Publications
276 results found
Zouganelis I, 2020, The Solar Orbiter Science Activity Plan: translating solar and heliospheric physics questions into action, Astronomy & Astrophysics, Vol: 642, Pages: 1-19, ISSN: 0004-6361
Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission’s science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit’s science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, w
Müller D, Cyr OCS, Zouganelis I, et al., 2020, The solar orbiter mission. science overview, Astronomy & Astrophysics, Vol: 642, Pages: 1-31, ISSN: 0004-6361
Aims. Solar Orbiter, the first mission of ESA’s Cosmic Vision 2015–2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.Methods. The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.Results. Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission’s science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.
Owen CJ, Bruno R, Livi S, et al., 2020, The Solar Orbiter Solar Wind Analyser (SWA) suite, ASTRONOMY & ASTROPHYSICS, Vol: 642, ISSN: 0004-6361
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- Citations: 113
Walsh AP, Horbury TS, Maksimovic M, et al., 2020, Coordination of the in situ payload of Solar Orbiter, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-7, ISSN: 0004-6361
Solar Orbiter’s in situ coordination working group met frequently during the development of the mission with the goal of ensuring that its in situ payload has the necessary level of coordination to maximise science return. Here we present the results of that work, namely how the design of each of the in situ instruments (EPD, MAG, RPW, SWA) was guided by the need for coordination, the importance of time synchronisation, and how science operations will be conducted in a coordinated way. We discuss the mechanisms by which instrument sampling schemes are aligned such that complementary measurements will be made simultaneously by different instruments, and how burst modes are scheduled to allow a maximum overlap of burst intervals between the four instruments (telemetry constraints mean different instruments can spend different amounts of time in burst mode). We also explain how onboard autonomy, inter-instrument communication, and selective data downlink will be used to maximise the number of transient events that will be studied using high-resolution modes of all the instruments. Finally, we briefly address coordination between Solar Orbiter’s in situ payload and other missions.
Rouillard AP, Pinto RF, Vourlidas A, et al., 2020, Models and data analysis tools for the Solar Orbiter mission, ASTRONOMY & ASTROPHYSICS, Vol: 642, ISSN: 0004-6361
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- Citations: 42
Velli M, Harra LK, Vourlidas A, et al., 2020, Understanding the origins of the heliosphere: integrating observations and measurements from Parker Solar Probe, Solar Orbiter, and other space- and ground-based observatories, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-13, ISSN: 0004-6361
Context. The launch of Parker Solar Probe (PSP) in 2018, followed by Solar Orbiter (SO) in February 2020, has opened a new window in the exploration of solar magnetic activity and the origin of the heliosphere. These missions, together with other space observatories dedicated to solar observations, such as the Solar Dynamics Observatory, Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations from WIND and ACE, and ground based multi-wavelength observations including the DKIST observatory that has just seen first light, promise to revolutionize our understanding of the solar atmosphere and of solar activity, from the generation and emergence of the Sun’s magnetic field to the creation of the solar wind and the acceleration of solar energetic particles.Aims. Here we describe the scientific objectives of the PSP and SO missions, and highlight the potential for discovery arising from synergistic observations. Here we put particular emphasis on how the combined remote sensing and in situ observations of SO, that bracket the outer coronal and inner heliospheric observations by PSP, may provide a reconstruction of the solar wind and magnetic field expansion from the Sun out to beyond the orbit of Mercury in the first phases of the mission. In the later, out-of-ecliptic portions of the SO mission, the solar surface magnetic field measurements from SO and the multi-point white-light observations from both PSP and SO will shed light on the dynamic, intermittent solar wind escaping from helmet streamers, pseudo-streamers, and the confined coronal plasma, and on solar energetic particle transport.Methods. Joint measurements during PSP–SO alignments, and magnetic connections along the same flux tube complemented by alignments with Earth, dual PSP–Earth, and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will allow a better understanding of the in situ evolution of solar-wind plasma flows and the full three-dimensional distribution of
Rodriguez-Pacheco J, Wimmer-Schweingruber RF, Mason GM, et al., 2020, The energetic particle detector: energetic particle instrument suite for the Solar Orbiter mission, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-35, ISSN: 0004-6361
After decades of observations of solar energetic particles from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼30° heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere.
