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NASA NTRS1985

Solar wind-magnetosphere coupling and the distant magnetotail: ISEE-3 observations

ISEE-3 Geotail observations are used to investigate the relationship between the interplanetary magnetic field, substorm activity, and the distant magnetotail. Magnetic field and plasma observations are used to present evidence for the existence of a quasi-permanent, curved reconnection neutral line in the distant tail. The distance to the neutral line varies from absolute value of X = 120 to 140 R/sub e near the center of the tail to beyond absolute value of X = 200 R/sub e at the flanks. Downstream of the neutral line the plasma sheet magnetic field is shown to be negative and directly proportional to negative B/sub z in the solar wind as observed by IMP-8. V/sub x in the distant plasma sheet is also found to be proportional to IMF B/sub z with southward IMF producing the highest anti-solar flow velocities. A global dayside reconnection efficiency of 20 +- 5% is derived from the ISEE-3/IMP-8 magnetic field comparisons. Substorm activity, as measured by the AL index, produces enhanced negative B/sub z and tailward V/sub x in the distant plasma sheet in agreement with the basic predictions of the reconnection-based models of substorms. The rate of magnetic flux transfer out of the tail as a function of AL is found to be consistent with previous near-Earth studies. Similarly, the mass and energy fluxes carried by plasma sheet flow down the tail are consistent with theoretical mass and energy budgets for an open magnetosphere. In summary, the ISEE-3 Geotail observations appear to provide good support for reconnection models of solar wind-magnetosphere coupling and substorm energy rates.

Slavin, J. A.↗

NASA NTRS1986

Magnetohydrodynamic instability

There have been major advances in the theory of magnetic reconnection and of magnetic instability, with important implications for the observations, as follows: (1) Fast and slow magnetic shock waves are produced by the magnetohydrodynamics of reconnection and are potential particle accelerators. (2) The impulsive bursty regime of reconnection gives a rapid release of magnetic energy in a series of bursts. (3) The radiative tearing mode creates cool filamentary structures in the reconnection process. (4) The stability analyses imply that an arcade can become unstable when either its height or twist of plasma pressure become too great.

Priest, E. R.↗

NASA NTRS1986

Solar wind-magnetosphere coupling and the distant magnetotail - ISEE-3 observations

ISEE-3 Geotail observations are used to investigate the relationship between the interplanetary magnetic field, substorm activity, and the distant magnetotail. Magnetic field and plasma observations are used to present evidence for the existence of a quasi-permanent, curved reconnection neutral line in the distant tail. The distance to the neutral line varies from absolute value of X = 120 to 140 R/sub e near the center of the tail to beyond absolute value of X = 200 R/sub e at the flanks. Downstream of the neutral line the plasma sheet magnetic field is shown to be negative and directly proportional to negative B/sub z in the solar wind as observed by IMP-8. V/sub x in the distant plasma sheet is also found to be proportional to IMF B/sub z with southward IMF producing the highest anti-solar flow velocities. A global dayside reconnection efficiency of 20 + or - 5 percent is derived from the ISEE-3/IMP-8 magnetic field comparisons. Substorm activity, as measured by the AL index, produces enhanced negative B/sub z and tailward V/sub x in the distant plasma sheet in agreement with the basic predictions of the reconnection-based models of substorms. The rate of magnetic flux transfer out of the tail as a function of AL is found to be consistent with previous near-earth studies. Similarly, the mass and energy fluxes carried by plasma sheet flow down the tail are consistent with theoretical mass and energy budgets for an open magnetosphere. In summary, the ISEE-3 Geotail observations appear to provide good support for reconnection models of solar wind-magnetosphere coupling and substorm energy rates.

Slavin, J. A.↗

NASA NTRS1987

Kinetic aspects of tail dynamics - Theory and simulation

Kinetic theories relevant to the geomagnetic tail are reviewed. The topics discussed include kinetic instabilities, simulations, and current-sheet particle acceleration. Tearing mode and reconnection theories are emphasized. Kinetic treatment is appropriate for these topics since the tail plasma is collisionless. Fluid calculations are appropriate when stochastic processes dominate and for studies where long wavelengths are important. However, fluid treatments of tearing modes and reconnection require a finite resistivity in the diffusion region. Thus, although 'anomalous resistivity' can be guessed or in some cases calculated, ideally the kinetic treatment is often to be preferred. Particle motion and acceleration in the current sheet can give rise to beam-like distributions in the plasma-sheet boundary layer. Studies of current-sheet particle motion have also been used as the basis for 'kinetic' tail equilibrium models. Furthermore, quite recently current-sheet particle motion is used directly in Coroniti's explosive tail reconnection model. The 'inertial conductivity' from the equilibrium models provides the 'dissipation' necessary for reconnection.

