[Report published in Astronomy & Geophysics, 44, 3.33-3.34 (2003)]
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Magnetic reconnection may be defined as a change in connectivity of lines of force of a magnetic field in an electrically neutral mixture of ions and electrons known as a plasma. The importance of magnetic reconnection is that through this change in topology, magnetic energy can be released into heat and plasma flow. Its importance is universal since most of the universe is a plasma and most plasmas generate magnetic fields. Magnetic reconnection occurs at the Sun and other stars, in accretion disks and in the near-earth space environment - geospace.
Geospace is the only natural environment in which reconnection can be observed both locally by in-situ spacecraft measurements and globally by remote sensing from the ground. The latter uses the dipole magnetic field as a lens that focuses signatures of reconnection from a vast region on the outer boundary of geospace (the magnetopause) onto relatively small areas of the upper atmosphere (ionosphere) in the polar regions. Possible signatures include the aurora, that can be observed by optical instruments, and plasma convection and associated electrical currents that can be measured by radars and magnetometers (see Figure 1).
Figure1: Ionospheric signatures of magnetopause reconnection. The left-
hand panel shows an image of the aurora over the north polar cap, taken
from the POLAR spacecraft. The right-hand panel is a plot of the
ionospheric convection streamlines in the north polar region, combining
data from ground-based radar and low-altitude spacecraft.
The RAS meeting aimed to improve our understanding of the physical processes and factors controlling reconnection in general by addressing two key specific questions: "What controls the location of magnetopause reconnection?" and "What controls the temporal variation of magnetopause reconnection?" Competing answers to these questions have been advanced through a range of satellite and ground-based observations and theoretical modelling. The meeting brought together about 50 interested researchers to discuss these ideas and identify ways toward resolving these questions through any approach. The meeting comprised three review talks, four talks on recent hot topics, two keynote talks by international speakers, and two posters. These have been summarised by the presenters as follows:
Stan Cowley (Leicester University).
Magnetic reconnection at the Earth's magnetopause: Theory and
modelling. (Review)
The principal focus of this presentation was on how magnetic geometry and the motion of open flux tubes result in theoretical restrictions on where steady reconnection can occur on the magnetopause. The results of simple theoretical models were reviewed, together with more recent models which use more complete descriptions of the field and flow in the magnetosheath and outer magnetosphere. These models show that steady reconnection can occur in principle over wide regions of the magnetopause (though not everywhere) when the IMF points southward, such that the question then remains as to where it occurs most frequently in that case. The two main schools of thought are that reconnection is either mainly subsolar, or occurs mainly where the fields are anti-parallel, which means generally at high latitudes in the vicinity of the cusps. Observations at the magnetopause and in the ionosphere that potentially can distinguish between these scenarios were outlined.
Mike Lockwood (Rutherford Appleton Laboratory/Southampton University).
Reconnection variations in space and time: What can we learn by
combining remote sensing and in-situ satellite observations? (Review)
Lockwood discussed the spatiotemporal characteristics of reconnection, with particular reference to combined data sets from POLAR images and low-altitude spacecraft. No summary of this talk was provided.
Mike Pinnock (British Antarctic Survey).
Imaging the full extent of the magnetopause X-line: the challenge to
ground-based experimenters. (Review)
Ionospheric observations in the polar regions provide a technique for obtaining a global view of the rate of magnetic reconnection. By determining the ionospheric footprint of the magnetopause and measuring the plasma motion across this boundary (in the rest frame of the boundary) the magnetic reconnection rate can be determined. Figure 1 shows one such dataset, constructed from ground-based radars and polar orbiting spacecraft. Pinnock provided a review of the development over the last 10 years of the techniques for doing this and of the results that have been obtained. Such measurements have confirmed the temporal variability of the reconnection rate, as first discovered by single meridian observations by radars and satellites. But their principle contribution has been to provide data on the spatial variability of the reconnection rate. One case study has shown that most of the dayside magnetopause (~40 Earth radii in extent) is involved in reconnection during IMF southward conditions. Whilst some of the results fit accepted models (such as control by the east-west component of the IMF), some are unexpected. For example, one study has reported a peak in reconnection rate on the dusk flank of the magnetopause for purely southward IMF conditions, an unexpected result that does not fit with theory.
