**3. Scientific Background**

**3.1 Introduction**

The Sun\'s atmosphere and heliosphere represent uniquely accessible
domains of space, where fundamental physical processes common to solar,
astrophysical and laboratory plasmas can be studied in detail under
conditions impossible to duplicate in Earth-based laboratories or to
study at astronomical distances.

The Solar Orbiter will, in an unprecedented way through a clever orbital
design, explore *in-situ* the innermost heliosphere and scrutinise
through high-resolution remote-sensing the Sun and its atmosphere, while
flying closer to the Sun than any other spacecraft in the past and out
of the ecliptic to higher latitudes. From there, the Solar Orbiter will
provide the first observations of the entire polar regions of the Sun
and the whole equatorial corona. The results from missions such as
Helios and Ulysses, and Yohkoh, SOHO and TRACE have brought enormous
progress in our understanding of the solar corona and the associated
solar wind and three-dimensional heliosphere. Yet, we have reached the
point where further *in-situ* measurements, now much closer to the Sun,
together with high-resolution imaging and spectroscopy from a near-Sun
and out-of-ecliptic perspective, promise to bring revolutionary
breakthroughs in solar and heliospheric physics.

There are four totally novel aspects to the Solar Orbiter mission, which
allow unique science investigations to be performed. These are:

-   Close-up remote sensing observations of the magnetised solar
    atmosphere providing unprecedented high-resolution.

-   Unique heliosynchronous observations enabling us to understand the
    links between solar and heliospheric processes.

-   In-situ measurements of the unexplored inner heliosphere.

-   First out-of-ecliptic imaging and spectroscopic observations of the
    solar poles

and equatorial corona from high latitudes.

The next four sections describe the new science and advances which will
be made in these four areas.

**3.2 The Sun\'s magnetised plasma: close-up observations of the solar
atmosphere.**

The high-resolution imaging of the solar atmosphere will represent a
major step forward by providing an order of magnitude improvement in
spatial resolution over past missions. The Solar Orbiter instruments in
concert will enable us to analyse thoroughly the time-variability,
evolution and fine-scale structure of the dynamic chromosphere,
transition region and corona, to study the Sun\'s magnetic activity on
multiple scales, to investigate energetic particle acceleration,
confinement and release, and to reveal plasma and radiation processes
underlying the heating of the chromosphere and corona. The Sun is the
only star that can be resolved at the level at which the physical
processes responsible for magnetic activity take place.

**3.2.1 Photospheric magnetic flux elements**

While our understanding of the equilibrium properties of the Sun has
been validated by helioseismology, most notably by the impressive
results from SOHO, our understanding of the non-equilibrium properties
-- primarily associated with magnetic fields -- remains poor. Magnetic
fields play a crucial role throughout astrophysics, ranging from the
formation of stars to the extraction of energy from supermassive black
holes in galactic nuclei. The Sun provides a natural laboratory for the
study of cosmic magnetism on scales not accessible on Earth and not
resolvable in distant astronomical objects.

A key goal of the Solar Orbiter is to advance our understanding of solar
magnetism by measuring the structure and dynamics of the magnetic field
at the solar surface down to its fundamental length scale. The major
part of the magnetic flux permeating the solar photosphere outside
sunspots is concentrated in small flux tubes of kilo-gauss field
strength. The scale of these magnetic flux tubes is believed to be
determined by the pressure scale height, which is about 100 km. This is
also near the typical photon mean free path at the photosphere whose
dynamics is controlled largely by radiation processes at this scale.

The structure and dynamics of these fundamental elements of the
near-surface magnetic field has profound implications for a number of
basic questions:

-   How do magnetic foot-point motion, wave excitation, flux
    cancellation and reconnection contribute to the flux of mechanical
    energy into the corona?

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-   In what way do the emergence, evolution and removal of magnetic flux
    elements determine the magnetic flux budget of the Sun? Is there a
    local dynamo operating on the scale of granulation?

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-   What is the origin of the facular contribution to the variability of
    the solar constant?

-   What is the physics of the interaction between convection and
    magnetism?

Answers to these questions require the study of magnetic flux elements
on their intrinsic spatial scale (\<100 km). Solar Orbiter will allow us
to achieve this resolution. The Solar Orbiter high-resolution
visible-light imager and magnetograph is intended to observe magnetic
field emergence, dynamics, twist, shearing, mutual interactions and
possible coalescence and subduction in order to follow the evolution and
understand the life cycles of magnetic flux elements.

**3.2.2 Basic processes and fundamental scales in the magnetised solar
atmosphere**

We wish to understand the basic processes on all scales in the solar
atmosphere, e.g. magnetic processes leading to particle acceleration or
heating. It has been recently realised that very small-scale processes
are at work in the solar atmosphere. For instance, theoreticians pointed
at the difficulty of having both large-scale and rapid magnetic
reconnection. This led to the concept of heating through numerous,
globally distributed, small-scale "events" for which observers have
candidates (e.g. SOHO observations of explosive events). Numerical
simulations have been performed in the frame of Self-Organized
Criticality or Cell Automation models whose outputs, in terms of
temporal fluctuations and statistical properties, compare well with
observations. Moreover, it seems that the observed distributions laws
with SOHO have self-similarity properties which point at sub-resolution
processes.

The nanoflare model of Parker is just an example of such models.
Whatever the scale, magnetic reconnection leads to particle acceleration
and wave generation. Particle acceleration and wave dissipation have the
net effect on the lower solar atmosphere of a local kinetic energy
increase which can be revealed through EUV imaging and spectroscopy at
high resolution. Wave propagation can be traced from the source site to
the region of dissipation through observations of EUV-line broadenings
and Doppler shifts.

Between the very small scales implied by the above processes and the
sizes of the smallest observed features, there are intermediate plasma
scales. Indications of these intermediate scales are provided by the
very small values of the filling factor derived from observations made
of a large variety of solar structures. Thus, the transition region and
inner corona are highly structured at scales even below those presently
resolvable by SOHO and TRACE and involve hot and cold plasmas at all
temperatures between 10^4^ K and 10^7^ K. Widely differing temperature
plasmas often co-exist side by side in a hierarchy of filamentary
structures, channelled by the magnetic field, which indeed form the
solar atmosphere.

With the high spatial resolution due to its proximity to the Sun,
combined with high temporal and spectral resolution, and
multi-wavelength coverage, the Solar Orbiter will measure the signatures
of the basic processes and the elementary structures of the solar
atmosphere providing plasma diagnostic information over the full
temperature range existing in the solar atmosphere.

Solar Orbiter will specifically address these issues by identifying the
basic physical processes and structures. Proper plasma diagnosis will
enable us to understand their nature and importance in the mass and
energy budget of the solar atmosphere. For example, Solar Orbiter will
be used to measure:

-   Outflow and accelerated particles from magnetic reconnection sites
    on all scales;

-   Proper motion, line broadening and Doppler shifts associated with
    wave propagation;

-   Scale properties of MHD turbulence;

-   Scales and evolution of the elementary structures.

