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Auger A.-T.,Aix - Marseille University | Auger A.-T.,University Paris - Sud | Groussin O.,Aix - Marseille University | Jorda L.,Aix - Marseille University | And 53 more authors.
Astronomy and Astrophysics | Year: 2015

Context. Since August 2014, the OSIRIS Narrow Angle Camera (NAC) onboard the Rosetta spacecraft has acquired high spatial resolution images of the nucleus of comet 67P/Churyumov-Gerasimenko, down to the decimeter scale. This paper focuses on the Imhotep region, located on the largest lobe of the nucleus, near the equator. Aims. We map, inventory, and describe the geomorphology of the Imhotep region. We propose and discuss some processes to explain the formation and ongoing evolution of this region. Methods. We used OSIRIS NAC images, gravitational heights and slopes, and digital terrain models to map and measure the morphologies of Imhotep. Results. The Imhotep region presents a wide variety of terrains and morphologies: smooth and rocky terrains, bright areas, linear features, roundish features, and boulders. Gravity processes such as mass wasting and collapse play a significant role in the geomorphological evolution of this region. Cometary processes initiate erosion and are responsible for the formation of degassing conduits that are revealed by elevated roundish features on the surface. We also propose a scenario for the formation and evolution of the Imhotep region; this implies the presence of large primordial voids inside the nucleus, resulting from its formation process. © ESO, 2015. Source


Lee S.H.,University of Alaska Fairbanks | Lee S.H.,NASA | Zhang H.,University of Alaska Fairbanks | Zong Q.-G.,Beijing Institute of Technology | And 4 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2016

We present a statistical study of plasmaspheric plumes and ionospheric outflows observed by the Cluster spacecraft near the dayside magnetopause. Plasmaspheric plumes are identified when the low-energy ions (<1 keV) with 90 pitch angle distributions are observed by the Cluster Ion Spectrometer/Hot Ion Analyzer instrument. The ionospheric outflows are characterized by unidirectional or bidirectional field-aligned pitch angle distributions of low-energy ions observed in the dayside magnetosphere. Forty-three (10%) plasmaspheric plume events and 32 (7%) ionospheric outflow events were detected out of the 442 times that C3 crossed the dayside magnetopause between 2007 and 2009. The occurrence rate of plumes at duskside is significantly higher than that at dawnside. The occurrence rate of outflows shows a weak dawn-dusk asymmetry. We investigate the dependence of the occurrence rates of plumes and ionospheric outflows on geomagnetic activity and on solar wind/interplanetary magnetic field (IMF) conditions. The plume events tend to occur during southward IMF (duskward solar wind electric field) and moderate geomagnetic activity (Kp = 3,-30≤Dst <- 10 nT). However, the ionospheric outflow events tend to occur during northward IMF (dawnward solar wind electric field). The ionospheric outflows do not occur when Kp = 0, and the occurrence rate of the ionospheric outflows does not have a clear Dst dependence. Seventy-five percent (46%) of the outflows are observed in the duskside for negative (positive) IMF By. Conversely, 54% (25%) of the outflows are observed in the dawnside for positive (negative) IMF By. Finally, the occurrence rates of both plumes and outflows increase with solar wind dynamic pressure. ©2016. American Geophysical Union. All Rights Reserved. Source


Lee S.H.,University of Alaska Fairbanks | Zhang H.,University of Alaska Fairbanks | Zong Q.-G.,Beijing Institute of Technology | Otto A.,University of Alaska Fairbanks | And 6 more authors.
Journal of Geophysical Research: Space Physics | Year: 2014

The factors controlling asymmetric reconnection and the role of the cold plasma population in the reconnection process are two outstanding questions. We present a case study of multipoint Cluster observations demonstrating that the separatrix and flow boundary angles are greater on the magnetosheath than on the magnetospheric side of the magnetopause, probably due to the stronger density than magnetic field asymmetry at this boundary. The motion of cold plasmaspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection in the diffusion region near the subsolar magnetopause, the colder ions are simply entrained by E×B drifts at high latitudes on the recently reconnected magnetic field lines. This indicates that plasmaspheric ions can sometimes play only a very limited role in asymmetric reconnection, in contrast to previous simulation studies. Three cold ion populations (probably H+, He+, and O+) appear in the energy spectrum, consistent with ion acceleration to a common velocity. Key Points The cold ions (H+, He+, and O+) are picked up by the reconnected magnetic field The cold ions are accelerated perpendicular to the magnetic field Asymmetric reconnection geometry is caused by the stronger asymmetry in density ©2014. American Geophysical Union. All Rights Reserved. Source


Lee S.H.,University of Alaska Fairbanks | Zhang H.,University of Alaska Fairbanks | Zong Q.-G.,Beijing Institute of Technology | Wang Y.,Beijing Institute of Technology | And 4 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

An important source of the terrestrial magnetospheric plasma is the Earth's ionospheric outflows from the high-latitude regions of both hemispheres. The ionospheric ion outflows have rarely been observed at the dayside magnetopause. We report Cluster observations of the ionospheric ion outflows observed at the dayside magnetopause. The low-energy (up to 1.5 keV) electrons are detected with bidirectional pitch angle distributions indicating that the magnetic field lines are closed. The unidirectional cold ions (< 200 eV) are observed in the magnetosphere by both C1 and C3. The pitch angle distributions (0°-75°) of the cold ions (< 1 keV) at the dayside magnetopause indicate that these cold ions are the ionospheric outflows coming only from the Southern Hemisphere. The cold ions (< 200 eV) fluxes are modulated by the ULF wave electric field. Two different species (possibly H+ and He+) are observed in the magnetosphere. Our results suggest that the ionospheric outflows can directly reach the dayside magnetopause region and may participate in the reconnection process. Key Points The unidirectional cold ions (<200 eV) are observed at the dayside magnetopause The cold ions are the ionospheric outflow coming from the Southern Hemisphere The cold ion fluxes are modulated by the ULF wave electric field ©2015. American Geophysical Union. All Rights Reserved. Source

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