Magnetopause, polar cusp, transients and FTEs
Formation of the low-latitude boundary layer during northward IMF
Several investigators discussed the formation of the low-latitude boundary layer (LLBL) and cold-dense plasma sheet (CDPS). GEOTAIL, Cluster, and THEMIS multi-spacecraft observations revealed the presence of a thick LLBL and CDPS adjacent to the dayside and flank magnetopause during northward interplanetary magnetic field (IMF) conditions. The presence of a thick LLBL and CDPS indicates substantial solar wind entry into the magnetosphere. The question is which of the processes (magnetic reconnection, Kelvin-Helmholtz instability or diffusive entry) is dominant. A global MHD simulation presented at the Workshop was able to reproduce the THEMIS observation of a 1-Earth-radius thick and high-density LLBL on closed field lines near the subsolar region. In the simulation (see Figure 1), the dominant process that led to the solar wind entry across the dayside magnetopause was reconnection occurring at the high-latitude magnetopause in both hemispheres. The simulation also predicted the presence of an open-field region sunward of the magnetopause with equatorward plasma flow (opposing the magnetosheath flow) resulting from reconnection in a single hemisphere.
Cluster observations, on the other hand, seem to indicate that the Kelvin-Helmholtz instability may be playing an increasingly important role in solar wind entry into the magnetosphere further down the dawn and dusk flanks of the magnetosphere.
Extreme magnetopause motion due to solar wind discontinuity interaction with the bow shock
Several investigators presented THEMIS observations indicating extremely fast outward expansion of the dayside magnetopause. In one study, the magnetopause expansion resulted in the consecutive crossings of the bow shock and magnetopause within 5 minutes, instead of the more typical 1-2 hours separation. Such events seem to occur in association with the arrival of solar wind discontinuities. In another study, extreme outward motion of the magnetopause of ~800 km/s was directly measured. Consistent with this speed, the four Cluster spacecraft encounters of the magnetopause indicated an outward magnetopause expansion of 5 Earth radii in 60 seconds. Interestingly, this extreme magnetopause motion was not caused by changes in the total pressure of the pristine solar wind. Instead, the motion was associated with the interaction of a solar wind discontinuity with the bow shock resulting in a strong deflection of solar wind flow and a significant drop in the total pressure.
Several other interesting topics were also discussed at the meeting. These include:
- MHD-based reconstruction of a Cluster reconnection event at the magnetopause. The analysis revealed an X-line orientation that is close to the Swisdak-Drake (2007) prediction.
- Theoretical study of the scaling of the subsolar magnetopause reconnection rate and whether the reconnection rate is controlled primarily by the large scale solar wind conditions or by the microphysics in the diffusion region.
- Successful capture of the magnetopause reconnection diffusion regions with the automated burst trigger onboard the THEMIS spacecraft. High-resolution plasma measurements reveal multiple interpenetrating ion populations in reconnection layers.
- Cluster observations of kinetic physics along the reconnection separatrices.
- Cluster observations of time-varying reconnection rate at the magnetopause and the possibility that mirror waves in the magnetosheath might be the cause of the variable reconnection rate.
- Near-simultaneous observations by TC-1 and Cluster of component and anti-parallel reconnection at the low and high-latitude magnetopause.
- THEMIS observations of standing Alfven waves at the magnetopause driven by upstream pulsations and modulated by internal magnetospheric cavity modes.
The polar cusp is one of the magnetospheric regions where coupling between the solar wind and the magnetosphere has the most visible effects since the cusp is in direct contact with the magnetosheath plasma. The effect of transient solar wind density was found to increase plasma density in the mid- and high-altitude polar cusp crossings with Cluster. The polar cusp density is seen to increase significantly reaching 100 cm-3, nearly equal to magnetosheath values. These transients are associated with "flat-top" particle distribution functions, ionospheric outflows and strong electromagnetic waves emissions.
Any changes in the interplanetary magnetic field will change plasma precipitation in the polar cusp. Cluster data revealed that the evolution of the ion dispersion observed by Cluster in the polar cusp is following an abrupt change of IMF direction from Southward to Northward-Westward. The dispersion changed from a "normal" dispersion (ion energy decreases with increasing latitude) to a "V" shaped dispersion (an energy decrease followed by an energy increase). This could be explained by a change in the mode of reconnection from the subsolar point to double reconnection in the lobes. Simulation of this event was performed using a three-dimensional magnetohydrodynamic (MHD) model and large-scale particle (LSK) simulation. Particle precipitation and in particular the ion dispersions could be well reproduced by the models.
