500 research outputs found
Radiative transfer in cylindrical threads with incident radiation. VII. Multi-thread models
Aims. Our aim is to improve on previous radiative transfer calculations in illuminated cylindrical threads in order to better understand
the physical conditions in cool solar chromospheric and coronal structures commonly observed in hydrogen and helium lines.
Methods. We solve the radiative transfer and statistical equilibrium equations in a two-dimensional cross-section of a cylindrical
structure oriented horizontally and lying above the solar surface. The cylinder is filled with a mixture of hydrogen and helium, and
is illuminated at a given altitude from the solar disc. We construct simple models made from a single thread, or from an ensemble
of several threads along the line of sight. This first use of 2D multi-thread fine structure modelling combining hydrogen and helium
radiative transfer allows us to compute synthetic emergent spectra from cylindrical structures and to study the effect of line-of-sight
integration of an ensemble of threads under a range of physical conditions. We analyse the effects of variations in temperature
distribution and in gas pressure.We consider the effect of multi-thread structures within a given field of view and the effect of peculiar
velocities between the structures in a multi-thread model. These new models are compared to the single thread model, and tested with
varying parameters.
Results. The presence of a temperature gradient, with temperature increasing towards the edge of the cylindrical thread, reduces the
relative importance of the incident radiation coming from the solar disc on the emergent intensities of most hydrogen and helium
lines. We also find that when assuming randomly displaced threads in a given field of view, the integrated intensities of optically
thick and thin transitions behave considerably differently. In optically thin lines, the emergent intensity increases proportionally with
the number of threads, and the spatial variation of the intensity becomes increasingly homogeneous. Optically thick lines however
saturate after only a few threads. As a consequence, the spatial variation of the intensity retains much similarity with that of the first
few threads. The multi-thread model produces complex line profiles with significant asymmetries if randomly generated line-of-sight
velocities are added for each thread.
Conclusions. These new computations show for the first time the effect of integrating the radiation emitted in H and He lines by
several cylindrical threads static or moving along the line of sight. They can be used to interpret high-spatial and spectral resolutions
of cylindrical structures found in the solar atmosphere, such as cool coronal loops or prominence threads
Structure of prominence legs: Plasma and magnetic field
We investigate the properties of a `solar tornado' observed on 15 July 2014,
and aim to link the behaviour of the plasma to the internal magnetic field
structure of the associated prominence. We made multi-wavelength observations
with high spatial resolution and high cadence using SDO/AIA, the IRIS
spectrograph and the Hinode/SOT instrument. Along with spectropolarimetry
provided by the THEMIS telescope we have coverage of both optically thick
emission lines and magnetic field information. AIA reveals that the two legs of
the prominence are strongly absorbing structures which look like they are
rotating, or oscillating in the plane of the sky. The two prominence legs,
which are both very bright in Ca II (SOT), are not visible in the IRIS Mg II
slit-jaw images. This is explained by the large optical thickness of the
structures in Mg II which leads to reversed profiles, and hence to lower
integrated intensities at these locations than in the surroundings. Using lines
formed at temperatures lower than 1 MK, we measure relatively low Doppler
shifts on the order of +/- 10 km/s in the tornado-like structure. Between the
two legs we see loops in Mg II, with material flowing from one leg to the
other, as well as counterstreaming. It is difficult to interpret our data as
showing two rotating, vertical structures which are unrelated to the loops.
This kind of `tornado' scenario does not fit with our observations. The
magnetic field in the two legs of the prominence is found to be preferentially
horizontal.Comment: 13 pages, 14 figures, one tabl
Simulated road following using neuroevolution
This paper describes a methodology wherein genetic algorithms were used to evolve neural network controllers for application in automatic road driving. The simulated controllers were capable of dynamically varying the mixture of colour components in the input image to ensure the ability to perform well across the entire range of possible environments. During the evolution phase, they were evaluated in a set of environments carefully designed to encourage the development of flexible and general-purpose solutions. Successfully evolved controllers were capable of navigating simulated roads across challenging test environments, each with different geometric and colour distribution properties. These controllers proved to be more robust and adaptable compared to the previous work done using this evolutionary approach. This was due to their improved dynamic colour perception capabilities, as they were now able to demonstrate feature extraction in three (red, green and blue) colour channels
Magnetic field in atypical prominence structures: Bubble, tornado and eruption
Spectropolarimetric observations of prominences have been obtained with the
THEMIS telescope during four years of coordinated campaigns. Our aim is now to
understand the conditions of the cool plasma and magnetism in `atypical'
prominences, namely when the measured inclination of the magnetic field
departs, to some extent, from the predominantly horizontal field found in
`typical' prominences. What is the role of the magnetic field in these
