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Magnetic field line random walk and solar energetic particle path lengths. Stochastic theory and PSP/IS⊙IS observations

Chhiber, R. and Matthaeus, W. H. and Cohen, C. M. S. and Ruffolo, D. and Sonsrettee, W. and Tooprakai, P. and Seripienlert, A. and Chuychai, P. and Usmanov, A. V. and Goldstein, M. L. and McComas, D. J. and Leske, R. A. and Szalay, J. R. and Joyce, C. J. and Cummings, A. C. and Roelof, E. C. and Christian, E. R. and Mewaldt, R. A. and Labrador, A. W. and Giacalone, J. and Schwadron, N. A. and Mitchell, D. G. and Hill, M. E. and Wiedenbeck, M. E. and McNutt, R. L. and Desai, M. I. (2021) Magnetic field line random walk and solar energetic particle path lengths. Stochastic theory and PSP/IS⊙IS observations. Astronomy and Astrophysics, 650 . Art. No. A26. ISSN 0004-6361. doi:10.1051/0004-6361/202039816.

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Context. In 2020 May-June, six solar energetic ion events were observed by the Parker Solar Probe/IS⊙IS 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 path length at an event onset greatly exceeds the radial distance from the Sun for these events. Methods. We developed analytical estimates of the average increase in path length of random-walking magnetic field lines, relative to the unperturbed mean field. Monte Carlo simulations of field line and particle trajectories in a model of solar wind turbulence were used to validate the formalism and study the path lengths of particle guiding-center and full-orbital trajectories. The formalism was implemented in a global solar wind model, and the results are compared with ion path lengths inferred from IS⊙IS observations. Results. Both a simple estimate and a rigorous theoretical formulation are obtained for field-lines’ path length increase as a function of path length along the large-scale field. From simulated field line and particle trajectories, we find that particle guiding centers can have path lengths somewhat shorter than the average field line path length, while particle orbits can have substantially longer path lengths 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 due to pitch angle scattering. 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.

Item Type:Article
Related URLs:
URLURL TypeDescription ItemIS⊙IS data and visualization tools ItemNASA Space Physics Data Facility
Cohen, C. M. S.0000-0002-0978-8127
Leske, R. A.0000-0002-0156-2414
Cummings, A. C.0000-0002-3840-7696
Roelof, E. C.0000-0002-2270-0652
Christian, E. R.0000-0003-2134-3937
Mewaldt, R. A.0000-0003-2178-9111
Labrador, A. W.0000-0001-9178-5349
Giacalone, J.0000-0002-0850-4233
Schwadron, N. A.0000-0002-3737-9283
Mitchell, D. G.0000-0003-1960-2119
Hill, M. E.0000-0002-5674-4936
Wiedenbeck, M. E.0000-0002-2825-3128
McNutt, R. L.0000-0002-4722-9166
Desai, M. I.0000-0002-7318-6008
Additional Information:© ESO 2021. Article published by EDP Sciences. Received 30 October 2020; Accepted 17 January 2021. Published online 02 June 2021. We thank Junxiang Hu for useful discussions. This research is partially supported by the Parker Solar Probe mission and the IS⊙IS project (contract NNN06AA01C) and a subcontract to University of Delaware from Princeton University (SUB0000165). Additional support is acknowledged from the NASA Living With a Star (LWS) program (NNX17AB79G) and HSR program (80NSSC18K1210 & 80NSSC18K1648) and Thailand Science Research and Innovation (RTA6280002). M.L.G. acknowledges support from the PSP FIELDS MAG team. The IS⊙IS data and visualization tools are available to the community at; data are also available via the NASA Space Physics Data Facility. PSP was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NASA’s LWS program (contract NNN06AA01C). Support from the LWS management and technical team has played a critical role in the success of the PSP mission.
Funding AgencyGrant Number
Princeton UniversitySUB0000165
Thailand Science Research and InnovationRTA6280002
Subject Keywords:turbulence – solar wind – Sun: magnetic fields – diffusion – Sun: flares – acceleration of particles
Record Number:CaltechAUTHORS:20210608-140607023
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109449
Deposited By: Tony Diaz
Deposited On:09 Jun 2021 18:11
Last Modified:09 Jun 2021 18:11

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