The solar wind is a weakly collisional ionized gas experiencing collective effects due to long-range electromagnetic forces. Unlike a collisionally mediated fluid like Earth’s atmosphere, the solar wind is not in thermodynamic or thermal equilibrium. That is, the particles are not in equipartition of energy (i.e., different species have different temperatures) and there is nearly always a finite heat flux present in the solar wind plasma near 1 AU [e.g., (Wilson et al., 2018; Wilson et al., 2019a; Wilson et al., 2019b; Wilson et al., 2020)].

Weakly collisional in this context refers to the large separation between Coulomb collisional mean free paths and relevant spatial scales like thermal gyroradii, ρ _cs , and/or inertial lengths, λ _s . The collisional mean free path of thermal particles near Earth are on the order of one astronomical unit or 1 AU ∼ 1.5 × 10⁸ km (Wilson et al., 2018; Wilson et al., 2019b). The typical thermal proton gyroradius and inertial length near Earth satisfy 30 km ≲ ρ _cp ≲ 190 km and 50 km ≲ λ _p ≲ 130 km [e.g., (Wilson et al., 2021)], i.e., nearly six orders of magnitude smaller than the collisional mean free path. It is therefore surprising that particle-particle collisional signatures are actually observed in the particle velocity distribution functions (VDFs) [e.g., (Salem et al., 2003; Maruca et al., 2013)]. Yet there are just as many studies showing evidence of VDF modification due to turbulence and/or instabilities ¹ [e.g., (Maruca et al., 2012; Kasper et al., 2013; Wilson et al., 2020)].

The fundamental issue is that research is often limited to fluid velocity moments of a VDF and rarely do we have full 3D VDFs available. If we do, the VDFs are often of limited resolution or counting statistics. The solar wind ion VDFs are of particular interest but are the least well resolved of all VDFs that instruments have measured to date. The ions can dominate in momentum and energy transport ² and often control the dynamics of fundamental plasma phenomena, but we are still greatly limited in our ability to resolve the core solar wind because it is such a fast and cold beam. We are also greatly limited in our observations of electron VDFs in the solar wind. Note that higher order velocity moments like the heat flux tensor require high resolution (i.e., energy and angle) measurements to properly calculate. The electron heat flux is very difficult to properly calculate but the ion species-dependent heat fluxes are even more problematic with low resolution or crude methods [e.g., see discussion by (Scudder, 2015)].

While the detailed shape/profile of any given VDF may seem to be a nuanced issue, it is well known that instabilities can be very sensitive to deviations in the VDF away from a Maxwellian, for instance [e.g., (Goldman et al., 2007; Lazar et al., 2012; Lazar et al., 2013; Lazar et al., 2014; Lazar et al., 2017; Lazar et al., 2018; Shaaban et al., 2018; Lazar et al., 2019; Lee et al., 2019; Shaaban et al., 2019)]. Even if we ignore non-Maxwellian features, the presence of multiple Maxwellians will alter instability thresholds and our interpretation of the corresponding velocity moments [e.g., (Chen et al., 2016; Husidic et al., 2020; Verniero et al., 2020; Klein et al., 2021)]. If we cannot resolve a VDF with sufficient accuracy to elucidate these features, we will both find an apparent lack of correspondence between observations and theory. Further, under resolved VDFs preclude our ability to properly document even the macroscopic energy inputs into critical systems like collisionless shock waves (Schwartz et al., 2022). Therefore, it is of fundamental importance for all aspects of plasma physics that we properly resolve the VDFs in any given region of space being examined.

The following is an expanded discussion of the White Paper by (Wilson et al., 2022) submitted to the Decadal Survey for Solar and Space Physics (Heliophysics) 2024–2033. The paper is outlined as follows: Section 3 describes our current understanding; Section 4 describes some current and future measurements for reference; Section 5 illustrates why high resolution measurements are necessary by comparing to MMS observations; Section 6 discusses what needs to be done to properly resolve solar wind distributions; Section 7 illustrates how low resolution and/or incomplete measurements can result in very inaccurate velocity moments; and Section 8 summarizes our discussion and conclusions.

In the solar wind near Earth, the electrons are comprised of four primary populations: a cold dense core with energies typically below ∼15 eV; a hot, tenuous halo with energies from several 10s of eV to ∼1–2 keV; an anti-sunward, field-aligned beam called the strahl with energies from a few 10s of eV up to ∼1–2 keV; and a higher energy superhalo with energies ≳1 keV [e.g., (Wilson et al., 2019a; Wilson et al., 2019b; Wilson et al., 2020), and references therein]. In contrast, the ion VDF is comprised of dozens of mass per charge species but the primary constituents are a cold dense proton core, a differentially drifting proton beam, and a differentially drifting alpha-particle beam [e.g., (Alterman et al., 2018), and references therein].

