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Author name code: wentzel
ADS astronomy entries on 2022-09-14
author:"Wentzel, Donat G."
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Title: Division XII / Commission 46 / Program Group World-Wide
Development of Astronomy
Authors: Hearnshaw, John B.; Batten, Alan H.; Alsabti, A. Athem;
Batten, Alan H.; Fierro, Julieta; Gray, Richard O.; Hemenway, Mary
Kay M.; Kozai, Yoshihide; Levato, Hugo; Malasan, Hakim L.; Martinez,
Peter; Narlikar, Jayant V.; Wentzel, Donat G.; White, James C.
2007IAUTB..26..234H Altcode:
The Program Group for the World-wide Development of Astronomy (PG-WWDA)
is one of nine Commission 46 program groups engaged with various aspects
of astronomical education or development of astronomy education and
research in the developing world. In the case of PG-WWDA, its goals
are to promote astronomy education and research in the developing
world through a variety of activities, including visiting astronomers
in developing countries and interacting with them by way of giving
encouragement and support.
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Title: Mathematical Proofs
Authors: Wentzel, D. G.
2004dbss.book..501W Altcode:
No abstract at ADS
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Title: Thermal Radiation
Authors: Wentzel, D. G.
2004dbss.book..449W Altcode:
No abstract at ADS
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Title: High-Energy Astrophysics, Electromagnetic Radiation
Authors: Wentzel, D. G.
2004dbss.book..489W Altcode:
No abstract at ADS
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Title: More Mechanics of the Solar System
Authors: Wentzel, D. G.
2004dbss.book..423W Altcode:
No abstract at ADS
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Title: Mechanics: Orbits and Kepler's Third Law
Authors: Wentzel, D. G.
2004dbss.book..399W Altcode:
No abstract at ADS
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Title: Astrophysics for University Physics Courses: Introduction
Authors: Wentzel, D. G.
2004dbss.book..391W Altcode:
Chapters 8.1 to 8.9 respond to the question posed by many universities:
"How can we introduce some astrophysics in our physics courses?" The
question is often qualified: "We cannot teach a whole course in
astrophysics." In these Chapters I present an array of astrophysical
problems, any one or a few of which can be selected and used within
existing physics courses on elementary mechanics, or on heat and
radiation, kinetic theory, electrical currents, and in some more
advanced courses. Answers are provided to all problems. These
astrophysics problems are designed to be an interesting and
challenging extension of existing physics courses, to test the
student's understanding of physics by testing it in new realms,
and to stretch the student's imagination. A brief tutorial on the
astrophysics is provided with each problem, enough so that the physics
professor can present the problem in class. The higher-level problems
start with a brief introduction to the physics. <P />All the problems
seek compact algebraic and numerical solutions that can easily be
translated into physics. For many problems, the solution is shorter
than the statement of the problem. A few mathematical proofs appear
in Chapter 8.9. The problems on mechanics (Chapter 8.2 to 8.4) are
nearly independent of each other, so that any one of them can be used
in an appropriate introductory physics course. However, the seven
problems in Chapter 8.2, "Mechanics - Orbits and Kepler's third law"
can be used together as a mini-course on many of the interesting topics
in modern astronomy and astrophysics, ranging from the solar system
to black holes in galaxies. The problems in Chapter 8.5 on thermal
radiation can be selected independently of each other. The problems on
the lives of stars (Chapter 8.6) are best done in sequence, starting
with the introduction, but it is easy to stop at any point without
reaching the (academically more challenging) end of the Chapter. It
is quite possible to create a mini-course on the Sun by selecting
the solar problems in Chapter 8.5 to 8.7. The problems on the cosmic
magnetic fields and high-energy astrophysics (Chapter 8.7 and 8.8) are
academically more advanced. However, although their backgrounds involve
Maxwell's equations and special relativity, the actual problems deal
with quantities that are physically intuitive. A mini-course on neutron
stars and pulsars can be constructed using problems in Chapters 8.4,
8.5, 8.7, and 8.8.
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Title: More Mechanics: Neutron Stars and Clusters of Galaxies
Authors: Wentzel, D. G.
2004dbss.book..437W Altcode:
No abstract at ADS
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Title: Cosmic Magnetic Fields
Authors: Wentzel, D. G.
2004dbss.book..477W Altcode:
No abstract at ADS
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Title: The Lives of Stars
Authors: Wentzel, D. G.
2004dbss.book..459W Altcode:
No abstract at ADS
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Title: A renewal of astronomy education in Vietnam
Authors: Wentzel, Donat G.
2001IAUGA..24...46W Altcode: 2000IAUSS..24E...4W
Vietnam was scientifically completely isolated for almost 30 years. With
French help, several lecture courses have introduced modern astrophysics
to Vietnamese physicists, and four students are studying abroad. The
IAU program "Teaching for Astronomy Development" (TAD) has concentrated
on modernizing the on-going astronomy course for students in the third
year of the pedagogical universities. Three one-to-two week "Teachers'
Workshops" have served to introduce selected up-to-date astronomical
topics and a few modern teaching methods. The TAD program has also
provided appropriate journals, books, a PC and educational software. A
new text, Astrophysics, in Vietnamese and English on facing pages and
with color pictures - apparently a first for any textbook in Vietnam
- will first be used starting in September 2000. Future aditional
activities: collaboration to plan a new astronomy course in the
twelfth grade of the natural science branch of the secondary schools;
collaboration so that the 41-cm telescope and astronomers in Hanoi
can produce some simple quality science; helping the only planetarium
in Vietnam to acquire a wider range of offerings; helping to create a
set of B.Sc.-level astrophysics courses for three universities; and
supporting the Vietnamese Astronomical Society in effective public
outreach.
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Title: Commission 46: Teaching of Astronomy: (Enseignement de
L'astronomie)
Authors: Fierro, Julieta; Isobe, Syuzo; Jones, B.; Batten, A.;
Arellano, A.; Gervaldi, M.; Guinan, E.; Tush, W.; Hoff, D.; Martinez,
P.; McNally, D.; Norton, A.; Narlikar, J.; Pasachoff, J.; Percy, J.;
Wentzel, D.
2000IAUTA..24..423F Altcode:
No abstract at ADS
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Title: Working Group for the Worldwide Development of Astronomy:
(Groupe de Travail Pour le Developpement Mondial de L'Astronomie)
Authors: Batten, A. H.; Fierro, J.; Hearnshaw, J. B.; Hidayat, B.;
Kozai, Y.; McNally, D.; Pineda de Carias, M.; Roberts, M. S.; Wentzel,
D. G.; Raither, S.
2000IAUTA..24..442B Altcode:
No abstract at ADS
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Title: Astronomy Education and the American Astronomical Society
Authors: Fraknoi, Andrew; Wentzel, Donat
1999aasf.book..194F Altcode:
This is a history of the role that astronomy education played in the
meetings and publications of the American Astronomical Society, during
the first century of its existence. We show that interest in education
at the Society was cyclical. It ebbed and flowed as small groups of
energetic astronomers got interested in education and pushed on the
governing councils to expand its role, and then retired or lost interest
and new governing groups returned to an emphasis on research concerns.
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Title: International Educational Projects
Authors: Wentzel, D. G.
1998HiA....11..901W Altcode:
No abstract at ADS
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Title: International Schools for Young Astronomers, Astronomically
developing countries and Lonely Astronomers
Authors: Wentzel, D.
1998ntat.coll...27W Altcode: 1998IAUCo.162...27W
No abstract at ADS
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Title: The Ann Arbor Community
Authors: Wentzel, Donat G.
1998ASSL..222...40W Altcode: 1998ream.conf...40W
No abstract at ADS
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Title: Type I Radio Bursts Deflected by Lower Hybrid Waves
Authors: Wentzel, Donat G.
1997SoPh..175..175W Altcode:
What are the observational effects if type I bursts are deflected
by lower-hybrid waves? The deflection creates an elliptical radio
mirage, with the direct source at one end of the minor axis. Unlike a
`scattering disk', the mirage has considerable polarization structure
which should be observable at high angular resolution. The lower-hybrid
waves have four attractive features to explain existing observations
of type I bursts: they naturally yield zero polarization for limb
bursts; they preserve a high directivity even for limb bursts; they
can explain the observed constancy of polarization during partially
polarized bursts; and they depolarize at a sufficient height so that
the resulting two circular polarizations arrive nearly simultaneously,
as observed. The small time interval between the arrival of the two
polarizations requires that we normally observe only the mirage and
that the source be asymmetric with respect to azimuth around the
magnetic field. The waves appear to be restricted to the vicinity of
type I sources.
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Title: On the Polarization of Type I Radio Bursts
Authors: Wentzel, Donat G.
1997SoPh..175..191W Altcode:
Circularly polarized radio radiation maintains its polarization
even where the magnetic field reverses its sign relative to the
ray (QT region) if the reversal is sufficiently abrupt (strong
QT region). Bastian (1995) suggested that coronal turbulence
scatters radiation, such as type I bursts, sufficiently to make
the reversal abrupt where it would otherwise not be. However, the
observed directivity of type I bursts sets an upper limit on the
scattering. This limit implies that the turbulent scattering is not
sufficient to maintain the circular polarization as in a strong QT
region. The conclusion is strengthened by an analytical calculation
of the polarization. Apparently, the fully polarized type I bursts,
near disk center, encounter no horizontal magnetic fields, at least
not until high enough in the corona that the QT region is strong anyway.
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Title: Astronomy Education Programmes
Authors: Wentzel, D. G.
1997IAUJD..20E..20W Altcode:
No abstract at ADS
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Title: Solar Active Regions as a Percolation Phenomenon. II.
Authors: Seiden, Philip E.; Wentzel, Donat G.
1996ApJ...460..522S Altcode:
The persistence of solar active regions was previously modeled
in terms of percolation releasing magnetic flux from deep in the
solar interior (Wentzel & Seiden 1992). Harvey's (1993) data on
the size distribution of active regions at the end of emergence are
reinterpreted in terms of a power law at small sizes and exponential at
larger sizes. This combination is characteristic of percolation near
the critical point. We have added a simple model for the magnetic
polarity of the released loops and for their diffusion once they
reach the surface. We present simulated active-region maps and size
and age distributions.
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Title: Unusual wave phenomena near interplanetary shocks at high
latitudes
Authors: Thejappa, G.; Wentzel, Donat G.; MacDowall, R. J.; Stone,
R. G.
1995GeoRL..22.3421T Altcode:
We report on several interplanetary shocks that are unusual because
waves at about 10 Hz are highly electrostatic in the upstream region
yet highly electromagnetic in the downstream region. These shocks,
detected by the Unified Radio and Plasma Wave Experiment (URAP) on
Ulysses are supercritical reverse shocks, which occurred predominantly
at high heliographic latitudes. The level of wave activity is observed
to be independent of the angle between the magnetic field and the shock
normal, the Mach number, and the change in the ratio of ion thermal
pressure to magnetic pressure from upstream to downstream regions. Since
the energy in the upstream electrostatic waves is small compared to the
downstream electromagnetic waves, mode conversion from electrostatic
to electromagnetic at the shock is not a viable mechanism. The upstream
and downstream waves are likely to be generated by separate mechanisms,
with lower hybrid waves being the most probable candidates for the
upstream waves and electromagnetic lower hybrid or whistlers for the
downstream waves.
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Title: Low-frequency waves associated with Langmuir waves in
solar wind
Authors: Thejappa, G.; Wentzel, Donat G.; Stone, R. G.
1995JGR...100.3417T Altcode:
The Ulysses spacecraft has detected several events of low-frequency
electromagnetic waves in association with Langmuir waves in the
solar wind. The high time resolution observations show that the
Langmuir waves are very intense and occur as broad peaks superposed by
collapsing millisecond spikes. The low-frequency waves are identified
as electomagnetic lower hybrid waves. The observed energy densities of
these waves often exceed the strong turbulence thresholds. It is shown
that none of the parametric decay instabilities involving Langmuir
and low-frequency waves are energetically favorable to explain the
present observations. The low-frequency waves are proposed to arise
from currents associated with gradients in the electron beam originating
at sites where Langmuir waves scatter the beam electrons.
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Title: Heating of the Solar Corona - Theory
Authors: Wentzel, D.
1995pist.conf..118W Altcode:
No abstract at ADS
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Title: IAU Programs and Projects in the Developing Countries
Authors: Wentzel, D. G.
1995HiA....10..147W Altcode:
No abstract at ADS
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Title: Solar Active Regions as a Percolation Phenomenon
Authors: Wentzel, D. G.; Seiden, P. E.
1994AAS...185.9208W Altcode: 1994BAAS...26.1473W
We have previously suggested that the appearance of magnetic active
regions on the surface of the sun arises from a percolation of flux out
of the dynamo layer (1992, Wentzel and Seiden, ApJ 390, 280). We found
an exponential distribution of cluster sizes similar to that found
by Tang, et al. (1984, Solar Phys. 91, 75). Recently Harvey (1993,
Thesis, Univ. Utrecht) and Harvey and Zwaan (1993, Solar Phys. 148,
85) have shown that for very small regions the distribution is a
power law in cluster size with an exponent of about -2. This type of
distribution, power law at small sizes and exponential for large sizes
is, just what is expected for a percolation problem. We have extended
our model to take into account the bipolar nature of the flux and to
allow for the buildup and diffusion of flux on the surface. The model
yields results in good accord with the observations. It has a power
law for small clusters with the same exponent as the observations
and becomes more nearly exponential like at large sizes. As the size
gets larger the exponential scale length increases, in accord with
observations selected according to various longevity criteria. The
original motivation for percolation, i.e., new active regions arise
disproportionately in existing active regions, is confirmed by the
present model. However, the physics of percolation, i.e., the dual
nature of the size distribution, is demonstrated best upon ignoring
internal regions.
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Title: Electromagnetic Radiation from a Strong DC Electric Field
Authors: Guedel, Manuel; Wentzel, Donat G.
1993ApJ...415..750G Altcode:
Computer simulations of electrons accelerated by a strong DC electric
field show not only very efficient generation of beam waves but also
emission of o-mode radiation. We present a set of particle simulations
for which we study the behavior of wave generation under varying
conditions. We determine that the o-mode arises from a combination of
beam waves and of z-mode waves that are themselves generated by beam
waves. We estimate how these interactions depend on beam density.
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Title: Group Activities in Introductory Astronomy Courses for
Non-science Majors
Authors: Deming, G. L.; Wentzel, D. G.
1993AAS...182.2106D Altcode: 1993BAAS...25..823D
No abstract at ADS
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Title: A New Explanation of Solar Radio Millisecond “Spikes”
Authors: Wentzel, Donat G.
1993ApJ...407..380W Altcode:
The study proposes an explanation for solar radio spikes that involves
an electromagnetic instability in fast electrons at a frequency
of greater than 2 Omega (Omega = cyclotron frequency), so that the
radiation can escape, and in which the unstable anisotropy is easily
regenerated. The instability occurs after scattering of an electron beam
by Langmuir waves. The radiation grows best along surfaces of constant
magnetic field strength and has very narrow intrinsic bandwidth.
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Title: A Radiation Mechanism for Solar Millisecond Radio Bursts
Authors: Wentzel, D. G.
1992AAS...180.0512W Altcode: 1992BAAS...24..735W
No abstract at ADS
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Title: Solar Active Regions as a Percolation Phenomenon
Authors: Wentzel, Donat G.; Seiden, Philip E.
1992ApJ...390..280W Altcode:
The appearance of solar active regions is modeled using percolation
theory. An attempt is made to bundle all the very complicated magnetic
phenomena into two dimensionless parameters. The main parameter is
the probability, Pst, that the release and rise of one flux tube
stimulates the subsequent release and rise of a neighboring flux
tube. A second parameter measures the lifetime of flux once it has
arrived at the surface. This hypothesis is used to reproduce several
properties of the distribution of active regions on the sun. (1) The
active regions persist for a long time. Magnetic flux emerges mostly
where there is flux already. (2) There are persistent empty regions,
reminiscent of coronal holes. (3) The dependence on Pst is that of a
phase transition. (4) The size distribution of the active regions is
close to exponential, as observed.
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Title: Structure of Sunspot Penumbrae: Fallen Magnetic Flux Tubes
Authors: Wentzel, Donat G.
1992ApJ...388..211W Altcode:
A model is presented of a sunspot penumbra involving magnetic flux tubes
that have fallen into the photosphere and float there. An upwelling
at the inner end of a fallen tube continuously provides additional
gas. This gas flows along and lengthens the tube and is observable
as the Evershed flow. Fallen flux tubes may appear as bright streaks
near the upwelling, but they become dark filaments further out. The
model is corroborated by recent optical high-resolution magnetic
data regarding the penumbral filaments, by the 12-micron magnetic
measurements relevant to the height of the temperature minimum, and
by photographs of the umbra/penumbra boundary.
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Title: Solar and Stellar Radio Spikes: Limits on the Saturation of
the Electron-Cyclotron Maser
Authors: Wentzel, Donat G.; Aschwanden, Markus J.
1991ApJ...372..688W Altcode:
The solar millisecond radio 'spikes' have been explained in terms of
X-mode radiation generated by a maser near the electron gyrofrequency,
acting on fast coronal electrons with a loss cone. This maser is a
phenomenon described by quasi-linear theory. It is sensitive to the
small first-relativistic correction to the gyrofrequency. Thus, it might
be disrupted rather easily by nonlinear effects. The maximum radiation
density that can be reached before the radiation entrains (phase-locks)
the electrons and saturates the maser is discussed. If the observed
durations of solar radio spikes are a measure of the rate of scattering
into the loss-cone, then the inferred energy density is at least two
orders of magnitude less than the energy density at which entrainment
sets in. Also, maser emission from auroral kilometric radiation does not
reach wave energies critical for electron entrainment. Maser emissions
from flare stars, however, show 3-4 orders of magnitude higher radio
fluxes and brightness temperatures than for the solar case and are
likely to be saturated by entrainment.
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Title: Direct Radiation from a Strong DC Electric Field
Authors: Wentzel, Donat G.
1991ApJ...373..285W Altcode:
Simulations have shown that a strong dc electric field yields,
after electrostatic transients, a beam of electrons that is nearly
flat in velocity distribution yet emits strong electrostatic and
electromagnetic waves. Electric energy is converted 'directly' into
radiation. This radiation is derived analytically in order to learn
how the radiation may depend on field strength and beam density in
astronomical objects. Any wave with a significant electric component
along the dc electric field grows exponentially in time, with the
growth rate proportional to the number of electrons in the beam. The
lower limits on the dc electric field and the beam density are set by
competing plasma processes. The results are related to solar millisecond
radio bursts.
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Title: The Solar Chimes - Searching for Oscillations Inside the Sun
Authors: Wentzel, D. G.
1991Mercu..20...77W Altcode:
No abstract at ADS
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Title: Active Regions as a Percolation Phenomenon
Authors: Wentzel, D. G.; Seiden, P. E.
1991BAAS...23.1034W Altcode:
No abstract at ADS
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Title: Book-Review - the Restless Sun
Authors: Wentzel, D. G.; Bumba, V.
1990SSRv...54R.450W Altcode:
No abstract at ADS
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Title: Diverse Structures, same Science
Authors: Wentzel, Donat G.
1990teas.conf....1W Altcode: 1990IAUCo.105....1W
No abstract at ADS
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Title: Programs of IAU Commission 46 for Developing Countries
Authors: Wentzel, Donat G.
1990teas.conf..405W Altcode: 1990IAUCo.105..405W
No abstract at ADS
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Title: The Penetration of a Very Hot Gas into a Warm Interstellar
Cloud
Authors: Wentzel, D. G.
1989BAAS...21..763W Altcode:
No abstract at ADS
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Title: Magnetohydrodynamic Wave Conversion and Solar-Wind Acceleration
in Coronal Holes
Authors: Wentzel, Donat G.
1989ApJ...336.1073W Altcode:
The conversion of Alfven waves to fast-mode waves and the subsequent
dissipation due to Landau damping near the height where the coronal-hole
nozzle diverges rapidly are examined. The parameters which describe the
conditions in which the Alfven wave will be converted are discussed. It
is found that refraction can cause the conversion of up to half of
the Alfven-wave energy to fast-mode energy, or vice versa. It is shown
that MHD wave conversion useful for wind acceleration near the sonic
point requires wave refraction caused by the variation of the Alfven
speed across the coronal hole.
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Title: The restless sun
Authors: Wentzel, Donat G.
1989resu.book.....W Altcode: 1989QB521.W46......
No abstract at ADS
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Title: Solar-Wind Acceleration in Coronal Holes
Authors: Wentzel, D. G.
1988BAAS...20Q.715W Altcode:
No abstract at ADS
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Title: Solar Oscillations: Generation of a g-Mode by Two p-Modes
Authors: Wentzel, Donat G.
1987ApJ...319..966W Altcode:
Three modes of solar oscillations can be coupled resonantly by the
nonlinear terms in the equations of motion. A general integral for
the coupling rate was derived by Dziembowski. The author evaluates
the coupling of two p-modes, of nearly identical frequencies, so as
to generate a g-mode. The coupling occurs primarily in the convection
zone. A rather select set of g-modes of suitably low order and degree
and with weak linear damping may grow, but the modes saturate when
surface amplitudes are still unobservably small.
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Title: Solar Oscillations: A Method for Deriving Nonlinear Effects
Authors: Wentzel, Donat G.
1987ApJ...317..477W Altcode:
The frequencies of solar oscillations are so closely spaced that
nonlinear interactions among modes are probable. The rate of interaction
is proportional to an integral involving the eigenfunctions of the
interacting modes, which usually are known only numerically. An
approximation in which the eigenfunctions are strictly sinusoidal
functions of a suitably defined radial variable, and the numerical
details of stellar structure are banished to a coefficient in the
integrand, are here explored. The physical assumptions are the same as
in the asymptotic approximation of p- or g-modes. This method should
allow a general investigation as to likely nonlinear interactions and
which modes may participate in such interactions. The coupling of two
p-modes by possible large-scale internal magnetic fields is introduced
as an anisotropic pressure response to a displacement. Pairs of modes
differing in frequency by less than the fraction magnetic/thermal
pressure are strongly coupled, and energy appears to oscillate slowly
between the two associated spherical harmonics. Potentially, upper
limits may be derived for internal magnetic fields.
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Title: A test for large-angle radio scattering in the solar corona
Authors: Wentzel, D. G.; Zlobec, P.; Messerotti, M.
1986A&A...159...40W Altcode:
A significant fraction of all solar type-I radio bursts are only
partially or negligibly polarized. The polarization during such bursts
is shown to remain so constant that the delay during coronal propagation
of one circularly polarized mode relative to the other is at most 4
milliseconds. This leads to the deductions (1) that the radiation is
emitted fully polarized, because partially polarized emission anywhere
near the plasma level would yield vastly longer delays, and (2) that
the depolarization occurs by large-angle scattering at least as high
in the corona as the harmonic level. Any roughly isotropic scattering
is excluded, because it cannot yield bursts of sufficiently constant
polarization. One model of highly directional scattering in which
the radiation is 'reflected' by the magnetic field, much like from a
half-silvered mirror, is discussed. Bursts within a group arise from
a single source.
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Title: Solar Oscillations: Interactions Among p-Modes
Authors: Wentzel, D. G.
1986BAAS...18..663W Altcode:
No abstract at ADS
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Title: A Theory for the Solar Type-I Radio Continuum
Authors: Wentzel, D. G.
1986SoPh..103..141W Altcode:
The type-I radio continuum may arise from the combination of two
electrostatic waves, both directed nearly normal to the magnetic
field. One wave, near the upper-hybrid frequency, is generated by
gyroresonance with superthermal electrons and comes into equilibrium
with these electrons. The other wave, at the lower-hybrid frequency, is
generated by the loss-cone instability of trapped superthermal protons
in those wave directions for which the lower-hybrid frequency is an
exact multiple of the proton gyrofrequency. The brightness temperature
of the continuum indicates both the energy of the superthermal electrons
and the existance of at least a small number of superthermal protons.
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Title: Solar Activity and the Coupling of g-Mode Oscillations
Authors: Wentzel, D. G.
1986ApJ...300..824W Altcode:
The solar g-mode oscillations depend strongly on the difference between
actual and adiabatic temperature gradients in the solar interior. Solar
activity anchored at the base of the convection zone disturbs this
difference and couples modes with similar eigenfrequencies. The author
computes this coupling and discusses the possible observational effects
at the photosphere. These include a scatter in the frequencies deduced
for weakly coupled modes and the apparent temporal behavior of strongly
coupled modes.
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Title: Solar Oscillations: a Method for Deriving Nonlinear Effects
Authors: Wentzel, D. G.
1985BAAS...17..896W Altcode:
No abstract at ADS
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Title: Langmuir waves from trapped superthermal electrons
Authors: Wentzel, D. G.
