Sensitive Test for Ion-Cyclotron Resonant Heating in the Solar Wind

Plasma carrying a spectrum of counterpropagating field-aligned ion-cyclotron waves can strongly and preferentially heat ions through a stochastic Fermi mechanism. Such a process has been proposed to explain the extreme temperatures, temperature anisotropies, and speeds of ions in the solar corona and solar wind. We quantify how differential flow between ion species results in a Doppler shift in the wave spectrum that can prevent this strong heating. Two critical values of differential flow are derived for strong heating of the core and tail of a given ion distribution function. Our comparison of these predictions to observations from the Wind spacecraft reveals excellent agreement. Solar wind helium that meets the condition for strong core heating is nearly 7 times hotter than hydrogen on average. Ion-cyclotron resonance contributes to heating in the solar wind, and there is a close link between heating, differential flow, and temperature anisotropy.

Figure 1

The dispersion relation for parallel propagating ion-cyclotron waves (green curve) and the resonance criteria given in Eq. (2) for ${\mathrm{H}}^{+}$ (top, blue) and ${\mathrm{He}}^{2+}$ (bottom, red) streaming along $\mathbf{B}$ at different $\delta v_{\parallel i}$. Gold circles indicate wave resonances.

Figure 2

$T_{\perp \alpha }/T_{\perp p}$ in the ${\beta }_{\parallel p}-\delta v_{\alpha p}$ plane. Dashed lines indicate $\pm \delta v_{\alpha p}^c$, the speed below which the core of the ${\mathrm{He}}^{2+}$ VDF should experience multiple resonance. Solid lines indicate $\delta v_{\alpha p}^t$, the speed beyond which only the tail of the ${\mathrm{He}}^{2+}$ is heated.

Figure 3

${\mathrm{H}}^{+}$ temperature anisotropy $T_{\perp p}/T_{\parallel p}$ in the ${\beta }_{\parallel p}-\delta v_{\alpha p}$ plane. Threshold conditions are the same as in Fig. 2. When $|\delta v_{\alpha p}|>\delta v_{\alpha p}^t$ and the ${\mathrm{He}}^{2+}$ become nonresonant, the ${\mathrm{H}}^{+}$ take on a large anisotropy consistent with cyclotron resonance.