Imperial College London

Dr Julia E. Stawarz

Faculty of Natural SciencesDepartment of Physics

Royal Society University Research Fellow
 
 
 
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Contact

 

+44 (0)20 7594 7766j.stawarz

 
 
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Location

 

6M71Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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83 results found

Stawarz J, Eastwood J, Phan T, Gingell I, Pyakurel P, Shay M, Robertson S, Russell C, Le Contel Oet al., 2022, Turbulence-driven magnetic reconnection and the magnetic correlation length: observations from magnetospheric multiscale in Earth's magnetosheath, Physics of Plasmas, Vol: 29, Pages: 1-20, ISSN: 1070-664X

Turbulent plasmas generate a multitude of thin current structures that can be sites for magnetic reconnection. The Magnetospheric Multiscale (MMS) mission has recently enabled the detailed examination of such turbulent current structures in Earth's magnetosheath and revealed that a novel type of reconnection, known as electron-only reconnection, can occur. In electron-only reconnection, ions do not have enough space to couple to the newly reconnected magnetic fields, suppressing ion jet formation and resulting in thinner sub-proton-scale current structures with faster super-Alfvénic electron jets. In this study, MMS observations are used to examine how the magnetic correlation length (λC) of the turbulence, which characterizes the size of the large-scale magnetic structures and constrains the length of the current sheets formed, influences the nature of turbulence-driven reconnection. We systematically identify 256 reconnection events across 60 intervals of magnetosheath turbulence. Most events do not appear to have ion jets; however, 18 events are identified with ion jets that are at least partially coupled to the reconnected magnetic field. The current sheet thickness and electron jet speed have a weak anti-correlation, with faster electron jets at thinner current sheets. When đœ†đ¶â‰Č20 ion inertial lengths, as is typical near the sub-solar magnetosheath, a tendency for thinner current sheets and potentially faster electron jets is present. The results are consistent with electron-only reconnection being more prevalent for turbulent plasmas with relatively short λC and may be relevant to the nonlinear dynamics and energy dissipation in turbulent plasmas.

Journal article

Nakamura TKM, Blasl KA, Hasegawa H, Umeda T, Liu Y-H, Peery SA, Plaschke F, Nakamura R, Holmes JC, Stawarz JE, Nystrom WDet al., 2022, Multi-scale evolution of Kelvin–Helmholtz waves at the Earth's magnetopause during southward IMF periods, Physics of Plasmas, Vol: 29, Pages: 012901-012901, ISSN: 1070-664X

Journal article

Blasl KA, Nakamura TKM, Plaschke F, Nakamura R, Hasegawa H, Stawarz JE, Liu Y-H, Peery S, Holmes JC, Hosner M, Schmid D, Roberts OW, Volwerk Met al., 2022, Multi-scale observations of the magnetopause Kelvin–Helmholtz waves during southward IMF, Physics of Plasmas, Vol: 29, Pages: 012105-012105, ISSN: 1070-664X

Journal article

Adhikari S, Parashar TN, Shay MA, Matthaeus WH, Pyakurel PS, Fordin S, Stawarz JE, Eastwood JPet al., 2021, Energy transfer in reconnection and turbulence, Physical Review E, Vol: 104, ISSN: 2470-0045

Journal article

Matteini L, Laker R, Horbury T, Woodham L, Bale SD, Stawarz JE, Woolley T, Steinvall K, Jones GH, Grant SR, Afghan Q, Galand M, O'Brien H, Evans V, Angelini V, Maksimovic M, Chust T, Khotyaintsev Y, Krasnoselskikh V, Kretzschmar M, Lorfevre E, Plettemeier D, Soucek J, Steller M, Stverak S, Travnicek P, Vaivads A, Vecchio A, Wimmer-Schweingruber RF, Ho GC, Gomez-Herrero R, Rodriguez-Pacheco J, Louarn P, Fedorov A, Owen CJ, Bruno R, Livi S, Zouganelis I, Muller Det al., 2021, Solar Orbiter's encounter with the tail of comet C/2019 Y4 (ATLAS): Magnetic field draping and cometary pick-up ion waves, ASTRONOMY & ASTROPHYSICS, Vol: 656, ISSN: 0004-6361

