113 research outputs found
Microphysics of Waves and Instabilities in the Solar Wind and Their Macro Manifestations in the Corona and Interplanetary Space
Investigations of the physical processes responsible for coronal heating and the acceleration of the solar wind were pursued with the use of our recently developed 2D MHD solar wind code and our 1D multifluid code. In particular, we explored: (1) the role of proton temperature anisotropy in the expansion of the solar (2) the role of plasma parameters at the coronal base in the formation of high (3) a three-fluid model of the slow solar wind (4) the heating of coronal loops (5) a newly developed hybrid code for the study of ion cyclotron resonance in wind, speed solar wind streams at mid-latitudes, the solar wind
Are solar maximum fan streamers a consequence of twisting sheet structures?
Context.Fan streamers are often observed at low to mid latitudes in the corona at solar maximum, appearing narrow in latitudinal extent near the Sun, and fanning out with height, adopting an approximately linear, but not necessarily radial, configuration above ~3Â .
Aims.We offer arguments to support the conjecture that such structures may sometimes consist of high density, non-uniform sheets, viewed edge-on near the Sun, and twisting to a more face-on alignment by 3Â .
Methods.EUV and white light observations of a fan streamer observed on 2000/12/05 are analyzed. A simple 3D density model is used to recreate the streamer structure.
Results.EIT images show a thin bright sheet at the base of the streamer. The continuation of this structure through the EIT, MLSO MKIV coronameter, and LASCO C2 fields of view, suggests that this sheet is formed mostly of open magnetic field lines. The overall large-scale appearance of the streamer is well simulated by a simple model of a twisting high-density sheet. If the twisting-sheet conjecture is valid, there is a correlation between the distribution of enhanced rays within the streamer viewed in white light, and the distribution of small regions of enhanced brightness seen on the disk in EIT 171 Å at the position of the streamer base.
Conclusions.We suggest that the apparent poleward divergence of equatorial coronal rays, or threads, seen during solar maximum above active regions, may sometimes be a consequence of such a twisting sheet topology
An empirical 3D model of the large-scale coronal structure based on the distribution of Hα filaments on the solar disk
Despite the wealth of solar data currently available, the explicit connection
between coronal streamers and features on the solar disk remains unresolved. An
empirical three-dimensional model, which reproduces the evolution of the
large-scale coronal structure starting from the solar surface, is presented.
The model is based on the view that the source of the large-scale coronal
structure, namely streamers, is a consequence of the evolution of twisted
sheet-like structures originating from prominences (or, equivalently,
filaments) at the base of the corona. The high-density sheets evolve and merge
with height into a final radial configuration constrained by the oberved
position of streamers stalks higher up in the corona. The observational
constraints are provided by white light observations from the LASCO/C2 data
during the declining phase of solar activity, spanning the end of Carrington
Rotation (CR) 2005 and the start of CR 2006, i.e. July-August 2003, and the
position of filaments from the corresponding H synoptic maps of the
Paris-Meudon Observatory. The 3D model thus derived yields a reasonable
agreement with the observed large-scale coronal structure, in particular the
shape of large helmet streamers. These results give confidence in the
underlying assumption that large helmet streamers can be the result of the
convergence of two or more sheet-like structures originating from a
distribution of prominences on the solar disk. The model supports the view that
streamers, during that time of the solar cycle, are often associated with
multiple current sheets
The Depiction of Coronal Structure in White-Light Images
The very sharp decrease of density with heliocentric distance makes imaging
of coronal density structures out to a few solar radii challenging. The radial
gradient in brightness can be reduced using numerous image processing
techniques, thus quantitative data are manipulated to provide qualitative
images. Introduced in this study is a new normalizing radial graded filter
(NRGF), a simple filter for removing the radial gradient to reveal coronal
structure. Applied to polarized brightness observations of the corona, the NRGF
produces images which are striking in their detail. Total brightness white
light images include contributions from the F corona, stray light and other
instrumental contributions which need to be removed as effectively as possible
to properly reveal the electron corona structure. A new procedure for
subtracting this background from LASCO C2 white light total brightness images
is introduced. The background is created from the unpolarized component of
total brightness images and is found to be remarkably time-invariant, remaining
virtually unchanged over the solar cycle. By direct comparison with polarized
brightness data, we show that the new background subtracting procedure is
superior in depicting coronal structure accurately, particularly when used in
conjunction with the NRGF. The effectiveness of the procedures is demonstrated
on a series of LASCO C2 observations of a coronal mass ejection (CME).Comment: 10 pages, 5 figure
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