Iain Coleman (British Antarctic Survey).
Inferring the macroscopic physics of magnetopause reconnection from
radar observations. (Hot topic)
The ionospheric signatures of reconnection are conditioned by the location of reconnection regions on the magnetopause, and by spatial and temporal variations in the associated reconnection electric field. These factors in turn depend on the underlying physics of reconnection. Coleman described the work done by the British Antarctic Survey on using ionospheric signatures of reconnection, observed by SuperDARN radars, to distinguish between competing reconnection theories. The implications of the results for the large-scale physics of reconnection were discussed. The results suggest that the time history of reconnection may play an important role in influencing where and when new merging sites can be established, and Coleman proposed mechanisms by which this may occur.
Tim Yeoman, Leicester University.
Remote-sensing dayside reconnection processes in the polar ionosphere.
(Hot topic)
Ionospheric investigations of the convection and auroral dynamics associated with magnetopause reconnection have the advantage that in principle large regions of space can be remotely-sensed, whereas in situ spacecraft can only provide point measurements. To get a truly global picture of the reconnection process, however, the more ionosphere that can be surveyed the better. In the past, most observations of the auroral signature of reconnection have employed ground-based imagers, but these suffer from several drawbacks. Chief amongst these are the relatively small viewing area provided and the inability to make measurements in sunlight or moonlight, limiting observations to a two- or three-week window near winter solstice. Ultraviolet observations from space, such as with the UVI imager onboard Polar or the FUV instrument on IMAGE, overcome these shortcomings to a large degree. When combined with SuperDARN radar observations of the global convection pattern, a powerful diagnostic tool is provided. SuperDARN can provide information not only on convection electric fields, but also the ionospheric irregularities created by cusp precipitation, and the associated field aligned current systems. This presentation showed UVI and SuperDARN observations of reconnection processes occurring during southward IMF conditions which for the first time provide compelling evidence for the truly large- scale nature of solar-terrestrial coupling, with reconnection bursts suggested to sweep across many hours of local time along the magnetopause from the noon sector to the magnetopause flanks. Yeoman discussed the implications of these observations, and outlined directions for future work.
Katie Throp (Southampton University).
The use of auroral imagery to define variations of magnetopause
reconnection in time and space. (Hot topic)
Throp presented the results of a study into the coupling of reconnection variations in space and time with signatures observed in the proton aurora. Data from two transient events seen at around 8 UT and 15:20 UT on the 26th November 2000, both of which were seen by a range of satellite-borne and ground-based instruments, were compared. The first event, believed to be a pair of travelling convection vortices, was linked to a brief southward excursion of the IMF accompanied by a significant increase in the solar wind density and pressure. The second event, characterised by a brief brightening of Lyman-alpha emission in the cusp, was caused by a short-lived southward turning in the IMF with no change to the solar wind pressure. The results of this analysis show that the fraction of emission produced by secondary electrons (resulting from proton precipitation), as opposed to that from direct electron precipitation, can be used to test for anti-parallel and component (subsolar) reconnection. The determination of this fraction is the subject of modelling work, and comparison with measurements from the ground-based imaging spectrograph on Svalbard.
Chris Owen (Mullard Space Science Laboratory).