Speiser, T. W.↗

NASA NTRS1991

Avalanches and the distribution of solar flares

The solar coronal magnetic field is proposed to be in a self-organized critical state, thus explaining the observed power-law dependence of solar-flare-occurrence rate on flare size which extends over more than five orders of magnitude in peak flux. The physical picture that arises is that solar flares are avalanches of many small reconnection events, analogous to avalanches of sand in the models published by Bak and colleagues in 1987 and 1988. Flares of all sizes are manifestations of the same physical processes, where the size of a given flare is determined by the number of elementary reconnection events. The relation between small-scale processes and the statistics of global-flare properties which follows from the self-organized magnetic-field configuration provides a way to learn about the physics of the unobservable small-scale reconnection processes. A simple lattice-reconnection model is presented which is consistent with the observed flare statistics. The implications for coronal heating are discussed and some observational tests of this picture are given.

Lu, Edward T.↗

NASA NTRS1992

Model of the formation of the low-latitude boundary layer for strongly northward interplanetary magnetic field

A model for the formation of the low-boundary layer in which a magnetosheath flux tube reconnects in the north and south beyond the cusp when the IMF is strongly northward is presented. For northward IMF the geomagnetic field captures solar wind flux tubes through intermittent reconnection at the cusp region. These newly captured flux tubes shorten and sink into the magnetosphere while the flux tubes reorient themselves as they become assimilated into the magnetosphere. There is no significant acceleration for the particles within the flux tubes. In the magnetosphere the interchange instability disperses the flux tube azimuthally along the magnetopause to form a boundary layer. Subsequent reconnection forms sublayers of the boundary layer and different sublayers represent different ages after reconnection. The interchange instability is stable radially, which keeps sharp boundaries between sublayers and between the boundary layer and magnetosphere.

Song, P.↗

NASA NTRS1992

Is there a near-Earth neutral line?

The high correlation between magnetic activity and the interplanetary magnetic field provides strong evidence that magnetic reconnection causes substorms. Rapid changes in magnetospheric configuration following a southward turning suggest that nightside reconnection is delayed relative to the dayside. The near Earth neutral line model postulates that it begins in the near Earth plasma sheet near midnight at expansion onset. Evidence supporting this model is reviewed. Observations of current disruption near synchronous orbit were recently interpreted as proof that reconnection is not necessary in the expansion phase, and that nightside reconnection is a recovery phase phenomenon. Problems with magnetic field mapping make this conclusion premature. Furthermore, the spatial distribution of delays in plasmoid signatures are incompatible with such a late start. Speculation is made concerning a mechanism whereby current disruption might be caused by an x-line that forms further from the Earth in the late growth phase.

Mcpherron, Robert L.↗

NASA NTRS1992

A global magnetohydrodynamic simulation of magnetospheric dynamics when the IMF is southward: Mapping to the auroral zone

A high resolution global magnetohydrodynamic simulation model is used to investigate magnetospheric dynamics during intervals with southward interplanetary magnetic field (IMF). When the southward IMF reaches the dayside magnetopause reconnection begins and magnetic flux is convected into the tail lobes. After about 35 m, reconnection begins within the plasma sheet near midnight at x = -14 RE. Later the x-line moves towards the magnetopause. The reconnection occurs just tailward of the region where the tail attaches onto the dipole dominated inner magnetosphere. Later when all the plasma sheet field lines have reconnected a plasmoid moves down the tail. The region of the ionosphere where the energy flux from the magnetosphere is greatest is calculated. The energy flux is confined to a region which approximates the auroral oval.

Walker, Raymond J.↗

NASA NTRS1995

Topological structure of the magnetotail as a function of interplanetary magnetic field direction

Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. As the angle between the IMF and the geomagnetic field is changed at the bow of the magnetosphere, the topological record of the location of the reconnection region should be recorded in the magnetosheath and on the magnetopause along the flanks of the tail, because the super fast flow freezes strong magnetic gradients formed in the bow reconnection regions into the plasma downstream. In this report, we present results from a three-dimensional, magnetohydrodynamic (MHD), global numerical simulation code for the location of the separatrix between unconnected IMF magnetosheath field lines and reconnected field lines which penetrate the magnetopause and connect to the polar ionosphere. The angle between the IMF direction and the line where the separatrix crosses the magnetopause is shown to be a sensitive function of the IMF clock angle. We also explain how this behavior can be used to derive an approximate relation for the dependence of the cross-polar voltage on the IMF clock angle. We conclude with a note of caution concerning the importance of physical boundary conditions in magnetoplasma simulations.

