ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the... more ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the dynamics and composition of the magnetosphere and the high latitude ionosphere. There are two competing hypotheses for such reconnection, sub-solar and anti-parallel. In this paper, evidence is provided that suggests that anti-parallel reconnection occurs, at least under some circumstances. Some of the consequences of anti-parallel reconnection are considered. These include the absence of a low latitude boundary layer during southward IMF, the relative timing of reconnection events as observed in the ionosphere, the time-dependence of ion outflow and the implications for the composition of the magnetosheath, and the transient nature of reconnection. Some suggestions for tests that would allow differentiation between the anti-parallel and sub-solar reconnection are also made.
ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the... more ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the dynamics and composition of the magnetosphere and the high latitude ionosphere. There are two competing hypotheses for such reconnection, sub-solar and anti-parallel. In this paper, evidence is provided that suggests that anti-parallel reconnection occurs, at least under some circumstances. Some of the consequences of anti-parallel reconnection are considered. These include the absence of a low latitude boundary layer during southward IMF, the relative timing of reconnection events as observed in the ionosphere, the time-dependence of ion outflow and the implications for the composition of the magnetosheath, and the transient nature of reconnection. Some suggestions for tests that would allow differentiation between the anti-parallel and sub-solar reconnection are also made.
Mervyn Freeman, Gareth Chisham, Iain Coleman and Mike Pinnock report on the November discussion m... more Mervyn Freeman, Gareth Chisham, Iain Coleman and Mike Pinnock report on the November discussion meeting ``Temporal and spatial structure of magnetic reconnection on the Earth's magnetopause'', the only natural environment in which this universal astrophysical phenomenon can be observed locally and globally.
M agnetic reconnection may be defined as a change in connectivity of lines of force of a magnetic... more M agnetic reconnection may be defined as a change in connectivity of lines of force of a magnetic field in a plasma, an electrically neutral mixture of ions and electrons. Through this change, magnetic energy can be released, into heat and plasma flow. Reconnection is universally important: most of the universe is plasma and most plasmas generate magnetic fields. Magnetic reconnection occurs at the Sun and other stars, in accretion disks and in near-Earth space -geospace.
An extensive variety of instruments, including Geotail, DMSP F11, SuperDARN, and IMP-8, were moni... more An extensive variety of instruments, including Geotail, DMSP F11, SuperDARN, and IMP-8, were monitoring the dayside magnetosphere and ionosphere between 14:00 and 18:00 UT on 18 January 1999. The location of the instruments provided an excellent opportunity to study in detail the direct coupling between the solar wind, the magnetosphere, and the ionosphere. Flux transfer events were observed by Geotail near the magnetopause in the dawn side magnetosheath at about 4 magnetic local time during exclusively northward interplanetary magnetic field conditions. Excellent coverage of the entire dayside high-latitude ionosphere was achieved by the Northern Hemisphere Su-perDARN radars. On the large scale, temporally and spatially, the dayside magnetosphere convection remained directly driven by the interplanetary magnetic field, despite the highly variable interplanetary magnetic field conditions, including long periods of northward field. The SuperDARN radars in the dawn sector also measured small-scale temporally varying convection velocities, which are indicative of flux transfer event activity, in the vicinity of the magnetic footprint of Geotail. DMSP F11 in the Southern Hemisphere measured typical cusp precipitation simultaneously with and magnetically conjugate to a single flux transfer event signature detected by Geotail. A study of the characteristics of the DMSP ion spectrogram revealed that the source plasma from the reconnection site originated downstream of the subsolar point. Detailed analyses of locally optimised coordinate systems for individual flux transfer events at Geotail are consistent with a series of flux tubes protruding from the magnetopause, and originating from a high-latitude reconnection site in the Southern Hemisphere. This high-latitude reconnection site agrees with plasma injected away from the sub-Correspondence to: K. A. McWilliams ([email protected]) solar point. This is the first simultaneous and independent determination from ionospheric and space-based data of the location of magnetic reconnection.
