Destruction of cyclotron resonances in weakly collisional, inhomogeneous plasmas

It is shown, both analytically and numerically, that cyclotron resonances can be destroyed in dense (ωp≳Ω, where ωp is the plasma frequency and Ω is the cyclotron frequency), weakly collisional, inhomogeneous plasmas when (ν/Ω) k2r2L≳1, where ν is the collision frequency and rL 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 (me/mi)1/2ω/Ωi≳νii/ Ωi ≳me/mi for Te≈Ti 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.