arXiv subjects

Steven R. Cranmer

Publications and source records attributed to Steven R. Cranmer.

81 records · Page 5Linked to original sources

arXiv2007

Winds of Main-Sequence Stars: Observational Limits and a Path to Theoretical Prediction

It is notoriously difficult to measure the winds of solar-type stars. Traditional spectroscopic and radio continuum techniques are sensitive to mass loss rates at least two to three orders of magnitude stronger than the Sun's relatively feeble wind. Much has been done with these methods to probe the more massive outflows of younger (T Tauri) and older (giant, AGB, supergiant) cool stars, but the main sequence remains terra incognita. This presentation reviews the limits on traditional diagnostics and outlines more recent ideas such as Lyman alpha astrospheres and charge-exchange X-ray emission. In addition, there are hybrid constraints on mass loss rates that combine existing observables and theoretical models. The Sackmann/Boothroyd conjecture of a more massive young Sun (and thus a much stronger ZAMS wind) is one such idea that needs to be tested further. Another set of ideas involves a strong proposed coupling between coronal heating and stellar mass loss rates, where the former is easier to measure in stars down to solar-like values. The combined modeling of stellar coronae and stellar winds is developing rapidly, and it seems to be approaching a level of development where the only remaining ``free parameters'' involve the sub-photospheric convection. This talk will also summarize these theoretical efforts to predict the properties of solar-type main-sequence winds.

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arXiv2005

A Statistical Study of Threshold Rotation Rates for the Formation of Disks around Be Stars

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 percent 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 to 60 percent up to 100 percent 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.

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arXiv2005

Why is the Fast Solar Wind Fast and the Slow Solar Wind Slow? A Survey of Geometrical Models

Four decades have gone by since the discovery that the solar wind at 1 AU seems to exist in two relatively distinct states: slow and fast. There is still no universal agreement concerning the primary physical cause of this apparently bimodal distribution, even in its simplest manifestation at solar minimum. In this presentation we review and extend a series of ideas that link the different states of solar wind to the varying superradial geometry of magnetic flux tubes in the extended corona. Past researchers have emphasized different aspects of this relationship, and we attempt to disentangle some of the seemingly contradictory results. We apply the hypothesis of Wang and Sheeley (as well as Kovalenko) that Alfven wave fluxes at the base are the same for all flux tubes to a recent model of non-WKB Alfven wave reflection and turbulent heating, and we predict coronal heating rates as a function of flux tube geometry. We compare the feedback of these heating rates on the locations of Parker-type critical points, and we discuss the ranges of parameters that yield a realistic bifurcation of wind solutions into fast and slow. Finally, we discuss the need for next-generation coronagraph spectroscopy of the extended corona - especially measurements of the electron temperature above 1.5 solar radii - in order to confirm and refine these ideas.

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arXiv2005

Capabilities of UV Coronagraphic Spectroscopy for Studying the Source Regions of Solar Energetic Particles and the Solar Wind

We summarize the unique capabilities of UV coronagraphic spectroscopy for determining the detailed plasma properties (e.g., density, temperature, outflow speed, composition) of the source regions of both transient phenomena such as CMEs, flares, and solar energetic particles (SEPs) and more time-steady solar wind streams. UVCS/SOHO observations have provided the first detailed diagnostics of the physical conditions of CME plasma in the extended corona. It provided new insights into the roles of shock waves, reconnection, and magnetic helicity in CME eruptions. We summarize past observations and discuss the diagnostic potential of UV coronagraphic spectroscopy for characterizing two possible sites of SEP production: CME shocks and reconnection current sheets. UVCS/SOHO has also led to fundamentally new views of the acceleration region of the solar wind. Understanding the physical processes in this region, which ranges from the low corona (1.1 to 1.5 Rsun) past the sonic points (greater than 5 Rsun), is key to linking the results of solar imaging to in situ particle and field detection. Despite the advances that have resulted from UVCS/SOHO, more advanced instrumentation could determine properties of additional ions with a wider sampling of charge/mass combinations. This would provide much better constraints on the specific kinds of waves that are present as well as the specific collisionless damping modes. Electron temperatures and departures from Maxwellian velocity distributions could also be measured. The instrumentation capable of making the above observations will be described.

