In this review we will focus on a topic of fundamental importance for both plasma physics and astrophysics, namely the occurrence of large-amplitude low-frequency fluctuations of the fields that describe the plasma state. This subject will be treated within the context of the expanding solar wind and the most meaningful advances in this research field will be reported emphasizing the results obtained in the past decade or so. As a matter of fact, Ulysses’ high latitude observations and new numerical approaches to the problem, based on the dynamics of complex systems, brought new important insights which helped to better understand how turbulent fluctuations behave in the solar wind. In particular, numerical simulations within the realm of magnetohydrodynamic (MHD) turbulence theory unraveled what kind of physical mechanisms are at the basis of turbulence generation and energy transfer across the spectral domain of the fluctuations. In other words, the advances reached in these past years in the investigation of solar wind turbulence now offer a rather complete picture of the phenomenological aspect of the problem to be tentatively presented in a rather organic way.

/pdf/the-solar-wind-as-a-turbulence-laboratory-4mad4q4xdr.pdf

The Solar Wind as a Turbulence Laboratory

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

In a previous study (Cane and Richardson, J. Geophys. Res.
 108(A4), SSH6-1, 2003), we investigated the occurrence of interplanetary coronal mass ejections in the near-Earth solar wind during 1996 – 2002, corresponding to the increasing and maximum phases of solar cycle 23, and provided a “comprehensive” catalog of these events. In this paper, we present a revised and updated catalog of the ≈300 near-Earth ICMEs in 1996 – 2009, encompassing the complete cycle 23, and summarize their basic properties and geomagnetic effects. In particular, solar wind composition and charge state observations are now considered when identifying the ICMEs. In general, these additional data confirm the earlier identifications based predominantly on other solar wind plasma and magnetic field parameters. However, the boundaries of ICME-like plasma based on charge state/composition data may deviate significantly from those based on conventional plasma/magnetic field parameters. Furthermore, the much studied “magnetic clouds”, with flux-rope-like magnetic field configurations, may form just a substructure of the total ICME interval.

Near-Earth Interplanetary Coronal Mass Ejections During Solar Cycle 23 (1996 - 2009): Catalog and Summary of Properties

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

Energetic particles are accelerated in rich profusion at sites throughout the heliosphere. They come from solar flares in the low corona, from shock waves driven outward by coronal mass ejections (CMEs), from planetary magnetospheres and bow shocks. They come from corotating interaction regions (CIRs) produced by high-speed streams in the solar wind, and from the heliospheric termination shock at the outer edge of the heliospheric cavity. We sample all these populations near Earth, but can distinguish them readily by their element and isotope abundances, ionization states, energy spectra, angular distributions and time behavior. Remote spacecraft have probed the spatial distributions of the particles and examined new sources in situ. Most acceleration sources can be "seen" only by direct observation of the particles; few photons are produced at these sites. Wave-particle interactions are an essential feature in acceleration sources and, for shock acceleration, new evidence of energetic-proton-generated waves has come from abundance variations and from local cross-field scattering. Element abundances often tell us the physics the source plasma itself, prior to acceleration. By comparing different populations, we learn more about the sources, and about the physics of acceleration and transport, than we can possibly learn from one source alone.

/pdf/particle-acceleration-at-the-sun-and-in-the-heliosphere-2yha5t1u73.pdf

Particle Acceleration at the Sun and in the Heliosphere

We present experimental and theoretical evidence suggesting that the mean free path of cosmic-ray electrons and protons may be fundamentally different at low to intermediate (less than 50 MV) rigidities. The experimental evidence is from Helios observations of solar energetic particles, which show that the mean free path of 1.4 MV electrons is often similar to that of 187 MV protons, even though proton mean free paths continue to decrease comparatively rapidly with decreasing rigidty down to the lowest channels (about 100 MV) observed. The theoretical evidence is from computations of particle scattering in dynamical magnetic turbulence, which predict that electrons will have a larger mean free path than protons of the same rigidity. In the light of these new results, 'consensus' ideas about cosmic-ray mean free paths may require drastic revision.

