References of "Milan, S. E"
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See detailHubble Space Telescope Observations of Variations in Ganymede's Oxygen Atmosphere and Aurora
Molyneux, P. M.; Nichols, J. D.; Bannister, N. P. et al

in Journal of Geophysical Research. Space Physics (2018), 123

We present high‐sensitivity Hubble Space Telescope (HST) Cosmic Origins Spectrograph and HST Space Telescope Imaging Spectrograph measurements of atmospheric OI 130.4‐nm and OI] 135.6‐nm emissions at ... [more ▼]

We present high‐sensitivity Hubble Space Telescope (HST) Cosmic Origins Spectrograph and HST Space Telescope Imaging Spectrograph measurements of atmospheric OI 130.4‐nm and OI] 135.6‐nm emissions at Ganymede, which exhibit significant spatial and temporal variability. These observations represent the first observations of Ganymede using HST Cosmic Origins Spectrograph and of both the leading and trailing hemispheres within a single HST campaign, minimizing the potential influence of long‐term changes in the Jovian plasma sheet or in Ganymede's atmosphere on the comparison of the two hemispheres. The mean disk‐averaged OI] 135.6‐nm/OI 130.4‐nm observed intensity ratio was 2.72 ± 0.57 on the leading hemisphere and 1.42 ± 0.16 on the trailing hemisphere. The observed leading hemisphere ratios are consistent with an O2 atmosphere, but we show that an atomic oxygen component of ~10% is required to produce the observed trailing hemisphere ratios. The excess 130.4‐nm emission on the trailing hemisphere relative to that expected for an O2 atmosphere was ~11 R. The O column density required to produce this excess is determined based on previous estimates of the electron density and temperature at Ganymede and exceeds the limit for an optically thin atmosphere. The implication that the O atmosphere is optically thick may be investigated in future by observing Ganymede as it moves into eclipse or by determining the ratio of the individual components within the 130.4‐nm triplet. [less ▲]

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See detailTranspolar arcs observed simultaneously in both hemispheres
Carter, J. A.; Milan, S. E.; Fear, R. C. et al

in Journal of Geophysical Research. Space Physics (2017), 122

Two coexisting transpolar arcs are observed on 31 August 2005. We track the formation and motion of the arcs in both the Northern and Southern Hemispheres, using data from two independent satellites ... [more ▼]

Two coexisting transpolar arcs are observed on 31 August 2005. We track the formation and motion of the arcs in both the Northern and Southern Hemispheres, using data from two independent satellites (Imager for Magnetopause to Aurora Global Exploration and a Defence Meteorological Satellite Program satellite). The observations are supported by supplementary ground-based ionospheric convection data from the Super Dual Auroral Radar Network. The two arcs form during a period of northward interplanetary magnetic field. Following a change in the direction of the interplanetary magnetic field B[SUB]Y[/SUB] component from negative to positive, the dawnside arc traverses the polar cap to the duskside in the Northern Hemisphere. Over the same time period and in the Southern Hemisphere, the duskside arc traverses the polar cap to the dawnside. A complex magnetic field line topology resulting in the coexistence of two tongues of closed field lines protruding into the otherwise open polar cap is implied. We discuss these observations in terms of magnetic conjugacy and a model of transpolar arcs formation. [less ▲]

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See detailTesting nowcasts of the ionospheric convection from the expanding and contracting polar cap model
Walach, M. T.; Milan, S. E.; Yeoman, T. K. et al

in Space Weather (2017)

The expanding/contracting polar cap (ECPC) model, or the time-dependent Dungey cycle, provides a theoretical framework for understanding solar wind-magnetosphere-ionosphere coupling. The ECPC describes ... [more ▼]

