2008 Workshop:Space Weather Effects and Aeronomy Studies at the Plasmaspheric Boundary Layer
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Space Weather Effects and Aeronomy Studies at the Plasmaspheric Boundary Layer
Location
Basel
Date/Time
1600-1800 Wednesday 18 June 2008
Conveners
Format of the Workshop
a few short tutorial background talks, followed by a number of shorter talks, with group discussion at the end
Duration
2 hours (default)
Estimated attendance
30-40
Conflicts with other workshops
IS Radar sessions, in particular AMISR workshops, SuperDARN sessions, World Day Scheduling meetings, DASI sessions
Special technology requests
internet
Forum
Comments, Questions, Discussion Forum
Brief Initial Description
The plasmasphere boundary layer (PBL) [Carpenter, 2004] is the region characterized by dynamic interaction between the plasmas of the inner and outer (auroral) magnetosphere. It is also a region where significant magnetosphere-ionosphere (MI) coupling occurs. For example, during geomagnetic storms, the ionosphere plays a critical role in the development of fast, latitudinally narrow plasma flows into the plasmapause region. Some of the critical interactions that occur in this region include the development of electric fields which couple the ionosphere, plasmasphere, and magnetosphere, the structuring and redistribution of thermal plasmas, and the formation of different scale-sizes of irregularities.
Many significant mid-latitude space weather events are associated with the geophysics of the PBL region. These include the formation of the plume of ionization associated with storm enhanced density and the production of mid-latitude irregularities observed by the new mid-latitude SuperDARN HF radars. Understanding the nature of these effects, their magnitude, and their spatial and temporal characteristics is critical to characterizing the PBL region and its underlying geophysics. Such knowledge is required in order to forecast and nowcast mid-latitude space weather events.
The goal of this workshop is to begin to build a framework that can be used to understand this complex region and to interpret experimental observations. We hope to formulate several unsolved questions involving this interaction region and to identify the observations and investigations required to address them. With this objective, the workshop will begin with a few short tutorial talks, followed by a number of contributed talks. The goal is to have a forum which includes both modelers and experimenters interested in this dynamic region.
Short talks solicited for this workshop include those which describe models and experimental observations of PBL geophysical processes, including M-I coupling. Time will be allocated at the end of the workshop for group discussion.
Student Summary
This workshop is focused on the plasmaspheric boundary layer (PBL) [Carpenter and Lemaire, 2004], which is a region of significant M-I coupling that is not well understood. It is a dynamic region between two distinct plasma flow regimes, one convecting sunward, the other co-rotating. Some of the consequences of the dynamics in this region result in significant space weather features, such as the storm enhanced density plume of ionization and SAR arcs. Some of the critical interactions that occur in this region include the development of electric fields which couple the ionosphere, plasmasphere, and magnetosphere, the structuring and redistribution of thermal plasmas, and the formation of different scale-sizes of irregularities. The goal of this workshop is to begin to build a framework that can be used to understand this complex region and to interpret experimental observations. We hope to formulate several unsolved questions involving this interaction region and to identify the observations and investigations required to address them. With this objective, the workshop will begin with a few short tutorial talks, followed by a number of contributed talks. We want to provide a forum which includes both modelers and experimenters interested in this dynamic region.
Further Background
Ionospheric scientists typically think of the plasmasphere as the region above the ionosphere that extends from approximately 1000 km on up and consider it to be a region that has relatively few electrons/ions. Magnetospheric scientists think of the plasmasphere differently. For them, the plasmasphere is a torus (an odd-shaped donut) of plasma that is dense (tens to thousands of particles per cubic centimeter) and cool (~1ev).
In fact the plasmasphere can be thought of as both an extension of the ionosphere and as a part of the inner magnetosphere. The plasmasphere is filled with ionospheric plasma from the mid- and low latitudes that has moved upward along magnetic field lines until the plasma gas pressure has been equalized along the entire field line. This plasma co-rotates with the Earth and its motion is dominated by the geomagnetic field. The outer boundary of the plasmasphere is known as the plasmapause (Carpenter, 1963). At this boundary there is an order of magnitude drop in the plasma density. This is because the magnetic field lines associated with higher latitudes (those approximately above 60 degrees geomagnetic latitude) are convected to the magnetopause and are thus open to the interplanetary medium (where the plasmaspheric plasma is lost). Under very quiet solar conditions, the plasmapause may extend to nearly seven Earth radii, and with very disturbed conditions it contracts to about three (sometimes even less) Earth radii.
The plasma that is observed outside of the plasmapause is associated with the central sheet of the magnetosphere. It is subject to convection electric fields, and thus has a very different circulation pattern than the plasma inside the plasmapause. The plasma outside of the plasmapause flows sunward toward the dayside magnetopause. The plasma inside the plasmasphere is co-rotating with the Earth. The workshop is looking at the physics that occurs at the boundary between these two flow regimes.
References
Carpenter, D. L., Whistler evidence of a 'knee' in the magnetospheric ionization density profile,(1963), J. Geophys. Res., 68, 1675-1682.
