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Interstellar Gas Dynamics by S.A. Kaplan and F.D. Kahn (Auth.)

By S.A. Kaplan and F.D. Kahn (Auth.)

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H e r e t h e m a g n e t i c f o r c e d e n s i t y (j x H)jc v a n i s h e s , a n d t h e r e f o r e t h i s field d o e s n o t c h a n g e t h e s t a t e o f m o t i o n . T h e d i s s i p a t i o n i s s m a l l e r i n a force-free m a g n e t i c field t h a n i n o t h e r c o n figurations. T h e field a l s o h a s t h e m i n i m u m e n e r g y . T h e r e f o r e , o n e m i g h t t h i n k t h a t t h e force-free field m a y p r e v a i l in s y s t e m s w h e r e t h e kinetic energy h a s been dissipated (for instance, b y shocks), a n d o n l y t h e m a g n e t i c e n e r g y is left [15].

4) - - 2 - 2 x t 1 0 " 3 1r . 9) C o m p a r i n g this q u a n t i t y with t h e value of G o r L given a b o v e , w e 2 5 see t h a t f o r 1 0 < T < 1 0 d e g . 4) is a l w a y s c o n s i d e r a b l y less t h a n L o r G. T h i s difference i n creases with increasing Q . W e m a y therefore conclude t h a t t h e i n t e r a c t i o n b e t w e e n g a s a n d r a d i a t i o n i n i n t e r s t e l l a r s p a c e is s o strong that in m a n y cases t h e t h e r m a l balance of the g a s depends a l m o s t exclusively o n t h i s i n t e r a c t i o n [ 1 2 ] .

I f t h e m o t i o n o f t h e g a s is c h a o t i c ( t u r b u l e n t ) i n c h a r a c t e r , i t will e n t r a i n t h e m a g n e t i c lines o f f o r c e , t h u s " e n t a n g l i n g " t h e m a n d c o n s e q u e n t l y i n c r e a s i n g t h e m a g n e t i c e n e r g y d e n s i t y . 9) 2 w h e r e v is t h e m e a n s q u a r e v e l o c i t y . I f t h e m a g n e t i c e n e r g y density were t o b e c o m e higher t h a n t h e kinetic energy density, t h e g a s m o t i o n w o u l d b e d e t e r m i n e d b y t h e m a g n e t i c field; t h e m a g n e tic lines o f force w o u l d t e n d t o " d i s e n t a n g l e " , a n d t h e m a g n e t i c energy density would thereby decrease, while t h e kinetic energy w o u l d increase.

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