Publications
128 results found
Cargill P, 2023, Sydney Chapman, Oxford's Sedleian Professors of Natural Philosophy, Editors: Hollings, McCartney, Publisher: OUP, ISBN: 9780192843210
Sydney Chapman FRS was the Sedleian professor between 1946 and 1953. He was also one of the outstanding geophysicists of the 20th century. This chapter reviews his career with particular reference to his work on solar terrestrial relations.
Cargill PJ, Bradshaw SJ, Klimchuk JA, et al., 2021, Static and dynamic solar coronal loops with cross-sectional area variations, Monthly Notices of the Royal Astronomical Society, Vol: 509, Pages: 4420-4429, ISSN: 0035-8711
The Enthalpy Based Thermal Evolution of Loops approximate model for static and dynamic coronal loops is developed to include the effect of a loop cross-sectional area which increases from the base of the transition region (TR) to the corona. The TR is defined as the part of a loop between the top of the chromosphere and the location where thermal conduction changes from an energy loss to an energy gain. There are significant differences from constant area loops due to the manner in which the reduced volume of the TR responds to conductive and enthalpy fluxes from the corona. For static loops with modest area variation the standard picture of loop energy balance is retained, with the corona and TR being primarily a balance between heating and conductive losses in the corona, and downward conduction and radiation to space in the TR. As the area at the loop apex increases, the TR becomes thicker and the density in TR and corona larger. For large apex areas, the coronal energy balance changes to one primarily between heating and radiation, with conduction playing an increasingly unimportant role, and the TR thickness becoming a significant fraction of the loop length. Approximate scaling laws are derived that give agreement with full numerical solutions for the density, but not the temperature. For non-uniform areas, dynamic loops have a higher peak temperature and are denser in the radiative cooling phase by of order 50 per cent than the constant area case for the examples considered. They also show a final rapid cooling and draining once the temperature approaches 1 MK. Although the magnitude of the emission measure will be enhanced in the radiative phase, there is little change in the important observational diagnostic of its temperature dependence.
Reid J, Cargill PJ, Johnston CD, et al., 2021, Linking computational models to follow the evolution of heated coronal plasma, Monthly Notices of the Royal Astronomical Society, Vol: 505, Pages: 4141-4150, ISSN: 0035-8711
A ‘proof of principle’ is presented, whereby the Ohmic and viscous heating determined by a three-dimensional (3D) MHD model of a coronal avalanche are used as the coronal heating input for a series of field-aligned, one-dimensional (1D) hydrodynamic models. Three-dimensional coronal MHD models require large computational resources. For current numerical parameters, it is difficult to model both the magnetic field evolution and the energy transport along field lines for coronal temperatures much hotter than 1MK, because of severe constraints on the time step from parallel thermal conduction. Using the 3D MHD heating derived from a simulation and evaluated on a single field line, the 1D models give coronal temperatures of 1MK and densities 1014--1015m−3 for a coronal loop length of 80Mm. While the temperatures and densities vary smoothly along the field lines, the heating function leads to strong asymmetries in the plasma flows. The magnitudes of the velocities in the 1D model are comparable with those seen in 3D reconnection jets in our earlier work. Advantages and drawbacks of this approach for coronal modelling are discussed.
Del Zanna G, Andretta V, Cargill PJ, et al., 2021, High resolution soft X-ray spectroscopy and the quest for the hot (5-10 MK) plasma in solar active regions, Frontiers in Astronomy and Space Sciences, Vol: 8, Pages: 1-19, ISSN: 2296-987X
We discuss the diagnostics available to study the 5–10 MK plasma in the solar corona, which is key to understanding the heating in the cores of solar active regions. We present several simulated spectra, and show that excellent diagnostics are available in the soft X-rays, around 100 Å, as six ionization stages of Fe can simultaneously be observed, and electron densities derived, within a narrow spectral region. As this spectral range is almost unexplored, we present an analysis of available and simulated spectra, to compare the hot emission with the cooler component. We adopt recently designed multilayers to present estimates of count rates in the hot lines, with a baseline spectrometer design. Excellent count rates are found, opening up the exciting opportunity to obtain high-resolution spectroscopy of hot plasma.
