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• 801. Wilder, F. D.
KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
The nonlinear behavior of whistler waves at the reconnecting dayside magnetopause as observed by the Magnetospheric Multiscale mission: A case study2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 5, s. 5487-5501Artikkel i tidsskrift (Fagfellevurdert)

We show observations of whistler mode waves in both the low-latitude boundary layer (LLBL) and on closed magnetospheric field lines during a crossing of the dayside reconnecting magnetopause by the Magnetospheric Multiscale (MMS) mission on 11 October 2015. The whistlers in the LLBL were on the electron edge of the magnetospheric separatrix and exhibited high propagation angles with respect to the background field, approaching 40°, with bursty and nonlinear parallel electric field signatures. The whistlers in the closed magnetosphere had Poynting flux that was more field aligned. Comparing the reduced electron distributions for each event, the magnetospheric whistlers appear to be consistent with anisotropy-driven waves, while the distribution in the LLBL case includes anisotropic backward resonant electrons and a forward resonant beam at near half the electron-Alfvén speed. Results are compared with the previously published observations by MMS on 19 September 2015 of LLBL whistler waves. The observations suggest that whistlers in the LLBL can be both beam and anisotropy driven, and the relative contribution of each might depend on the distance from the X line.

• 802.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
Cosmic ray antiprotons at high energies2017Inngår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 2, artikkel-id 048Artikkel i tidsskrift (Fagfellevurdert)

Cosmic ray antiprotons provide a powerful tool to probe dark matter annihilations in our galaxy. The sensitivity of this important channel is, however, diluted by sizable uncertainties in the secondary antiproton background. In this work, we improve the calculation of secondary antiproton production with a particular focus on the high energy regime. We employ the most recent collider data and identify a substantial increase of antiproton cross sections with energy. This increase is driven by the violation of Feynman scaling as well as by an enhanced strange hyperon production. The updated antiproton production cross sections are made publicly available for independent use in cosmic ray studies. In addition, we provide the correlation matrix of cross section uncertainties for the AMS-02 experiment. At high energies, the new cross sections improve the compatibility of the AMS-02 data with a pure secondary origin of antiprotons in cosmic rays.

• 803. Wiseman, D. R.
KTH, Skolan för teknikvetenskap (SCI), Fysik.
Probe of triple shape coexistence in neutron deficient polonium nuclei2006Inngår i: AIP Conf. Proc., 2006, s. 560-562Konferansepaper (Fagfellevurdert)

γ-ray transitions in the neutron deficient 190,197Po nuclei have been identified. The yrast band of 190Po has been extended up to a spin and parity of 14+ and is found to display similar systematic behaviour to isotones 186Hg and 188Pb above the 4+ level, thus confirming its prolate nature. In 197Po the band built upon the 13/2+ isomer has been extended up to a spin and parity of 33/2+, while the non-yrast side-band has been observed for the first time. The behaviour of 197Po is found to be similar to that of the nearby even-mass isotopes, which is consistent with the model in which the i13/2 neutron is weakly coupled to the states in the even-even core.

• 804.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF. KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
Band inversion and the topological phase transition in (Pb,Sn)Se2014Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, nr 16, s. 161202-Artikkel i tidsskrift (Fagfellevurdert)

The recent discovery of a topological phase transition in IV-VI narrow-gap semiconductors has revitalized the decades-old interest in the bulk band inversion occurring in these materials. Here we systematically study the (001) surface states of Pb1-xSnxSe mixed crystals by means of angle-resolved photoelectron spectroscopy in the parameter space 0 <= x <= 0.37 and 300 K >= T >= 9 K. Using the surface-state observations, we monitor directly the topological phase transition in this solid solution and gain valuable information on the evolution of the underlying fundamental band gap of the system. In contrast to common model expectations, the band-gap evolution appears to be nonlinear as a function of the studied parameters, resulting in the measuring of a discontinuous band-inversion process. This finding signifies that the anticipated gapless bulk state is in fact not a stable configuration and that the topological phase transition therefore exhibits features akin to a first-order transition.

