The scientific goals of the SO mission are summarized in four fundamendal top-level objectives:

These science goals emphasise and fully exploit the unique mission characteristics of and Solar Orbiter, namely: Possibilities for combined investigations of the EUI data together with multi-point data taken by Solar Probe Plus are unprecedented. Here below a few science cases are outlined which gives a flavour of the science return expected form the mission.

What are the origins of the solar wind streams and the heliospharic magnetic fileds?

Funnels in the polar coronal hole


It is now accepted that coronal holes (CH) generally are the sources of the fast solar wind. The most prominent features within polar coronal holes are plumes: ray-like structures that extend over several solar radii. But whether the fast solar wind streams originate from plumes or mostly from inter-plume regions is still a matter of debate. It can be expected that near the Solar Orbiter perihelion (around 48 solar radii) the fast solar wind will not be as uniform as observed by Ulysses at greater distances. The unique out-of-ecliptic and the close-up viewpoints of Solar Orbiter will allow us to study plumes in unprecedented detail removing the ambiguity due to the line of sight integration of the present day observations (from the ecliptic plane). This will provide crucial insight into the elusive acceleration mechanism of the solar wind. more ...

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.

What are the sources, acceleration mechanisms, and transport processes of solar energetic particles?

X-Flare as seen by Trace 195 Å superimposed with hard X-Ray contours form RHESSI.


Solar energetic particles (SEPs) events display great variability in their physical characteristics, including particle flux, peak intensity, duration and composition, as well as in the arrival time with respect to their source location. The mechanisms responsible for their acceleration are poorly understood but it is commonly accepted that more than one process (acting on different temporal and spatial scales) may be involved in the formation of large SEP events. Two types of solar sources of SEPs are usually distinguished: flares, and shocks driven by coronal mass ejections (CMEs), although the details of the link to these solar activity manifestations are still uncertain.
The EUI will provide a complete perspective on the position of flares and CME source regions in the low corona along with the coronal configuration in the flaring active regions and around them. more ...

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 HRILy-α. High cadence (sub-second) is essential to observe the evolution of flare ribbons in detail.

How do coronal mass ejections evolve in the inner heliosphere?

Eruptive prominence abserved in the He II 304 Å bandpass


The observations of CME initiation in the solar atmosphere will be necessary to complement multi-point in situ measurements of interplanetary CME counterparts (ICMEs) by Solar Orbiter. CMEs will be detected by the COR instrument, and the EUI will provide information on both their source region and evolution of coronal structures before, during and in the aftermath of the CME initiation. The 174 Å and 304 Å channels of the FSI are well suited for the identification of the CME source region as successfully achieved with SOHO/EIT data through observations of coronal dimmings, EIT waves, erupting prominences (erupting filaments when observed on the disk) and post-eruption arcades.
Using the measurements made by the FSI combined with COR and the heliospheric imager (GHI) data, we will have an extensive picture of the CME origin and evolution from close to the Sun into the interplanetary medium. more ...

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.

Explore, at all latitudes, the energetics, dynamics and fine-scale structure of the Sun's magnetized atmosphere.

H I Lyman-α image of the solar limb at 250 km spatial resolution taken during the second flight of the VAULT rocket telescope.


The solar atmosphere appears extremely structured and dynamic when observed at the best present spatial resolution: about 700 km in the corona (TRACE), 250 km in the transition region (VAULT), about 100 km in the photosphere and chromosphere (Hinode/SOT and ground- based telescopes). In all these cases, structures appear at the limit of the instruments' spatial resolution. Therefore, it is likely that elementary structures are not yet resolved. The spatio-temporal structuring of plasma and magnetic fields in the solar atmosphere controls the dominant fundamental physical processes of energy dissipation that lead to plasma heating, cooling, radiation, motion and wave generation, solar wind and energetic particle acceleration. more ...

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.