#1 WHAT DRIVES THE SOLAR WIND AND WHERE DOES THE CORONAL MAGNETIC FIELD ORIGINATE FROM?

    Non-stop streams of electrons and ions pour from the Sun in all directions, in slow and fast electric wind streams travelling at speeds of 300 to 800 kilometres per second. This solar wind and its variations have a profound effect on the Earth's space environment, as investigated by the Cluster and Double Star missions.

    At large scales, the structure of the solar wind and heliospheric magnetic field and their mapping to the solar corona are reasonably well understood. The origin of the slow solar wind, however, is not yet clear, and the mechanisms that heat the corona and accelerate the solar wind remain a mystery. To understand the effects of the Sun on the heliosphere, we need to understand the physics which connects the plasma at the solar surface and the heating and acceleration of the escaping solar wind. This will improve understanding in the future of how stars in general lose mass and angular momentum to stellar winds.

    Moving relatively slowly over the solar surface near perihelion, Solar Orbiter will measure how the properties of the solar wind vary, depending on the changing properties of its source region. The results should help distinguish between competing models of solar wind generation. It will be possible to reconstruct the coronal magnetic field by extrapolation, with well-defined boundary conditions, using the photospheric magnetic field measurements, together with those made in situ.

     

     #2 HOW DO SOLAR TRANSIENTS DRIVE HELIOSPHERIC VARIABILITY?

    The Sun exhibits transient phenomena, such as flares, coronal mass ejections (CMEs), eruptive prominences, and shock waves. These events affect the structure and dynamics of the outflowing solar wind and so eventually affect Earth’s magnetosphere and upper atmosphere, creating what is known as 'space weather'.

    There are fundamental questions to be answered about these transient events and how they develop if in the future we are to attempt to predict such occurrences or their effects on geospace and the heliosphere. This will also help our understanding of other stars that exhibit transient behaviour such as flaring.

    Operating close to the solar sources of transients, Solar Orbiter will be able to sample the fields and plasmas in their pristine state. This will allow Solar Orbiter both to determine the input to the heliosphere, and to measure the heliospheric consequences of eruptive events.

     

    #3 HOW DO SOLAR ERUPTIONS PRODUCE ENERGETIC PARTICLE RADIATION THAT FILLS THE HELIOSPHERE?

    The Sun is the most powerful particle accelerator in the Solar System. It routinely produces energetic particle radiation at speeds close to the speed of light. This radiation is sufficiently energetic to be detected on Earth even under the protection of our magnetic field and atmosphere. Solar Energetic Particle (SEP) events can severely affect space hardware, disrupt radio communications, and cause re-routing of commercial air traffic away from polar regions.

    Almost the entire Solar Orbiter payload contributes to revealing the origins of SEP events. By examining the particles themselves and by taking measurements and images at different wavelengths of the environment they come from, Solar Orbiter will reveal which of the candidate mechanisms for SEP production comes closest to nature.

     

    #4 HOW DOES THE SOLAR DYNAMO WORK AND DRIVE CONNECTIONS BETWEEN THE SUN AND THE HELIOSPHERE?

    The Sun's magnetic field dominates the solar atmosphere. It produces all the observed energetic phenomena, and displays an 11-year activity cycle. Despite notable advances in our knowledge and understanding of solar magnetism using observations from the Ulysses, SOHO, and Hinode missions - and recent theoretical models and numerical simulations – the details of the so-called "solar dynamo" processes that power the Sun's magnetic activity cycle are not yet fully understood.

    The Sun's global magnetic field is generated by a dynamo generally believed to be seated in the tachocline, the shear layer at the base of the convection zone. According to flux-transport dynamo models, meridional circulation and other near-surface flows transport magnetic flux from decaying active regions to the poles. There subduction carries it to the tachocline to be reprocessed for the next cycle.

    A key objective of the Solar Orbiter mission is to measure and characterize the flows that transport the solar magnetic fields: complex near-surface flows, the meridional flow, and the differential rotation at all latitudes and radii. We need detailed knowledge of magnetic flux transport near the poles to understand the solar dynamo and the regular polarity reversal of the global magnetic field. Solar Orbiter's imaging of the properties and dynamics of the polar region during the out-of-the-ecliptic phase of the mission (reaching heliographic latitudes of 25° during the nominal mission and as high as 34° during the extended mission) will provide vital new constraints for models of the solar dynamo.