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See detailEL - a possible indicator to monitor the magnetic field stretching at global scale during substorm expansive phase: Statistical study
Meurant, M.; Gérard, Jean-Claude ULg; Blockx, Caroline ULg et al

in Journal of Geophysical Research. Space Physics (2007), 112

An interesting open question of magnetospheric physics is the understanding of the dynamics of the magnetotail. The question of the field stretching is even more challenging during substorm periods ... [more ▼]

An interesting open question of magnetospheric physics is the understanding of the dynamics of the magnetotail. The question of the field stretching is even more challenging during substorm periods, mainly because of the short time scales involved during such explosive events. In this study, we asses the ability of global scale proton auroral imaging to provide information on the tail stretching during active periods. We base our investigation on more than 250 isolated substorms observed by IMAGE-SI12 between 2000 and 2002. Applying the algorithm proposed by Donovan et al. (2003) for ground based observations to IMAGE-SI12 data, we determine the Equatorial Limit (EL) of the oval and propose to use it as an indicator of the tail stretching. Simultaneous comparison with GOES-8 allows us to estimate how strong is the relationship between the EL position deduced from SI12 and the magnetic field stretching. The EL indicator is shown to be consistent with previous studies (Sergeev and Gvozdevsky (1995) and Blockx et al. (2005)) and is found to be located in average ~1 degree equatorward of the limit deduced from DMSP measurements. The time evolution of the EL magnetic latitude is also presented for different local times relative to the onset position. This evolution of the EL index presents an asymmetric shape following the time of onset, suggesting a more important stretching of the tail duskward of the onset position. This asymmetric stretching is consistent with GOES-8 in situ measurements. [less ▲]

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See detailGlobal morphology of substorm growth phases observed by the IMAGE-SI12 imager
Blockx, Caroline ULg; Gérard, Jean-Claude ULg; Coumans, Valérie ULg et al

Poster (2007)

Growth phases are observed to start from less than 30 minutes to over an hour before the substorm onset. The sector of maximum proton precipitation during the growth phase is generally located around 2200 ... [more ▼]

Growth phases are observed to start from less than 30 minutes to over an hour before the substorm onset. The sector of maximum proton precipitation during the growth phase is generally located around 2200 MLT. It rapidly moves in local time by about 1.2 hour toward midnight at the time of the onset. The open magnetic flux increases by as much as a 33% during the growth phase. The mean value of the open flux at the end of the growth phase, immediately preceding the substorm, onset is about 0.74. GWb for substorms triggered by external (solar wind) factors and 0.67 GWb for non-triggered substorms. The open magnetic flux generally drops following the onset of triggered substorms but continues to increase for non-triggered events. We interpret this behavior as an indication that the rate of opening of closed field lines on the dayside can exceed that of the nightside reconnection after the onset in non-triggered substorms. By contrast, flux closure is more efficient while the flux opening rate drops in the case of externally triggered onset, so that the closure rate exceeds that of field line opening on the dayside. The rate of equatorward displacement is typically ~ 3 deg/hour. It is statistically correlated with the magnitude of the southward Bz component of the IMF measured by the ACE satellite. It is also correlated with transfer functions describing the efficiency of solar wind energy transfer which involve the transverse electric field carried by the solar wind. The equatorward motion may be global, restricted to local time sectors or a combination of both. No nightside local time sector appears favored where the motion of the equatorial boundary would be more pronounced. The maximum displacement of the polar boundary is statistically located around midnight MLT. [less ▲]

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See detailA comparison between FUV remote sensing of magnetotail stretching and the T01 model during quiet conditions and growth phases
Blockx, Caroline ULg; Gérard, Jean-Claude ULg; Coumans, Valérie ULg et al

in Annales Geophysicae [= ANGEO] (2007), 25(1), 161-170

In a previous study, Blockx et al. (2005) showed that the SI12 camera on board the IMAGE spacecraft is an excellent tool to remotely determine the position of the isotropy boundary (IB) in the ionosphere ... [more ▼]

