L. Teriaca

Observations of a rotating macrospicule associated with an X-ray jet

Aims. We attempt to understand the driving mechanism of a macrospicule and its relationship with a coronal jet. Methods. We study the dynamics of a macrospicule and an associated coronal jet captured by multi-spacecraft observations. Doppler velocities in both the macrospicule and the coronal jet are determined by EIS and SUMER spectra. Their temporal evolution is studied using X-ray and He II λ304 images. Results. A blueshift of -120 ± 15 km s -1 is detected on one side of the macrospicule, while a redshift of 50 ± 6 km s -1 is found at the base of the other side. The inclination angle of the macrospicule inferred from a stereoscopic analysis with STEREO suggests that the measured Doppler velocities can be attributed to a rotating motion of the macrospicule rather than a radial flow or an expansion. Conclusions. The macrospicule is driven by the unfolding motion of a twisted magnetic flux rope, while the associated X-ray jet is a radial outflow.

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.

Propagating waves in polar coronal holes as seen by SUMER & EIS

Context. To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode. Aims. Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties. Methods. The observations analysed here consist of SUMER spectra of the Ne VIII 770 angstrom line (T = 0.6 MK) and EIS slot images in the Fe XII 195 angstrom line (T = 1.3 MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions. Results. We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km s(-1) (Ne VIII) to 125 km s(-1)(Fe XII). These velocities are subsonic and roughly in the same ratio as the respective sound speeds. Conclusions. We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind.

Propagating MHD Waves in Coronal Holes

Coronal holes are the coolest and darkest regions of the upper solar atmosphere, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. During the years of the solar minima, coronal holes are generally confined to the Sun’s polar regions, while at solar maxima they can also be found at lower latitudes. Waves, observed via remote sensing and detected in-situ in the wind streams, are most likely responsible for the wind and several theoretical models describe the role of MHD waves in the acceleration of the fast solar wind. This paper reviews the observational evidences of detection of propagating waves in these regions. The characteristics of the waves, like periodicities, amplitude, speed provide input parameters and also act as constraints on theoretical models of coronal heating and solar wind acceleration.

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.

Helium line formation and abundance in a solar active region

An observing campaign (SOHO JOP 139), coordinated between ground-based and Solar and Heliospheric Observatory (SOHO) instruments, has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines. The chromospheric lines Ca II K, Hα, and Na I D, as well as He I 10830, 5876, 584, and He II 304 A lines have been observed. The EUV radiation in the range λ 1 × 104 K. This region, between the chromosphere and transition region, has been indicated as a good candidate for processes that might be responsible for strong variations of [He]. The set of our observables can still be well reproduced in both cases, changing the atmospheric structure mainly in the low transition region. This implies that, to choose between different values of [He], it is necessary to constrain the transition region with different observables, independent of the He lines.

The Ly-α profile and center-to-limb variation of the quiet Sun

Aims. We study the emission of the hydrogen Lyman-α line in the quiet Sun, its center-to-limb variation, and its radiance distribution. We also compare quasi-simultaneous Ly-α and Ly-β line profiles. Methods. We used the high spectral and spatial resolution of the SUMER spectrometer and completed raster scans at various locations along the disk. For the first time, we used a method to reduce the incoming photon flux to a 20%-level by partly closing the aperture door. We also performed a quasi-simultaneous observation of both Ly-α and Ly-β at the Sun center in sit-and-stare mode. We infer the flow characteristic in the Ly-α map from variations in the calibrated λ 1206 Si iii line centroids. Results. We present the average profile of Ly-α, its radiance distribution, its center-to-limb behaviour, and the signature of flows on the line profiles. Little center-to-limb variation and no limb brightening are observed in the profiles of the Ly-α line. In contrast to all other lines of the Lyman series, which have a red-horn asymmetry, Ly-α has a robust and – except for dark locations – dominating blue-horn asymmetry. There appears to be a brightness-to-asymmetry relationship. A similar and even clearer trend is observed in the downflow-to-asymmetry relationship. This important result is consistent with predictions from models that include flows. However, the absence of a clear center-to-limb variation in the profiles may be more indicative of an isotropic field than a mainly radial flow. Conclusions. It appears that the ubiquitous hydrogen behaves in a similar way to a filter that dampens all signatures of the line formation by processes in both the chromosphere and transition region.

SPECTROSCOPIC OBSERVATIONS OF Fe XVIII IN SOLAR ACTIVE REGIONS

The large uncertainties associated with measuring the amount of high temperature emission in solar active regions represents a significant impediment to making progress on the coronal heating problem. Most current observations at temperatures of 3 MK and above are taken with broad band soft X-ray instruments. Such measurements have proven difficult to interpret unambiguously. Here we present the first spectroscopic observations of the Fe XVIII 974.86 AA emission line in an on-disk active region taken with then SUMER instrument on SOHO. Fe XVIII has a peak formation temperature of 7.1 MK and provides important constraints on the amount of impulsive heating in the corona. Detailed evaluation of the spectra and comparison of the SUMER data with soft X-ray images from the XRT on Hinode confirm that this line is unblended. We also compare the spectroscopic data with observations from the AIA 94 AA channel on SDO. The AIA 94 AA channel also contains Fe XVIII, but is blended with emission formed at lower temperatures. We find that is possible to remove the contaminating blends and form relatively pure Fe XVIII images that are consistent with the spectroscopic observations from SUMER. The observed spectra also contain the Ca XIV 943.63 AA line that, although a factor 2 to 6 weaker than the Fe XVIII 974.86 AA line, allows us to probe the plasma around 3.5 MK. The observed ratio between the two lines indicates (isothermal approximation) that most of the plasma in the brighter Fe XVIII active region loops is at temperatures between 3.5 and 4 MK.

The widths of vacuum-ultraviolet spectral lines in the equatorial solar corona observed with CDS and SUMER

Observations of the solar equatorial corona between heights of 36 Mm and 184 Mm above the limb obtained by the SOHO spectrometers CDS and SUMER in December 2003 are presented and discussed with special emphasis on the widths of the spectral lines Mg  at 62.50 nm, Al  at 55.00 nm and 56.82 nm, Ca  at 55.78 nm, and Si  at 58.09 nm. SUMER observed, in addition, the lines Mg  60.98 nm, Ca  57.40 nm, Fe  124.20 nm, Fe  115.31 nm, and Ca  113.37 nm. The Si  52.11 nm line was only observed by CDS. A different behaviour of the line width of Mg  62.50 nm as a function of height above the limb had been found in studies carried out independently with both instruments at different times. It is the aim of this joint investigation to (a) study instrumental effects on line-width results; and (b) provide a thorough analysis of line profiles with altitude for the new campaign.

Solar magnetism eXplorer (SolmeX)

The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona—that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200 m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. SolmeX integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations.