T. D. Phan

Transport of solar wind into Earth's magnetosphere through rolled-up Kelvin-Helmholtz vortices.

Establishing the mechanisms by which the solar wind enters Earths magnetosphere is one of the biggest goals of magnetospheric physics, as it forms the basis of space weather phenomena such as magnetic storms and aurorae. It is generally believed that magnetic reconnection is the dominant process, especially during southward solar-wind magnetic field conditions when the solar-wind and geomagnetic fields are antiparallel at the low-latitude magnetopause. But the plasma content in the outer magnetosphere increases during northward solar-wind magnetic field conditions, contrary to expectation if reconnection is dominant. Here we show that during northward solar-wind magnetic field conditions—in the absence of active reconnection at low latitudes—there is a solar-wind transport mechanism associated with the nonlinear phase of the Kelvin–Helmholtz instability. This can supply plasma sources for various space weather phenomena.

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.

The magnetosheath region adjacent to the dayside magnetopause: AMPTE/IRM observations

We have studied 38 low-latitude, dayside (0800-1600 LT) magnetopause crossings by the AMPTE/IRM satellite to investigate the variations of key plasma parameters and the magnetic field in the magnetosheath region adjacent to the dayside magnetopause. We find that the structures of the key plasma parameters and the magnetic field and the dynamics of plasma flows in this region depend strongly on the magnetic shear across the magnetopause, that is, on the angle between the magnetosheath magnetic field and the geomagnetic field. When the magnetic shear is low ( 1. When the magnetic shear across the magnetopause is high (>60°), the near-magnetopause magnetosheath is more disturbed. The magnetic field in this case does not pile up in the immediate vicinity of the magnetopause, and no systematic variations in the plasma parameters are observed in this region until the encounter of the magnetopause current layer; that is, there is no magnetosheath transition layer. Also in contrast to the low-shear case, the mirror instability threshold is marginally satisfied throughout the magnetosheath. The plasma flow pattern in the magnetosheath region adjacent to the dayside magnetopause is also found to depend strongly on the magnetic shear across the magnetopause: the magnetosheath flow component tangential to the magnetopause is enhanced and rotates to become more perpendicular to the local magnetic field as the low-shear magnetopause is approached. This flow behavior may be consistent with the formation of a stagnation line instead of a stagnation point at the subsolar magnetopause. Enhancement and rotation of the magnetosheath flow on approach to the magnetopause are rarely observed when the magnetic shear across the magnetopause is high. In essence, our observations provide evidence for high (low) rate of transfer of magnetic flux and mass across the magnetopause when the magnetic shear is high (low). The relationships between the electron and proton temperature anisotropies and β in the near-magnetopause magnetosheath region are also examined. It is found that Te⊥/Te∥ remains close to 1 for the entire range of βe, whereas Tp⊥/Tp∥ is generally anticorrelated with βp∥. However, no universal relationship seems to exist between Tp⊥/Tp∥ and βp∥.

Low‐latitude dayside magnetopause and boundary layer for high magnetic shear: 2. Occurrence of magnetic reconnection

Quantitative comparisons of the flow velocity change across the magnetopause (MP) with the prediction from local tangential stress balance in a one-dimensional time-stationary rotational discontinuity, that is, with the Walen relation, have been performed on 69 Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) low-latitude ( 45°) MP crossings. It is found that in 61% of the crossings the observed flow changes agree with the prediction to better than 50%. No dependence of the occurrence of reconnection flows on local magnetic shear, local time/latitude, local tangential magnetosheath flow speed, and local magnetosheath Alfven Mach number is found. We confirm an earlier result that the agreement with the Walen relation becomes worse with increasing magnetosheath plasma β (β is the ratio of plasma pressure to magnetic pressure) but find that the velocity change itself, predicted by the Walen relation, decreases with increasing β. Moreover, the motion and thickness of the boundary also depend on β: the higher the β value, the faster the speed and the smaller the thickness. These effects combine to make velocity changes in high β events more difficult to measure accurately, which may contribute to the poor agreement with the Walen relation in these events. The 42 events which exhibit plasma flows in reasonable agreement with the Walen relation include 21 cases where the flow direction is inconsistent with a single X line hinged at the subsolar point. The discrepancies between the former result arid dayside X line locations reported earlier may be due to a bias in selection of reconnection events in earlier studies. An average (dimensionless) reconnection rate that is substantially lower than 0.1 is inferred for the 42 events.

Continuous magnetic reconnection at Earth's magnetopause

The most important process that allows solar-wind plasma to cross the magnetopause and enter Earths magnetosphere is the merging between solar-wind and terrestrial magnetic fields of opposite sense—magnetic reconnection. It is at present not known whether reconnection can happen in a continuous fashion or whether it is always intermittent. Solar flares and magnetospheric substorms—two phenomena believed to be initiated by reconnection—are highly burst-like occurrences, raising the possibility that the reconnection process is intrinsically intermittent, storing and releasing magnetic energy in an explosive and uncontrolled manner. Here we show that reconnection at Earths high-latitude magnetopause is driven directly by the solar wind, and can be continuous and even quasi-steady over an extended period of time. The dayside proton auroral spot in the ionosphere—the remote signature of high-latitude magnetopause reconnection—is present continuously for many hours. We infer that reconnection is not intrinsically intermittent; its steadiness depends on the way that the process is driven.