Maksimovic M, Bale SD, Chust T, et al., 2020, The Solar Orbiter Radio and Plasma Waves (RPW) instrument, ASTRONOMY & ASTROPHYSICS, Vol: 642, ISSN: 0004-6361
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- Citations: 64
Phan TD, Bale SD, Eastwood JP, et al., 2020, Parker solar probe In situ observations of magnetic reconnection exhausts during encounter 1, The Astrophysical Journal Supplement, Vol: 246, Pages: 34-34, ISSN: 0067-0049
Magnetic reconnection in current sheets converts magnetic energy into particle energy. The process may play an important role in the acceleration and heating of the solar wind close to the Sun. Observations from Parker Solar Probe (PSP) provide a new opportunity to study this problem, as it measures the solar wind at unprecedented close distances to the Sun. During the first orbit, PSP encountered a large number of current sheets in the solar wind through perihelion at 35.7 solar radii. We performed a comprehensive survey of these current sheets and found evidence for 21 reconnection exhausts. These exhausts were observed in heliospheric current sheets, coronal mass ejections, and regular solar wind. However, we find that the majority of current sheets encountered around perihelion, where the magnetic field was strongest and plasma β was lowest, were Alfvénic structures associated with bursty radial jets, and these current sheets did not appear to be undergoing local reconnection. We examined conditions around current sheets to address why some current sheets reconnected while others did not. A key difference appears to be the degree of plasma velocity shear across the current sheets: the median velocity shear for the 21 reconnection exhausts was 24% of the Alfvén velocity shear, whereas the median shear across 43 Alfvénic current sheets examined was 71% of the Alfvén velocity shear. This finding could suggest that large, albeit sub-Alfvénic, velocity shears suppress reconnection. An alternative interpretation is that the Alfvénic current sheets are isolated rotational discontinuities that do not undergo local reconnection.
Horbury T, Woolley T, Laker R, et al., 2020, Sharp Alfvenic impulses in the near-Sun solar wind, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 246, Pages: 1-8, ISSN: 0004-637X
Measurements of the near-Sun solar wind by Parker Solar Probe have revealed the presence of largenumbers of discrete Alfv ́enic impulses with an anti-Sunward sense of propagation. These are similarto those previously observed near 1 AU, in high speed streams over the Sun’s poles and at 60 solarradii. At 35 solar radii, however, they are typically shorter and sharper than seen elsewhere. Inaddition, these spikes occur in “patches” and there are also clear periods within the same stream whenthey do not occur; the timescale of these patches might be related to the rate at which the spacecraftmagnetic footpoint tracks across the coronal hole from which the plasma originated. While the velocityfluctuations associated with these spikes are typically under 100 km/s, due to the rather low Alfv ́enspeeds in the streams observed by the spacecraft to date, these are still associated with large angulardeflections of the magnetic field - and these deflections are not isotropic. These deflections do notappear to be related to the recently reported large scale, pro-rotation solar wind flow. Estimates ofthe size and shape of the spikes reveal high aspect ratio flow-aligned structures with a transverse scaleof≈104km. These events might be signatures of near-Sun impulsive reconnection events.
Stansby D, Matteini L, Horbury TS, et al., 2020, The origin of slow Alfvenic solar wind at solar minimum, Monthly Notices of the Royal Astronomical Society, Vol: 492, Pages: 39-44, ISSN: 0035-8711
Although the origins of slow solar wind are unclear, there is increasing evidence that at least some of it is released in a steady state on overexpanded coronal hole magnetic field lines. This type of slow wind has similar properties to the fast solar wind, including strongly Alfvénic fluctuations. In this study, a combination of proton, alpha particle, and electron measurements are used to investigate the kinetic properties of a single interval of slow Alfvénic wind at 0.35 au. It is shown that this slow Alfvénic interval is characterized by high alpha particle abundances, pronounced alpha–proton differential streaming, strong proton beams, and large alpha-to-proton temperature ratios. These are all features observed consistently in the fast solar wind, adding evidence that at least some Alfvénic slow solar wind also originates in coronal holes. Observed differences between speed, mass flux, and electron temperature between slow Alfvénic and fast winds are explained by differing magnetic field geometry in the lower corona.
