NASA's Juno Peers Inside a Giant

Scientists will use the twin magnetometers aboard NASA's Juno spacecraft to gain a better understanding about how Jupiter's magnetic field is generated. Credit: NASA Goddard Space Flight Center.  › Larger view

Exploring Jupiter's Magnetic Field

Youtube Nasa Goddard/Youtube video

NASA's Juno spacecraft will make its long anticipated arrival at Jupiter on July 4. Coming face-to-face with the gas giant, Juno will begin to unravel some of the greatest mysteries surrounding our solar system's largest planet, including the origin of its massive magnetosphere.

Magnetospheres are the result of a collision between a planet's intrinsic magnetic field and the supersonic solar wind. Jupiter's magnetosphere -- the volume carved out in the solar wind where the planet's magnetic field dominates --extends up to nearly 2 million miles (3 million kilometers). If it were visible in the night sky, Jupiter's magnetosphere would appear to be about the same size as Earth's full moon. By studying Jupiter's magnetosphere, scientists will gain a better understanding about how Jupiter's magnetic field is generated.

They also hope to determine whether the planet has a solid core, which will tell us how Jupiter formed during the earliest days of our solar system.

In order to look inside the planet, the science team equipped Juno with a pair of magnetometers. The magnetometers, which were designed and built by an in-house team of scientists and engineers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, will allow scientists to map Jupiter's magnetic field with high accuracy and observe variations in the field over time.

"The best way to think of a magnetometer is like a compass," said Jack Connerney, deputy principal investigator and head of the magnetometer team at Goddard. "Compasses record the direction of a magnetic field. But magnetometers expand on that capability and record both the direction and magnitude of the magnetic field."

The magnetometer sensors rest on a boom attached to one of the solar arrays, placing them about 40 feet (12 meters) from the body of the spacecraft. This helps ensure that the rest of the spacecraft does not interfere with the magnetometer.

However, the sensor orientation changes in time with the mechanical distortion of the solar array and boom resulting from the extremely cold temperatures of deep space. This distortion would limit the accuracy of the magnetometer measurements if not measured.

To ensure that the magnetometers retain their high accuracy, the team paired the instruments with a set of four cameras. These cameras measure the distortion of the magnetometer sensors in reference to the stars to determine their orientation.

"This is our first opportunity to do very precise, high-accuracy mapping of the magnetic field of another planet," Connerney said. "We are going to be able to explore the entire three-dimensional space around Jupiter, wrapping Jupiter in a dense net of magnetic field observations completely covering the sphere."

One of the mysteries the team hopes to answer is how Jupiter's magnetic field is generated. Scientists expect to find similarities between Jupiter's magnetic field and that of Earth.

Magnetic fields are produced by what are known as dynamos -- convective motion of electrically conducting fluid inside planets. As a planet rotates, the electrically susceptible liquid swirls around and drives electric currents, inducing a magnetic field. Earth's magnetic field is generated by liquid iron in the planet's core.

"But with Jupiter, we don't know what material is producing the planet's magnetic field," said Jared Espley, Juno program scientist for NASA Headquarters, Washington. "What material is present and how deep down it lies is one of the questions Juno is designed to answer."

The observations made by Juno's magnetometers will also add to our understanding of Earth's dynamo, the source of our planet's magnetic field, which lies deep beneath a magnetized layer of rocks and iron.

Imagine Earth's crust strewn with refrigerator magnets as you try to peer beneath the surface to observe the dynamo. The magnetization of Earth's crust will skew your measurements of the magnetic field.

"One of the reasons that the Juno mission is so exciting is because we can map Jupiter's magnetic field without having to look through the crustal magnetic fields, which behave like a jumble of refrigerator magnets," Connerney said. "Jupiter has a gaseous envelope about it made of hydrogen and helium that gives us a clear and unobstructed view of the dynamo."

These observations will also add to the general understanding of how dynamos generate magnetic fields, including here on Earth.

"Any time we understand anything about another planet, we can take that knowledge and apply it to our knowledge about our own planet," Espley said. "We'll be looking at Juno's observations in a big-picture perspective."

NASA's Jet Propulsion Laboratory in Pasadena, California, manages the Juno mission for the principal investigator, Scott Bolton, of Southwest Research Institute in San Antonio. Juno is part of NASA's New Frontiers Program, which is managed at NASA's Marshall Space Flight Center in Huntsville, Alabama, for NASA's Science Mission Directorate. Lockheed Martin Space Systems, Denver, built the spacecraft.

