EUI & Its Scientific Impact

EUS and HRI together with VIM will be able to observe the birth of the solar wind inside funnels, faning out in the corona, at very small scales and study other possible source regions of solar wind (e.g. in the quiet Sun) in much finer detail than was previously possible. Images obtained by the three HRIs will be used (in combination with the configuration and evolution of the photospheric magnetic field measured by VIM) to study the morphology and dynamics of coronal magnetic field. FSI images will be combined with the COR white light images of the inner corona (using the overlap of the two fields of view) to map and follow the evolution of coronal holes and their fine structures.
A major challenge is to understand the solar origin of turbulence, ranging from kinetic to fluid scales, and of convected structures and shocks in the solar wind, thus identifying the multiple links between activity on the Sun's surface and the resulting imprints in the inner heliosphere. The presence of Alfvén fluctuations prevailing at all radial distances is a main characteristic of the fast solar wind. A new result from HINODE has demonstrated the existence of Alfvén waves in the chromosphere but they also could be produced in situ in the corona or in the solar wind as well. To help test wave and turbulence generation scenarios and evolution hypotheses, the Lyman-α HRI will provide high-resolution images of the chromospheric network (where Alfvén waves may be generated) complementary to the photospheric motion observations of VIM.

FSI will also detect small-scale coronal jets that may be a source of impulsive SEP events, and HRI will observe them in detail when they occur in its FOV. FSI will also image possible signatures of large-scale shock waves in the corona (EIT waves). Additionally, during the observations of the far side of the Sun (as seen from the Earth), the EUI will image the sources of far-side particle events, acting as a far-side sentinel as well.
The HRIs will provide crucial information on the evolution of active regions's magnetic configurations during flares, similarly to TRACE images that are successfully used to provide the coronal context to RHESSI observations of hard X-ray emission. Foot points of post-eruption loops have been shown to be sources of hard X-ray emission produced by thick-target bremsstrahlung of accelerated electrons in the chromosphere. The hard X-ray sources will be detected by STIX, post-eruption arcades will be well visible in both coronal HRI bandpasses, and the ribbons at their foot points will be observed by the HRI_Ly-α. High cadence (sub-second) is essential to observe the evolution of flare ribbons in detail.

The issue of the common plasma between CMEs and ICMEs is of prime importance for understanding the ICME measurements and their solar origins. While there is a close correspondence between prominence eruptions and CMEs, prominence material is very rarely observed in ICMEs. This is most likely due to the fact that the prominence is heated substantially during the eruption, and, additionally, constitutes only a minor part of the CME and ICME volume. HRI images of prominences in the Lyman-α, 174 Å and 335 Å bandpasses will allow us to assess how quickly the filament is heated. FSI, COR and GHI will provide a continuous observation of the prominence propagation through the corona and inner heliosphere. During the SO mission Solar Probe Plus is expected to fly in the ecliptic plane, allowing the in situ detection of the prominence material identified by compositional signatures. Establishing the correspondence between CME and ICME parameters remains a challenge with observations available in 2007, the Solar Orbiter mission is hence expected to give a major contribution to this unresolved issue.

These fundamental processes, in turn, influence the fine structure of the corona. The dominant spatial and temporal scales of energy storage and dissipation still remain elusive. The HRI, with its 160 km spatial resolution (i.e. 80 km px size at perihelion) will allow significant progress to be made in the study of the fine structure and dynamical processes in the solar transition region (TR) and corona.
Yohkoh/SXT, EIT and TRACE images indicate that coronal loops do not expand systematically with height. They have instead a uniform cross section, contrary to the geometry of magnetic flux tubes that can be derived from potential or force-free extrapolations of photospheric magnetograms. It has been suggested that loops with uniform width might be compatible with a complex internal loop structure of tangled elementary flux tubes with non-uniform geometry. We expect that the HRI₁₇₄ bandpass will be able to resolve more elementary loops (strands) and other fine scale structures and thus to clarify this issue. The 174 Å and 335 Å channels of the HRI cover the emission of plasma in adjacent temperature ranges (around 0.8-1.2 and 1.2-1.6 MK respectively). They are thus fully complementary for studies of the thermal structure of active region loops at very small scales. HRI observations will also be used to perform statistical studies of nano- and possibly pico-flare energy distributions.
The interaction between the corona and the lower atmosphere continues to be a mystery. SOHO/SUMER spectro-heliograms and Lyman-α observations made by VAULT reveal that, in the quiet Sun, the lower to mid TR is formed by many loop-like structures located along and across network boundaries (and also in the network cell centers), with widths at or below the prensent instrument spatial resolution. These observations support the unresolved fine structure concept of the TR suggesting that the quiescent corona is magnetically disconnected from most of the transition region. Simultaneous images at high spatio-temporal resolution in three HRI channels will form the long-awaited dataset to confirm or falsify the current scenarios, and perhaps motivate a new paradigm.