The global circulation of the open magnetic flux of the Sun, the component of the solar magnetic field that opens into the heliosphere, and the consequences of the global circulation were proposed by ...Fisk and coworkers in the early 2000s. The Parker Solar Probe, on its initial encounters with the Sun, has provided direct confirmation of both the global circulation and the physical mechanism by which the circulation occurs, transport by interchange reconnection between open magnetic flux and large coronal loops. The implications of this confirmation of the global circulation of open magnetic flux and the importance of interchange reconnection is discussed.
The first two orbits of the Parker Solar Probe spacecraft have enabled the first in situ measurements of the solar wind down to a heliocentric distance of 0.17 au (or 36 ). Here, we present an ...analysis of this data to study solar wind turbulence at 0.17 au and its evolution out to 1 au. While many features remain similar, key differences at 0.17 au include increased turbulence energy levels by more than an order of magnitude, a magnetic field spectral index of −3/2 matching that of the velocity and both Elsasser fields, a lower magnetic compressibility consistent with a smaller slow-mode kinetic energy fraction, and a much smaller outer scale that has had time for substantial nonlinear processing. There is also an overall increase in the dominance of outward-propagating Alfvénic fluctuations compared to inward-propagating ones, and the radial variation of the inward component is consistent with its generation by reflection from the large-scale gradient in Alfvén speed. The energy flux in this turbulence at 0.17 au was found to be ∼10% of that in the bulk solar wind kinetic energy, becoming ∼40% when extrapolated to the Alfvén point, and both the fraction and rate of increase of this flux toward the Sun are consistent with turbulence-driven models in which the solar wind is powered by this flux.
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
Astronomical wide-field imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new ...wide-field interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependences of casa's w-projection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarization correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2–12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.
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.
A study of the structure of 145 low‐Mach number (M≤3), low‐beta (β≤1), quasi‐perpendicular interplanetary collisionless shock waves observed by the Wind spacecraft has provided strong evidence that ...these shocks have large‐amplitude whistler precursors. The common occurrence and large amplitudes of the precursors raise doubts about the standard assumption that such shocks can be classified as laminar structures. This directly contradicts standard models. In 113 of the 145 shocks (∼78%), we observe clear evidence of magnetosonic‐whistler precursor fluctuations with frequencies ∼0.1–7 Hz. We find no dependence on the upstream plasma beta, or any other shock parameter, for the presence or absence of precursors. The majority (∼66%) of the precursors propagate at ≤45° with respect to the upstream average magnetic field and most (∼87%) propagate ≥30° from the shock normal vector. Further, most (∼79%) of the waves propagate at least 20° from the coplanarity plane. The peak‐to‐peak wave amplitudes (δBpk‐pk) are large with a range of maximum values for the 113 precursors of ∼0.4–13 nT with an average of ∼2 nT. When we normalize the wave amplitudes to the upstream averaged magnetic field and the shock ramp amplitude, we find average values of ∼40% and ∼220%, respectively.
Plain Language Summary
We present new results that suggest that the magnetic structure of collisionless shock waves is not a smooth, step‐like transition but rather riddled with large‐amplitude waves as large or larger than the shock itself. These results have implications for the dynamics of weak shocks from propagation and evolution to particle acceleration and heating.
Key Points
Low‐Mach number, low‐beta, quasi‐perpendicular shocks are not laminar, step‐like, magnetic structures
Whistler precursor amplitudes are on average 50% and 80% of the upstream average magnetic field and shock ramp amplitude, respectively
Whistler precursors propagate obliquely to the upstream magnetic field, shock normal vector, and coplanarity plane
The high temperatures and strong magnetic fields of the solar corona form streams of solar wind that expand through the Solar System into interstellar space. At 09:33 UT on 28 April 2021 Parker Solar ...Probe entered the magnetized atmosphere of the Sun 13 million km above the photosphere, crossing below the Alfvén critical surface for five hours into plasma in casual contact with the Sun with an Alfvén Mach number of 0.79 and magnetic pressure dominating both ion and electron pressure. The spectrum of turbulence below the Alfvén critical surface is reported. Magnetic mapping suggests the region was a steady flow emerging on rapidly expanding coronal magnetic field lines lying above a pseudostreamer. The sub-Alfvénic nature of the flow may be due to suppressed magnetic reconnection at the base of the pseudostreamer, as evidenced by unusually low densities in this region and the magnetic mapping.
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.
ABSTRACT We present a study of magnetic field fluctuations in a slow solar wind stream, close to ion scales, where an increase of the level of magnetic compressibility is observed. Here, the nature ...of these compressive fluctuations is found to be characterized by coherent structures. Although previous studies have shown that current sheets can be considered the principal cause of intermittency at ion scales, here we show for the first time that, in the case of the slow solar wind, a large variety of coherent structures contributes to intermittency at proton scales, and current sheets are not the most common. Specifically, we find compressive ( ), linearly polarized structures in the form of magnetic holes, solitons, and shock waves. Examples of Alfvénic structures ( ) are identified as current sheets and vortex-like structures. Some of these vortices have , as in the case of Alfvén vortices, but the majority of them are characterized by . Thanks to multi-point measurements by the Cluster spacecraft, for about 100 structures we could determine the normal, the propagation velocity, and the spatial scale along this normal. Independently of the nature of the structures, the normal is always perpendicular to the local magnetic field, meaning that k > k . The spatial scales of the studied structures are found to be between two and eight times the proton gyroradius. Most of them are simply convected by the wind, but 25% propagate in the plasma frame. Possible interpretations of the observed structures and the connection with plasma heating are discussed.
The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9
R
⊙
, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than ...previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP’s fourth solar encounter, which was likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with a spectral index close to –5/3 rather than –3/2), a lower Alfvénicity, and a ‘1∕
f
’ break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ≈4° from the HCS, suggesting ≈8° as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.
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