M. Øieroset

In situ detection of collisionless reconnection in the Earth's magnetotail

Magnetic reconnection is the process by which magnetic field lines of opposite polarity reconfigure to a lower-energy state, with the release of magnetic energy to the surroundings. Reconnection at the Earths dayside magnetopause and in the magnetotail allows the solar wind into the magnetosphere. It begins in a small ‘diffusion region’, where a kink in the newly reconnected lines produces jets of plasma away from the region. Although plasma jets from reconnection have previously been reported, the physical processes that underlie jet formation have remained poorly understood because of the scarcity of in situ observations of the minuscule diffusion region. Theoretically, both resistive and collisionless processes can initiate reconnection, but which process dominates in the magnetosphere is still debated. Here we report the serendipitous encounter of the Wind spacecraft with an active reconnection diffusion region, in which are detected key processes predicted by models of collisionless reconnection. The data therefore demonstrate that collisionless reconnection occurs in the magnetotail.

A magnetic reconnection X-line extending more than 390 Earth radii in the solar wind

Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory, space and astrophysical contexts. It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature. Frequent reports of small-scale flux ropes and flow channels associated with reconnection in the Earths magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii (RE), even though the associated current sheets span many tens or hundreds of RE. Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390RE (or 2.5 × 106 km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earths magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.

THE DEPENDENCE OF MAGNETIC RECONNECTION ON PLASMA β AND MAGNETIC SHEAR: EVIDENCE FROM SOLAR WIND OBSERVATIONS

We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, θ, across the current sheet and the difference in the plasma β values on the two sides of the current sheet, Δβ. For low Δβ, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large Δβ: Events with shears as low as 11° were observed for Δβ 1.5 only events with θ > 100° were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high β plasmas at low magnetic shears due to super-Alfvenic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-Δβ dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind.

Hot diamagnetic cavities upstream of the Martian bow shock

We present Mars Global Surveyor (MGS) observations of hot diamagnetic cavities upstream of the Martian bow shock. The events are characterized by a region of turbulent magnetic field, high electron temperature, and densities at or below the ambient solar wind density. The region of high temperature is flanked by layers of high magnetic field and electron density and is accompanied by a rotation in the interplanetary magnetic field (IMF). We suggest that the events are generated by interplanetary discontinuities interacting with the Martian bow shock, i.e., the events are the Martian counterpart to the hot flow anomalies observed upstream of the Earths bow shock.

Walén and variance analyses of high‐speed flows observed by Wind in the midtail plasma sheet: Evidence for reconnection

We have analyzed 4 days of Wind observations of high-speed convective flows of up to ∼800 km s -1 in the plasma sheet at X GSE ∼ -60 R E during late March and early April 1999. Both earthward and tailward flows were observed. The high-speed flows had a duration of several hours, unlike the shorter-lasting (tens of minutes) bursty bulk flows, which are typically observed closer to the Earth. We have analyzed in detail a ∼10 hour interval of high-speed flows detected during rather low geomagnetic activity and northward interplanetary magnetic field. Our analysis indicates that the fast flows are produced by magnetic reconnection and that the observed flow reversals are consistent with the passage of a reconnection X line. First, the results of the shear stress balance test (the Walen test) indicate that the flow in the deHoffmann-Teller frame, which is aligned with the magnetic field, has an average flow speed that is ∼60% of the Alfven speed, consistent with the presence of slow shocks in the magnetotail reconnection layer. Furthermore, the slope of the Walen regression line switches sign at the flow reversals, as expected from reconnection. Consistent with the passage of a reconnection X line, the magnetic field component normal to the neutral sheet also reverses sign at the flow reversals. For this event the tailward flowing plasma is hotter than the earthward flowing plasma, consistent with the two plasmas being magnetically disconnected. Our observations imply that quasi-steady reconnection can occur in the midmagnetotail region during periods of persistent northward interplanetary magnetic field.

Ion-scale secondary flux ropes generated by magnetopause reconnection as resolved by MMS.

Abstract New Magnetospheric Multiscale (MMS) observations of small‐scale (~7 ion inertial length radius) flux transfer events (FTEs) at the dayside magnetopause are reported. The 10 km MMS tetrahedron size enables their structure and properties to be calculated using a variety of multispacecraft techniques, allowing them to be identified as flux ropes, whose flux content is small (~22 kWb). The current density, calculated using plasma and magnetic field measurements independently, is found to be filamentary. Intercomparison of the plasma moments with electric and magnetic field measurements reveals structured non‐frozen‐in ion behavior. The data are further compared with a particle‐in‐cell simulation. It is concluded that these small‐scale flux ropes, which are not seen to be growing, represent a distinct class of FTE which is generated on the magnetopause by secondary reconnection.