**3.2.3 Waves in the corona**

Magnetohydrodynamic waves generated in the photosphere by convective
motion are primarily of low frequency. Small-scale magnetic activity is
expected to continually produce energetic particles, rapidly-moving
plasma and waves. Their dissipation could involve cyclotron damping,
which is observed to operate in the distant solar wind. The Solar
Orbiter will make the first observations of such key plasma processes
from a near-Sun perspective and thus address the extended heating of the
outer corona. In the small-scale magnetic structures of the strongly
inhomogeneous network fields higher-frequency waves could be excited up
to the kilo-Hertz range. Such waves would transfer their energy, e.g.
into the transverse kinetic degrees of freedom of the protons, and
particularly the heavy ions, and thereby heat the particles very
effectively to high kinetic temperatures, a process for which the UVCS
and SUMER instruments on SOHO have recently found evidence in the strong
Doppler broadenings of emission lines.

Due to its proximity to the heating sites in the corona and the high
sensitivity and spatial resolution of its instrumentation, the Solar
Orbiter will for the first time be able to

-   see very dim emissions, which are not visible from 1 AU due to the
    low density of the gas and the high contrast in emissivity, when
    compared with plasma confined in small loops,

-   resolve and diagnose dilute plasma on open fields and the outflow in
    coronal funnels,

-   search for high-frequency waves in the corona (with reasonable
    chances of success due to the high resolution).

    **3.2.4 Coronal magnetic field**

A key scientific objective of Solar Orbiter is to study the emergence
and the cancellation of photospheric magnetic flux (the latter is the
disappearance of opposite polarity regions in close contact), and to
investigate the consequences of such processes for the overlying global
coronal magnetic loops and for the chromospheric and transition region
magnetic network. Flux cancellations are known to be associated with or
at the origin of various active phenomena, such as filament formation
and eruption, evolution of small points of emission bright in radio,
ultraviolet or X rays, or the occurrence of flares.

Magnetograms combined with EUV and soft-X-ray images as well as EUV
spectra are the key data necessary to understand the influence that
small-scale magnetic activity has on the corona. Such coordinated
observations could reveal basic processes at work throughout the solar
atmosphere which are drivers of the solar wind acceleration or coronal
heating. Thus, the detailed understanding of the fine-scale processes in
the magnetic fields at the base of the solar atmosphere is an essential
goal of this mission.

![](media/image1.png){width="6.5256944444444445in"
height="5.042361111111111in"}

Figure 3.1: Images of gas in loops observed on the Sun. In addition to
hydrogen, the Sun\'s atmosphere contains atoms of common elements like
helium, oxygen and magnesium. The loops have been rendered more easily
visible by observing them when they occur near the limb of the Sun, and
hence they are highlighted against the dark background of space. The
elements and their characteristic temperatures are indicated on the
individual images. One of the surprises that the new SOHO/CDS data show
is that loops at different temperatures can co-exist in the same regions
of the Sun\'s atmosphere. The disk plotted on the oxygen image shows the
Earth to the same scale.

On a larger scale, coronal loops are the very building blocks of the
outer solar atmosphere, ranging from small loops in active regions,
through loop arcades to active region interconnecting loops and helmet
streamers. Observations from Yohkoh, SOHO and TRACE in extreme
ultraviolet and X-ray wavelengths have revealed a truly complex, highly
dynamic environment with loops confining plasmas at widely varying
temperatures even in adjacent loops. Of course, it is within such loop
systems that we witness the most dramatic of solar phenomena, the solar
flares and mass ejections, as magnetic loop systems interact or become
unstable. The study of the interaction and evolution of loop systems,
through high-resolution EUV mapping combined with fine-scale diagnostic
analysis using high-resolution spectroscopic observations, will be a
major activity for Solar Orbiter scientists in particular in an effort
to understand processes leading to mass ejection and flaring.

The TRACE extreme ultraviolet observations in particular illustrate the
existence of fine-scale structures in the coronal loops, and reveal
continuous dynamic activity in particular at the smallest scales. There
is strong evidence that the size of the actual brightness structures
lies well below the best current spatial resolution. This points to the
need for still higher spatial resolution, which, as in the visible, can
be obtained by modest means with the Solar Orbiter.

The wide coverage of coronal temperatures by the telescopes on the Solar
Orbiter will enable complete images to be obtained in fast cadence. From
these the density, temperature and flow distributions can reliably be
derived for the full range of magnetic structures, allowing detailed
studies such that coronal heating processes in current sheets, shock
fronts, or acceleration in small explosive events and rapid plasma jets
might become clearly visible and be resolved in time and space for the
first time.

**3.3 Linking the photosphere and corona to the heliosphere: corotation
observations**

The Solar Orbiter mission provides unique possibilities to do
complementary remote sensing and in-situ observations of the Sun from
close distances (perihelion of 45 R~s~). Unprecedented spatial
resolution at scales below 100 km will be achieved in the images
obtained in various wavelength bands. Solar rotation will have a
negligible influence on the observations during the heliosynchronous
orbital phase of the spacecraft. This will allow us to separate spatial
from temporal variations. Therefore, time variability of the magnetic
field and its optical and interplanetary-particles manifestations can be
studied extensively for many days at a given heliographic longitude. The
favourable vantage points of Solar Orbiter along a heliosynchronous
trajectory are unattainable by any other means.

The scientific success of the Solar Orbiter mission in this area is
ensured by a comprehensive and well-focused suite of *in-situ* and
remote-sensing instruments, which are required to be able to connect the
local plasma and field environment with its plasma and magnetic sources
on the Sun, and thus to establish spatial and temporal links with the
location and evolution of these source regions in the highly structured
and widely varying solar atmosphere. The Solar Orbiter will fly through
field lines with foot points in the fields of view of its imagers, and
therefore the characterisation of the properties of the plasma and its
sources can be made, essentially by virtue of the corotation of the
spacecraft with the Sun during the perihelion passes.

**3.3.1 Global coronal sources of the solar wind**

Helios and Ulysses have clarified the global origin of the fast and slow
solar wind in the large-scale corona. Yet, the detailed origin of the
slow wind remains unclear. There are two main alternative ideas about
the origin of the slow solar wind. One is that it arises from the outer
edges of coronal holes, in magnetic field regions continuously open to
interplanetary space. The other is that the slow wind arises in the
middle and lower corona from the tops of helmet streamers, which are
primarily closed magnetic field regions but do open intermittently to
release the flow at heights beyond about two solar radii in the form of
plasma blobs or plasmoids. Perhaps the slow wind consists of these two
types of flow that intermingle at a few solar radii. The remote sensing
observations of the corona combined with *in situ* solar wind
observations made by Solar Orbiter during the corotational passes will
allow us to distinguish between these two hypotheses, by providing
accurate connections of in-situ solar wind structures with
remotely-sensed coronal structures.