Traditionally, the core electrons have been modeled as a bi-Maxwellian, the halo and/or strahl as bi-kappas, and the superhalo as a power-law VDF [e.g., (Wang et al., 2012; Wilson et al., 2019a), and references therein]. However, recent analysis of electron VDFs near collisionless shocks have revealed that the core electron population is better modeled by what is called a self-similar distribution (Wilson et al., 2019a; Wilson et al., 2019b; Wilson et al., 2020). These functions can be used to model the so called “flattop” or “flat-top” electron VDF. Recent Parker Solar Probe measurements have shown that even the core proton population can exhibit a flattop profile (Martinović et al., 2020). These observations suggest the proton core shares the same multitude of populations as the electrons below ∼1 keV in the solar wind.

What’s perhaps more confusing is how to reconcile the implications for the self-similar distribution, given that the deviation from a bi-Maxwellian is evidence of inelastic collisions (Wilkinson and Edwards, 1982; Martin and Piasecki, 1999; Alvarez Laguna et al., 2022). This realization was made when comparing to studies of flow through disordered porous media and shocks in coarse grain media (Villani, 2006; Siena et al., 2014; Matyka et al., 2016).

The solutions to the Vlasov equation (i.e., collisionless Boltzmann equation) are, by construction, time-reversible functions. Similarly, the solutions to the original Boltzmann equation are time-irreversible functions. The standard Boltzmann collision operator assumes two-particle elastic collisions and molecular chaos ³ , among other things. It is through this latter assumption that the entire operator can generate a time-irreversible result (Bhatnagar et al., 1954; Gressman and Strain, 2010). These two equations are commonly used to model the evolution of plasmas in simulations.

The surprising aspect of the recent electron observations was the realization that the deviation from Maxwellian to a self-similar VDF is a quantified measure of inelastic collisions [e.g., see discussions in (Wilson et al., 2019a; Wilson et al., 2019b; Wilson et al., 2020)]. Inelastic collisions are intrinsically a time-irreversible process. Self-similar distributions were predicted in quasi-linear and nonlinear theory of wave-particle interactions decades ago [e.g., (Vedenov, 1963; Sagdeev, 1966; Dum et al., 1974; Dum, 1975)]. An analogous operator to that generating inelastic collisions can arise indirectly through approximations like coarse grain averaging, but what is lost through such averaging? While the connection to a fundamentally irreversible phenomena remains unknown, it does seem possible for irreversible processes to arise in plasmas [e.g., (Villani, 2014)].

Thus, the realization that the particle VDFs in the solar wind can exhibit signatures of inelastic collisions is of fundamental importance to kinetic theory, space plasma physics, and astrophysics. The question is, how do inelastic collisions occur? Further, if such inelastic processes are important near collisionless shocks, are they important everywhere in the solar wind? Are these irreversible processes unknowingly hidden in our estimates of collisionality? What is more important for the evolution of VDFs, collisional effects or waves/turbulence?

Further, our inability to properly resolve the thermal ion populations precludes our ability to address outstanding questions like the partition of energy amongst the constituent particle populations in any fundamental plasma phenomena. This is especially problematic for collisionless shock waves. If we cannot properly account for the ion populations incident on the shock, we have no proper baseline for the most important input to the system, i.e., the upstream ion kinetic energy density. Further, if our measurements effectively “smear out” the ion VDF, we cannot discriminate whether the ion VDF changes shape across the shock or how it evolves. This leaves us with crude parameters like the total ion temperature, which can be extremely misleading in such kinetic processes [e.g., (Wilson et al., 2014a; Wilson et al., 2014b; Goldman et al., 2020; Wilson et al., 2020; Goldman et al., 2021; Argall et al., 2022)].

