MHD Turbulence Revisited

Kraichnan (1965) proposed that MHD turbulence occurs as a result of collisions between oppositely directed Alfv\'en wave packets. Recent work has generated some controversy over the nature of non linear couplings between colliding Alfv\'en waves. We find that the resolution to much of the confusion lies in the existence of a new type of turbulence, intermediate turbulence, in which the cascade of energy in the inertial range exhibits properties intermediate between those of weak and strong turbulent cascades. Some properties of intermediate MHD turbulence are: (i) in common with weak turbulent cascades, wave packets belonging to the inertial range are long lived; (ii) however, components of the strain tensor are so large that, similar to the situation in strong turbulence, perturbation theory is not applicable; (iii) the breakdown of perturbation theory results from the divergence of neighboring field lines due to wave packets whose perturbations in velocity and magnetic fields are localized, but whose perturbations in displacement are not; (iv) 3--wave interactions dominate individual collisions between wave packets, but interactions of all orders $n\geq 3$ make comparable contributions to the intermediate turbulent energy cascade; (v) successive collisions are correlated since wave packets are distorted as they follow diverging field lines; (vi) in common with the weak MHD cascade, there is no parallel cascade of energy, and the cascade to small perpendicular scales strengthens as it reaches higher wave numbers; (vii) For an appropriate weak excitation, there is a natural progression from a weak, through an intermediate, to a strong cascade.Comment: 25 pages, to appear in The Astrophysical Journa

On the Dynamical Stability of the Solar System

A long-term numerical integration of the classical Newtonian approximation to the planetary orbital motions of the full Solar System (sun + 8 planets), spanning 20 Gyr, was performed. The results showed no severe instability arising over this time interval. Subsequently, utilizing a bifurcation method described by Jacques Laskar, two numerical experiments were performed with the goal of determining dynamically allowed evolutions for the Solar System in which the planetary orbits become unstable. The experiments yielded one evolution in which Mercury falls onto the Sun at ~1.261Gyr from now, and another in which Mercury and Venus collide in ~862Myr. In the latter solution, as a result of Mercury's unstable behavior, Mars was ejected from the Solar System at ~822Myr. We have performed a number of numerical tests that confirm these results, and indicate that they are not numerical artifacts. Using synthetic secular perturbation theory, we find that Mercury is destabilized via an entrance into a linear secular resonance with Jupiter in which their corresponding eigenfrequencies experience extended periods of commensurability. The effects of general relativity on the dynamical stability are discussed. An application of the bifurcation method to the outer Solar System (Jupiter, Saturn, Uranus, and Neptune) showed no sign of instability during the course of 24Gyr of integrations, in keeping with an expected Uranian dynamical lifetime of 10^(18) years.Comment: 37 pages, 18 figures, accepted for publication in the Astrophysical Journa

The Heider balance - a continuous approach

The Heider balance (HB) is investigated in a fully connected graph of $N$ nodes. The links are described by a real symmetric array r(i,j), i,j=1,...,N. In a social group, nodes represent group members and links represent relations between them, positive (friendly) or negative (hostile). At the balanced state, r(i,j)r(j,k)r(k,i)>0 for all the triads (i,j,k). As follows from the structure theorem of Cartwright and Harary, at this state the group is divided into two subgroups, with friendly internal relations and hostile relations between the subgroups. Here the system dynamics is proposed to be determined by a set of differential equations. The form of equations guarantees that once HB is reached, it persists. Also, for N=3 the dynamics reproduces properly the tendency of the system to the balanced state. The equations are solved numerically. Initially, r(i,j) are random numbers distributed around zero with a symmetric uniform distribution of unit width. Calculations up to N=500 show that HB is always reached. Time to get the balanced state varies with the system size N as N^{-1/2}. The spectrum of relations, initially narrow, gets very wide near HB. This means that the relations are strongly polarized. In our calculations, the relations are limited to a given range around zero. With this limitation, our results can be helpful in an interpretation of somestatistical data.Comment: 9 pages, 4 figures. Int. J. Mod. Phys. C (2005), in prin

First Results from SPARO: Evidence for Large-Scale Toroidal Magnetic Fields in the Galactic Center

We have observed the linear polarization of 450 micron continuum emission from the Galactic center, using a new polarimetric detector system that is operated on a 2 m telescope at the South Pole. The resulting polarization map extends ~ 170 pc along the Galactic plane and ~ 30 pc in Galactic latitude, and thus covers a significant fraction of the central molecular zone. Our map shows that this region is permeated by large-scale toroidal magnetic fields. We consider our results together with radio observations that show evidence for poloidal fields in the Galactic center, and with Faraday rotation observations. We compare all of these observations with the predictions of a magnetodynamic model for the Galactic center that was proposed in order to explain the Galactic Center Radio Lobe as a magnetically driven gas outflow. We conclude that the observations are basically consistent with the model.Comment: 11 pages, 2 figures, 1 table, submitted to ApJ Let

The automation of a stellar proper motion measuring system Annual report, 1 Jul. 1966 - 30 Jun. 1967

Design and operation of automated stellar proper motion measuring syste

First Results from Viper: Detection of Small-Scale Anisotropy at 40 GHZ

Results of a search for small-scale anisotropy in the cosmic microwave background (CMB) are presented. Observations were made at the South Pole using the Viper telescope, with a .26 degree (FWHM) beam and a passband centered at 40 GHz. Anisotropy band-power measurements in bands centered at l = 108, 173, 237, 263, 422 and 589 are reported. Statistically significant anisotropy is detected in all bands.Comment: 5 pages, 4 figures, uses emulateapj.sty, submitted to ApJ Letter

Anisotropy in the Cosmic Microwave Background at Degree Angular Scales: Python V Results

Observations of the microwave sky using the Python telescope in its fifth season of operation at the Amundsen-Scott South Pole Station in Antarctica are presented. The system consists of a 0.75 m off-axis telescope instrumented with a HEMT amplifier-based radiometer having continuum sensitivity from 37-45 GHz in two frequency bands. With a 0.91 deg x 1.02 deg beam the instrument fully sampled 598 deg^2 of sky, including fields measured during the previous four seasons of Python observations. Interpreting the observed fluctuations as anisotropy in the cosmic microwave background, we place constraints on the angular power spectrum of fluctuations in eight multipole bands up to l ~ 260. The observed spectrum is consistent with both the COBE experiment and previous Python results. There is no significant contamination from known foregrounds. The results show a discernible rise in the angular power spectrum from large (l ~ 40) to small (l ~ 200) angular scales. The shape of the observed power spectrum is not a simple linear rise but has a sharply increasing slope starting at l ~ 150.Comment: 5 page

H. influenzae potentiates airway epithelial cell responses to rhinovirus by increasing ICAM‐1 and TLR3 expression

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154674/1/fsb2fj065806fje.pd