![](media/image2.jpeg){width="4.081944444444445in"
height="3.8854166666666665in"}

Figure 3.2: SUMER observations of the solar-wind source regions and
magnetic structure of the chromospheric network. The insert shows the
measured Doppler shifts of Neon ions, indicating blue-shifts, i.e.
outflow, at the network cell boundaries and lane junctions below the
polar coronal hole, and red-shifts (downflow) in the network regions
underlying the globally closed corona

**3.3.2 Magnetic network and origin of the solar wind**

The supergranulation network, which dominates the chromospheric plasma
dynamics, is apparent in the EUV emission pattern as seen by the SUMER
instrument on SOHO (see Figure 3.2). Magnetograms from SOHO have
revealed the ubiquitous appearance of small magnetic bipoles at the
solar surface. After emergence, the polarities separate and are carried
to the network boundaries by the supergranular flow, where they merge
with the pre-existing network flux. This leads to flux cancellation,
submergence and reconnection events. The magnetograms also show that the
magnetic field exists in the network in two components side-by-side,
i.e. in uncanceled unipolar fields together with a carpet of closed
loops and flux tubes. The small loops will either emerge or contract
downwards and collide, and thus constitute a permanent source of energy,
which can be tapped by the particles through magnetic field dissipation.
Numerical simulations suggest that many of the bipolar structures have
scales smaller than 100 km.

**3.3.3 Boundaries and fine structures in the corona and solar wind**

The boundaries and gradients between the fast and slow solar wind have
been well characterised by the Helios and Ulysses spacecraft at 0.3 AU
and beyond, but little is known about their microphysics and dynamics.
For example, instabilities can be caused by velocity shear across a
boundary and become a source of turbulence and friction, and thus alter
the nature of the wind in the vicinity of stream boundaries and
interaction regions. There are plenty of other fine-scale structures in
the solar wind and corona, one example of which are the conspicuous
plumes over the poles, which have been studied by SOHO in considerable
detail. Plumes have been observed to extend beyond many solar radii, yet
their *in situ* signatures in the wind plasma are elusive. Material in
plumes has been found to flow at much lower speeds than in the ambient
darker lanes throughout the altitudes observed in coronal holes. By
combining the Solar Orbiter data from the *in situ* and remote-sensing
instruments, we will be able to identify the genuine contributions of
plumes to the fast wind.

Little is known currently about the continuation of coronal
substructures into the outer corona and near-Sun solar wind. There are
many other meso-scale structures in the corona and particularly the slow
solar wind, which is intrinsically variable in space and time and
convects structures such as tangential discontinuities, for example, the
coronal origin of which remain a mystery. Many fine structures exist in
the corona at a wide variety of length scales. Structures very similar
to the polar plumes have also been observed in inter-streamer regions.
At even smaller spatial scales, coronal spikes and EUV spicules with
widths of order 10^3^ km have been observed. Ample evidence of fine
structure also comes from interplanetary radio observations.
Ground-based white light eclipse images show elongated and thin,
threadlike structures that extend outward in the innermost corona and
seem to be ubiquitous outside of coronal holes. The Solar Orbiter by its
proximity to the corona will be able to resolve such features with
sufficient brightness contrast. It will trace their extensions from the
solar atmosphere into space by measuring the remnant signatures in the
more distant solar wind during the perihelion passes. The Solar Orbiter
will examine fine-scale structures in the corona at much higher spatial
resolution than ever employed before and associate them with their
interplanetary manifestations.

**3.3.3 Connections between the internal plasma states of the solar
corona and the solar wind**

There are many closely-connected heliospheric and solar science
objectives of Solar Orbiter, which can be addressed in particular during
the corotation passes enabling steady observational conditions from a
fixed vantage point. The principal goals are: to determine the
relationship between coronal and solar wind structure on all scales that
can be resolved; to identify the coronal energy source for the solar
wind and trace by remote-sensing diagnostics and imaging the flow of
energy through the different layers of the atmosphere. Remnants of the
processes that heat and accelerate the ions will still be detectable in
the in-situ measured microscopic features of the solar wind particle
velocity distributions functions (such as double ion beam, minor ion
differential streaming, pronounced temperature anisotropies indicating
cyclotron or Landau resonance between plasma waves and particles, or
heat flux-carrying suprathermal tails, as already observed by Helios and
Ulysses) and will allow clear inferences be made on the coronal plasma
processes. A related goal is to determine the role of turbulence and
waves in heating the corona, accelerating the solar wind, and energising
particles and to determine how the particle populations, plasma waves,
and magnetohydrodynamic turbulence evolve together with heliocentric
distance in the innermost heliosphere.

**3.3.4 Coronal and solar wind abundances and fractionation effects**

Understanding the abundances of elements in the Sun and solar wind is an
important issue in solar physics. The solar wind carries the material
delivered from the outer convective zone of the Sun into the
heliosphere. With the exception of the H and He isotopes, the elemental
composition of the outer convective zone is close to the proto-solar
nebula composition. The element helium, which is not visible in the
photosphere, is highly variable and rather under-abundant in the solar
wind and solar energetic particles. The two types of solar wind, fast
and slow, differ considerably in their charge-state composition (a
coronal signature) and elemental abundance (a chromospheric signature).
Apparently, the elemental fractionation processes are fundamentally
different in the fast and slow solar wind, although fractionation on the
basis of the First Ionization Potential (FIP) is present to a different
degree in both types of wind. The FIP effect seems to be active also in
the chromosphere beneath the polar coronal holes, in the sources of the
fast wind, but at a reduced strength. Charge-state spectra are also
differing in the two type of streams: thermal equilibrium spectra
prevail in fast streams, indicating a simple freezing-in process. The
slow wind charge-state distribution, on the other hand, shows an excess
of high charge states and indicates that different coronal temperature
regimes may coexist in the source region.

Solar Orbiter, while taking high-resolution images and making
spectroscopic measurements of solar wind source regions that are
magnetically linked to the spacecraft location and doing simultaneous
in-situ measurements over the long corotation intervals, is ideally
suited to address the critical issues of the chromospheric fractionation
process. The coronagraph on the Solar Orbiter will determine for the
first time the helium abundance (high FIP element) in those atmospheric
layers where the acceleration of the slow and fast streams actually
occurs, and where the charge states freeze in. The results expected will
provide keys for the understanding of the processes at the origin of the
solar wind and of the elemental composition in the heliosphere.

For the Solar Orbiter a new type of UV and visible-light coronagraph is
envisioned which aims at measuring directly and characterising in detail
the properties of the two most abundant elements, hydrogen and helium.
In particular, this coronagraph will by its innovative design provide
new and better views of the solar corona than possible with SOHO
presently and STEREO in the future. In particular, it will be able to
provide

-   the first UV images of the full corona for the two most abundant
    elements,

-   the first global maps of the solar wind outflow,

-   the first images of the He II coronal emission.

The Solar Orbiter will allow us to do these new measurements of the
corona also from out of the ecliptic, and will thus provide the first
view of the entire equatorial corona and global observations of the
coronal expansion near the equator.

**3.3.5 Global solar corona and solar wind**

There are two characteristic types of solar corona and wind, prevailing
at different heliographic latitude regions of the Sun and heliosphere.
The high-speed flow is the basic equilibrium state of the solar wind. It
is most conspicuous near solar minimum, when it emanates steadily from
the magnetically open coronal holes around the poles, whereas the slow
wind originates in an unsteady fashion from the equatorial streamers.
These are for most of the time magnetically closed, but seem to open
intermittently to release abruptly mass ejections (CMEs) (see the
subsequent Figure 3.3) and more continuously the slow wind embedding the
heliospheric current sheet. The transient nature is also evident in the
high variability of its abundances in helium and other heavier elements
found in association with large-scale magnetic activity on the Sun. The
solar magnetic field in the equatorial regions reveals a rich morphology
and many fine-scale structures, such as the low-latitude rays resembling
the polar plumes, which are clearly evident in visible light coronagraph
pictures and in the EIT images. Remnants of these features are even
found in the meso-scale stream variations of the solar wind, as observed
in situ by Helios and Ulysses and through interplanetary scintillations.

