Reconnection Exhausts In Solar Wind Research Articles

The Nonrelaxation of Magnetic Field Lines in Solar Wind Magnetic Reconnection Exhausts

Abstract We present a case study using data from multiple spacecraft and statistics from the ACE spacecraft to investigate the relaxation of reconnected magnetic field lines in solar wind magnetic reconnection exhausts, which constitute one of the two outflow regions resulting from magnetic reconnection, and which are often characterized by a plasma jet, magnetic strength depletion, and density and temperature enhancements. The normal magnetic fractions (∣B n /B∣) of the reconnection exhausts are used to indicate the degrees of relaxation of the magnetic field lines. 97 out of 98 reconnection exhausts have a relatively small ∣B n /B∣ (<0.3), while only one exhaust associated with a relatively small interplanetary coronal mass ejection has a large ∣B n /B∣ of 0.85. This result demonstrates the nonrelaxed nature of reconnected magnetic field lines in solar wind, due to the open boundary condition. As a consequence, the magnetic tension of such nonrelaxed magnetic field lines can accelerate the local plasma to produce the observed jet within the reconnection exhaust. Our results support the understanding that most solar wind reconnection exhausts are probably initiated near the Sun, where the plasma beta is low, and that they are not ongoing when they are observed at 1 au, because of the solar wind expansion from near the Sun to 1 au.

Bifurcated outflow jet in a solar wind reconnection exhaust

We present new observations of the reconnection outflow jet obtained from the crossing of a solar wind reconnection exhaust by the Wind spacecraft on 5 March 2001. The outflow jet is characterized by a bifurcated configuration, which is different from typical reconnection exhausts. The two outflow jets are mainly distributed in the region close to the exhaust boundaries. Between the two jets, the bulging of the magnetic field and the slight depression of the plasma density are observed. The possible mechanism for the bifurcation of the outflow jet has also been investigated by using a magnetohydrodynamics (MHD) simulation of Petschek-type reconnection. The dynamic patterns of the reconnection exhaust are analyzed in detail. We find that the bifurcated jet is one of the transient configurations produced in the magnetic reconnection. Other observed features of the exhaust show similar agreement with the predictions from the simulation. The study of the reconnection exhaust, over multiple spatial scales, provides an insight into the dynamic evolution of solar wind reconnection exhausts.

Solar Wind Reconnection Exhausts in the Inner Heliosphere Observed by Helios and Detected via Machine Learning

Abstract Reconnecting current sheets in the solar wind play an important role in the dynamics of the heliosphere and offer an opportunity to study magnetic reconnection exhausts under a wide variety of inflow and magnetic shear conditions. However, progress in understanding reconnection can be frustrated by the difficulty of finding events in long time-series data. Here we describe a new method to detect magnetic reconnection events in the solar wind based on machine learning, and apply it to Helios data in the inner heliosphere. The method searches for known solar wind reconnection exhaust features, and parameters in the algorithm are optimized to maximize the Matthews Correlation Coefficient using a training set of events and non-events. Applied to the whole Helios data set, the trained algorithm generated a candidate set of events that were subsequently verified by hand, resulting in a database of 88 events. This approach offers a significant reduction in construction time for event databases compared to purely manual approaches. The database contains events covering a range of heliospheric distances from ∼0.3 to ∼1 au, and a wide variety of magnetic shear angles, but is limited by the relatively coarse time resolution of the Helios data. Analysis of these events suggests that proton heating by reconnection in the inner heliosphere depends on the available magnetic energy in a manner consistent with observations in other regimes such as at the Earth’s magnetopause, suggesting this may be a universal feature of reconnection.

Statistical properties of solar wind reconnection exhausts

AbstractThe solar wind provides an excellent opportunity to study the exhausts that form as a result of symmetric guide field reconnection, where spacecraft rapidly cross the exhausts far downstream of the X line. We study the statistical properties of solar wind exhausts through a superposed epoch analysis of 188 events observed at 1 AU using the Wind spacecraft. These events span a range of guide fields of 0 to 10 times the reconnecting magnetic field and inflow region plasma beta of 0.1 to 6.6. This analysis reveals that the out‐of‐plane magnetic field is enhanced within solar wind exhausts. Furthermore, the amount by which the plasma density and ion temperature increase from inflow region to exhaust region is found to be a function of the inflow region plasma beta and reconnection guide field, which explains the lack of these enhancements in a subset of previous observations. This dependence is consistent with the scaling of ion heating with inflow region Alfven speed, which is measured to be consistent with previous observations in the solar wind and at the magnetopause.

