A theory of local and global processes which affect solar wind electrons 2. Experimental support
Authors: Scudder Jack D. , Olbert Stanislaw 
Published: February 2008

We have extended the theoretical considerations of Scudder and Olbert (1979) (hereafter called paper 1) to show from the microscopic characteristics of the Coulomb cross section that there are three natural subpopulations for plasma electrons: the subthermals with local kinetic energy E <kTc, the transthermals with kTc <E <7kTc, and the extrathermals E >7kTc. We present experimental support from three experimental groups on three different spacecraft over a radial range in the interplanetary medium for the five interrelations projected in paper 1 between solar wind electron properties and changes in the interplanetary medium: (1) subthermals respond primarily to local changes (compressions and rarefactions) in stream dynamics; (2) the extrathermal fraction of the ambient electron density should be anticorrelated with the asymptotic bulk speed; (3) the extrathermal ‘temperature’ should be anticorrelated with the local wind speed at 1 AU; (4) the heat flux carried by electrons should be anticorrelated with the local bulk speed; and (5) the extrathermal differential ‘temperature’ should be nearly independent of radius within 1 AU. From first principles and the spatial inhomogeneity of the plasma we show that the velocity dependence of Coulomb collisions in the solar wind plasma produces a bifurcation in the solar wind electron distribution function at a transition energy E*. This energy is theoretically shown to scale with the local thermal (that is, approximately the core) temperature as E*(r) ≃7kTc (r). This scaling is observationally supported over the radial range from 0.45 to 0.9 AU by Mariner 10 data and Imp data acquired at 1 AU. The extrathermals, defined on the basis of Coulomb collisions, are synonymous with the subpopulation previously labeled in the literature as the ‘halo’ or ‘hot’ component. If the transition energy should be required to equal the polarization potential energy eΦ(r) for all radii beyond the observer, (as motivated by Mariner 10 data) the thermal electrons should obey a polytrope law with index γ=7/6. In this circumstance the polarization potential is equal to the specific enthalpy: eΦ(r)=[γ/(γ ‐ 1)] kTc (r). This relation probably does not obtain in the corona, inside r <10–20Rs (cf. paper 1). In the asymptotic spherically symmetric solar wind the inverse power law index for radial variation required for the thermal electrons should be α=1/3, which is consistent with the recent in situ determinations between 0.45 and ∼3 AU. We thus provide the first self‐consistent argument for associating E*(r) with eΦ(r). This theoretical asymptotic (r → ∞) thermal electron temperature variation is between the conduction dominated (2/7) and the inviscid solution (2/5). In the proximity of the ecliptic the ratio of the thermal electron Coulomb mean free path to scale length is shown to decrease with increasing radial distance. By contrast, over the solar poles the same asymptotic temperature and density variations imply that the Coulomb thermal mean free path will increase with increasing radial distance. A straightforward extension of our model to slow time dependent situations shows that the fundamental scaling of E* with temperature and, by inference, the polytrope law for thermal electrons is theoretically maintained within compression and rarefaction regions. An illustrative calculation is given of the extrathermal density's response in stream compressions near 1 AU. These calculations show that the extrathermal electron density can either increase or decrease depending on whether the core electrons compress more nearly as an isothermal gas or an adiabatic one. On the basis of these calculations and a reexamination of published data we suggest that the extrathermal density is probably enhanced within stream compression regions. This implies (as observed) that (1) the core electrons should compress more nearly as an isothermal (γ ≃1) rather than as an adiabatic (γ ≃5/3) gas and (2) a very large electron heat flux should be observed leaving the compression region along the magnetic field. We thus conclude that the global and local Coulomb effects discussed in paper 1 and elaborated here are essential aspects of the solar wind plasma as it is observed. The experimental support which the predictions of the theory in paper 1 enjoy strongly suggests that the further development of that theory is warranted with the ultimate objective of understanding the impact that global heat transport has on the physics of coronal expansions.