GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 8, PAGES 797-800, APRIL 15, 1996 Lower hybrid turbulence and ponderomotive force effects in space plasmas subjected to large-amplitude low- frequency waves G. V. Khazanov and T. E. Moore Space Sciences Laboratory, Huntsville,Alabama E. N. Krivorutsky, and J. L. Horwitz Department of Physics andCenter for Space Plasma andAeronomic Research, The University of Alabama in Huntsville M. W. Liemohn Space Physics Research Laboratory, University of Michigan, Ann Arbor, Michigan Abstract. It is demonstratedthat large-amplitude low-fre- quency waves (LFW) can generate lower hybrid waves (LHW) in the auroral zone and ring current region. The LHW could then heat the ions. The ion energization due to the LHW may be comparable with that produced by the ponderomotive force of the LFW. Introduction Wave-particle interactions are of crucial importance to magnetospheric and ionospheric plasma behavior. Space plasmas support a wide variety of large-amplitude waves. For example,in the region of overlap between the ring currentand the outer plasmasphere numerous observations have shown large-amplitude waves with frequencies below the local proton cyclotron frequency [McPherron et al., 1972; Taylor et al., 1975; Kintner and Gurnett, 1977; Younget el., 1981; Perraut, 1982; Roux et al., 1982; Fraser, 1985; Labelle et al., 1988]. These observations indicate the presence of waves with both left-hand polarization (Alfv6n waves), and right-hand polar- ization (fast magnetosonic waves). It is known that the gen- eration of ion cyclotron waves may occur as a result of an in- stability in the high energy anisotropic component of the ring current during interaction with the plasmasphere [Corn- wall, 1965, 1966; Kennel and Petschek, 1966; Liemohn, 1967; McKean et al., 1994]. In the auroralregion, simultane- ous occurrences of upward-flowingions and field-alignedelec- trons have been observed by the Viking satellite. The occur- renceis strongly correlated with large amplitude low-frequency fluctuations of the electric field [Hultqvistet al., 1988; Lundin et el., 1990]. Large-amplitudeshearAlfv6n waves were also observedby soundingrockets [Boehrn et al., 1990] in the auroral ionosphere. When such low-frequency waves (LFW) propagate in a plasma,a ponderomotive force is produced which may lead to significanteffects at the plasma. Boehmet el. [1990] sug- Copyright 1996by theAmerican Geophysical Union. Paper number 96GL00844 0094-8534/96/96 G L-00844505.00 gested that this ponderomotive forcecan be the cause of sig-. nificantdensity perturbations. Allan [1993] has investigated mass transport caused by the ponderomotive forceof hydrody- namic waves in the middle magnetosphere. It was also sug- gested that significant plasma energization could occur in theseregions. Li and Ternerin [1993] have studiedthe ion en- ergization by the ponderomotiveforce of large-amplitude Alfv6n waves(E> 100 mV/m; oKfl i ) in the auroral zone. It is well known [eg. Akhiezer et el., 1975] that in the field of a LFW, electrons and ions move with different velocities. A significantion-electron relative velocity may lead to a current instability which may produce, for example, lower hybrid waves (LHW). The purpose of this paper is to consider the possibility of suchLHW generation by large-amplitude LFW in a space plasmaand to compare the effectsof LHW and the ponderomotive force in the processes of heating and accelera- tion in the plasma. Below we discuss the possibility of LHW generation in the ring currentregion and in the auroralzone and mechanisms for LH instability saturation. The LHW powerlevel andthe result- ing plasma energization due to the LH turbulence are esti- mated. The plasmaenergization due to LH turbulence is com- paredwith that due to the ponderomotive force. Analysis Lower Hybrid Current Instability Due to Low- Frequency Waves If a plasma is subjectedto a variable electric field E (for instance, the electric field of an Alfv6n or fast magnetosonic wave, which has a component at a right angle to the external magnetic field B o), the electrons and ions will acquirediffer- ent velocities •e and •i under the action of thatfield. If the frequencyco o of the field is considerably lower than the elec- tron cyclotron frequency, the electron and ion velocities will be given by [eg., Akhiezer et al., 1975]' U e =C -- Bø i me -. e { /•) + iCOo ((• x /•)} U i=mi ((OB2i -- (0 o ) (1) where /7, isa unit vector inthe direction ofthe external mag- 797