For more information about the Juno mission, visit: www.nasa.gov/juno


News Media Contact 

DC Agle
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-9011
agle@jpl.nasa.gov

Source:  JPL-Caltech/News

Cassini Catches Titan Naked in the Solar Wind

This diagram depicts conditions observed by NASA's Cassini spacecraft during a flyby in Dec. 2013, when Saturn's magnetosphere was highly compressed, exposing Titan to the full force of the solar wind. Image credit: NASA/JPL-Caltech.   › Full image and caption

Researchers studying data from NASA's Cassini mission have observed that Saturn's largest moon, Titan, behaves much like Venus, Mars or a comet when exposed to the raw power of the solar wind. The observations suggest that unmagnetized bodies like Titan might interact with the solar wind in the same basic ways, regardless of their nature or distance from the sun.

Titan is large enough that it could be considered a planet if it orbited the sun on its own, and a flyby of the giant moon in Dec. 2013 simulated that scenario, from Cassini's vantage point. The encounter was unique within Cassini's mission, as it was the only time the spacecraft has observed Titan in a pristine state, outside the region of space dominated by Saturn's magnetic field, called its magnetosphere. 

"We observed that Titan interacts with the solar wind very much like Mars, if you moved it to the distance of Saturn," said Cesar Bertucci of the Institute of Astronomy and Space Physics in Buenos Aires, who led the research with colleagues from the Cassini mission. "We thought Titan in this state would look different. We certainly were surprised," he said.

The solar wind is a fast-flowing gale of charged particles that continually streams outward from the sun, flowing around the planets like islands in a river. Studying the effects of the solar wind at other planets helps scientists understand how the sun's activity affects their atmospheres. These effects can include modification of an atmosphere's chemistry as well as its gradual loss to space.

Titan spends about 95 percent of the time within Saturn's magnetosphere. But during a Cassini flyby on Dec. 1, 2013, the giant moon happened to be on the sunward side of Saturn when a powerful outburst of solar activity reached the planet. The strong surge in the solar wind so compressed the sun-facing side of Saturn's magnetosphere that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to, and unprotected from, the raging stream of energetic solar particles. 

Using its magnetometer instrument, which is akin to an equisitely sensitive compass, Cassini has observed Titan many times during the mission's decade in the Saturn system, but always within Saturn's magnetosphere. The spacecraft has not been able to detect a magnetic field coming from Titan itself. In its usual state, Titan is cloaked in Saturn's magnetic field.

This time the influence of Saturn was not present, allowing Cassini's magnetometer to observe Titan as it interacted directly with the solar wind. The special circumstance allowed Bertucci and colleagues to study the shockwave that formed around Titan where the full-force solar wind rammed into the moon's atmosphere.

At Earth, our planet's powerful magnetic field acts as a shield against the solar wind, helping to protect our atmosphere from being stripped away. In the case of Venus, Mars and comets -- none of which is protected by a global magnetic field -- the solar wind drapes around the objects themselves, interacting directly with their atmospheres (or in the comet's case, its coma). Cassini saw the same thing at Titan. 

Researchers thought they would have to treat Titan's response to the solar wind with a unique approach because the chemistry of the hazy moon's dense atmosphere is highly complex. But Cassini's observations of a naked Titan hinted at a more elegant solution. "This could mean we can use the same tools to study how vastly different worlds, in different parts of the solar system, interact with the wind from the sun," Bertucci said.

Bertucci noted that the list of similarly unmagnetized bodies might include the dwarf planet Pluto, to be visited this year for the first time by NASA's New Horizons spacecraft.

"After nearly a decade in orbit, the Cassini mission has revealed once again that the Saturn system is full of surprises," said Michele Dougherty, principal investigator of the Cassini magnetometer at Imperial College, London. "After more than a hundred flybys, we have finally encountered Titan out in the solar wind, which will allow us to better understand how such moons maintain or lose their atmospheres."

The new research is published today in the journal Geophysical Review Letters.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The magnetometer team is based at Imperial College, London, U.K. 

More information about Cassini:  http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov

Media Contact

Preston Dyches
NASA's Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston.dyches@jpl.nasa.gov

Source: JPL-Caltech/News

 

How NASA Watches CMEs

Two main types of explosions occur on the sun: solar flares and coronal mass ejections. Unlike the energy and x-rays produced in a solar flare – which can reach Earth at the speed of light in eight minutes – coronal mass ejections are giant clouds of solar material that take one to three days to reach Earth. Once at Earth, these ejections, also called CMEs, can impact satellites in space or interfere with radio communications. During CME Week from Sept. 22 to 26, 2014, we explore different aspects of these giant eruptions that surge out from the star we live with.

