Description of the objective:
At any given time, magnetic field lines stretch out from the Sun deep into interplanetary space, carried by the solar wind. However, these fields must eventually disconnect from the Sun, resulting in a complex, tangled magnetic topology in the heliosphere, with important consequences for energetic particle propagation. It is essential to quantify the large-scale connectivity, but this is remarkably difficult. The magnetic field polarity is an important indicator of the solar region of origin of a packet of solar wind, but cannot determine its connectivity alone. Signatures of connectivity such as suprathermal electron streaming (e.g. Zurbuchen and Richardson, 2006) are difficult to interpret. Indeed, (Owens and Crooker, 2007) suggests that electron scattering and fading can result in signatures which look like disconnected field lines, but are actually due to interchange reconnection. Only by simultaneously measuring the streaming electrons and magnetic field polarity over a wide range of distances, and particularly close to the Sun, can we distinguish dis- and re-connections from electron fade-out.
Evidence of reconnection at solar wind magnetic discontinuities (e.g. Gosling et al., 2007) demonstrates that the connectivity of the IMF can change in space. How often does this occur close to the Sun, where the solar wind is much more dynamic than at 1 AU? There is also evidence for folds in the magnetic field, from cross-helicity (Balogh et al., 1999) and proton-alpha streaming data (Yamauchi et al., 2004): what is their origin? Are they related to chromospheric reconnection features such as jets (Shibata et al., 2007) or velocity shears (Landi et al., 2006)?
We have to search for local reconnection events using MAG and SWA in order to determine their radial distribution and significance for the connectivity of the solar wind.
We will use signatures of the varying connectivity of the solar wind (e.g. suprathermal electrons), combined with the magnetic field orientation and other measures of source polarity (e.g. alpha/proton streaming and the normalized cross helicity) to search for bends, folds and small scale polarity reversals. In this way, we will determine the small-scale polarity structure within coronal hole flows and its relation to the global field.
This is an in situ objective and can be addressed with the I_DEFAULT SOOP as well as during the connectivity operating plans for the fast and slow winds, e.g. L_SMALL_HRES_HCAD_Fast-Wind, L_SMALL_HRES_HCAD_Slow-Wind-Connection, L_SMALL_MRES_MCAD_Connection-Mosaic.
Relevant SOOPs: I_DEFAULT , L_SMALL_MRES_MCAD_Connection-Mosaic
Observational analysis of coronal inflows from Sheeley and Wang, Apj, 2001 (doi:10.1086/338104) and more theoretical arguments from Owens, Crooker and Lockwood, JGR, 2011 (doi:10.1029/2010JA016039) suggest that disconnection of open solar flux occurs preferentially when the heliospheric current sheet is highly inclined to the solar equator. Thus observational campaigns to measure this phenomenon using the remote sensing instruments should focus on such regions. Determining when the spacecraft is directly connected to such a region would allow in situ measurement.
Mathew Owens, 14/11/17