Journal article

Eastwood JP, Stawarz JE, Phan TD, Laker R, Robertson S, Zhao L-L, Zank GP, Lavraud B, Shay MA, Evans V, Angelini V, O'Brien H, Horbury TSet al., 2021, Solar Orbiter observations of an ion-scale flux rope confined to a bifurcated solar wind current sheet, Astronomy & Astrophysics, Vol: 656, Pages: 1-8, ISSN: 0004-6361

Context. Flux ropes in the solar wind are a key element of heliospheric dynamics and particle acceleration. When associated withcurrent sheets, the primary formation mechanism is magnetic reconnection and flux ropes in current sheets are commonly used astracers of the reconnection process.Aims. Whilst flux ropes associated with reconnecting current sheets in the solar wind have been reported, their occurrence, sizedistribution, and lifetime are not well understood.Methods. Here we present and analyse new Solar Orbiter magnetic field data reporting novel observations of a flux rope confined toa bifurcated current sheet in the solar wind. Comparative data and large-scale context is provided by Wind.Results. The Solar Orbiter observations reveal that the flux rope, which does not span the current sheet, is of ion scale, and in areconnection formation scenario, existed for a prolonged period of time as it was carried out in the reconnection exhaust. Wind is alsofound to have observed clear signatures of reconnection at what may be the same current sheet, thus demonstrating that reconnectionsignatures can be found separated by as much as ∼ 2 000 Earth radii, or 0.08 au.Conclusions. The Solar Orbiter observations provide new insight into the hierarchy of scales on which flux ropes can form, and showthat they exist down to the ion scale in the solar wind. The context provided by Wind extends the spatial scale over which reconnectionsignatures have been found at solar wind current sheets. The data suggest the local orientations of the current sheet at Solar Orbiterand Wind are rotated relative to each other, unlike reconnection observed at smaller separations; the implications of this are discussedwith reference to patchy vs. continuous reconnection scenarios.

Journal article

Kieokaew R, Lavraud B, Yang Y, Matthaeus WH, Ruffolo D, Stawarz JE, Aizawa S, Foullon C, GĂ©not V, Pinto RF, Fargette N, Louarn P, Rouillard A, Fedorov A, Penou E, Owen CJ, Horbury T, O'Brien H, Evans V, Angelini Vet al., 2021, Solar Orbiter Observations of the Kelvin-Helmholtz Instability in the Solar Wind, Astronomy and Astrophysics: a European journal, Vol: 656, ISSN: 0004-6361

The Kelvin-Helmholtz instability (KHI) is a nonlinear shear-driveninstability that develops at the interface between shear flows in plasmas. KHIhas been inferred in various astrophysical plasmas and has been observed insitu at the magnetospheric boundaries of solar-system planets and throughremote sensing at the boundaries of coronal mass ejections. While it washypothesized to play an important role in the mixing of plasmas and intriggering solar wind fluctuations, its direct and unambiguous observation inthe solar wind was still lacking. We report in-situ observations of ongoing KHIin the solar wind using Solar Orbiter during its cruise phase. The KHI is foundin a shear layer in the slow solar wind in the close vicinity of theHeliospheric Current Sheet, with properties satisfying linear theory for itsdevelopment. An analysis is performed to derive the local configuration of theKHI. A 2-D MHD simulation is also set up with empirical values to test thestability of the shear layer. In addition, magnetic spectra of the KHI eventare analyzed. We find that the observed conditions satisfy the KHI onsetcriterion from the linear theory analysis, and its development is furtherconfirmed by the simulation. The current sheet geometry analyses are found tobe consistent with KHI development. Additionally, we report observations of anion jet consistent with magnetic reconnection at a compressed current sheetwithin the KHI interval. The KHI is found to excite magnetic and velocityfluctuations with power-law scalings that approximately follow $k^{-5/3}$ and$k^{-2.8}$ in the inertial and dissipation ranges, respectively. Theseobservations provide robust evidence of KHI development in the solar wind. Thissheds new light on the process of shear-driven turbulence as mediated by theKHI with implications for the driving of solar wind fluctuations.

