E. J. SMITH ,let Propulsion Laboratory, California Institute of Technology, PasadenaNormally the •> 80-eV electrons which carry the solar wind electron heat flux are collimated along the interplanetary magnetic field (IMF) in the direction pointing outward away from the sun. Occasionally, however, collimated fluxes of •> 80-eV electrons are observed traveling both parallel and antiparallel to the IMF. Here we present the results of a survey of such bidirectional electron heat flux events as observed with the plasma and magnetic field experiments aboard ISEE 3 at times when the spacecraft was not magnetically connected to the earth's bow shock. The onset of a bidirectional electron heat flux at ISEE 3 usually signals spacecraft entry into a distinct solar wind plasma and field entity, most often characterized by anomalously low proton and electron temperatures, a strong, smoothly varying magnetic field, a low plasma beta, and a high total pressure. Significant field rotations often occur at the beginning and/or end of bidirectional heat flux events, and, at times, the large field rotations characteristic of "magnetic clouds" are present. Approximately half of all bidirectional heat flux events are associated with and follow interplanetary shocks, while the other events have no obvious shock associations. When shock associated, the delay from shock passage typically is ~ 13 hours, corresponding to a radial separation of ,-•0.16 AU. Independent of any shock association, bidirectional heat flux events typically are ~0.13 AU thick in the radial direction, although considerable variability is evident from one event to another. Near solar activity maximum, bidirectional heat flux events occurred at a rate of •3 per month, and the solar wind electron heat flux was bidirectional ~ 5% of the time. Bidirectional heat flux events often contain strong out-of-the-ecliptic field components and thus can be effective in producing geomagnetic disturbances. This is particularly true for shock-associated events where the intrinsically strong fields in the leading portions of the events are amplified by compression in transit from the sun and where strong out-of-the-ecliptic field components resulting from compression and draping of the ambient field are often present within the shocked plasma immediately ahead. Consistent with previous work we interpret the bidirectional heat flux as evidence for a closed field topology in interplanetary space. Further, we suggest that these events are one of the more prominent signatures of coronal mass ejection events in the solar wind at 1 AU.On occasion it is observed that solar wind electrons with energies •<80eV are collimated both parallel and antiparallel to the IMF. At such times the phase space densities of •> 80-eV electrons traveling in opposite directions along the field are roughly comparable, though not necessarily equal.

The formation of the slow solar wind has been debated for many years. In this Letter we show evidence of persistent outflow at the edges of an active region as measured by the EUV Imaging Spectrometer on board Hinode. The Doppler velocity ranged between 20 and 50 km s Ϫ1 and was consistent with a steady flow seen in the X-Ray Telescope. The latter showed steady, pulsing outflowing material and some transverse motions of the loops. We analyze the magnetic field around the active region and produce a coronal magnetic field model. We determine from the latter that the outflow speeds adjusted for line-of-sight effects can reach over 100 km s Ϫ1 . We can interpret this outflow as expansion of loops that lie over the active region, which may either reconnect with neighboring large-scale loops or are likely to open to the interplanetary space. This material constitutes at least part of the slow solar wind.

Hinode's EUV Imaging Spectrometer (EIS) has discovered ubiquitous outflows of a few to 50 km s −1 from active regions (ARs). These outflows are most prominent at the AR boundary and appear over monopolar magnetic areas. They are linked to strong non-thermal line broadening and are stronger in hotter EUV lines. The outflows persist for at least several days. Using Hinode EIS and X-Ray Telescope observations of AR 10942 coupled with magnetic modeling, we demonstrate that the outflows originate from specific locations of the magnetic topology where field lines display strong gradients of magnetic connectivity, namely quasi-separatrix layers (QSLs), or in the limit of infinitely thin QSLs, separatrices. We found the strongest AR outflows to be in the vicinity of QSL sections located over areas of strong magnetic field. We argue that magnetic reconnection at QSLs separating closed field lines of the AR and either large-scale externally connected or 'open' field lines is a viable mechanism for driving AR outflows which are likely sources of the slow solar wind.

