RAS/MIST discussion meeting
10th October 2003

Society of Antiquaries Lecture Theatre, Burlington House, Piccadilly, London

[Report published in Astronomy & Geophysics, 45, 3.36-3.38 (2004)]


A. Programme B. Report C. Abstracts


Identifying the open-closed field line boundary in magnetospheric and ionospheric data sets

Organisers: Dr Gareth Chisham (BAS), Dr Gary Abel (BAS) and Dr Steve Milan (Leicester)


Programme:

10:00 - 10:30 Coffee

Morning Session (Chair: Steve Milan)

10:30 Gareth Chisham (BAS): Welcome and Introduction

10:35 Thomas Sotirelis and Patrick T. Newell (JHU/APL): The poleward boundary of the auroral oval in particle precipitation.

11:05 Gareth Chisham, Mervyn P. Freeman (BAS), and Thomas Sotirelis (JHU/APL): The SuperDARN spectral width boundary and how it relates to the open-closed field line boundary.

11:30 Mervyn P. Freeman and Gareth Chisham (BAS): On the probability distributions of SuperDARN radar spectral width and their use in identifying the open-closed magnetic field line boundary.

11:45 Joran Moen (Oslo/Svalbard): Identification of the open-closed field line boundary by multi-instrument techniques.

12:15 Richard W. Sims and S. Eleri Pryse (Aberystwyth): Summertime ESR observations of the adiaroic boundary during IMF Bz>0 and the open/closed magnetic field boundary.

12:30 Tom Stallard (UCL), S. Miller, and H. Melin: Identifying the open-closed field line boundary in the ionospheric data sets of the giant planets.

12:45 Gary A. Abel, Gareth Chisham, and Mervyn P. Freeman (BAS): Dot-to-dot: How do we join up sparse measurements of the open-closed field line boundary?

13:00 - 14:00 Lunch

Afternoon Session (Chair: Gary Abel)

14:00 Malcolm W. Dunlop (RAL): Analysing structures and interfaces using spatially-distributed data.

14:25 Andrew N. Fazakerley (MSSL): Identification of the open?closed field line boundary using particle observations made by mid? and high altitude spacecraft.

14:50 James A. Wild, Steve E. Milan (Leicester), Chris J. Owen (MSSL), J.M. Bosqued (CESR/CNRS), Mark Lester, Darren M. Wright (Leicester), Harold Frey, C.W. Carlson (Berkeley), Andrew N. Fazakerley (MSSL), and H. Rème (CESR/CNRS): On the determination of the open-closed magnetic field line boundary location in the dusk sector auroral ionosphere: global auroral imaging, coherent scatter radar and energetic particle observations.

15:05 Steve E. Milan (Leicester): Determining the open flux content of the magnetosphere from observations of the polar ionosphere.

15:30 - 16:00 Tea (at the Geological Society).

16:00 - 18:00 RAS Monthly A&G (Ordinary) Meeting.


Report:

by G Chisham (British Antarctic Survey), G A Abel (British Antarctic Survey) and S E Milan (University of Leicester)

Published in Astronomy & Geophysics 45, 3.36-3.38 (2004)

The Earth sits protected from the continually expanding atmosphere of the Sun, known as the solar wind, inside the magnetic cavity known as the magnetosphere. Most of the geomagnetic field lines that form this cavity are 'closed', i.e., both ends of the field line are attached to the Earth. However, around each pole there is a region where the geomagnetic field lines interconnect with the interplanetary magnetic field and the field lines are 'open'. Such field lines have one end attached to the Earth, while the other extends into interplanetary space, eventually mapping back to the Sun or into the heliosphere. The open flux content of the Earth's magnetosphere focuses from two large regions in the outer magnetosphere to two relatively small areas within the Earth's polar ionospheres. These areas are termed the 'polar caps', and the boundary between the open and closed magnetic field line regions is termed the 'polar cap boundary' or the 'open-closed field line boundary' (OCB).

The ability to identify and track the location of the OCB allows the electrodynamics of the Earth's magnetospheric system to be investigated. Magnetic reconnection is responsible for the transfer of flux (as well as momentum and energy) across this boundary from closed to open regions of the magnetosphere (and vice versa). Hence, studying the dynamics of the polar cap and the OCB provides indirect measurements of the balance between reconnection on the dayside and nightside of the magnetosphere. Measurements of the transfer of flux across the OCB also provide direct measurements of the rate of magnetic reconnection. Magnetic reconnection is a universally important phenomena, occurring at the Sun and other stars, and in accretion disks. However, geospace remains the only natural environment within which reconnection can be observed and studied in detail.