Journal article

Lavraud B, Kieokaew R, Fargette N, Louarn P, Fedorov A, André N, Fruit G, Génot V, Réville V, Rouillard AP, Plotnikov I, Penou E, Barthe A, Prech L, Owen CJ, Bruno R, Allegrini F, Berthomier M, Kataria D, Livi S, Raines JM, D'Amicis R, Eastwood JP, Froment C, Laker R, Maksimovic M, Marcucci F, Perri S, Perrone D, Phan TD, Stansby D, Stawarz J, Redondo ST, Vaivads A, Verscharen D, Zouganelis I, Angelini V, Evans V, Horbury TS, O'brien Het al., 2021, Magnetic reconnection as a mechanism to produce multiple protonpopulations and beams locally in the solar wind, Journal of Astrophysics and Astronomy, Vol: 656, Pages: 1-8, ISSN: 0250-6335

Context. Spacecraft observations early revealed frequent multiple protonpopulations in the solar wind. Decades of research on their origin have focusedon processes such as magnetic reconnection in the low corona and wave-particleinteractions in the corona and locally in the solar wind.Aims.This study aimsto highlight that multiple proton populations and beams are also produced bymagnetic reconnection occurring locally in the solar wind. Methods. We use highresolution Solar Orbiter proton velocity distribution function measurements,complemented by electron and magnetic field data, to analyze the association ofmultiple proton populations and beams with magnetic reconnection during aperiod of slow Alfv\'enic solar wind on 16 July 2020. Results. At least 6reconnecting current sheets with associated multiple proton populations andbeams, including a case of magnetic reconnection at a switchback boundary, arefound during this day. This represents 2% of the measured distributionfunctions. We discuss how this proportion may be underestimated, and how it maydepend on solar wind type and distance from the Sun. Conclusions. Althoughsuggesting a likely small contribution, but which remains to be quantitativelyassessed, Solar Orbiter observations show that magnetic reconnection must beconsidered as one of the mechanisms that produce multiple proton populationsand beams locally in the solar wind.

Journal article

Woolley T, Matteini L, McManus MD, Bercic L, Badman ST, Woodham LD, Horbury TS, Bale SD, Laker R, Stawarz JE, Larson DEet al., 2021, Plasma properties, switchback patches, and low alpha-particle abundance in slow Alfvenic coronal hole wind at 0.13 au, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 508, Pages: 236-244, ISSN: 0035-8711

Journal article

Laker R, Horbury TS, Bale SD, Matteini L, Woolley T, Woodham LD, Stawarz JE, Davies EE, Eastwood JP, Owens MJ, O'Brien H, Evans V, Angelini V, Richter I, Heyner D, Owen CJ, Louarn P, Fedorov Aet al., 2021, Multi-spacecraft study of the solar wind at solar minimum: Dependence on latitude and transient outflows, Astronomy and Astrophysics: a European journal, Vol: 652, Pages: 1-10, ISSN: 0004-6361

Context. The recent launches of Parker Solar Probe, Solar Orbiter (SO), and BepiColombo, along with several older spacecraft, have provided the opportunity to study the solar wind at multiple latitudes and distances from the Sun simultaneously.Aims. We take advantage of this unique spacecraft constellation, along with low solar activity across two solar rotations between May and July 2020, to investigate how the solar wind structure, including the heliospheric current sheet (HCS), varies with latitude.Methods. We visualise the sector structure of the inner heliosphere by ballistically mapping the polarity and solar wind speed from several spacecraft onto the Sun’s source surface. We then assess the HCS morphology and orientation with the in situ data and compare this with a predicted HCS shape.Results. We resolve ripples in the HCS on scales of a few degrees in longitude and latitude, finding that the local orientations of sector boundaries were broadly consistent with the shape of the HCS but were steepened with respect to a modelled HCS at the Sun. We investigate how several CIRs varied with latitude, finding evidence for the compression region affecting slow solar wind outside the latitude extent of the faster stream. We also identified several transient structures associated with HCS crossings and speculate that one such transient may have disrupted the local HCS orientation up to five days after its passage.Conclusions. We have shown that the solar wind structure varies significantly with latitude, with this constellation providing context for solar wind measurements that would not be possible with a single spacecraft. These measurements provide an accurate representation of the solar wind within ±10° latitude, which could be used as a more rigorous constraint on solar wind models and space weather predictions. In the future, this range of latitudes will increase as SO’s orbit becomes more inclined.

