39,824 research outputs found

    The dependence of the helicity bound of force-free magnetic fields on boundary conditions

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    This paper follows up on a previous study showing that in an open atmosphere such as the solar corona the total magnetic helicity of a force-free field must be bounded and the accumulation of magnetic helicity in excess of its upper bound would initiate a non-equilibrium situation resulting in an expulsion such as a coronal mass ejection (CME). In the current paper, we investigate the dependence of the helicity bound on the boundary condition for several families of nonlinear force-free fields. Our calculation shows that the magnitude of the helicity upper bound of force-free fields is non-trivially dependent on the boundary condition. Fields with a multipolar boundary condition can have a helicity upper bound ten times smaller than those with a dipolar boundary condition when helicity values are normalized by the square of their respective surface poloidal fluxes. This suggests that a coronal magnetic field may erupt into a CME when the applicable helicity bound falls below the already accumulated helicity as the result of a slowly changing boundary condition. Our calculation also shows that a monotonic accumulation of magnetic helicity can lead to the formation of a magnetic flux rope applicable to kink instability. This suggests that CME initiations by exceeding helicity bound and by kink instability can both be the consequences of helicity accumulation in the corona. Our study gives insights into the observed associations of CMEs with the magnetic features at their solar surface origins.Comment: accepted by Ap

    On approximation of Markov binomial distributions

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    For a Markov chain X={Xi,i=1,2,...,n}\mathbf{X}=\{X_i,i=1,2,...,n\} with the state space {0,1}\{0,1\}, the random variable S:=i=1nXiS:=\sum_{i=1}^nX_i is said to follow a Markov binomial distribution. The exact distribution of SS, denoted LS\mathcal{L}S, is very computationally intensive for large nn (see Gabriel [Biometrika 46 (1959) 454--460] and Bhat and Lal [Adv. in Appl. Probab. 20 (1988) 677--680]) and this paper concerns suitable approximate distributions for LS\mathcal{L}S when X\mathbf{X} is stationary. We conclude that the negative binomial and binomial distributions are appropriate approximations for LS\mathcal{L}S when VarS\operatorname {Var}S is greater than and less than ES\mathbb{E}S, respectively. Also, due to the unique structure of the distribution, we are able to derive explicit error estimates for these approximations.Comment: Published in at http://dx.doi.org/10.3150/09-BEJ194 the Bernoulli (http://isi.cbs.nl/bernoulli/) by the International Statistical Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm

    Four Facets of Privacy and Intellectual Freedom in Licensing Contracts for Electronic Journals

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    This is a study of the treatment of library patron privacy in licenses for electronic journals in academic libraries. We begin by distinguishing four facets of privacy and intellectual freedom based on the LIS and philosophical literature. Next, we perform a content analysis of 42 license agreements for electronic journals, focusing on terms for enforcing authorized use and collection and sharing of user data. We compare our findings to model licenses, to recommendations proposed in a recent treatise on licenses, and to our account of the four facets of intellectual freedom. We find important conflicts with each