89 research outputs found
Influence of misalignments on performance of externally occulted solar coronagraphs. Application to PROBA-3/ASPIICS
ASPIICS is a novel externally occulted coronagraph that will be launched
onboard the PROBA-3 mission of ESA. The external occulter (EO) will be placed
on one satellite ~150 m ahead of the second satellite with an optical
instrument. During part of each orbit, the satellites will fly in a precise
formation, constituting a giant externally occulted coronagraph. Large distance
between the EO and the primary objective will allow observations of the
white-light solar corona starting already from ~1.1RSun. We analyze influence
of shifts of the satellites and misalignments of optical elements on diffracted
light. Based on the quantitative influence of misalignments on diffracted
light, we will provide a "recipe" for choosing the size of the internal
occulter (IO) to achieve a trade-off between the minimal height of observations
and sustainability to possible misalignments. We implement a numerical model of
the diffracted light and its propagation through the optical system, and
compute intensities of diffracted light throughout the instrument. Our
numerical model extends axi-symmetrical model of Rougeot et al. 2017 to
non-symmetrical cases. The computations fully confirm main properties of the
diffracted light obtained from semi-analytical consideration. Results: relative
influences of various misalignments are significantly different. We show that:
the IO with R=1.1RSun is large enough to compensate possible misalignments in
ASPIICS, apodizing the edge of the IO leads to additional suppression of the
diffracted light. Conclusions: the most important misalignment is the tilt of
the telescope WRT the line connecting the center of the EO and the entrance
aperture. Special care should be taken to co-align the EO and the coronagraph,
i.e. co-aligning the diffraction fringe from the EO and the IO. We suggest that
the best orientation strategy is to point the coronagraph to the center of the
EO.Comment: 13 pages, 15 figure
Modeling and removal of optical ghosts in the PROBA-3/ASPIICS externally occulted solar coronagraph
Context: ASPIICS is a novel externally occulted solar coronagraph, which will
be launched onboard the PROBA-3 mission of the European Space Agency. The
external occulter will be placed on the first satellite approximately 150 m
ahead of the second satellite that will carry an optical instrument. During 6
hours per orbit, the satellites will fly in a precise formation, constituting a
giant externally occulted coronagraph. Large distance between the external
occulter and the primary objective will allow observations of the white-light
solar corona starting from extremely low heights 1.1RSun. Aims: To analyze
influence of optical ghost images formed inside the telescope and develop an
algorithm for their removal. Methods: We implement the optical layout of
ASPIICS in Zemax and study the ghost behaviour in sequential and non-sequential
regimes. We identify sources of the ghost contributions and analyze their
geometrical behaviour. Finally we develop a mathematical model and software to
calculate ghost images for any given input image. Results: We show that ghost
light can be important in the outer part of the field of view, where the
coronal signal is weak, since the energy of bright inner corona is
redistributed to the outer corona. However the model allows to remove the ghost
contribution. Due to a large distance between the external occulter and the
primary objective, the primary objective does not produce a significant ghost.
The use of the Lyot spot in ASPIICS is not necessary.Comment: 14 pages, 13 figure
Nonlinear evolution of short-wavelength torsional Alfvén waves
We analyze nonlinear evolution of torsional AlfvĂ©n waves in a straight magnetic flux tube filled in with a low-ÎČ plasma, and surrounded with a plasma of lower density. Such magnetic tubes model, in particular, a segment of a coronal loop or a polar plume. The wavelength is taken comparable to the tube radius. We perform a numerical simulation of the wave propagation using ideal magnetohydrodynamics. We find that a torsional wave nonlinearly induces three kinds of compressive flows: the parallel flow at the AlfvĂ©n speed, which constitutes a bulk plasma motion along the magnetic field, the tube wave, and also transverse flows in the radial direction, associated with sausage fast magnetoacoustic modes. In addition, the nonlinear torsional wave steepens and its propagation speed increases. The latter effect leads to the progressive distortion of the torsional wave front, i.e., nonlinear phase mixing. Because of the intrinsic non-uniformity of the torsional wave amplitude across the tube radius, the nonlinear effects are more pronounced in regions with higher wave amplitudes. They are always absent at the axes of the flux tube. In the case of a linear radial profile of the wave amplitude, the nonlinear effects are localized in an annulus region near the tube boundary. Thus, the parallel compressive flows driven by torsional AlfvĂ©n waves in the solar and stellar coronae, are essentially non-uniform in the perpendicular direction. The presence of additional sinks for the wave energy reduces the efficiency of the nonlinear parallel cascade in torsional AlfvĂ©n waves
Short-Period Internal Waves under an Ice Cover in Van Mijen Fjord, Svalbard
Temperature and velocity fluctuations measured in Van Mijen Fjord in Svalbard and interpreted as the fluctuations induced by internal waves revealed the existence of short-period internal waves with an amplitude of approximately 1âm and a period of approximately 5â10âmin that correlate with the ice cover fluctuations of the same period with an amplitude of a few millimeters
LUCI onboard Lagrange, the Next Generation of EUV Space Weather Monitoring
LUCI (Lagrange eUv Coronal Imager) is a solar imager in the Extreme
UltraViolet (EUV) that is being developed as part of the Lagrange mission, a
mission designed to be positioned at the L5 Lagrangian point to monitor space
weather from its source on the Sun, through the heliosphere, to the Earth. LUCI
will use an off-axis two mirror design equipped with an EUV enhanced active
pixel sensor. This type of detector has advantages that promise to be very
beneficial for monitoring the source of space weather in the EUV. LUCI will
also have a novel off-axis wide field-of-view, designed to observe the solar
disk, the lower corona, and the extended solar atmosphere close to the
Sun-Earth line. LUCI will provide solar coronal images at a 2-3 minute cadence
in a pass-band centred on 19.5 nm. Observations made through this pass-band
allow for the detection and monitoring of semi-static coronal structures such
as coronal holes, prominences, and active regions; as well as transient
phenomena such as solar flares, limb Coronal Mass Ejections (CMEs), EUV waves,
and coronal dimmings. The LUCI data will complement EUV solar observations
provided by instruments located along the Sun-Earth line such as PROBA2-SWAP,
SUVI-GOES and SDO-AIA, as well as provide unique observations to improve space
weather forecasts. Together with a suite of other remote-sensing and in-situ
instruments onboard Lagrange, LUCI will provide science quality operational
observations for space weather monitoring
Prominence eruption observed in He II 304 Ă up to >6 Râ by EUI/FSI aboard Solar Orbiterâ
Aims. We report observations of a unique, large prominence eruption that was observed in the He II 304 Ă
passband of the Extreme Ultraviolet Imager/Full Sun Imager telescope aboard Solar Orbiter on 15â16 February 2022.
Methods. Observations from several vantage points â Solar Orbiter, the Solar-Terrestrial Relations Observatory, the Solar and Heliospheric Observatory, and Earth-orbiting satellites â were used to measure the kinematics of the erupting prominence and the associated coronal mass ejection. Three-dimensional reconstruction was used to calculate the deprojected positions and speeds of different parts of the prominence. Observations in several passbands allowed us to analyse the radiative properties of the erupting prominence.
Results. The leading parts of the erupting prominence and the leading edge of the corresponding coronal mass ejection propagate at speeds of around 1700 km sâ1 and 2200 km sâ1, respectively, while the trailing parts of the prominence are significantly slower (around 500 km sâ1). Parts of the prominence are tracked up to heights of over 6âRâ. The He II emission is probably produced via collisional excitation rather than scattering. Surprisingly, the brightness of a trailing feature increases with height.
Conclusions. The reported prominence is the first observed in He II 304 Ă
emission at such a great height (above 6 Râ)
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