Majority of Solar Wind Intervals Support Ion-Driven Instabilities.

Area of Science:

• Space Physics
• Plasma Physics
• Astrophysics

Background:

• The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, constantly flowing through the solar system.
• Understanding the stability of the solar wind is crucial for comprehending space weather phenomena and plasma behavior in astrophysical environments.
• Previous studies often focused on single free-energy sources, potentially oversimplifying the complex dynamics of solar wind plasma.

Purpose of the Study:

• To statistically assess the stability of the solar wind at 1 Astronomical Unit (AU) against multiple ion free-energy sources.
• To investigate the influence of proton and Helium (He^{2+}) temperature anisotropy and relative drifts on solar wind stability.
• To compare results with traditional threshold models by employing Nyquist's instability criterion.

Main Methods:

• Utilized Nyquist's instability criterion to analyze solar wind plasma stability.
• Modeled solar wind ion components (proton core, proton beam, He^{2+}) as drifting bi-Maxwellians.
• Analyzed 309 randomly selected spectra from the Wind spacecraft data.

Main Results:

• 53.7% of analyzed spectra were found to be unstable when considering multiple free-energy sources.
• Only 4.5% of spectra exhibited instability towards long-wavelength perturbations.
• Instabilities were predominantly associated with resolved proton beams, large He^{2+} drift speeds, and proton temperature anisotropy.

Conclusions:

• The solar wind exhibits significant instability when multiple free-energy sources, including temperature anisotropies and ion drifts, are considered.
• Long-wavelength instabilities are rare, suggesting a relative stability against large-scale perturbations.
• Observed instabilities generally possess slow growth rates, indicating they may not significantly impact short-term kinetic-scale processes.