Perrone D, D'Amicis R, De Marco R, et al., 2020, Highly Alfvenic slow solar wind at 0.3 au during a solar minimum: Helios insights for Parker Solar Probe and Solar Orbiter, Astronomy and Astrophysics: a European journal, Vol: 633, Pages: 1-7, ISSN: 0004-6361
Alfvénic fluctuations in solar wind are an intrinsic property of fast streams, while slow intervals typically have a very low degree of Alfvénicity, with much more variable parameters. However, sometimes a slow wind can be highly Alfvénic. Here we compare three different regimes of solar wind, in terms of Alfvénic content and spectral properties, during a minimum phase of the solar activity and at 0.3 au. We show that fast and Alfvénic slow intervals share some common characteristics. This would suggest a similar solar origin, with the latter coming from over-expanded magnetic field lines, in agreement with observations at 1 au and at the maximum of the solar cycle. Due to the Alfvénic nature of the fluctuations in both fast and Alfvénic slow winds, we observe a well-defined correlation between the flow speed and the angle between magnetic field vector and radial direction. The high level of Alfvénicity is also responsible of intermittent enhancements (i.e. spikes), in plasma speed. Moreover, only for the Alfvénic intervals do we observe a break between the inertial range and large scales, on about the timescale typical of the Alfvénic fluctuations and where the magnetic fluctuations saturate, limited by the magnitude of the local magnetic field. In agreement with this, we recover a characteristic low-frequency 1/f scaling, as expected for fluctuations that are scale-independent. This work is directly relevant for the next solar missions, Parker Solar Probe and Solar Orbiter. One of the goals of these two missions is to study the origin and evolution of slow solar wind. In particular, Parker Solar Probe will give information about the Alfvénic slow wind in the unexplored region much closer to the Sun and Solar Orbiter will allow us to connect the observed physics to the source of the plasma.
Klein KG, Martinovic M, Stansby D, et al., 2019, Linear Stability in the Inner Heliosphere: <i>Helios</i> Re-evaluated, ASTROPHYSICAL JOURNAL, Vol: 887, ISSN: 0004-637X
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- Citations: 15
Kasper JC, Bale SD, Belcher JW, et al., 2019, Alfvenic velocity spikes and rotational flows in the near-Sun solar wind, Nature, Vol: 576, Pages: 228-233, ISSN: 0028-0836
The prediction of a supersonic solar wind1 was first confirmed by spacecraft near Earth2,3 and later by spacecraft at heliocentric distances as small as 62 solar radii4. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy5,6,7. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams8. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii9,10,11, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second—considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age12,13,14.
Bale SD, Badman ST, Bonnell JW, et al., 2019, Highly structured slow solar wind emerging from an equatorial coronal hole, Nature, Vol: 576, Pages: 237-242, ISSN: 0028-0836
During the solar minimum, when the Sun is at its least active, the solar wind1,2 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 wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near coronal hole boundaries7,8, or within coronal holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. 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 Probe15 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-instabilities10,16 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.
Matthaeus WH, Bandyopadhyay R, Brown MR, et al., 2019, [Plasma 2020 Decadal] The essential role of multi-point measurements in turbulence investigations: the solar wind beyond single scale and beyond the Taylor Hypothesis, Publisher: Arxiv
This paper briefly reviews a number of fundamental measurements that need tobe made in order to characterize turbulence in space plasmas such as the solarwind. It has long been known that many of these quantities require simultaneousmultipoint measurements to attain a proper characterization that would revealthe fundamental physics of plasma turbulence. The solar wind is an ideal plasmafor such an investigation, and it now appears to be technologically feasible tocarry out such an investigation, following the pioneering Cluster and MMSmissions. Quantities that need to be measured using multipoint measurementsinclude the two-point, two-time second correlation function of velocity,magnetic field and density, and higher order statistical objects such as thirdand fourth order structure functions. Some details of these requirements aregiven here, with a eye towards achieving closure on fundamental questionsregarding the cascade rate, spectral anisotropy, characteristic coherentstructures, intermittency, and dissipation mechanisms that describe plasmaturbuelence, as well as its variability with plasma parameters in the solarwind. The motivation for this discussion is the current planning for a proposedHelioswarm mission that would be designed to make these measurements,leading tobreakthrough understanding of the physics of space and astrophysicalturbulence.
