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  • Journal article
    Papini E, Cicone A, Franci L, Piersanti M, Landi S, Hellinger P, Verdini Aet al., 2021,

    Spacetime Hall-MHD Turbulence at Sub-ion Scales: Structures or Waves?

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 917, ISSN: 2041-8205
  • 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
    Ceppi P, Nowack P, 2021,

    Observational evidence that cloud feedback amplifies global warming

    , Proceedings of the National Academy of Sciences, Vol: 118, ISSN: 0027-8424

    Global warming drives changes in Earth’s cloud cover, which, in turn, may amplify or dampen climate change. This “cloud feedback” is the single most important cause of uncertainty in Equilibrium Climate Sensitivity (ECS)—the equilibrium global warming following a doubling of atmospheric carbon dioxide. Using data from Earth observations and climate model simulations, we here develop a statistical learning analysis of how clouds respond to changes in the environment. We show that global cloud feedback is dominated by the sensitivity of clouds to surface temperature and tropospheric stability. Considering changes in just these two factors, we are able to constrain global cloud feedback to 0.43 ± 0.35 W⋅m<jats:sup>−2</jats:sup>⋅K<jats:sup>−1</jats:sup> (90% confidence), implying a robustly amplifying effect of clouds on global warming and only a 0.5% chance of ECS below 2 K. We thus anticipate that our approach will enable tighter constraints on climate change projections, including its manifold socioeconomic and ecological impacts.
  • 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
    Thomas C, Voulgarakis A, Lim G, Haigh J, Nowack Pet al., 2021,

    An unsupervised learning approach to identifying blocking events:the case of European summer

    , Weather and Climate Dynamics, Vol: 2, ISSN: 2698-4016

    Atmospheric blocking events are mid-latitudeweather patterns, which obstruct the usual path of the polar jet streams. They are often associated with heat wavesin summer and cold snaps in winter. Despite being centralfeatures of mid-latitude synoptic-scale weather, there is nowell-defined historical dataset of blocking events. Variousblocking indices (BIs) have thus been suggested for automatically identifying blocking events in observational and inclimate model data. However, BIs show significant regionaland seasonal differences so that several indices are typicallyapplied in combination to ensure scientific robustness. Here,we introduce a new BI using self-organizing maps (SOMs),an unsupervised machine learning approach, and compare itsdetection skill to some of the most widely applied BIs. Toenable this intercomparison, we first create a new groundtruth time series classification of European blocking basedon expert judgement. We then demonstrate that our method(SOM-BI) has several key advantages over previous BIs because it exploits all of the spatial information provided in theinput data and reduces the dependence on arbitrary thresholds. Using ERA5 reanalysis data (1979–2019), we find thatthe SOM-BI identifies blocking events with a higher precision and recall than other BIs. In particular, SOM-BI alreadyperforms well using only around 20 years of training data sothat observational records are long enough to train our newmethod. We present case studies of the 2003 and 2019 European heat waves and highlight that well-defined groups ofSOM nodes can be an effective tool to diagnose such weatherevents, although the domain-based approach can still lead toerrors in the identification of certain events in a fashion similar to the other BIs. We further test the red blocking detectionskill of SOM-BI depending on the meteorological variableused to study blocking, including geopotential height, sealevel pressure and four variables related to potential vorticity,and t
  • Journal article
    Thomas C, Voulgarakis A, Lim G, Haigh J, Nowack Pet al., 2021,

    An unsupervised learning approach to identifying blocking events: the case of European summer

