Author name code: hollweg
ADS astronomy entries on 2022-09-14
author:"Hollweg, Joseph V."
------------------------------------------------------------------------
Title: Radio Occultation Observations of the Solar Corona Over
1.60-1.86 R⊙: 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.
Bibcode: 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⊙
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.
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.
Bibcode: 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 ⊙. 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 ⊙. 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 ⊙. 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.
Title: Quasilinear Consequences of Turbulent Ion Heating by Magnetic
Moment Breaking
Authors: Isenberg, Philip A.; Vasquez, Bernard J.; Hollweg, Joseph V.
Bibcode: 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 & 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.
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.
Bibcode: 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.
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.
Bibcode: 2017SPD....4830102W
Altcode:
Study of coronal MHD wave energetics relies upon accurate representation
of plasma particle number densities (ne) 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 (Rs) 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 ne 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 ne
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 ne 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-2 added. Back-calculating
ne along the LOS from the TEC curves, we found that the
equivalent ne 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.
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.
Bibcode: 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.
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.
Bibcode: 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}{flow} 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}{crit}, and the rotational force controls
the deceleration of alpha particles at r\gt {r}{crit},
where {r}{crit}≃ 2.5 {AU} in the fast solar wind in
the ecliptic plane. We find that {Q}{flow} is positive
at r\lt {r}{crit} and {Q}{flow}=0 at r≥slant
{r}{crit}, consistent with the previous finding that the
rotational force does not lead to a release of energy. We compare
the value of {Q}{flow} at r\lt {r}{crit}
with empirical heating rates for protons and alpha particles, denoted
{Q}p and {Q}α , deduced from in situ measurements
of fast-wind streams from the Helios and Ulysses spacecraft. We find
that {Q}{flow} exceeds {Q}α at r\lt 1 {AU},
and that {Q}{flow}/{Q}p 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}{crit} 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.
Title: Magnetohydrodynamic Slow Mode with Drifting He++:
Implications for Coronal Seismology and the Solar Wind
Authors: Hollweg, Joseph V.; Verscharen, Daniel; Chandran, Benjamin
D. G.
Bibcode: 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++) 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 Te /Tp Gt 1.
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.
Bibcode: 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.
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.
Bibcode: 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.
Title: Alfvén Waves in Shear Flows Revisited
Authors: Hollweg, Joseph V.; Kaghashvili, Edisher Kh.
Bibcode: 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.
Title: Observational Constraints on the Role of Kinetic Alfven Waves
in the Solar Wind
Authors: Smith, C. W.; Vasquez, B. J.; Hollweg, J. V.
Bibcode: 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.
Title: The Argument Against Kinetic Alfven Waves Forming the Whole
of the Dissipation Range
Authors: Smith, Charles William; Vasquez, Bernard J.; Hollweg,
Joseph V.
Bibcode: 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.
Title: Observational Constraints on the Role of Kinetic Alfven Waves
in the Solar Wind
Authors: Smith, Charles William; Vasquez, Bernard J.; Hollweg,
Joseph V.
Bibcode: 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.
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.
Bibcode: 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.
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.
Bibcode: 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.
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.
Bibcode: 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.
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.
Bibcode: 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 2 bottom + 1.5 v 2/3
A|v par|4/3. Here, v bottom
and v par are the velocity components perpendicular and
parallel to the background magnetic field, and v A 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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 + and z - of Alfvénic fluctuations
propagating parallel and antiparallel to the background magnetic field
B 0, and the correlation length (outer scale) measured
perpendicular to B 0, denoted L bottom. We argue
that if z + Gt z - and if the z -
fluctuations are sustained by the reflection of z +
fluctuations in a strong background magnetic field, then epsilon ~
ρ(z +)2 z -/L bottom
as in previous studies by Hossain, Matthaeus, Dmitruk, Lithwick,
Goldreich, Sridhar, and others. On the other hand, if the minority
wave type (z -) is sustained by some form of forcing
that is uncorrelated with or only weakly correlated with the z
+ fluctuations, then epsilon can be much less than ρ(z
+)2 z -/L bottom, 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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+5 in coronal holes: Importance
of sunward propagating waves
Authors: Hollweg, Joseph V.
