SM22A-01 INVITED
Development of the Near Earth Neutral Line Model
The concept of a magnetospheric substorm has developed over the years into a phenomenological model called the near Earth neutral line model (NENL). In this talk we review the development of this model and some of its testable predictions. The initial motivation for the NENL model was the observation that major auroral zone activity only occurs when the interplanetary magnetic field (IMF) is southward. Dungey had anticipated this possibility and suggested that reconnection between the IMF both on the dayside magnetopause and in the tail could account for auroral motions and ionospheric convection. The NENL model incorporates a sequence of physical processes through which magnetic flux from the dayside is moved to the nightside and stored in the tail lobes. A sequence of changes in configuration of the magnetopause and plasma sheet during the growth phase lead to the onset of reconnection in the plasma sheet. In the expansion phase the energy stored in the lobe magnetic field is released by reconnection heating the plasma and causing it to jet Earthward (and tailward) releasing a flux rope that is ejected from the tail. Plasma flowing Earthward piles up in the inner region and is then diverted towards the flanks. Precipitation of this heated and turbulent plasma creates an auroral disturbance that propagates poleward as the pileup moves tailward. At the same time the pressure gradients and vorticity of the plasma flow produce the substorm current wedge. The outward current in the current wedge accelerates electrons into the ionosphere generating bright discrete aurora in the westward surge. When the pileup of plasma and flux approaches the location of the neutral line the neutral line begins to move tailward. This initiates the recovery phase of the substorm with a global expansion of the plasma sheet and a broadening of the auroral oval over most of the night side. The initial cause and location of the plasma sheet instability is a subject of intense controversy. Some believe that a process other than reconnection first occurs at the inner edge of the plasma sheet and only later triggers reconnection further down the tail. The NASA Themis (Greek Goddess of Justice) mission successfully launched in February 2007 is designed to resolve this controversy. It will test the NENL concept by timing the onset of different phenomena in the tail as a function of distance. Themis is supported by the most extensive collection of ground magnetometers, all sky cameras, riometers, and radars ever deployed. This ground instrumentation will help to time and to localize the onset of various ionospheric phenomena for comparison with space observations.
SM22A-02 INVITED
The Near-Earth Neutral Line Model: Advanced Insights From Simulations
A major attraction of the early neutral line model of substorms was that one mechanism, namely magnetic reconnection caused by the local breakdown of ideal MHD, seemed to provide an explanation for various aspects of substorms: fast flows, plasmoid severance, current disruption and deviation, and the acceleration of energetic particles. However, extensive observations in the near and mid tail then cast doubt on this relationship, because particularly the signs of current disruption and the acceleration of energetic particles appeared to happen in the transition region between dipole and tail, clearly earthward from the inferred average reconnection site. Here we review insights, obtained from simulations of plasma and particle dynamics in the tail, about the relationship between reconnection, the current redistribution in the substorm current wedge, and particle acceleration. We also dicuss further insights into the role of entropy loss in the earthward transport and a possible coupling between small-scale structures and auroral arcs.
SM22A-03
In search of the near-Earth neutral line: How much a role can ground-based observations play?
The launch of THEMIS has raised the expectation that new progress will be made on the assessment and evaluation of competing substorm models. While the THEMIS spacecraft will offer much improved capability to delimit and localize substorm triggers, they will measure primarily "perpendicular" signatures of a near-Earth neutral line (e.g., BBFs). For "parallel" signatures of NENL, ground-based observations should prove more advantageous. However, there has been relatively little attention to what effects can be uniquely attributed to a would-be NENL, as a given auroral signature can receive more than one theoretical explanations. There is therefore a need to systematize and quantify our approach to the use of ground-based observations to detect, constrain, and characterize NENL. In this talk, we will review the ground-based observations that will be made in conjunction with THEMIS during its tail operation, identify some candidate signatures that might be related to the NENL, and outline steps to evolve this line of research through focused theory and modeling efforts.
