10,393 research outputs found
Additions, combinations, and synonyms for the Bolivian moss flora
Fifty-five mosses are newly recorded for Bolivia. Additional collection data are given for twelve mosses considered little known or rare in the country. Six new synonyms are recognized, five from Bolivia, one from Brazil: Hookeria scabripes Müll. Hal. [Callicostella scabripes (Müll. Hal.) Broth.] = Callicostella pallida (Hornsch.) Ångstr.; Leucobryum fragile Herzog = Leucobryum subobtusifolium (Broth.) B.H. Allen; Macromitrium pinnulatum Herzog = Macromitrium microstomum (Hook. & Grev.) Schwägr.; Schlotheimia vesiculata Herzog [Macromitrium vesiculatum (Herzog) Herzog] = Macromitrium stellulatum (Hornsch.) Brid.; Cyclodictyon breve Herzog = Cyclodictyon albicans (Hedw.) Kuntze; and from Brazil: Callicostella paludicola Broth. = Callicostella merkelii (Hornsch.) A. Jaeger. Three new combinations are proposed: Entosthodon subaloma (Herzog) S.P. Churchill (Goniobryum subaloma Herzog), Syntrichia xerophila (Herzog) S.P. Churchill (Tortula xerophila Herzog), Thamniopsis lepidopiloides (Herzog) S.P. Churchill (Hookeriopsis lepidopiloides Herzog)
A simple model for the evolution of multi-stranded coronal loops
We develop and analyze a simple cellular automaton (CA) model that reproduces
the main properties of the evolution of soft X-ray coronal loops. We are
motivated by the observation that these loops evolve in three distinguishable
phases that suggest the development, maintainance, and decay of a
self-organized system. The model is based on the idea that loops are made of
elemental strands that are heated by the relaxation of magnetic stress in the
form of nanoflares. In this vision, usually called "the Parker conjecture"
(Parker 1988), the origin of stress is the displacement of the strand
footpoints due to photospheric convective motions. Modeling the response and
evolution of the plasma we obtain synthetic light curves that have the same
characteristic properties (intensity, fluctuations, and timescales) as the
observed cases. We study the dependence of these properties on the model
parameters and find scaling laws that can be used as observational predictions
of the model. We discuss the implications of our results for the interpretation
of recent loop observations in different wavelengths.Comment: 2010, accepted for publication in Ap
THE ECONOMICS OF INCREASING SPEED IN SEA TRANSPORTATION: THE CASE FOR THE SOUTHERN U.S., MEXICO, CENTRAL AMERICA AND THE CARIBBEAN
Public Economics,
Are constant loop widths an artifact of the background and the spatial resolution?
We study the effect of the coronal background in the determination of the
diameter of EUV loops, and we analyze the suitability of the procedure followed
in a previous paper (L\'opez Fuentes, Klimchuk & D\'emoulin 2006) for
characterizing their expansion properties. For the analysis we create different
synthetic loops and we place them on real backgrounds from data obtained with
the Transition Region and Coronal Explorer (\textit{TRACE}). We apply to these
loops the same procedure followed in our previous works, and we compare the
results with real loop observations. We demonstrate that the procedure allows
us to distinguish constant width loops from loops that expand appreciably with
height, as predicted by simple force-free field models. This holds even for
loops near the resolution limit. The procedure can easily determine when loops
are below resolution limit and therefore not reliably measured. We find that
small-scale variations in the measured loop width are likely due to
imperfections in the background subtraction. The greatest errors occur in
especially narrow loops and in places where the background is especially bright
relative to the loop. We stress, however, that these effects do not impact the
ability to measure large-scale variations. The result that observed loops do
not expand systematically with height is robust.Comment: Accepted for publication in Ap
Coronal Loop Expansion Properties Explained Using Separators
One puzzling observed property of coronal loops is that they are of roughly
constant thickness along their length. Various studies have found no consistent
pattern of width variation along the length of loops observed by TRACE and
SOHO. This is at odds with expectations of magnetic flux tube expansion
properties, which suggests that loops are widest at their tops, and
significantly narrower at their footpoints. Coronal loops correspond to areas
of the solar corona which have been preferentially heated by some process, so
this observed property might be connected to the mechanisms that heat the
corona. One means of energy deposition is magnetic reconnection, which occurs
along field lines called separators. These field lines begin and end on
magnetic null points, and loops forming near them can therefore be relatively
wide at their bases. Thus, coronal energization by magnetic reconnection may
replicate the puzzling expansion properties observed in coronal loops. We
present results of a Monte Carlo survey of separator field line expansion
properties, comparing them to the observed properties of coronal loops.Comment: 16 pages, 9 figures, to be submitted to Ap
- …