1985ApJ...296..278W Altcode:
A 'gap' distribution of trapped superthermal but nonrelativistic
electrons may generate Langmuir waves with a high brightness temperature
1/2 mc-squared, which has been invoked to explain the solar type
I radio continuum. The conditions under which the Langmuir waves
can actually become this intense when the electrons are treated as
magnetized with a gyrofrequency smaller than the plasma frequency
are evaluated. For isotropic electrons at a single velocity, there
arise many instabilities, but most disappear for a slightly broadened
velocity distribution. A serious Cerenkov instability, equivalent to the
two-stream instability, occurs for electrons of small pitch angles. It
is avoided by a modest loss cone. Then the Langmuir waves within 45 deg
of the magnetic field can indeed achieve the expected high brightness
temperature. The solar type I radio continuum can be explained in
terms of a nonrelativistic electron gap distribution. Superthermal
microwave gyroradiation implies sufficient superthermal electrons
that their Langmuir waves might cause observable microwave emission
at twice the plasma frequency.
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Title: A Test for Large-Angle Radio Scattering in the Solar Corona
Authors: Wentzel, D. G.; Zlobec, P.; Messerotti, M.
1985BAAS...17..632W Altcode:
No abstract at ADS
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Title: Self-confined cosmic rays
Authors: Wentzel, D. G.
1985IAUS..107..341W Altcode:
The theory of self-confinement explains the isotropy of the bulk
of the cosmic rays but not of cosmic rays above 10<SUP>3</SUP>GeV;
it has been a stimulus to the theory for cosmic-ray acceleration
at supernova shocks; and, on inclusion of diffusion in a galactic
wind, it may explain the uniform cosmic-ray density out to 18 kpc
in our galaxy. Rapidly streaming electrons in clusters of galaxies,
in supernova remnants, and near solar flares are accommodated by the
theory when it is expanded to include the effects of hot plasmas and
other wave modes. A "resonance gap" may prevent the turning backwards
of streaming particles and thus allow streaming near the particle speed.
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Title: Heating of the Upper Solar Corona over Active Regions
Authors: Wentzel, D. G.
1984BAAS...16..527W Altcode:
No abstract at ADS
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Title: Polarization of Fundamental Type-Iii Radio Bursts
Authors: Wentzel, D. G.
1984SoPh...90..139W Altcode:
The fundamental of type III bursts is only partially polarized, yet
all theory for emission near the plasma frequency predicts pure o-mode
emission. I argue depolarization is inherent in the burst itself. The
o-mode radiation is intensely scattered and mode-converted when
it temporarily falls behind its own source and finds itself in the
medium that is already disturbed by the electron beam. In particular,
mode conversion is very efficient and yet causes only modest angular
scattering at the height were ω≃ω<SUB>p</SUB> + 0.5Ω.
---------------------------------------------------------
Title: Coronal evolution and solar type I radio bursts: an
ion-acoustic wavemodel.
Authors: Benz, A. O.; Wentzel, D. G.
1984ost1.conf..127B Altcode:
No abstract at ADS
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Title: Kinetic and dynamic problems
Authors: Wentzel, D. G.
1984ost1.conf..187W Altcode:
No abstract at ADS
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Title: Summary of the Session on Educational Astronomy
Authors: Wentzel, D. G.
1984aprm.conf..247W Altcode:
No abstract at ADS
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Title: Needs and Opportunities
Authors: Wentzel, D. G.
1984aprm.conf..213W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar radio emission very near the plasma frequency
Authors: Wentzel, D. G.
1983ApJ...270..250W Altcode:
The propagation of signals from the solar corona emitted very near
the plasma frequency is investigated. The group velocity near the
plasma frequency is analyzed; near the edge of the cone of emission,
the bandwidth of the escaping radiation is much smaller than the
inherent bandwidth of emission. Finite burst duration is observed
because of the differential delays in the various parts of the signal,
even when there is no subsequent coronal scattering. A formula for
the burst duration when the density gradient and magnetic field are
parallel is presented. For a homogeneous corona, the burst duration is
0.1 s longer than observed. The signal can be shortened by invoking
a smaller density scale height near the plasma level or letting the
direction of the density gradient lie outside the cone of emission. A
coronal model to account for the observations is suggested.
---------------------------------------------------------
Title: Solar radio storms. Proceedings of the 4th CESRA workshop on
solar noise storms, held at Trieste, Italy, August 9 - 13, 1982.
Authors: Benz, A. O.; Zlobec, P.; Wentzel, D. G.; House, L. L.; Dulk,
G. A.; Tlamicha, A.; Melrose, D. B.
1983srs..work.....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New ideas on noise storm theories.
Authors: Wentzel, D. G.
1983srs..work..145W Altcode:
The author summarizes the qualitative aspects of noise storm
theories. He outlines a working model for type I bursts and the
continuum. The model invokes weak shocks, traversing the corona and
accelerating fast electrons. The shocks and electrons together yield
the observed radiation. Questions related to observed bursts of very
short duration are briefly outlined.
---------------------------------------------------------
Title: Is there a common explanation for scattering of type III
radio bursts and solar radar?
Authors: Wentzel, D. G.
1982SoPh...79..375W Altcode:
The apparently common source heights of type III fundamental and
harmonic components and the source height of the solar 38 MHz radar
echoes may all refer to scattering at a coronal level where (radio
wavenumber) × (electron gyroradius) ≃ 1, that is, where radio
frequency = (2 to 5) × plasma frequency.
---------------------------------------------------------
Title: On the solar type III radio burst emission process
Authors: Wentzel, D. G.
1982ApJ...256..271W Altcode:
The application of a version of the nonlinear plasma theories involving
plasma solitons to the corona is investigated. A fairly compact beam
is assumed in order to initiate nonlinear plasma phenomena in the
corona. The beam's survival is favored by open magnetic fields, and
the solitons occur in a front about 1 km thick. The fundamental is
emitted at this front and depends strongly on the beam energy through
its dependence on the narrow angle of emission. The variability
of the fundamental indicates the beam evolution at the beam's
front. Observations addressed by the theory include correlations of
type III bursts with coronal structure and temperature, the emission of
fundamental and harmonic from different source volumes, the polarization
of the fundamental, and the starting and disappearance of bursts in
the corona, especially type IIIb bursts.
---------------------------------------------------------
Title: Radio Emission Very Near the Plasma Frequency
Authors: Wentzel, D. G.
1982BAAS...14..607W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New ideas on noise storm theories
Authors: Wentzel, D. G.
1982srs..work..145W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Summary of discussions and conclusions of theoretical
working group
Authors: Wentzel, D. G.
1982srs..work..327W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Teaching of Astronomy
Authors: Wentzel, D. G.
1982IAUTA..18..633W Altcode: 1982IAUT...18..633W
No abstract at ADS
---------------------------------------------------------
Title: Radio emission from the Seyfert galaxy NGC 5548
Authors: Ulvestad, J. S.; Wilson, A. S.; Wentzel, D. G.
1982IAUS...97..191U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A new interpretation of James's solar radar echoes involving
lower-hybrid waves
Authors: Wentzel, D. G.
1981ApJ...248.1132W Altcode:
In the surprisingly large-bandwidth and high-intensity solar radar
echoes observed by James (1968, 1970) at 38 MHz, the radar beam is
scattered by density fluctuations high in the corona, typically at 2.2
solar radii. At such heights, the plasma wave mode most suitable for
interaction with the radar is the lower hybrid mode, whose waves are
driven at sites of steep density gradients and associated electrical
currents in the corona. The hypothetical coherence of the lower-hybrid
waves makes the required coronal activity and energy supply reasonable,
so that the radar may be considered as a probe for coherent plasma waves
in the corona. Alternative explanations for the radar echoes are shown
to be improbable, and it is pointed out that unusually intense radar
echoes also require a suitable geometrical alignment of the density
gradients with the large-scale coronal structure.
---------------------------------------------------------
Title: Solar type I radio bursts - Shock model
Authors: Wentzel, D. G.
1981A&A...100...20W Altcode:
The brief intense emission of type I bursts is taken to be evidence
for localized strong electrical currents in the corona. The radiation
mechanism is considered to be the combination of upper-hybrid and
lower-hybrid waves. The upper-hybrid waves, driven by the loss-cone
of trapped fast electrons, are highly directional and can yield the
observed high brightness temperature. The lower-hybrid waves provide
a high radiative opacity and are most probably generated by shocks.
---------------------------------------------------------
Title: Coronal heating.
Authors: Wentzel, D. G.
1981NASSP.450..331W Altcode: 1981suas.nasa..331W
Theoretical arguments for coronal heating that take into account coronal
structure are reviewed. Heating by the dissipation of electrical
currents and heating by dissipation of waves are discussed. Loop
structures in the solar corona are described.
---------------------------------------------------------
Title: Solar Type III Radio Emission Process
Authors: Wentzel, D. G.
1981BAAS...13..543W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book-Review - Wave Instabilities in Space Plasmas
Authors: Palmadesso, P. J.; Papadopoulos, K.; Wentzel, D. G.
1981ApL....21..128P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal evolution and solar type I radio bursts - an
ion-acoustic wave model
Authors: Benz, A. O.; Wentzel, D. G.
1981A&A....94..100B Altcode:
A model is proposed for type I burst emission that can accommodate
both the main burst observations and an origin for the continuum. It
is assumed that ion-acoustic waves are generated in the burst source
by a current that is related to the coronal magnetic evolution, in
particular to magnetic nonequilibrium caused by photospheric changes
(e.g. emerging magnetic field) in active regions. Radio emission
arises from coalescence of ion-acoustic and plasma waves. Contrary
to other plasma wave models, emission at the harmonic of the plasma
frequency is below the present detection threshold (not greater
than 0.1%). The ion-acoustic wave density, having a high saturation
value, determines the optical depth, which reaches unity within a
few meters. The brightness temperature is thus entirely given by
the level of Langmuir waves. These waves may be produced by trapped
non-thermal electrons from previous burst sources. The same population
also provides sufficient plasma waves for the type I continuum, which
may arise from interactions with low-frequency waves present in the
corona during times of type I activity.
---------------------------------------------------------
Title: A Model for Solar Type I Radio Bursts Based on Lower-hybrid
Waves
Authors: Wentzel, D. G.
1980BAAS...12..816W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A New Look at James' Solar Radar Results
Authors: Wentzel, D. G.
1980BAAS...12..505W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Type-I Radio Bursts - an Ion-Acoustic Wave Model
Authors: Benz, A. O.; Wentzel, D. G.
1980IAUS...86..251B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the fundamental emission of type III solar radio bursts
Authors: Wentzel, D. G.
1979A&A....80..268W Altcode:
Radio emission from plasma waves which are generated by a beam of
electrons and form nonlinear structures ('antennas' of finite size) is
discussed. One-dimensional solitons emit negligible radiation at or near
the plasma frequency, and can produce type III radio bursts only at the
harmonic. However, fundamental emission at about 1.1 times the plasma
frequency in the undisturbed medium (omega<SUB>p)</SUB> is possible if
the plasma waves 'collapse' in 2 or 3 dimensions. It is estimated that
emission from finite-sized plasma structures at 1.1 omega<SUB>p</SUB>
yields a reasonable polarization, which is proportional to the Alfven
speed, independent of the details of the emission process. The ratio of
the polarizations in harmonic and fundamental emission for F-H pairs
depends on the structure of the electron beam and should show strong
center-limb dependence when it is in the observed range of values. A
correlation between polarization and drift speed indicates that the
plasma is at most marginally in the 2-D regime.
---------------------------------------------------------
Title: The Restless Solar Corona: Type I Radio Bursts and the Magnetic
Evolution of Coronal Loops
Authors: Wentzel, D. G.
1979BAAS...11..679W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The dissipation of hydromagnetic surface waves.
Authors: Wentzel, D. G.
1979ApJ...233..756W Altcode:
When hydromagnetic surface waves travel along a surface with a thin but
finite boundary layer, velocities within this layer become singular
when computed according to the ideal MHD equations. The present
paper computes the corresponding rate of wave damping. Sufficiently
weak surface waves are dissipated either by a resonant conversion
into kinetic Alfven waves or by viscosity. Astrophysically important
surface waves may involve such large velocity amplitudes outside the
narrow zone of linear dissipation that nonlinear phenomena limit the
singularity and the method of dissipation. Even these, however, are
confined to extremely narrow layers.
---------------------------------------------------------
Title: Hydromagnetic surface waves on cylindrical fluxtubes.
Authors: Wentzel, D. G.
1979A&A....76...20W Altcode:
Hydromagnetic surface waves resulting from a magnetic discontinuity are
analyzed for the case of cylindrical geometry. The results are applied
to cylindrical magnetic flux tubes in the solar photosphere. Physical
properties of the hydromagnetic surface waves are discussed, with
emphasis on differences between such waves on plane and cylindrical
surfaces. It is concluded that possible linear mode conversion between
surface and body waves is not affected by cylindrical curvature.