Observations of Hall Reconnection Physics Far Downstream of the X Line.

Observations made using the Wind spacecraft of Hall magnetic fields in solar wind reconnection exhausts are presented. These observations are consistent with the generation of Hall fields by a narrow ion inertial scale current layer near the separatrix, which is confirmed with an appropriately scaled particle-in-cell simulation that shows excellent agreement with observations. The Hall fields are observed thousands of ion inertial lengths downstream from the reconnection X line, indicating that narrow regions of kinetic dynamics can persist extremely far downstream.

Development of bifurcated current sheets in solar wind reconnection exhausts

AbstractPetschek‐type reconnection is expected to result in bifurcations of reconnection current sheets. In contrast, Hall reconnection simulations show smooth changes in the reconnecting magnetic field. Here we study three solar wind reconnection events where different spacecraft sample oppositely directed reconnection exhausts from a common reconnection site. The spacecraft's relative separations and measurements of the exhaust width are used to geometrically calculate each spacecraft's distance from the X line. We find that in all cases spacecraft farthest from the X line observe clearly bifurcated reconnection current sheets, while spacecraft nearer to the X line do not. These observations suggest that clear bifurcations of reconnection current sheets occur at large distances from the X line (~1000 ion skin depths) and that Petschek‐type signatures are less developed close to the reconnection site. This may imply that fully developed bifurcations of reconnection current sheets are unlikely to be observed in the near‐Earth magnetotail.

A statistical survey of reconnection exhausts in the solar wind based on the Riemannian decay of current sheets

AbstractWe present a statistical study of the magnetic reconnection exhausts in solar wind. Observational data are compared with the analytical model based on the Riemann analysis of tangential discontinuity decay forced by finite X‐line reconnection of skewed magnetic fields. Statistical analysis is based on 51 events of the solar wind reconnection listed in Phan et al. (2009). The best agreement of the observed and analytically predicted values is achieved for the rotational angle of the tangential magnetic field component with correlation coefficient reaching the value of 0.97. The lowest correlation coefficient of 0.87 is obtained for the exhaust flow plasma temperature. It is found that proton temperature increases at the exhaust boundary while electron temperature stays unchanged. This may indicate that heating and acceleration processes operate on the proton scale. Exhaust boundaries are identified as tangential discontinuities, except one particular event, where Alfvén discontinuity and slow shock were detected instead. Hence, the impulsive reconnection may be supposed in that case rather than steady state one. Exhaust regions extending up to 690RE, registered in some observations, do not necessarily imply X‐lines of similar length. They could be explained alternatively by reconnection of skewed magnetic fields. The numerical modeling of the interplanetary coronal mass ejection (ICME) propagating in the solar wind reveals that the resistance force, impeding the ICME motion, may be reduced significantly (three times in our simulations) by means of the magnetic reconnection at the leading edge. Thus, reconnection may substantially increase ICME velocity and travel distance.

ON MULTIPLE RECONNECTIONX-LINES AND TRIPOLAR PERTURBATIONS OF STRONG GUIDE MAGNETIC FIELDS

We report new multi-spacecraft Cluster observations of tripolar guide magnetic field perturbations at a solar wind reconnection exhaust in the presence of a guide field B-M. which is almost four ti ...

Direct evidence for kinetic effects associated with solar wind reconnection.

Kinetic effects resulting from the two-fluid physics play a crucial role in the fast collisionless reconnection, which is a process to explosively release massive energy stored in magnetic fields in space and astrophysical plasmas. In-situ observations in the Earth's magnetosphere provide solid consistence with theoretical models on the point that kinetic effects are required in the collisionless reconnection. However, all the observations associated with solar wind reconnection have been analyzed in the context of magnetohydrodynamics (MHD) although a lot of solar wind reconnection exhausts have been reported. Because of the absence of kinetic effects and substantial heating, whether the reconnections are still ongoing when they are detected in the solar wind remains unknown. Here, by dual-spacecraft observations, we report a solar wind reconnection with clear Hall magnetic fields. Its corresponding Alfvenic electron outflow jet, derived from the decouple between ions and electrons, is identified, showing direct evidence for kinetic effects that dominate the collisionless reconnection. The turbulence associated with the exhaust is a kind of background solar wind turbulence, implying that the reconnection generated turbulence has not much developed.

