At solar minimum, the solar wind is observed at high solar latitudes as a predominantly fast (> 500 km/s), highly Alfvenic, rarefied stream of plasma originating deep within coronal holes, while near ...the ecliptic plane it is interspersed with a more variable slow (< 500 kms) wind. The precise origins of the slow wind streams are less certain, with theories and observations supporting sources from the tips of helmet streamers, interchange reconnection near coronal hole boundaries, and origins within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also an open question and candidate mechanisms include Alfven wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients1. At 1 au, the wind is mixed and evolved and much of the diagnostic structure of these sources and processes has been lost. Here we present new measurements from Parker Solar Probe at 36 to 54 solar radii that show clear evidence of slow, Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals associated with jets of plasma and enhanced Poynting flux and interspersed in a smoother and less turbulent flow with near-radial magnetic field. Furthermore, plasma wave measurements suggest electron and ion velocity-space micro-instabilities that have been identified with plasma heating and thermalization processes. Our measurements suggest an impulsive mechanism associated with solar wind energization and a heating role for micro-instabilities and provide strong evidence for low latitude coronal holes as a significant contribution to the source of the slow solar wind.
The Axial Double Probe (ADP) instrument measures the DC to ∼100 kHz electric field along the spin axis of the Magnetospheric Multiscale (MMS) spacecraft (Burch et al., Space Sci. Rev.,
2014, this ...issue
), completing the vector electric field when combined with the spin plane double probes (SDP) (Torbert et al., Space Sci. Rev.,
2014, this issue
, Lindqvist et al., Space Sci. Rev.,
2014, this issue
). Two cylindrical sensors are separated by over 30 m tip-to-tip, the longest baseline on an axial DC electric field ever attempted in space. The ADP on each of the spacecraft consists of two identical, 12.67 m graphite coilable booms with second, smaller 2.25 m booms mounted on their ends. A significant effort was carried out to assure that the potential field of the MMS spacecraft acts equally on the two sensors and that photo- and secondary electron currents do not vary over the spacecraft spin. The ADP on MMS is expected to measure DC electric field with a precision of ∼1 mV/m, a resolution of ∼25 μV/m, and a range of ∼±1 V/m in most of the plasma environments MMS will encounter. The Digital Signal Processing (DSP) units on the MMS spacecraft are designed to perform analog conditioning, analog-to-digital (A/D) conversion, and digital processing on the ADP, SDP, and search coil magnetometer (SCM) (Le Contel et al., Space Sci. Rev.,
2014, this issue
) signals. The DSP units include digital filters, spectral processing, a high-speed burst memory, a solitary structure detector, and data compression. The DSP uses precision analog processing with, in most cases, >100 dB in dynamic range, better that −80 dB common mode rejection in electric field (
E
) signal processing, and better that −80 dB cross talk between the
E
and SCM (
B
) signals. The A/D conversion is at 16 bits with ∼1/4 LSB accuracy and ∼1 LSB noise. The digital signal processing is powerful and highly flexible allowing for maximum scientific return under a limited telemetry volume. The ADP and DSP are described in this article.
Bursty bulk flow (BBF) events, frequently observed in the magnetotail, carry significant energy and mass from the tail region at distances that are often greater than 20 RE into the near‐Earth plasma ...sheet at ∼10 RE where the flow is slowed and/or diverted. This region at ∼10 RE is referred to as the BBF braking region. A number of possible channels are available for the transfer or dissipation of energy in BBF events including adiabatic heating of particles, the propagation of Alfvén waves out of the BBF braking region and into the auroral region, diverted flow out of the braking region, and energy dissipation within the braking region itself. This study investigates the generation of intense high‐frequency electric field activity observed within the braking region. When present, these intense electric fields have power above the ion cyclotron frequency and almost always contain nonlinear structures such as electron phase space holes and double layers, which are often associated with field‐aligned currents. A hypothesis in which the observed high‐frequency electric field activity is generated by field‐aligned currents resulting from turbulence in the BBF braking region is considered. Although linear Alfvén waves can generate field‐aligned currents, based on theoretical calculations, the required currents are likely not the result of linear waves. Observations from the Time History of Events and Macroscale Interactions during Substorms satellites support the picture of a turbulent plasma leading to the generation of nonlinear kinetic structures. This work provides a possible mechanism for energy dissipation in turbulent plasmas.
Key Points
Strong electric field activity generated by intense field‐aligned currents
Intense currents generated by turbulence in bursty bulk flow braking region
This work provides mechanism for energy dissipation in turbulent plasmas
We aimed to identify recipient, donor and transplant risk factors associated with graft failure and patient mortality following donation after cardiac death (DCD) liver transplantation. These ...estimates were derived from Scientific Registry of Transplant Recipients data from all US liver‐only DCD recipients between September 1, 2001 and April 30, 2009 (n = 1567) and Cox regression techniques. Three years post‐DCD liver transplant, 64.9% of recipients were alive with functioning grafts, 13.6% required retransplant and 21.6% died. Significant recipient factors predictive of graft failure included: age ≥ 55 years, male sex, African–American race, HCV positivity, metabolic liver disorder, transplant MELD ≥ 35, hospitalization at transplant and the need for life support at transplant (all, p ≤ 0.05). Donor characteristics included age ≥ 50 years and weight >100 kg (all, p ≤ 0.005). Each hour increase in cold ischemia time (CIT) was associated with 6% higher graft failure rate (HR 1.06, p < 0.001). Donor warm ischemia time ≥ 35 min significantly increased graft failure rates (HR 1.84, p = 0.002). Recipient predictors of mortality were age ≥ 55 years, hospitalization at transplant and retransplantation (all, p ≤ 0.006). Donor weight >100 kg and CIT also increased patient mortality (all, p ≤ 0.035). These findings are useful for transplant surgeons creating DCD liver acceptance protocols.