---------------------------------------------------------
Title: Type I Solar Radio Bursts
Authors: Benz, A. O.; Wentzel, D. G.
1979BAAS...11..441B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hydromagnetic surface waves.
Authors: Wentzel, D. G.
1979ApJ...227..319W Altcode:
Plane and filamentary structures aligned with a magnetic field abound
on the sun and in both interplanetary and interstellar space. When
the Alfven speed changes across such boundaries, hydromagnetic
surface waves can travel along them, carry energy, and provide
heating. This paper surveys the nature of such surface waves, with
emphasis on the dispersion relations, the spatial extent of the waves,
the degree of gas compression, and the possibility of coupling to
ordinary hydromagnetic waves. Explicit results are provided for the
cases where the gas pressure is either much smaller or much larger
than the magnetic pressure on either side of the surface. The waves
are shear waves wherever the gas pressure is much smaller than the
magnetic pressure. All surface waves involve finite gas compression,
but this compression is negligible when the product of wave vector
times magnetic-field vector is much smaller than the product of wave
number times magnetic-field strength.
---------------------------------------------------------
Title: Discussion
Authors: Hirayama, T.; van Tend, W.; Wentzel, D.
1979phsp.coll..305H Altcode: 1979IAUCo..44..305H
No abstract at ADS
---------------------------------------------------------
Title: Discussion
Authors: Wentzel, D.; Leer, E.
1979phsp.coll..231W Altcode: 1979IAUCo..44..231W
No abstract at ADS
---------------------------------------------------------
Title: Heating of the solar corona: a new outlook.
Authors: Wentzel, D. G.
1978RvGSP..16..757W Altcode: 1978RvGeo..16..757W
Observations of the last few years show the corona to be highly
inhomogeneous. The corona consists of loops of gas aligned with magnetic
fields. The most viable theories for heating the corona are those that
explicitly incorporate the structure of the magnetic loops. Coronal
heating may be due to dissipation of electrical currents running along
the loops and/or dissipation of hydromagnetic surface waves. The two
theories have different implications for the heating of coronal holes.
---------------------------------------------------------
Title: Wave reflection and wave disorder in the solar transition
zone and corona.
Authors: Wentzel, D. G.
1978SoPh...58..307W Altcode:
The reflection coefficient for sound or Alfvén waves reaching
the transition zone is evaluated. A family of temperature profiles,
including T<SUP>5/2</SUP> dT/dz = constant, permits analytical solutions
for the velocity and yields the reflection coefficient as a function
of both the wavelength and the temperature jump across the zone. When
the temperature jump is large, even waves appreciably shorter than
the zone thickness are reflected efficiently.
---------------------------------------------------------
Title: Rigidity-independent propagation of cosmic rays in the
solar corona
Authors: Newkirk, G., Jr.; Wentzel, D. G.
1978JGR....83.2009N Altcode:
The delay in arrival times at the earth of solar cosmic rays has
traditionally been explained in terms of diffusive transport within the
corona to locations distant from the flare site and subsequent escape
followed by interplanetary propagation more or less along the spiral
field. The mechanism of such diffusive coronal transport has often
been thought to be pitch angle scattering by irregularities in the
coronal magnetic field. Recently, Ma-Sung et al. have noted that the
time delays attributed to the coronal portion of the propagation for
electrons between 0.5 and 12 MeV and protons between 4 and 100 MeV are
independent of rigidity and energy. It is demonstrated that the observed
delays, their independence of rigidity over 2 orders of magnitude,
and the absence of significant losses of particles in traveling large
distances in the corona are inconsistent with propagation by continuous
diffusion by pitch angle scattering for any reasonable topology of
the coronal magnetic field. We propose an alternate mechanism for the
coronal transport of solar cosmic rays: the bird cage model, in which
particles are normally transferred between adjacent flux tubes by
field line reconnection produced by the rearrangement of the field in
the supergranulation network. The observed size of coronal flux loops
and the rate of reorganization of flux in the supergranulation network
lead to 'diffusion' rates which are both independent of rigidiaty and
energy and consistent with the observed propagation rates.
---------------------------------------------------------
Title: Fermi Acceleration and Particle Pitch Angle Scattering
Authors: Scott, John S.; Cocke, W. J.; Chevalier, R. A.; Wentzel,
Donat G.
1978Ap&SS..53..421S Altcode:
We suggest that sharp velocity gradients will exist in fluid-like
turbulence in nearly collisionless plasma. This implies effective
quenching of Fermi acceleration of thermal particles, but the Fermi
acceleration coefficient for relativistic particles remains essentially
unchanged.
---------------------------------------------------------
Title: Do changes in coronal emission structure imply magnetic
reconnection?
Authors: Nolte, J. T.; Gerassimenko, M.; Krieger, A. S.; Petrasso,
R. D.; Svestka, Z.; Wentzel, D. G.
1977SoPh...55..401N Altcode:
We examine three major possible interpretations of observed
reconfigurations of coronal X-ray and XUV emitting structures on a scale
comparable to the size of the structures themselves. One possibility
is that little change in the large-scale magnetic field configuration
is associated with the change in emission. The other two possibilities
are processes by which the magnetic field structure can change.
---------------------------------------------------------
Title: Isotropy of cosmic rays caused by magnetic discontinuities.
Authors: Wentzel, D. G.
1977ApJ...216L..59W Altcode:
When cosmic rays stream past a bend in a magnetic field, they acquire
a correlation in their phase of gyration about the magnetic field. The
resulting plasma instability generates Alfven waves and simultaneously
reduces the cosmic rays' mean streaming velocity along the field. The
growth rate of the instability is proportional to a fractional power
of the cosmic-ray density and is rapid even for cosmic rays above
10 to the 11th power eV. A few interstellar shocks or interplanetary
rotational discontinuities suffice to reduce the streaming anisotropy
to the local Alfven velocity. Cosmic rays of 10 to the 11th power eV
may be made nearly isotropic even within the solar system.
---------------------------------------------------------
Title: On the role of hydrodynamic waves in the corona and the base
of the solar wind.
Authors: Wentzel, D. G.
1977SoPh...52..163W Altcode:
Hydromagnetic waves are of interest for heating the corona or coronal
loops and for accelerating the solar wind. This paper enumerates some
of the limitations that must be considered before hydromagnetic waves
are taken seriously. In the lowest part of the corona, waves interact
so that a significant fraction of the coronal wave flux should have
periods as ≈10 s. If the problem of interest determines either a
flux of wave energy or a dissipation rate, the distance that each wave
mode can travel can be specified, and for at least one mode it must be
consistent with the size and location of the region where the waves are
to act. Heating of coronal loops observed by X-rays can be explained if
the strength of the magnetic field along the loop lies within a rather
narrow range and if the wave period is sufficiently short. In general,
Alfvén waves travel furthest and reach high into the corona and into
the solar wind. The radial variation of the magnetic field is the most
important parameter determining where the waves are dissipated. Heating
of coronal helmets by Alfvén waves is probable.
---------------------------------------------------------
Title: Isotropy of Cosmic Rays Caused by Magnetic Discontinuities.
Authors: Wentzel, D. G.
1977BAAS....9Q.349W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the momentum and energy deposited by hydromagnetic waves
in the solar wind
Authors: Wentzel, D. G.
1977JGR....82..714W Altcode:
The ratio of thermal energy to momentum (Q/D) deposited in the solar
wind by hydromagnetic waves is determined primarily by the wind and
Alfvén speeds and by the rate of wave dissipation. When dissipation of
Alfvén waves dominates, then Q/D = 2(V + A). This restricts models for
the possible acceleration of the solar wind by hydromagnetic waves. It
also affects the manner in which resonant cosmic rays can interact
with the solar wind and the interstellar medium.
---------------------------------------------------------
Title: Coronal heating by Alfvén waves. II.
Authors: Wentzel, D. G.
1976SoPh...50..343W Altcode:
I extend a previous paper which argued that Alfvén waves traveling
up a large coronal loop may heat this loop at the top and increase
its visibility. This heating is now evaluated more completely, taking
into account the changes along the loop in field strength, gas density
and flux of waves. The location and efficiency of the heating depend
very non-linearly on the intensity of the waves, which allows rapid
changes in the visibility of a loop. Observational and theoretical
conditions for the applicability of the theory are summarized. Alfvén
waves preferentially heat the upper portions of coronal helmets, but a
measurable excess temperature on a loop requires somewhat implausibly
high wave fluxes. Radiation losses from low-lying loops with strong
magnetic fields cannot be explained without modifying the theory.
---------------------------------------------------------
Title: Conditions for "storage" of energetic particles in the
solar corona.
Authors: Wentzel, D. G.
1976ApJ...208..595W Altcode:
Energetic electrons and protons stored on a coronal magnetic loop
develop a 'loss-cone' distribution in pitch-angle which is unstable. The
particles are lost quickly unless the resulting waves are damped. Under
coronal conditions, Landau damping of the waves stabilizes the particles
if the field lines are sufficiently curved. Solar cosmic rays (electrons
above 0.5 MeV, protons above 4 MeV) may be confined for hours on solar
magnetic loops if the field curvature within these loops is roughly
10,000 km. However, the instability quickly destroys any distribution
of electrons with unique pitch-angles, such as is invoked for some
models of nonthermal X-ray bursts.
---------------------------------------------------------
Title: Do Changes in Coronal Emission Structure Imply Magnetic
Reconnection?
Authors: Nolte, J.; Gerassimenko, M.; Krieger, A.; Petrasso, R.;
Wentzel, D.
1976BAAS....8..367N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Limitations on the Roles of Hydromagnetic Waves in the
Solar Corona
Authors: Wentzel, D. G.
1976BAAS....8R.367W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of envelopes of planetary nebulae.
Authors: Wentzel, D. G.
1976ApJ...204..452W Altcode:
The paper evaluates some temporal changes in interstellar temperature
and ionization which are associated with multiple ejections of
planetary nebulae. The dynamics of multiple ejections is analyzed
in terms of isothermal shocks, ionization fronts, and expansion
waves. The ionization of an initially cold nebula that had been
ejected by a central star is examined in the case where the star
ejects a second nebula. It is found that the first nebula may be
broken up into dense swarm of globules surrounding the second nebula
if it was not originally ionized by the star, but will resemble a H
II region if it had been. The repeated ejection of planetary nebulae
which merge with each other is investigated, and it is shown that
the main characteristic of such repeated ejections is a thin shell
of ionized gas with an electron density of the order of 100 per cu
cm. It is concluded that a filamentary H II region will be produced
if the radiation field reappears within a reasonable time period.
---------------------------------------------------------
Title: Report on the solar physics - plasma physics workshop. held
at Stanford University, 17 - 20 September 1974.
Authors: Sturrock, P. A.; Baum, P. J.; Beckers, J. M.; Newman, C. E.;
Priest, E. R.; Rosenberg, H.; Smith, D. F.; Wentzel, D. G.
1976SoPh...46..411S Altcode:
This report summarizes the proceedings of a meeting held on
17-20 September 1974, at Stanford University. The purpose was to
explore plasma physics problems which arise in the study of solar
physics. Sessions were concerned with specific questions including
the following: Is the solar plasma thermal or non-thermal? What
spectroscopic data are required? What types of magnetic field structures
exist? Do MHD instabilities occur? Do resistive or non-MHD instabilities
occur? What mechanisms of particle acceleration have been proposed? What
information do we have concerning shock waves? Very few questions were
answered categorically but, for each question, there was discussion
concerning the observational evidence, theoretical analyses, and
existing or potential laboratory and numerical experiments.
---------------------------------------------------------
Title: On the relation between CO and gamma-ray observations, cosmic
rays, and the thickness of the galactic disk.
Authors: Wentzel, D. G.; Jackson, P. D.; Rose, W. K.; Sinha, R. P.
1975ApJ...201L...5W Altcode:
Recent CO observations suggest that dense clouds provide a high mean
gas density (mostly H2) at approximately 5 kpc from the galactic
center. Gamma-ray observations have been explained as resulting
primarily from the interaction of cosmic rays with interstellar
matter. The relationship between cosmic-ray density and mean gas
density is discussed, and it is argued that the CO clouds must be
dynamically independent of the gas that is observed in 21-cm line
radiation. A plausible upper limit to the cosmic-ray density at 5 kpc
from the galactic center is derived which is based on measurements
of galactic-disk thickness and hydrogen turbulent velocities from
21-cm line data. An estimate is also made of the 'distributed'
gas density which confines the cosmic rays to the disk. This upper
limit is approximately five times the cosmic-ray density near the
sun. Observations of the thickness of the galactic disk suggest that
the cosmic-ray density increases with distributed gas density.
