J. T. Gosling

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.

Previous work has shown that solar wind suprathermal electrons can display a number of features in terms of their anisotropy. Of importance is the occurrence of counter-streaming electron patterns, i.e., with "beams" both parallel and anti-parallel to the local magnetic field, which is believed to shed light on the heliospheric magnetic field topology. In the present study, we use STEREO data to obtain the statistical properties of counter-streaming suprathermal electrons (CSEs) in the vicinity of corotating interaction regions (CIRs) during the period March–December 2007. Because this period corresponds to a minimum of solar activity, the results are unrelated to the sampling of large-scale coronal mass ejections, which can lead to CSE owing to their closed magnetic field topology. The present study statistically confirms that CSEs are primarily the result of suprathermal electron leakage from the compressed CIR into the upstream regions with the combined occurrence of halo depletion at 90° pitch angle. The occurrence rate of CSE is found to be about 15–20% on average during the period analyzed (depending on the criteria used), but superposed epoch analysis demonstrates that CSEs are preferentially observed both before and after the passage of the stream interface (with peak occurrence rate >35% in the trailing high speed stream), as well as both inside and outside CIRs. The results quantitatively show that CSEs are common in the solar wind during solar minimum, but yet they suggest that such distributions would be much more common if pitch angle scattering were absent. We further argue that (1) the formation of shocks contributes to the occurrence of enhanced counter-streaming sunward-directed fluxes, but does not appear to be a necessary condition, and (2) that the presence of small-scale transients with closed-field topologies likely also contributes to the occurrence of counter-streaming patterns, but only in the slow solar wind prior to CIRs.

Using Advanced Composition Explorer (ACE) 64 s data at 1 AU we find that Alfvénic fluctuations propagating outward from the Sun along the magnetic field, B , in the solar wind often produce one-sided variations in one of the equatorial components of B and velocity, V . This is a natural consequence of the fact that the Alfvénic fluctuations are transverse fluctuations in which |B| remains nearly constant. Thus, fluctuations in the field component that defines the underlying background field direction are always relative to a base value rather than to an average value. This suggests that conclusions derived from statistical analyses of fluctuations in the solar wind that assume the fluctuations in all field components are relative to average values need to be re-examined. We also find that discrete, sunward-propagating Alfvénic fluctuations or rotational discontinuities are extremely rare in the pristine solar wind; thus far we have identified such discrete events in ACE data only in association with events identified as magnetic reconnection exhausts and/or in association with backstreaming ions from reverse shocks, including Earth's bow shock.

We report observations from the Wind spacecraft of Petschek‐like magnetic reconnection exhausts and thin current sheets in the solar wind on 19 and 20 November 2007, encompassing a solar wind disturbance driven by a magnetic cloud and followed by a corotating high‐speed stream. We have identified an unusually large number (11) of reconnection exhausts in this 2‐day interval using 3‐s plasma and magnetic field data. Despite the relatively smooth large‐scale field rotation associated with the magnetic cloud, five of the exhausts occurred within the cloud; three of those exhausts were associated with extremely small (<18°) local field shear angles. All 11 exhausts contained double‐step magnetic field rotations; such double‐step rotations are called bifurcated current sheets since they result from the splitting of reconnecting current sheets as an after‐effect of the reconnection process. We have also identified 27 current sheets in this 2‐day interval that were too thin to be adequately resolved by the 3‐s plasma measurement cadence. All of these thin current sheets were well resolved by the 92 ms magnetic field measurement. At least three, and possibly six, of these relatively thin current sheets had double‐step magnetic field rotations, indicating the underlying current sheets had probably been disrupted by magnetic reconnection. Current sheets thinner than ∼3 ion inertial lengths were not present in this data set. The relative lack of such ultrathin current sheets in the solar wind in general suggests that such current sheets usually are quickly disrupted by magnetic reconnection.

Magnetic reconnection plays a central role in the interpretation of a wide variety of observed solar, space, astrophysical, and laboratory plasma phenomena. The relatively recent discovery that reconnection is common at thin current sheets in the solar wind opens up a new laboratory for studying this fundamental plasma process and its after-effects. Here we provide a brief overview of some of the new insights on reconnection derived from observations of reconnection exhaust jets in the solar wind.

Observations of oppositely directed plasma jets within an extended, bifurcated current sheet in the solar wind by a flotilla of five well‐separated spacecraft (STEREO A and B, ACE, Wind and Geotail) on 11 March 2007 demonstrate that magnetic reconnection X‐lines in the solar wind can extend to distances at least as great as 4.26 × 10⁶ km (0.0284 AU) in the presence of a significant and variable guide field. The observations also indicate that reconnection in the solar wind can persist for at least 5 hours and 20 minutes. These minimum values are the largest yet obtained from direct measurements of reconnection exhaust flows in a space plasma. Both dynamic processes in the reconnection region and the spherical expansion of the solar wind probably contribute to the production of long X‐lines in the solar wind.