We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.

We report on transport properties of Josephson junctions in hybrid superconducting topological insulator devices, which show two striking departures from the common Josephson junction behavior: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model which expands on the existing theory for topological insulator Josephson junctions.The Majorana fermion, a charge-neutral particle that is its own antiparticle, was proposed theoretically almost 75 years ago [1]. Electronic excitations in certain condensed matter systems have recently been predicted to act as Majorana fermions [1]. One such system is a three-dimensional topological insulator (TI) where superconducting correlations between particles are introduced, producing a "topological superconductor" [2]. When two superconductors are connected by a TI, the TI "weak link" superconducts due to its proximity to the superconducting leads. This produces a Josephson junction (JJ) but with several important distinctions compared to a conventional JJ, where the weak link is typically an ordinary metal or insulator. Fu and Kane have predicted [2] a one-dimensional (1D) mode of Majorana fermions at the interface between a conventional superconductor and a superconducting topological surface state. Hence, JJs formed with a TI weak link are expected to have two 1D modes at the two superconductor-TI interfaces [arrows in Fig. 1(a)], which fuse to form a 1D wire of Majorana fermions [shown in purple in Fig. 1(a)] running along the width of the device [2]. The energy spectrum of these Majorana fermions is characterized by states within the superconducting gap, which cross at zero energy when the phase difference ϕ between the two superconducting leads is π.To probe this exotic state, recent experiments have investigated transport in TI JJs, finding good agreement with conventional JJ behavior [3][4][5][6][7]. Two characteristic properties are typically reported for JJs. The first is the product I C R N , where I C is the critical current and R N is the normal state resistance. I C R N should be of order ∆/e (where ∆ is the superconducting gap of the leads and e is the charge of the electron) and independent of device geometry [8]. The second characteristic property is the "Fraunhofer-like" magnetic diffraction pattern, i.e. the decaying, oscillatory response of the supercurrent to the magnetic field B, applied perpendicular to the flow of the supercurrent. The first minimum in I C should occur at B = B C , when one quantum of flux Φ 0 = h/2e (where h is Planck's constant) is passed through the area of the device. Recent reports on TI JJs [6,7] match this expectation.In this Letter we report on transport properties of nanoscale Josephson junctions fabricated using Bi 2 Se 3 as the weak link material. The main experimental results of this Report are two departures from conventional Josephson junction behavior in these devices: a...

Solar flares occur in complex sunspot groups, but it remains unclear how the probability of producing a flare of a given magnitude relates to the characteristics of the sunspot group. Here, we use Geostationary Operational Environment Satellite X-ray flares and McIntosh group classifications from solar cycles 21 and 22 to calculate average flare rates for each McIntosh class and use these to determine Poisson probabilities for different flare magnitudes. Forecast verification measures are studied to find optimum thresholds to convert Poisson flare probabilities into yes/no predictions of cycle 23 flares. A case is presented to adopt the true skill statistic (TSS) as a standard for forecast comparison over the commonly used Heidke skill score (HSS). In predicting flares over 24 hr, the maximum values of TSS achieved are 0.44 (C-class), 0.53 (M-class), 0.74 (X-class), 0.54 ( M1.0), and 0.46 ( C1.0). The maximum values of HSS are 0.38 (C-class), 0.27 (M-class), 0.14 (X-class), 0.28 ( M1.0), and 0.41 ( C1.0). These show that Poisson probabilities perform comparably to some more complex prediction systems, but the overall inaccuracy highlights the problem with using average values to represent flaring rate distributions.

Solar flares produce radiation which can have an almost immediate effect on the near-Earth environment, making it crucial to forecast flares in order to mitigate their negative effects. The number of published approaches to flare forecasting using photospheric magnetic field observations has proliferated, with varying claims about how well each works. Because of the different analysis techniques and data sets used, it is essentially impossible to compare the results from the literature. This problem is exacerbated by the low event rates of large solar flares. The challenges of forecasting rare events have long been recognized in the meteorology community, but have yet to be fully acknowledged by the space weather community. During the interagency workshop on "all clear" forecasts held in Boulder, CO in 2009, the performance of a number of existing algorithms was compared on common data sets, specifically line-of-sight magnetic field and continuum intensity images from MDI, with consistent definitions of what constitutes an event. We demonstrate the importance of making such systematic comparisons, and of using standard verification statistics to determine what constitutes a good prediction scheme. When a comparison was made in this fashion, no one method clearly outperformed all others, which may in part be due to the strong correlations among the parameters used by different methods to characterize an active region. For M-class flares and above, the set of methods tends towards a weakly positive skill score (as measured with several distinct metrics), with no participating method proving substantially better than climatological forecasts.

The kinematics of a globally propagating disturbance (also known as an ``EIT wave") is discussed using Extreme UltraViolet Imager (EUVI) data Solar Terrestrial Relations Observatory (STEREO). We show for the first time that an impulsively generated propagating disturbance has similar kinematics in all four EUVI passbands (304, 171, 195, and 284 A). In the 304 A passband the disturbance shows a velocity peak of 238+/-20 kms-1 within ~28 minutes of its launch, varying in acceleration from 76 ms-2 to -102 ms-2. This passband contains a strong contribution from a Si XI line (303.32 A) with a peak formation temperature of ~1.6 MK. The 304 A emission may therefore be coronal rather than chromospheric in origin. Comparable velocities and accelerations are found in the coronal 195 A passband, while lower values are found in the lower cadence 284 A passband. In the higher cadence 171 A passband the velocity varies significantly, peaking at 475+/-47 kms-1 within ~20 minutes of launch, with a variation in acceleration from 816 ms-2 to -413 ms-2. The high image cadence of the 171 A passband (2.5 minutes compared to 10 minutes for the similar temperature response 195 A passband) is found to have a major effect on the measured velocity and acceleration of the pulse, which increase by factors of ~2 and ~10, respectively. This implies that previously measured values (e.g., using EIT) may have been underestimated. We also note that the disturbance shows strong reflection from a coronal hole in both the 171 and 195 A passbands. The observations are consistent with an impulsively generated fast-mode magnetoacoustic wave.Comment: 4 pages 4 figure