prominence types? Are plasma dynamics more important in these cases than the
magnetic support? We focus our study on three types of `atypical' prominences
(tornadoes, bubbles and jet-like prominence eruptions) that have all been
observed by THEMIS in the He I D_3 line, from which the Stokes parameters can
be derived. The magnetic field strength, inclination and azimuth in each pixel
are obtained by using the Principal Component Analysis inversion method on a
model of single scattering in the presence of the Hanle effect. The magnetic
field in tornadoes is found to be more or less horizontal, whereas for the
eruptive prominence it is mostly vertical. We estimate a tendency towards
higher values of magnetic field strength inside the bubbles than outside in the
surrounding prominence. In all of the models in our database, only one magnetic
field orientation is considered for each pixel. While sufficient for most of
the main prominence body, this assumption appears to be oversimplified in
atypical prominence structures. We should consider these observations as the
result of superposition of multiple magnetic fields, possibly even with a
turbulent field component.Comment: 13 pages, 9 figure
Modeling of the hydrogen Lyman lines in solar flares
The hydrogen Lyman lines (91.2 nm < λ < 121.6 nm) are significant contributors to the radiative losses of the solar chromosphere, and they are enhanced during flares. We have shown previously that the Lyman lines observed by the Extreme Ultraviolet Variability instrument onboard the Solar Dynamics Observatory exhibit Doppler motions equivalent to speeds on the order of 30 km s−1. However, contrary to expectations, both redshifts and blueshifts were present and no dominant flow direction was observed. To understand the formation of the Lyman lines, particularly their Doppler motions, we have used the radiative hydrodynamic code, RADYN, along with the radiative transfer code, RH, to simulate the evolution of the flaring chromosphere and the response of the Lyman lines during solar flares. We find that upflows in the simulated atmospheres lead to blueshifts in the line cores, which exhibit central reversals. We then model the effects of the instrument on the profiles, using the Extreme Ultraviolet Variability Experiment (EVE) instrument's properties. What may be interpreted as downflows (redshifted emission) in the lines, after they have been convolved with the instrumental line profile, may not necessarily correspond to actual downflows. Dynamic features in the atmosphere can introduce complex features in the line profiles that will not be detected by instruments with the spectral resolution of EVE, but which leave more of a signature at the resolution of the Spectral Investigation of the Coronal Environment instrument onboard the Solar Orbiter
Three-dimensional view of ultrafast dynamics in photoexcited bacteriorhodopsin in the multiphoton regime and biological relevance
How does chemistry scale in complexity to unerringly direct biological functions? Nass Kovacs et al. have shown that bacteriorhodopsin undergoes structural changes tantalizingly similar to the expected pathway even under excessive excitation. Is the protein structure so highly evolved that it directs all deposited energy into the designed function
The development of lower-atmosphere turbulence early in a solar flare
We present the first observational study of the onset and evolution of solar
flare turbulence in the lower solar atmosphere on an unprecedented time scale
of 1.7 s using the Interface Region Imaging Spectrograph observing plasma at a
temperature of 80,000 K. At this time resolution, nonthermal spectral line
broadening, indicating turbulent velocity fluctuations, precedes the flare
onset at this temperature and is coincident with net blue-shifts. The
broadening decreases as the flare brightens and then oscillates with a period
of ~10 s. These observations are consistent with turbulence in the lower solar
atmosphere at the flare onset, heating that region as it dissipates. This
challenges the current view of energy release and transport in the standard
solar flare model, suggesting that turbulence partly heats the lower
atmosphere.Comment: Published in Science Advances (5th December 2018
Visibility of prominences using the He i D3 line filter on PROBA-3/ASPIICS coronagraph
We determine the optimal width and shape of the narrow-band filter centered on the He i D3 line for prominence and coronal mass ejection (CME) observations with the ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) coronagraph onboard the PROBA-3 (Project for On-board Autonomy) satellite, to be launched in 2020. We analyze He i D3 line intensities for three representative non-local thermal equilibrium prominence models at temperatures 8, 30, and 100 kK computed with a radiative transfer code and the prominence visible-light (VL) emission due to Thomson scattering on the prominence electrons. We compute various useful relations at prominence line-of-sight velocities of 0, 100, and 300 km s−1 for 20 Å wide flat filter and three Gaussian filters with a full-width at half-maximum (FWHM) equal to 5, 10, and 20 Å to show the relative brightness contribution of the He i D3 line and the prominence VL to the visibility in a given narrow-band filter. We also discuss possible signal contamination by Na i D1 and D2 lines, which otherwise may be useful to detect comets. Our results mainly show that i) an optimal narrow-band filter should be flat or somewhere between flat and Gaussian with an FWHM of 20 Å in order to detect fast-moving prominence structures, ii) the maximum emission in the He i D3 line is at 30 kK and the minimal at 100 kK, and iii) the ratio of emission in the He i D3 line to the VL emission can provide a useful diagnostic for the temperature of prominence structures. This ratio is up to 10 for hot prominence structures, up to 100 for cool structures, and up to 1000 for warm structures
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