; Landi, E.; Matthaeus, W. H.; Murphy,
   N. A.; Oughton, S.; Raymond, J. C.; Reisenfeld, D. B.; Suess, S. T.;
   van Ballegooijen, A. A.; Wood, B. E.
2010arXiv1011.2469C    Altcode:
  Understanding the physical processes responsible for accelerating the
  solar wind requires detailed measurements of the collisionless plasma
  in the extended solar corona. Some key clues about these processes
  have come from instruments that combine the power of an ultraviolet
  (UV) spectrometer with an occulted telescope. This combination enables
  measurements of ion emission lines far from the bright solar disk,
  where most of the solar wind acceleration occurs. Although the UVCS
  instrument on SOHO made several key discoveries, many questions remain
  unanswered because its capabilities were limited. This white paper
  summarizes these past achievements and also describes what can be
  accomplished with next-generation instrumentation of this kind.

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Title: Coronal Faraday Rotation Fluctuations and a
    Wave/Turbulence-driven Model of the Solar Wind
Authors: Hollweg, Joseph V.; Cranmer, Steven R.; Chandran, Benjamin
   D. G.
2010ApJ...722.1495H    Altcode:
  Some recent models for coronal heating and the origin of the solar wind
  postulate that the source of energy and momentum consists of Alfvén
  waves of solar origin dissipating via MHD turbulence. We use one of
  these models to predict the level of Faraday rotation fluctuations
  (FRFs) that should be imposed on radio signals passing through the
  corona. This model has the virtue of specifying the correlation length
  of the turbulence, knowledge of which is essential for calculating the
  FRFs; previous comparisons of observed FRFs with models suffered from
  the fact that the correlation length had to be guessed. We compare the
  predictions with measurements of FRFs obtained by the Helios radio
  experiment during occultations in 1975 through 1977, close to solar
  minimum. We show that only a small fraction of the FRFs are produced by
  density fluctuations; the bulk of the FRFs must be produced by coronal
  magnetic field fluctuations. The observed FRFs have periods of hours,
  suggesting that they are related to Alfvén waves which are observed
  in situ by spacecraft throughout the solar wind; other evidence also
  suggests that the FRFs are due to coronal Alfvén waves. We choose
  a model field line in an equatorial streamer which has background
  electron concentrations that match those inferred from the Helios
  occultation data. The predicted FRFs are found to agree very well
  with the Helios data. If the FRFs are in fact produced by Alfvén
  waves with the assumed correlation length, our analysis leads us to
  conclude that wave-turbulence models should continue to be pursued
  with vigor. But since we cannot prove that the FRFs are produced by
  Alfvén waves, we state the more conservative conclusion, still subject
  to the correctness of the assumed correlation length, that the corona
  contains long-period magnetic fluctuations with sufficient energy to
  heat the corona and drive the solar wind.

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Title: Resonant Interactions Between Protons and Oblique
    Alfvén/Ion-cyclotron Waves in the Solar Corona and Solar Flares
Authors: Chandran, Benjamin D. G.; Pongkitiwanichakul, Peera; Isenberg,
   Philip A.; Lee, Martin A.; Markovskii, Sergei A.; Hollweg, Joseph V.;
   Vasquez, Bernard J.
2010ApJ...722..710C    Altcode:
  We consider interactions between protons and Alfvén/ion-cyclotron
  (A/IC) waves in collisionless low-β plasmas in which the proton
  distribution function f is strongly modified by wave pitch-angle
  scattering. If the angle θ between the wave vector and background
  magnetic field is zero for all the waves, then strong scattering
  causes f to become approximately constant on surfaces of constant η,
  where η ~= v <SUP>2</SUP> <SUB>bottom</SUB> + 1.5 v <SUP>2/3</SUP>
  <SUB>A</SUB>|v <SUB>par</SUB>|<SUP>4/3</SUP>. Here, v <SUB>bottom</SUB>
  and v <SUB>par</SUB> are the velocity components perpendicular and
  parallel to the background magnetic field, and v <SUB>A</SUB> is the
  Alfvén speed. If f = f(η), then A/IC waves with θ = 0 are neither
  damped nor amplified by resonant interactions with protons. In this
  paper, we argue that if some mechanism generates high-frequency A/IC
  waves with a range of θ values, then wave-particle interactions
  initially cause the proton distribution function to become so
  anisotropic that the plasma becomes unstable to the growth of waves with
  θ = 0. The resulting amplification of θ = 0 waves leads to an angular
  distribution of A/IC waves that is sharply peaked around θ = 0 at the
  large wavenumbers at which A/IC waves resonate with protons. Scattering
  by this angular distribution of A/IC waves subsequently causes f to
  become approximately constant along surfaces of constant η, which in
  turn causes oblique A/IC waves to be damped by protons. We calculate
  the proton and electron contributions to the damping rate analytically,
  assuming Maxwellian electrons and f = f(η). Because the plasma does
  not relax to a state in which proton damping of oblique A/IC waves
  ceases, oblique A/IC waves can be significantly more effective at
  heating protons than A/IC waves with θ = 0.

---------------------------------------------------------
Title: Alfvén Wave Reflection and Turbulent Heating in the Solar
Wind from 1 Solar Radius to 1 AU: An Analytical Treatment
Authors: Chandran, Benjamin D. G.; Hollweg, Joseph V.
2009ApJ...707.1659C    Altcode: 2009arXiv0911.1068C
  We study the propagation, reflection, and turbulent dissipation of
  Alfvén waves in coronal holes and the solar wind. We start with
  the Heinemann-Olbert equations, which describe non-compressive
  magnetohydrodynamic fluctuations in an inhomogeneous medium with a
  background flow parallel to the background magnetic field. Following
  the approach of Dmitruk et al., we model the nonlinear terms in these
  equations using a simple phenomenology for the cascade and dissipation
  of wave energy and assume that there is much more energy in waves
  propagating away from the Sun than waves propagating toward the Sun. We
  then solve the equations analytically for waves with periods of hours
  and longer to obtain expressions for the wave amplitudes and turbulent
  heating rate as a function of heliocentric distance. We also develop a
  second approximate model that includes waves with periods of roughly one
  minute to one hour, which undergo less reflection than the longer-period
  waves, and compare our models to observations. Our models generalize the
  phenomenological model of Dmitruk et al. by accounting for the solar
  wind velocity, so that the turbulent heating rate can be evaluated
  from the coronal base out past the Alfvén critical point—that is,
  throughout the region in which most of the heating and acceleration
  occurs. The simple analytical expressions that we obtain can be used
  to incorporate Alfvén-wave reflection and turbulent heating into
  fluid models of the solar wind.

---------------------------------------------------------
Title: Driven Waves as a Diagnostics Tool in the Solar Corona
Authors: Kaghashvili, Edisher Kh.; Quinn, Richard A.; Hollweg,
   Joseph V.
2009ApJ...703.1318K    Altcode:
  Detecting the signature of Alfvén waves in the solar atmosphere
  remains an observational challenge. At the same time, it could also
  be an important key to gaining critical understanding of the solar
  wind and especially of the near-Earth space weather formation. Here,
  we investigate the plausibility of using inhomogeneous flow-driven
  compressional fluctuations as a diagnostics tool for Alfvén waves in
  the solar corona. The nature of the fluctuations driven by transverse
  Alfvén waves in inhomogeneous flows was recently investigated by
  Kaghashvili et al., and analytical solutions that accurately link driven
  waves to the Alfvénic driver were found. The novelty of this mechanism
  is that the analysis of the detected compressional fluctuations can
  provide a clue about the Alfvén waves that are otherwise difficult
  to detect. We review this physical process in a low-β approximation
  relevant to solar coronal conditions and outline basic reasons why it
  can be one of the major processes that comes about as outflowing plasma
  emerges from divergent coronal holes. After establishing a quantitative
  link, we consider an example with coronal hole plasma parameters
  similar to the ones reported recently where evidence for Alfvén waves
  in solar X-ray jets was discussed. We show how this diagnostics tool
  can be used to analyze the detected intensity fluctuations.

---------------------------------------------------------
Title: Strong MHD Turbulence with Cross Helicity
Authors: Chandran, Benjamin D. G.; Quataert, Eliot; Howes, Gregory;
   Hollweg, Joseph; Dorland, Bill
2009shin.confE..63C    Altcode:
  Velocity and magnetic field fluctuations in the solar wind are often
  strongly correlated. This correlation, or cross helicity, indicates
  that much of the fluctuation energy is in Alfven waves propagating
  away from the Sun in the solar-wind frame. Although cross helicity has
  been studied in the solar-wind context for many years, the effects
  of cross helicity on solar-wind turbulence remain only partially
  understood. This poster presents some new theoretical results on how
  cross helicity affects the energy cascade rate and power spectra in
  strong MHD turbulence.

---------------------------------------------------------
Title: The Turbulent Heating Rate in Strong Magnetohydrodynamic
    Turbulence with Nonzero Cross Helicity
Authors: Chandran, Benjamin D. G.; Quataert, Eliot; Howes, Gregory G.;
   Hollweg, Joseph V.; Dorland, William
2009ApJ...701..652C    Altcode: 2009arXiv0905.3382C
  Different results for the cascade power epsilon in strong,
  incompressible magnetohydrodynamic turbulence with nonzero cross
  helicity appear in the literature. In this paper, we discuss the
  conditions under which these different results are valid. Our
  conclusions can be expressed in terms of the density ρ, the rms
  amplitudes z <SUP>+</SUP> and z <SUP>-</SUP> of Alfvénic fluctuations
  propagating parallel and antiparallel to the background magnetic field
  B <SUB>0</SUB>, and the correlation length (outer scale) measured
  perpendicular to B <SUB>0</SUB>, denoted L <SUB>bottom</SUB>. We argue
  that if z <SUP>+</SUP> Gt z <SUP>-</SUP> and if the z <SUP>-</SUP>
  fluctuations are sustained by the reflection of z <SUP>+</SUP>
  fluctuations in a strong background magnetic field, then epsilon ~
  ρ(z <SUP>+</SUP>)<SUP>2</SUP> z <SUP>-</SUP>/L <SUB>bottom</SUB>
  as in previous studies by Hossain, Matthaeus, Dmitruk, Lithwick,
  Goldreich, Sridhar, and others. On the other hand, if the minority
  wave type (z <SUP>-</SUP>) is sustained by some form of forcing
  that is uncorrelated with or only weakly correlated with the z
  <SUP>+</SUP> fluctuations, then epsilon can be much less than ρ(z
  <SUP>+</SUP>)<SUP>2</SUP> z <SUP>-</SUP>/L <SUB>bottom</SUB>, as in
  previous studies by Dobrowolny, Lazarian, Chandran, and others. The
  mechanism for generating the minority wave type strongly affects the
  cascade power because it controls the coherence time for interactions
  between oppositely directed wave packets at the outer scale.

---------------------------------------------------------
Title: Proton Heating by Nonlinear Field-Aligned Alfvén Waves in
    Solar Coronal Holes
Authors: Markovskii, S. A.; Vasquez, Bernard J.; Hollweg, Joseph V.
2009ApJ...695.1413M    Altcode:
  Field-aligned Alfvén waves are often viewed as a source of the
  proton heating that accelerates the fast solar wind. However,
  the energy that they can inject into the protons in the limit of
  cyclotron-resonant quasi-linear diffusion is insufficient to account
  for the observed acceleration. To test the validity of this limit in
  coronal holes, nonlinear Alfvén waves are modeled using a hybrid
  code. It is found that the nonlinearity is particularly strong
  when the intensity of antisunward-propagating waves is comparable
  to that of sunward waves. The sunward waves can be generated by the
  proton distribution as it evolves with the heliocentric distance. The
  ponderomotive force and beat interaction are identified as the most
  important nonlinear effects. The nonlinearity of the field-aligned
  Alfvén waves produces density fluctuations. In the simulations, the
  amplitude of the density fluctuations was kept within the observed
  constraints from the interplanetary scintillation measurements in the
  corona. In this case, the characteristic time of the proton heating
  is almost 2 orders of magnitude smaller than the solar wind expansion
  time. Therefore, it can contribute to the energization of the solar
  wind on the global scale. The nonlinear wave damping operating alone
  cannot be responsible for the energization because it only causes
  particle diffusion parallel to the magnetic field. However, it can
  relax the limitation on the perpendicular diffusion imposed by the
  cyclotron resonance condition. The nonlinear damping combined with
  the linear one can then inject the additional thermal energy needed
  to accelerate the solar wind.

---------------------------------------------------------
Title: The solar wind: Our current understanding and how we got here
Authors: Hollweg, Joseph V.
2008JApA...29..217H    Altcode:
  In the original theory for the solar wind, the electron pressure
  gradient was the principal accelerating force. This was soon recognized
  to be insufficient to drive the high-speed streams. Subsequently,
  the discovery of Alfvén waves in the solar wind led to a long series
  of models in which wave pressure provided additional acceleration,
  but these wavedriven models ultimately failed to explain the rapid
  acceleration of the fast wind close to the Sun. An alternate view was
  that the pressure of hot protons close to the Sun could explain the
  rapid acceleration, with the proton heating coming from the cyclotron
  resonance. SOHO has provided remarkable data which have verified some
  of the predictions of this view, and given impetus to ongoing studies
  of the ion-cyclotron resonance in the fast wind. After a historical
  review, we discuss the basic ideas behind current research, emphasizing
  the importance of particle kinetics. We conclude with some guesses as
  to how work might proceed in the future.

---------------------------------------------------------
Title: Hybrid simulations of anisotropic proton distributions in
    solar coronal holes
Authors: Hollweg, J.; Markovskii, S.; Vasquez, B.
2007AGUFMSH22B..06H    Altcode:
  The plasma in solar coronal holes is likely to be energized by the
  resonant damping of proton cyclotron waves. In this case, the protons
  can develop considerable temperature anisotropy in the region where
  the solar wind becomes collisionless. The temperature anisotropy can
  give us important information about the processes in the solar corona
  based on in situ observations. The extrapolation of the values of the
  anisotropy from one heliocentric distance to another is not necessarily
  a valid procedure because the plasma heating and expansion, which
  contribute to the anisotropy, may operate differently there. However,
  the observations suggest that the mean anisotropy in the fast wind as
  a function of the plasma beta obeys the same scaling law at different
  distances. This can provide a link between widely separated regions
  of the solar wind. We will carry out hybrid simulations to determine
  how the proton anisotropy is affected by the interplay of the proton
  energization (perhaps by more than one mechanism), plasma instabilities
  self-driven by the distribution, and the solar wind expansion. We will
  verify if the observed beta scaling can be reproduced in a numerical
  experiment for typical coronal hole parameters.

---------------------------------------------------------
Title: Reflection of Alfvén waves in the corona and solar wind:
    An impulse function approach
Authors: Hollweg, Joseph V.; Isenberg, P. A.
2007JGRA..112.8102H    Altcode: 2007JGRA..11208102H
  We consider the reflection of Alfvén waves in the corona and solar
  wind, using variables f and g which follow sunward and antisunward
  characteristics, respectively. We show that the basic equations for
  f and g have the same structure as the Klein-Gordon equation. Unlike
  previous studies which used a harmonic analysis, we emphasize the
  impulse response of the system. This is equivalent to finding the
  Green's function, but it may have direct application to situations
  where Alfvén waves are launched impulsively. We provide an approximate
  analysis which can be used to understand most features that appear
  in detailed numerical solutions. The analysis reveals the origin of
  a previous result that f and g each has both sunward and antisunward
  propagating phase in a harmonic analysis, even though f (g) follows
  only the sunward (antisunward) characteristic. We numerically study the
  propagation of an antisunward moving impulse in the corona and solar
  wind. We find that the sunward moving “wake” tends to become more
  important at greater distances beyond the Alfvén critical point,
  possibly providing a natural explanation of the observation that
  outward propagating waves become less dominant at greater distances
  from the Sun. There is an extended region behind the initial impulse
  in which magnetic energy dominates kinetic energy; it is not clear,
  however, whether our result can explain the observed dominance of
  magnetic energy throughout many decades of frequency in the observed
  power spectrum. We also find that the outgoing wake has a tendency
  to “ring,” with periods of the order of 15-30 min. The ringing is
  associated mainly with propagation through a structured Alfvén speed
  profile rather that with the cutoff in the Klein-Gordon equation. These
  oscillation periods seem too short to explain why Alfvén waves in
  the solar wind have most power at periods of hours, but other Alfvén
  speed profiles could yield longer periods. We also investigate whether
  the same approach can be used for acoustic-gravity waves propagating
  along magnetic flux tubes in the solar atmosphere.

---------------------------------------------------------
Title: On the behavior of O<SUP>+5</SUP> in coronal holes: Importance
    of sunward propagating waves
Authors: Hollweg, Joseph V.
2006JGRA..11112106H    Altcode:
  The high thermal anisotropy of O<SUP>+5</SUP> in coronal holes, as
  observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on the
  Solar and Heliospheric Observatory (SOHO), suggests that these ions
  are being heated by the cyclotron resonance. The observations indicate
  that the O<SUP>+5</SUP> temperature steadily increases from r ≈ 1.5
  r<SUB>S</SUB> (where the ions become almost collisionless) out to r
  ≈ 3.5 r<SUB>S</SUB> (the outer limit of the observations). Previous
  models have not been able to reproduce even the qualitative result of
  a steady temperature rise. We suggest that the problem has been that
  previous models have considered O<SUP>+5</SUP> resonating only with
  outward propagating waves. Once the ions are heated perpendicularly to
  the background magnetic field, they are accelerated to high outward flow
  speeds by the mirror force. As a result, they resonate with outgoing
  waves having higher (normalized) wave numbers, where there is presumably
  less power; they may even drop out of resonance altogether. We suggest
  here that resonances with inward (i.e., sunward) propagating waves
  may be the key to explaining the observed O<SUP>+5</SUP> temperature
  rise. In that case, as the ions are accelerated by the mirror force,
  they never drop out of resonance, and they resonate with ingoing waves
  having lower (normalized) wave numbers where there is presumably
  more power. We offer a very simple strawman model to illustrate
  the differences between oxygen resonances with ingoing and outgoing
  waves and to show that the UVCS/SOHO results can be approximately
  reproduced if the ingoing wave power spectrum in the resonant range
  varies as k<SUP>-γ</SUP>, with γ ≈ 5/3. We point out that it is
  really necessary to take into account the fact that O<SUP>+5</SUP>
  (and other heavy ions) can resonate with ingoing and outgoing waves
  simultaneously, which can only be studied via full kinetic solutions
  for the ion distribution functions; however, it is possible that once
  the ions are accelerated by the mirror force, the resonances with
  sunward propagating waves will be dominant.

---------------------------------------------------------
Title: Erratum: “Dissipation of the Perpendicular Turbulent Cascade
    in the Solar Wind” (<A href="/abs/2006ApJ...639.1177M">ApJ, 639,
    1177 [2006]</A>)
Authors: Markovskii, S. A.; Vasquez, Bernard J.; Smith, Charles W.;
   Hollweg, Joseph V.
2006ApJ...648.1291M    Altcode:
  The citation given as Hui et al. (2001) on pages 1177 (Introduction,
  paragraph 3), 1178 (§ 1.2, paragraph 2), and 1182 (§ 3, paragraph 8)
  should instead read Li et al. (2001), and the correct full reference
  should read <A href="/abs/2006ApJ...639.1177M">ApJ, 639, 1177 [2006]</A>
  (instead of Hui, L., Gary, S. P., &amp; Stawicki, O.). The authors
  sincerely regret the error.

---------------------------------------------------------
Title: The Solar Wind, Then and Now
Authors: Hollweg, Joseph V.
2006SPD....37.1501H    Altcode: 2006BAAS...38..244H
  Early spacecraft data in the 1960s revealed solar wind properties,
  which could not be well explained by models in which the electron
  pressure gradient was the principal accelerating force. The Alfven waves
  discovered around 1970 were thought for a while to provide additional
  energy and momentum, but they ultimately failed to explain the rapid
  acceleration of the fast wind close to the Sun. By the late 1970s,
  various data were suggesting the importance of the ion-cyclotron
  resonance far from the Sun. This notion was soon applied to the
  acceleration region close to the Sun. The models that resulted
  suggested that the fast wind could be driven mainly by the proton
  pressure gradient. Since the mid-1990s, the Solar and Heliospheric
  Observatory has provided remarkable data, which have verified some
  of the predictions of these theories, and given impetus to studies of
  the ion-cyclotron resonance as the principal mechanism for heating the
  coronal holes, and ultimately driving the fast wind. After a historical
  review, we discuss the basic ideas behind current research, emphasizing
  the particle kinetics. We discuss remaining problems, especially the
  source of the ion-cyclotron resonant waves.

---------------------------------------------------------
Title: Dissipation of the Perpendicular Turbulent Cascade in the
    Solar Wind
Authors: Markovskii, S. A.; Vasquez, Bernard J.; Smith, Charles W.;
   Hollweg, Joseph V.
2006ApJ...639.1177M    Altcode:
  The core solar wind protons are observed to be heated perpendicularly
  to the magnetic field. This is taken to be a signature of the
  cyclotron damping of the turbulent fluctuations, which are thought
  to be responsible for the heating. At the same time, it is commonly
  accepted that the turbulent cascade produces mostly highly oblique
  (quasi-two-dimensional) fluctuations, which cannot be immediately
  cyclotron resonant with the ions because of their low frequencies
  and small parallel wavenumbers. To address this problem, we propose
  a new, indirect mechanism for damping the quasi-two-dimensional
  fluctuations. The mechanism involves a plasma instability, which
  excites ion cyclotron resonant waves. As the cascade proceeds to higher
  wavenumbers, it generates increasingly high velocity shear associated
  with the turbulent fluctuations. The shear eventually becomes unstable
  to waves near harmonics of the ion cyclotron frequency. Once the
  frequency of the waves is upshifted, they can heat ions perpendicularly,
  extracting the energy from the quasi-two-dimensional fluctuations. The
  dissipation rates of quasi-two-dimensional fluctuations are incorporated
  into a model of the energy transfer in the turbulent cascade. Our
  analysis of the observed spectra shows that the spectral break
  separating the inertial and dissipation ranges of the turbulence,
  where the dissipation sets in, corresponds to the same shear under a
  wide range of plasma conditions, in agreement with the prediction of the
  theory. The observed turbulence spectra often have power-law dissipation
  ranges with an average spectral index of -3. We demonstrate that this
  fact is simply a consequence of a marginal state of the instability
  in the dissipation range.

---------------------------------------------------------
Title: Drivers of the solar wind: then and now
Authors: Hollweg, Joseph V.
2006RSPTA.364..505H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Solar Wind: Then and Now
Authors: Hollweg, Joseph V.
2006GMS...167...19H    Altcode:
  In the original formulation of the solar wind, the electron pressure
  gradient was the principal accelerating force. This was soon recognized
  to be insufficient to drive the observed flow speeds, especially of
  the high-speed streams. The discovery of Alfvén waves in the solar
  wind led to a long series of models in which wave pressure provided
  additional acceleration, but these wave-driven models ultimately failed
  to explain the rapid acceleration of the fast wind close to the Sun. An
  alternate view was that the pressure of hot protons close to the Sun
  could explain the rapid acceleration, with the proton (and ion) heating
  coming from the cyclotron resonance. SOHO has provided remarkable
  data which have verified some of the predictions of this view, and
  given impetus to ongoing studies of the ion cyclotron resonance in
  the fast wind. After a historical review, we discuss the basic ideas
  behind current research, emphasizing the particle kinetics.

---------------------------------------------------------
Title: A Mechanism of Dissipation of the Perpendicular Turbulent
    Cascade
Authors: Markovskii, S. A.; Vasquez, B. J.; Smith, C. W.; Hollweg,
   Joseph V.
2005ESASP.592..177M    Altcode: 2005soho...16E..27M; 2005ESASP.592E..27M
  No abstract at ADS

---------------------------------------------------------
Title: Coronal loop oscillations. Calculation of resonantly damped
    MHD quasi-mode kink oscillations of longitudinally stratified loops
Authors: Andries, J.; Goossens, M.; Hollweg, J. V.; Arregui, I.;
   Van Doorsselaere, T.
2005A&A...430.1109A    Altcode:
  The observed coronal loop oscillations and their damping are often
  theoretically described by the use of a very simple coronal loop
  model, viz. a straight, longitudinally invariant, axi-symmetric, and
  pressureless flux tube with a different density inside and outside
  of the loop. In this paper we generalize the model by including
  longitudinal density stratification and we examine how the longitudinal
  density stratification alters the linear eigenmodes of the system,
  their oscillation frequencies, and the damping rates by resonant
  absorption. <P />Appendix A is only available in electronic form at
  http://www.edpsciences.org

---------------------------------------------------------
Title: Deceleration of relative streaming between proton components
    among nonlinear low-frequency Alfvén waves
Authors: Kaghashvili, Edisher K.; Vasquez, Bernard J.; Zank, Gary P.;
   Hollweg, Joseph V.
2004JGRA..10912101K    Altcode:
  Proton distributions in fast solar winds often have a beam component
  with a differential streaming speed near the local Alfvén speed. The
  Alfvén speed and differential streaming speed decrease with increasing
  distance from the Sun. Thus the beam decelerates, especially within
  1 AU where β (which is ratio of plasma to magnetic pressure) can be
  significantly smaller than unity. We present 2 1/2-dimensional hybrid
  numerical simulation results of the evolution of particle proton
  components streaming relative to each other for moderate relative
  beam densities (up to 50%) for initially isotropic distributions
  with mostly equal beam and main proton temperatures and small plasma
  β(=0.2). Electrons are treated as a fluid. We consider cases without
  and with initial nonlinear low-frequency (nearly dispersionless) shear
  Alfvén waves propagating in the direction of the beam. Without initial
  waves, a strong linear beam instability can occur for streaming speeds
  above the Alfvén speed generating oblique proton-proton cyclotron waves
  through both cyclotron and Landau resonances. The initial beam speed
  can decelerate and saturate at speeds below the Alfvén speed. When
  nonlinear Alfvén waves are included in simulations, we find that the
  deceleration rates are enhanced. Deceleration is especially strong
  for initial super-Alfvénic speeds where we interpret the results
  with initial waves to be due to a wave amplification of the linear
  beam instability.

