Flux tubes and energetic particles in Parker Solar Probe orbit 5: magnetic helicity - PVI method and ISOIS observations

Plasma turbulence can be viewed as a magnetic landscape populated by large- and small-scale coherent structures, consisting notionally of magnetic flux tubes and their boundaries. Such structures exist over a wide range of scales and exhibit diverse morphology and plasma properties.  Moreover, interactions of particles with turbulence may involve temporary trapping in, as well as exclusion from, certain regions of space, generally controlled by the topology and connectivity of the magnetic field.  In some cases, such as SEP "dropouts'' the influence of the magnetic structure is dramatic; in other cases, it is more subtle, as in edge effects in SEP confinement. With Parker Solar Probe now closer to the sun than any previous mission, novel opportunities are available for examination of the relationship between magnetic flux structures and energetic particle populations. 

We present a method that is able to characterize both the large- and small-scale structures of the turbulent solar wind, based on the combined use of a filtered magnetic helicity (H_m) and the partial variance of increments (PVI). The synergistic combination with energetic particle measurements suggests whether these populations are either trapped within or excluded from the helical structure.

This simple, single-spacecraft technique exploits the natural tendency of flux tubes to assume a cylindrical symmetry of the magnetic field about a central axis. Moreover, large helical magnetic tubes might be separated by small-scale magnetic reconnection events (current sheets) and present magnetic discontinuity with the ambient solar wind. The method was first validated via direct numerical simulations of plasma turbulence and then applied to data from the Parker Solar Probe (PSP) mission. In particular, ISOIS energetic particle (EP) measurements along with FIELDS magnetic field measurements and SWEAP plasma moments, are enabling characterization of observations of EPs closer to their sources than ever before.
 
This novel analysis, combining H_m and PVI methods, reveals that a large number of flux tubes populate the solar wind and continuously merge in contact regions where magnetic reconnection and particle acceleration may occur. Moreover, the detection of boundaries, correlated with high-energy particle measurements, gives more insights into the nature of such helical structures as "excluding barriers'' suggesting a strong link between particle properties and fields topology. This research is partially supported by the Parker Solar Probe project.