---------------------------------------------------------
Title: Cosmic-ray Propagation through the Solar Corona.
Authors: Wentzel, D. G.
1975BAAS....7R.474W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Heating by Alfvén Waves
Authors: Wentzel, D. G.
1974SoPh...39..129W Altcode:
If Alfvén waves are responsible for the heating of the solar corona,
what are the various dissipation processes, under what conditions
are they important, and what observational consequences may be
expected? For wave periods longer than roughly one minute, the corona
appears to the waves as turbulent and dissipation is efficient, but
little more can be said. For shorter wave periods, a bend or twist
in the magnetic field can convert Alfvén waves into fast-mode waves,
which then dissipate. But, if the waves travel nearly along the field,
the main dissipation occurs via nonlinear interactions among Alfvén
waves, near the top of magnetic loops. Once the wave intensity is
sufficient so that wave dissipation exceeds radiative energy losses,
the temperature rise (ΔT) is limited by electron heat conduction. The
basic result is that ΔT is independent of the wave intensity and
it is also independent of the electron density. Therefore, regions
high and low in the corona are heated similarly. Since ΔT is also
proportional to the magnetic pressure, the heating by Alfvén waves
naturally outlines regions (loops) of enhanced magnetic fields. A
numerical approximation, for wave period τ≲100 s, is ΔT/T ≈
10<SUP>2.5</SUP>B<SUP>2</SUP>(2×10<SUP>6</SUP>/T)<SUP>5/2</SUP> τ/30s.
---------------------------------------------------------
Title: Task Group on Education in Astronomy (TGEA): Annual report
for 1974.
Authors: Wentzel, D. G.; Verschuur, G.
1974BAAS....6..491W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Alfven Waves Heating the Solar Corona.
Authors: Wentzel, D. G.
1974BAAS....6..346W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Break-up of Pulsar Low-frequency Radiation into Filaments
Authors: Wentzel, D. G.
1974BAAS....6..272W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar physics. Plasma physics workshop.
Authors: Baum, P. J.; Beckers, J. M.; Newman, C. E.; Priest, E. R.;
Rosenberg, H.; Smith, D. F.; Sturrock, P. A.; Wentzel, D. G.
1974sppp.book.....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Cosmic-ray propagation in the Galaxy: collective effects.
Authors: Wentzel, D. G.
1974ARA&A..12...71W Altcode:
It is pointed out that most cosmic rays in the energy range from 0.1
to 1000 GeV are believed to be created in our Galaxy and to escape
from it after a few million years. Cosmic rays are basically a very
hot collisionless plasma. Questions regarding the resonant scattering
of the rays by static magnetic fields are investigated along with
problems concerning a diffusion through interstellar space, the energy
loss by cosmic rays, and the self-confinement of cosmic rays in the
Galaxy. The validity of approximations and physical assumptions is
explored and details of cosmic-ray propagation are examined. Effects
of cosmic rays on galactic gasdynamics are also studied.
---------------------------------------------------------
Title: Self-Focussing of Pulsar Low-Frequency Radiation
Authors: Wentzel, Donat G.
1973ApL....15...67W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Adiabatic Deceleration of Cosmic Rays near Their Sources
Authors: Wentzel, Donat G.
1973Ap&SS..23..417W Altcode:
Every plausible source of cosmic rays yields a high flux of cosmic
rays near the source. The high flux leads to plasma effects that cause
scattering of the cosmic rays, coupling to the interstellar gas and
hence to adiabatic deceleration. The cosmic rays are released from the
gas only when their pressure has fallen to the cosmic-ray pressure
near the Sun multiplied by a factor between 10 and 100. I discuss
a model aimed to minimize the deceleration before the cosmic rays
are released. The volume which cosmic rays occupy before scattering
is maximized by injection into a large but thin disk. Even then,
deceleration is reduced only to a factor of two. Such deceleration
should cause quasi-supernova remnants somewhat resembling the Cygnus
loop but associated with much younger pulsars. Since both the required
model and the predicted observations cause difficulties, the problem
of adiabatic deceleration remains severe.
---------------------------------------------------------
Title: Astronomy Education
Authors: Hoff, Darrel; Wentzel, Donat G.
1973AmJPh..41..578H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ultraviolet Stars and the Interstellar Gas
Authors: Rose, William K.; Wentzel, Donat G.
1973ApJ...181..115R Altcode:
If luminous red giants evolve to white dwarfs via a very hot stage
(ultraviolet star), then these stars may have a significant influence on
the interstellar medium. We extend the analysis of Hills by considering
the evolution of ultraviolet stars and how these stars affect the
dynamics of the resulting H ii regions. Compared to 0 stars, the
ultraviolet stars are more uniformly distributed and in regions of
lower density so that the initial spheres are larger. H ii regions
due to ultraviolet stars are very short-lived and hence unimportant in
regions with hydrogen density n0 > 1 cm 2; but if most ultraviolet
stars occur in a rather uniform intercloud medium with n0 0.1 cm-2,
then their H ii regions fill about one-third of this space, produce a
mean electron density about 0.03 cm -2, comparable to that indicated
by pulsar dispersions, and cause acceleration of interstellar gas
adequate to explain the observed interstellar motions. Subject headings:
early-type stars interstellar matter - nebulae - ultraviolet
---------------------------------------------------------
Title: Teaching of Astronomy
Authors: Wentzel, D. G.
1973BASI....1...40W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Editorial: The TGEA-A New Educational Venture of the AAS
Authors: Wentzel, Donat G.; Verschurr, Gerrit
1972BAAS....4..405W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ultraviolet Stars and the Interstellar Gas.
Authors: Rose, W. K.; Wentzel, D. G.
1972BAAS....4R.318R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear Dissipation of Alfvén Waves
Authors: Chin, Yuan-Chung; Wentzel, Donat G.
1972Ap&SS..16..465C Altcode:
Alfvén waves are generated easily in many cosmic plasmas, but they
possess no linear damping mechanism since they are not compressive. The
most prominent nonlinear damping occurs when one Alfvén wave decays
into another plus a slow magnetosonic wave, or two Alfvén waves combine
into one fast magnetosonic wave; the resulting magnetosonic waves can
then be dissipated. The nonlinear coupling rates are presented, with
special emphasis on the astrophysically important case of sound speed
≪Alfvén speed. Streaming cosmic rays generate Alfvén waves moving in
the direction of streaming, but they reabsorb the backward moving waves
then produced by wave decay. The possible steady states for this system
of cosmic rays and Alfvén waves turn out to be highly restricted.
---------------------------------------------------------
Title: Interstellar Motions: Minuet or Rock?
Authors: Wentzel, Donat G.
1972PASP...84..225W Altcode:
Our knowledge of interstellar space has been revolutionized by many
recent observations. This article reviews some of the forces that we now
believe control the motions of interstellar gas, and its variations in
density and temperature. The observed degree of ionization has led to
the cloud-intercloud model of interstellar gas. It also implies that
the gas may be in a very dynamic, turbulent state, so that hydrostatic
equations of state for this gas must be used with caution. Cosmic rays
contribute an important force to interstellar gas dynamics and are an
additional source of heat. They are most effective near cosmic-ray
sources and at the edge of the galactic disk. Galactic structure
affects gas dynamics, not only through the shock that bounds spiral
arms, but also by a large-scale, ordered magnetic field in a way that
encourages escape of gas, cosmic rays, and magnetic flux out of the
galactic disk. But this field amplffication also depends on local
"turbulence." Minuet or Rock? Large-scale order or turbulence? You
don't have one without the other! Key words: interstellar clouds -
cosmic rays - magnetic dynamo
---------------------------------------------------------
Title: Scattering of high-energy cosmic rays.
Authors: Wentzel, D. G.
1972BAAS....4..261W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The TGEA (Task Group for Education in Astronomy) - a new
educational venture of the AAS.
Authors: Verschuur, G.; Wentzel, D. G.
1972BAAS....4..405V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Scattering of High-Energy Cosmic Rays
Authors: Wentzel, D. G.
1972ApL....10..167W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: University level astronomy education for nonscience
concentrators. A case for astronomy.
Authors: Wentzel, D. G.
1972NYASA.198..109W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High-Speed Interstellar Gas Dynamics: Shocks Moderated by
Cosmic Rays
Authors: Wentzel, Donat G.
1971ApJ...170...53W Altcode:
Cosmic rays interact with the thermal interstellar gas through
a cosmic-ray sound speed" of the order of (cosmic-ray energy
density/thermal gas density)1'2, typically km . Gas motions
faster than the thermal sound speed may he moderated by the cosmic
rays. The cosmic-ray sound speed and the jump conditions across
cosmic-ray-dominated shocks are derived here on the assumption that the
cosmic rays and the interstellar gas interact via resonant hydromagnetic
waves. Cosmic-ray shocks occur where gas is driven through the cosmic
rays by gravity, for instance at the "high-velocity" clouds if these
are gas falling into our Galaxy, or where cosmic rays experience a
density gradient, for instance around supernovae. In all cases, the
cosmic rays lose energy to the gas, affecting our understanding of
both the cosmic rays (is there any need to store them in a galactic
"halo"?) and interstellar gas dynamics (are cosmic rays the missing
source of kinetic energy for interstellar clouds?). In case the
high-velocity clouds are subsonic, however, the cosmic rays may cushion
their infall so that no shock occurs at all.
---------------------------------------------------------
Title: Interstellar motion: Minuet or Rock?
Authors: Wentzel, D. G.
1971BAAS....3..497W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interstellar Motions: Minuet or Rock?
Authors: Wentzel, Donat G.
1971BAAS....3..381W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Acceleration and Heating of Interstellar Gas by Cosmic Rays
Authors: Wentzel, Donat G.
1971ApJ...163..503W Altcode:
When cosmic rays try to stream along the magnetic field of the
Galaxy, they are scattered and must diffuse down a cosmic-ray density
gradient. The associated pressure gradient acts on the interstellar
gas, with the hydromagnetic waves that cause the scattering acting as
a clutch which couples gas and cosmic rays. When the cosmic rays have
diffused down a density scale height, they have lost a major fraction
of their momentum and energy. Therefore, they may be a major source
of the observed interstellar kinetic and thermal energies. This energy
transfer is a collective, plasma-physical process that does not ionize
the gas. For applications, one must distinguish the cosmic rays that
are "engaged by the clutch" from those that are not. The former have
energies up to a few GeV and carry much of the energy and pressure of
the cosmic rays. The engaged cosmic rays may raise neutral hydrogen
several hundred parsecs above spiral arms, as observed. They may aid in
star formation. Their confinement to spiral arms helps to explain the
pressure balance in inter-arm regions, and perhaps in the galactic halo
---------------------------------------------------------
Title: Science Education: A Case for Astronomy
Authors: Wentzel, Donat G.
1971AmJPh..39...27W Altcode:
An astronomy course can demonstrate to the nonscientist that science
is a human endeavor, involving social judgments and philosophical
implications as well as established facts.
---------------------------------------------------------
Title: "Friction" between cosmic rays and the interstellar gas.
Authors: Wentzel, D. G.
1970BAAS....2..287W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Can Cosmic Rays Push Interstellar Gas Far Out of the Galactic
Plane?.
Authors: Wentzel, Donat G.
1970BAAS....2R.355W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interaction Between Cosmic-Ray Electrons and Cosmic-Ray Protons
Authors: Melrose, D. B.; Wentzel, D. G.
1970ApJ...161..457M Altcode:
Anisotropic cosmic-ray protons generate hydromagnetic waves. In
this generation process, the momentum transfer scatters the
protons. Anisotropic cosmic-ray electrons scatter either by amplifying
or by absorbing the waves generated by the protons. Results are. (1)
Streaming cosmic-ray protons tend to drag the electrons along
with them, affecting the confinement of electrons to the galactic
disk. (2) Waves generated by protons scatter the electrons made
anisotropic by synchrotron radiation, so that a future measurement
of electron anisotropy may inform us of the dynamics of cosmic-ray
protons. (3) In a varying magnetic field, electrons absorb some of the
proton-generated waves and are thereby accelerated, while all cosmic
rays experience a Fermi-type acceleration. These accelerations are
probably not significant. An appendix treats the range in the waves'
angles of propagation and criteria for the usual assumption of parallel
propagation.
---------------------------------------------------------
Title: On the Instability of a Stellar Envelope due to Radiation
Pressure
Authors: Wentzel, Donat G.
