The global distribution of magnetic helicity in the solar corona

By defining an appropriate field line helicity, we apply the powerful concept of magnetic helicity to the problem of global magnetic field evolution in the Sun's corona. As an ideal-magnetohydrodynamic invariant, the field line helicity is a meaningful measure of how magnetic helicity is distributed within the coronal volume. It may be interpreted, for each magnetic field line, as a magnetic flux linking with that field line. Using magneto-frictional simulations, we investigate how field line helicity evolves in the non-potential corona as a result of shearing by large-scale motions on the solar surface. On open magnetic field lines, the helicity injected by the Sun is largely output to the solar wind, provided that the coronal relaxation is sufficiently fast. But on closed magnetic field lines, helicity is able to build up. We find that the field line helicity is non-uniformly distributed, and is highly concentrated in twisted magnetic flux ropes. Eruption of these flux ropes is shown to lead to sudden bursts of helicity output, in contrast to the steady flux along the open magnetic field lines.</p

Introduction to Field Line Helicity

Field line helicity measures the net linking of magnetic flux with a single magnetic field line. It offers a finer topological description than the usual global magnetic helicity integral, while still being invariant in an ideal evolution unless there is a flux of helicity through the domain boundary. In this chapter, we explore how to appropriately define field line helicity in different volumes in a way that preserves a meaningful topological interpretation. We also review the time evolution of field line helicity under both boundary motions and magnetic reconnection.Comment: 23 pages, 8 figures, to appear as chapter of AGU book "Helicities in Geophysics, Astrophysics and Beyond", published by Wiley, ISBN 111984168

Optimal unstirred state of a passive scalar

This work was supported by Leverhulme Trust grant PRG-2017-169.Given a passive tracer distribution, what is the simplest unstirred pattern that may be reached under incompressible advection? This question is partially motivated by recent studies of three-dimensional (3-D) magnetic reconnection, in which the patterns of a topological invariant called the field line helicity greatly simplify until reaching a relaxed state. We test two approaches: a variational method with minimal constraints, and a magnetic relaxation scheme where the velocity is determined explicitly by the pattern of. Both methods achieve similar convergence for simple test cases. However, the magnetic relaxation method guarantees a monotonic decrease in the Dirichlet seminorm of, and is numerically more robust. We therefore apply the latter method to two complex mixed patterns modelled on the field line helicity of 3-D magnetic braids. The unstirring separates into a small number of large-scale regions determined by the initial topology, which is well preserved during the computation. Interestingly, the velocity field is found to have the same large-scale topology as. Similarity to the simplification found empirically in 3-D magnetic reconnection simulations supports the idea that advection is an important principle for field line helicity evolution.Publisher PDFPeer reviewe

Evolution of field line helicity in magnetic relaxation

This work was facilitated by Leverhulme Trust under Grant No. PRG-2017–169, with additional support from Science and Technology Facilities Council (UK) under consortium Grants Nos. ST/N000714, ST/N000781, and ST/S000321.Plasma relaxation in the presence of an initially braided magnetic field can lead to self-organization into relaxed states that retain non-trivial magnetic structure. These relaxed states may be in conflict with the linear force-free fields predicted by the classical Taylor theory, and remain to be fully understood. Here, we study how the individual field line helicities evolve during such a relaxation, and show that they provide new insights into the relaxation process. The line helicities are computed for numerical resistive-magnetohydrodynamic simulations of a relaxing braided magnetic field with line-tied boundary conditions, where the relaxed state is known to be non-Taylor. First, our computations confirm recent analytical predictions that line helicity will be predominantly redistributed within the domain, rather than annihilated. Second, we show that self-organization into a relaxed state with two discrete flux tubes may be predicted from the initial line helicity distribution. Third, for this set of line-tied simulations we observe that the sub-structure within each of the final tubes is a state of uniform line helicity. This uniformization of line helicity is consistent with Taylor theory applied to each tube individually. However, it is striking that the line helicity becomes significantly more uniform than the force-free parameter.Publisher PDFPeer reviewe

