The aim of this study is to investigate the impact of the cosmic ray and the ambipolar diffusion on the thermal instability in a weakly ionized gas. The cosmic ray propagates along the magnetic-field line where the ambipolar diffusion is present. The weakly ionized gas and the cosmic ray are considered as two different interacting fluids. Using the linear perturbation analysis, we consider the occurrence of the thermal instability to obtain a dispersion relation in the presence of both phenomena. This equation describes the stable and unstable modes in the terms of some parameters, which depend on the cosmic ray and the ambipolar diffusion. The results show that the angle between the direction of the perturbation propagation and the magnetic-field lines plays an important role on the domains of stability and instability. Furthermore, we found that there is a characteristic wavelength related to the ambipolar diffusivity that explains whether the ambipolar diffusion increases the magnetic support of the cloud against the collapse, or decreases it against the collapse. Finally, the results address some new points in the study of the structure formation within the clumpy molecular clouds as well as the formation of the low-mass stars in the interstellar medium.
Regular and irregular variations in total electron content
(TEC) are one of the most significant observables in ionospheric
studies. During the solar cycle 24, the variability of ionosphere is
studied using global positioning system derived TEC at a
mid-latitude station, Tehran (35.70N, 51.33E). Based on solar radio
flux and seasonal and local time-dependent features of TEC values, a
semi-empirical model is developed to represent its monthly/hourly
mean values. Observed values of TEC and the results of our
semi-empirical model then are compared with estimated values of a
standard plasmasphere–ionosphere model. The outcome of this model
is an expected mean TEC value considering the monthly/hourly regular
effects of solar origin. Thus, it is possible to use it for
monitoring irregular effects induced by solar events. As a result,
the connection of TEC variations with solar activities are studied
for the case of coronal mass ejections accompanying extreme
solar flares. TEC response to solar flares of class X is well
reproduced by this model. Our resulting values show that the most
powerful flares (i.e. class X) induce a variation of more than 20
percent in daily TEC extent.
In the transition from classical to modern physics, the idea of taking some certain quantities as distinct or bounded values and keeping the rest continuous has proved useful in dealing with many problems. In this paper we assume an upper bound on the velocity of classical particles and indicate that applying this assumption to electromagnetism leads to a maximum strength for the magnetic field that shows acceptable agreement with the highest observed strengths in the cosmos, especially in magnetars. Despite its invalidity because of the complete ignoring of quantum physics, this approach can serve as an enjoyable and instructive exercise in introductory physics.
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