41,089 research outputs found
The dependence of the helicity bound of force-free magnetic fields on boundary conditions
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
For a Markov chain with the state space
, the random variable is said to follow a Markov
binomial distribution. The exact distribution of , denoted ,
is very computationally intensive for large (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
when is stationary. We conclude that the negative binomial and
binomial distributions are appropriate approximations for when
is greater than and less than ,
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
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
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