Soil acidity on high rainfall pastures

Most soils of the high rainfall area of south-western Western Australia are naturally acis. The most acid group of soils, the peaty sands. have been routinely limed before subterranean clover pastures were established since research in the 1950s showed that poor Rhizobium nodulation could be overcome with the application of about 2 tonnes per hectare of coastal limesand

Transition to a weaker Sun: Changes in the solar atmosphere during the decay of the Modern Maximum

Context. The Sun experienced a period of unprecedented activity during the 20th century, now called the Modern Maximum (MM). The decay of the MM after its maximum in cycle 19 has changed the Sun, the heliosphere, and the planetary environments in many ways. However, studies disagree on whether this decay has proceeded synchronously in different solar parameters or not.Aims. One of the related key issues is if the relation between two long parameters of solar activity, the sunspot number and the solar 10.7 cm radio flux, has remained the same during this decay. A recent study argues that there is an inhomogeneity in the 10.7 cm radio flux in 1980, which leads to a step-like jump (“1980 jump”) in this relation. If true, this result would reduce the versatility of possible long-term studies of the Sun during the MM. Here we aim to show that the relation between sunspot number and 10.7 cm radio flux does indeed vary in time, not due to an inhomogeneous radio flux but due to physical changes in the solar atmosphere.Methods. We used radio flux measurements made in Japan at four different wavelengths, and studied their long-term relation with the sunspot number and the 10.7 cm radio flux during the decay of MM. We also used two other solar parameters, the MgII index and the number of solar active regions, in order to study the nature of the observed long-term changes in more detail.Results. We find that the 1980 jump is only the first of a series of 1–2-year “humps” that mainly occur during solar maxima. All five radio fluxes depict an increasing trend with respect to the sunspot number from the 1970s to 2010s. These results exclude the interpretation of the 1980 jump as an inhomogeneity in the 10.7 cm flux, and reestablish the 10.7 cm flux as a homogeneous measure of solar activity. The fluxes of the longer radio waves are found to increase with respect to the shorter waves, which suggests a long-term change in the solar radio spectrum. We also find that the MgII index of solar UV irradiance and the number of active regions also increased with respect to the sunspot number, further verifying the difference in the long-term evolution in chromospheric and photospheric parameters.Conclusions. Our results provide evidence for important structural changes in solar magnetic fields and the solar atmosphere during the decay of the MM, which have not been reliably documented so far. We also emphasize that the changing relation between the different (e.g., photospheric and chromospheric) solar parameters should be taken into account when using the sunspot number or any single parameter in long-term studies of solar activity

Soil acidity - high rainfall pastures.

Aims of the Project (i) To establish the current pH of the cultivated soils of the high rainfall areas of south-west Western Australia, and the extent to which pH has altered since clearing. (ii) To examine the responsiveness of old land pastures with low current soil pH levels (\u3c 5.5 water) to applied lime. (iii) To relate the responsiveness of subterranean clover-based pastures to measured soil parameters. 80BU14, 81AL10, 81AL12, 81BU18, 81BY18, 81BY25, 81BY26, 82AL4, 82AL5, 82AL55, 82BU7, 82HA35, 82HA36, 82PE1, 82MA20, 83AL7, 83AL9, 83AL10, 83ALll, 83BY29, 84BU9, 84BU10, 84BY37, 84HA21, 84HA37, 84MA21

A Near-half-century Simulation of the Solar Corona

We present an overview of results from a magnetofrictional model of the entire solar corona over a period of 47 yr. The simulation self-consistently reproduces decades of solar phenomena, varying in duration between rapid eruptions and the long-term solar cycles, from an input of observed active regions emerging at the photosphere. We have developed a geometric approach to use magnetic helicity to identify and localize the frequent eruptions that occur in the simulation. This method allows us to match our results to extreme-ultraviolet observations of transient events. We have analyzed the evolving magnetic topology by computing the squashing factor and segmenting the corona into discrete magnetic domains bounded by the Separatrix-Web. The simulations show a more dynamic structure to the Separatrix-Web than is predicted by potential field models, which may explain solar wind observations