Space weather models combined with real time observations help scientists track CMEs. These images were produced from a model known as ENLIL named after the Sumerian storm god. It shows the journey of a CME on March 5, 2013, as it moved toward Mars.  Image Credit: NASA/Goddard/SWRC/CCMC. Download video

A March 5, 2013, CME as seen by the Solar and Heliospheric Observatory. Combining the information gleaned from such imagery with state-of-the-art models helps scientists better understand how CMEs move toward, and affect, Earth. Image Credit: ESA/NASA/SOHO/Jhelioviewer 

Those who study Earth's weather have a luxury of data points to study. From thousands of weather stations measuring temperature and rainfall to satellites tracking storm fronts up in space, meteorologists can watch detailed maps of the weather as it sweeps across land or sea.

Compared to this, the study of space weather – including CMEs – is a much younger science, with far fewer observatories available. However, our resources have grown dramatically in the last decade: NASA currently flies 18 missions to study the sun's effects at Earth and on the entire solar system, a field known as heliophysics, and additionally launches numerous short-flight rockets for observations of solar impacts in and above Earth's atmosphere. Coupled with improved computer modeling, keeping an eye on – and getting a better understanding of – CMEs has taken a giant leap forward in the 21^st century.

"Over the past ten years, we have had a major breakthrough in understanding space weather," said Antti Pulkkinen a space weather scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We can now track the basic properties of CMEs. When our solar observatories see a CME, we can tell what direction it's going in and how fast it's traveling."

Improved observations combined with improved models has led to hybrid descriptions of a CME, relying partially on computer simulations and partially on actual observations. NASA houses a collection of space weather models available for public access at the Community Coordinated Modeling Center at Goddard. Together with observations they can provide a holistic picture of any given CME.

For example, NASA's Solar and Terrestrial Relations Observatory, or STEREO, might see a CME erupt on the sun. When that imagery is combined with observations from the European Space Agency and NASA's Solar and Heliospheric Observatory, or SOHO, scientists can create a 3-dimensional picture of the giant cloud. Scientists then input this data into a model and then track how the CME unfolded and spread through space until it passed by NASA observatories closer to Earth. These observatories can directly measure the magnetic fields and speed of the CME as it passes by, as well as see how it affected Earth's own magnetic fields – the magnetosphere.

By gathering data from numerous observatories, scientists can create models and explore what-if scenarios about what would happen near Earth due to a given CME. Watch the video to learn more about what scientists can see in these models. Image Credit: NASA/Bridgman/Duberstein. Download video

Such information on the CME's entire path opens the door to understanding why any given characteristic of the CME near the sun might lead to a given effect near Earth. Each additional piece of the puzzle helps us better understand just what causes these giant eruptions -- and whether or not any particular CME could pose a hazard to astronauts as well as technology in space and on the ground.

Related Links
 

• More on CME Week    

• More on STEREO  

• More on SOHO 

• Other NASA Sun-Earth missions

Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Md.

'Tis the Season -- for Plasma Changes at Saturn

This is an artist's concept of the Saturnian plasma sheet based on data from Cassini magnetospheric imaging instrument. It shows Saturn's embedded "ring current," an invisible ring of energetic ions trapped in the planet's magnetic field. Image credit: NASA/JPL/JHUAPL.   › Full image and caption

Researchers working with data from NASA's Cassini spacecraft have discovered one way the bubble of charged particles around Saturn -- known as the magnetosphere -- changes with the planet's seasons. The finding provides an important clue for solving a riddle about the planet's naturally occurring radio signal. The results might also help scientists better understand variations in Earth's magnetosphere and Van Allen radiation belts, which affect a variety of activities at Earth, ranging from space flight safety to satellite and cell phone communications.

The paper, just published in the Journal of Geophysical Research, is led by Tim Kennelly, an undergraduate physics and astronomy major at the University of Iowa, Iowa City, who is working with Cassini's radio and plasma wave science team.

In data collected by Cassini from July 2004 to December 2011, Kennelly and his colleagues examined "flux tubes," structures composed of hot, electrically charged gas called plasma, which funnel charged particles in towards Saturn. Focusing on the tubes when they initially formed and before they had a chance to dissipate under the influence of the magnetosphere, the scientists found that the occurrence of the tubes correlates with radio wave patterns in the northern and southern hemisphere depending upon the season. This seasonal effect is roughly similar to the way Earth's northern lights appear more frequently in the spring and autumn months.

Radio emissions have been used to measure Jupiter's rotation period reliably, and scientists thought it would also help them determine Saturn's rotation period. To their chagrin, however, the pattern has varied over the visits by different spacecraft and even in radio emissions originating in the northern and southern hemispheres. The new results could help scientists hone in on why these signals vary the way they do.

For more on the finding, go to: http://now.uiowa.edu/2013/03/telling-time-saturn .

