경희대학교 BK21

 

아래와 같이 세미나를 진행하오니
관심있는 분들의 많은 참여를 바랍니다.
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● 날짜 : 2017년 03월 30일 (목요일)
● 시간 : 16시 30분
● 장소 : 천문대 영상실
● 강사 : Dr. George Parks (SSL, UC Berkeley)
 
Title : Unresolved Issues about Heating of Space Plasmas

 
Abstract :
This talk will address some issues in space plasma heating that have not been adequately resolved. Space plasmas are treated as collisionless and the Vlasov theory and concepts have been used to interpret the observations. The Vlasov equation is time symmetric and can describe only reversible processes. However, space plasmas observations often include heating of plasmas from the SW to the distant geomagnetic tail. We first present new observations of particle heating across the bow shock. Then we discuss two ideas that have been around since the sixties about heating collisionless plasmas:  wave interaction with particles and collisions among the particles. Both processes scatter and change the velocity of the particles causing particle diffusion. However, wave-particle interaction under the guise of Vlasov equation is reversible and still inadequate. Analytical studies of wave particle-interaction begin with perturbation around the equilibrium distribution and examine the dispersion relation of the excited wave. As an example, the perturbed Vlasov equation (B = 0) to first order has been studied by Bernstein and Tehran (1960). They have shown that in addition to a damping solution (Landau damping), there is also growing solutions depending on the initial condition. The amplitude of growing waves can become nonlinear and the suggestion is that the nonlinear waves can break like steepened water waves releasing energy to heat the particles. Schmidt (1966) discusses another example, a beam distribution that somehow gets scattered into a spiral narrow shape, conserving entropy (Liouville theorem), but the spiral is spread over larger velocities and the temperature increases because the integration is performed over larger velocity limits. Since the mean free path for collisions in the SW is nearly 1 AU the physical collisions are rare. But we show that collisions may be important for some problems that still remain unresolved. There is also a dilemma is that PIC codes have reproduced numerous observations of heating. But particles in PIC codes move around obeying the Lorentz force, which is time symmetric and thus cannot produce heating. One possible speculative idea to this dilemma is that the particles and waves in PIC codes are in inhomogeneous plasmas and can phase mix producing small-scale structures. These small-scale structures are unstable and can dissipate and produce even smaller scale structures covering a large velocity space. Hence phase mixing can mimick diffusion in velocity space resulting in higher temperatures. But this heating mechanism is not physical but an artifact of the code. How do we resolve this question about heating space plasmas? Your thoughts and criticisms will be much appreciated.