This RAS meeting aimed to review the different techniques used to determine the location of the OCB using the wide range of instrumentation available to magnetospheric and ionospheric scientists. It is important to understand the strengths and weaknesses of the varying techniques and to investigate how well the boundary identifications from the different techniques correlate. The main objective of the meeting was to allow researchers to better address the science questions for which an accurate identification of the OCB is crucial. The meeting brought together about 60 researchers to share ideas and to identify ways forward in the investigation of the OCB. Much of the work presented by the 11 speakers is summarised below.

Identifying the OCB in magnetospheric and ionospheric data sets has been an active research topic for over 30 years. Spacecraft, in situ in the magnetosphere, regularly observe the boundary; ion and electron distributions change markedly between open and closed field line regions. Andrew Fazakerley (MSSL) reviewed observations of the particle signatures typically measured during high-altitude spacecraft transitions of the boundary. The closed field line region is characterised by isotropic hot ions and electrons. As a spacecraft passes from the outer magnetosphere into the magnetopause boundary layer on the dayside of the Earth, an initial signature of this transition is the loss of magnetospheric electrons, escaping from their trapped location on closed magnetospheric field lines. Electrons from the magnetosheath are also observed travelling down the field lines towards the ionosphere. This 'electron edge' represents the best proxy for the OCB from in situ spacecraft observations in the dayside magnetosphere. However, recent observations by the 4-spacecraft CLUSTER mission have identified an electron-scale current layer (~20 km) that may represent a more definite signature of the boundary. Ion signatures are observed deeper within the boundary layer and provide a cruder estimate of the boundary location. In the magnetospheric tail on the nightside of the Earth, earthward-moving ions, energised by reconnection, can be used to locate the reconnection site and hence infer the location of the OCB.

Andrew Fazakerley (MSSL) also reviewed the Lockwood model linking high-altitude particle signatures to those observed by mid and low- altitude spacecraft. It is at low altitudes that the particle signatures are best understood due to the multitude of low-altitude spacecraft observations, such as those made by the DMSP spacecraft. Tom Sotirelis (APL/JHU) reviewed the particle signatures typically observed by low-altitude spacecraft and how they relate to the OCB. On the dayside several well-defined particle precipitation regions have been identified and classified according to the magnetospheric regions to which they map best. Hence, the open field line regions have precipitation signatures classified as cusp, mantle and polar rain, and the closed field line regions have precipitation regions classified as boundary plasma sheet and central plasma sheet. The Lockwood model (as presented by Andrew Fazakerley (MSSL)) describes how these magnetospheric regions overlap at low altitude due to the finite time of flight of precipitating particles from the reconnection site to the ionosphere and the convection of the newly- opened field lines following reconnection. Hence, the true location of the OCB within these precipitation regions is still a matter of intense debate. One of the main bones of contention is when the precipitation region classified as relating to the low latitude boundary layer is located on open or closed field lines. Tom Sotirelis (APL/JHU) also reviewed low-altitude particle precipitation signatures on the nightside of the magnetosphere. Here, the so-called 'b6' boundary (marking a transition from sub-visual drizzle on the last closed field lines to polar rain on open field lines) appears to represent the best proxy for the OCB.

The main weakness of spacecraft observations is that they are limited by their single point sampling of the boundary. Even multiple spacecraft missions such as CLUSTER are limited to boundary observations on small spatial and temporal scales (as shown in presentations by Andrew Fazakerley (MSSL) and Malcolm Dunlop (RAL)). However, Malcolm Dunlop (RAL) showed how CLUSTER can distinguish unambiguously between spatial and temporal variations in the boundary motion for the first time. Although spacecraft observations often provide very detailed high-resolution information about particular boundary crossings, the global picture of the OCB can only be determined from ionospheric observations.

