The Earth’s magnetosphere and the solar wind provide two valuable opportunities to bring systems science into the field of plasma physics. Both the magnetospheric system and the solar-wind system are comprised of multiple electron and ion populations with interactions between the populations occurring largely via plasma waves. Many of the plasma-wave interactions of the magnetosphere were uncovered and quantified by Richard Thorne in over 4 decades of research (e.g. Thorne, 1968; Thorne et al., 2017) and many of the plasma-wave interactions of the solar wind were uncovered and quantified by Peter Gary in over 4 decades of research (e.g. Gary, 1974; Gary et al., 2020). Without theses two visionary researchers, these two coherent plasma-systems pictures could not be created.

Both the magnetosphere and the solar wind exhibit multiple populations of ions and electrons that are co-located (i.e. that reside on the same magnetic-field lines) so that the multiple populations easily interact with each other. The evolution of magnetized plasmas often leads to anisotropies giving rise to instabilities and wave-particle interactions: for the magnetosphere the particle anisotropies arise from solar-wind-driven magnetospheric internal convection pushing particle populations into the strong magnetic field of the dipolar regions and for the solar wind the particle anisotropies arise from the solar-wind expansion into weaker magnetic fields away from the Sun. For every plasma-wave instability, wave-particle interactions act in two fashions: in driving the waves and in dissipating the waves. In this manner two diverse particle populations can interact with each other.

In this perspective article the plasma system science of the magnetosphere and the plasma system science of the solar wind will be briefly outlined. The magnetosphere is a driven closed system (more or less), with the multiple particle populations trapped in dipolar (or stretched) magnetic flux tubes. The solar wind is an open system with multiple particle populations born in the solar corona and moving outward at different speeds from the Sun in open magnetic flux tubes.

This paper is organized as follows. In The Magnetospheric System Section the magnetospheric plasma system is described. In The Solar-Wind System Section the solar-wind plasma system is described. Discussion Section contains discussions of 1) previous system-science analysis that has been done for the magnetosphere and the solar wind, 1) the availability of data for future studies, and 3) some advantages that systems analysis may provide.

The Earth’s magnetosphere is a system of multiple trapped particle populations with free energy added from the solar wind, which drives internal convection. Convection of particle populations into the stronger field of Earth’s dipole drives particle anisotropies and free energy for plasma waves. The resulting multiple plasma instabilities gives rise to multiple couplings between the diverse particle populations, resulting in a complex evolution of those diverse particle populations. A thorough review of the Earth’s magnetosphere as a system wherein the multiple ion and electron populations interact via plasma waves appears in Borovsky and Valdivia (2018).

The key to the interaction of the different particle populations is the fact that they are co-located on the same magnetic field lines. There are three major regions in the Earth’s magnetosphere and the three regions contain differing particle populations. The first region is the stretched-field “magnetotail” on the nightside of the Earth. The major particle populations of the magnetotail are the ion plasma sheet, the electron plasma sheet, the cusp-mantle (ions and electrons), and the polar wind (ions and electrons). The second region is the “outer dipole”: this region contains the ion plasma sheet (ions), the electron plasma sheet (electrons), substorm-injected ions, substorm-injected electrons, the electron radiation belt, the ion radiation belt, cloak ions, cloak electrons, and polar-wind ions and electrons. The third region is the “inner dipole” closest to the Earth: this region contains the plasmasphere (ions and electrons), the ion plasma sheet, substorm-injected ions, the electron radiation belt, and the ion radiation belt. The boundary between the inner and outer dipole regions is the plasmapause, which is the outer boundary of the dense, cold plasmasphere.

As an example of the co-location of multiple particle populations, Figure 1 provides a sketch of the distribution functions of the ions and electrons in the inner dipolar region at local midnight. The plot is a log-log plot, sketching each particle distribution as a Maxwellian, which in general they are not. (The hotter populations often have anisotropies and biteouts). The populations range in energy from the ionosphere (T_i ≈ T_e ∼ 0.1 eV) and plasmasphere (T_i ≈ T_e ∼ 0.5 eV) to the ion radiation belt (which has a tail of particles in the 1-MeV range of energies) and the electron radiation belt (which has a tail of particles in the 10-MeV range).

The plasma populations of the magnetosphere interact via plasma waves. The typical types of plasma waves observed depend on the region of the magnetosphere and the ion and electron populations in that region that can drive the waves. Figure 2 is a classic sketch from Thorne (2010) that has been reproduced in numerous scientific publications: the sketch depicts the equatorial plane of the Earth’s magnetosphere and the major types of plasma waves that impact the electron radiation belt observed in the various regions. The types of plasma waves seen inside the plasmapause (inner dipole) differ from the types seen outside the plasmapause (outer dipole), with drastically different consequences for the particle populations in those two regions. Critical to the evolution of the electron radiation belt, Figure 2 depicts that whistler-mode chorus waves exist outside of the plasmasphere (outer dipole), whistler-mode hiss waves exist inside of the plasmasphere (inner dipole), and electromagnetic ion-cyclotron (EMIC) waves tend to exist inside the plasmasphere (inner dipole).