In a previous study, Blockx et al. (2005) showed that the SI12 camera on board the IMAGE spacecraft is an excellent tool to remotely determine the position of the isotropy boundary (IB) in the ionosphere, and thus is able to provide a reasonable estimate of the amount of stretching of the magnetic field lines in the magetotail. By combining an empirical model of the magnetospheric configuration with Sergeev's criterion for non-adiabatic motion, it is also possible to obtain a theoretical position of IB in the ionosphere, for known conditions in the solar wind. Earlier studies have demonstrated the inadequacy of the Tsyganenko-1989 (T89) model to quantitatively reproduce the field line stretching, particularly during growth phases. In this study, we reexamine this question using the T01 model which considers the time history of the solar wind parameters. We compare the latitude of IB derived from SI12 global images near local midnight with that calculated from the T01 model and the Sergeev's criterion. Observational and theoretical results are found to frequently disagree. We use in situ measurements of the magnetic field with the GOES-8 satellite to discriminate which of the two components in the calculation of the theoretical position of the IB (the T01 model or Sergeev's criterion) induces the discrepancy. For very quiet magnetic conditions, we find that statistically the T01 model approximately predicts the correct location of the maximum proton precipitation. However, large discrepancies are observed in individual cases, as demonstrated by the large scatter of predicted latitudes. For larger values of the AE index, the model fails to predict the observed latitude of the maximum proton intensity, as a consequence of the lack of consideration of the cross-tail current component which produces a more elongated field configuration at the location of the proton injection along the field lines. We show that it is possible to match the observed location of the maximum proton precipitation by decreasing the current sheet half-thickness D parameter. We thus conclude that underestimation of the field line stretching leads to inadequately prediction of the boundary latitude of the non-adiabatic proton precipitation region. [less ▲]

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See detailMagnetotail topology around substorm onset time
Meurant, M.; Donovan, E. F.; Gérard, Jean-Claude ULg et al

Conference (2006, December 01)

The time evolution of the magnetotail topology during minutes preceding and following substorm onset is an important question of the magnetospheric dynamic. The substorm onset is a local and explosive ... [more ▼]

The time evolution of the magnetotail topology during minutes preceding and following substorm onset is an important question of the magnetospheric dynamic. The substorm onset is a local and explosive phenomenon, which makes it difficult to describe with in situ data. In this study, we investigate this problem thanks the remote sensing data provided by the FUV camera onboard the IMAGE spacecraft. In the first part of this study, we use a set of IMAGE-FUV and GOES-8 simultaneous observations obtained during substorms periods to develop a model of the magnetic field elevation angle at geosynchronous orbit. In the second part, we use a set of 259 substorms observed by the IMAGE spacecraft between 2000 and 2002. Taking advantage of the global scale of the observations provided by IMAGE-FUV, we describe the time evolution of the magnetotail topology provided by the model. An interesting result shows that the field is symmetrically stretched around the onset position before the onset time and this symmetry is broken after onset, with a more important stretching duskward to the onset. [less ▲]

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See detailDayside and nightside reconnection rates inferred from IMAGE FUV and Super Dual Auroral Radar Network data
Hubert, Benoît ULg; Milan, S. E.; Grocott, A. et al

in Journal of Geophysical Research. Space Physics (2006), 111(A3),

The spectrographic imager at 121.8 nm ( SI12) of the far ultraviolet ( FUV) experiment onboard the IMAGE spacecraft produces global images of the Doppler- shifted Lyman alpha emission of the proton aurora ... [more ▼]

The spectrographic imager at 121.8 nm ( SI12) of the far ultraviolet ( FUV) experiment onboard the IMAGE spacecraft produces global images of the Doppler- shifted Lyman alpha emission of the proton aurora. This emission is solely due to proton precipitation and is not contaminated by dayglow, allowing us to monitor the auroral oval on the dayside as well as on the nightside. Remote sensing of the polar aurora can be advantageously supplemented by use of ground- based data from the Super Dual Auroral Radar Network ( SuperDARN) that monitors the ionospheric convective flow pattern in the polar region. In the present study, the SI12 images are used to determine the location of the open/ closed field line boundary and to monitor its movement. The SuperDARN data are then used to compute the ionospheric electric field at the location of the open/ closed boundary. The total electric field is then computed along the boundary accounting for its movement via Faraday's law so that the dayside and nightside reconnection voltages can be derived. This procedure is applied to several substorm intervals observed simultaneously with IMAGE FUV and SuperDARN. The dayside reconnection voltage feeds the magnetosphere with open flux, which is later closed by nightside reconnection. The calculated dayside reconnection rate is consistent with the solar wind properties measured by the Geotail, Wind, and ACE satellites. We identify the presence of nightside reconnection due to pseudobreakups taking place during the growth phase. In several cases, we establish that the nightside reconnection rate is maximum at the time of the substorm expansion phase onset or shortly after, reaching similar to 120 kV, and then slowly returns to undisturbed values of similar to 30 kV. The flux closure rate can also start intensifying prior to expansion phase onset, producing pseudobreakups. [less ▲]

