The scientific goals of the SO mission are summarized in four
fundamendal top-level objectives:
Determine the properties, dynamics and interactions of plasma, fields and
particles in the near-Sun heliosphere
Investigate the links between the solar surface, corona and inner heliosphere
Explore, at all latitudes, the energetics, dynamics and fine-scale structure of
the Sun's magnetized atmosphere
Probe the solar dynamo by observing the Sun's high-latitude field, flows and
These science goals emphasise and fully exploit the unique mission characteristics of and Solar Orbiter, namely:
- close-up view point,
- out-of-ecliptic view point,
- co-rotating orbital phases by Solar Orbiter,
Possibilities for combined investigations of the
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
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
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
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
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
X-Flare as seen by Trace 195 Å superimposed with hard X-Ray contours
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.
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.
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
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.
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
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
The interaction between the corona and the lower atmosphere continues to be a
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.