Perrone D, Stansby D, Horbury TS, et al., 2019, Thermodynamics of pure fast solar wind: radial evolution of the temperature-speed relationship in the inner heliosphere, Monthly Notices of the Royal Astronomical Society, Vol: 488, Pages: 2380-2386, ISSN: 0035-8711
A strong correlation between speed and proton temperature has been observed, across many years, on hourly averaged measurements in the solar wind. Here, we show that this relationship is also observed at a smaller scale on intervals of a few days, within a single stream. Following the radial evolution of a well-defined stream of coronal-hole plasma, we show that the temperature–speed (T–V) relationship evolves with distance, implying that the T–V relationship at 1 au cannot be used as a proxy for that near the Sun. We suggest that this behaviour could be a combination of the anticorrelation between speed and flux-tube expansion factor near the Sun and the effect of a continuous heating experienced by the plasma during the expansion. We also show that the cooling index for the radial evolution of the temperature is a function of the speed. In particular, T⊥ in faster wind, although higher close to the Sun, decreases more quickly with respect to slower wind, suggesting that it has less time to interact with the mechanism(s) able to heat the plasma. Finally, we predict the expected T–V relationship in fast streams closer to the Sun with respect to the Helios observations, which Parker Solar Probe will explore in the near future.
Matteini L, Stansby D, Horbury TS, et al., 2019, The rotation angle distribution underlying magnetic field fluctuations in the 1/f range of solar wind turbulent spectra, Il Nuovo Cimento C – Colloquia and Communications in Physics, Vol: 42, ISSN: 2037-4909
We discuss properties of large amplitude magnetic field fluctuationsduring fast Alfv ́eenic solar wind streams, focussing on the statistics of the rotationangle between consecutive magnetic field vector measurements for different scalesin the plasma. Since in the fast solar wind fluctuations preserve the modulus ofthe magnetic field to a good approximation, the tip of the magnetic field vector isobserved to move on a sphere of approximately constant radius|B|.Wethencom-pare statistics of solar wind measurements with that of a simple model of a randomwalk bounded on a spherical surface. The analogy consists in the fact that in bothsystems the geometrical constraint imposes a limiting amplitude at large separa-tions and thus introduces a break scale in the power spectrum of the fluctuations,leading to a shallower slope for scales where the fluctuations amplitude becomesscale-independent. However, while in the case of the random walk the saturationof the fluctuations occurs when the pattern becomes uniform on the sphere (flatdistribution of the cosine of the rotation angle), transitioning then to a white noiseregime, in the solar wind magnetic field fluctuations saturate in amplitude maintain-ing a preferential direction. We suggest that this behaviour is due to the presence ofthe background interplanetary magnetic field, which keeps some long-range memoryin the system also when the fluctuations becomes independent of the scale. Thislong-range correlation is a necessary ingredient in order to produce the 1/fspectrumobserved at large scales in the solar wind.
Klein KG, Alexandrova O, Bookbinder J, et al., 2019, Multipoint measurements of the solar wind: A proposed advance for studying magnetized turbulence
A multi-institutional, multi-national science team will soon submit a NASAproposal to build a constellation of spacecraft to fly into the near-Earthsolar wind in a swarm spanning a multitude of scales in order to obtaincritically needed measurements that will reveal the underlying dynamics ofmagnetized turbulence. This white paper, submitted to the Plasma 2020 DecadalSurvey Committee, provides a brief overview of turbulent systems thatconstitute an area of compelling plasma physics research, including why thismission is needed, and how this mission will achieve the goal of revealing howenergy is transferred across scales and boundaries in plasmas throughout theuniverse.