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See detailProton precipitation during substorm growth observed by IMAGE-FUV: a case study
Coumans, V.; Gérard, Jean-Claude ULg; Blockx, Caroline ULg et al

Poster (2006, March)

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See detailEL-a possible indicator to monitor the magnetic field stretching at global scale during substorm expansive phase: case study
Meurant, M.; Blockx, Caroline ULg; Spanswick, E. et al

in Syrjasuo, M.; Donovan, E. (Eds.) Substorms VIII (2006, March)

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See detailFar ultraviolet remote sensing of the isotropy boundary and magnetotail stretching
Blockx, Caroline ULg; Gérard, Jean-Claude ULg; Meurant, M. et al

in Journal of Geophysical Research. Space Physics (2005), 110(A11),

Several studies have attempted to identify the isotropy boundary (IB) defining the limit between the adiabatic and nonadiabatic trajectories of the trapped protons along closed magnetic field lines. This ... [more ▼]

Several studies have attempted to identify the isotropy boundary (IB) defining the limit between the adiabatic and nonadiabatic trajectories of the trapped protons along closed magnetic field lines. This boundary is an indicator of the amount of magnetic field line stretching in the magnetotail. Previous studies were based on in situ measurements, resulting in spatially and temporally restricted samples. To overcome these limitations, we propose to use global data obtained with the FUV spectrographic proton auroral imager (SI12) on board the IMAGE satellite. We determine at each magnetic local time the position of an optical boundary related to the IB and thereby to the stretching of the magnetic field lines. We show that the correspondence between the latitude of the maximum proton precipitation observed by SI12 and the IB measured by Defense Meteorological Satellite Program satellites is statistically established and depends on the magnetic local time. The relation between the position of the maximum proton precipitation as well as the intensity of this maximum and the magnetic field distortion is determined by comparison with GOES 8 data. We suggest that SI12 images can be used as a tool to globally determine the isotropy boundary and to monitor the level of stretching in the magnetotail. [less ▲]

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See detailComparison of intense nightside shock-induced precipitation and substorm activity
Meurant, M.; Gérard, Jean-Claude ULg; Blockx, Caroline ULg et al

in Journal of Geophysical Research. Space Physics (2005), 110(A7),

[1] Sudden variations of the solar wind dynamic pressure frequently induce dayside enhancements of auroral activity with features such as high-latitude arcs, low-latitude proton flashes, and enhancement ... [more ▼]

[1] Sudden variations of the solar wind dynamic pressure frequently induce dayside enhancements of auroral activity with features such as high-latitude arcs, low-latitude proton flashes, and enhancement of auroral precipitation propagating dawnward and duskward from noon to the night sector. In some cases, these shocks also induce enhanced activity during which the power precipitated into the night sector may reach values as high as observed during substorms. Several studies have shown that the triggering of nightside-enhanced precipitation is more likely during periods of southward interplanetary magnetic field (IMF) components. Early works showed that substorm-like activity is not frequent after a shock and suggested that shocks may not be considered as substorm triggers. We examine up to what point substorm-like nightside activity triggered by a shock is comparable to an isolated substorm. For this purpose, we analyze three events morphologically similar to substorms and occurring within less than 20 min after the arrival of a pressure pulse on the front of the magnetosphere. Different features of these events such as the mean energy of precipitated electrons, the latitudinal motion of boundaries before and after onset, and the power precipitated into the nightside sector are compared with isolated substorms. We conclude that the characteristics of shock-induced substorms appear very similar to those of isolated substorms. Shocks are able to trigger substorms when they hit an unstable magnetosphere. The interpretation is that the perturbation due to the shock induces a substorm by closure of the plasma sheet magnetic field. For the events presented in this study, the instability result from a period of southward IMF or stretching of the magnetic tail induced by a previous shock. [less ▲]

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See detailFUV remote sensing of the isotropic boundary and magnetotail stretching
Blockx, Caroline ULg; Gérard, Jean-Claude ULg; Meurant, M. et al