Carpenter, D. L. and J. Lemaire, The Plasmasphere Boundary Layer Annales Geophysicae (2004) 22: 4291–4298
Workshop Agenda
Introduction
Anthea Coster
Jan Sojka (modeling)
John Foster (physics)
PBL Observations
Mike Ruohoniemi – mid-latitude SuperDARN HF radars
Michael Mendillo - SAR arcs
Phil Erickson – plasma physics
Susan Skone - space weather
Discussion
Mike Ruohoniemi
Workshop Summary
The plasmasphere encompasses the doughnut-shaped region of closed, dipolar field lines in the innermost portion of Earth’s magnetosphere. Previously viewed as a relatively inactive component of the couple solar wind – magnetosphere system, new measurement techniques have revealed a host of dramatic effects that indicate the importance of understanding the dynamics of the inner magnetosphere and magnetically conjugate subauroral ionosphere. This workshop was convened to introduce the CEDAR community to the new results and to identify areas of potential collaboration between experimentalists, modelers, and the GEM community. Several review talks were followed by presentations on focused science results and a lively discussion of outstanding scientific questions.
Anthea Coster introduced the workshop by briefly describing the plasmasphere and plasmapause. She illustrated the appearance of the plasmasphere from deep space with the aid of pictures from the IMAGE satellite. These showed the erosion of the plasmasphere during storm conditions and the emergence of plumes. Jan Sojka then explained that the current models do not describe the plasmasphere very well. In particular, the models do not explain how electric fields reach inside the plasmasphere nor can they account for rapid time variations. As of now, only empirical relationships exist to describe the interactions of cool and hot particles and the interactions of particles with waves. T. Grebowsky commented from the floor that the interaction of the ring current with waves is an open problem and that the MHD models are inadequate to describe these processes.
John Foster emphasized the role of the boundary layer of the plasmasphere as the interface between the auroral magnetosphere and the IT-dominated inner region. The layer extends across ~1 L-shell and has active exchange of wave-particle energy and momentum. He reviewed the range of effects associated with perturbation of the plasmaspheric boundary layer, including SAPS, SED, SAID, and SAR arcs, and recalled early studies that explained whistler chorus in terms of wave-particle interaction at the PBL. He described how the disturbed ring current drives field-aligned current into the subauroral ionosphere and that flow and thermal gradients at the PBL lead to instability.
In the second part of the workshop the presentations shifted to application of specific techniques to observe effects related to the PBL. J. Michael Ruohoniemi showed examples from the new mid-latitude SuperDARN radars. In addition to the expansion of auroral electric fields to mid-latitudes during storms, these showed the spatial extent and temporal dynamics of SAPS and SAID events. During less disturbed periods the radars routinely detect backscatter from plasmapause irregularities; a joint Millstone Hill ISR – Wallops SuperDARN study has shown that these are due to the temperature gradient instability (TGI). The plasmapause backscatter renders views of the ionospheric electric field in the subauroral ionosphere and a movie was presented that showed the impact of substorm activity over several hours of MLT using simultaneous observations from the Wallops and Blackstone radars.
Michael Mendillo described ground-based observations of stable auroral red (SAR) arcs, which arise when the ring current encounters the edge of the plasmasphere during energization events. The arcs, despite their name, are structured and time variable. Of particular note, he described a case-study of a single SAR arc on the night of 29 Oct 1991 (Baumgardner et al., Annales Geophysicae, 25,2593-2608,2007). This SAR arc could possibly have been the brightest one since 1975. The Boston University CEDAR Optical Facility at the Millstone Hill/Haystack Observatory measured a peak value of above-atmosphere brightness of 13,500 R +- 20 %. This is compared to long-term averages of SAR arcs measured at the Millstone Hill location of 500 R +- 270R (and 20% uncertainty). It was concluded that the extremely high brightness levels observed were due to the heating of electron densities not in the trough but with the higher Ne values equatorward of the trough. This activity expresses important aspects of the dynamics of the inner magnetosphere, and of the relationship between the ring-current plasmasphere boundary and the mid-latitude trough. .
Phil Erickson spoke on ionospheric plasma waves generated in the vicinity of the PBL and observed on a sidelobe of the Millstone Hill ISR. The ISR measurements and new techniques utilizing passive radio observations show fingers of SAIDs moving poleward due to conductivity variations. The variability of the electric fields is being studied and the complex interaction of field-aligned currents and conductivity variations that underly this variability needs to be explored.
Susan Skone offered a view of space weather effects in subauroral ionosphere. She described the impact of scintillations on GPS applications and reviewed advances in GPS technology that offer mitigation. She discussed the error imparted to WAAS navigation from ionospheric perturbations and traced the source of the errors to the large gradients that develop near plumes.
The workshop concluded with an exchange of views on basic questions posed from the floor, such as the reasons for inconsistency between the magnetospheric and ionospheric manifestations of PBL processes and the limiting conditions for SAPS electric fields. The participants felt that the CEDAR community can provide necessary measurements and insight into the coupling and interdependence of magnetospheric processes with the subauroral ionosphere and neutral atmosphere and that future workshops - possibly to include GEM researchers - should be organized on this theme.
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