The expanding/contracting polar cap (ECPC) model, or the time-dependent Dungey cycle, provides a theoretical framework for understanding solar wind-magnetosphere-ionosphere coupling. The ECPC describes the relationship between magnetopause reconnection and substorm growth phase, magnetotail reconnection and substorm expansion phase, associated changes in auroral morphology, and ionospheric convective motions. Despite the many successes of the model, there has yet to be a rigorous test of the predictions or nowcasts made regarding ionospheric convection, which remains a final hurdle for the validation of the ECPC. In this study we undertake a comparison of ionospheric convection, as measured in situ by ion drift meters on board DMSP (Defense Meteorological Satellite Program) satellites and from the ground by SuperDARN (Super Dual Auroral Radar Network), with motions nowcasted by a theoretical model. The model is coupled to measurements of changes in the size of the polar cap made using global auroral imagery from the IMAGE FUV (Imager for Magnetopause to Aurora Global Exploration Far Ultraviolet) instrument, as well as the dayside reconnection rate, estimated using the OMNI data set. The results show that we can largely nowcast the magnitudes of ionospheric convection flows using the context of our understanding of magnetic reconnection at the magnetopause and in the magnetotail. [less ▲]

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See detailComparative study of large-scale auroral signatures of substorms, steady magnetospheric convection events, and sawtooth events
Walach, M.-T.; Milan, S. E.; Murphy, K. R. et al

in Journal of Geophysical Research. Space Physics (2017)

This paper investigates the auroral evolution during different magnetospheric modes: substorms, steady magnetospheric convection, and sawtooth events. We undertake a superposed epoch analysis using data ... [more ▼]

This paper investigates the auroral evolution during different magnetospheric modes: substorms, steady magnetospheric convection, and sawtooth events. We undertake a superposed epoch analysis using data from the Imager for Magnetopause-to-Aurora Global Exploration Far Ultraviolet spectrographic imager and wideband imaging camera for each of these event types. We find that the auroral oval narrows and shows an equatorward movement prior to substorm onset. At substorm onset, the auroral oval brightens explosively near 23 magnetic local time (MLT). After this the aurorae expand poleward and the brightening stretches duskward and dawnward, with the duskward expansion being faster. Approximately 20 min after substorm onset, the aurorae begin to dim. Steady magnetospheric convection events with preceding substorms initially show the same signatures as substorms, but instead of the recovery after 20 min postonset, the aurorae stay bright for an extended period of time (at least 4 h after onset). Despite continued dayside driving of the system during steady magnetospheric convection events, we see a reconfiguration in the nightside auroral activity, taking place between 120 to 150 min after onset. Sawtooth events show very similar signatures to substorms, except for the auroral emission being much brighter, covering a wider MLT extent, and taking significantly less time to recover. The proton aurorae during substorms take ˜2-4 h to dim, during sawtooth events this process takes less than 1 h, despite enhanced reconnection rates. A similar effect is seen in the electron aurorae, albeit not as extreme. [less ▲]

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See detailWhat controls the local time extent of flux transfer events?
Milan, S. E.; Imber, S. M.; Carter, J. A. et al

in Journal of Geophysical Research. Space Physics (2016), 121

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See detailHubble Space Telescope observations of variation of the O I 135.6 nm/ O I 130.4 nm ratio in Ganymede’s atmosphere
Molyneux, P. M.; Nichols, J. D.; Bannister, N. P. et al

Poster (2015, June)

We present new high-sensitivity HST/COS measurements of the atmospheric O I 135.6 nm/ O I 130.4 nm ratio at Ganymede, which we show exhibits significant spatial and temporal variability. Specifically, the ... [more ▼]

We present new high-sensitivity HST/COS measurements of the atmospheric O I 135.6 nm/ O I 130.4 nm ratio at Ganymede, which we show exhibits significant spatial and temporal variability. Specifically, the ratios observed on Ganymede’s leading hemispheres vary between 2.14±0.03 and 2.67±0.02, while on the trailing hemisphere the ratios are observed to be between 0.98±0.02 and 1.53±0.03. These high-sensitivity observations increase the signal to noise of these measurements by an order of magnitude over previous HST/STIS observations of the same [1], thus confirming that the temporal variation suggested by these previous observations is real. The emissions are excited through electron-impact excitation of Ganymede’s oxygen atmosphere by electrons which are locally accelerated within its magnetosphere [2,3]. The variation in the ratio magnitude may be explained either by variations in the ratio of atomic to molecular oxygen in the atmosphere or by a change in the temperature of the electrons exciting the emissions. An increase in the proportion of molecular oxygen acts to increase the ratio, as does a cooler electron temperature.References [1] Feldman, P. D., McGrath, M. A., Strobel, D. F., Moos, H. W., Retherford, K. D. and Wolven, B. C., HST/STIS ultraviolet imaging of polar aurora on Ganymede, Astrophys. J., Vol. 535, pp. 1085-1090, 2000. [2] Hall, D. T., Feldman, P. D., McGrath, M. A. and Strobel, D. F., The far-ultraviolet oxygen airglow of Europa and Ganymede, Astrophys. J., Vol. 499, pp. 475-481, 1998. [3] Eviatar, A., Strobel, D. F., Wolven, B. C., Feldman, P. D., McGrath, M. A. and Williams, D. J., Excitation of the Ganymede ultraviolet aurora, Astrophys. J., Vol. 555, pp. 1013-1019, 2001. [less ▲]

