Self-interacting vectors are seeing a burst of interest where various groups demonstrated that the field evolution ends in finite time. Two nonequivalent criteria have been offered to identify this ...breakdown: (i) the vector constraint equation cannot be satisfied beyond a point where the breakdown occurs, (ii) the dynamics is governed by an effective metric that becomes singular at the breakdown. We show that (i) identifies a coordinate singularity, and can be removed by a change of coordinates. Hence, it does not signify a physical problem, and cannot determine the validity of a theory.
Four formation-flying spacecraft reveal the 3D electron physics of magnetic reconnection
That plasmas don't mix well is a key aspect of this magnetized, highly conducting fourth state of matter. ...Magnetized plasmas are ubiquitous in our solar system, in astrophysics, and in the lab. The same fundamental process is common to solar flares, coronal mass ejections, the solar wind, the magnetospheres of Earth and other planets, comet tails, magnetars, and tokamaks. We know that plasma regions with embedded magnetic fields actually do spontaneously mix via the process of magnetic reconnection, with explosive results—we see the effects of particle acceleration in the aurora and solar energetic particles. On page 1189 of this issue, Burch
et al.
(
1
) present results that help shed light on the process of magnetic reconnection. They have probed a magnetic reconnection site in Earth's magnetosphere using a constellation of four spacecraft in close proximity, with unprecedentedly fast electron measurements to reveal what triggers reconnection and the vital role that electrons play on the small scale.
Aerosol growth in Titan’s ionosphere Lavvas, Panayotis; Yelle, Roger V.; Koskinen, Tommi ...
Proceedings of the National Academy of Sciences - PNAS,
02/2013, Letnik:
110, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on ...atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan’s upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.
The cuscuton is a scalar field with infinite speed of propagation. It was introduced in the context of cosmology but it has also been claimed to resemble Hořava gravity in a certain limit. Here we ...revisit the cuscuton theory as a Lorentz-violating gravity theory. We clarify its relation with Hořava gravity and Einstein-æther theory, analyze its causal structure, and consider its initial value formulation. Finally, we discuss to which extent the cuscuton theory can be used as a proxy for Hořava gravity in the context of gravitational collapse and formation of universal horizons.
Ionisation of neutrals plays a key role in the plasma interactions of comets and moons, and in magnetospheres. Following ionisation, ions and electrons are 'picked up' in the plasma flow, initially ...accelerating along the electric field and then gyrating around the magnetic field. For an ion with mass m (amu) this leads to an acceleration of up to 4m times the energy of the flowing plasma. Further scattering in pitch angle and energy may increase the acceleration further. Here, we illustrate the process at work with relevant plasma measurements from comets including Rosetta's comet 67P, from Titan and near Enceladus and Rhea.
Interaction of Titan's ionosphere with Saturn's magnetosphere Coates, Andrew J
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
02/2009, Letnik:
367, Številka:
1889
Journal Article
Recenzirano
Titan is the only Moon in the Solar System with a significant permanent atmosphere. Within this nitrogen-methane atmosphere, an ionosphere forms. Titan has no significant magnetic dipole moment, and ...is usually located inside Saturn's magnetosphere. Atmospheric particles are ionized both by sunlight and by particles from Saturn's magnetosphere, mainly electrons, which reach the top of the atmosphere. So far, the Cassini spacecraft has made over 45 close flybys of Titan, allowing measurements in the ionosphere and the surrounding magnetosphere under different conditions. Here we review how Titan's ionosphere and Saturn's magnetosphere interact, using measurements from Cassini low-energy particle detectors. In particular, we discuss ionization processes and ionospheric photoelectrons, including their effect on ion escape from the ionosphere. We also discuss one of the unexpected discoveries in Titan's ionosphere, the existence of extremely heavy negative ions up to 10 000 amu at 950 km altitude.
Abstract
Mercury has a global dayside exosphere, with measured densities of 10
−2
cm
−3
at ~1500 km. Here we report on the inferred enhancement of neutral densities (<10
2
cm
−3
) at high altitudes ...(~5300 km) by the MESSENGER spacecraft. Such high-altitude densities cannot be accounted for by the typical exosphere. This event was observed by the Fast-Imaging Plasma Spectrometer (FIPS), which detected heavy ions of planetary origin that were recently ionized, and “picked up” by the solar wind. We estimate that the neutral density required to produce the observed pickup ion fluxes is similar to typical exospheric densities found at ~700 km altitudes. We suggest that this event was most likely caused by a meteroid impact. Understanding meteoroid impacts is critical to understanding the source processes of the exosphere at Mercury, and the use of plasma spectrometers will be crucial for future observations with the Bepi-Colombo mission.
We present the first observation of a flux rope at Saturn's dayside magnetopause. This is an important result because it shows that the Saturnian magnetopause is conducive to multiple X‐line ...reconnection and flux rope generation. Minimum variance analysis shows that the magnetic signature is consistent with a flux rope. The magnetic observations were well fitted to a constant‐α force‐free flux rope model. The radius and magnetic flux content of the rope are estimated to be 4600–8300 km and 0.2–0.8 MWb, respectively. Cassini also observed five traveling compression regions (remote signatures of flux ropes), in the adjacent magnetosphere. The magnetic flux content is compared to other estimates of flux opening via reconnection at Saturn.
Key Points
We present the first observation of a flux transfer event at Saturn which occurred during an increase in solar wind dynamic pressure
Conditions at Saturn's magnetopause are at times conducive to multiple X‐line reconnection and flux rope generation
MVA and modeling are used to determine that the FTE had a flux rope structure and a magnetic flux content of 0.2–0.8 MWb
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