Ustilospores of Tilletia ehrhartae, a smut of Ehrharta calycina, are common contaminants of Australian wheat grain, and a potential source of confusion with Tilletia indica, the cause of Karnal bunt of wheat

Australian wheat consigned for export from Australian ports was surveyed in March 2004 using a national diagnostic protocol for detection and identification of Tilletia indica. No ustilospores of T. indica were detected, confirming previous surveys which have failed to detect T. indica in Australia. However, the survey detected moderate levels of the common smuts Tilletia caries (syn. Tilletia tritici), Tilletia laevis and Urocystis agropyri, and very low levels (average fewer than six ustilospores per 150 g sample) of an unidentified dark, tuberculate-spored Tilletia in ≈ 60% of samples tested. Comparison with herbarium specimens enabled identification of the majority of the tuberculate ustilospores as Tilletia ehrhartae, a smut fungus known to infect only Ehrharta calycina (perennial veldt grass) and which is common in southern Australia. A smaller number of tuberculate smut ustilospores were identified as Tilletia walkeri, a smut of Lolium spp. recorded in Australia but apparently uncommon. Both T. ehrhartae and T. walkeri bear sufficient resemblance to T. indica for misidentifications to be possible where only a very few ustilospores are seen, although T. ehrhartae ustilospores are always <25 µm in diameter. The frequent presence of ustilospores of both T. ehrhartae and T. walkeri as contaminants of Australian wheat grain exports has significance for diagnosticians testing Australian export wheat, as it demonstrates the potential for tuberculate ustilospores of species other than those covered in existing diagnostic protocols to be misidentified as T. indica. This paper describes T. ehrhartae in detail, and provides criteria for its differentiation from T. indica, T. walkeri and some other species

A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. I. Theory

We derive an analytical approximation of nonlinear force-free magnetic field solutions (NLFFF) that can efficiently be used for fast forward-fitting to solar magnetic data, constrained either by observed line-of-sight magnetograms and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph data. The derived NLFFF solutions provide the magnetic field components $B_x({\bf x})$, $B_y({\bf x})$, $B_z({\bf x})$, the force-free parameter $\alpha({\bf x})$, the electric current density ${\bf j}({\bf x})$, and are accurate to second-order (of the nonlinear force-free $\alpha$-parameter). The explicit expressions of a force-free field can easily be applied to modeling or forward-fitting of many coronal phenomena.Comment: Solar Physics (in press), 26 pages, 11 figure

Deterministically Driven Avalanche Models of Solar Flares

We develop and discuss the properties of a new class of lattice-based avalanche models of solar flares. These models are readily amenable to a relatively unambiguous physical interpretation in terms of slow twisting of a coronal loop. They share similarities with other avalanche models, such as the classical stick--slip self-organized critical model of earthquakes, in that they are driven globally by a fully deterministic energy loading process. The model design leads to a systematic deficit of small scale avalanches. In some portions of model space, mid-size and large avalanching behavior is scale-free, being characterized by event size distributions that have the form of power-laws with index values, which, in some parameter regimes, compare favorably to those inferred from solar EUV and X-ray flare data. For models using conservative or near-conservative redistribution rules, a population of large, quasiperiodic avalanches can also appear. Although without direct counterparts in the observational global statistics of flare energy release, this latter behavior may be relevant to recurrent flaring in individual coronal loops. This class of models could provide a basis for the prediction of large solar flares.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in Solar Physic

The Timing of Application and Inclusion of a Surfactant Are Important for Absorption and Translocation of Foliar Phosphoric Acid by Wheat Leaves

Published: 22 November 2019Introduction: Foliar applied phosphorus (P) has the potential to provide a more tactical approach to P fertilization that could enhance P use efficiency. The aims of this study were to investigate the influence of adjuvant choice and application timing of foliar applied phosphoric acid on leaf wettability, foliar uptake, translocation, and grain yield of wheat plants. Materials and Methods: We measured the contact angles of water and fertilizers on wheat leaves, and the uptake, translocation and wheat yield response to isotopicallylabelled phosphoric acid in combination with five different adjuvants when foliar-applied to wheat at either early tillering or flag leaf emergence. Results: There was high foliar uptake of phosphoric acid in combination with all adjuvants that contained a surfactant, but only one treatment resulted in a 12% increase in grain yield and two treatments resulted in a decrease in grain yield. Despite the wettability of all foliar fertilizers being markedly different, foliar uptake was similar for all treatments that contained a surfactant. The translocation of phosphorus from foliar sources was higher when applied at a later growth stage than when applied at tillering despite the leaf surface properties that affect wettability being similar across all leaves at both growth stages. Discussion: Both the timing of foliar application and the inclusion of a surfactant in the formulation are important for absorption and translocation of phosphoric acid by wheat leaves, however high foliar uptake and translocation will not always translate to a yield increase.Courtney A. E. Peirce, Therese M. McBeath, Craig Priest and Michael J. McLaughli

Investigation of quasi-periodic varaiations in hard X-rays of solar flares

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by a good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong impulses. The advantage of our model of oscillating magnetic traps is that it can explain both the impulses of electron acceleration and quasi-periodicity of their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic

Study of flare energy release using events with numerous type III-like bursts in microwaves

The analysis of narrowband drifting of type III-like structures in radio bursts dynamic spectra allows to obtain unique information about primary energy release mechanisms in solar flares. The SSRT spatially resolved images and a high spectral and temporal resolution allow direct determination not only the positions of its sources but also the exciter velocities along the flare loop. Practically, such measurements are possible during some special time intervals when the SSRT (about 5.7 GHz) is observing the flare region in two high-order fringes; thus, two 1D scans are recorded simultaneously at two frequency bands. The analysis of type III-like bursts recorded during the flare 14 Apr 2002 is presented. Using-muliwavelength radio observations recorded by SSRT, SBRS, NoRP, RSTN we study an event with series of several tens of drifting microwave pulses with drift rates in the range from -7 to 13 GHz/s. The sources of the fast-drifting bursts were located near the top of the flare loop in a volume of a few Mm in size. The slow drift of the exciters along the flare loop suggests a high pitch-anisotropy of the emitting electrons.Comment: 16 pages, 6 figures, Solar Physics, in press, 201

Is null-point reconnection important for solar flux emergence?

The role of null-point reconnection in a 3D numerical MHD model of solar emerging flux is investigated. The model consists of a twisted magnetic flux tube rising through a stratified convection zone and atmosphere to interact and reconnect with a horizontal overlying magnetic field in the atmosphere. Null points appear as the reconnection begins and persist throughout the rest of the emergence, where they can be found mostly in the model photosphere and transition region, forming two loose clusters on either side of the emerging flux tube. Up to 26 nulls are present at any one time, and tracking in time shows that there is a total of 305 overall, despite the initial simplicity of the magnetic field configuration. We find evidence for the reality of the nulls in terms of their methods of creation and destruction, their balance of signs, their long lifetimes, and their geometrical stability. We then show that due to the low parallel electric fields associated with the nulls, null-point reconnection is not the main type of magnetic reconnection involved in the interaction of the newly emerged flux with the overlying field. However, the large number of nulls implies that the topological structure of the magnetic field must be very complex and the importance of reconnection along separators or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc

The Flare-energy Distributions Generated by Kink-unstable Ensembles of Zero-net-current Coronal Loops

It has been proposed that the million degree temperature of the corona is due to the combined effect of barely-detectable energy releases, so called nanoflares, that occur throughout the solar atmosphere. Alas, the nanoflare density and brightness implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release and so we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field transitions to a lower energy state, which is modelled by relaxation theory, i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with a critical magnetic twist value for every point along the instability threshold

Time-dependent Stochastic Modeling of Solar Active Region Energy

A time-dependent model for the energy of a flaring solar active region is presented based on a stochastic jump-transition model (Wheatland and Glukhov 1998; Wheatland 2008; Wheatland 2009). The magnetic free energy of the model active region varies in time due to a prescribed (deterministic) rate of energy input and prescribed (random) flare jumps downwards in energy. The model has been shown to reproduce observed flare statistics, for specific time-independent choices for the energy input and flare transition rates. However, many solar active regions exhibit time variation in flare productivity, as exemplified by NOAA active region AR 11029 (Wheatland 2010). In this case a time-dependent model is needed. Time variation is incorporated for two cases: 1. a step change in the rates of flare jumps; and 2. a step change in the rate of energy supply to the system. Analytic arguments are presented describing the qualitative behavior of the system in the two cases. In each case the system adjusts by shifting to a new stationary state over a relaxation time which is estimated analytically. The new model retains flare-like event statistics. In each case the frequency-energy distribution is a power law for flare energies less than a time-dependent rollover set by the largest energy the system is likely to attain at a given time. For Case 1, the model exhibits a double exponential waiting-time distribution, corresponding to flaring at a constant mean rate during two intervals (before and after the step change), if the average energy of the system is large. For Case 2 the waiting-time distribution is a simple exponential, again provided the average energy of the system is large. Monte Carlo simulations of Case~1 are presented which confirm the analytic estimates. The simulation results provide a qualitative model for observed flare statistics in active region AR 11029.Comment: 25 pages, 9 figure

Radio Observations of the January 20, 2005 X-Class Event

We present a multi-frequency and multi-instrument study of the 20 January 2005 event. We focus mainly on the complex radio signatures and their association with the active phenomena taking place: flares, CMEs, particle acceleration and magnetic restructuring. As a variety of energetic particle accelerators and sources of radio bursts are present, in the flare-ejecta combination, we investigate their relative importance in the progress of this event. The dynamic spectra of {Artemis-IV-Wind/Waves-Hiras with 2000 MHz-20 kHz frequency coverage, were used to track the evolution of the event from the low corona to the interplanetary space; these were supplemented with SXR, HXR and gamma-ray recordings. The observations were compared with the expected radio signatures and energetic-particle populations envisaged by the {Standard Flare--CME model and the reconnection outflow termination shock model. A proper combination of these mechanisms seems to provide an adequate model for the interpretation of the observational data.Comment: Accepted for publication in Solar Physic