Sensitive test for ion-cyclotron resonant heating in the solar wind

Area of Science:

• Plasma physics
• Space physics
• Astrophysics

Background:

• Plasma waves, specifically counterpropagating field-aligned ion-cyclotron waves, are proposed to heat ions via a stochastic Fermi mechanism.
• This mechanism is hypothesized to explain extreme ion temperatures, anisotropies, and speeds observed in the solar corona and solar wind.

Purpose of the Study:

• To quantify the impact of differential flow between ion species on the stochastic Fermi heating mechanism.
• To determine critical differential flow values that govern strong ion heating in the core and tail of ion distributions.
• To compare theoretical predictions with observational data from the Wind spacecraft.

Main Methods:

• Theoretical analysis to derive critical values of differential ion flow.
• Doppler shift analysis of the wave spectrum due to differential flow.
• Comparison of derived predictions with in-situ measurements from the Wind spacecraft.

Main Results:

• Differential ion flow can introduce a Doppler shift that inhibits strong stochastic Fermi heating.
• Two critical differential flow values are identified for effective heating of the core and tail populations.
• Observations from the Wind spacecraft show excellent agreement with these predictions, particularly for solar wind helium.

Conclusions:

• Differential flow is a crucial factor that can prevent efficient ion heating by ion-cyclotron waves in space plasmas.
• Helium ions in the solar wind meeting core heating conditions are significantly hotter (approx. 7x) than hydrogen ions.
• A strong correlation exists between ion heating, differential flow, and temperature anisotropy in the solar wind, mediated by ion-cyclotron resonance.