132 research outputs found
Stability of latitudinal differential rotation in stars
The question is addressed whether stellar differentially rotating radiative
zones (like the solar tachocline) excite nonaxisymmetric r-modes which can be
observed. To this end the hydrodynamical stability of latitudinal differential
rotation is studied. The amount of rotational shear required for the
instability is estimated in dependence of the character of radial
stratification and the flow patterns excited by the instability are found. The
eigenvalue equations for the nonaxisymmetric disturbances are formulated in 3D
and then solved numerically. Radial displacements and entropy disturbances are
included. The equations contain the 2D approximation of strictly horizontal
displacements as a special limit. The critical magnitude of the latitudinal
differential rotation for onset of the instability is considerably reduced in
the 3D theory compared to the 2D approximation. The instability requires a
subadiabatic stratification. It does not exist in the bulk of convection zone
with almost adiabatic stratification but may switch on near its base in the
region of penetrative convection. Growth rates and symmetry types of the modes
are computed in dependence on the rotation law parameters. The S1 mode with its
transequatorial toroidal vortices is predicted as the dominating instability
mode. The vortices show longitudinal drift rates retrograde to the basic
rotation which are close to that of the observed weak r-mode signatures at the
solar surface.Comment: 5 pages, 6 figure
North-South Distribution of Solar Flares during Cycle 23
In this paper, we investigate the spatial distribution of solar flares in the
northern and southern hemisphere of the Sun that occurred during the period
1996 to 2003. This period of investigation includes the ascending phase, the
maximum and part of descending phase of solar cycle 23. It is revealed that the
flare activity during this cycle is low compared to previous solar cycle,
indicating the violation of Gnevyshev-Ohl rule. The distribution of flares with
respect to heliographic latitudes shows a significant asymmetry between
northern and southern hemisphere which is maximum during the minimum phase of
the solar cycle. The present study indicates that the activity dominates the
northern hemisphere in general during the rising phase of the cycle
(1997-2000). The dominance of northern hemisphere is shifted towards the
southern hemisphere after the solar maximum in 2000 and remained there in the
successive years. Although the annual variations in the asymmetry time series
during cycle 23 are quite different from cycle 22, they are comparable to cycle
21.Comment: 6 pages, 2 figures, 1 table; Accepted for the publication in the
proceedings of international solar workshop held at ARIES, Nainital, India on
"Transient Phenomena on the Sun and Interplanetary Medium" in a special issue
of "Journal of Astrophysics and Astronomy (JAA)
Measurement of Muon Capture on the Proton to 1% Precision and Determination of the Pseudoscalar Coupling g_P
The MuCap experiment at the Paul Scherrer Institute has measured the rate L_S
of muon capture from the singlet state of the muonic hydrogen atom to a
precision of 1%. A muon beam was stopped in a time projection chamber filled
with 10-bar, ultra-pure hydrogen gas. Cylindrical wire chambers and a segmented
scintillator barrel detected electrons from muon decay. L_S is determined from
the difference between the mu- disappearance rate in hydrogen and the free muon
decay rate. The result is based on the analysis of 1.2 10^10 mu- decays, from
which we extract the capture rate L_S = (714.9 +- 5.4(stat) +- 5.1(syst)) s^-1
and derive the proton's pseudoscalar coupling g_P(q^2_0 = -0.88 m^2_mu) = 8.06
+- 0.55.Comment: Updated figure 1 and small changes in wording to match published
versio
Measurement of the Rate of Muon Capture in Hydrogen Gas and Determination of the Proton's Pseudoscalar Coupling
The rate of nuclear muon capture by the proton has been measured using a new
experimental technique based on a time projection chamber operating in
ultra-clean, deuterium-depleted hydrogen gas at 1 MPa pressure. The capture
rate was obtained from the difference between the measured
disappearance rate in hydrogen and the world average for the decay
rate. The target's low gas density of 1% compared to liquid hydrogen is key to
avoiding uncertainties that arise from the formation of muonic molecules. The
capture rate from the hyperfine singlet ground state of the atom is
measured to be , from which the induced
pseudoscalar coupling of the nucleon, , is
extracted. This result is consistent with theoretical predictions for
that are based on the approximate chiral symmetry of QCD.Comment: submitted to Phys.Rev.Let
Designing Building Skins with Biomaterials
This chapter presents several successful examples of biomaterial facade design. It discusses facade function from aesthetical, functional, and safety perspectives. Special focus is directed on novel concepts for adaptation and special functionalities of facades. Analysis of the structure morphologies and aesthetic impressions related to the bio-based building facades is supported with photographs collected by authors in various locations. Finally, particular adaptations and special functionalities of bio-based facades going beyond traditional building envelope concept are supported by selected case studies
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