Solar Wind Temperature Isotropy

Reliable models of the solar wind in the near-Earth space environment may constrain conditions close to the Sun. This is relevant to NASA’s contemporary innerheliospheric mission Parker Solar Probe. Among the outstanding issues is how to explain the solar wind temperature isotropy. Perpendicular and parallel proton and electron temperatures near 1 AU are theoretically predicted to be unequal, but in situ observations show quasi-isotropy sufficiently below the instability threshold condition. This has not been satisfactorily explained. The present Letter shows that the dynamical coupling of electrons and protons via collisional processes and instabilities may contribute toward the resolution of this problem.

Figure 1

Proton data distribution, which shows that the marginal mirror instability condition defines the upper-right boundary, although the electromagnetic ion-cyclotron (EMIC) threshold condition could also partially contribute. For the lower-right boundary, proton (PFH for parallel and OFH for oblique) firehose instability threshold conditions can be associated with. Superposed is the progression of proton beta and temperature anisotropy from initial (open circles) to final state (red circles) computed theoretically. The proton data set comes from Wind spacecraft Solar Wind Experiment Faraday cup observations.

Figure 2

Electron data distribution, which shows that the upper-right boundary is defined by the electromagnetic electron-cyclotron (EMEC) instability threshold, while the electron firehose (EFH) instability curve defines the lower-right boundary. While they circumscribe the distribution for large ${\beta }_{\parallel e}$, data points are located significantly far away from the marginal instability curves. Superposed is the progression of the electron beta and temperature anisotropy from initial (open circles) to final state (red circles) computed theoretically. The electron data set comes from the Wind spacecraft 3DP instrument. The magnetic field was determined using the magnetic field investigation (MFI) magnetometers.

Figure 3

The same as Fig. 1, except that a lower value of $g$ parameter corresponding to ${10}^{-5}$ is adopted. Nonlinear progression of proton beta and temperature anisotropy from initial (open circles) to final state (red circles) shows that the proton isotropization is achieved.

Figure 4

Nonlinear progression of electron beta and temperature anisotropy from initial (open circles) to final state (red circles). Final states are all close to the EFH marginal stability.