Journal article

Masters A, Dunn W, Stallard T, Manners H, Stawarz Jet al., 2021, Magnetic reconnection near the planet as a possible driver of Jupiter's mysterious polar auroras, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-10, ISSN: 2169-9380

Auroral emissions have been extensively observed at the Earth, Jupiter, and Saturn. These planets all have appreciable atmospheres and strong magnetic fields, and their auroras predominantly originate from a region encircling each magnetic pole. However, Jupiter’s auroras poleward of these “main” emissions are brighter and more dynamic, and the drivers responsible for much of these mysterious polar auroras have eluded identification to date. We propose that part of the solution may stem from Jupiter’s stronger magnetic field. We model large-scale Alfvénic perturbations propagating through the polar magnetosphere toward Jupiter, showing that the resulting <0.1° deflections of the magnetic field closest to the planet could trigger magnetic reconnection as near as ∼0.2 Jupiter radii above the cloud tops. At Earth and Saturn this physics should be negligible, but reconnection electric field strengths above Jupiter’s poles can approach ∼1 V m−1, typical of the solar corona. We suggest this near-planet reconnection could generate beams of high-energy electrons capable of explaining some of Jupiter’s polar auroras.

Journal article

Nakamura TKM, Hasegawa H, Genestreti KJ, Denton RE, Phan TD, Stawarz JE, Nakamura R, Nystrom WDet al., 2021, Fast cross‐scale energy transfer during turbulent magnetic reconnection, Geophysical Research Letters, Vol: 48, Pages: 1-8, ISSN: 0094-8276

Magnetic reconnection is a key fundamental process in collisionless plasmas that explosively converts magnetic energy to plasma kinetic and thermal energies through a change of magnetic field topology in a central electron-scale region called the electron diffusion region (EDR). Past simulations and observations demonstrated that this process causes efficient energy conversion through the formation of multiple macro-scale or micro-scale magnetic islands/flux ropes. However, the coupling of these phenomena on different spatiotemporal scales is still poorly understood. Here, based on a new large-scale fully-kinetic simulation with a realistic, initially-fluctuating magnetic field, we demonstrate that macro-scale evolution of turbulent reconnection involving merging of macro-scale islands induces repeated, quick formation of new electron-scale islands within the EDR which soon grow to larger scales. This process causes an efficient cross-scale energy transfer from electron- to larger-scales, and leads to strong electron energization within the growing islands.

Journal article

Verscharen D, Wicks RT, Alexandrova O, Bruno R, Burgess D, Chen CHK, D'Amicis R, De Keyser J, de Wit TD, Franci L, He J, Henri P, Kasahara S, Khotyaintsev Y, Klein KG, Lavraud B, Maruca BA, Maksimovic M, Plaschke F, Poedts S, Reynolds CS, Roberts O, Sahraoui F, Saito S, Salem CS, Saur J, Servidio S, Stawarz JE, Stverak S, Told Det al., 2021, A case for electron-astrophysics, Experimental Astronomy: an international journal on astronomical instrumentation and data analysis, Pages: 1-47, ISSN: 0922-6435

The smallest characteristic scales, at which electron dynamics determines the plasma behaviour, are the next frontier in space and astrophysical plasma research. The analysis of astrophysical processes at these scales lies at the heart of the research theme of electron-astrophysics. Electron scales are the ultimate bottleneck for dissipation of plasma turbulence, which is a fundamental process not understood in the electron-kinetic regime. In addition, plasma electrons often play an important role for the spatial transfer of thermal energy due to the high heat flux associated with their velocity distribution. The regulation of this electron heat flux is likewise not understood. By focussing on these and other fundamental electron processes, the research theme of electron-astrophysics links outstanding science questions of great importance to the fields of space physics, astrophysics, and laboratory plasma physics. In this White Paper, submitted to ESA in response to the Voyage 2050 call, we review a selection of these outstanding questions, discuss their importance, and present a roadmap for answering them through novel space-mission concepts.

Journal article

Woodham L, Horbury T, Matteini L, Woolley T, Laker R, Bale S, Nicolaou G, Stawarz J, Stansby D, Hietala H, Larson D, Livi R, Verniero J, McManus M, Kasper J, Korreck K, Raouafi N, Moncuquet M, Pulupa Met al., 2021, Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere, Astronomy and Astrophysics: a European journal, Vol: 650, Pages: 1-7, ISSN: 0004-6361

Context. Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the helio-sphere. Recent observations by Parker Solar Probe(PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by ‘quieter’ radial fields. Aims. We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona. Methods. We fit 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, Tp,‖, and perpendicular, Tp,⊥, temperature. Results. We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in Tp,‖, while Tp,⊥remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. Conclusions. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced Tp,‖that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.

