Free Newsletters - Space - Defense - Environment - Energy - Solar - Nuclear
..
. 24/7 Space News .




SOLAR SCIENCE
Cluster reveals Earth bow shock is remarkably thin
by Staff Writers
Paris, France (ESA) Nov 28, 2011


Astrophysical shocks are known to be extremely efficient sites for particle acceleration. But the mechanisms through which particles gain such large amounts of energy by interacting with a shock are still unclear. The very early phase of cosmic particle acceleration is one area where questions remain unanswered. While the mechanisms by which particles at a relatively high energy threshold are accelerated to even higher energies are fairly well understood, figuring out how particles can reach this threshold in the first place is much more complex. In other words, how are particles injected into cosmic accelerators?

A new study based on data from ESA's Cluster mission has revealed that the bow shock formed by the solar wind as it encounters Earth's magnetic field is remarkably thin: it measures only 17 kilometres across.

Thin astrophysical shocks such as this are candidate sites for early phases of particle acceleration. The finding thus sheds new light on the much debated issue of particle injection in the context of cosmic ray acceleration.

Most baryonic matter in the Universe consists of charged particles and ions - an ionised state of matter known as plasma. Streams of plasma move across the cosmos on all scales, permeating interplanetary space as well as the immense distances that separate stars in a galaxy and galaxies from one another.

Shock waves arise when supersonic flows of plasma are faced with an obstacle, such as a planet or a star with a magnetic field, or when they encounter a slower moving flow.

These abrupt transitions between supersonic and subsonic flows have been observed in a variety of cosmic environments, most notably in stellar winds gusting from young and massive stars, in the shell-like remnants of supernova explosions and in the proximity of the lobes and jets emanating from radio galaxies.

Observations at radio frequencies, as well as in X-rays and gamma rays, have revealed that shocks are also intimately connected to the origin of cosmic rays, the most energetic particles in the Universe.

Astrophysical shocks are known to be extremely efficient sites for particle acceleration. But the mechanisms through which particles gain such large amounts of energy by interacting with a shock are still unclear. The very early phase of cosmic particle acceleration is one area where questions remain unanswered.

While the mechanisms by which particles at a relatively high energy threshold are accelerated to even higher energies are fairly well understood, figuring out how particles can reach this threshold in the first place is much more complex. In other words, how are particles injected into cosmic accelerators?

"A unique opportunity to tackle such questions is represented by Earth's bow shock, the standing shock wave that forms when the solar wind encounters the magnetosphere of our planet," explains Steven Schwartz from Imperial College London, UK.

Schwartz led a team that used data from ESA's Cluster mission to obtain pioneering measurements of the thickness of this transition layer. The thickness of the bow shock is a crucial parameter in investigating the physical processes taking place in that region.

"It turns out that the bow shock is remarkably thin - only about 17 kilometres across. This means that it may be a far more efficient particle accelerator than we suspected," he adds. These results are reported in a paper to appear in the 18 November 2011 issue of Physical Review Letters.

Unlike shock waves around faraway stars and galaxies, the Earth's bow shock can be studied in situ by spacecraft flying through it. "The bow shock is an extraordinary laboratory to directly probe plasma dynamics and to explore scales that are inaccessible to astronomical observations," comments Matt Taylor, Cluster Project Scientist at ESA.

"With four spacecraft flying in formation, Cluster is the only space mission able to disentangle temporal and spatial dependencies in its data, and measure spatial variations in the temperature and other physical properties of particles in the plasma that surrounds our planet," he adds.

In 2003, an analysis based on Cluster measurements set an upper limit to the bow shock's thickness, hinting that it is at most 100 kilometres across. The new study led by Schwartz successfully exploited a particularly favourable set of data to precisely constrain the width of the shock. The results demonstrate that the width is about one-fifth of previous estimates.

"We dug into the massive archive of the Cluster mission to look for events characterised by a slow crossing of the shock by the spacecraft," notes Schwartz. A slow crossing means that the bow shock, which undergoes fluctuations due to changes in the solar wind, is relatively stable as the spacecraft fly through it. This, in turn, means that the scientists can sample the particle population more accurately, with better temporal and, consequently, better spatial resolution.