    , Weather and Climate Dynamics, Vol: 2, Pages: 581-608, ISSN: 2698-4016

    Atmospheric blocking events are mid-latitude weather patterns, which obstruct the usual path of the polar jet streams. They are often associated with heat waves in summer and cold snaps in winter. Despite being central features of mid-latitude synoptic-scale weather, there is no well-defined historical dataset of blocking events. Various blocking indices (BIs) have thus been suggested for automatically identifying blocking events in observational and in climate model data. However, BIs show significant regional and seasonal differences so that several indices are typically applied in combination to ensure scientific robustness. Here, we introduce a new BI using self-organizing maps (SOMs), an unsupervised machine learning approach, and compare its detection skill to some of the most widely applied BIs. To enable this intercomparison, we first create a new ground truth time series classification of European blocking based on expert judgement. We then demonstrate that our method (SOM-BI) has several key advantages over previous BIs because it exploits all of the spatial information provided in the input data and reduces the dependence on arbitrary thresholds. Using ERA5 reanalysis data (1979–2019), we find that the SOM-BI identifies blocking events with a higher precision and recall than other BIs. In particular, SOM-BI already performs well using only around 20 years of training data so that observational records are long enough to train our new method. We present case studies of the 2003 and 2019 European heat waves and highlight that well-defined groups of SOM nodes can be an effective tool to diagnose such weather events, although the domain-based approach can still lead to errors in the identification of certain events in a fashion similar to the other BIs. We further test the red blocking detection skill of SOM-BI depending on the meteorological variable used to study blocking, including geopotential height, sea level pressure and four variables related to
  • Journal article
    Zazzeri G, Xu X, Graven H, 2021,

    Efficient sampling of atmospheric methane for radiocarbon analysis and quantification of fossil methane.

    , Environmental Science and Technology (Washington), Vol: 55, Pages: 8535-8541, ISSN: 0013-936X

    Radiocarbon (14C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of 14C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ14CH4). By observing the changes in Δ14CH4, the fossil fraction of methane emissions can be quantified. Presently, there are very few published Δ14CH4 measurements, mainly because it is challenging to collect and process the large volumes of air needed for radiocarbon measurements. We present a new sampling system that collects enough methane carbon for high precision Δ14CH4 measurements without having to transport large volumes of air. The system catalytically combusts CH4 into CO2 and adsorbs the combustion-derived CO2 onto a molecular sieve trap, after first removing CO2, CO, and H2O. Tests using reference air show a Δ14CH4 measurement repeatability of 5.4‰, similar or better than the precision in the most recent reported measurements. We use the system to produce the first Δ14CH4 measurements in central London and show that day-to-day differences in Δ14CH4 in these samples can be attributed to fossil methane input. The new system could be deployed in a range of settings to investigate CH4 sources.
  • Journal article
    Runov A, Grandin M, Palmroth M, Battarbee M, Ganse U, Hietala H, Hoilijoki S, Kilpua E, Pfau-Kempf Y, Toledo-Redondo S, Turc L, Turner Det al., 2021,

    Ion distribution functions in magnetotail reconnection: global hybrid-Vlasov simulation results

    , ANNALES GEOPHYSICAE, Vol: 39, Pages: 599-612, ISSN: 0992-7689
  • Journal article
    Chen Y-J, Hwang Y-T, Ceppi P, 2021,

    The impacts of cloud-radiative changes on poleward atmospheric and oceanic energy transport in a warmer climate

    , Journal of Climate, Vol: 34, Pages: 7857-7874, ISSN: 0894-8755

    Based on theory and climate model experiments, previous studies suggest most of the uncertainties in projected future changes in meridional energy transport and zonal mean surface temperature can be attributed to cloud feedback. To investigate how radiative and dynamical adjustments modify the influence of cloud-radiative changes on energy transport, this study applies a cloud-locking technique in a fully-coupled climate model, CESM. Under global warming, the impacts of cloud-radiative changes on the meridional energy transport are asymmetric in the two hemispheres. In the Northern Hemisphere, the cloud-radiative changes have little impact on energy transport, because 89% of the cloud-induced heating is balanced locally by increasing outgoing longwave radiation. In the Southern Hemisphere, on the other hand, cloud-induced dynamical changes in the atmosphere and the ocean cause enhanced poleward energy transport, accounting for most of the increase in energy transport under warming. Our experiments highlight that the local longwave radiation adjustment induced by temperature variation can partially offset the impacts of cloud-radiative changes on energy transport, making the estimated impacts smaller than those obtained from directly integrating cloud-radiative changes in previous studies. It is also demonstrated that the cloud-radiative impacts on temperature and energy transport can be significantly modulated by the oceanic circulation, suggesting the necessity of considering atmospheric-oceanic coupling when estimating the impacts of cloud-radiative changes on the climate system.
  • Journal article
    Joyce CJ, McComas DJ, Schwadron NA, Vourlidas A, Christian ER, McNutt RL, Cohen CMS, Leske RA, Mewaldt RA, Stone EC, Mitchell DG, Hill ME, Roelof EC, Allen RC, Szalay JR, Rankin JS, Desai M, Giacalone J, Matthaeus WH, Niehof JT, de Wet W, Winslow RM, Bale SD, Kasper JCet al., 2021,

