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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 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 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 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 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 detailObservations of significant flux closure by dual lobe reconnection
Imber, S. M.; Milan, S. E.; Hubert, Benoît ULg

in Annales Geophysicae (2007), 25(7), 1617-1627

We present an interval of dual lobe reconnection during which interplanetary magnetic field lines are captured by the magnetosphere by reconnecting at high latitudes in both the Northern and the Southern ... [more ▼]

We present an interval of dual lobe reconnection during which interplanetary magnetic field lines are captured by the magnetosphere by reconnecting at high latitudes in both the Northern and the Southern Hemispheres. This event was identified using measurements of the ionospheric convection flow and observations of the aurora using the Super-DARN radars and the IMAGE spacecraft. A cusp spot, characteristic of northward IMF, is clearly visible for a 30 min period enabling the ionospheric footprint of the Northern Hemisphere merging gap to be accurately determined. During the interval a strong burst of sunward flow across the dayside open/closed field line boundary (OCB) is observed, which we interpret as the reconfiguration of the magnetosphere following a burst of reconnection. Noon-midnight and dawn-dusk keograms of the aurora show that the polar cap shrinks during the interval indicating that a large amount of flux was closed by the reconnection. Using the SuperDARN potential maps it is possible to calculate that the amount of flux closed during the interval is 0.13 GWb which represents approximately 10% of the pre-existing polar cap. The number of ions captured by the burst of dual lobe reconnection was calculated to be similar to 2.2x10(31), more than sufficient to populate a cold, dense plasma sheet. That a dense plasma sheet was not subsequently observed is discussed in terms of subsequent changes in the IMF. [less ▲]

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See detailThe auroral and ionospheric flow signatures of dual lobe reconnection
Imber, S. M.; Milan, S. E.; Hubert, Benoît ULg

in Annales Geophysicae (2006), 24(11), 3115-3129

We present the first substantial evidence for the occurrence of dual lobe reconnection from ionospheric flows and auroral signatures. The process of dual lobe reconnection refers to an interplanetary ... [more ▼]

We present the first substantial evidence for the occurrence of dual lobe reconnection from ionospheric flows and auroral signatures. The process of dual lobe reconnection refers to an interplanetary magnetic field line reconnecting with lobe field lines in both the northern and southern hemispheres. Two bursts of sunward plasma flow across the noon portion of the open/closed field line boundary (OCB), indicating magnetic flux closure at the dayside, were observed in SuperDARN radar data during a period of strongly northward IMF. The OCB is identified from spacecraft, radar backscatter, and auroral observations. In order for dual lobe reconnection to take place, we estimate that the interplanetary magnetic field clock angle must be within +/- 10 degrees of zero (North). The total flux crossing the OCB during each burst is small (1.8% and 0.6% of the flux contained within the polar cap for the two flows). A brightening of the noon portion of the northern auroral oval was observed as the clock angle passed through zero, and is thought to be due to enhanced precipitating particle fluxes due to the occurrence of reconnection at two locations along the field line. The number of solar wind protons captured by the flux closure process was estimated to be similar to 2.5 x 10(30) (4 tonnes by mass), sufficient to populate the cold, dense plasma sheet observed following this interval. [less ▲]

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See detailFlux closure during a substorm observed by Cluster, Double Star, IMAGE FUV, SuperDARN, and Greenland magnetometers
Milan, S. E.; Wild, J. A.; Hubert, Benoît ULg et al

in Annales Geophysicae (2006), 24(2), 751-767

We examine magnetic flux closure during an extended substorm interval on 29 August 2004 involving a two-stage onset and subsequent re-intensifications. Cluster and Double Star provide observations of ... [more ▼]

We examine magnetic flux closure during an extended substorm interval on 29 August 2004 involving a two-stage onset and subsequent re-intensifications. Cluster and Double Star provide observations of magnetotail dynamics, while the corresponding auroral evolution, convection response, and substorm current wedge development are monitored by IMAGE FUV, SuperDARN, and the Greenland magnetometer chain, respectively. The first stage of onset is associated with the reconnection of closed flux in the plasma sheet; this is accompanied by a short-lived auroral intensification, a modest substorm current wedge magnetic bay, but no significant ionospheric convection enhancement. The second stage follows the progression of reconnection to the open field lines of the lobes; accompanied by prolonged auroral bulge and westward-travelling surge development, enhanced magnetic bays and convection. We find that the tail dynamics are highly influenced by ongoing dayside creation of open flux, leading to flux pile-up in the near-tail and a step-wise down-tail motion of the tail reconnection site. In all, 5 dipolarizations are observed, each associated with the closure of similar to 0.1 GWb of flux. Very simple calculations indicate that the X-line should progress down-tail at a speed of 20 km s(-1) or 6 R-E between each dipolarization. [less ▲]

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See detailCoulomb modeling of Marmara Sea earthquakes since 1700; implications on the earthquake hazard of the Istanbul region
Barka, A; Nalbant, S.B.; Hubert, Aurelia ULg et al

in Annales Geophysicae (1998, April), 16(Suppl. 1), 138

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See detailThe redesign of the Belgian permanent GPS network for future (near) real-time applications
Muls, Alain; Sleewaegen, Jean-Marie; Bruyninx, Carine et al

in Annales Geophysicae (1998), 16(Suppl. I, C394),

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See detailThe transport of thermospheric nitric oxide into the mesosphere
Gérard, Jean-Claude ULg; Deneye, E. J. F.

in Annales Geophysicae (1984), 2

A ratio phi/K of vertical nitric oxide flux to the eddy diffusion coefficient is derived to estimate the magnitude of nitric oxide transport from the thermosphere into the mesosphere. Rocket measurements ... [more ▼]

A ratio phi/K of vertical nitric oxide flux to the eddy diffusion coefficient is derived to estimate the magnitude of nitric oxide transport from the thermosphere into the mesosphere. Rocket measurements of NO vertical profiles are used to show the existence of a latitudinal-seasonal dependence on the efficiency of thermosphere-mesosphere coupling. One peak is observed in each auroral zone at 100 km, but there is evidence of only one peak in the mesospause. This trend is considered to be in good agreement with the results of a recent two-dimensional odd nitrogen model and is explained by the combined effects of particle precipitation at high winter latitudes and photodissociation of the NO downward flow in the sunlit regions. [less ▲]

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