Measurements of the HI 21-cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the line-of-sight direction of any observed volume. We use numerical as well as semi-numerical simulations of reionization
in a cubic volume of 607 Mpc across to study this so-called light cone effect on the HI 21-cm power spectrum. We find that the light cone effect has the largest impact at two different stages of reionization: one when reionization is $sim 20%$ and other when it is $sim 80%$ completed. We find a factor of $sim 4$ amplification of the power spectrum at the largest scale available in our simulations. We do not find any significant anisotropy in the 21-cm power spectrum due to the light cone effect. We argue that for the power spectrum to become anisotropic, the light cone effect would have to make the ionized bubbles significantly elongated or compressed along the line-of-sight, which would require extreme reionization scenarios. We also calculate the two-point correlation functions parallel and perpendicular to the line-of-sight and find them to differ. Finally, we calculate an optimum frequency bandwidth below which the light cone effect can be neglected when extracting power spectra from observations. We find that if one is willing to accept a $10 %$ error due to the light cone effect, the optimum frequency bandwidth for $k= 0.056 , rm{Mpc}^{-1}$ is $sim 7.5$ MHz. For $k = 0.15$ and $0.41 , rm{Mpc}^{-1}$ the optimum bandwidth is $sim 11$ and $sim 16$ MHz respectively.
The forces acting on solar Coronal Mass Ejections (CMEs) in the interplanetary medium have been evaluated so far in terms of an empirical drag coefficient $C_{rm D} sim 1$ that quantifies the role of the aerodynamic drag experienced by a typical CME
due to its interaction with the ambient solar wind. We use a microphysical prescription for viscosity in the turbulent solar wind to obtain an analytical model for the drag coefficient $C_{rm D}$. This is the first physical characterization of the aerodynamic drag experienced by CMEs. We use this physically motivated prescription for $C_{rm D}$ in a simple, 1D model for CME propagation to obtain velocity profiles and travel times that agree well with observations of deceleration experienced by fast CMEs.
A complete understanding of Doppler shift in active region loops can help probe the basic physical mechanism involved into the heating of those loops. Here we present observations of upflows in coronal loops detected in a range of temperature tempera
tures (log T=5.8 - 6.2). The loop was not discernible above these temperatures. The speed of upflow was strongest at the footpoint and decreased with height. The upflow speed at the footpoint was about 20 km/s in Fe VIII which decreased with temperature being about 13 km/s in Fe X, about 8 km/s in Fe XII and about 4 km/s in FeXIII. To the best of our knowledge this is the first observation providing evidence of upflow of plasma in coronal loop structures at these temperatures. We interpret these observations as evidence of chromospheric evaporation in quasi-static coronal loops.
[Abridged] We present a detailed study of the largest sample of intervening O VI systems in the redshift range 1.9 < z < 3.1 detected in high resolution (R ~ 45,000) spectra of 18 bright QSOs observed with VLT/UVES. Based on Voigt profile and apparen
t optical depth analysis we find that (i) the Doppler parameters of the O VI absorption are usually broader than those of C IV (ii) the column density distribution of O VI is steeper than that of C IV (iii) line spread (delta v) of the O VI and C IV are strongly correlated (at 5.3sigma level) with delta v(O VI) being systematically larger than delta v(C IV) and (iv) delta v(O VI) and delta v(C IV) are also correlated (at > 5sigma level) with their respective column densities and with N(H I) (3 and 4.5 sigma respectively). These findings favor the idea that C IV and O VI absorption originate from different phases of a correlated structure and systems with large velocity spread are probably associated with overdense regions. The velocity offset between optical depth weighted redshifts of C IV and O VI absorption is found to be in the range 0 < |Delta v (O VI - CIV)| < 48 km/s with a median value of 8 km/s. We compare the properties of O VI systems in our sample with that of low redshift (z < 0.5) samples from the literature and find that (i) the O VI components at low-z are systematically wider than at high-z with an enhanced non-thermal contribution to their b-parameter, (ii) the slope of the column density distribution functions for high and low-z are consistent, (iii) range in gas temperature estimated from a subsample of well aligned absorbers are similar at both high and low-z, and (iv) Omega_{O VI} = (1.0 pm 0.2) times10^{-7} for N(O VI) > 10^{13.7} cm^{-2}, estimated in our high-z sample, is very similar to low-z estimations.
We seek to reconcile observations of small source sizes in the solar corona at 327 MHz with predictions of scattering models that incorporate refractive index effects, inner scale effects and a spherically diverging wavefront. We use an empirical pre
scription for the turbulence amplitude $C_{N}^{2}(R)$ based on VLBI observations by Spangler and coworkers of compact radio sources against the solar wind for heliocentric distances $R approx$ 10--50 $R_{odot}$. We use the Coles & Harmon model for the inner scale $l_{i}(R)$, that is presumed to arise from cyclotron damping. In view of the prevalent uncertainty in the power law index that characterizes solar wind turbulence at various heliocentric distances, we retain this index as a free parameter. We find that the inclusion of spherical divergence effects suppresses the predicted source size substantially. We also find that inner scale effects significantly reduce the predicted source size. An important general finding for solar sources is that the calculations substantially underpredict the observed source size. Three possible, non-exclusive, interpretations of this general result are proposed. First and simplest, future observations with better angular resolution will detect much smaller sources. Consistent with this, previous observations of small sources in the corona at metric wavelengths are limited by the instrument resolution. Second, the spatially-varying level of turbulence $C_{N}^{2}(R)$ is much larger in the inner corona than predicted by straightforward extrapolation Sunwards of the empirical prescription, which was based on observations between 10--50 $R_{odot}$. Either the functional form or the constant of proportionality could be different. Third, perhaps the inner scale is smaller than the model, leading to increased scattering.
We present the results from a study of wide profile pulsars using high sensitivity multifrequency observations with the GMRT. Since the line of sight samples a large region of the polar cap in case of the wide profile pulsars, presence of simultaneou
s multiple drift regions is quite probable (as seen in PSR B0826-34 and PSR B0818-41). We solve the aliasing problem of PSR B0818-41 using the observed phase relationship of the drift regions, and determine its pattern rotation period P4 to be ~ 10s, which makes it the fastest known carousel. We find that, for all the pulsars showing drifting in multiple rings of emission, the drift pattern from the rings are phase locked. This can constraint the theoretical models of pulsar emission as it favors a pan magnetospeheric radiation mechanism.
In this article, we develop a formalism which is different from the standard lensing scenario and is necessary for understanding lensing by gravitational fields which arise as solutions of the effective Einstein equations on the brane. We obtain gene
ral expressions for measurable quantities such as time delay, deflection angle, Einstein ring and magnification. Subsequently, we estimate the deviations (relative to the standard lensing scenario) in the abovementioned quantities by considering the line elements for clusters and spiral galaxies obtained by solving the effective Einstein equations on the brane. Our analysis reveals that gravitational lensing can be a useful tool for testing braneworld gravity as well as the existence of extra dimensions.
