Online Preprints and Reprints

On this page are abstracts, preprints, and reprints of papers submitted or accepted for publication in refereed journals. Online copies of other published papers and meeting presentations are linked (where available) on other pages.

* Solar Papers:

``Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence,'' Cranmer, S. R., van Ballegooijen, A. A., and Edgar, R. J. 2007, Ap. J. Suppl., 171, 520. [see also arXiv preprint: astro-ph/0703333]

This paper is available as:

ABSTRACT:

We present a series of models for the plasma properties along open magnetic flux tubes rooted in solar coronal holes, streamers, and active regions. These models represent the first self-consistent solutions that combine: (1) chromospheric heating driven by an empirically guided acoustic wave spectrum, (2) coronal heating from Alfven waves that have been partially reflected, then damped by anisotropic turbulent cascade, and (3) solar wind acceleration from gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure. The only input parameters are the photospheric lower boundary conditions for the waves and the radial dependence of the background magnetic field along the flux tube. We have not included multifluid or collisionless effects (e.g., preferential ion heating) which are not yet fully understood. For a single choice for the photospheric wave properties, our models produce a realistic range of slow and fast solar wind conditions by varying only the coronal magnetic field. Specifically, a two-dimensional model of coronal holes and streamers at solar minimum reproduces the latitudinal bifurcation of slow and fast streams seen by Ulysses. The radial gradient of the Alfven speed affects where the waves are reflected and damped, and thus whether energy is deposited below or above the Parker critical point. As predicted by earlier studies, a larger coronal ``expansion factor'' gives rise to a slower and denser wind, higher temperature at the coronal base, less intense Alfven waves at 1 AU, and correlative trends for commonly measured ratios of ion charge states and FIP-sensitive abundances that are in general agreement with observations. These models offer supporting evidence for the idea that coronal heating and solar wind acceleration (in open magnetic flux tubes) can occur as a result of wave dissipation and turbulent cascade.

``On the Generation, Propagation, and Reflection of Alfven Waves from the Solar Photosphere to the Distant Heliosphere,'' Cranmer, S. R., and van Ballegooijen, A. A. 2005, Ap. J. Suppl., 156, 265. [see also arXiv preprint astro-ph/0410639]

This paper is available as:

ABSTRACT:

We present a comprehensive model of the global properties of Alfven waves in the solar atmosphere and the fast solar wind. Linear non-WKB wave transport equations are solved from the photosphere to a distance past the orbit of the Earth, and for wave periods ranging from 3 seconds to 3 days. We derive a radially varying power spectrum of kinetic and magnetic energy fluctuations for waves propagating in both directions along a superradially expanding magnetic flux tube. This work differs from previous models in three major ways. (1) In the chromosphere and low corona, the successive merging of flux tubes on granular and supergranular scales is described using a two-dimensional magnetostatic model of a network element. Below a critical flux-tube merging height the waves are modeled as thin-tube kink modes, and we assume that all of the kink-mode wave energy is transformed into volume-filling Alfven waves above the merging height. (2) The frequency power spectrum of horizontal motions is specified only at the photosphere, based on prior analyses of G-band bright point kinematics. Everywhere else in the model the amplitudes of outward and inward propagating waves are computed with no free parameters. We find that the wave amplitudes in the corona agree well with off-limb nonthermal line-width constraints. (3) Nonlinear turbulent damping is applied to the results of the linear model using a phenomenological energy loss term. A single choice for the normalization of the turbulent outer-scale length produces both the right amount of damping at large distances (to agree with in situ measurements) and the right amount of heating in the extended corona (to agree with empirically constrained solar wind acceleration models). In the corona, the modeled heating rate differs by more than an order of magnitude from a rate based on isotropic Kolmogorov turbulence.

``New views of the solar wind with the Lambert W function,'' Cranmer, S. R. 2004, American J. Phys., 72, 1397. [see also arXiv preprint astro-ph/0406176]

This paper is available as:

ABSTRACT:

This paper presents closed-form analytic solutions to two illustrative problems in solar physics that have been considered not solvable in this way previously. Both the outflow speed and the mass loss rate of the solar wind of plasma particles ejected by the Sun are derived analytically for certain illustrative approximations. The calculated radial dependence of the flow speed applies to both Parker's isothermal solar wind equation and Bondi's equation of spherical accretion. These problems involve the solution of transcendental equations containing products of variables and their logarithms. Such equations appear in many fields of physics and are solvable by use of the Lambert W function, which is briefly described. This paper is an example of how new functions can be applied to existing problems.

``Alfvenic Turbulence in the Extended Solar Corona: Kinetic Effects and Proton Heating,'' Cranmer, S. R. and van Ballegooijen, A. A., 2003, Ap. J., 594, 573.

