Verification of operational solar flare forecast: Case of Regional Warning Center Japan [SSA]

In this article, we discuss a verification study of an operational solar flare forecast in the Regional Warning Center (RWC) Japan. The RWC Japan has been issuing four-categorical deterministic solar flare forecasts for a long time. In this forecast verification study, we used solar flare forecast data accumulated over 16 years (from 2000 to 2015). We compiled the forecast data together with solar flare data obtained with the Geostationary Operational Environmental Satellites (GOES). Using the compiled data sets, we estimated some conventional scalar verification measures with 95% confidence intervals. We also estimated a multi-categorical scalar verification measure. These scalar verification measures were compared with those obtained by the persistence method and recurrence method. As solar activity varied during the 16 years, we also applied verification analyses to four subsets of forecast-observation pair data with different solar activity levels. We cannot conclude definitely that there are significant performance difference between the forecasts of RWC Japan and the persistence method, although a slightly significant difference is found for some event definitions. We propose to use a scalar verification measure to assess the judgment skill of the operational solar flare forecast. Finally, we propose a verification strategy for deterministic operational solar flare forecasting.

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Y. Kubo, M. Den and M. Ishii
Wed, 26 Jul 17

Comments: 29 pages, 7 figures and 6 tables. Accepted for publication in Journal of Space Weather and Space Climate (SWSC)

Big Data vs. complex physical models: a scalable inference algorithm [CL]

The data torrent unleashed by current and upcoming instruments requires scalable analysis methods. Machine Learning approaches scale well. However, separating the instrument measurement from the physical effects of interest, dealing with variable errors, and deriving parameter uncertainties is usually an after-thought. Classic forward-folding analyses with Markov Chain Monte Carlo or Nested Sampling enable parameter estimation and model comparison, even for complex and slow-to-evaluate physical models. However, these approaches require independent runs for each data set, implying an unfeasible number of model evaluations in the Big Data regime. Here we present a new algorithm based on nested sampling, deriving parameter probability distributions for each observation. Importantly, in our method the number of physical model evaluations scales sub-linearly with the number of data sets, and we make no assumptions about homogeneous errors, Gaussianity, the form of the model or heterogeneity/completeness of the observations. Our method has immediate application in speeding up analyses of large surveys, integral-field-unit observations, and Monte Carlo simulations.

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J. Buchner
Mon, 17 Jul 17

Comments: Submitted to MNRAS. Comments welcome. Figure 6 demonstrates the scaling. Implementation at this https URL

Computing Entropies With Nested Sampling [CL]

The Shannon entropy, and related quantities such as mutual information, can be used to quantify uncertainty and relevance. However, in practice, it can be difficult to compute these quantities for arbitrary probability distributions, particularly if the probability mass functions or densities cannot be evaluated. This paper introduces a computational approach, based on Nested Sampling, to evaluate entropies of probability distributions that can only be sampled. I demonstrate the method on three examples: a simple gaussian example where the key quantities are available analytically; (ii) an experimental design example about scheduling observations in order to measure the period of an oscillating signal; and (iii) predicting the future from the past in a heavy-tailed scenario.

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B. Brewer
Thu, 13 Jul 17

Comments: Submitted to Entropy. 18 pages, 3 figures. Software available at this https URL

Radio-flaring Ultracool Dwarf Population Synthesis [SSA]

Over a dozen ultracool dwarfs (UCDs), low-mass objects of spectral types $\geq$M7, are known to be sources of radio flares. These typically several-minutes-long radio bursts can be up to 100\% circularly polarized and have high brightness temperatures, consistent with coherent emission via the electron cyclotron maser operating in $\sim$kG magnetic fields. Recently, the statistical properties of the bulk physical parameters that describe these UCDs have become adequately described to permit synthesis of the population of radio-flaring objects. For the first time, I construct a Monte Carlo simulator to model the population of these radio-flaring UCDs. This simulator is powered by Intel Secure Key (ISK)- a new processor technology that uses a local entropy source to improve random number generation that has heretofore been used to improve cryptography. The results from this simulator indicate that only $\sim$5% of radio-flaring UCDs within the local interstellar neighborhood ($<$25 pc away) have been discovered. I discuss a number of scenarios which may explain this radio-flaring fraction, and suggest that the observed behavior is likely a result of several factors. The performance of ISK as compared to other pseudorandom number generators is also evaluated, and its potential utility for other astrophysical codes briefly described.