• 805.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurements of cosmic ray antiprotons with PAMELA and studies of propagation models2012Doktoravhandling, monografi (Annet vitenskapelig)

Studying the acceleration and propagation mechanisms of Galactic cosmic rays can provide information regarding astrophysical sources, the properties of our Galaxy, and possible exotic sources such as dark matter. To understand cosmic ray acceleration and propagation mechanisms, accurate measurements of different cosmic ray elements over a wide energy range are needed. The PAMELA experiment is a satellite-borne apparatus which allows different cosmic ray species to be identified over background.

Measurements of the cosmic ray antiproton flux and the antiproton-to-proton flux ratio from 1.5 GeV to 180 GeV are presented in this thesis, employing the data collected between June 2006 and December 2008. Compared to previous experiments, PAMELA extends the energy range of antiproton measurements and provides significantly higher statistics. During about 800 days of data collection, PAMELA identified approximately 1300 antiprotons including 61 above 31.7 GeV. A dramatic improvement of statistics is evident since only 2 events above 30 GeV are reported by previous experiments. The derived antiproton flux and antiproton-to-proton flux ratio are consistent with previous measurements and generally considered to be produced as secondary products when cosmic ray protons and helium nuclei interact with the interstellar medium.

To constrain cosmic ray acceleration and propagation models, the antiproton data measured by PAMELA were further used together with the proton spectrum reported by PAMELA, as well as the B/C data provided by other experiments. Statistical tools were interfaced with the cosmic ray propagation package GALPROP to perform the constraining analyses.

Different diffusion models were studied. It was shown in this work that only current PAMELA data, i.e. the antiproton-to-proton ratio and the proton flux, are not able to place strong constraints on propagation parameters. Diffusion models with a linear diffusion coefficient and modified diffusion models with a low energy dependence of the diffusion coefficient were studied in the $\chi^{2}$ study. Uncertainties on the parameters and the goodness of fit of each model were given. Some models are further studied using the Bayesian inference. Posterior means and errors of the parameters base on our prior knowledge on them were obtained in the Bayesian framework. This method also allowed us to understand the correlation between parameters and compare models.

Since the B/C ratio used in this analysis is from experiments other than PAMELA, future PAMELA secondary-to-primary ratios (B/C, $^{2}$H/$^{4}$He and $^{3}$He/$^{4}$He) can be used to avoid the data sets inconsistencies between different experiments and to minimize uncertainties on the solar modulation parameters. More robust and tighter constraints are expected. The statistical techniques have been demonstrated useful to constrain models and can be extended to other observations, e.g. electrons, positrons, gamma rays etc. Using these channels, exotic contributions from, for example, dark matter will be further investigated in future.

• 806.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden..
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden.
A Study of Background Conditions for Sphinx-The Satellite-Borne Gamma-Ray Burst Polarimeter2018Inngår i: Galaxies, E-ISSN 2075-4434, Vol. 6, nr 2, artikkel-id 50Artikkel i tidsskrift (Fagfellevurdert)

SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy range 50-500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray burst prompt emission through polarisation measurements. Optimising the signal-to-background ratio for SPHiNX is an important task during the design phase. The Geant4 Monte Carlo toolkit is used in this work. From the simulation, the total background outside the South Atlantic Anomaly (SAA) is about 323 counts/s, which is dominated by the cosmic X-ray background and albedo gamma rays, which contribute similar to 60% and similar to 35% of the total background, respectively. The background from albedo neutrons and primary and secondary cosmic rays is negligible. The delayed background induced by the SAA-trapped protons is about 190 counts/s when SPHiNX operates in orbit for one year. The resulting total background level of similar to 513 counts/s allows the polarisation of similar to 50 GRBs with minimum detectable polarisation less than 30% to be determined during the two-year mission lifetime.