Horbury TS, OBrien H, Carrasco Blazquez I, et al., 2020, The Solar Orbiter magnetometer, Astronomy & Astrophysics, Vol: 642, Pages: A9-A9, ISSN: 0004-6361
The magnetometer instrument on the Solar Orbiter mission is designed to measure the magnetic field local to the spacecraft continuously for the entire mission duration. The need to characterise not only the background magnetic field but also its variations on scales from far above to well below the proton gyroscale result in challenging requirements on stability, precision, and noise, as well as magnetic and operational limitations on both the spacecraft and other instruments. The challenging vibration and thermal environment has led to significant development of the mechanical sensor design. The overall instrument design, performance, data products, and operational strategy are described.
Johnston CD, Cargill PJ, Hood AW, et al., 2020, Modelling the solar transition region using an adaptive conduction method, Astronomy & Astrophysics, Vol: 635, Pages: A168-A168, ISSN: 0004-6361
Modelling the solar Transition Region with the use of an Adaptive Conduction (TRAC) method permits fast and accurate numerical solutions of the field-aligned hydrodynamic equations, capturing the enthalpy exchange between the corona and transition region, when the corona undergoes impulsive heating. The TRAC method eliminates the need for highly resolved numerical grids in the transition region and the commensurate very short time steps that are required for numerical stability. When employed with coarse spatial resolutions, typically achieved in multi-dimensional magnetohydrodynamic codes, the errors at peak density are less than 5% and the computation time is three orders of magnitude faster than fully resolved field-aligned models. This paper presents further examples that demonstrate the versatility and robustness of the method over a range of heating events, including impulsive and quasi-steady footpoint heating. A detailed analytical assessment of the TRAC method is also presented, showing that the approach works through all phases of an impulsive heating event because (i) the total radiative losses and (ii) the total heating when integrated over the transition region are both preserved at all temperatures under the broadening modifications of the method. The results from the numerical simulations complement this conclusion.
Reid J, Cargill PJ, Hood AW, et al., 2020, Coronal energy release by MHD avalanches: Heating mechanisms, Astronomy and Astrophysics: a European journal, Vol: 633, Pages: 1-16, ISSN: 0004-6361
The plasma heating associated with an avalanche involving three twisted magnetic threads within a coronal loop is investigated using three-dimensional magnetohydrodynamic simulations. The avalanche is triggered by the kink instability of one thread, with the others being engulfed as a consequence. The heating as a function of both time and location along the strands is evaluated. It is shown to be bursty at all times but to have no preferred spatial location. While there appears to be a level of “background” heating, this is shown to be comprised of individual, small heating events. A comparison between viscous and resistive (Ohmic) heating demonstrates that the strongest heating events are largely associated with the Ohmic heating that arises when the current exceeds a critical value. Viscous heating is largely (but not entirely) associated with smaller events. Ohmic heating dominates viscous heating only at the time of the initial kink instability. It is also demonstrated that a variety of viscous models lead to similar heating rates, suggesting that the system adjusts to dissipate the same amount of energy.
Johnston C, Cargill P, antolin P, et al., 2019, The effects of numerical resolution, heating timescales and background heating on thermal non-equilibrium in coronal loops, Astronomy and Astrophysics, Vol: 625, ISSN: 0004-6361
Thermal non-equilibrium (TNE) is believed to be a potentially important process in understanding some properties ofthe magnetically closed solar corona. Through one-dimensional hydrodynamic models, this paper addresses the importanceof the numerical spatial resolution, footpoint heating timescales and background heating on TNE. Inadequatetransition region (TR) resolution can lead to significant discrepancies in TNE cycle behaviour, with TNE being suppressedin under-resolved loops. A convergence on the periodicity and plasma properties associated with TNE requiredspatial resolutions of less than 2 km for a loop of length 180 Mm. These numerical problems can be resolved using anapproximate method that models the TR as a discontinuity using a jump condition, as proposed by Johnston et al.(2017a,b). The resolution requirements (and so computational cost) are greatly reduced while retaining good agreementwith fully resolved results. Using this approximate method we (i) identify different regimes for the response of coronalloops to time-dependent footpoint heating including one where TNE does not arise and (ii) demonstrate that TNE in aloop with footpoint heating is suppressed unless the background heating is sufficiently small. The implications for thegenerality of TNE are discussed.
