96 research outputs found
Signatures of Secondary Collisionless Magnetic Reconnection Driven by Kink Instability of a Flux Rope
The kinetic features of secondary magnetic reconnection in a single flux rope
undergoing internal kink instability are studied by means of three-dimensional
Particle-in-Cell simulations. Several signatures of secondary magnetic
reconnection are identified in the plane perpendicular to the flux rope: a
quadrupolar electron and ion density structure and a bipolar Hall magnetic
field develop in proximity of the reconnection region. The most intense
electric fields form perpendicularly to the local magnetic field, and a
reconnection electric field is identified in the plane perpendicular to the
flux rope. An electron current develops along the reconnection line in the
opposite direction of the electron current supporting the flux rope magnetic
field structure. Along the reconnection line, several bipolar structures of the
electric field parallel to the magnetic field occur making the magnetic
reconnection region turbulent. The reported signatures of secondary magnetic
reconnection can help to localize magnetic reconnection events in space,
astrophysical and fusion plasmas
Approaching the Coverability Problem Continuously
The coverability problem for Petri nets plays a central role in the
verification of concurrent shared-memory programs. However, its high
EXPSPACE-complete complexity poses a challenge when encountered in real-world
instances. In this paper, we develop a new approach to this problem which is
primarily based on applying forward coverability in continuous Petri nets as a
pruning criterion inside a backward coverability framework. A cornerstone of
our approach is the efficient encoding of a recently developed polynomial-time
algorithm for reachability in continuous Petri nets into SMT. We demonstrate
the effectiveness of our approach on standard benchmarks from the literature,
which shows that our approach decides significantly more instances than any
existing tool and is in addition often much faster, in particular on large
instances.Comment: 18 pages, 4 figure
Attractive Potential around a Thermionically Emitting Microparticle
We present a simulation study of the charging of a dust grain immersed in a
plasma, considering the effect of electron emission from the grain (thermionic
effect). It is shown that the OML theory is no longer reliable when electron
emission becomes large: screening can no longer be treated within the
Debye-Huckel approach and an attractive potential well forms, leading to the
possibility of attractive forces on other grains with the same polarity. We
suggest to perform laboratory experiments where emitting dust grains could be
used to create non-conventional dust crystals or macro-molecules.Comment: 3 figures. To appear on Physical Review Letter
Future beam experiments in the magnetosphere with plasma contactors: The electron collection and ion emission routes
Experiments where a highâvoltage electron beam emitted by a spacecraft in the lowâdensity magnetosphere is used to probe the magnetospheric configuration could greatly enhance our understanding of the nearâEarth environment. Their challenge, however, resides in the fact that the background magnetospheric plasma cannot provide a return current that balances the electron beam current without charging the spacecraft to such high potential that in practice prevents beam emission. In order to overcome this problem, a possible solution is based on the emission of a highâdensity contactor plasma by the spacecraft prior to and after the beam. We perform particleâinâcell simulations to investigate the conditions under which a highâvoltage electron beam can be emitted from a magnetospheric spacecraft, comparing two possible routes that rely on the highâdensity contactor plasma. The first is an âelectron collectionâ route, where the contactor has lower current than the electron beam and is used with the goal of connecting to the background plasma and collecting magnetospheric electrons over a much larger area than that allowed by the spacecraft alone. The second is an âion emissionâ route, where the contactor has higher current than the electron beam. Ion emission is then enabled over the large quasiâspherical area of the contactor cloud, thus overcoming the space charge limits typical of ion beam emission. Our results indicate that the ion emission route offers a pathway for performing beam experiments in the lowâdensity magnetosphere, while the electron collection route is not viable because the contactor fails to draw a large neutralizing current from the background.Key PointsThe ion emission route is credible for beam experiments in the magnetosphereThe electron collection route is not viableThe background plasma facilitates beam emissionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111985/1/jgra51700.pd
Smart rogaining for computer science orientation
In this paper, we address the problem of designing new formats of computer science orientation activities to be offered during high school students internships in Computer Science Bachelor degrees. In order to cover a wide range of computer science topics as well to deal with soft skills and gender gap issues, we propose a teamwork format, called smart rogaining, that offer engaging introductory activities to prospective students in a series of checkpoints dislocated along the different stages of a rogaine. The format is supported by a smart mobile and web application. Our proposal is aimed at stimulating the interest of participants in different areas of computer science and at improving digital and soft skills of participants and, as a side effect, of staff members (instructors and university students). In the paper, we introduce the proposed format and discuss our experience in the editions organized at the University of Genoa before the COVID-19 pandemic (2019 and 2020 waves)
Expressive Equivalence and Succinctness of Parametrized Automata with respect to Finite Memory Automata
International audienceWe compare parametrized automata, a class of automata recently introduced by the authors, against finite memory automata with non-deterministic assignment, an existing class of automata used to model services. We prove that both classes have the same expressive power, while parametrized automata can be exponentially succinct in some cases. We then prove that deciding simulation preorder for parametrized automata is EXPTIME-complete, extending an earlier result showing it in EXPTIME
Is lazy abstraction a decision procedure for broadcast protocols?
Lazy abstraction builds up an abstract reachability tree by locally refining abstractions in order to eliminate spurious counterexamples in smaller and smaller subtrees. The method has proven useful to verify systems code. It is still open how good the method is as a decision procedure, i.e., whether the method terminates for already known decidable verification problems. In this paper, we answer the question positively for broadcast protocols and other infinite-state models in the class of so-called well-structured systems. This extends an existing result on systems with a finite bisimulation quotient
Verifying parameterized timed security protocols
Quantitative timing is often explicitly used in systems for better security, e.g., the credentials for automatic website logon often has limited lifetime. Verifying timing relevant security protocols in these systems is very challenging as timing adds another dimension of complexity compared with the untimed protocol verification. In our previous work, we proposed an approach to check the correctness of the timed authentication in security protocols with fixed timing constraints. However, a more difficult question persists, i.e., given a particular protocol design, whether the protocol has security flaws in its design or it can be configured secure with proper parameter values? In this work, we answer this question by proposing a parameterized verification framework, where the quantitative parameters in the protocols can be intuitively specified as well as automatically analyzed. Given a security protocol, our verification algorithm either produces the secure constraints of the parameters, or constructs an attack that works for any parameter values. The correctness of our algorithm is formally proved. We implement our method into a tool called PTAuth and evaluate it with several security protocols. Using PTAuth, we have successfully found a timing attack in Kerberos V which is unreported before.No Full Tex
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