1 research outputs found
Magnetic Field Line Random Walk and Solar Energetic Particle Path Lengths: Stochastic Theory and PSP/ISoIS Observation
Context:In 2020 May-June, six solar energetic ion events were observed by the
Parker Solar Probe/ISoIS instrument suite at 0.35 AU from the Sun. From
standard velocity-dispersion analysis, the apparent ion path length is 0.625 AU
at the onset of each event. Aims:We develop a formalism for estimating the path
length of random-walking magnetic field lines, to explain why the apparent ion
pathlength at event onset greatly exceeds the radial distance from the Sun for
these events. Methods:We developed analytical estimates of the average increase
in pathlength of random-walking magnetic field lines, relative to the
unperturbed mean field. Monte Carlo simulations of fieldline and particle
trajectories in a model of solar wind turbulence are used to validate the
formalism and study the path lengths of particle guiding-center and
full-orbital trajectories. The formalism is implemented in a global solar wind
model, and results are compared with ion pathlengths inferred from ISoIS
observations. Results:Both a simple estimate and a rigorous theoretical
formulation are obtained for fieldlines' pathlength increase as a function of
pathlength along the large-scale field. From simulated fieldline and particle
trajectories, we find that particle guiding centers can have pathlengths
somewhat shorter than the average fieldline pathlength, while particle orbits
can have substantially larger pathlengths due to their gyromotion with a
nonzero effective pitch angle. Conclusions:The long apparent path length during
these solar energetic ion events can be explained by 1) a magnetic field line
path length increase due to the field line random walk, and 2) particle
transport about the guiding center with a nonzero effective pitch angle. Our
formalism for computing the magnetic field line path length, accounting for
turbulent fluctuations, may be useful for application to solar particle
transport in general