Isabelle F. Scholl

Total Solar Eclipse Observations of Hot Prominence Shrouds

Using observations of the corona taken during the total solar eclipses of 2006 March 29 and 2008 August 1 in broadband white light and in narrow bandpass filters centered at Fe X 637.4 nm, Fe XI 789.2 nm, Fe XIII 1074.7 nm, and Fe XIV 530.3 nm, we show that prominences observed off the solar limb are enshrouded in hot plasmas within twisted magnetic structures. These shrouds, which are commonly referred to as cavities in the literature, are clearly distinct from the overlying arch-like structures that form the base of streamers. The existence of these hot shrouds had been predicted by model studies dating back to the early 1970s, with more recent studies implying their association with twisted magnetic flux ropes. The eclipse observations presented here, which cover a temperature range of 0.9 to 2 ×106 K, are the first to resolve the long-standing ambiguity associated with the temperature and magnetic structure of prominence cavities.

Mapping the Distribution of Electron Temperature and Fe Charge States in the Corona with Total Solar Eclipse Observations

The inference of electron temperature from the ratio of the intensities of emission lines in the solar corona is valid only when the plasma is collisional. Once collisionless, thermodynamic ionization equilibrium no longer holds, and the inference of an electron temperature and its gradient from such measurements is no longer valid. At the heliocentric distance where the transition from a collision-dominated to a collisionless plasma occurs, the charge states of different elements are established, or frozen-in. These are the charge states which are subsequently measured in interplanetary space. We show in this study how the 2006 March 29 and 2008 August 1 eclipse observations of a number of Fe emission lines yield an empirical value for a distance, which we call Rt , where the emission changes from being collisionally to radiatively dominated. Rt ranges from 1.1 to 2.0 R ☉, depending on the charge state and the underlying coronal density structures. Beyond that distance, the intensity of the emission reflects the distribution of the corresponding Fe ion charge states. These observations thus yield the two-dimensional distribution of electron temperature and charge state measurements in the corona for the first time. The presence of the Fe X 637.4 nm and Fe XI 789.2 nm emission in open magnetic field regions below Rt , such as in coronal holes and the boundaries of streamers, and the absence of Fe XIII 1074.7 nm and Fe XIV 530.3 nm emission there indicate that the sources of the solar wind lie in regions where the electron temperature is less than 1.2 × 106 K. Beyond Rt , the extent of the Fe X [Fe9+] and Fe XI emission [Fe10+], in comparison with Fe XIII [Fe12+] and Fe XIV [Fe13+], matches the dominance of the Fe10+ charge states measured by the Solar Wind Ion Composition Spectrometer, SWICS, on Ulysses, at –43° latitude at 4 AU, in March-April 2006, and Fe9+ and Fe10+ charge states measured by SWICS on the Advanced Composition Explorer, ACE, in the ecliptic plane at 1 AU, at the time of both eclipses. The remarkable correspondence between these two measurements establishes the first direct link between the distribution of charge states in the corona and in interplanetary space.

MEASURING THE SOLAR RADIUS FROM SPACE DURING THE 2003 AND 2006 MERCURY TRANSITS

The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8. Contact times between Mercury and the solar limb have been used since the seventeenth century to derive the Suns size but this is the first time that high-quality imagery from space, above the Earths atmosphere, has been available. Unlike other measurements, this technique is largely independent of optical distortion. The true solar radius is still a matter of debate in the literature as measured differences of several tenths of an arcsecond (i.e., about 500 km) are apparent. This is due mainly to systematic errors from different instruments and observers since the claimed uncertainties for a single instrument are typically an order of magnitude smaller. From the MDI transit data we find the solar radius to be 96012 ± 009 (696, 342 ± 65 km). This value is consistent between the transits and consistent between different MDI focus settings after accounting for systematic effects.

The Precise Solar Shape and Its Variability

Our Constant Sun The exact shape of the Sun provides information on its internal structure. Based on data obtained by the Helioseismic and Magnetic Imager aboard NASAs Solar Dynamics Observatory, Kuhn et al. (p. 1638, published online 16 August; see the Perspective by Gough) measured the solar shape during a 2-year period in which the Sun evolved from a minimum to a maximum of sunspot activity. Against expectations, the Suns oblate shape was found to be constant and not to vary with the 11-year solar cycle. Observations with NASA’s Solar Dynamics Observatory show that the shape of the Sun does not vary with the 11-year solar cycle. The precise shape of the Sun has not been convincingly determined, despite half a century of modern photoelectric observations. The expected deviation of the solar-limb shape from a perfect circle is very small, but such asphericity is sensitive to the Sun’s otherwise invisible interior conditions, as well as the solar atmosphere. We use evidence from a long-running experiment based in space to show that, when analyzed with sufficiently high spatial resolution, the Sun’s oblate shape is distinctly constant and almost completely unaffected by the solar-cycle variability seen on its surface. The solar oblateness is significantly lower than theoretical expectations by an amount that could be explained by a slower differential rotation in the outer few percent of the Sun.

The European grid of solar observations

The European Grid of Solar Observations (EGSO) is a Grid test-bed that will change the way users analyze solar data. One of the major hurdles in the analysis of solar data is finding what data are available and retrieving those required. EGSO is integrating the access to solar data by building a Grid including solar archives around the world. The Grid relies on metadata and tools for selecting, processing and retrieving distributed and heterogeneous solar data. EGSO is also creating a solar feature catalogue giving for the first time the ability to select solar data based on phenomena and events. In essence, EGSO is providing the fabric of a virtual observatory. Since the first release of EGSO in September 2003, members of the solar community have been involved in product testing. The constant testing and feedback allows us to assure the usability of the system. The capabilities of the latest release will be described, and the scientific problems that it addresses discussed. EGSO is funded under the IST (Information Society Technologies) thematic priority of the European Commissions Fifth Framework Programme (FP5) – it started in March 2002 and will last for three years. The EGSO Consortium comprises 11 institutes from Europe and the US and is led by the Mullard Space Science Laboratory of University College London. EGSO is collaborating with other groups in the US who are working on similar virtual observatory projects for solar and heliospheric data with the objective of providing integrated access to these data.