The focussing of the open field line regions of the magnetosphere to small regions of the polar ionospheres allows multiple measurements of the OCB to be made from ground-based instruments or from spacecraft measuring auroral images of the ionosphere. The wide range of ground-based instrumentation used to identify the OCB includes optical all-sky cameras and coherent and incoherent scatter radars. Probably the earliest estimates of the OCB from the ground were made using optical instrumentation. Joran Moen (University of Oslo) reviewed how optical emissions seen in the dark dayside ionosphere relate to the OCB. Closed field lines are characterised by 557 nm green-line auroral emissions, relating to particle precipitation from the central plasma sheet. Newly-opened field lines are characterised by 630 nm red-line auroral emissions relating to particle precipitation in the cusp region. The equatorward edge of the cusp red-line emissions is often used as a proxy for the OCB in the dayside ionosphere. In the nightside ionosphere the poleward edge of auroral emissions is often used as an OCB proxy. However, this can often be inaccurate as optical instruments are not sensitive to the sub-visual particle precipitation region that often exists just equatorward of the OCB in the nightside ionosphere (as highlighted by Tom Sotirelis (APL/JHU)).

Ground-based optical imagers typically have limited fields of view (~1000 km) and so can only image small regions of the polar ionospheres and then only when the ionosphere is dark. Spacecraft ultra-violet imagers, however, are able to take global images of the polar ionosphere, showing auroral emissions in both the day and nightside ionosphere. Although the spatial resolution of the spacecraft imager data is not as good as that of ground-based imagers, they often allow a global estimate of the OCB to be determined at good temporal resolution (as shown by Steve Milan (University of Leicester)). Spacecraft imagers are even being used to make the first observations of the OCB on other planets in the solar system. Tom Stallard (UCL) showed optical observations of auroral luminosity in the polar regions of Saturn and suggested that the Dark Polar Region (DPR) might be synonymous with the polar cap. His results showed that while regions of high luminosity corotated with the planet, the DPR sub-corotates, as would be expected for a region of the ionosphere held stationary by field lines interconnected with the solar wind.

Ground-based radars are being increasingly used to identify the OCB. As highlighted by Joran Moen (University of Oslo) it is still unclear what the best proxy for the OCB is in incoherent scatter radar observations. Latitudinal reversals in the ion velocity typically provide a good estimate of the convection reversal boundary (as shown by Richard Sims (University of Wales, Aberystwyth)) but this is typically displaced from the OCB in regions where reconnection is ongoing. Present studies are focussing on gradients in the ion and electron temperature, and whether there is a clear gradient that correlates with the OCB. However, incoherent scatter radar observations are highly restricted as a global tool by their limited fields of view.

Coherent scatter radars, such as the SuperDARN HF radar network with its near-complete polar field of view, provide the potential for making global estimates of the OCB from the ground. The SuperDARN spectral width boundary is a very common feature of SuperDARN radar data, dividing backscatter characterised by high spectral widths at high latitudes from backscatter characterised by low spectral widths at low latitudes. Gareth Chisham (BAS) reviewed the history of the spectral width boundary and presented details of an accurate technique for automating the determination of the boundary location. The spectral width boundary has long been known to represent a good proxy for the OCB in the dayside ionosphere where it has often been used as such.

The precise physical mechanisms that are responsible for the enhanced spectral width have always been poorly understood. Mervyn Freeman (BAS) presented a study of the occurrence distributions of spectral width values in different ionospheric regions, and showed that they can all be described as log-Levy distributions, but with different distribution moments in different regions. He suggested that it is possible that the log-Levy distribution arises as a natural consequence of the microphysics occurring within the radar scattering volume. Hence, investigation of the parameterization of the observed log-Levy distributions might reveal new information regarding different geophysical regions, as well as understanding of the enhancement of the spectral width values.

With the multitude of OCB determinations from different instruments being combined to give global pictures of the OCB, inter-calibrations of boundary determinations from different instruments and using different techniques, is a topic of immense importance. For instance, although SuperDARN spectral width boundaries are observed at all magnetic local times, how these boundaries relate to boundaries in the magnetospheric system in the nightside ionosphere has always been a matter of debate. Gareth Chisham (BAS) showed, in a statistical comparison of spectral width boundaries and 'b6' boundaries from low- altitude spacecraft, that the spectral width boundary proved a good proxy for the OCB in the pre-midnight local time sector. However, James Wild (University of Leicester) presented conflicting evidence which showed that the spectral width boundary in the morning/dawn sector ionosphere was located at lower latitude than the OCB determined from low and high altitude particle measurements and from UV auroral observations. He showed that the spectral width boundary was located near the poleward edge of the high luminosity main auroral oval. Tom Sotirelis (APL/JHU) showed that the poleward boundary of the UV oval in the nightside ionosphere was often consistent with the 'b5' particle precipitation boundary observed by low-altitude spacecraft, placing it a small distance equatorward of the best particle precipitation proxy for the OCB, the 'b6' boundary.

