3,008 research outputs found
Reconnection of a kinking flux rope triggering the ejection of a microwave and hard X-ray source. II. Numerical Modeling
Numerical simulations of the helical () kink instability of an
arched, line-tied flux rope demonstrate that the helical deformation enforces
reconnection between the legs of the rope if modes with two helical turns are
dominant as a result of high initial twist in the range . Such
reconnection is complex, involving also the ambient field. In addition to
breaking up the original rope, it can form a new, low-lying, less twisted flux
rope. The new flux rope is pushed downward by the reconnection outflow, which
typically forces it to break as well by reconnecting with the ambient field.
The top part of the original rope, largely rooted in the sources of the ambient
flux after the break-up, can fully erupt or be halted at low heights, producing
a "failed eruption." The helical current sheet associated with the instability
is squeezed between the approaching legs, temporarily forming a double current
sheet. The leg-leg reconnection proceeds at a high rate, producing sufficiently
strong electric fields that it would be able to accelerate particles. It may
also form plasmoids, or plasmoid-like structures, which trap energetic
particles and propagate out of the reconnection region up to the top of the
erupting flux rope along the helical current sheet. The kinking of a highly
twisted flux rope involving leg-leg reconnection can explain key features of an
eruptive but partially occulted solar flare on 18 April 2001, which ejected a
relatively compact hard X-ray and microwave source and was associated with a
fast coronal mass ejection.Comment: Solar Physics, in pres
Is Radiation of Quantized Black Holes Observable?
If primordial black holes (PBH) saturate the present upper limit on the dark
matter density in our Solar system and if their radiation spectrum is discrete,
the sensitivity of modern detectors is close to that necessary for detecting
this radiation. This conclusion is not in conflict with the upper limits on the
PBH evaporation rate.Comment: 6 pages, 1 figure (reproduced properly in pdf file
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The behaviour of betting and currency markets on the night of the EU referendum
We study the behaviour of the Betfair betting market and the sterling/dollar exchange rate (futures price) during 24 June 2016, the night of the EU referendum. We investigate how the two markets responded to the announcement of the voting results. We employ a Bayesian updating methodology to update prior opinion about the likelihood of the final outcome of the vote. We then relate the voting model to the real time evolution of the market determined prices. We find that although both markets appear to be inefficient in absorbing the new information contained in vote outcomes, the betting market is apparently less inefficient than the FX market. The different rates of convergence to fundamental value between the two markets leads to highly profitable arbitrage opportunities
Multi-fluid simulations of chromospheric magnetic reconnection in a weakly ionized reacting plasma
We present results from the first self-consistent multi-fluid simulations of
chromospheric magnetic reconnection in a weakly ionized reacting plasma. We
simulate two dimensional magnetic reconnection in a Harris current sheet with a
numerical model which includes ion-neutral scattering collisions, ionization,
recombination, optically thin radiative loss, collisional heating, and thermal
conduction. In the resulting tearing mode reconnection the neutral and ion
fluids become decoupled upstream from the reconnection site, creating an excess
of ions in the reconnection region and therefore an ionization imbalance. Ion
recombination in the reconnection region, combined with Alfv\'{e}nic outflows,
quickly removes ions from the reconnection site, leading to a fast reconnection
rate independent of Lundquist number. In addition to allowing fast
reconnection, we find that these non-equilibria partial ionization effects lead
to the onset of the nonlinear secondary tearing instability at lower values of
the Lundquist number than has been found in fully ionized plasmas.These
simulations provide evidence that magnetic reconnection in the chromosphere
could be responsible for jet-like transient phenomena such as spicules and
chromospheric jets.Comment: 8 Figures, 32 pages tota
Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot
The discrete nature of the solar magnetic field as it emerges into the corona
through the photosphere indicates that it exists as isolated flux tubes in the
convection zone, and will remain as discrete flux tubes in the corona until it
collides and reconnects with other coronal fields. Collisions of these flux
tubes will in general be three dimensional, and will often lead to
reconnection, both rearranging the magnetic field topology in fundamental ways,
and releasing magnetic energy. With the goal of better understanding these
dynamics, we carry out a set of numerical experiments exploring fundamental
characteristics of three dimensional magnetic flux tube reconnection. We first
show that reconnecting flux tubes at opposite extremes of twist behave very
differently: in some configurations, low twist tubes slingshot while high twist
tubes tunnel. We then discuss a theory explaining these differences: by
assuming helicity conservation during the reconnection one can show that at
high twist, tunneled tubes reach a lower magnetic energy state than slingshot
tubes, whereas at low twist the opposite holds. We test three predictions made
by this theory. 1) We find that the level of twist at which the transition from
slingshot to tunnel occurs is about two to three times higher than predicted on
the basis of energetics and helicity conservation alone, probably because the
dynamics of the reconnection play a large role as well. 2) We find that the
tunnel occurs at all flux tube collision angles predicted by the theory. 3) We
find that the amount of magnetic energy a slingshot or a tunnel reconnection
releases agrees reasonably well with the theory, though at the high
resistivities we have to use for numerical stability, a significant amount of
magnetic energy is lost to diffusion, independent of reconnection.Comment: 21 pages, 15 figures, submitted to Ap
Averaging of moment condition estimators
We establish the consistency and asymptotic normality for a class of estimators that are linear combinations of a set of √ n-consistent estimators whose cardinality increases with sample size. A special case of our framework corresponds to the conditional moment restriction and the implied estimator in that case is shown to achieve the semiparametric efficiency bound. The proofs do not rely on smoothness of underlying criterion functions
The Cross-Quantilogram: Measuring Quantile Dependence and Testing Directional Predictability between Time Series
This paper proposes the cross-quantilogram to measure the quantile dependence between two time series. We apply it to test the hypothesis that one time series has no directional predictability to another time series. We establish the asymptotic distribution of the cross quantilogram and the corresponding test statistic. The limiting distributions depend on nuisance parameters. To construct consistent confidence intervals we employ the stationary bootstrap procedure; we show the consistency of this bootstrap. Also, we consider the self-normalized approach, which is shown to be asymptotically pivotal under the null hypothesis of no predictability. We provide simulation studies and two empirical applications. First, we use the cross-quantilogram to detect predictability from stock variance to excess stock return. Compared to existing tools used in the literature of stock return predictability, our method provides a more complete relationship between a predictor and stock return. Second, we investigate the systemic risk of individual financial institutions, such as JP Morgan Chase, Goldman Sachs and AIG. This article has supplementary materials online
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The cross-quantilogram: Measuring quantile dependence and testing directional predictability between time series
This paper proposes the cross-quantilogram to measure the quantile dependence between two time series. We apply it to test the hypothesis that one time series has no directional predictability to another time series. We establish the asymptotic distribution of the cross-quantilogram and the corresponding test statistic. The limiting distributions depend on nuisance parameters. To construct consistent confidence intervals we employ a stationary bootstrap procedure; we establish consistency of this bootstrap. Also, we consider a self-normalized approach, which yields an asymptotically pivotal statistic under the null hypothesis of no predictability. We provide simulation studies and two empirical applications. First, we use the cross-quantilogram to detect predictability from stock variance to excess stock return. Compared to existing tools used in the literature of stock return predictability, our method provides a more complete relationship between a predictor and stock return. Second, we investigate the systemic risk of individual financial institutions, such as JP Morgan Chase, Morgan Stanley and AIG.Cambridge INETThis is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.jeconom.2016.03.00
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