Phenomenological Theory of the Photoevaporation Front Instability

The dynamics of photoevaporated molecular clouds is determined by the ablative pressure acting on the ionization front. An important step in the understanding of the ensuing motion is to develop the linear stability theory for an initially flat front. Despite the simplifications introduced by linearization, the problem remains quite complex and still draws a lot of attention. The complexity is related to the large number of effects that have to be included in the analysis: acceleration of the front, possible temporal variation of the intensity of the ionizing radiation, the tilt of the radiation flux with respect to the normal to the surface, and partial absorption of the incident radiation in the ablated material. In this paper, we describe a model where all these effects can be taken into account simultaneously, and a relatively simple and universal dispersion relation can be obtained. The proposed phenomenological model may prove to be a helpful tool in assessing the feasibility of the laboratory experiments directed towards scaled modeling of astrophysical phenomena. PACS Numbers: 98.38.Dq, 98.38.Hv, 52.38.Mf, 5257.FG, 52.72.+v     

The Plasma and Magnetic Field Characteristics of a Double Discontinuity in Interplanetary Space

A double discontinuity is a rarely observed compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer in the downstream region. In this paper, we report the observations of a double discontinuity detected by Wind on May 15, 1997. This double discontinuity is found to be the front boundary of a magnetic cloud boundary layer. We strictly identify the shock layer and the rotational discontinuity layer by using the high-resolution plasma and magnetic field data from Wind. The observed jump conditions of the upstream and downstream region of the slow shock layer are in good agreement with the Rankine – Hugoniot relations. The flow speeds in the shock frame U _n <V _Acos θ _Bn on both sides of the slow shock layer. In the upstream region, the slow Mach number M _s1=U _n1/V _s1 is 1.95 (above unity), and in the downstream region, the slow Mach number M _s2=U _n2/V _s2 is 0.31 (below unity). Here V _A and V _s represent the Alfvén speed and the local slow magnetosonic speed, respectively, and θ _Bn is the angle between the direction of the magnetic field and the shock normal. The magnetic cloud boundary layer observed by Wind was also detected by Geotail 48 min later when the spacecraft was located outside the bow shock of the magnetosphere. However, Geotail observations showed that its front boundary was no longer a double discontinuity and the rotational discontinuity layer disappeared, indicating that this double discontinuity was unstable when propagating from Wind to Geotail.     

CCD photometry of RV tau stars. VI. TX ophiuchii

Photometric observations of the variable star TX Oph were performed with CCD photometers during the observing seasons of 1999, 2000, and 2002. Analysis of these observations together with published data has confirmed the mean period of the main variability cycle P = 135^d. 2613 over almost 70 years. A secondary cycle with a period of about 10–12 thousand days has also been found.     

Formation of the spectrum of ultra-high-energy cosmic rays: Effect of extragalactic magnetic fields and expected contribution of individual sources

The effect of the extragalactic magnetic field on the propagation of ultra-high-energy cosmic rays (UHECRs) is investigated. We use the infrared galaxy catalog IRAS PSCz to reconstruct the magnetic field distribution in the Local Universe. The magnetic field induction is considered as a power function of the galactic infrared luminosity density: B = Kρ^β. In contrast to some earlier studies in which the exponent β = 2/3 corresponded to the freezing-in condition, the parameters K and β are estimated from the field inductions normalized by the expected maximum inductions (strong field) and minimum inductions (weak field) in galaxy clusters and voids, respectively. Maps of angular deflections of UHECRs are presented for these magnetic field models. We found that the protons with energies E > 4 × 10¹⁹ eV are not significantly deflected from their sources in a sphere with a radius of 100 Mpc only in the case of the weak magnetic field model (the deflections are comparable to the errors of modern detectors). The effect of the extragalactic magnetic field on the UHECR spectrum is investigated, with Virgo A and Arp 299 taken as potential sources.     

Development dynamics of intense solar proton flares

We consider the relationship of electromagnetic radiation in the three most intense flares of solar cycle 23, more specifically, those of October 28, 2003, January 20, 2005, and September 7, 2005, to the acceleration and release of protons into interplanetary space. The impulsive phase of these flares lasted ～ 20 min and consisted of at least three energy release episodes, which differed by their manifestation in the soft (1–8 Å, GOES) and hard (>150 keV, INTEGRAL) X-ray ranges as well as at radio frequencies of 245 MHz and 8.8 GHz. The protons and electrons were accelerated in each episode, but with a different efficiency; the relativistic protons were accelerated only after 5–6min of impulsive-phase development after the onset of a coronal mass ejection. It is at this time that maximum hard X-ray fluxes were observed in the September 7, 2005 event, which exceeded severalfold those for the other two flares considered. We associate the record fluxes of protons with energies > 200MeV observed in the heliosphere in the September 7, 2005 event with the dynamics of the impulsive phase. The extreme intensities of the microwave emission in the October 28, 2003 and January 20, 2005 events were probably attributable to the high-energy electron trapping conditions and did not reflect the acceleration process.     

Five dimensional cosmological models in Lyra geometry with time dependent displacement field

In this paper exact solutions of the five-dimensional vacuum cosmological field equations based on Lyra geometry are obtained. Further it is shown that neither dust distribution nor perfect fluid distributions survive for the model. Some properties of the vacuum model are also discussed.