When the interplanetary magnetic field (IMF) encounters the Earth's magnetosphere, it is compress... more When the interplanetary magnetic field (IMF) encounters the Earth's magnetosphere, it is compressed and distorted. This distortion is known as draping, and plays an important role in the interaction between the IMF and the geomagnetic field. This paper considers a particular aspect of draping, namely how the orientation of the IMF in a plane perpendicular to the Sun-Earth line (the clock angle) is altered by draping in the magnetosheath close to the day-side magnetopause. The clock angle of the magnetosheath field is commonly estimated from the interplanetary magnetic field (IMF) measured by upstream monitoring spacecraft either by assuming that the draping process does not significantly alter the clock angle (" perfect draping ") or that the change in clock angle is reasonably approximated by a gas dynamic model. In this paper, the magnetosheath clock angles measured during 36 crossings of the magnetopause by the Geotail and Interball-Tail spacecraft are compared to the upstream IMF clock angles measured by the Wind spacecraft. Overall, about 30% of data points exhibit perfect draping within ±10 • , and 70% are within 30 •. The differences between the IMF and magnetosheath clock angles are not, in general, well-ordered in any systematic fashion which could be accounted for by hydrodynamic draping. The draping behaviour is asymmetric with respect to the y-component of the IMF, and the form of the draping distribution function is dependent on solar wind pressure. While the average clock angle observed in the magnetosheath does reflect the orientation of the IMF to within ∼30 • or less, the assumption that the magnetosheath field direction at any particular region of the magnetopause at any instant is approximately similar to the IMF direction is not justified. This study shows that re-connection models which assume laminar draping are unlikely to accurately reflect the distribution of reconnection sites across the dayside magnetopause.
This paper presents the results of a multi-spacecraft survey of the draping of magnetic field lin... more This paper presents the results of a multi-spacecraft survey of the draping of magnetic field lines at the dayside magnetopause. A database of approximately forty magne- topause crossings from the Geotail and Interball-Tail spacecraft was analysed, and compared with upstream solar wind data from the Wind spacecraft. This paper shows the relationship between the magnetic field orientation observed in the solar wind and that observed at the magnetopause, as a function of location on the magnetopause and of solar wind conditions. These results are compared to the predictions of field line draping models.
There has been interest in recent years in the fractal properties of structures on the magnetosph... more There has been interest in recent years in the fractal properties of structures on the magnetosphere, including reconnection structures, and in investigating these properties via ionospheric observations. This presentation concernes how fractal properties are affected by the magnetosphere-ionosphere mapping process, and how observed properties in the ionosphere relate to source structures in the magnetosphere.
Large-scale properties of reconnection structures on the magnetopause can be explained successful... more Large-scale properties of reconnection structures on the magnetopause can be explained successfully by simple models incorporating laminar magnetosheath flow with antiparallel reconnection. However, such models are inconsistent with the highly turbulent nature of the magnetosheath flow adjacent to the magnetopause. This presentation proposes a fractal reconnection model that resolves this contradiction by replacing the laminar magnetosheath flow with a turbulent flow which has realistic levels of fluctuation. The resultant fractal reconnection structures preserve the large-scale behaviour of simpler models, consistent with ground-based observations, but have small-scale fluctuations consistent with those observed in situ by spacecraft. We also present observations of evidence for scale-free fluctuations in ionospheric convection associated with dayside reconnection. This scale free behaviour may well arise as a consequence of the fractal reconnection model proposed.
The role of radar observations in resolving the ambiguities between spatial and temporal interpre... more The role of radar observations in resolving the ambiguities between spatial and temporal interpretations of the double cusp will be outlined, and related to the wider context of understanding the large-scale physics of reconnection. Our recent work has established that dayside reconnection under conditions of significant IMF y-component can lead to a split reconnection x-line on the magnetopause, which is observable in the polar ionosphere when the geometry of field-line mapping to the ionosphere is favourable. In this presentation, simultaneous DMSP particle data and SuperDARN radar data will be shown from a split x-line event, demonstrating the connection between split x-line radar signatures and double-cusp particle signatures. Separated reconnection sites will in general have different histories and velocity filter characteristics: the observable consequences of this in the double cusp signature will be discussed. Both temporal and spatial variability in the reconnection site can give rise to a particle cusp discontinuity, and the utility of ground-based data in determining which of these is more important in any particular case will be assessed.
We present observations of spatiotemporal variations in the dayside reconnection rate measured in... more We present observations of spatiotemporal variations in the dayside reconnection rate measured in the ionosphere during an extended interval of northward IMF. The convection electric field during this interval is measured in the high-latitude ionosphere using the northern hemisphere SuperDARN HF radar network. This is combined with an estimation of the location of the ionospheric footprint of the reconnection X-line, as determined by DMSP spacecraft overpasses and SuperDARN measurements, to determine the spatial variations in the reconnection electric field. We also present the temporal variation in the integrated reconnection voltage for an extended time interval which illustrates the transient nature of the reconnection process under these conditions.