---------------------------------------------------------
Title: The Generation of Ion Cyclotron Turbulence by the Intermittent
    Heat Flux in Coronal Holes
Authors: Markovskii, S. A.; Hollweg, J. V.
2004ESASP.575..192M    Altcode: 2004soho...15..192M
  No abstract at ADS

---------------------------------------------------------
Title: A mechanism of dissipation of the perpendicular turbulent
    cascade in the solar wind
Authors: Markovskii, S. A.; Vasquez, B. J.; Smith, C. W.; Hollweg,
   J. V.
2004AGUFMSH44A..02M    Altcode:
  We discuss a mechanism of dissipation that allows us to explain several
  key features of the turbulent fluctuations in the solar wind. The
  observational data suggest that the solar wind turbulence is dominated
  by fluctuations with wavevectors nearly perpendicular to the background
  magnetic field. This is in agreement with numerical simulations
  showing that the turbulent cascade tends to produce small spatial
  scales across the magnetic field rather than along it. The dissipation
  of the turbulent fluctuations is thought to be responsible for the
  observed perpendicular heating of the solar wind protons. The problem,
  however, is that the perpendicular heating is usually a signature of
  the cyclotron resonance, while the cross-field fluctuations cannot
  be immediately cyclotron-resonant with the protons. We suggest that
  the velocity shear associated with the cross-field fluctuations can
  excite a proton cyclotron instability. These unstable waves will then
  transfer the energy from the cross-field fluctuations to the protons
  thus dissipating the cascade and producing the perpendicular heating. We
  analyze the observed turbulence spectra and show that the threshold
  of the instability is consistent with the spectral break separating
  the inertial and dissipation ranges of the turbulence. In particular,
  during the periods of strong variation of the plasma beta in the solar
  wind, the threshold scales as the proton inertial length rather than the
  proton gyroradius, in agreement with the prediction of the theory. The
  observed turbulence spectra often have power-law dissipation ranges
  with an average spectral index of -3. We demonstrate that this fact
  is simply a consequence of a marginal state of the instability in the
  dissipation range.

---------------------------------------------------------
Title: Cross-field energy transfer of a body Alfvén wave propagating
    along and across a pressure-balanced structure
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
2004GeoRL..3114803V    Altcode:
  We conduct hybrid numerical simulations with particle protons and fluid
  electrons of a low-frequency, planar (body) and shear Alfvén wave
  imbedded in a smoothly varying cross-field pressure-balanced structure
  which provides a wave-speed gradient. We consider wave propagation
  directions starting at 90°, which resembles the case of a surface
  wave, and less than 90° with respect to the gradient direction. We
  find that the planar Alfvén wave undergoes resonant absorption. When
  the propagation direction is less than 90°, we show that there are
  resonant field lines which can actually lose wave energy to other
  neighboring resonant field lines, which is a situation that has not
  been encountered in previous work with surface waves. A consequence
  of this process is an overall faster development time for smaller
  scales perpendicular to the magnetic field through phase mixing and
  potentially faster dissipation of these generated scales in coronal
  and solar wind plasma.

---------------------------------------------------------
Title: Intermittent Heating of the Solar Corona by Heat Flux-generated
    Ion Cyclotron Waves
Authors: Markovskii, S. A.; Hollweg, Joseph V.
2004ApJ...609.1112M    Altcode:
  Recently, we suggested that the source of ion heating in solar coronal
  holes is small-scale reconnection events (microflares) at the coronal
  base. The microflares launch intermittent heat flux up into the corona
  exciting ion cyclotron waves through a plasma microinstability. The
  ions are heated by these waves during the microflare bursts
  and then evolve with no energy input between the bursts. In this
  paper, we show that the structure of the proton distribution in the
  relatively long time periods between the microflares is determined
  by collisions at small heliocentric distances. At greater distances,
  the collisional processes can be replaced by similar processes due to
  secondary instabilities. These are excited by the distortion of the
  distribution under the action of the mirror force. At the same time,
  the heating during the microflare bursts is not affected by either the
  collisions or the secondary instabilities because of the short duration
  of the bursts. We demonstrate that in each intermittent heating event
  the protons diffuse approximately along one-dimensional curves in
  the phase space and can develop a quasi-plateau. The corresponding
  temperature increase can then be calculated without solving the
  diffusion equations. The overall coronal heating by this mechanism
  is a summed effect of all microflare bursts during the expansion time
  of the solar wind and adiabatic cooling between the microflares. The
  calculations for the collision-dominated region suggest that the
  overall heating is efficient enough to account for the acceleration
  of the fast solar wind in this region.

---------------------------------------------------------
Title: Nonlinear Alfvén waves: 2. The influence of wave advection
    and finite wavelength effects
Authors: Vasquez, Bernard J.; Markovskii, Sergei A.; Hollweg, Joseph V.
2004JGRA..109.5104V    Altcode:
  Using a hybrid code, we examine the effects of mildly oblique,
  low-frequency Alfvén waves on cross-field pressure-balanced structures
  of varying scales. We show that the evolution is organized by a
  parameter ξ = A<SUB>w</SUB>c<SUB>A</SUB>k<SUB>⊥</SUB>/ω, where
  A<SUB>w</SUB> is the relative wave amplitude, k<SUB>⊥</SUB> is the
  characteristic wave number of a cross-field structure, and ω is a
  characteristic wave frequency. This parameter is a measure of the
  relative displacement of the structure by the wave. When ξ ≪ 1,
  agreement with small-amplitude solutions and linear wave theory is
  good. Waves can refract and undergo cross-field energy transfer,
  which is a finite wavelength effect. When ξ is large, advection
  of the structure is significant and small-amplitude solutions are
  not valid. The rate of wave refraction diminishes as the effective
  Alfvén speed gradient is diluted over a wave period by significant
  advection. When the structure is purely magnetic and polarized like an
  Alfvén wave, significant advection converts the structure into waves
  which have an Alfvénic character but do not always satisfy energy
  equipartition. Nonlinear wave interactions occur between Alfvén and
  gradient-derived waves, wherein the Alfvén wave undergoes partial
  reflection and continued cross-field energy transfer. When sufficiently
  small parallel scales are produced by advection, proton cyclotron
  resonant heating and damping occurs on all field lines so that both
  Alfvén and gradient-derived waves undergo dissipation.

---------------------------------------------------------
Title: Nonlinear Alfvén waves: 1. Interactions between outgoing
    and ingoing waves according to an amplitude expansion
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
2004JGRA..109.5103V    Altcode:
  We expand the compressible magnetohydrodynamic (MHD) equations in
  terms of the amplitude of first-order Alfvén waves. Incompressible
  MHD equations are treated as a special case. We examine the evolution
  of mildly oblique Alfvén waves consisting of forward and backward
  propagating waves so that these would correspond to the interactions
  of outgoing and ingoing Alfvén waves in the solar wind. We obtain
  the solutions of these equations through second order. We find and
  distinguish three types of second-order modes which have zero frequency
  and are generated in association with nonsecular interactions of mildly
  oblique first-order Alfvén waves. Two of these modes have wave vectors
  which are perpendicular to the background magnetic field. Both are
  associated with magnetic energy but they do not have kinetic energy.

---------------------------------------------------------
Title: Intermittent coronal heating due to heat flux generated ion
    cyclotron waves
Authors: Markovskii, S. A.; Hollweg, J. V.
2003AGUFMSH21B0112M    Altcode:
  Recently, we suggested [Hollweg and Markovskii, JGR, 107, No. 6,
  2002; Markovskii and Hollweg, GRL, 29, No. 17, 2002] that the source
  of ion heating in coronal holes is small-scale reconnection events
  (microflares) at the coronal base. The microflares launch intermittent
  heat flux up into the corona exciting ion cyclotron waves through a
  plasma microinstability. The ions are heated by these waves during
  the microflare bursts and then evolve with no energy input between the
  bursts. We show that the structure of the proton distribution in the
  relatively long time periods between the microflares is determined by
  collisions at small heliocentric distances. At greater distances, the
  collisional processes are replaced by similar processes due to secondary
  instabilities. These are excited by the distortion of the distribution
  under the action of the mirror force. At the same time, the heating
  during the microflare bursts is not considerably affected either by
  the collisions or by the secondary instabilities, because of the short
  duration of the bursts. The overall coronal heating by this mechanism
  is a summed effect of all microflare bursts during the expansion time
  of the solar wind and adiabatic cooling between the microflares. Our
  calculations for the collision-dominated region suggest that the overall
  heating is efficient enough to account for the acceleration of the fast
  solar wind in this region. Further development of the model including
  the collisionless region will be reported elsewhere.

---------------------------------------------------------
Title: Origin of the Fast Solar Wind: From an Electron - Driven Wind
    to Cyclotron Resonances
Authors: Hollweg, Joseph V.
2003AIPC..679...14H    Altcode:
  Even before the discovery of the fast solar wind in the mid - 1970s, it
  was known that even the average solar wind could not be well explained
  by models in which electron heat conduction was the energy source
  and the electron pressure gradient was the principal accelerating
  force. The outward - propagating Alfvén waves discovered around 1970
  were thought for a while to provide the sought - after additional energy
  and momentum, but their wave pressure ultimately failed to explain
  the rapid acceleration of the fast wind close to the Sun in coronal
  holes. By the late 1970s, various in situ data were suggesting that
  protons and heavy ions were being heated and accelerated by the ion -
  cyclotron resonance far from the Sun. This notion was soon applied to
  the acceleration region in coronal holes close to the Sun. The models
  which resulted suggested that the fast wind could be driven mainly by
  the proton pressure gradient (which is mainly the mirror force if the
  anisotropy is large), and that the high temperatures and flow speeds of
  heavy ions could originate within a few solar radii of the coronal base;
  these models also emphasized the importance of treating the extended
  coronal heating and solar wind acceleration on an equal footing. By
  the mid 1990s, SOHO, especially the UVCS (Ultraviolet Coronagraph
  Spectrometer), provided remarkable data which have given great impetus
  to studies of the ion cyclotron resonance as the principal mechanism
  for heating the plasma in coronal holes, and ultimately driving the
  fast wind. We will discuss the basic ideas behind current research,
  emphasizing the particle kinetics. We will discuss remaining problems
  such as the source of the ion - cyclotron resonant waves (direct
  launching, turbulence, microinstabilities), problems concerning OVI
  and MgX, the roles of inward - propagating waves and instabilities,
  the importance of oblique propagation, and the electron heating. Some
  alternatives, such as shock heating and turbulence - driven magnetic
  reconnection, will also be reviewed.

---------------------------------------------------------
Title: Ion Heating Due to Plasma Microinstabilities in Coronal Holes
    and the Fast Solar Wind
Authors: Markovskii, S. A.; Hollweg, Joseph V.
2003AIPC..679..307M    Altcode:
  There is growing evidence that the heating of ions in coronal holes
  and the fast solar wind is due to cyclotron resonant damping of
  ion cyclotron waves. At the same time, the origin of these waves
  is much less understood. We suggest that the source of the waves in
  the coronal holes is a heat flux coming from the Sun. The heat flux
  generates ion cyclotron waves through plasma microinstability, and
  then the waves heat the ions. We use a new view according to which
  the heat flux is launched intermittently by small-scale reconnection
  events (nanoflares) at the coronal base. This allows the heat flux to
  be sporadically large enough to drive the instabilities, while at the
  same time to satisfy the time-averaged energy requirements of the solar
  wind. Depending on the plasma parameters, the heat flux can excite
  shear Alfvén and electrostatic ion cyclotron waves. We show that,
  for reasonable parameters, the heat flux is sufficient to drive the
  instability that results in significant heating of protons and heavy
  ions in the inner corona.

---------------------------------------------------------
Title: Deceleration of streaming alpha particles interacting with
    waves and imbedded rotational discontinuities
Authors: Kaghashvili, Edisher K.; Vasquez, Bernard J.; Hollweg,
   Joseph V.
2003JGRA..108.1036K    Altcode:
  Alphas (He<SUB>4</SUB><SUP>2+</SUP>) and other ions in the
  interplanetary medium show a tendency to stream near or below the
  local Alfvén speed (V<SUB>A</SUB>) relative to the main component of
  protons. Because V<SUB>A</SUB> decreases with increasing distance from
  the Sun, forces must exist to slow the heavier ions with increasing
  distance. We have conducted hybrid simulations in a plasma with
  particle protons and alphas and with a quasineutralizing electron
  fluid. Simulation runs with other streaming minor ions were also
  performed. In our simulations, a group of Alfvén waves steepen and
  generate imbedded rotational discontinuities (RDs) and compressional
  waves. We examine cases of almost steady waveforms and RDs and ones
  with evolving waveforms and RDs due to a significantly nonuniform
  background. When alphas stream with the waves and imbedded RDs faster
  than the protons, they decelerate more rapidly from higher speeds and
  heat. We have concluded that alphas do not remain at one streaming speed
  due to nonlinear Lorentz forces from the wave compressional component
  and the presence of collisionless dissipation, which dissipates this
  component so that bulk alpha kinetic energy is ultimately deposited
  into alpha thermal energy. Imbedded RDs play no significant role in the
  overall deceleration of alphas. Protons heat similarly to cases without
  alphas and are slightly accelerated so that the total ion momentum
  along B<SUB>0</SUB> is nearly conserved. For small streaming speeds
  (≲0.5 Alfvén speeds), the deceleration rate can be relatively small
  because the loss of streaming energy competes with the gain in wave
  kinetic energy required by large-amplitude Alfvén waves. Less proton
  and more alpha heating is also found since alphas can resonate with
  the left-handed portion of oblique waves. Starting from rest, alphas
  and protons can develop a small differential flow in which Lorentz and
  pressure forces become balanced. The simulation behavior of alphas
  for streaming speeds near the Alfvén speed is fairly consistent
  with solar wind observations in high-speed streams. Turbulent
  Alfvénic fluctuations do have a small compressional component and
  so might be responsible for the observed deceleration and heating
  of alphas. Simulations with other streaming minor ions also gave
  deceleration, suggesting that the behavior of a wide range of solar wind
  minor ions might be explained by the same processes that affect alphas.

---------------------------------------------------------
Title: On the Origin of Perpendicular Cascades Among Nonlinear
    Alfvén Waves
Authors: Vasquez, B. J.; Markovskii, S. A.; Hollweg, J. V.
2002AGUFMSH12A0408V    Altcode:
  We examine the effects of mildly oblique, low-frequency Alfvén waves
  on cross-field magnetic structures of varying scales. The magnetic
  structures correspond to the type of zero-frequency modes which
  can arise when ingoing and outgoing Alfvén waves interact with one
  another. Such modes are important to the develop of nearly perpendicular
  cascades in magnetohydrodynamic (MHD) turbulence. In linear theory,
  the structure remains static and the Alfvén wave generally refracts
  into the perpendicular direction and is resonantly absorbed. Resonant
  absorption is a pependicular cascade since small perpendicular wave
  features are generated. However, it differs from a turbulent cascade
  in that it is an phase-ordered process. Given a small wave amplitude,
  linear theory is valid when the structure's gradient scale is of order
  or larger than the Alfvén wavelength. However, when the gradient scale
  becomes smaller than the wavelength, nonlinear effects appear due to the
  Alfvén wave advecting and bending the magnetic structure. The magnetic
  structure acquires parallel scales and also a velocity field since
  moving the magnetic field of the structure induces an electric field and
  so also a velocity field. This renders the structure indistinct from
  a wave at a given monent although there is no net propagation. The
  Alfvén wave still undergoes resonant absorption but now without
  the refraction of the Alfvén wave and at a reduced rate. Instead
  of refraction, the Alfvén wave reflects and generates a backward
  Alfvén wave flux because advection of the structure gives Alfvén
  speed gradients along the background magnetic field. The reflection
  saturates when the wave flux in each direction along B<SUB>0</SUB>
  becomes equal. In this case, we have a mildly oblique Alfvén wave with
  the nonlinear capability of cascading power perpendicularly without
  changing its wave vector orientation and without its entrainment
  into the nearly perpendicular direction. We will also show that power
  accumulation along the advecting resonant field lines can be quenched
  by ion cyclotron resonant heating when small parallel scales develop
  in association with the advection. We relate this evolution to MHD
  simulations of isotropic spectra of Alfvén waves which develop nearly
  perpendicular cascades. We suggest that the cascade originates not
  simply from the development of zero-frequency modes but rather from
  the nonlinear modifications of resonant absorption.

---------------------------------------------------------
Title: Ion heating due to plasma microinstabilities in coronal holes
    and the generation of the fast solar wind
Authors: Markovskii, S. A.; Hollweg, J. V.
2002AGUFMSH12A0397M    Altcode:
  There is growing evidence that the heating of ions in coronal holes is
  due to cyclotron resonant damping of ion cyclotron waves. At the same
  time, the origin of these waves is much less understood. Recently,
  we suggested [Eos Trans. AGU, 82(47), Fall Meet. Suppl., Abstract
  SH11A-0695, 2001] that the source of the waves in the coronal holes
  is a heat flux coming from the Sun. The heat flux excites shear Alfven
  and electrostatic ion cyclotron waves through plasma microinstability,
  and then the waves heat the ions. We used a new view according to which
  the heat flux is launched intermittently by small-scale reconnection
  events (nanoflares) at the coronal base. This allows the heat flux to
  be sporadically large enough to drive the instabilities, while at the
  same time to satisfy the time-averaged energy requirements of the solar
  wind. Here we develop this model further. We show that the unstable
  waves can become strongly ion-resonant. This results in ion heating in
  the inner corona that is efficient enough, in the quasilinear limit,
  to generate the fast solar wind.

---------------------------------------------------------
Title: Parametric cross-field current instability in solar coronal
    holes
Authors: Markovskii, S. A.; Hollweg, Joseph V.
2002JGRA..107.1329M    Altcode:
  We consider a parametric instability of fast and Alfvén waves
  with length scales of the order of the proton inertial length. The
  instability is driven by currents that are associated with these pump
  waves and that flow in the direction perpendicular to the background
  magnetic field. The initial pump waves generate secondary ion cyclotron
  waves with length scales below the proton gyroradius. The important
  property of the cross-field current instability is that it can exist
  at relatively low amplitudes of the pump waves. As a result, it can
  be excited in the solar corona, where turbulent fluctuations are
  much smaller than in the distant solar wind. The instability of the
  pump wave provides an additional mechanism of wave damping compared
  to direct cyclotron damping. This mechanism starts to operate in the
  region of spatial scales where the direct cyclotron damping is weak. We
  show that the wave amplitudes, derived from an observed spectrum
  of density fluctuations, are sufficient to excite the cross-field
  current instability and that the turbulent heating associated with the
  instability is fast enough to provide a heating mechanism for protons
  in the coronal holes.

---------------------------------------------------------
Title: Electron heat flux instabilities in coronal holes: Implications
    for ion heating
Authors: Markovskii, S. A.; Hollweg, Joseph V.
2002GeoRL..29.1843M    Altcode: 2002GeoRL..29q..24M
  There is growing evidence that the heating of the ions in solar coronal
  holes and the resulting generation of the fast solar wind is due to
  cyclotron-resonant damping of ion cyclotron waves. At the same time,
  the origin of these waves is not understood. In this paper, it is
  suggested that the waves in the proton cyclotron frequency range are
  generated by a plasma microinstability due to a heat flux coming from
  the Sun. A new view is used according to which heat flux is launched
  intermittently by small-scale reconnection events (nanoflares) at
  the coronal base. This allows the heat flux to be sporadically large
  enough to drive the instabilities, while at the same time satisfying the
  time-averaged energy requirements of the solar wind. It is shown that
  in a low-beta coronal plasma an electrostatic heat-flux instability
  has a comparable threshold and much greater growth rate than a shear
  Alfvén heat flux instability. The implications of these instabilities
  for the ion heating in the coronal holes are discussed.

---------------------------------------------------------
Title: Generation of the fast solar wind: A review with emphasis on
    the resonant cyclotron interaction
Authors: Hollweg, Joseph V.; Isenberg, Philip A.
2002JGRA..107.1147H    Altcode:
  In situ measurements of the solar wind and remote observations of
  coronal holes have strongly implicated the resonant interaction with
  ion cyclotron waves as the responsible mechanism for heating and
  accelerating coronal hole ions to generate the fast solar wind. We
  review the current observational and theoretical knowledge of this
  mechanism and the progress that has been made in modeling the solar wind
  properties that result from this interaction. We begin by examining
  the observational and theoretical motivations for the continued
  study of this mechanism, including a brief historical review of these
  ideas. We then discuss the interplay of the resonance condition and
  the wave dispersion relation, which determines which ions can exchange
  energy with the waves. The physical basis for the interaction is then
  described, and we derive simple expressions for the ion response to the
  resonant wave dissipation. The complicated topic of oblique propagation
  is dealt with next, including how the resonant interaction operates
  for obliquely propagating waves. The plasma response to the resonant
  dissipation is often approximated by treating the ion populations as
  fluids, and we examine the solar wind models which incorporate various
  versions of this interaction. We then present a sample model that
  illustrates many of the properties and shortcomings of the current
  fluid results in this area. However, the resonant interaction is most
  accurately treated with a kinetic description of the ions and the
  recent attempts to construct kinetic models are explored. We close
  with a discussion of the many observational and theoretical gaps in
  our understanding that remain, including a presentation of alternative
  mechanisms and some speculations on future developments in this field.

---------------------------------------------------------
Title: Cyclotron resonances of ions with obliquely propagating waves
    in coronal holes and the fast solar wind
Authors: Hollweg, Joseph V.; Markovskii, S. A.
2002JGRA..107.1080H    Altcode:
  There is a growing consensus that cyclotron resonances play important
  roles in heating protons and ions in coronal holes where the fast solar
  wind originates and throughout interplanetary space as well. Most work
  on cyclotron resonant interactions has concentrated on the special,
  but unrealistic, case of propagation along the ambient magnetic field,
  B<SUB>0</SUB>, because of the great simplification it gives. This
  paper offers a physical discussion of how the cyclotron resonances
  behave when the waves propagate obliquely to B<SUB>0</SUB>. We show
  how resonances at harmonics of the cyclotron frequency come about, and
  how the physics can be different depending on whether E<SUB>⊥</SUB>
  is in or perpendicular to the plane containing k and B<SUB>0</SUB>
  (k is wave vector, and E<SUB>⊥</SUB> is the component of the wave
  electric field perpendicular to B<SUB>0</SUB>). If E<SUB>⊥</SUB>
  is in the k-B<SUB>0</SUB> plane, the resonances are analogous to
  the Landau resonance and arise because the particle tends to stay
  in phase with the wave during the part of its orbit when it is
  interacting most strongly with E<SUB>⊥</SUB>. If E<SUB>⊥</SUB>
  is perpendicular to the k-B<SUB>0</SUB> plane, then the resonances
  depend on the fact that the particle is at different positions during
  the parts of its orbit when it is interacting most strongly with
  E<SUB>⊥</SUB>. Our main results are our &lt;cross-ref refid="df10"
  type="formula"&gt;equations (10)&lt;/cross-ref&gt;, &lt;cross-ref
  refid="df11" type="formula"&gt;(11)&lt;/cross-ref&gt;, and &lt;cross-ref
  refid="df13" type="formula"&gt;(13)&lt;/cross-ref&gt; for the secular
  rate of energy gain (or loss) by a resonant particle and the unfamiliar
  result that ions can resonate with a purely right-hand circularly
  polarized wave if the propagation is oblique. We conclude with some
  speculations about the origin of highly obliquely propagating ion
  resonant waves in the corona and solar wind. We point out that there
  are a number of instabilities that may generate such waves locally in
  the corona and solar wind.

---------------------------------------------------------
Title: Heating and Deceleration of Differentially Streaming Ions
    Among Nonlinear Waves
Authors: Vasquez, B. J.; Kaghashvili, E. K.; Hollweg, J. V.
2002AGUSMSH21C..05V    Altcode:
  Coronal and solar wind ions show important kinetic behavior. First,
  we examine the behavior of minor ions which stream differentially
  relative to protons. With increasing distance in the solar wind,
  minor ions decelerate. We have found from hybrid simulations with
  particle ions and fluid electrons that streaming Helium and Oxygen
  ions can be decelerated among Alfvén waves which have steepened
  and produced imbedded RDs. We have examined the causes of this
  deceleration. We have eliminated demagnetization within RD layers
  as a cause. Moreover, we have found reflecting ions near RDs, but
  these do not explain why the bulk of ions decelerate. At present,
  we have identified a source of nonequilibrium among two and half
  dimensional (2(1)/(2)-D) waveforms in which the generalized Reynolds
  stresses are unbalanced at RD layers when minor ions are included in
  simulations but are balanced when only protons are simulated. This
  might explain why minor ions fail to find an equilibrium within the
  simulated waveforms. We compare the expectations of this theory with
  simulation results at a various streaming streaming speeds and for
  minor ions of differing charge to mass ratio. Furthermore, we also
  compare behavior with 1(1)/(2)-D imbedded RDs where the generalized
  Reynolds stresses vanish identically. A second hybrid study is carried
  out examining minor ion and proton heating among potentially resonant
  waves produced by a cascade.

---------------------------------------------------------
Title: Heating and acceleration of the solar wind in coronal holes:
    cyclotron resonances
Authors: Hollweg, Joseph V.
2002AdSpR..30..469H    Altcode:
  Cyclotron resonances were first studied to explain observations
  showing that solar wind heavy ions flow faster and are hotter than
  the protons. About ten years ago the resonances were applied to the
  corona. Those models predicted high proton temperatures, and were
  discounted. The SOHO/UVCS data now indicate that coronal protons are
  in fact hot, and that heavy ions are more than mass-proportionally
  hotter; protons and ions are hotter than the electrons. The ions,
  and probably the protons, are heated primarily perpendicularly to the
  magnetic field. These are all indications that the corona is heated by
  ion-cyclotron resonances, and that the wind is largely driven by the
  proton pressure. We will discuss the basic physics of the cyclotron
  interaction, and show in simple terms how it can qualitatively explain
  the observations. Illustrative models will be used to show that the idea
  works quantitatively, though there may be problems with simultaneously
  reproducing the UVCS proton and oxygen data. We will emphasize that
  there are basic questions concerning the wave source: does the Sun
  directly launch high-frequency (kHz) waves, or does the Sun launch
  lower frequency waves which subsequently undergo a turbulent cascade
  to the resonant frequencies? We will further emphasize that a full
  description must be in terms of the particle distribution functions,
  which will evolve in such a way as to become unstable to the generation
  of sunward-propagating waves, which may lead to novel effects.

---------------------------------------------------------
Title: Electron Heat Flux Instabilities in Coronal Holes: Implications
    for Ion Heating
Authors: Markovskii, S. A.; Hollweg, J. V.
2001AGUFMSH11A0695M    Altcode:
  We show that in a low-beta coronal hole plasma the electrostatic ion
  cyclotron instability driven by an electron heat flux can have a larger
  growth rate and a lower threshold than shear Alfven, magnetosonic,
  and whistler instabilities, with the most competitive instability
  being the shear Alfven instability. We will discuss the implications
  of this result for the heating of protons and heavier ions in coronal
  holes. To model the electron heat flux, we use a three-component plasma
  consisting of protons, core electrons, and halo electrons drifting with
  respect to each other. We consider distribution functions that are
  stable to higher-frequency and potentially faster-growing electron
  instabilities. We demonstrate that in this case the heat flux is
  sufficient to drive the ion cyclotron and shear Alfven instabilities
  with growth rates that are high enough to give significant heating of
  protons and heavy ions.

