193 research outputs found
Coherence retrieval using trace regularization
The mutual intensity and its equivalent phase-space representations quantify
an optical field's state of coherence and are important tools in the study of
light propagation and dynamics, but they can only be estimated indirectly from
measurements through a process called coherence retrieval, otherwise known as
phase-space tomography. As practical considerations often rule out the
availability of a complete set of measurements, coherence retrieval is usually
a challenging high-dimensional ill-posed inverse problem. In this paper, we
propose a trace-regularized optimization model for coherence retrieval and a
provably-convergent adaptive accelerated proximal gradient algorithm for
solving the resulting problem. Applying our model and algorithm to both
simulated and experimental data, we demonstrate an improvement in
reconstruction quality over previous models as well as an increase in
convergence speed compared to existing first-order methods.Comment: 28 pages, 10 figures, accepted for publication in SIAM Journal on
Imaging Science
Why does the apparent mass of a coronal mass ejection increase?
Mass is one of the most fundamental parameters characterizing the dynamics of
a coronal mass ejection (CME). It has been found that CME apparent mass
measured from the brightness enhancement in coronagraph images shows an
increasing trend during its evolution in the corona. However, the physics
behind it is not clear. Does the apparent mass gain come from the mass outflow
from the dimming regions in the low corona, or from the pileup of the solar
wind plasma around the CME when it propagates outwards from the Sun? We
analyzed the mass evolution of six CME events. Their mass can increase by a
factor of 1.6 to 3.2 from 4 to 15 Rs in the field of view (FOV) of the
coronagraph on board the Solar Terrestrial Relations Observatory (STEREO). Over
the distance about 7 to 15 Rs, where the coronagraph occulting effect can be
negligible, the mass can increase by a factor of 1.3 to 1.7. We adopted the
`snow-plough' model to calculate the mass contribution of the piled-up solar
wind in the height range from about 7 to 15 Rs. For 2/3 of the events, the
solar wind pileup is not sufficient to explain the measured mass increase. In
the height range from about 7 to 15 Rs, the ratio of the modeled to the
measured mass increase is roughly larger than 0.55. Although the ratios are
believed to be overestimated, the result gives evidence that the solar wind
pileup probably makes a non-negligible contribution to the mass increase. It is
not clear yet whether the solar wind pileup is a major contributor to the final
mass derived from coronagraph observations. However, our study suggests that
the solar wind pileup plays increasingly important role in the mass increase as
a CME moves further away from the Sun.Comment: 27 pages, 2 tables, 9 figures, accepted by Ap
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