The slow and transient solar wind can only be resolved and understood
properly by a direct probing close to its sources at a fixed
heliographic longitude. It is a key advantage of the Solar Orbiter that
from the corotational vantage point the temporal and spatial variations
of the solar wind can unambiguously be disentangled. The observations to
be made by the Solar Orbiter will concentrate on the slow streams and on
solar disturbances associated with magnetic solar activity, flares,
loops, erupting prominences, and CMEs, when the Solar Orbiter is close
to the ecliptic, and on the fast streams, when the Solar Orbiter is
close to maximum inclination. The temporal evolution and spatial
structure of such phenomena at the coronal base will for the first time
be measured at very high spatial (\< 100 km) and temporal resolution.

It is well known that the solar wind carries angular momentum from the
Sun through kinetic and magnetic stresses. This will lead ultimately to
a spin-down of the Sun\'s rotation within its remaining life time, a
conclusion derived from the Helios measurements of the angular momentum
as carried away by the wind near the ecliptic plane. To determine the
overall angular momentum loss associated with the solar wind outflow at
higher latitudes is an important goal for Solar Orbiter. Also, the
analysis of the partial corotation of the outer corona and the
detachment of the wind from solar rotation near the Alfven surface may
be possible by exploiting the corotational passes. Such measurements
will produce novel results about the global angular momentum loss of the
Sun as a star, which is an important issue in astrophysics.

**3.3.6 Coronal transients**

A subject that has recently attained much attention is "space weather".
It is concerned with transient events such as flares, coronal mass
ejections (CMEs), eruptive prominences, and shock waves and their
impacts upon the Earth\'s magnetosphere and atmosphere. Fundamental
questions in this context remain to be solved, e.g.,

-   What are the indicators for imminent violent eruptions? Can they be
    predicted?

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-   Why are there so extremely different types of solar transients?

-   What determines their propagation properties?

-   How far around the Sun do the resulting shock waves reach?

The Solar Orbiter will be ideally located, being closer to the sources
of transients in the solar atmosphere, to measure the input into the
heliosphere and determine the boundary conditions near the Sun. These
dramatic events can literally shatter the whole heliosphere, and their
effects can be felt at all planets. The Solar Orbiter will be a key link
in a chain of solar terrestrial observatories to be stationed in
Earth\'s orbit and at the libration points in that it provides
near-real-time event alerts from its unique orbit close to the Sun.

![](media/image3.png){width="3.9479166666666665in"
height="2.7708333333333335in"}

Figure 3.3**:** Giant coronal mass ejection as seen by LASCO on SOHO in
1998. The filament material of the ejected prominence exhibits twisted
helix-shaped structures. The Solar Orbiter at its perihelion of 45 R~s~
would be able to measure the ejecta *in situ* and out of the ecliptic in
much more detail than possible from the Earth\'s orbit.

The Solar Orbiter will provide high resolution images of the solar
atmosphere as well as the determination of plasma diagnostics using
spectroscopy. These, combined with a capability to determine the
photospheric fields at the footpoints of coronal loops provide the tools
for a thorough, detailed analysis of the processes leading to eruptive
events. The influence of the eruptions of the high coronal can be
determined with coronagraph instrumentation and the associated effects
on the interplanetary medium can be studied with in-situ instrumentation
without significant delays or transport-related changes of the particles
and fields. The multi-wavelength, multi-disciplinary approach of Solar
Orbiter, combined with the novel location, produces a powerful tool for
studies of the influence of eruptive events such as CMEs on
interplanetary space.

**3.4 Particles and fields: *in-situ* measurements in the inner
heliosphere.**

According to recent SOHO findings, one must conclude that the coronal
expansion arises because of the high temperatures of the coronal ions,
with the minor species reaching even 10^8^ K at a few solar radii. In
contrast, electrons are comparatively cool, in fact they are found to
hardly reach the canonical coronal temperature of 10^6^ K, and
consequently the electric field (related to the electron partial
pressure gradient) has a minor role in accelerating the ions. The high
pressure of the coronal ions and the low pressure of the local
interstellar medium lead to a supersonic solar wind extending to long
distances from the Sun to 100 AU. Yet, even after the SOHO mission the
detailed physical mechanisms that heat the corona and accelerate the
plasma to supersonic speed remain poorly understood, because the
resolution of the SOHO imagers and spectrometers were still not
sufficient, and because the solar wind plasma has never been directly
sampled closer to the Sun than the Helios perihelion (0.3 AU).

Solar Orbiter will provide the first opportunity of going closer to the
Sun and into the inner heliosphere to 0.21 AU, but unlike Helios and
Ulysses Solar Orbiter will also carry powerful, high-resolution optical
instruments together with the in-situ instruments. In particular, the
plasma and field instruments will have high temporal resolutions,
ranging between 0.01 s and 1 s, and offer unique chances for resolving
physical processes at their intrinsic scales. Therefore, this mission
will reveal new insights in the plasma kinetic processes that structure
the Sun\'s atmosphere, heat the extended corona and accelerate the solar
wind as well as energetic particles.

**3.4.1 Microstate of the interplanetary solar wind**

The ultimate causes of interplanetary kinetic phenomena are to be found
in the dynamic corona itself. The closer a S/C comes to the Sun the more
likely it is to detect remnants of coronal heating and related plasma
processes, occurring on a broad range of scales in space and time. The
radial evolution of the internal state of the expanding wind resembles a
complicated relaxation process, in which free energy stored in the form
of stream structure and shear as well as in non-Maxwellian particle
velocity distributions (see Figure 3.4) is converted into wave and
turbulence energy.

![](media/image4.png){width="4.20625in" height="3.095833333333333in"}

Plasma waves in the collisionless solar corona and wind play a role
analogous to collisions in ordinary fluids. These wave modes can
theoretically be excited by a variety of free energy sources, including
drifts, currents, temperature anisotropies and beams, which must be
resolved in detail. All the wave modes of primary importance together
with the ions and electrons will be measured by Solar Orbiter at high
time resolution, in order to provide the comprehensive wave and
particles diagnostics necessary to study the wave-particle interactions
and kinetic processes.

Solar Orbiter, while approaching the Sun to about 0.2 AU with its plasma
and wave analysers, will enable high-resolution measurements of kinetic
processes be made, which holds much promise of unexpected revelations.
It will address fundamental solar wind science and key plasma-physics
questions such as

-   How do particle distributions develop velocity-space gradients and
    deviations from Maxwellians?

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-   What processes drive plasma instabilities and cause wave growth and
    damping?

-   What regulates transport and ensures the observed fluid behaviour in
    the collisionless solar wind?

-   What are the radial, latitudinal and longitudinal gradients of
    plasma parameters in the inner heliosphere?

-   What is the microstructure of, e.g., stream interfaces and boundary
    layers near the Sun?

-   How does the chemical and charge-state composition of the plasma
    vary spatially?

