2,236 research outputs found
A new view of quiet-Sun topology from Hinode/SOT
Context.
With the recent launch of the Hinode satellite our view of the nature and evolution of quiet-Sun regions has been improved. In light of the new high resolution observations, we revisit the study of the quiet Sun's topological nature.
Aims.
Topology is a tool to explain the complexity of the magnetic field, the occurrence of reconnection processes, and the heating of the corona. This Letter aims to give new insights to these different topics.
Methods.
Using a high-resolution Hinode/SOT observation of the line-of-sight magnetic field on the photosphere, we calculate the three dimensional magnetic field in the region above assuming a potential field. From the 3D field, we determine the existence of null points in the magnetic configuration.
Results.
From this model of a continuous field, we find that the distribution of null points with height is significantly different from that reported in previous studies. In particular, the null points are mainly located above the bottom boundary layer in the photosphere (54%) and in the chromosphere (44%) with only a few null points in the corona (2%). The density of null points (expressed as the ratio of the number of null points to the number of photospheric magnetic fragments) in the solar atmosphere is estimated to be between 3% and 8% depending on the method used to identify the number of magnetic fragments in the observed photosphere.
Conclusions.
This study reveals that the heating of the corona by magnetic reconnection at coronal null points is unlikely. Our findings do not rule out the heating of the corona at other topological features. We also report the topological complexity of the chromosphere as strongly suggested by recent observations from Hinode/SOT
Intersexual conflict influences female reproductive success in a female-dispersing primate
In group-living mammals, individual efforts to maximize reproductive success result in conflicts and compromises between the sexes. Females utilize counterstrategies to minimize the costs of sexual coercion by males, but few studies have examined the effect of such behaviors on female reproductive success. Secondary dispersal by females is rare among group-living mammals, but in western gorillas, it is believed to be a mate choice strategy to minimize infanticide risk and infant mortality. Previous research suggested that females choose males that are good protectors. However, how much female reproductive success varies depending on male competitive ability and whether female secondary dispersal leads to reproductive costs or benefits has not been examined. We used data on 100 females and 229 infants in 36 breeding groups from a 20-year long-term study of wild western lowland gorillas to investigate whether male tenure duration and female transfer rate had an effect on interbirth interval, female birth rates, and offspring mortality. We found that offspring mortality was higher near the end of malesâ tenures, even after excluding potential infanticide when those males died, suggesting that females suffer a reproductive cost by being with males nearing the end of their tenures. Females experience a delay in breeding when they dispersed, having a notable effect on birth rates of surviving offspring per female if females transfer multiple times in their lives. This study exemplifies that female counterstrategies to mitigate the effects of male-male competition and sexual coercion may not be sufficient to overcome the negative consequences of male behavior
Solar Magnetic Tracking. IV. The Death of Magnetic Features
The removal of magnetic flux from the quiet-sun photosphere is important for
maintaining the statistical steady-state of the magnetic field there, for
determining the magnetic flux budget of the Sun, and for estimating the rate of
energy injected into the upper solar atmosphere. Magnetic feature death is a
measurable proxy for the removal of detectable flux. We used the SWAMIS feature
tracking code to understand how nearly 20000 detected magnetic features die in
an hour-long sequence of Hinode/SOT/NFI magnetograms of a region of quiet Sun.
Of the feature deaths that remove visible magnetic flux from the photosphere,
the vast majority do so by a process that merely disperses the
previously-detected flux so that it is too small and too weak to be detected.
The behavior of the ensemble average of these dispersals is not consistent with
a model of simple planar diffusion, suggesting that the dispersal is
constrained by the evolving photospheric velocity field. We introduce the
concept of the partial lifetime of magnetic features, and show that the partial
lifetime due to Cancellation of magnetic flux, 22 h, is 3 times slower than
previous measurements of the flux turnover time. This indicates that prior
feature-based estimates of the flux replacement time may be too short, in
contrast with the tendency for this quantity to decrease as resolution and
instrumentation have improved. This suggests that dispersal of flux to smaller
scales is more important for the replacement of magnetic fields in the quiet
Sun than observed bipolar cancellation. We conclude that processes on spatial
scales smaller than those visible to Hinode dominate the processes of flux
emergence and cancellation, and therefore also the quantity of magnetic flux
that threads the photosphere.Comment: Accepted by Ap
Spontaneous reconnection at a separator current layer : I. Nature of the reconnection
Magnetic separators, which lie on the boundary between four topologically-distinct flux domains, are prime locations in three-dimensional magnetic fields for reconnection, especially in the magnetosphere between the planetary and interplanetary magnetic fields and also in the solar atmosphere. Little is known about the details of separator reconnection and so the aim of this paper, which is the first of two, is to study the properties of magnetic reconnection at a single separator. Three-dimensional, resistive magnetohydrodynamic numerical experiments are run to study separator reconnection starting from a magnetohydrostatic equilibrium which contains a twisted current layer along a single separator linking a pair of opposite-polarity null points. The resulting reconnection occurs in two phases. The first is short involving rapid reconnection in which the current at the separator is reduced by a factor of around 2.3. Most 75% of the magnetic energy is converted during this phase, via Ohmic dissipation, directly into internal energy, with just 0.1% going into kinetic energy. During this phase the reconnection occurs along most of the separator away from its ends (the nulls), but in an asymmetric manner which changes both spatially and temporally over time. The second phase is much longer and involves slow impulsive-bursty reconnection. Again Ohmic heating dominates over viscous damping. Here the reconnection occurs in small localized bursts at random anywhere along the separator.Publisher PDFPeer reviewe
Spontaneous reconnection at a separator current layer : 2. Nature of the waves and flows
JEHS would like to thank STFC for financial support during her Ph.D and continued support after on the St Andrews SMTGâs STFC consortium grant. CEP also acknowledges support from this same grant.Sudden destabilizations of the magnetic field, such as those caused by spontaneous reconnection, will produce waves and/or flows. Here we investigate the nature of the plasma motions resulting from spontaneous reconnection at a 3âD separator. In order to clearly see these perturbations, we start from a magnetohydrostatic equilibrium containing two oppositely signed null points joined by a generic separator along which lies a twisted current layer. The nature of the magnetic reconnection initiated in this equilibrium as a result of an anomalous diffusivity is discussed in detail in Stevenson and Parnell (2015). The resulting sudden loss of force balance inevitably generates waves that propagate away from the diffusion region carrying the dissipated current. In their wake a twisting stagnation flow, in planes perpendicular to the separator, feeds flux back into the original diffusion site (the separator) in order to try to regain equilibrium. This flow drives a phase of slow weak impulsive bursty reconnection that follows on after the initial fastâreconnection phase.Publisher PDFPeer reviewe
Induced radioactivity in LDEF components
A systematic study of the induced radioactivity of the Long Duration Exposure Facility (LDEF) is being carried out in order to gather information about the low earth orbit radiation environment and its effects on materials. The large mass of the LDEF spacecraft, its stabilized configuration, and long mission duration have presented an opportunity to determine space radiation-induced radioactivities with a precision not possible before. Data presented include preliminary activities for steel and aluminum structural samples, and activation subexperiment foils. Effects seen in the data show a clear indication of the trapped proton anisotropy in the South Atlantic Anomaly and suggest contributions from different sources of external radiation fluxes
Nonlinear wave propagation and reconnection at magnetic X-points in the Hall MHD regime
The highly dynamical, complex nature of the solar atmosphere naturally
implies the presence of waves in a topologically varied magnetic environment.
Here, the interaction of waves with topological features such as null points is
inevitable and potentially important for energetics. The low resistivity of the
solar coronal plasma implies that non-MHD effects should be considered in
studies of magnetic energy release in this environment. This paper investigates
the role of the Hall term in the propagation and dissipation of waves, their
interaction with 2D magnetic X-points and the nature of the resulting
reconnection. A Lagrangian remap shock-capturing code (Lare2d) is used to study
the evolution of an initial fast magnetoacoustic wave annulus for a range of
values of the ion skin depth in resistive Hall MHD. A magnetic null-point
finding algorithm is also used to locate and track the evolution of the
multiple null-points that are formed in the system. Depending on the ratio of
ion skin depth to system size, our model demonstrates that Hall effects can
play a key role in the wave-null interaction. In particular, the initial
fast-wave pulse now consists of whistler and ion-cyclotron components; the
dispersive nature of the whistler wave leads to (i) earlier interaction with
the null, (ii) the creation of multiple additional, transient nulls and, hence,
an increased number of energy release sites. In the Hall regime, the relevant
timescales (such as the onset of reconnection and the period of the oscillatory
relaxation) of the system are reduced significantly, and the reconnection rate
is enhanced.Comment: 13 pages, 10 figure
Elementary Heating Events - Magnetic Interactions Between Two Flux Sources. III Energy Considerations
The magnetic field plays a crucial role in heating the solar corona, but the
exact energy release mechanism(s) is(are) still unknown. Here, we investigate
in detail, the process of magnetic energy release in a situation where two
initially independent flux systems are forced into each other. Work done by the
foot point motions goes in to building a current sheet in which magnetic
reconnection takes place. The scaling relations of the energy input and output
are determined as functions of the driving velocity and the strength of fluxes
in the independent flux systems. In particular, it is found that the energy
injected into the system is proportional to the distance travelled not the rate
of travel. Similarly, the rate of Joule dissipation is related to the distance
travelled. Hence, rapidly driven foot points lead to bright, intense, but
short-lived events, whilst slowly driven foot points produce weaker, but
longer-lived brightenings. Integrated over the lifetime of the events both
would produce the same heating if all other factors were the same. A strong
overlying field has the affect of creating compact flux lobes from the sources.
These appear to lead to a more rapid injection of energy, as well as a more
rapid release of energy. Thus, the stronger the overlying field the more
compact and more intense the heating. This means observers must know the rate
of movement of the magnetic fragments involved in an events, as well as
determine the strength and orientation of the surrounding field to be able to
predict anything about the energy dissipated.Comment: A&A accepted, 24 pages, 11 figure
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