TenBarge JM, Alexandrova O, Boldyrev S, et al., 2019, Disentangling the spatiotemporal structure ofturbulence using multi-spacecraft data
This white paper submitted for 2020 Decadal Assessment of Plasma Scienceconcerns the importance of multi-spacecraft missions to address fundamentalquestions concerning plasma turbulence. Plasma turbulence is ubiquitous in theuniverse, and it is responsible for the transport of mass, momentum, and energyin such diverse systems as the solar corona and wind, accretion discs, planetformation, and laboratory fusion devices. Turbulence is an inherentlymulti-scale and multi-process phenomenon, coupling the largest scales of asystem to sub-electron scales via a cascade of energy, while simultaneouslygenerating reconnecting current layers, shocks, and a myriad of instabilitiesand waves. The solar wind is humankind's best resource for studying thenaturally occurring turbulent plasmas that permeate the universe. Sincelaunching our first major scientific spacecraft mission, Explorer 1, in 1958,we have made significant progress characterizing solar wind turbulence. Yet,due to the severe limitations imposed by single point measurements, we areunable to characterize sufficiently the spatial and temporal properties of thesolar wind, leaving many fundamental questions about plasma turbulenceunanswered. Therefore, the time has now come wherein making significantadditional progress to determine the dynamical nature of solar wind turbulencerequires multi-spacecraft missions spanning a wide range of scalessimultaneously. A dedicated multi-spacecraft mission concurrently covering awide range of scales in the solar wind would not only allow us to directlydetermine the spatial and temporal structure of plasma turbulence, but it wouldalso mitigate the limitations that current multi-spacecraft missions face, suchas non-ideal orbits for observing solar wind turbulence. Some of thefundamentally important questions that can only be addressed by in situmultipoint measurements are discussed.
Stansby D, Horbury TS, Wallace S, et al., 2019, Predicting Large-scale Coronal Structure for Parker Solar Probe Using Open Source Software, Research Notes of the AAS, Vol: 3, Pages: 57-57
Perrone D, Stansby D, Horbury T, et al., 2019, Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations, Monthly Notices of the Royal Astronomical Society, Vol: 483, Pages: 3730-3737, ISSN: 0035-8711
Spacecraft observations have shown that the proton temperature in the solar wind falls off with radial distance more slowly than expected for an adiabatic prediction. Usually, previous studies have been focused on the evolution of the solar-wind plasma by using the bulk speed as an order parameter to discriminate different regimes. In contrast, here, we study the radial evolution of pure and homogeneous fast streams (i.e. well-defined streams of coronal-hole plasma that maintain their identity during several solar rotations) by means of re-processed particle data, from the HELIOS satellites between 0.3 and 1 au. We have identified 16 intervals of unperturbed high-speed coronal-hole plasma, from three different sources and measured at different radial distances. The observations show that, for all three streams, (i) the proton density decreases as expected for a radially expanding plasma, unlike previous analysis that found a slower decrease; (ii) the magnetic field deviates from the Parker prediction, with the radial component decreasing more slowly and the tangential more quickly than expected; (iii) the double-adiabatic invariants are violated and an increase of entropy is observed; (iv) the collisional frequency is not constant, but decreases as the plasma travels away from the Sun. This work provides an insight into the heating problem in pure fast solar wind, fitting in the context of the next solar missions, and, especially for Parker Solar Probe, it enables us to predict the high-speed solar-wind environment much closer to the Sun.
