Transport coefficients of low‐energy cosmic rays in interplanetary space

Abstract

The propagation of energetic particles along and across the interplanetary magnetic field is governed by the large-scale field geometry and by scattering in small-scale turbulent fields. Values of the scattering mean free path parallel to the field, h•(R), are reviewed in prompt solar bursts and nonimpulsive (corotating) events. Analysis of intensity and anisotropy profiles in combination is a powerful tool for elucidating X•(R). A consensus is found: at 1 AU, X• = 0.08-0.3 AU over a wide range of rigidity, R = 5 x 10 -4 to 5 GV. Efforts to explain the discrepancy between empirical values of X• and scattering theory are discussed. Quantitative measures of x• in rare scatter-free events, where h• >• 1 AU, are discussed because they can provide important details of the scattering process and the magnetic power spectra. Cross-field diffusion due to random walk of field lines is revisited. Recent values deduced from magnetic power spectra in interplanetary space, magnetic diffusion at the sun, Jovian electron propagation, and cosmic ray events are evaluated. Again, a consensus is sought, and a reasonable mean is K•_r/13 = 102• cm 2 s -•. Previous arguments against a significant Kñ r are reassessed, including the problem of the persistence of intensity fluctuations in cosmic ray events. Combining the consensus for K•_r/13 with that for h• gives K•_r/K• < 0.1 at 1 AU, and thus neglect of K_• r in the modeling of solar cosmic ray events appears justified (although account needs to be taken of coronal propagation). The outlook for the future includes better empirical values of h• down to Ep ---10 keV and Ee '" 1 keV, comparison with scattering theories at these energies, and comparison between empirical and theoretical X• in other regions such as the magnetosheath and upstream solar wind. A. terms of K•_r. 1977. Zwickl and Webber [1977] studied several different characteristics in many flare events and reached an important consensus for the value of K• at low proton energies, 3-30 MeV.

Latitudinal and radial gradients of anomalous and galactic cosmic rays in the outer heliosphere

“…Using this scaling for the range of Ki/~ in Ki/~ = 0.5-2 X 10 21 cm 2 /sec at 1 AU, consistent with the range of values summarized by Palmer [1982]. Latitudinal gradients may also result from a latitudinal variation in the solar wind velocity (see, e.g., Newkirk and Fisk [1985] and McDonald and Lal [1986]).…”

The latitudinal transport of energetic particles associated with corotating interaction regions

“…Two different mean free paths of 1-MeV particles are considered in this study: 0.2 AU and 1.0 AU. The former is consistent with Palmer "consensus" values [Palmer, 1982], while the latter is considered only to determine the effect of using a long mean free path.…”

Early propagation of energetic particles across the mean field in turbulent plasmas

“…They remain on the fieldlines from hours up to tens of hours for protons of 1-100 MeV, in turbulence consistent with proton parallel mean free paths λ ∼ 0.1 − 1 AU, range consistent with those measured during solar energetic particle events (e.g. Palmer 1982;Torsti, Riihonen & Kocharov 2004) and interplanetary turbulence conditions (r.g. Pei et al 2010;Laitinen et al 2016;Strauss, Dresing & Engelbrecht 2017).…”

“…Pei et al 2010;Laitinen et al 2016;Strauss, Dresing & Engelbrecht 2017) and SEP observation analysis (e.g. Palmer 1982;Torsti, Riihonen & Kocharov 2004).…”