A conjunction between THEMIS and FAST under Northward IMF conditions was presented. The cusp field aligned currents and flows were studied as well as the "reversed" ion dispersion. Finally the topic of cusp particle energization to MeV energies was addressed. Local cyclotron resonant acceleration was shown to be a strong candidate while on the other hand cusp examples revealed where the bow shock could be the origin. More work is necessary to quantify these different processes and Cluster observations in the polar cusp together with THEMIS at the magnetopause, bow shock and solar wind should be a key data set to address that topic.
Transients and FTEs
|Report on a Plenary Session at the first joint Cluster THEMIS workshop, University of New Hampshire, Durham, USA, 23-26 September 2008|
|Report prepared by D. Sibeck, NASA/GSFC, USA|
Due to its internal magnetic field, the Earth is protected from the continuous stream of particles blown outward from the Sun: the solar wind. The boundary layer separating the magnetosphere - the Earth's magnetic sphere of influence - from the solar wind is the magnetopause. However, this is not an impenetrable shield and the means by which mass, energy and momentum are transferred across is a central issue in space physics. Consequences of this transport are of major concern for space weather specialists worldwide, who monitor Earth's space environment and technology infrastructure.
One way for solar wind material to enter our environment is via interconnected solar wind and magnetospheric magnetic field lines. Thus, a focus of space scientists is to understand and reproduce interconnections that result in magnetic transient events commonly observed in the vicinity of the Earth’s magnetopause. Some are caused by the arrival of interplanetary magnetic field discontinuities carried by the solar wind, while others are caused by transient bursts of reconnection at the magnetopause called Flux Transfer Events (FTEs).
|Figure 2: Animation of the magnetic field strength and direction in a plane parallel to the equator from the high-spatial and high-temporal resolution Center for Space Environment Modeling of the University of Michigan BATS-R- US simulation as run at the NASA/GSFC Coordinated Community Modeling Center. Various forms of transient events can be seen, some with weak core magnetic field strengths, others with strong core magnetic field strengths. Credit: David Sibeck, NASA/GSFC, USA|
Global MHD simulations have now reached the point of accurately simulating these processes, and a number of successful comparisons between model results and observations were presented. For instance, results from a simulation of flux transfer events that occurred on 20 May 2007 were shown. Real solar wind observations were used as input to the BATS-R-US model developed by the University of Michigan and run at the NASA/GSFC Community Coordinated Modeling Center. As illustrated in Figure 2, the model generated a series of flux transfer events with highly variable characteristics. Some events exhibited strong core fields, while others exhibited weak core fields. All displaced draped magnetosheath magnetic fields, resulting in bipolar magnetic field signatures normal to the nominal magnetopause and enhanced magnetosheath magnetic field strengths. The signatures of some events were very similar to those for a crater FTE observed by the 5 THEMIS spacecraft on this day during an interval when the spacecraft were nearly radially aligned across the magnetopause. As Grad-Shafranov reconstructions reveal, the THEMIS spacecraft at the magnetopause observed the strongest magnetic field enhancement, which was bounded by weaker magnetic fields resulting from a bifurcated magnetopause current layer.
Results from the global simulation indicated a bent FTE structure, with events extending from high latitudes across the dayside magnetopause to the opposite hemisphere. Instead of event axes assuming a single orientation parallel to the nominal tilted subsolar reconnection line, a variety of tilts were observed, even within single FTEs. Different parts of the same event move in different directions. Some FTEs were followed by magnetic cavities of the type recently described by Prof. Chris Owen (MSSL, University College of London, UK) and co-workers in Cluster observations.
Other topics discussed in this session include:
- Evidence for a two-step response to an interplanetary tangential discontinuity, allowing effects like the Kelvin-Helmholtz instability to be isolated.
- Evidence that bipolar velocity and magnetic field signatures normal to the magnetopause result from flux tubes in active reconnection layers, while bifurcated current sheets occur in the wake of an FTE.
- Observations indicating that drift magnetosonic waves generated by the ions heat electrons in FTEs via inverse Landau damping.
- Combined ground and multipoint spacecraft observations demonstrate that FTEs move away from a tilted line passing through the subsolar point at speeds comparable to those predicted by a simple analytical model. However, at least at formation, well-developed FTE signatures do not extend beyond 5 RE along this tilted line.
- Evidence indicating that the Kelvin-Helmholtz instability broadens boundary layers on the flanks at low-latitudes, with the boundary layer thickening downstream, towards lower latitudes and for northward IMF orientations.