We describe a method for determining the limb polarization and limb darkening of stars in eclipsi... more We describe a method for determining the limb polarization and limb darkening of stars in eclipsing binary systems, by inverting photometric and polarimetric light curves. Because of the ill-conditioning of the problem, we use the \relax Backus-Gilbert\relax method to control the resolution and stability of the recovered solution, and to make quantitative estimates of the maximum accuracy possible. Using this method we confirm that the limb polarization can indeed be recovered, and demonstrate this with simulated data, thus determining the level of observational accuracy required to achieve a given accuracy of reconstruction. This allows us to set out an optimal observational strategy, and to critcally assess the claimed detection of limb polarization in the Algol system. The use of polarization in stars has been proposed as a diagnostic tool in microlensing surveys by \cite[Simmons et al. (1995)]{simm95}, and we discuss the extension of this work to the case of microlensing of extended sources.
During an interval of southward IMF we use observations from the SuperDARN HF radar network and t... more During an interval of southward IMF we use observations from the SuperDARN HF radar network and the DMSP satellites to determine the location of the ionospheric projection of the dayside magnetopause reconnection X-line. From the flow of plasma across the polar cap boundary we derive the reconnection electric field and thus de- termine the reconnection rate over a wide range of longitudes. Using the Tsyganenko 96 magnetic field model the ionospheric data are mapped to the magnetopause in or- der to provide an estimate of the extent of the reconnection X-line. Our results are compared with previously reported encounters by the Equator-S and Geotail space- craft with a reconnecting magnetopause near the dawn flank for the same period. The SuperDARN observations allow these satellite observations to be set in the context of the whole magnetopause reconnection X-line. Our results show that the dayside merging gap in the ionosphere was 10 h of magnetic local time (MLT) in extent and mapped to a magnetopause reconnection X-line of 39 Re extent, the largest reported in the literature. The reconnection rate was accurately determined over 7 h of MLT and was exceptionally stable over the 30 minute study period. However, there was a clear spatial variation in the reconnection rate across the dayside ionosphere, with a distinct minimum at 14 h MLT. The total potential associated with dayside reconnection was ~150kV. Whilst the reconnection signatures detected by the Equator-S at ~0730 MLT mapped to a region showing flows across the polar cap boundary, the Geotail satellite at~06 MLT mapped to a region in the ionosphere in which no reconnection flows were measured. From this it appears that terminator and post-terminator reconnection sites may not couple their momentum to the ionosphere.
Magnetic reconnection is the most significant process that re-4 sults in the transport of magneti... more Magnetic reconnection is the most significant process that re-4 sults in the transport of magnetised plasma into, and out of, the Earth's magnetosphere- 5 ionosphere system. There is also compelling observational evidence that it 6 plays a major role in the dynamics of the solar corona, and it may also be important for understanding cosmic rays, accretion disks, magnetic dynamos, 8 and star formation. The Earth's magnetosphere and ionosphere are presently 9 the most accessible natural plasma environments where magnetic reconnec-10 tion and its consequences can be measured, either in situ, or by remote sens-11 ing. This paper presents a complete methodology for the remote sensing of 12 magnetic reconnection in the magnetosphere from the ionosphere. This method 13 combines measurements of ionospheric plasma convection and the ionospheric 14 footprint of the reconnection separatrix. Techniques for measuring both the 15 ionospheric plasma flow and the location and motion of the reconnection sep-16 aratrix are reviewed, and the associated assumptions and uncertainties as-17 sessed, using new analyses where required. Application of the overall method-18 ology is demonstrated by the study of a 2-hour interval from 26 December 19 2000 using a wide range of spacecraft and ground-based measurements of the 20 northern hemisphere ionosphere. This example illustrates how spatial and 21 temporal variations in the reconnection rate, as well as changes in the bal-22 ance of magnetopause (dayside) and magnetotail (nightside) reconnection, 23 can be routinely monitored, affording new opportunities for understanding 24 the universal reconnection process and its influence on all aspects of space 25 weather.
