2,360 research outputs found
Evidence for Wave Heating of the Quiet Sun Corona
We have measured the energy and dissipation of Alfvenic waves in the quiet
Sun. A magnetic field was used to infer the location and orientation of the
magnetic field lines along which the waves are expected to travel. The waves
were measured using spectral lines to infer the wave amplitude. The waves cause
a non-thermal broadening of the spectral lines, which can be expressed as a
non-thermal velocity v_nt. By combining the spectroscopic measurements with
this magnetic field model we were able to trace the variation of v_nt along the
magnetic field. At the footpoints of the quiet Sun loops we find that waves
inject an energy flux in the range of 1.2-5.2 x 10^5 erg cm^-2 s^-1. At the
minimum of this range, this amounts to more than 80% of the energy needed to
heat the quiet Sun. We also find that these waves are dissipated over a region
centered on the top of the loops. The position along the loop where the damping
begins is strongly correlated with the length of the loop, implying that the
damping mechanism depends on the global loop properties rather than on local
collisional dissipation.Comment: Submitted to the Astrophysical Journa
Observational Quantification of the Energy Dissipated by Alfv\'en Waves in a Polar Coronal Hole: Evidence that Waves Drive the Fast Solar Wind
We present a measurement of the energy carried and dissipated by Alfv\'en
waves in a polar coronal hole. Alfv\'en waves have been proposed as the energy
source that heats the corona and drives the solar wind. Previous work has shown
that line widths decrease with height in coronal holes, which is a signature of
wave damping, but have been unable to quantify the energy lost by the waves.
This is because line widths depend on both the non-thermal velocity v_nt and
the ion temperature T_i. We have implemented a means to separate the T_i and
v_nt contributions using the observation that at low heights the waves are
undamped and the ion temperatures do not change with height. This enables us to
determine the amount of energy carried by the waves at low heights, which is
proportional to v_nt. We find the initial energy flux density present was 6.7
+/- 0.7 x 10^5 erg cm^-2 s^-1, which is sufficient to heat the coronal hole and
acccelerate the solar wind during the 2007 - 2009 solar minimum. Additionally,
we find that about 85% of this energy is dissipated below 1.5 R_sun,
sufficiently low that thermal conduction can transport the energy throughout
the coronal hole, heating it and driving the fast solar wind. The remaining
energy is roughly consistent with what models show is needed to provide the
extended heating above the sonic point for the fast solar wind. We have also
studied T_i, which we found to be in the range of 1 - 2 MK, depending on the
ion species.Comment: Accepted for the Astrophysical Journa
Inferring the Coronal Density Irregularity from EUV Spectra
Understanding the density structure of the solar corona is important for
modeling both coronal heating and the solar wind. Direct measurements are
difficult because of line-of-sight integration and possible unresolved
structures. We present a new method for quantifying such structure using
density-sensitive EUV line intensities to derive a density irregularity
parameter, a relative measure of the amount of structure along the line of
sight. We also present a simple model to relate the inferred irregularities to
physical quantities, such as the filling factor and density contrast. For quiet
Sun regions and interplume regions of coronal holes, we find a density contrast
of at least a factor of three to ten and corresponding filling factors of about
10-20%. Our results are in rough agreement with other estimates of the density
structures in these regions. The irregularity diagnostic provides a useful
relative measure of unresolved structure in various regions of the corona.Comment: Submitted to the Astrophysical Journa
Functoriality and the Inverse Galois Problem
We prove that there are infinitely many finite simple groups of symplectic
Lie type, of any specified characteristic and rank, which appear as Galois
groups over the field of rational numbers. This generalizes a result of Wiese,
which inspired this paper.Comment: This is a major revision in which several errors are corrected at the
cost of a somewhat weakened main theorem. 32 page
Relative Abundance Measurements in Plumes and Interplumes
We present measurements of relative elemental abundances in plumes and
interplumes. Plumes are bright, narrow structures in coronal holes that extend
along open magnetic field lines far out into the corona. Previous work has
found that in some coronal structures the abundances of elements with a low
first ionization potential (FIP) < 10 eV are enhanced relative to their
photospheric abundances. This coronal-to-photospheric abundance ratio, commonly
called the FIP bias, is typically 1 for element with a high-FIP (> 10 eV). We
have used EIS spectroscopic observations made on 2007 March 13 and 14 over an
~24 hour period to characterize abundance variations in plumes and interplumes.
To assess their elemental composition, we have used a differential emission
measure (DEM) analysis, which accounts for the thermal structure of the
observed plasma. We have used lines from ions of iron, silicon, and sulfur.
From these we have estimated the ratio of the iron and silicon FIP bias
relative to that for sulfur. From the results, we have created FIP-bias-ratio
maps. We find that the FIP-bias ratio is sometimes higher in plumes than in
interplumes and that this enhancement can be time dependent. These results may
help to identify whether plumes or interplumes contribute to the fast solar
wind observed in situ and may also provides constraints on the formation and
heating mechanisms of plumes.Comment: 21 pages; 3 tables; 12 figure
Evidence of Wave Damping at Low Heights in a Polar Coronal Hole
We have measured the widths of spectral lines from a polar coronal hole using
the Extreme Ultraviolet Imaging Spectrometer onboard Hinode. Polar coronal
holes are regions of open magnetic field and the source of the fast solar wind.
We find that the line widths decrease at relatively low heights. Previous
observations have attributed such decreases to systematic effects, but we find
that such effects are too small to explain our results. We conclude that the
line narrowing is real. The non-thermal line widths are believed to be
proportional to the amplitude of Alfven waves propagating along these open
field lines. Our results suggest that Alfven waves are damped at unexpectedly
low heights in a polar coronal hole. We derive an estimate on the upper limit
for the energy dissipated between 1.1 and 1.3 solar radii and find that it is
enough to account for up to 70% of that required to heat the polar coronal hole
and accelerate the solar wind.Comment: Accepted for publication in the Astrophysical Journal, April 201
Spectroscopic measurements of the ion velocity distribution at the base of the fast solar wind
In situ measurements of the fast solar wind reveal non-thermal distributions of electrons, protons, and minor ions extending from 0.3 au to the heliopause. The physical mechanisms responsible for these non-thermal properties and the location where these properties originate remain open questions. Here, we present spectroscopic evidence, from extreme ultraviolet spectroscopy, that the velocity distribution functions (VDFs) of minor ions are already non-Gaussian at the base of the fast solar wind in a coronal hole, at altitudes of <1.1 R ⊙. Analysis of Fe, Si, and Mg spectral lines reveals a peaked line-shape core and broad wings that can be characterized by a kappa VDF. A kappa distribution fit gives very small kappa indices off-limb of κ ≈ 1.9–2.5, indicating either (a) ion populations far from thermal equilibrium, (b) fluid motions such as non-Gaussian turbulent fluctuations or non-uniform wave motions, or (c) some combination of both. These observations provide important empirical constraints for the source region of the fast solar wind and for the theoretical models of the different acceleration, heating, and energy deposition processes therein. To the best of our knowledge, this is the first time that the ion VDF in the fast solar wind has been probed so close to its source region. The findings are also a timely precursor to the upcoming 2018 launch of the Parker Solar Probe, which will provide the closest in situ measurements of the solar wind at approximately 0.04 au (8.5 solar radii)
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