On spectral density of Neumann matrices

In hep-th/0111281 the complete set of eigenvectors and eigenvalues of Neumann matrices was found. It was shown also that the spectral density contains a divergent constant piece that being regulated by truncation at level L equals (log L)/(2\pi). In this paper we find an exact analytic expression for the finite part of the spectral density. This function allows one to calculate finite parts of various determinants arising in string field theory computations. We put our result to some consistency checks.Comment: 10 pages, 2 figures, LaTeX 2e; typos correction

Magnetic dipole radiation tailored by substrates: numerical investigation

Nanoparticles of high refractive index materials can possess strong magnetic polarizabilities and give rise to artificial magnetism in the optical spectral range. While the response of individual dielectric or metal spherical particles can be described analytically via multipole decomposition in the Mie series, the influence of substrates, in many cases present in experimental observations, requires different approaches. Here, the comprehensive numerical studies of the influence of a substrate on the spectral response of high- index dielectric nanoparticles were performed. In particular, glass, perfect electric conductor, gold, and hyperbolic metamaterial substrates were investigated. Optical properties of nanoparticles were characterized via scattering cross-section spectra, electric field profiles, and induced electric and magnetic moments. The presence of substrates was shown to introduce significant impact on particle's magnetic resonances and resonant scattering cross-sections. Variation of substrate material provides an additional degree of freedom in tailoring properties of emission of magnetic multipoles, important in many applications.Comment: 10 page, 28 figure

Cosmic Rays near Proxima Centauri b

Cosmic rays are an important factor of space weather determining radiation conditions near the Earth and it seems to be essential to clarify radiation conditions near extrasolar planets too. Last year a terrestrial planet candidate was discovered in an orbit around Proxima Centauri. Here we present our estimates on parameters of stellar wind from the Parker model, possible fluxes and fluencies of galactic and stellar cosmic rays based on the available data of the Proxima Centauri activity and its magnetic field. We found that galactic cosmic rays will be practically absent near Proxima b up to energies of 1 TeV due to the modulation by the stellar wind. Stellar cosmic rays may be accelerated in Proxima Centauri events, which are able to permanently maintain density of stellar cosmic rays in the astrosphere comparable to low energy cosmic ray density in the heliosphere. Maximal proton intensities in extreme Proxima events should be by 3–4 orders more than in solar events

About long term modulation of cosmic rays in the 23-24 solar activity cycles

Recently, there has been a significant trend in magnetic fields on the Sun. The total magnetic field of the Sun from the end of the 22nd cycle of solar activity (SA) has more than halved and this decrease continues. Chan- ges in the magnetic field are the key to all the active phenomena occurring on the Sun and in the heliosphere and, accordingly, to processes in cosmic rays. In long-term CR variations in 23-24 cycles of SA the attenuation of the solar magnetic field is displayed and these variations turned out to be the smallest for the entire time of CR observations. Model calculations of CR modulation for 21-22 and 23-24 cycles of SA showed: with a slight difference in the regression characteristics obtained, the distribution of contributions to the generated CR modulation from the effects of various SA indices is strongly varies in the analyzed periods. Possible reasons for the features of the last two CA cycles are discussed

Comparison of long-term variations of the cosmic ray flux from the network of ground-based detectors, PAMELA and AMS-02 data

The paper presents preliminary results of a comparison of long-term variations of the cosmic ray flux using data from the network of ground-based detectors with direct flux measurements on the PAMELA and AMS-02 magnetic spectrometers and a series of balloon stratospheric soundings. The analysis showed good agreement for the entire period of continuous ground-based monitoring of cosmic ray variations

Scattering Suppression from Arbitrary Objects in Spatially-Dispersive Layered Metamaterials