    **3.4.2 Solar wind ions as tracers of coronal structures**

The solar wind carries along information on its coronal source regions,
e.g., through its elemental and isotope composition, and the ionisation
states of the various atoms. At the coronal base, a compositional bias
is introduced according to the first ionisation potential (FIP) of the
elements. This FIP effect appears to be significantly different for slow
and fast solar wind flows. Further, the ionisation state of various
species indicates that slow wind must have undergone substantially more
heating than fast wind. These signatures will be used for the first time
to resolve fine structures and boundaries in the solar wind. This
diagnostic technique works most reliably close to the Sun where the flow
has not yet been processed (compressed, deflected, etc.) by interactions
between streams of different speed. With Helios it was found that at
least in high-speed wind a basic flow-tube structure was still
recognisable in situ at 0.3 AU. The scale size observed matches that of
supergranules fairly well, if an appropriate flux-tube expansion is
taken into account. The Solar Orbiter, using modern ion-composition
instruments and being closer to the Sun, will

-   reveal, through precise determination of compositional variations,
    the fine structures in all types of solar wind,

-   link the flow tubes directly with the underlying chromospheric
    network observed remotely,

-   identify pick-up ions stemming from dust and interplanetary sources.

    **3.4.3 Magnetohydrodynamic turbulence**

The solar wind plasma is in a highly turbulent state composed of various
components. The energetically dominant component consists of largely
incompressible Alfvénic fluctuations (see Figure 3.5 for the power
spectrum). The minor component has a much lower amplitude than the
Alfvénic fluctuations, is compressible and clearly enhanced in the mixed
low-speed flows. Solar Orbiter will scan a belt ranging roughly from
-40° to 40° in heliographic latitude, while being within 0.3 AU of the
Sun. From these vantage points Solar Orbiter will be able to answer
important questions such as:

-   How does MHD turbulence evolve spatially at higher latitudes near
    the Sun?

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-   What are the crucial conditions for in-situ turbulence generation?

-   How does the turbulence pattern vary with stream structure closer to
    the Sun?

The overall radial trends as seen by Ulysses and Helios suggest strong
variations of the local production rate of the Alfvénic fluctuations in
the region just inside 0.3 AU. The reduction of the perihelion distance
from 0.3 AU (Helios) to 0.2 AU (Solar Orbiter) will offer unique
opportunities to study the local generation, nonlinear coupling and
spatial evolution of MHD waves near the Sun. Measuring Alfvénic
fluctuations and MHD turbulence *in situ* represents also a means of
diagnosing their coronal sources. Solar Orbiter will address these
issues from its unique vantage point and help to answer basic questions
such as:

-   How and where are Alfvénic fluctuations generated in the solar
    corona?

-   How does MHD turbulence evolve radially and dissipate in the inner
    heliosphere?

-   Do the spectra contain indications or relics of high-frequency wave
    heating of the corona?

The solar wind is the only available plasma "laboratory" where detailed
studies on MHD turbulence can be carried out free from interference with
spatial boundaries, and in the important domain of very large magnetic
Reynolds numbers. Detailed comparison between experimental in situ data
and theoretical concepts will allow to put MHD turbulence theory on more
solid physical ground, which will be of paradigmatic importance to
understand the solar (stellar) coronal heating mechanism and the role or
turbulence in the solar (a stellar) wind.

![](media/image5.png){width="3.854861111111111in"
height="3.3055555555555554in"}

**3.4.4 Acceleration and transport of solar energetic particles**

A continuing source of difficulty has been our inability to predict the
intensity of solar energetic particles at the Earth from observed
transient activity on the Sun. An important part of the problem is that
we do not know the suprathermal population that feeds the acceleration
processes near the Sun. The efficiency for transferring energy from
flares to energetic particles cannot be inferred from remote
observations, because an unknown fraction of the accelerated ions
remains trapped by strong magnetic fields near the Sun for a significant
time after acceleration. Subsequent γ-ray and neutron emissions
resulting from their eventual loss to the atmosphere are often too low
in flux density to be observable. Present observations indicate that
small transients occur sufficiently often to allow a determination of
the efficiency, e.g., by neutrons as proxies for the magnetically bound
component. Our ability to solve this issue will be greatly enhanced by
Solar Orbiter, because during its multiple perihelion passages we will

-   gain a better knowledge of the source spectrum,

-   obtain new observations on particle motion in the hypothetical
    storage region,

-   measure changes in the spectrum as the ions and electrons propagate
    from the Sun to the spacecraft after escape from the trapping
    region.

The Solar Orbiter will for the first time investigate the particles\'
environment in close proximity to the different source regions on the
Sun, such as coronal holes, streamers, coronal mass ejections (CMEs) and
associated shocks, active regions, and flare locations. Concerning CMEs
in particular, the Solar Orbiter will

-   determine the solar source conditions for different particle species
    (e.g., e, p, ^3^He, heavy ions, p/He ratios) from composition
    measurements, energy spectra and time evolution,

-   distinguish clearly between gradual (shock-associated) CME events
    and impulsive flare-type events related with magnetic reconnection,

-   study the effects of particle acceleration and turbulence-moderated
    propagation at different locations with respect to the CME centre,

-   find the differences between the particle signatures associated with
    parallel and perpendicular shocks at the east and west flanks of
    CMEs,

-   probe the effects of magnetic reconfigurations in the aftermath of
    CME launches, at times when the acceleration processes still occur
    in the corona,

-   detect perhaps for the first time energetic particle populations
    from microflares, a measurement which is not possible further away
    from Sun due to background problems.

    ![](media/image6.png){width="5.728472222222222in"
    height="6.352083333333334in"}Figure 3.6: A series of solar energetic
    particle events observed in July 1996 with the COSTEP-EPHIN
    experiment onboard the SOHO. The upper panel shows images of the Sun
    taken on July 9, 12, 14 with the SOHO-EIT in Fe XV (28.4 nm). The
    bottom panel shows intensities of 0.25-0.7 MeV electron, 4.3-7.8 MeV
    proton and 4.3-7.8 MeV/n helium nuclei measured with the
    COSTEP-EPHIN. The intensity increases of the energetic particles at
    SOHO were caused by flares and coronal mass ejections in an active
    region in the Sun\'s western hemisphere.

    In addition, we can study with Solar Orbiter important global
    aspects of the Sun and heliosphere (see the Figure 3.6) by

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-   utilising the energetic particles as probes for the coronal and
    heliospheric magnetic field,

-   analysing the propagation of solar particles and modulation of
    galactic cosmic rays.

Generally, the processes which accelerate particles to very high
energies are of great interest in astrophysics. Observations of
energetic particles close to their sources on the Sun will allow us to
study our nearest star**,** the Sun, as a particle accelerator.

