Author name code: wentzel ADS astronomy entries on 2022-09-14 author:"Wentzel, Donat G." ------------------------------------------------------------------------ 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. Bibcode: 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. Title: Mathematical Proofs Authors: Wentzel, D. G. Bibcode: 2004dbss.book..501W Altcode: No abstract at ADS Title: Thermal Radiation Authors: Wentzel, D. G. Bibcode: 2004dbss.book..449W Altcode: No abstract at ADS Title: High-Energy Astrophysics, Electromagnetic Radiation Authors: Wentzel, D. G. Bibcode: 2004dbss.book..489W Altcode: No abstract at ADS Title: More Mechanics of the Solar System Authors: Wentzel, D. G. Bibcode: 2004dbss.book..423W Altcode: No abstract at ADS Title: Mechanics: Orbits and Kepler's Third Law Authors: Wentzel, D. G. Bibcode: 2004dbss.book..399W Altcode: No abstract at ADS Title: Astrophysics for University Physics Courses: Introduction Authors: Wentzel, D. G. Bibcode: 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.

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. Title: More Mechanics: Neutron Stars and Clusters of Galaxies Authors: Wentzel, D. G. Bibcode: 2004dbss.book..437W Altcode: No abstract at ADS Title: Cosmic Magnetic Fields Authors: Wentzel, D. G. Bibcode: 2004dbss.book..477W Altcode: No abstract at ADS Title: The Lives of Stars Authors: Wentzel, D. G. Bibcode: 2004dbss.book..459W Altcode: No abstract at ADS Title: A renewal of astronomy education in Vietnam Authors: Wentzel, Donat G. Bibcode: 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. 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. Bibcode: 2000IAUTA..24..423F Altcode: No abstract at ADS 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. Bibcode: 2000IAUTA..24..442B Altcode: No abstract at ADS Title: Astronomy Education and the American Astronomical Society Authors: Fraknoi, Andrew; Wentzel, Donat Bibcode: 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. Title: International Educational Projects Authors: Wentzel, D. G. Bibcode: 1998HiA....11..901W Altcode: No abstract at ADS Title: International Schools for Young Astronomers, Astronomically developing countries and Lonely Astronomers Authors: Wentzel, D. Bibcode: 1998ntat.coll...27W Altcode: 1998IAUCo.162...27W No abstract at ADS Title: The Ann Arbor Community Authors: Wentzel, Donat G. Bibcode: 1998ASSL..222...40W Altcode: 1998ream.conf...40W No abstract at ADS Title: Type I Radio Bursts Deflected by Lower Hybrid Waves Authors: Wentzel, Donat G. Bibcode: 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. Title: On the Polarization of Type I Radio Bursts Authors: Wentzel, Donat G. Bibcode: 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. Title: Astronomy Education Programmes Authors: Wentzel, D. G. Bibcode: 1997IAUJD..20E..20W Altcode: No abstract at ADS Title: Solar Active Regions as a Percolation Phenomenon. II. Authors: Seiden, Philip E.; Wentzel, Donat G. Bibcode: 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. Title: Unusual wave phenomena near interplanetary shocks at high latitudes Authors: Thejappa, G.; Wentzel, Donat G.; MacDowall, R. J.; Stone, R. G. Bibcode: 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. Title: Low-frequency waves associated with Langmuir waves in solar wind Authors: Thejappa, G.; Wentzel, Donat G.; Stone, R. G. Bibcode: 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. Title: Heating of the Solar Corona - Theory Authors: Wentzel, D. Bibcode: 1995pist.conf..118W Altcode: No abstract at ADS Title: IAU Programs and Projects in the Developing Countries Authors: Wentzel, D. G. Bibcode: 1995HiA....10..147W Altcode: No abstract at ADS Title: Solar Active Regions as a Percolation Phenomenon Authors: Wentzel, D. G.; Seiden, P. E. Bibcode: 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. Title: Electromagnetic Radiation from a Strong DC Electric Field Authors: Guedel, Manuel; Wentzel, Donat G. Bibcode: 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. Title: Group Activities in Introductory Astronomy Courses for Non-science Majors Authors: Deming, G. L.; Wentzel, D. G. Bibcode: 1993AAS...182.2106D Altcode: 1993BAAS...25..823D No abstract at ADS Title: A New Explanation of Solar Radio Millisecond ``Spikes'' Authors: Wentzel, Donat G. Bibcode: 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. Title: A Radiation Mechanism for Solar Millisecond Radio Bursts Authors: Wentzel, D. G. Bibcode: 1992AAS...180.0512W Altcode: 1992BAAS...24..735W No abstract at ADS Title: Solar Active Regions as a Percolation Phenomenon Authors: Wentzel, Donat G.; Seiden, Philip E. Bibcode: 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. Title: Structure of Sunspot Penumbrae: Fallen Magnetic Flux Tubes Authors: Wentzel, Donat G. Bibcode: 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. Title: Solar and Stellar Radio Spikes: Limits on the Saturation of the Electron-Cyclotron Maser Authors: Wentzel, Donat G.; Aschwanden, Markus J. Bibcode: 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. Title: Direct Radiation from a Strong DC Electric Field Authors: Wentzel, Donat G. Bibcode: 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. Title: The Solar Chimes - Searching for Oscillations Inside the Sun Authors: Wentzel, D. G. Bibcode: 1991Mercu..20...77W Altcode: No abstract at ADS Title: Active Regions as a Percolation Phenomenon Authors: Wentzel, D. G.; Seiden, P. E. Bibcode: 1991BAAS...23.1034W Altcode: No abstract at ADS Title: Book-Review - the Restless Sun Authors: Wentzel, D. G.; Bumba, V. Bibcode: 1990SSRv...54R.450W Altcode: No abstract at ADS Title: Diverse Structures, same Science Authors: Wentzel, Donat G. Bibcode: 1990teas.conf....1W Altcode: 1990IAUCo.105....1W No abstract at ADS Title: Programs of IAU Commission 46 for Developing Countries Authors: Wentzel, Donat G. Bibcode: 1990teas.conf..405W Altcode: 1990IAUCo.105..405W No abstract at ADS Title: The Penetration of a Very Hot Gas into a Warm Interstellar Cloud Authors: Wentzel, D. G. Bibcode: 1989BAAS...21..763W Altcode: No abstract at ADS Title: Magnetohydrodynamic Wave Conversion and Solar-Wind Acceleration in Coronal Holes Authors: Wentzel, Donat G. Bibcode: 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. Title: The restless sun Authors: Wentzel, Donat G. Bibcode: 1989resu.book.....W Altcode: 1989QB521.W46...... No abstract at ADS Title: Solar-Wind Acceleration in Coronal Holes Authors: Wentzel, D. G. Bibcode: 1988BAAS...20Q.715W Altcode: No abstract at ADS Title: Solar Oscillations: Generation of a g-Mode by Two p-Modes Authors: Wentzel, Donat G. Bibcode: 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. Title: Solar Oscillations: A Method for Deriving Nonlinear Effects Authors: Wentzel, Donat G. Bibcode: 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. Title: A test for large-angle radio scattering in the solar corona Authors: Wentzel, D. G.; Zlobec, P.; Messerotti, M. Bibcode: 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. Title: Solar Oscillations: Interactions Among p-Modes Authors: Wentzel, D. G. Bibcode: 1986BAAS...18..663W Altcode: No abstract at ADS Title: A Theory for the Solar Type-I Radio Continuum Authors: Wentzel, D. G. Bibcode: 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. Title: Solar Activity and the Coupling of g-Mode Oscillations Authors: Wentzel, D. G. Bibcode: 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. Title: Solar Oscillations: a Method for Deriving Nonlinear Effects Authors: Wentzel, D. G. Bibcode: 1985BAAS...17..896W Altcode: No abstract at ADS Title: Langmuir waves from trapped superthermal electrons Authors: Wentzel, D. G. Bibcode: 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. Title: A Test for Large-Angle Radio Scattering in the Solar Corona Authors: Wentzel, D. G.; Zlobec, P.; Messerotti, M. Bibcode: 1985BAAS...17..632W Altcode: No abstract at ADS Title: Self-confined cosmic rays Authors: Wentzel, D. G. Bibcode: 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 103GeV; 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. Title: Heating of the Upper Solar Corona over Active Regions Authors: Wentzel, D. G. Bibcode: 1984BAAS...16..527W Altcode: No abstract at ADS Title: Polarization of Fundamental Type-Iii Radio Bursts Authors: Wentzel, D. G. Bibcode: 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≃ωp + 0.5Ω. Title: Coronal evolution and solar type I radio bursts: an ion-acoustic wavemodel. Authors: Benz, A. O.; Wentzel, D. G. Bibcode: 1984ost1.conf..127B Altcode: No abstract at ADS Title: Kinetic and dynamic problems Authors: Wentzel, D. G. Bibcode: 1984ost1.conf..187W Altcode: No abstract at ADS Title: Summary of the Session on Educational Astronomy Authors: Wentzel, D. G. Bibcode: 1984aprm.conf..247W Altcode: No abstract at ADS Title: Needs and Opportunities Authors: Wentzel, D. G. Bibcode: 1984aprm.conf..213W Altcode: No abstract at ADS Title: Solar radio emission very near the plasma frequency Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 1983srs..work.....B Altcode: No abstract at ADS Title: New ideas on noise storm theories. Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1982BAAS...14..607W Altcode: No abstract at ADS Title: New ideas on noise storm theories Authors: Wentzel, D. G. Bibcode: 1982srs..work..145W Altcode: No abstract at ADS Title: Summary of discussions and conclusions of theoretical working group Authors: Wentzel, D. G. Bibcode: 1982srs..work..327W Altcode: No abstract at ADS Title: Teaching of Astronomy Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1980BAAS...12..816W Altcode: No abstract at ADS Title: A New Look at James' Solar Radar Results Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 1980IAUS...86..251B Altcode: No abstract at ADS Title: On the fundamental emission of type III solar radio bursts Authors: Wentzel, D. G. Bibcode: 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 (omegap) 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 omegap 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. Bibcode: 1979BAAS...11..679W Altcode: No abstract at ADS Title: The dissipation of hydromagnetic surface waves. Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 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. Bibcode: 1979BAAS...11..441B Altcode: No abstract at ADS Title: Hydromagnetic surface waves. Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 1979phsp.coll..305H Altcode: 1979IAUCo..44..305H No abstract at ADS Title: Discussion Authors: Wentzel, D.; Leer, E. Bibcode: 1979phsp.coll..231W Altcode: 1979IAUCo..44..231W No abstract at ADS Title: Heating of the solar corona: a new outlook. Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 1978SoPh...58..307W Altcode: The reflection coefficient for sound or Alfvén waves reaching the transition zone is evaluated. A family of temperature profiles, including T5/2 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1976BAAS....8..367N Altcode: No abstract at ADS Title: Limitations on the Roles of Hydromagnetic Waves in the Solar Corona Authors: Wentzel, D. G. Bibcode: 1976BAAS....8R.367W Altcode: No abstract at ADS Title: Dynamics of envelopes of planetary nebulae. Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1975BAAS....7R.474W Altcode: No abstract at ADS Title: Coronal Heating by Alfvén Waves Authors: Wentzel, D. G. Bibcode: 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 ≈ 102.5B2(2×106/T)5/2 τ/30s. Title: Task Group on Education in Astronomy (TGEA): Annual report for 1974. Authors: Wentzel, D. G.; Verschuur, G. Bibcode: 1974BAAS....6..491W Altcode: No abstract at ADS Title: Alfven Waves Heating the Solar Corona. Authors: Wentzel, D. G. Bibcode: 1974BAAS....6..346W Altcode: No abstract at ADS Title: Break-up of Pulsar Low-frequency Radiation into Filaments Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 1974sppp.book.....B Altcode: No abstract at ADS Title: Cosmic-ray propagation in the Galaxy: collective effects. Authors: Wentzel, D. G. Bibcode: 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. Bibcode: 1973ApL....15...67W Altcode: No abstract at ADS Title: Adiabatic Deceleration of Cosmic Rays near Their Sources Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 1973AmJPh..41..578H Altcode: No abstract at ADS Title: Ultraviolet Stars and the Interstellar Gas Authors: Rose, William K.; Wentzel, Donat G. Bibcode: 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. Bibcode: 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 Bibcode: 1972BAAS....4..405W Altcode: No abstract at ADS Title: Ultraviolet Stars and the Interstellar Gas. Authors: Rose, W. K.; Wentzel, D. G. Bibcode: 1972BAAS....4R.318R Altcode: No abstract at ADS Title: Nonlinear Dissipation of Alfvén Waves Authors: Chin, Yuan-Chung; Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1972BAAS....4..405V Altcode: No abstract at ADS Title: Scattering of High-Energy Cosmic Rays Authors: Wentzel, D. G. Bibcode: 1972ApL....10..167W Altcode: No abstract at ADS Title: University level astronomy education for nonscience concentrators. A case for astronomy. Authors: Wentzel, D. G. Bibcode: 1972NYASA.198..109W Altcode: No abstract at ADS Title: High-Speed Interstellar Gas Dynamics: Shocks Moderated by Cosmic Rays Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 1971BAAS....3..497W Altcode: No abstract at ADS Title: Interstellar Motions: Minuet or Rock? Authors: Wentzel, Donat G. Bibcode: 1971BAAS....3..381W Altcode: No abstract at ADS Title: Acceleration and Heating of Interstellar Gas by Cosmic Rays Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1969BAAS....1Q.354M Altcode: No abstract at ADS Title: The Propagation and Anisotropy of Cosmic Rays. II. Electrons Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1969lhea.conf...85W Altcode: No abstract at ADS Title: Particle acceleration in astrophysics Authors: Wentzel, D. G. Bibcode: 1969pia..conf..271W Altcode: No abstract at ADS Title: Plasma instabilities in astrophysics Authors: Wentzel, Donat G.; Tidman, Derek A. Bibcode: 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. Bibcode: 1968ApJ...153..331W Altcode: No abstract at ADS Title: Hydromagnetic Waves Excited by Slowly Streaming Cosmic Rays Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1967IAUS...31..131W Altcode: No abstract at ADS Title: Interstellar Clouds and Petschek's Mechanism for Dissipating Magnetic Fields Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 1966IAUTB..12..411W Altcode: No abstract at ADS Title: Fermi Acceleration of Solar Cosmic Rays Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1963ARA&A...1..195W Altcode: No abstract at ADS Title: Dissipation of Magnetic Energy in a Solar Flare. Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1960BAN....15..103W Altcode: No abstract at ADS Title: Hydromagnetic Equilibria. Authors: Wentzel, Donat G. Bibcode: 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 Bibcode: 1960LowOB...4..264D Altcode: No abstract at ADS Title: Hydromagnetic equilibria Authors: Wentzel, Donat Gotthard Bibcode: 1960PhDT........39W Altcode: No abstract at ADS Title: Equilibria of magnetic stars. Authors: Wentzel, Donat G. Bibcode: 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. Bibcode: 1959BAN....14..335W Altcode: No abstract at ADS Title: Motion of Charged Particles in a Force-Free Magnetic Field Authors: Wentzel, Donat G. Bibcode: 1957ApJ...126..559W Altcode: No abstract at ADS