    Energetic particle evolution during coronal mass ejection passage from 0.3 to 1 AU

    , ASTRONOMY & ASTROPHYSICS, Vol: 651, ISSN: 0004-6361
  • Journal article
    Greaves JS, Richards AMS, Bains W, Rimmer PB, Sagawa H, Clements DL, Seager S, Petkowski JJ, Sousa-Silva C, Ranjan S, Drabek-Maunder E, Fraser HJ, Cartwright A, Mueller-Wodarg I, Zhan Z, Friberg P, Coulson I, Lee E, Hoge Jet al., 2021,

    Phosphine gas in the cloud deck of Venus (vol 5, pg 655, 2021)

    , Nature Astronomy, Vol: 5, Pages: 726-728, ISSN: 2397-3366
  • Journal article
    Halekas JS, Bercic L, Whittlesey P, Larson DE, Livi R, Berthomier M, Kasper JC, Case AW, Stevens ML, Bale SD, MacDowall RJ, Pulupa MPet al., 2021,

    The Sunward Electron Deficit: A Telltale Sign of the Sun's Electric Potential

    , ASTROPHYSICAL JOURNAL, Vol: 916, ISSN: 0004-637X
  • Journal article
    Wang R, Vasko IY, Mozer FS, Bale SD, Kuzichev IV, Artemyev AV, Steinvall K, Ergun R, Giles B, Khotyaintsev Y, Lindqvist P-A, Russell CT, Strangeway Ret al., 2021,

    Electrostatic Solitary Waves in the Earth's Bow Shock: Nature, Properties, Lifetimes, and Origin

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 126, ISSN: 2169-9380
  • Journal article
    Yao Z, Dunn WR, Woodfield EE, Clark G, Mauk BH, Ebert RW, Grodent D, Bonfond B, Pan D, Rae IJ, Ni B, Guo R, Branduardi-Raymont G, Wibisono AD, Rodriguez P, Kotsiaros S, Ness J-U, Allegrini F, Kurth WS, Gladstone GR, Kraft R, Sulaiman AH, Manners H, Desai RT, Bolton SJet al., 2021,

    Revealing the source of Jupiter's x-ray auroral flares

    , SCIENCE ADVANCES, Vol: 7, ISSN: 2375-2548
  • Journal article
    Duan D, He J, Bowen TA, Woodham LD, Wang T, Chen CHK, Mallet A, Bale SDet al., 2021,

    Anisotropy of solar wind turbulence in the inner heliosphere at kinetic scales: PSP observations