**3.4.5 Neutral particles from the Sun**

The Solar Orbiter provides the opportunity of observing for the first
time neutral particles emitted from the Sun, the neutral solar wind.
Thus we obtain an independent means of determining the solar wind
velocity profile in the outer corona and the electron temperature close
to the Sun. In addition, Energetic Neutral Atoms (ENAs) can be produced
by charge-exchange processes in interplanetary space. They might be
detected by the Solar Orbiter for the first time near the Sun. ENAs may
be used for imaging the outer solar corona. The ENAs originating from a
seed-ion population or background neutral gas will allow one to derive
properties of the background gas as well as to remotely sense the energy
distribution, spatial distribution and temporal evolution of the seed
populations. The neutral gas could be either of interstellar origin or a
neutral component stemming from the solar wind itself, being produced,
e.g., by neutralisation of solar wind ions on interplanetary dust
particles close to the Sun. Possible seed ion populations are the solar
wind protons, energetic solar particles and anomalous cosmic rays.
Measuring the neutrals means probing and inferring the physical
properties of the

circumsolar dust. Such measurements possess a potential for unexpected
discoveries.

Neutral atoms are widespread in the heliosphere and their energies are
in the range from a few eV to more than 100 keV. Contrary to charged
particles, they can travel large distances through space, undisturbed by
the interplanetary magnetic field. In the solar corona, the neutral
hydrogen atoms are closely coupled to the emerging solar wind plasma and
give rise to the prominent Lα corona. The ratio of the densities of
hydrogen and protons is very low, 10^-6^ to 10^-7^, and the neutral
particles therefore have a negligible impact on the plasma. The neutral
atoms are a trace particle population originating from the solar wind
plasma by charge exchange.

The in-situ observation of neutral hydrogen by a space-borne instrument
in the vicinity of the Sun would be a \"first\" and an essential
contribution to our knowledge of the solar corona. Direct observation of
the neutral atoms will help to refine the physical models of the Lα
corona, by providing information complementing the remote-sensing
observations of the coronagraph on Solar Orbiter, and thus will enhance
our understanding of the coronal plasma processes and wave-particle
interactions out to a few solar radii. Further outward, the neutral
atoms become within a few solar radii more and more de-coupled from the
plasma. The resulting neutral solar wind constitutes an in-situ trace
particle population and should be observable at 0.21 AU (perihelion of
the Solar Orbiter).

Coronal mass ejections (CMEs) are known to bear a wide range of ion
charge distributions, originating from very cold to very hot coronal
plasma. Up to now, there are no direct observations of the energetic
neutral particles envisaged to exist in CMEs. When being detected, they
could give a clue to the understanding of the trigger process of the
CMEs. This would be another \"first\" and important observation of the
neutral atom detector on Solar Orbiter.

**3.4.6 Circumsolar and interplanetary dust**

The Solar Orbiter will also measure the sizes and distribution of dust
particles surrounding the Sun. They are known to produce the zodiacal
light and the F-corona and may have a prolific source in comets grazing
or falling into the Sun, as the SOHO coronagraphs have shown us. The
dust distribution in the inner heliosphere has not been measured since
the Helios mission, although substantial modelling efforts have been
made and new data are urgently needed.

The observed distribution of interplanetary dust particles (IDPs), i.e.,
micrometeorites of submicron to millimetre size, forms a flat spheroid
centred on the ecliptic plane. Its spatial density decreases with
increasing heliocentric distance and increasing latitude. Different
forces and effects are acting on IDPs: gravitational attraction,
radiation pressure, corpuscular pressure, magnetic forces, planetary
perturbations, erosion processes (sputtering, sublimation), and mutual
catastrophic collisions. IDPs originate from comets and asteroids or
even from interstellar space. They approach the Sun on time scales of
10000 to 100000 years due to the deceleration by the Poynting-Robertson
effect. In the vicinity of the Sun, IDPs suffer, as their temperature
becomes high, a complicated evolution caused by their progressive
sublimation, which depends on the particle properties and chemical
composition.

On the other hand, these processes are the source of an additional ion
population in the solar wind: outgassing, sublimation and sputtering
produce neutrals which are then ionised and picked up by the solar wind.
The study of these particles will contribute significantly to the
understanding of the evolution of interplanetary dust in the vicinity of
the Sun. The interaction between the plasma and the solar magnetic field
with IDPs makes this picture even more complex: far-reaching, dense
coronal structures as well as the changing pattern of the magnetic field
can perturb the IDPs dynamics.

The present understanding of IDPs results from Zodiacal Light
observations, *in-situ* impact detectors and laboratory analysis of
collected particles. The processes taking place in the circumsolar
region remain poorly understood. Their description is, to a large
extent, based on theoretical studies. The possibilities of remote
sensing experiments are limited due to instrumental problems. Especially
the chemical composition and the dynamics of submicron grains can only
be investigated by *in-situ* methods. The dust observations on Solar
Orbiter will

-   render possible the first *in-situ* detection of grains which have
    suffered extreme radiation and corpuscular impacts, processes
    relevant for star formation,

-   help in determining the extent of the dust-free zone near the Sun,

-   provide the potential for discovery of a dust disc fed partly by
    Sun-grazing comets,

-   deliver data relevant for understanding the physics of
    protoplanetary discs.

    **3.4.7 Solar neutrons**

A large fraction of the particles accelerated by flares (and
microflares) interact in closed magnetic field lines of the solar
atmosphere (at heights much less than 3 R~s~) and do not escape into
interplanetary space. They are detected through the X-ray, γ-ray,
microwave and decimetric-metric radio emissions they produce, and
through secondary particles, such as neutrons produced in interactions
with the dense layers of the solar atmosphere. The neutron flux varies
dramatically with the distance because of the beta decay. Note that the
maximum distance neutrons can reach is given by the product of their
speed and their mean lifetime (which is only eight minutes for
non-relativistic neutrons!). In fact, at the Earth's orbit neutrons with
energies less than 100 MeV could not be detected at all. Very few solar
neutron events have been reported as compared with several hundreds of
γ-ray events. During the perihelion passages of Solar Orbiter, it will
be possible to directly measure also the less energetic neutrons for the
first time. Also, the neutron fluxes at energies above 100 MeV will
increase by orders of magnitude.

The γ-rays and neutrons are produced by interactions between the
primarily accelerated ions and the constituents of the solar atmosphere.
There is important information about the composition and charge states
of energetic particles and about the existence of distinct phases of
proton acceleration. This process is either very prompt and occurs
within seconds of the release of flare energy, or it is delayed and
associated with the shock wave possibly generated by the flare ejecta.
Solar Orbiter will provide new information about nuclear reactions in
the close vicinity of the Sun and thus open up a new window: solar
neutron astronomy.

**3.4.8 Solar radio emission**

The Sun is an intense source of thermal as well as nonthermal radio
emission. The nonthermal radiation is generated by suprathermal and/or
highly energetic electrons produced by sudden releases of magnetic
energy in the active corona. Nonthermal radiation at frequencies below 1
GHz is generally assumed to be plasma radiation emitted near the local
electron plasma frequency which is a unique function of the local
electron density. Consequently, the higher and lower frequencies
correspond to the lower and higher corona, respectively, and remote
sensing of the radial electron density profile is possible.

In the corona the nonthermal radio radiation is generated by energetic
electrons. The measurement of this radiation will allow us to study the
plasma processes associated with such electron populations and to
address questions as:

-   What is the elementary process of magnetic energy release?

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-   How are ions and electrons accelerated up to high energies within a
    few seconds?

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-   What are the relationships between flares, coronal mass ejections,
    radio bursts, interplanetary shocks, and energetic particle events?

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-   How does the quiet Sun produce suprathermal particles?