Journal article

Quijia P, Fraternale F, Stawarz JE, Vasconez CL, Perri S, Marino R, Yordanova E, Sorriso-Valvo Let al., 2021, Comparing turbulence in a Kelvin-Helmholtz instability region across the terrestrial magnetopause (vol 503, pg 4815, 2021), MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 503, Pages: 4828-4828, ISSN: 0035-8711

Journal article

Quijia P, Fraternale F, Stawarz J, VĂĄsconez C, Perri S, Marino R, Yordanova E, Sorriso-Valvo Let al., 2021, Comparing turbulence in a Kelvin-Helmholtz instability region across the terrestrial magnetopause, Monthly Notices of the Royal Astronomical Society, Vol: 503, Pages: 4815-4827, ISSN: 0035-8711

The properties of turbulence observed within the plasma originating from the magnetosheath and the magnetospheric boundary layer, which have been entrained within vortices driven by the Kelvin–Helmholtz Instability (KHI), are compared. The goal of such a study is to determine similarities and differences between the two different regions. In particular, we study spectra, intermittency and the third-order moment scaling, as well as the distribution of a local energy transfer rate proxy. The analysis is performed using the Magnetospheric Multiscale data from a single satellite that crosses longitudinally the KHI. Two sets of regions, one set containing predominantly magnetosheath plasma and the other containing predominantly magnetospheric plasma, are analysed separately, thus allowing us to explore turbulence properties in two portions of very different plasma samples. Results show that the dynamics in the two regions is different, with the boundary layer plasma presenting a shallower spectra and larger energy transfer rate, indicating an early stage of turbulence. In both regions, the effect of the KHI is evidenced.

Journal article

Robertson SL, Eastwood JP, Stawarz JE, Hietala H, Phan TD, Lavraud B, Burch JL, Giles B, Gershman DJ, Torbert R, Lindqvist P, Ergun RE, Russell CT, Strangeway RJet al., 2021, Electron trapping in magnetic mirror structures at the edge of magnetopause flux ropes, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-17, ISSN: 2169-9380

Flux ropes are a proposed site for particle energization during magnetic reconnection, with several mechanisms proposed. Here, Magnetospheric Multiscale mission observations of magnetic mirror structures on the edge of two ion‐scale magnetopause flux ropes are presented. Donut‐shaped features in the electron pitch angle distributions provide evidence for electron trapping in the structures. Furthermore, both events show trapping with extended 3D structure along the body of the flux rope. Potential formation mechanisms, such as the magnetic mirror instability, are examined and the evolutionary states of the structures are compared. Pressure and force analysis suggest that such structures could provide an important electron acceleration mechanism for magnetopause flux ropes, and for magnetic reconnection more generally.

Journal article

Stawarz JE, Matteini L, Parashar TN, Franci L, Eastwood JP, Gonzalez CA, Gingell IL, Burch JL, Ergun RE, Ahmadi N, Giles BL, Gershman DJ, Le Contel O, Lindqvist P, Russell CT, Strangeway RJ, Torbert RBet al., 2021, Comparative analysis of the various generalized Ohm's law terms in magnetosheath turbulence as observed by magnetospheric multiscale, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-14, ISSN: 2169-9380

Decomposing the electric field (E) into the contributions from generalized Ohm's law provides key insight into both nonlinear and dissipative dynamics across the full range of scales within a plasma. Using high‐resolution, multi‐spacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent E spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to E over the accessible length scales, which extend to scales smaller than the electron inertial length at the greatest extent, with the Hall and electron pressure terms dominating at sub‐ion scales. The strength of the non‐ideal electron pressure contribution is stronger than expected from linear kinetic Alfvén waves and a partial anti‐alignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents in the turbulence. The relative contribution of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping E for the intervals examined in this study. Overall, the sum of the Ohm's law terms and measured E agree to within ∼ 20% across the observable scales. These results both confirm general expectations about the behavior of E in turbulent plasmas and highlight features that should be explored further theoretically.

