Joseph E. Borovsky

There are 27-day patterns and solar-cycle patterns to the passage of different solar-wind plasma types at Earth given off by the different regions of the rotating, changing Sun. The four types of solar-wind plasma are coronal-hole-origin plasma, streamer-belt-origin plasma, sector-reversal-region plasma, and ejecta. The different plasma types have systematically different properties (e.g., solar-wind speed, magnetic-field strength, Mach number, and Alfvénicity) The different plasmas produce different environments for the Earth and each plasma type drives the Earth differently, producing 27-day and solar-cycle temporal patterns of geomagnetic activity. Geomagnetic storms occur mainly during coronal-hole-origin plasma (modulated by the Russell-McPherron effect) and during ejecta and geomagnetic calms occur during sector-reversal-region plasma and streamer-belt-origin plasma. For coronal-hole-origin plasma at Earth, an interplay between the hemispheric-number asymmetry of sunspots and the 22-year Hale magnetic cycle has been seen that will affect the occurrence of high-speed-stream-driven storms at Earth and the intensity of the electron radiation belt by biasing the Russell-McPherron effect. For the different solar-wind plasmas, the difference in driving and the different ambient environments both effect the particle populations of the Earth’s magnetosphere such as the cold plasmasphere, the magnetotail plasma sheet, and the electron radiation belt. The interactions of solar-wind plasmas, geomagnetic activity, and magnetospheric particle populations are explored in the present study, with particular attention paid to 27-day and solar-cycle variations.

Open heliospheric magnetic-field lines wander along the Parker-spiral direction, and the length scale of the wandering provides information about the parallel wavelength of solar wind turbulence and affects scatter-free transport of energetic charged particles in the solar wind. While single-spacecraft measurement of length scales along the mean magnetic-field directions (e.g. along the Parker spiral) has long been known to be challenging, occasions when magnetic-field lines connect a solar-wind spacecraft to the Earth’s bow shock enable an estimation of the length scales. A large set of Advanced Composition Explorer (ACE)–Earth magnetic-connection events, identified by the ACE detection of backstreaming strahl electrons from the Earth’s bow shock, is used to estimate the length scales associated with the wandering heliospheric magnetic field at 1 AU. Our estimates indicate that the local radius of curvature of the magnetic-field lines is ∼0.016 AU and that the global length scales of the field-line variations along the Parker-spiral direction are ∼0.072 AU. These estimations are necessarily coarse due to the simplification of the geometry of magnetic-field lines to circular arcs or sine curves, and we assume that backstreaming electrons from the Earth’s bow shock mainly come from the bow shock nose. Nevertheless, here, we provide a novel perspective to make use of single-spacecraft measurement of particles and fields to probe structures parallel to the mean heliospheric magnetic field. The global length scales estimated here agree with estimations of field-aligned mean-free-paths for cosmic ray scattering, with braiding lengths associated with granule/supergranule motion on the photosphere/corona and lengths associated with the wandering of the open flux at the Sun produced by interchange reconnection in the low corona.

Motivated by knowledge that directional discontinuities (strong current sheets) in the solar-wind magnetic time series have dominating effects on the magnetic power spectral density of the solar wind, the question is raised as to what are the effects of the magnetic fluctuations that reside in between those discontinuities. A methodology is developed to remove the effects of discontinuities in the magnetic time series and the resulting modified time series is Fourier examined. At 1 AU, an interval of fast Alfvénic coronal-hole-origin wind and an interval of slow non-Alfvénic streamer-belt-origin wind are analyzed. For both types of solar wind, the amplitude of the trace-B power spectral density in the inertial range is about an order of magnitude smaller for the time series of fluctuations between the discontinuities than it is for the raw (with discontinuities) time series, and the spectral index is shallower when the discontinuities are removed. For future global MHD computer simulations of the “upstream turbulence effect” on the solar-wind-driven Earth’s magnetosphere, we have a methodology to determine the effects of discontinuities on the Earth’s magnetosphere versus the effects of between-the-discontinuity magnetic fluctuations.