Stansby D, Perrone D, Matteini L, et al., 2019, Alpha particle thermodynamics in the inner heliosphere fast solar wind, Astronomy and Astrophysics, Vol: 623, ISSN: 0004-6361
Context. Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties ofdifferent ion species. However, most in-situ observations of positive ions in the solar wind are taken at 1 AU, where information ontheir solar source properties may have been irreversibly erased.Aims. In this study we aimed to use the properties of alpha particles at heliocentric distances between 0.3 and 1 AU to study plasmaprocesses occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previousremote sensing observations of the plasma closer to the Sun.Methods. We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computedthe alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radialvariation of alpha particle temperatures in fast solar wind observed between 0.3 and 1 AU.Results. Between 0.3 and 1 AU alpha particles are heated in the magnetic field perpendicular direction, and cooled in the magneticfield parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possiblemechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that thealpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AUmay be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.
Stansby D, Horbury T, Matteini L, 2019, Diagnosing solar wind origins using in situ measurements in the inner heliosphere, Monthly Notices of the Royal Astronomical Society, Vol: 482, Pages: 1706-1714, ISSN: 0035-8711
Robustly identifying the solar sources of individual packets of solar wind measured in interplanetary space remains an open problem. We set out to see if this problem is easier to tackle using solar wind measurements closer to the Sun than 1 au, where the mixing and dynamical interaction of different solar wind streams is reduced. Using measurements from the Helios mission, we examined how the proton core temperature anisotropy and cross-helicity varied with distance. At 0.3 au there are two clearly separated anisotropic and isotropic populations of solar wind that are not distinguishable at 1 au. The anisotropic population is always Alfvénic and spans a wide range of speeds. In contrast the isotropic population has slow speeds, and contains a mix of Alfvénic wind with constant mass fluxes and non-Alfvénic wind with large and highly varying mass fluxes. We split the in situ measurements into three categories according these observations, and suggest that these categories correspond to wind that originated in the core of coronal holes, in or near active regions or the edges of coronal holes, and as small transients form streamers or pseudo-streamers. Although our method by itself is simplistic, it provides a new tool that can be used in combination with other methods for identifying the sources of solar wind measured by Parker Solar Probe and Solar Orbiter.
Pudney M, King S, Horbury T, et al., 2019, SOLAR ORBITER STRATEGIES FOR EMC CONTROL AND VERIFICATION, ESA Workshop on Aerospace EMC (Aerospace EMC), Publisher: IEEE
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- Citations: 7
Matteini L, Stansby D, Horbury TS, et al., 2018, On the 1/f spectrum in the solar wind and its connection with magnetic compressibility, Letters of the Astrophysical Journal, Vol: 869, ISSN: 2041-8205
We discuss properties of Alfvénic fluctuations with large amplitude in plasmas characterized by low magnetic field compression. We note that in such systems power laws cannot develop with arbitrarily steep slopes at large scales, i.e., when $| \delta {\boldsymbol{B}}| $ becomes of the order of the background field $| {\boldsymbol{B}}| $. In such systems there is a scale l 0 at which the spectrum has to break due to the condition of weak compressibility. A very good example of this dynamics is offered by solar wind fluctuations in Alfvénic fast streams, characterized by the property of constant field magnitude. We show here that the distribution of $\delta B=| \delta {\boldsymbol{B}}| $ in the fast wind displays a strong cutoff at $\delta B/| {\boldsymbol{B}}| \lesssim 2$, as expected for fluctuations bounded on a sphere of radius $B=| {\boldsymbol{B}}| $. This is also associated with a saturation of the rms of the fluctuations at large scales and introduces a specific length l 0, above which the amplitude of the fluctuations becomes independent on the scale l. Consistent with that, the power spectrum at l > l 0 is characterized by a −1 spectral slope, as expected for fluctuations that are scale-independent. Moreover, we show that the spectral break between the 1/f and inertial range in solar wind spectra indeed corresponds to the scale l 0 at which $\langle \delta B/B\rangle \sim 1$. Such a simple model provides a possible alternative explanation of magnetic spectra observed in interplanetary space, also pointing out the inconsistency for a plasma to simultaneously maintain $| {\boldsymbol{B}}| \sim \mathrm{const}.$ at arbitrarily large scales and satisfy a Kolmogorov scaling.
