Publications

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.

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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

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

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Citations: 13

Journal article

Cohen CMS, Christian ER, Cummings AC, Davis AJ, Desai M, de Nolfo GA, Giacalone J, Hill ME, Joyce CJ, Labrador AW, Leske RA, Matthaeus WH, McComas DJ, McNutt RL, Mewaldt RA, Mitchell DG, Mitchell JG, Rankin JS, Roelof EC, Schwadron NA, Stone EC, Szalay JR, Wiedenbeck ME, Vourlidas A, Bale SD, Pulupa M, MacDowall RJet al., 2021,

Parker Solar Probe observations of He/H abundance variations in SEP events inside 0.5 au

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 19

Journal article

Akhavan-Tafti M, Kasper J, Huang J, Bale Set al., 2021,

Discontinuity analysis of the leading switchback transition regions

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 20

Journal article

Cattell C, Short B, Breneman A, Halekas J, Whittesley P, Larson D, Kasper JC, Stevens M, Case T, Moncuquet M, Bale S, Bonnell J, de Wit TD, Goetz K, Harvey P, MacDowall R, Malaspina D, Maksimovic M, Pulupa M, Goodrich Ket al., 2021,

Narrowband oblique whistler-mode waves: comparing properties observed by Parker Solar Probe at <0.3 AU and STEREO at 1 AU

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 24

Journal article

Finley AJ, McManus MD, Matt SP, Kasper JC, Korreck KE, Case AW, Stevens ML, Whittlesey P, Larson D, Livi R, Bale SD, de Wit TD, Goetz K, Harvey PR, MacDowall RJ, Malaspina DM, Pulupa Met al., 2021,

The contribution of alpha particles to the solar wind angular momentum flux in the inner heliosphere

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 12

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

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Citations: 20

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

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Citations: 12

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.

Journal article

Chen CHK, Chandran BDG, Woodham LD, Jones SI, Perez JC, Bourouaine S, Bowen TA, Klein KG, Moncuquet M, Kasper JC, Bale SDet al., 2021,

The near-Sun streamer belt solar wind: turbulence and solar wind acceleration

, Astronomy & Astrophysics, Vol: 650, Pages: 1-6, ISSN: 0004-6361

The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 R⊙, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP’s fourth solar encounter, which was likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with a spectral index close to –5/3 rather than –3/2), a lower Alfvénicity, and a ‘1∕f’ break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ≈4° from the HCS, suggesting ≈8° as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.

Journal article

Liu M, Issautier K, Meyer-Vernet N, Moncuquet M, Maksimovic M, Halekas JS, Huang J, Griton L, Bale S, Bonnell JW, Case AW, Goetz K, Harvey PR, Kasper JC, MacDowall RJ, Malaspina DM, Pulupa M, Stevens MLet al., 2021,

Solar wind energy flux observations in the inner heliosphere: first results from Parker Solar Probe

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 21

Journal article

Zhao L-L, Zank GP, Hu Q, Telloni D, Chen Y, Adhikari L, Nakanotani M, Kasper JC, Huang J, Bale SD, Korreck KE, Case AW, Stevens M, Bonnell JW, de Wit TD, Goetz K, Harvey PR, MacDowall RJ, Malaspina DM, Pulupa M, Larson DE, Livi R, Whittlesey P, Klein KG, Raouafi NEet al., 2021,

Detection of small magnetic flux ropes from the third and fourth Parker Solar Probe encounters

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 49

Journal article

Larosa A, Krasnoselskikh V, de Wit TD, Agapitov O, Froment C, Jagarlamudi VK, Velli M, Bale SD, Case AW, Goetz K, Harvey P, Kasper JC, Korreck KE, Larson DE, MacDowall RJ, Malaspina D, Pulupa M, Revillet C, Stevens MLet al., 2021,

Switchbacks: statistical properties and deviations from Alfvenicity

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 51

Journal article

Badman ST, Bale SD, Rouillard AP, Bowen TA, Bonnell JW, Goetz K, Harvey PR, MacDowall RJ, Malaspina DM, Pulupa Met al., 2021,

Measurement of the open magnetic flux in the inner heliosphere down to 0.13 AU

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 29

Journal article

Bandyopadhyay R, Matthaeus WH, McComas DJ, Joyce CJ, Szalay JR, Christian ER, Giacalone J, Schwadron NA, Mitchell DG, Hill ME, McNutt RL, Desai M, Bale SD, Bonnell JW, de Wit TD, Goetz K, Harvey PR, MacDowall RJ, Malaspina DM, Pulupa M, Kasper JC, Stevens Met al., 2021,

Energetic particle behavior in near-Sun magnetic field switchbacks from PSP

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 16

Journal article

Joyce CJ, McComas DJ, Schwadron NA, Christian ER, Wiedenbeck ME, McNutt RL, Cohen CMS, Leske RA, Mewaldt RA, Stone EC, Labrador AW, Davis AJ, Cummings AC, Mitchell DG, Hill ME, Roelof EC, Allen RC, Szalay JR, Rankin JS, Desai M, Giacalone J, Matthaeus WH, Bale SD, Kasper JCet al., 2021,

Time evolution of stream interaction region energetic particle spectra in the inner heliosphere

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 16

Journal article

Froment C, Krasnoselskikh V, de Wit TD, Agapitov O, Fargette N, Lavraud B, Larosa A, Kretzschmar M, Jagarlamudi VK, Velli M, Malaspina D, Whittlesey PL, Bale SD, Case AW, Goetz K, Kasper JC, Korreck KE, Larson DE, MacDowall RJ, Mozer FS, Pulupa M, Revillet C, Stevens MLet al., 2021,