Conference (2004, December 01)

Several studies attempted to identify the Isotropic Boundary (IB) defining the limit between the adiabatic and non-adiabatic trajectories of the trapped protons along closed magnetic field lines. This ... [more ▼]

Several studies attempted to identify the Isotropic Boundary (IB) defining the limit between the adiabatic and non-adiabatic trajectories of the trapped protons along closed magnetic field lines. This boundary is an indicator of the amount of magnetic field lines' stretching in the magnetotail. These studies are based on in situ measurements, resulting in spatially and temporally restricted samples. To avoid this limitation, we propose to use global data obtained with the FUV-SI12 proton imager on board IMAGE spacecraft. We determine at each magnetic local time the position of an optical boundary equivalent to IB and thereby the stretching of the magnetic field lines. We show that the correspondence between the latitude of the maximum proton precipitation observed by SI12 and the IB measured by DMSP satellites is statistically established and depends on the magnetic local time. The relation between the position of the maximum proton precipitation as well as the intensity of this maximum and the magnetic field's distortion is determined by comparison with GOES-8 data. We thus suggest that SI12 can be used as a tool for the global determination of the isotropic boundary and to monitor the amount of stretching in the magnetotail. [less ▲]

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See detailComparison of intense nightside shock induced aurora and substorms activity
Meurant, M.; Gérard, Jean-Claude ULg; Blockx, Caroline ULg et al

Conference (2004, December 01)

Variations of the solar wind dynamic pressure induce perturbation of magnetospheric processes. These perturbations frequently induce dayside enhancements of activity with particular features such as low ... [more ▼]

Variations of the solar wind dynamic pressure induce perturbation of magnetospheric processes. These perturbations frequently induce dayside enhancements of activity with particular features such as low latitude proton flash, low latitude arcs and aurora propagating eastward and westward from noon to the night sector. In some cases, these shocks may also induce an enhancement of the nightside activity during which the power precipitated in the night sector may reach values as high as observed during substorms. Various studies have shown that high precipitated powers are more likely during period of negative values of the North - South IMF components. Liou et al (2003) have shown that substorm-like activity is not frequent after a shock and they concluded that a shock may not be considered as a substorm trigger. The question addressed in this study is to know up to what point the substorm like nightside activity triggered by a shock is comparable to a classical substorm. For this purpose, we analyze four events presenting nightside activity morphologically similar to substorms and occurring within a short time (less than 20 minutes) after the arrival of a pressure pulse on the front of the magnetosphere. Different features of these events such as the mean energy of precipitated electrons, the motion of boundaries before and after onset and the power precipitated in the nightside region are compared to typical substorms. Except for the absence of southward motion of activity before onset, shock induced substorms appear very similar to isolated substorms. We investigate the ability of a shock to trigger a substorm during periods characterized by particular conditions. We suggest that the sign of B[SUB]z[/SUB] plays an important role as well as the history of solar wind and interplanetary magnetic field and the resulting state of the magnetosphere. [less ▲]

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See detailPropagation of electron and proton shock-induced aurora and the role of the interplanetary magnetic field and solar wind
Meurant, M.; Gérard, Jean-Claude ULg; Blockx, Caroline ULg et al

in Journal of Geophysical Research. Space Physics (2004), 109

Shock-induced aurora observed with satellite-borne ultraviolet imagers shows distinct characteristics from the more common and extensively studied aurora generated during magnetospheric substorms. It is ... [more ▼]

Shock-induced aurora observed with satellite-borne ultraviolet imagers shows distinct characteristics from the more common and extensively studied aurora generated during magnetospheric substorms. It is initiated in the noon sector immediately following dynamic pressure pulses associated with the arrival of enhanced solar wind plasma at the front of the magnetosphere. The auroral brightening rapidly propagates toward the dawn and dusk sectors and may eventually trigger the development of an auroral substorm on the nightside. The FUV imaging system on board the IMAGE satellite has the ability to discriminate between proton and electron precipitation. This feature has been used to study the morphology and dynamics of the electron and proton precipitation following pulse-induced magnetospheric perturbations. A different dynamic is observed for aurora caused by electron and proton precipitation, as well as the important role played by the north-south component of the interplanetary magnetic field. The propagation from the noon to the night sector mainly occurs through the afternoon region for proton precipitation and the morning sector for electron aurora, as expected from azimuthal drift of newly injected plasma. The asymmetry of the precipitation distribution around the noon-midnight axis is more pronounced during negative B[SUB]z[/SUB] periods, when activity is the most important. The magnitude of both the interplanetary magnetic field and the solar wind speed appears well correlated with the precipitated power, by contrast with the solar wind density and the magnitude of the dynamic pressure, which appear to play a minor role. It is suggested that adiabatic compression and plasma waves play an important role on the locations of electron and proton precipitation in the dayside. [less ▲]