    , Letters of the Astrophysical Journal, Vol: 915, Pages: 1-7, ISSN: 2041-8205

    The anisotropy of solar wind turbulence is a critical issue in understanding the physics of energy transfer between scales and energy conversion between fields and particles in the heliosphere. Using the measurement of Parker Solar Probe (PSP), we present an observation of the anisotropy at kinetic scales in the slow, Alfvénic, solar wind in the inner heliosphere. The magnetic compressibility behaves as expected for kinetic Alfvénic turbulence below the ion scale. A steepened transition range is found between the inertial and kinetic ranges in all directions with respect to the local background magnetic field direction. The anisotropy of k⊥ ≫ k∥ is found evident in both transition and kinetic ranges, with the power anisotropy P⊥/P∥ > 10 in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from αt∥ = −5.7 ± 1.0 to αt⊥ = −3.7 ± 0.3 in the transition range and αk∥ = −3.12 ± 0.22 to αk⊥ = −2.57 ± 0.09 in the kinetic range. The corresponding wavevector anisotropy has the scaling of ${k}_{\parallel }\sim {k}_{\perp }^{0.71\pm 0.17}$ in the transition range, and changes to ${k}_{\parallel }\sim {k}_{\perp }^{0.38\pm 0.09}$ in the kinetic range, consistent with the kinetic Alfvénic turbulence at sub-ion scales.
  • Journal article
    Chen L, Ma B, Wu D, Zhao G, Tang J, Bale SDet al., 2021,

    An Interplanetary Type IIIb Radio Burst Observed by Parker Solar Probe and Its Emission Mechanism

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 915, ISSN: 2041-8205
  • Journal article
    Madanian H, Schwartz SJ, Fuselier SA, Burgess D, Turner DL, Chen L-J, Desai MI, Starkey MJet al., 2021,

    Direct Evidence for Magnetic Reflection of Heavy Ions from High Mach Number Collisionless Shocks

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 915, ISSN: 2041-8205
  • Journal article
    Farrell WM, Rasca AP, MacDowall RJ, Gruesbeck JR, Bale SD, Kasper JCet al., 2021,

    Switchback Boundary Dissipation and Relative Age

    , ASTROPHYSICAL JOURNAL, Vol: 915, ISSN: 0004-637X
  • Journal article
    Kuhn- Regnier A, Voulgarakis A, Nowack P, Forkel M, Prentice IC, Harrison Set al., 2021,

    The importance of antecedent vegetation and drought conditions as global drivers of burnt areas

    , Biogeosciences, Vol: 18, Pages: 3861-3879, ISSN: 1726-4170

    The seasonal and longer-term dynamics of fuel accumulation affect fire seasonality and the occurrence of extreme wildfires. Failure to account for their influence may help to explain why state-of-the-art fire models do not simulate the length and timing of the fire season or interannual variability in burnt area well. We investigated the impact of accounting for different timescales of fuel production and accumulation on burnt area using a suite of random forest regression models that included the immediate impact of climate, vegetation, and human influences in a given month and tested the impact of various combinations of antecedent conditions in four productivity-related vegetation indices and in antecedent moisture conditions. Analyses were conducted for the period from 2010 to 2015 inclusive. Inclusion of antecedent vegetation conditions representing fuel build-up led to an improvement of the global, climatological out-of-sample R2 from 0.579 to 0.701, but the inclusion of antecedent vegetation conditions on timescales ≥ 1 year had no impact on simulated burnt area. Current moisture levels were the dominant influence on fuel drying. Additionally, antecedent moisture levels were important for fuel build-up. The models also enabled the visualisation of interactions between variables, such as the importance of antecedent productivity coupled with instantaneous drying. The length of the period which needs to be considered varies across biomes; fuel-limited regions are sensitive to antecedent conditions that determine fuel build-up over longer time periods (∼ 4 months), while moisture-limited regions are more sensitive to current conditions that regulate fuel drying.
  • Journal article
    Kuhn-Régnier A, Voulgarakis A, Nowack P, Forkel M, Prentice IC, Harrison SPet al., 2021,

    Quantifying the Importance of antecedent fuel-related vegetationproperties for burnt area using random forests

