213 research outputs found
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
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
Properties of Turbulence in the Very Local Interstellar Clouds
We have investigated the degree to which turbulence in the Very Local
Interstellar Clouds resembles the highly-studied turbulence in the solar corona
and the solar wind. The turbulence diagnostics for the Local Clouds are the
absorption line widths measured along 32 lines of sight to nearby stars,
yielding measurements for 53 absorption components (Redfield and Linsky 2004).
We have tested whether the Local Cloud turbulence has the following properties
of turbulence in the solar corona or the solar wind: (a) velocity fluctuations
mainly perpendicular to the average magnetic field, (b) a temperature
anisotropy in the sense that the perpendicular temperature is larger than the
parallel temperature (or at least enhanced relative to expectation), and (c) an
ion temperature which is dependent on the ion Larmor radius, in the sense that
more massive ions have higher temperatures. Our analysis of the data does not
show compelling evidence for any of these properties in Local Cloud turbulence,
indicating possible differences with heliospheric plasmas. In the case of
anisotropy of velocity fluctuations, although the expected observational
signature is not seen, we cannot exclude the possibility of relatively high
degrees of anisotropy (anisotropy parameter ), if
some other process in the the Local Clouds is causing variations in the
turbulent line width from one line of sight to another. We briefly consider
possible reasons for differences between coronal and solar wind turbulence and
that in the Local Clouds.Comment: Submitted to the Astrophysical Journa
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