Concealing objects by making them invisible to an external electromagnetic probe is coined by the term cloaking. Cloaking devices, having numerous potential applications, are still face challenges in realization, especially in the visible spectral range. In particular, inherent losses and extreme parameters of metamaterials required for the cloak implementation are the limiting factors. Here, we numerically demonstrate nearly perfect suppression of scattering from arbitrary shaped objects in spatially dispersive metamaterial acting as an alignment-free concealing cover. We consider a realization of a metamaterial as a metal-dielectric multilayer and demonstrate suppression of scattering from an arbitrary object in forward and backward directions with perfectly preserved wavefronts and less than 10% absolute intensity change, despite spatial dispersion effects present in the composite metamaterial. Beyond the usual scattering suppression applications, the proposed configuration may serve as a simple realisation of scattering-free detectors and sensors

Tailoring and enhancing spontaneous two-photon emission processes using resonant plasmonic nanostructures

The rate of spontaneous emission is known to depend on the environment of a light source, and the enhancement of one-photon emission in a resonant cavity is known as the Purcell effect. Here we develop a theory of spontaneous two-photon emission for a general electromagnetic environment including inhomogeneous dispersive and absorptive media. This theory is used to evaluate the two-photon Purcell enhancement in the vicinity of metallic nanoparticles and it is demonstrated that the surface plasmon resonances supported by these particles can enhance the emission rate by more than two orders of magnitude. The control over two-photon Purcell enhancement given by tailored nanostructured environments could provide an emitter with any desired spectral response and may serve as an ultimate route for designing light sources with novel properties

Precursory signals of Forbush decreases with and without shock wave

Many previous studies have shown that before the beginning of a Forbush Decrease (FD) of the cosmic ray intensity, a precursor signal can be observed. All these surveys were focused on FDs that are associated with a sudden storm com- mencement (SSC). In this work we demonstrate that precursors could also be observed in events without a SSC that is determined by an abrupt increase of the interplanetary magnetic field. The type of precursory signals and their diversity among the events are the main purpose of this study. We try to figure out similarities and differences on the signals and the associated events from both categories in the last fifty years, from 1969 to 2019, using the same selection criteria of the under investigation FDs. Simultaneously the orientation of the upcoming solar disturbances in comparison to the way they configure the increase of the interplanetary magnetic field and create these signals are discussed

Magnetospheric effects on cosmic rays during the magnetic storm of March 2015

Cosmic ray variations of magnetospheric origin during the magnetic storm on 17th of March 2015 were studied. Cosmic ray intensity data were obtained from the neutron monitor database (NMDB) and the data of the Dst index were taken from World Data Center for Geomagnetism, Kyoto. The global survey method was employed for the calculation of changes in the cutoff rigidities throughout the storm. A correlation analysis between the Dst index and the calculated cutoff rigidity variations was performed for each cosmic ray station. The most essential decrease in cutoff rigidities occured when the Dst index was around the value of -234nT. A latitudinal distribution of the cutoff rigidities was acquired, showing that the maximum effect took place at mid-latitude stations with rigidities around 8-10GV. During the examined event the maximum change in cutoff rigidity was observed at Athens station where the decrease of the cutoff rigidity reached the value of 1.07GV. Furthermore, corrections of cosmic ray intensity due to the magnetospheric effect were calculated using the derived cutoff rigidities showing a discperancy with the observed values at mid- and low- latitude stations

Relationship of the characteristics of large Forbush decreases and the heliolongitude of their sources

In this investigation the different features and characteristics of Forbush decreases, with emphasis on large For- bush decreases (≥4%) and their association to solar sources, are being examined. According to the heliolongitude of the solar source, the events under study were separated into three subcategories: western (21º ≤ heliolongitude ≤ 60º), eastern (-60º ≤ heliolongitude ≤ -21º) and central (-20º ≤ heliolongitude ≤ 20º). The selected events cover the time period 1967 - 2017. The ‘Global Survey Method’ was used for analyzing the Forbush decreases, along with data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. In ad - dition, the superimposed epoch method was applied in order to plot the time profiles for the aforementioned group of events. This detailed analysis reveals interesting results concerning the features of cosmic ray decreases in re- gard to the heliolongitude of the solar sources. Moreover, it is also shown that large Forbush decreases, regardless of the heliolongitude of the solar source, are accompanied by increased geomagnetic activity and increased aniso- tropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases