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@article{Laker2021a,
archivePrefix = {arXiv}, arxivId = {2103.0023},
author = {R. Laker, T. S. Horbury, S. D. Bale, L. Matteini, T. Woolley, L. D. Woodham, J. E. Stawarz, E. E. Davies, J. P. Eastwood, M. J. Owens, H. OBrien, V. Evans, V. Angelini, I. Richter, D. Heyner, C. J. Owen, P. Louarn, A. Federov},
eprint = {2103.00230},
journal = {Astronomy & Astrophysics},
title = {{Multi-spacecraft Study of the Solar Wind at Solar Minimum: Dependence on Latitude and Transient Outflows}},
year = {2021}
}

The recent launches of Parker Solar Probe (PSP), 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. 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. We visualise the sector structure of the inner heliosphere by ballistically mapping the polarity and solar wind speed from several spacecraft onto the Suns source surface. We then assess the HCS morphology and orientation with the in situ data and compare with a predicted HCS shape. We resolve ripples in the HCS on scales of a few degrees in longitude and latitude, finding that the local orientation 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. 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 degrees 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 SOs orbit becomes more inclined.

@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{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.