Correlations between the electron fluxes in the Earth’s central plasma sheet and various solar wind and magnetospheric parameters are studied. Higher-energy fluxes (above approximately 2 keV) are the focus of the analysis. Flux measurements are taken during 950 current-sheet crossings in the central plasma sheet using 3-s-resolution electron measurements from the THEMIS-B and THEMIS-C spacecraft in the solar minimum years 2007–2010. The current-sheet crossings were from 10.8–30.6 R _E downtail from the Earth. Plots of the Pearson linear correlation coefficient r _corr as a function of the instrument channel energy are examined. Even though all the solar wind variables are intercorrelated, which complicates the situation, indications are that the same element in the solar wind that drives magnetospheric activity is also what drives the fluxes of energetic electrons in the magnetotail plasma sheet. The correlation curves show that the energetic electron fluxes are related to the electron bulk temperature of the central plasma sheet. Accounting for the fact that magnetospheric activity moves the plasma sheet plasma earthward and systematically and adiabatically heats the plasma as a function of downtail distance, it is found that the electron temperature at a given downtail distance from the Earth is related to the level of magnetospheric activity at the time that the temperature is measured. The correlations of energetic electron fluxes with various common solar wind coupling functions are examined. The impact of measurement shot noise in the data is examined, and future work is outlined.

The simple problems of the solar wind number density driving the plasma sheet number density and the solar wind speed driving the plasma sheet temperature are examined. To ensure that the central plasma sheet is studied, 1017 current sheet crossings in the plasma sheet are collected from 10.8 R_E to 76.9 R_E downtail, and 3-s resolution ion and electron measurements from THEMIS-B and THEMIS-C are compared with time-lagged solar wind number densities and speeds. The central plasma sheet measurements are taken from the solar minimum years of 2007–2009. Three correlation methods are used: (1) Pearson (univariate) linear correlations; (2) multivariate linear correlations; (3) canonical correlation analysis. For both ions and electrons, knowing the solar wind speed adds insignificant information to the solar wind density versus plasma sheet density correlations. Likewise, for both ions and electrons, knowing the solar wind density adds insignificant information to the solar wind speed versus plasma sheet temperature correlations. The standard two problems (1) the density of the solar wind driving the density of the plasma sheet and (2) the velocity of the solar wind driving the temperature of the plasma sheet, appear to be completely unrelated, even though the solar wind density and the solar wind velocity have a strong anticorrelation. Future work is outlined.

A systems science approach based on canonical correlation analysis (CCA) is applied as a new, behavioral way to validate global geospace models. The biggest novelty of the technique is that it validates models at a system level, whereby a side‐by‐side comparison is performed of CCA applied to a 30‐day observational and the corresponding simulation data sets comprising quiet, moderate and active times. The simulation used the Multiscale Atmosphere‐Geospace Environment (MAGE) model. It is shown that (a) CCA must be combined with sensitivity analysis to be effective, (b) the MAGE model generally reproduces the observed behavior (more so for quieter time intervals), quantified by the intercorrelations between different variables and (c) the technique identifies the SuperMAG SML index as a quantity for which refinements of the model are needed.

To explore some properties of small-scale magnetic structures in the solar wind at 1 AU, measurements from the THEMIS-ARTEMIS mission with two spacecraft in orbit around the Earth’s moon are used. The small-scale magnetic structures are categorized into four types: discontinuities (current sheets), magnetic flux tubes (including twisted flux tubes), magnetic holes, and small-amplitude magnetic-field fluctuations. The large-scale solar-wind structures in which the small-scale structures are investigated are noted. The two-spacecraft measurements are used to examined differences in the structures over spatial scales of a few R_E. Further, a two-spacecraft methodology was used to examine the planarity of discontinuities over scales of a few Earth radii.

A systems science approach based on canonical correlation analysis (CCA) is applied as a new, behavioral way to validate global geospace models. The biggest novelty of the technique is that it validates models at a system level, whereby a side-by-side comparison is performed of CCA applied to a 30-day observational and the corresponding simulation datasets comprising quiet, moderate and active times. The simulation used the Multiscale Atmosphere-Geospace Environment (MAGE) model. It is shown that (1) CCA must be combined with sensitivity analysis to be effective, (2) the MAGE model generally reproduces the observed behavior (more so for quieter time intervals), quantified by the intercorrelations between different variables and (3) the technique identifies the SuperMAG SML index as a quantity for which model refinements are needed.

Editorial: The future of space physics 2022