2,114 research outputs found
Joule Heating and Anomalous Resistivity in the Solar Corona
Recent radioastronomical observations of Faraday rotation in the solar corona
can be interpreted as evidence for coronal currents, with values as large as
Amperes (Spangler 2007). These estimates of currents are used
to develop a model for Joule heating in the corona. It is assumed that the
currents are concentrated in thin current sheets, as suggested by theories of
two dimensional magnetohydrodynamic turbulence. The Spitzer result for the
resistivity is adopted as a lower limit to the true resistivity. The calculated
volumetric heating rate is compared with an independent theoretical estimate by
Cranmer et al (2007). This latter estimate accounts for the dynamic and
thermodynamic properties of the corona at a heliocentric distance of several
solar radii. Our calculated Joule heating rate is less than the Cranmer et al
estimate by at least a factor of . The currents inferred from
the observations of Spangler (2007) are not relevant to coronal heating unless
the true resistivity is enormously increased relative to the Spitzer value.
However, the same model for turbulent current sheets used to calculate the
heating rate also gives an electron drift speed which can be comparable to the
electron thermal speed, and larger than the ion acoustic speed. It is therefore
possible that the coronal current sheets are unstable to current-driven
instabilities which produce high levels of waves, enhance the resistivity and
thus the heating rate.Comment: Submitted to Nonlinear Processes in Geophysics as a contribution from
the 7th International Workshop on Nonlinear Waves and Turbulence in Space
Plasmas", Beaulieu, France, April 21-25, 200
A study of shock-associated magnetohydrodynamic waves in the solar wind
Three major topics were addressed, one theoretical and two observational. The topics were: (1) an attempt to understand the evolution of the large-amplitude magnetohydrodynamic (MHD) waves in the foreshock, using a nonlinear wave equation called the Derivative Nonlinear Schrodinger equation (henceforth DNLS) as a model, (2) using the extensive set of ISE data to test for the presence of various nonlinear wave processes which might be present, and (3) a study of plasma turbulence in the interstellar medium which might be physically similar to that in the solar wind. For these investigations we used radioastronomical techniques. Good progress was made in each of these areas and a separate discussion of each is given
Shock-associated plasma density fluctuations in the interstellar medium
Theories of diffuse shock acceleration of the cosmic rays that predict that there should be regions of intense Alfven waves both upstream and downstream of the shock front are considered. Observations of similar waves near solar system shock waves show that they produce substantial density fluctuations. Such density fluctuations might produce observable scattering of radio waves. Observations which searched for angular broadening or blurring of radio sources whose lines of sight pass close to or through supernova remnants are discussed. No definite cases of remnant-associated scattering are detected. However, the source CL 4, which is viewed through the Cygnus Loop supernova remnant, may be such an object and merits further observation
Calculation of the longitudinal aerodynamic characteristics of upper-surface-blown wing-flap configurations
An engineering method for predicting the longitudinal aerodynamic characteristics of wing-flap configurations with upper surface blowing (USB) was developed. Potential flow models were incorporated into the prediction method: a wing and flap lifting surface model and a jet wake model. The wing-flap model used a vortex-lattice to represent the wing and flaps. The wing had an arbitrary planform and camber and twist, and the flap system was made up of a Coanda flap and other flap segments of arbitrary size. The jet wake model consisted of a series of closely spaced rectangular vortex rings. The wake was positioned such that it was tangent to the upper surface of the wing and flap between the exhaust nozzle and the flap trailing edge. It was specified such that the mass, momentum, and spreading rates were similar to actual USB jet wakes. Comparisons of measured and predicted pressure distributions, span load distributions, and total lift and pitching-moment coefficients on swept and unswept USB configurations are included. A wide range of thrust coefficients and flap deflection angles were considered at angles of attack up to the onset of stall
Ion-Neutral Collisions in the Interstellar Medium: Wave Damping and Elimination of Collisionless Processes
Most phases of the interstellar medium contain neutral atoms in addition to
