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We compare hybrid (kinetic proton, fluid electron) and particle-in-cell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steady-state results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasi-perpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particle-in-cell simulation, likely due to differences in the cross-shock electric field arising from electron kinetic-scale effects. The higher fluxes may help address recent discrepancies noted between observations and large-scale hybrid simulations. [ABSTRACT FROM AUTHOR]

We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma- β and high-Alfvén-speed regime of the Earth's magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electron β e covered by this study are 800–4000 km s−1 and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, Δ T e, is correlated with the inflow Alfvén speed, V Ax,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, Δ T e is linearly proportional to the inflowing magnetic energy per particle, m i V Ax , in 2 , and the best fit to the data produces the empirical relation Δ T e = 0.020 m i V Ax , in 2 , i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electron β, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range of V A ∼ 10–4000 km s−1 and β e ∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma. [ABSTRACT FROM AUTHOR]

A variety of energy sources, ranging from dynamic processes, such as magnetic reconnection and waves, to quasi-steady terms, such as plasma pressure, may contribute to the acceleration of the solar wind. We utilize a combination of charged particle and magnetic field observations from the Parker Solar Probe (PSP) to attempt to quantify the steady-state contribution of the proton pressure, the electric potential, and the wave energy to the solar wind proton acceleration observed by PSP between 13.3 and ∼100 solar radii (R ☉). The proton pressure provides a natural kinematic driver of the outflow. The ambipolar electric potential acts to couple the electron pressure to the protons, providing another definite proton acceleration term. Fluctuations and waves, while inherently dynamic, can act as an additional effective steady-state pressure term. To analyze the contributions of these terms, we utilize radial binning of single-point PSP measurements, as well as repeated crossings of the same stream at different distances on individual PSP orbits (i.e., fast radial scans). In agreement with previous work, we find that the electric potential contains sufficient energy to fully explain the acceleration of the slower wind streams. On the other hand, we find that the wave pressure plays an increasingly important role in the faster wind streams. The combination of these terms can explain the continuing acceleration of both slow and fast wind streams beyond 13.3 R ☉. [ABSTRACT FROM AUTHOR]

We investigate the detailed properties of electron inflow in an electron-only reconnection event observed by the four Magnetospheric Multiscale (MMS) spacecraft in the Earth's turbulent magnetosheath downstream of the quasi-parallel bow shock. The lack of ion coupling was attributed to the small-scale sizes of the current sheets, and the observed bidirectional super-Alfvénic electron jets indicate that the MMS spacecraft crossed the reconnecting current sheet on both sides of an active X-line. Remarkably, the MMS spacecraft observed the presence of large asymmetries in the two electron inflows, with the inflows (normal to the current sheet) on the two sides of the reconnecting current layer differing by as much as a factor of four. Furthermore, even though the four MMS spacecraft were separated by less than seven electron skin depths, the degree of inflow asymmetry was significantly different at the different spacecraft. The asymmetry in the inflow speeds was larger with increasing distances downstream from the reconnection site, and the asymmetry was opposite on the two sides of the X-line. We compare the MMS observations with a 2D kinetic particle-in-cell (PIC) simulation and find that the asymmetry in the inflow speeds stems from in-plane currents generated via the combination of reconnection-mediated inflows and parallel flows along the magnetic separatrices in the presence of a large guide field. [ABSTRACT FROM AUTHOR]

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 (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is 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 Alfvén 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. [ABSTRACT FROM AUTHOR]

We present observations of ≳10â€"100 keV nucleonâˆ'1 suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of <0.1 au from the Sun. Our key findings are as follows: (1) very few heavy ions are detected during the first full crossing, the heavy-ion intensities are reduced during the second partial crossing and peak just after the second crossing; (2) ion arrival times exhibit no velocity dispersion; (3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ∼10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic farther from the HCS; (4) the He spectrum steepens either side of the HCS, and the He, O, and Fe spectra exhibit power laws of the form ∼ E âˆ'4â€" E 6; and (5) maximum energies E X increase with the ion’s charge-to-mass (Q / M) ratio as E X / E H ∝ (Q X / M X) δ , where δ ∼ 0.65â€"0.76, assuming that the average Q states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-Sun coronal-mass-ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to E / Q but also for local diffusive and magnetic-reconnection-driven acceleration models. Reevaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions. [ABSTRACT FROM AUTHOR]

