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@article{Laker2021,
archivePrefix = {arXiv},
arxivId = {2010.10211},
author = {Laker, R. and Horbury, T. S. and Bale, S. D. and Matteini, L. and Woolley, T. and Woodham, L. D. and Badman, S. T. and Pulupa, M. and Kasper, J. C. and Stevens, M. and Case, A. W. and Korreck, K. E.},
doi = {10.1051/0004-6361/202039354},
eprint = {2010.10211},
journal = {Astronomy & Astrophysics},
title = {{Statistical analysis of orientation, shape, and size of solar wind switchbacks}},
year = {2021}
}

One of the main discoveries from the first two orbits of Parker Solar Probe (PSP) was the presence of magnetic switchbacks, whose deflections dominated the magnetic field measurements. Determining their shape and size could provide evidence of their origin, which is still unclear. Previous work with a single solar wind stream has indicated that these are long, thin structures although the direction of their major axis could not be determined. We investigate if this long, thin nature extends to other solar wind streams, while determining the direction along which the switchbacks within a stream were aligned. We try to understand how the size and orientation of the switchbacks, along with the flow velocity and spacecraft trajectory, combine to produce the observed structure durations for past and future orbits. We searched for the alignment direction that produced a combination of a spacecraft cutting direction and switchback duration that was most consistent with long, thin structures. The expected form of a long, thin structure was fitted to the results of the best alignment direction, which determined the width and aspect ratio of the switchbacks for that stream. The switchbacks had a mean width of 50,000 km, with an aspect ratio of the order of 10. We find that switchbacks are not aligned along the background flow direction, but instead aligned along the local Parker spiral, perhaps suggesting that they propagate along the magnetic field. Since the observed switchback duration depends on how the spacecraft cuts through the structure, the duration alone cannot be used to determine the size or influence of an individual event. For future PSP orbits, a larger spacecraft transverse component combined with more radially aligned switchbacks will lead to long duration switchbacks becoming less common.

@article{Woodham2021,
author = {Woodham, L and Horbury, T and Matteini, L and Woolley, T and Laker, R and Bale, S and Nicolaou, G and Stawarz, J and Stansby, D and Hietala, H and Larson, D and Livi, R and Verniero, J and McManus, M and Kasper, J and Korreck, K and Raouafi, N and Moncuquet, M and Pulupa, M},
doi = {10.1051/0004-6361/202039415},
journal = {Astronomy & Astrophysics},
title = {{Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere}},
year = {2021} }

Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the heliosphere. 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. 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. We fitted 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,k , and perpendicular, Tp,⊥, temperature. 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,k , while Tp,⊥ remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a largerscale structure identified by enhanced Tp,k that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.

@Woolley2020,
author = {Woolley, Thomas and Matteini, Lorenzo and Horbury, Timothy S and Bale, Stuart D and Woodham, Lloyd D and Laker, Ronan and Alterman, Benjamin L and Bonnell, John W and Case, Anthony W and Kasper, Justin C and Klein, Kristopher G and Martinovic, Mihailo M and Stevens, Michael},
doi = {10.1093/mnras/staa2770},
issn = {0035-8711},
journal = {Monthly Notices of the Royal Astronomical Society},
number = {4},
pages = {5524--5531},
title = {{Proton core behaviour inside magnetic field switchbacks}},
url = {https://doi.org/10.1093/mnras/staa2770},
volume = {498},
year = {2020}
}

During Parker Solar Probes first two orbits, there are widespread observations of rapid magnetic field reversals known as switchbacks. These switchbacks are extensively found in the near-Sun solar wind, appear to occur in patches, and have possible links to various phenomena such as magnetic reconnection near the solar surface. As switchbacks are associated with faster plasma flows, we questioned whether they are hotter than the background plasma and whether the microphysics inside a switchback is different to its surroundings. We have studied the reduced distribution functions from the Solar Probe Cup instrument and considered time periods with markedly large angular deflections to compare parallel temperatures inside and outside switchbacks. We have shown that the reduced distribution functions inside switchbacks are consistent with a rigid velocity space rotation of the background plasma. As such, we conclude that the proton core parallel temperature is very similar inside and outside of switchbacks, implying that a temperature–velocity (T–V) relationship does not hold for the proton core parallel temperature inside magnetic field switchbacks. We further conclude that switchbacks are consistent with Alfvenic pulses travelling along open magnetic field lines. The origin of these pulses, however, remains unknown. We also found that there is no obvious link between radial Poynting flux and kinetic energy enhancements suggesting that the radial Poynting flux is not important for the dynamics of switchbacks.

