2,664 research outputs found
Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot
The discrete nature of the solar magnetic field as it emerges into the corona
through the photosphere indicates that it exists as isolated flux tubes in the
convection zone, and will remain as discrete flux tubes in the corona until it
collides and reconnects with other coronal fields. Collisions of these flux
tubes will in general be three dimensional, and will often lead to
reconnection, both rearranging the magnetic field topology in fundamental ways,
and releasing magnetic energy. With the goal of better understanding these
dynamics, we carry out a set of numerical experiments exploring fundamental
characteristics of three dimensional magnetic flux tube reconnection. We first
show that reconnecting flux tubes at opposite extremes of twist behave very
differently: in some configurations, low twist tubes slingshot while high twist
tubes tunnel. We then discuss a theory explaining these differences: by
assuming helicity conservation during the reconnection one can show that at
high twist, tunneled tubes reach a lower magnetic energy state than slingshot
tubes, whereas at low twist the opposite holds. We test three predictions made
by this theory. 1) We find that the level of twist at which the transition from
slingshot to tunnel occurs is about two to three times higher than predicted on
the basis of energetics and helicity conservation alone, probably because the
dynamics of the reconnection play a large role as well. 2) We find that the
tunnel occurs at all flux tube collision angles predicted by the theory. 3) We
find that the amount of magnetic energy a slingshot or a tunnel reconnection
releases agrees reasonably well with the theory, though at the high
resistivities we have to use for numerical stability, a significant amount of
magnetic energy is lost to diffusion, independent of reconnection.Comment: 21 pages, 15 figures, submitted to Ap
The rise and emergence of untwisted toroidal flux ropes on the sun
Magnetic flux ropes (MFRs) rising buoyantly through the Sun's convection zone are thought to be subject to viscous forces preventing them from rising coherently. Numerous studies have suggested that MFRs require a minimum twist in order to remain coherent during their rise. Furthermore, even MFRs that get to the photosphere may be unable to successfully emerge into the corona unless they are at least moderately twisted, since the magnetic pressure gradient needs to overcome the weight of the photospheric plasma. To date, however, no lower limit has been placed on the critical minimum twist required for an MFR to rise coherently through the convection zone or emerge through the photosphere. In this paper, we simulate an untwisted toroidal MFR that is able to rise from the convection zone and emerge through the photosphere as an active region that resembles those observed on the Sun. We show that untwisted MFRs can remain coherent during their rise and then pile up near the photosphere, triggering undular instability, allowing the MFR to emerge through the photosphere. We propose that the toroidal geometry of our MFR is critical for its coherent rise. Upon emergence, a pair of lobes rises into the corona. The two lobes then interact and reconnect, resulting in a localized high speed jet. The resulting photospheric magnetogram displays the characteristic salt-and-pepper structure often seen in observations. Our major result is that MFRs need not be twisted to rise coherently through the convection zone and emerge through the photosphere. © 2021. The American Astronomical Society. All rights reserved
The Power of Light Zine 2 - Why does life exist? - an epistemically insightful way to explore the nature of science and research at Diamond Light Source, UK
In the STFC funded Epistemic Insight Initiative project, The Power of Light, a series of resources have been designed informed by co-creation activities, pilot lessons, and workshops that involved children in schools and with their families in community spaces. Through this project with Diamond, we brought into classrooms and community spaces how light can be used to help investigate the world around us, address real-world problems and inform our thinking about Big Questions. The resources we develop support teachers' and their students' sense of agency when exploring 'how knowledge works' and how knowledge is built through different disciplines (including the natural sciences, the arts, and the humanities).
This 'zine', with its focus on how scientists have been working with paleotonologists to investigate evidence, found inside the fossilised leg of a thescelosaurus, of the cataclysmic event that led to the extinction of dinosaurs. Zine 2 'Why does life exist?' has been developed through co-creative activities involving research scientists at Diamond Light Source (UK), academics, primary school teachers, STEM ambassadors, and Diamond's public engagement team.
Zines use an appealing combination of text and images to create a concise comic-like narrative format to generate enthusiasm about a particular area of interest - the series of zines designed for this project focuses on research taking place at the Diamond facility. The Diamond Light Source facility houses a synchrotron which is used to conduct research in a variety of applied fields of science and technology.
This zine is designed to be accessible to ages 8+, and works well with a short animation (available in both Zenodo and on the Epistemic Insight You Tube channel) that has been created with additional funding from STFC. Teaching notes are available for this zine, with guidance and activity sheets to support working with the Power of Light resources.
