References of "Radioti, Aikaterini"
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See detailCorotating Magnetic Reconnection Site in Saturn's Magnetosphere
Yao, Zhonghua ULiege; Coates, A. J.; Ray, L. C. et al

in Astrophysical Journal Letters (2017), 846(2),

Using measurements from the Cassini spacecraft in Saturn's magnetosphere, we propose a 3D physical picture of a corotating reconnection site, which can only be driven by an internally generated source ... [more ▼]

Using measurements from the Cassini spacecraft in Saturn's magnetosphere, we propose a 3D physical picture of a corotating reconnection site, which can only be driven by an internally generated source. Our results demonstrate that the corotating magnetic reconnection can drive an expansion of the current sheet in Saturn's magnetosphere and, consequently, can produce Fermi acceleration of electrons. This reconnection site lasted for longer than one of Saturn's rotation period. The long-lasting and corotating natures of the magnetic reconnection site at Saturn suggest fundamentally different roles of magnetic reconnection in driving magnetospheric dynamics (e.g., the auroral precipitation) from the Earth. Our corotating reconnection picture could also potentially shed light on the fast rotating magnetized plasma environments in the solar system and beyond. © 2017. The American Astronomical Society. All rights reserved. [less ▲]

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See detailAn isolated, bright cusp aurora at Saturn
Kinrade, J.; Badman, S. V.; Bunce, E. J. et al

in Journal of Geophysical Research. Space Physics (2017), 122(6), 6121-6138

Saturn's dayside aurora displays a number of morphological features poleward of the main emission region. We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014 (day 165 ... [more ▼]

Saturn's dayside aurora displays a number of morphological features poleward of the main emission region. We present an unusual morphology captured by the Hubble Space Telescope on 14 June 2014 (day 165), where for 2 h, Saturn's FUV aurora faded almost entirely, with the exception of a distinct emission spot at high latitude. The spot remained fixed in local time between 10 and 15 LT and moved poleward to a minimum colatitude of ~4°. It was bright and persistent, displaying intensities of up to 49 kR over a lifetime of 2 h. Interestingly, the spot constituted the entirety of the northern auroral emission, with no emissions present at any other local time—including Saturn's characteristic dawn arc, the complete absence of which is rarely observed. Solar wind parameters from propagation models, together with a Cassini magnetopause crossing and solar wind encounter, indicate that Saturn's magnetosphere was likely to have been embedded in a rarefaction region, resulting in an expanded magnetosphere configuration during the interval. We infer that the spot was sustained by reconnection either poleward of the cusp or at low latitudes under a strong component of interplanetary magnetic field transverse to the solar wind flow. The subsequent poleward motion could then arise from either reconfiguration of successive open field lines across the polar cap or convection of newly opened field lines. We also consider the possible modulation of the feature by planetary period rotating current systems. ©2017. The Authors. [less ▲]

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See detailMechanisms of Saturn's Near-Noon Transient Aurora: In Situ Evidence From Cassini Measurements
Yao, Zhonghua ULiege; Radioti, Aikaterini ULiege; Rae, I. J. et al

in Geophysical Research Letters (2017)

Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in ... [more ▼]

Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images). ©2017. American Geophysical Union. [less ▲]

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See detailStagnation of Saturn's auroral emission at noon
Radioti, Aikaterini ULiege; Grodent, Denis ULiege; Gérard, Jean-Claude ULiege et al

in Journal of Geophysical Research. Space Physics (2017)

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See detailResponse of Jupiter's auroras to conditions in the interplanetary medium as measured by the Hubble Space Telescope and Juno
Nichols, J. D.; Badman, S. V.; Bagenal, F. et al

in Geophysical Research Letters (2017)

We present the first comparison of Jupiter's auroral morphology with an extended, continuous and complete set of near-Jupiter interplanetary data, revealing the response of Jupiter's auroras to the ... [more ▼]

We present the first comparison of Jupiter's auroral morphology with an extended, continuous and complete set of near-Jupiter interplanetary data, revealing the response of Jupiter's auroras to the interplanetary conditions. We show that for ∼1-3 days following compression region onset the planet's main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiter's magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought. [less ▲]

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See detailEnergy Dissipation in Saturn’s Magnetotail: A Comparative Magnetotail Approach
Yao, Zhonghua ULiege; Coates, Andrew; Ray, Licia et al

Poster (2016, December 16)