Detection of small‐scale folds at a solar wind reconnection exhaust

AbstractObservations of reconnection in the solar wind over the last few years appear to indicate that the majority of large‐scale reconnecting current sheets are roughly planar, and that reconnection itself is quasi‐steady. Most studies of solar wind exhausts have used spacecraft with large separations and relatively low time cadence ion measurements. Here we present multipoint Cluster observations of a reconnection exhaust and the associated current sheet at ACE and Wind, enabling it to be studied on multiple length scales and at high time resolution. While analysis shows that on large scales the current sheet is planar, detailed measurements using the four closely spaced Cluster spacecraft show that the trailing edge of the reconnection jet is nonplanar with folds orthogonal to the reconnection plane, with length scales of approximately 230 ion inertial lengths. Our findings thus suggest that while solar wind current sheets undergoing reconnection may be planar on large scales, they may also exhibit complex smaller‐scale structure. Such structure is difficult to observe and has rarely been detected because exhausts are rapidly convected past the spacecraft in a single cut; there is therefore a limited set of spacecraft trajectories through the exhaust which would allow the nonplanar features to be intercepted. We consider how such nonplanar reconnection current sheets can form and the processes which may have generated the 3‐D structure that was observed.

CORE ELECTRON HEATING IN SOLAR WIND RECONNECTION EXHAUSTS

We present observational evidence of core electron heating in solar wind reconnection exhausts. We show two example events, one which shows clear heating of the core electrons within the exhaust, and one which demonstrates no heating. The event with heating occurred during a period of high inflow Alfven speed (V AL ), while the event with no heating had a low V AL . This agrees with the results of a recent study of magnetopause exhausts, and suggests that similar core electron heating can occur in both symmetric (solar wind) and asymmetric (magnetopause) exhausts.

On the signatures of magnetic islands and multiple X-lines in the solar wind as observed by ARTEMIS and WIND

We report the first observation consistent with a magnetic reconnection generated magnetic island at a solar wind current sheet that was observed on 10 June 2012 by the two ARTEMIS satellites and the upstream WIND satellite. The evidence consists of a core magnetic field within the island which is formed by enhanced Hall magnetic fields across a solar wind reconnection exhaust. The core field at ARTEMIS displays a local dip coincident with a peak plasma density enhancement and a locally slower exhaust speed which differentiates it from a regular solar wind exhaust crossing. Further indirect evidence of magnetic island formation is presented in the form of a tripolar Hall magnetic field, which is supported by an observed electron velocity shear, and plasma density depletion regions which are in general agreement with multiple reconnection X-line signatures at the same current sheet on the basis of predicted signatures of magnetic islands as generated by a kinetic reconnection simulation for solar wind-like conditions. The combined ARTEMIS and WIND observations of tripolar Hall magnetic fields across the same exhaust and Grad–Shrafranov reconstructions of the magnetic field suggest that an elongated magnetic island was encountered which displayed a 4RE normal width and a 43RE extent along the exhaust between two neighboring X-lines.

Kelvin‐Helmholtz stability of reconnection exhausts in the solar wind

Three solar wind reconnection exhaust events observed by the Wind satellite are compared to the analytical solution of the Riemannian decay of a current sheet due to reconnection of skewed magnetic fields. This process leads to high speed flows inside an exhaust region which is bounded not only by Alfvénic and slow mode waves but also, to a large extent, by tangential discontinuities (TDs). The TD portions of the exhaust boundary expand with distance from the X‐line and therefore a long exhaust does not necessarily imply a long X‐line. In some cases, portions of the exhaust show oscillations which might be connected to a Kelvin‐Helmholtz instability in agreement with analytical estimates.

Triggering of magnetic reconnection in a magnetosheath current sheet due to compression against the magnetopause

[1] We report in-situ measurements by three THEMIS spacecraft showing the evolution of reconnection in a solar wind current sheet as the current sheet transited from the solar wind across the bow shock and close to the magnetopause on July 11, 2008. The observations suggest that the solar wind reconnection exhaust within the current sheet was disrupted by its interaction with the bow shock, while the subsequent compression of the current sheet against the magnetopause significantly reduced both the current sheet thickness and the plasma β and initiated reconnection at a new X-line located within the magnetosheath. Furthermore, electrons were heated at the center of the magnetosheath exhaust, in contrast to the previously reported absence of electron heating in solar wind exhausts, but consistent with electron heating occasionally observed in association with magnetopause reconnection. This suggests that the level of electron heating in reconnection exhausts depends strongly on the boundary conditions.