We analyze the structure and evolution of ribbons from the M7.3 SOL2014-04-18T13 flare using ultraviolet images from the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA), magnetic data from the SDO/Helioseismic and Magnetic Imager, hard X-ray (HXR) images from the Reuven Ramaty High Energy Solar Spectroscopic Imager, and light curves from the Fermi/Gamma-ray Burst Monitor, in order to infer properties of coronal magnetic reconnection. As the event progresses, two flare ribbons spread away from the magnetic polarity inversion line. The width of the newly brightened front along the extension of the ribbon is highly intermittent in both space and time, presumably reflecting nonuniformities in the structure and/or dynamics of the flare current sheet. Furthermore, the ribbon width grows most rapidly in regions exhibiting concentrated nonthermal HXR emission, with sharp increases slightly preceding the HXR bursts. The light curve of the ultraviolet emission matches the HXR light curve at photon energies above 25 keV. In other regions the ribbon-width evolution and light curves do not temporally correlate with the HXR emission. This indicates that the production of nonthermal electrons is highly nonuniform within the flare current sheet. Our results suggest a strong connection between the production of nonthermal electrons and the locally enhanced perpendicular extent of flare ribbon fronts, which in turn reflects the inhomogeneous structure and/or reconnection dynamics of the current sheet. Despite this variability, the ribbon fronts remain nearly continuous, quasi-one-dimensional features. Thus, although the reconnecting coronal current sheets are highly structured, they remain quasi-two-dimensional and the magnetic energy release occurs systematically, rather than stochastically, through the volume of the reconnecting magnetic flux. [ABSTRACT FROM AUTHOR]

The origin of switchbacks in the solar wind is discussed in two classes of theory that differ in the location of the source being either near the transition region near the Sun or in the solar wind itself. The two classes of theory differ in their predictions of the switchback rate (the number of switchbacks observed per hour) as a function of distance from the Sun. To distinguish between these theories, one-hour averages of Parker Solar Probe data were averaged over five orbits to find the following: (1) The hourly averaged switchback rate was independent of distance from the Sun. (2) The average switchback rate increased with solar wind speed. (3) The switchback size perpendicular to the flow increased as R, the distance from the Sun, while the radial size increased as R2, resulting in an increasing switchback aspect ratio with distance from the Sun. (4) The hourly averaged and maximum switchback rotation angles did not depend on the solar wind speed or distance from the Sun. These results are consistent with switchback formation in the transition region because their increase of tangential size with radius compensates for the radial falloff of their equatorial density to produce switchback rates that are independent of radial distance. This constant switchback rate is inconsistent with an in situ source. The switchback size and aspect ratio, but not their hourly average or maximum rotation angle, increased with radial distance to 100 solar radii. Additionally, quiet intervals between switchback patches occurred at the lowest solar wind speeds. [ABSTRACT FROM AUTHOR]

The shape of the heliosphere is thought to resemble a long, comet tail, however, recently it has been suggested that the heliosphere is tailless with a two-lobe structure. The latter study was done with a three-dimensional (3D) magnetohydrodynamic code, which treats the ionized and neutral hydrogen atoms as fluids. Previous studies that described the neutrals kinetically claim that this removes the two-lobe structure of the heliosphere. In this work, we use the newly developed Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model. SHIELD is a self-consistent kinetic-MHD model of the outer heliosphere that couples the MHD solution for a single plasma fluid from the BATS-R-US MHD code to the kinetic solution for neutral hydrogen atoms solved by the Adaptive Mesh Particle Simulator, a 3D, direct simulation Monte Carlo model that solves the Boltzmann equation. We use the same boundary conditions as our previous simulations using multi-fluid neutrals to test whether the two-lobe structure of the heliotail is removed with a kinetic treatment of the neutrals. Our results show that despite the large difference in the neutral hydrogen solutions, the two-lobe structure remains. These results are contrary to previous kinetic-MHD models. One such model maintains a perfectly ideal heliopause and does not allow for communication between the solar wind and interstellar medium. This indicates that magnetic reconnection or instabilities downtail play a role for the formation of the two-lobe structure. [ABSTRACT FROM AUTHOR]

In many space, astrophysical, and laboratory plasmas the energy contained in the magnetic field or plasma flow exceeds the thermal energy. Magnetic field () annihilation, often enabled by magnetic reconnection, transfers magnetic energy to particles. Shocks transfer bulk flow energy to particles. If there is a sufficiently large energy transfer, strong turbulence (∣ ∣/∣ B ∣ ∼ 1) develops, which, in turn, can result in nonthermal acceleration. In this article, we investigate acceleration in a finite-sized region of strong turbulence driven by magnetic reconnection with analytical modeling and test-particle simulations. This research is based on detailed observations in the Earth's magnetotail. We find that the primary transfer of magnetic energy to particle energy is advanced by large-amplitude electric field structures () generated by the strong turbulence. To no surprise, ion energization is dominated by intense DC , near the ion cyclotron frequency (fci), and/or variations at scales near the ion gyroradius. Electron energization comes from higher-frequency. The turbulent cascade continuously regenerates near fci and higher frequencies. Importantly, the turbulence also creates magnetic depletions that can trap particles and considerably increase their dwell time in regions of strong energization, which substantially enhances nonthermal acceleration. Moreover, energization is primarily perpendicular to , so particles have difficulty escaping regions of depleted , which can lead to near runaway acceleration. We discuss how this process may be active in large-scale settings such as supernova shells and may contribute, at least in in part, to the development of the cosmic ray spectrum. [ABSTRACT FROM AUTHOR]

Taken together, the Voyager 1 and 2 (V1 and V2) spacecraft have collected over 11 yr of data in the heliosheath. Despite extensive study, energetic particles and magnetic fields measured in the heliosheath have not been reconciled by existing models. In particular, the differences between the energetic particle intensity variations at V1 and V2 are unexplained. While energetic particle intensities at V1 change gradually over 7 yr in the heliosheath, those at V2 vary by a factor ∼10 in 1 yr. Energetic particle intensities at V2 show temporally coherent variations over a broad range of species and energies: from suprathermal ions (10s of keV) to galactic cosmic rays (>1 GeV), as well as electrons from 10s of keV to >100 MeV, corresponding to a range ∼104 in particle gyroradii. Here we suggest that many of the intensity variations of energetic particle populations in the heliosheath are organized by their proximity to two fundamentally different regions—the unipolar heliosheath (UHS) and the sectored heliosheath (SHS). The SHS is a region of enhanced particle intensities, wherein particle transport, acceleration, and magnetic connectivity differ from those in the UHS. The SHS may serve as either a reservoir of energetic particles or as a region of enhanced transport, depending on the particle species and energy. Comparatively, particle intensities in the UHS are greatly reduced. We propose that the boundary between the SHS and UHS plays as important a role in the physics of heliosheath particles and fields as do the termination shock and heliopause. [ABSTRACT FROM AUTHOR]

In observations of flare-heated electrons in the solar corona, a longstanding problem is the unexplained prolonged lifetime of the electrons compared to their transit time across the source. This suggests confinement. Recent particle-in-cell (PIC) simulations, which explored the transport of pre-accelerated hot electrons through ambient cold plasma, showed that the formation of a highly localized electrostatic potential drop, in the form of a double layer (DL), significantly inhibited the transport of hot electrons. The effectiveness of confinement by a DL is linked to the strength of the DL as defined by its potential drop. In this work, we investigate the scaling of the DL strength with the hot electron temperature by PIC simulations and find a linear scaling. We demonstrate that the strength is limited by the formation of parallel shocks. Based on this, we analytically determine the maximum DL strength, and also find a linear scaling with the hot electron temperature. The DL strength obtained from the analytic calculation is comparable to that from the simulations. At the maximum strength, the DL is capable of confining a significant fraction of hot electrons in the source. [ABSTRACT FROM AUTHOR]

The plasma β (the ratio of the plasma pressure to the magnetic pressure) of a system can have a large effect on its dynamics as high β enhances the effects of pressure anisotropies. We investigate the effects of β in a system of stacked current sheets that break up into magnetic islands due to magnetic reconnection, which is analogous to the compressed heliospheric current sheet in the heliosheath. We find significant differences between systems with low and high initial values of β. At low β, growing magnetic islands are modestly elongated and become round as contraction releases magnetic stress and reduces magnetic energy. At high β, the increase of the parallel pressure in contracting islands causes saturation of modestly elongated islands as island cores approach the marginal firehose condition. Only highly elongated islands reach finite size. The anisotropy within these islands prevents full contraction, leading to a final state of highly elongated islands in which further reconnection is suppressed. The elongation of islands at finite β is further enhanced by reducing the electron-to-ion mass ratio to more realistic values. The results are directly relevant to reconnection in the sectored region of the heliosheath where there is evidence that elongated islands are present, and possibly to other high-β systems such as astrophysical accretion flows and the magnetosphere of Saturn. [ABSTRACT FROM AUTHOR]