Cluster observations at the magnetopause: New insights into the
reconnection process? (Hot topic)
Owen presented a 'crater' flux transfer event observed by the 4- spacecraft Cluster mission. Following the ISEE and AMPTE missions, such signatures were the subject of a debate on whether they are due to magnetopause reconnection processes or solar wind pressure pulses driving the magnetopause boundary layers inwards over a spacecraft. For this event, the latter explanation initially appeared to be supported, as the spacecraft located closest to the magnetopause, briefly moves out into the magnetosheath. However, the signatures at the 3 spacecraft that remain within the magnetosphere are consistent with the spacecraft sampling different parts of the velocity-dispersed electron and ion boundary layers expected downstream from an active reconnection site, clearly indicating that this FTE has a reconnection origin. The use of such multi-spacecraft data promises further progress in our understanding of these effects.
Karlheinz Trattner (Lockheed Martin).
Cusp structures as a tool to evaluate reconnection location and rate at
the magnetopause. (Keynote)
Downward precipitating ions in the cusp regularly exhibit sudden changes in the ion energy distributions, forming distinctive structures that are used to study the temporal/spatial nature of reconnection at the magnetopause. The observation of such cusp structures observed simultaneously with the Polar, FAST and Interball satellites revealed remarkable similar features. These cusp structures could be observed up to several hours during stable solar wind conditions. Their similarities led to the conclusion that large-scale cusp structures are spatial structures related to global ionospheric convection pattern set up by magnetic merging and not the result of temporal variations in reconnection parameters. The launch of the Cluster fleet allows the analysis of cusp structures in great detail and during changing solar wind conditions using three spacecraft with identical instrumentation. The compact orbit configuration of the Cluster spacecraft operating at about the same altitude allows for delay times of about 45 minutes in crossing the cusp. To link Cluster observations with convection cells the ion observations were combined with SuperDARN radar observations that are used to derive convection pattern in the ionosphere. The combination of satellite observations with ground observations during variable solar wind conditions shows that large- scale cusp structures are in agreement with both, spatial and temporal structures. Cusp structures could be described as spatial features caused by satellites crossing into spatially separated flux tubes. These flux tubes were formed by spatially separated convection cells, emanating from multiple reconnection lines at the magnetopause. Cusp structures could be also observed as poleward traveling features within the same convection cell, caused most probably by variations in the reconnection rate at the magnetopause.
Nelson C. Maynard (Mission Research Corporation).
Coupling the solar-wind/IMF to the ionosphere: Global- and meso-scale
dynamics of magnetic merging. (Keynote)
Interpretations of the global and meso-scale temporal and spatial dynamics of magnetic merging can be constrained by correlating measurements from diverse locations and using large scale MHD models to put the measurements in context. Evidence from spacecraft measurements by the Polar and Cluster satellites in the magnetosphere and magnetosheath, and SuperDARN and sounding rockets in the ionosphere, demonstrates that merging often occurs at high latitudes in the vicinity of the cusps. The location is in part controlled by the clock angle in the IMF YZ plane. In fact, BY bifurcates the cusp relative to source regions. Merging is temporally variable and may be occurring at multiple sites simultaneously. BX controls the timing of the interaction, and merging sites in each hemisphere may respond at temporally different times. Correlation times are variable and are controlled by the dynamics of the tilt of the interplanetary electric field phase plane. The newly opened field lines may couple to the ionosphere at MLT locations of as much as 3 hours away from local noon. All-sky photometer observations of 557.7 nm emissions in the cusp region provide a "television picture" of the merging process and can be used to monitor the temporal and spatial variability of merging, tied to variations in the IMF.
Stuart Bale, Forrest Mozer, and Tai Phan (Space Sciences Laboratory,
University of California, Berkeley).
Observation of lower hybrid drift instability in the diffusion region
at a reconnecting magnetopause. (Poster)
Intense lower hybrid waves are observed in the Hall current region of a reconnecting current sheet at the earth's magnetopause. Large measured cross-field drifts and density gradients are the likely sources of free energy through a lower hybrid drift instability (LHDI), which is stabilized near the neutral point where the plasma beta is large. The authors fitted Harris neutral sheet and linear models to the data and compared the resulting inferred current density profiles to the observed current density from particle measurements. The contribution of LHDI anomalous resistivity to the parallel electric field was estimated, and shown to be more than 100 times smaller than the measured parallel field at the separator boundaries and essentially zero near the neutral point. This leaves the gradient electron pressure tensor and inertial terms, or anomalous resistivity from higher frequency instabilities, to support any parallel fields there and hence control the reconnection process.