---------------------------------------------------------
Title: Nature of fluctuations on directional discontinuities inside
a solar ejection: Wind and IMP 8 observations
Authors: Vasquez, Bernard J.; Farrugia, Charles J.; Markovskii,
   Sergei A.; Hollweg, Joseph V.; Richardson, Ian G.; Ogilvie, Keith W.;
   Lepping, Ronald P.; Lin, Robert P.; Larson, Davin
2001JGR...10629283V    Altcode:
  A solar ejection passed the Wind spacecraft between December 23 and 26,
  1996. On closer examination, we find a sequence of ejecta material, as
  identified by abnormally low proton temperatures, separated by plasmas
  with typical solar wind temperatures at 1 AU. Large and abrupt changes
  in field and plasma properties occurred near the separation boundaries
  of these regions. At the one boundary we examine here, a series of
  directional discontinuities was observed. We argue that Alfvénic
  fluctuations in the immediate vicinity of these discontinuities
  distort minimum variance normals, introducing uncertainty into
  the identification of the discontinuities as either rotational or
  tangential. Carrying out a series of tests on plasma and field data
  including minimum variance, velocity and magnetic field correlations,
  and jump conditions, we conclude that the discontinuities are
  tangential. Furthermore, we find waves superposed on these tangential
  discontinuities (TDs). The presence of discontinuities allows the
  existence of both surface waves and ducted body waves. Both probably
  form in the solar atmosphere where many transverse nonuniformities
  exist and where theoretically they have been expected. We add to prior
  speculation that waves on discontinuities may in fact be a common
  occurrence. In the solar wind, these waves can attain large amplitudes
  and low frequencies. We argue that such waves can generate dynamical
  changes at TDs through advection or forced reconnection. The dynamics
  might so extensively alter the internal structure that the discontinuity
  would no longer be identified as tangential. Such processes could
  help explain why the occurrence frequency of TDs observed throughout
  the solar wind falls off with increasing heliocentric distance. The
  presence of waves may also alter the nature of the interactions of
  TDs with the Earth's bow shock in so-called hot flow anomalies.

---------------------------------------------------------
Title: Hybrid Simulation Studies of Wave-Ion Interactions With
    Application to the Corona and Solar Wind
Authors: Vasquez, B. J.; Hollweg, J. V.; Kaghashvili, E. K.
2001AGUFMSH21A0723V    Altcode:
  Strong ion heating and bulk flow acceleration is either inferred or
  directly observed in the corona and solar wind. A likely source of this
  heating and acceleration comes from Alfvénic fluctuations evolving
  turbulently and cascading wave energy to small scales where direct
  interactions with ions occur. We present a number of studies using a
  hybrid numerical simulation with particle ions and a quasineutralizing
  electron fluid. We examine several situations which could arise in
  turbulence. First, we examine a cascade among large-amplitude waves
  with imbedded abrupt field rotations called rotational discontinuities
  (RDs). This situation is appropriate to the solar wind. We show how
  the cascade and interactions with RDs can heat ions and also decelerate
  streaming populations of helium relative to protons. This deceleration
  of helium is observed in the solar wind. We then examine a class of
  partially driven simulations meant to mimic the dissipation range of
  turbulence in the corona and solar wind. Finally, we show simulation
  results of the reconnection or merger of magnetic fields and examine
  its effect on ions.

---------------------------------------------------------
Title: Cyclotron Resonances of Ions with Obliquely-Propagating Waves
    in Coronal Holes and the Fast Solar Wind
Authors: Hollweg, J. V.; Markovskii, S. A.
2001AGUSM..SH22E05H    Altcode:
  UVCS/SOHO has provided observations of protons and ions in coronal
  holes which suggest the operation of ion-cyclotron heating and
  acceleration. Many models have concentrated on the interactions of
  particles with parallel-propagating ion-cyclotron waves. There is of
  course no reason to expect parallel propagation in the corona, so we
  consider here some consequences of oblique propagation. Following Stix
  (1992), we analytically calculate the energy absorbed by an ion moving
  in an obliquely-propagating electromagnetic wave. Resonances occur at
  harmonics of the gyro frequency, though we will show that the physical
  interpretations are quite different for electric field polarizations in,
  or perpendicular to, the plane containing k and Bo (k is wavenumber
  and Bo is the ambient magnetic field). Surprisingly, a resonance at
  the fundamental frequency can occur even if the wave is right-hand
  circularly polarized (i.e. opposite to the sense of the gyromotion). We
  suggest, therefore, that resonances with the fast/whistler branch,
  which are often overlooked, may play a role in the heating of ions
  and protons in coronal holes as long as the waves are oblique. We
  will discuss possible sources of such waves. We will also summarize
  other consequences of oblique propagation for the resonant heating
  of coronal holes and the origin of the fast solar wind. Stix, T.H.,
  Waves in Plasmas, AIP, New York, 1992.

---------------------------------------------------------
Title: The Interaction of Alpha Particles With Alfven Waves and
    Imbedded Rotational Discontinuities
Authors: Kaghashvili, E.; Vasquez, B. J.; Hollweg, J. V.
2001AGUSM..SH41B15K    Altcode:
  Heavy ions in the interplanetary medium show a statistical tendency
  to stream at the local Alfven speed (V<SUB>A</SUB>) relative to
  protons. Because V<SUB>A</SUB> decreases with increasing distance from
  the Sun, forces must exist to slow the heavier ions to V<SUB>A</SUB>
  with increasing distance. We have conducted hybrid simulations in a
  plasma with particle protons and alphas and with a quasineutralizing
  electron fluid. In our simulations, a group of Alfven waves steepen
  and generate imbedded rotational discontinuities (RDs). Cross-field
  nonuniformities are also induced by wave nonlinearity. This ultimately
  brings about a cascade of wave energy to dissipative scales and the
  dissipation of some of the RDs. When alphas stream at speeds above
  V<SUB>A</SUB> relative to protons, we find that the streaming speed
  of alphas decreases to approximately V<SUB>A</SUB> and the alphas
  are heated by their interactions with waves and RDs. We present these
  simulation results and examine the role that wave cascades and RDs can
  play in regulating streaming speeds and in heating alphas and other
  heavy ions in the interplanetary medium.

---------------------------------------------------------
Title: Ion Cyclotron Instabilities in Multicomponent Coronal Hole
    Plasma with Cross-Field Streaming Species
Authors: Markovskii, S. A.; Hollweg, J. V.
2001AGUSM..SH22E10M    Altcode:
  It is well known that ion cyclotron instability driven by cross-field
  streaming of the plasma components, with zero or nonzero net
  current, can be excited at relative velocities of the species much
  smaller than the ion thermal velocity. We use this instability as a
  mechanism of generation of ion cyclotron waves in coronal holes. In
  previous work, we have demonstrated that, given the scales of spatial
  inhomogeneity suggested by observations and reasonably large magnetic
  field fluctuations, the instability can be excited by the cross-field
  current in an electron-proton plasma. We now extend our work to include
  other ion species. We show that the abundance of helium is large enough
  for the generation of helium cyclotron waves, while the excitation of
  minor-ion cyclotron waves would require unrealistically large streaming
  velocities. Nevertheless, the minor ions can resonantly interact
  with the waves existing in the electron-hydrogen-helium plasma. The
  important property of the waves induced by the cross-field streaming
  is that they propagate almost perpendicular to the background magnetic
  field. One of the consequences of this fact is a considerable Doppler
  shift of the wave frequency in the frames of reference connected with
  different plasma components. We will discuss how the resonant wave
  particle interaction is modified in this case compared to the case of
  parallel-propagating waves with no streaming of the species.

---------------------------------------------------------
Title: The Evolution of Very Small-Scale Cross-Field Structures
    Interacting With Very Low-Frequency Alfven Waves
Authors: Vasquez, B. J.; Markovskii, S. A.; Hollweg, J. V.
2001AGUSM..SH41B16V    Altcode:
  We examine the affects of very low-frequency Alfven waves in an energy
  containing range on well separated and very small length scales in
  the inertial and dissipative ranges. These waves cause dynamics at the
  smallest scales which can mimic quasi-2-D dynamics without restriction
  on the wave vector direction of the Alfven waves. Using a hybrid
  code, we have followed the evolution of Alfven waves and cross-field
  structures with scales of the ion inertial length. Despite background
  gradients of the Alfven speed, the Alfven waves are not resonantly
  absorbed. Instead, the cross-field structures are advected and undergo
  topological changes. We compare simulation results with solutions of the
  magnetohydrodynamic (MHD) equations using the method of characteristics
  which show good agreement. We also examine the scales below ion
  inertial length where parallel electric fields become important and
  where drift waves develop. In the solar wind, Alfvenic fluctuations
  can have very low frequencies and are well separated from the inertial
  and dissipative ranges. In a spherically expanding solar wind, it
  is difficult to restrict their wave vectors to quasi-perpendicular
  directions. We find that these fluctuations can behave compressively
  (k<SUB>∥ </SUB> ~ k<SUB>perpendicular to </SUB>), while forcing
  quasi-2-D and weakly compressive dynamics on very small scales.

---------------------------------------------------------
Title: Evolution and dissipation of imbedded rotational
    discontinuities and Alfvén waves in nonuniform plasma and the
    resultant proton heating
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
2001JGR...106.5661V    Altcode:
  We show that nonuniform Alfvén speed gradients across field lines
  generally arise from the evolution of Alfvén waves. The evolution
  of a group of nonlinear Alfvén waves with the same sign of parallel
  wavenumber generate small-amplitude pressure-balanced structures (PBSs)
  which cause the speed variations. This always causes refraction. In
  most cases, the Alfvén waves also couple to magnetosonic waves and
  acquire a weak compressional component and can undergo resonant
  absorption or transfer, wherein wave energy can propagate across
  field lines. At large amplitudes the waves also generate imbedded
  rotational discontinuities (RDs). Some of these RDs can be dissipated
  owing to resonant transfer. This process could partly contribute to
  the observed decrease of solar wind RDs with increasing distance from
  the Sun. Resonant transfer also triggers a cascade due to steepening,
  which leads to sustained proton heating. The cascade produces oblique
  and large wavenumber waves which travel in different directions and have
  associated compressions. Protons interact with these by pitch angle
  scattering. They gain energy from second-order Fermi acceleration and
  from Landau and transit time damping. Oblique waves are inferred to be
  present in the dissipation range of Alfvénic fluctuations at 1 AU. We
  argue that the process of proton heating should proceed similarly to
  simulation results. We also propose a role for the wave imbedded RD
  in coronal heating through its formation in the chromosphere and its
  likely dissolution in the corona where wave amplitudes are very small.

---------------------------------------------------------
Title: The kinetic shell model of coronal heating and acceleration
by ion cyclotron waves: 1. Outward propagating waves
Authors: Isenberg, Philip A.; Lee, Martin A.; Hollweg, Joseph V.
2001JGR...106.5649I    Altcode:
  We introduce a new kinetic treatment of the heating of the magnetically
  open solar corona and the acceleration of the fast solar wind by the
  cyclotron resonant interaction of coronal protons with ion cyclotron
  waves. In this “kinetic shell” formalism we approximate the evolution
  of the collisionless coronal proton distribution by the assumption
  that the pitch angle diffusion due to the resonant ion cyclotron
  waves is much faster than the other processes taking place. Under
  this assumption the resonant protons uniformly populate velocity space
  surfaces, or shells, of constant energy in the frame moving with the
  wave phase speed. These resonant shells then evolve slowly in response
  to the nonresonant large-scale forces in the system. For this initial
  demonstration of the kinetic shell concept, we additionally take the
  resonant waves to be solely outward propagating and dispersionless. In
  this case the resonant shells are spherical sections in velocity space
  which are confined to the sunward half of the proton distribution. We
  then calculate the radial evolution of collisionless protons in a
  coronal hole using this simplified system, which also includes the
  effects of gravity, the charge-separation electric field, and the
  mirror force. We find that a fast solar wind can be generated by this
  process using reasonable values of the physical parameters. However,
  we also prove that the proton distribution generated by the interaction
  with only outward propagating waves will necessarily be unstable to
  the generation of inward propagating waves. Thus this illustrative
  calculation is incomplete and will have to be extended to include
  waves in both propagation directions.

---------------------------------------------------------
Title: Cyclotron resonance in coronal holes: 3. A five-beam
    turbulence-driven model
Authors: Hollweg, Joseph V.
2000JGR...10515699H    Altcode:
  Following Hollweg and Johnson [1988], Isenberg [1990], and Li et
  al. [1999a], we postulate that the Sun launches a flux of low-frequency
  Alfvén waves, which dissipate via a turbulent cascade to high
  frequencies where the energy is absorbed by ion cyclotron resonant
  interactions. The plasma consists of two proton beams, which are
  proxies for the resonant and nonresonant halves of their distribution
  function, two He<SUP>++</SUP> beams, which are proxies for the strongly
  and weakly resonant halves of their distribution, and a single beam of
  O<SUP>+5</SUP> with vanishing density. The level of the power spectrum
  at the high resonant frequencies is determined by the condition that
  the protons and He<SUP>++</SUP> resonantly absorb energy at the same
  rate at which the low-frequency waves are dissipating. Once the level
  of the high-frequency power spectrum is determined, the resonant
  heating and acceleration of the O<SUP>+5</SUP> can be calculated. For
  both Kolmogorov and Kraichnan scalings of the turbulent dissipation
  the model yields results for the protons that are in reasonably good
  agreement with the UVCS/SOHO results. The He<SUP>++</SUP> becomes more
  than mass proportionally heated and flows faster than the protons, close
  to the Sun. However, our model is unable to reproduce the UVCS/SOHO
  observation that the O<SUP>+5</SUP> temperature is still increasing
  with heliocentric distance r out to 3.5r<SUB>s</SUB>. Instead, the
  O<SUP>+5</SUP> becomes very hot initially, experiences a strong mirror
  force, and accelerates to high speed, which in turn leads to rapid
  adiabatic cooling. Put another way, the O<SUP>+5</SUP> observations
  imply that (dT<SUB>⊥</SUB>/dt)<SUB>res</SUB> must be an increasing
  function of r, while it is the nature of the resonant interactions to
  make (dT<SUB>⊥</SUB>/dt)<SUB>res</SUB> decrease with increasing r.

---------------------------------------------------------
Title: Compressibility of ion cyclotron and whistler waves: Can
    radio measurements detect high-frequency waves of solar origin in
    the corona?
Authors: Hollweg, Joseph V.
2000JGR...105.7573H    Altcode:
  The ultraviolet coronagraph spectrometer on Solar and Heliospheric
  Observatory (SOHO) has provided several lines of evidence strongly
  suggesting that coronal holes and the high-speed solar wind are
  heated by resonant interactions with ion cyclotron waves. Related
  evidence has also been provided by the solar ultraviolet measurements
  of emitted radiation instrument on SOHO. However, the source of the
  waves is still unclear. Hollweg [1986], Hollweg and Johnson [1988],
  and Isenberg [1990] developed models in which the high-frequency waves
  are the result of a turbulent cascade from lower-frequency waves that
  are launched by the Sun. Axford and McKenzie [1992] suggested that
  solar reconnection events launch the high-frequency waves directly;
  the frequencies of these waves must be in the kHz range if they are
  to resonate with the coronal protons. In this paper we point out
  that the waves suggested by Axford and McKenzie can in principle be
  detected using interplanetary scintillation (IPS) techniques. If the
  ion cyclotron waves are obliquely propagating, they will be compressive,
  and the corresponding density fluctuations will induce phase, intensity,
  and Faraday rotation fluctuations on radio signals passing through the
  corona. Tu and Marsch [1997] and Marsch and Tu [1997] provided some
  detailed models based on Axford and McKenzie's suggestion, including
  the wave magnetic power spectrum. From the latter we calculate the
  associated density power spectrum at 5R<SUB>S</SUB>, which at high
  wavenumbers turns out to be above the actual generic density power
  spectrum at 5R<SUB>S</SUB> inferred from IPS by Coles and Harmon
  [1989]. The predicted spectrum is even farther above an inferred
  density spectrum in coronal holes based on Coles et al. [1995]. It
  is tempting to conclude that the density fluctuations implied by the
  models of Tu and Marsch are not present and thus that the postulated
  ion cyclotron waves of solar origin are not present. However, we offer
  several reasons why such a conclusion, though we believe it is likely,
  would be premature. We do suggest, though, that IPS has the potential
  to verify or refute whether the Sun launches very high frequency waves
  into the coronal holes.

---------------------------------------------------------
Title: A Kinetic Model of Coronal Heating and Acceleration by
Ion-Cyclotron Waves: Preliminary Results
Authors: Isenberg, Philip A.; Lee, Martin A.; Hollweg, Joseph V.
2000SoPh..193..247I    Altcode:
  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.

---------------------------------------------------------
Title: Commission 49: Interplanetary Plasma and Heliosphere:
    (Plasma Interplanetaire et Heliosphere)
Authors: Verheest, F.; Vandas, M.; Buti, B.; Cramer, N. F.; Dryer, M.;
   Habbal, S. R.; Hollweg, J. V.; Huber, M. C. E.; Kojima, M.; Ripken, H.
2000IAUTA..24...77V    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Cyclotron resonance in coronal holes: 1. Heating and
    acceleration of protons, O<SUP>5+</SUP>, and Mg<SUP>9+</SUP>
Authors: Hollweg, Joseph V.
1999JGR...10424781H    Altcode:
  The resonant heating and acceleration of protons and selected heavy
  ions in coronal holes are investigated by calculating trajectories
  of individual test particles under the influence of gravity, the
  electrostatic electric field, the mirror force, and the resonant
  acceleration due to interaction with dispersive ion cyclotron waves. The
  transverse heating due to the resonance is also included. We show
  in general terms how heavy ions can be more than mass proportionally
  heated, emphasizing that wave dispersion may play an important part
  in producing very hot heavy ions. We pay particular attention to the
  ultraviolet coronagraph spectrometer (UVCS) SOHO observation that
  the transverse temperature of O<SUP>5+</SUP> is still increasing out
  to the outer limit of observation at ~3.5 solar radii. Using both
  approximate analytical expressions and the trajectory calculations,
  we find that this observation can only be reproduced if the magnetic
  power spectrum falls off at least as steeply as k<SUP>-2</SUP>,
  where k is wavenumber. Surprisingly, this conclusion holds even when
  the power spectrum consists of two power laws, if the inner scale
  is proportional to the proton inertial length. Once the particles
  are heated transversely by the resonance, the mirror force provides
  the dominant outward acceleration and leads to heavy ions which flow
  faster than the protons. It is shown that it is possible to construct
  a model which gives reasonable agreement with the UVCS/SOHO data for
  both protons and O<SUP>5+</SUP>. Overall, we conclude that it is highly
  likely that the cyclotron resonance is responsible for heating protons
  and heavy ions in coronal holes. However, we also briefly discuss some
  data for Mg<SUP>9+</SUP>, which do not fit the overall picture.

---------------------------------------------------------
Title: Cyclotron resonance in coronal holes: 2. A two-proton
    description
Authors: Hollweg, Joseph V.
1999JGR...10424793H    Altcode:
  In a cold plasma, the ion-cyclotron mode does not extend above the
  proton cyclotron frequency. As a consequence, for waves propagating
  outward from the Sun, only protons that are moving slower than the
  mean proton wind speed can resonate with this mode. Thus only roughly
  half of the proton distribution function can be in resonance at any
  instant of time. The proton distribution function is then expected to
  depart significantly from a bi-Maxwellian, which is usually assumed
  to provide closure to a set of fluid equations. Here we consider the
  effects of the ion-cyclotron resonance on protons in a coronal hole. We
  calculate the trajectories of individual protons in the electric,
  magnetic, and gravitational fields, and we include the resonant heating
  and acceleration for an average particle that is diffusing in phase
  space. For closure we consider two protons, which are proxies for
  the resonant and nonresonant halves of the distribution. Elementary
  arguments show that the two protons tend to approach nearly the same
  radial velocity. When the waves are dispersive, this means that the
  resonant wavenumber k<SUB>res</SUB> increases. For a power spectrum
  that is a power law in wavenumber, and if the dissipation is determined
  only by the resonant particles, then the resonant effects become very
  weak as k<SUB>res</SUB> becomes large and there is a little heating
  or acceleration of the coronal plasma. On the other hand, if the
  dissipation is determined by a turbulent cascade, k<SUB>res</SUB>
  mainly controls the relative importance of resonant acceleration and
  resonant heating. Such models can yield good agreement with what is
  known about the behavior of protons in coronal holes.

---------------------------------------------------------
Title: Resonant Heating and Acceleration of Protons and Ions in
Coronal Holes : Two-Proton Closure
Authors: Hollweg, Joseph V.
1999ESASP.446..357H    Altcode: 1999soho....8..357H
  UVCS/SOHO has provided remarkable evidence that protons and heavy ions
  in coronal holes are heated by the ion-cyclotron resonance. We will
  review some of the basic physical principles governing the resonant
  interactions, emphasizing the difficulty that only about half of
  the protons can be in resonance with the ion-cyclotron mode. For
  quantitative results, we calculate the trajectories of individual
  protons and ions in the electric, magnetic, and gravitational
  fields, and we include the resonant heating and acceleration for
  the average particle which is diffusing in phase space. To provide
  closure we consider two protons, which are proxies for the resonant
  and non-resonant halves of the distribution. Elementary arguments
  show that the two protons tend to approach nearly the same radial
  velocity. When the waves are dispersive, this means that the resonant
  wavenumber, k<SUB>res</SUB>, increases. For a power spectrum which
  is a power law in wavenumber, and if the dissipation is determined
  only by the resonant particles, then the resonant effects become very
  weak as k<SUB>res</SUB> becomes large, and there is little heating
  or acceleration of the coronal plasma. On the other hand, if the
  dissipation is determined by a turbulent cascade, k<SUB>res</SUB>
  mainly controls the relative importance of resonant acceleration
  and resonant heating. Such models yield good agreement with what is
  known about the behavior of protons in coronal holes. We will also
  emphasize the importance of the UVCS/SOHO observations of Oxygen+5,
  whose temperature is still increasing after the proton temperature has
  leveled off. This provides an important constraint on the steepness
  of the power spectrum at the resonant wave numbers.

---------------------------------------------------------
Title: Kinetic Alfvén wave revisited
Authors: Hollweg, Joseph V.
1999JGR...10414811H    Altcode:
  We develop a series of new analytical expressions describing the
  physical properties of the kinetic Alfvén wave. The wave becomes
  strongly compressive when k<SUB>⊥</SUB><SUP>-1</SUP> is of the order
  of the ion inertial length. Thus, in a low-β plasma, the kinetic
  Alfvén wave can be compressive at values of k<SUB>⊥</SUB> for which
  the dispersion relation departs only slightly from that of the usual
  MHD Alfvén wave. The compression is accompanied by a magnetic field
  fluctuation δB<SUB>∥</SUB> such that the total pressure perturbation
  δp<SUB>tot</SUB>~0. Thus the wave undergoes transit-time damping
  as well as Landau damping; the two effects are comparable if the ion
  thermal speed is of the order of the Alfvén speed. We find that the
  transverse electric field is elliptically polarized but rotating in
  the electron sense; this surprising behavior of the polarization of the
  Alfvén branch was discovered numerically by Gary [1986]. We derive a
  new dispersion relation which explicitly shows how the kinetic Alfvén
  wave takes on some properties of the large-k<SUB>⊥</SUB> limit of
  the slow mode. We also derive approximate dispersion relations valid
  for a multi-ion plasma with differential streaming. We suggest that the
  kinetic Alfvén wave may be responsible for the flattening of density
  fluctuation spectra observed at large wavenumbers in the corona and in
  the solar wind. We also find that our derived properties of the kinetic
  Alfvén wave are consistent with its presence in the dissipation range
  of MHD turbulence [Leamon et al., 1998a, b].

---------------------------------------------------------
Title: Proton temperature anisotropy in the fast solar wind:
    Turbulence-driven dispersive ion cyclotron waves
Authors: Li, Xing; Habbal, Shadia R.; Hollweg, Joseph V.; Esser, Ruth
1999AIPC..471..531L    Altcode: 1999sowi.conf..531L
  The effects of parallel propagating ion cyclotron waves on the solar
  wind plasma are investigated in an attempt to reproduce the observed
  proton temperature anisotropy. The model calculations presented
  here assume that a nonlinear cascade process, at the Kolmogorov rate,
  transports energy from low-frequency Alfvén waves to the ion cyclotron
  resonant range. The energy is then picked up by the plasma through the
  resonant cyclotron interaction. Ion cyclotron waves are found to play
  an important role in shaping the proton temperature anisotropy starting
  in the inner corona and extending to interplanetary space. Dispersive
  ion cyclotron waves are able to cool protons more significantly than
  nondispersive ones.

---------------------------------------------------------
Title: The cyclotron resonance: Heating of protons and oxygen in
    coronal holes
Authors: Hollweg, Joseph V.
1999AIPC..471..369H    Altcode: 1999sowi.conf..369H
  The UVCS/SOHO data have offered remarkable evidence that the coronal
  holes and acceleration region of the fast solar wind are heated
  by ion-cyclotron waves. We here summarize the basic physics of the
  cyclotron resonance, and show why ions such as O<SUP>+5</SUP> can be
  heated to more than mass-proportional temperatures compared to the
  protons. The mirror force provides the main acceleration out of the
  corona, yielding heavy ions which flow faster than the protons. We
  quantify these ideas by following an average test particle. Agreement
  with observation is achieved, but only if we take a steep power
  spectrum. Particular attention is given to the behavior of T⊥ for
  O<SUP>+5</SUP>, which seems to increase with distance from the Sun
  out to the limits of this the observations; this observation is a
  major constraint.

---------------------------------------------------------
Title: Nonlinear evolution of Alfvén waves and RDs-hybrid simulations
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1999AIPC..471..167V    Altcode: 1999sowi.conf..167V
  We review our use of hybrid simulations in the study nonlinear Alfvén
  waves. The simulations treat ions as particles and electrons as a
  fluid. We address how wave nonlinearity and ion kinetics can influence
  the polarization, compressional component, and imbedded rotational
  discontinuities seen in association with Alfvénic fluctuations in
  the solar wind.

---------------------------------------------------------
Title: Formation of pressure-balanced structures and fast waves from
    nonlinear Alfvén waves
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1999JGR...104.4681V    Altcode:
  In the solar wind, Alfvénic fluctuations are typically observed in
  association with small fluctuations of the density (ρ) and magnetic
  field strength (B), which tend to be anticorrelated and in approximate
  pressure balance. One would not expect any finite δρ and δB among
  pure Alfvén waves propagating strictly outward from the Sun. Our
  paper shows how Alfvén waves can nonlinearly produce structures in
  pressure balance. We present a second-order analysis of the pure
  magnetohydrodynamic equations and hybrid simulations which show
  that nonlinear Alfvén waves traveling in different directions but
  with equal group velocity can generate pressure-balanced structures
  with wave vectors perpendicular to the background magnetic field
  B<SUB>0</SUB>. Homogeneous fast waves are also generated in this
  direction in order to satisfy initial conditions. They cannot be Landau
  or transit-time damped and so cause the values of B and ρ to vary
  with time as they beat with the pressure-balanced structures. However,
  we find δρδB&lt;0 is satisfied most of the time, and this can partly
  explain the tendency for anticorrelation observed in the solar wind. In
  directions away from the perpendicular one, Alfvén waves produce driven
  fast waves which give constant B and ρ to second order. Homogeneous
  fast and slow waves are also produced in these directions but Landau
  damp away in large β plasmas. Thus an equilibrium or steady propagating
  waveform at second order can be produced where B and ρ vary only in the
  perpendicular direction. If transverse magnetic structures with wave
  vectors perpendicular to B<SUB>0</SUB> are included at the same order
  as the initial Alfvén waves, then these evolve to pressure-balanced
  structures and can also coexist with the Alfvén waves. However,
  an equilibrium is obtained generally only when these structures also
  have velocity fluctuations equivalent of those of the Alfvén waves.

