Free Energy Source of the Mirror Instability: Nonresonant Particles

The mirror instability is a fundamental phenomenon in plasma physics. Given the historical discussion regarding the role of resonant particles in driving this instability-which is at odds with its presence in magnetohydrodynamic (MHD) models that only describe fluid plasma behavior-we seek to clarify the free energy source of this instability through a detailed investigation of velocity space. We introduce a method for calculating the wave-particle energy transfer rate. This diagnostic identifies that the free energy source of the mirror instability originates from nonresonant particles. This leads to a more coherent understanding of the origins of the instability within both MHD theory and plasma kinetic theory. Because of the opposing diamagnetic drift motions of the nonresonant and resonant particle populations, energy from nonresonant particles is transferred into the mirror mode waves, while resonant particles absorb energy from these waves. Consequently, both particle populations play critical roles in determining the growth rate of the linear mirror instability.