Monthly Notices of The Royal Astronomical Society, 2002
We discuss the flux and polarization signals obtained from the microlensing of stars with extende... more We discuss the flux and polarization signals obtained from the microlensing of stars with extended circumstellar envelopes by a single point-mass lens. A previous paper considered the case of main-sequence stars, and showed that microlensing of the stellar photosphere could produce a small net polarization (up to 0.1 per cent). In this paper, we show that stars with extensive envelopes will show a much higher level of variable polarization (up to 10 per cent), even if they are spherically symmetric. Since the stellar envelopes most likely to be lensed are produced by red giant winds, we also investigate the effects of a central cavity (representing the dust condensation radius) within the scattering envelope, as well as different radial density distributions. Our study has led to three major results. (i) For optically thin envelopes with an inner cavity, the lensing light curve for the polarization can be of longer duration than that for the total flux, the latter being dominated by the much brighter, unpolarized photospheric emission. If observed, the differing time-scales would provide direct evidence of a thin circumstellar envelope with a cavity. (ii) The combination of photometric and polarimetric light curves determines the lens impact parameter even if the event is not a photospheric transit. (iii) The variation of the polarization position angle determines both the magnitude and direction of the relative proper motion. Finally, we derive an asymptotic limit of the lensing polarization for large Einstein radii. This limit greatly simplifies the determination of the lensing parameters.
It has already been shown that microlensing can give rise to a non-zero variable polarisation sig... more It has already been shown that microlensing can give rise to a non-zero variable polarisation signal. Here we use realistic simulations to demonstrate the additional information that can be gained from polarimetric observations of lensing events.
We investigate the feasibility of reconstructing the radial intensity profile of extended stellar... more We investigate the feasibility of reconstructing the radial intensity profile of extended stellar sources by inverting their microlensed light curves. Using a simple, linear, limb darkening law as an illustration, we show that the intensity profile can be accurately determined, at least over the outer part of the stellar disc, with realistic light curve sampling and photometric errors. The principal requirement is that the impact parameter of the lens be less than or equal to the stellar radius. Thus, the analysis of microlensing events provides a powerful method for testing stellar atmosphere models.
ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the... more ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the dynamics and composition of the magnetosphere and the high latitude ionosphere. There are two competing hypotheses for such reconnection, sub-solar and anti-parallel. In this paper, evidence is provided that suggests that anti-parallel reconnection occurs, at least under some circumstances. Some of the consequences of anti-parallel reconnection are considered. These include the absence of a low latitude boundary layer during southward IMF, the relative timing of reconnection events as observed in the ionosphere, the time-dependence of ion outflow and the implications for the composition of the magnetosheath, and the transient nature of reconnection. Some suggestions for tests that would allow differentiation between the anti-parallel and sub-solar reconnection are also made.
ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the... more ABSTRACT Reconnection at the dayside magnetopause is of fundamental importance in determining the dynamics and composition of the magnetosphere and the high latitude ionosphere. There are two competing hypotheses for such reconnection, sub-solar and anti-parallel. In this paper, evidence is provided that suggests that anti-parallel reconnection occurs, at least under some circumstances. Some of the consequences of anti-parallel reconnection are considered. These include the absence of a low latitude boundary layer during southward IMF, the relative timing of reconnection events as observed in the ionosphere, the time-dependence of ion outflow and the implications for the composition of the magnetosheath, and the transient nature of reconnection. Some suggestions for tests that would allow differentiation between the anti-parallel and sub-solar reconnection are also made.
Mervyn Freeman, Gareth Chisham, Iain Coleman and Mike Pinnock report on the November discussion m... more Mervyn Freeman, Gareth Chisham, Iain Coleman and Mike Pinnock report on the November discussion meeting ``Temporal and spatial structure of magnetic reconnection on the Earth's magnetopause'', the only natural environment in which this universal astrophysical phenomenon can be observed locally and globally.
M agnetic reconnection may be defined as a change in connectivity of lines of force of a magnetic... more M agnetic reconnection may be defined as a change in connectivity of lines of force of a magnetic field in a plasma, an electrically neutral mixture of ions and electrons. Through this change, magnetic energy can be released, into heat and plasma flow. Reconnection is universally important: most of the universe is plasma and most plasmas generate magnetic fields. Magnetic reconnection occurs at the Sun and other stars, in accretion disks and in near-Earth space -geospace.