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M. Route
Mon, 10 Jul 17

Comments: Accepted for publication in ApJ; 18 pages, 4 figures

Charged particle tracking without magnetic field: optimal measurement of track momentum by a Bayesian analysis of the multiple measurements of deflections due to multiple scattering [CL]

We revisit the precision of the measurement of track parameters (position, angle) with optimal methods in the presence of detector resolution, multiple scattering and zero magnetic field. We then obtain an optimal estimator of the track momentum by a Bayesian analysis of the filtering innovations of a series of Kalman filters applied to the track.
This work could pave the way to the development of autonomous high-performance gas time-projection chambers (TPC) or silicon wafer gamma-ray space telescopes and be a powerful guide in the optimisation of the design of the multi-kilo-ton liquid argon TPCs that are under development for neutrino studies.

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M. Frosini and D. Bernard
Tue, 20 Jun 17

Comments: 39 pages, 12 figures, submitted to Nuclear Inst. and Methods in Physics Research, A

Statistical properties of coronal hole rotation rates: Are they linked to the solar interior? [SSA]

The present paper discusses results of a statistical study of the characteristics of coronal hole (CH) rotation in order to find connections to the internal rotation of the Sun. The goal is to measure CH rotation rates and study their distribution over latitude and their area sizes. In addition, the CH rotation rates are compared with the solar photospheric and inner layer rotational profiles. We study coronal holes observed within $\pm 60$ latitude and longitude degrees from the solar disc centre during the time span from the 1 January 2013 to 20 April 2015, which includes the extended peak of solar cycle 24.We used data created by the Spatial Possibilistic Clustering Algorithm (SPoCA), which provides the exact location and characterisation of solar coronal holes using SDO=AIA 193 {\AA} channel images. The CH rotation rates are measured with four-hour cadence data to track variable positions of the CH geometric centre. North-south asymmetry was found in the distribution of coronal holes: about 60 percent were observed in the northern hemisphere and 40 percent were observed in the southern hemisphere. The smallest and largest CHs were present only at high latitudes. The average sidereal rotation rate for 540 examined CHs is $13:86 (\pm 0:05)$ degrees/d. Conclusions. The latitudinal characteristics of CH rotation do not match any known photospheric rotation profile. The CH angular velocities exceed the photospheric angular velocities at latitudes higher than 35-40 degrees. According to our results, the CH rotation profile perfectly coincides with tachocline and the lower layers of convection zone at around 0.71 $R_{\odot}$; this indicates that CHs may be linked to the solar global magnetic field, which originates in the tachocline region.

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S. Bagashvili, B. Shergelashvili, D. Japaridze, et. al.
Thu, 15 Jun 17

Comments: 8 pages, 8 figures, Accepted for publication in A&A

New radiographic image processing tested on the simple and double-flux platform at OMEGA [CL]

Ablation fronts and shocks are two radiative/hydrodynamic features ubiquitous in inertial confinement fusion (ICF). A specially designed shock-tube experiment was tested on the OMEGA laser facility to observe these two features evolve at once and to assess thermodynamical and radiative properties. It is a basic science experiment aimed at improving our understanding of shocked and ablated matter which is critical to ICF design. At all time, these two moving “interfaces” separate the tube into three distinct zones where matter is either ablated, shocked or unshocked. The {\it simple-flux} or {\it double-flux} experiments, respectively one or two halfraum-plus-tube, have been thought up to observe and image these zones using x-ray and visible image diagnostic. The possibility of observing all three regions at once was instrumental in our new radiographic image processing used to remove the backlighter background otherwise detrimental to quantitative measurement. By so doing, after processing the radiographic images of the 15 shots accumulated during the 2013 and 2015 campaigns, a quantitative comparison between experiments and our radiative hydrocode simulations was made possible. Several points of the principal Hugoniot of the aerogel used as a light material in the shock-tube were inferred from that comparison. Most surprisingly, rapid variations of relative-transmission in the ablated region were observed during radiographic irradiations while it remained constant in the shocked region. This effect might be attributed to the spectral distribution variability of the backlighter during the radiographic pulse. Numerically, that distribution is strongly dependent upon NLTE models and it could potentially be used as a mean to discriminate among them.

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O. Poujade, M. Ferri and I. Geoffray
Tue, 30 May 17

Comments: N/A