172 research outputs found
A Model for the Stray Light Contamination of the UVCS Instrument on SOHO
We present a detailed model of stray-light suppression in the spectrometer
channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO
spacecraft. The control of diffracted and scattered stray light from the bright
solar disk is one of the most important tasks of a coronagraph. We compute the
fractions of light that diffract past the UVCS external occulter and
non-specularly pass into the spectrometer slit. The diffracted component of the
stray light depends on the finite aperture of the primary mirror and on its
figure. The amount of non-specular scattering depends mainly on the
micro-roughness of the mirror. For reasonable choices of these quantities, the
modeled stray-light fraction agrees well with measurements of stray light made
both in the laboratory and during the UVCS mission. The models were constructed
for the bright H I Lyman alpha emission line, but they are applicable to other
spectral lines as well.Comment: 19 pages, 5 figures, Solar Physics, in pres
Air Temperature and Vapor Pressure Changes Caused by Sprinkler Irrigation
The downwind effect of evaporation from sprinkler
spray was studied in the field to determine if air temperature
and vapor pressure were changed enough to
influence plant growth and water use. Wet-bulb and
dry-bulb temperature profiles were measured upwind
and at three distances downwind from a sprinkler lateral
before and during sprinkling. Wind-speed and direction
were also measured. Air temperature generally was reduced
less than 1 C, and vapor pressure in the air was
increased less than 0.8 mb. This amount of change in
the air temperature and humidity is not likely to be sufficient
to cause any significant change in plant growth or
evaporative loss of water
Onion Seed Yield and Quality as Affected by Irrigation Management
The effects of soil moisture, sprinkler irrigation, and
misting (5 min at 30-min intervals) on hybrid onion-seed
(Allium cepa L.) production were evaluated at Kimberly,
Idaho. Sprinkling and misting treatments were included
to determine if sprinkler irrigation could be used for
onion-seed production, and if evaporative cooling (misting)
would alleviate the scalding of umbels at excessively
high temperatures. Results showed that reasonable onionseed
yields could be obtained with sprinkler irrigation in
spite of anticipated adverse effects on pollination. In
areas with high daytime temperatures, misting could be
used to cool the umbels, as much as 15 C for about 20
min, although it does not seem necessary for the low 32
to 35 C daytime temperatures at Kimbqly. The highest
anion-seed yields were obtained with the furrow-irrigation
treatment where water was applied when tensiometers at
20-cm depth read about 0.5 bar soil moisture tension (furrow
adequate). The addition of misting to furrow irrigation
decreased seed yields 19%. Furrow irrigating at
0.4 bar decreased yields 26% below those obtained with irrigation
at 0.5 bar. While overirrigation adversely affected
yield, irrigation treatment had only a slight effect on
seed weight and vigor
Coronal Diagnostics from Narrowband Images around 30.4 nm
Images taken in the band centered at 30.4 nm are routinely used to map the
radiance of the He II Ly alpha line on the solar disk. That line is one of the
strongest, if not the strongest, line in the EUV observed in the solar
spectrum, and one of the few lines in that wavelength range providing
information on the upper chromosphere or lower transition region. However, when
observing the off-limb corona the contribution from the nearby Si XI 30.3 nm
line can become significant. In this work we aim at estimating the relative
contribution of those two lines in the solar corona around the minimum of solar
activity. We combine measurements from CDS taken in August 2008 with
temperature and density profiles from semiempirical models of the corona to
compute the radiances of the two lines, and of other representative coronal
lines (e.g., Mg X 62.5 nm, Si XII 52.1 nm). Considering both diagnosed
quantities from line ratios (temperatures and densities) and line radiances in
absolute units, we obtain a good overall match between observations and models.
We find that the Si XI line dominates the He II line from just above the limb
up to ~2 R_Sun in streamers, while its contribution to narrowband imaging in
the 30.4 nm band is expected to become smaller, even negligible in the corona
beyond ~2 - 3 R_Sun, the precise value being strongly dependent on the coronal
temperature profile.Comment: 26 pages, 11 figures; to be published in: Solar Physic
Non-destructive, dynamic detectors for Bose-Einstein condensates
We propose and analyze a series of non-destructive, dynamic detectors for
Bose-Einstein condensates based on photo-detectors operating at the shot noise
limit. These detectors are compatible with real time feedback to the
condensate. The signal to noise ratio of different detection schemes are
compared subject to the constraint of minimal heating due to photon absorption
and spontaneous emission. This constraint leads to different optimal operating
points for interference-based schemes. We find the somewhat counter-intuitive
result that without the presence of a cavity, interferometry causes as much
destruction as absorption for optically thin clouds. For optically thick
clouds, cavity-free interferometry is superior to absorption, but it still
cannot be made arbitrarily non-destructive . We propose a cavity-based
measurement of atomic density which can in principle be made arbitrarily
non-destructive for a given signal to noise ratio
TomograPy: A Fast, Instrument-Independent, Solar Tomography Software
Solar tomography has progressed rapidly in recent years thanks to the
development of robust algorithms and the availability of more powerful
computers. It can today provide crucial insights in solving issues related to
the line-of-sight integration present in the data of solar imagers and
coronagraphs. However, there remain challenges such as the increase of the
available volume of data, the handling of the temporal evolution of the
observed structures, and the heterogeneity of the data in multi-spacecraft
studies.