An extensive variety of instruments, including Geotail, DMSP F11, SuperDARN, and IMP-8, were moni... more An extensive variety of instruments, including Geotail, DMSP F11, SuperDARN, and IMP-8, were monitoring the dayside magnetosphere and ionosphere between 14:00 and 18:00 UT on 18 January 1999. The location of the instruments provided an excellent opportunity to study in detail the direct coupling between the solar wind, the magnetosphere, and the ionosphere. Flux transfer events were observed by Geotail near the magnetopause in the dawn side magnetosheath at about 4 magnetic local time during exclusively northward interplanetary magnetic field conditions. Excellent coverage of the entire dayside high-latitude ionosphere was achieved by the Northern Hemisphere Su-perDARN radars. On the large scale, temporally and spatially, the dayside magnetosphere convection remained directly driven by the interplanetary magnetic field, despite the highly variable interplanetary magnetic field conditions, including long periods of northward field. The SuperDARN radars in the dawn sector also measured small-scale temporally varying convection velocities, which are indicative of flux transfer event activity, in the vicinity of the magnetic footprint of Geotail. DMSP F11 in the Southern Hemisphere measured typical cusp precipitation simultaneously with and magnetically conjugate to a single flux transfer event signature detected by Geotail. A study of the characteristics of the DMSP ion spectrogram revealed that the source plasma from the reconnection site originated downstream of the subsolar point. Detailed analyses of locally optimised coordinate systems for individual flux transfer events at Geotail are consistent with a series of flux tubes protruding from the magnetopause, and originating from a high-latitude reconnection site in the Southern Hemisphere. This high-latitude reconnection site agrees with plasma injected away from the sub-Correspondence to: K. A. McWilliams ([email protected]) solar point. This is the first simultaneous and independent determination from ionospheric and space-based data of the location of magnetic reconnection.
When the interplanetary magnetic field (IMF) encounters the Earth's magnetosphere, it is compress... more When the interplanetary magnetic field (IMF) encounters the Earth's magnetosphere, it is compressed and distorted. This distortion is known as draping, and plays an important role in the interaction between the IMF and the geomagnetic field. This paper considers a particular aspect of draping, namely how the orientation of the IMF in a plane perpendicular to the Sun-Earth line (the clock angle) is altered by draping in the magnetosheath close to the day-side magnetopause. The clock angle of the magnetosheath field is commonly estimated from the interplanetary magnetic field (IMF) measured by upstream monitoring spacecraft either by assuming that the draping process does not significantly alter the clock angle (" perfect draping ") or that the change in clock angle is reasonably approximated by a gas dynamic model. In this paper, the magnetosheath clock angles measured during 36 crossings of the magnetopause by the Geotail and Interball-Tail spacecraft are compared to the upstream IMF clock angles measured by the Wind spacecraft. Overall, about 30% of data points exhibit perfect draping within ±10 • , and 70% are within 30 •. The differences between the IMF and magnetosheath clock angles are not, in general, well-ordered in any systematic fashion which could be accounted for by hydrodynamic draping. The draping behaviour is asymmetric with respect to the y-component of the IMF, and the form of the draping distribution function is dependent on solar wind pressure. While the average clock angle observed in the magnetosheath does reflect the orientation of the IMF to within ∼30 • or less, the assumption that the magnetosheath field direction at any particular region of the magnetopause at any instant is approximately similar to the IMF direction is not justified. This study shows that re-connection models which assume laminar draping are unlikely to accurately reflect the distribution of reconnection sites across the dayside magnetopause.
This paper presents the results of a multi-spacecraft survey of the draping of magnetic field lin... more This paper presents the results of a multi-spacecraft survey of the draping of magnetic field lines at the dayside magnetopause. A database of approximately forty magne- topause crossings from the Geotail and Interball-Tail spacecraft was analysed, and compared with upstream solar wind data from the Wind spacecraft. This paper shows the relationship between the magnetic field orientation observed in the solar wind and that observed at the magnetopause, as a function of location on the magnetopause and of solar wind conditions. These results are compared to the predictions of field line draping models.
There has been interest in recent years in the fractal properties of structures on the magnetosph... more There has been interest in recent years in the fractal properties of structures on the magnetosphere, including reconnection structures, and in investigating these properties via ionospheric observations. This presentation concernes how fractal properties are affected by the magnetosphere-ionosphere mapping process, and how observed properties in the ionosphere relate to source structures in the magnetosphere.