• 807. Xing, G.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
Thermal-diffusional instability in white dwarf flames: Regimes of flame pulsation2017Inngår i: 10th U.S. National Combustion Meeting, Eastern States Section of the Combustion Institute , 2017Konferansepaper (Fagfellevurdert)

Thermal-diffusional pulsation behaviors in planar as well as outwardly and inwardly propagating white dwarf carbon flames are systematically studied. It is shown that different equations of state in non-degenerate and degenerate matters in white dwarfs lead to different criterions of flame pulsation, with the critical Zel'dovich number in the later twice as large as in the former. For realistic physical conditions in white dwarf carbon flames, the asymptotic degenerate equation of state is adopted and the simplified one-step reaction rate for nuclear reactions are used to study the flame propagation. Flame front pulsation behaviors in different environmental densities and temperatures are obtained to form the regime diagram of pulsation, showing that carbon flames pulsate in the typical density and temperature. While being stable at higher temperatures, in relatively lower temperatures the amplitude of the flame pulsation becomes larger. In outwardly propagating spherical flames the pulsation instability is enhanced and flames are also easier to quench, while the inwardly propagating flames are more stable.

• 808. Yee, H. C.
KTH, Skolan för datavetenskap och kommunikation (CSC), Numerisk Analys och Datalogi, NADA.
Development of low dissipative high order filter schemes for multiscale Navier-Stokes/MHD systems2006Inngår i: Numerical Modeling of Space Plasma Flows: Astronum-2006 / [ed] Pogorelov, N; Zank, GP, SAN FRANCISCO: ASTRONOMICAL SOC PACIFIC , 2006, Vol. 359, s. 47-61Konferansepaper (Fagfellevurdert)

Recent progress in the development of a class of low dissipative high order filter schemes for multiscale Navier-Stokes, and ideal and non-ideal magnetohydrodynamics (MHD) systems is described. The four main features of this schemes are: (a) multiresolution wavelet decomposition of the computed flow data as sensors for adaptive numerical dissipative control, (b) multistep filter to accommodate efficient application of different numerical dissipation models and different spatial high order base schemes, (c) solving the ideal conservative MHD system without having to deal with an incomplete eigensystern set while at the same time ensuring that correct shock speeds and locations are computed, and (d) minimization of the divergence of the magnetic field (del(.) B) numerical error. By design, the flow sensors, different choice of high order base schemes and numerical dissipation models are stand-alone modules. A whole class of low dissipative high order schemes can be derived at ease, making the resulting computer software very flexible with widely applicable.

• 809. Yu, Hoi-Fung
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Synchrotron cooling in energetic gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor2015Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, artikkel-id A81Artikkel i tidsskrift (Fagfellevurdert)

Context. We study the time-resolved spectral properties of energetic gamma-ray bursts (GRBs) with good high-energy photon statistics observed by the Gamma-Ray Burst Monitor ((IBM) onboard the Fermi Gamma-Ray Space Telescope. Aims. We aim to constrain in detail the spectral properties of GRB prompt emission on a time-resolved basis and to discuss the theoretical implications of the fitting results in the context of various prompt emission models. Methods. Our sample comprises eight GRBs observed by the Fermi (IBM in its first five years of mission, with 1 keV-1 MeV fluence f > 1.0 x 10(-4) erg cm(-2) and a signal-to-noise ratio level of S/N >= 10.0 above 900 keV. We performed a time-resolved spectral analysis using a variable temporal binning technique according to optimal S/N criteria, resulting in a total of 299 time-resolved spectra. We performed Band function fits to all spectra and obtained the distributions for the low-energy power-lay index alpha, the high-energy power-law index beta, the peak energy in the observed nu F-nu, spectrum E-p, and the difference between the low- and high-energy power-law indices Delta s = alpha-beta. We also applied a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test the prompt emission for consistency with a synchrotron origin and obtain the distributions for the two break energies E-b,E-1 and E-b,E-2 the middle segment power-law index beta, and the Planck function temperature kT. Results. The Band function parameter distributions are alpha = -0.73(-0.21)(+0.16), beta = -2.13(-0.56)(+0.28), E-p = 374.47(-187.7)(+307.3) keV (log(10) E-p = 2.577(-0.30)(+0.26)), and Delta s = 1.38(-0.31)(+0.54), with average errors sigma(alpha) similar to 0.1, sigma(beta) similar to 0.2, and sigma(Ep) similar to 0.1E(p). Using the distributions of Delta s and beta, the electron population index p is found to be consistent with the "moderately fast" scenario, in which fast- and slow-cooling scenarios cannot be distinguished. The physically motivated synchrotron-fitting function parameter distributions are E-b,E-1 = 129.6(-32.4)(+132.2) keV, E-b,E-2 = 631.4(-309.6)(+582) keV, beta = 1.721(-0.25)(+0.48), and kT = 10.4(-3.7)(+4.9) keV, with average errors sigma(beta) similar to 0.2, sigma E-b,E-1 similar to 0.1E(b,1), sigma E-b,E-2 similar to 0.4E(b,2,) and sigma(kT) similar to 0.1kT. This synchrotron function requires the synchrotron injection and cooling break (i.e., E-min and E-cool) to be close to each other within a factor of ten, often in addition to a Planck function. Conclusions. A synchrotron model is found that is consistent with most of the time-resolved spectra for eight energetic Fermi (IBM bursts with good high-energy photon statistics as long as both the cooling and injection break are included and the leftmost spectral slope is lifted either by including a thermal component or when an evolving magnetic field is accounted for.

