امواج سالیتونی غبار-صوتی در پلاسماهای غباری فضایی با توزیع غیرتعادلی

نوع مقاله : پژوهشی

نویسندگان

1 استادیار، گروه فیزیک، دانشکد علوم پایه، دانشگاه نیشابور، نیشابور، ایران

2 دانش‌آموخته کارشناسی ارشد، گروه فیزیک، دانشکد علوم پایه، دانشگاه نیشابور، نیشابور، ایران

چکیده

در این مقاله، با به‌کارگیری جدیدترین یافته‌ها در مدل توزیع کاپا برای پلاسماهای فضایی غیرتعادلی، امواج غبار-صوتی در یک پلاسمای غباری شامل ذرات سنگین غبار با بار منفی و توزیعی از الکترون‌ها و پوزیترون‌های فوق‌گرمایی مطالعه شده‌اند. توزیع سرعت‌ها در این مدل توسط یک شاخص طیفی ناوردا ( ) که مستقل از تعداد درجات‌آزادی سیستم است، و پارامتر  که معرف تعداد درجات‌آزادی سیستم است، برچسب زده می‌شود. در تحلیل خطی، رابطه پاشندگی امواج غبار-صوتی مطالعه شده است و در آنجا یک سرعت صوت تعمیم‌یافته وابسته به متغیرهای مسأله و دربرگیرنده حالت‌های تعادلی و غیرتعادلی محاسبه شده است. در تحلیل غیر‌خطی و با به‌کار گیری سرعت صوت به‌دست آمده، امواج غبار-صوتی سالیتونی با استخراج یک معادله انتگرال انرژی و تحلیل آن مطالعه شده‌اند. شرایط تشکیل چاه‌پتانسیل برای انتشار امواج سالیتونی غبار-صوتی، محدوده مجاز دقیق عدد ماخ و نقش پارامترهای سرعت سالیتون، شاخص طیفی و درجات‌آزادی وابسته به پتانسیل اختلالی ( ) در انتشار امواج سالیتونی غبار-صوتی به‌صورت پارامتری و عددی بررسی شده‌اند. نشان دادیم که آستانه عدد ماخ به شاخص آدیاباتیک ذرات با توزیع کاپا ارتباط دارد. به‌علاوه، وقوع سالیتون‌های غبار-صوتی با پلاریته منفی امکان‌پذیر است. حالت‌های نزدیک و دور از تعادل‌گرمایی مطالعه شده‌اند و نتیجه شده است که در نواحی دور از تعادل‌گرمایی، امکان وقوع سالیتون‌های زیرصوتی وجود دارد و در آن نواحی امواج غبار-صوتی سالیتونی با دامنه و شیب پالس بیشتر منتشر می‌شوند.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Dust-acoustic Solitary Waves in Space Dusty Plasmas with Nonequilibrium Distribution

نویسندگان [English]

  • Ehsan Saberian 1
  • Rasoul Khoosheh Shahi 2
1 Assistant Professor, Department of Physics, Faculty of Basic Sciences, University of Neyshabur, Neyshabur, Iran
2 M.Sc. Graduated, Department of Physics, Faculty of Basic Sciences, University of Neyshabur, Neyshabur, Iran
چکیده [English]

In this paper by using the most recent findings in the field of the Kappa distribution statistics for the non-equilibrium space plasmas, dust-acoustic waves have been studied in a dusty plasma comprising of the inertial dust particles with negative charges and suprahermal distributions of electrons and positrons. The velocity distribution function for stationay state of the plasma in this model is labeled by an invariant Kappa index () which is independent of the numbers of degrees of freedom, and the parameter  which represents the the numbers of degrees of freedom. In linear analysis, the dispersion relation of dust-acoustic waves is studied, whrere the true sound speed of the problem is derived. The derived dust-sound speed is a generalized one which depends on the polytropic index of Kappa distributed paricles ( ), which itself depends on the spectral index  and the potential degrees of freedom ( ). Generally, the dust-sound speed has its maximum in an equilibrium plamsa with Maxwellian distribution or isothermal electrons ( ), and it reduces by approaching to the anti-equilibrium regions with sub-isothermal electrons
( ).
On the other hand, in the non-linear analysis, the dust-acoustic solitary waves have been studied by deriving an energy-integral equation, where we have used the true dust-sound speed for defining the true Mach number (the fractional wave speed to the sound speed). The formation conditions of the potential well, the true Mach number domains, and the effects of the parameters of soliton speed, the spectral index  and the potential degrees of freedom via the perturbation ( ) in the propagation of dust-acoustic solitary waves have been studied analytically and numerically.
 In such a plasma, only the negative polarity solitons are possible. The reason is the negative charge of dust paricles via the attracted electrons, which causes the formation of negative potential solitons.
The structure of dust-acoustic solitons are examined in the near-equilibrium states, where the spectral indices are distributed with the values of , and also in the far-from-thermal equilibrium states which are labeled by the spectral indices with the values of .
It is found that the threshold Mach nmber is proportional to the square root of the polytropic index of Kappa distributed paricles which vaies in the range . So, the threshold Mach number increases by approaching to the equilibrium state and it reduces in far-from-thermal equilibrium states. 
It is shown that the subsonic solitons are possible in the far-from-thermal equilibrium plasmas. On the other hand, in an equilibrium plasma, corresponding to the asymptotic limit of , only the altrasonic solitons are possible which confirms the classical theory of solitons in equilibrium statistical mechanics.
It is found that the amplitude and steepening of the dust-acoustic solitons grows in far-from-thermal equilibrium states, which corresponds to the lower values of the spectral index . It is because of the impact the suprathermal particles on dust-acoustic solitons in that regions. Furthermore, an increase in Mach number results in the propagation of dust-acoustic solitons with more amplitude and steepening, in agreement with the standard theory of solitary waves. Moreover, decreasing the potential degrees of freedom causes an increase in the maximum amplitude and pulse steepening of dust-acoustic solitons.

کلیدواژه‌ها [English]

  • Dust-acostic wave
  • Space plasma
  • Invariant kappa index
  • Potential degrees of freedom
  • Soliton
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