The magnetic field experiment on ACE provides continuous measurements of the local magnetic field in the interplanetary medium. These measurements are essential in the interpretation of simultaneous ACE observations of energetic and thermal particles distributions. The experiment consists of a pair of twin, boom-mounted, triaxial fluxgate sensors which are located 165 inches (= 4.19 m) from the center of the spacecraft on opposing solar panels. The electronics and digital processing unit (DPU) is mounted on the top deck of the spacecraft. The two triaxial sensors provide a balanced, fully redundant vector instrument and permit some enhanced assessment of the spacecraft’s magnetic field. The instrument provides data for Browse and high-level products with between 3 and 6 vector s-1 resolution for continuous coverage of the interplanetary magnetic field. Two high-resolution snapshot buffers each hold 297 s of 24 vector s-1 data while on-board Fast Fourier Transforms extend the continuous data to 12 Hz resolution. Real-time observations with 1-s resolution are provided continuously to the Space Environmental Center (SEC) of the National Oceanographie and Atmospheric Association (NO A A) for near-instantaneous, world-wide dissemination in service to space weather studies. As has been our team’s tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the interplanetary medium in support of the fundamental goals of the ACE mission and cooperative studies with other ACE investigators using the combined ACE dataset as well as other ISTP spacecraft involved in the general program of Sun—Earth Connections.

/pdf/the-ace-magnetic-fields-experiment-p814wtufjt.pdf

The ace magnetic fields experiment

We present a new analysis of the transport of cosmic rays in a turbulent magnetic field that varies in all three spatial dimensions. The analysis utilizes a numerical simulation that integrates the trajectories of an ensemble of test particles from which we obtain diffusion coefficients based on the particle motions. We find that the diffusion coefficient parallel to the mean magnetic field is consistent with values deduced from quasi-linear theory, in agreement with earlier work. The more interesting and less understood transport perpendicular to the average magnetic field is found to be enhanced (above the classical scattering result) by the random walk, or braiding, of the magnetic field. The value of κ⊥ obtained is generally larger than the classical scattering value but smaller than the quasi-linear value. The computed values of κ⊥/κ∥, for a representation of the interplanetary magnetic field, are 0.02-0.04; these values are of the same general magnitude as those assumed in recent numerical simulations of cosmic-ray modulation and transport in the heliosphere, and give reasonable agreement with spacecraft observations of cosmic rays. Some consequences of these results for the interpretation of heliospheric observations are discussed.

https://iopscience.iop.org/article/10.1086/307452/pdf

The Transport of Cosmic Rays across a Turbulent Magnetic Field

Two new methods for distinguishing two-dimensional (2D) turbulence from slab turbulence are applied to Helios magnetometer data. Two-component models with varying slab and 2D ingredients are considered. Both methods indicate that solar wind magnetic turbulence possesses a dominant (∼85 % by energy) 2D component. The presence of such a large 2D component provides a natural solution to the long-standing problem of “too small” cosmic ray mean free paths derived from quasilinear scattering theory when using the slab model.

Dominant two‐dimensional solar wind turbulence with implications for cosmic ray transport

The interaction of the solar wind with the local interstellar medium (LISM) is attracting renewed interest, thanks to the possibility that the Voyager spacecraft may, in the not too distant future, cross the heliospheric termination shock. This has spurred the development of increasingly sophisticated models which attempt to describe various aspects of the physics underlying the interaction of the solar wind and the LISM. A comprehensive review of the subject is presented here.

/pdf/interaction-of-the-solar-wind-with-the-local-interstellar-wg8st723sd.pdf

Interaction of the solar wind with the local interstellar medium: a theoretical perspective

Solar energetic particles, or SEPs, from suprathermal (few keV) up to relativistic (
 $$\sim $$
 few GeV) energies are accelerated near the Sun in at least two ways: (1) by magnetic reconnection-driven processes during solar flares resulting in impulsive SEPs, and (2) at fast coronal-mass-ejection-driven shock waves that produce large gradual SEP events. Large gradual SEP events are of particular interest because the accompanying high-energy (
 $${>}10$$
 s MeV) protons pose serious radiation threats to human explorers living and working beyond low-Earth orbit and to technological assets such as communications and scientific satellites in space. However, a complete understanding of these large SEP events has eluded us primarily because their properties, as observed in Earth orbit, are smeared due to mixing and contributions from many important physical effects. This paper provides a comprehensive review of the current state of knowledge of these important phenomena, and summarizes some of the key questions that will be addressed by two upcoming missions—NASA’s Solar Probe Plus and ESA’s Solar Orbiter. Both of these missions are designed to directly and repeatedly sample the near-Sun environments where interplanetary scattering and transport effects are significantly reduced, allowing us to discriminate between different acceleration sites and mechanisms and to isolate the contributions of numerous physical processes occurring during large SEP events.

Large gradual solar energetic particle events

Spatial structure and field-line diffusion in transverse magnetic turbulence.

The diffusion coefficient associated with random walk of magnetic field lines is computed by direct calculation of a large number of field lines from a specified statistical ensemble. Two-component magnetic field models are examined, consisting of a mixture of slab and two-dimensional fluctuations. The scaling of the diffusion coefficient with magnetic field strength is established numerically for a wide range of relative fluctuation amplitudes ( δb/B0). The results confirm the recent nonperturbative theory of field line random walk (Phys. Rev. Lett., 75, 2136, 1995), but depart considerably from the quasilinear B0−2scaling, for models thought to be appropriate to the solar wind.

Scaling of field‐line random walk in model solar wind fluctuations

We generalize the derivation of the average gradient/curvature-drift for a flux tube filled with an isotropic distribution of particles at specified kinetic energy. The present treatment is restricted to a two-dimensional magnetic field with zero electric field, but it includes all chaotic and Speiser orbits, which do not correspond to the simple picture of gradient/curvature drift. We assume that particles are evenly distributed throughout the regions of phase space allowed by their energy and canonical momentum. This assumption is closely related but not exactly equivalent to the assumption of isotropic pitch-angle distribution. Our derivation assumes that the maximum Larmor radius is small compared to the scale length for equatorial variations in the flux tube volume, but it does not involve any restrictions on the curvature of the field line. The resulting expression for the drift rate is valid for situations where the particle drift velocity is comparable to the thermal speed in some regions. The apparent implication of this generalized treatment is that the existence of very complex non-adiabatic particle trajectories in the plasma sheet may not invalidate previous estimates of the average rate of particle drift out the sides of the tail, estimates that were made under the assumption of simple guiding-center drifts.

Particle drift in the Earth's plasma sheet

Magnetic surfaces and field lines in turbulence with fluctuations transverse to a uniform mean magnetic behave exactly as a passive tracer in two dimensional incompressible hydrodynamics. This broad class of turbulence models includes the two-component model for magnetic fluctuations, consisting of fluctuations having both slab and 2D symmetry. An 80% 2D/20% slab model with δB/B0≈1 and a −5/3 spectral law is thought to approximately represent a snapshot of solar wind turbulence. Using this model, a recent calculation of the diffusion coefficient for field line random walk finds a functional form quite different from the standard result. This discrepancy is especially important in the small amplitude limit and affects also the theory of perpendicular diffusion of charged particles. Discussion here emphasizes the analogy with passive scalar dynamics and the nature of the approximations that lead to the new result for the diffusion coefficient.

D. H. Pontius

Papers

Spatial structure and field-line diffusion in transverse magnetic turbulence.

Scaling of field‐line random walk in model solar wind fluctuations

Particle drift in the Earth's plasma sheet

Diffusion of field lines and magnetic surfaces in models of solar wind turbulence