    , Biogeosciences, Vol: 8, ISSN: 1726-4170

    The seasonal and longer-term dynamics of fuel accumulation affect fire seasonality and the occurrence of extreme wildfires. Failure to account for their influence mayhelp to explain why state-of-the-art fire models do not simulate the length and timing of the fire season or interannual variability in burnt area well. We investigated the impact of accounting for different timescales of fuel production and accumulation on burnt area using a suite of random forest regression models that included the immediateimpact of climate, vegetation, and human influences in agiven month and tested the impact of various combinationsof antecedent conditions in four productivity-related vegetation indices and in antecedent moisture conditions. Analyses were conducted for the period from 2010 to 2015 inclusive. Inclusion of antecedent vegetation conditions representing fuel build-up led to an improvement of the global,climatological out-of-sample R2from 0.579 to 0.701, but theinclusion of antecedent vegetation conditions on timescales≥ 1 year had no impact on simulated burnt area. Currentmoisture levels were the dominant influence on fuel drying. Additionally, antecedent moisture levels were importantfor fuel build-up. The models also enabled the visualisationof interactions between variables, such as the importanceof antecedent productivity coupled with instantaneous drying. The length of the period which needs to be consideredvaries across biomes; fuel-limited regions are sensitive to antecedent conditions that determine fuel build-up over longertime periods (∼ 4 months), while moisture-limited regionsare more sensitive to current conditions that regulate fuel drying.
  • Journal article
    Hall RJ, Mitchell DM, Seviour WJM, Wright CJet al., 2021,

    Persistent Model Biases in the CMIP6 Representation of Stratospheric Polar Vortex Variability

    , JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 126, ISSN: 2169-897X
  • Journal article
    Pal S, Kilpua E, Good S, Pomoell J, Price DJet al., 2021,

    Uncovering erosion effects on magnetic flux rope twist

    , ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361
  • Journal article
    Ke Q, Yin J, Bricker JD, Savage N, Buonomo E, Ye Q, Visser P, Dong G, Wang S, Tian Z, Sun L, Toumi R, Jonkman SNet al., 2021,

    An integrated framework of coastal flood modelling under the failures of sea dikes: a case study in Shanghai

    , NATURAL HAZARDS, Vol: 109, Pages: 671-703, ISSN: 0921-030X
  • Journal article
    Wang S, Toumi R, Ye Q, Ke Q, Bricker J, Tian Z, Sun Let al., 2021,

    Is the tropical cyclone surge in Shanghai more sensitive to landfall location or intensity change?

    , ATMOSPHERIC SCIENCE LETTERS, Vol: 22, ISSN: 1530-261X
  • Journal article
    Malaspina DM, Wilson LB, Ergun RE, Bale SD, Bonnell JW, Goodrich K, Goetz K, Harvey PR, MacDowall RJ, Pulupa M, Halekas J, Case A, Kasper JC, Larson D, Stevens M, Whittlesey Pet al., 2021,

    Electron Bernstein waves and narrowband plasma waves near the electron cyclotron frequency in the near-Sun solar wind

    , ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361
  • Journal article
    Harra L, Brooks DH, Bale SD, Mandrini CH, Barczynski K, Sharma R, Badman ST, Dominguez SV, Pulupa Met al., 2021,

    The active region source of a type III radio storm observed by Parker Solar Probe during encounter 2

    , ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361
  • Journal article
    Schwadron NA, Joyce CJ, Aly A, Cohen CMS, Desai M, McComas DJ, Niehof JT, Mobius E, Lee M, Bower J, Bale S, Case A, Christian ER, Davis AJ, de Wet W, Goetz K, Giacalone J, Hill ME, Allen R, Kasper JC, Korreck K, Leske RA, Malandraki O, Matthaeus WH, McNutt RL, Mewaldt RA, Mitchell DG, Pulupa M, Rankin JS, Roelof EC, Stone EC, Szalay JR, Wiedenbeck MEet al., 2021,

    A new view of energetic particles from stream interaction regions observed by Parker Solar Probe

    , ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361
  • Journal article
    Quaas J, Gryspeerdt E, Vautard R, Boucher Oet al., 2021,

    Climate impact of aircraft-induced cirrus assessed from satellite observations before and during COVID-19