Fedder, J. A.↗

NASA NTRS1993

A mechanism for the multiple brightenings of dayside poleward-moving auroral forms

Magnetic reconnection plays an important role in the magnetosphere-ionosphere coupling and may lead to the formation of the dayside auroras. Observations of the dayside auroras may provide clues to the reconnection process at the magnetopause. Recently Fasel et al. (1992) observed poleward-moving dayside auroral forms that rebrighten during their lifetimes. Lee et. al. (1993) examined the topology of magnetic flux ropes formed by the patchy multiple X-line reconnection (PMXR), in which a magnetic flux tube reconnects with other flux tubes at multiple sites of limited extent. This paper develops the connection between the evolution of the magnetic flux ropes and the multiple brighentings of poleward-moving dayside auroral forms.

Fasel, G. J.↗

NASA NTRS1994

Midtail plasma flows and the relationship to near-Earth substorm activity: A case study

Recent simulations of magnetotail reconnection have pointed to a link between plasma flows, dipolarization, and the substorm current wedge. In particular, Hesse and Birn (1991) have proposed that earthward jetting of plasma from the reconnection region transports flux into the near-Earth region. At the inner edge of the plasma sheet this flux piles up, producing a dipolarization of the magnetic field. The vorticity produced by the east-west deflection of the flow at the inner edge of the plasma sheet gives rise to field-aligned currents that have region 1 polarity. Thus in this scenario the earthward flow from the reconnection region produces the dipolarization ad the current wedge in a self-consistent fashion. In this study we examine observations made on April 8, 1985 by the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Ion Release Module (IRM), the geosynchronous satellites 1979-053, 1983-019, and 1984-037, and Syowa station, as well as AE. This event is unique because IRM was located near the neutral sheet in the midnight sector for am extended period of time. Ground data show that there was ongoing activity in the IRM local time sector for several hours, beginning at 1800 UT and reaching a crescendo at 2300 UT. This activity was also accompanied by energetic particle variations, including injections, at geosynchronous orbit in the nighttime sector. Significantly, there were no fast flows at the neutral sheet until the great intensification of activity at 2300 UT. At that time, IRM recorded fast eartheard flow simultaneous with a dipolatization of the magetic field. We conclude that while the aforementioned scenario for the creation of the current wedge encounters serious problems explaining the earlier activity, the observations at 2300 UT are consistent with the scenario of Hesse and Birn (1191). On that basis it is argued that the physics of substorms is not exclusively rooted in the development of a global tearing mode. Processes at the inner edge of the cross-tail current that cause a disruption of the current and a consequent dipolarization and current wedge may be unrelated to the formation of a macroscale reconnection region. Thus the global evolution of a substorm is probably a complicated superposition of such processes operating on a very localized scale and a global macroscale process that allows for such things as releasing te energy stored in lobe flux and creation of plasmoids.

Lopez, R. E.↗

NASA NTRS1995

Flow-aligned jets in the magnetospheric cusp: Results from the Geospace Environment Modeling Pilot program

The extended flight of the Airborne Ionospheric Observatory during the Geospace Environment Modeling (GEM) Pilot program on January 16, 1990, allowed continuous all-sky monitoring of the two-dimensional ionospheric footprint of the northward interplanetary magnetic field (IMF) cusp in several wavelengths. Especially important in determining the locus of magnetosheath electron precipitation was the 630.0-nm red line emission. The most striking morphological change in the images was the transient appearance of zonally elongated regions of enhanced 630.0-nm emission which resembled 'rays' emanating from the centroid of the precipitation. The appearance of these rays was strongly correlated with the Y component of the IMF: when the magnitude of B(sub y) was large compared to B(sub z), the rays appeared; otherwise, the distribution was relatively unstructured. Late in the flight the field of view of the imager included the field of view of flow measurements from the European incoherent scatter radar (EISCAT). The rays visible in 630.0-nm emission exactly aligned with the position of strong flow jets observed by EISCAT. We attribute this correspondence to the requirement of quasi-neutrality; namely, the soft electrons have their largest precipitating fluxes where the bulk of the ions precipitate. The ions, in regions of strong convective flow, are spread out farther along the flow path than in regions of weaker flow. The occurrence and direction of these flow bursts are controlled by the IMF in a manner consistent with newly opened flux tubes; i.e., when absolute value of B(sub y) greater than absolute value of B(sub z), tension in the reconnected field lines produce east-west flow regions downstream of the ionospheric projection of the x line. We interpret the optical rays (flow bursts), which typically last between 5 and 15 min, as evidence of periods of enhanced dayside (or lobe) reconnection when absolute value of B(sub y) greater than absolute value of B(sub z). The length of the reconnection pulse is difficult to determine, however, since strong zonal flows would be expected to persist until the tension force in the field line has decayed, even if the duration of the enhanced reconnection was relatively short.