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arXiv2004

Coronal Heating versus Solar Wind Acceleration

Parker's initial insights from 1958 provided a key causal link between the heating of the solar corona and the acceleration of the solar wind. However, we still do not know what fraction of the solar wind's mass, momentum, and energy flux is driven by Parker-type gas pressure gradients, and what fraction is driven by, e.g., wave-particle interactions or turbulence. SOHO has been pivotal in bringing these ideas back to the forefront of coronal and solar wind research. This paper reviews our current understanding of coronal heating in the context of the acceleration of the fast and slow solar wind. For the fast solar wind, a recent model of Alfven wave generation, propagation, and non-WKB reflection is presented and compared with UVCS, SUMER, radio, and in-situ observations at the last solar minimum. The derived fractions of energy and momentum addition from thermal and nonthermal processes are found to be consistent with various sets of observational data. For the more chaotic slow solar wind, the relative roles of steady streamer-edge flows (as emphasized by UVCS abundance analysis) versus bright blob structures (seen by LASCO) need to be understood before the relation between streamer heating and and slow-wind acceleration can be known with certainty. Finally, this presentation summarizes the need for next-generation remote-sensing observations that can supply the tight constraints needed to unambiguously characterize the dominant physics.

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arXiv2004

New Insights into Solar Wind Physics from SOHO

The Solar and Heliospheric Observatory (SOHO) was launched in December 1995 with a suite of instruments designed to answer long-standing questions about the Sun's internal structure, its extensive outer atmosphere, and the solar wind. This paper reviews the new understanding of the physical processes responsible for the solar wind that have come from the past 8 years of SOHO observations, analysis, and theoretical work. For example, the UVCS instrument on SOHO has revealed the acceleration region of the fast solar wind to be far from simple thermal equilibrium. Evidence for preferential acceleration of ions, 100 million K ion temperatures, and marked departures from Maxwellian velocity distributions all point to specific types of collisionless heating processes. The slow solar wind, typically associated with bright helmet streamers, has been found to share some of the nonthermal characteristics of the fast wind. Abundance measurements from spectroscopy and visible-light coronagraphic movies from LASCO have led to a better census of the plasma components making up the slow wind. The origins of the solar wind in the photosphere and chromosphere have been better elucidated with disk spectroscopy from the SUMER and CDS instruments. Finally, the impact of the solar wind on spacecraft systems, ground-based technology, and astronauts has been greatly aided by having continuous solar observations at the Earth-Sun L1 point, and SOHO has set a strong precedent for future studies of space weather.

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arXiv2004

New views of the solar wind with the Lambert W function

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.

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arXiv2003

Observational Aspects of Wave Acceleration in Open Magnetic Regions

This paper reviews the latest observational evidence for the existence of propagating waves in the open magnetic flux tubes of the solar corona. SOHO measurements have put tentative limits on the fluxes of various types of magnetohydrodynamic (MHD) waves in the acceleration region of the solar wind. Also, continually improving measurements of fluctuations at larger distances (i.e., in situ detection and radio scintillation) continue to provide significant constraints on the dominant types of plasma oscillation throughout the corona and wind. The dissipation of MHD fluctuations of some kind, probably involving anisotropic turbulent cascade, is believed to dominate the heating of the extended corona. Spectroscopic observations from the UVCS instrument on SOHO have helped to narrow the field of possibilities for the precise modes, generation mechanisms, and damping channels. This presentation will also review some of the collisionless, kinetic aspects of wave heating and acceleration that are tied closely to the observational constraints.

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arXiv2002

Solar Wind Acceleration in Coronal Holes

This paper reviews the current state of our understanding of high-speed solar wind acceleration in coronal holes. Observations by SOHO, coupled with interplanetary particle measurements going back several decades, have put strong constraints on possible explanations for how the protons, electrons, and minor ions receive their extreme kinetic properties. The asymptotic plasma conditions of the wind depend on energy and momentum deposition both at the coronal base (where, e.g., the mass flux is determined) and in the extended acceleration region between 2 and 10 solar radii (where the plasma becomes collisionless and individual particle species begin to exhibit non-Maxwellian velocity distributions with different moments). The dissipation of magnetohydrodynamic fluctuations (i.e., waves, turbulence, and shocks) is believed to dominate the heating in the extended corona, and spectroscopic observations from the UVCS instrument on SOHO have helped to narrow the field of possibilities for the precise modes, generation mechanisms, and damping channels. We will survey recent theoretical and observational results that have contributed to new insights, and we will also show how next-generation instruments can be designed to identify and characterize the dominant physical processes to an unprecedented degree.

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