Walen and slow-mode shock analyses in the near-Earth magnetotail in connection with a substorm onset on 27 August 2001

plasma density N � 0.3 cm � 3 and the plasma ion b � 1.5. The Walen analysis applied to the tailward flow interval from 0400:36 UT to 0403:14 UT is consistent with the ions being accelerated to � 73% of the Alfven speed across a slow-mode shock connected to a near-Earth neutral line located on the earthward side of the spacecraft. The occurrence of a small plasmoid-type magnetic flux rope during the leading edge of the tailward flows provides further support in favor of an active region of magnetic reconnection earthward of Cluster. The more field-aligned earthward flows between 0406 UT and 0408 UT, however, failed to satisfy the Walen test. Rankine-Hugoniot analyses of upstream and downstream plasma and magnetic field parameters confirm the presence of a slow-mode shock in connection with the passage of the tailward flow region but not with the 0406 UT to 0408 UT earthward flow interval. The confirmed shock satisfies the critical slow-mode requirements: MI * � 1.0 and MSM * > 1.0 on the upstream side and MSM * � 19 RE of the near-Earth magnetotail. INDEX TERMS: 7835 Space Plasma Physics: Magnetic reconnection; 7851 Space Plasma Physics: Shock waves; 2744 Magnetospheric Physics: Magnetotail; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: magnetic reconnection, shock waves, magnetotail boundary layers

Ion bulk heating in magnetic reconnection exhausts at Earth's magnetopause: Dependence on the inflow Alfvén speed and magnetic shear angle

We surveyed 87 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of ion bulk heating depend on the inflow boundary conditions. We find that the heating, ΔTi, is correlated with the asymmetric Alfven speed, VAL,asym, based on the reconnecting magnetic field and the plasma number density measured in both inflow regions. Best fit to the data produces the empirical relation ΔTi = 0.13 miVAL,asym2, where mi is the proton mass, indicating that the increase in the ion internal (thermal) energy, 3ΔTi/2, is 20% of the available magnetic energy per proton-electron pair. The observed parallel heating generally exceeds perpendicular heating (by a factor of ~2), and there are some indications that the heating is reduced in the presence of a strong guide field. Finally, the ratio of ion to electron bulk heating is ~8 on average.

Energetic ion outflow from the dayside ionosphere: Categorization, classification, and statistical study

The energetic ion outflow (40 eV – 1.2 keV) from the dayside ionosphere has been investigated using 2 weeks (70 orbits) of Viking ion data when the AE index showed little dependence on the interplanetary magnetic field (IMF) Bz component. The data cover altitudes between 6000 and 13,500 km, 0600–1800 magnetic local time (MLT), and 65°–90° invariant latitude (ILAT). We present an automated algorithm using principal component analysis to categorize and classify the upflowing ions into beams, conies, and hybrids. Conies dominate the ion outflow (number flux) in the cusp region, while beams are the main contributor to the outflow prenoon and postnoon outside the cusp region. Generally, the highest average number flux is found in the cusp region. The outflow intensity and the region of energetic ion outflow are different for positive and negative IMF Bz. However, this difference is not as strong as the difference in outflow intensity and active region observed for AE 200. The results imply that the dayside energetic ion outflow is controlled partly by nightside (driven by the release of stored energy in the magnetotail, as observed in the AE index) and partly by dayside (directly driven, as observed in the polarity in the IMF Bz component) activity. The effect of the directly driven component (IMF Bz) is strongest in the cusp and the postnoon region.

Influence of asymmetries and guide fields on the magnetic reconnection diffusion region in collisionless space plasmas

Collisionless magnetic reconnection is considered to be one of the most important plasma phenomena because it governs the transport of energy, momentum and plasma in a wide variety of situations. In particular, understanding the central diffusion region is crucial to gaining a full understanding of the physics of reconnection. Although most diffusion region studies have historically focussed on simple reconnection geometries (antiparallel fields and symmetric reconnecting plasmas), in recent years significant progress has been made in understanding the impact of plasma asymmetries, guide fields and flow shear on collisionless diffusion region physics. Here we present a review of this recent progress, which is based both on supercomputer simulations and increasingly detailed multi-point satellite measurements of collisionless magnetic reconnection in space plasmas. (Some figures may appear in colour only in the online journal)