@article{Horbury2020,
author = {Horbury, Timothy S and Woolley, Thomas and Laker, Ronan and Matteini, Lorenzo and Eastwood, Jonathan and Bale, Stuart D and Velli, Marco and Chandran, Benjamin D G and Phan, Tai and Raouafi, Nour E and Goetz, Keith and Harvey, Peter R and Pulupa, Marc and Klein, K G and de Wit, Thierry Dudok and Kasper, Justin C and Korreck, Kelly E and Case, A W and Stevens, Michael L and Whittlesey, Phyllis and Larson, Davin and MacDowall, Robert J and Malaspina, David M and Livi, Roberto},
doi = {10.3847/1538-4365/ab5b15},
journal = {The Astrophysical Journal Supplement Series},
number = {2},
pages = {45},
publisher = {American Astronomical Society},
title = {{Sharp Alfvenic Impulses in the Near-Sun Solar Wind}},
volume = {246},
year = {2020} }

Measurements of the near-Sun solar wind by the Parker Solar Probe have revealed the presence of large numbers of discrete Alfvenic impulses with an anti-sunward sense of propagation. These are similar to those previously observed near 1 au, in high speed streams over the Suns poles and at 60 solar radii. At 35 solar radii, however, they are typically shorter and sharper than seen elsewhere. In addition, these spikes occur in patches and there are also clear periods within the same stream when they do not occur; the timescale of these patches might be related to the rate at which the spacecraft magnetic footpoint tracks across the coronal hole from which the plasma originated. While the velocity fluctuations associated with these spikes are typically under 100 km s−1, due to the rather low Alfven speeds in the streams observed by the spacecraft to date, these are still associated with large angular deflections of the magnetic field—and these deflections are not isotropic. These deflections do not appear to be related to the recently reported large-scale, pro-rotation solar wind flow. Estimates of the size and shape of the spikes reveal high aspect ratio flow-aligned structures with a transverse scale of 10000 km. These events might be signatures of near-Sun impulsive reconnection events.

@article{Bale2019,
author = {Bale, S D and Badman, S T and Bonnell, J W and Bowen, T A and Burgess, D and Case, A W and Cattell, C A and Chandran, B D G and Chaston, C C and Chen, C H K and Drake, J F and de Wit, T Dudok and Eastwood, J P and Ergun, R E and Farrell, W M and Fong, C and Goetz, K and Goldstein, M and Goodrich, K A and Harvey, P R and Horbury, T S and Howes, G G and Kasper, J C and Kellogg, P J and Klimchuk, J A and Korreck, K E and Krasnoselskikh, V V and Krucker, S and Laker, R and Larson, D E and MacDowall, R J and Maksimovic, M and Malaspina, D M and Martinez-Oliveros, J and McComas, D J and Meyer-Vernet, N and Moncuquet, M and Mozer, F S and Phan, T D and Pulupa, M and Raouafi, N E and Salem, C and Stansby, D and Stevens, M and Szabo, A and Velli, M and Woolley, T and Wygant, J R},
doi = {10.1038/s41586-019-1818-7},
issn = {14764687},
journal = {Nature},
number = {7786},
pages = {237--242},
publisher = {Springer US},
title = {{Highly structured slow solar wind emerging from an equatorial coronal hole}},
volume = {576},
year = {2019} }

During the solar minimum, when the Sun is at its least active, the solar wind is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain; theories and observations suggest that they may originate at the tips of helmet streamers, from interchange reconnection near coronal hole boundaries or within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind.