Electromagnetic velocity gradient instabilities in the magnetosphere

Abstract

The velocity gradients of the contrastreaming electron beams observed in the Earth's magnetosphere can excite three types of ordinary mode instabilities, namely (i) ∇B-resonance electron instability, (ii) ion cyclotron instability, and (iii) unmagnetized ion instability. The ∇B-resonance electron instability occurs at small values of the shear parameter 10⁻⁴<S<10⁻³, whereS = [(1/Ωe){dU _o(x)}/(dx)] (U ₀(x) and Ω _e being the streaming velocity of the electron beams and the electron cyclotron frequency, respectively). Near the equatorial plane of the bouncing electron beams region, this instability can generate electromagnetic waves having frequenciesf∼(0.045–0.2) Hz and wavelentghsλ _⊥ ∼ (0.5–10)km, and the wave magnetic field is polarised in a radial direction. This instability can also occur in the plasma sheet region during the earthwards and tailwards plasma flows events and can generate waves, with wave magnetic field polarised along north-south direction, in the frequency rangef∼(0.007–0.02) Hz withλ _⊥ ∼ (10–100)km nearR=−35R _E . For 10⁻³<S<10⁻², the ion cyclotron instability is excited and it can generate waves up to 5th harmonic or so of ion cyclotron frequency. ForS>10⁻², the unmagnetized ion instability is excited which can generate electromagnetic waves having frequences from 5 to 50 Hz and typical wavelengthsλ _⊥ ∼ (0.5–6)km. The growth rates of all the three velocity shear driven instabilities are reduced in the presence of cold background plasma. The turbulence generated by these instabilities may give rise to enhanced effective electron-electron and electron-ion collisions and broaden the bouncing electron beams.