This paper is available as:

ABSTRACT:

We present a model of magnetohydrodynamic (MHD) turbulence in the extended solar corona that contains the effects of collisionless dissipation and anisotropic particle heating. Recent observations have shown that preferential heating and acceleration of positive ions occurs in the first few solar radii of the high-speed solar wind. Measurements made by the Ultraviolet Coronagraph Spectrometer aboard SOHO have revived interest in the idea that ions are energized by the dissipation of ion cyclotron resonant waves, but such high-frequency (i.e., small wavelength) fluctuations have not been observed. A turbulent cascade is one possible way of generating small-scale fluctuations from a pre-existing population of low-frequency MHD waves. We model this cascade as a combination of advection and diffusion in wavenumber space. The dominant spectral transfer occurs in the direction perpendicular to the background magnetic field. As expected from earlier models, this leads to a highly anisotropic fluctuation spectrum with a rapidly decaying tail in the parallel wavenumber direction. The wave power that decays to high enough frequencies to become ion cyclotron resonant depends on the relative strengths of advection and diffusion in the cascade. For the most realistic values of these parameters, though, there is insufficient power to heat protons and heavy ions. The dominant oblique fluctuations (with dispersion properties of kinetic Alfven waves) undergo Landau damping, which implies strong parallel electron heating. We discuss the probable nonlinear evolution of the electron velocity distributions into parallel beams and discrete phase-space holes (similar to those seen in the terrestrial magnetosphere) which can possibly heat protons via stochastic interactions.

``Coronal Holes and the High-Speed Solar Wind,'' Cranmer, S. R. 2002, Space Science Reviews, 101, 229 (65-page review paper).

This paper is available as:

ABSTRACT:

Coronal holes are the lowest density plasma components of the Sun's outer atmosphere, and are associated with rapidly expanding magnetic fields and the acceleration of the high-speed solar wind. Spectroscopic and polarimetric observations of the extended corona, coupled with interplanetary particle and radio sounding measurements going back several decades, have put strong constraints on possible explanations for how the plasma in coronal holes receives its extreme kinetic properties. The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft has revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies for positive ions in coronal holes. We review recent observations, modeling techniques, and proposed heating and acceleration processes for protons, electrons, and heavy ions. We emphasize that an understanding of the acceleration region of the wind (in the nearly collisionless extended corona) is indispensable for building a complete picture of the physics of coronal holes.

``Ion Cyclotron Diffusion of Velocity Distributions in the Extended Solar Corona,'' Cranmer, S. R. 2001, J. Geophys. Res., 106, 24937.

A preprint of this paper is available as:

ABSTRACT:

The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) has revealed strong kinetic anisotropies and extremely large perpendicular temperatures of heavy ions in the extended solar corona. These observations have revived interest in the idea that the high-speed solar wind is heated and accelerated by the dissipation of ion cyclotron resonant Alfven waves. This process naturally produces departures from Maxwellian and bi-Maxwellian velocity distributions. Here it is argued that these departures must be taken into account in order to understand the resonant velocity-space diffusion, the wave damping, and the formation of ultraviolet emission lines. Time-dependent ion velocity distributions are computed for a fixed spectrum of waves in a homogeneous plasma, and the moments of the distributions are compared with simple bi-Maxwellian models. The existence of a boundary, in parallel velocity space, between resonance and nonresonance produces an effective saturation of the velocity-space diffusion that bi-Maxwellian models could not predict. The damping of an input wave spectrum is computed for a coronal population of 1000 ion species with the above saturation effect included. For realistic levels of fluctuation power, it is concluded that waves propagating solely from the coronal base would not be able to heat and accelerate the ions that have been observed to exhibit strong energization, and that local wave generation is required. Ultraviolet emission line profiles are computed for the derived non-Maxwellian distributions, and possible unique identifiers of the ion cyclotron resonance mechanism are noted.

``Ion Cyclotron Wave Dissipation in the Solar Corona: The Summed Effect of More than 2000 Ion Species,'' Cranmer, S. R. 2000, Ap. J., 532, 1197.

The paper is available as:

ABSTRACT:

In this paper the dissipation of ion cyclotron resonant Alfven waves in the extended solar corona is examined in detail. For the first time, the wave damping arising from more than 2000 low-abundance ion species is taken into account. Useful approximations for the computation of coronal ionization equilibria for elements heavier than nickel are presented. Also, the Sobolev approximation from the theory of hot-star winds is applied to the resonant wave dissipation in the solar wind, and the surprisingly effective damping ability of ``minor'' ions is explained in simple terms. High-frequency (10 to 10,000 Hz) waves propagating up from the base of the corona are damped significantly when they resonate with ions having charge-to-mass ratios of about 0.1, and negligible wave power would then be available to resonate with higher charge-to-mass ratio ions at larger heights. This result confirms preliminary suggestions from earlier work that the waves that heat and accelerate the high-speed solar wind must be generated throughout the extended corona. The competition and eventual equilibrium between wave damping and wave replenishment may explain observed differences in coronal O VI and Mg X emission line widths.