The primary current thermal plasma instruments making measurements in the solar wind near Earth are from Wind 3DP and SWE (Wilson et al., 2021), THEMIS electrostatic analyzers or ESAs (McFadden et al., 2008a; McFadden et al., 2008b), and MMS FPI (Pollock et al., 2016). The THEMIS and MMS instruments were designed to measure the hot, slow magnetospheric and magnetosheath plasmas. Thus, they tend to over estimate the temperature and underestimate the number density of particles in the solar wind [e.g., (Wilson et al., 2014a; Artemyev et al., 2018; Roberts et al., 2021)]. The PESA Low instrument from Wind/3DP measures ions with an angular resolution of ∼ 5.6 ° and an energy resolution of ∼20%. The EESA Low instrument from Wind/3DP measures electrons with an angular resolution of ∼ 11.25 ° and an energy resolution of ∼20%. The SWE Faraday cups measure ions with an energy resolution of ∼6.5–13% and angular resolution of ∼ 8 ° . The best time cadence of both of the 3DP ESAs ∼3 s whereas the SWE Faraday cups are at ∼92 s [e.g., see (Wilson et al., 2021), and references therein].

Newer thermal particle instruments on Parker Solar Probe (PSP) (Kasper et al., 2016) and Solar Orbiter (SolO) (Nicolaou et al., 2019) have made improvements with angular resolutions of ∼ 5 ° –6° for SolO and ∼ 5 ° –11.25° for PSP. Instrumentation on both spacecraft have energy resolutions of ∼7%. The nominal time resolutions ⁴ are ∼3.5 and ∼4 s, respectively. Since both PSP and SolO are significantly closer to the Sun than Wind, the typical time scales of interest (e.g., cyclotron period) are much shorter. Thus, the increased time resolution on SolO compared to, say, Wind, is not as advantageous as it would be were the spacecraft located near Earth. The future IMAP mission (McComas et al., 2018) will have both thermal electron and ion particle measurements. The electron instrument will have an angular resolution of ≳ 12° and an energy resolution of ∼12% at a cadence of ∼15 s. The ion instrument will have an energy resolution of ∼8.5% at a cadence of ∼15 s and is meant for measuring bulk solar wind properties. Therefore, the IMAP instruments make some improvements in energy resolution over Wind, but the time resolution is five times lower.

In short, there are no current or future planned instrumentation that will properly resolve the thermal solar wind plasma populations.

The purpose of this section is to help illustrate that what can appear to be a minor nuance (i.e., changes or deviations in a VDF or low counting statistics or under resolved peaks) actually becomes a major problem in some regions of space. Although the MMS FPI instrument is unparalleled in the magnetosheath and magnetosphere, it was not designed for the cold, fast solar wind. There are a great deal of extra efforts “behind the scenes” by the FPI team to improve and validate the data products in these regions, which can sometimes hide the issue at hand or give false confidence to an unknowing researcher. Below we outline why a great deal of caution is necessary when using FPI data in the solar wind and why MMS cannot address the issue of solar wind measurements.

Both the DES and DIS detectors (Pollock et al., 2016), part of the fast plasma investigation (FPI) on MMS, systematically over estimate the temperature, T _s,tot , and under estimate the number density, n _s , in the solar wind [e.g., (Roberts et al., 2021)]. The reason being that the energy and angular resolution of the detectors were designed for a slow-moving, hot plasma. The solar wind, by comparison to the magnetosphere, is a fast-moving, very cold plasma. Further, under most bow shock crossings, the MMS FPI instrument is not in its solar wind mode thus the minimum energy bin, E _min, is above the spacecraft potential, ϕ _SC . This means the instrument is missing part or most of the solar wind core electrons, depending on conditions. Further, due to the often large number of secondary and photoelectrons (Gershman et al., 2017), the signal-to-noise ratio (SNR) of any given DES energy-angle bin in the solar wind can be quite small.

Figure 1 shows an illustrative comparison between a Wind 3DP EESA Low (Wilson et al., 2021) and MMS DES (Pollock et al., 2016) electron VDF. In both cases, the data are corrected for ϕ _SC and transformed into the bulk flow rest frame following the procedures discussed in the appendices of Wilson III et al. (Wilson et al., 2019a). The MMS DES VDF had the secondary and photoelectrons removed (Gershman et al., 2017), following the procedures outlined in the SPEDAS software (Angelopoulos et al., 2019). The Wind 3DP VDF was chosen roughly 1 hour earlier than the DES VDF to correspond to the propagation time of the solar wind. The SNR shown here is illustrated by the ratio between the 1D cuts (i.e., red or blue lines in the bottom panels) to that of the Poisson statistics (i.e., magenta lines). One can see that DES only has a few energy-angle bins with sufficient count rates ⁵ to be statistically significant. In contrast, all energy-angle bins have sufficient count rates for the Wind 3DP VDF.