Bogdan Hnat, Sandra Chapman, George Rowlands (University of Warwick),
Nick Watkins and Mervyn Freeman (British Antarctic Survey).
A statistical comparison between fluctuations in epsilon and in
geomagnetic indices. (Poster)
Solar wind-magnetosphere coupling via reconnection is a fundamental aspect of magnetospheric dynamics. Geomagnetic indices are of particular interest in this context as they provide a global measure of magnetospheric output and are evenly sampled over a long time interval. The epsilon parameter, also used in this study, represents energy input from the solar wind into the magnetosphere. The authors applied the finite size scaling technique to fluctuations in the AU, AL and AE indices and in the epsilon parameter. This allowed them to make a quantitative comparison of the statistical properties of the solar wind input and the magnetospheric response. They found that the exponents needed to rescale the probability density functions (PDF) of the fluctuations are the same to within experimental error for all four quantities. This self-similarity persists for time scales up to ~4 hours for AU, AL and epsilon and up to ~2 hours for AE. On shorter time scales than these, the flucuations were found to have similar PDF: this long-tailed (leptokurtic) PDF is consistent with an underlying turbulent process.
The organisers would like to thank all the participants for their contributions to the meeting. Mervyn Freeman would also like to thank the RAS for inviting him to give a talk at the Ordinary Meeting of the RAS after the Discussion meeting. The talk entitled, "Recent Advances in Understanding Magnetic Reconnection Using Remote-Sensing and In-Situ Observations From Geospace" is summarised in The Observatory magazine.
Magnetic Reconnection at the Earth's Magnetopause: Theory and Modelling
S W H Cowley ; Physics & Astronomy, University of Leicester
An overview will be presented of theoretical and modelling studies which are germane to the temporal and spatial structure of magnetic reconnection at the Earth's magnetopause, involving signatures at the magnetopause and in the dayside cusp, together with their counterparts in the high-latitude ionosphere. Particular emphasis will be placed on the signatures of pulsed reconnection and their relation to observed 'flux transfer events'.
Inferring the macroscopic physics of magnetopause reconnection from radar observations
Iain J. Coleman
British Antarctic Survey, Cambridge.
The ionospheric signatures of reconnection are conditioned by the location of reconnection regions on the magnetopause, and by spatial and temporal variations in the associated reconnection electric field. These factors in turn depend on the underlying physics of reconnection. This talk will describe our work on using ionospheric signatures of reconnection, observed by SuperDARN radars, to distinguish between competing reconnection theories. The implications of our results for the large-scale physics of reconnection will be discussed. Our work suggests that the time history of reconnection may play an important role in influencing where and when new merging sites can be established, and this talk will propose mechanisms by which this may occur.
Remote-sensing dayside reconnection processes in the polar ionosphere
Tim Yeoman, Steve Milan, and Kathryn McWilliams
Physics and Astronomy, University of Leicester.