1970ApJ...160..373W Altcode:
The gradient of radiation pressure in a stellar envelope may cause
a density inversion locally. The main instability of such an inverted
layer is normal convection, not a Rayleigh-Taylor instability. Radiation
also may damp the convection in some supergiant atmospheres that are
normally considered to be convective.
---------------------------------------------------------
Title: Interaction between Cosmic-Ray Electrons and Cosmic-Ray Protons
Authors: Melrose, D. B.; Wentzel, D. G.
1969BAAS....1Q.354M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Propagation and Anisotropy of Cosmic Rays. II. Electrons
Authors: Wentzel, Donat G.
1969ApJ...157..545W Altcode:
Synchrotron radiation makes cosmic-ray electrons anisotropic. We
evaluate the electrons' distribu- tion function in pitch angle, in case
the electrons steadily create-and are scattered by-hydromagnetic waves
in an ionized plasma. We find that the radiation from radio galaxies is
generated by nearly iso- tropic electrons. In our Galaxy, the anisotropy
increases with energy and is 1 percent at ,_~4O2 GeV. However, the
scattering of cosmic-ray electrons in the galactic disk is dominated
by waves from other poorly known sources, primarily the streaming of
cosmic-ray protons
---------------------------------------------------------
Title: Pulsar Scintillations due to Distant Streaming Cosmic Rays
Authors: Wentzel, Donat G.
1969ApJ...156L..91W Altcode:
Scintillation of pulsar radio emission indicates the existence of
interstellar hydromagnetic waves that interact with and probably are
caused by cosmic rays. Measurable characteristics of the scintillation
can determine the spectrum of the hydromagnetic waves. Observation that
most waves are shorter than roughly 1012 cm could inform us about the
mean streaming velocity of distant cosmic rays, the cause of their
streaming, and the degree of interstellar ionization in that region
of space. An observation of wave- lengths of 1011 cm would demonstrate
the existence of interstellar protons below 1 MeV or of electrons near
25 MeV
---------------------------------------------------------
Title: The Propagation and Anisotropy of Cosmic Rays. I. Theory for
Steady Streaming
Authors: Wentzel, Donat G.
1969ApJ...156..303W Altcode:
We consider cosmic rays steadily streaming along a uniform magnetic
field and down a cosmic-ray density gradient. The anisotropy generates
hydromagnetic waves which, in turn, scatter the cosmic rays. We
derive both the shape of the cosmic-ray distribution function and
the wave spectrum for a steady state with a low cosmic-ray streaming
velocity. The results are summarized in § VI. Limits of validity are
discussed in terms of a numerical example
---------------------------------------------------------
Title: Plasma Aspects of Cosmic Rays (Notes taken by D. Hall)
Authors: Wentzel, D.
1969lhea.conf...85W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Particle acceleration in astrophysics
Authors: Wentzel, D. G.
1969pia..conf..271W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Plasma instabilities in astrophysics
Authors: Wentzel, Donat G.; Tidman, Derek A.
1969pia..conf.....W Altcode: 1969QB461.P55......
No abstract at ADS
---------------------------------------------------------
Title: The Growth Rate of the Interstellar Garden-Hose Instability
due to Cosmic Rays
Authors: Wentzel, Donat G.
1968ApJ...153..331W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hydromagnetic Waves Excited by Slowly Streaming Cosmic Rays
Authors: Wentzel, Donat G.
1968ApJ...152..987W Altcode:
Cosmic rays streaming along a magnetic field generate hydromagnetic
waves with a wide range in 0, the angle between the field and the
direction of wave propagation. An expansion in terms of VA! (c cos 0)
<<1, where VA is the Alfvén speed, is used to show that such
waves are not influenced by mix- ing with electrostatic wave modes
and that they are generated when the mean velocity of the cosmic rays
exceeds (approximately) VA/COS 0. The growth rate of the unstable waves
is greater than the rate at which cosmic rays escape the galactic disk
---------------------------------------------------------
Title: Evidence from Cosmic-Ray Anisotropy Concerning the Local
Interstellar Medium.
Authors: Wentzel, Donat G.
1968AJS....73S..39W Altcode:
Cosmic-ray anisotropy is quite small. Whatever the mechanism that causes
the anisotropy, it must be counteracted steadily by some scattering
process. Shocks are inefficient. Instead, the cosmic rays can be
scattered by plasma waves, which are generated by the anisotropic cosmic
rays themselves. Leading candidates among the plasma waves are the
magnetosonic (Alfve'n) waves (Lerche, Astrophys. J. 147, 689, 1967). A
cosmic-ray distribution even in pitch angle makes circularly polarized
magnetosonic waves resonantly unstable. In a curved magnetic field,
a steady state is set up wherein the anisotropy is adjusted such that
the unstable rate of growth of the waves is equal to their rate of loss
of circular polarization, which occurs when the waves travel at even a
small angle relative to the magnetic field. (Cosmic rays with a mean
streaming velocity or other more general distribution in pitch angle
also make linearly polarized waves unstable.) Measurements by Jacklyn
(Nature 211, 690, 1966) show that 100-GeV cosmic rays have a sidereal
anisotropy of about 0.05% and a distribution nearly even in pitch
angle. If the measurement really represents an interstellar anisotropy,
as seems probable, then one can deduce from the above theory: (i)
In the scattering region, the radius of curvature of the magnetic
field is about 20 pc, with a possible error by a factor of 3. (ii)
The density of neutral hydrogen atoms there must be less than 0.08
cm-3. This implies an H II scattering region. (iii) To avoid Landau
damping by thermal gas, ne&0.1 cm-3 there. (iv) Since scattered
particles must have access to the sun, the field strength between the
scattering region and the sun may not exceed the value just outside
the solar system. Therefore the scattering region is probably quite
near. If it surrounds the solar system, then the solar wind is bounded
by an H II region. This work was supported by NSF Grant GP 7001.
---------------------------------------------------------
Title: An Upper Limit on the Abundance of H_{2} Formed by
Chemical-Exchange Reactions
Authors: Wentzel, Donat G.
1967ApJ...150..453W Altcode:
Stecher and Williams proposed the formation of interstellar H2 by
chemical-exchange reactions on graphite grains. The fraction of
hydrogen atoms converted to molecules, f, is computed according
to their theory, but taking into account the radiative cooling by
the molecules. During each cloud collision, f increases according
to A J2 < 3 X 1 thereafter the temperature drops so as to stop
further thermal reactions. Additional reactions on fast grains produce
another increase Af < O.4p (grains)/p (atoms). If H2 is formed by
chemical-exchange reactions, its abundance in typical clouds is less
than 10 per cent. f The main body of this paper concerns the structure
of the shocks in which the molecules form during cloud collisions. They
are "isothermal" shocks whose "front" itself consists of an adiabatic
shock and a region in which H2 both forms and radiates.
---------------------------------------------------------
Title: An Upper Limit on the Abundance of Interstellar H2 Formed by
Chemical-Exchange Reactions.
Authors: Wentzel, Donat G.
1967AJ.....72R.837W Altcode:
146, 88,1966) derived the rate at which interstellar H2 is formed on
graphite grains by chemical-exchange reactions. The reactions require
an activation energy of 0.18 eV. For H I regions one must, therefore,
invoke cloud collisions. If the gas in a colliding cloud remains
at a temperature T~ 1030K for a time r, then the ratio of molecules
to neutral atoms f increases by roughly Af= r/(106 yr) during this
collision. Since the molecules are collisionally excited and radiate
efficiently, the needed temperature lasts only on the order of r =
102/f yr. Therefore, one cloud collision yields fAflO-4. Even if the
cloud initially contains no molecules at all, the 1 % H2 that is formed
during the first 10~ yr after the gas is heated cools the gas in the
next 10~ yr so that no more H2 is formed. An initial abundance of H2 or
other cooling agents further decreases the production of new H2. Some
additional chemical-exchange reactions on fast grains may yield Af~
O.4p (grain/ p atoms). Since all molecules are destroyed near hot stars
in intervals of roughly 1O~ yr, the interstellar medium contains less
than 10 % H2 molecules formed by chemical-exchange reactions. This
limit is independent of the value of the interstellar magnetic field.
---------------------------------------------------------
Title: On Mixing in Stellar Interiors Caused by Magnetic Fields
Authors: Gurm, Hardev S.; Wentzel, Donat G.
1967ApJ...149..139G Altcode:
Convection cells may rise in zones of radiative equilibrium when
magnetic fields within the cells provide cohesion and buoyancy. The
motion of such cells is evaluated both far from and near the edge of
a radiative zone. For example, cells of radius <10 km rise through
the radiative zone of stars on the upper main sequence in a time less
than the of these stars if the magnetic pressure in the cells is as
small as 10- times the gas pressure.
---------------------------------------------------------
Title: On the Motion of Shocks Through the Solar Chromosphere
Authors: Wentzel, Donat G.; Solinger, Alan B.
1967ApJ...148..877W Altcode:
Whitham's theory for the motion of shocks through inhomogeneous
media is applied to the chromosphere where magnetic fields can guide
shocks. The effects of both the changing ambient gas pressure and
the guidance by arbitrarily shaped magnetic fields can be evaluated
analytically in case both the medium and the shock are isothermal In
general, the strength of an isothermal shock increases much less with
decreasing ambient pressure than does the strength of an adiabatic
shock. Parker's suggestion for the origin of spicules is evaluated in
some detail. The shocked gas emerging from the lower chromosphere can
be identified with spicules and the necessary guiding magnetic fields
are less than about 60 gauss only if the shocks can be guided through
the unstable regime where V, d <VAIf < V,h ,k. In this case,
the spicules are strongly correlated with magnetic field strength but
are nearly independent of the magnitude of photospheric velocities. The
theory is also applied to a sudden, flare- caused coronal heating over
an undisturbed region of the chromosphere.
---------------------------------------------------------
Title: On Petschek's mechanism for dissipating interstellar magnetic
fields
Authors: Wentzel, D. G.
1967IAUS...31..131W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interstellar Clouds and Petschek's Mechanism for Dissipating
Magnetic Fields
Authors: Wentzel, Donat G.
1966ApJ...145..595W Altcode:
Petschek's mechanism ("P.M.") for reconnecting and dissipating
magnetic fields is summarized, critically evaluated (§ II), and
subsequently applied to interstellar clouds. It leads to the following
observational and theoretical consequences: (1) If P.M. occurs in H
i regions, they involve streams of gas with densities 100 atoms and
speeds + 3 km/sec. streams should be recognizable in 21-cm emission,
even more in 21-cm absorption. They actually dominate the absorption
lines of Ca+ and Na, requiring a recomputation of the abundances of
interstellar Ca and Na. The inhomogeneous density distribution also
allows significant variations in temperature. (2) The clouds may be
disconnected from the general galactic magnetic field. It follows that
spiral arms must be gravitational in origin and the galactic field
must be nearly force-free. The alignment of interstellar polarization
implies slightly elongated clouds. (3) The evolution of clouds can be
meaningfully discussed. A lifetime of the order of 10 years may start
at the time of a supernova, involves gradual dissipation of internal
magnetic fields and a consequent increase in density, and may end by
star formation.
---------------------------------------------------------
Title: The Effect of a Solar Wind Blast Wave on the Chromosphere.
Authors: Wentzel, Donat G.; Solinger, Alan B.
1966AJ.....71R.402W Altcode:
The enhanced solar wind following some solar flares is probably caused
by coronal heating. The energy involved in the enhanced wind suggests
that flare-induced coronal heating occurs over an area extending far
beyond the flare area. The resulting disturbance of the underlying
nonflaring chromosphere is computed on the assumption that the corona is
heated suddenly. In this case, a shock moves downward from the heated
region. The increase in the undisturbed chromospheric density ahead of
the shock (p) tends to weaken the shock. In addition, magnetic fields
may act as a guide for the shock. An increase in field strength along
the shock's path constricts the area of the shock front (A) and tends
to strengthen the shock. Both effects can be evaluated analytically,
using Whitham's theory for shock propagation in inhomogeneous media
(J. Fluid Mech. 4, 337, 1958), if both the shock and the chromosphere
are considered to be isothermal. The change in Mach number (M) with p
and A is given by 1 1 2 dp dA 1+ +-+ dM+-+-= 0. M2 M M-1 p A For plane
stratification or vertical magnetic fields (dA =0), the shock penetrates
gas increasing in density by one to two orders of magnitude before
attenuation becomes significant. Its effect may relate to the flare
nimbus and to the disappearance of stria- tion patterns. The strength
of a shock entering the field of a sunspot (dA/dp<0) is attenuated
much less. Flaring over sunspot umbrae may be initiated by such a shock.