A major challenge in determining global estimates of the OCB remains that of combining the diverse measurements of the OCB from different data sets and filling in the gaps in data coverage. Gary Abel (BAS) discussed possible methods of combining sparse measurements of the OCB to give an indication of the overall size and shape of the polar cap. The latitude of the OCB as a function of magnetic local time can be described as a Fourier series, where the order of the series should be chosen to be appropriate for the number of observations of the boundary. For a limited number of irregularly-spaced observations, fits of order 3 or 4 seem to work best. When near- continuous measurements of the boundary exist it is possible to use a filtered Fourier series. This involves the Fourier transform of the boundary, the removal of the high frequency terms and the transform back to give the best global estimate of the boundary. Steve Milan (University of Leicester) presented an example of the combination of a UV auroral image, low-altitude particle measurements and SuperDARN spectral width observations to determine a global estimate of the OCB using this method. This global boundary estimate can then be used to determine the open flux content of the magnetosphere.

Steve Milan (University of Leicester) rounded off the meeting by showing the strength of global measurements of the OCB in understanding the consequences of magnetic reconnection. He presented measurements of the open flux content of the magnetosphere that showed how the polar cap expanded in response to intervals of dayside reconnection and contracted in response to nightside reconnection. This technique allowed the reconnection rates associated with both the day and nightside reconnection processes to be quantified. In one example the size of the polar cap and upstream solar wind conditions were employed to estimate changes in the radius of the magnetospheric tail. These estimates were shown to match closely observations made by the IMP-8 spacecraft located in this region. These results show that knowledge of the polar cap size allows the large-scale structure of the magnetosphere to be investigated.

The meeting left a number of open questions in the study of the OCB:

The organizers thank all participants for their contributions to the meeting.


Abstracts:

Identifying the open-closed field line boundary in magnetospheric and ionospheric data sets

10:35 Thomas Sotirelis and Patrick T. Newell (JHU/APL): The poleward boundary of the auroral oval in particle precipitation

Abstract: Various algorithms for identifying the poleward boundary of the auroral oval are discussed with an emphasis on automated methods. The methods make use of DMSP particle precipitation observations. Algorithm details are reviewed, and comparisons made with other boundary schemes based on auroral imagery and radar observations. Applications, such as empirical modeling and the APL auroral particles and imagery group's OVATION tool are discussed.


11:05 Gareth Chisham, Mervyn P. Freeman (BAS), and Thomas Sotirelis (JHU/APL): The SuperDARN spectral width boundary and how it relates to the open-closed field line boundary

Abstract: The transition from low spectral width values at low latitudes to high spectral width values at high latitudes, as measured by the SuperDARN radars, is now routinely used as a proxy for the open-closed field line boundary in the locale of the cusp in the dayside ionosphere. Similar spectral width transitions are observed at all magnetic local times, although it is unclear how these transitions relate to the open-closed field line boundary away from the cusp. In this presentation we discuss the best methods for identifying latitudinal spectral width transitions in SuperDARN data sets. We also show, by comparing these transitions with particle precipitation observations from the DMSP low-altitude spacecraft, how these spectral width transitions relate to the open-closed field line boundary in the nightside ionosphere. This allows the SuperDARN spectral width parameter to be used to identify the open-closed field line boundary at nearly all magnetic local times.


11:30 Mervyn P. Freeman and Gareth Chisham (BAS): On the probability distributions of SuperDARN radar spectral width and their use in identifying the open-closed magnetic field line boundary

Abstract: The spectral width of Doppler backscatter from the SuperDARN radars has been routinely used to identify the boundary between open and closed magnetic field lines. The probability distribution of spectral width values has been described as Gaussian in the cusp and exponential equatorward of it. In this paper, we re-examine these distributions and show that in fact they are both described by a single universal function whose moments change in the latitudinal transition from cusp to non-cusp. This universal function is the log-Levy function. We show that the log-Levy function also describes the spectral width distributions in latitudinal transitions from low to high spectral widths at other magnetic local times. We discuss what this may indicate about the physics of spectral width fluctuations and how these results may be used to identify better the open-closed magnetic field line boundary.


11:45 Joran Moen (Oslo/Svalbard): Identification of the open-closed field line boundary by multi-instrument techniques

Abstract: - Precise determination of the open-closed field-line boundary (OCB) is of critical importance in the continuous monitoring of solar-terrestrial interactions by ground-based remote sensing techniques. It is well known that the F2 region electron temperature measured by the incoherent scatter radar technique is sensitive to magnetosheath particle precipitation. This is because the electron gas is effectively heated by low-energy magnetosheath electrons. Ion temperature gradients near the flow reversal boundary, or associated with narrow flow channels, have been suggested as possible markers for the flow reversal boundary (FRB) or OCB. On November 23, 1999 EISCAT VHF observed a continuous band of high ion temperature, which persisted for about 8 hours in the dayside auroral ionosphere (7-15 MLT), ideal to test the test the potential role of Ti and Te gradients as OCB markers. The continuous band of high Ti was situated on open flux. However, the equatorward Ti boundary was in general located poleward of the magnetosheath electron edge (high Te). The distance between the arrival of the first (super-Alfvénic) magnetosheath electrons (high Te) and the transfer of momentum carried by Alfvén waves (high Ti) will naturally depend on the field-aligned distance to the reconnection site and the convection speed. In the case when the release of magnetic tension gives rise to significant Joule heating, the equatorward Ti boundary is a good reference boundary for calculating the reconnection rate. Although Te appears sensitive to magnetosheath electron fluxes it is not found to be a suitable parameter for routine tracking of the open-closed boundary, as it involves case dependent analysis of the thermal balance. At the end the presentation will change focus towards the coherent HF radar technique and its capability to track the cusp equatorward boundary. One of the mysteries is to understand the underlying physical mechanism(s) for the generation of decametre scale backscatter irregularities in the cusp. The sounding rocket ICI-1 (Investigation of Cusp irregularities) to be launched from Svalbard this winter has been dedicated to in-situ measurements of the backscatter targets.


12:15 Richard W. Sims and S. Eleri Pryse (Aberystwyth): Summertime ESR observations of the adiaroic boundary during IMF Bz>0 and the open/closed magnetic field boundary

Abstract: Results are presented from a multi-instrument investigation of the summer dayside ionosphere during an interval of small clock angle. The adiaroic boundary is identified by converging line-of-sight ion flows observed by the EISCAT Svalbard Radar, whose latitude varied with |Bz| near magnetic noon. Increased ion temperatures observed north of the region of convergence are attributed to the rapid convection of flux tubes away from the high-latitude reconnection site. Radio tomography imaged the background ionospheric electron density over an extended latitudinal region. Satellite particle detectors indicate that the open/closed field line boundary was poleward of the adiaroic boundary. The observations are discussed in light of the Tsyganenko magnetic field model.


12:30 Tom Stallard (UCL), S. Miller, and H. Melin: Identifying the open-closed field line boundary in the ionospheric data sets of the giant planets

Abstract: The study of the magnetospheres of the Giant Planets has progessed significantly over the last decade, through both in situ and ground based observations. As these magnetospheres become better understood, the similarities and differences with Earth's magnetosphere have become an important debating point. We present observations of the ionospheres of the giant planets that conclusively show the location of the boundary between regions of closed and open magnetic field lines for the first time. The magnetospheres of the giant planets are far larger than that of Earth. There is also significant internal sources for plasma, especially in the case of Jupiter, which has a large plasma disk formed from material stripped from the volcanic moon of Io. This results in a magnetosphere at the very least partially controlled my the internal plasma environment, significantly reducing the direct influence of the solar wind on the ionosphere itself.


12:45 Gary A. Abel, Gareth Chisham, and Mervyn P. Freeman (BAS): Dot-to-dot: How do we join up sparse measurements of the open-closed field line boundary?

Abstract: Knowledge of the location and motion of the boundary between open and closed field lines is vital to a range of studies in magnetospheric physics, particularly on the global scale. However, most determinations of this boundary are made as single point measurements or over a limited MLT window. Often we have more than one measurement at our disposal and how best to combine these measurements presents a challenge. In this paper we discuss various possible methods for estimating the position of the boundary between sparse measurements. Whist satisfactory methods for estimating the boundary between measurements at a single time exist, a satisfactory method for tracking the temporal of this estimation does not. We discuss the requirements and problems involved with such a method.