This should not be surprising as 3DP was designed for the solar wind and accumulates for a full spacecraft spin period (∼3 s) whereas DES was not designed for the solar wind and only accumulates for ∼0.03 s (i.e., ∼100 times shorter duration). Note that the FPI team does some extra work to help “fill in” the gap at low energies and help correct the electron velocity moments to account for these issues (e.g., several personal communications with FPI team, circa 2017–2018). This is why the level-2 electron velocity moments can still be reliable in the solar wind and are often more reliable than the level-2 ion velocity moments.

Figure 2 shows an illustrative comparison between a Wind 3DP PESA Low (Wilson et al., 2021) and MMS DIS (Pollock et al., 2016) ion VDF ⁶ . The VDFs are taken from different dates due to limited burst mode observations from PESA Low, but both are representative and will serve the comparison purpose for this discussion. As previously noted, the FPI instrument was not designed to measure the cold, fast incident solar wind. This is why the ion VDF shown in the third panel of Figure 2 has a much broader peak (see 1D cuts in bottom row) than that shown in the PESA Low panels. In fact, the solar wind is often so cold that the DIS detector will both under estimate n _i and over estimate T _i,tot . Similar issues were observed with THEMIS illustrating the short time cadence of the instruments is not the primary cause of the discrepancy [e.g., (Wilson et al., 2014a; Artemyev et al., 2018)].

There is more to examine than just the superficial difference in thermal spread here as well. If one looks closely at the lowest energy (in the core ion rest frame) part of the VDF, specifically the perpendicular cut (blue line) in the left-hand panel, it becomes clear there are effectively two cold populations. When one “zooms out” to a larger velocity range (i.e., middle and right-hand panels), neither 3DP or DIS resolve this colder population ⁷ . While it may seem like a nuanced issue stuck in the minutiae, this coldest part of the VDF carries the majority of the number density, and thus, the majority of the kinetic energy density. Further, it was known at the time of launch that 3DP would not fully resolve the solar wind core, due to data/telemetry/technological constraints. Thus, even the “zoomed-in” view in the left-hand panels of Figure 2 contain more detail than is currently shown. The instrument does capture what is effectively a proton strahl in the anti-parallel direction (i.e., red line for negative velocities). However, such features are lost in the DIS measurement or the “zoomed-out” view from 3DP. Previous work has shown that treating the VDF as a single population results in a grossly incorrect picture when viewed from velocity moments [e.g., (Wilson et al., 2014a)]. Further, the inability to properly resolve the ion populations precludes us from examining whether any energy is used to “reshape” the VDF, similar to what is seen in electron VDFs [e.g., (Wilson et al., 2020)], versus to heat the VDF. We are also unable to accurately measure any associated skewness or heat flux in the ions for most intervals [e.g., (Scudder, 2015; Schwartz et al., 2022)]. The efficiencies and pathways through which these would occur are different and it is critical we understand them because they are fundamental processes not limited to shock physics.

A recent study (Schwartz et al., 2022) attempted to budget all the energy inputs and outputs across the terrestrial bow shock using MMS to illustrate both its capabilities and limitations. They showed that the limitations of the FPI instruments in the solar wind required their reliance on time-shifted Wind observations (something not unique to this study). They also noted that the inability to properly measure and separate heavy ions like alpha-particles greatly inhibits our ability to properly determine fundamental but basic properties of a plasma, like the equation of state.

In summary, the purpose of this discussion is to illustrate that caution is necessary when using DES and DIS data in the solar wind when the instrument is in burst mode and not in its solar wind energy table mode. This section is also meant to convey that while the MMS FPI detectors are unparalleled in the magnetosheath and magnetosphere, they were not designed for the solar wind and are thus not accurate in that region.

However, a critical lesson we should recognize from MMS observations is that higher resolution measurements can and do result in paradigm shifting discoveries. It is not difficult to see that higher energy, angular, and time resolution measurements in the solar wind near 1 AU would provide substantial advances in our understanding of weakly collisional plasmas. There are fundamental processes at play in the solar wind, in its evolution of the particle VDFs, that are critical to our understanding of kinetic physics and statistical mechanics. Some of these issues, were they to be properly addressed, would reach the level of major physics prizes/awards and could upend the foundations of our conceptual thinking of kinetic physics and its broader implications.