• 810.
Beihang Univ, Sch Space & Environm, Beijing, Peoples R China..
Los Alamos Natl Lab, Los Alamos, NM USA.. Los Alamos Natl Lab, Los Alamos, NM USA.. KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, Parallelldatorcentrum, PDC. KTH, Skolan för elektroteknik och datavetenskap (EECS), Beräkningsvetenskap och beräkningsteknik (CST).
PIC simulations of wave-particle interactions with an initial electron velocity distribution from a kinetic ring current model2018Inngår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 177, s. 169-178Artikkel i tidsskrift (Fagfellevurdert)

Whistler wave-particle interactions play an important role in the Earth inner magnetospheric dynamics and have been the subject of numerous investigations. By running a global kinetic ring current model (RAM-SCB) in a storm event occurred on Oct 23-24 2002, we obtain the ring current electron distribution at a selected location at MLT of 9 and L of 6 where the electron distribution is composed of a warm population in the form of a partial ring in the velocity space (with energy around 15 keV) in addition to a cool population with a Maxwellian-like distribution. The warm population is likely from the injected plasma sheet electrons during substorm injections that supply fresh source to the inner magnetosphere. These electron distributions are then used as input in an implicit particle-in-cell code (iPIC3D) to study whistler-wave generation and the subsequent wave-particle interactions. We find that whistler waves are excited and propagate in the quasi-parallel direction along the background magnetic field. Several different wave modes are instantaneously generated with different growth rates and frequencies. The wave mode at the maximum growth rate has a frequency around 0.62 omega(ce), which corresponds to a parallel resonant energy of 2.5 keV. Linear theory analysis of wave growth is in excellent agreement with the simulation results. These waves grow initially due to the injected warm electrons and are later damped due to cyclotron absorption by electrons whose energy is close to the resonant energy and can effectively attenuate waves. The warm electron population overall experiences net energy loss and anisotropy drop while moving along the diffusion surfaces towards regions of lower phase space density, while the cool electron population undergoes heating when the waves grow, suggesting the cross-population interactions.

