Showing papers by "Pawan Kumar published in 1991"

Reconstructing the primordial spectrum of fluctuations of the universe from the observed nonlinear clustering of galaxies

Abstract: It was discovered that the nonlinear evolution of the two point correlation function in N-body experiments of galaxy clustering with Omega = 1 appears to be described to good approximation by a simple general formula. The underlying form of the formula is physically motivated, but its detailed representation is obtained empirically by fitting to N-body experiments. In this paper, the formula is presented along with an inverse formula which converts a final, nonlinear correlation function into the initial linear correlation function. The inverse formula is applied to observational data from the CfA, IRAs, and APM galaxy surveys, and the initial spectrum of fluctuations of the universe, if Omega = 1.

Implications of Solar p-Mode Frequency Shifts

Abstract: We relate entropy and magnetic field perturbations to variations of solar p-mode eigenfrequencies. The frequency variations result from changes in path length and propagation speed. These produce shifts of opposite sign. Path length changes dominate for entropy perturbations, and propagation speed changes dominate for most types of magnetic field perturbations. The p-mode frequencies increased along with solar activity between 1986 and 1989. The frequency shifts exhibit a rapid rise with increasing frequency followed by a precipitous drop. The positive component signals a strengthening of the photospheric magnetic field to an rms value of order 200 G. The sudden drop at high frequency is due to a combination of a resonance and an increase in temperature in the chromospheric cavity. The magnetic stress perturbation decays above the top of the convection zone on a length scale comparable to the pressure scale height and grows gradually with depth below. The former characteristic implies that the stress is mainly due to small magnetic elements of the enhanced network, a conclusion supported by our analysis of Kitt Peak magnetograms. The latter property suggests that the flux tubes which pierce the photosphere strengthen with depth, at least to a pressure level of 10^8 dynes cm^(-2). The presence of a resonance in the chromospheric cavity means that the transition layer maintains enough coherence to partially reflect acoustic waves even near cycle maximum. The fractional chromospheric temperature rise implies a much larger fractional increase in the rate of mechanical heating, as indicated by the variation of the Ca II H and K lines.

The location of the source of high-frequency solar acoustic oscillations

Abstract: Recently Libbrecht and Jefferies et al. have reported regular peaks in the solar oscillation power spectrum extending well above 5.3 mHz, the maximum frequency of trapped acoustic modes. Kumar et al. argued that these peaks are primarily due to the interference of traveling waves which are excited due to acoustic emission from turbulent convection. In contrast with the standing wave P-mode frequencies below 5.3 mHz, the positions of the high-frequency interference peaks (HIPs) are dependent on the location of the source of the acoustic oscillations. In the present work, Kumar et al.'s argument is strengthened, and more importantly, use is made of the above dependence to determine the acoustic source strength as a function of depth. It is found that the acoustic source profile, and thus the convective velocity, is peaked about 200 km deeper than what is expected from standard mixing length theory.

Thermal and mechanical damping of solar p-modes

Abstract: Nonadiabatic effects associated with the transfer of energy and with turbulent stresses add small imaginary parts, ω_i^(1) and ω_i^(2), to solar p-mode eigenfrequencies. Numerical calculations have shown that these quite different processes make comparable contributions to ωi at frequencies well below the acoustic cutoff at ω_(ac). We derive analytic expressions which reveal the connection between ω_i^(1) and ω_i^(2). Our estimates yield ω_i ∝ ω^8 for ω « ω_(ac), in good agreement with the numerical calculations. However, the observed line width is proportional to ω^(4.2) at low frequencies. We suspect that there is an unmodeled component of perturbed convective energy transport or of turbulent viscosity that makes an important contribution to ω_i at ω « ω_(ac).