---------------------------------------------------------
Title: Interpretation of the Observations in Terms of Gas Dynamics
Authors: Wentzel, D.
1966IAUTB..12..411W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Fermi Acceleration of Solar Cosmic Rays
Authors: Wentzel, Donat G.
1965JGR....70.2716W Altcode:
Fermi acceleration of solar relativistic protons has sometimes been
discounted because `second order' Fermi acceleration is inefficient
and is considered to generate an energy-dependent spectrum, whereas
the spectrums observed at the earth seem to show a greater simplicity
when expressed in terms of rigidity [Webber, 1964]. A more efficient
`first-order' Fermi mechanism has been suggested recently [Wentzel,
1964a]. This note is to point out several features favoring
Fermi acceleration. In particular, a great variety of energy- or
rigidity-dependent spectrums can be generated by the Fermi mechanism,
the spectral shape depending on the manner of escape from the region
of acceleration.
---------------------------------------------------------
Title: Interstellar Lines and the Dissipation of Interstellar Magnetic
Fields by Petschek's Mechanism.
Authors: Wentzel, Donat G.
1965AJ.....70S.696W Altcode:
An efficient method of dissipating magnetic fields has been proposed by
Petschek (The Physics of Solar Flares, NASA SP-50, p. 425, 1964). If
such dissipation occurs within an interstellar cloud which radiates
efficiently, it generates a thin layer of gas with a density of roughly
100 atoms/cm3 and a streaming speed of *3 km/sec. This inhomogeneity
appears in 21-cm emission as a symmetrical pattern in the sky or as a
Doppler profile with several, probably three, resolvable components. It
is more visible in 21-cm absorption because the dense layers are cooled
efficiently. The inhomogeneity may actually dominate the absorption
lines of Ca+ and Na0, the abundances of which are proportional to
the density. Observed ratios of line strengths (Howard, Wentzel,
and McGee, Astrophys. J. 138, 988, 1963) can be made consistent
with a uniform abundance of free Ca and Na in low-velocity clouds;
the underabundances relative to the sun become a factor 300 for Ca,
6 for Na. In analogy to the geomagnetic field, Petschek's mechanism
may disconnect interstellar H I clouds from the general galactic
field. If so, clouds move independently of each other and of the
galactic field; consequently spiral arms must be gravitational
in origin and the galactic field, constrained only by negligible
intercloud mass, must be nearly force-free. An elongation of the
clouds along the confining galactic field can explain the alignment
of interstellar polarization. The evolution of clouds may consist of
their formation near a supernova, dissipation of internal magnetic
fields and a compensating increase in density during some 108 yr,
and finally star formation or simply disintegration.
---------------------------------------------------------
Title: Solar Flares Caused by the Skin Effect in Twisted Magnetic
Fields.
Authors: Wentzel, Donat G.
1964ApJ...140.1563W Altcode:
An optical flare may be caused by the collapse of an unstable
chromospheric region, under the action of magnetic forces, toward a
surface of minimum but finite magnetic field strength. The compression
of the magnetic field during the collapse results in forces which
stop the collapse, in a time comparable to the rise time of flare
light-curves Turbulence is likely to occur if the velocities of
collapse can become comparable to Alfven speeds. The turbulence provides
sufficient ohmic dissipation of the magnetic fields to explain the rate
of optical emission. The emission is l6calixed in thin surfaces at the
eddy boundaries. The total depth of the emitting surfaces crossed by a
line of sight is in agreement with inferences from observations. If the
same kind of collapse which causes an optical flare in the chromosphere
occurs at greater heights, the density is insufficient to radiate
away the thermal energy derived from ohmic dissipation. The result is
heating, with attendant radio, X-ray, and particle events, instead of
an optical flare. The conditions under which turbulence may occur are
investigated Some of the consequences of the theory relate to flare
light-curves, homologous flares, and rates of expansion of flare areas
---------------------------------------------------------
Title: Motion across Magnetic Discontinuities and Fermi Acceleration
of Charged Particles.
Authors: Wentzel, Donat G.
1964ApJ...140.1013W Altcode:
The motion of charged particles across stationary magnetic
discontinuities is investigated. The constancy of the magnetic moment,
a result of the guiding center approximation (GCA), is shown to hold
for both weak shocks and shocks of arbitrary strength whose fronts
are nearly parallel to the lines of force. For most other shocks,
"scattering" of the pitch angle, relative to the change across
the shock expected from the GCA, is limited to less than the angle
between the lines of force at the shock. Only rather strong shocks
can significantly randomize the pitch angles, and these result in
non-uniform distributions in phase angle. Charged particles with Larmor
radii large compared to the thickness of shock fronts but small compared
to the length of the shock disturbances can be efficiently Fermi
accelerated between series of converging shocks with equal amplitudes:
A particle between diverging shocks cannot be trapped and decelerated,
but, upon entering the region between converging shocks, it may be
trapped and accelerated. The average efficiency of acceleration is
estimated to be comparable to that of "first-order" Fermi acceleration.
---------------------------------------------------------
Title: Formation of Solar Flares without Magnetic Neutral Points.
Authors: Wentzel, Donat G.
1964AJ.....69..151W Altcode:
A flare may be caused by the collapse of an unstable chromospheric
region, under the action of magnetic forces, toward a surface of minimum
but finite magnetic field strength. The compression of the magnetic
field during the collapse results in forces which stop the collapse,
in a time comparable to the rise time of flare light curves. Turbulence
is likely to occur if the velocities of collapse become comparable to
Alfv6n speeds. If turbulence occurs, it can cause sufficient Ohmic
dissipation of the magnetic fields to explain the rate of optical
emission (Wentzel, Astron. J. 68, 299(A), 1963). The dissipation and
emission are localized in thin surfaces at eddy boundaries. The total
depth of the emitting surfaces crossed by a line of sight through the
flare is of the order of 10 km, in agreement with inferences from
observations (Suemoto, Hiei, and Hirayama, J. Phys. Soc. Japan 17,
Suppl. A-2, 231). if the same kind of collapse which causes an optical
flare in the chromosphere occurs at greater heights, the density is
insufficient to radiate away the thermal energy derived from Ohmic
dissipation. The result is heating, with attendant favorable conditions
for the generation of radio, x-ray, and particle events, instead of
an optical flare. Depending on the extent of the collapsing region,
optical and heating events may occur alone or together. The conditions
under which turbulence may occur and the corresponding gas densities
have been investigated numerically. Some of the consequences of the
theory relate to flare heights and light curves, increases in density
before optical flaring starts, rates of expansion of flare areas,
homologous flares, and apparently simultaneous flares.
---------------------------------------------------------
Title: The Origin of Solar Flares and the Acceleration of Charged
Particles
Authors: Wentzel, D. G.
1964NASSP..50..397W Altcode: 1964psf..conf..397W
No abstract at ADS
---------------------------------------------------------
Title: On a Correlation Between the Radial Velocities of Optical
and Radio Interstellar Lines.
Authors: Howard, William E., III; Wentzel, Donat G.; McGee, Richard X.
1963ApJ...138..988H Altcode:
Optical and 21-cm data pertaining to interstellar clouds are
analyzed. The radial velocity correlation coefficient pertaining to
the directions toward 39 stars with b > 15 and single component Ca
ii lines is +0.68 + 0.09 (m.e.). The dispersion of the histogram of
the individual velocity differences is +3 km/sec at both high and low
latitudes. A study of the radio and optical line intensities shows an
agreement between the theoretical and observed ratios but a discrepancy
between the theoretical and observed ,, ratios. Observation and theory
can be reconciled by taking the sodium-tohydrogen abundance ratio equal
to its value in the sun and by taking the free calcium-to-hydrogen
abundance ratio to be of the order of 1/60 of its solar value The
possibility of differential calciumhydrogen streaming is briefly
discussed.
---------------------------------------------------------
Title: Fermi Acceleration of Charged Particles.
Authors: Wentzel, Donat G.
1963ApJ...137..135W Altcode:
Two efficient types of Fermi acceleration by hydromagnetic shocks
are presented. One is based on Parker's (1958a) work on acceleration
between shocks which cross each other. An equivalent situation is
that of a single shock crossing a closed magnetic line of force. It
is shown that a net gain in particle energy occurs even if the shocks
are weak A second type is the acceleration by a shock which moves into
stronger magnetic fields. While the particle is accelerated ahead of
the shock and is able to penetrate stronger fields, the shock field
may become sufficiently stronger that the particle is unable to escape
the trap and is accelerated for a long time No such prolonged trapping
occurs for the same particle decelerated behind a similar shock moving
into weaker fields, so that the net efficiency of acceleration is
high. Conditions on acceleration of thermal particles, specifically
electrons, are discussed. One condition is that the particle must move
faster than the shock.
---------------------------------------------------------
Title: Magnetic Fields and Spiral Structure
Authors: Wentzel, Donat G.
1963ARA&A...1..195W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dissipation of Magnetic Energy in a Solar Flare.
Authors: Wentzel, Donat G.
1963AJ.....68Q.299W Altcode:
A flare is thought to occur because of a collapse of chromospheric
gases toward a magnetically neutral region. Where the rate of collapse
reaches the hydromagnetic velocity, inhomogeneities permit the gas to
twist the magnetic fields and turbulence may be set up. Effects of this
turbulence are investigated. First, the turbulence enhances the rate
at which magnetic energN is dissipated: Opposing fields are pressed
together at eddy boundaries during a time D/v (D is the eddy size; v
the eddy velocity), permitting dissipatioi~ of the fields only within
a skin depth, L, given bN D/v = 4~jL2. Hence the turbulence maintains
a localized but high rate of dissipation. The total dissipation time is
(D/v) (D/L) = (4ir~D3/v)-. Second, the turbulence cushions the collapse
and finall~ stops it. At this time, the fields are most twisted ai~d
are dissipating most rapidly. If the flare is dei~se enough so that
the dissipated energy is rapidly radiated away, then the end of the
collapse mark-s the maximum of the flare light curve. Correspondingly,
the total dissipation time of the twisted magnetic fields is to be
identified with the decay time of the light curve. For v~200 km/sec
and plausible values for j (in an ionized gas), a decay time __10~
sec requires D40 km. Third, during the rise time the collapse drives
the turbulent velocities to exceed the hydromagnetic velocity, causing
shocks and perhaps acceleration of charged particles; thereafter, the
turbulence is driven only by the already twisted fields and is less
violent, in accord with the few available observations. Fourth, since
turbulence and flaring occur only where the rate of collapse can reach
and has reached the hydromagnetic velocity, the possibility of flare
occurrence and the increase in flare area with time may be computed
for any initial model. Fifth, after the twisted fields are dissipated,
the remaining large-scale field is similar to that existing before
the flare, so that a very similar flare may occur at a later time,
as is sometimes observed.
---------------------------------------------------------
Title: On a Correlation between the Radial Velocities of Optical
and Radio Interstellar Lines.
Authors: Howard, William E., III; Wentzel, Donat G.
1962AJ.....67..273H Altcode:
The radial velocities of the interstellar K line of Ca ii in a total
of 39 high-latitude stars ( b ~ 150) having single interstellar line
components have been analyzed in conjunction with the corresponding
H I radial velocities of the 21-cm line profile. The correlation
coefficient is found to be +0.65~*0.09 (m.e.). No radial velocity in
the sample, either radio or optical, exceeds ~ 12 km/sec. When the
H I radial velocity is subtracted from the corresponding optical
value and a histogram of the points is plotted, the mean value of
the difference (~ AV>) is +0.3t0.6 (m.e.) km/sec, showing that the
large- scale motions of H I and Ca ii are statistically the same. The
standard deviation of the AV histogram is 3.5*~0.4 (m.e.) km/sec. After
this figure is corrected for instrumental effects, we find a cosmic
dispersion of approximately 3 ~ 1 km/sec, which we attribute in varying
degrees either to cloud rotation, to differential Ca-H streaming, or to
turbulent motions on a scale that is smaller than the solid angle of the
20 radio beamwidth but large enough to include the optical solid angle
through the interstellar cloud. The last alternative seems the most
probable. From the area under the 21-cm line profile we can derive the
number of H I atoms per cm 2in the line of sight (NH). Then, by adopting
a hydrogen-tocalcium abundance ratio, we can estimate Nea (em-2). The
optical lines are unsaturated with K/H ratio ~1.6. We can derive Ne
ii (cm-2) from the usual curve-of-growth analysis given by Str6mgren
(Stro~mgren, B., Astrophys. J. 108, 242,1948). The range of the ratio
Nea/Nca ~~ over ten stars is 190/XY to 4500/XY, with a mean value of
1400/NY; the same ratio predicted theoretically is 83 Z/nu Pv, where
N--the factor by which calcium is underabundant compared to the solar
value (NH/NCa -6.5 X 10 N) Y the total H I optical depth/H I optical
depth in front of the star; 7--the factor of overionization compared
to Seaton's estimate, viz., flea iiifle =0.025Z cm-3 AT~a iii-Nea; flCa
II ~V--the ratio of true-to-estimated electron density (n6-~3X10~4nu W
cm-3). Most of the fluctuation in the observed ratio of N~a/N~a ii may
be attributed to fluctuations in either the neutral hydrogen density 13~
or the ionization field 7 from star to star. However, on the average,
the observed and predicted values disagree considerably in the sense
that less Ca ii is observed than is predicted. An underabundance of
calcium could account for this disagreement. A repetition of this
analysis over five stars shows agreement between the observed and
predicted NNa/NNa ratio. We note with interest that Spitzer and Routly
(Spitzer, L., Jr., and Routly, P. M. Astrophys. I. 115, 227, 1952) have
shown that calcium is underabundant relative to sodium in lowvelocity
interstellar clouds.