Journal article

Eastwood JP, Goldman M, Phan TD, Stawarz JE, Cassak PA, Drake JF, Newman D, Lavraud B, Shay MA, Ergun RE, Burch JL, Gershman DJ, Giles BL, Lindqvist PA, Torbert RB, Strangeway RJ, Russell CTet al., 2020, Energy flux densities near the electron dissipation region in asymmetric magnetopause reconnection, Physical Review Letters, Vol: 125, Pages: 1-6, ISSN: 0031-9007

Magnetic reconnection is of fundamental importance to plasmas because of its role in releasing and repartitioning stored magnetic energy. Previous results suggest that this energy is predominantly released as ion enthalpy flux along the reconnection outflow. Using Magnetospheric Multiscale data we find the existence of very significant electron energy flux densities in the vicinity of the magnetopause electron dissipation region, orthogonal to the ion energy outflow. These may significantly impact models of electron transport, wave generation, and particle acceleration.

Journal article

Pouquet A, Rosenberg D, Stawarz JE, 2020, Interplay between turbulence and waves: large-scale helical transfer, and small-scale dissipation and mixing in fluid and Hall-MHD turbulence, ATTI Della Accademia Nazionale Dei Lincei Rendiconti Lincei Scienze Fisiche e Naturali, Vol: 31, Pages: 949-961, ISSN: 2037-4631

Novel features of turbulent flows have been analyzed recently, for example: (1) the possibility of an ideal invariant, such as the energy, to be transferred both to the small scales and to the large scales, in each case with a constant flux; (2) the existence of non-Gaussian wings in Probability Distribution Functions of kinetic, magnetic, and temperature fluctuations, together with their gradients, thus displaying large-scale as well as small-scale intermittency; and (3) the linear dependence on the control parameter of the effective dissipation in turbulence when non-linear eddies and waves interact. We shall briefly review these results with examples stemming from Solar Wind data, the atmosphere and the ocean with either magnetic fields, stratification, and/or rotation. In a second part, we shall examine numerically the inverse cascades of magnetic and of generalized helicity for Hall-MHD in the presence of forcing. These helical invariants in the ideal non-dissipative case involve various cross-correlations between the velocity and vorticity, the magnetic field, and the magnetic potential. For an ion inertial length larger than the forcing scale, the effect of the waves is significant. It leads to an exponential attenuation of the inverse cascade to large scales, since, through the velocity and vorticity, small scales play an increasing dynamical role for a strong Hall current.

Journal article

Desai R, Zhang H, Davies E, Stawarz J, Mico-Gomez J, Ivåñez-Ballesteros Pet al., 2020, Three dimensional simulations of solar wind preconditioning and the 23 July 2012 Interplanetary Coronal Mass Ejection, Solar Physics: a journal for solar and solar-stellar research and the study of solar terrestrial physics, Vol: 295, Pages: 1-14, ISSN: 0038-0938

Predicting the large-scale eruptions from the solar corona and theirpropagation through interplanetary space remains an outstanding challenge in solar- and helio-physics research. In this article, we describe three dimensional magnetohydrodynamic simulations of the inner heliosphere leading up to and including the extreme interplanetary coronal mass ejection (ICME) of 23 July 2012, developed using the code PLUTO. The simulations are driven using the output of coronal models for Carrington rotations 2125 and 2126 and, given the uncertainties in the initial conditions, are able to reproduce an event of comparable magnitude to the 23 July ICME, with similar velocity and densityprofi les at 1 au. The launch-time of this event is then varied with regards to an initial 19 July ICME and the effects of solar wind preconditioning are found to be signi ficant for an event of this magnitude and to decrease over a time-window consistent with the ballistic re filling of the depleted heliospheric sector. These results indicate that the 23 July ICME was mostly unaffected by events prior, but would have travelled even faster had it erupted closer in time to the 19 July event where it would have experienced even lower drag forces. We discuss this systematic study of solar wind preconditioning in the context of space weatherforecasting.

Journal article

Ergun RE, Ahmadi N, Kromyda L, Schwartz SJ, Chasapis A, Hoilijoki S, Wilder FD, Stawarz JE, Goodrich KA, Turner DL, Cohen IJ, Bingham ST, Holmes JC, Nakamura R, Pucci F, Torbert RB, Burch JL, Lindqvist P-A, Strangeway RJ, Le Contel O, Giles BLet al., 2020, Observations of Particle Acceleration in Magnetic Reconnection-driven Turbulence, ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X

Journal article

Ergun RE, Ahmadi N, Kromyda L, Schwartz SJ, Chasapis A, Hoilijoki S, Wilder FD, Cassak PA, Stawarz JE, Goodrich KA, Turner DL, Pucci F, Pouquet A, Matthaeus WH, Drake JF, Hesse M, Shay MA, Torbert RB, Burch JLet al., 2020, Particle Acceleration in Strong Turbulence in the Earth's Magnetotail, ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X