Large-scale properties of reconnection structures on the magnetopause can be explained successful... more Large-scale properties of reconnection structures on the magnetopause can be explained successfully by simple models incorporating laminar magnetosheath flow with antiparallel reconnection. However, such models are inconsistent with the highly turbulent nature of the magnetosheath flow adjacent to the magnetopause. This presentation proposes a fractal reconnection model that resolves this contradiction by replacing the laminar magnetosheath flow with a turbulent flow which has realistic levels of fluctuation. The resultant fractal reconnection structures preserve the large-scale behaviour of simpler models, consistent with ground-based observations, but have small-scale fluctuations consistent with those observed in situ by spacecraft. We also present observations of evidence for scale-free fluctuations in ionospheric convection associated with dayside reconnection. This scale free behaviour may well arise as a consequence of the fractal reconnection model proposed.
The role of radar observations in resolving the ambiguities between spatial and temporal interpre... more The role of radar observations in resolving the ambiguities between spatial and temporal interpretations of the double cusp will be outlined, and related to the wider context of understanding the large-scale physics of reconnection. Our recent work has established that dayside reconnection under conditions of significant IMF y-component can lead to a split reconnection x-line on the magnetopause, which is observable in the polar ionosphere when the geometry of field-line mapping to the ionosphere is favourable. In this presentation, simultaneous DMSP particle data and SuperDARN radar data will be shown from a split x-line event, demonstrating the connection between split x-line radar signatures and double-cusp particle signatures. Separated reconnection sites will in general have different histories and velocity filter characteristics: the observable consequences of this in the double cusp signature will be discussed. Both temporal and spatial variability in the reconnection site can give rise to a particle cusp discontinuity, and the utility of ground-based data in determining which of these is more important in any particular case will be assessed.
We present observations of spatiotemporal variations in the dayside reconnection rate measured in... more We present observations of spatiotemporal variations in the dayside reconnection rate measured in the ionosphere during an extended interval of northward IMF. The convection electric field during this interval is measured in the high-latitude ionosphere using the northern hemisphere SuperDARN HF radar network. This is combined with an estimation of the location of the ionospheric footprint of the reconnection X-line, as determined by DMSP spacecraft overpasses and SuperDARN measurements, to determine the spatial variations in the reconnection electric field. We also present the temporal variation in the integrated reconnection voltage for an extended time interval which illustrates the transient nature of the reconnection process under these conditions.
We describe a method for determining the limb polarization and limb darkening of stars in eclipsi... more We describe a method for determining the limb polarization and limb darkening of stars in eclipsing binary systems, by inverting photometric and polarimetric light curves. Because of the ill-conditioning of the problem, we use the \relax Backus-Gilbert\relax method to control the resolution and stability of the recovered solution, and to make quantitative estimates of the maximum accuracy possible. Using this method we confirm that the limb polarization can indeed be recovered, and demonstrate this with simulated data, thus determining the level of observational accuracy required to achieve a given accuracy of reconstruction. This allows us to set out an optimal observational strategy, and to critcally assess the claimed detection of limb polarization in the Algol system. The use of polarization in stars has been proposed as a diagnostic tool in microlensing surveys by \cite[Simmons et al. (1995)]{simm95}, and we discuss the extension of this work to the case of microlensing of extended sources.
During an interval of southward IMF we use observations from the SuperDARN HF radar network and t... more During an interval of southward IMF we use observations from the SuperDARN HF radar network and the DMSP satellites to determine the location of the ionospheric projection of the dayside magnetopause reconnection X-line. From the flow of plasma across the polar cap boundary we derive the reconnection electric field and thus de- termine the reconnection rate over a wide range of longitudes. Using the Tsyganenko 96 magnetic field model the ionospheric data are mapped to the magnetopause in or- der to provide an estimate of the extent of the reconnection X-line. Our results are compared with previously reported encounters by the Equator-S and Geotail space- craft with a reconnecting magnetopause near the dawn flank for the same period. The SuperDARN observations allow these satellite observations to be set in the context of the whole magnetopause reconnection X-line. Our results show that the dayside merging gap in the ionosphere was 10 h of magnetic local time (MLT) in extent and mapped to a magnetopause reconnection X-line of 39 Re extent, the largest reported in the literature. The reconnection rate was accurately determined over 7 h of MLT and was exceptionally stable over the 30 minute study period. However, there was a clear spatial variation in the reconnection rate across the dayside ionosphere, with a distinct minimum at 14 h MLT. The total potential associated with dayside reconnection was ~150kV. Whilst the reconnection signatures detected by the Equator-S at ~0730 MLT mapped to a region showing flows across the polar cap boundary, the Geotail satellite at~06 MLT mapped to a region in the ionosphere in which no reconnection flows were measured. From this it appears that terminator and post-terminator reconnection sites may not couple their momentum to the ionosphere.
