Solar Probe Plus (SPP) will be the first spacecraft to fly into the low solar corona. SPP’s main science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, ...understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Understanding these fundamental phenomena has been a top-priority science goal for over five decades, dating back to the 1958 Simpson Committee Report. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. The mission design and the technology and engineering developments enable SPP to meet its science objectives to: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. The SPP mission was confirmed in March 2014 and is under development as a part of NASA’s Living with a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and will perform 24 orbits over a 7-year nominal mission duration. Seven Venus gravity assists gradually reduce SPP’s perihelion from 35 solar radii (
R
S
) for the first orbit to
<
10
R
S
for the final three orbits. In this paper we present the science, mission concept and the baseline vehicle for SPP, and examine how the mission will address the key science questions
Who Needs Turbulence? Matthaeus, W. H.; Velli, M.
Space science reviews,
10/2011, Letnik:
160, Številka:
1-4
Journal Article
Recenzirano
The significant influences of turbulence in neutral fluid hydrodynamics are well accepted but the potential for analogous effects in space and astrophysical plasmas is less widely recognized. This ...situation sometimes gives rise to the question posed in the title; “Who need turbulence?” After a brief overview of turbulence effects in hydrodynamics, some likely effects of turbulence in solar and heliospheric plasma physics are reviewed here, with the goal of providing at least a partial answer to the posed question.
The structure of magnetic flux ropes injected into the solar wind during reconnection in the coronal atmosphere is explored with particle-in-cell simulations and compared with in situ measurements of ...magnetic “switchbacks” from the Parker Solar Probe. We suggest that multi-x-line reconnection between open and closed flux in the corona injects flux ropes into the solar wind and that these flux ropes convect outward over long distances before eroding due to reconnection. Simulations that explore the magnetic structure of flux ropes in the solar wind reproduce the following key features of the switchback observations: a rapid rotation of the radial magnetic field into the transverse direction, which is a consequence of reconnection with a strong guide field; and the potential to reverse the radial field component. The potential implication of the injection of large numbers of flux ropes in the coronal atmosphere for understanding the generation of the solar wind is discussed.
The Highly Structured Outer Solar Corona DeForest, C. E.; Howard, R. A.; Velli, M. ...
The Astrophysical journal,
07/2018, Letnik:
862, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We report on the observation of fine-scale structure in the outer corona at solar maximum, using deep-exposure campaign data from the Solar Terrestrial Relations Observatory-A (STEREO-A)/COR2 ...coronagraph coupled with postprocessing to further reduce noise and thereby improve effective spatial resolution. The processed images reveal radial structure with high density contrast at all observable scales down to the optical limit of the instrument, giving the corona a "woodgrain" appearance. Inferred density varies by an order of magnitude on spatial scales of 50 Mm and follows an f−1 spatial spectrum. The variations belie the notion of a smooth outer corona. They are inconsistent with a well-defined "Alfvén surface," indicating instead a more nuanced "Alfvén zone"-a broad trans-Alfvénic region rather than a simple boundary. Intermittent compact structures are also present at all observable scales, forming a size spectrum with the familiar "Sheeley blobs" at the large-scale end. We use these structures to track overall flow and acceleration, finding that it is highly inhomogeneous and accelerates gradually out to the limit of the COR2 field of view. Lagged autocorrelation of the corona has an enigmatic dip around 10 R , perhaps pointing to new phenomena near this altitude. These results point toward a highly complex outer corona with far more structure and local dynamics than has been apparent. We discuss the impact of these results on solar and solar-wind physics and what future studies and measurements are necessary to build upon them.
Abstract
Motivated by prior remote observations of a transition from striated solar coronal structures to more isotropic “flocculated” fluctuations, we propose that the dynamics of the inner solar ...wind just outside the Alfvén critical zone, and in the vicinity of the first
surface, is powered by the relative velocities of adjacent coronal magnetic flux tubes. We suggest that large-amplitude flow contrasts are magnetically constrained at lower altitude but shear-driven dynamics are triggered as such constraints are released above the Alfvén critical zone, as suggested by global magnetohydrodynamic (MHD) simulations that include self-consistent turbulence transport. We argue that this dynamical evolution accounts for features observed by Parker Solar Probe (PSP) near initial perihelia, including magnetic “switchbacks,” and large transverse velocities that are partially corotational and saturate near the local Alfvén speed. Large-scale magnetic increments are more longitudinal than latitudinal, a state unlikely to originate in or below the lower corona. We attribute this to preferentially longitudinal velocity shear from varying degrees of corotation. Supporting evidence includes comparison with a high Mach number three-dimensional compressible MHD simulation of nonlinear shear-driven turbulence, reproducing several observed diagnostics, including characteristic distributions of fluctuations that are qualitatively similar to PSP observations near the first perihelion. The concurrence of evidence from remote sensing observations, in situ measurements, and both global and local simulations supports the idea that the dynamics just above the Alfvén critical zone boost low-frequency plasma turbulence to the level routinely observed throughout the explored solar system.
