Laissez Les Bon Recherche de LLBL Rouler

P. T. Newell

The Johns Hopkins University Applied Physics Laboratory

Laurel, Maryland, 20723

T. G. Onsager

NOAA Space Environment Center, 325 Broadway,

Boulder, CO 80303

Submitted to EOS.

May 9, 2001.

Revised and Resubmitted August 16, 2001

Eighty scientists gathered in New Orleans from April 16-20, 2001 to discuss the “The Low-Latitude Boundary Layer And Its Dynamic Interaction with the Solar Wind and Magnetosphere”. Patrick T. Newell, Michael Lockwood, and Walter J. Heikkila served as convenors. Although the topic was somewhat more narrowly defined than most previous Chapman Meetings, the turnout was respectable, albeit in part because many scientists ventured into neighboring issues – notably merging – which have dominated previous related meetings such as the 1994 Chapman conference on the “Physics of the Magnetopause”. Discussions were unusually lengthy and lively, and fortunately were conducted with little rancor. An early highlight came in Tim Eastman’s historical review of early LLBL research, establishing, for instance, when certain terms first entered the literature (“LLBL” was first used by Haerendel and colleagues in 1978, although earlier observations of this region exist).

The three key LLBL-specific research topics focused upon were (1) Whether the LLBL is open or closed; (2) Whether it forms by merging, diffusion through wave-particle action such as the Kelvin-Helmholtz instability, or by impulsive penetration; and (3) The role of the LLBL as a plasma source and sink for the rest of the magnetosphere. Obviously it is unrealistic to expect active controversies to crystallize into consensus views over the course of a single meeting. Nonetheless, such a formidable gathering of scientists working on a single subject does help provide insight into the current status of these problems. In some areas agreement has developed and the debate has narrowed, while others remain as disputed as ever.

In reviewing the first issue, Stephen Fuselier declared that the evidence showed that the LLBL on the frontside of the magnetopause is largely open, although some closed LLBL field lines could be found there. Fuselier did agree that instances of largely closed LLBL appear much more common along the flanks and further downstream, although that issue has been less studied. It seems significant that Fuselier’s summary received little opposition (as such a statement likely would have a few years ago). For example Newell agreed that low-altitude particle signatures which can be construed as the closed LLBL occur mostly 2 hours or more away from noon (certainly for southward IMF), and thus probably map to the flanks or beyond at high altitude. The issue is not settled, but it appears a tentative new standard picture is emerging, with the LLBL mostly open on the frontside (especially for southward IMF, but also for weakly northward IMF) and mostly closed along the flanks and further downtail.

On the second issue, the formation of the LLBL, far more work remains, yet even here a partial answer is emerging. Terry Onsager presented Polar observations which appear to convincingly demonstrate that reverse merging poleward of the cusp for northward IMF can create both open and closed frontside LLBL field lines. This type of Song-Russell mechanism, which involves simultaneous uncorrelated merging in opposite hemispheres, was still considered quite controversial even a few years ago. The new Polar observations appear to have demonstrated that the Song-Russell mechanism does work, at least some of the time. However given the quasi-permanent nature of the LLBL, extending over huge volumes of space under a variety of IMF conditions, this single mechanism is unlikely to be the only means of LLBL formation, and may not be the major mechanism. For example, a great deal of evidence was presented that wave-particle interactions can transport plasma across the magnetopause flanks, creating a closed boundary layer. Masaki Fujimoto’s presentation of Geotail observations of this effect were particularly compelling, although it is not clear whether the surface waves observed along the magnetopause are really Kelvin-Helmholtz or are actually some other type.

On the third major LLBL-specific topic, the extent to which the LLBL acts as a plasma source and sink for the rest of the magnetosphere, observational evidence is mounting that the LLBL is indeed a major source for the magnetotail. For example, Marit Øieroset presented Wind and Geotail observations showing that a dense cold plasma population appears along the magnetotail flanks, especially for northward IMF. This cold dense population is found within the plasma sheet, and covers an extended region. Other source and sink roles for the magnetospheric boundary layers, such as energetic ion production in the cusp (Jiasheng Chen), appear much more controversial.

As previously mentioned, several topics which are related to the LLBL but are not LLBL specific were discussed, especially that perenial favorite, merging. Here the argument was mainly between antiparallel merging and component merging. Jim Drake presented theoretical calculations that for a low-beta plasma, component merging is as fast as anti-parallel, while for high beta plasma, component merging is suppressed, and only anti-parallel merging occurs.

Observationally, Fuselier, using results from the recently launched Image satellite, argued for component merging for southward IMF, then asked why it should be different for northward. However several presentations, including Nelson Maynard and Alan Rodger presented results such as a bifurcated cusp which they believe are consistent with antiparallel but not component merging, even under weakly southward (but large IMF B_y) conditions. The number of supporters of antiparallel merging appeared larger at this meeting than at previous meetings.

A few argued against merging altogether, notably Walter Heikkila and Joseph Lemaire. These individuals advocate impulsive penetration models, claiming the standard merging model does not give kinetic effects (Newton’s laws) equal treatment with Maxwell’s equations. However the overwhelming majority of scientists present continued to accept merging as the dominant form of magnetopause-solar wind interaction. Some scientists are willing to regard impulsive penetration as a topic of interesting but distinctly secondary importance. Young researchers continue to produce closely related work. At this meeting Helene Stenuit presented some fairly impressive InterBall results showing plasma injections along the LLBL flanks into closed auroral zone field lines, especially for weakly northward IMF conditions. These plasma injections have magnetosheathlike energies, and occur in conjunction with solar wind pressure pulses.

Some of the issues debated at the 2001 New Orleans meeting were debated 20 years ago and some may be debated again 20 years hence. Nonetheless, on each of the three key LLBL-specific issues, a clearer picture has emerged. The LLBL definitely includes both open and closed field lines, and is very likely mostly open on the frontside for southward (or even weakly northward) IMF. It is probably mostly closed downstream of the dawn-dusk meridian. The Song-Russell mechanism does create both open and closed field lines for northward IMF, although it is far too early to say that the major means of LLBL formation has been identified. Mounting evidence indicates that the LLBL acts as a significant source of cold dense plasma for the magnetotail.

Blessed by unusually good weather and long spirited discussions, the tightly focussed New Orleans meeting achieved its goal in illuminating the current state of research on this controversial topic. P. T. Newell and T. G. Onsager will edit a book deriving from this AGU Chapman Conference on the Low-Latitude Boundary Layer. As our title urges, “Let the good LLBL research roll!”