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TOPIC: Solar wind
Blobrana


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RE: Solar wind
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Title: Solar Wind at 33 AU: Setting Bounds on the Pluto Interaction for New Horizons
Author: F. Bagenal, P.A. Delamere, H.A. Elliott, M.E. Hill, C.M. Lisse, D.J. McComas, R.L McNutt, Jr., J.D. Richardson, C.W. Smith, D.F. Strobel

The NASA New Horizons spacecraft flies past Pluto on July 14, 2015, carrying two instruments that detect charged particles. Pluto has a tenuous, extended atmosphere that is escaping the weak gravity of the planet. The interaction of the solar wind with the escaping atmosphere of Pluto depends on solar wind conditions as well as the vertical structure of the atmosphere. We have analysed Voyager 2 particles and fields measurements between 25 and 39 AU and present their statistical variations. We have adjusted these predictions to allow for the declining activity of the Sun and solar wind output. We summarize the range of SW conditions that can be expected at 33 AU and survey the range of scales of interaction that New Horizons might experience. Model estimates for the solar wind stand-off distance vary from approximately 7 to 1000 RP with our best estimate being around 40 RP (where we take the radius of Pluto to be RP=1184 km).

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Blobrana


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Title: Turbulence in the solar wind: spectra from Voyager 2 data at 5 AU
Author: F. Fraternale (1), L. Gallana (1), M. Iovieno (1), M. Opher (2), J. D. Richardson (3), D. Tordella (1) ((1) Ingegneria Meccanica e Aerospaziale, Politecnico di Torino, (2) Astronomy Department, Boston University, (3) Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology)

The solar wind spectral properties are far from uniformity and evolve with the increasing distance from the sun. Most of the available spectra of solar wind turbulence were computed at 1 astronomical unit, while accurate spectra on wide frequency ranges at larger distances are still few. In this paper we consider solar wind spectra derived from the data recorded by the Voyager 2 mission during 1979 at about 5 AU from the sun. Voyager 2 data are an incomplete time series with a voids/signal ratio that typically increases as the spacecraft moves away from the sun (45% missing data in 1979), making the analysis challenging. In order to estimate the uncertainty of the spectral slopes, different methods are tested on synthetic turbulence signals with the same gap distribution as V2 data. Spectra of all variables show a power law scaling with exponents between -2.1 and -1.1, depending on frequency subranges. PDFs and correlations indicate that the flow has a significant intermittency.

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Blobrana


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Electric Asteroids
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New NASA Model Gives Glimpse into the Invisible World of Electric Asteroids

A solar wind blown from the surface of the sun at about a million miles per hour flows around all solar system objects, forming swirling eddies and vortices in its wake. Magnetic fields carried by the solar wind warp, twist, and snap as they slam into the magnetic fields around other objects in our solar system, blasting particles to millions of miles per hour and sending electric currents surging in magnetic storms that, around Earth, can damage sensitive technology like satellites and power grids.
On airless objects like moons and asteroids, sunlight ejects negatively charged electrons from matter, giving sunlit areas a strong positive electric charge. The solar wind is an electrically conducting gas called plasma where matter has been torn apart into electrons, which are relatively light, and positively charged ions, which are thousands of times more massive. While areas in sunlight can charge positive, areas in shadow get a strong negative charge when electrons in the solar wind rush in ahead of heavier ions to fill voids created as the solar wind flows by.
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Blobrana


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RE: Solar wind
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Solar Wind Energy Source Discovered

Using data from an aging NASA spacecraft, researchers have found signs of an energy source in the solar wind that has caught the attention of fusion researchers. NASA will be able to test the theory later this decade when it sends a new probe into the sun for a closer look.
The discovery was made by a group of astronomers trying to solve a decades-old mystery: What heats and accelerates the solar wind? 
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Blobrana


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NASA Study Using Cluster Reveals New Insights Into Solar Wind

A new study based on data from European Space Agency's Cluster mission shows that it is easier for the solar wind to penetrate Earth's magnetic environment, the magnetosphere, than had previously been thought. Scientists from NASA's Goddard Space Flight Centre in Greenbelt, Md. have, for the first time, directly observed the presence of certain waves in the solar wind - called Kelvin-Helmholtz waves that can help transfer energy into near-Earth space - under circumstances when previous theories predicted they were not expected.
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Blobrana