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See detailThe complex behavior of the satellite footprints at Jupiter: the result of universal processes?
Bonfond, Bertrand ULiege; Grodent, Denis ULiege; Badman, Sarah V. et al

Poster (2016, December 14)

At Jupiter, some auroral emissions are directly related to the electromagnetic interaction between the moons Io, Europa and Ganymede on one hand and the rapidly rotating magnetospheric plasma on the other ... [more ▼]

At Jupiter, some auroral emissions are directly related to the electromagnetic interaction between the moons Io, Europa and Ganymede on one hand and the rapidly rotating magnetospheric plasma on the other hand. Out of the three, the Io footprint is the brightest and the most studied. Present in each hemisphere, it is made of at least three different spots and an extended trailing tail. The variability of the brightness of the spots as well as their relative location has been tentatively explained with a combination of Alfvén waves’ partial reflections on density gradients and bi-directional electron acceleration at high latitude. Should this scenario be correct, then the other footprints should also show the same behavior. Here we show that all footprints are, at least occasionally, made of several spots and they all display a tail. We also show that these spots share many characteristics with those of the Io footprint (i.e. some significant variability on timescales of 2-3 minutes). Additionally, we present some Monte-Carlo simulations indicating that the tails are also due to Alfvén waves electron acceleration rather than quasi-static electron acceleration. Even if some details still need clarification, these observations strengthen the scenario proposed for the Io footprint and thus indicate that these processes are universal. In addition, we will present some early results from Juno-UVS concerning the location and morphology of the footprints during the first low-altitude observations of the polar aurorae. These observations, carried out in previously unexplored longitude ranges, should either confirm or contradict our understanding of the footprints. [less ▲]

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See detailJupiter’s auroras during the Juno approach phase as observed by the Hubble Space Telescope
Nichols, Jonathan D; Clarke, John T; Orton, Glennn S et al

Conference (2016, December 13)

We present movies of the Hubble Space Telescope (HST) observations of Jupiter’s FUV auroras observed during the Juno approach phase and first capture orbit, and compare with Juno observations of the ... [more ▼]

We present movies of the Hubble Space Telescope (HST) observations of Jupiter’s FUV auroras observed during the Juno approach phase and first capture orbit, and compare with Juno observations of the interplanetary medium near Jupiter and inside the magnetosphere. Jupiter’s FUV auroras indicate the nature of the dynamic processes occurring in Jupiter’s magnetosphere, and the approach phase provided a unique opportunity to obtain a full set of interplanetary data near to Jupiter at the time of a program of HST observations, along with the first simultaneous with Juno observations inside the magnetosphere. The overall goal was to determine the nature of the solar wind effect on Jupiter’s magnetosphere. HST observations were obtained with typically 1 orbit per day over three intervals: 16 May – 7 June, 22-30 June and 11-18 July, i.e. while Juno was in the solar wind, around the bow shock and magnetosphere crossings, and in the mid-latitude middle-outer magnetospheres. We show that these intervals are characterised by particularly dynamic polar auroras, and significant variations in the auroral power output caused by e.g. dawn storms, intense main emission and poleward forms. We compare the variation of these features with Juno observations of interplanetary compression regions and the magnetospheric environment during the intervals of these observations. [less ▲]

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See detailDynamics of the flares in the active polar region of Jupiter
Bonfond, Bertrand ULiege; Grodent, Denis ULiege; Badman, S. V. et al

in Geophysical Research Letters (2016)

The dusk-side of the polar region of Jupiter's UV aurorae, called the active region, sometimes exhibits quasi-periodic (QP) flares on time-scales of 2-3 minutes. Based on Hubble Space Telescope Far-UV ... [more ▼]

The dusk-side of the polar region of Jupiter's UV aurorae, called the active region, sometimes exhibits quasi-periodic (QP) flares on time-scales of 2-3 minutes. Based on Hubble Space Telescope Far-UV time-tag images, we show for the first time that the northern hemisphere also displays QP-flares. The area covered by these flares can reach up to 2.4 × 108 km2 (i.e. the whole active region), but often only involves an area an order of magnitude smaller. Using a magnetic field mapping model, we deduced that these areas correspond to the dayside outer magnetosphere. In our dataset, quasi-periodic features are only seen on half of the cases and even on a given observation, a region can be quiet for one half and blinking on the other half. Consecutive observations in the two hemispheres show that the brightening can occur in phase. Combined with the size and location of the flares, this behaviour suggests that the QP-flares most likely take place on closed magnetic field lines. [less ▲]