Bibcode: 2006JGRA..11112106H
Altcode:
The high thermal anisotropy of O+5 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+5 temperature steadily increases from r ≈ 1.5
rS (where the ions become almost collisionless) out to r
≈ 3.5 rS (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+5 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+5 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-γ, with γ ≈ 5/3. We point out that it is
really necessary to take into account the fact that O+5
(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'' (ApJ, 639,
1177 [2006])
Authors: Markovskii, S. A.; Vasquez, Bernard J.; Smith, Charles W.;
Hollweg, Joseph V.
Bibcode: 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 ApJ, 639, 1177 [2006]
(instead of Hui, L., Gary, S. P., & Stawicki, O.). The authors
sincerely regret the error.
Title: The Solar Wind, Then and Now
Authors: Hollweg, Joseph V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 2006RSPTA.364..505H
Altcode:
No abstract at ADS
Title: The Solar Wind: Then and Now
Authors: Hollweg, Joseph V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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. 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 = AwcAk⊥/ω, where
Aw is the relative wave amplitude, k⊥ 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 2003JGRA..108.1036K
Altcode:
Alphas (He42+) and other ions in the
interplanetary medium show a tendency to stream near or below the
local Alfvén speed (VA) relative to the main component of
protons. Because VA 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 B0 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.
Bibcode: 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 B0
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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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,
B0, 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 B0. We show
how resonances at harmonics of the cyclotron frequency come about, and
how the physics can be different depending on whether E⊥
is in or perpendicular to the plane containing k and B0
(k is wave vector, and E⊥ is the component of the wave
electric field perpendicular to B0). If E⊥
is in the k-B0 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⊥. If E⊥
is perpendicular to the k-B0 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⊥. Our main results are our <cross-ref refid="df10"
type="formula">equations (10)</cross-ref>, <cross-ref
refid="df11" type="formula">(11)</cross-ref>, and <cross-ref
refid="df13" type="formula">(13)</cross-ref> 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 2001AGUSM..SH41B15K
Altcode:
Heavy ions in the interplanetary medium show a statistical tendency
to stream at the local Alfven speed (VA) relative to
protons. Because VA decreases with increasing distance from
the Sun, forces must exist to slow the heavier ions to VA
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
VA relative to protons, we find that the streaming speed
of alphas decreases to approximately VA 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.
Bibcode: 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.
Bibcode: 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∥ ~ kperpendicular to ), 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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++ beams, which are proxies for the strongly
and weakly resonant halves of their distribution, and a single beam of
O+5 with vanishing density. The level of the power spectrum
at the high resonant frequencies is determined by the condition that
the protons and He++ 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+5 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++ 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+5 temperature is still increasing
with heliocentric distance r out to 3.5rs. Instead, the
O+5 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+5 observations
imply that (dT⊥/dt)res must be an increasing
function of r, while it is the nature of the resonant interactions to
make (dT⊥/dt)res 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.
Bibcode: 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 5RS, which at high
wavenumbers turns out to be above the actual generic density power
spectrum at 5RS 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.
Bibcode: 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.
Bibcode: 2000IAUTA..24...77V
Altcode:
No abstract at ADS
Title: Cyclotron resonance in coronal holes: 1. Heating and
acceleration of protons, O5+, and Mg9+
Authors: Hollweg, Joseph V.
Bibcode: 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 O5+ 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-2,
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 O5+. 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 Mg9+, which do not fit the overall picture.
Title: Cyclotron resonance in coronal holes: 2. A two-proton
description
Authors: Hollweg, Joseph V.
Bibcode: 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 kres 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 kres 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, kres
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.
Bibcode: 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, kres, 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 kres 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, kres
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.
Bibcode: 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⊥-1 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⊥ 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∥ such that the total pressure perturbation
δptot~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⊥ 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
Bibcode: 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.