Ionospheric investigations of the convection and auroral dynamics associated with magnetopause reconnection have the advantage that in principle large regions of space can be remotely-sensed, whereas in situ spacecraft can only provide point measurements. To get a truly global picture of the reconnection process, however, the more ionosphere that can be surveyed the better. In the past, most observations of the auroral signature of reconnection have employed ground-based imagers, but these suffer from several drawbacks. Chief amongst these are the relatively small viewing area provided and the inability to make measurements in sunlight or moonlight, limiting observations to a two- or three-week window near winter solstice. Ultraviolet observations from space, such as with the UVI imager onboard Polar or the FUV instrument on IMAGE, overcome these shortcomings to a large degree. When combined with SuperDARN radar observations of the global convection pattern, a powerful diagnostic tool is provided. SuperDARN can provide information not only on convection electric fields, but also the ionospheric irregularities created by cusp precipitation, and the associated field aligned current systems. We show UVI and SuperDARN observations of reconnection processes occurring during southward IMF conditions which for the first time provide compelling evidence for the truly large-scale nature of solar-terrestrial coupling, with reconnection bursts suggested to sweep across many hours of local time along the magnetopause from the noon sector to the magnetopause flanks. The implications of this will be discussed, and directions for future work will be outlined.
Observation of lower hybrid drift instability in the diffusion region at a reconnecting magnetopause
S. D. Bale, F. S. Mozer, and T. Phan
Space Sciences Laboratory, University of California, Berkeley
Intense lower hybrid waves are observed in the Hall current region of a reconnecting current sheet at the earth's magnetopause. Large measured cross-field drifts and density gradients are the likely sources of free energy through a lower hybrid drift instability (LHDI), which is stabilized near the neutral point where the plasma beta is large. Harris neutral sheet and linear models are fitted to the data and inferred current density profiles are compared to the observed current density from particle measurements. We estimate the contribution of LHDI anomalous resistivity to the parallel electric field and show that it is more than 100 times smaller than the measured parallel field at the separator boundaries and essentially zero near the neutral point, leaving gradient electron pressure tensor and inertial terms or anomalous resistivity from higher frequency instabilities to support any parallel fields there and, hence, control the reconnection process.
A statistical comparison between fluctuations in epsilon and in geomagnetic indices
B. Hnat¹, S. C. Chapman¹ , G. Rowlands¹ , N. W. Watkins² and M. P. Freeman²
1. University of Warwick.
2. British Antarctic Survey, Cambridge.
Solar wind-magnetosphere coupling via reconnection is a fundamental aspect of magnetospheric dynamics. Geomagnetic indices are of particular interest in this context as they provide a global measure of magnetospheric output and are evenly sampled over a long time interval. The epsilon parameter, also used in our studies, represents energy input from the solar wind into the magnetosphere. We apply the finite size scaling technique to fluctuations in the AU, AL and AE indices and in the epsilon parameter. This allows us to make a quantitative comparison of the statistical properties of the solar wind input and the magnetosperic response. We find that the exponents needed to rescale the probability density functions (PDF) of the fluctuations are the same to within experimental error for all four quantities. This self-similarity persists for time scales up to ~4 hours for AU, AL and epsilon and up to ~2 hours for AE. On shorter time scales than these, the flucuations are found to have similar PDF, this long-tailed (leptokurtic) PDF is consistent with an underlying turbulent process.
Coupling the solar-wind/IMF to the ionosphere: Global- and meso-scale dynamics of magnetic merging
Nelson C. Maynard
Mission Research Corporation
Magnetic merging is a primary means for coupling energy from the solar wind into the magnetosphere-ionosphere system. The location and nature of the process remain as open questions. By correlating measurements from diverse locations and using large scale MHD models to put the measurements in context, it is possible to constrain our interpretations of the global and meso-scale dynamics of magnetic merging. Recent evidence demonstrates that merging often occurs at high latitudes in the vicinity of the cusps. The location is in part controlled by the clock angle in the IMF YZ plane. In fact, BY bifurcates the cusp relative to source regions. Merging is temporally variable and may be occurring at multiple sites simultaneously. BX controls the timing of the interaction and merging sites in each hemisphere may respond at temporally different times. Correlation times are variable and are controlled by the dynamics of the tilt of the interplanetary electric field phase plane. The newly opened field lines may couple to the ionosphere at MLT locations of as much as 3 hr away from local noon. SuperDARN and all-sky photometer observations can be used to monitor the temporal and spatial variability of merging.