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See detailFirst Hubble Space Telescope Movies of Jupiter’s Ultraviolet Aurora During the NASA Juno Prime Mission
Grodent, Denis ULiege; Gladstone, G. Randall; Clarke, John T. et al

Poster (2016, December)

The primary goal of this HST campaign is to complement Juno-UVS (Ultraviolet Spectrograph) observations. This complementarity is four-fold as HST observes Jupiter’s aurora when: 1) Juno-UVS is turned off ... [more ▼]

The primary goal of this HST campaign is to complement Juno-UVS (Ultraviolet Spectrograph) observations. This complementarity is four-fold as HST observes Jupiter’s aurora when: 1) Juno-UVS is turned off, that is about 98% of Juno’s 14-day orbit, and Juno’s in situ instruments are in operation. 2) Juno-UVS is operating, but observes the opposite hemisphere of Jupiter. 3) UVS is on in the same hemisphere, but too close to Jupiter to have a global, contextual, view of the aurora and/or UVS is affected by the noise induced by Jupiter’s radiation belts. 4) Juno is too far from Jupiter to get a detailed view of the aurora. In addition, HST will observe the auroral and airglow emissions of the Galilean moons Io, Ganymede and Europa, when UVS is measuring their auroral footprints in Jupiter’s ionosphere. During this campaign, HST is obtaining 45-min STIS time-tag images -movies- of both hemispheres of Jupiter and STIS/COS spectra of Jupiter's moons. These observations are taking place during 4 sequences of Juno's orbit (Figure: typical orbit in magnetic coordinates): 1) Perijove segment: a 6-hour sequence bracketing the time of Juno's closest approach of Jupiter. 2) Crossing segments: few hours periods during which Juno is crossing the magnetic equator of Jupiter and in situ instruments are observing the plasma sheet particles. 3) Perijove +/- 1 Jovian rotation (or more), to provide a context for the auroral activity before and after perijove. 4) Apojove segment: a 12-hour period bracketing the time when Juno is farthest from Jupiter and Juno-UVS is continuously monitoring the global auroral UV power of Jupiter. During Juno orbit PJ5, between 28 Nov. and 07 Dec. 2016, HST obtains 9 STIS movies: 3 movies of the northern aurora near perijove, 1 movie (north) one Jovian rotation before and 2 movies (south- north) one and two Jovian rotations after perijove, 2 movies (north) during two close CS crossings, and 1 movie near apojove. These movies will be commented during this presentation. [less ▲]

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See detailPulsations of the polar cusp aurora at Saturn
Palmaerts, Benjamin ULiege; Radioti, Aikaterini ULiege; Roussos, E. et al

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

The magnetospheric cusp is a region connecting the interplanetary environment to the ionosphere and enabling solar wind particles to reach the ionosphere. We report the detection of several isolated high ... [more ▼]

The magnetospheric cusp is a region connecting the interplanetary environment to the ionosphere and enabling solar wind particles to reach the ionosphere. We report the detection of several isolated high-latitude auroral emissions with the Ultraviolet Imaging Spectrograph of the Cassini spacecraft. We suggest that these auroral spots, located in the dawn-to-noon sector and poleward of the main emission, are the ionospheric signatures of the magnetospheric cusp, in agreement with some previous observations with the Hubble Space Telescope. The high-latitude cusp auroral signature has been associated with high-latitude lobe reconnection in the presence of a southward interplanetary magnetic field. The occurrence rate of the polar cusp aurora suggests that lobe reconnection is frequent at Saturn. Several auroral imaging sequences reveal a quasiperiodic brightening of the polar cusp aurora with a period in the range of 60 to 70 min. Similar pulsations in the energetic electron fluxes and in the azimuthal component of the magnetic field are simultaneously observed by Cassini instruments, suggesting the presence of field-aligned currents. Pulsed dayside magnetopause reconnection is a likely common triggering process for the cusp auroral brightenings at Saturn and the quasiperiodic pulsations in the high-latitude energetic electron fluxes. [less ▲]

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See detailStatistical study of Saturn's auroral electron properties with Cassini/UVIS FUV spectral images
Gustin, Jacques ULiege; Grodent, Denis ULiege; Radioti, Aikaterini ULiege et al

in Icarus (2016)

About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean ... [more ▼]