The Fortran code used to compute the ionization equilibrium of ions from hydrogen to uranium, along with sample output files, is publically available as a gzipped UNIX tar file (112 K). The uncompressed files are also available individually:

* Hot-Star Papers:

``A Statistical Study of Threshold Rotation Rates for the Formation of Disks around Be Stars,'' Cranmer, S. R. 2005, Ap. J., 634, 585. [see arXiv preprint: astro-ph/0507718]

ABSTRACT:

This paper presents a detailed statistical determination of the equatorial rotation rates of classical Be stars. The rapid rotation of Be stars is likely to be linked to the ejection of gas that forms dense circumstellar disks. The physical origins of these disks are not understood, though it is generally believed that the ability to spin up matter into a Keplerian disk depends on how close the stellar rotation speed is to the critical speed at which the centrifugal force cancels gravity. There has been recent disagreement between the traditional idea that Be stars rotate between 50% and 80% of their critical speeds and new ideas (inspired by the tendency for gravity darkening to mask rapid rotation at the equator) that their rotation may be very nearly critical. This paper utilizes Monte Carlo forward modeling to simulate distributions of the projected rotation speed v sin i, taking into account gravity darkening, limb darkening, and observational uncertainties. A chi-squared minimization procedure was used to find the distribution parameters that best reproduce observed v sin i distributions from R. Yudin's database. Early-type (O7e-B2e) Be stars were found to exhibit a roughly uniform spread of intrinsic rotation speed that extends from 40-60% up to 100% of critical. Late-type (B3e-A0e) Be stars exhibit progressively narrower ranges of rotation speed as the effective temperature decreases; the lower limit rises to reach critical rotation for the coolest Be stars. The derived lower limits on equatorial rotation speed represent conservative threshold rotation rates for the onset of the Be phenomenon. The significantly subcritical speeds found for early-type Be stars represent strong constraints on physical models of angular momentum deposition in Be star disks.

``A Multiwavelength Campaign on gamma Cassiopeiae. IV. The Case for Illuminated Disk-Enhanced Wind Streams,'' Cranmer, S. R., Smith, M. A., and Robinson, R. D. 2000, Ap. J., 537, 433.

The paper is available as:

ABSTRACT:

On 1996 March 14-15 we conducted a campaign with the Hubble Space Telescope GHRS to observe the Si IV 1394, 1403 lines of the B0.5e star gamma Cas at high temporal and spectral resolution. As a part of this 22 hour campaign, the Rossi X-ray Timing Explorer (RXTE) was also used to monitor this star's copious and variable X-ray emission. In this fourth paper of a series we present an analysis of the rapid variations of the Discrete Absorption Components (DACs) of the Si IV doublet. The DACs attain a maximum absorption at -1280 km/s, taper at higher velocities, and extend to -1800 km/s. The DACs in this star's resonance lines have been shown to be correlated with a > 6 year cycle in the Balmer line emission V/R ratio, and in 1996 this DAC strength was near its maximum. We derive hydrogen densities of 10^(9)-10^(10) cm^(-3) in the DAC material using a curve of growth method and find that the plasma becomes marginally optically thick near -1280 km/s. The ``mean DAC'' probably represents a broad ``plateau'' with a volume density intermediate between the star's mid-latitude wind and equatorial disk. We also follow the blueward evolution of subfeatures in the DACs. These features appear to emanate primarily from one or two discrete azimuths on the star and accelerate much slower than expected for the background wind, thereby exhibiting an enhanced opacity spiral stream pattern embedded within the structure forming the DAC. In Papers I and II we suggested that active X-ray centers are associated with at least two major cool clouds forced into corotation. Several correlations of flickering in the Si IV DACs are found in our data which support the idea that changes in X-ray ionizing flux cause changes in the ionization of material at various sectors along the spiral pattern. We demonstrate that similar flickering is visible in archival IUE data from 1982 and may also be responsible for earlier reports from Copernicus of rapid changes in this star's UV and optical lines. Finally, we discovered that flickering of the DAC fluxes in the 1982 data is correlated with rotation phase and shows a modulation with a 7.5 hour cyclical cessation of X-ray flares that was observed recently by RXTE. This confirms our basic picture that lulls in X-ray activity close to the star's surface cause both a lower Si V ionization fraction and an increase in Si IV variability within the DAC structures.

Some of the Hubble Space Telescope data that was used in this paper (and in the preceding papers in this series) are archived by the Multi-Mission Archive at Space Telescope (MAST) at this URL.

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