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. 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. 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.

/pdf/the-solar-orbiter-mission-science-overview-27zlqx0qyv.pdf

The Solar Orbiter mission -- Science overview

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.

/pdf/the-solar-orbiter-mission-science-overview-1edys0hbk4.pdf

The Solar Orbiter mission. Science overview

Simulations of a solar coronal jet driven by filament ejection demonstrate that magnetic reconnection underlies the energy release, implying that a universal ‘breakout’ model explains both tiny jets and huge mass ejections from the Sun. Eruptions on the Sun, from small- to large-scale, have frequently been associated with magnetic ejections. It has previously been thought that these different-scale eruptions also differ in the mechanism through which they arise. However, recent interpretations of new observations led to the conclusion that all small coronal jets are driven by magnetic ejections, just like large coronal mass ejections, suggesting that the two phenomena have the same physical origin and therefore could arise through the same mechanism. Peter Wyper et al. report simulations of coronal jets that suggest that magnetic reconnection underlies the small-scale jets. Their model of the ejection process is universal across multiple scales in the Sun's corona, and so suggests a common mechanism for different-scale eruptions—assuming that they are indeed of the same origin—thereby bringing theory and observation into agreement. Magnetically driven eruptions on the Sun, from stellar-scale coronal mass ejections1 to small-scale coronal X-ray and extreme-ultraviolet jets2,3,4, have frequently been observed to involve the ejection of the highly stressed magnetic flux of a filament5,6,7,8,9. Theoretically, these two phenomena have been thought to arise through very different mechanisms: coronal mass ejections from an ideal (non-dissipative) process, whereby the energy release does not require a change in the magnetic topology, as in the kink or torus instability10,11; and coronal jets from a resistive process2,12 involving magnetic reconnection. However, it was recently concluded from new observations that all coronal jets are driven by filament ejection, just like large mass ejections13. This suggests that the two phenomena have physically identical origin and hence that a single mechanism may be responsible, that is, either mass ejections arise from reconnection, or jets arise from an ideal instability. Here we report simulations of a coronal jet driven by filament ejection, whereby a region of highly sheared magnetic field near the solar surface becomes unstable and erupts. The results show that magnetic reconnection causes the energy release via ‘magnetic breakout’—a positive-feedback mechanism between filament ejection and reconnection. We conclude that if coronal mass ejections and jets are indeed of physically identical origin (although on different spatial scales) then magnetic reconnection (rather than an ideal process) must also underlie mass ejections, and that magnetic breakout is a universal model for solar eruptions.

/pdf/a-universal-model-for-solar-eruptions-4nlzrw315t.pdf

A universal model for solar eruptions

Magnetic flux emergence from the solar convection zone into the overlying atmosphere is the driver of a diverse range of phenomena associated with solar activity. In this article, we introduce theoretical concepts central to the study of flux emergence and discuss how the inclusion of different physical effects (e.g., magnetic buoyancy, magnetoconvection, reconnection, magnetic twist, interaction with ambient field) in models impact the evolution of the emerging field and plasma.

/pdf/flux-emergence-theory-45pkofupwr.pdf

Flux Emergence (Theory)

Over the last two decades the uninterrupted, high resolution observations of the Sun, from the excellent range of telescopes aboard many spacecraft complemented with observations from sophisticated ground-based telescopes have opened up a new world producing significantly more complete information on the physical conditions of the solar atmosphere than before. The interface between the lower solar atmosphere where energy is generated by subsurface convection and the corona comprises the chromosphere, which is dominated by jet-like, dynamic structures, called mottles when found in quiet regions, fibrils when found in active regions and spicules when observed at the solar limb. Recently, space observations with Hinode have led to the suggestion that there should exist two different types of spicules called Type I and Type II which have different properties. Ground-based observations in the Ca ii H and K filtergrams reveal the existence of long, thin emission features called straws in observations close to the limb, and a class of short-lived events called rapid blue-shifted excursions characterized by large Doppler shifts that appear only in the blue wing of the Ca ii infrared line. It has been suggested that the key to understanding how the solar plasma is accelerated and heated may well be found in the studies of these jet-like, dynamic events. However, while these structures are observed and studied for more than 130 years in the visible, but also in the UV and EUV emission lines and continua, there are still many questions to be answered. Thus, despite their importance and a multitude of observations performed and theoretical models proposed, questions regarding their origin, how they are formed, their physical parameters, their association with the underlying photospheric magnetic field, how they appear in the different spectral lines, and the interrelationship between structures observed in quiet and active regions on the disk and at the limb, as well as their role in global processes has not yet received definitive answers. In addition, how they affect the coronal heating and solar wind need to be further explored. In this review we present observations and physical properties of small-scale jet-like chromospheric events observed in active and quiet regions, on the disk and at the limb and discuss their interrelationship.