Stansby D, Salem C, Matteini L, et al., 2018, A new inner heliosphere proton parameter dataset from the Helios mission, Solar Physics, Vol: 293, ISSN: 0038-0938
In the near future, Parker Solar Probe and Solar Orbiter will provide the first comprehensive in-situ measurements of the solar wind in the inner heliosphere since the Helios mission in the 1970s. We describe a reprocessing of the original Helios ion distribution functions to provide reliable and reproducible data to characterise the proton core population of the solar wind in the inner heliosphere. A systematic fitting of bi-Maxwellian distribution functions was performed to the raw Helios ion distribution function data to extract the proton core number density, velocity, and temperatures parallel and perpendicular to the magnetic field. We present radial trends of these derived proton parameters, forming a benchmark to which new measurements in the inner heliosphere will be compared. The new dataset has been made openly available for other researchers to use, along with the source code used to generate it.
Horbury TS, Matteini L, Stansby D, 2018, Short, large-amplitude speed enhancements in the near-Sun fast solar wind, Monthly Notices of the Royal Astronomical Society, Vol: 478, Pages: 1980-1986, ISSN: 0035-8711
We report the presence of intermittent, short discrete enhancements in plasma speed in the near-Sun high speed solar wind. Lasting tens of seconds to minutes in spacecraft measurements at 0.3 AU, speeds inside these enhancements can reach 1000 km/s, corresponding to a kinetic energy up to twice that of the bulk high speed solar wind. These events, which occur around 5% of the time, are Alfvenic in nature with large magnetic field deflections and are the same temperature as the surrounding plasma, in contrast to the bulk fast wind which has a well-established positive speed-temperature correlation. The origin of these speed enhancements is unclear but they may be signatures of discrete jets associated with transient events in the chromosphere or corona. Such large short velocity changes represent a measurement and analysis challenge for the upcoming Parker Solar Probe and Solar Orbiter missions.
Stansby D, Horbury T, 2018, Number density structures in the inner heliosphere, Astronomy and Astrophysics, Vol: 613, ISSN: 0004-6361
Aims.The origins and generation mechanisms of the slow solar wind are still unclear. Part of the slow solar wind is populated by“number density structures”, discrete patches of increased number density that are frozen in to and move with the bulk solar wind. Inthis paper we aimed to provide the first in-situ statistical study of number density structures in the inner heliosphere.Methods.We reprocessed in-situ ion distribution functions measured by Helios in the inner heliosphere to provide a new reliable setof proton plasma moments for the entire mission. From this new data set we looked for number density structures measured within0.5 AU of the Sun and studied their properties.Results.We identified 140 discrete areas of enhanced number density. The structures occurred exclusively in the slow solar wind andspanned a wide range of length scales from 50 Mm to 2000 Mm, which includes smaller scales than have been previously observed.They were also consistently denser and hotter that the surrounding plasma, but had lower magnetic field strengths, and thereforeremained in pressure balance.Conclusions.Our observations show that these structures are present in the slow solar wind at a wide range of scales, some of whichare too small to be detected by remote sensing instruments. These structures are rare, accounting for only 1% of the slow solar windmeasured by Helios, and are not a significant contribution to the mass flux of the solar wind.
Owens MJ, Riley P, Horbury T, 2017, The Role of Empirical Space-Weather Models (in a World of Physics-Based Numerical Simulations), IAU Symposia IAUS 335: Space Weather of the Heliosphere: Processes and Forecasts, Pages: 254-257, ISSN: 1743-9213
Copyright © International Astronomical Union 2018. Advanced forecasting of space weather requires prediction of near-Earth solar-wind conditions on the basis of remote solar observations. This is typically achieved using numerical magnetohydrodynamic models initiated by photospheric magnetic field observations. The accuracy of such forecasts is being continually improved through better numerics, better determination of the boundary conditions and better representation of the underlying physical processes. Thus it is not unreasonable to conclude that simple, empirical solar-wind forecasts have been rendered obsolete. However, empirical models arguably have more to contribute now than ever before. In addition to providing quick, cheap, independent forecasts, simple empirical models aid in numerical model validation and verification, and add value to numerical model forecasts through parameterization, uncertainty estimation and 'downscaling' of sub-grid processes.
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