Reid J, Hood AW, Parnell CE, et al., 2018, Coronal energy release by MHD avalanches: continuous driving, Astronomy and Astrophysics: a European journal, Vol: 615, Pages: 1-10, ISSN: 0004-6361
Previous work has confirmed the concept of a magnetohydrodynamic (MHD) avalanche in pre-stressed threads within a coronal loop. We undertook a series of full, three-dimensional MHD simulations in order to create three threads by twisting the magnetic field through boundary motions until an instability ensues. We find that, following the original instability, one unstable thread can disrupt its neighbours with continued driving. A “bursty” heating profile results, with a series of ongoing energy releases, but no evident steady state. For the first time using full MHD, we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions.
Goldstraw EE, Hood AW, Browning PK, et al., 2018, Comparison of methods for modelling coronal magnetic fields, Astronomy and Astrophysics, Vol: 610, ISSN: 0004-6361
Aims. Four different approximate approaches used to model the stressing of coronal magnetic fields due to an imposed photospheric motion are compared with each other and the results from a full time-dependent magnetohydrodynamic (MHD) code. The assumptions used for each of the approximate methods are tested by considering large photospheric footpoint displacements.Methods. We consider a simple model problem, comparing the full non-linear MHD, determined with the Lare2D numerical code, with four approximate approaches. Two of these, magneto-frictional relaxation and a quasi-1D Grad-Shafranov approach, assume sequences of equilibria, whilst the other two methods, a second-order linearisation of the MHD equations and Reduced MHD, are time dependent.Results. The relaxation method is very accurate compared to full MHD for force-free equilibria for all footpoint displacements, but has significant errors when the plasma β0 is of order unity. The 1D approach gives an extremely accurate description of the equilibria away from the photospheric boundary layers, and agrees well with Lare2D for all parameter values tested. The linearised MHD equations correctly predict the existence of photospheric boundary layers that are present in the full MHD results. As soon as the footpoint displacement becomes a significant fraction of the loop length, the RMHD method fails to model the sequences of equilibria correctly. The full numerical solution is interesting in its own right, and care must be taken for low β0 plasmas if the viscosity is too high.
Johnston CD, Hood AW, Cargill PJ, et al., 2017, A new approach for modelling chromospheric evaporation in response to enhanced coronal heating, Astronomy & Astrophysics, Vol: 605, Pages: A8-A8, ISSN: 0004-6361
We proposed that the use of an approximate “jump condition” at the solar transition region permits fast and accurate numerical solutions of the one dimensional hydrodynamic equations when the corona undergoes impulsive heating. In particular, it eliminates the need for the very short timesteps imposed by a highly resolved numerical grid. This paper presents further examples of the applicability of the method for cases of non-uniform heating, in particular, nanoflare trains (uniform in space but non-uniform in time) and spatially localised impulsive heating, including at the loop apex and base of the transition region. In all cases the overall behaviour of the coronal density and temperature shows good agreement with a fully resolved one dimensional model and is significantly better than the equivalent results from a 1D code run without using the jump condition but with the same coarse grid. A detailed assessment of the errors introduced by the jump condition is presented showing that the causes of discrepancy with the fully resolved code are (i) the neglect of the terms corresponding to the rate of change of total energy in the unresolved atmosphere; (ii) mass motions at the base of the transition region and (iii) for some cases with footpoint heating, an over-estimation of the radiative losses in the transition region.