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See detailDynamics of global scale electron and proton precipitation induced by a solar wind pressure pulse
Meurant, M.; Gérard, Jean-Claude ULg; Hubert, Benoît ULg et al

in Geophysical Research Letters (2003), 30(20),

[1] On April 28 2001, simultaneous global images of electron and proton aurora were obtained by IMAGE- FUV following a sudden increase of solar wind dynamic pressure. The local time and intensity ... [more ▼]

[1] On April 28 2001, simultaneous global images of electron and proton aurora were obtained by IMAGE- FUV following a sudden increase of solar wind dynamic pressure. The local time and intensity distribution of both types of precipitation are examined and compared. It is found that the electron and the proton precipitation both start in the post noon sector and expand concurrently, but the expansion into the nightside starts sooner for the protons than for the electrons. The characteristic rise time in the onset sector is on the order of 6 minutes. A distinct dynamics and morphology of electron and proton precipitation is observed in the nightside sector. DMSP electron measurements in the afternoon sector indicate that the shock has a significant effect on the electron spectral characteristics. It is suggested that the various Alfven frequencies generated by the shock account for the two different speeds of propagation of the disturbance. [less ▲]

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See detailElectron and proton shock aurora observed by IMAGE-FUV
Meurant, M.; Gérard, Jean-Claude ULg; Hubert, Benoît ULg et al

in EGS - AGU - EUG Joint Assembly (2003, April 01)

The FUV instrument on the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite monitors the aurora in three different spectral regions. The Wideband Imaging Camera (WIC) observes the ... [more ▼]

The FUV instrument on the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite monitors the aurora in three different spectral regions. The Wideband Imaging Camera (WIC) observes the molecular N_2 LBH and the atomic NI emissions at 140-180 nm. The two channels of the Spectrographic Imager (SI) observe the Doppler shifted Lyman-α emission at 121.8 nm due to precipitating protons (SI12) and the electron auroral emission of OI at 135.6 nm (SI13). Three simultaneous snapshots are recorded each 2 minutes. In this study, the FUV instrument allows a global viewing of the aurora with a high temporal resolution both in proton and electron. It is used to study the shock aurora resulting from the disturbance caused by the arrival of a coronal mass ejection on the front of the magnetosphere. A comparison between electron and proton injection features at global scale is performed for different isolated events with positive and negative interplanetary B_z. A correlation with IMF and solar wind parameters is presented as well as a description of the magnetosphere morphology given by the Tsyganenko model in the shock aurora period. [less ▲]

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See detailElectron and proton shock aurora observed by IMAGE-FUV
Meurant, M.; Gérard, Jean-Claude ULg; Hubert, Benoît ULg et al

in EGS - AGU - EUG Joint Assembly (2003, April 01)

The FUV instrument on the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite monitors the aurora in three different spectral regions. The Wideband Imaging Camera (WIC) observes the ... [more ▼]

The FUV instrument on the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite monitors the aurora in three different spectral regions. The Wideband Imaging Camera (WIC) observes the molecular N_2 LBH and the atomic NI emissions at 140-180 nm. The two channels of the Spectrographic Imager (SI) observe the Doppler shifted Lyman-α emission at 121.8 nm due to precipitating protons (SI12) and the electron auroral emission of OI at 135.6 nm (SI13). Three simultaneous snapshots are recorded each 2 minutes. In this study, the FUV instrument allows a global viewing of the aurora with a high temporal resolution both in proton and electron. It is used to study the shock aurora resulting from the disturbance caused by the arrival of a coronal mass ejection on the front of the magnetosphere. A comparison between electron and proton injection features at global scale is performed for different isolated events with positive and negative interplanetary B_z. A correlation with IMF and solar wind parameters is presented as well as a description of the magnetosphere morphology given by the Tsyganenko model in the shock aurora period. [less ▲]

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