132 research outputs found
An implementation plan for priorities in solar-system space physics
The scientific objectives and implementation plans and priorities of the Space Science Board in areas of solar physics, heliospheric physics, magnetospheric physics, upper atmosphere physics, solar-terrestrial coupling, and comparative planetary studies are discussed and recommended programs are summarized. Accomplishments of Skylab, Solar Maximum Mission, Nimbus-7, and 11 other programs are highlighted. Detailed mission plans in areas of solar and heliospheric physics, plasma physics, and upper atmospheric physics are also described
Observation and Modeling of the Solar Transition Region: II. Solutions of the Quasi-Static Loop Model
In the present work we undertake a study of the quasi-static loop model and
the observational consequences of the various solutions found. We obtain the
most general solutions consistent with certain initial conditions. Great care
is exercised in choosing these conditions to be physically plausible (motivated
by observations). We show that the assumptions of previous quasi-static loop
models, such as the models of Rosner, Tucker and Vaiana (1978) and Veseckey,
Antiochos and Underwood (1979), are not necessarily valid for small loops at
transition region temperatures. We find three general classes of solutions for
the quasi-static loop model, which we denote, radiation dominated loops,
conduction dominated loops and classical loops. These solutions are then
compared with observations. Departures from the classical scaling law of RTV
are found for the solutions obtained. It is shown that loops of the type that
we model here can make a significant contribution to lower transition region
emission via thermal conduction from the upper transition region.Comment: 30 pages, 3 figures, Submitted to ApJ, Microsoft Word File 6.0/9
Search for photospheric footpoints of quiet Sun transition region loops
CONTEXT:The footpoints of quiet Sun Transition Region (TR) loops do not seem
to coincide with the photospheric magnetic structures appearing in traditional
low-sensitivity magnetograms.
AIMS: To look for the so-far unidentified photospheric footpoints of TR loops
using G-band bright points (BPs) as proxies for photospheric magnetic field
concentrations.
METHODS: Comparison of TR measurements with SoHO/SUMER and photospheric
magnetic field observations obtained with the Dutch Open Telescope.
RESULTS: Photospheric BPs are associated with bright TR structures, but they
seem to avoid the brightest parts of the structure. BPs appear in regions that
are globally redshifted, but they avoid extreme velocities. TR explosive events
are not clearly associated with BPs.
CONCLUSIONS: The observations are not inconsistent with the BPs being
footpoints of TR loops, although we have not succeeded to uniquely identify
particular BPs with specific TR loops.Comment: Accepted for publication in A&A. 10 pages, 10 figures. Due to size
limitations, the quality of fig3 is not goo
Chromospheric jets around the edges of sunspots
Aims. Evidence is beginning to be put forward that demonstrates the role of the chromosphere in supplying energy and mass to the corona. We aim to asses the role of chromospheric jets in active region dynamics.
Methods. Using a combination of the Hinode/SOT Ca II H and TRACE 1550 Å and 1600 Å filters we examine chromospheric jets situated at the edge of a sunspot.
Results. Analysis reveals a near continuous series of jets, that raise chromospheric material into the low corona above a sunspot. The jets have average rise speeds of 30 km/s
and a range of 10−100 km/s. Enhanced emission observed at the jets leading edge suggests the formation of a shock front. Increased emission in TRACE bandpasses above the sunspot and the disappearance of the jets from the Ca II filter suggests that some of the chromospheric jet material is at least heated to ∼ 0.1 MK. The evidence suggests that the jets could be a mechanism which provides a steady, low-level heating for active region features
Enhancement of the helium resonance lines in the solar atmosphere by suprathermal electron excitation II: non-Maxwellian electron distributions
In solar EUV spectra the He I and He II resonance lines show unusual
behaviour and have anomalously high intensities compared with other transition
region lines. The formation of the helium resonance lines is investigated
through extensive non-LTE radiative transfer calculations. The model
atmospheres of Vernazza, Avrett & Loeser are found to provide reasonable
matches to the helium resonance line intensities but significantly
over-estimate the intensities of other transition region lines. New model
atmospheres have been developed from emission measure distributions derived by
Macpherson & Jordan, which are consistent with SOHO observations of transition
region lines other than those of helium. These models fail to reproduce the
observed helium resonance line intensities by significant factors. The
possibility that non-Maxwellian electron distributions in the transition region
might lead to increased collisional excitation rates in the helium lines is
studied. Collisional excitation and ionization rates are re-computed for
distribution functions with power law suprathermal tails which may form by the
transport of fast electrons from high temperature regions. Enhancements of the
helium resonance line intensities are found, but many of the predictions of the
models regarding line ratios are inconsistent with observations. These results
suggest that any such departures from Maxwellian electron distributions are not
responsible for the helium resonance line intensities.Comment: 23 pages, 11 figures, accepted to appear in MNRAS, LaTeX uses mn.st
Spectroscopy at the solar limb: I. Average off-limb profiles and Doppler shifts of Ca II H
We present constraints on the structure of the chromosphere from observations
of the Ca II H line profile near and off the solar limb. We obtained a data set
of the Ca II H line in a field of view extending 20" across the limb. We
analyzed the spectra for the properties of off-limb spectra. We used tracers of
the Doppler shifts, such as the location of the absorption core, the ratio of
the two emission peaks H2V and H2R, and intensity images at a fixed wavelength.
The average off-limb profiles show a smooth variation with increasing limb
distance. The line width increases up to a height of about 2 Mm above the limb.
The profile shape is fairly symmetric with nearly identical H2V and H2R
intensities; at a height of 5 Mm, it changes into a single Gaussian without
emission peaks. We find that all off-limb spectra show large Doppler shifts
that fluctuate on the smallest resolved spatial scales. The variation is more
prominent in cuts parallel to the solar limb than on those perpendicular to it.
As far as individual structures can be unequivocally identified at our spatial
resolution, we find a specific relation between intensity enhancements and
Doppler shifts: elongated brightenings are often flanked all along their
extension by velocities in opposite directions. The average off-limb spectra of
Ca II H present a good opportunity to test static chromospheric atmosphere
models because they lack the photospheric contribution that is present in
disk-center spectra. We suggest that the observed relation between intensity
enhancements and Doppler shifts could be caused by waves propagating along the
surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave
propagation inclined to the tube axis would cause a helical shape of the
Doppler excursion, visible as opposite velocity at the sides of the flux tube.Comment: 11 pages, 11 figures + 3 pages Appendix, accepted by A&
On the statistical detection of propagating waves in polar coronal holes
Waves are important for the heating of the solar corona and the acceleration
of the solar wind. We have examined a long spectral time series of a northern
coronal hole observed on the 20th October 1996, with the SUMER spectrometer
onboard SoHO. The observations were obtained in a transition region N IV 765 A
line and in a low coronal Ne VIII 770 A line. Our observations indicate the
presence of compressional waves with periods of ~25 min. Using Fourier
techniques, we measured the phase delays between intensity as well as velocity
oscillations in the two chosen lines. From this we are able to measure the
travel time of the propagating oscillations and, hence, the propagation speeds
of the waves producing the oscillations. We found that there is a difference in
the nature of the propagation in bright ('network') and dark ('internetwork')
regions with the latter sometimes showing evidence for downwardly propagating
waves that is not seen in the former. As, in all cases, the measured
propagation speeds are subsonic, we concluded that the detected waves are slow
magnetoacoustic in nature.Comment: 7 pages, 7 figure
What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models
One of the major discoveries of the Extreme ultraviolet Imaging Telescope
(EIT) on SOHO were intensity enhancements propagating over a large fraction of
the solar surface. The physical origin(s) of the so-called `EIT' waves is still
strongly debated. They are considered to be either wave (primarily fast-mode
MHD waves) or non-wave (pseudo-wave) interpretations. The difficulty in
understanding the nature of EUV waves lies with the limitations of the EIT
observations which have been used almost exclusively for their study. Their
limitations are largely overcome by the SECCHI/EUVI observations on-board the
STEREO mission. The EUVI telescopes provide high cadence, simultaneous
multi-temperature coverage, and two well-separated viewpoints. We present here
the first detailed analysis of an EUV wave observed by the EUVI disk imagers on
December 07, 2007 when the STEREO spacecraft separation was .
Both a small flare and a CME were associated with the wave cadence, and single
temperature and viewpoint coverage. These limitations are largely overcome by
the SECCHI/EUVI observations on-board the STEREO mission. The EUVI telescopes
provide high cadence, simultaneous multi-temperature coverage, and two
well-separated viewpoints. Our findings give significant support for a
fast-mode interpretation of EUV waves and indicate that they are probably
triggered by the rapid expansion of the loops associated with the CME.Comment: Solar Physics, 2009, Special STEREO Issue, in pres
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
Solar low-lying cool loops and their contribution to the transition region EUV output
In the last 30 years, the existence of small and cool magnetic loops (height
< 8 Mm, T < 10^5 K) has been proposed and debated to explain the increase of
the DEM (differential emission measure) towards the chromosphere. We present
hydrodynamic simulations of low-lying cool loops to study their conditions of
existence and stability, and their contribution to the transition region EUV
output. We find that stable, quasi-static cool loops (with velocities < 1 km/s)
can be obtained under different and more realistic assumptions on the radiative
losses function with respect to previous works. A mixture of the DEMs of these
cool loops plus intermediate loops with temperatures between 10^5 and 10^6 K
can reproduce the observed emission of the lower transition region at the
critical turn-up temperature point (T ~ 2x10^5 K) and below T = 10^5 K.Comment: Accepted for publication in A&A on Nov 25th 201
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