1,051 research outputs found
Are Coronal Loops Isothermal or Multithermal? Yes!
Surprisingly few solar coronal loops have been observed simultaneously with
TRACE and SOHO/CDS, and even fewer analyses of these loops have been conducted
and published. The SOHO Joint Observing Program 146 was designed in part to
provide the simultaneous observations required for in-depth temperature
analysis of active region loops and determine whether these loops are
isothermal or multithermal. The data analyzed in this paper were taken on 2003
January 17 of AR 10250. We used TRACE filter ratios, emission measure loci, and
two methods of differential emission measure analysis to examine the
temperature structure of three different loops. TRACE and CDS observations
agree that Loop 1 is isothermal with Log T 5.85, both along the line of
sight as well as along the length of the loop leg that is visible in the CDS
field of view. Loop 2 is hotter than Loop 1. It is multithermal along the line
of sight, with significant emission between 6.2 Log T 6.4, but the loop
apex region is out of the CDS field of view so it is not possible to determine
the temperature distribution as a function of loop height. Loop 3 also appears
to be multithermal, but a blended loop that is just barely resolved with CDS
may be adding cool emission to the Loop 3 intensities and complicating our
results. So, are coronal loops isothermal or multithermal? The answer appears
to be yes
The Distance to High-Velocity Cloud Complex M
21-cm HI4PI survey data are used to study the anomalous-velocity hydrogen gas
associated with high-velocity cloud Complex M. These high-sensitivity,
high-resolution, high-dynamic-range data show that many of the individual
features, including MI, MIIa, and MIIb, are components of a long, arched
filament that extends from about (l, b) = (105{\deg}, 53{\deg}) to (l, b) =
(196{\deg}, 55{\deg}). Maps at different velocities, results from Gaussian
analysis, and observations of associated high-energy emission make a compelling
case that the MI cloud and the arched filament are physically interacting. If
this is the case, we can use the distance to MI, 150 pc as reported by Schmelz
& Verschuur (2022), to set the distance to Complex M. The estimated mass of
Complex M is then about 120 solar masses and the energy implied using the
observed line-of-sight velocity, -85 km/s, is 8.4 x 10^48 ergs. Integrating
over 4{\pi} steradians, the total energy for a spherically symmetrical
explosion is estimated to be 1.9 x 10^50 ergs, well within the energy budget of
a typical supernova
Neon Lights Up a Controversy: the Solar Ne/O Abundance
The standard solar model was so reliable that it could predict the existence
of the massive neutrino. Helioseismology measurements were so precise that they
could determine the depth of the convection zone. This agreement between theory
and observation was the envy of all astrophysics -- until recently when
sophisticated three-dimensional hydrodynamic calculations of the solar
atmosphere reduced the metal content by a factor of almost two. Antia & Basu
(2005) suggested that a higher value of the solar neon abundance, Ne/O = 0.52,
would resolve this controversy. Drake & Testa (2005) presented strong evidence
in favor of this idea from a sample of 21 Chandra stars with enhanced values of
the neon abundance, Ne/O = 0.41. In this paper, we have analyzed solar active
region spectra from the archive of the Flat Crystal Spectrometer on Solar
Maximum Mission, a NASA mission from the 1980s, as well as full-Sun spectra
from the pioneering days of X-ray astronomy in the 1960s. These data seem
consistent with the standard neon-to-oxygen abundance value, Ne/O = 0.15
(Grevesse & Sauval 1998). If these results prove to be correct, than the
enhanced-neon hypothesis will not resolve the current controversy.Comment: submitted to ApJ Letter
Multithermal Analysis of a CDS Coronal Loop
The observations from 1998 April 20 taken with the Coronal Diagnostics
Spectrometer CDS on SOHO of a coronal loop on the limb have shown that the
plasma was multi-thermal along each line of sight investigated, both before and
after background subtraction. The latter result relied on Emission Measure Loci
plots, but in this Letter, we used a forward folding technique to produce
Differential Emission Measure curves. We also calculate DEM-weighted
temperatures for the chosen pixels and find a gradient in temperature along the
loop as a function of height that is not compatible with the flat profiles
reported by numerous authors for loops observed with EIT on SOHO and TRACE. We
also find discrepancies in excess of the mathematical expectation between some
of the observed and predicted CDS line intensities. We demonstrate that these
differences result from well-known limitations in our knowledge of the atomic
data and are to be expected. We further show that the precision of the DEM is
limited by the intrinsic width of the ion emissivity functions that are used to
calculate the DEM. Hence we conclude that peaks and valleys in the DEM, while
in principle not impossible, cannot be confirmed from the data.Comment: 12 pages, 3 figures, Accepted by ApJ Letter
All Coronal Loops are the Same: Evidence to the Contrary
The 1998 April 20 spectral line data from the Coronal Diagnostics
Spectrometer (CDS) on the {\it Solar and Heliospheric Observatory} (\SOHO)
shows a coronal loop on the solar limb. Our original analysis of these data
showed that the plasma was multi-thermal, both along the length of the loop and
along the line of sight. However, more recent results by other authors indicate
that background subtraction might change these conclusions, so we consider the
effect of background subtraction on our analysis. We show Emission Measure (EM)
Loci plots of three representative pixels: loop apex, upper leg, and lower leg.
