Cosmic axions from cosmic strings

The possibility of a new constraint on the Peccei-Quinn symmetry breaking scale, arising from the decay of cosmic axion strings, is discussed.     

Searching for energetic cosmic axions in a laboratory experiment

Astrophysical sources of energetic gamma rays provide the right conditions for maximal mixing between (pseudo)scalar (axion-like) particles and photons if their coupling is strong enough. This is independent of whether or not the axion interaction is standard at all energies or becomes suppressed in the extreme conditions of the stellar interior. The flux of such particles through the Earth could be observed using a metre long, Tesla strength superconducting solenoid. The rate of events in CAST caused by axions from the Crab pulsar is also estimated. PACS 14.80.Mz; 98.70.Rz; 95.85.Ry     

Kinetic helicity,magnetic helicity and fast dynamo action

The influence of the flow helicity on kinematic fast dynamo action is considered. Three different flows are studied, possessing identical chaotic properties but very different distributions of helicity (maximal helicity, zero net helicity and zero helicity density). All three flows provide strong evidence of fast dynamo action, indicating that helicity is not a crucial feature of fast dynamo flows. Comparisons are made between the magnetic fields generated by the three flows and it is established how certain key quantities scale with the magnetic Reynolds number. In particular, it is shown that the relative magnetic helicity tends to zero as the magnetic Reynolds number tends to infinity.     

Detecting solar axions using Earth's magnetic field

We show that solar axion conversion to photons in the Earth's magnetosphere can produce an x-ray flux, with average energy omega approximately 4 keV, which is measurable on the dark side of the Earth. The smallness of the Earth's magnetic field is compensated by a large magnetized volume. For axion masses m(a) less, similar10(-4) eV, a low-Earth-orbit x-ray detector with an effective area of 10(4) cm(2), pointed at the solar core, can probe the photon-axion coupling down to 10(-11) GeV-1, in 1 yr. Thus, the sensitivity of this new approach will be an order of magnitude beyond current laboratory limits.     

Emission of axions by a hydrogen-like atom in an ultrastrong magnetic field

The resonance effect of emission of axions by a hydrogen-like atom in an ultrastrong magnetic field B ≫ B ₀ = m ²/e = 4.41 × 10¹³ Gs, which is induced by polarization of electron-positron vacuum, is considered. The emission probability and the radiation intensity are on the order of (B/B ₀) × 10⁻¹² of electromagnetic radiation characteristics, which exceeds the conventional ratio by many orders of magnitude. It is shown that, at the temperature of early Universe ≲(Zα)² m, the contribution from the resonance mechanism prevails. However, the relation between the concentrations of relic photons and axions cannot explain the origin of cold dark matter. The axion energy density in “our epoch” is 10⁻⁴(B/B ₀) eV/cm³.     

Large-scale amplification of a magnetic field by turbulent helicity fluctuations

In this paper the procedure of large-scale averaging of the magnetic-field diffusion equation with the α-term curlα(r,t)B(r,t) is used to show that a nonuniform distribution of the turbulent helicity fluctuations (more precisely, the fluctuations of the coefficient α) with a zero average value gives rise to large-scale amplification of the initial magnetic field. A detailed study is carried out of the dependence of the resulting large-scale α effect on the characteristics of the correlator 〈〈α(r, t)α(r″,t″)〉〉 in a rotating medium with a nonuniform distribution of the angular velocity ω=ω(ρ,z) (ρ is the distance for the rotation axis z). The effect of helicity fluctuations and the diffusion coefficient on the turbulent diffusion process is also investigated. Zh. éksp. Teor. Fiz. 116, 85–104 (July 1999)     

Search for axions emitted in nuclear magnetic transitions

Searches for invisible axions emitted in the electromagnetic transitions of isomeric nuclei are continued. It is found experimentally that the probability of axion emission in an M1 transition in ^125m Te is less than 8.5×10^?6 (90% C.L.).     

Electric control of spin helicity in a magnetic ferroelectric

Magnetic ferroelectrics or multiferroics, which are currently extensively explored, may provide a good arena to realize a novel magnetoelectric function. Here we demonstrate the genuine electric control of the spiral magnetic structure in one such magnetic ferroelectric, TbMnO3. A spin-polarized neutron scattering experiment clearly shows that the spin helicity, clockwise or counterclockwise, is controlled by the direction of spontaneous polarization and hence by the polarity of the small electric field applied on cooling.     

Increasing the magnetic helicity content of a plasma by pulsing a magnetized source

By operating a magnetized coaxial gun in a pulsed mode it is possible to produce large voltage pulses of duration approximately 500 mus while reaching a few kV, giving a discrete input of helicity into a spheromak. In the sustained spheromak physics experiment (SSPX), it is observed that pulsing serves to nearly double the stored magnetic energy and double the temperature. We discuss these results by comparison with 3D MHD simulations of the same phenomenon.     

Current helicity pattern of large-scale magnetic field on the photosphere

Following Pevtsov and Latushko, we study the current helicity pattern of the large-scale magnetic field on the photosphere. We use the same technique as theirs to derive the vector magnetic field (B _r , B _θ , B _ϕ ) from full-disk longitudinal magnetograms based on the assumption that large-scale magnetic fields evolve rather slowly and the variations of the longitudinal magnetic fields within certain time duration are caused by the changing position angles only. Different from their study, we have calculated the current helicity maps directly from the derived vector magnetograms, rather than from obtaining the latitudinal variation of h _c by ignoring the role of B _θ component and averaging B _r and B _ϕ over all solar longitudes. This approach significantly strengthens the evidence of the hemispheric rule presented in the reconstructed vector magnetic field. Our study shows that the established hemispheric sign rule, that is, positive helicity sign in the southern hemisphere and negative helicity sign in the northern hemisphere, is applicable everywhere in the global magnetic field, namely, also evident in weak fields outside active regions, and that the obtained sign pattern is independent of the longitudinal magnetograms and the parameters that we have used.     

Photogeneration of neutrinos and axions under the stimulating effect of a strong magnetic field

Photogeneration of neutrinos and axions at nuclei, $\gamma (Ze) \uparrow \gamma (\nu \bar \nu ),\gamma a$ , and inelastic photon-photon scattering, $\gamma \gamma \uparrow \gamma (\nu \bar \nu ),\gamma a$ , are considered in the 2D covariant theory being developed for calculating matrix elements of Feynman diagrams in a strong magnetic field. Since the matrix elements of four-pole diagrams are linear functions of the magnetic induction B, the contribution of the radiative photogeneration of neutrinos at nuclei to the luminosity of magnetic neutron stars at early stages of their evolution may compete with URCA processes for values of B ～ (10³–10⁴)B ₀ (B ₀ = m ₂ ^e /|e| = 4.41 × 10¹³ G). The upper estimate of the axion mass obtained from the condition of dominance of the neutrino luminosity over the axion luminosity for the proposed values of temperature and magnetic induction is in accord with other independent results.