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.

a is the plaama di s persion func t i on (3?xied a n d C o n t e , 1961), IQ is the m o d i fied B essel function, v 4 a n d v_ are the ion a n d the electron neutral colli- 1 Ѳ aion frequenoy, 5 i{, are the corresponding gyrofrequencies, and n^ 0 and q^ e are the densities and charges of the ion and electrons, к is the m a g n i tude of the w a y e v ector w i t h the components perpendicular a n d кц paral lel to the magnetic field; u»*=wR+ijr are the wave frequency a nd the g r ow t h rate of the plasma waves. T he d r i ving force of the instability is Ѵд ^ - ѵ ^ , the relative e lectron-ion drift velocity, as al r e ad y mentioned. Strong e l e c t r o n density gradients £ = 7 / can also contribute to the instability; if they dominate, formula ( 1 ) describes the gradient-drift plasma instability. The MTS I is gene r a ll y ex c ited in the altitude range 100-120 km. The con finement to this height range has two reasons: B e l o w about 100 km, collisions of the ions a n d electrons w i t h neutral m o lecules become more a nd more impor tant w h i c h re s ults i n a damping of the unstable waves. Above 120 k m on the other hand, ѵ^«=ѵе-ѵ^ becomes smaller and smaller due to the decrease of the i on neutral colli s i on freq u e nc y . A t altitudes above about 2 0 0 k m -* -*■ is so small that v e becomes equal to v^ res u l ti n g in v d <w 0 . The developed MISI is observable i n many ways. In the following three methods wil l be discussed in detail, b y w h i c h the MTSI has b e e n studied in recent years. R EC E N T EXPERIM E N TA L RE S ULTS AN D THE I R T HEORETICAL EXPLANATION. A. El e c t r o n temperature enhancements in the E - r e g i o n . The electron tempera ture at the relevant E - r e g i o n h ei ghts (100-200 km) is norma l l y close to the neutral temperature, n a m e l y ar o u nd 250 to 300 K. D u ri n g times of hi g h magne- tospheric convection (high v ^ ) , however, m u c h h i g he r electron temperatures are observed, sometimes more than 1000 К (Wickwar et al.,1981). The c o rrela tio n of the d e c t r o n temperature i n the E - r e g i o n a n d the excitation of the MTSI was first demonstrated b y Sohlegel and St-Maurioe (1 9 8 1 )wtth the hel p of incoherent scatter observations. Figure 2 shows a n example. As lon g as the drift v e l o ci t y v^ is b e l o w or not too far a w a y fr o m the threshold of about 4 0 0 m/s, the electron temperature at 1 1 2 km exhibits "normal" values around 300-400 K. For hi g h drift velocities, however, T fi increases strongly. ТЕМ 1150 Figure 2. C o r r e l ation of the electron temperature at 1 1 2 k m altitude and the r e l a tive drift velo ci t y + + 750 + + (from Schlegel and S t - M a u r i c e ,1981). 2 5 0 J__ I— I— I— I 0 400 000 1200 W(M/S) 80