Comparisons of the original and background-subtracted intensities show that the
EM Loci are more tightly clustered after background subtraction, but that the
plasma is still not well represented by an isothermal model. Our results taken
together with those of other authors indicate that a variety of temperature
structures may be present within loops.Comment: Accepted for publication in ApJ Letter
Deriving Plasma Densities and Elemental Abundances from SERTS Differential Emission Measure Analysis
We use high-resolution spectral emission line data obtained by the SERTS instrument during three rocket flights to demonstrate a new approach for constraining electron densities of solar active region plasma.We apply differential emission measure (DEM) forward-fitting techniques to characterize the multithermal solar plasma producing the observed EUV spectra, with constraints on the high-temperature plasma from the Yohkoh Soft X-ray Telescope. In this iterative process, we compare line intensities predicted by an input source distribution to observed line intensities for multiple iron ion species, and search a broad range of densities to optimize chi-square simultaneously for the many available density-sensitive lines. This produces a density weighted by the DEM, which appears to be useful for characterizing the bulk of the emitting plasma over a significant range of temperature. This "DEM-weighted density" technique is complementary to the use of density-sensitive line ratios and less affected by uncertainties in atomic data and ionization fraction for any specific line. Once the DEM shape and the DEM-weighted density have been established from the iron lines, the relative elemental abundances can be determined for other lines in the spectrum. We have also identified spectral lines in the SERTS wavelength range that may be problemati
Transverse oscillations of a multi-stranded loop
We investigate the transverse oscillations of a line-tied multi-stranded
coronal loop composed of several parallel cylindrical strands. First, the
collective fast normal modes of the loop are found with the T-matrix theory.
There is a huge quantity of normal modes with very different frequencies and a
complex structure of the associated magnetic pressure perturbation and velocity
field. The modes can be classified as bottom, middle, and top according to
their frequencies and spatial structure. Second, the temporal evolution of the
velocity and magnetic pressure perturbation after an initial disturbance are
analyzed. We find complex motions of the strands. The frequency analysis
reveals that these motions are a combination of low and high frequency modes.
The complexity of the strand motions produces a strong modulation of the whole
tube movement. We conclude that the presumed internal fine structure of a loop
influences its transverse oscillations and so its transverse dynamics cannot be
properly described by those of an equivalent monolithic loop.Comment: Accepted in Ap
Supernovae Origin for the Low-Latitude-Intermediate-Velocity Arch and the North-Celestial-Pole Loop
Supernova explosions attributed to the unseen companion in several binary
systems identified by the Third Gaia Data Release (Gaia DR3) may be responsible
for a number of well-known and well-studied features in the radio sky,
including the Low-Latitude-Intermediate-Velocity Arch and the
North-Celestial-Pole Loop. Slices from the Longitude-Latitude-Velocity data
cube of the -21-cm galactic neutral hydrogen HI4PI survey (HI4PI
Collaboration et al. 2016) show multiple signatures of an expanding shell. The
source of this expansion, which includes the Low-Latitude-Intermediate-Velocity
Arch on the approaching side, may be the neutron star candidate in the Gaia DR3
1093757200530267520 binary. If we make the simplifying assumptions that the
expansion of the cavity is uniform and spherically symmetric, then the
explosion took place about 700,000 years ago. The momentum is in reasonable
agreement with recent model estimates for a supernova this old. The HI on the
receding side of this cavity is interacting with the gas approaching us on the
near side of a second cavity. The North-Celestial-Pole Loop appears to be
located at the intersection of these two expanding features. The neutron star
candidate in the Gaia DR3 1144019690966028928 binary may be (in part)
responsible for this cavity. Explosions from other candidates may account for
the observed elongation along the line of sight of this second cavity. We can
use the primary star in these binaries to anchor the distances to the
Low-Latitude-Intermediate-Velocity Arch and North-Celestial-Pole Loop, which
are about 167 and about 220 pc, respectively.Comment: Published in The Astrophysical Journa
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