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Garching bei München, Germany

Phan T.D.,University of California at Berkeley | Shay M.A.,University of Delaware | Gosling J.T.,University of Colorado at Boulder | Fujimoto M.,ISAS | And 5 more authors.
Geophysical Research Letters | Year: 2013

We surveyed 79 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of electron bulk heating produced by reconnection depend on the inflow boundary conditions. We find that the amount of heating, ΔTe, is correlated with the asymmetric Alfvén speed, VAL,asym, based on the reconnecting magnetic field and the plasma density measured in both the high-density magnetosheath and low-density magnetospheric inflow regions. Best fit to the data produces the empirical relation ΔTe = 0.017 miV AL,asym 2, indicating that the amount of heating is proportional to the inflowing magnetic energy per proton-electron pair, with ∼1.7% of the energy being converted into electron heating. This finding, generalized to symmetric reconnection, could account for the lack of electron heating in typical solar wind exhausts at 1 AU, as well as strong heating to keV energies common in magnetotail exhausts. We also find that the guide field suppresses perpendicular heating. Key Points Electron bulk heating controlling factors revealed Heating depends on the total incoming magnetic energy per particle 1.7% of magnetic energy is converted into electron bulk heating ©2013. American Geophysical Union. All Rights Reserved.

Roediger E.,Jacobs University Bremen | Bruggen M.,Jacobs University Bremen | Simionescu A.,Stanford University | Bohringer H.,MPE | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We perform hydrodynamical simulations of minor-merger-induced gas sloshing and the subsequent formation of cold fronts in the Virgo cluster. Comparing to observations, we show for the first time that the sloshing scenario can reproduce the radii and the contrasts in X-ray brightness, projected temperature and metallicity across the cold fronts quantitatively. The comparison suggests a third cold front 20kpc north-west of the Virgo core. We identify several new features typical for sloshing cold fronts: an alternating distribution of cool, metal-enriched X-ray brightness excess regions and warm brightness deficit regions of reduced metallicity; a constant or radially decreasing temperature accompanied by a plateau in metallicity inside the cold fronts; a warm rim outside the cold fronts and a large-scale brightness asymmetry. We can trace these new features not only in Virgo, but also in other clusters exhibiting sloshing cold fronts. By comparing synthetic and real observations, we estimate that the original minor-merger event took place about 1.5 Gyr ago when a subcluster of 1-4 × 1013M⊙ passed the Virgo core at 100-400kpc distance, where a smaller mass corresponds to a smaller pericentre distance, and vice versa. From our inferred merger geometry, we derive the current location of the disturbing subcluster to be about 1-2Mpc east of the Virgo core. A possible candidate is M60. Additionally, we quantify the metal redistribution by sloshing and discuss its importance. We verify that the subcluster required to produce the observed cold fronts could be completely ram-pressure-stripped before reaching the Virgo centre, and discuss the conditions required for this to be achieved. Finally, we demonstrate that the bow shock of a fast galaxy passing the Virgo cluster at ~400kpc distance also causes sloshing and leads to very similar cold front structures. The responsible galaxy would be located about 2Mpc north of the Virgo centre. A possible candidate is M85. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Phan T.D.,University of California at Berkeley | Gosling J.T.,University of Colorado at Boulder | Paschmann G.,MPE | Pasma C.,University of California at Berkeley | And 7 more authors.
Astrophysical Journal Letters | Year: 2010

We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, θ, across the current sheet and the difference in the plasma β values on the two sides of the current sheet, δβ. For low.β, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large.β: Events with shears as low as 11° were observed for.β < 0.1, but for.β > 1.5 only events with. > 100. were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high β plasmas at low magnetic shears due to super-Alfv́enic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-.β dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Thi W.-F.,MPE
EPJ Web of Conferences | Year: 2015

The chemical species in protoplanetary disks react with each other. The chemical species control part of the thermal balance in those disks. How the chemistry proceeds in the varied conditions encountered in disks relies on detailed microscopic understanding of the reactions through experiments or theoretical studies. This chapter strives to summarize and explain in simple terms the different types of chemical reactions that can lead to complex species. The first part of the chapter deals with gas-phase chemistry and the second part introduces chemical reactions occurring on grain surfaces. Several terms pertaining to astrochemistry are introduced. © 2015 Owned by the authors, published by EDP Sciences.

Thi W.-F.,MPE
EPJ Web of Conferences | Year: 2015

This chapter discusses the fundamental ideas of how chemical networks are build, their strengths and limitations. The chemical reactions that occur in disks combine the cold phase reactions used to model cold molecular clouds with the hot chemistry applied to planetary atmosphere models. With a general understanding of the different types of reactions that can occur, one can proceed in building a network of chemical reactions and use it to explain the abundance of species seen in disks. One on-going research subject is finding new paths to synthesize species either in the gas-phase or on grain surfaces. Specific formation routes for water or carbon monoxide are discussed in more details. © 2015 Owned by the authors, published by EDP Sciences.

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