In the interactions of co-located particle populations via plasma waves in the Earth’s magnetosphere, typically a warm (keV) population of particles with medium density is the driver for the waves and typically a very-energetic populations (100’s of keV) with a very-low-density is the absorber of the wave energy. And often there is a high-density cold population of ions or electrons that regulates the plasma-wave dispersion relation, determining what waves can be driven and determining the resonance conditions for those waves (cf. Delzanno et al., 2021).

In Figure 3 a map is displayed of the basic web of interactions between the multiple electron and ion populations of the Earth’s magnetosphere and the type of plasma wave that mediates each interaction. (The various plasma waves in Figure 3 are elaborated upon in Section 6 of Borovsky and Valdivia (2018).). In the map of Figure 3 the direction of each arrow indicates the transfer of wave action: from the driving population to the receiving population. For the receiving population, the waves can lead to energization and/or to pitch-angle scattering, with pitch-angle scattering often leading to scattering into the atmospheric loss cone where the scattered ions or electrons are lost from the magnetospheric system into the atmosphere. Note in Figure 3 that a number of “circular” wave interactions are indicated: here one portion of a single population will drive waves that affect another portion of the same population. This often produces pitch-angle scattering into the atmospheric loss cone, resulting in a loss from the magnetospheric system and perhaps producing aurora in the upper atmosphere.

The complicated system of instability-driven wave-particle interactions that drive the evolution of the multiple ion and electron populations of the Earth’s magnetosphere were largely worked out by Richard Thorne. Focused review articles on these various interactions can be found in Thorne and Kennel (1971), Thorne et al., (1973, 1979, 2010, 2013, 2017), Thorne (1974, 2010), Thorne and Summers (1991), and Thorne and Horne (1992, 1994).

One hallmark of a “complex system” is that the system exhibits “emergence”, new things “emerge”, i.e. the system creates something from nothing. Examples of emergence are given in Borovsky and Valdivia (2018). On example of emergence in the Earths magnetospheric system is the electron radiation belt: here the system takes medium-energy short-lifetime substorm-injected electrons and, via a complex web of interactions, creates a population of high-energy long-lifetime relativistic electrons.

The magnetic structure of the solar-wind plasma resembles a spaghetti of magnetic flux tubes with strong current sheets forming the boundaries between adjacent flux tubes (Michel, 1967; Bruno et al., 2001; Borovsky, 2008; Greco et al., 2008; Pecora et al., 2019). The field of most of the solar wind is “open”, with field lines magnetically connecting the Sun to the distant heliosphere. The multiple solar-wind particle populations are born in the solar corona and move outward (at different speeds) along the magnetic field in the open flux tubes. Note, however, that many of the details of the physical processes in the corona that are acting in the birth of the solar wind are not known (cf. Cranmer et al., 2017).

The solar-wind plasma in the inner heliosphere has 7 major particle populations: protons, alpha particles, highly charged heavy ions, an antisunward proton beam, core electrons, halo electrons, and a field-aligned energetic electron strahl. Minor populations can include a sunward streaming proton beam.

As the fast energetic strahl electrons move outward from the Sun they produce an ambipolar electric field that pulls ions outward from the Sun against the Sun’s gravity (Jockers, 1970; Lemaire, 2010). This magnetic-field-aligned electrostatic “interplanetary electric field”, in part, accelerates the solar wind to high velocities. (A similar process occurs on the sunlit ionosphere of Earth where multi-eV photoelectrons from the atmosphere produce an ambipolar electric field that pulls sub-eV ions out against the Earth’s gravity: this is the “polar wind” (Schunk, 2007).) The field-aligned potential drop from the Sun to infinity is on the order of 1 kV, with most of that drop occurring between the Sun and the orbit of Mercury. As the ion and electron populations of the solar wind move outward from the Sun (at different speeds), they interact and evolve. Some of the plasma-wave interactions between the major particle populations are mapped out in Figure 4.

Free energy sources for the system evolution are (cf. Borovsky and Gary, 2014; Smith and Vasquez, 2021) 1) electron heat flux, 2) large-amplitude Alfvén waves, 3) the relative drift between alpha particles and protons, and 4) anisotropies driven by the changing magnetic-field strength. Well beyond 1 AU other sources are interplanetary shocks (Smith et al., 1985) and interstellar pickup ions (Lee, 2018).

It is thought that whistle-mode scattering of the field-aligned strahl electrons gives rise to the quasi-isotropic energetic electron halo population (Gary and Saito, 2007): as evidence, with distance from the Sun the strahl population becomes fractionally less dense and the halo population becomes fractionally more dense (Stverak et al., 2009).

A key source of energy in the solar wind resides in outward propagating Alfvénic fluctuations from the corona. About half of the solar wind at 1 AU is Alfvénic, with strong correlations between the vector changes in the magnetic field and the vector changes in the proton flow velocity. In the Alfvénic solar wind, it is observed that the magnetic structure moves en masse relative to the proton plasma at a speed of about 0.7 v_A (Borovsky, 2020a; Nemecek et al., 2020), where v_A is the measured Alfvén speed. In the reference frame of the magnetic structure, to within measurement error all proton flows v are parallel to the local magnetic-field direction B ; with v_⊥ = 0 there is (to within measurement error) no time evolution of the magnetic structure as it moves outward through the inner heliosphere. This is an example of the Chadrasekhar dynamic equilibrium (“CDE”) (cf. Fig. 7.1 of Parker (1979)) where a nonlinear tangle (spaghetti) of magnetic field will propagate en masse without evolution provided that the flow is everywhere parallel to the local field. The alpha particles of the solar wind (and perhaps also the heavy ions) reside nearly at rest in the reference frame of the magnetic structure (Nemecek et al., 2020): it is not known why. The outward-propagating Alfvénic structure interacts with the particle populations 1) via parametric instabilities (e.g. Malara et al., 2001; Vasquez and Hollweg, 1996) and 2) via an MHD-turbulence cascade where the Alfvén waves transfer energy into electrons and ions via mode conversion (Gary and Smith, 2009; Gary et al., 2020) followed by kinetic (cyclotron or Landau) damping (Leamon et al., 1998; Gary and Borovsky, 2004, 2008).

Because of the relative motion of the alpha particles with respect to the proton population, ion-ion streaming instabilities can couple the two populations (Gary, 1991; Gary et al., 2000a). In particular, the alpha-proton magnetosonic instability acts to heat both populations in a fashion such that the protons and alpha particles both have the same thermal speed (Gary et al., 2000b), which is observed in the solar wind (Feynman, 1975).

As the particle populations move outward from the Sun into weaker magnetic fields, conservation of the particle first adiabatic invariants v² _perp/B decreases the perpendicular temperatures of the populations and leads to anisotropy. For the protons the firehose instability driven by T_parallel > T_perp acts to return the proton population toward isotropy (Gary et al., 1998, 2001). Similar instabilities act on the alpha-particle anisotropy (Gary et al., 2003) and on the electron anisotropy (Gary and Nishimura, 2003).

Core electrons originate from the low-energy portion of the strahl population (e.g. Boldyrev et al., 2020). The core electrons are locally trapped along the magnetic field (Marsch, 2006): 1) moving toward the Sun the core electrons mirror as the interplanetary magnetic field gets stronger nearer to the Sun and 2) moving away from the Sun the interplanetary electric field pulls them back. The core-electron temperature reflects the local potential of the plasma with respect to infinity (away from the Sun) (Feldman et al., 1975).

Review articles on the diverse types of solar-wind plasma waves and their impact on the solar-wind particle populations can be found in Gary (1991, 1992, 1999), Gary et al. (1975a,b, 1976, 1984, 1994, 1999, 2000a), and Gary and Karimabadi (2006) and in the monograph Gary (1993).

Here we see the system, with the strahl driving the interplanetary potential that drives the solar wind outward. As the populations move through each other at different speeds (all moving outward), they interact via plasma waves (cf. Figure FF03).

The magnetic-field structure of the solar wind resembles a spaghetti of magnetic flux tubes separated by current sheets. The diameters of the flux tubes vary, but a typical flux tube at 1 AU has a diameter of about 4 × 10⁵ km, and a spacecraft crosses from one flux tube to the next every 10 or 20 min. From tube to tube there can be differences in the plasma properties: proton specific entropy, ion composition, magnetic-field strength, plasma beta, etc. Important for this system-science picture, the strahl intensity (Borovsky, 2020b) and the electron temperature (Borovsky et al., 2021) can vary from tube to tube with sudden jumps in the values as the current-sheet wall between flux tubes is crossed. The electron-temperature jumps indicate that the interplanetary electric potential differs from flux tube to flux tube. This implies that each flux tube is an independent evolving system, and that as a solar-wind spacecraft crosses from tube to tube it is measuring different realizations of system evolution. This opens the possibility of statistical plasma system science.

This section discusses previous system-science analysis that has been done for the magnetosphere and the solar wind, the availability of quality data for future studies, and the advantages that systems analysis may provide.