ALBERT

Description: Space weather is listed as a natural disaster by the Federal Emergency Management Agency of the United States ( www.ready.gov/space-weather ). The advancement of space weather research has become imperative since the advent of technologies vulnerable to its impacts. Astronauts during space station operations, satellite and radio communications, to name a few, are adversely affected by space weather. In Japan, early works (1950s) focused on the ionospheric disturbances that disrupt radio wave communications. During the International Geophysical Year (1957–1958), scientists at the Radio Research Laboratory discovered a correlation between solar bursts and short-wave fadeouts [ Nishida , 2010]. This made the sun an important subject matter of investigation. In 1998, the National Astronomical Observatory of Japan established a program on solar studies utilizing the facilities at Nobeyama Radio Observatory [ Kumar , 2007]. In order to support the development of an early warning system for space weather irregularities and space debris problems, the Space Environment Research Center (SERC) of Kyushu University (KU) was established in 2002 to carry out basic science research under the direction of Kiyohumi Yumoto. In 2005, the construction of a global array of magnetometers called MAGnetic Data Acquisition System/Circum-pan Pacific Magnetometer Network (MAGDAS/CPMN) was initiated [ Yumoto and STPP Sub-Committee , 2009]. As of 2014, there are 72 stations worldwide monitoring the Earth's magnetic field globally (Figure 1). Also in 2005, joining in the research effort is the project " Basic Study of Space Weather prediction " , which was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan. Kazunari Shibata led this 5-year project to find answers to fundamental questions of space weather science in order to develop a physical model of the coupling processes in the solar-terrestrial interaction [Shibata and Kamide, 2007]. Researches in forecasting space weather phenomena continued to rise [ Tsubouchi and Omura , 2007; Yamamoto and Sakurai , 2009; Nóse et al ., 2012; Sakaguchi et al ., 2013]. At present, the Space Weather and Environment Informatics Laboratory of the National Institute of Information and Communications Technology (NICT) is the regional warning center of International Space Environment Service in Japan. Japanese users mainly rely on NICT for the daily space weather forecasting [ Nagatsuma , 2013]. However, an accurate forecasting system has not yet been achieved. According to Akasofu [2007], the success of space weather prediction research lies on strong collaboration between solar, magnetospheric, interplanetary and upper atmospheric scientists. There is still much work to be done on this aspect. However, the education and training of potential researchers can build not only stronger collaborations, but also sustain data acquisition for a longer period of time, which is essential for solar-terrestrial research. Thus, in this paper, capacity building is proposed as a new tool for the advancement of space weather research.

Description: 〈span class="paragraphSection"〉〈div class="boxTitle"〉Summary〈/div〉We describe three achievements for a ground motion simulation. First, we propose a kinematic modelling in which rupture delay time is governed by an eikonal equation on a Riemannian manifold and develop a coupling method between the ground motion simulation and the eikonal solver. In general the rupture velocity distribution is not spatially uniform and the rupture propagation depends on a fault shape. So we derive the eikonal equation by considering the Riemannian metric of the fault surface and give a detailed discretization of its difference scheme to deal with a curved surface fault. Next, in order to explain the effect of spatially discontinuous non-uniformity of rupture velocity, we introduce an isochrones jumping intensity and obtain a new decomposed isochrones formula in general settings. It is known that the representation theorem with the Green's function can be rewritten into an expression with a contour integral by the isochrones theory. The new formula says that the known isochrones formula for ground velocity can be decomposed into a trend component and a disturbance component. The disturbance component consists of the isochrones jumping intensity. Finally, by applying our ground motion simulation coupled with the eikonal solver and the decomposed isochrones formula, we investigate some relations between the non-uniformity of the rupture velocity and pulse-like disturbance of the ground motion velocity. Our simulations show that the disturbance of velocity waveform corresponds with that of rate of change of isochrones band area. It turns out that the pulse-like disturbance of velocity waveform occurs when isochrones move across the region where rupture velocity varies discontinuously. Thus we can explain that the pulse-like disturbance of the ground motion velocity occurs when the isochrones jumping intensity has nonzero value. Moreover, as another example of application of our simulation and formula, we show a distinctive dependence of peak ground velocity upon parameters such as the rupture velocity and the distance between a fault and an observer.〈/span〉

Description: Context. Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are the most important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a direct comparison with observational data. Aims. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. Methods. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a millimetre VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects, compared to the addition of thermal noise only. Using synthetic data based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M 87, we performed case studies to assess the image quality that can be obtained with the current and future EHT array for different weather conditions. Results. Our synthetic observations show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of our GRMHD source models can be recovered robustly with the EHT2017 array after performing calibration steps, which include fringe fitting, a priori amplitude and network calibration, and self-calibration. With the planned addition of new stations to the EHT array in the coming years, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images.

Description: Very Long Baseline Interferometry observations at 86 GHz reveal an almost hollow jet in M87 with a forked morphology. The detailed analysis presented here indicates that the spectral luminosity of the central spine of the jet in M87 is a few percent of that of the surrounding hollow jet 200–400 μ as from the central black hole. Furthermore, recent jet models indicate that a hollow “tubular” jet can explain a wide range of plausible broadband spectra originating from jetted plasma located within ~30 μ as of the central black hole, including the 230 GHz correlated flux detected by the Event Horizon Telescope. Most importantly, these hollow jets from the inner accretion flow have an intrinsic power capable of energizing the global jet out to kiloparsec scales. Thus motivated, this paper considers new models of the event horizon magnetosphere (EHM) in low luminosity accretion systems. Contrary to some models, the spine is not an invisible powerful jet. It is an intrinsically weak jet. In the new EHM solution, the accreted poloidal magnetic flux is weak and the background photon field is weak. It is shown how this accretion scenario naturally results in the dissipation of the accreted poloidal magnetic flux in the EHM not the accumulation of poloidal flux required for a powerful jet. The new solution indicates less large scale poloidal magnetic flux (and jet power) in the EHM than in the surrounding accretion flow and cannot support significant EHM driven jets.

Description: The mechanisms producing fast variability of the γ-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E  〉  100 GeV) γ-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5 ± 0.3) × 10−10 photons cm−2 s−1 and halving time of 26 ± 8 min. The MAGIC observations were triggered by a high state in the optical and high-energy (HE, E  〉  100 MeV) γ-ray bands. In this paper we present the MAGIC VHE γ-ray data together with multi-wavelength data from radio, optical, X-rays, and HE γ rays from 2015 May 1 to July 31. Well-sampled multi-wavelength data allow us to study the variability in detail and compare it to the other epochs when fast, VHE γ-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays, and HE γ-rays is very similar to two other observed VHE γ-ray flares. In particular, also during this flare there was an indication of rotation of the optical polarization angle and of activity at the 43 GHz core. These repeating patterns indicate a connection between the three events. We also test modelling of the spectral energy distribution based on constraints from the light curves and VLBA observations, with two different geometrical setups of two-zone inverse Compton models. In addition we model the γ-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE γ-ray flare, but all have some tension with the multi-wavelength observations.