Proton and Electron Mean Free Paths: The Palmer Consensus Revisited

We show that the variations of the interplanetary magnetic field strength (B) over a 22-year period are tracked by the inverted profile of the cosmic ray density measured by neutron monitors. We suggest that global changes in the Sun's magnetic field are more important for long-term modulation than magnetic field enhancements resulting from the merging of high-speed flows and coronal mass ejections in the outer heliosphere. The unexpectedly close relationship that we find between the “tilt angle” of the heliospheric current sheet and the cosmic ray density away from solar minimum for both polarity states of the solar magnetic field may be accounted for by the anticorrelation between the cosmic ray density and field strength variations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GL900032

Cosmic ray modulation and the solar magnetic field

We have used >60 MeV/amu particle data from Helios 1 and 2 to demonstrate the close association between ejecta (as defined, for example, by regions of depressed solar wind proton temperature) and short-term (<3 days duration) particle decreases. Of 84 short-term decreases of ≥4%, we find that 88% were associated with an ejecta, and 70% of these were also associated with a shock. It is clear that the presence of a particle decrease is a robust signature for identifying ejecta in the ecliptic, at least within 1 AU. Conversely, ejecta are evidently important in the production of short-term cosmic ray depressions. The absence of a sample of well-defined ejecta without an associated particle decrease suggests that extended regions of open field geometry are rare inside ejecta, at least on the ∼0.005 AU scale sizes probed by these particles. Sixty-three percent of the ejecta decreases were associated with smooth magnetic field rotations characteristic of magnetic clouds. Our results suggest that there is no fundamental difference in the particle response to ejecta with or without magnetic cloud signatures. We find that some ejecta observed at multiple spacecraft have a magnetic cloud signature at one spacecraft but not at another. The most likely explanation is that magnetic clouds are a substructure of ejecta and the field structure observed depends on where the ejecta is intercepted. We also find that ejecta probably typically extend much less in longitude than the 100° inferred from single-spacecraft studies.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA00149

Helios 1 and 2 observations of particle decreases, ejecta, and magnetic clouds

The velocity dispersion for a large number of solar proton events is analyzed in the energy regime of 10–60 MeV It is found for all events that the time from the flare to particle maximum t
m is well represented by a sum of two components The first component which is energy independent describes the propagation in the solar atmosphere, the second component describes the propagation in the interplanetary medium giving a velocity dispersion v × t
m = const The additional study of time intensity profiles, onset times, and multispaceprobe observations reveals that the propagation in the solar atmosphere consists of three processes: (1) A rapid transport process in the initial (≲ 1 h) phase after the event fills up a “fast propagation region (FPR)”, which may extend up to ≈ 60° from the flare site and which is tentatively identified with a large unipolar magnetic cell as seen on Hα synoptic charts, (2) a large-scale drift process which is energy independent with drift velocities v
D in the range 1° ⩽ v
D ⩽ 4°h-1, and simultaneously (3) a diffusion process which yields the general broadening of the intensity time profiles for eastern hemisphere events, which is, however, of less importance than previously assumed

Propagation of flare protons in the solar atmosphere

We examine the detailed relationship between recurrent cosmic ray depressions and corotating high-speed streams in the inner heliosphere near the ecliptic plane using counting rates from anticoincidence guards of instruments on the IMP 8, Helios 1, and Helios 2 spacecraft. These rates indicate the density of >60 MeV/amu ions with reasonable time resolution (∼15 min) and high counting statistics. Essentially all corotating streams are accompanied by significant particle depressions. The particle decrease commences most frequently (∼63% of events) at the leading edge of the stream which is typically colocated with the stream interface inside the corotating interaction region (CIR). If the depression starts ahead of the interface, there is usually an additional abrupt decrease at the interface. In ∼61% of events the onset of the depression is closely associated with the onset of enhanced field turbulence which typically occurs near the stream leading edge. Minimum particle densities are generally found in the vicinity of the maximum solar wind speed in the high-speed stream. The density recovers during the declining phase of the stream. The observations are most consistent with modulation of the cosmic ray density in high-speed streams by the increase in solar wind speed. Enhanced scattering by turbulence in the CIR may also contribute near the onset of the depression, in particular in cases where the decrease commences ahead of the stream interface. The absence of a consistent relationship between the depressions and magnetic field enhancements suggests that localized particle drifts in the enhanced magnetic fields of CIRs do not produce the particle depressions. The observation of a depression is not dependent on the presence of a heliospheric current sheet crossing in the CIR, in contrast to previous reports.

The relationship between recurring cosmic ray depressions and corotating solar wind streams at 1 AU : IMP 8 and Helios 1 and 2 anticoincidence guard rate observations

G. Wibberenz

Papers

Proton and Electron Mean Free Paths: The Palmer Consensus Revisited

Cosmic ray modulation and the solar magnetic field

Helios 1 and 2 observations of particle decreases, ejecta, and magnetic clouds

Propagation of flare protons in the solar atmosphere

The relationship between recurring cosmic ray depressions and corotating solar wind streams at 1 AU : IMP 8 and Helios 1 and 2 anticoincidence guard rate observations