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See detailRelationship between interplanetary parameters and the magnetopause reconnection rate quantified from observations of the expanding polar cap
Milan, S. E.; Gosling, J. S.; Hubert, Benoit ULiege

in Journal of Geophysical Research. Space Physics (2012), 117

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See detailAutomated detection of magnetospheric modes to investigate solar wind-magnetosphere coupling
Milan, S. E.; Imber, S. M.; Coxon, J. et al

Conference (2012)

The magnetosphere is known to respond to solar wind driving by displaying a variety of behaviours known as magnetospheric modes, including the substorm cycle, sawtooth intervals, and steady magnetospheric ... [more ▼]

The magnetosphere is known to respond to solar wind driving by displaying a variety of behaviours known as magnetospheric modes, including the substorm cycle, sawtooth intervals, and steady magnetospheric convection. Other behaviour includes geomagnetic storms and periods of relative magnetospheric quiescence associated with northward interplanetary magnetic field. There have been many studies of the changes in magnetospheric open flux content during different modes to investigate the role of magnetic reconnection at the magnetopause and in the magnetotail in determining the magnetospheric response. In this study we attempt an automated recognition of different modes by their characteristic signatures in geomagnetic indices and other magnetospheric state parameters, including open flux. This allows us to investigate the interplanetary conditions that give rise to different behaviours. [less ▲]

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See detailParameterizing the Magnetopause Reconnection Rate from Observations of the Expanding Polar Cap
Milan, S. E.; Gosling, J. S.; Hubert, Benoit ULiege et al

in EGU General Assembly Conference Abstracts (2012)

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See detailA superposed epoch investigation of the relation between magnetospheric solar wind driving and substorm dynamics with geosynchronous particle injection signatures
Boakes, P. D.; Milan, S. E.; Abel, G. A. et al

in Journal of Geophysical Research. Space Physics (2011), 116

We report a superposed epoch analysis of the hemispheric open magnetic flux, maximum nightside auroral intensity, geomagnetic activity, and solar wind and interplanetary magnetic field conditions around ... [more ▼]

We report a superposed epoch analysis of the hemispheric open magnetic flux, maximum nightside auroral intensity, geomagnetic activity, and solar wind and interplanetary magnetic field conditions around the time of substorm onset for three distinct categories of substorms defined by their energetic particle injection signatures. Substorms identified from global auroral imagery are classified into one of three categories based on their energetic particle injection signatures as seen at geosynchronous orbit by the Los Alamos National Laboratory spacecraft. Category 1 events are associated with a “classic” substorm injection, category 2 events show varied activity (i.e., energetic enhancements not following the evolution expected for classic substorms), and category 3 events show no apparent injection activity. The superposed epoch analysis reveals that the three distinct particle injection categories exhibit distinct differences in the level and continuity of magnetospheric driving by the solar wind, such that category 1 events can be described as classic substorm events, category 2 as continuously driven events, and category 3 as weak events. The results of this study suggest that the level and continuity of the dayside solar wind driving of the magnetosphere during substorms have a direct impact on the injection of energetic particles to geosynchronous orbit at substorm onset. These results could have considerable value in empirical predictions of the space weather environment. [less ▲]

