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Correcting for interplanetary scattering in velocity dispersion analysis of solar energetic particles  

Timo Laitinen   Submitted: 2015-04-24 06:41

To understand the origin of Solar Energetic Particles (SEPs), we must study their injection time relative to other solar eruption manifestations. Traditionally the injection time is determined using the Velocity Dispersion Analysis (VDA) where a linear fit of the observed event onset times at 1 AU to the inverse velocities of SEPs is used to derive the injection time and path length of the first-arriving particles. VDA does not, however, take into account that the particles that produce a statistically observable onset at 1 AU have scattered in the interplanetary space. We use Monte Carlo test particle simulations of energetic protons to study the effect of particle scattering on the observable SEP event onset above pre-event background, and consequently on VDA results. We find that the VDA results are sensitive to the properties of the pre-event and event particle spectra as well as SEP injection and scattering parameters. In particular, a VDA-obtained path length that is close to the nominal Parker spiral length does not imply that the VDA injection time is correct. We study the delay to the observed onset caused by scattering of the particles and derive a simple estimate for the delay time by using the rate of intensity increase at the SEP onset as a parameter. We apply the correction to a magnetically well-connected SEP event of June 10 2000, and show it to improve both the path length and injection time estimates, while also increasing the error limits to better reflect the inherent uncertainties of VDA.

Authors: T. Laitinen, K. Huttunen-Heikinmaa, E. Valtonen, S. Dalla
Projects: GOES X-rays ,SoHO-LASCO

Publication Status: Accepted for publication in Astrophysical Journal
Last Modified: 2015-04-25 18:48
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Energetic particle cross-field propagation early in a solar event  

Timo Laitinen   Submitted: 2013-07-25 02:53

Solar energetic particles (SEPs) have been observed to easily spread across heliographic longitudes, and the mechanisms responsible for this behaviour remain unclear. We use full-orbit simulations of a 10 MeV proton beam in a turbulent magnetic field to study to what extent the spread across the mean field can be described as diffusion early in a particle event. We compare the full-orbit code results to solutions of a Fokker-Planck equation including spatial and pitch angle diffusion, and of one including also propagation of the particles along random-walking magnetic field lines. We find that propagation of the particles along meandering field lines is the key process determining their cross-field spread at 1 AU at the beginning of the simulated event. The mean square displacement of the particles an hour after injection is an order of magnitude larger than that given by the diffusion model, indicating that models employing spatial cross-field diffusion cannot be used to describe early evolution of an SEP event. On the other hand, the diffusion of the particles from their initial field lines is negligible during the first 5 hours, which is consistent with the observations of SEP intensity dropouts. We conclude that modelling SEP events must take into account the particle propagation along meandering field lines for the first 20 hours of the event.

Authors: T. Laitinen, S. Dalla, and M.S. Marsh
Projects: None

Publication Status: Accepted for publication in Astrophysical Journal Letters
Last Modified: 2013-07-25 09:55
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Energetic Particle Diffusion In Critically Balanced Turbulence  

Timo Laitinen   Submitted: 2013-04-25 05:00

Observations and modelling suggest that the fluctuations in magnetised plasmas exhibit scale-dependent anisotropy, with more energy in the fluctuations perpendicular to the mean magnetic field than in the parallel fluctuations and the anisotropy increasing at smaller scales. The scale-dependence of the anisotropy has not been studied in full-orbit simulations of particle transport in turbulent plasmas so far. In this paper, we construct a model of critically balanced turbulence, as suggested by cite{GoSr1995}, and calculate energetic particle spatial diffusion coefficients using full-orbit simulations. The model uses an enveloped turbulence approach, where each 2-dimensional wave mode with wavenumber k_perp is packed into envelopes of length L following the critical balance condition, Lpropto k_perp-2/3, with the wave mode parameters changing between envelopes. Using full-orbit particle simulations, we find that both the parallel and perpendicular diffusion coefficients increase by a factor 2, compared to previous models with scale-independent anisotropy.

Authors: Laitinen, T., Dalla, S., Kelly, J., and Marsh, M.
Projects: None

Publication Status: Published in ApJ, 764:168, 2013, doi:10.1088/0004-637X/764/2/168
Last Modified: 2013-04-25 09:52
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Energetic Particle Diffusion In Structured Turbulence  

Timo Laitinen   Submitted: 2012-04-02 07:49

In the full-orbit particle simulations of energetic particle transport in plasmas, the plasma turbulence is typically described as a homogeneous superposition of linear Fourier modes. The turbulence evolution is, however, typically a nonlinear process, and, particularly in the heliospheric context, the solar wind plasma is inhomogeneous due to the transient structures, as observed by remote and in-situ measurements. In this work, we study the effects of the inhomogeneities on energetic particle transport by using spatially distributed, superposed turbulence envelopes. We find that the cross-field transport is significantly reduced, when compared to the results obtained with homogeneous turbulence. The reduction can reach an order of magnitude when the enveloping breaks the wave phase coherence along the mean magnetic field direction.

Authors: Laitinen, T., Dalla, S. and Kelly, J.
Projects: None

Publication Status: ApJ (Published)
Last Modified: 2012-04-04 08:13
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Abstracts by Author
Correcting for interplanetary scattering in velocity dispersion analysis of solar energetic particles
Energetic particle cross-field propagation early in a solar event
Energetic Particle Diffusion In Critically Balanced Turbulence
Energetic Particle Diffusion In Structured Turbulence

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