We present diffusion Monte Carlo calculations of low-lying rotational states of 4 He N SF 6 which confirm recent experiments suggesting that dopant molecules trapped inside 4 He clusters behave as free rotors. Analysis of the rotational wave functions leads to a clear physical explanation for this effect based on angular momentum coupling arguments; a fraction of the helium density is found to follow the rotation of the SF 6 molecule adiabatically. This leads to a free-rotor spectrum and to a reduction in the effective rotational constant that is in excellent quantitative agreement with experiment. PACS numbers: 36.40.Mr, 02.70.Lq, 33.20.Sn The rotational dynamics of a solute molecule in a classical liquid are rather complicated: collisions scramble the molecule's angular momentum and, as a result, it is rarely possible to observe sharp rotational lines. In recent experiments, however, rotationally resolved spectra of single molecules in a quantum liquid have been obtained: the "dopant" molecules are introduced into droplets of superfluid 4 He using pickup techniques in a molecular beam which are then studied by laser spectroscopy [1][2][3][4][5][6][7][8]. The rotational spectra so obtained can be fit rather well by assuming a free rotor Hamiltonian, usually of the same symmetry as the gas phase molecule, but with a reduced rotational constant. By contrast, rotationally resolved spectra are not observed in fermionic 3 He clusters which, at the temperatures achieved in the experiments (T ϳ 0.15 0.4 K), lie above the corresponding superfluid transition temperature. These experimental findings have been taken to imply that free rotation of the dopant molecules is a consequence of the superfluidity of the bosonic 4 He clusters [3].SF 6 was the first molecule to be studied using high resolution spectroscopy in a 4 He N droplet [1]; accordingly, this Letter provides a molecular explanation for the experimental observation of a free rotor energy level pattern in SF 6 -doped 4 He clusters. We also provide a physical description of the reduction in the effective rotational constant, B. This is accomplished through explicit quantum calculations of the low-lying cluster rotational energy level structure for 4 He N SF 6 , with 1 # N # 20, which spans the first solvation shell of the molecule [9]. These calculations elucidate the generality of the phenomenon of apparent free rotation in 4 He N by demonstrating that the spectroscopically relevant excited states correspond to J ϳ j, where J and j denote the total cluster and dopant angular momentum quantum numbers, respectively. Despite the anisotropy introduced by the dopant, these excitations distribute almost no angular momentum among the 4 He atoms and j becomes a quasi-good quantum number. This is a consequence of the boson statistics of 4 He, and would not be true for 3 He N clusters, in which the 3 He atoms would carry some angular momentum, even in an isotropic (central field) potential, due to fermion statistics. SF 6 is a prototype for molecules that are relatively...

The Cluster and Double Star satellites recently observed plasma density holes upstream of Earth’s collisionless bow shock to apogee distances of ∼19 and 13 earth radii, respectively. A survey of 147 isolated density holes using 4s time resolution data shows they have a mean duration of ∼17.9±10.4s, but holes as short as 4s are observed. The average fractional density depletion (δn∕n) inside the holes is ∼0.68±0.14. The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with the solar wind are important.

[1] Cluster encountered a thin current sheet in the magnetotail during a substorm on Oct. 1, 2001. The phase space distributions observed by Cluster CIS showed large gradients in the phase space with ions existing only in one hemisphere of the phase space. It has been suggested that these non-gyrotropic distributions come from remote sensing of a thin current sheet by spacecrafts outside the current sheet. We present the results of a test particle simulation that confirm non-gyrotropic distributions can arise from remote sensing of a thin current sheet. These nongyrotropic distributions can yield large velocity moments even when the plasma is stationary. The large velocity moments are not in the convective flow direction that would be normal to the current sheet. Instead they are along the n  B direction where n is normal to the boundary. These modeling results reinforce the importance of examining full particle distributions when studying plasma sheet dynamics.

We have previously shown that ExoU, a type III secreted cytotoxin of Pseudomonas aeruginosa, causes acute cytotoxicity towards corneal epithelial cells in vitro, and contributes to corneal disease pathology and ocular colonization in vivo. Subsequently, we reported that ExoU represses phagocyte infiltration of infected corneas in vivo. ExoU has patatin-like phospholipase activity that is required for cytotoxic activity in vitro (mammalian cell injury and death) and for disease in a murine model of pneumonia. We hypothesized that the phospholipase activity was required for ExoU-mediated corneal disease and ocular colonization. Using the murine scarification model, corneal disease pathology was examined after inoculation with ~10 6 cfu of a P. aeruginosa effector mutant (PA103ΔexoUexoT::Tc) complemented with either exoU (pUCPexoU), phospholipase-inactive exoU (pUCPexoUD344A) or a plasmid control (pUCP18). Eyes were photographed and disease severity scored at 24 and 48 h post-infection. Viable bacteria colonizing infected eyes were quantified at 6 and 48 h. Complementation with exoU caused significantly more pathology (increased disease severity scores) and enabled bacteria to better colonize (bỹ 1000-fold) at 48 h as compared to phospholipase-inactive exoU which did not differ from plasmid control. Surprisingly, exoU did not contribute to early (6 h) colonization. In-vitro assays confirmed that the phospholipase domain of exoU was required for cytotoxicity towards human corneal epithelial cells. Taken together these data show that the phospholipase activity of the P. aeruginosa cytotoxin, ExoU, plays a role in the pathogenesis of corneal infection via mechanism(s) occurring after initial colonization of a susceptible cornea.

[1] Energy spectra of electron microbursts in the energy range 170-360 keV have been measured in the outer radiation zone by the low-altitude (680 km), polarorbiting Korean satellite STSAT-1. These electrons are the lower energy population of relativistic microbursts. Our observations show microburst energy spectra of precipitated electrons inside the loss cone (precipitated) have higher e-folding energies during disturbed times than quiet times. The loss cone at these energies is empty except when microbursts abruptly appear and fill the loss cone in less than 50 msec. This fast pitch angle diffusion requires diffusion coefficients larger than $3.5 Â 10 À2 rad 2 /sec, while $1.5 Â 10 À5 rad 2 /sec was proposed by a wave particle interaction theory. The source of microbursts remains unknown as our observations are not adequately explained by wave and particle resonant interaction models.