V. Domingo

The Sunrise Mission

The first science flight of the balloon-borne Sunrise telescope took place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset Island in northern Canada. We describe the scientific aims and mission concept of the project and give an overview and a description of the various hardware components: the 1-m main telescope with its postfocus science instruments (the UV filter imager SuFI and the imaging vector magnetograph IMaX) and support instruments (image stabilizing and light distribution system ISLiD and correlating wavefront sensor CWS), the optomechanical support structure and the instrument mounting concept, the gondola structure and the power, pointing, and telemetry systems, and the general electronics architecture. We also explain the optimization of the structural and thermal design of the complete payload. The preparations for the science flight are described, including AIV and ground calibration of the instruments. The course of events during the science flight is outlined, up to the recovery activities. Finally, the in-flight performance of the instrumentation is discussed.

Evidence of small-scale magnetic concentrations dragged by vortex motion of solar photospheric plasma

Vortex-type motions have been measured by tracking bright points in high-resolution observations of the solar photosp here. These small-scale motions are thought to be determinant in the evolution of magnetic footpoints and their interaction with pl asma and therefore likely to play a role in heating the upper solar atm osphere by twisting magnetic flux tubes. We report the observ ation of magnetic concentrations being dragged towards the center of a convective vortex motion in the solar photosphere from high-resolution ground-based and space-borne data. We describe this event by analyzing a series of images at different solar atmospheric layers. By computing horizontal proper motions, we detect a vortex whose center appears to be the draining point for the magnetic concentrations detected in magnetograms and well-correlated with the locations of bright points seen in G-band and CN images.

Spatial distribution and statistical properties of small-scale convective vortex-like motions in a quiet-Sun region

High-resolution observations of a quiet Sun internetwork r egion taken with the Solar 1-m Swedish Telescope in La Palma are analyzed. We determine the location of small-scale vortex motions in the solar photospheric region by computing the horizontal proper motions of smallscale structures on time series of images. These plasma convectively-driven swirl motions are associated to: (1) downdrafts (that have been commonly explained as corresponding to sites where the plasma is cooled down and hence returned to the interior below the visible photospheric level), and (2) horizontal velocity vectors c onverging into a central point. The sink cores are proved to be the final destination of passive flo ats tracing plasma flows towards the center of each vortex. We establish the occurrence of the se events to be 1.4× 10 −3 and 1.6 × 10 −3 vortices Mm −2 min −1 respectively for two time series analyzed here.

The intensity contrast of solar photospheric faculae and network elements II. Evolution over the rising phase of solar cycle 23

We studied the radiative properties of small magnetic elements (active region faculae and the network) during the rising phase of solar cycle 23 from 1996 to 2001, determining their contrasts as a function of heliocentric angle, magnetogram signal, and the solar cycle phase. We combined near-simultaneous full disk images of the line-of-sight magnetic field and photospheric continuum intensity provided by the MDI instrument on board the SOHO spacecraft. Sorting the magnetogram signal into different ranges allowed us to distinguish between the contrast of different magnetic structures. We find that the contrast center-to-limb variation (CLV) of these small magnetic elements is independent of time with a 10% precision, when measured during the rising phase of solar cycle 23. A 2-dimensional empirical expression for the contrast of photospheric features as a function of both the position on the disk and the averaged magnetic field strength was determined, showing its validity through the studied time period. A study of the relationship between magnetogram signal and the peak contrasts shows that the intrinsic contrast (maximum contrast per unit of magnetic flux) of network flux tubes is higher than that of active region faculae during the solar cycle.

Magnetic Fiel Emergence in Mesogranular-Sized Exploring Granules Observed with SUNRISE/IMaX Data

We report on magnetic field emergences covering significant areas of exploding granules. The balloon-borne mission Sunrise provided high spatial and temporal resolution images of the solar photosphere. Continuum images, longitudinal and transverse magnetic field maps and Dopplergrams obtained by IMaX onboard Sunrise are analyzed by local correlation traking (LCT), divergence calculation and time slices, Stokes inversions and numerical simulations are also employed. We characterize two mesogranular-scale exploding granules where ∼10 18 Mx of magnetic flux emerges. The emergence of weak unipolar longitudinal fields (∼100 G) start with a single visible magnetic polarity, occupying their respective granules’ top and following the granular splitting. After a while, mixed polarities start appearing, concentrated in downflow lanes. The events last around 20 min. LCT analyses confirm mesogranular scale expansion, displaying a similar pattern for all the physical properties, and divergence centers match between all of them. We found a similar behaviour with the emergence events in a numerical MHD simulation. Granule expansion velocities are around 1 km s −1 while magnetic patches expand at 0.65 km s −1 . One of the analyzed events evidences the emergence of a loop-like structure. Advection of the emerging magnetic flux features is dominated by convective motion resulting from the exploding granule due to the magnetic field frozen in the granular plasma. Intensification of the magnetic field occurs in the intergranular lanes, probably because of being directed by the downflowing plasma.

Evolution of Small-Scale Magnetic Elements in the Vicinity of Granular-Sized Swirl Convective Motions

Advances in solar instrumentation have led to widespread use of time series to study the dynamics of solar features, especially at small spatial scales and at very fast cadences. Physical processes at such scales are important as building blocks for many other processes occurring from the lower to the upper layers of the solar atmosphere and beyond, ultimately for understanding the larger picture of solar activity. Ground-based (Swedish Solar Telescope) and space-borne (Hinode) high-resolution solar data are analyzed in a quiet-Sun region that displays negative-polarity small-scale magnetic concentrations and a cluster of bright points observed in G-band. The region is characterized by two granular-sized convective vortex-type plasma motions, one of which appears to be affecting the dynamics of magnetic features and bright points in its vicinity and is therefore the main target of our investigations. We followed the evolution of the bright points, intensity variations at different atmospheric height, and the magnetic evolution for a set of interesting selected regions. We describe the evolution of the photospheric plasma motions in the region near the convective vortex and some plausible cases for convective collapse detected in Stokes profiles.

Detailed design of the imaging magnetograph experiment (IMaX): a visible imager magnetograph for the Sunrise mission

In this work, it is described the Imaging Magnetograph eXperiment, IMaX, one of the three postfocal instruments of the Sunrise mission. The Sunrise project consists on a stratospheric balloon with a 1 m aperture telescope, which will fly from the Antarctica within the NASA Long Duration Balloon Program. IMaX will provide vector magnetograms of the solar surface with a spatial resolution of 70 m. This data is relevant for understanding how the magnetic fields emerge in the solar surface, how they couple the photospheric base with the million degrees of temperature of the solar corona and which are the processes that are responsible of the generation of such an immense temperatures. To meet this goal IMaX should work as a high sensitivity polarimeter, high resolution spectrometer and a near diffraction limited imager. Liquid Crystal Variable Retarders will be used as polarization modulators taking advantage of the optical retardation induced by application of low electric fields and avoiding mechanical mechanisms. Therefore, the interest of these devices for aerospace applications is envisaged. The spectral resolution required will be achieved by using a LiNbO3 Fabry-Perot etalon in double pass configuration as spectral filter before the two CCDs detectors. As well phase-diversity techniques will be implemented in order to improve the image quality. Nowadays, IMaX project is in the detailed design phase before fabrication, integration, assembly and verification. This paper briefly describes the current status of the instrument and the technical solutions developed to fulfil the scientific requirements.