The radio spectrometer data are particularly valuable since they will be
obtained in connection with complementary data from the optical
telescopes imaging the activity site, and data of the particles and
fields instruments onboard. Detailed correlative studies between the
coronal/interplanetary electrons and the radio waves they generate will
be possible for the first time from a near-Sun and out-of-ecliptic
vantage point on Solar Orbiter.

**3.5 The Sun\'s polar regions and equatorial corona: excursion out of
the ecliptic**

Despite the great achievements of Ulysses and SOHO, significant
scientific questions remain unanswered, which can be answered by the
Solar Orbiter with its high latitude vantage points. For example,
progress in understanding the solar dynamo will depend on how well we
understand differential rotation and the circumpolar and meridional
flows near the poles of the Sun. Torsional oscillations associated with
the magnetically-active latitudes seem to originate close to the polar
regions and then propagate toward the equator. Are the meridional flows
observed at lower latitudes and these torsional oscillations associated?
The poles appear to rotate comparatively slowly, but the polar vortex is
still not well characterised due to the serious limitations of
in-ecliptic observations. Solar Orbiter will provide the first
opportunity to measure directly the magnetic field at the poles, as well
as the surface and subsurface flows there, and thus we anticipate being
able to answer some of the outstanding questions related to solar
magnetism.

The unipolar character of the magnetic field in coronal holes leads to
the fast solar wind which super-radially expands in the heliosphere. The
Solar Orbiter offers the opportunity of sampling the fast wind at much
closer distances than Ulysses where the plasma has kept memory of
accelerating processes.

Concerning the sites of acceleration, SOHO spectroscopic measurements
have provided some of the special role of the network, but through very
difficult Doppler measurements affected by grazing LOS. Going at high
latitudes allows to provide a definite answer to this question.

Even with new dedicated missions such as STEREO, it is impossible to
determine the mass distribution of large-scale structures such as
streamers and the longitudinal extent of Coronal Mass Ejections (CMEs).
The Solar Orbiter will provide the first observations of the complete
equatorial corona and its expansion in the equatorial plane.
Furthermore, it will provide the third dimension of CMEs.

The solar irradiance measurements performed, up-to-now, in the ecliptic
plane are strongly influenced by the mid-latitude activity phenomena.
The high latitude irradiance is totally unknown although it is an
important component of the solar luminosity.

**3.5.1 The Sun\'s polar magnetic field and the dynamo**

Knowledge of the nature and evolution of the magnetic field near the
solar poles is a key for understanding the solar dynamo mechanism. The
amplitude of geomagnetic disturbances during solar minimum, which are
caused by recurrent fast solar wind streams originating from the polar
coronal holes, is well correlated with the height of the following
activity maximum. This suggests that the polar field is directly related
to the dynamo process, presumably as a source of poloidal field to be
wound up by the differential rotation at the shear layer at the base of
the convection zone. Theoretical models indicate that the meridional
circulation pattern found by SOHO could be crucial for the solar dynamo,
by introducing the equatorward drift of the activity belts and
transporting surface magnetic flux toward the poles and downward to the
shear layer. Such models predict a substantial magnetic flux
concentration near the poles. The Solar Orbiter will allow us

-   to measure reliably the strength of the polar field for the first
    time and thus

-   to provide a new and crucial observational constraint for dynamo
    models.

Studies of the evolution of solar features such as active regions,
loops, prominences or sunspots are greatly complicated by the fact that
their evolution time scales are comparable to the solar rotation period.
Thus the evolution is entangled with other effects such as
centre-to-limb variation, foreshortening and projection effects. In
order to disentangle these effects it is necessary to co-rotate with the
Sun. The Solar Orbiter will for the first time provide such an
opportunity and will thus help resolve old and otherwise intractable
problems related to the solar dynamo and the diffusion of the magnetic
field across the solar surface.

**3.5.2 Polar rotation and internal flows**

The internal structure and dynamics of the near polar regions of the Sun
is of paramount importance and perhaps the key to our understanding of
the solar cycle. There are strong empirical indications that the
magnetic properties of the polar regions at solar minimum determine the
strength of the forthcoming solar maximum. It was found that the value
of the geomagnetic aa index at its minimum was related to the sunspot
number during the ensuing maximum. At solar minimum this geomagnetic
index mostly depends on the solar wind from the polar regions. This
means that a new sunspot cycle starts in the polar regions. This idea
was successfully used to make a correct prediction of the current solar
cycle from direct measurements of the polar magnetic field. He called
this the 'Solar Dynamo Amplitude index method'. However, our
understanding of how the solar dynamo actually operates is still very
poor. Studying the internal properties of the polar regions will help to
solve the puzzle of the solar cycle, and it will provide a better
understanding of how the solar dynamo works and how other stars generate
magnetic fields.

The polar regions are extremely dynamical. There is evidence that the
differential rotation near the poles may vary on a short scale. Evidence
was found for a significant variation of the subsurface high-latitude
rotation with the solar cycle. While there are large uncertainties in
these measurements because of our limited view of the polar regions, it
is evident that mapping the internal flows near the poles will improve
our understanding of the mechanism of the differential rotation and its
role in the global circulation of the Sun. Various dynamic phenomena in
the polar regions of the solar atmosphere and corona also show close
connection to the solar cycle, e.g. the \"rush\" of H-alpha filaments to
the poles near the periods of magnetic polarity reversals and the sudden
increase of the coronal temperature during these periods.

However, there is no basic understanding of how these variations are
related to each other and to the internal dynamo processes. Therefore,
studying the polar regions of the Sun is fundamental to one of the key
open questions of solar physics and astrophysics: **How and why does the
Sun vary?**

With the advent of helioseismology from space, our knowledge of the
solar interior has made considerable progress. SOHO has provided
helioseismic data of extreme precision. Large-scale differential
rotation has been mapped in most of the convection zone, and temporal
variations in the zonal flows beneath the Sun\'s surface have been
detected as the present activity cycle unfolds. There have been
unexpected and still controversial discoveries, such as a buried
jet-like flow and a surprisingly slow rotation rate at high latitudes
(see Figure 3.7a).

To complement the techniques of global seismology, which has no
resolution in longitude and cannot distinguish the northern from the
southern hemisphere, recent techniques of local helioseismology have
been developed. In particular, time-distance helioseismology gives the
opportunity to make 3-D tomographic maps of the sub-photospheric
temperature inhomogeneities, magnetic field and flows. An important
result concerns the observation at depth of the poleward meridional
circulation (see Figure 3.7b).

Solar Orbiter offers a unique opportunity to learn about the polar
regions of the Sun, which remain largely unexplored. The main objective
will be to map rotation and meridional flows near the poles using
time-distance helioseismology. One week of data is sufficient to derive
the rotation rate in the upper convection zone, and confirm or refute
the existence of the high-speed jet at 75 degrees. We will also be able
to study the convergence of the meridional flow at the poles and observe
how and where the solar plasma dives back into the Sun. Convective
motions over the poles, in particular supergranulation, will also be
studied. Figure (3.7a) shows an incomplete view of the north pole
simulated with MDI Dopplergrams, where supergranular cells can easily be
identified. Figure (3.7b) demonstrates that only a few hours of
observations are necessary to make time-distance measurements of
supergranules.

