Lucia Abbo

Morphology, dynamics and plasma parameters of plumes and inter-plume regions in solar coronal holes

Coronal plumes, which extend from solar coronal holes (CH) into the high corona and—possibly—into the solar wind (SW), can now continuously be studied with modern telescopes and spectrometers on spacecraft, in addition to investigations from the ground, in particular, during total eclipses. Despite the large amount of data available on these prominent features and related phenomena, many questions remained unanswered as to their generation and relative contributions to the high-speed streams emanating from CHs. An understanding of the processes of plume formation and evolution requires a better knowledge of the physical conditions at the base of CHs, in plumes and in the surrounding inter-plume regions. More specifically, information is needed on the magnetic field configuration, the electron densities and temperatures, effective ion temperatures, non-thermal motions, plume cross sections relative to the size of a CH, the plasma bulk speeds, as well as any plume signatures in the SW. In spring 2007, the authors proposed a study on ‘Structure and dynamics of coronal plumes and inter-plume regions in solar coronal holes’ to the International Space Science Institute (ISSI) in Bern to clarify some of these aspects by considering relevant observations and the extensive literature. This review summarizes the results and conclusions of the study. Stereoscopic observations allowed us to include three-dimensional reconstructions of plumes. Multi-instrument investigations carried out during several campaigns led to progress in some areas, such as plasma densities, temperatures, plume structure and the relation to other solar phenomena, but not all questions could be answered concerning the details of plume generation process(es) and interaction with the SW.

SPECTROSCOPIC SIGNATURE OF ALFVÉN WAVES DAMPING IN A POLAR CORONAL HOLE UP TO 0.4 SOLAR RADII

Between February 24-25, 2009, the EIS spectrometer onboard the Hinode spacecraft performed special “sit & stare” observations above the South po lar coronal hole continuously over more than 22 hours. Spectra were acquired with the 1” slit pla ced off-limb covering altitudes up to 0.48 R⊙ (3.34× 102 Mm) above the Sun surface, in order to study with EIS the non-t hermal spectral line broadenings. Spectral lines such as Fe xii λ186.88, Fexii λ193.51, Fexii λ195.12 and Fexiii λ202.04 are observed with good statistics up to high altitude s and they have been analyzed in this study. Results show that the FWHM of Fe xii λ195.12 line increases up to ≃ 0.14 R⊙, then decreases higher up. EIS stray light has been estimate d and removed. Derived electron density and non-thermal velocity profiles have bee n used to estimate the total energy flux transported by Alfvén waves o ff-limb in polar coronal hole up to≃ 0.4 R⊙. The computed Alfvén wave energy flux densityfw progressively decays with altitude fromfw ≃ 1.2 · 106 erg cm−2 s−1 at 0.03 R⊙ down to fw ≃ 8.5 · 103 erg cm−2 s−1 at 0.4 R⊙, with an average energy decay rate∆ fw/∆h ≃ −4.5·10−5 erg cm−3 s−1. Hence, this result suggests energy deposition by Alfvén waves in a polar coronal hole, thus providing a signi ficant source for coronal heating. Subject headings: Sun: corona; Sun: oscillations; Sun: UV radiation; line: pr ofiles; waves

Slow wind and magnetic topology in the solar minimum corona in 1996–1997

This study examines the physical conditions of the outer solar corona in order to identify the regions where the slow solar wind is accelerated and to investigate the latitudinal transition from slow to fast wind during the minimum of the solar cycle. The analysis is based on observations of six streamers obtained during the years of solar minimum, 1996 and 1997, with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO). The outflow velocity of the oxygen ions and the electron density of the coronal plasma are determined in altitude ranging from 1.5 to 3.5 solar radii (R ○. ). The adopted diagnostic method, based on spectroscopic analysis of the OVI 1032 and 1038 A lines, fully accounts for the large expansion factor of the magnetic field lines expected in the regions surrounding the streamers. The analysis leads to the conclusion that the slow corona] wind is observed (i) in the region external to and running along the streamer boundary; and (ii) in the region above the streamer core beyond 2.7 R ○. , where the transition between closed and open magnetic field lines takes place and the heliospheric current sheet forms. Regions in the immediate vicinity of the streamer boundary can be identified with the edges of the large polar coronal holes that characterize solar minimum. Results point to gradual variations of the properties of a coronal hole from the streamer boundary to its polar core, most likely related to the topology of the coronal magnetic field.