---------------------------------------------------------
Title: Heating and cooling of protons by turbulence-driven ion
    cyclotron waves in the fast solar wind
Authors: Li, Xing; Habbal, Shadia R.; Hollweg, Joseph V.; Esser, Ruth
1999JGR...104.2521L    Altcode:
  The effects of parallel propagating nondispersive ion cyclotron
  waves on the solar wind plasma are investigated in an attempt
  to reproduce the observed proton temperature anisotropy, namely,
  T<SUB>p⊥</SUB>&gt;&gt;T<SUB>p∥</SUB> in the inner corona and
  T<SUB>p⊥</SUB>&lt;T<SUB>p∥</SUB> at 1 AU. Low-frequency Alfvén
  waves are assumed to carry most of the energy needed to accelerate and
  heat the fast solar wind. The model calculations presented here assume
  that nonlinear cascade processes, at the Kolmogorov and Kraichnan
  dissipation rates, transport energy from low-frequency Alfvén waves
  to the ion cyclotron resonant range. The energy is then picked up
  by the plasma through the resonant cyclotron interaction. While
  the resonant interaction determines how the heat is distributed
  between the parallel and perpendicular degrees of freedom, the level
  of turbulence determines the net dissipation. Ion cyclotron waves
  are found to produce a significant temperature anisotropy starting
  in the inner corona, and to limit the growth of the temperature
  anisotropy in interplanetary space. In addition, this mechanism heats
  or cools protons in the direction parallel to the magnetic field. While
  cooling in the parallel direction is dominant, heating in the parallel
  direction occurs when T<SUB>p⊥</SUB>&gt;&gt;T<SUB>p∥</SUB>. The
  waves provide the mechanism for the extraction of energy from the
  parallel direction to feed into the perpendicular direction. In
  our models, both Kolmogorov and Kraichnan dissipation rates yield
  T<SUB>p⊥</SUB>&gt;&gt;T<SUB>p∥</SUB> in the corona, in agreement
  with inferences from recent ultraviolet coronal measurements, and
  predict temperatures at 1 AU which match in situ observations. The
  models also reproduce the inferred rapid acceleration of the fast
  solar wind in the inner corona and flow speeds and particle fluxes
  measured at 1 AU. Since this mechanism does not provide direct energy
  to the electrons, and the electron-proton coupling is not sufficient
  to heat the electrons to temperatures at or above 10<SUP>6</SUP>K,
  this model yields electron temperatures which are much cooler than
  those currently inferred from observations.

---------------------------------------------------------
Title: Solar Wind Nine
Authors: Habbal, Shadia Rifai; Esser, Ruth; Hollweg, Joseph V.;
   Isenberg, Philip A.
1999AIPC..471.....H    Altcode: 1999sowi.conf.....H
  No abstract at ADS

---------------------------------------------------------
Title: Potential wells, the cyclotron resonance, and ion heating in
    coronal holes
Authors: Hollweg, Joseph V.
1999JGR...104..505H    Altcode:
  We consider the motions of protons and O<SUP>5+</SUP> ions in coronal
  holes. We first consider the effects of a potential well, which arises
  from the combination of gravity, the electrostatic electric field, and
  the mirror force. We show that if the potential well is time dependent,
  then ions which are initially trapped will undergo a time-averaged
  energy gain. They can eventually gain enough energy to escape out of
  the potential well and be ejected out of the corona. The process is
  analogous to Fermi acceleration of cosmic rays by reflections off of
  moving magnetic clouds, except here the trapped ions can be regarded as
  reflecting off of moving walls. There is evidence that the trajectories
  of the particles are chaotic. However, the timescales are long, the
  potential wells are not very deep, and the process is probably not
  important for coronal heating. We also point out that the potential
  wells can provide a population of particles which are moving inward
  relative to waves which are propagating outward from the Sun. These
  particles are the ones which can interact most strongly with ion
  cyclotron waves, since they resonate with the lowest frequency waves
  which have the highest phase speeds and presumably the most power. We
  present some simple arguments, invoking energy-conserving pitch angle
  scattering in the wave frame, which show how O<SUP>5+</SUP> ions can
  in principle acquire perpendicular temperatures which are more than
  mass-proportionally hotter than the protons. The basic principles
  are demonstrated by calculating trajectories for average particles
  interacting with dispersive ion cyclotron waves. We also present a
  strongly driven case which gives perpendicular energies and parallel
  flow speeds qualitatively resembling those believed to exist in coronal
  holes, but there are significant differences between the model results
  and the SOHO/UVCS data. In this case the particles are not trapped in
  a potential well.

---------------------------------------------------------
Title: Formation of spherically polarized Alfvén waves and imbedded
    rotational discontinuities from a small number of entirely oblique
    waves
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1998JGR...103..335V    Altcode:
  We present two-and-one-half-dimensional
  (2&lt;fraction shape="case" style="single"
  align="center"&gt;&lt;num&gt;1&lt;/num&gt;&lt;den&gt;2&lt;/den&gt;&lt;/fraction&gt;-D)
  hybrid numerical simulations of a small group of oblique Alfvén
  waves with linear polarization. These are the first simulations of
  nonplanar Alfvén waves which evolve to a nearly constant-B state
  (spherical polarization) with imbedded rotational discontinuities
  (RDs). Initially, B varies with position in the wave group,
  and we consider only cases where the Fourier components of
  B<SUP>2</SUP> are entirely oblique to the background magnetic
  field &lt;bold&gt;B&lt;/bold&gt;<SUB>0</SUB>. When propagating
  in different directions but with group speeds in the same
  direction along &lt;bold&gt;B&lt;/bold&gt;<SUB>0</SUB>, Alfvén
  waves generally evolve nonplanar waveforms with nearly constant
  magnetic intensity B. In this waveform, the magnetic field vectors
  move on a sphere of radius B and have a spherical rather than
  arc polarization. Most Alfvénic fluctuations in the solar wind
  are spherically polarized. We also find analytical second-order
  solutions from the magnetohydrodynamic equations for linearly
  polarized Alfvén waves in 2&lt;fraction shape="case" style="single"
  align="center"&gt;&lt;num&gt;1&lt;/num&gt;&lt;den&gt;2&lt;/den&gt;&lt;/fraction&gt;-D
  and 3-D. For moderate-wave amplitudes
  (|δ&lt;bold&gt;B&lt;/bold&gt;|/B<SUB>0</SUB>&lt;~0.5),
  these show that the second-order driven wave solution can
  only remove variations of B when the Fourier components of
  B<SUP>2</SUP> are oblique to B<SUB>0</SUB>. Large-amplitude
  (|δ&lt;bold&gt;B&lt;/bold&gt;|/B<SUB>0</SUB>~1) waves also
  evolve to constant magnetic intensity, but higher-order terms
  produce imbedded RDs with properties similar to those seen in the
  solar wind. The RDs are steady, and their normals are oblique to
  &lt;bold&gt;B&lt;/bold&gt;<SUB>0</SUB>. The transverse extent of the
  RDs is approximately of order of the average wavelength of the Alfvén
  waves, and the RDs are either locally planar or fully nonplanar. RDs can
  appear at less than twice per cycle in waveforms composed of two or more
  waves. If the magnetic fields are sampled along a single line through
  the simulation box, as a single spacecraft would observe Alfvénic
  fluctuations in the solar wind, the rate depends on the direction that
  the line takes through the box. We also suggest that all Alfvénic
  fluctuations are spherically polarized, and instances of arc-polarized
  fluctuations occur when spacecraft sample locally planar structures.

---------------------------------------------------------
Title: Formation of imbedded rotational discontinuities with nearly
    field aligned normals
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1998JGR...103..349V    Altcode:
  We present hybrid numerical simulations of a small number of
  low-frequency (much less than the proton gyrofrequency) Alfvén
  waves and, for the first time, show how to produce imbedded rotational
  discontinuities (RDs) with a small angle (θ<SUB>Bn</SUB>) between their
  asymptotic and normal fields which are stable against dispersion. When
  the initial waves are linearly polarized and give a waveform whose
  Fourier components of B<SUP>2</SUP> have wave vectors within ~10° of
  the background magnetic field &lt;bold&gt;B&lt;/bold&gt;<SUB>0</SUB>,
  the waveform tends to steepen even for small amplitudes
  (|δ&lt;bold&gt;B&lt;/bold&gt;|/B&lt;&lt;1). This produces RDs
  with normals nearly along &lt;bold&gt;B&lt;/bold&gt;<SUB>0</SUB>
  and nearly constant B between the RDs. However, the waveform is not
  steady because these RDs widen continually due to the presence of
  dispersive intermediate or ion cyclotron modes which are permanently
  attached to their edges. This commonly occurs in simulations of
  isolated RDs which have small θ<SUB>Bn</SUB>. When the initial
  waveform is modified by including a transverse field component with a
  wavenumber equaling that from the Fourier component of B<SUP>2</SUP>
  along &lt;bold&gt;B&lt;/bold&gt;<SUB>0</SUB> so that the field moves
  on an “elliptical” (B not constant) arc, the dispersive intermediate
  modes no longer develop, and the waveform and imbedded RDs evolve
  toward a steady state with nearly constant B and a “circular” arc
  polarization. Spherically polarized waveforms can be made in a similar
  manner. Only this type of waveform produces RDs without intermediate
  modes on their edges for all parameters. Oblique fast modes do form,
  but these have phase speeds which are faster than the Alfvén phase
  speed and so detach from the RD's edge and do not cause the RD to
  widen continually. The appearance of RDs with small θ<SUB>Bn</SUB>
  varies greatly with the time of development, and this can explain
  the diversity of hodogram shapes of such RDs in the solar wind. We
  finally conclude that Alfvénic fluctuations with imbedded RDs must
  evolve through a succession of arc-polarized or spherically polarized
  waveforms as they travel outward from the Sun: Otherwise, the RDs with
  small θ<SUB>Bn</SUB> in the solar wind would have widen to the point
  where they are beyond recognition.

---------------------------------------------------------
Title: Comment on “Gravitational Damping of Alfvén Waves in Stellar
    Atmospheres and Winds”
Authors: Hollweg, Joseph V.
1997ApJ...488..895H    Altcode:
  Khabibrakhmanov &amp; Mullan considered Alfvén wave propagation in a
  gravitational field. They pointed out that an ion can drift along the
  wave electric field, and gain energy if the wave propagates upward. They
  interpreted this result in terms of plasma heating and wave damping. We
  offer an alternative interpretation. We show that the Lorentz force
  associated with the drift in question is the force that guides the
  particle along the magnetic field lines. This force is analogous to
  the surface force exerted by an inclined plane on an object which is
  sliding on it. The electric field implies a moving magnetic field line,
  which is analogous to a moving inclined plane. The energy gained as
  a particle drifts along the electric field is shown to be analogous
  to the work done by the surface force which is exerted by a moving
  inclined plane. Thus the energy gain does not represent heating. It
  is simply the work done by the force which guides a particle along a
  moving magnetic field line.

---------------------------------------------------------
Title: A simple mechanical model for resonance absorption: The
    Alfvén resonance
Authors: Hollweg, Joseph V.
1997JGR...10224127H    Altcode:
  We consider resonance absorption of magnetohydrodynamic waves, and the
  Alfvén resonance layer in particular. We show that the dissipative
  layer can be modeled as a simple mechanical system consisting of a few
  harmonic oscillators which are coupled by friction. The mechanical
  model reproduces known results for the externally driven system in
  steady state, such as the structure of the dissipative layer, the
  “waves” of heating which propagate across the layer, and the fact
  that the total heating is independent of time. The total work done on
  the oscillators by the driver is always positive; the external driver
  sees the total system as a single damped oscillator driven exactly at
  resonance. Nonetheless, some of the oscillators return energy back to
  the driver. The total kinetic energy of all the oscillators and the
  total potential energy are nearly independent of time, because the
  integrals, across the dissipative layer, of the square of the velocity
  and the square of the displacement, are truly constants in time. Waves
  of kinetic and potential energy propagate across the system in the
  same sense as the waves of heating. We also investigate an initial
  value problem in which the driver is turned on at t=0. There is no
  single number representing the time required for the dissipative
  layer to reach a steady state. The waves of heating which are found
  in the steady state are also present in the buildup phase. However,
  if the driver is turned off after the system has reached a steady
  state, then the waves of heating are less obvious. We consider the
  effects of a nonlinear frictional coupling between the oscillators,
  designed to mimic the effects of Kelvin-Helmholtz instabilities. The
  nonlinear coupling has surprisingly little effect on the system. The
  total steady state heating rate is the same as in the linear system;
  even with nonlinear dissipation, the dissipative layer adjusts itself
  to absorb a predetermined amount of energy being pumped in by the
  external driver. The waves of heating which are found in the linear
  system are still present. We find no evidence of chaotic behavior.

---------------------------------------------------------
Title: Hot protons in the inner corona and their effect on the flow
    properties of the solar wind
Authors: Esser, Ruth; Habbal, Shadia R.; Coles, William A.; Hollweg,
   Joseph V.
1997JGR...102.7063E    Altcode:
  Following recent observations which indicate the
  presence of extremely high flow speeds in the inner
  corona, 700-800kms<SUP>-1</SUP> below 10R<SUB>S</SUB>,
  and the possible presence of very high proton temperatures,
  3×10<SUP>6</SUP>&lt;=T<SUB>p</SUB>&lt;=8.5×10<SUP>6</SUP>K,
  we present a parameter study which shows that if the high proton
  temperatures in the inner corona are genuine, then flow speeds of
  700 to 800kms<SUP>-1</SUP> can readily be achieved at 10<SUB>S</SUB>
  or even closer to the coronal base. If one allows for both heat and
  momentum deposition in the inner corona, the rapid acceleration close
  to the coronal base can be achieved with proton temperatures well
  below the upper limit placed by the observations.

---------------------------------------------------------
Title: The solar corona and solar wind: Theoretical issues
Authors: Hollweg, Joseph V.; Esser, Ruth
1997AIPC..385..169H    Altcode: 1997recs.conf..169H
  The mechanisms responsible for the solar corona and the high-speed
  solar wind streams are still not known. Traditional wave-driven solar
  wind models do not seem capable of producing the rapid acceleration of
  the wind close to the Sun, which is implied by recent remote sensing
  data. A variety of new ideas have been put forth. In one class of
  models, the coronal protons (and perhaps also the heavier ions, such as
  helium nuclei) are preferentially heated to temperatures substantially
  in excess of the electrons; high-frequency ion-cyclotron waves, MHD
  shocks, and/or MHD turbulence may induce the ion heating. Another
  class of models invokes transitory field-aligned jets at the coronal
  base. In both of these types of models, magnetic reconnection events
  may be the fundamental energy source. In another class of models,
  electric fields and gravity act as velocity filters which allow only
  high-speed particles to reach the corona and solar wind. The Solar
  Probe should be able to detect remnants and signatures of the processes
  which heat the corona and accelerate the solar wind.

---------------------------------------------------------
Title: A Spacecraft Going Behind the Sun Will Support SOHO
Authors: Ruzmaikin, A.; Anderson, J. D.; Asmar, S.; Bird, M.; Cassiani,
   A.; Coles, W.; Feynman, J.; Harvey, J.; Harvey, K.; Hollweg, J.;
   Linker, K.; Mikic, Z.; Pätzold, M.; Smith, E. J.
1997ESASP.404..653R    Altcode: 1997cswn.conf..653R
  No abstract at ADS

---------------------------------------------------------
Title: Non-WKB Alfvén waves in the solar wind: propagation and
    reflection of pulses
Authors: Hollweg, Joseph V.
1996AIPC..382..327H    Altcode:
  We study the propagation and reflection of waves in the solar wind
  using the system impulse response. We find that the ingoing Elsässer
  variable tends to accumulate as a random walk with a `memory' of
  several days, and that the corona has a tendency to `ring' with a
  period of several hours.

---------------------------------------------------------
Title: The making of an Alfvénic fluctuation: The resolution of a
    second-order analysis
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1996AIPC..382..331V    Altcode:
  We perform 112-D hybrid numerical simulations of initially linearly
  polarized and obliquely propagating Alfvén waves. These simulations
  can outline the actual role of ion kinetics. We find that moderate
  (δB/B&lt;~0.5) amplitude wave trains in warm ion plasmas have a
  tendency to evolve to a B-constant state with arc polarization and
  are well described by a weakly nonlinear treatment. However, the
  generated magnetosonic fast modes are only weakly damped for small
  plasma β(&lt;~0.5). As a result, B does not asymptote to a constant
  over many wave train periods. These results can be used to explain
  the formation of arc polarized Alfvénic fluctuations in the solar wind.

---------------------------------------------------------
Title: Formation of arc-shaped Alfvén waves and rotational
    discontinuities from oblique linearly polarized wave trains
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1996JGR...10113527V    Altcode:
  The forms of Alfvénic fluctuations in the solar wind sometimes possess
  nearly constant magnetic intensities but have an approximate arc rather
  than circular polarization. They are also associated with layers of
  abrupt field rotation called rotational discontinuities (RDs) where
  the field changes direction by &lt;180°. Ion-sense and electron-sense
  rotations are observed in approximately equal numbers. To explore the
  origin of this form, we conduct a one-and-one-half-dimensional hybrid
  numerical simulation study of the evolution of obliquely propagating,
  low-frequency (&lt;&lt;ion cyclotron) Alfvén wave trains. Starting
  from a linearly polarized wave train, an approximate arc polarization
  evolves rapidly where the magnetic field moves to and fro on a less
  than semicircular arc. Large-amplitude (|δB|/B~1) wave trains steepen
  and produce RDs which always rotate the field by &lt;180° with no
  preference for ion or electron sense of rotation. These properties
  correspond to those of Alfvénic fluctuations in the solar wind, and
  our model is the first which offers an explanation of the observed
  arc-shaped waves and imbedded RDs. At early times, a large density
  signal is also generated. For large plasma β, the signal rapidly
  damps, and the waveform varies little with time. For small plasma β,
  the generated constant-B Alfvén wave is parametrically unstable and
  causes the density signal to grow further before the instability
  saturates. The wave train and density signal beat strongly giving
  a periodic time variation of the wave amplitude and waveform. Ion
  heating from steepening, RD formation, relaxation to constant B, and
  parametric processes occurs mainly parallel to the background magnetic
  field and cannot explain the perpendicular heating of ions observed
  in the solar wind.

---------------------------------------------------------
Title: Comment on “Nonlinear studies of coronal heating by the
    resonant absorption of Alfvén waves” by Ofman et al.
Authors: Hollweg, Joseph V.
1995GeoRL..22.2677H    Altcode:
  Ofman et al. [1994] claim to have simulated the nonlinear development of
  the resonant absorption of Alfvén waves. We argue that linear resonant
  absorption does not occur in the configuration they studied and we
  suggest that phase mixing (refraction) is the dominant linear effect
  in their simulations. We point out, however, that their demonstrtion
  of the Kelvin-Helmholz instability of refracting Alfvén waves may
  have important implications for solar coronal heating.

---------------------------------------------------------
Title: Non-WKB Alfven waves in the solar wind: Propagation and
    reflection of pulses
Authors: Hollweg, J. V.
1995sowi.conf...78H    Altcode:
  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-propagating 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.

---------------------------------------------------------
Title: The making of an Alfvenic fluctuation: The resolution of a
    second-order analysis
Authors: Vasquez, Bernard J.; Hollweg, Joseph V.
1995sowi.conf...78V    Altcode:
  Ulysses observations of the high speed polar streams show that they
  are largely occupied by very large amplitude Alfvenic fluctuations
  accompanied by many rotational discontinuities. These fluctuations
  have a nearly constant magnetic intensity or amplitude, and the
  magnetic field direction per wave cycle sweeps only through a limited
  arc, much as a car wiperblade would do. Barnes and Hollweg (JGR,
  79, 2302, 1974) suggested that this unusual waveform could arise
  from an obliquely propagating and linearly polarized Alfven wave of
  finite amplitude. From a second-order analysis, they showed that the
  existence of a particular solution with a constant amplitude but could
  not resolve the outcome of the homogeneous solution which consisted of
  fast waves. They suggested that Landau damping of these fast waves may
  be needed to get the observed waveform. We present a 1 1/2 D hybrid
  simulation which is fully nonlinear and correctly describes the ion
  kinetics for an initially monochromatic and linearly polarized Alfven
  wave propagating obliquely to the background magnetic field. The
  wave has a large amplitude and a wavelength so long that it can
  be considered dispersionless for simulation times. At early times,
  the second harmonic in density and in magnetic field transverse to
  the initial wave magnetic field are generated and have more power
  than other harmonics. Steepening is observed with a weak fast shock
  emerging, but no rotational discontinuity is left behind, and instead a
  constant amplitude and an arc-shaped waveform is made. The compressional
  component which develops after the shocks have dissipated is to zeroth
  order better described as a pure acoustic wave than as a fast wave. This
  might be explained by the relaxing of the Alfven wave to a state where
  its ponderomotive force vanishes so that the compressional component
  can travel almost independently of it.

---------------------------------------------------------
Title: Dissipative MHD solutions for resonant AlfvÉn waves in
    1-dimensional magnetic flux tubes
Authors: Goossens, Marcel; Ruderman, Michail S.; Hollweg, Joseph V.
1995SoPh..157...75G    Altcode:
  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<SUB>r</SUB>, 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<SUB>r</SUB>,
  andP' in terms of double integrals of Hankel functions of complex
  argument of order 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.

---------------------------------------------------------
Title: The Alfvénic Impulse Response of the Open Solar Corona and
    Solar Wind
Authors: Hollweg, J. V.
1995SPD....26..906H    Altcode: 1995BAAS...27R.974H
  No abstract at ADS

---------------------------------------------------------
Title: Beat, modulational, and decay instabilities of a circularly
    polarized Alfvén wave
Authors: Hollweg, Joseph V.
1994JGR....9923431H    Altcode:
  A circularly polarized low-frequency electromagnetic pump
  wave propagating along an ambient magnetic field is known to be
  unstable to the growth of several parallel-propagating parametric
  instabilities. If ion-cyclotron effects are retained in a two-fluid
  description, the dispersion relation is a sixth-order polynomial. We
  present a series of new analytical approximations to this dispersion
  relation. We emphasize new results for the beat instability that occurs
  as an interaction of the forward propagating upper sideband with the
  backward propagating lower sideband. The nature of the beat instability
  depends on β=(υ<SUB>sound</SUB>/υ<SUB>A</SUB>)<SUP>2</SUP> and
  on the sense of polarization of the pump wave. The beat and decay
  instabilities can occur together if the pump is left-handed (i.e.,
  ion resonant) and if β&lt;~1, but they cannot occur together if the
  pump is right-handed. For a left-handed pump the beat mode is the
  only instability if β&gt;1. If the pump is right-handed and β&gt;1,
  then the beat instability exists only when the pump amplitude exceeds a
  threshold value, and the beat will be the only instability if the pump
  amplitude is large enough to stabilize the modulational instability. If
  the pump is left-handed and β&lt;~1, then the beat mode is stabilized
  when the pump amplitude becomes sufficiently large. The beat instability
  primarily produces a forward propagating transverse wave in the upper
  sideband. Thus if β&gt;1, the instabilities considered here do not
  produce the backward propagating waves which are thought to affect
  turbulence and the evolution of cross helicity in the solar wind. New
  analytical results are presented also for the decay and modulational
  instabilities when β~1.

---------------------------------------------------------
Title: Growth rates of new parametric instabilities occurring in a
    plasma with streaming He<SUP>++</SUP>
Authors: Jayanti, V.; Hollweg, Joseph V.
1994JGR....9923449J    Altcode:
  We consider parametric instabilities of a circularly polarized pump
  Alfvén wave, which propagates parallel to the ambient magnetic field;
  the daughter waves are also parallel-propagating. We follow Hollweg
  et al. (1993) and consider several new instabilities that owe their
  existence to the presence of streaming alpha particles. One of the new
  instabilities is similar to the familiar decay instability, but the
  daughter waves are a forward going alpha sound wave and a backward
  going Alfvén wave. The growth rate of this instability is usually
  small if the alpha abundance is small. The other three new instabilities
  occur at high frequencies and small wavelengths. We find that the new
  instability which involves the proton cyclotron wave and alpha sound
  (i.e., the (+f, αs) mode) can be the fastest growing instability if
  β~1. However, if β is small, then the instability which can compete
  with the decay instability is the (+f, -α) instability, which involves
  both the proton and alpha cyclotron resonances, but the pump wave must
  have low frequency and large amplitude. These instabilities may be a
  means of heating and accelerating alpha particles in the solar wind,
  but this claim is unproven until a fully kinetic study is carried out.

---------------------------------------------------------
Title: Parametric instabilities of parallel-propagating Alfvén waves:
    Some analytical results
Authors: Jayanti, V.; Hollweg, Joseph V.
1993JGR....9819049J    Altcode:
  We consider the stability of a circularly polarized Alfvén wave
  (the pump wave) which propagates parallel to the ambient magnetic
  field. Only parallel-propagating perturbations are considered, and
  we ignore dispersive effects due to the ion cyclotron frequency. The
  dissipationless MHD equations are used throughout; thus possibly
  important effects arising from Landau and transit time damping
  are omitted. We derive a series of analytical approximations to the
  dispersion relation using A=(ΔB/B<SUB>0</SUB>)<SUP>2</SUP> as a small
  expansion parameter; ΔB is the pump amplitude, and B<SUB>0</SUB> is the
  ambient magnetic field strength. We find that the plasma β (the square
  of the ratio of the sound speed to the Alfvén speed) plays a crucial
  role in determining the behavior of the parametric instabilities of the
  pump. If 0&lt;β&lt;1 we find the familiar result that the pump decays
  into a forward propagating sound wave and a backward propagating Alfvén
  wave with maximum growth rate γ<SUB>max</SUB>~A<SUP>1/2</SUP>, but β
  cannot be too close to 0 or to 1. If β~1, we find γ<SUB>max</SUB>,
  ~A<SUP>3/4</SUP>; if β&gt;1, we find γ<SUB>max</SUB>~A<SUP>3/2</SUP>,
  while if β~0, we obtain γ<SUB>max</SUB>~A<SUP>1/3</SUP> moreover,
  if β~0 there is a nearly purely growing instability. In contrast to
  the familiar decay instability, for which the backward propagating
  Alfvén wave has lower frequency and wavenumber than the pump, we find
  that if β&gt;1 the instability is really a beat instability which is
  dominated by a transverse wave which is forward propagating and has
  frequency and wavenumber which are nearly twice the pump values. Only
  the decay instability for 0&lt;β&lt;1 can be regarded as producing
  two recognizable normal modes, namely, a sound wave and an Alfvén
  wave. We discuss how the different characteristics of the instabilities
  may affect the evolution of Alfvén waves in the solar wind. However,
  for a solar wind in which β~1 the growth times of the instabilities
  are probably too long for these instabilities to have an appreciable
  effect inside 1 AU.