Bibcode: 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+5 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+5, 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.
Bibcode: 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.
Bibcode: 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
B0. 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<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 B0 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
Bibcode: 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,
Tp⊥>>Tp∥ in the inner corona and
Tp⊥<Tp∥ 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 Tp⊥>>Tp∥. 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
Tp⊥>>Tp∥ 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 106K,
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.
Bibcode: 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.
Bibcode: 1999JGR...104..505H
Altcode:
We consider the motions of protons and O5+ 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 O5+ 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.
Bibcode: 1998JGR...103..335V
Altcode:
We present two-and-one-half-dimensional
(2<fraction shape="case" style="single"
align="center"><num>1</num><den>2</den></fraction>-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
B2 are entirely oblique to the background magnetic
field <bold>B</bold>0. When propagating
in different directions but with group speeds in the same
direction along <bold>B</bold>0, 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<fraction shape="case" style="single"
align="center"><num>1</num><den>2</den></fraction>-D
and 3-D. For moderate-wave amplitudes
(|δ<bold>B</bold>|/B0<~0.5),
these show that the second-order driven wave solution can
only remove variations of B when the Fourier components of
B2 are oblique to B0. Large-amplitude
(|δ<bold>B</bold>|/B0~1) 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
<bold>B</bold>0. 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.
Bibcode: 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 (θBn) 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 B2 have wave vectors within ~10° of
the background magnetic field <bold>B</bold>0,
the waveform tends to steepen even for small amplitudes
(|δ<bold>B</bold>|/B<<1). This produces RDs
with normals nearly along <bold>B</bold>0
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 θBn. When the initial
waveform is modified by including a transverse field component with a
wavenumber equaling that from the Fourier component of B2
along <bold>B</bold>0 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 θBn
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 θBn 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.
Bibcode: 1997ApJ...488..895H
Altcode:
Khabibrakhmanov & 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.
Bibcode: 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.
Bibcode: 1997JGR...102.7063E
Altcode:
Following recent observations which indicate the
presence of extremely high flow speeds in the inner
corona, 700-800kms-1 below 10RS,
and the possible presence of very high proton temperatures,
3×106<=Tp<=8.5×106K,
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-1 can readily be achieved at 10S
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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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<~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(<~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.
Bibcode: 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 <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 (<<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 <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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
forξr, andP' across the dissipative layer, are correct. In
addition, the present paper replaces the complicated dissipative MHD
solutions obtained by Sakurai, Goossens, and Hollweg forξr,
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.
Bibcode: 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.
Bibcode: 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=(υsound/υA)2 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<~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>1. If the pump is right-handed and β>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<~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>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++
Authors: Jayanti, V.; Hollweg, Joseph V.
Bibcode: 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.
Bibcode: 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/B0)2 as a small
expansion parameter; ΔB is the pump amplitude, and B0 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<β<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 γmax~A1/2, but β
cannot be too close to 0 or to 1. If β~1, we find γmax,
~A3/4; if β>1, we find γmax~A3/2,
while if β~0, we obtain γmax~A1/3 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>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<β<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.
Bibcode: 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.
Bibcode: 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++
Authors: Hollweg, Joseph V.; Esser, R.; Jayanti, V.
Bibcode: 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++) which drifts relative to the protons. Even
though its abundance is small, the He++ 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++ 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1991BAAS...23.1037H
Altcode:
No abstract at ADS
Title: Alfvén Waves
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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+≠0 and δz-≠0. Thus
if δz- corresponds to the outgoing mode in a homogeneous
background, an observation of δz+≠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=vA)
we find that δz+≠0. Thus incompressible MHD turbulence,
which requires both δz+≠0 and δz-≠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+ and δz- 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.
Bibcode: 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 K2x
(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 -2, where α 2
[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 - 8/3 [case (1)] or α -
4/3 [case (2)] for large α 2. 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.
Bibcode: 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.
Bibcode: 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 B0 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1989BAAS...21..831Y
Altcode:
No abstract at ADS
Title: Coronal heating: theoretical ideas
Authors: Hollweg, J. V.
