Present state of knowledge:
Magnetic flux is transported into the heliosphere both by the solar wind, in the form of open flux carried mostly by the fast wind from polar coronal holes, and by coronal mass ejections, which drag closed flux with them as they propagate into the heliosphere. At some point, the closed flux introduced by CMEs must be opened to avoid an unsustainable buildup of magnetic flux in the heliosphere. Measurements of the magnetic flux content of the heliosphere from near the Earth show that the total amount of magnetic flux in the solar system changes over the solar cycle. Longer-term variations are also known to occur. Proxies such as geomagnetic activity and cosmic ray fluxes provide evidence that the average IMF strength has increased substantially in the last 100 years, perhaps by as much as a factor of 2. Surprisingly, however, during the recent solar minimum, the IMF strength is lower than at any time since the beginning of the space age.
The relative contribution of the solar wind and CMEs to the heliospheric magnetic flux budget is an unresolved question, as is the process by which the flux added by the CMEs is removed. Models to explain the solar cycle variation assume a background level of open flux, to which CMEs add extra flux during solar maximum, increasing the intensity of the IMF. The exceptionally low intensity of the IMF during the last minimum has been attributed to the low rate of CME occurrence [Owens et al. 2008]. Alternatively, there may simply be no ‘background’ open flux level.
There is evidence that the flux introduced into the heliosphere by CMEs may be removed by magnetic reconnection within the trailing edges of CMEs, which disconnects the CME from the Sun or by interchange reconnection closer to the solar surface [e.g., Owens and Crooker 2006]. Recent observations show that the reconnection process occurs quite often in the solar wind, even when the magnetic field is not under compression. However, the rate and/or locations at which reconnection generally removes open flux are not at present known.
Together with magnetic flux, the solar wind and CMEs carry magnetic helicity away from the sun. Helicity is a fundamental property of magnetic fields in natural plasmas, where it plays a special role because it is conserved not only by the ideal dynamics but also during the relaxation which follows instabilities and dissipation. Helicity is injected into the corona when sunspots and active regions emerge, via the twisting and braiding of magnetic flux. During the coronal heating process, the overall helicity is conserved and tends to accumulate at the largest possible scales. It is natural to assume that critical helicity thresholds may be involved in the triggering of CMEs, but how solar eruptions depend on the relative amounts of energy and helicity injection during active region emergence and evolution is unknown. Yet this understanding could be a crucial element in the prediction of large solar events.
How Solar Orbiter will address this question:
Fundamental to the question of the contribution of CMEs to the heliospheric flux budget is the flux content of individual events. Encountering CMEs close to the Sun before interplanetary dynamics affect their structure, Solar Orbiter will measure their magnetic flux content directly; comparisons with remote-sensing measurements of their source regions will clarify the relation between CME flux and the eruption process. As Solar Orbiter moves through the inner heliosphere, it will encounter CMEs at different solar distances, making it possible to quantify the effect of interplanetary dynamics on their apparent flux content.
The flux carried outwards by CMEs must eventually disconnect completely from the Sun, or interchange reconnects with existing open field lines. Solar Orbiter will diagnose the magnetic connectivity of the solar wind and CME plasma using suprathermal electron and energetic particle measurements. These particles, which stream rapidly along the magnetic field from the Sun, indicate whether a magnetic flux tube is connected to the Sun at one end, at both ends, or not at all. These particles disappear when the field is completely disconnected, or may reverse their flow direction as a result of interchange reconnection. However, scattering and reflection due to curved, tangled, or compressed magnetic field lines act to smear out these signatures with increasing solar distance, leading to ambiguity in connectivity measurements. Solar Orbiter, by traveling close to the Sun before this scattering is significant, will determine the original level of magnetic connectivity; covering a wide range of distances in the inner heliosphere, the spacecraft will measure how the connectivity changes as field lines are carried away from the Sun.
Solar Orbiter will also directly sample reconnection regions in the solar wind as they pass the spacecraft, determining their occurrence rates in the inner heliosphere as a function of distance and testing theories of CME disconnection by searching for reconnection signatures in the tails of CMEs.
The contribution to the heliospheric magnetic flux of small scale plasmoids, ejected from the tops of streamers following reconnection events, is unclear. Solar Orbiter, slowly moving above the solar surface during perihelion passes, will determine the magnetic structure, connectivity, and plasma properties including the composition of these ejecta, using spectroscopic imaging observations to unambiguously link them to their source regions.
To assess the role of CMEs in maintaining the solar magnetic helicity balance, Solar Orbiter will compare the helicity content of active regions as determined from remote sensing of the photospheric magnetic field with that of magnetic clouds measured in situ. Such a comparison requires both extended remote-sensing observations of the same active region over the region’s lifetime and in-situ measurements of magnetic clouds from a vantage point as close to the solar source as possible. Around its perihelia, Solar Orbiter will ‘dwell’ over particular active regions and observe the emergent flux for a longer interval (more than 22 days) than is possible from 1 AU, where perspective effects complicate extended observations. The resulting data will be used to calculate the helicity content of an active region, track its temporal variation, and determine the change in helicity before and after the launch of any CMEs. Should a magnetic cloud result from an eruptive event in the active region over which Solar Orbiter is dwelling, the relatively small heliocentric distances between the solar source and the spacecraft will make it highly probable that Solar Orbiter will directly encounter the magnetic cloud soon after its release. Determination of the cloud’s properties and connectivity through Solar Orbiter’s in-situ particle-and-fields measurements will enable the first-ever comparison of a magnetic cloud in a relatively unevolved state with the properties of the solar source, an impossibility with measurements made at 1 AU. The comparison of the helicity change in the source region with the value measured in the magnetic cloud will provide insight into the role of CMEs in the helicity balance of the Sun.
Detailed sub-objectives:
- 2.2.1 How do CMEs contribute to the global evolution of magnetic flux in the heliosphere?
- 2.2.2 What is the role of ICMEs in the Sun’s magnetic cycle?