Solar Fine-Scale Structures. I. Spicules and Other Small-Scale, Jet-Like Events at the Chromospheric Level: Observations and Physical Parameters

Aims. We aim to improve our picture of the low chromosphere in the quiet-Sun internetwork by investigating the intensity, horizontal velocity, size and lifetime variations of small bright points (BPs; diameter smaller than 0.3 arcsec) observed in the Ca ii H 3968 A passband along with their magnetic field parameters, derived from photospheric magnetograms. Methods. Several high-quality time series of disc-centre, quiet-Sun observations from the Sunrise balloon-borne solar telescope, with spatial resolution of around 100 km on the solar surface, have been analysed to study the dynamics of BPs observed in the Ca ii H passband and their dependence on the photospheric vector magnetogram signal. Results. Parameters such as horizontal velocity, diameter, intensity and lifetime histograms of the isolated internetwork and magn

/pdf/structure-and-dynamics-of-isolated-internetwork-ca-ii-h-3hqefb62qr.pdf

Structure and dynamics of isolated internetwork Ca II H bright points observed by SUNRISE

Context. Except for the resonance lines, fibrils are ubiquitously present in most high-resolution observations of chromospheric lines.Aims. We show that fibrils are also a prevailing feature in K, provided the spatial-resolution is sufficiently high.Methods. We present high spatial resolution observations of an active region in the K line from the Swedish Solar Telescope. Through a comparison between photospheric intensity and magnetic field data, we study the connection between bright chromospheric fibrils and photospheric structures. Additionally, using Fourier analysis we study how the fibrils are linked to the observed dynamics.Results. We find that very narrow, bright fibrils are a prevailing feature over large portions of the observed field. We also find a clear connection between the fibril footpoints and photospheric magnetic features. We show that the fibrils play two distinct roles in the observed dynamics: depending on their location they can act as a canopy suppressing oscillations or they can channel low-frequency oscillations into the chromosphere.Conclusions. The K fibrils share many characteristics with fibrils observed in other chromospheric lines, but some features, such as the very small widths, are unique to these observations.

/pdf/bright-fibrils-in-ca-ii-k-1ste7q234w.pdf

Bright fibrils in Ca II K

We study the velocity structure of penumbral filaments in the deep photosphere to obtain direct evidence for the convective nature of sunspot penumbrae. A sunspot was observed at high spatial resolution with the 1 m Swedish Solar Telescope in the deep photospheric C I 5380 A absorption line. The Multi-Object Multi-Frame Blind Deconvolution method is used for image restoration and straylight is filtered out. We report here the discovery of clear redshifts in the C I 5380 A line at multiple locations in sunspot penumbral filaments. For example, bright head of filaments show larger concentrated blueshift and are surrounded by darker, redshifted regions, suggestive of overturning convection. Elongated downflow lanes are also located beside bright penumbral fibrils. Our results provide the strongest evidence yet for the presence of overturning convection in penumbral filaments and highlight the need to observe the deepest layers of the penumbra in order to uncover the energy transport processes taking place there.

Convective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere

We present high spatial and temporal resolution Ca II 8542 A observations of a kink wave in an on-disk chromospheric active region fibril. The properties of the wave are similar to those observed in off-limb spicules. From the observed phase and period of the wave we determine a lower limit for the field strength in the chromospheric active region fibril located at the edge of a sunspot to be a few hundred gauss. We find indications that the event was triggered by a small-scale reconnection event higher up in the atmosphere.

https://iopscience.iop.org/article/10.1088/0004-637X/739/2/92/pdf

Kink waves in an active region dynamic fibril

Context. The migration of magnetic bright point-like features (MBP) in the lower solar atmosphere reflects the dispersal of magnetic flux as well as the horizontal flows of the atmospheric layer they are embedded in.Aims. We analyse trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca ii H MBPs (mean lifetime 461 ± 9 s) to obtain their diffusivity behaviour.Methods. We use seeing-free high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca ii H 3968 A passband of the Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar observatory. Small MBPs in the internetwork are automatically tracked. The trajectory of each MBP is then calculated and described by a diffusion index (γ ) and a diffusion coefficient (D ). We also explore the distribution of the diffusion indices with the help of a Monte Carlo simulation.Results. We find γ  = 1.69 ± 0.08 and D  = 257 ± 32 km2  s-1 averaged over all MBPs. Trajectories of most MBPs are classified as super-diffusive, i.e. γ  > 1, with the determined γ being the largest obtained so far to our knowledge. A direct correlation between D and timescale (τ ) determined from trajectories of all MBPs is also obtained. We discuss a simple scenario to explain the diffusivity of the observed, relatively short-lived MBPs while they migrate within a small area in a supergranule (i.e. an internetwork area). We show that the scatter in the γ values obtained for individual MBPs is due to their limited lifetimes.Conclusions. The super-diffusive MBPs can be described as random walkers (due to granular evolution and intergranular turbulence) superposed on a large systematic (background) velocity, caused by granular, mesogranular, and supergranular flows.

/pdf/migration-of-ca-ii-h-bright-points-in-the-internetwork-41mxc3az6b.pdf

Migration of Ca II H bright points in the internetwork

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