Bibcode: 1989HiA.....8..517H
Altcode:
No abstract at ADS
Title: Resonance Absorption of Solar p-Modes by Sunspots
Authors: Hollweg, Joseph V.
Bibcode: 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
Bibcode: 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
Te=Tp in r<=2 RS but drops
electron-proton coupling in r>2 Rs. The dissipated wave
energy is assumed to heat only the protons. Classical heat conduction
is used in r>10 RS, and an electron polytrope is used
in r>10 RS. The models have the right qualitative
features: a steep temperature rise to T>106 K and
acceleration to supersonic speeds. But models with base pressures
neT>2×1014 (cgs) are too
slow: v(1 AU)<280 kms-1. Models with v(1
AU)>400 kms-1 have lower base pressures:
1014<neT<2×1014
(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<=r/RS<=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
Rs. 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.
Bibcode: 1988BAAS...20..910H
Altcode:
No abstract at ADS
Title: Resonance absorption of compressible magnetohydrodynamic
waves at thin ``surfaces''
Authors: Hollweg, Joseph V.; Yang, G.
Bibcode: 1988JGR....93.5423H
Altcode:
If an MHD surface wave is supported by a true discontinuity, then the
total pressure fluctuation, δPtot, is constant across the
discontinuity. If the discontinuity is replaced by a thin transition
layer, then δPtot 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 δPtot as known, and write
the relevant equations in forms in which δPtot 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, B0. The
Alfven resonance affects the velocity and magnetic field components
normal to B0. 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.
Bibcode: 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.
Bibcode: 1988BAAS...20..690S
Altcode:
No abstract at ADS
Title: Resonance Absorption of Magnetohydrodynamic Surface Waves:
Viscous Effects
Authors: Hollweg, Joseph V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 106K,
and a substantial solar wind mass flux in excess of 3.5×108
cm-2s-1 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1985JGR....90.7620H
Altcode:
We provide a physically motivated alternate derivation of the
ɛ0 terms in the viscous stress tensor given by Braginskii
(1965) and others. We show that the ɛ0 terms are fully
accounted for by a gyrotropic diagonal pressure tensor; off-diagonal
terms do not contribute to the ɛ0 part of the viscous
stress. The ɛ0 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.
Bibcode: 1985BAAS...17..637H
Altcode:
No abstract at ADS
Title: Spicule Dynamics: Long Time Behavior
Authors: Sterling, A. C.; Hollweg, J. V.
Bibcode: 1985BAAS...17Q.631S
Altcode:
No abstract at ADS
Title: Alfvénic Pulses in the Solar Atmosphere
Authors: Mariska, J. T.; Hollweg, J. V.
Bibcode: 1985BAAS...17..643M
Altcode:
No abstract at ADS
Title: Alfvénic heating of the chromosphere and corona.
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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, υA. 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
υA; 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.
Bibcode: 1984BAAS...16..527H
Altcode:
No abstract at ADS
Title: Alfvenic Heating: An Interpretation of Coronal Loop Data
Authors: Sterling, A. C.; Hollweg, J. V.
Bibcode: 1984BAAS...16Q.527S
Altcode:
No abstract at ADS
Title: Resonances of coronal loops
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1983NASCP2280..105H
Altcode:
No abstract at ADS
Title: Impulse response of the corona.
Authors: Suess, S. T.; Hollweg, J. V.
Bibcode: 1983NASCP2280...61S
Altcode:
No abstract at ADS
Title: Coronal heating by waves.
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1983swfp.book.....N
Altcode:
No abstract at ADS
Title: Surface Waves on solar wind tangential discontinuities
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 Bo. The pulses typically have amplitudes of 60
km s−1 or so and durations of a few tens of seconds. Such
features may have been observed, suggesting that the corona is in fact
heated by fast shocks. (3) Alfvén waves exhibit a strong tendency to
drive upward flows, with many of the properties of spicules. Spicules,
and the observed corrugated nature of the transition region, may
therefore be by-products of magnetic heating of the corona. (4)
It is qualitatively suggested that Alfvén waves may heat the upper
chromosphere indirectly by exerting time-dependent forces on the plasma,
rather than by directly depositing heat into the plasma.
Title: On rotational forces in the solar wind
Authors: Hollweg, J. V.; Isenberg, P. A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
nα/np versus υp correlation and the
negative nα/np versus npυp
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 nα/np
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.
Bibcode: 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.
Bibcode: 1981Sci...212..787D
Altcode:
No abstract at ADS
Title: On the Origin of Solar Spicules
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 ni>na>np,
where i refers to ions heavier than He++;
(3) dispersive waves have a slight tendency to yield
∂Ti/∂t>(mi/ma)∂Ta/∂t,
while non-dispersive waves have a slight tendency to yield
∂Ti/∂t<(mi/ma)∂Ta/∂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.
Bibcode: 1981sowi.conf..414H
Altcode:
No abstract at ADS
Title: Alfvén waves in sunspots
Authors: Nye, A. H.; Hollweg, J. V.
Bibcode: 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 107 erg
cm−2 s−1. 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.
Bibcode: 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.
Bibcode: 1980MitAG..50...42E
Altcode:
No abstract at ADS
Title: A new resonance in the solar atmosphere. I. Theory.
Authors: Hollweg, J. V.
Bibcode: 1979SoPh...62..227H
Altcode:
We consider a horizontally stratified isothermal model of the
solar atmosphere, with vertical and uniform B0, and
vA2≫vs2. The equations
of motion are linearized about a background which is in hydrostatic
equilibrium. A homogeneous wave equation results for the motions
perpendicular to B0; 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.
Bibcode: 1979JGR....84.2141H
Altcode:
No abstract at ADS
Title: Alfven Waves on Open and Closed Solar Magnetic Flux Tubes
Authors: Hollweg, J. V.
Bibcode: 1979BAAS...11..409H
Altcode:
No abstract at ADS
Title: Some physical processes in the solar wind.
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1978JGR....83.3905H
Altcode:
JGREA,82,563,1978
Title: Geometrical MHD wave coupling
Authors: Hollweg, J. V.; Lilliequist, C. G.
Bibcode: 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 (<0.1AU).
Title: Experimental Search for Coronal Alfven Waves.
Authors: Querfeld, C. W.; Hollweg, J. V.
Bibcode: 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 B0
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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 nH = 5.2 × 1016 cm−3, and if
B⊙ = 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 J0, but depend
on B⊙, 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++ 3. Effects of
resonant and nonresonant interactions with transverse waves
Authors: Hollweg, J. V.; Turner, J. M.
Bibcode: 1978JGR....83...97H
Altcode:
Simple models are described which investigate the combined effects on
solar wind He++ of resonant and nonresonant acceleration by
left-hand transverse waves. The principal points are the following. (1)
For a wide range of parameters (να-νρ) at 1 AU
is close to the effective phase speed of the left-hand waves. (2) The
most important factor in determining να/νρ
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 να/νρ at the sun
(and of na/nρ at 1 AU) are obtained for a
power law index α?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++
abundance is greater or less than that at 1 AU. (5) Some of the models
show a positive correlation between nα/nρ and
νρ at 1 AU, roughly as has been observed. (6) The models
suggest that variations in nα/nρ 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 (να/νρ) 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.
Bibcode: 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.
Bibcode: 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 Rs,
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.
Bibcode: 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, -KSH▽∥Te,
or as the collisionless heat conduction flux (Hollweg, 1974a),
1.5nekTe(Vsw - 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 ne2/3(1+α) 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.
Bibcode: 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 B0. 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.
Bibcode: 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 υα/υp =
0.7-0.8 at 1 AU. But the model calculations still contain two important
problems: (1) we do not obtain υα ≥ υp
at 1 AU, as is frequently observed; and (2) the model calculations
give values of υα 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 <δB²>1/2 are larger in the
compression region of the stream than in the rarefaction region. (2)
The relative amplitudes <δB²>1/2/B0
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
B0, whereas in the compression region, k turns away from the
direction of B0. 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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<=3.6×10-11 cm-1,
mph<=1.3×10-48 g, and a stronger, but
less certain upper limit μ<3.1×10-12 cm-1,
mph<1.1×10-49 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.