• 811. Zackrisson, Erik
KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
Finding high-redshift dark stars with the James Webb Space Telescope2010Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 717, nr 1, s. 257-267Artikkel i tidsskrift (Fagfellevurdert)

The first stars in the history of the universe are likely to form in the dense central regions of similar to 10(5)-10(6) M-circle dot cold dark matter halos at z approximate to 10-50. The annihilation of dark matter particles in these environments may lead to the formation of so-called dark stars, which are predicted to be cooler, larger, more massive, and potentially more long-lived than conventional population III stars. Here, we investigate the prospects of detecting high-redshift dark stars with the upcoming James Webb Space Telescope (JWST). We find that all dark stars with masses up to 10(3) M-circle dot are intrinsically too faint to be detected by JWST at z > 6. However, by exploiting foreground galaxy clusters as gravitational telescopes do, certain varieties of cool (T-eff <= 30,000 K) dark stars should be within reach at redshifts up to z approximate to 10. If the lifetimes of dark stars are sufficiently long, many such objects may also congregate inside the first galaxies. We demonstrate that this could give rise to peculiar features in the integrated spectra of galaxies at high redshifts, provided that dark stars make up at least similar to 1% of the total stellar mass in such objects.

• 812. Zackrisson, Erik
KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
Observational constraints on supermassive dark stars2010Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 407, nr 1, s. L74-L78Artikkel i tidsskrift (Fagfellevurdert)

Some of the first stars could be cooler and more massive than standard stellar models would suggest, due to the effects of dark matter annihilation in their cores. It has recently been argued that such objects may attain masses in the 10(4)-10(7) M-circle dot range and that such supermassive dark stars should be within reach of the upcoming James Webb Space Telescope. Notwithstanding theoretical difficulties with this proposal, we argue here that some of these objects should also be readily detectable with both the Hubble Space Telescope and ground-based 8-10 m class telescopes. Existing survey data already place strong constraints on 10(7) M-circle dot dark stars at z approximate to 10. We show that such objects must be exceedingly rare or short lived to have avoided detection.

• 813. Zender, J. J.
KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
Segmentation of photospheric magnetic elements corresponding to coronal features to understand the EUV and UV irradiance variability2017Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, artikkel-id A41Artikkel i tidsskrift (Fagfellevurdert)

Context. The magnetic field plays a dominant role in the solar irradiance variability. Determining the contribution of various magnetic features to this variability is important in the context of heliospheric studies and Sun-Earth connection. Aims. We studied the solar irradiance variability and its association with the underlying magnetic field for a period of five years (January 2011-January 2016). We used observations from the Large Yield Radiometer (LYRA), the Sun Watcher with Active Pixel System detector and Image Processing (SWAP) on board PROBA2, the Atmospheric Imaging Assembly (AIA), and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Methods. The Spatial Possibilistic Clustering Algorithm (SPoCA) is applied to the extreme ultraviolet (EUV) observations obtained from the AIA to segregate coronal features by creating segmentation maps of active regions (ARs), coronal holes (CHs) and the quiet sun (QS). Further, these maps are applied to the full-disk SWAP intensity images and the full-disk (FD) HMI line-of-sight (LOS) magnetograms to isolate the SWAP coronal features and photospheric magnetic counterparts, respectively. We then computed fulldisk and feature-wise averages of EUV intensity and line of sight (LOS) magnetic flux density over ARs/CHs/QS/FD. The variability in these quantities is compared with that of LYRA irradiance values. Results. Variations in the quantities resulting from the segmentation, namely the integrated intensity and the total magnetic flux density of ARs/CHs/QS/FD regions, are compared with the LYRA irradiance variations. We find that the EUV intensity over ARs/CHs/QS/FD is well correlated with the underlying magnetic field. In addition, variations in the full-disk integrated intensity and magnetic flux density values are correlated with the LYRA irradiance variations. Conclusions. Using the segmented coronal features observed in the EUV wavelengths as proxies to isolate the underlying magnetic structures is demonstrated in this study. Sophisticated feature identification and segmentation tools are important in providing more insights into the role of various magnetic features in both the short-and long-term changes in the solar irradiance.

