References of "Milan, S. E"
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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 ULg; Grodent, Denis ULg; Gérard, Jean-Claude ULg 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 detailA superposed epoch analysis of auroral evolution during substorms: Local time of onset region
Milan, S. E.; Grocott, A.; Hubert, Benoît ULg

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 ULg

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 ULg; 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 ULg; Blockx, Caroline ULg; 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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See detailOpen magnetic flux and magnetic flux closure during sawtooth events
Hubert, Benoît ULg; Milan, S. E.; Grocott, A. et al

in Geophysical Research Letters (2008), 35

We use IMAGE-FUV observations of the polar aurora and measurements of the ionospheric convection from the SuperDARN radar network to study several sawtooth events previously reported in the literature. We ... [more ▼]

We use IMAGE-FUV observations of the polar aurora and measurements of the ionospheric convection from the SuperDARN radar network to study several sawtooth events previously reported in the literature. We estimate the amount of open magnetic flux in the Earth magnetosphere during a significant part of these sawtooth intervals as well as the magnetic flux opening and closure rates, that is, the dayside and nightside reconnection rates. We find that during the sawtooth intervals the magnetosphere is highly loaded with open flux as a result of the strongly southward IMF carried by the solar wind during these intervals. The magnetosphere tries to relax to a less loaded configuration through a sequence of substorm expansions. However, these substorms do not necessarily evolve to their end before reintensification of nightside reconnection occurs in response to continued loading of the magnetosphere on the dayside. [less ▲]

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See detailResponse of the expanding/contracting polar cap to weak and strong solar wind driving: Implications for substorm onset
Milan, S. E.; Boakes, P. D.; Hubert, Benoît ULg

in Journal of Geophysical Research. Space Physics (2008), 113

We quantify the amount of open magnetic flux in the magnetosphere from observations of the auroral polar cap on a near-continuous basis for a period of 18 days, 20 August to 6 September 2005. This ... [more ▼]

We quantify the amount of open magnetic flux in the magnetosphere from observations of the auroral polar cap on a near-continuous basis for a period of 18 days, 20 August to 6 September 2005. This interval encompasses periods of weak, moderate, and strong solar wind driving, including two geomagnetic storms. We identify 49 substorms during the interval and determine the response of the polar cap to growth and expansion phases of the substorms. We find that the frequency of substorms and the flux closed by substorms both increase during enhanced solar wind driving, each approximately as the square root of the dayside reconnection rate. In addition, the average size of the polar cap increases during intervals when there is strong driving and especially when the SYM-H index indicates that the ring current is enhanced. We suggest that this occurs for two reasons: because there is a delay between substorm onset and the closure of open magnetic flux in the magnetotail (while closed flux is pinched off), during which dayside reconnection can lead to further growth in the size of the polar cap, and also because the magnetotail is more stable to reconnection when the ring current is enhanced. [less ▲]

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See detailOn the use of IMAGE FUV for estimating the latitude of the open/closed magnetic field line boundary in the ionosphere
Boakes, P. D.; Milan, S. E.; Abel, G. A. et al

in Annales Geophysicae (2008), 26

A statistical comparison of the latitude of the open/closed magnetic field line boundary (OCB) as estimated from the three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband ... [more ▼]

A statistical comparison of the latitude of the open/closed magnetic field line boundary (OCB) as estimated from the three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband Imaging camera, WIC, and the Spectrographic Imagers, SI-12 and SI-13) has been carried out over all magnetic local times. A total of over 400 000 OCB estimations were compared from December 2000 and January and December of 2001 2002. The modal latitude difference between the FUV OCB proxies from the three detectors is small, <1°, except in the predawn and evening sectors, where the SI-12 OCB proxy is found to be displaced from both the SI-13 and WIC OCB proxies by up to 2° poleward in the predawn sector and by up to 2° equatorward in the evening sector. Comparing the IMAGE FUV OCB proxies with that determined from particle precipitation measurements by the Defense Meteorological Satellites Program (DMSP) also shows systematic differences. The SI-12 OCB proxy is found to be at higher latitude in the predawn sector, in better agreement with the DMSP OCB proxy. The WIC and SI-13 OCB proxies are found to be in better agreement with the DMSP OCB proxy at most other magnetic local times. These systematic offsets may be used to correct FUV OCB proxies to give a more accurate estimate of the OCB latitude. [less ▲]

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See detailFlux transport and tail dynamics during a prolonged substorm interval
Milan, S. E.; Wild, J. A.; Hubert, Benoît ULg et al

in Cluster and Double Star Symposium (2006, January 01)