---------------------------------------------------------
Title: On the dispersion relations for parametric instabilities of
    parallel-progagating Alfvén waves
Authors: Jayanti, Venku; Hollweg, Joseph V.
1993JGR....9813247J    Altcode:
  We consider the dispersion relation for the parametric instabilities
  of large-amplitude, circularly polarized Alfvén waves, propagating
  parallel to the ambient magnetic field. A linear perturbation
  analysis is employed, and the perturbations are taken to propagate
  along the ambient field. The standard analysis which has been used
  previously assumes that density perturbations vary as exp[i(kz-ωt)]
  this defines the meaning of ω and k. However, the differential
  equations have periodic coefficients, implying that Floquet analysis
  should be used. We here present an analysis based on Floquet's
  theorem. The result is a hierarchy of dispersion relations. However,
  all the dispersion relations are found to be equivalent to the one
  obtained via the standard analysis; the differences between them
  are due only to how ω and k are defined. Thus we conclude that
  physically there is really only one dispersion relation, namely the
  “electrostatic dispersion relation,” which is in agreement with
  earlier works. However, we disagree with Viñas and Goldstein (1991b),
  who obtained additional dispersion relations which they have called the
  “electromagnetic dispersion relations.” Their additional dispersion
  relations are a consequence of first truncating the dispersion relation
  for obliquely propagating perturbations and then taking the limit of
  parallel-propagating perturbations.

---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
    Flux Tubes - Part Four
Authors: Goossens, Marcel; Hollweg, Joseph V.
1993SoPh..145...19G    Altcode:
  Resonant absorption of MHD waves on a nonuniform flux tube is
  investigated as a driven problem for a 1D cylindrical equilibrium. The
  variation of the fractional absorption is studied as a function of
  the frequency and its relation to the eigenvalue problem of the MHD
  radiating eigenmodes of the nonuniform flux tube is established. The
  optimal frequencies producing maximal fractional absorption are
  determined and the condition for total absorption is obtained. This
  condition defines an impedance matching and is fulfilled for an
  equilibrium that is fine tuned with respect to the incoming wave. The
  variation of the spatial wave solutions with respect to the frequency
  is explained as due to the variation of the real and imaginary parts
  of the dispersion relation of the MHD radiating eigenmodes with respect
  to the real driving frequency.

---------------------------------------------------------
Title: Modulational and decay instabilities of Alfvén waves:
    Effects of streaming He<SUP>+</SUP><SUP>+</SUP>
Authors: Hollweg, Joseph V.; Esser, R.; Jayanti, V.
1993JGR....98.3491H    Altcode:
  We consider parametric instabilities of a circularly polarized Alfvén
  wave propagating along the background magnetic field. The perturbations
  too are assumed to propagate along the background field. The
  new feature of this work is the presence of a second ion species
  (He<SUP>+</SUP><SUP>+</SUP>) which drifts relative to the protons. Even
  though its abundance is small, the He<SUP>+</SUP><SUP>+</SUP> modifies
  the dispersion relation of the “pump” Alfvén wave and introduces a
  new sound wave (alpha sound) in addition to the usual sound wave carried
  primarily by the electrons and protons. These features modify the wave
  couplings leading to instability and introduce new wave couplings which
  lead to several new instabilities which may be of interest in the solar
  wind. In particular, we will find instabilities which are close to
  the He<SUP>+</SUP><SUP>+</SUP> gyroresonance. This may provide a means
  of directly transferring Alfvén wave energy to the alpha particles,
  if the alphas are able to resonantly extract energy from the unstable
  waves, without quenching the instability altogether. We will also find
  instabilities which are close to the alpha particle sound speed. The
  alpha particles in this case will tend to absorb energy via Landau
  damping, and this may again represent a mechanism of transferring
  Alfvén wave energy to the alpha particles, if the Landau damping does
  not suppress the instability. Finally, we will find new instabilities
  close to the proton cyclotron resonance, which may serve as a new
  mechanism for transferring wave energy into the protons. Some similar
  results have been recently obtained independently by Goldstein (1990).

---------------------------------------------------------
Title: Theoretical studies of the physics of the solar atmosphere
Authors: Hollweg, Joseph V.
1992nhud.rept.....H    Altcode:
  Significant advances in our theoretical basis for understanding several
  physical processes related to dynamical phenomena on the sun were
  achieved. We have advanced a new model for spicules and fibrils. We
  have provided a simple physical view of resonance absorption of MHD
  surface waves; this allowed an approximate mathematical procedure
  for obtaining a wealth of new analytical results which we applied to
  coronal heating and p-mode absorption at magnetic regions. We provided
  the first comprehensive models for the heating and acceleration of
  the transition region, corona, and solar wind. We provided a new view
  of viscosity under coronal conditions. We provided new insights into
  Alfven wave propagation in the solar atmosphere. And recently we have
  begun work in a new direction: parametric instabilities of Alfven waves.

---------------------------------------------------------
Title: Total Absorption of Sound Waves by Solar Magnetic Flux Tubes
Authors: Hollweg, J. V.; Goossens, M.
1992AAS...180.1702H    Altcode: 1992BAAS...24R.753H
  No abstract at ADS

---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
    Flux Tubes - Part Three
Authors: Goossens, Marcel; Hollweg, Joseph V.; Sakurai, Takashi
1992SoPh..138..233G    Altcode:
  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.

---------------------------------------------------------
Title: Alfvenically Driven Slow Shocks in the Solar Chromosphere
    and Corona
Authors: Hollweg, Joseph V.
1992ApJ...389..731H    Altcode:
  The nonlinear evolution of an Alfvenic impulse launched from the
  photosphere and its dynamical effects on the chromosphere, transition
  region (TR), and corona are investigated using a simple 1D model. It
  is found that the leading edge of the torsional pulse can steepen
  into a fast shock in the chromosphere if the pulse is of sufficiently
  large amplitude and short duration. A slow shock which develops behind
  the Alfvenic pulse can reflect downgoing Alfven waves back up to the
  corona. The upgoing reflected wave can induce a significant upward
  ejection of the TR. Nonlinear dynamics are found to lead to very
  impulsive behavior at later times. It is suggested that impulsive
  events occurring in the TR or corona need not be interpreted in terms
  of reconnection-driven microflares. It is also found that B(0) in the
  chromosphere can be amplified when the TR and chromosphere fall.

---------------------------------------------------------
Title: Status of solar wind modeling from the transition region
    outwards
Authors: Hollweg, J. V.
1992sws..coll...53H    Altcode:
  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.

---------------------------------------------------------
Title: The Solar Wind Ion Composition Spectrometer
Authors: Gloeckler, G.; Geiss, J.; Balsiger, H.; Bedini, P.; Cain,
   J. C.; Fischer, J.; Fisk, L. A.; Galvin, A. B.; Gliem, F.; Hamilton,
   D. C.; Hollweg, J. V.; Ipavich, F. M.; Joos, R.; Livi, S.; Lundgren,
   R. A.; Mall, U.; McKenzie, J. F.; Ogilvie, K. W.; Ottens, F.; Rieck,
   W.; Tums, E. O.; von Steiger, R.; Weiss, W.; Wilken, B.
1992A&AS...92..267G    Altcode:
  The Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses
  is designed to determine uniquely the elemental and ionic-charge
  composition, and the temperatures and mean speeds of all major
  solar-wind ions, from H through Fe, at solar wind speeds ranging from
  175 km/s (protons) to 1280 km/s (Fe(8+)). The instrument, which covers
  an energy per charge range from 0.16 to 59.6 keV/e in about 13 min,
  combines an electrostatic analyzer with post-acceleration, followed
  by a time-of-flight and energy measurement. The measurements made
  by SWICS will have an impact on many areas of solar and heliospheric
  physics, in particular providing essential and unique information on:
  (1) conditions and processes in the region of the corona where the solar
  wind is accelerated; (2) the location of the source regions of the solar
  wind in the corona; (3) coronal heating processes; (4) the extent and
  causes of variations in the composition of the solar atmosphere; (5)
  plasma processes in the solar wind; (6) the acceleration of energetic
  particles in the solar wind; (7) the thermalization and acceleration
  of interstellar ions in the solar wind, and their composition; and (8)
  the composition, charge states, and behavior of the plasma in various
  regions of the Jovian magnetosphere.

---------------------------------------------------------
Title: The effects of velocity shear on the resonance absorption of
MHD surface waves: Cold plasma
Authors: Hollweg, Joseph V.
1991JGR....9613807H    Altcode:
  Magnetohydrodynamic (MHD) surface waves may decay via a process called
  “resonance absorption,” which is a candidate for solar coronal
  heating. Recently, Hollweg, Yang, Cadez, and Gakovic studied the effects
  of velocity shear on the rate of resonance absorption of incompressible
  MHD surface waves. We extend the theory to a compressible but cold
  plasma, which is the case more applicable to the solar corona. We
  find that the rate of resonance absorption can be either increased or
  decreased significantly by velocity shear. We also find that there can
  exist resonances which lead to instability of the surface mode at values
  of the velocity shear below the Kelvin-Helmholtz threshold. However,
  the resonance instability usually occurs when the density ratio
  across the surface is less than about 0.1. The resonant instability
  may therefore not be important in the solar corona, though resonance
  absorption remains an attractive possibility for coronal heating.

---------------------------------------------------------
Title: The effects of velocity shear on the resonance absorption of
    MHD surface waves - Cold plasma
Authors: Yang, Guang; Hollweg, Joseph V.
1991JGR....9613807Y    Altcode:
  Magnetohydrodynamic (MHD) surface waves can decay via a process
  called 'resonance absorption', which is a candidate for solar
  coronal heating. Recently, the effects of velocity shear on the
  rate of resonance absorption of incompressible MHD surface waves
  was studied. The theory is extended to a compressible but cold
  plasma, which is the case more applicable to the solar corona. The
  rate of resonance absorption can be either increased or decreased
  significantly by velocity shear. There can exist resonances which
  lead to instability of the surface mode at values of the velocity
  shear below the Kelvin-Helmholtz threshold. However, the resonance
  instability usually occurs when the density ratio across the surface
  is less than about 0.1. The resonant instability can therefore be
  unimportant in the solar corona, though resonance absorption remains
  an attractive possibility for corona heating.

---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
    Flux Tubes - Part One
Authors: Sakurai, Takashi; Goossens, Marcel; Hollweg, Joseph V.
1991SoPh..133..227S    Altcode:
  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.

---------------------------------------------------------
Title: Resonant Behaviour of Magnetohydrodynamic Waves on Magnetic
    Flux Tubes II. Absorption of Sound Waves by Sunspots
Authors: Sakurai, Takashi; Goossens, Marcel; Hollweg, Joseph V.
1991SoPh..133..247S    Altcode:
  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.

---------------------------------------------------------
Title: Alfvénically-Driven Slow Shocks in the Solar Chromosphere
    and Corona
Authors: Hollweg, J. V.
1991BAAS...23.1037H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfvén Waves
Authors: Hollweg, J. V.
1991mcch.conf..423H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Theoretical studies on the chromosphere, corona, and solar wind
Authors: Hollweg, Joseph V.; Lee, Martin A.
1990unh..rept.....H    Altcode:
  Papers published during the period from Jun. 1980 - Dec. 1990 are
  listed.

---------------------------------------------------------
Title: On WKB expansions for Alfvén waves in the solar wind
Authors: Hollweg, Joseph V.
1990JGR....9514873H    Altcode:
  We reexamine the WKB expansion for “toroidal” Alfvén waves
  in the solar wind, as described by equations (9) of Heinemann and
  Olbert (1980). Our principal conclusions are as follows: (1) The WKB
  expansion used by Belcher (1971) and Hollweg (1973) is nonuniformly
  convergent. (2) Using the method of multiple scales (Nayfeh, 1981),
  we obtain an expansion which is uniform. (3) The uniform expansion
  takes into account the small modification to the Alfvén wave phase
  speed due to spatial gradients of the background. (4) Both the uniform
  and nonuniform expansions reveal that each “normal mode” has both
  Elsässer variable δz<SUP>+</SUP>≠0 and δz<SUP>-</SUP>≠0. Thus
  if δz<SUP>-</SUP> corresponds to the outgoing mode in a homogeneous
  background, an observation of δz<SUP>+</SUP>≠0 does not necessarily
  imply the presence of the inward propagating mode, as is commonly
  assumed. (5) Even at the Alfvén critical point (where V=v<SUB>A</SUB>)
  we find that δz<SUP>+</SUP>≠0. Thus incompressible MHD turbulence,
  which requires both δz<SUP>+</SUP>≠0 and δz<SUP>-</SUP>≠0,
  can proceed at the Alfvén critical point (cf. Roberts, 1989). (6)
  With very few exceptions, the predictions of these calculations do
  not agree with recent observations (Marsch and Tu, 1990) of the power
  spectra of δz<SUP>+</SUP> and δz<SUP>-</SUP> in the solar wind. Thus
  the evolution of Alfvén waves in the solar wind is governed by dynamics
  not included in the Heinemann and Olbert equations.

---------------------------------------------------------
Title: Resonance absorption of propagating fast waves in a cold plasma
Authors: Hollweg, Joseph V.
1990P&SS...38.1017H    Altcode:
  Resonance absorption of MHD surface waves has received considerable
  attention recently, but rather little attention has been paid to the
  absorption of propagating waves impinging on a "surface" in which
  the plasma and magnetic field may change. Here we examine in some
  depth a very simple but instructive problem: the plasma is cold, the
  magnetic field is uniform, and the density in the "surface" varies
  linearly from zero at the left end to some finite value at the right
  end, beyond which the density is constant. We consider two cases:
  (1) the plasma is a vacuum everywhere to the left of the surface, or
  (2) the plasma density jumps to a very large value to the left of the
  surface. Case (1) may correspond to coronal conditions, while case
  (2) may mimic the magnetosphere with the dense region at the left
  corresponding to the plasmasphere. The goals of the paper are to study
  the parametric behavior of the absorption coefficient numerically, and
  to provide several useful analytical approximations. We find that the
  parametric dependence of the absorption is far richer than implied by
  the single curve appearing in Fig. 2 of Kivelson and Southwood (1986,
  J. geophys. Res.91, 4345), although we do recover that curve as a
  limiting case in which the waves are essentially WKB at the right end
  of the "surface" and a dimensionless parameter K<SUP>2</SUP><SUB>x</SUB>
  (defined in Section 5) is moderately large. In case (1) we find that the
  absorption coefficient is always less than about 50 %, but in case (2)
  the absorption can approach 100 %. Thus the boundary condition at the
  left critically affects the results. We also find that the thickness
  of the surface affects the parametric dependence of the absorption
  coefficient. For example, a thin surface yields an absorption
  coefficient scaling as α <SUP>-2</SUP>, where α <SUP>2</SUP>
  [defined in equation (4)] is a measure of the steepness of the density
  ramp. On the other hand, if the surface is thick enough so that the
  waves are essentially WKB as they start down the density ramp, then
  the absorption scales as α <SUP>- 8/3</SUP> [case (1)] or α <SUP>-
  4/3</SUP> [case (2)] for large α <SUP>2</SUP>. Our numerical results
  are pre in a format which reveals the dependence of the absorption on
  the propagation direction of an incident wave. The absorption depends
  on the angle of incidence with respect to the surface, maximizing at
  moderately large angles of incidence [around 70° in case (1)]. The
  absorption depends also on the angle between the magnetic field and
  the plane of incidence [there is a broad maximum around 30° in case
  (1)]. Finally, along the way we offer two other analytical results:
  (1) we show that the mathematical discontinuity in the Poynting
  flux which occurs in the present steady-state analysis is precisely
  equivalent to the rate at which energy is pumped into the resonant
  layer as calculated by Hollweg and Yang (1988, J. geophys. Res.93,
  5423) using a simple harmonic oscillator model; (2) we show that
  a convenient approximation scheme used by us for calculating the
  absorption of propagating waves in another context (Hollweg, 1988,
  Astrophys. J.335, 1005) has a useful domain of validity.

---------------------------------------------------------
Title: Heating of the solar corona.
Authors: Hollweg, J. V.
1990CoPhR..12..205H    Altcode:
  The author reviews a number of models which are currently being
  considered for coronal heating, but he considers also heating of
  the chromosphere. There are basically two types of models, which are
  motivated by a variety of observations. 1. Models which invoke MHD waves
  generated by the convective motions are motivated by observations of
  the ubiquitous presence of Alfvén waves in the solar wind. The solar
  wind provides one example of wave heating. Waves have the advantage
  of being able to heat the chromosphere and photospheric magnetic flux
  tubes on their way to the corona. A problem with wave theories is that
  the waves tend to be reflected by the steep Alfvén speed gradient
  in the chromosphere and transition region. 2. Models which invoke the
  gradual buildup of coronal magnetic energy due to random walks of the
  photospheric flux tubes, and the subsequent release of that energy via
  current sheet formation and reconnection, are supported by observations
  indicating that localized impulsive heating and dynamic events occur
  in the transition region and corona. These models cannot explain the
  chromospheric heating or the coronal heating on open field lines. A
  third possibility, which has not been studied in detail, is that the
  chromospheric and coronal heating is associated with emergence and
  cancellation of magnetic flux.

---------------------------------------------------------
Title: Resonant decay of global MHD modes at “thick” interfaces
Authors: Hollweg, Joseph V.
1990JGR....95.2319H    Altcode:
  A global surface mode can exist on a “surface” which is not a true
  discontinuity, but without sufficient dissipation it is not a normal
  mode and it decays in time via resonance absorption. The decay rate can
  be calculated analytically when the “surface” is thin. The goal of
  this paper is to numerically estimate the decay rate when the surface
  is not thin. We consider a cold plasma in a uniform magnetic field,
  and we take the density to vary linearly across the “surface.” In our
  linearized calculation, the global surface mode is driven in steady
  state by an antenna located in one of the uniform regions external
  to the surface. The frequency is a free parameter, and resonance
  curves are computed numerically without approximations. The widths
  of the resonance curves are used to estimate the free decay times
  of undriven surface modes, via the uncertainty principle, equation
  (1). This procedure agrees with the analytical results for a thin
  surface. The thin surface results are found to break down when kT~0.3,
  where T is the surface thickness and k is the wave number along the
  surface. When the angle between k and B<SUB>0</SUB> exceeds about
  40 degrees, the decay rates show distinct maxima at kT~0.5-1.0. When
  applied to the active solar corona, the decay rates are large enough
  to account for the coronal heating, but is should be kept in mind that
  the role of nonlinearity in resonance absorption is still undetermined.

---------------------------------------------------------
Title: MHD waves on solar magnetic flux tubes - Tutorial review
Authors: Hollweg, Joseph V.
1990GMS....58...23H    Altcode:
  Some of the highly simplified models that have been developed for
  solar magnetic flux tubes, which are intense photospheric-level
  fields confined by external gas pressure but able to vary rapidly with
  height, are presently discussed with emphasis on the torsional Alfven
  mode's propagation, reflection, and non-WKB properties. The 'sausage'
  and 'kink' modes described by the thin flux-tube approximation are
  noted. Attention is also given to the surface waves and resonance
  absorption of X-ray-emitting loops, as well as to the results of
  recent work on the resonant instabilities that occur in the presence
  of bulk flows.

---------------------------------------------------------
Title: Surface Waves in an Incompressible Fluid: Resonant Instability
    Due to Velocity Shear
Authors: Hollweg, Joseph V.; Yang, G.; Cadez, V. M.; Gakovic, B.
1990ApJ...349..335H    Altcode:
  The effects of velocity shear on the resonance absorption of
  incompressible MHD surface waves are studied. It is found that there
  are generally values of the velocity shear for which the surface wave
  decay rate becomes zero. In some cases, the resonance absorption goes
  to zero even for very small velocity shears. It is also found that the
  resonance absorption can be strongly enhanced at other values of the
  velocity shear, so the presence of flows may be generally important
  for determining the effects of resonance absorption, such as might
  occur in the interaction of p-modes with sunspots. Resonances leading
  to instability of the global surface mode can exist, and instability
  can occur for velocity shears significantly below the Kelvin-Helmholtz
  threshold. These instabilities may play a role in the development or
  turbulence in regions of strong velocity shear in the solar wind or
  the earth's magnetosphere.

---------------------------------------------------------
Title: Slow twists of solar magnetic flux tubes and the polar magnetic
    field of the Sun
Authors: Hollweg, Joseph V.; Lee, Martin A.
1989GeoRL..16..919H    Altcode:
  The solar wind model of Weber and Davis (1967) is generalized to
  compute the heliospheric magnetic field resulting from solar rotation
  or a steady axisymmetric twist including a geometrical expansion which
  is more rapid than spherical. The calculated increase in the ratio
  of the toroidal to poloidal field components with heliocentric radial
  distance r clarifies an expression derived recently by Jokipii and Kota
  (1989). Magnetic field components transverse to r do not in general
  grow to dominate the radial component at large r. Our analysis also
  yields expressions for the Poynting flux associated with the steady
  twists. These results are regarded as indicative of the Poynting flux
  associated with very low frequency Alfven waves, and it is shown how
  the Poynting flux and the spatial evolution of the wave amplitude differ
  from the usual WKB result. It is found that the low-frequency Poynting
  flux at the base of a coronal hole can be about 50 percent larger than
  the WKB flux inferred from spectral observations of coronal motions
  (e.g., Hassler et al., 1988).

---------------------------------------------------------
Title: A Rebound Shock Mechanism for Solar Fibrils
Authors: Sterling, Alphonse C.; Hollweg, Joseph V.
1989ApJ...343..985S    Altcode:
  Flows along a rigid solar magnetic flux tube which is horizontal over
  a substantial portion of its length are numerically investigated. A
  single, quasi-impulsive force near the base of the first vertical
  segment drives a series of upward propagating rebound shocks on the
  flux tube. When the horizontal segment is in the corona, the shocks
  raise the transition region onto the horizontal segment and eventually
  onto the coronal vertical segment. The material behind the displaced
  transition region resembles a fibril on the horizontal segment, and a
  short spicule on the second vertical segment. A full-sized spicule does
  not develop. The resulting density of the material on the horizontal
  segment is 10 to the -14th g/cu cm, which is consistent with the
  observed densities in fibrils. When the horizontal segment is in the
  chromosphere, the motions and densities induced on the horizontal
  segment do not resemble those of observed fibrils, and a full-sized
  spicule again does not develop.

---------------------------------------------------------
Title: Resonant Decay of MHD "Surface" Waves on a Thick "Surface"
Authors: Hollweg, J. V.
1989BAAS...21..830H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Surface Waves in an Incompressible Fluid: Resonant Instability
    due to Velocity Shear
Authors: Yang, G.; Hollweg, J.; Cadez, V. M.; Gakovic, B.
1989BAAS...21..831Y    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Coronal heating: theoretical ideas
Authors: Hollweg, J. V.
1989HiA.....8..517H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Resonance Absorption of Solar p-Modes by Sunspots
Authors: Hollweg, Joseph V.
1988ApJ...335.1005H    Altcode:
  Braun, Duvall, and LaBonte have reported recently that the power in
  outgoing p-modes in the vicinity of sunspots is significantly less than
  the incoming power. Here, the author considers the possibility that
  the energy deficit is due to resonance absorption, which occurs when
  the sunspot boundary has a nonzero thickness. A simple planar analysis
  is used to examine the conditions required for resonance absorption,
  and to estimate the absorption coefficient. It is found that resonance
  absorption can be significant under certain circumstances, but that,
  on the whole, it probably cannot explain the substantial loss of p-mode
  power observed.

---------------------------------------------------------
Title: Transition region, corona, and solar wind in coronal holes:
    Some two-fluid models
Authors: Hollweg, Joseph V.; Johnson, Walter
1988JGR....93.9547H    Altcode:
  We consider the possibility that the heating of the corona and the
  heating and acceleration of the solar wind can be described by a
  single process, namely, the turbulent dissipation of solar-generated
  Alfven waves at the Kolmogorov rate. The model assumes that
  T<SUB>e</SUB>=T<SUB>p</SUB> in r&lt;=2 R<SUB>S</SUB> but drops
  electron-proton coupling in r&gt;2 R<SUB>s</SUB>. The dissipated wave
  energy is assumed to heat only the protons. Classical heat conduction
  is used in r&gt;10 R<SUB>S</SUB>, and an electron polytrope is used
  in r&gt;10 R<SUB>S</SUB>. The models have the right qualitative
  features: a steep temperature rise to T&gt;10<SUP>6</SUP> K and
  acceleration to supersonic speeds. But models with base pressures
  n<SUB>e</SUB>T&gt;2×10<SUP>1</SUP><SUP>4</SUP> (cgs) are too
  slow: v(1 AU)&lt;280 kms<SUP>-</SUP><SUP>1</SUP>. Models with v(1
  AU)&gt;400 kms<SUP>-</SUP><SUP>1</SUP> have lower base pressures:
  10<SUP>1</SUP><SUP>4</SUP>&lt;n<SUB>e</SUB>T&lt;2×10<SUP>1</SUP><SUP>4</SUP>
  (cgs). A difficulty with the model is that line-of-sight proton random
  velocities (thermal plus wave) are larger than values deduced from Lyman
  α resonant scattering observations in 1.5&lt;=r/R<SUB>S</SUB>&lt;=4,
  and they do not fall off with r to the extent observed. Large random
  velocities are due in part to a proton temperature peak at r~3-4
  R<SUB>s</SUB>. On the whole, this model seems unsatisfactory, but
  several possible resolutions are discussed.

---------------------------------------------------------
Title: Resonance Absorption of Solar p-Modes by Sunspots
Authors: Hollweg, J. V.
1988BAAS...20..910H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Resonance absorption of compressible magnetohydrodynamic
    waves at thin “surfaces”
Authors: Hollweg, Joseph V.; Yang, G.
1988JGR....93.5423H    Altcode:
  If an MHD surface wave is supported by a true discontinuity, then the
  total pressure fluctuation, δP<SUB>tot</SUB>, is constant across the
  discontinuity. If the discontinuity is replaced by a thin transition
  layer, then δP<SUB>tot</SUB> will be approximately constant across
  the transition layer, with a value approximately the same as the
  value obtained for the case of a true discontinuity. We use this
  approximation to study the behavior of the plasma and fields in the
  transition layer. We regard δP<SUB>tot</SUB> as known, and write
  the relevant equations in forms in which δP<SUB>tot</SUB> appears as
  driving terms. Two resonances appear. The cusp resonance affects the
  density and pressure fluctuations, and the velocity and magnetic field
  components along the background magnetic field, B<SUB>0</SUB>. The
  Alfven resonance affects the velocity and magnetic field components
  normal to B<SUB>0</SUB>. We concentrate on the Alfven resonance, and
  show in a simple way how energy is pumped out of the surface wave into
  thin layers surrounding the resonant field lines. We consider also
  the effects of three types of viscosity on the Alfven resonance. Only
  classical shear viscosity is able to absorb the energy which is pumped
  into the thin resonant layer. In the steady state, the net viscous
  heating is independent of the viscosity coefficient, if the heating
  occurs in a sufficiently thin layer. We suggest that the large velocity
  shears which occur in the vicinity of the resonant field lines can
  lead to Kelvin-Helmholtz instabilities, which can in turn lead to an
  effective eddy viscosity, which we estimate to be large enough in the
  solar corona to distribute heat throughout large portions of coronal
  active region loops. We show also that coronal heating by the Alfven
  resonance is compatible with a variety of coronal data.

