Present state of knowledge:
Meridional circulation transports the surface magnetic flux toward the poles, where a concentration of magnetic flux is expected to occur. However, because of the directional sensitivity of the Zeeman effect and magnetic polarity cancellation resulting from geometric foreshortening, present-day observations from the ecliptic at 1 AU can provide only a poor representation of the polar magnetic field. The high resolution of Hinode’s Solar Optical Telescope (SOT) can partly overcome the second disadvantage (Tsuneta et al. 2008), but not the first. Consequently, an accurate quantitative estimate of the polar magnetic field remains a major and as yet unattained goal.
The polar field is directly related to the dynamo process, presumably as a source of poloidal field that is wound up by the differential rotation in the shear layer at the base of the convection zone. The distribution of the magnetic field at the poles drives the formation and evolution of polar coronal holes, polar plumes, X-ray jets, and other events and structures that characterize the polar corona. Polar coronal holes have been intensively studied from the non-ideal vantage point offered by the ecliptic, but never imaged from outside the ecliptic. Consequently the distribution of the polar field and the origin of polar structures are only poorly determined. The fast solar wind is associated with open field lines inside coronal holes, whereas at least parts of the slow solar wind are thought to emanate from the coronal hole boundaries. Understanding the interaction of open and closed field lines across these boundaries is of paramount importance for elucidating the connection between the solar magnetic field and the heliosphere.
The magnetic flux in the heliosphere varies with the solar cycle (Owens et al. 2008). There is evidence that the heliospheric magnetic flux has increased substantially in the last hundred years, perhaps by as much as a factor of two (Lockwood et al. 1999; Rouillard et al. 2007), possibly due to a long-term change in the Sun’s dynamo action. As already noted, however, the interplanetary magnetic field was dramatically lower than expected during the last solar minimum. Models based on the injection of flux into the heliosphere by coronal mass ejections cannot explain this reduction, and it is becoming clear that the processes by which flux is added to and removed from the heliosphere are more complex than previously thought.
How Solar Orbiter will address this question:
Solar Orbiter’s comprehensive imaging instruments will characterize the properties and dynamics of the polar regions for the first time, including magnetic fields, plasma flows, and temperatures. Solar Orbiter will make the first reliable measurements of the amount of polar magnetic flux, its spatial distribution and its evolution (by comparing results from different orbits), providing an independent constraint on the strength and direction of the meridional flow near the pole. The evolution of Solar Orbiter’s orbit to higher heliographic latitudes will make it possible to study the transport of magnetic flux from the activity belts toward the poles, which drives the polarity reversal of the global magnetic field (see Wang et al. 1989; Sheeley 1991; Makarov et al. 2003). From its viewpoint outside the ecliptic, Solar Orbiter will probe the cancellation processes that take place when flux elements of opposite polarity meet as part of the polarity reversal process. Joint observations from Solar Orbiter and spacecraft in the ecliptic will determine, with high accuracy, the transversal magnetic field, which is notoriously difficult to measure, along with derived quantities such as the electric current density.
Solar Orbiter will measure the photospheric magnetic field at the poles, while simultaneously imaging the coronal and heliospheric structure at visible and EUV wavelengths. In addition, as the spacecraft passes through the mid-latitude slow/fast wind boundary at around 0.5 AU, the field and plasma properties of the solar wind will be measured. With the help of magnetic field extrapolation methods these observations will, for the first time, allow the photospheric and coronal magnetic field in polar coronal holes to be studied simultaneously and the evolution of polar coronal hole boundaries and other coronal structures to be investigated.
Solar Orbiter’s observations from progressively higher heliographic latitudes (25° by the end of the nominal mission) will enable the first coordinated investigation (jointly with spacecraft in the ecliptic) of the three-dimensional structure of the inner heliosphere. These observations will reveal the links between the Sun’s polar regions and the properties of the solar wind and interplanetary magnetic field, in particular the heliospheric current sheet, which is used as a proxy for the tilt of the solar magnetic dipole. In addition, Solar Orbiter will pass both north and south of the solar equatorial plane in each orbit, with repeated transits through the equatorial streamer belt and through the slow/fast wind boundary at mid-latitudes into the polar wind, making it possible to follow the evolution of the solar wind and interplanetary magnetic field as well as of the sources in the polar coronal holes. Ulysses has shown that poleward of the edge of coronal holes the properties of the solar wind are relatively uniform, so that Solar Orbiter only needs to reach heliographic latitudes just above the coronal hole edge to enter the high-speed solar wind. The orbital inclination of 25° reached during the nominal mission is sufficiently high to satisfy this constraint.
- 4.2.1 Probability density function (PDF) of solar high-latitude magnetic field structures.
- 4.2.2 Basic properties of solar high-latitude magnetic field structures
- 4.2.3 Probe the structure in deep layers of the Sun