Journal article

Franci L, Stawarz JE, Papini E, Hellinger P, Nakamura T, Burgess D, Landi S, Verdini A, Matteini L, Ergun R, Contel OL, Lindqvist P-Aet al., 2020, Modeling MMS observations at the Earth's magnetopause with hybrid simulations of Alfvénic turbulence, The Astrophysical Journal, Vol: 898, ISSN: 0004-637X

Magnetospheric Multiscale (MMS) observations of plasma turbulence generated by a Kelvin–Helmholtz (KH) event at the Earth's magnetopause are compared with a high-resolution two-dimensional (2D) hybrid direct numerical simulation of decaying plasma turbulence driven by large-scale balanced Alfvénic fluctuations. The simulation, set up with four observation-driven physical parameters (ion and electron betas, turbulence strength, and injection scale), exhibits a quantitative agreement on the spectral, intermittency, and cascade-rate properties with in situ observations, despite the different driving mechanisms. Such agreement demonstrates a certain universality of the turbulent cascade from magnetohydrodynamic to sub-ion scales, whose properties are mainly determined by the selected parameters, also indicating that the KH instability-driven turbulence has a quasi-2D nature. The fact that our results are compatible with the validity of the Taylor hypothesis, in the whole range of scales investigated numerically, suggests that the fluctuations at sub-ion scales might have predominantly low frequencies. This would be consistent with a kinetic Alfvén wave-like nature and/or with the presence of quasi-static structures. Finally, the third-order structure function analysis indicates that the cascade rate of the turbulence generated by a KH event at the magnetopause is an order of magnitude larger than in the ambient magnetosheath.

Journal article

AkhavanTafti M, Palmroth M, Slavin JA, Battarbee M, Ganse U, Grandin M, Le G, Gershman DJ, Eastwood JP, Stawarz JEet al., 2020, Comparative analysis of the vlasiator simulations and MMS observations of multiple X‐line reconnection and flux transfer events, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-22, ISSN: 2169-9380

The Vlasiator hybrid‐Vlasov code was developed to investigate global magnetospheric dynamics at ion‐kinetic scales. Here, we focus on the role of magnetic reconnection in the formation and evolution of the magnetic islands at the low‐latitude magnetopause, under southward interplanetary magnetic field (IMF) conditions. The simulation results indicate that: 1) the magnetic reconnection ion kinetics, including the Earthward‐pointing Larmor electric field on the magnetospheric‐side of an X‐point and anisotropic ion distributions, are well‐captured by Vlasiator, thus enabling the study of reconnection‐driven magnetic island evolution processes, 2) magnetic islands evolve due to continuous reconnection at adjacent X‐points, ‘coalescence’ which refers to the merging of neighboring islands to create a larger island, ‘erosion’ during which an island loses magnetic flux due to reconnection, and ‘division’ which involves the splitting of an island into smaller islands, and 3) continuous reconnection at adjacent X‐points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross‐sectional growth rates up to +0.3 RE2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion‐scale flux transfer events (FTEs) sandwiched between two dominant X‐lines. The MMS measurements similarly reveal: 1) anisotropic ion populations, and 2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion‐scale FTEs may grow Earth‐sized within ~10 minutes, which is comparable to the average transport time for FTEs formed in the subsolar region to the high‐latitude magnetopause. Future simulations shall revisit reconnection‐driven island evolution processes with improved spatial resolutions.

Journal article

Adhikari S, Shay MA, Parashar TN, Pyakurel PS, Matthaeus WH, Godzieba D, Stawarz JE, Eastwood JP, Dahlin JTet al., 2020, Reconnection from a turbulence perspective, Physics of Plasmas, Vol: 27, Pages: 1-10, ISSN: 1070-664X