About 2000 FUV spectra of different regions of Saturn's aurora, obtained with Cassini/UVIS from December 2007 to October 2014 have been examined. Two methods have been employed to determine the mean energy 〈E〉 of the precipitating electrons. The first is based on the absorption of the auroral emission by hydrocarbons and the second uses the ratio between the brightness of the Lyman-α line and the H2 total UV emission (Lyα/H2), which is directly related to 〈E〉 via a radiative transfer formalism. In addition, two atmospheric models obtained recently from UVIS polar occultations have been employed for the first time. It is found that the atmospheric model related to North observations near 70° latitude provides the results most consistent with constraints previously published. On a global point of view, the two methods provide comparable results, with 〈E〉 mostly in the 7–17 keV range with the hydrocarbon method and 〈E〉 in the 1–11 keV range with the Lyα/H2 method. Since hydrocarbons have been detected on ∼20% of the auroral spectra, the Lyα/H2 technique is more effective to describe the primary auroral electrons, as it is applicable to all spectra and allows an access to the lowest range of energies (≤5 keV), unreachable by the hydrocarbon method. The distribution of 〈E〉 is found fully compatible with independent HST/ACS constraints (emission peak in the 840–1450 km range) and FUSE findings (emission peaking at pressure level ≤0.2 µbar). In addition, 〈E〉 exhibits enhancements in the 3 LT–10 LT sector, consistent with SKR intensity measurements. An energy flux–electron energy diagram built from all the data points strongly suggests that acceleration by field-aligned potentials as described by Knight's theory is a main mechanism responsible for electron precipitation creating the aurora. Assuming a fixed electron temperature of 0.1 keV, a best-fit equatorial electron source population density of 3 × 103 m−3 is derived, which matches very well to the plasma properties observed with Cassini MAG and CAPS/ELS instruments. However, several auroral regions are characterized by relatively high 〈E〉 and low energy flux, suggesting that additional processes such as plasma injections or magnetic reconnections must be accounted for to explain the emission in these regions. The Lyα/H2 ratio technique can be used to build maps of 〈E〉 from single spectral images. As expected, preliminary results show that the spatial distribution of 〈E〉 is not uniform, as seen on Jupiter. Our study reveals that a fraction of the aurora is due to very low energy electrons (<1 keV). Even in this case, comparisons between observed and modeled spectra show that 100 eV is a suitable value to represent the average energy of the secondary electrons. [less ▲]

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See detailCassini UVIS Auroral Observations in 2016
Pryor, Wayne R.; Jouchoux, Alain; Esposito, Larry et al

Conference (2016, October)

In June of 2016, the Cassini Saturn orbiter began a series of high inclination orbits that will continue until September 2017 when the mission ends as Cassini enters the Saturn atmosphere. These orbits ... [more ▼]

In June of 2016, the Cassini Saturn orbiter began a series of high inclination orbits that will continue until September 2017 when the mission ends as Cassini enters the Saturn atmosphere. These orbits present excellent views of Saturn's polar regions suitable for auroral imaging at the closest distances to date, with the additional prospect of simultaneous particle and fields measurements within the sources of Saturn Kilometric Radiation (SKR) associated with ultraviolet auroral emissions and/or acceleration regions likely coinciding with them. We will present new Cassini Ultraviolet Imaging Spectrograph (UVIS) auroral images, spectra and movies obtained during the summer and fall of 2016 and put them in the context of auroral data collected since Cassini orbit insertion in 2004. Included in the new data will be UVIS south polar observations obtained simultaneously with Hubble Space Telescope observations of the north polar region on June 29, 2016 and August 19, 2016. [less ▲]

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See detailMulti-instrument overview of the 1-hour pulsations in Saturn's magnetosphere and auroral emissions
Palmaerts, Benjamin ULiege; Roussos, Elias; Radioti, Aikaterini ULiege et al

Conference (2016, May 11)

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See detailJupiter’s magnetopause: A search for reconnection and wave signatures
Bonfond, Bertrand ULiege; Kivelson, Margaret; Khurana, Krishan et al

Conference (2016, April 26)

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See detailMulti-instrument overview of the 1-hour pulsations in Saturn's magnetosphere and auroral emissions
Palmaerts, Benjamin ULiege; Roussos, Elias; Radioti, Aikaterini ULiege et al

Conference (2016, April 19)

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See detailAuroral evidence of flux tube blockage near noon at Saturn’s magnetosphere
Radioti, Aikaterini ULiege; Grodent, Denis ULiege; Gérard, Jean-Claude ULiege et al

Poster (2016, April)

We discuss plasma circulation in Saturn’s magnetosphere on the basis of auroral observations. Auroral enhance- ments in the dawn region are suggested to be related to intense field-aligned currents ... [more ▼]