Impact of Active Regions on Coronal Hole Outflows

Establishing the sources of the fast and slow solar wind is important for understanding their drivers and their subsequent interaction in interplanetary space. Although coronal holes continue to be viewed as the main source of the fast solar wind, there is recent evidence that the quiet Sun provides other spatially concentrated sources. To identify the underlying physical characteristics of the outflow from coronal holes, solar disk observations from the Solar and Heliospheric Observatory (SOHO) are considered. These observations encompass photospheric line-of-sight magnetic field measurements from the Michelson Doppler Imager (MDI), Fe X 171 A passband imaging from the Extreme-ultraviolet Imaging Telescope (EIT), and Ne VIII 770 A spectral observations with outflows inferred from their corresponding Doppler blueshifts, at solar minimum and maximum and at different latitudes, from the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument. The sharp variations of outflows within the SUMER field of view, referred to as velocity gradients, are introduced as a new diagnostic. It is shown that, in general, coronal holes are indistinguishable from the quiet Sun, whether in their outflows or their gradients. Surprisingly, however, when enhanced unbalanced magnetic flux from active regions extends into neighboring coronal holes, both outflows and their gradients become significantly enhanced within the coronal holes and along their boundaries. The same effect is observed in the quiet Sun, albeit to a lesser extent. These findings point to the possibility that active regions can lead to enhanced plasma outflows in neighboring coronal holes.

EGSO in need for a global schema

The European Grid of Solar Observations (EGSO) is a project to develop a virtual observatory for the solar physics community. Like in all such projects, a vital component is a schema that adequately describes the data in the distributed data sets. Here, we discuss the schema in general terms, and present a draft example of a portion of a possible XML schema.

Cryogenic near infrared spectropolarimeter for the Daniel K. Inouye Solar Telescope

The Cryogenic Near Infrared Spectropolarimeter for the Daniel K Inouye Solar Telescope is designed to measure polarized light from 0.5 to 5 μm. It uses an almost all reflective design for high throughput and an R2 echelle grating to achieve the required resolution of up to R = 100,000. The optics cooled to cryogenic temperatures reduce the thermal background allowing for IR observations of the faint solar corona. Both the spectrograph and its context imager use H2RG detector arrays with a newly designed controller to allow synchronized exposures at frame rates up to 10 Hz. All hardware has been built and tested and the key components met their design goals. 1) The cryogenic system uses mechanical closed cycle coolers which introduce vibrations. Our design uses a two stage approach with a floating mounting disk and flexible cold links to reduce these. The vibration amplitudes on all critical stages were measured and are smaller than 1μm. 2) The grating stage of the spectrograph uses a double stack of harmonic drives and an optical encoder to provide sub-arcsecond resolution and a measured repeatability of better than 0.5 arcsec.

Optical and IR observations of planetary and exoplanetary atmospheres

The spatial and temporal variations of planetary atmospheric phenomena (e.g., auroras on Jupiter and Io, as well as the polarization of expolanetary atmospheres) are extremely complicated. Indeed, the wide range of these phenomena, and the cross-scale coupling processes that exist between them, means that is difficult to understand their underlying mechanisms. To study and understand such planetary atmospheric phenomena it is therefore essential to perform continuous and flexible monitoring with a suitable telescope. From previous studies it is known that Jupiter’s auroras are generated by two separate mechanisms: a rapid 10-hour rotation of the magnetic field (known as the ‘internal source’) and the variation of the solar wind (known as the ‘external source’).1 Although much effort has been made to determine which of these sources plays the major role in causing the spatial and temporal variations of Jupiter’s auroras, the mechanism has still not been resolved.2 In addition, it has been shown that active volcanoes on Io (a moon of Jupiter) intermittently inject large amounts of gases into the magnetosphere and can modulate the auroral activity.3 Past studies have also revealed that the light from exoplanet host stars is polarized with the variation period of the planet’s orbital motion. This result suggests that the polarization is caused by Rayleigh scattering in the exoplanetary atmosphere.4 The examples of exoplanetary atmospheres that have so far been studied, however, have been very limited. Further observations are thus required to fully characterize the atmospheres of different types of exoplanets. Figure 1. The 60cm Cassegrain and Coude telescope (T60) installed at the summit of Mount Haleakala, Hawaii.

A new way to look at observations with EGSO

The European Grid of Solar Observations (EGSO) is a Solar virtual observatory (see Hill et al., 2002). It has been funded through the 5th Framework Program of the European Community. A dozen of laboratories, mixing Solar Physics and Information Technology, in Great Britain, France, Italy and Swiss have been involved in this project during 3 years. A grid accessing several dozens of databases and archives scattered all around the world has been developped as well as a Solar Event Catalogue and a Solar Feature Catalogue. The original aspect of this work consists in the possibility not only to search through the characteristics of observations, but also search for available data corresponding to specific kinds of events. So it is now very important to be able to follow the Sun 24 hours a day in order to enrich the events database for future queries. More informations on EGSO, catalogues and user interface can be accessd through the web site: http://www.egso.org/