Context. Observations and simulations show that reconnection will take place when a flux tube emerges into a coronal hole, which is characterised by magnetic fieldlines "open" towards interplanetary space. Although the mechanism by which reconnection is initiated has been thoroughly studied, the long-term evolution of this reconnecting magnetic system remains unreported. Aims. We aim to understand the long-term evolution of the reconnecting flux tube and coronal hole system and, in particular, to ascertain whether it can reach an equilibrium state in which all reconnection has ceased. By determining the evolution in this particular scenario, we aim to be able to select a subset from the broad spectrum of reconnecting systems, which will undergo the same progression to equilibrium. Methods. Using a 2.5-dimensional numerical magnetohydrodynamic (MHD) code, we evolve a simple stratified atmospheric domain, which is endowed with a vertical magnetic field, representing the interior of a coronal hole, and a horizontal buoyant flux tube that is placed near the bottom of the domain. To investigate the long-term evolution of the system, we continue to study the domain long after the flux tube has emerged and reconnection has commenced between the magnetic fields of the flux tube and coronal hole. Results. We find that a series of reconnection reversals (or oscillatory reconnection) takes place, whereby reconnection occurs in distinct bursts and the inflow and outflow magnetic fields of one burst of reconnection become the outflow and inflow fields in the following burst of reconnection, respectively. During each burst of reconnection the gas pressure in the bounded outflow regions increases above the level of that in the inflow regions and, eventually, gives rise to a reconnection reversal. In consecutive bursts of reconnection, the contrast in the gas pressure across the boundaries of the inflow and outflow regions decreases and, over time, the system settles towards equilibrium. Once the equilibrium state is reached, all reconnection ceases. This is the first reported instance of oscillatory reconnection initiated in a self-consistent manner, and the signatures of the mechanism compare favourably with observations of select flux emergence events and with solar and stellar flares. Conclusions. Across the broader spectrum of reconnecting systems, oscillatory reconnection will only occur if the outflow regions are quasi-bounded during each burst of reconnection. The swaying outflow jet and periodic heating signatures of oscillatory reconnection are exceedingly similar to those exhibited by MHD modes and, in many observations, distinction between the two mechanisms may be impossible.

Context. The Extreme Ultraviolet Imager (EUI) is part of the remote sensing instrument package of the ESA/NASA Solar Orbiter mission that will explore the inner heliosphere and observe the Sun from vantage points close to the Sun and out of the ecliptic. Solar Orbiter will advance the “connection science” between solar activity and the heliosphere. Aims. With EUI we aim to improve our understanding of the structure and dynamics of the solar atmosphere, globally as well as at high resolution, and from high solar latitude perspectives. Methods. The EUI consists of three telescopes, the Full Sun Imager and two High Resolution Imagers, which are optimised to image in Lyman-α and EUV (17.4 nm, 30.4 nm) to provide a coverage from chromosphere up to corona. The EUI is designed to cope with the strong constraints imposed by the Solar Orbiter mission characteristics. Limited telemetry availability is compensated by state-of-the-art image compression, onboard image processing, and event selection. The imposed power limitations and potentially harsh radiation environment necessitate the use of novel CMOS sensors. As the unobstructed field of view of the telescopes needs to protrude through the spacecraft’s heat shield, the apertures have been kept as small as possible, without compromising optical performance. This led to a systematic effort to optimise the throughput of every optical element and the reduction of noise levels in the sensor. Results. In this paper we review the design of the two elements of the EUI instrument: the Optical Bench System and the Common Electronic Box. Particular attention is also given to the onboard software, the intended operations, the ground software, and the foreseen data products. Conclusions. The EUI will bring unique science opportunities thanks to its specific design, its viewpoint, and to the planned synergies with the other Solar Orbiter instruments. In particular, we highlight science opportunities brought by the out-of-ecliptic vantage point of the solar poles, the high-resolution imaging of the high chromosphere and corona, and the connection to the outer corona as observed by coronagraphs.