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See detailMagnetic Flux Closure Directly Induced by Interplanetary Shocks: Observations Using IMAGE-FUV and SuperDARN, and Modelling With GUMICS-4.
Hubert, Benoît ULg; Palmroth, M.; Milan, S. E. et al

Conference (2005, December 01)

A method has been developed to monitor the dayside and nightside reconnection rates using FUV remote sensing of the proton aurora and ionospheric convection patterns. Global images of the proton aurora ... [more ▼]

A method has been developed to monitor the dayside and nightside reconnection rates using FUV remote sensing of the proton aurora and ionospheric convection patterns. Global images of the proton aurora are obtained using the SI12 instrument of the FUV experiment on board the IMAGE satellite, and used to identify the open/closed (o/c) field line boundary. SuperDARN data are used to determine the ionospheric convection velocity, and the associated electric field. The dayside and nightside reconnection voltages are then determined accounting for the ionospheric electric field and the motion of the o/c boundary. This method is used to compute the dayside and nightside reconnection voltages during two interplanetary shocks for which the IMF was mostly northward, so that the amount of open magnetic flux was so small that no significant substorm expansion phase could develop. The flux closure voltage shows a sharp signature when the interplanetary shocks sweep by the nightside magnetosphere. MHD simulations conducted using the GUMICS-4 model for similar conditions show a similar signature in the nightside flux closure rate. We suggest that this flux closure event is directly induced by the compression of the magnetotail. [less ▲]

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See detailOpen Flux Estimates and Reconnection Rates in Saturn's Magnetosphere, Derived Using HST and Cassini Data
Badman, S. V.; Bunce, E. J.; Clarke, J. T. et al

Conference (2005, December 01)

During January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated with measurements of the upstream interplanetary conditions made ... [more ▼]

During January 2004, a sequence of 68 UV images of Saturn's southern aurora was obtained by the Hubble Space Telescope (HST), coordinated with measurements of the upstream interplanetary conditions made by the Cassini spacecraft. Using the poleward edge of the observed aurora as a proxy for the open-closed field line boundary, the open flux content of the southern polar region has been estimated. It is found to range from 15 to 50 GWb during the interval, such a large variation providing evidence of a significant magnetospheric interaction with the solar wind, in particular with structures associated with corotating interaction regions (CIRs). Cassini measurements of the upstream interplanetary conditions have been used to estimate the rate of open flux production at the magnetopause. By comparison with the open flux values obtained from the images, the rate of open flux closure in Saturn's magnetotail is also estimated. We find that the rate of open flux production is high (200 kV) in the high-field strength intervals following the onset of CIR-related compressions, and elevated (30-40 kV) in an intermediate-field strength rarefaction region. High flux closure rates (100-200 kV) are estimated in association with the onset of solar wind compressions and are intermittently increased (30-60 kV) during a weak-field rarefaction region. The disturbed auroral forms seen in the images are discussed in relation to the estimated reconnection rates. Recent studies have suggested that Saturn Kilometric Radiation emissions are also disturbed by the onset of CIR compressions at Saturn. [less ▲]

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See detailModulation of Dayside Reconnection During Northward IMF
Provan, G.; Lester, M.; Cowley, S. W. et al

Poster (2005, December 01)

On 17 September 2000 the IMF was directed continuously northward for more than three hours. Density fluctuations in the solar wind resulted in quasi-periodic variations in the solar wind dynamic pressure ... [more ▼]

On 17 September 2000 the IMF was directed continuously northward for more than three hours. Density fluctuations in the solar wind resulted in quasi-periodic variations in the solar wind dynamic pressure, and correlated fluctuations also occurred in the IMF Bz component. The Northern hemisphere SuperDARN radars observed bursts of high-latitude high-velocity plasma flow during this northward IMF interval, both when ionospheric signatures consistent with low-latitude merging were observed, and when lobe merging was occurring. On average the recurrence period of these flow bursts was ~22 min. During this time the SI-12 spectrographic imager channel on the IMAGE spacecraft observed the dayside proton auroral spot continuously (Frey et al., 2003). The brightness of the auroral spot varied over time. Here we find a direct correlation between the occurrence of bursts of plasma flow and periodic fluctuations in the brightness of the proton auroral spot. Our results suggest that correlated fluctuations in the solar wind dynamic pressure and IMF Bz component modulated ionospheric precipitation and dayside reconnection, resulting in fluctuations in the brightness of the proton auroral spot and periodic variations in the dayside high-latitude plasma flow. [less ▲]

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See detailOpen flux estimates in Saturn's magnetosphere during the January 2004 Cassini-HST campaign, and implications for reconnection rates
Badman, S. V.; Bunce, E. J.; Clarke, J. T. et al

Conference (2005, August 07)

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