    , Environmental Research Letters, Vol: 16, Pages: 1-6, ISSN: 1748-9326

    Aircraft produce condensation trails, which are thought to increase high-level cloudiness under certain conditions. Howeverthe magnitude of such an effect and whether this contributes substantially to the radiative forcing due to the aviation sectorremain uncertain. The very substantial, near-global reduction in air traffic in response to the COVID-19 outbreak offers anunprecedented opportunity to identify the anthropogenic contribution to the observed cirrus coverage and thickness. Here weshow, using an analysis of satellite observations for the period March-May 2020, that in the 20% of the Northern Hemispheremid-latitudes with the largest air traffic reduction, cirrus fraction was reduced by ~9 ± 1.5% on average, and cirrus emissivitywas reduced by ~2 ±5% relative to what they should have been with normal air traffic. The changes are corroborated by aconsistent estimate based on linear trends over the period 2011 – 2019. The change in cirrus translates to a global radiativeforcing of 61 ±39 mWm-2. This estimate is somewhat smaller than previous assessments.
  • Journal article
    Phan TD, Lavraud B S J, Halekas, Øieroset M, Drake JF, Eastwood JP, Shay MA, Bale SD, Larson D, Livi R, Whittlesey PL, Rahmati A, Pulupa M, McManus MD, Verniero JL, Bonnell JW, Stevens M, Case AWet al., 2021,

    Prevalence of magnetic reconnection in the near-Sun heliospheric current sheet

    , Astronomy & Astrophysics, Vol: 650, Pages: 1-14, ISSN: 0004-6361

    During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale Heliospheric Current Sheet (HCS)multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the commondetections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5-107 solar radii during Encounters1, 4 and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts,PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic fieldlines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent withthe reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests thatreconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossingsof the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solarradii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is muchthicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absenceof reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which areassociated with ‘switchbacks’. Thus, the PSP findings suggest that large-scale dynamics either locally in the solar wind or within thecoronal source of the HCS (at the tip of helmet streamers) plays a critical role in triggering reconnection onset.
  • Journal article
    Fargette N, Lavraud B, Rouillard A, Eastwood JP, Bale SD, Phan T, Øieroset M, Halekas JS, Kasper J, Berthomier M, Case AW, Korreck KE, Larson DE, Louarn P, Malaspina D, Pulupa M, Stevens ML, Whittlesey PLet al., 2021,

    Magnetic increases with central current sheets: Observations with Parker Solar Probe

    , Astronomy & Astrophysics, Vol: 650, Pages: 1-12, ISSN: 0004-6361

    Aims. We report the observation by Parker Solar Probe (PSP) of magnetic structures in the solar wind that present a strong peak intheir magnetic field magnitude with an embedded central current sheet. Similar structures have been observed, either at the Earth’smagnetopause and called interlinked flux tubes, or in the solar wind and called interplanetary field enhancements.Methods. In this work, we first investigate two striking events in detail; one occurred in the regular slow solar wind on November 2,2018 and the other was observed during a heliospheric current sheet crossing on November 13, 2018. They both show the presenceof a central current sheet with a visible ion jet and general characteristics consistent with the occurrence of magnetic reconnection.We then performed a survey of PSP data from encounters 1 to 4 and find 18 additional events presenting an increase in the magneticfield magnitude of over 30% and a central current sheet. We performed a statistical study on the 20 "magnetic increases with centralcurrent sheet" (MICCS), with 13 observed in the regular slow solar wind with a constant polarity (i.e., identical strahl direction), and7 which were specifically observed near a heliospheric current sheet (HCS) crossing.Results. We analyze and discuss the general properties of the structures, including the duration, location, amplitude, and magnetictopology, as well as the characteristics of their central current sheet. We find that the latter has a preferential orientation in the TNplane of the RTN frame. We also find no significant change in the dust impact rate in the vicinity of the MICCS under study, leadingus to conclude that dust probably plays no role in the MICCS formation and evolution. Our findings are overall consistent with adouble flux tube-configuration that would result from initially distinct flux tubes which interact during solar wind propagation.

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

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