Direct evidence for magnetic reconnection at the boundaries of magnetic switchbacks with Parker Solar Probe

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 36

Journal article

Jagarlamudi VK, de Wit TD, Froment C, Krasnoselskikh V, Larosa A, Bercic L, Agapitov O, Halekas JS, Kretzschmar M, Malaspina D, Moncuquet M, Bale SD, Case AW, Kasper JC, Korreck KE, Larson DE, Pulupa M, Stevens ML, Whittlesey Pet al., 2021,

Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 30

Journal article

Shi C, Velli M, Panasenco O, Tenerani A, Reville V, Bale SD, Kasper J, Korreck K, Bonnell JW, de Wit TD, Malaspina DM, Goetz K, Harvey PR, MacDowall RJ, Pulupa M, Case AW, Larson D, Verniero JL, Livi R, Stevens M, Whittlesey P, Maksimovic M, Moncuquet Met al., 2021,

Alfvenic versus non-Alfvenic turbulence in the inner heliosphere as observed by Parker Solar Probe

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 42

Journal article

Halekas JS, Whittlesey PL, Larson DE, McGinnis D, Bale SD, Berthomier M, Case AW, Chandran BDG, Kasper JC, Klein KG, Korreck KE, Livi R, MacDowall RJ, Maksimovic M, Malaspina DM, Matteini L, Pulupa MP, Stevens MLet al., 2021,

Electron heat flux in the near-Sun environment

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 32

Journal article

Allen RC, Ho GC, Jian LK, Vines SK, Bale SD, Case AW, Hill ME, Joyce CJ, Kasper JC, Korreck KE, Malaspina DM, McComas DJ, McNutt R, Mostl C, Odstrcil D, Raouafi N, Schwadron NA, Stevens MLet al., 2021,

A living catalog of stream interaction regions in the Parker Solar Probe era

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 28

Journal article

Drake JF, Agapitov O, Swisdak M, Badman ST, Bale SD, Horbury TS, Kasper JC, MacDowall RJ, Mozer FS, Phan TD, Pulupa M, Szabo A, Velli Met al., 2021,

Are switchbacks signatures of magnetic flux ropes generated by interchange reconnection in the corona?

, Astronomy and Astrophysics: a European journal, Vol: 650, Pages: 1-8, ISSN: 0004-6361

The structure of magnetic flux ropes injected into the solar wind duringreconnection in the coronal atmosphere is explored with particle-in-cellsimulations and compared with {\it in situ} measurements of magnetic"switchbacks" from the Parker Solar Probe. We suggest that multi-x-linereconnection between open and closed flux in the corona will inject flux ropesinto the solar wind and that these flux ropes can convect outward over longdistances before disintegrating. Simulations that explore the magneticstructure of flux ropes in the solar wind reproduce key features of the"switchback" observations: a rapid rotation of the radial magnetic field intothe transverse direction (a consequence of reconnection with a strong guidefield); and the potential to reverse the radial field component. The potentialimplication of the injection of large numbers of flux ropes in the coronalatmosphere for understanding the generation of the solar wind is discussed.

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

Laker R, Horbury TS, Bale SD, Matteini L, Woolley T, Woodham LD, Badman ST, Pulupa M, Kasper JC, Stevens M, Case AW, Korreck KEet al., 2021,

Statistical analysis of orientation, shape, and size of solar wind switchbacks

, Astronomy & Astrophysics, Vol: 650, Pages: 1-7, ISSN: 0004-6361

One of the main discoveries from the first two orbits of Parker Solar Probe(PSP) was the presence of magnetic switchbacks, whose deflections dominated themagnetic field measurements. Determining their shape and size could provideevidence of their origin, which is still unclear. Previous work with a singlesolar wind stream has indicated that these are long, thin structures althoughthe direction of their major axis could not be determined. We investigate ifthis long, thin nature extends to other solar wind streams, while determiningthe direction along which the switchbacks within a stream were aligned. We tryto understand how the size and orientation of the switchbacks, along with theflow velocity and spacecraft trajectory, combine to produce the observedstructure durations for past and future orbits. We searched for the alignmentdirection that produced a combination of a spacecraft cutting direction andswitchback duration that was most consistent with long, thin structures. Theexpected form of a long, thin structure was fitted to the results of the bestalignment direction, which determined the width and aspect ratio of theswitchbacks for that stream. The switchbacks had a mean width of $50,000 \,\rm{km}$, with an aspect ratio of the order of $10$. We find that switchbacksare not aligned along the background flow direction, but instead aligned alongthe local Parker spiral, perhaps suggesting that they propagate along themagnetic field. Since the observed switchback duration depends on how thespacecraft cuts through the structure, the duration alone cannot be used todetermine the size or influence of an individual event. For future PSP orbits,a larger spacecraft transverse component combined with more radially alignedswitchbacks will lead to long duration switchbacks becoming less common.

Journal article

Vech D, Martinovic MM, Klein KG, Malaspina DM, Bowen TA, Verniero JL, Paulson K, de Wit TD, Kasper JC, Huang J, Stevens ML, Case AW, Korreck K, Mozer FS, Goodrich KA, Bale SD, Whittlesey PL, Livi R, Larson DE, Pulupa M, Bonnell J, Harvey P, Goetz K, MacDowall Ret al., 2021,

Wave-particle energy transfer directly observed in an ion cyclotron wave

, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361

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Citations: 19

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