Destruction of cyclotron resonances in weakly collisional, inhomogeneous plasmas

It is shown, both analytically and numerically, that cyclotron resonances can be destroyed in dense (ω_p≳Ω, where ω_p is the plasma frequency and Ω is the cyclotron frequency), weakly collisional, inhomogeneous plasmas when (ν/Ω) k²r²_L≳1, where ν is the collision frequency and r_L is the mean Larmor radius. The theory is based upon a model Fokker–Planck equation. It is found that the particles make a transition from magnetized to unmagnetized behavior. This is an important result since it indicates that the ion‐and electron‐cyclotron‐drift instabilities transform into their unmagnetized counterparts, the lower‐hybrid‐drift instability and the ion‐acoustic instability, respectively. The ion‐cyclotron‐drift (or drift‐cyclotron instability) is examined in detail and is found to become the lower‐hybrid‐drift instability in the region of maximum growth when (m_e/m_i)^1/2ω/Ω_i≳ν_ii/ Ω_i ≳m_e/m_i for T_e≈T_i plasmas. The first inequality is required to overcome electron viscuous damping, while the second allows the ions to become ’’unmagnetized.’’ Applications to the equatorial F region of the ionosphere and the tandem mirror experiment (TMX) are discussed.