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See detailBifurcations of the main auroral ring at Saturn: ionospheric signatures of consecutive reconnection events at the magnetopause
Radioti, Aikaterini ULiege; Grodent, Denis ULiege; Gérard, Jean-Claude ULiege et al

in Journal of Geophysical Research. Space Physics (2011), 116

This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the ... [more ▼]

This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously. [less ▲]

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See detailParameterizing the Magnetopause Reconnection Rate From Observations of the Expanding Polar Cap
Milan, S. E.; Gosling, J. S.; Hubert, Benoît ULiege

Conference (2011)

The expanding/contracting polar cap (ECPC) paradigm, or the time-dependent Dungey cycle, provides a theoretical framework for understanding solar wind-magnetosphere-ionosphere coupling. The ECPC describes ... [more ▼]

The expanding/contracting polar cap (ECPC) paradigm, or the time-dependent Dungey cycle, provides a theoretical framework for understanding solar wind-magnetosphere-ionosphere coupling. The ECPC describes the relationship between magnetopause reconnection and substorm growth phase, magnetotail reconnection and substorm expansion phase, associated changes in auroral morphology, and ionospheric convective motions. Despite the many successes of the model, there has yet to be a rigorous test of the predictions made regarding ionospheric convection, which remains a final hurdle for the validation of the ECPC. In this study we undertake a comparison of ionospheric convection, as measured by ion driftmeters on board DMSP (Defense Meteorological Satellite Program) satellites, with motions predicted by a theoretical model (Milan, 2013). The model is coupled to measurements of changes in the size of the polar cap made using global auroral imagery from the IMAGE FUV (Imager for Magnetopause to Aurora Global Exploration Far Ultraviolet) instrument, as well as the dayside reconnection rate, calculated using the OMNI dataset. The results show that we can largely predict the magnitudes of ionospheric convection flows using the context of our understanding of magnetic reconnection at the magnetopause and in the magnetotail. [less ▲]

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See detailA superposed epoch analysis of auroral evolution during substorms: Local time of onset region
Milan, S. E.; Grocott, A.; Hubert, Benoît ULiege

in Journal of Geophysical Research. Space Physics (2010), 115

Previous workers have shown that the magnetic local time (MLT) of substorm onset depends on the prevailing east-west component of the interplanetary magnetic field (IMF). To investigate the influence of ... [more ▼]

Previous workers have shown that the magnetic local time (MLT) of substorm onset depends on the prevailing east-west component of the interplanetary magnetic field (IMF). To investigate the influence of the onset MLT on the subsequent auroral response we perform a superposed epoch analysis of the auroral evolution during approximately 2000 substorms using observations from the FUV instrument on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. We subdivide the substorms by onset latitude and onset local time before determining average auroral images before and after substorm onset, for both electron and proton aurorae. We find that during the growth phase there is preexisting auroral emission in the MLT sector of the subsequent onset. After onset the auroral bulge expands eastward and westward, but remains centered on the onset sector. Approximately 30 min after onset, during the substorm recovery phase, the peaks in electron and proton auroral emission move into the postnoon and prenoon sectors, respectively, reflecting the “average” auroral precipitation patterns determined by previous studies. Superposed epoch analysis of the interplanetary magnetic field for the substorms under study suggests that the B[SUB]Y[/SUB] component of the IMF must be biased toward positive or negative values for up to a day prior to onset for the onset MLT to be influenced. [less ▲]

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See detailAverage auroral configuration parameterized by geomagnetic activity and solar wind conditions
Milan, S. E.; Evans, T. A.; Hubert, Benoît ULiege

in Annales Geophysicae (2010), 28

Average proton and electron auroral images are compiled from three years of observations by the IMAGE spacecraft, binned according to concurrent K[SUB]P[/SUB] and upstream solar wind conditions measured ... [more ▼]