![](media/image7.png){width="3.24375in" height="3.372916666666667in"}

![](media/image8.bmp)

Figure 3.7: (a) Solar internal rotation. Lines are contours of constant
angular velocity (in units of nHz). The uncertainty is large in the
shaded region. Note the slow near-surface rotation at high latitudes,
and the jet stream at latitude 75 degrees. (b) Meridional circulation
measured by time-distance helioseismology as a function of latitude. The
scale on the left vertical axis gives the travel time difference in
seconds between waves travelling in the southward and northward
directions. On the right axis is the corresponding velocity, which is an
average in the 34 Mm below the surface.

**3.5.3 Polar observations of CMEs**

The high inclinations to be reached by the Solar Orbiter will allow
unique observations of CMEs and their distributions. This will allow us:

-   to determine for the first time the longitudinal extent of CMEs,

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-   to provide in conjunction with a spacecraft in Earth orbit and
    ground-based observations a 360°-longitude observation of the entire
    Sun,

-   to monitor global effects and to determine the extension of coronal
    waves associated with CMEs,

The high-latitude vantage point will allow us to determine for the first
time the longitudinal extent of CME\'s, a parameter which has never been
measured yet is a basic measure of CME topology and is crucial for the
identification of CME source regions. The high-latitude observations
also allow a complete view of the equatorial corona and thus a global
view of CME activity. This has never been done and is extremely
important for the determination of CME longitudinal distribution, the
occurrence of sympathetic CME activity and the identification of large,
or even global (360 degree), CMEs. Global and large-area effects which
may be related to CMEs can also be determined for the first time, such
as the extent and timing of coronal waves and dimming in coronal lines.

**3.5.4 Solar luminosity variations**

The irradiance of the Sun (i.e. its brightness as measured above the
Earth's atmosphere) is known to vary by 0.1% over the solar cycle. There
is also some evidence for a longer term, secular variation. In spite of
its small magnitude, the irradiance variation is a potential cause for
climate change. Two basic questions need to be answered before we can
reach an understanding of the causes of this variability:

-   How does the solar luminosity (i.e. the radiation escaping in all
    directions) vary? Does it change or is a brightening at the equator
    compensated by a darkening over the poles?

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-   Why is the irradiance variability of the Sun a factor of three
    smaller than that of Sun-like stars?

By carrying out irradiance studies from out of the ecliptic and on the
side away from the Earth, the Solar Orbiter will provide a unique
opportunity to answer these questions. For example, one major difference
between the Sun and Sun-like stars is that whereas we observe the Sun
nearly equator on, stars are on average seen from a latitude of
approximately 30^o^. The Solar Orbiter will reach this latitude in the
course of the extended mission and test whether this inclination effect
really is responsible for the Sun\'s anomalous behaviour.

The \"solar constant\", or total solar irradiance, is a measure of the
total radiative output of the Sun from the X-rays over the visible to
the infrared wavelengths and, as we have learned from satellite
measurements, is changing on two different time-scales. The solar output
and activity exhibit variations that are anti-correlated over days, when
large dark sunspots cross the disk, but they are correlated over the
solar cycle. This apparent contradiction can be solved by fully
understanding the role that the excess energy in faculae and the deficit
energy in sunspots are playing on the different time scales. The
variability of solar irradiance over a solar cycle, approximately 0.1 %,
is too small to affect significantly the Earth\'s climate, however
changes over longer periods might be responsible for larger global
climatic variations.

The influence of faculae and sunspots on the value of the solar constant
value be eliminated only when observing the Sun above the poles, where
these features are not expected to occur. Therefore, the Solar Orbiter,
at its highest inclination with respect to the ecliptic, will allow a
measurement of the \"true solar irradiance\" (TSI), as well as a
quantification of the effects of solar activity through a comparison of
these high-latitude measurements with the traditional irradiance
measurements simultaneously made in the ecliptic.

The TSI record is not only a highly sensitive diagnostic tool for the
study of solar cycle related changes of the Sun, but also an important
contribution to the issues of natural versus anthropogenic global
change. For the latter, information over time scales of centuries is
needed, which obviously cannot be assessed from measurements during a
few 11-year solar activity cycles. Thus, the understanding of how the
Sun modulates the irradiance and possibly its luminosity is of paramount
importance.

**3.5.5 Polar observation of the fast solar wind**

The Solar Orbiter will for the first time image directly the polar
regions, where the fast solar wind is formed, from an out-of-ecliptic
position (with inclinations ranging from 6.7° to 23.4° during the
nominal mission and up to 38.3° at the end of the extended mission). The
almost radial expansion of the fast solar wind has not yet been
determined, since the expansion over the poles is essentially
perpendicular to the line of sight (LOS) of spectroscopic instruments
observing from the ecliptic, and therefore the Doppler effect is
negligible. With increasing inclination angle of its orbit, the Solar
Orbiter will perform out-of-ecliptic measurements and

-   render possible velocity measurements due to a sizable flow
    component along the LOS,

-   image the border of polar coronal holes and their deformation in
    response to the coronal waves (discovered with SOHO) that develop
    during the CMEs,

-   observe globally the entire streamer belt in the extended corona
    (beyond 2.6 R~s~ at the end of the nominal mission and beyond 1.3
    R~s~ at the end of the extended mission),

-   find the magnetic signal associated with coronal plumes and clarify
    their nature.

    **3.5.6 Secular variation of the heliospheric magnetic flux**

The solar wind drags magnetic flux out of the Sun into the heliosphere
in the form of the weak interplanetary magnetic field. Magnetic flux is
also carried into the heliosphere by the sporadic and violent coronal
mass ejections, that are frequently observed also during the minimum of
solar activity. Some of the solar magnetic energy is then transferred to
the near-Earth environment through magnetic reconnection of the
interplanetary and local Earth\'s magnetic field. Recently, it has been
shown that the total magnetic flux leaving the Sun has increased by a
factor 1.4 since 1964. Indirect interplanetary magnetic field
measurements seem to indicate that the increase has been a factor of 2.3
over a century. Consequently, a doubling of the Sun\'s coronal magnetic
field may have occurred during the past century. The change in magnetic
flux may be related to a long-term change in the Sun\'s dynamo action.
The possible influence of this secular magnetic flux variation on the
global climate on Earth has not yet been quantified.

The Solar Orbiter will determine with unprecedented accuracy the
relative contributions of the solar wind and coronal mass ejections to
the interplanetary magnetic field flux. From the out-of-ecliptic vantage
point the Solar Orbiter for the first time will in fact allow us to
derive the longitudinal extent of coronal mass ejections and therefore
give a much more accurate assessment of the magnetic flux dragged into
the heliosphere during such huge events. Present space-borne and
ground-based coronagraphs can only measure the latitudinal extent of
coronal mass ejections. Furthermore, in the periods of corotation with
the Sun, the ability of Solar Orbiter to observe the long-term evolution
of the magnetic configuration of a coronal streamer, that remains in the
plane of the sky for many days, will allow us to study the processes
that lead to magnetic reconnection and to determine the contribution to
the heliospheric magnetic flux of the plasmoids (discovered with SOHO),
when detaching from the streamers as a consequence of reconnection
events.