All current global models of the heliosphere are based on the assumption that the magnetic field in the heliosheath, in the region close to the heliopause (HP), is laminar. We argue that in that region the heliospheric magnetic field is not laminar but instead consists of magnetic bubbles. We refer to it as the bubble-dominated heliosheath region. Recently, we proposed that the annihilation of the "sectored" magnetic field within the heliosheath as it is compressed on its approach to the HP produces anomalous cosmic rays and also energetic electrons. As a product of the annihilation of the sectored magnetic field, densely packed magnetic islands (which further interact to form magnetic bubbles) are produced. These magnetic islands/bubbles will be convected with ambient flows as the sector region is carried to higher latitudes filling the heliosheath. We further argue that the magnetic islands/bubbles will develop upstream within the heliosheath. As a result, the magnetic field in the heliosheath sector region will be disordered well upstream of the HP. We present a three-dimensional MHD simulation with very high numerical resolution that captures the north-south boundaries of the sector region. We show that due to the high pressure of the interstellar magnetic field a north-south asymmetry develops such that the disordered sectored region fills a large portion of the northern part of the heliosphere with a smaller extension in the southern hemisphere. We suggest that this scenario is supported by the following changes that occurred around 2008 and from 2009.16 onward: (1) the sudden decrease in the intensity of low energy electrons (0.02-1.5 MeV) detected by Voyager 2, (2) a sharp reduction in the intensity of fluctuations of the radial flow, and (3) the dramatic differences in intensity trends between galactic cosmic ray electrons (3.8-59 MeV) at Voyager 1 and 2. We argue that these observations are a consequence of Voyager 2 leaving the sector region of disordered field during these periods and crossing into a region of unipolar laminar field. [ABSTRACT FROM AUTHOR]

Observations of flare-heated electrons in the corona typically suggest confinement of electrons. The confinement mechanism, however, remains unclear. The transport of coronal hot electrons into ambient plasma was recently investigated by particle-in-cell (PIC) simulations. Electron transport was significantly suppressed by the formation of a highly localized, nonlinear electrostatic potential in the form of a double layer (DL). In this work large-scale PIC simulations are performed to explore the dynamics of DLs in larger systems where, instead of a single DL, multiple DLs are generated. The primary DL accelerates return current electrons, resulting in high velocity electron beams that interact with ambient ions. This forms a Buneman unstable system that spawns more DLs. Trapping of heated return current electrons between multiple DLs strongly suppresses electron transport. DLs also accelerate ambient ions and produce strong ion flows over an extended region. This clarifies the mechanism by which hot electrons in the corona couple to and accelerate ions to form the solar wind. These new dynamics in larger systems reveal a more likely picture of DL development and their impact on the ambient plasma in the solar corona. They are applicable to the preparation for in situ coronal space missions like the Solar Probe Plus. [ABSTRACT FROM AUTHOR]

During flares and coronal mass ejections, energetic electrons from coronal sources typically have very long lifetimes compared to the transit times across the systems, suggesting confinement in the source region. Particle-in-cell simulations are carried out to explore the mechanisms of energetic electron transport from the corona to the chromosphere and possible confinement. We set up an initial system of pre-accelerated hot electrons in contact with ambient cold electrons along the local magnetic field and let it evolve over time. Suppression of transport by a nonlinear, highly localized electrostatic electric field (in the form of a double layer) is observed after a short phase of free-streaming by hot electrons. The double layer (DL) emerges at the contact of the two electron populations. It is driven by an ion-electron streaming instability due to the drift of the back-streaming return current electrons interacting with the ions. The DL grows over time and supports a significant drop in temperature and hence reduces heat flux between the two regions that is sustained for the duration of the simulation. This study shows that transport suppression begins when the energetic electrons start to propagate away from a coronal acceleration site. It also implies confinement of energetic electrons with kinetic energies less than the electrostatic energy of the DL for the DL lifetime, which is much longer than the electron transit time through the source region. [ABSTRACT FROM AUTHOR]

Since April of 2010, Voyager 1 has been immersed in a region of near zero radial flows, where the solar wind seems to have stopped. The existence of this region contradicts current models that predict that the radial flows will go to zero only at the heliopause. These models, however, do not include the sector region (or include it in a kinematic fashion), where the solar magnetic field periodically reverses polarity. Here we show that the presence of the sector region in the heliosheath, where reconnection occurs, fundamentally alters the flows, giving rise to a Flow Transition Region (FTR), where the flow abruptly turns and the radial velocity becomes near zero or negative. We estimate, based on a simulation, that at the Voyager 1 location, the thickness of the FTR is around 7-11 AU. [ABSTRACT FROM AUTHOR]