The spectral properties associated with laminar, anti-parallel reconnection are examined using a 2.5D kinetic particle in cell simulation. Both the reconnection rate and the energy spectrum exhibit three distinct phases: an initiation phase where the reconnection rate grows, a quasi-steady phase, and a declining phase where both the reconnection rate and the energy spectrum decrease. During the steady phase, the energy spectrum exhibits approximately a double power law behavior, with a slope near −5/3 at wave numbers smaller than the inverse ion inertial length and a slope steeper than −8/3 for larger wave numbers up to the inverse electron inertial length. This behavior is consistent with a Kolmogorov energy cascade and implies that laminar reconnection may fundamentally be an energy cascade process. Consistent with this idea is the fact that the reconnection rate exhibits a rough correlation with the energy spectrum at wave numbers near the inverse ion inertial length. The 2D spectrum is strongly anisotropic with most energy associated with the wave vector direction normal to the current sheet. Reconnection acts to isotropize the energy spectrum, reducing the Shebalin angle from an initial value of 70° to about 48° (nearly isotropic) by the end of the simulation. The distribution of energy over length scales is further analyzed by dividing the domain into spatial subregions and employing structure functions.

Journal article

Nakamura TKM, Stawarz JE, Hasegawa H, Narita Y, Franci L, Wilder FD, Nakamura R, Nystrom WDet al., 2020, Effects of Fluctuating Magnetic Field on the Growth of the Kelvin-Helmholtz Instability at the Earth's Magnetopause, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380

Journal article

Pouquet A, Stawarz JE, Rosenberg D, 2020, Coupling large eddies and waves in turbulence: Case study of magnetic helicity at the ion inertial scale, Publisher: arXiv

In turbulence, for neutral or conducting fluids, a large ratio of scales isexcited because of the possible occurrence of inverse cascades to large, globalscales together with direct cascades to small, dissipative scales, as observedin the atmosphere and oceans, or in the solar environment. In this context,using direct numerical simulations with forcing, we analyze scale dynamics inthe presence of magnetic fields with a generalized Ohm's law including a Hallcurrent. The ion inertial length epsilon_H serves as the control parameter atfixed Reynolds number. Both the magnetic and generalized helicity -- invariantsin the ideal case -- grow linearly with time, as expected from classicalarguments. The cross-correlation between the velocity and magnetic field growsas well, more so in relative terms for a stronger Hall current. We find thatthe helical growth rates vary exponentially with epsilon_H, provided the ioninertial scale resides within the inverse cascade range. These exponentialvariations are recovered phenomenologically using simple scaling arguments.They are directly linked to the wavenumber power-law dependence of generalizedand magnetic helicity, k^(-2), in their inverse ranges. This illustrates andconfirms the important role of the interplay between large and small scales inthe dynamics of turbulent flows.

Working paper

Gingell I, Schwartz SJ, Eastwood JP, Stawarz JE, Burch JL, Ergun RE, Fuselier SA, Gershman DJ, Giles BL, Khotyaintsev YV, Lavraud B, Lindqvist P, Paterson WR, Phan TD, Russell CT, Strangeway RJ, Torbert RB, Wilder Fet al., 2020, Statistics of reconnecting current sheets in the transition region of earth's bow shock, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-14, ISSN: 2169-9380

We have conducted a comprehensive survey of burst mode observations of Earth's bow shock by the Magnetospheric Multiscale mission to identify and characterize current sheets associated with collisionless shocks, with a focus on those containing fast electron outflows, a likely signature of magnetic reconnection. The survey demonstrates that these thin current sheets are observed within the transition region of approximately 40% of shocks within the burst mode data set of Magnetospheric Multiscale. With only small apparent bias toward quasi‐parallel shock orientations and high Alfvén Mach numbers, the results suggest that reconnection at shocks is a universal process, occurring across all shock orientations and Mach numbers. On examining the distributions of current sheet properties, we find no correlation between distance from the shock, sheet width, or electron jet speed, though the relationship between electron and ion jet speed supports expectations of electron‐only reconnection in the region. Furthermore, we find that robust heating statistics are not separable from background fluctuations, and thus, the primary consequence of reconnection at shocks is in relaxing the topology of the disordered magnetic field in the transition region.

Journal article

Ergun RE, Hoilijoki S, Ahmadi N, Schwartz SJ, Wilder FD, Burch JL, Torbert RB, Lindqvist P-A, Graham DB, Strangeway RJ, Le Contel O, Holmes JC, Stawarz JE, Goodrich KA, Eriksson S, Giles BL, Gershman D, Chen LJet al., 2019, Magnetic Reconnection in Three Dimensions: Observations of Electromagnetic Drift Waves in the Adjacent Current Sheet, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 10104-10118, ISSN: 2169-9380

Journal article

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