Asymmetric shear flow effects on magnetic field configuration within oppositely directed solar wind reconnection exhausts

We present observations of the nearly oppositely directed solar wind reconnection jets from the same reconnection X line that swept by the Wind and STEREO‐A spacecraft on 11 March 2007. The event was characterized by an external plasma density asymmetry and a jet‐aligned shear flow across the exhausts. A well‐defined pair of asymmetric current sheets bounded both outflow regions. The observed current sheet asymmetries and differences in the magnitude of the antiparallel magnetic field components within the two exhausts were consistent with the sense of the shear flow and the predictions from two‐dimensional MHD simulations of reconnection outflows for the Earth's asymmetric flank magnetopause.

Observation of a Complex Solar Wind Reconnection Exhaust from Spacecraft Separated by over 1800 R E

We analyze Wind, ACE, and STEREO (ST-A and ST-B) plasma and magnetic field data in the vicinity of the heliospheric current sheet (HCS) crossed by all spacecraft between 22:15 UT on 31 March and 01:25 UT on 1 April 2007 corresponding to its observation at ST-A and ST-B, which were separated by over 1800 R E (or over 1200 R E across the Sun – Earth line). Although only Wind and ACE provided good ion flow data in accord with a solar wind magnetic reconnection exhaust at the HCS, the magnetic field bifurcation typical of such exhausts was clearly observed at all spacecraft. They also all observed unambiguous strahl mixing within the exhaust, consistent with the sunward flow deflection observed at Wind and ACE and thus with the formation of closed magnetic field lines within the exhaust with both ends attached to the Sun. The strong dawnward flow deflection in the exhaust is consistent with the exhaust and X-line orientations obtained from minimum variance analysis at each spacecraft so that the X-line is almost along the GSE Z-axis and duskward of all the spacecraft. The observation of strahl mixing in extended and intermittent layers outside the exhaust by ST-A and ST-B is consistent with the formation of electron separatrix layers surrounding the exhaust. This event also provides further evidence that balanced parallel and antiparallel suprathermal electron fluxes are not a necessary condition for identification of closed field lines in the solar wind. In the present case the origin of the imbalance simply is the mixing of strahls of substantially different strengths from a different solar source each side of the HCS. The inferred exhaust orientations and distances of each spacecraft relative to the X-line show that the exhaust was likely nonplanar, following the Parker spiral orientation. Finally, the separatrix layers and exhausts properties at each spacecraft suggest that the magnetic reconnection X-line location and/or reconnection rate were variable in both space and time at such large scales.

Interpretation of solar wind reconnection exhaust in terms of kinetic Alfvén wave group‐velocity cones

Recent multi‐satellite measurements of reconnection exhausts in the solar wind have allowed for the first time to determine the reconnection rate (R) and the exhaust wedge angle 2θw. We compare such observations of R and θw with the theoretical predictions based on the half‐cone angles (θg) of the group velocity vectors (Vg) of the kinetic Alfvén (KA) and slow MHD (SMHD) wave modes. We find that for these recently observed reconnection events in the solar wind the exhaust wedge angle and the reconnection rate are determined by the KA waves and not by the SMHD wave mode. Our study suggests a theoretical basis for determining the reconnection rate based on group‐velocity cone angle of certain wave modes depending on the MHD or kinetic reconnection regimes.

Wind/WAVES observations of high‐frequency plasma waves in solar wind reconnection exhausts

This paper studies high‐frequency plasma waves during 28 encounters of the Wind spacecraft with solar wind reconnection exhausts. We use measurements by the Thermal Noise Receiver (TNR) and Time Domain Sampler (TDS) experiments on Wind/WAVES to survey characteristics of electron plasma waves and the most common regions where they are found. TNR spectrograms showed intense emission bursts ∼4 kHz (corresponding to the ion acoustic range) during 79% of the events while around the local electron plasma frequency intense emission bursts were recorded for 39% of the events. The TDS instrument, operating at 120,000 samples per second, detected three kinds of electric waveforms: Langmuir waves, electron solitary waves (ESW), and wave packets with frequencies between ion and electron plasma frequencies typically interpreted as Doppler‐shifted ion acoustic waves. Except for one very intense ESW event, the average amplitudes for all these three waveform types were similar, below 1 mV/m. We found an increased probability to observe intense plasma wave activity when the reconnection X‐line region was approached. Wave activity may be present anywhere in the reconnection exhaust, although the boundary region of the exhaust appears to be the most dynamic region for wave activity. In particular, the majority of the captured ESW occurred near the exhaust boundaries.