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/sub p/>Ω, where ω/sub p/ is the plasma frequency and Ω is the cyclotron frequency), weakly collisional, inhomogeneous plasmas when (;/Ω) k2r2/sub L/> or approx. =1, where ; is the collision frequency and r/sub 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/sub e//m/sub i/)/sup 1/2/ω/Ω/sub i/> or approx. =ν/sub i/i/Ω/sub i/ > or approx. =m/sub e//m/sub i/ for T/sub e/approx. =T/sub 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