Wave modes excited by photospheric p-modes and mode conversion in a multi-loop system

Context. Waves are ubiquitous in the solar corona and there are indications that they are excited by photospheric p-modes. However, it is unclear how p-modes in coronal loops are converted to sausage modes and transverse (kink) modes, which are observed in the corona. Aims. We aim to investigate how those wave modes are excited in the lower corona by photospheric acoustic waves. Methods. We built 3D magnetohydrostatic loop systems with multiple inclinations spanning from the photosphere to the lower corona. We then simulated these atmospheres with the MANCHA code, in which we perturb the equilibrium with a p-mode driver at the bottom of the domain. By splitting the velocity perturbation into components longitudinal, normal, and azimuthal to the magnetic flux surfaces we can study wave behavior. Results. In vertical flux tubes, we find that deformed fast sausage surface waves and slow sausage body waves are excited. In inclined flux tubes fast kink surface waves, slow sausage body waves, and either a fast sausage surface wave or a plane wave are excited. In addition, we calculate a wave conversion factor (0 $\le$ C $\le$ 1) from acoustic to magnetic wave behavior by taking the ratio of the mean magnetic energy flux to the sum of the mean magnetic and acoustic energy flux and compare it to a commonly used theoretical conversion factor. We find that between magnetic field inclinations of 10$^\circ$ to 30$^\circ$ those two methods lie within 40%. For smaller inclinations the absolute deviation is smaller than 0.1.Comment: 14 pages, 14 figure

Efficient Coordination in Weakest-Link Games

Existing experimental research on behavior in weakest-link games shows overwhelmingly the inability of people to coordinate on the efficient equilibrium, especially in larger groups. We hypothesize that people will be able to coordinate on efficient outcomes, provided they have sufficient freedom to choose their interaction neighborhood. We conduct experiments with medium sized and large groups and show that neighborhood choice indeed leads to coordination on the fully efficient equilibrium, irrespective of group size. This leads to substantial welfare effects. Achieved welfare is between 40 and 60 percent higher in games with neighborhood choice than without neighborhood choice. We identify exclusion as the simple but very effective mechanism underlying this result. In early rounds, high performers exclude low performers who in consequence ‘learn’ to become high performers.efficient coordination, weakest-link, minimum effort, neighborhood choice, experiment

Observation of the Pairing Gap in a Strongly Interacting Fermi Gas

We study fermionic pairing in an ultracold two-component gas of $^6$Li atoms by observing an energy gap in the radio-frequency excitation spectra. With control of the two-body interactions via a Feshbach resonance we demonstrate the dependence of the pairing gap on coupling strength, temperature, and Fermi energy. The appearance of an energy gap with moderate evaporative cooling suggests that our full evaporation brings the strongly interacting system deep into a superfluid state.Comment: 18 pages, 3 figure

Dynamics of a strongly interacting Fermi gas: the radial quadrupole mode

We report on measurements of an elementary surface mode in an ultracold, strongly interacting Fermi gas of 6Li atoms. The radial quadrupole mode allows us to probe hydrodynamic behavior in the BEC-BCS crossover without being influenced by changes in the equation of state. We examine frequency and damping of this mode, along with its expansion dynamics. In the unitarity limit and on the BEC side of the resonance, the observed frequencies agree with standard hydrodynamic theory. However, on the BCS side of the crossover, a striking down shift of the oscillation frequency is observed in the hydrodynamic regime as a precursor to an abrupt transition to collisionless behavior; this indicates coupling of the oscillation to fermionic pairs.Comment: 11 pages, 11 figures v2: minor change

Exploring the BEC-BCS Crossover with an Ultracold Gas of 6^6Li Atoms

We present an overview of our recent measurements on the crossover from a Bose-Einstein condensate of molecules to a Bardeen-Cooper-Schrieffer superfluid. The experiments are performed on a two-component spin-mixture of $^6$Li atoms, where a Fesh\-bach resonance serves as the experimental key to tune the s-wave scattering length and thus to explore the various interaction regimes. In the BEC-BCS crossover, we have characterized the interaction energy by measuring the size of the trapped gas, we have studied collective excitation modes, and we have observed the pairing gap. Our observations provide strong evidence for superfluidity in the strongly interacting Fermi gas.Comment: Proceedings of ICAP-2004 (Rio de Janeiro). Review on Innsbruck BEC-BCS crossover experiments with updated Feshbach resonance positio

Finite-Temperature Collective Dynamics of a Fermi Gas in the BEC-BCS Crossover

We report on experimental studies on the collective behavior of a strongly interacting Fermi gas with tunable interactions and variable temperature. A scissors mode excitation in an elliptical trap is used to characterize the dynamics of the quantum gas in terms of hydrodynamic or near-collisionless behavior. We obtain a crossover phase diagram for collisional properties, showing a large region where a non-superfluid strongly interacting gas shows hydrodynamic behavior. In a narrow interaction regime on the BCS side of the crossover, we find a novel temperature-dependent damping peak, suggesting a relation to the superfluid phase transition