---------------------------------------------------------
Title: The Rebound Shock Model for Solar Spicules: Dynamics at
    Long Times
Authors: Sterling, Alphonse C.; Hollweg, Joseph V.
1988ApJ...327..950S    Altcode:
  The spicule model due to Hollweg is extended and developed. The
  dynamics is emphasized here; radiative and ionization losses, heat
  conduction, and nonshock heat input, are not included. In the model,
  a series of rebound shocks results in chromospheric material with
  spicule-like properties below a raised transition region. The shocks
  result from a single quasi-impulsive source in the photosphere. It
  is found that at long times, the model approaches a new hydrostatic
  equilibrium with the transition region remaining raised, and with a
  region of shock-heated chromosphere below it. Attention is given to
  the variation of the properties of the model in response to different
  values for the magnitude and location of the source, and to different
  initial transition region heights. It is concluded that the model
  is capable of generating structures with properties consistent with
  observations of spicules (with the exception of temperature) when only
  the dynamics is considered.

---------------------------------------------------------
Title: A Rebound Shock Model for Solar Fibrils
Authors: Sterling, A. C.; Hollweg, J. V.
1988BAAS...20..690S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Resonance Absorption of Magnetohydrodynamic Surface Waves:
    Viscous Effects
Authors: Hollweg, Joseph V.
1987ApJ...320..875H    Altcode:
  The effects of viscosity on the resonance absorption of incompressible
  MHD surface waves, which occurs when the waves are supported by
  a thin 'transition layer' rather than by a discontinuous surface,
  are considered. The behavior of the plasma and fields inside the
  transition layer is considered, allowing for classical viscosity. An
  inhomogeneous Airy equation for the velocity component along the
  propagative direction is obtained in the vicinity of the resonant
  field line. The viscous stress tensor for a magnetized plasma is
  considered, and a simple algebraic steady state equation is obtained
  for the velocity component along the background magnetic field. The
  net heating rate is evaluated and found to be independent of the
  viscosity coefficient, and to correspond to the surface wave 'decay
  rate' obtained from ideal MHD equations.

---------------------------------------------------------
Title: Small-scale MHD wave processes in the solar atmosphere and
    solar wind.
Authors: Hollweg, Joseph V.
1987ESASP.275..161H    Altcode: 1987sspp.symp..161H
  Solar wind observations suggesting wave-particle interactions via
  ion-cyclotron resonances are reviewed. The required power at high
  frequencies is presumably supplied via a turbulent cascade. Tu's (1987)
  model, which considers a turbulent cascade explicitly, is outlined. In
  the solar atmosphere, resonance absorption is considered. The meanings
  of the cusp and Alfven resonances are discussed, and it is shown
  how energy gets pumped into small scales. It is shown that resonance
  absorption can heat the corona and spicules in a manner consistent
  with observations, if turbulence provides an eddy viscosity.

---------------------------------------------------------
Title: Incompressible Magnetohydrodynamic Surface Waves: Nonlinear
    Aspects
Authors: Hollweg, Joseph V.
1987ApJ...317..918H    Altcode:
  The nonlinear properties of MHD surface waves in the solar atmosphere
  are investigated analytically, assuming that the fluid is incompressible
  and that the waves are confined to a single surface, with semiinfinite
  regions on both sides. The governing equations are derived in detail,
  and qualitative results are presented in a graph. For propagating
  waves, second-order terms in the wave amplitude are found to lead
  to wave steepening at leading or trailing edges, the steepening rate
  becoming very large as the threshold for the linear Kelvin-Helmholtz
  instability is approached. Second-order effects on standing waves
  include crest and trough sharpening (increasing with time), a current
  independent of distance on the surface but decreasing exponentially
  with distance from the surface, and pressure-field fluctuations of
  infinite extent. It is suggested that these effects could account for
  a large fraction of solar-atmosphere heating.

---------------------------------------------------------
Title: Resonance Absorption of Magnetohydrodynamic Surface Waves:
    Physical Discussion
Authors: Hollweg, Joseph V.
1987ApJ...312..880H    Altcode:
  It is shown how the phenomenon of MHD surface wave resonance absorption
  can be described in simple terms, both physically and mathematically,
  by applying the 'thin flux tube equations' to the finite-thickness
  transition layer which supports the surface wave. The thin flux tubes
  support incompressible slow-mode waves that are driven by fluctuations
  in the total pressure which exist due to the presence of the surface
  wave. It is shown that the equations for the slow-mode waves can
  be reduced to a simple equation, equivalent to a driven harmonic
  oscillator. Certain field lines within the transition layer are
  equivalent to a harmonic oscillator driven at resonance, and neighboring
  field lines are effectively driven at resonance as long as a given
  condition is satisfied. Thus, a layer which secularly extracts energy
  from the surface wave develops. The analysis indicates that nonlinear
  effects may destroy the resonance which is crucial to the whole effect.

---------------------------------------------------------
Title: Viscosity and the Chew-Goldberger-Low Equations in the
    Solar Corona
Authors: Hollweg, J. V.
1986ApJ...306..730H    Altcode:
  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.

---------------------------------------------------------
Title: Transition region, corona, and solar wind in coronal holes
Authors: Hollweg, J. V.
1986JGR....91.4111H    Altcode:
  Previous wave-driven solar wind models (Hollweg, 1978) have been
  extended by including a new hypothesis for the nonlinear wave
  dissipation. The hypothesis is that the waves dissipate via a turbulent
  cascade at the rate given by (1) and the waves evolve according to
  (16). A subhypothesis is that the relevant correlation length scales as
  the distance between magnetic field lines. This hypothesis allows us
  to treat the corona and the solar wind on an equal footing; unlike in
  previous wave-driven models, we do not assume that the coronal heating
  takes place below the base of the model. The models exhibit the correct
  qualitative features, viz., a steep temperature rise (the transition
  region) to a maximum coronal temperature in excess of 10<SUP>6</SUP>K,
  and a substantial solar wind mass flux in excess of 3.5×10<SUP>8</SUP>
  cm<SUP>-</SUP><SUP>2</SUP>s<SUP>-</SUP><SUP>1</SUP> at 1 AU. However,
  the model fails in detail. Parameters that yield a high-speed flow
  at 1 AU have base pressures that are too low; parameters that yield
  correct base pressures have low solar wind flow speeds. However,
  the model “comes close.” Thus although we have not shown that the
  initial hypothesis is consistent with available data, we feel that
  there are sufficient uncertainties both in the model and in the data
  to preclude outright rejection of the hypothesis altogether.

---------------------------------------------------------
Title: Nonlinear development of phase-mixed alfvén waves
Authors: Nocera, L.; Priest, E. R.; Hollweg, J. V.
1986GApFD..35..111N    Altcode:
  We derive an equation governing the nonlinear propagation of a
  linearly polarized Alfvén wave in a two-dimensional, anisotropic,
  slightly compressible, highly magnetized, viscous plasma, where
  nonlinearities arise from the interaction of the Alfvén wave with fast
  and slow magnetoacoustic waves. The phase mixing of such a wave has
  been suggested as a mechanism for heating the outer solar atmosphere
  (Heyvaerts and Priest, 1983). We find that cubic wave damping dominates
  shear linear dissipation whenever the Alfvén wave velocity amplitude
  vy exceeds a few times ten metres per second. In the nonlinear regime,
  phase-mixed waves are marginally stable, while non-phase-mixed waves
  of wavenumber ka are damped over a timescale kuRe0|δ vy/vA|-2, Re0
  being the Reynolds number corresponding to the Braginskij viscosity
  coefficient η0 and vA the Alfvén speed. Dissipation is most effective
  where β = (vs/vA)2 ≈ 1, vs being the speed of sound.

---------------------------------------------------------
Title: Alfvenic pulses in the solar atmosphere
Authors: Mariska, J. T.; Hollweg, J. V.
1985ApJ...296..746M    Altcode:
  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.

---------------------------------------------------------
Title: Viscosity in a magnetized plasma: Physical interpretation
Authors: Hollweg, J. V.
1985JGR....90.7620H    Altcode:
  We provide a physically motivated alternate derivation of the
  ɛ<SUB>0</SUB> terms in the viscous stress tensor given by Braginskii
  (1965) and others. We show that the ɛ<SUB>0</SUB> terms are fully
  accounted for by a gyrotropic diagonal pressure tensor; off-diagonal
  terms do not contribute to the ɛ<SUB>0</SUB> part of the viscous
  stress. The ɛ<SUB>0</SUB> part of the viscous stress results from
  the plasma's tendency to develop small thermal anisotropies as it
  evolves. Collisions oppose the production of anisotropy and ultimately
  lead to irreversible heating.

---------------------------------------------------------
Title: The Transition Region, Corona, and Solar Wind in a Coronal Hole
Authors: Hollweg, J. V.; Pollock, C. J.
1985BAAS...17..637H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Spicule Dynamics: Long Time Behavior
Authors: Sterling, A. C.; Hollweg, J. V.
1985BAAS...17Q.631S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfvénic Pulses in the Solar Atmosphere
Authors: Mariska, J. T.; Hollweg, J. V.
1985BAAS...17..643M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfvénic heating of the chromosphere and corona.
Authors: Hollweg, J. V.
1985cdm..proc..235H    Altcode:
  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.

---------------------------------------------------------
Title: Energy and Momentum Transport by Waves in the Solar Atmosphere
Authors: Hollweg, J. V.
1985aspp.conf...77H    Altcode:
  The author explores the point of view that the heating and acceleration
  of the solar wind, corona, and chromosphere may all be due to the
  same physical processes. Solar wind observations are used as a guide,
  suggesting that Alfvén or Alfvénic surface waves may be the prime
  candidates for energy and momentum flux, and that turbulence may be
  the principal dissipation mechanism.

---------------------------------------------------------
Title: Surface solitary waves and solitons
Authors: Hollweg, J. V.; Roberts, B.
1984JGR....89.9703H    Altcode:
  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.

---------------------------------------------------------
Title: Alfvenic resonances on solar spicules
Authors: Sterling, A. C.; Hollweg, J. V.
1984ApJ...285..843S    Altcode:
  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.

---------------------------------------------------------
Title: Resonant heating - an interpretation of coronal loop data
Authors: Hollweg, J. V.; Sterling, A. C.
1984ApJ...282L..31H    Altcode:
  The authors show that the resonant heating theory of Hollweg can be
  used to organize the coronal loop data of Golub et al. When combined
  with a reasonable form for the input power spectrum, the resonant
  heating theory is fully compatible with the loop data.

---------------------------------------------------------
Title: Alfvénic resonant cavities in the solar atmosphere: Simple
    aspects
Authors: Hollweg, J. V.
1984SoPh...91..269H    Altcode:
  We investigate the propagation of Alfvén waves in a simple medium
  consisting of three uniform layers; each layer is characterized by a
  different value for the Alfvén speed, υ<SUB>A</SUB>. We show how
  the central layer can act as a resonant cavity under quite general
  conditions. If the cavity is driven externally, by an incident wave
  in one of the outer layers, there result resonant transmission peaks,
  which allow large energy fluxes to enter the cavity from outside. The
  transmission peaks result from the destructive interference between a
  wave which leaks out of the cavity, and a directly reflected wave. We
  show that there are two types of resonances. The first type occurs when
  the cavity has the largest (or smallest) of the three Alfvén speeds;
  this situation occurs on coronal loops. The second type occurs when the
  cavity Alfvén speed is intermediate between the other two values of
  υ<SUB>A</SUB>; this situation may occur on solar spicules. Significant
  heating of the cavity can occur if the waves are damped. We show that
  if the energy lost to heat greatly exceeds the energy lost by leakage
  out of the cavity, then the cavity heating can be independent of the
  damping rate. This conclusion is shown to apply to coronal resonances
  and to the spicule resonances. This conclusion agrees with a point
  made by Ionson (1982) in connection with the coronal resonances. Except
  for a numerical factor of order unity, we recover Ionson's expression
  for the coronal heating rate. However, Ionson's qualities are much
  too large. For solar parameters, the maximum quality is of the order
  of 100, but the heating is independent of the damping rate only when
  dissipation reduces the quality to less than about 10.

---------------------------------------------------------
Title: Coronal Loop Heating: Theory and Data
Authors: Hollweg, J. V.; Sterling, A. C.
1984BAAS...16..527H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfvenic Heating: An Interpretation of Coronal Loop Data
Authors: Sterling, A. C.; Hollweg, J. V.
1984BAAS...16Q.527S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Resonances of coronal loops
Authors: Hollweg, J. V.
1984ApJ...277..392H    Altcode:
  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.

---------------------------------------------------------
Title: MHD waves and turbulence in the sun and interplanetary medium.
Authors: Barnes, A.; Goldstein, M.; Hollweg, J.; Mariska, J.;
   Matthaeus, W.; Smith, C.; Smith, E.; Stein, R.; Withbroe, G.; Woo, R.
1984NASRP1120....4B    Altcode:
  Contents: Introduction. Global oscillations of the sun. Observations
  related to waves or turbulence in the solar atmosphere. Local waves
  in the solar atmosphere: theoretical considerations. Interplanetary
  hydromagnetic fluctuations. Recent studies of the interplanetary plasma
  based on turbulence theory. Effects of waves and turbulence of the
  solar wind.

---------------------------------------------------------
Title: Surface waves on solar wind TD's.
Authors: Hollweg, J. V.
1983NASCP2280..105H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Impulse response of the corona.
Authors: Suess, S. T.; Hollweg, J. V.
1983NASCP2280...61S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Coronal heating by waves.
Authors: Hollweg, J. V.
1983NASCP.2280...5H    Altcode: 1983sowi.conf....5H
  The author shows that Alfvén waves or Alfvénic surface waves can
  carry enough energy into the corona to provide the coronal energy
  requirements. Coronal loop resonances are an appealing means by which
  large energy fluxes can 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.

---------------------------------------------------------
Title: Reply
Authors: Hollweg, Joseph V.; Isenberg, Philip A.
1983JGR....88.7253H    Altcode:
  Alfven waves and rotational forces in the solar wind are commented upon.

---------------------------------------------------------
Title: Resonances of Solar Spicules
Authors: Hollweg, J. V.; Sterling, A. C.
1983BAAS...15R.994H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the preferential acceleration and heating of solar wind
    heavy ions
Authors: Isenberg, P. A.; Hollweg, J. V.
1983JGR....88.3923I    Altcode:
  We investigate the preferential heating and acceleration of
  solar wind heavy ions by the resonant cyclotron interaction with
  parallel-propagating left-polarized hydromagnetic waves. We set up
  a scenario whereby the energy for this interaction is taken from
  saturated low-frequency Alfven waves via a cascade to the higher,
  resonant frequencies. In order to utilize the existing theoretical
  work, the particles are taken to be thermally isotropic, and the waves
  are taken to be dispersionless. This scenario is incorporated into
  a numerical solar wind code describing the flow from an inner radius
  (taken to be 10 solar radii) to 1 AU. Thus we present the first model
  of a wave-driven, three-fluid, supersonic solar wind. By varying the
  model parameters we test the ability of the resonant interaction in
  this model to produce the excess speeds and temperatures of heavy ions
  that are observed. We find that unrealistically steep wave spectra
  are required to produce differential speeds of the order of the 4.7,
  where γ is the power law spectral index. Ions of oxygen or iron,
  with larger mass-per-charge ratios, are accelerated more readily than
  helium, but still require steeper spectra than are observed. This model
  is also unable to produce the mass-proportional heavy ion temperatures
  that are observed. We show that the production of mass-proportional
  temperatures is inconsistent with preferential acceleration of heavy
  ions by this mechanism. The model also produces a heavy ion-to-proton
  temperature ratio at 1 AU which is anticorrelated with solar wind speed,
  in contradiction to the observed behavior.

---------------------------------------------------------
Title: Collisional damping of surface waves in the solar corona
Authors: Gordon, B. E.; Hollweg, J. V.
1983ApJ...266..373G    Altcode:
  It has been suggested that surface waves may be able to heat the
  solar corona. These waves can propagate into the corona and supply the
  required energies, and because they are linearly compressive they can
  be dissipated by ion viscosity and electron heat conduction. In this
  paper the authors evaluate the damping of surface waves by viscosity and
  heat conduction. It is found that surface waves dissipate efficiently
  only if their periods are shorter than a few tens of seconds and only
  if the background magnetic field is less than about 10 gauss. Heating
  of quiet coronal regions is possible if the coronal waves have short
  periods, but they cannot heat regions of strong magnetic field, such
  as coronal active region loops.

---------------------------------------------------------
Title: Solar wind five. Proceedings of a conference held in Woodstock,
    Vermont, November 1 - 5, 1982.
Authors: Neugebauer, M.; Hollweg, J. V.; Barnes, A.; MacQueen, R.;
   Rosner, R.; Eddy, J. A.
1983swfp.book.....N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Surface Waves on solar wind tangential discontinuities
Authors: Hollweg, J. V.
1982JGR....87.8065H    Altcode:
  We demonstrate that (tangential) discontinuities in the magnetic field
  direction can support MHD surface waves. The surface waves resemble
  the usual Alfvén wave, but there are some important differences: (1)
  The surface waves exhibit a low-frequency cutoff. (2) The velocity and
  magnetic field fluctuations are elliptically, and sometimes circularly,
  polarized. They may account for the solar wind helicity spectrum. (3)
  The surface waves are compressive, but there are special cases where
  they are noncompressive. (4) The wave vector k, the local normals to the
  surfaces of constant phase, and the magnetic minimum variance direction
  (mvd) do not all coincide. (5) There is a tendency for the mvd to align
  itself with the mean magnetic field direction. (6) The waves can be
  intrinsically nonplanar. (7) Equipartition between magnetic and kinetic
  energies is not obeyed locally. These properties of the surface waves
  lead us to believe that surface waves may be common in the solar wind.

---------------------------------------------------------
Title: Resonances of Coronal Loops
Authors: Hollweg, J. V.
1982BAAS...14..977H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Finite amplitude Alfvén waves in a multi-ion plasma:
    Propagation, acceleration, and heating
Authors: Isenberg, P. A.; Hollweg, J. V.
1982JGR....87.5023I    Altcode:
  We derive an expression for the wave action flux of finite amplitude
  Alfvén waves in a multi-ion plasma. The expression is valid in
  the presence of dissipative forces and allows an arbitrary angle
  between the average mangetic field and the wave vector. Aplying the
  conservation of wave action and the first law of thermodynamics yields
  the following results for a multi-ion plasma: (1) an expression for
  the spatial evolution of Alfvén wave amplitude in the absence of
  dissipation. (2) the relationship between the wave amplitude and the
  dissipative heating, and (3) an expression for the acceleration of an
  ion species by finite amplitude Alfvén waves. The acceleration consists
  of two terms: 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. These
  results are discussed in the context of the observations of heavy ions
  in the solar wind.

---------------------------------------------------------
Title: On the preferential acceleration and heating of solar wind
    heavy ions
Authors: Isenberg, P. A.; Hollweg, J. V.
1982STIN...8230212I    Altcode:
  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.

---------------------------------------------------------
Title: Collisional damping of surface waves in the solar corona
Authors: Gordon, B. E.; Hollweg, J. V.
1982STIN...8230213G    Altcode:
  The damping of surface waves by viscosity and heat conduction is
  evaluated. For the solar corona, it is found that surface waves
  dissipate efficiently only if their periods are shorter than a few
  tens of seconds and only if the background magnetic field is less
  than about 10 Gauss. Heating of quiet coronal regions is possible if
  the coronal waves have short periods, but they cannot heat regions of
  strong magnetic field, such as coronal active region loops.

---------------------------------------------------------
Title: On the origin of solar spicules
Authors: Hollweg, J. V.
1982ApJ...257..345H    Altcode:
  The nonlinear evolution of vertical motions on intense solar
  magnetic flux tubes is considered. It is shown that a quasi-impulsive
  source in the photosphere can excite a train of upward-propagating
  rebound shocks in the chromosphere. The rebound shock train is the
  nonlinear development of oscillations of the atmosphere at its natural
  frequency. The rebound shocks impinge on the transition region and
  thrust the underlying chromosphere upward. It is found that the rebound
  shock train leads naturally to structures which can be identified with
  the solar spicules.

---------------------------------------------------------
Title: Heating of the corona and solar wind by switch-on shocks
Authors: Hollweg, J. V.
1982ApJ...254..806H    Altcode:
  The possibility is examined that the corona is heated by a train of
  weak switch-on shocks which are formed in the chromosphere from a
  train of Alfven waves, and which subsequently enter the corona from
  below. New results for the shock train propagation and dissipation
  and the resultant coronal heating are derived. It is shown that most
  of the energy in the shock train can be dissipated within one or
  two solar radii above the coronal base. A train of switch-on shocks
  therefore represents a viable coronal heating mechanism. The results are
  generalized to switch-on shocks in the solar wind. It is shown that such
  shocks can dissipate rapidly, but it is concluded that they are not the
  dominant factor governing the evolution of the solar wind turbulence.

---------------------------------------------------------
Title: Possible evidence for coronal Alfvén waves
Authors: Hollweg, J. V.; Bird, M. K.; Volland, H.; Edenhofer, P.;
   Stelzried, C. T.; Seidel, B. L.
1982JGR....87....1H    Altcode:
  The 2.29 GHz S band carrier signals of the two Helios spacecraft are
  used to probe the magnetic and density structures of the solar corona
  inside 0.05 AU. In this paper we analyze the observed fluctuations of
  the electron content and Faraday rotation. A simple statistical ray
  analysis is employed. We conclude that (1) the observed Faraday rotation
  fluctuations cannot be solely due to electron density fluctuations in
  the corona unless the coronal magnetic field is some 5 times stronger
  than suggested by current estimates, and (2) the observed Faraday
  rotation fluctuations are consistent with the hypothesis that the sun
  radiates Alfvén waves with sufficient energies to heat and accelerate
  high-speed solar wind streams.

---------------------------------------------------------
Title: A new resonance in the solar atmosphere
Authors: Hollweg, J. V.
1982SoPh...75...79H    Altcode:
  It is shown that the solar atmosphere resonance between fast MHD and
  gravito-acoustic waves reported by Hollweg (1979) is spurious, in the
  absence of genuine forcing terms. Without such external forcing terms,
  the equations can only yield a dispersion relation from which it is
  possible to determine the natural modes of the system. The discussion
  presented is confined to the isothermal case, although the arguments
  are valid for the adiabatic case as well.

---------------------------------------------------------
Title: Alfven Waves in the Solar Atmospheres - Part Three - Nonlinear
    Waves on Open Flux Tubes
Authors: Hollweg, J. V.; Jackson, S.; Galloway, D.
1982SoPh...75...35H    Altcode:
  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<SUB>o</SUB>. The pulses typically have amplitudes of 60
  km s<SUP>−1</SUP> 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.

---------------------------------------------------------
Title: On rotational forces in the solar wind
Authors: Hollweg, J. V.; Isenberg, P. A.
1981JGR....8611463H    Altcode:
  We present a new and simpler derivation of the rotational forces on
  minor ions in the solar wind. We show that the rotational forces can be
  interpreted as potential forces affecting all particles equally. As
  such, they do not invole interactions between different particle
  species and do not represent an equilibrating process.

---------------------------------------------------------
Title: Bound oscillations on thin magnetic flux tubes - Convective
    instability and umbral oscillations
Authors: Hollweg, J. V.; Roberts, B.
1981ApJ...250..398H    Altcode:
  The possibility that 'tube waves' can be trapped on slender solar
  magnetic flux tubes is investigated. For rigid isothermal flux tubes, it
  is found that the flux tube geometry can by itself lead to waves which
  are trapped on the part of the tube that expands with height. Some
  geometries lead to trapped modes with eigenperiods near 180 s, if
  parameters appropriate to sunspot umbrae are chosen. It is possible
  that the umbral oscillations are a manifestation of such trapped waves,
  if sunspot umbrae consist of an assembly of slender flux tubes, as
  in the spaghetti model of Parker (1979). For flux tubes which have a
  constant ratio of Alfven speed to sound speed, it is found that it is
  primarily the variation of temperature with height which determines
  whether trapped waves can exist. Certain temperature profiles lead to
  disturbances for which omega squared is less than zero, corresponding
  to convective instability or Rayleigh-Taylor instability.

---------------------------------------------------------
Title: Minor ions in the low corona
Authors: Hollweg, J. V.
1981JGR....86.8899H    Altcode:
  The ability of Coulomb friction to drag minor ions out of the
  subsonic region of the low corona is examined analytically. With some
  assumptions, we obtain new analytical expressions for the 'minimum
  proton flux' that is required to drag minor ions out of the corona
  and for the velocity, relative to the protons, at which the minor ions
  are dragged out. We use these new results to suggest that the positive
  n<SUB>α</SUB>/n<SUB>p</SUB> versus υ<SUB>p</SUB> correlation and the
  negative n<SUB>α</SUB>/n<SUB>p</SUB> versus n<SUB>p</SUB>υ<SUB>p</SUB>
  correlation observed for solar wind helium at 1 AU are due to variations
  in the temperature of the low corona: hotter coronal regions produce
  solar wind flows that are slower but that have a greater proton
  flux density relative to cooler coronal regions, but because of the
  temperature dependence of the Coulomb friction the hotter coronal
  regions result in smaller values of n<SUB>α</SUB>/n<SUB>p</SUB>
  at 1 AU. We also suggest that the charge dependence of the Coulomb
  friction may lead to errors in determining the coronal temperature
  from measurements of the ionization state at 1 AU.

---------------------------------------------------------
Title: The energy balance of the solar wind.
Authors: Hollweg, J. V.
1981NASSP.450..355H    Altcode: 1981suas.nasa..355H
  The effects of modifying some of the 'classical' assumptions underlying
  many of the solar wind models constructed over the past 20 years are
  examined in an effort to obtain both a better fit with the observations
  and a deeper understanding of the relevant physical processes.

---------------------------------------------------------
Title: Solar Phenomena. (Views of a Watershed: Solar and
    Interplanetary Dynamics)
Authors: Hollweg, Joseph V.
1981Sci...212..787D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the Origin of Solar Spicules
Authors: Hollweg, J. V.
1981BAAS...13..914H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfven Waves in the Solar Atmosphere - Part Two - Open and
    Closed Magnetic Flux Tubes
Authors: Hollweg, J. V.
1981SoPh...70...25H    Altcode:
  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.

---------------------------------------------------------
Title: The physical interpretation of Alfven wave flux in the
    solar corona
Authors: Hollweg, J. V.
1981MNRAS.194..381H    Altcode:
  A recent analysis of coronal hole acceleration by McWhirter and Kopp
  (1979) is examined. It is shown that the approach adopted by these
  authors is invalid on physical grounds.