METIS: a novel coronagraph design for the Solar Orbiter mission

METIS (Multi Element Telescope for Imaging and Spectroscopy) METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The mission profile will bring the Solar Orbiter spacecraft as close to the Sun as 0.3 A.U., and up to 35° out-of-ecliptic providing a unique platform for helio-synchronous observations of the Sun and its polar regions. METIS coronagraph is designed for multi-wavelength imaging and spectroscopy of the solar corona. This presentation gives an overview of the innovative design elements of the METIS coronagraph. These elements include: i) multi-wavelength, reflecting Gregorian-telescope; ii) multilayer coating optimized for the extreme UV (30.4 nm, HeII Lyman-α) with a reflecting cap-layer for the UV (121.6 nm, HI Lyman-α) and visible-light (590-650); iii) inverse external-occulter scheme for reduced thermal load at spacecraft peri-helion; iv) EUV/UV spectrograph using the telescope primary mirror to feed a 1st and 4th-order spherical varied line-spaced (SVLS) grating placed on a section of the secondary mirror; v) liquid crystals electro-optic polarimeter for observations of the visible-light K-corona. The expected performances are also presented.

Characterization of the slow wind in the outer corona

The study concerns the streamer belt observed at high spectral resolution during the minimum of solar cycle 23 with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO. On the basis of a spectroscopic analysis of the O VI doublet, the solar wind plasma parameters are inferred in the extended corona. The analysis accounts for the coronal magnetic topology, extrapolated through a 3D magneto-hydrodynamic model, in order to define the streamer boundary and to analyse the edges of coronal holes. The results of the analysis allow an accurate identification of the source regions of the slow coronal wind that are confirmed to be along the streamer boundary in the open magnetic field region.

Solar Wind Outflow in Polar Plumes from 1.05 to 2.4 R

An earlier publication (Paper I), which measured the outflow velocity in solar plumes out to 1.35 R☉ using the Doppler dimming technique, has here been extended out to 2.4 R☉ by including observations from SOHO UVCS. It is shown that plume outflow velocities, greater than interplumes at lower heights, have lower acceleration and fall below interplume velocities at heights greater than 1.6 R☉. This analysis resolves what has been an apparent disagreement between previously published work. The mass flow rate in plumes is shown to decrease with height, presumably through mass transfer to the interplume regions.

Oxygen Abundance and Energy Deposition in the Slow Coronal Wind

Observations of the extended corona obtained with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO) during the solar minimum years 1996 and 1997 have been analyzed to derive the oxygen abundance in the outer corona. A comparison of the absolute coronal abundance, measured in the coronal regions surrounding the quiescent solar minimum streamers, to the heliospheric values confirms that these regions are the dominant sources of the slow solar wind. However, the inferred coronal abundances are consistent with the heliospheric values only in case the ion velocity distribution is anisotropic and enhanced across the coronal magnetic field. Thus this analysis also leads to the conclusion that energy is deposited in the slow coronal wind at least up to 2.7 R☉ and that the efficiency of energy deposition is likely to be related to the local coronal magnetic topology.

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.

MULTI-FLUID MODEL OF A STREAMER AT SOLAR MINIMUM AND COMPARISON WITH OBSERVATIONS

We present the results of a time-dependent 2.5-dimensional three-fluid magnetohydrodynamic model of the coronal streamer belt, which is compared with the slow solar wind plasma parameters obtained in the extended corona by the UV spectroscopic data from the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO during the past minimum of solar activity (Carrington Rotation 1913). Our previous three-fluid streamer model has been improved by considering the solar magnetic field configuration relevant for solar minimum conditions, and preferential heating for O5 + ions. The model was run until a fully self-consistent streamer solution was obtained in the quasi-steady state. The plasma parameters from the multi-fluid model were used to compute the expected UV observables from H I Lyα 1216 A and O VI 1032 A spectral lines, and the results were compared in detail with the UVCS measurements. A good agreement between the model and the data was found. The results of the study provide insight into the acceleration and heating of the multi-ion slow solar wind.

Novel space coronagraphs: METIS, a flexible optical design for multi-wavelength imaging and spectroscopy

This presentation outlines a general optical design for coronagraphs working in both the visible-light (VL) and UV/EUV wavelength ranges by combining the use of reflective, multilayer-coated or interference-coated optics with Lyot stops. This design has been successfully applied to a sub-orbital coronagraph. Another version of this novel design for visiblelight/ EUV coronagraphs uses an inverted-occultation design in order to minimize the solar flux entering the instrument. This design has been used for the coronagraph – METIS - on board the ESA Solar Orbital mission. The current optical configuration of METIS adopted for the Solar Orbiter mission includes Visible-light and UV imaging. However, the innovative inverted-occultation concept is flexible enough that it can also accommodate a EUV spectrograph maintaining the same basic optical layout. The paper also describes the potential capabilities of the inverted-occulter coronagraph as a VL/UV imager and EUV spectrograph for future solar missions.