SM22A-04 INVITED
Onset and Cessation of Magnetic Reconnection: Missing Links of Substorm Evolution
Magnetic reconnection determines the overall evolution of a magnetic substorm. As the primary conversion process of magnetic energy to particle bulk kinetic and thermal energy, it also provides the new magnetic flux, which is threading the main current sheet during the substorm dipoloarization. Owing to the critical importance of reconnection to substorm evolution, the questions of when and how reconnection starts, and how reconnection ceases require answers if magnetospheric evolution is to be described quantitatively. In this presentation, we present a brief overview of research into both questions. Specifically, we will show that thin current sheets play a critical role in reconnection, and hence, substorm initiation, and we will describe a number of candidate causes for reconnection cessation.
SM22A-05
Simultaneous Observations of Substorm Onset by the Alignment of Cluster, IMAGE, GOES, and Canadian Ground Based Instruments.
Several recent studies of substorm onset suggested that in fact this process may comprise two distinct activations seen both in situ and on the ground. It has been asserted that the one activation is associated with the near-Earth breakup and the other - with the mid-tail plasma sheet reconnection (or NENL). We will present an event study of onset with a fortunate alignment of several satellites and ground-based observatories. GOES 8 and Cluster (19 RE down the tail) were in the near-midnight magnetotail conjugate to hi-resolution photometers and magnetometers in the Canadian sector. The IMAGE satellite provided contemporaneous global auroral images throughout the interval. During this event, double onset was clearly seen by all instruments. The first one, detected by ground-based magnetometers, GOES, and IMAGE, was a near-Earth breakup (or equator-most auroral breakup) which followed by a tailward flow, bipolar magnetic field variations, and extreme thinning of the current sheet observed by Cluster. The second, observed roughly 5 min later on, led to a much larger activation with a fully developed WTS and electrojet. This onset was observed by Cluster as a sharp flow reversal from tailward to Earthward with a quadrupolar magnetic field structure and intensive ion heating. This can be interpreted as the Hall-type CPS reconnection tailward of the spacecraft. Prior to this reconnection, the current sheet half-thickness decreased to a minimum on the order of 1000 km. At the time of reconnection a large auroral vortex, on the scale of the auroral oval width, formed. It was rapidly followed by the formation of a double oval and gradual transition to the recovery phase. We assert that we have, for this event, identified ionospheric signatures of mid-tail reconnection in the substorm. Further, we assert that these ionospheric and ground-based signatures of the mid-tail reconnection are distinct from those of the near-Earth breakup.
SM22A-06 INVITED
Critical Issues of the Near-Earth Neutral-Line Model
The substorm trigger is often addressed in terms of the formation of a near-Earth neutral-line (NENL). The currently popular idea, the outside-in model, proposes that the substorm is triggered as the fast plasma flow caused by near-Earth reconnection changes the current and plasma distributions in the near-Earth magnetotail. In this presentation I shall discuss some fundamental features of the substorm onset that this model does not explain explicitly. The list of such features includes the auroral expansion, pseudobreakup, and northward turning. Critical tests of the outside-in model in terms of those features should be essential for advancing our understanding of the substorm trigger mechanism.
SM22A-07
Substorns without Dayside Reconnection: Lessons from the Outer Planets
The fundamental attribute of the Near-Earth neutral line model is the accumulation of stored magnetic energy in the tail during a growth phase, and the sudden release of that energy during an expansion phase, followed by a recovery. The existence of substorms at both Jupiter and Saturn enables us to explore this model under varied boundary conditions. In particular the source of plasma convection is quite different at Earth, Jupiter and Saturn, being clearly solar-wind driven at Earth, internally driven at Jupiter, and under debate at Saturn. We use observations of the magnetotails of Jupiter and Saturn to show that analogous processes occur therein with both similarities to and differences with Earth. The rate of tail reconnection appears to be faster at Jupiter and Saturn, producing very strong north-south field, and the growth phases appear to be much longer. Despite the expected weakness of dayside reconnection with the interplanetary magnetic field, plasmoids are formed in the tail and the tail field strength increases and decreases as seen at Earth.