• 814.
Chinese Acad Sci, Key Lab Solar Act, Natl Astron Observ, Beijing 100012, Peoples R China..
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80303 USA.;Univ Colorado, Dept Astrophys & Planetary Sci, Boulder, CO 80303 USA.;Stockholm Univ, AlbaNova Univ Ctr, Dept Astron, SE-10691 Stockholm, Sweden.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA..
Solar Kinetic Energy and Cross Helicity Spectra2018Inngår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 862, nr 2, artikkel-id L17Artikkel i tidsskrift (Fagfellevurdert)

We develop a formalism that treats the calculation of solar kinetic energy and cross helicity spectra in an equal manner to that of magnetic energy and helicity spectra. The magnetic helicity spectrum is shown to be equal to the vertical part of the current helicity spectrum divided by the square of the wavenumber. For the cross helicity, we apply the recently developed two-scale approach globally over an entire active region to account for the sign change between the two polarities. Using vector magnetograms and Dopplergrams of NOAA. 11158 and 12266, we show that kinetic and magnetic energy spectra have similar slopes at intermediate wavenumbers, where the contribution from the granulation velocity has been removed. At wavenumbers around 0.3 Mm(-1), the magnetic helicity is found to be close to its maximal value. The cross helicity spectra are found to be within about 10% of the maximum possible value. Using the two-scale method for NOAA. 12266, the global cross helicity spectrum is found to be particularly steep, similarly to what has previously been found in theoretical models of spot generation. In the quiet Sun, by comparison, the cross helicity spectrum is found to be small.

• 815. Zhang, Hongqi
Nordita, Sweden; Stockholm University, Sweden; University of Colorado, United States;.
EVOLUTION OF MAGNETIC HELICITY AND ENERGY SPECTRA OF SOLAR ACTIVE REGIONS2016Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 819, nr 2, artikkel-id 146Artikkel i tidsskrift (Fagfellevurdert)

We adopt an isotropic representation of the Fourier-transformed two-point correlation tensor of the magnetic field to estimate the magnetic energy and helicity spectra as well as current helicity spectra of two individual active regions (NOAA. 11158 and NOAA. 11515) and the change of the spectral indices during their development as well as during the solar cycle. The departure of the spectral indices of magnetic energy and current helicity from 5/ 3 are analyzed, and it is found that it is lower than the spectral index of the magnetic energy spectrum. Furthermore, the fractional magnetic helicity tends to increase when the scale of the energy-carrying magnetic structures increases. The magnetic helicity of NOAA. 11515 violates the expected hemispheric sign rule, which is interpreted as an effect of enhanced field strengths at scales larger than 30-60Mm with opposite signs of helicity. This is consistent with the general cycle dependence, which shows that around the solar maximum the magnetic energy and helicity spectra are steeper, emphasizing the large-scale field.

• 816. Zhang, S. -N
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Introduction to the high energy cosmic-radiation detection (HERD) facility onboard China's future space station2017Inngår i: Proceedings of Science, Sissa Medialab Srl , 2017Konferansepaper (Fagfellevurdert)

The High Energy cosmic-Radiation Detection (HERD) facility is one of several space astronomy payloads onboard China's Space Station, which is planned for operation starting around 2025 for about 10 years. The main scientific objectives of HERD are searching for signals of dark matter annihilation products, precise cosmic electron (plus positron) spectrum and anisotropy measurements up to 10 TeV, precise cosmic ray spectrum and composition measurements up to the knee energy, and high energy gamma-ray monitoring and survey. HERD is composed of a 3-D cubic calorimeter (CALO) surrounded by microstrip silicon trackers (STKs) from five sides except the bottom. CALO is made of about 7,500 cubes of LYSO crystals, corresponding to about 55 radiation lengths and 3 nuclear interaction lengths, respectively. The top STK microstrips of six X-Y layers are sandwiched with tungsten converters to make precise directional measurements of incoming electrons and gamma-rays. In the baseline design, each of the four side STKs is made of only three layers microstrips. All STKs will also be used for measuring the charge and incoming directions of cosmic rays, as well as identifying back scattered tracks. With this design, HERD can achieve the following performance: energy resolution of 1% for electrons and gamma-rays beyond 100 GeV and 20% for protons from 100 GeV to 1 PeV; electron/proton separation power better than 10-5; effective geometrical factors of &gt;3 m2sr for electron and diffuse gamma-rays, &gt;2 m2sr for cosmic ray nuclei. R&amp;D is under way for reading out the LYSO signals with optical fiber coupled to image intensified IsCMOS and CALO prototype of 250 LYSO crystals.