Average proton and electron auroral images are compiled from three years of observations by the IMAGE spacecraft, binned according to concurrent K[SUB]P[/SUB] and upstream solar wind conditions measured by the ACE spacecraft. The solar wind parameters include solar wind velocity, density, and pressure, interplanetary magnetic field (IMF) magnitude and orientation, and an estimate of the magnetopause reconnection rate. We use both (a) the overall variation in brightness in the images and (b) the variation in location of the aurorae with respect to the binning parameters to determine which parameters best order the auroral response. We find that the brightness varies by a factor of ~50 with K[SUB]P[/SUB], a similar amount with estimated dayside reconnection voltage, ~15 with the IMF, ~3 with solar wind density, ~2 with solar wind velocity, and ~5 with pressure. Clearly, geomagnetic activity as measured by K[SUB]P[/SUB] and auroral dynamics are closely associated. In terms of the solar wind-magnetosphere coupling that drives auroral dynamics, the IMF is of paramount importance in modulating this, with solar wind speed and density playing a lesser role. Dayside reconnection voltage, derived from the solar wind velocity and IMF magnitude and orientation, orders the data almost as well as K[SUB]P[/SUB], though we find a plateau in the auroral response between voltages of 100 and 150 kV. We also discuss changes in configuration and overall size of the average auroral oval with upstream conditions. [less ▲]

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See detailComparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations
Hubert, Benoît ULiege; Aikio, A. T.; Amm, O. et al

in Annales Geophysicae (2010), 28

We compare the location of the polar cap boundary (PCB) determined using two different techniques, and use them as proxies for the open-closed field line boundary (OCB). Electron temperatures from ... [more ▼]

We compare the location of the polar cap boundary (PCB) determined using two different techniques, and use them as proxies for the open-closed field line boundary (OCB). Electron temperatures from observations of the EISCAT radar facility are used to estimate the latitude of the PCB along the meridian of the EISCAT VHF beam. The second method utilizes global images of proton aurora obtained by the IMAGE satellite FUV SI12 instrument. These methods are applied to three different intervals. In two events, the agreement between the methods is good and the mean of the difference is within the resolution of the observations. In a third event, the PCB estimated from EISCAT data is located several degrees poleward of that obtained from the IMAGE FUV SI12 instrument. Comparison of the reconnection electric field estimated from the two methods shows that high-resolution measurements both in time and space are needed to capture the variations in reconnection electric field during substorm expansion. In addition to the two techniques introduced above to determine the PCB location, we also use a search for the location of the reversal of the east-west component of the equivalent current known as the magnetic convection reversal boundary (MCRB). The MCRB from the MIRACLE magnetometer chain mainly follows the motion of the polar cap boundary during different substorm phases, but differences arise near the Harang discontinuity. [less ▲]

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See detailAn initial investigation of the magnetosphere at a system level using auroral oval radius and ring current intensity as state variables
Milan, S. E.; Hutchinson, J.; Boakes, P. D. et al

Conference (2009, December 01)

One approach to understanding the magnetosphere at a system level is to select a number of magnetospheric state variables and to examine statistically their inter-relationships and the temporal evolution ... [more ▼]

One approach to understanding the magnetosphere at a system level is to select a number of magnetospheric state variables and to examine statistically their inter-relationships and the temporal evolution of the magnetosphere through state-space. This talk outlines a first attempt at such a study, using the radius of the auroral oval, a proxy for the open flux content of the magnetosphere, and the Sym-H index, a measure of the intensity of the ring current, as the primary state variables. Using observations from the two-year period June 2000 to May 2002, the response of the state of the magnetosphere to differing solar wind inputs, and the evolution of the system state during geomagnetic storms is investigated. Our main finding is a characteristic evolution of magnetospheric state through the initial, main, and recovery phases of geomagnetic storms. We discuss our findings within the context of the expanding/contracting polar cap paradigm, in terms of a modification of substorm onset conditions by the magnetic perturbation associated with the ring current. [less ▲]

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See detailInfluences on the radius of the auroral oval
Milan, S. E.; Hutchinson, J.; Boakes, P. D. et al

in Annales Geophysicae (2009), 27

We examine the variation in the radius of the auroral oval, as measured from auroral images gathered by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft, in response to solar ... [more ▼]

We examine the variation in the radius of the auroral oval, as measured from auroral images gathered by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft, in response to solar wind inputs measured by the Advanced Composition Explorer (ACE) spacecraft for the two year interval June 2000 to May 2002. Our main finding is that the oval radius increases when the ring current, as measured by the Sym-H index, is intensified during geomagnetic storms. We discuss our findings within the context of the expanding/contracting polar cap paradigm, in terms of a modification of substorm onset conditions by the magnetic perturbation associated with the ring current. [less ▲]