We discuss plasma circulation in Saturn’s magnetosphere on the basis of auroral observations. Auroral enhance- ments in the dawn region are suggested to be related to intense field-aligned currents generated by hot tenuous plasma carried inward in fast moving flux tubes as they return from tail reconnection site to the dayside. Here we demonstrate that the rotation of the auroral emission in the dawn sector is occasionally (in half of the auroral sequences examined) slowed down and blocked near noon for a couple of hours. When the blockage is prominent and persistent, we observe auroral evidence of dayside magnetopause reconnection and openign of flux. A pos- sible interpretation for our observations could be that depleted flux tubes at large radial distances, which rotate around Saturn are blocked in the prenoon sector between the heavy Vasyliunas cycle flux tubes on one side, and the magnetopause on the other side. These depleted flux tubes have to move above or below the current sheet to pass this blockage. The blockage of the field lines close to midday will bend them and trigger reconnection, which opens the flux tubes and allows for solar wind material to enter the magnetosphere. Secondly, we suggest that the circulation pattern of depleted flux tubes close to noon in Saturn’s magnetosphere alternates between a ’blocked’ and ’unblocked’ state, depending on the solar wind dynamic pressure and the internal processes. [less ▲]

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See detailThe color ratio-intensity relation in the Jovian aurora: Hubble observations of auroral components
Gérard, Jean-Claude ULiege; Bonfond, Bertrand ULiege; Grodent, Denis ULiege et al

in Planetary and Space Science (2016), 131

Spectral observations made with the long slit of the Space Telescope Imaging Spectrograph (STIS) on board Hubble have been used to construct spectral maps of the FUV Jovian aurora. They reveal that the ... [more ▼]

Spectral observations made with the long slit of the Space Telescope Imaging Spectrograph (STIS) on board Hubble have been used to construct spectral maps of the FUV Jovian aurora. They reveal that the amount of absorption by overlying methane shows significant spatial variations. In this report, we examine the relationship between the auroral brightness of the unabsorbed H2 emission that is proportional to the precipitated electron energy flux, and the ultraviolet color ratio, a proxy of the mean electron energy. We find that it varies significantly between the different components of the aurora and in the polar region. Although no global dependence can be found, we show that the two quantities are better organized in some auroral components such as regions of the main aurororal emission. By contrast, the dependence of the electron characteristic energy in high-latitude and diffuse aurora regions on the auroral energy input is generally more scattered. We conclude that the various auroral components are associated with different electron acceleration processes, some of which are not governed by a simple relation linking the value of a field-aligned acceleration potential with the parallel currents flowing from the ionosphere. [less ▲]

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See detailPlanetary Space Weather: Scientific Aspects and Future Perspectives
Plainaki, C.; Lilensten, J.; Radioti, Aikaterini ULiege et al

in Journal of Space Weather and Space Climate (2016)

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See detailHST-Juno synergistic approach of Jupiter's magnetosphere and ultraviolet auroras
Grodent, Denis ULiege; Bonfond, Bertrand ULiege; Gérard, Jean-Claude ULiege et al

E-print/Working paper (2016)

Jupiter's system is not only fundamental to our understanding of the solar system but also of planetary systems around other stars as well as more distant astrophysical bodies, not accessible to a ... [more ▼]

Jupiter's system is not only fundamental to our understanding of the solar system but also of planetary systems around other stars as well as more distant astrophysical bodies, not accessible to a detailed investigation. Fully exploiting any rare opportunity to explore the Jovian system through synergistic observations is thus critical, as it will impact significantly across wider astronomical studies. Such an exceptional opportunity will occur in Cycle 24, when the NASA Juno spacecraft will achieve its prime mission around Jupiter. Since Juno will literally fly through the auroral acceleration regions, the combination of HST auroral observations with Juno in situ measurements will allow us to finally unravel the origins and consequences of Jupiter's powerful and highly variable ultraviolet auroras. This occasion has never occurred before and is unlikely to ever repeat. Juno will address key scientific issues related to unexplored regions of the Jovian magnetosphere. The auroral signatures associated with these magnetospheric processes will be precisely observed with STIS and COS. This program responds to the UV initiative and is only possible during Cycle 24. Indeed, HST is the only observatory capable of making these high spatial and temporal resolution FUV observations during the Juno mission. This ambitious campaign will yield high-impact results and significantly augment the science return of the NASA Juno mission. [less ▲]

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