Recent studies have been performed to examine new technologies and/or new algorithms to improve solar wind observations for future missions [e.g., (De Marco et al., 2016; Cara et al., 2017; De Keyser et al., 2018)]. The mission concept called Turbulent Heating ObserveR or THOR (Vaivads et al., 2016) would have had a thermal particle instrument with an angular resolution of ∼ 3 ° and an energy resolution of ∼5–8%. This would have been the first instrument to properly resolve the ions in the near-Earth solar wind. Unfortunately, THOR was not selected by ESA and is no longer funded. Another mission concept was recently proposed to help address this inability to properly resolve the solar wind ions as well (Borovsky and Raines, 2022).

Table 1 shows direct instrument capabilities comparisons between some current missions and two concepts, THOR and MAKOS. The table is separated into ions and electrons, as the measurements and technological limitations are different for measuring the different species. The table lists the nominal or typical values for the various instruments, taken from the instrument papers or direct observations as for the Wind 3DP detectors. The table lists the cadence (i.e., time between full VDF measurements), energy resolution (ΔE/E), azimuthal angular resolution (Δϕ), poloidal/latitudinal angular resolution (Δθ), and energy range. While Wind was designed to measure the solar wind VDFs, it was greatly limited by technological and telemetry constraints when it was designed and built (compared to modern missions). Even so, as we have shown, it can do much better than some modern missions precisely because it was designed to measure the solar wind. Note we did not show the IMAP instrument comparisons in the table as the instrument papers are not yet published and IMAP is not being designed for in situ science.

The ideal state of the art advancements in solar wind VDF measurements over the next decade would increase time resolutions for full 3D 4π ion distributions in the solar wind down to < 100 ms, with energy resolutions < 5% and angular resolutions < 5°. These values should reduce to < 10 ms with energy resolutions < 2.5% and angular resolutions < 2.5° over the next 3 decades. The justification for such constraints arise from the analysis performed by Cara et al. (2017); De Keyser et al. (2018); De Marco et al. (2016). One can see by examining their simulated reconstruction of the VDFs that even with THOR’s advanced capabilities, we would still not fully resolve some features.

The resolutions referenced above derive from the need to resolve things like fast/magnetosonic-whistler precursors [e.g., (Wilson et al., 2017)] with ion measurements, shock ramps, and short-duration reconnection processes [e.g., (Wang et al., 2020)] with ion and electron measurements. Since many electron-scale fluctuations have spacecraft frame in the 100s–1000s of Hz, there is little hope to provide full 3D VDFs on those time scales. However, many of these phenomena (e.g., lion roars/whistler mode waves, magnetosonic-whistlers, etc.) grow a much slower rates, on the order of the lower hybrid resonance frequency (∼10 Hz in the solar wind near 1 AU). For these phenomena, we do have a finite probability of observing some of their evolution in time from, for instance, an instability [e.g., (Akimoto et al., 1993; Forslund et al., 1970; Forslund et al., 1971; Gary, 1993; Matsukiyo and Scholer, 2006)]. Some previous work has even shown that MMS has sufficient time resolution (but not quite the energy-angular resolution) to observe part of the evolution of shock-reflected ion beams near the bow shock [e.g., (Goodrich et al., 2019)]. There is also the issue of trying to properly resolve the particle VDFs during/within a turbulent region of space. The electric and magnetic fields are often capable of examining down to electron-scales, but the particle data are either too slow or do not properly resolve the VDFs [e.g., see discussion in (Wilson et al., 2021), and references therein].

Electron VDF measurements can have slightly more relaxed energy and angular resolution due to their larger thermal-to-bulk speed ratios. However, the time resolution is a much stricter constraint because electron time scales are so much shorter than any ion time scale. Finally, it is imperative that the lowest energy bin for electron measurements be no more than ∼1–2 eV to ensure the instrument observes photoelectrons for correction/removal (Lavraud and Larson, 2016).

To further illustrate the necessity of improved solar wind measurements, we provide a case example for both ions and electrons. In the following, we will generate a generic, model energy distribution function (EDF) for each population and then interpolate particle instruments to these model EDFs to illustrate the sparsity of measurements. We will also provide an example of a future instrument concept that would address the current measurement deficit (e.g., see the MAKOS science and mission concept white papers submitted to the Heliophysics Decadal Survey led by Prof. Katherine Goodrich).

Note that the purpose of examining these model distributions in this way is to illustrate that without high resolution measurements, even the velocity moments of the entire distribution (i.e., not separating out species or subcomponents) are highly inaccurate. Further, although the MAKOS mission concept intends to use a time-of-flight (ToF) component to its solar wind ion instrument, we do not separate out protons and alpha-particles in Section 7.1.2 so as to directly compare with Wind and MMS.