"As the spacecraft make the transition into the shocked region of the plasma, they record how the electrons experience a dramatic and abrupt rise in temperature over scales of only about 17 kilometres," continues Schwartz.

Such a sharp transition is close to the limit set by wave dispersion and could hardly be any steeper, implying that the shock layer is as thin as it can be. "And thin shocks make better accelerators," Schwartz adds.

In the vicinity of very thin shocks, a particle acceleration mechanism known as multiple reflection, or surfing, becomes particularly efficient. Ions that are initially slow, with energies of only a few keV, are energised gradually by repeatedly 'bouncing off' the shock.

The shock initially presents as a discontinuity that the ions cannot cross. After several rebounds, the ions gain enough energy (of the order of 0.5 MeV or beyond) to pass through the shock.

This mechanism may be a solution to the injection problem in cosmic accelerators. If a shock can be this thin, particles 'surfing' along it may be accelerated to a sufficiently high energy threshold that they can then be fed to different mechanisms that accelerate them to very high energies, well beyond 1 GeV, such as those reported in cosmic ray studies.

"These results show how local studies in the Solar System can have a major impact on the understanding of cosmic particle acceleration, an ubiquitous phenomenon which is active on a wide range of scales across the Universe," concludes Taylor.

S. Schwartz, et al., "Electron Temperature Gradient Scale at Collisionless Shocks", 2011, Physical Review Letters, 107, 215002, DOI: 10.1103/PhysRevLett.107.215002

.


Related Links
Cluster at ESA
Solar Science News at SpaceDaily






Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks
del.icio.usdel.icio.us DiggDigg RedditReddit GoogleGoogle




Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News





SOLAR SCIENCE
Cluster reveals Earth's bow shock is remarkably thin
Paris (ESA) Nov 17, 2011
A new study based on data from ESA's Cluster mission has revealed that the bow shock formed by the solar wind as it encounters Earth's magnetic field is remarkably thin: it measures only 17 kilometres across. Thin astrophysical shocks such as this are candidate sites for early phases of particle acceleration. The finding thus sheds new light on the much debated issue of particle injection in the ... read more


SOLAR SCIENCE
Schafer Corp Signs Licensing Agreement with MoonDust Technologies

Russia wants to focus on Moon if Mars mission fails

Flying over the three-dimensional Moon

LRO Camera Team Releases High Resolution Global Topographic Map of Moon

SOLAR SCIENCE
NASA Launches Most Capable and Robust Rover to Mars

Did US climate weapon knock-out Russian probe

Russia's Medvedev evokes Stalin ahead of elections

The Martian Chronicles Continues With Russian Bit Part

SOLAR SCIENCE
Looking for a Space Job

Thanksgiving in space may one day come with all the trimmings

More U.S. science degrees by foreign-born

ULA Completes Milestone Toward Certifying Atlas V For Human Spaceflight

SOLAR SCIENCE
15 patents granted for Chinese space docking technology

China plans major effort in pursuing manned space technology

Tiangong-1 orbiter enters long-term operation management

China launches two satellites: state media

SOLAR SCIENCE
Satellite junk no threat to space station crew

Space Station Trio Lands Safely in Kazakhstan

Russian Soyuz brings astronauts safely back to Earth

New Trio Welcomed Aboard Station, Gets to Work

SOLAR SCIENCE
Assembly milestone reached with Ariane 5 to launch next ATV

Russia launches Chinese satellite

AsiaSat 7 Spacecraft Separation Successfully Completed

Pleiades 1 is readied for launch

SOLAR SCIENCE
Habitable Does not Mean 'Earth-Like'

Exo planet count tops 700

Giant planet ejected from the solar system

Three New Planets and a Mystery Object Discovered Outside Our Solar System

SOLAR SCIENCE
Kindle sales quadrupled on Black Friday: Amazon

Mapheus-3 - spherules, metals and microgravity

Recycle this: Bolivian turns waste into high fashion

Carbon nanotube forest camouflages 3d objects




The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement