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Influence of misalignments on the performance of externally occulted solar coronagraphs. Application to PROBA-3/ASPIICS  

Sergei Shestov   Submitted: 2018-01-23 00:54

Context: The ASPIICS instrument is a novel externally occulted coronagraph that will be launched on board the PROBA-3 mission of the European Space Agency. The external occulter will be placed on one satellite ∼ 150 m ahead of the second satellite that will carry an optical instrument. During 6 hours out of 19.38 hours of orbit, the satellites will fly in a precise (accuracy around a few millimeters) formation, constituting a giant externally occulted coronagraph. The large distance between the external occulter and the primary objective will allow observations of the white-light solar corona starting from extremely low heights ∼ 1.1R . Aims: We intend to analyze influence of shifts of the satellites and misalignments of optical elements on the ASPIICS performance in terms of diffracted light. Based on the quantitative influence of misalignments on diffracted light, we 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. Methods: We considered different types of misalignments and analyzed their influence from optical and computational points of view. We implemented a numerical model of the diffracted light and its propagation through the optical system and computed intensities of diffracted light throughout the instrument. Our numerical approach is based on the axisymmetrical Fourier-based model of the coronagraph, published recently. Here we extend the model to include nonsymmetrical cases and possible misalignments. Results: The numerical computations fully confirm the main properties of the diffracted light that we obtained from semi-analytical consideration. We obtain that relative influences of various misalignments are significantly different. We show that the internal occulter with RIO = 1.694 mm = 1.1R is large enough to compensate possible misalignments expected to occur in PROBA-3/ASPIICS. Besides that we show that apodizing the edge of the internal occulter leads to additional suppression of the diffracted light. Conclusions: We conclude that the most important misalignment is the tilt of the telescope with respect to the line connecting the center of the external occulter and the entrance aperture. Special care should be taken to co-align the external occulter and the coronagraph, which means co-aligning the diffraction fringe from the external occulter and the internal occulter. We suggest that the best orientation strategy is to point the coronagraph to the center of the external occulter.

Authors: S. V. Shestov, A. N. Zhukov
Projects: None

Publication Status: In print
Last Modified: 2018-01-23 11:51
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Nonlinear evolution of short-wavelength torsional Alfvén waves ?  

Sergei Shestov   Submitted: 2017-05-08 03:18

We analyze nonlinear evolution of torsional Alfvén waves in a straight magnetic flux tube filled in with a low-β 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 Alfvén 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 Alfvén 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 Alfvén waves.

Authors: S.V. Shestov and V. M. Nakariakov and A. S. Ulyanov and A. A. Reva and S. V. Kuzin
Projects: None

Publication Status: published
Last Modified: 2017-05-10 09:17
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Influence of misalignments on the performance of externally occulted solar coronagraphs. Application to PROBA-3/ASPIICS
Nonlinear evolution of short-wavelength torsional Alfven waves ?

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