Several recipient, donor, and transplant‐related factors are associated with increased graft failure and mortality risk following DCD liver transplant, but this study found that donor warm ischemia time greater than 35 minutes was particularly noteworthy.
We report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric ...fields (E) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations (δB) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the |δE|/|δB| ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m2. This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m2, which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.
Key Points
Observations of large‐amplitude electric fields associated with BBF events
Intense currents of kinetic Alfvén waves generate large‐amplitude E fields
Alfvénic Poynting flux from the BBF braking region as energy source for aurora
We report observations of turbulent dissipation and particle acceleration from large‐amplitude electric fields (E) associated with strong magnetic field (B) fluctuations in the Earth's plasma sheet. ...The turbulence occurs in a region of depleted density with anti‐earthward flows followed by earthward flows suggesting ongoing magnetic reconnection. In the turbulent region, ions and electrons have a significant increase in energy, occasionally >100 keV, and strong variation. There are numerous occurrences of |E| >100 mV/m including occurrences of large potentials (>1 kV) parallel to B and occurrences with extraordinarily large J · E (J is current density). In this event, we find that the perpendicular contribution of J · E with frequencies near or below the ion cyclotron frequency (fci) provide the majority net positive J · E. Large‐amplitude parallel E events with frequencies above fci to several times the lower hybrid frequency provide significant dissipation and can result in energetic electron acceleration.
Plain Language Summary
The Magnetospheric Multiscale mission is able to examine dissipation associated with magnetic reconnection with unprecedented accuracy and frequency response. The observations show that roughly 80% of the dissipation is from the perpendicular currents and electric fields. However, large‐amplitude parallel electric fields appear to play a strong role in turbulent dissipation into electrons and in electron acceleration.
Key Points
MMS observations reveal characteristics of turbulent dissipation and particle acceleration associated with magnetic reconnection
Perpendicular electric fields and large‐amplitude parallel electric fields structures have dominant roles in turbulent dissipation
Turbulent electric fields in a magnetic structure is shown to play a key role in accelerating electrons to greater than 100 keV energies
Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many ...astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA's Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth's magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.
Switchbacks (rotations of the magnetic field) are observed on the Parker Solar Probe. Their evolution, content, and plasma effects are studied in this paper. The solar wind does not receive a net ...acceleration from switchbacks that it encountered upstream of the observation point. The typical switchback rotation angle increased with radial distance. Significant Poynting fluxes existed inside, but not outside, switchbacks, and the dependence of the Poynting flux amplitude on the switchback radial location and rotation angle is explained quantitatively as being proportional to (B sin(θ))2. The solar wind flow inside switchbacks was faster than that outside due to the frozen-in ions moving with the magnetic structure at the Alfvén speed. This energy gain results from the divergence of the Poynting flux from outside to inside the switchback, which produces a loss of electromagnetic energy on switchback entry and recovery of that energy on exit, with the lost energy appearing in the plasma flow. Switchbacks contain 0.3-10 Hz waves that may result from currents and the Kelvin-Helmholtz instability that occurs at the switchback boundaries. These waves may combine with lower frequency magnetohydrodynamic waves to heat the plasma.
The Magnetospheric Multiscale Mission observes, in detail, charged particle heating and substantial nonthermal acceleration in a region of strong turbulence ( , where is the magnetic field) that ...surrounds a magnetic reconnection X-line. Magnetic reconnection enables magnetic field annihilation in a volume that far exceeds that of the diffusion region. The formidable magnetic field annihilation breaks into strong, intermittent turbulence with magnetic field energy as the driver. The strong, intermittent turbulence appears to generate the necessary conditions for nonthermal acceleration. It creates intense, localized currents ( ) and unusually large-amplitude electric fields ( ). The combination of turbulence-generated and results in a significant net positive mean of , which signifies particle energization. Ion and electron heating rates are such that they experience a fourfold increase from their initial temperature. Importantly, the strong turbulence also generates magnetic holes or depletions in that can trap particles. Trapping considerably increases the dwell time of a subset of particles in the turbulent region, which results in significant nonthermal particle acceleration. The direct observation of strong turbulence that is enabled by magnetic reconnection with nonthermal particle acceleration has far-reaching implications, since turbulence in plasmas is pervasive and may occupy significant volumes of the interstellar medium and intergalactic space. For example, strong turbulence from magnetic field annihilation in the supernova nebulae may dominate large volumes. As such, this observed energization process could plausibly contribute to the supply and development of the cosmic-ray spectrum.
Context.
The first studies with Parker Solar Probe (PSP) data have made significant progress toward understanding of the fundamental properties of ion cyclotron waves in the inner heliosphere. The ...survey mode particle measurements of PSP, however, did not make it possible to measure the coupling between electromagnetic fields and particles on the time scale of the wave periods.
Aims.
We present a novel approach to study wave-particle energy exchange with PSP.
Methods.
We used the Flux Angle operation mode of the Solar Probe Cup in conjunction with the electric field measurements and present a case study when the Flux Angle mode measured the direct interaction of the proton velocity distribution with an ion cyclotron wave.
Results.
Our results suggest that the energy transfer from fields to particles on the timescale of a cyclotron period is equal to approximately 3–6% of the electromagnetic energy flux. This rate is consistent with the hypothesis that the ion cyclotron wave was locally generated in the solar wind.
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