---------------------------------------------------------
Title: Mechanisms of energy supply.
Authors: Hollweg, J. V.
1981sars.work..277H    Altcode:
  Current ideas on the physical mechanisms responsible for the energy
  supply of solar active regions are summarized. Taking into account the
  existence of waves, loops and structure, large-scale reconnection,
  cool cores, flows and the violation of hydrostatic equilibrium and
  steady-state heating, as well as the global nature of the problem,
  the means by which energy rises from the convection zone, processes
  of energy dissipations and possible observational consequences
  are examined in relation to various currently tractable physical
  processes. The processes include the heating of coronal active
  region loops by anomalous dissipation of parallel currents in thin
  sheaths or minireconnections, wave processes in active region loops,
  coronal heating and loop formation influenced by refraction and
  Landau/transit-time damping of fast-mode MHD waves, and energy
  transfer by means of propagating twists (Alfven waves) on open and
  closed magnetic flux tubes.

---------------------------------------------------------
Title: Ion-cyclotron heating and acceleration of solar wind minor ions
Authors: Dusenbery, P. B.; Hollweg, J. V.
1981JGR....86..153D    Altcode:
  The resonant acceleration and heating of solar wind minor ions via
  interactions with a spectrum of dispersive or nondispersive undamped
  ion-cyclotron waves are investigated. The principal goal is to determine
  how the dimensionless parameters of the problem affect the heating and
  acceleration processes. In addition, simple physical interpretations of
  the heating and acceleration are given. The exact numerical evaluations
  of the heating and acceleration rates imply that (1) the total heating
  rate is roughly proportional to the mass of the ion; (2) dispersive
  waves should lead to n<SUB>i</SUB>&gt;n<SUB>a</SUB>&gt;n<SUB>p,</SUB>
  where i refers to ions heavier than He<SUP>++</SUP>;
  (3) dispersive waves have a slight tendency to yield
  ∂T<SUB>i</SUB>/∂t&gt;(m<SUB>i</SUB>/m<SUB>a</SUB>)∂T<SUB>a</SUB>/∂t,
  while non-dispersive waves have a slight tendency to yield
  ∂T<SUB>i</SUB>/∂t&lt;(m<SUB>i</SUB>/m<SUB>a</SUB>)∂T<SUB>a</SUB>/∂t.
  The predictions of this model are compared with observations and may
  offer an explanation for some observed properties of minor ions.

---------------------------------------------------------
Title: Helium and Heavy Ions
Authors: Hollweg, J. V.
1981sowi.conf..414H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfvén waves in sunspots
Authors: Nye, A. H.; Hollweg, J. V.
1980SoPh...68..279N    Altcode:
  The propagation of Alfvén waves in a simple model of a sunspot is
  considered. The vertical structure near the center of the umbra
  is modelled realistically, but the horizontal structure is not
  considered. The full wave equation is solved, without recourse to
  the WKB approximation. Only wave propagation in the vicinity of the
  central field line in an axially symmetric spot is examined, and it is
  assumed that this field line is open. By taking wave reflections into
  account, we find that the observations of non-thermal motions near the
  temperature minimum (Beckers, 1976) and in the corona (Beckers and
  Schneeberger, 1977) are both consistent with an upward-propagating
  Alfvénic energy flux density of a few times 10<SUP>7</SUP> erg
  cm<SUP>−2</SUP> s<SUP>−1</SUP>. This flux density is too small
  to cool the sunspot, but it is large enough to supply the energy
  requirements of the transition region and corona above a sunspot. This
  conclusion depends on the assumptions that the observed motions are
  indeed Alfvénic with periods near 180 s.

---------------------------------------------------------
Title: Spicules and Coronal Heating: A Unified View
Authors: Hollweg, J. V.
1980BAAS...12..909H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Indirekte Beobachtungen magnetohydrodynamischer
    Wellenaktivität in der Sonnenkorona
Authors: Edenhofer, P.; Bird, M. K.; Volland, H.; Hollweg, J. V.
1980MitAG..50...42E    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A new resonance in the solar atmosphere. I. Theory.
Authors: Hollweg, J. V.
1979SoPh...62..227H    Altcode:
  We consider a horizontally stratified isothermal model of the
  solar atmosphere, with vertical and uniform B<SUB>0</SUB>, and
  v<SUB>A</SUB><SUP>2</SUP>≫v<SUB>s</SUB><SUP>2</SUP>. 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<SUB>0</SUB>; 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.

---------------------------------------------------------
Title: Reply
Authors: Hollweg, Joseph V.; Turner, M.
1979JGR....84.2141H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Alfven Waves on Open and Closed Solar Magnetic Flux Tubes
Authors: Hollweg, J. V.
1979BAAS...11..409H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Some physical processes in the solar wind.
Authors: Hollweg, J. V.
1978RvGSP..16..689H    Altcode: 1978RvGeo..16..689H
  The 'standard physics' of the solar wind is reviewed, and arguments
  that this standard physics is inadequate are summarized. A variety
  of suggestions for modifying the physics of the solar wind are then
  reviewed, with emphasis on effects of MHD waves of solar origin and
  collisionless or instability-limited electron heat conduction. The basic
  effects of the modified physics are demonstrated in a two-fluid model
  of the solar wind flow. The predictions of this model are carefully
  compared with observations, and the need for further observations
  is emphasized. The review concludes with suggestions for future
  theoretical efforts.

---------------------------------------------------------
Title: Fast wave evanescence in the solar corona
Authors: Hollweg, J. V.
1978GeoRL...5..731H    Altcode:
  Using a horizontally stratified model of the solar atmosphere, we
  argue that appreciable fast wave fluxes of solar origin may not be
  able to propagate into the solar wind. Fast waves may thus not be a
  significant source of extended heating and acceleration of the solar
  wind. This conclusion could be altered by nonlinearity and/or by
  complex structure of the solar atmosphere, however.

---------------------------------------------------------
Title: Correction
Authors: Hollweg, Joseph V.
1978JGR....83.3905H    Altcode:
  JGREA,82,563,1978

---------------------------------------------------------
Title: Geometrical MHD wave coupling
Authors: Hollweg, J. V.; Lilliequist, C. G.
1978JGR....83.2030H    Altcode:
  Refraction and/or magnetic field curvature can lead to 'geometrical
  mode coupling' between the MHD modes. The effect is linear, but it is a
  finite-wavelength effect. It is studied here for a simple configuration
  which is amenable to analysis and which illustrates the basic features
  of the coupling. In the solar wind the geometrical coupling may be
  operative only in the solar corona or in the interaction regions of
  high-speed streams. In the later case the geometrical coupling may
  provide an explanation for the non-Alfvénic fluctuations but only for
  long-period waves (greater than several hours) in small interaction
  regions (&lt;0.1AU).

---------------------------------------------------------
Title: Experimental Search for Coronal Alfven Waves.
Authors: Querfeld, C. W.; Hollweg, J. V.
1978BAAS...10..431Q    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A quasi-linear WKB kinetic theory for nonplanar waves in a
    nonhomogeneous warm plasma 1. Transverse waves propagating along
    axisymmetric B<SUB>0</SUB>
Authors: Hollweg, J. V.
1978JGR....83..563H    Altcode:
  A new set of quasi-linear kinetic equations is presented for transverse
  waves propagating along an axisymmetric magnetic field configuration. A
  WKB expansion is used to include effects of nonhomogeneity and
  nonplanarity of the waves. The equations allow simultaneous calculation
  of the spatial (and temporal) evolution of wave power spectra and the
  spatial (and temporal) evolution of particle distribution functions,
  including for the first time important wave-particle interactions
  which depend explicitly on the nonplanarity of the waves and the
  nonhomogeneity of the plasma and fields. The usefulness of the
  equations is demonstrated for a cold plasma, where a number of new
  results for wave propagation and acceleration of the plasma have been
  obtained. Along the way, a new quasi-linear separation of the average
  distribution function has been introduced, the usefulness of which
  is demonstrated by comparison with fluid theory. The principal new
  results are briefly summarized in section 6 of the paper.

---------------------------------------------------------
Title: Alfvén waves in the solar atmosphere.
Authors: Hollweg, J. V.
1978SoPh...56..305H    Altcode:
  We examine the propagation of Alfvén waves in the solar atmosphere. The
  principal theoretical virtues of this work are: (i) The full wave
  equation is solved without recourse to the small-wavelength eikonal
  approximation (ii) The background solar atmosphere is realistic,
  consisting of an HSRA/VAL representation of the photosphere and
  chromosphere, a 200 km thick transition region, a model for the upper
  transition region below a coronal hole (provided by R. Munro), and the
  Munro-Jackson model of a polar coronal hole. The principal results are:
  If the wave source is taken to be near the top of the convection zone,
  where n<SUB>H</SUB> = 5.2 × 10<SUP>16</SUP> cm<SUP>−3</SUP>, and if
  B<SUB>⊙</SUB> = 10.5 G, then the wave Poynting flux exhibits a series
  of strong resonant peaks at periods downwards from 1.6 hr. The resonant
  frequencies are in the ratios of the zeroes of J<SUB>0</SUB>, but depend
  on B<SUB>⊙</SUB>, and on the density and scale height at the wave
  source. The longest period peaks may be the most important, because
  they are nearest to the supergranular periods and to the observed
  periods near 1 AU, and because they are the broadest in frequency.

---------------------------------------------------------
Title: Acceleration of solar wind He<SUP>++</SUP> 3. Effects of
    resonant and nonresonant interactions with transverse waves
Authors: Hollweg, J. V.; Turner, J. M.
1978JGR....83...97H    Altcode:
  Simple models are described which investigate the combined effects on
  solar wind He<SUP>++</SUP> of resonant and nonresonant acceleration by
  left-hand transverse waves. The principal points are the following. (1)
  For a wide range of parameters (ν<SUB>α</SUB>-ν<SUB>ρ</SUB>) at 1 AU
  is close to the effective phase speed of the left-hand waves. (2) The
  most important factor in determining ν<SUB>α</SUB>/ν<SUB>ρ</SUB>
  at 1 AU is close is whether the high-frequency left-hand waves
  are predominantly outward propagating, inward propagating, inward
  propagating, or a mix of both. The resonant acceleration may be more
  important than the effects of heating or stream-stream interactions. (3)
  Reasonable values of ν<SUB>α</SUB>/ν<SUB>ρ</SUB> at the sun
  (and of n<SUB>a</SUB>/n<SUB>ρ</SUB> at 1 AU) are obtained for a
  power law index α<SUP>?</SUP>1.5 in the wave power spectrum if the
  effective phase speed of the resonant waves near the sun is not small
  in comparison to the Alfvén speed there. This requires a substantial
  level of high-frequency power in outward going waves at the sun, which
  cannot come from heat-conduction-driven instabilities. (4) The present
  models do not allow one to decide whether the coronal He<SUP>++</SUP>
  abundance is greater or less than that at 1 AU. (5) Some of the models
  show a positive correlation between n<SUB>α</SUB>/n<SUB>ρ</SUB> and
  ν<SUB>ρ</SUB> at 1 AU, roughly as has been observed. (6) The models
  suggest that variations in n<SUB>α</SUB>/n<SUB>ρ</SUB> at 1 AU can
  result from variations in the wave properties near the sun and not
  neccessarily from variations in the coronal abundance. (7) Some models
  indicate that (ν<SUB>α</SUB>/ν<SUB>ρ</SUB>) may decrease with
  increasing r in the vicinity of 1 AU. (8) The resonant acceleration
  is more efficient than Coulomb friction in the sense that it does not
  exhibit a runaway effect. (9) Observations of minor species may be
  used to deduce wave properties and plasma processes in the solar wind.

---------------------------------------------------------
Title: Low-frequency instabilities of a warm plasma in a magnetic
field: Part 1. Instabilities driven by field-aligned currents
Authors: Smith, D. F.; Hollweg, J. V.
1977JPlPh..17..105S    Altcode:
  The marginal stability of a plasma carrying current along the
  static magnetic field with isotropic Maxwellian ions and isotropic
  Maxwellian electrons drifting relative to the ions is investigated. The
  complete electromagnetic dispersion relation is studied using numerical
  techniques; the electron sums are restricted to three terms which limits
  the analysis to frequencies much less than the electron gyro-frequency,
  but includes frequencies somewhat above the ion gyro-frequency. A
  ‘kink-like’ instability and an instability of the Alfvén mode
  are found to have the lowest threshold drift velocities in most
  cases. In fact the threshold drift for the kink-like instability can
  be significantly less than the ion thermal speed. Electrostatic and
  electromagnetic ion-cyclotron instabilities are also found as well
  as the electro-static ion-acoustic instability. No instability of the
  fast magnetosonic mode was found. The stability analysis provides only
  threshold drift velocities and gives no information about growth rates.

---------------------------------------------------------
Title: Fokker-Planck theory for cosmic ray diffusion in the presence
    of Alfvén waves 2. Model stream calculation
Authors: Skadron, G.; Hollweg, J. V.
1976JGR....81.5887S    Altcode:
  We investigate the cosmic ray radial diffusion coefficient resulting
  from linearly polarized Alfvén waves propagating outward through an
  azimuthally structured solar wind. The analysis utilizes a diffusion
  coefficient derived from quasi-linear Fokker-Planck theory and a model
  solar wind stream in which the solar wind velocity varies linearly
  with azimuth. It is found that beyond 1 AU the stream significantly
  reduces the diffusion coefficient below that for a spherically symmetric
  solar wind. The diffusion coefficient is also found to reach a minumum
  value at a heliucentric distance of approximately 75 R<SUB>s</SUB>,
  and this minimum moves outward with increasing steepness of the wave
  spectrum. The diffusion coefficient is a separable function of radius
  and rigidity below approximately 0.5 GV, but at higher rigidities it
  is found that the separability fails. Finally, it is concluded that
  the present diffusion theory is consistent with a cosmic ray gradient
  which decreases slowly with r and has a mean value, between 1 and 5 AU ,
  of about 3%/AU for 1-GeV galactic protons.

---------------------------------------------------------
Title: Collisionless electron heat conduction in the solar wind
Authors: Hollweg, J. V.
1976JGR....81.1649H    Altcode:
  The point of view that heat-conduction-driven plasma instabilities
  may not be capable of directly modifying the electron heat
  conduction flux in the solar wind is explored. The electron heat
  conduction flux is written either as the usual collision-dominated
  Spitzer-Härm flux, -K<SUB>SH</SUB>▽<SUB>∥</SUB>T<SUB>e</SUB>,
  or as the collisionless heat conduction flux (Hollweg, 1974a),
  1.5n<SUB>e</SUB>kT<SUB>e</SUB>(V<SUB>sw</SUB> - ω × r)α. The
  factor α is of order unity but is only estimated. The former
  expression pertains close to the sun and far from the sun, where
  collisions are important, while the latter expression pertains in
  the intermediate region; the divisions between regions are taken
  to occur where the radial component of the mean free path equals
  the radial trapping distance, which is taken to be r/2. Perkins'
  (1973) term is omitted for three reasons: it is often smaller than
  the collisionless heat conduction flux; it can be reduced by plasma
  instabilities; and it is not observed by in situ measurements at 1
  AU. The electron-proton coupling term is also omitted; this means
  that the electron temperature is overestimated and that the proton
  temperature must be specified ad hoc. In comparison with solar wind
  models which use the Spitzer-Härm flux throughout, the present
  computations yield the following new features: (1) The electron
  temperature is elevated in the collision-dominated region close to
  the sun. (2) The electron temperature falls off more rapidly in the
  region where the collisionless heat conduction flux is used, varying
  as n<SUB>e</SUB><SUP>2/3(1+α)</SUP> if α = const. (3) The electron
  temperature and heat conduction flux at 1 AU are lower in the present
  computations than in models which use only the Spitzer-Härm flux. (4)
  The elevated electron temperature close to the sun results in higher
  solar wind flow speeds at 1 AU.

---------------------------------------------------------
Title: Current-driven Alfvén instability
Authors: Hollweg, J. V.; Smith, D. F.
1976JPlPh..15..245H    Altcode:
  The current-driven Alfvén instability is discussed analytically. The
  instability is driven by the resonant Landau and transit-time terms
  for propagation not parallel to B<SUB>0</SUB>. The critical current
  for instability is slightly less than the corresponding one for
  the ‘k-like’instability thus, the instability discussed in this
  paper may be the more important for the parameter range in which our
  approximations are valid. The effect of current flow on the magnetosonic
  mode is also discussed, but no instability is found.

---------------------------------------------------------
Title: Alfvénic acceleration of solar wind helium 2. Model
    calculations
Authors: Chang, S. C.; Hollweg, J. V.
1976JGR....81.1659C    Altcode:
  A previous paper (Hollweg, 1974a) discussed a new physical mechanism
  for accelerating α particles in the solar wind, via their interaction
  with Alfvén waves. This paper presents numerical calculations of a
  simple three-fluid solar wind model, which incorporates the new physical
  process. The principal result is that the Alfvén waves do effect an
  important additional coupling between protons and α particles, which
  tends to equalize their flow speeds at 1 AU. This is in sharp contrast
  to previous calculations, which gave υ<SUB>α</SUB>/υ<SUB>p</SUB> =
  0.7-0.8 at 1 AU. But the model calculations still contain two important
  problems: (1) we do not obtain υ<SUB>α</SUB> ≥ υ<SUB>p</SUB>
  at 1 AU, as is frequently observed; and (2) the model calculations
  give values of υ<SUB>α</SUB> which are unreasonably low near the
  sun. Thus we conclude that the new physical mechanism is probably
  sufficiently important to warrant its inclusion in future solar wind
  models but also that there must be other physical processes working,
  which still remain to be discovered.

---------------------------------------------------------
Title: The interplanetary plasma and the heliosphere (Plasma
    interplanétaire et de l'héliosphère).
Authors: Axford, W. I.; Hollweg, J. V.; Suess, S. T.; Blum, P. W.;
   Fahr, H. J.
1976IAUTA..16a.175A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Fokker-Planck theory for cosmic ray diffusion in the presence
    of Alfvén waves 1. Theory
Authors: Hollweg, J. V.; Skadron, G.
1975JGR....80.2701H    Altcode:
  Using the standard quasi-linear Fokker-Planck approach and an expansion
  of the cosmic ray distribution function in Legendre polynomials, we
  derive a complete set of Fokker-Planck coefficients and expressions
  for the complete cosmic ray diffusion tensor due to linearly polarized
  planar Alfvén waves propagating at an angle to the average magnetic
  field. We find that although an off-diagonal term formally appears
  in the diffusion tensor, it is identically zero. In addition, the
  cross-field diffusion coefficient is identically the same as that found
  for wave propagation exactly parallel to the average magnetic field.

---------------------------------------------------------
Title: Alfvén wave refraction in high-speed solar wind streams
Authors: Hollweg, J. V.
1975JGR....80..908H    Altcode:
  We use a simple physical theory to calculate the variation of Alfvén
  wave amplitudes and the wave refraction in a schematic model for
  a high-speed solar wind stream. The results are as follows. (1)
  The wave amplitudes &lt;δB²&gt;<SUP>1/2</SUP> are larger in the
  compression region of the stream than in the rarefaction region. (2)
  The relative amplitudes &lt;δB²&gt;<SUP>1/2</SUP>/B<SUB>0</SUB>
  are larger in the rarefaction region than in the compression region,
  this result indicating that nonlinear effects may be more important
  in the rarefaction region. (3) The azimuthal velocity gradient in the
  stream leads to the result that k is no longer nearly radial at 1 AU,
  in contrast to predictions based on a spherically symmetric solar wind
  structure. In the rarefaction region, k turns into the direction of
  B<SUB>0</SUB>, whereas in the compression region, k turns away from the
  direction of B<SUB>0</SUB>. This predicted result in the rarefaction
  region agrees with direct in situ observations at 1 AU. (4) Waves that
  start near the sun with different k all tend to be refracted into
  the same direction by the time that they reach 0.5 AU. This result
  indicates that plane wave analyses will be appropriate beyond 0.5 AU.

---------------------------------------------------------
Title: Waves and instabilities in the solar wind.
Authors: Hollweg, J. V.
1975RvGSP..13..263H    Altcode: 1975RvGeo..13..263H
  We present a review of waves and instabilities in the solar wind,
  concentrating on those aspects that are likely to play important roles
  in influencing the dynamic and thermodynamic states of the general solar
  wind expansion. We consider in particular the roles played by various
  waves and instabilities in influencing the heating and expansion of
  the solar wind, the angular momentum of the solar wind, the solar
  wind thermal anisotropy, the heating and flow of alpha particles in
  the solar wind, the interstellar neutral particles that become ionized
  in the solar wind, and the 'fluidlike behavior' of the solar wind. We
  include a brief review of the properties of the hydromagnetic wave
  modes, concentrating particularly on the Alfven mode, which has been
  observed to contribute significantly to the microscale fluctuations
  of the solar wind. But we also present a summary of observational
  evidence pertaining to the presence and action in the solar wind of
  waves and instabilities that are not among the hydromagnetic modes.

---------------------------------------------------------
Title: Hydromagnetic Waves in Interplanetary Space
Authors: Hollweg, J. V.
1974PASP...86..561H    Altcode:
  Review of recent theoretical and observational work attempting
  to explain the origin and nonlinear properties of interplanetary
  hydromagnetic waves and the role played by these waves in modifying
  the thermal and dynamic characteristics of the solar wind. Attention
  is restricted to propagating hydromagnetic waves which are intrinsic
  to the solar wind itself. An initial straightforward analysis of
  small-amplitude hydromagnetic waves serves as a guide for interpreting
  the data and as a reference point for examining the more complex
  nonlinear phenomena. Representative data illustrate the appearance of
  the various hydromagnetic wave modes in the solar wind. The various
  effects which the waves may have on the solar wind itself are finally
  discussed.

---------------------------------------------------------
Title: Improved Limit on Photon Rest Mass
Authors: Hollweg, Joseph V.
1974PhRvL..32..961H    Altcode:
  Recent observations of Alfvén waves in the interplanetary medium
  provide an improved upper limit on the photon rest mass. We find a
  reliable upper limit μ&lt;=3.6×10<SUP>-11</SUP> cm<SUP>-1</SUP>,
  m<SUB>ph</SUB>&lt;=1.3×10<SUP>-48</SUP> g, and a stronger, but
  less certain upper limit μ&lt;3.1×10<SUP>-12</SUP> cm<SUP>-1</SUP>,
  m<SUB>ph</SUB>&lt;1.1×10<SUP>-49</SUP> g. These represent improvements
  on the heretofore best reliable estimate by 0.5 and 1.5 orders of
  magnitude, respectively.

---------------------------------------------------------
Title: Alfvénic acceleration of solar wind helium and related
    phenomena 1. Theory
Authors: Hollweg, Joseph V.
1974JGR....79.1357H    Altcode:
  We present a new physical mechanism by which helium nuclei can be
  preferentially accelerated by Alfvén waves in the solar wind. The
  mechanism works as follows. The acceleration of the solar wind
  by Alfvén wave pressure is basically a δJ × δB force; but the
  wave-associated current δJ carried by a given plasma species depends,
  via the Lorentz transformation, on the bulk velocity of that species,
  and thus species that move at different bulk velocities experience
  different accelerations. In the solar wind this differential
  acceleration can for the most part be interpreted as an additional
  frictional interaction between protons and helium nuclei. Numerical
  estimates indicate that this interaction is important at 1 AU. It is
  expected that this additional friction will be able to account for the
  observed fact that helium nuclei almost never flow slower than protons
  in the solar wind; detailed numerical calculations are deferred to
  a later paper, however. Although our calculation is done for Alfvén
  waves, we argue that the basic physics of our mechanism is applicable
  to a wide variety of waves, and we suggest that it may be important
  for cosmic rays, interstellar gas, comet tails, and the earth's (and
  Jupiter’) geomagnetic tail.

---------------------------------------------------------
Title: Large-amplitude hydromagnetic waves
Authors: Barnes, Aaron; Hollweg, Joseph V.
1974JGR....79.2302B    Altcode:
  We examine several aspects of the theory of large-amplitude
  hydromagnetic waves and their behavior in the interplanetary medium. We
  consider the characteristic modes of the full (i.e., nonlinearized) MHD
  equations and their modification by collisionless and finite-frequency
  effects. We give special attention to the transverse Alfvén mode, which
  is undamped and characterized by strictly constant pressure, density,
  and |B|; this seems to be the predominant propagating fluctuation
  at 1 AU. We show that its propagation in the small-wavelength (WKB)
  approximation is essentially identical to that of the small-amplitude
  Alfvén wave of linearized theory. We also suggest that its presence
  at 1 AU may provide a natural explanation of the observed power
  anisotropy of the fluctuations. We use a second-order analysis to study
  fluctuations that are not characteristic modes. We find that for a small
  range of propagation directions, and subject to third-order effects,
  a finite-amplitude wave can exist that is linearly polarized with
  δB perpendicular to both B<SUB>0</SUB> and k; such a wave can damp
  nonlinearly. But the situation is different for other directions of
  propagation, and our analysis suggests a possible explanation for the
  presence of the transverse Alfvén mode at 1 AU. Our results are used
  to discuss several possible mechanisms by which hydromagnetic waves
  may heat the solar wind.

---------------------------------------------------------
Title: Waves and instabilities in the solar wind.
Authors: Hollweg, J. V.
1974sowi.conf..333H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Transverse Alfvén waves in the solar wind: Arbitrary k,
    v <SUB>0</SUB>, B <SUB>0</SUB>, and |δB|
Authors: Hollweg, Joseph V.
1974JGR....79.1539H    Altcode:
  Using a simple analysis based on energy conservation, we derive
  expressions for the spatial variation of the amplitudes of transverse
  Alfvén waves in the solar wind. We make no assumptions about the
  solar wind geometry or the directions of propagation, and we do not
  require that the wave amplitudes be small.

---------------------------------------------------------
Title: On electron heat conduction in the solar wind
Authors: Hollweg, Joseph V.
1974JGR....79.3845H    Altcode:
  First, it is pointed out that Perkins' work on the helioclassical
  electron conductivity in the solar wind is missing an important
  term. This term essentially removes Perkins' helioclassical reduction
  of the heat flux below the classical Spitzer-Härm value. Second, a
  speculative discussion of an alternative approach is presented. It is
  argued that heat current instabilities may be effective in substantially
  reducing the heat flux, and approximate formulas are presented for
  the heat flux when the instabilities are operating. However, these
  formulas must be regarded merely as speculations as to the end result
  of a difficult and as yet unsolved problem in nonlinear plasma physics.