We study the onset and evolution of the Alfvén wave parametric decay instability within the Accelerating Expanding Box model in the framework of a one‐fluid description of the plasma. As we are ...interested in understanding wave propagation and dissipation in the inner heliosphere and solar wind, the expansion of the solar wind itself may not be neglected. In this sense, the Accelerating Expanding Box provides a useful and simple model to mimic the effects that the expansion of the underlying atmosphere has on wave propagation and plasma dynamics. In the simulations, we follow the evolution of Alfvén waves along a fast solar wind stream, from the sub‐Alfvénic region up to a maximum heliocentric distance of nearly 4 AU. We consider exact solutions of the compressible MHD system given by circularly polarized Alfvén waves which propagate in the radial direction, along the mean magnetic field. Both monochromatic waves and a nonmonochromatic wave are considered. Monochromatic waves have periods ranging from a few minutes to a few hours, the latter being stabilized by the expansion. The nonmonochromatic wave has a central period of the order of a few minutes, with a broad spectrum containing frequencies near the threshold of the instability. In this case the Alfvén wave partly decays into backward daughter Alfvén waves up to the instability saturation, then giving rise to a nonlinear cascade of incompressible and compressible modes.
Key Points
Study of Alfven wave parametric decay from subalfvenic to superalfveinc wind
Effects of the solar wind expansion: stabilizing effects on the parametric decay
ABSTRACT
We use 2.5D magnetohydrodynamic simulations to investigate the spectral signatures of the non-linear disruption of a tearing unstable current sheet via the generation of multiple secondary ...current sheets and magnetic islands. During the non-linear phase of tearing mode evolution, there develops a regime in which the magnetic energy density shows a spectrum with a power law close to B(k)2 ∼ k−0.8. Such an energy spectrum is found in correspondence of the neutral line, within the diffusion region of the primary current sheet, where energy is conveyed towards smaller scales via a ‘recursive’ process of fast tearing-type instabilities. Far from the neutral line, we find that magnetic energy spectra evolve towards slopes compatible with the ‘standard’ Kolmogorov spectrum. Starting from a self-similar description of the non-linear stage at the neutral line, we provide a model that predicts a reconnecting magnetic field energy spectrum scaling as k−4/5, in good agreement with numerical results. An extension of the predicted power law to generic current sheet profiles is also given and possible implications for turbulence phenomenology are discussed. These results provide a step forward to understand the ‘recursive’ generation of magnetic islands (plasmoids), which has been proposed as a possible explanation for the energy release during flares, but which, more in general, can have an impact on the subsequent turbulent evolution of unstable sheets that naturally form in the high Lundquist number and collisionless plasmas found in most of the astrophysical environments.
We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism ...that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (i.e., a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, are ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite polarity magnetic fields producing a short-lived jet of hot plasma and Alfv´en waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.
Comparative studies of fast and slow solar wind streams performed over the past decades have illustrated several differences between the plasma regimes for these different flows, examples including ...features such as temperatures, particle distribution function anisotropies, and the nature of the embedded turbulence, specifically the Alfvénicity of the fluctuations. Though this two‐state classification of the solar wind primarily based on flow speed has been widely adopted, more in depth studies have found that slow solar wind should be further categorized, flow speed not being a sufficient descriptor of the plasma state. Within this framework, slow solar wind streams with a strong Alfvénic character have been identified and characterized, showing that in many ways they resemble fast solar wind. The similarities between fast and slow Alfvénic wind regimes have been explained in terms of a similar solar origin, with the latter corresponding to slow winds emanating from rapidly diverging low‐latitude small coronal holes. The aim of this review is to describe the state of art of our understanding of Alfvénic slow solar wind streams. The results presented cover observations performed at different heliocentric distances spanning from Wind at L1 to Helios and Parker Solar Probe in the inner heliosphere, as well as a discussion of their source regions.
Key Points
Alfvenicity is a crucial element, in addition to composition, for classifying the solar‐wind in situ and identifying the source regions
Similarities between Alfvenic slow and fast wind have been found at different radial distances during different phases of solar activity
The solar sources of the Alfvenic slow wind streams have been recognized as regions of over‐expanded magnetic flux tubes
The Solar Orbiter mission Müller, D.; St. Cyr, O. C.; Zouganelis, I. ...
Astronomy and astrophysics (Berlin),
10/2020, Letnik:
642
Journal Article
Recenzirano
Odprti dostop
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.
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