---------------------------------------------------------
Title: Acceleration of Electrons near Solar Flares.
Authors: Wentzel, Donat G.
1962AJ.....67Q.285W Altcode:
Two efficient versions of Fermi acceleration by hydromagnetie
shocks are presented. One version is based on the work by Parker
(Phys. Rev. 109,1328, 1958) on acceleration between approaching
shocks. It is shown that the mechanism is efficient even for moderate
shock strengths. Efficient acceleration is also shown to occur ahead
of a shock moving into stronger fields. As the particle is accelerated
by the shock it is able to penetrate stronger fields, but the shock
field also becomes stronger, so that the particle remains trapped even
while it gains energy. Doubling of the energy by each passing shock is
possible in plausible magnetic fields. Two conditions of acceleration
are that the particle moves faster than the shock and that collisional
energy losses are negligible. For solar thermal electrons the conditions
of acceleration hold only in and above the upper chromosphere in
the high-energy tail of the velocity distribution. With Tc=3X105 0K,
ne=109 em2, and an estimated shock frequency of 10 sec-', about 10-2
of all electrons are accelerated. The thermal tail is replenished
nearly as rapidly as it is depleted by shocks, and in about ~ min the
whole electron gas is heated to roughly 30 kev. Further acceleration is
inefficient since the electron magnetic moment is then not constant. The
electrons emit bremsstrahlung by collisions among themselves and the
intensity, risetime, energy, and spectrum of the observed x rays can
be adequately explained. Motion of the fast electrons downwards to nc
8 x 1010 cm- may heat the electron gas there to several 1~ 0K and may
explain some cm-wave radio bursts with apparently thermal spectra. The
hot region is ejected into the corona, where it expands under its
own forces.
---------------------------------------------------------
Title: Hydromagnetic Oscillations of a Self-Gravitating Fluid.
Authors: Wentzel, Donat G.
1962ApJ...135..593W Altcode:
Hydromagnetic modes of oscillation of a self-gravitating fluid are
investigated by means of the tensor virial theorem. For a spherically
symmetric density distribution and an axisymmetric magnetic field,
the equations yield the radial homologous pulsation and the Kelvin
oscillation, with the periods modified by the magnetic field. The
effect on these oscillations of an axisymmetric distortion of the
configuration of equilibrium is evaluated In the case of resonance
between the two modes, the poles and the equator oscillate independently
of each other and with slightly different periods There are also four
modes of oscillation which do not preserve axisymmetry.
---------------------------------------------------------
Title: Hydromagnetic Waves and the Trapped Radiation, 3, Effects on
Protons above the Proton Belt
Authors: Wentzel, Donat G.
1962JGR....67..485W Altcode:
In parts 1 and 2 [Wentzel, 1961] it was shown that hydromagnetic
waves in the magnetosphere cause a systematic drift of the mirror
points of energetic protons. Above the proton belt these waves do
not travel parallel to the dipole field. The systematic changes of
the magnetic moment due to such waves are computed numerically for a
variety of particle orbits and waves. The results are qualitatively
similar to those for transverse waves unless the Larmor gyration
carries the particle across a major part of a wave, in which case
systematic changes are small. The hm waves above the proton belt are
estimated to be nearly sinusoidal and are estimated to travel at a
typical angle of 45° relative to the dipole field. It is found that
such waves systematically raise the mirror points of proton orbits
above the proton belt, especially if the mirror points are near the
atmosphere. The numerical estimates indicate that this process is
about as efficient as the lowering of the mirror points by random
walk [Dragt, 1961], but the uncertain nature of the hm waves permits
only the conclusion that the time needed to remove a proton into the
atmosphere is longer than that estimated from random walk alone. The
only mirror points that appear to be systematically lowered are near
the upper boundary of the proton belt, which is determined jointly by
systematic and random-walk effects.
---------------------------------------------------------
Title: Galactic Motions in a Large-Scale Magnetic Field
Authors: Wentzel, Donat G.
1961Natur.189..907W Altcode:
RECENT observations on spiral galaxies have shown that their
interstellar material cannot be in purely rotational motion. The
evidence points to a circulation of interstellar gas between the
galactic plane and the galactic halo.
---------------------------------------------------------
Title: Galactic Motions in a Large-Scale Magnetic Field.
Authors: Wentzel, Donat G.
1961AJ.....66S..57W Altcode:
A large-scale galactic magnetiC field transfers angular momentum of
the interstellar gas among different regions of the galaxy and thereby
causes cumulative, rather than epicyclic, radial displacements of the
gas. The persistence of spiral structure in spite of differential
galactic rotation indicates that such a magnetic field exists. A
plausible field connecting the galactic plane with the halo can
exchange angular momentum between these regions and leads to two giant
circulations of the interstellar gas on each side of the galactic
plane. The observed radial motions of the "3-kpc arm" and the isolated
central region of the galaxy can be explained adequately.
---------------------------------------------------------
Title: Hydromagnetic Waves and the Trapped Radiation: 1, Breakdown
of the Adiabatic Invariance
Authors: Wentzel, Donat G.
1961JGR....66..359W Altcode:
The magnetic moment of a charged particle traveling through a
hydromagnetic wave does not remain constant. The motion through
small, static, transverse perturbations of a uniform magnetic field
is calculated. The changes of the magnetic moment are not random. They
depend strongly on the number of wavelengths traversed by the particle
during one Larmor period and on the pitch angle of the gyration. For
sinusoidal waves a resonant decrease of the moment may occur when the
particle traverses 0.5 wavelength per Larmor period. For sufficiently
peaked waves the moment generally decreases if the particle traverses
less than about 0.3 wavelength per Larmor period.
---------------------------------------------------------
Title: Hydromagnetic Waves and the Trapped Radiation: 2, Displacements
of the Mirror Points
Authors: Wentzel, Donat G.
1961JGR....66..363W Altcode:
Theoretical estimates on the nature of hydromagnetic waves in the
magnetosphere are reviewed. The calculations of part 1 are applicable
near the inner Van Allen belt. The magnetic moment of a fast proton
may change significantly in one day. After the mirror point of a
proton injected within the belt is raised or lowered by at most a few
hundred kilometers the magnetic moment remains constant. A stationary
velocity-dependent distribution of mirror points is established. The
existence and finite spatial extent of the inner Van Allen belt are
associated with the peak in the hydromagnetic-wave velocity in that
region. Results indicate that mirror points of protons above the belt
are continuously lowered into the atmosphere. The disappearance of
the Argus electrons during a geomagnetic storm is discussed.
---------------------------------------------------------
Title: On the Shape of Magnetic Stars.
Authors: Wentzel, Donat G.
1961ApJ...133..170W Altcode:
The integration of the simplest of the equations governing a
compressible hydromagnetic equilibrium (Woltjer 1960) with a density
distribution similar to that of actual stars results in a magnetic field
whose strength is roughly proportional to the density throughout most
of the star. Hence there may exist strong central magnetic fields whose
only observable effect is a distortion of the star as a whole. Surfaces
of constant density in distorted magnetic stars are expected to be
spheroidal. A variational calculation results in a simple formula for
the eccentricity of the spheroids, provided that it is small. On the
basis of particular models for the density, it is concluded that the
distortions are significant if the magnetic energy is 6 per cent or
more of the gravitational potential energy. For the models considered,
the spheroids are found to be prolate.
---------------------------------------------------------
Title: The distortion of a magnetized cloud by differential galactic
rotation
Authors: Wentzel, D. G.
1960BAN....15..103W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hydromagnetic Equilibria.
Authors: Wentzel, Donat G.
1960ApJS....5..187W Altcode:
Configurations of hydromagnetic equilibrium are calculated in detail,
using a variational method which is suitable for an incompressible and
infinitely conducting fluid with axisymmetric internal magnetic fields
and motions. The shape of the boundary of the fluid, as well as the
magnetic and the velocity fields, may be evaluated in terms of a set
of integrals of the motion, even when the magnetic and the kinetic
energies are comparable to the gravitational potential energy of the
fluid. The shape of the boundary is determined by a balance between
magnetic forces exerted on the boundary and magnetic and other forces
acting on the body of the fluid. If a magnetic field is force-free,
there necessarily exist magnetic forces acting at its boundary, and the
field is stable (even in a compressible medium) only if these forces
are balanced by means of other magnetic fields or by non-magnetic
forces. If the fluid is in equilibrium under its own gravitation, one
may assume that its boundary is an oblate spheroid. Its eccentricity
may be determined by the variational principle. In one configuration,
in which a poloidal magnetic field prevails with a strength such that
its energy is only 5 per cent of the gravitational potential energy,
the ratio of the major to the minor axis of this spheroid is 1.25. This
indicates that magnetic stars may be considerably distorted by their
magnetic fields. The variational procedure for a fluid rotating
as a solid body without magnetic fields results in the well-known
Maclaurin spheroids. The interaction of magnetic fields with the
rotation is discussed. The virial theorem is an explicit consequence
of the variational procedure only if the medium is assumed to be
compressible. It is satisfied if the total energy of a configuration
is a minimum with respect to the density of the medium. The limiting
procedure for an incompressible medium is discussed.
---------------------------------------------------------
Title: Polarization of starlight by the interstellar medium :
a conference sponsored by the National Science Foundation and the
Lowell Observatory, held at the Kitt Peak and Steward observatories
at Tucson, Arizona, on February 6, 1960
Authors: Davis, Leverett; Wentzel, Donat G.; Donn, Bertram; Platt,
John Rader; Greenberg, Jerome Mayo; Behr, Alfred; Serkowski, Krzysztof;
Gehrels, Tom; National Science Foundation; Lowell Observatory
1960LowOB...4..264D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hydromagnetic equilibria
Authors: Wentzel, Donat Gotthard
1960PhDT........39W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Equilibria of magnetic stars.
Authors: Wentzel, Donat G.
1959AJ.....64..348W Altcode:
A variational method has recently been developed by Chandrasekhar which
permits the calculation of equilibrium configurations of magnetic
stars. The magnetic and velocity fields and also the shape of the
boundary surface may be evaluated even when the magnetic and kinetic
energies are comparable to the gravitational potential energy of the
star. The theory is restricted to an infinitely conducting fluid of
uniform density under conditions of axisymmetry. The equilibria depend
on an infinite set of integrals of motion, and some of the simpler cases
have been investigated in detail. A magnetic field which lies completely
in meridional planes deforms the star into a doughnut- like object,
but when the fluid is in self-gravitational equilibrium, the boundary
surface is always an ellipsoid of revolution. The eccentricity and the
form of the magnetic fields may be evaluated. A special case is that of
the force- free fields. The effect of surface forces may be included
explicitly. If the fluid rotates as a solid body without magnetic
fields, the well known results on Maclaurin ellipsoids follow from the
variational method. In the presence of magnetic fields the treatment
may be generalized to include different modes of isorotation. If a
certain relation between the integrals of motion is satisfied, there
is equipartition between magnetic and kinetic energy everywhere in
the fluid at equilibrium. When the virial theorem is not satisfied,
instability appears to manifest itself primarily in a decrease in
density rather than large changes in boundary shape. Physics Department
University of Chicago Chicago, Ill.
---------------------------------------------------------
Title: Observations of M 32 at 21 cm
Authors: Wentzel, D. G.; van Woerden, H.
1959BAN....14..335W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Motion of Charged Particles in a Force-Free Magnetic Field
Authors: Wentzel, Donat G.
1957ApJ...126..559W Altcode:
No abstract at ADS
