Электродинамические процессы в высоких широтах: материалы междунарордного симпозиума «Полярные геомагнитные явления», 25-31 мая 1986 г., Суздаль, СССР / Междунар. геодез. и геофиз. союз, Междунар. ассоц. геомагнетизма и аэрономии ; Акад. наук СССР, Кол. фил. им. С. М. Кирова, Поляр. геофиз. ин-т ; [редкол.: О. М. Распопов (отв. ред.). и др.]. - Апатиты : Кольский филиал АН СССР, 1988.

the bulk velocity of the ions and the electrons. In the example, shown in the fight hand part of Figure 1, the velo c i ty is meas u r ed in the rest frame of the ions, and the electrons have a drift velocity v d relative to the ions. If we now have oscillations with a frequency ч >0 i n the plasma, then there are more particles w h i c h feed energy to the waves than particles wh i c h take energy away from the waves. As a result, the oscillations start to gro w and the plasma becomes unstable. This instability is na m ed "two stream instabili ty" because of the difference in the streaming velocities of the plasma constituents. To excite this instability, the drift veloc i t y difference has to exceed a threshold velocity of aeveral km/s, a situation whi c h is r ar e l y met in the ionospheric E-region. The threshold velocity, however, is strongly reduced if there is a magnetic field a cting upo n the plasma and if the drift velocity is directed perpendicular to this field. I n this special case the instability ie called the "modified" two stream instability or "Farley-Buneman" instability because it w as first described by F a r le y (1963) and Buneman (1963). The threshold veloc i t y to excite this modi f i ed two stream instabi lity (MTSI) is of the oreder of the ion-acoustic veloc i t y in the plasma (because the waves whioh are excited a re ion-acoustio waves) wh ic h is normally between 350 and 400 m /a in the E-region. The most general dispersion relation d e s cribing the MTSI is given by (Schlegel and S t - M au r i ce , 1983): 1+Ue where Furthermore, we have the ion and electron thermal velocity the ion and electron D ebye-1ength й е = о и к ±vd uj* =K1svt2e / 2 Ve- k 1 vd 2 79