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See detailStatistical properties of flux closure induced by solar wind dynamic pressure fronts
Hubert, Benoît ULiege; Blockx, Caroline ULiege; Milan, S. E. et al

in Journal of Geophysical Research. Space Physics (2009), 114

We present a statistical study of flux closure intervals induced by solar wind dynamic pressure fronts. We consider that a dynamic pressure front reaches the Earth when a dayside subauroral proton flash ... [more ▼]

We present a statistical study of flux closure intervals induced by solar wind dynamic pressure fronts. We consider that a dynamic pressure front reaches the Earth when a dayside subauroral proton flash is observed in the SI2 channel of the IMAGE-FUV experiment. This pragmatic criterion selects both weak and strong pressure fronts. It is found that the preconditioning of the magnetosphere prior to the pressure pulse arrival mainly governs the magnetospheric response to a weak solar wind dynamic pressure front. This preconditioning includes the amount of open magnetic flux available in the magnetosphere prior to the pressure front arrival and the size of the magnetospheric cavity. However, in the case of a strong pressure pulse, the magnetospheric response is more sensitive to the solar wind properties characterizing the dynamic pressure front. The pressure jump is not the only one important, but also the variation of the solar wind velocity and IMF magnitude. In overall terms, we find that a strong dynamic pressure front is typically characterized by a dynamic pressure increase larger than Ë 2.8 nPa that takes place on timescales of the order of a few minutes. [less ▲]

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See detailA superposed epoch analysis of auroral evolution during substorm growth, onset and recovery: open magnetic flux control of substorm intensity
Milan, S. E.; Grocott, A.; Forsyth, C. et al

in Annales Geophysicae (2009), 27

We perform two superposed epoch analyses of the auroral evolution during substorms using the FUV instrument on the Imager for Magnetopause-to-Aurora Global Explorer (IMAGE) spacecraft. The larger of the ... [more ▼]

We perform two superposed epoch analyses of the auroral evolution during substorms using the FUV instrument on the Imager for Magnetopause-to-Aurora Global Explorer (IMAGE) spacecraft. The larger of the two studies includes nearly 2000 substorms. We subdivide the substorms by onset latitude, a measure of the open magnetic flux in the magnetosphere, and determine average auroral images before and after substorm onset, for both electron and proton aurora. Our results indicate that substorms are more intense in terms of auroral brightness when the open flux content of the magnetosphere is larger, and that magnetic flux closure is more significant. The increase in auroral brightness at onset is larger for electrons than protons. We also show that there is a dawn-dusk offset in the location of the electron and proton aurora that mirrors the relative locations of the region 1 and region 2 current systems. Superposed epoch analyses of the solar wind, interplanetary magnetic field, and geomagnetic indices for the substorms under study indicate that dayside reconnection is expected to occur at a faster rate prior to low latitude onsets, but also that the ring current is enhanced for these events. [less ▲]

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See detailA statistical study of the open magnetic flux content of the magnetosphere at the time of substorm onset
Boakes, P. D.; Milan, S. E.; Abel, G. A. et al

in Geophysical Research Letters (2009), 36

In this paper we determine the probability of substorm onset as a function of open magnetic flux in the magnetosphere by comparing the occurrence distribution of open flux observed at all times with that ... [more ▼]

In this paper we determine the probability of substorm onset as a function of open magnetic flux in the magnetosphere by comparing the occurrence distribution of open flux observed at all times with that observed at the time of substorm onset. The open magnetic flux is measured in 12735 auroral images of the ionospheric polar cap from the IMAGE WIC detector. The probability of substorm onset is found to be negligible for fluxes below ~0.3 GWb, increases almost linearly until ~0.9 GWb, and is undefined above this. We also demonstrate that those substorms which show a clear particle injection signature at geosynchronous orbit, as measured by the LANL spacecraft, occur, on average, with higher values of open flux than those showing no activity. We discuss these results in the context of various hypotheses for substorm onset. [less ▲]

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