Johnson CD, Hood AW, de Moortel I, et al., 2017, A New Approach for Modelling Chromospheric Evaporation in Response to Enhanced Coronal Heating: 1 The Method, Astronomy & Astrophysics, Vol: 597, ISSN: 0004-6361
We present a new computational approach that addresses the difficulty of obtaining the correct interaction betweenthe solar corona and the transition region in response to rapid heating events. In the coupled corona/transition region/chromospheresystem, an enhanced downward conductive flux results in an upflow (chromospheric evaporation).However, obtaining the correct upflow generally requires high spatial resolution in order to resolve the transition region.With an unresolved transition region, artificially low coronal densities are obtained because the downward heatflux “jumps” across the unresolved region to the chromosphere, underestimating the upflows. Here, we treat the lowertransition region as a discontinuity that responds to changing coronal conditions through the imposition of a jumpcondition that is derived from an integrated form of energy conservation. To illustrate and benchmark this approachagainst a fully resolved one-dimensional model, we present field-aligned simulations of coronal loops in response to arange of impulsive (spatially uniform) heating events. We show that our approach leads to a significant improvement inthe coronal density evolution than just when using coarse spatial resolutions insufficient to resolve the lower transitionregion. Our approach compensates for the “jumping” of the heat flux by imposing a velocity correction that ensuresthat the energy from the heat flux goes into driving the transition region dynamics, rather than being lost throughradiation. Hence, it is possible to obtain improved coronal densities. The advantages of using this approach in bothone-dimensional hydrodynamic and three-dimensional magnetohydrodynamic simulations are discussed.
Barnes WT, Cargill PJ, Bradshaw SJ, 2016, Inference of heating properties from "hot" non-flaring plasmas in active region cores. II. nanoflare trains, Astrophysical Journal, Vol: 833, ISSN: 1538-4357
Despite its prediction over two decades ago, the detection of faint, high-temperature (\hot") emissiondue to nano are heating in non- aring active region cores has proved challenging. Using an e cienttwo- uid hydrodynamic model, this paper investigates the properties of the emission expected fromrepeating nano ares (a nano are train) of varying frequency as well as the separate heating of electronsand ions. If the emission measure distribution (EM(T)) peaks atT=Tm, we nd that EM(Tm) isindependent of details of the nano are train, and EM(T) above and belowTmre ects di erent aspectsof the heating. BelowTmthe main in uence is the relationship of the waiting time between successivenano ares to the nano are energy. AboveTmpower-law nano are distributions lead to an extensiveplasma population not present in a monoenergetic train. Furthermore, in some cases characteristicfeatures are present in EM(T). Such details may be detectable given adequate spectral resolution anda good knowledge of the relevant atomic physics. In the absence of such resolution we propose somemetrics that can be used to infer the presence of \hot" plasma.
Barnes WT, Cargill PJ, Bradshaw SJ, 2016, Inference of heating properties from "hot" non-flaring plasmas in active region cores i. single nanoflares, Astrophysical Journal, Vol: 829, Pages: 31-31, ISSN: 1538-4357
The properties expected of “hot” non-flaring plasmas due to nanoflare heating in active regions areinvestigated using hydrodynamic modeling tools, including a two-fluid development of the EBTELcode. Here we study a single nanoflare and show that while simple models predict an emission measuredistribution extending well above 10 MK that is consistent with cooling by thermal conduction,many other effects are likely to limit the existence and detectability of such plasmas. These include:differential heating between electrons and ions, ionization non-equilibrium and, for short nanoflares,the time taken for the coronal density to increase. The most useful temperature range to look for thisplasma, often called the “smoking gun” of nanoflare heating, lies between 106.6 and 107 K. Signaturesof the actual heating may be detectable in some instances.
Cargill PJ, DeMoortel I, Kiddie G, 2016, Coronal density structure and its role in wave damping in loops, Astrophysical Journal, Vol: 823, ISSN: 1538-4357
It has long been established that gradients in the Alfvén speed, and in particular the plasmadensity, are an essential part of the damping of waves in the magnetically closed solar coronaby mechanisms such as resonant absorption or phase mixing. While models of wave dampingoften assume a fixed density gradient, in this paper the self-consistency of such calculationsis assessed by examining the temporal evolution of the coronal density. It is shownconceptually that for some coronal structures, density gradients can evolve in a way that thewave damping processes are inhibited. For the case of phase mixing we argue that: (a) waveheating cannot sustain the assumed density structure and (b) inclusion of feedback of theheating on the density gradient can lead to a highly structured density, although on longtimescales. In addition, transport coefficients well in excess of classical are required tomaintain the observed coronal density. Hence, the heating of closed coronal structures byglobal oscillations may face problems arising from the assumption of a fixed density gradientand the rapid damping of oscillations may have to be accompanied by a separate (non-wavebased) heating mechanism to sustain the required density structuring.