Bibcode: 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.
Bibcode: 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 B0 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.
Bibcode: 1974sowi.conf..333H
Altcode:
No abstract at ADS
Title: Transverse Alfvén waves in the solar wind: Arbitrary k,
v 0, B 0, and |δB|
Authors: Hollweg, Joseph V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 <% 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.
Bibcode: 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 υϕ 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
<δB²>/B0² ≳ 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 υϕ at 1
AU by reducing the heliocentric distance of the critical point that
appears in the equation for υϕ. 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-1
≳ 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 RE for waves
with ω-1 ≳ 2 hours.
Title: ALFVÉNIC Motions in the Solar Atmosphere
Authors: Hollweg, Joseph V.
Bibcode: 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.
Bibcode: 1972NASSP.308..223H
Altcode: 1972sowi.conf..223H
No abstract at ADS
Title: Wavelength Dependence of the Interplanetary Scintillation
Index - Comments
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
<cos δθ>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.
Bibcode: 1972CoEl....2..423H
Altcode:
No abstract at ADS
Title: Nonlinear Landau Damping of Alfvén Waves
Authors: Hollweg, Joseph V.
Bibcode: 1971PhRvL..27.1349H
Altcode:
It is shown that large-amplitude linearly or elliptically polarized
Alfvén waves propagating parallel to B-->0 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<<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.
Bibcode: 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.
Bibcode: 1971JGR....76.7491H
Altcode:
We consider a two-component ‘model’ for the solar wind, in
which the protons become collisionless beyond r0≥10
RS, 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 Tp∥/Tp⊥ < 3 if r0 =
40 RS, while Tp∥/Tp⊥ < 2
if r0 = 55 RS. Wave-particle interactions may
therefore play a less significant role in destroying proton anisotropy
than has been heretofore thought. (2) The observed dependence of
Tp∥/Tp⊥ on solar-wind speed is consistent
with collisionless flow, and the double adiabatic equation of
state, beyond r0=55 RS, if r0 and
υ0 = υ(r0) do not change. (3) Solar rotation
leads to significantly lower mean proton temperatures, Tp =
( Tp∥ + 2Tp⊥)/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, Te∥/Te⊥ ≌ 1.2, in the region
r>10 RS 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.
Bibcode: 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 (Ap±ωr/Ωp) 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 4Ωp, 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
4×10-4 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.
Bibcode: 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 Ωp. 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⊥p⪞3T∥p 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
Ωp 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.
Bibcode: 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-2,
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-2.5, and the two length
scales are constant; or that the density varies as r-2,
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.
Bibcode: 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.
Bibcode: 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 106 °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 Te1/2 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∥/T⊥,
is predicted to be about 50.
Title: Lunar limb shock wave
Authors: Hollweg, J. V.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1968Natur.220..771H
Altcode:
THE discovery1 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.
Title: Interaction of the solar wind with the moon and formation of
a lunar limb shock wave
Authors: Hollweg, Joseph V.
Bibcode: 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 10-10 mho/m, while a 10-kilometer thick layer would
require a conductivity less than 10-7 mho/m to shield the
core; conductivities in this low range do not seem reasonable. We
thus establish an upper limit of 10-5 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 (10-4 < σ < 10-3
mho/m) of thickness between one-tenth and several kilometers.
Title: A statistical ray analysis of the scattering of radio waves
by anisotropically turbulent, non-homogeneous solar corona
Authors: Hollweg, Joseph Vincent
Bibcode: 1968PhDT.......106H
Altcode:
No abstract at ADS
Title: Properties of solar wind turbulence deduced by radio
astronomical measurements
Authors: Hollweg, Joseph V.; Harrington, J. V.
Bibcode: 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-2 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.