Low-latitude dusk flank magnetosheath, magnetopause, and boundary layer for low magnetic shear: Wind observations

We have studied in detail a Wind spacecraft crossing of the low-latitude dusk flank magnetosheath, magnetopause (MP), and the low-latitude boundary layer (LLBL) when the local magnetic shear across the MP was low (<30°) and the interplanetary magnetic field (IMF) was northward. We find that the magnetosheath flow tangential to the MP slows down initially as one moves from the bow shock toward the MP. However, close to the MP this flow speeds up as the MP is approached. The source of flow acceleration is likely to be the magnetic force associated with draping of the field lines around the MP. Magnetic flux pile-up and a plasma depletion layer are also observed next to the flank MP indicating that the level of magnetic flux transfer across the entire dayside low-latitude MP via reconnection is low. The MP is characterized by changes in the plasma properties. The electron parallel temperature is enhanced across the MP and continues to increase across the LLBL, while the perpendicular temperature is constant across the MP. This constancy of the perpendicular temperature suggests that the transfer of plasma takes place across the local MP. In the LLBL, the ion and electron temperatures are well correlated with the density. In addition, the flow direction in a substantial portion of the LLBL is nearly aligned with that in the magnetosheath, and the flow speed tangential to the MP decreases gradually with decreasing LLBL density. The behavior of the particle distributions suggests that the entire LLBL was on closed field lines. In essence, our findings on the topology and on the LLBL plasma characteristics suggest that even in the absence of reconnection at the local low-shear MP, the LLBL is locally coupled to the adjacent magnetosheath. The smooth variations of the plasma parameters with the density are consistent with the LLBL spatial profiles being gradual. This may suggest that diffusion processes play a role in the formation and dynamics of the LLBL. Finally, the magnetic field and the state of the plasma in the plasma sheet adjacent to the flank MP/LLBL appear to be functions of the IMF direction. Thus the IMF may control both the external (magnetosheath) and the internal (plasma sheet) boundary conditions for the flank MP processes.

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.

New observations of ion beams in the plasma sheet boundary layer

The Wind perigee passes covered tail distances from 6–24 RE. By use of bulk quantities and the parent distributions, we have found new features in the PSBL that had been missed previously. The PSBL consists of a unidirectional earthward streaming ion beam at the edge and another unidirectional beam inside this edge streaming in the tailward direction. Bidirectional beams are observed with higher densities, further inside the PSBL. The plasma in the region supporting the tailward streaming beams consists of the beam distribution plus an isotropic component, whereas the earthward streaming beams consists mainly of the beam distribution. These distributions yield fast flows (>400 km/s) in the earthward direction and slower flows (≈ 150 km/s) in the tailward direction. Both regions support counter streaming electron beams superposed on an isotropic component. These new findings are substantially different from previous observations and the interpretation of fast flows and ion beams in terms of a neutral line model needs to be reexamined.

Relationships between plasma depletion and subsolar reconnection

Observations in the subsolar magnetosheath show that the plasma depletion layer (PDL) is less pronounced for southward than for northward IMF. Since subsolar plasma depletion indicates pile-up of magnetic flux, the degree of plasma depletion should depend on the relative rates of flux transport via subsolar reconnection and flux advection by the solar wind flow. To identify the factors affecting plasma depletion, we consider the ratio D = Er/Esw where Er is the reconnection electric field and Esw is the imposed solar wind electric field. For a quasi-perpendicular subsolar bow shock D can be expressed in terms of magnetosheath parameters. Since PDL formation is suppressed when the subsolar bow shock is quasi-parallel, we restrict attention to quasi-perpendicular conditions. We show that D increases with increasing reconnection efficiency, magnetic shear at the magnetopause, and the magnetosheath magnetic field but decreases with increasing total perpendicular pressure (particle plus magnetic field) in the magnetosheath. By combining observations of the subsolar quasi-perpendicular magnetosheath from AMPTE/IRM and AMPTE/CCE we verify that the degree of plasma depletion is inversely correlated with D. Furthermore, we show that the greater prevalence of plasma depletion in the CCE data implies that the reconnection efficiency is lower for the CCE events and specifically that the reconnection efficiency depends roughly inversely on magnetosheath β, for β > 1. Finally, the results show that all of the changes brought about by plasma depletion act to increase Er so that some subsolar reconnection is likely to occur even for low magnetic shear. Thus the percentage of the magnetosheath magnetic field that participates in reconnection near the subsolar region does not act as a rectifier but remains positive for all shear angles, decreasing monotonically as the magnetic shear at the subsolar magnetopause changes from high to low shear. This implies that the equatorward polar cap convection cells observed during northward IMF and conventionally thought to be driven by a viscous interaction may be due at least in part if not wholly to subsolar low shear reconnection.

The Plasmaspheric Plume and Magnetopause Reconnection

We present near-simultaneous measurements from two THEMIS spacecraft at the dayside magnetopause with a 1.5 h separation in local time. One spacecraft observes a high-density plasmaspheric plume while the other does not. Both spacecraft observe signatures of magnetic reconnection, providing a test for the changes to reconnection in local time along the magnetopause as well as the impact of high densities on the reconnection process. When the plume is present and the magnetospheric density exceeds that in the magnetosheath, the reconnection jet velocity decreases, the density within the jet increases, and the location of the faster jet is primarily on field lines with magnetosheath orientation. Slower jet velocities indicate that reconnection is occurring less efficiently. In the localized region where the plume contacts the magnetopause, the high-density plume may impede the solar wind-magnetosphere coupling by mass loading the reconnection site.