Hood AW, Cargill PJ, Browning PK, et al., 2016, An MHD avalanche in a multi-threaded coronal loop, Astrophysical Journal, Vol: 817, ISSN: 1538-4357
Tam KV, Hood AW, Browning PK, et al., 2015, Coronal heating in multiple magnetic threads, Astronomy & Astrophysics, Vol: 580, ISSN: 1432-0746
Cargill PJ, Warren HP, Bradshaw SJ, 2015, Modelling nanoflares in active regions and implications for coronal heating mechanisms, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 373, ISSN: 1364-503X
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- Citations: 35
Fletcher L, Cargill PJ, Antiochos SK, et al., 2015, Structures in the Outer Solar Atmosphere, SPACE SCIENCE REVIEWS, Vol: 188, Pages: 211-249, ISSN: 0038-6308
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- Citations: 15
Cargill P, 2015, Magnetic Reconnection in the Solar Corona: Historical Perspective and Modern Thinking, MAGNETOSPHERIC PLASMA PHYSICS: THE IMPACT OF JIM DUNGEY'S RESEARCH, Editors: Southwood, Cowley, Mitton, Publisher: SPRINGER, Pages: 221-251, ISBN: 978-3-319-18358-9
Cargill PJ, 2014, ACTIVE REGION EMISSION MEASURE DISTRIBUTIONS AND IMPLICATIONS FOR NANOFLARE HEATING, ASTROPHYSICAL JOURNAL, Vol: 784, ISSN: 0004-637X
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- Citations: 61
Balogh A, Bykov A, Cargill P, et al., 2014, Microphysics of Cosmic Plasmas: Background, Motivation and Objectives, MICROPHYSICS OF COSMIC PLASMAS, Editors: Balogh, Bykov, Cargill, Dendy, DeWit, Raymond, Publisher: SPRINGER, Pages: 1-4, ISBN: 978-1-4899-7412-9
Balogh A, Bykov A, Cargill P, et al., 2013, Microphysics of Cosmic Plasmas: Background, Motivation and Objectives, SPACE SCIENCE REVIEWS, Vol: 178, Pages: 77-80, ISSN: 0038-6308
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- Citations: 2
Cargill PJ, Bradshaw SJ, 2013, THE COOLING OF CORONAL PLASMAS. IV. CATASTROPHIC COOLING OF LOOPS, ASTROPHYSICAL JOURNAL, Vol: 772, ISSN: 0004-637X
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- Citations: 26
Bradshaw SJ, Cargill PJ, 2013, THE INFLUENCE OF NUMERICAL RESOLUTION ON CORONAL DENSITY IN HYDRODYNAMIC MODELS OF IMPULSIVE HEATING, ASTROPHYSICAL JOURNAL, Vol: 770, ISSN: 0004-637X
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- Citations: 94
Cargill P, 2013, SOLAR PHYSICS Towards ever smaller length scales, NATURE, Vol: 493, Pages: 485-486, ISSN: 0028-0836
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- Citations: 10
Cargill PJ, Vlahos L, Baumann G, et al., 2012, Current Fragmentation and Particle Acceleration in Solar Flares, SPACE SCIENCE REVIEWS, Vol: 173, Pages: 223-245, ISSN: 0038-6308
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- Citations: 54
Cargill PJ, Bradshaw SJ, Klimchuk JA, 2012, ENTHALPY-BASED THERMAL EVOLUTION OF LOOPS. III. COMPARISON OF ZERO-DIMENSIONAL MODELS, ASTROPHYSICAL JOURNAL, Vol: 758, ISSN: 0004-637X
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- Citations: 54
Cargill PJ, Bradshaw SJ, Klimchuk JA, 2012, ENTHALPY-BASED THERMAL EVOLUTION OF LOOPS. II. IMPROVEMENTS TO THE MODEL, ASTROPHYSICAL JOURNAL, Vol: 752, ISSN: 0004-637X
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- Citations: 91
Cargill P, De Moortel I, 2011, SOLAR PHYSICS Waves galore, NATURE, Vol: 475, Pages: 463-464, ISSN: 0028-0836
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- Citations: 17
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