Magnetic reconnection is the most significant process that re-4 sults in the transport of magneti... more Magnetic reconnection is the most significant process that re-4 sults in the transport of magnetised plasma into, and out of, the Earth's magnetosphere- 5 ionosphere system. There is also compelling observational evidence that it 6 plays a major role in the dynamics of the solar corona, and it may also be important for understanding cosmic rays, accretion disks, magnetic dynamos, 8 and star formation. The Earth's magnetosphere and ionosphere are presently 9 the most accessible natural plasma environments where magnetic reconnec-10 tion and its consequences can be measured, either in situ, or by remote sens-11 ing. This paper presents a complete methodology for the remote sensing of 12 magnetic reconnection in the magnetosphere from the ionosphere. This method 13 combines measurements of ionospheric plasma convection and the ionospheric 14 footprint of the reconnection separatrix. Techniques for measuring both the 15 ionospheric plasma flow and the location and motion of the reconnection sep-16 aratrix are reviewed, and the associated assumptions and uncertainties as-17 sessed, using new analyses where required. Application of the overall method-18 ology is demonstrated by the study of a 2-hour interval from 26 December 19 2000 using a wide range of spacecraft and ground-based measurements of the 20 northern hemisphere ionosphere. This example illustrates how spatial and 21 temporal variations in the reconnection rate, as well as changes in the bal-22 ance of magnetopause (dayside) and magnetotail (nightside) reconnection, 23 can be routinely monitored, affording new opportunities for understanding 24 the universal reconnection process and its influence on all aspects of space 25 weather.
Monthly Notices of The Royal Astronomical Society, 2002
We discuss the flux and polarization signals obtained from the microlensing of stars with extende... more We discuss the flux and polarization signals obtained from the microlensing of stars with extended circumstellar envelopes by a single point-mass lens. A previous paper considered the case of main-sequence stars, and showed that microlensing of the stellar photosphere could produce a small net polarization (up to 0.1 per cent). In this paper, we show that stars with extensive envelopes will show a much higher level of variable polarization (up to 10 per cent), even if they are spherically symmetric. Since the stellar envelopes most likely to be lensed are produced by red giant winds, we also investigate the effects of a central cavity (representing the dust condensation radius) within the scattering envelope, as well as different radial density distributions. Our study has led to three major results. (i) For optically thin envelopes with an inner cavity, the lensing light curve for the polarization can be of longer duration than that for the total flux, the latter being dominated by the much brighter, unpolarized photospheric emission. If observed, the differing time-scales would provide direct evidence of a thin circumstellar envelope with a cavity. (ii) The combination of photometric and polarimetric light curves determines the lens impact parameter even if the event is not a photospheric transit. (iii) The variation of the polarization position angle determines both the magnitude and direction of the relative proper motion. Finally, we derive an asymptotic limit of the lensing polarization for large Einstein radii. This limit greatly simplifies the determination of the lensing parameters.
It has already been shown that microlensing can give rise to a non-zero variable polarisation sig... more It has already been shown that microlensing can give rise to a non-zero variable polarisation signal. Here we use realistic simulations to demonstrate the additional information that can be gained from polarimetric observations of lensing events.
We investigate the feasibility of reconstructing the radial intensity profile of extended stellar... more We investigate the feasibility of reconstructing the radial intensity profile of extended stellar sources by inverting their microlensed light curves. Using a simple, linear, limb darkening law as an illustration, we show that the intensity profile can be accurately determined, at least over the outer part of the stellar disc, with realistic light curve sampling and photometric errors. The principal requirement is that the impact parameter of the lens be less than or equal to the stellar radius. Thus, the analysis of microlensing events provides a powerful method for testing stellar atmosphere models.
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Papers by Iain Coleman