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Magnetic Turbulence Trumps Collisions to Heat Solar Wind

New research led by University of Warwick physicist Dr Kareem Osman has provided significant insight into how the solar wind heats up when it should not. The solar wind rushes outwards from the raging inferno that is our Sun, but from then on the wind should only get cooler as it expands beyond our solar system since there are no particle collisions to dissipate energy. However, the solar wind is surprisingly hotter than it should be, which has puzzled scientists for decades. Two new research papers led by Dr Osman may have solved that puzzle.
Turbulence pervades the universe, being found in stars, stellar winds, accretion disks, galaxies, and even the material between galaxies. It also plays a critical role in the evolution of many laboratory plasmas, causing diminished confinement times in fusion devices. Therefore, understanding plasma turbulence is essential to the interpretation of a large body of laboratory, space, and astrophysical observations. The solar wind and near-Earth environment provide an excellent laboratory for the study of turbulence, and are the only in-situ accessible astrophysical plasmas.
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Blobrana


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Solar Corona
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Title: Convective Instability Of The Solar Corona: Why The Solar Wind Blows
Authors: Joseph Lemaire

Chapman's (1957) conductive model of the solar corona is characterised by a temperature varying as r**(-2/7) with heliocentric distance r. The density distribution in this non-isothermal hydrostatic model has a minimum value at 123 RS, and increases with r above that altitude. It is shown that this hydrostatic model becomes convectively unstable above r = 35 RS, where the temperature lapse rate becomes superadiabatic. Beyond this radial distance heat conduction fails to be efficient enough to keep the temperature gradient smaller than the adiabatic lapse rate. We report the results obtained by Lemaire (1968) who showed that an additional mechanism is then required to transport the energy flux away from the Sun into interplanetary space. He pointed out that this additional mechanism is advection: i.e. the stationary hydrodynamic expansion of the corona. In other words the corona is unable to stay in hydrostatic equilibrium. The hydrodynamic solar wind expansion is thus a physical consequence of the too steep (superadiabatic) temperature gradient beyond the peak of coronal temperature that can be determined from white light brightness distributions observed during solar eclipses. The thermodynamic argument for the existence of a continuous solar wind expansion which is presented here, complements Parker's classical argument based on boundary conditions imposed to the solutions of the hydrodynamic equations for the coronal expansion: i.e. the inability of the mechanical forces to hold the corona in hydrostatic equilibrium. The thermodynamic argument presented here is based on the energy transport equation. It relies on the temperature distribution which becomes super-adiabatic above a certain altitude in the inner corona.

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Blobrana


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Title: The role Alfvén waves in the generation of Earth polar auroras
Authors: Fabrice Mottez (LUTH)

The acceleration of electrons at 1-10 keV energies is the cause of the polar aurora displays, and an important factor of magnetic energy transfer from the solar wind to the Earth. Two main families of acceleration processes are observed: those based on coherent quasi-static structures called double layers, and those based of the propagation of Alfvén Waves (AW). This paper is a review of the Alfvénic acceleration processes, and of their role in the global dynamics of the auroral zone.

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Blobrana


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Solar Storms Could 'Sandblast' the Moon

Solar storms and associated Coronal Mass Ejections (CMEs) can significantly erode the lunar surface according to a new set of computer simulations by NASA scientists. In addition to removing a surprisingly large amount of material from the lunar surface, this could be a major method of atmospheric loss for planets like Mars that are unprotected by a global magnetic field.
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Blobrana


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Why solar wind is rhombic-shaped

Why the temperatures in the solar wind are almost the same in certain directions, and why different energy densities are practically identical, was until now not clear. With a new approach to calculating instability criteria for plasmas, Bochum researchers lead by Prof. Dr. Reinhard Schlickeiser (Chair for Theoretical Physics IV) have solved both problems at once. They were the first to incorporate the effects of collisions of the solar wind particles in their model. This explains experimental data significantly better than previous calculations and can also be transferred to cosmic plasmas outside our solar system. The scientists report on their findings in Physical Review Letters.
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