• 817. Zhang, Y. C.
KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
Quantitative analysis of a Hall system in the exhaust of asymmetric magnetic reconnection2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 5, s. 5277-5289Artikkel i tidsskrift (Fagfellevurdert)

Taking advantage of high-resolution measurements from the MMS mission, we find evidence for a complete Hall system in the exhaust of asymmetric magnetic reconnection 40 Di downstream of the X line. The investigation of the fine structure of the Hall system reveals that it displays features in the exhaust similar to those reported previously in the ion diffusion region by simulations and observations. This finding confirms the importance of particle-scale processes in the reconnection exhaust as well. On the magnetospheric side of the exhaust, electrons are strongly accelerated by parallel electric fields. This process significantly contributes to feed the Hall current system, resulting in a nonnegligible Hall magnetic field signature on this side despite an otherwise lower density. Calculation of the induced out-of-plane magnetic field by in-plane currents (based on Biot-Savart law) provides direct quantitative evidence for the process of Hall magnetic field generation by the Hall current system. A strong normal Hall electric field is present only on the magnetospheric side of the exhaust region, consistent with previous works. Multipoint data analysis shows that the ion pressure gradient in the ion momentum equation produces this Hall electric field. This global pattern of the Hall system can be explained by kinetic Alfvén wave theory.

• 818.
Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China..
Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.. Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.. Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.. Nanchang Univ, Inst Space Sci & Technol, Nanchang, Jiangxi, Peoples R China.. UCLA, Dept Phys & Astron, Los Angeles, CA USA.. UCLA, Dept Earth Planetary & Space Sci, Los Angeles, CA USA.. UCLA, Dept Earth Planetary & Space Sci, Los Angeles, CA USA.. Katholieke Univ Leuven, Dept Math, Ctr Plasma Astrophys, Leuven, Belgium.. UCLA, Dept Earth Planetary & Space Sci, Los Angeles, CA USA.. Univ New Hampshire, Durham, NH 03824 USA.. Southwest Res Inst, San Antonio, TX USA.. NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.. NASA, Goddard Space Flight Ctr, Greenbelt, MD USA.. Swedish Inst Space Phys, Uppsala, Sweden.. Univ Colorado LASP, Boulder, CO USA.. KTH, Skolan för elektroteknik och datavetenskap (EECS), Rymd- och plasmafysik.
Observations of an Electron Diffusion Region in Symmetric Reconnection with Weak Guide Field2019Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 870, nr 1, artikkel-id 34Artikkel i tidsskrift (Fagfellevurdert)

The Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) in a symmetric reconnection in the Earth's magnetotail. The EDR contained a guide field of about 2 nT, which was 13% of the magnetic field in the inflow region, and its thickness was about 2 local electron inertial lengths. Intense energy dissipation, a super-Alfvenic electron jet, electron nongyrotropy, and crescent-shaped electron velocity distributions were observed in association with this EDR. These features are similar to those of the EDRs in asymmetric reconnection at the dayside magnetopause. Electrons gained about 50% of their energy from the immediate upstream to the EDR. Crescent electron distributions were seen at the boundary of the EDR, while highly curved magnetic field lines inside the EDR may have gyrotropized the electrons. The EDR was characterized by a parallel current that was carried by antiparallel drifting electrons that were probably accelerated by a parallel electric field along the guide field. These results reveal the essential electron physics of the EDR and provide a significant example of an EDR in symmetric reconnection with a weak guide field.