This zine explores these discussion questions: 1) What is needed for living things to exist on Earth? 2) What helps us to learn more about past events? 3) What enables us to be able say we 'know' something
20 and 3D Numerical Simulations of Flux Cancellation
Cancellation of magnetic flux in the solar photosphere and chromosphere has been linked observationally and theoretically to a broad range of solar activity, from filament channel formation to CME initiation. Because this phenomenon is typically measured at only a single layer in the atmosphere, in the radial (line of sight) component of the magnetic field, the actual processes behind this observational signature are ambiguous. It is clear that reconnection is involved in some way, but the location of the reconnection sites and associated connectivity changes remain uncertain in most cases. We are using numerical modeling to demystify flux cancellation, beginning with the simplest possible configuration: a subphotospheric Lundquist flux tube surrounded by a potential field, immersed in a gravitationally stratified atmosphere, spanning many orders of magnitude in plasma beta. In this system, cancellation is driven slowly by a 2-cell circulation pattern imposed in the convection zone, such that the tops of the cells are located around the beta= 1 level (Le., the photosphere) and the flows converge and form a downdraft at the polarity inversion line; note however that no flow is imposed along the neutral line. We will present the results of 2D and 3D MHD-AMR simulations of flux cancellation, in which the flux at the photosphere begins in either an unsheared or sheared state. In all cases, a lOW-lying flux rope is formed by reconnection at the polarity inversion line within a few thousand seconds. The flux rope remains stable and does not rise, however, in contrast to models which do not include the presence of significant mass loading
A Model for Patchy Reconnection in Three Dimensions
We show, theoretically and via MHD simulations, how a short burst of
reconnection localized in three dimensions on a one-dimensional current sheet
creates a pair of reconnected flux tubes. We focus on the post-reconnection
evolution of these flux tubes, studying their velocities and shapes. We find
that slow-mode shocks propagate along these reconnected flux tubes, releasing
magnetic energy as in steady-state Petschek reconnection. The geometry of these
three-dimensional shocks, however, differs dramatically from the classical
two-dimensional geometry. They propagate along the flux tube legs in four
isolated fronts, whereas in the two-dimensional Petschek model, they form a
continuous, stationary pair of V-shaped fronts.
We find that the cross sections of these reconnected flux tubes appear as
teardrop shaped bundles of flux propagating away from the reconnection site.
Based on this, we argue that the descending coronal voids seen by Yohkoh SXT,
LASCO, and TRACE are reconnected flux tubes descending from a flare site in the
high corona, for example after a coronal mass ejection. In this model, these
flux tubes would then settle into equilibrium in the low corona, forming an
arcade of post-flare coronal loops.Comment: 27 pages plus 16 figure
Evidence For Mixed Helicity in Erupting Filaments
Erupting filaments are sometimes observed to undergo a rotation about the
vertical direction as they rise. This rotation of the filament axis is
generally interpreted as a conversion of twist into writhe in a kink-unstable
magnetic flux rope. Consistent with this interpretation, the rotation is
usually found to be clockwise (as viewed from above) if the post-eruption
arcade has right-handed helicity, but counterclockwise if it has left-handed
helicity. Here, we describe two non--active-region filament events recorded
with the Extreme-Ultraviolet Imaging Telescope (EIT) on the {\it Solar and
Heliospheric Observatory} ({\it SOHO}), in which the sense of rotation appears
to be opposite to that expected from the helicity of the post-event arcade.
Based on these observations, we suggest that the rotation of the filament axis
is in general determined by the net helicity of the erupting system, and that
the axially aligned core of the filament can have the opposite helicity sign to
the surrounding field. In most cases, the surrounding field provides the main
contribution to the net helicity. In the events reported here, however, the
helicity associated with the filament ``barbs'' is opposite in sign to and
dominates that of the overlying arcade.Comment: ApJ, accepte
Patchy Reconnection in a Y-Type Current Sheet
We study the evolution of the magnetic field in a Y-type current sheet
subject to a brief, localized magnetic reconnection event. The reconnection
produces up- and down-flowing reconnected flux tubes which rapidly decelerate
when they hit the Y-lines and underlying magnetic arcade loops at the ends of
the current sheet. This localized reconnection outflow followed by a rapid
deceleration reproduces the observed behavior of post-CME downflowing coronal
voids. These simulations support the hypothesis that these observed coronal
downflows are the retraction of magnetic fields reconnected in localized
patches in the high corona.Comment: 4 pages, 3 figure
Modifying memory for a museum tour in older adults: reactivation-related updating that enhances and distorts memory is reduced in ageing
Memory reactivation, the activation of a latent memory trace when we are reminded of a past experience, strengthens memory but can also contribute to distortions if new information present during reactivation is integrated with existing memory. In a previous study in young adults (St. Jacques & Schacter, 2013; Psychological Science) we found that the quality of memory reactivation, manipulated using the principle of encoding specificity and indexed by recollection ratings, modulated subsequent true and false memories for events experienced during a museum tour. Here, we examined age-related changes in the quality of memory reactivation on subsequent memory. Young and older adults reactivated memories for museum stops immediately followed by the presentation of a novel lure photo from an alternate tour version (i.e., reactivation plus new information). There was an increase in subsequent true memories for reactivated targets and for subsequent false memories for lures that followed reactivated targets, when compared to baseline target and lure photos. However, the influence of reactivation on subsequent memories was reduced in older adults. These data reveal that aging alters reactivation-related updating processes that allow memories to be strengthened and updated with new information- consequently reducing memory distortions in older compared to young adults
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