ions and electrons. This introduces differences in plasma physics processes in
those media relative to the solar corona and the solar wind at a heliocentric
distance of 1 astronomical unit. In this paper, we consider two well-diagnosed,
partially-ionized interstellar plasmas. The first is the Diffuse Ionized Gas
(DIG) which is probably the extensive phase in terms of volume. The second is
the gas that makes up the Local Clouds of the Very Local Interstellar Medium
(VLISM). Ion-neutral interactions seem to be important in both media. In the
DIG, ion-neutral collisions are relatively rare, but sufficiently frequent to
damp magnetohydrodynamic (MHD) waves (as well as propagating MHD eddies) within
less than a parsec of the site of generation. This result raises interesting
questions about the sources of turbulence in the DIG. In the case of the VLISM,
the ion-neutral collision frequency is higher than that in the DIG, because the
hydrogen is partially neutral rather than fully ionized. We present results
showing that prominent features of coronal and solar wind turbulence seem to be
absent in VLISM turbulence. For example, ion temperature does not depend on ion
mass. This difference may be attributable to ion-neutral collisions, which
distribute power from more effectively heated massive ions such as iron to
other ion species and neutral atoms.Comment: Submitted to American Institute of Physics Conference Proceedings for
conference "Partially Ionized Plasmas Throughout the Cosmos", Dastgeer
Shaikh, edito
Theoretical Study of Ducted Fan Performance
Existing computer program improved capability for predicting performance of ducted fan in uniform axial flo
A Computer Program for the Prediction of Ducted Fan Performance
Manual for computer program for predicting performance of ducted fan
Observational Tests of the Properties of Turbulence in the Very Local Interstellar Medium
The Very Local Interstellar Medium (VLISM) contains clouds which consist of
partially-ionized plasma. These clouds can be effectively diagnosed via high
resolution optical and ultraviolet spectroscopy of the absorption lines they
form in the spectra of nearby stars. Among the information provided by these
spectroscopic measurements are the root-mean-square velocity fluctuation due to
turbulence in these clouds and the ion temperature, which may be partially
determined by dissipation of turbulence. We consider whether this turbulence
resembles the extensively studied and well-diagnosed turbulence in the solar
wind and solar corona. Published observations are used to determine if the
velocity fluctuations are primarily transverse to a large-scale magnetic field,
whether the temperature perpendicular to the large scale field is larger than
that parallel to the field, and whether ions with larger Larmor radii have
higher temperatures than smaller gyroradius ions. Although a thorough
investigation of the data is underway, a preliminary examination of the
published data shows neither evidence for anisotropy of the velocity
fluctuations or temperature, nor Larmor radius-dependent heating. These results
indicate differences between solar wind and Local Cloud turbulence.Comment: Paper submitted to Nonlinear Processes in Geophysic
A Technique for Measuring Electrical Currents in the Solar Corona
A technique is described for measuring electrical currents in the solar
corona. It uses radioastronomical polarization measurements of a
spatially-extended radio source viewed through the corona. The observations
yield the difference in the Faraday rotation measure between two closely-spaced
lines of sight through the corona, a measurement referred to as {\em
differential Faraday rotation}. It is shown that the expression for
differential Faraday rotation is proportional to the path integral where is the plasma density and is the
coronal magnetic field. The integral is around a closed loop (Amperian Loop) in
the corona. If the plasma density is assumed roughly constant, the differential
Faraday rotation is proportional to the current within the loop, via Ampere's
Law. The validity of the constant density approximation is discussed, and two
test cases are presented in which the associated error in the inferred current
is small, of order tens of percent or less. The method is illustrated with
observations of the radio source 3C228 with the Very Large Array (VLA) in
August, 2003. A measurement of a differential Faraday rotation ``event'' on
August 16, 2003, yields an estimate of Amperes in the
Amperian Loop. A smaller event on August 18 yields an enclosed current of Amperes. The implications of these currents for coronal heating
are briefly discussed.Comment: Revised version of paper; accepted for Astrophysical Journal, vol
670, November 20, 200
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