• 819.
Nanchang Univ, Inst Space Sci & Technol, Nanchang 330031, Jiangxi, Peoples R China.;Nanchang Univ, Sch Environm & Chem Engn, Minist Educ, Key Lab Poyang Lake Environm & Resource Utilizat, Nanchang 330031, Jiangxi, Peoples R China..
Nanchang Univ, Inst Space Sci & Technol, Nanchang 330031, Jiangxi, Peoples R China.;Nanchang Univ, Sch Sci, Dept Phys, Nanchang 330031, Jiangxi, Peoples R China.. Nanchang Univ, Inst Space Sci & Technol, Nanchang 330031, Jiangxi, Peoples R China.;Nanchang Univ, Sch Sci, Dept Phys, Nanchang 330031, Jiangxi, Peoples R China.. Nanchang Univ, Inst Space Sci & Technol, Nanchang 330031, Jiangxi, Peoples R China.. Nanchang Univ, Inst Space Sci & Technol, Nanchang 330031, Jiangxi, Peoples R China.;Nanchang Univ, Sch Resources Environm & Chem Engn, Nanchang 330031, Jiangxi, Peoples R China.. Univ Calif Los Angeles, Dept Earth Planetary & Space Sci, Los Angeles, CA USA.. NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA.. NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA.. KTH, Skolan för elektroteknik och datavetenskap (EECS), Rymd- och plasmafysik. Royal Inst Technol, SE-75121 Stockholm, Sweden.. Southwest Res Inst, San Antonio, TX 78238 USA..
Electron-scale Vertical Current Sheets in a Bursty Bulk Flow in the Terrestrial Magnetotail2019Inngår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 872, nr 2, artikkel-id L26Artikkel i tidsskrift (Fagfellevurdert)

We report Magnetospheric Multiscale observations of multiple vertical current sheets (CSs) in a bursty bulk flow in the near-Earth magnetotail. Two of the CSs were fine structures of a dipolarization front (DF) at the leading edge of the flow. The other CSs were a few Earth radii tailward of the DF; that is, in the wake of the DF. Some of these vertical CSs were a few electron inertial lengths thick and were converting energy from magnetic field to plasma. The currents of the CSs in the DF wake were carried by electrons that formed flow shear layers. These electron-scale CSs were probably formed during the turbulent evolution of the bursty bulk flow and are important for energy conversion associated with fast flows.

• 820.
KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
hi_class: Horndeski in the Cosmic Linear Anisotropy Solving System2017Inngår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 8, artikkel-id 019Artikkel i tidsskrift (Fagfellevurdert)

We present the public version of hi cl a ss (www . hiclass-code . net), an extension of the Boltzmann code CLASS to a broad ensemble of modifications to general relativity. In particular, hi cl a ss can calculate predictions for models based on Horndeski's theory, which is the most general scalar-tensor theory described by second-order equations of motion and encompasses any perfect-fluid dark energy, quintessence, Brans-Dicke, f( R) and covariant Galileon models. hi-class has been thoroughly tested and can be readily used to understand the impact of alternative theories of gravity on linear structure formation as well as for cosmological parameter extraction.

• 821. Řípa, J.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Cosmological effects on the observed flux and fluence distributions of gamma-ray bursts2011Konferansepaper (Fagfellevurdert)

Several claims have been put forward that an essential fraction of long-duration BATSE gamma-ray bursts should lie at redshifts larger than 5. This point-of-view follows from the natural assumption that fainter objects should, on average, lie at larger redshifts. However, redshifts larger than 5 are rare for bursts observed by Swift. The purpose of this article is to show that the most distant bursts in general need not be the faintest ones. We derive the cosmological relationships between the observed and emitted quantities, and arrive at a prediction that is tested on the ensembles of BATSE, Swift and Fermi bursts. This analysis is independent on the assumed cosmology, on the observational biases, as well as on any gamma-ray burst model. We arrive to the conclusion that apparently fainter bursts need not, in general, lie at large redshifts. Such a behaviour is possible, when the luminosities (or emitted energies) in a sample of bursts increase more than the dimming of the observed values with redshift. In such a case dP(z)/dz > 0 can hold, where P(z) is either the peak-flux or the fluence. This also means that the hundreds of faint, long-duration BATSE bursts need not lie at high redshifts, and that the observed redshift distribution of long Swift bursts might actually represent the actual distribution.

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