---------------------------------------------------------
Title: ALFVtN Waves in a Two-Fluid Model of the Solar Wind
Authors: Hollweg, Joseph V.
1973ApJ...181..547H    Altcode:
  We present a two-fluid model for the solar wind which includes
  the presence of Alfve'n waves which originate at the Sun. The
  effective pressure of the Alfve'n waves is included, as well as a
  model representation for proton heating via nonlinear damping of the
  Alfve'n waves. The effects of rotation in the solar equatorial plane
  are self-consistently included. Our principal results are summarized as
  follows: 1) Our calculations reproduce the correlation at 1 a.u. quite
  well for V &lt;% 450km 1 This supports the idea that Alfve'n waves of
  solar origin are responsible for the high-speed streams. 2) An Alfve'nic
  energy flux of 6000 ergs -1 at the Sun yields values for Tp, V, and nat
  1 a.u. which agree well with the data. 3) Wave pressure tends to produce
  a positive n-v correlation at 1 a.u., contrary to the observed negative
  correlation. This suggests that the cross-section of the high-speed
  streams may increase more rapidly than r2. 4) Including the spiral
  magnetic field in the electron energy equation worsens the disagreement
  between observed and calculated values of T at 1 a.u., if the electron
  heat conductivity is given by the collisional value. More work on the
  electron energy equation is needed. 5) Peripheral to our main theme,
  we present (a) general derivation of the conservation equations in
  the presence of Alfve'n waves; (b) a simple, new derivation for the
  properties of Alfve'n waves in a spiral field; and (c) a new power
  series for the electron temperature near r = , in the presence of a
  spiral magnetic field. Subject headings: hydromagnetics - solar wind

---------------------------------------------------------
Title: Alfvén waves in the solar wind: Wave pressure, poynting flux,
    and angular momentum
Authors: Hollweg, Joseph V.
1973JGR....78.3643H    Altcode:
  We consider three effects of Alfvén waves propagating in the
  solar wind. (1) Modification of the angular momentum balance of the
  solar wind by Alfvén waves in the presence of thermal anisotropy
  is considered. The Alfvén waves are found to reduce the azimuthal
  velocity υ<SUB>ϕ</SUB> at 1 AU. This effect occurs because the
  Alfvén waves are transverse and represent an additional component of
  the pressure perpendicular to the magnetic field. The effect is large if
  &lt;δB²&gt;/B<SUB>0</SUB>² ≳ ⅓, and it is concluded that thermal
  anisotropy cannot be invoked to explain the large azimuthal velocity
  of the solar wind. (2) Modification of the angular momentum balance
  of the solar wind by Alfvén waves by finite-wavelength (non-WKB)
  effects is considered. The Alfvén waves reduce υ<SUB>ϕ</SUB> at 1
  AU by reducing the heliocentric distance of the critical point that
  appears in the equation for υ<SUB>ϕ</SUB>. This effect occurs because
  the waves act like a Reynold's ‘viscosity,’ but the sign is such
  that the viscosity is negative, leading to antirotation of the solar
  wind. This effect is only important for waves with ω<SUP>-1</SUP>
  ≳ 10 hours. (3) Finite-wavelength modifications of the wave pressure
  are considered. It is found that the wave pressure is reduced close
  to the sun. This effect is important near 2 R<SUB>E</SUB> for waves
  with ω<SUP>-1</SUP> ≳ 2 hours.

---------------------------------------------------------
Title: ALFVÉNIC Motions in the Solar Atmosphere
Authors: Hollweg, Joseph V.
1972ApJ...177..255H    Altcode:
  The amplitude of bulk velocities associated with upward-propagating
  Alfve'n waves in the lower solar atmosphere is discussed. We
  show that for a given wave energy flux, the bulk velocities can
  be appreciably lower in cases when the wavelength is much larger
  than the scale height, than in situations where the wavelength is
  smaller than the scale height. In the chromosphere and lower corona,
  the former case pertains to waves with dominant timescales of hours,
  as is found for Alfven waves observed in the solar wind at 1 a.u.,
  or to supergranulation-driven waves with dominant timescales of the
  order of the supergranular lifetime, 20 hours. This has the important
  consequence that Alfven waves with energy fluxes of several thousand
  to a few tens of thousand ergs em 2 -1 at the Sun (such fluxes have
  been predicted for supergranulation-dnven Alfven waves, and can play
  a significant role in the dynamics of the solar wind) can have bulk
  velocity amplitudes 3 km s ' at heights less than 50,000km above
  the solar limb, if the general solar field strength is at least 2
  gauss. Such velocities are consistent with observational limits to
  bulk velocities in the lower solar atmosphere.

---------------------------------------------------------
Title: Model for Energy Transfer in the Solar Wind: Model Results
    - Comments
Authors: Hollweg, J. V.
1972NASSP.308..223H    Altcode: 1972sowi.conf..223H
  No abstract at ADS

---------------------------------------------------------
Title: Wavelength Dependence of the Interplanetary Scintillation
    Index - Comments
Authors: Hollweg, J. V.
1972NASSP.308..503H    Altcode: 1972sowi.conf..497H
  No abstract at ADS

---------------------------------------------------------
Title: Wavelength Dependence of the Interplanetary Scintillation Index
Authors: Hollweg, J. V.; Jokipii, J. R.
1972NASSP.308..494H    Altcode: 1972sowi.conf..488H
  No abstract at ADS

---------------------------------------------------------
Title: Supergranulation-driven Alfvén waves in the solar chromosphere
    and related phenomena.
Authors: Hollweg, J. V.
1972CosEl...2..423H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Heat conduction in a turbulent magnetic field, with application
    to solar-wind electrons
Authors: Hollweg, Joseph V.; Jokipii, J. R.
1972JGR....77.3311H    Altcode:
  We consider random, long-wavelength fluctuations in a turbulent magnetic
  field and show that they can appreciably decrease the heat conductivity
  of a plasma along the magnetic field. In simp1e cases of interest,
  the reduction along the average field is approximately by the factor
  &lt;cos δθ&gt;², where δθ is the angle of the local magnetic field
  relative to the average field. Application to solar-wind electrons
  indicates that this reduction in heat conductivity due to observed
  fluctuations in the interplanetary magnetic field may be of the order
  of a factor of 2. This may help to explain recent measurements which
  indicate a rather low electron heat flux in the solar wind.

---------------------------------------------------------
Title: Supergranulation Driven Alfvén Waves in the Solar Chromosphere
    and Related Phenomena
Authors: Hollweg, J. V.
1972CoEl....2..423H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Nonlinear Landau Damping of Alfvén Waves
Authors: Hollweg, Joseph V.
1971PhRvL..27.1349H    Altcode:
  It is shown that large-amplitude linearly or elliptically polarized
  Alfvén waves propagating parallel to B--&gt;<SUB>0</SUB> can be
  dissipated by nonlinear Landau damping. The damping is due to the
  longitudinal electric field associated with the ion sound wave which
  is driven (in second order) by the Alfvén wave. The damping rate can
  be large even in a cold plasma (β&lt;&lt;1, but not zero), and the
  mechanism which we propose may be the dominant one in many plasmas of
  astrophysical interest.

---------------------------------------------------------
Title: Density fluctuations driven by Alfvén waves
Authors: Hollweg, Joseph V.
1971JGR....76.5155H    Altcode:
  The equations for a linearly polarized Alfvén wave, propagating
  parallel to the direction of the average magnetic field in a perfectly
  conducting fluid, are solved to second order in the wave quanities
  for cases where the fluid obeys single adiabatic or double adiabatic
  equations of state. To this order, we find no change in the wave
  magnetic field or transverse wave velocity, but longitudinal wave
  velocity and density fluctuations appear, driven by gradients in the
  wave magnetic-field pressure. This is in contrast to the common belief
  that even large-amplitude Alfvén waves remain purely transverse. The
  density fluctuations can become quite large when the Alfvén speed
  is close to the ion sound speed in the fluid; this condition may at
  times exist in the solar wind at 1 AU. We suggest that part of the
  density fluctuations observed in the solar wind by satellites and
  interplanetary scintillation may be associated with large-amplitude
  Alfvén waves. Heating of the solar wind might result if the ion sound
  waves, which are driven by the Alfvén waves, are appreciably damped.

---------------------------------------------------------
Title: Collisionless solar wind, 2, Variable electron temperature
Authors: Hollweg, Joseph V.
1971JGR....76.7491H    Altcode:
  We consider a two-component ‘model’ for the solar wind, in
  which the protons become collisionless beyond r<SUB>0</SUB>≥10
  R<SUB>S</SUB>, where they are already highly supersonic. The
  proton temperatures are found from the double adiabatic equation
  of state. The electrons are highly subsonic, and their temperature
  profile is prescribed ad hoc. Solar rotation is considered in
  a semi-self-consistent fashion. The momentum equations for the
  electrons and protons are solved subject to the conditions of quasi
  neutrality and zero charge efflux from the sun. The principal
  results are the following. (1) The proton thermal anisotropy
  is substantially reduced when solar rotation is considered. We
  find T<SUB>p∥</SUB>/T<SUB>p⊥</SUB> &lt; 3 if r<SUB>0</SUB> =
  40 R<SUB>S</SUB>, while T<SUB>p∥</SUB>/T<SUB>p⊥</SUB> &lt; 2
  if r<SUB>0</SUB> = 55 R<SUB>S</SUB>. Wave-particle interactions may
  therefore play a less significant role in destroying proton anisotropy
  than has been heretofore thought. (2) The observed dependence of
  T<SUB>p∥</SUB>/T<SUB>p⊥</SUB> on solar-wind speed is consistent
  with collisionless flow, and the double adiabatic equation of
  state, beyond r<SUB>0</SUB>=55 R<SUB>S</SUB>, if r<SUB>0</SUB> and
  υ<SUB>0</SUB> = υ(r<SUB>0</SUB>) do not change. (3) Solar rotation
  leads to significantly lower mean proton temperatures, T<SUB>p</SUB> =
  ( T<SUB>p∥</SUB> + 2T<SUB>p⊥</SUB>)/3, than are obtained when the
  magnetic field is radial. Thus the apparent advantage of collisionless
  models in reproducing the observed solar-wind conditions does not
  persist when solar rotation is included. (4) Even slight electron
  anisotropy, T<SUB>e∥</SUB>/T<SUB>e⊥</SUB> ≌ 1.2, in the region
  r&gt;10 R<SUB>S</SUB> reduces the solar-wind speed at the earth by
  10-15%, thus worsening the disagreement between observations and the
  two-fluid model. (5) The electron temperature profile in the supersonic
  region is the primary parameter determining flow acceleration there;
  we urge that the details of the electron energy balance, and the
  possibility of electron heating, be carefully examined in future work.

---------------------------------------------------------
Title: Energy and momentum exchange in transverse plasma waves
Authors: Hollweg, Joseph V.; Völk, H. J.
1971JGR....76.7527H    Altcode:
  We calculate, by a perturbation analysis, the energy and momentum
  changes both of a single particle and of a distribution of particles
  moving in a transverse electromagnetic wave propagating parallel to
  the direction of the average magnetic field. It is shown that energy
  and momentum conservation of the particles and of the electromagnetic
  field leads to the same dispersion relation as has been obtained by
  Stix (1962) from solution of the Vlasov equation. More importantly,
  we are able to discuss the transfer of energy and momentum between
  particles and fields, in the context of stability or instability of the
  plasma. We emphasize that it is generally not sufficient to attempt to
  deduce the stability properties of a plasma from considerations of the
  energy gained or lost by the resonant particles alone. In general, one
  must also consider the additional constraint of momentum conservation
  and the effects of the nonresonant particles; this conclusion persists
  even in the limit of zero growth rate, when the resonant particles
  would be expected to play the dominant role. Two illustrations are
  provided. The real part of the frequency of the fire-hose instability
  is shown to be determined by momentum conservation, whereas the growth
  rate follows from energy conservation. For the new proton-resonant
  modes (Hollweg and Völk, 1970b), on the other hand, we find that
  the growth rate follows from momentum conservation, whereas energy
  conservation yields the real part of the frequency. We show that the
  earlier classification of the proton-resonant modes in accord with the
  sign of (A<SUB>p</SUB>±ω<SUB>r</SUB>/Ω<SUB>p</SUB>) is, in fact,
  a classification assigned in accord with the gain or loss of energy by
  the protons. The small-wavelength extension of the fire hose leads to
  absorption of energy by the protons, and this mode could simultaneously
  cool the electrons and heat the protons in the solar wind near 1 AU; the
  other two proton-resonant modes lead to loss of energy by the protons
  and would aggravate the discrepancy between the two-fluid solar-wind
  model and observation. Finally, it is shown that the small-wavelength
  extension of the fire hose tends to reduce solar-wind proton thermal
  anisotropies to the observed values only for frequencies less than
  about 4Ω<SUB>p</SUB>, whereas the other proton-resonant instabilities
  lead in all instances to reduction of the anisotropy.

---------------------------------------------------------
Title: Fluctuations in Times of Arrival of Pulsar Pulses
Authors: Hollweg, Joseph V.
1970ApJ...161L.225H    Altcode:
  Recent progress in the interpretation of scintillation and angular
  broadening of radio sources indicates that the correlation length for
  electron-density fluctuations in the interplanetary plasma may be four
  orders of magnitude larger than was earlier supposed. In that case
  we expect rms fluctuations in the times of arrival of pulsar pulses
  to be as large as 10 ms at 75 MHz, if the apparent distance of the
  ulsar from the solar center is 10 Ro. Detection of fluctuations in
  the times of arrival could provide valuab e new information about the
  nature of the solar coronal turbulence, since this effect is sensitive
  primarily to the longer wavelengths in the turbulence spectrum, in
  contrast to scintillation and angular broadening, which are sensitive
  to the shortest wavelengths.

---------------------------------------------------------
Title: Interplanetary Scintillations and the Structure of Solar-Wind
    Fluctuations
Authors: Jokipii, J. R.; Hollweg, Joseph V.
1970ApJ...160..745J    Altcode:
  It is demonstrated that the observed correlation scale of the
  interplanetary scintillation of radio sources is consistent with a
  plasma-density correlation length of 10 km or more. This result is
  in sharp contrast to previous analyses which inferred a correlation
  length of 100-200 km. Fluctuations in plasma density may therefore
  have a structure similar to that observed for the interplanetary
  magnetic field and plasma velocity. We find that in this case of a
  long correlation length the rms phase fluctuation produced in the
  radio wave by the solar plasma is very large ( 10 radians) and that
  the -km scale inferred in previous work is then closely related to the
  "inner scale" of the fluctuations, i.e., that wavelength below which
  there is little power.

---------------------------------------------------------
Title: Lunar conducting islands and formation of a lunar limb
    shock wave
Authors: Hollweg, Joseph V.
1970JGR....75.1209H    Altcode:
  We propose a mechanism for the creation of a weak solar-wind shock at
  the lunar limb, in which the fluctuating interplanetary magnetic field
  interacts with a thin highly conducting crust. The crust is assumed to
  be fractured and describable in terms of what we call ‘conducting
  islands.’ The fluctuating interplanetary field produces currents
  inside the conducting islands, which in turn produce magnetic fields
  capable of deflecting the solar wind. We find that conductivities of
  4×10<SUP>-4</SUP> mho/m and crust thicknesses of 5 km are consistent
  with the observed 3°-6° flow deflection if the ‘fractionation
  scale’ is of the order of 400 km. Higher conductivities and greater
  thicknesses imply a smaller fractionation scale.

---------------------------------------------------------
Title: New plasma instabilities in the solar wind
Authors: Hollweg, Joseph V.; Völk, H. J.
1970JGR....75.5297H    Altcode:
  We discuss the instability of transverse electromagnetic waves
  propagating parallel to the average magnetic field in an electron-proton
  plasma, and we examine the question of whether the instabilities may
  occur in the solar wind. Our discussion is based on Stix' well-known
  dispersion relation, but we depart from most previous analyses
  by assuming that the resonant protons lie near the peak of their
  distribution function. We find three instabilities, two of which are
  new. The first new instability is driven by anisotropic electrons in
  the same manner as is the firehose, and we believe that it represents an
  extension of the firehose instability to large wave numbers. The growth
  rates are large over a broad frequency range around Ω<SUB>p</SUB>. We
  suggest that the excitation of this instability results in a partial
  transfer of thermal energy from the electrons to the protons and, if
  it occurs in the solar wind, may therefore heat the solar-wind protons
  and reduce their anisotropy. The second, new instability is driven by
  anisotropic protons with T<SUB>⊥p</SUB>⪞3T<SUB>∥p</SUB> it may
  occur in local regions of the solar wind. The third instability is not
  new; it represents a continuation of the unstable whistler mode near
  Ω<SUB>p</SUB> to those cases where there are many resonant protons. In
  contrast to earlier papers, in which it is assumed that the number of
  resonant protons is small, our work demonstrates that this instability
  can in fact be effective in destroying proton anisotropies of the type
  observed in the solar wind.

---------------------------------------------------------
Title: Angular broadening of radio sources by solar wind turbulence
Authors: Hollweg, Joseph V.
1970JGR....75.3715H    Altcode:
  The observed apparent angular broadening of radio sources as they pass
  close to the sun is discussed in terms of statistical ray theory. Unlike
  earlier works in which the turbulent interplanetary plasma is assumed
  to possess a spectrum of Gaussian form, we here treat power spectra for
  fluctuations in electron density, which are assumed to be of the same
  form as have been obtained for magnetic field and velocity fluctuations
  by in situ measurements. Such spectra are distinctly non-Gaussian. We
  find that the data suggest that the solar wind turbulence is strong
  between 10 and 100 solar radii distance from the sun. This result
  disagrees with earlier analyses, which suggest that the fluctuations
  are of the order of only a few per cent of the mean, but it agrees
  with the large fluctuations in electron density indicated by direct
  satellite measurement near the orbit of the earth. The data usually
  imply that the solar wind electron density varies as r<SUP>-2</SUP>,
  and that the correlation length and inner scale (that wavelength
  below which there is little power) of the turbulence are constant
  with distance from the sun. At times, however, the data imply that
  the electron density varies as r<SUP>-2.5</SUP>, and the two length
  scales are constant; or that the density varies as r<SUP>-2</SUP>,
  and the two length scales increase linearly with distance from the sun.

---------------------------------------------------------
Title: Two New Plasma Instabilities in the Solar Wind
Authors: Hollweg, Joseph V.; Völk, H. J.
1970Natur.225..441H    Altcode:
  IN this communication we consider the left-hand mode in a hot
  plasma, propagating in the direction of the average magnetic field,
  and for frequencies near the proton gyrofrequency we find two
  new instabilities. The first is driven by strong electron thermal
  anisotropy and the second by strong proton thermal anisotropy in the
  presence of electron anisotropy. We believe that the first of these
  new instabilities may have important consequences for the development
  of the solar wind. This mode is similar to the low-frequency firehose
  instability but it involves only the electron anisotropy. The wavelength
  is short, and the destabilizing centrifugal force on the electrons is
  able to overwhelm the cyclotron damping due to the resonant protons. If
  this instability occurs in the solar wind, it results in an additional
  coupling between the electrons and protons, with the result that it
  can contribute to the proton heating.

---------------------------------------------------------
Title: Collisionless solar wind: 1. Constant electron temperature
Authors: Hollweg, Joseph V.
1970JGR....75.2403H    Altcode:
  A 2-fluid model for the solar wind is discussed, in which the electrons,
  at constant temperature, are treated hydrodynamically and the protons
  are assumed to become collisionless, at a distance from the sun at which
  they are already supersonic. A radial magnetic field is included, with
  radial gravitational and electric fields; the electric field arises
  from the condition of quasi-neutrality and is an important feature
  of the model. Quantities such as bulk velocity, concentration, and
  proton temperatures parallel and perpendicular to the magnetic field
  are discussed directly in terms of the proton distribution function;
  viscosity and heat conduction are thus automatically included. The
  principle result is that if the electron temperature in the vicinity
  of the point where the electrons become collisionless (10-20 solar
  radii from the sun) is of the order 10<SUP>6</SUP> °K, the flow
  velocity at the orbit of the earth is in excess of 300 km/sec, and
  the average proton temperature is of the order of 10,000 °K. Both
  of these figures are higher than those obtained by the 2-fluid
  hydrodynamical model. The velocity far from the base of the model
  varies as T<SUB>e</SUB><SUP>1/2</SUP> and is relatively insensitive to
  temperature or velocity at the base or to the location of the base. The
  thermal anisotropy of the protons, T<SUB>∥</SUB>/T<SUB>⊥</SUB>,
  is predicted to be about 50.

---------------------------------------------------------
Title: Lunar limb shock wave
Authors: Hollweg, J. V.
1969MitAG..27Q.222H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Occulting Disk of the Sun at Radio Wavelengths
Authors: Bracewell, R. N.; Eshleman, V. R.; Hollweg, Joseph V.
1969ApJ...155..367B    Altcode:
  The angular size of the occulting disk of the Sun at radio wavelengths,
  which depends on the deviation of slightly deviated rays traversing
  the outer solar corona, can be calculated simply without resorting to
  ray tracing

---------------------------------------------------------
Title: Stochastic heating of protons by fast hydromagnetic wave
Authors: Hollweg, Joseph V.
1969JGR....74.2899H    Altcode:
  The stochastic heating of protons by a random magnetosonic wave
  propagating normal to the magnetic field is suggested as an explanation
  of the observation that the protons are hotter than the electrons in
  the plasma sheet of the magnetotail. A perturbation analysis is used
  to find the proton trajectories under the influence of the electric
  and magnetic fields of the driving wave. The gradients of the field
  quantities across a proton orbit are included by using the first term
  in a Taylor series expansion. The inclusion of the field gradients and
  particle drifts gives the result that the heating depends not only on
  the energy in the wave spectrum at the gyrofrequency but also on that
  at the first harmonic of the gyrofrequency, and suggests, furthermore,
  that the faster protons are heated more strongly with the result
  that their distribution function can be broadened, consistent with
  observation. A nonlinear mechanism that can decouple the particles
  from the wave is suggested, and it is shown to yield proton energy
  gains of the correct order of magnitude for the plasma sheet.

---------------------------------------------------------
Title: A Statistical Ray Analysis of The Scattering of Radio Waves
    by the Solar Corona
Authors: Hollweg, Joseph V.
1968AJ.....73..972H    Altcode:
  The scattering of radio waves by an anisotropically turbulent solar
  corona exhibiting large-scale refraction (due to a radial gradient in
  average electron density) is discussed in terms of a statistical ray
  analysis similar to that of Chandrasekhar. The corona is assumed to
  be spherically symmetric throughout. The ray equations of geometrical
  optics are written in terms of the spherical coordinate system natural
  to the solar corona, and discussed for the case of an anisotropically
  turbulent corona for which the electron density may be known in only a
  statistical sense. A linear perturbation analysis is employed to obtain
  explicit solutions for the statistical fluctuations in the ray position,
  signal phase, and pulse propagation times. The general expressions thus
  obtained are discussed in particular for the special case of nearly
  linear rays. It is shown that at appropriate frequencies even very
  slight ray bending can have a significant effect on the fluctuations in
  the times of propagation of pulse signals across the corona. Throughout
  the work we seek to provide a proper analytical framework in which to
  interpret observed fluctuations in the apparent source position (or
  angular sixe), the arrival times of pulse signals, and variations in
  the signal bandwidth. Our attention is drawn specifically to deducing,
  as functions of distance from the sun, the mean-square fluctuations in
  electron density, the statistical correlation lengths, and the degree
  of anisotropy. We point out that the scattering data available at
  present is consistent, beyond some ten solar radii, with a coronal
  density behaving as , a degree of anisotropy nearly constant with
  distance from the sun, and a statistical correlation length which
  during solar minimum does not vary with (r), but which tends to
  increase linearly with (r) near solar maximum indicating that the
  interplanetary plasma develops a radial filamentary structure as solar
  maximum is approached. In the region three to six solar radii, we find
  a2/ ', where a is the correlation length in the radial direction and b
  the correlation length in the transverse direction. This behavior can
  result if both the anisotropy ratio a/b and the transverse correlation
  length vary linearly with r in that region.

---------------------------------------------------------
Title: Solar Coronal Effects on Pulsar Signals
Authors: Hollweg, Joseph V.
1968Natur.220..771H    Altcode:
  THE discovery<SUP>1</SUP> of the southern pulsar PSR 2045-16 suggests
  the possibility of detecting the influence of the solar corona on
  pulsar signals, for this pulsar is located only some 1.5° from the
  ecliptic, and will therefore at the end of January be located within
  some six solar radii of the solar centre. This letter presents estimates
  of the influence of the corona on pulsar signals by considering the
  delay which the corona will produce in the arrival times of the signal
  pulses, fluctuations in the arrival times of pulses due to statistical
  inhomogeneities in the coronal electron density, and the temporal
  “smearing” of pulse structure due to the propagation of waves over
  different electrical paths. Detection of the first and last of these
  effects may be possible for PSR 2045-16.

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Title: Interaction of the solar wind with the moon and formation of
    a lunar limb shock wave
Authors: Hollweg, Joseph V.
1968JGR....73.7269H    Altcode:
  The interaction of the solar wind with a two-layered moon is considered
  from the point of view of the induction generator model of Sonett
  and Colburn (1967). We show that a highly conducting lunar core,
  shielded by a thin insulating outer layer, cannot reasonably be
  consistent with the observed absence of a lunar bow shock, since a
  10-meter thick surface dust layer would require a conductivity less
  than 10<SUP>-10</SUP> mho/m, while a 10-kilometer thick layer would
  require a conductivity less than 10<SUP>-7</SUP> mho/m to shield the
  core; conductivities in this low range do not seem reasonable. We
  thus establish an upper limit of 10<SUP>-5</SUP> mho/m for the core
  conductivity, but point out that this figure can be consistent with
  the existence of a highly conducting surface layer. Two mechanisms are
  suggested for the formation of a lunar limb shock wave. It is shown
  that in the steady-state unipolar generator model a limb shock will
  be expected to form in the vicinity of the plane defined by the solar
  wind velocity and magnetic field directions, while a limb shock may be
  expected to form also in the nonsteady state if the moon possesses a
  conducting outer layer (10<SUP>-4</SUP> &lt; σ &lt; 10<SUP>-3</SUP>
  mho/m) of thickness between one-tenth and several kilometers.

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Title: A statistical ray analysis of the scattering of radio waves
    by anisotropically turbulent, non-homogeneous solar corona
Authors: Hollweg, Joseph Vincent
1968PhDT.......106H    Altcode:
  No abstract at ADS

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Title: Properties of solar wind turbulence deduced by radio
    astronomical measurements
Authors: Hollweg, Joseph V.; Harrington, J. V.
1968JGR....73.7221H    Altcode:
  A simple ray analysis is used to derive expressions for the angular
  spread and spectral broadening of coherent radio signals on traversing
  the turbulent interplanetary medium, when the anisotropy and radial
  gradient of the turbulence and the large-scale motions of the medium
  (solar wind) are important. The resulting expressions are used to
  interpret observations, reported in the literature, of the angular
  broadening of natural radio sources and of the spectral broadening
  of Mariner 4 during superior conjunction. In the region 10-100 solar
  radii our results are consistent with a coronal electron concentration
  behaving as r<SUP>-2</SUP> and a statistical correlation length which
  during solar minimum does not vary with r but which tends to increase
  linearly with r near solar maximum, indicating that the interplanetary
  plasma tends to develop a radial filamentary structure during solar
  maximum; we find no necessity to invoke nonradial outflow to explain
  the observations. In the region 3-6 solar radii the data imply a²/b
  ∼ r³, where a is the correlation length in the radial direction and
  b is the correlation length in the transverse direction. We point out
  that this behavior can result if both the transverse correlation length
  and the anisotropy ratio, a/b, increase linearly with r in that region.