Weiss, L. A.↗

NASA NTRS1997

Geotail Measurements Compared with the Motions of High-Latitude Auroral Boundaries during Two Substorms

Geotail plasma and field measurements at -95 R(sub E) are compared with extensive ground-based, near-Earth, and geosynchronous measurements to study relationships between auroral activity and magnetotail dynamics during the expansion phases of two substorms. The studied intervals are representative of intermittent, moderate activity. The behavior of the aurora and the observed effects at Geotail for both events are harmonized by the concept of the activation of near-Earth X lines (NEXL) after substorm onsets, with subsequent discharges of one or more plasmoids down the magnetotail. The plasmoids must be viewed as three-dimensional structures which are spatially limited in the dawn-dusk direction. Also, reconnection at the NEXL must proceed at variable rates on closed magnetic field lines for significant times before beginning to reconnect lobe flux. This implies that the plasma sheet in the near-Earth magnetotail is relatively thick in comparison with an embedded current sheet and that both the NEXL and distant X line can be active simultaneously. Until reconnection at the NEXL engages lobe flux, the distant X line maintains control of the poleward auroral boundary. If the NEXL remains active after reaching the lobe, the auroral boundary can move poleward explosively. The dynamics of high-latitude aurora in the midnight region thus provides a means for monitoring these processes and indicating when significant lobe flux reconnects at the NEXL.

Maynard, N. C.↗

NASA NTRS2005

Flare Emission Onset in the Slow-Rise and Fast-Rise Phases of an Erupting Solar Filament Observed with TRACE

We observe the eruption of an active-region solar filament of 1998 July 11 using high time cadence and high spatial resolution EUV observations from the TRACE sareiii'ce, along with soft X-ray images from the soft X-ray telescope (SXT) on the Yohkoh satellite, hard X-ray fluxes from the BATSE instrument on the (CGRO) satellite and from the hard X-ray telescope (HXT) on Yohkoh, and ground-based magnetograms. We concentrate on the initiation of the eruption in an effort to understand the eruption mechanism. First the filament undergoes slow upward movement in a "slow rise" phase with an approximately constant velocity of approximately 15 km/s that lasts about 10-min, and then it erupts in a "fast-rise" phase, reaching a velocity of about 200 km/s in about 5-min, followed by a period of deceleration. EUV brightenings begin just before the start of the filament's slow rise, and remain immediately beneath the rising filament during the slow rise; initial soft X-ray brightenings occur at about the same time and location. Strong hard X-ray emission begins after the onset of the fast rise, and does not peak until the filament has traveled a substantial altitude (to a height about equal to the initial length of the erupting filament) beyond its initial location. Our observations are consistent with the slow-rise phase of the eruption resulting from the onset of "tether cutting" reconnection between magnetic fields beneath the filament, and the fast rise resulting from an explosive increase in the reconnection rate or by catastrophic destabilization of the overlying filament-carrying fields. About two days prior to the event new flux emerged near the location of the initial brightenings, and this recently- emerged flux could have been a catalyst for initiating the tether-cutting reconnection. With the exception of the initial slow rise, our findings qualitatively agree with the prediction for erupting-flux-rope height as a function of time in a model discussed by Chen & Shibata (2000) based on reconnection between emerging flux and a flux rope.

Sterling, A. C.↗

NASA NTRS2007

Response of the Earth's Magnetosphere to Changes in the Solar Wind

The solar wind couples to the magnetosphere via dynamic pressure and electric field. Pressure establishes the size and shape of the system, while the electric field transfers energy, mass, and momentum to the magnetosphere. When the interplanetary magnetic field (IMF) is antiparallel to the dayside magnetic field, magnetic reconnection connects the IMF to the dipole field. Solar wind transport of the newly opened field lines to the nightside creates an internal convection system. These open field lines must ultimately be closed by reconnection on the nightside. For many decades, it was thought that a magnetospheric substorm was the process for accomplishing this and that all magnetic activity was a consequence of substorms. It is now recognized that there are a variety of modes of response of the magnetosphere to the solar wind. In this paper, we briefly describe these modes and the conditions under which they occur. They include substorms, pseudo-breakups, poleward boundary intensifications (PBI), steady magnetospheric convection (SMC), sawtooth injection events, magnetic storms, high-intensity long-duration continuous AE activities (HILDCAAs), and storm-time activations. There are numerous explanations for these different phenomena, some of which do not involve magnetic reconnection. However, we speculate that it is possible to interpret each mode in terms of differences in the way magnetic reconnection occurs on the nightside.