14:00 Malcolm W. Dunlop (RAL): Analysing structures and interfaces using spatially-distributed data

Abstract: Cluster has provided fully co?ordinated, spatially?distributed data for the first time. A number of multi?spacecraft techniques which measure, for example, boundary orientation, thickness and motion, and deviation from planarity, have been developed. In addition, the data allows the electric current structures, both within the boundary layers and associated with other structure, to be probed. We investigate and compare boundary crossings arising from spacecraft encounters with discontinuities in the solar wind, the bow shock, the magnetopause and the magnetotail, which are observed by Cluster on multiple scales (ranging from 6000?100 km). Behavior of the interfaces, such as distinguishing strong acceleration and thickness variations, can be made. Knowledge of boundary orientation and motion can accurately scale the plasma and field signatures across the boundary layer.


14:25 Andrew N. Fazakerley (MSSL): Identification of the open-closed field line boundary using particle observations made by mid and high altitude spacecraft

Abstract: The relationship between particle precipitation observed by low altitude satellites (~100's km altitude) and ionospheric signatures measured by ground-based techniques has been studied quite extensively. A fair understanding of how the various forms of precipitation relate to corresponding ionospheric effects has been developed. However the origins of the various particle populations in the boundary region between unambiguously magnetospheric plasma and unambiguously polar cap plasma, is a topic of ongoing debate. It is important to understand these populations and how they behave in order to use ionospheric observations to define the open-closed boundary. Models which seek to explain the low altitude observations generally discuss processes occurring at very high altitudes and thus have generated predictions that can only be tested by high altitude measurements. We use observations, mainly from Cluster, to illustrate typical particle behaviour at altitudes above 3 Earth radii and we discuss what can be learned from them.


14:50 James A. Wild, Steve E. Milan (Leicester), Chris J. Owen (MSSL), J.M. Bosqued (CESR/CNRS), Mark Lester, Darren M. Wright (Leicester), Harold Frey, C.W. Carlson (Berkeley), Andrew N. Fazakerley (MSSL), and H. Rème (CESR/CNRS): On the determination of the open-closed magnetic field line boundary location in the dusk sector auroral ionosphere: global auroral imaging, coherent scatter radar and energetic particle observations

Abstract: As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the instantaneous size of the polar cap, the region corresponding to ionospheric termini of open magnetic flux tubes, is of prime importance. Variations in the polar cap size indicate whether open magnetic flux is being appended to (increasing polar cap size), or removed from (decreasing polar cap size), the magnetospheric cavity. Consequently, the ability to reliably and globally remote sense the location of the polar cap boundary is highly desirable, and several potential techniques have been previously proposed. We present space- and ground-based observations of the dawn-sector auroral zone between 11:00?17:00 UT on 8th December 2001, and attempt to determine the location of the open/closed boundary (OCB) using a variety of techniques over several (~6) hours of magnetic local time. We find that the correspondence between the location of OCB inferred from in-situ particle observations made by Cluster, FAST and DMSP spacecraft, and the location of the poleward edge of broadband ultraviolet auroral emission observed by the IMAGE FUV imager is highly variable over the spatial scale of a few (2-3) hours of magnetic local time. In addition, we find that the Doppler spectral width boundary observed at similar local times by a subset of the SuperDARN radar network was approximately co-located with the region of brightest broadband UV auroral emission and not with the poleward edge of the auroral oval, or the ionospheric projection of the OCB. We therefore conclude that during the interval presented, neither the poleward edge of the auroral UV emission, nor the Doppler spectral width boundary were reliable indicators of the location of the OCB and recommend caution in the use of such proxies.


15:05 Steve E. Milan (Leicester): Determining the open flux content of the magnetosphere from observations of the polar ionosphere

Abstract: We employ observations from several sources to determine the location of the polar cap boundary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral images from the Ultraviolet Imager (UVI) instrument onboard the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing us to estimate the day- and nightside reconnection voltages associated with different IMF orientations and the occurrence of substorms. In addition, we discuss the ramifications for the magnetotail of changes in open flux content of the magnetosphere.


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