We present a generic software package that can perform fast tomographic
inversions that scales linearly with the number of measurements, linearly with
the length of the reconstruction cube (and not the number of voxels) and
linearly with the number of cores and can use data from different sources and
with a variety of physical models: TomograPy
(http://nbarbey.github.com/TomograPy/), an open-source software freely
available on the Python Package Index. For performance, TomograPy uses a
parallelized-projection algorithm. It relies on the World Coordinate System
standard to manage various data sources. A variety of inversion algorithms are
provided to perform the tomographic-map estimation. A test suite is provided
along with the code to ensure software quality. Since it makes use of the
Siddon algorithm it is restricted to rectangular parallelepiped voxels but the
spherical geometry of the corona can be handled through proper use of priors.
We describe the main features of the code and show three practical examples
of multi-spacecraft tomographic inversions using STEREO/EUVI and STEREO/COR1
data. Static and smoothly varying temporal evolution models are presented.Comment: 21 pages, 6 figures, 5 table
Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter
The Coronal Multichannel Polarimeter (CoMP) measures not only the
polarization of coronal emission, but also the full radiance profiles of
coronal emission lines. For the first time, CoMP observations provide
high-cadence image sequences of the coronal line intensity, Doppler shift and
line width simultaneously in a large field of view. By studying the Doppler
shift and line width we may explore more of the physical processes of CME
initiation and propagation. Here we identify a list of CMEs observed by CoMP
and present the first results of these observations. Our preliminary analysis
shows that CMEs are usually associated with greatly increased Doppler shift and
enhanced line width. These new observations provide not only valuable
information to constrain CME models and probe various processes during the
initial propagation of CMEs in the low corona, but also offer a possible
cost-effective and low-risk means of space weather monitoring.Comment: 6 figures. Will appear in the special issue of Coronal Magnetism,
Sol. Phy
A Statistical Study on the Morphology of Rays and Dynamics of Blobs in the Wake of Coronal Mass Ejections
In this paper, with a survey through the Large Angle and Spectrometric
Coronagraph (LASCO) data from 1996 to 2009, we present 11 events with plasma
blobs flowing outwards sequentially along a bright coronal ray in the wake of a
coronal mass ejection. The ray is believed to be associated with the current
sheet structure that formed as a result of solar eruption, and the blobs are
products of magnetic reconnection occurring along the current sheet. The ray
morphology and blob dynamics are investigated statistically. It is found that
the apparent angular widths of the rays at a fixed time vary in a range of
2.1-6.6 (2.0-4.4) degrees with an average of 3.5 (2.9) degrees at 3 (4) Rs,
respectively, and the observed durations of the events vary from 12 h to a few
days with an average of 27 h. It is also found, based on the analysis of blob
motions, that 58% (26) of the blobs were accelerated, 20% (9) were decelerated,
and 22% (10) moved with a nearly-constant speed. Comparing the dynamics of our
blobs and those that are observed above the tip of a helmet streamer, we find
that the speeds and accelerations of the blobs in these two cases differ
significantly. It is suggested that these differences of the blob dynamics stem
from the associated magnetic reconnection involving different magnetic field
configurations and triggering processes.Comment: 12 pages, 6 figures, accepted by Solar Physic
Roles of Fast-Cyclotron and Alfven-Cyclotron Waves for the Multi-Ion Solar Wind
Using linear Vlasov theory of plasma waves and quasi-linear theory of
resonant wave-particle interaction, the dispersion relations and the
electromagnetic field fluctuations of fast and Alfven waves are studied for a
low-beta multi-ion plasma in the inner corona. Their probable roles in heating
and accelerating the solar wind via Landau and cyclotron resonances are
quantified. We assume that (1) low-frequency Alfven and fast waves have the
same spectral shape and the same amplitude of power spectral density; (2) these
waves eventually reach ion cyclotron frequencies due to a turbulence cascade;
(3) kinetic wave-particle interaction powers the solar wind. The existence of
alpha particles in a dominant proton/electron plasma can trigger linear mode
conversion between oblique fast-whistler and hybrid alpha-proton cyclotron
waves. The fast-cyclotron waves undergo both alpha and proton cyclotron
resonances. The alpha cyclotron resonance in fast-cyclotron waves is much
stronger than that in Alfven-cyclotron waves. For alpha cyclotron resonance, an
oblique fast-cyclotron wave has a larger left-handed electric field
fluctuation, a smaller wave number, a larger local wave amplitude, and a
greater energization capability than a corresponding Alfven-cyclotron wave at
the same wave propagation angle \theta, particularly at < \theta <
. When Alfven-cyclotron or fast-cyclotron waves are present, alpha
particles are the chief energy recipient. The transition of preferential
energization from alpha particles to protons may be self-modulated by
differential speed and temperature anisotropy of alpha particles via the
self-consistently evolving wave-particle interaction. Therefore, fast-cyclotron
waves as a result of linear mode coupling is a potentially important mechanism
for preferential energization of minor ions in the main acceleration region of
the solar wind.Comment: 29 pages, 10 figures, 3 tables. Accepted for publication in Solar
Physic
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
- …