To start, we define the Maxwell-Boltzmann EDF for a system with three degrees of freedom in the presence of a potential, ϕ _s , is given by: f s E = 2 n s E − ϕ s π k B T s − 3 / 2 e − E − ϕ s k B T s (1) where k _B is Boltzmann’s constant, T _s is the temperature, and n _s is the number density of particle species s. Note that unlike a velocity distribution function (VDF), the units of the EDF are given as number per unit volume per unit energy, e.g., # cm⁻³ eV⁻¹. Normally Eq. 1 is shown in the limit where ϕ _s → 0 and then moments of f s E are integrated over all energies from zero to infinity. However, in this case in the presence of a potential, the moments are given as: ϵ N ≡ ∫ ϕ s ∞ d E E − ϕ s N f s E (2) where ϵ ^N is the Nth energy moment of f s E . We also note that, in general, the solar wind particle VDFs are not Maxwellian, i.e., even the low energy part of the VDF is better characterized by the more general self-similar VDF [e.g., see (Wilson et al., 2019a; Wilson et al., 2019b; Wilson et al., 2020)]. However, we forego this additional complication for the sake of simplicity since low resolution instruments can only rarely distinguish between Maxwellian and self-similar VDFs.

We can also define the kappa EDF for a system with three kinetic degrees of freedom in the absence of a potential energy [e.g., (Livadiotis, 2015a; Livadiotis, 2015b)] is given by: f s E = 2 n s E 1 / 2 Γ κ s + 1 π Γ κ s − 1 2 κ s − 3 2 k B T s − 3 / 2 × 1 + κ s − 3 2 − 1 E k B T s − κ s + 1 (3) where k _B is Boltzmann’s constant, κ _s is the kappa exponent, T _s is the temperature, and n _s is the number density of particle species s. Eq. 3 assumes that any kinetic energies are given by the Galilean representation, i.e., E = m s 2 v − v o s 2 where m _s is the mass and v _os is the drift velocity relative to relevant reference frame of v.

The purpose of this article is to illustrate that high resolution particle VDF measurements are an essential requirement for any in situ mission. We also show that although MMS is unparalleled in the magnetosphere and magnetosheath for measuring particle VDFs, it does not perform well in the solar wind (as it was not designed for this region of space). PSP and SolO have high resolution particle VDFs, but neither are near 1 AU and both have their own limitations and issues (e.g., heat shield obscures the ion VDFs for much of PSPs orbit). The focus of the future IMAP mission is not in situ science, so its particle instruments will be largely focused on providing velocity moments for space weather purposes. There is hope with plans to design and build the potential future mission currently called HelioSwarm (Klein et al., 2019) but the instrument specifics are still not published. We also hope that this article will motivate further studies to critically examine the limitations of relying upon velocity moments from under resolved VDF measurements and/or other instrumental/measurement limitations.

A critical examination of the core proton population could show as much diversity as the electron distributions below ∼1 keV, which would completely change our understanding of the solar wind and how it evolves. PSP is conveniently measuring very slow solar wind during its perihelion passes, which improves the velocity resolution measurements in the core of the solar protons. As such, they are seeing a great deal of structure in the ion VDFs that we just cannot resolve ¹³  at 1 AU [e.g., (Verniero et al., 2020)]. Multiple populations, multiple species, and numerous non-Maxwellian features all result in vastly different instabilities [e.g., (Chen et al., 2016; Husidic et al., 2020; Verniero et al., 2020; Klein et al., 2021)] and particle dynamics at boundaries like shocks or current sheets [e.g., (Gedalin and Ganushkina, 2022; Ha et al., 2022)]. All of these phenomena can greatly alter the evolution of the solar wind, since all are tied to its acceleration and propagation away from the Sun. Therefore, it is critical we properly resolve all the populations if we are to understand the acceleration and evolution of the solar wind, among numerous other kinetic physics topics of interest.

Since all the interesting, fundamental processes of interest to plasma physicists occur at collisionless shocks (e.g., reconnection, turbulence, particle acceleration, wave-particle interactions), it is particularly relevant to have proper solar wind measurements for such investigations. It is also critical that we design a mission focused on examining collisionless shocks to advance our understanding of these fundamental topics (e.g., see the MAKOS mission concept led by Prof. Katherine Goodrich).

Without such measurement advancements, we will not reach closure on multiple major topics such as wave-particle interactions, turbulence dissipation, energy dissipation in collisionless shocks, and particle acceleration by waves and/or shocks. All of these topics have implications for nonequilibrium statistical mechanics, plasma physics, and astrophysics.