McPherron, Robert L.↗

NASA NTRS2008

A Model for Solar Polar Jets

We propose a model for the jetting activity that is commonly observed in the Sun's corona, especially in the open-field regions of polar coronal holes. Magnetic reconnection is the process driving the jets and a relevant magnetic configuration is the well-known null point and fan separatrix topology. The primary challenge in explaining the observations is that reconnection must occur in a short-duration energetic burst rather than quasi-continuously as is implied by the observations of long-lived structures in coronal holes, such as polar plumes, for example. The key idea underlying our model for jets is that reconnection is forbidden for an axisymmetric null-point topology. Consequently, by imposing a twisting motion that maintains the axisymmetry, magnetic stress can be built up to large levels until an ideal instability breaks the symmetry and leads to an explosive release of energy via reconnection. Using 3D MHD simulations we demonstrate that this mechanism does produce jets with high speed and mass, driven by nonlinear Alfven waves. We discuss the implications of our results for observations of the solar corona.

Pariat, E.↗

NASA NTRS2007

Bifurcation and Hysteresis of the Magnetospheric Structure with a varying Southward IMF: Field Topology and Global Three-dimensional Full Particle Simulations

Using a three-dimensional full electromagnetic particle model (EMPM), we have performed global simulations of the interaction between the solar wind and the terrestrial magnetosphere, and have investigated its asymptotic stability. The distance between the dayside magnetopause subsolar point and the Earth center, R(sub mp) is measured, as the intensity of southward IMF |B(sub z)| is slowly varying. Based on the field topology theory, one analyzes the variation of R(sub mp) as a reference index of the dynamics of this interaction, when IMF |B(sub z)| successively increases and decreases to its original value. Two striking results are observed. First, as the IMF |B(sub z)| increases above a critical value, the variation of R(sub mp) suddenly changes (so called 'bifurcation' process in field topology). Above this critical value, the overall magnetic field topology changes drastically and is identified as being the signature of magnetic reconnection at the subsolar point on the magnetopause. Second, this subsolar point recovers its original location R(sub mp) by following different paths as the IMF |B(sub z)| value increases (from zero to a maximum fixed value) and decreases (from this maximum to zero) passing through some critical values. These different paths are the signature of 'hysteresis' effect, and are characteristic of the so-called 'subcritical-type' bifurcation. This hysteresis signature indicates that dissipation processes take place via an energy transfer from the solar wind to the magnetosphere by some irreversible way, which leads to a drastic change in the magnetospheric field topology. This hysteresis is interpreted herein as a consequence of the magnetic reconnection taking place at the dayside magnetopause. The field topology reveals to be a very powerful tool to analyze the signatures of three-dimensional magnetic reconnection without the obligation for determining the mechanisms responsible for, and the consequences of the reconnection on the overall magnetospheric dynamics.

Cai, DongSheng↗

NASA NTRS2008

Jet-Front Speed and the Origin of Jets in Polar Coronal Holes

The area-average strength of the open magnetic field in the polar coronal holes can be estimated from the radial component of the magnetic field measured by Ulysses in the solar wind, the fraction of the solar sphere covered by the polar coronal holes, and the fraction of the heliosphere filled by the fast solar wind from the polar coronal holes. For the present minimum phase of the solar cycle, the estimated strength is approximately 10 G. Using this strength for the ambient open field in the standard reconnection model for jets in coronal holes, we obtain for any given jet-front speed a lower bound on the initial temperature of the expanding jet-front plasma, and an upper bound on the ambient plasma density at the reconnection site. These two bounds indicate the following. For jet-front speeds of approximately 1000 km/s, (1) the reconnection site has to be in the low corona or upper transition region (n(e) is less than 10(exp 9) cm(exp -3)), not in the lower transition region or chromosphere, (2) the jet-front plasma is initially heated to T greater than approximately 10(exp 7) K, and (3) hence a compact X-ray flare is produced at the base of the jet. For jet-front speeds less than approximately 100 km/s, (1) the jet can be produced by reconnection in the lower transition region (approximately 10(exp 9) less than n(e) less than approximately 10(exp 10) cm(exp-3)) or upper chromosphere (approximately 10(exp 10) less than n(e) less than approximately 10(exp 12) cm-3), (2) the initial temperature of the jet-front plasma can be less than 10(exp 6) K, and (3) hence some EUV and H(alpha) jet-type macrospicules may be produced with no detectable X-ray emission.

Moore, Ron↗