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the mission for the agency's Science Mission Directorate in Washington. The radio and plasma wave science team is based at the University of Iowa, Iowa City, where the instrument was built. JPL is a division of the California Institute of Technology, Pasadena.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

Gary Galluzzo 319-384-0009
University of Iowa, Iowa City
gary-galluzzo@uiowa.edu

NASA’s Wind Mission Encounters ‘SLAMS’ Waves

Earth is surrounded by a giant magnetic bubble called the magnetosphere. As it travels through space, a complex system of charged particles from the sun and magnetic structures piles up in front of it. Scientists wish to better understand this area in front of the bow shock, known as the foreshock, as it can help explain how energy from the rest of space makes its way past this boundary into the magnetosphere. Credit: Credit: NASA/GSFC. › View larger -  › View unannotated version

As Earth moves around the sun, it travels surrounded by a giant bubble created by its own magnetic fields, called the magnetosphere. As the magnetosphere plows through space, it sets up a standing bow wave or bow shock, much like that in front of a moving ship. Just in front of this bow wave lies a complex, turbulent system called the foreshock. Conditions in the foreshock change in response to solar particles streaming in from the sun, moving magnetic fields and a host of waves, some fast, some slow, sweeping through the region.

To tease out what happens at that boundary of the magnetosphere and to better understand how radiation and energy from the sun can cross it and move closer to Earth, NASA launches spacecraft into this region to observe the changing conditions. From 1998 to 2002, NASA’s Wind spacecraft traveled through this foreshock region in front of Earth 17 times, providing new information about the physics there.

“I stumbled on some cool squiggles in the data,” says Lynn Wilson, who is deputy project scientist for Wind at NASA’s Goddard Space Flight Center in Greenbelt, Md. “They turned out to be a special kind of magnetic pulsations called short large amplitude magnetic structures, which we call SLAMS for short.”

SLAMS are waves with a single, large peak, a little like giant rogue waves that can develop in the deep ocean. By studying the region around the SLAMS and how they propagate, the Wind data showed SLAMS may provide an improved explanation for what accelerates narrow jets of charged particles back out into space, away from Earth. Tracking how any phenomenon catalyzes the movement of other particles is one of the crucial needs for modeling this region. In this case, understanding just how a wave can help initiate a fast-moving beam might also help explain what causes incredibly powerful rays that travel from other solar systems across interstellar space toward Earth. Wilson and his colleagues published a paper on these results in the Journal of Geophysical Research online on March 6, 2013.

The material pervading this area of space – indeed all outer space – is known as plasma. Plasma is much like a gas, but each particle is electrically charged so movement is governed as much by the laws of electromagnetics as it is by the fundamental laws of gravity and motion we more regularly experience on Earth.

“One of the unique things about space weather is how little things can have big effects,” says David Sibeck, a space scientist at Goddard who is a co-author on the paper. “An event might seem small and just generate local turbulence, but it can have profound effects downstream. The front of the magnetosphere is right in the line between sun and Earth, so it’s a crucial place to understand which small things can lead to big results.”

Since the 1970s, researchers have known that particles seem to be reflecting off the magnetosphere, creating intense particle jets called field aligned ion beams, but it’s not been clear how. Now, the Wind data helps provide a more detailed snapshot of how they form, as it travels through a slew of SLAMS and the ion beams.

The scientists’ job was to map where these events happen in space and time and to try to determine which events initiate which. Wilson says that the solar wind constantly moves toward Earth’s bow shock and then reflects off it.

“These structures get excited upstream and they start to grow and steepen, kind of like a water wave,” says Wilson. “But instead of breaking and tumbling over, they stand up, getting bigger and faster.” He says that the SLAMS attempt to move against the gale of solar wind streaming toward them, but ultimately get pushed back, creating a new messy boundary in front of the magnetosphere. “And then they effectively create their own new bow shock,” says Wilson.

Without the SLAMS, one would expect incoming particles from the solar wind to skip and slide along the outside of the bow shock, the way flowing water in a river might move around a large rock. But the SLAMS create a kind of magnetic mirror, causing the solar particles to reflect, attenuating them into one of these field-aligned ion beams, shooting out along magnetic fields back out and away from Earth.

Wind data does not inherently show which of these things create the other, it simply shows the presence of both. However, the ion beams were not seen in the space between the front of the true bow shock and the SLAMS -- only streaming away from the SLAMS out toward space. The beams also only appeared after the SLAMS had a chance to fully form. This strengthened the conclusion that the SLAMS themselves lead to the beams, acting as a magnetic mirror to reflect the particles outward.

The more we know about what happens in the frothy, turbulent area in front of Earth, the more we know about how the solar wind and other material bursting off the sun may be able to penetrate into near Earth-space.

“What happens to Earth’s magnetic field depends on what’s happening here at the front of the bow shock,” says Sibeck. “And what’s happening there is dramatic. It’s going to affect how much energy moves into the magnetosphere. Once inside the magnetosphere, it can create powerful solar storms and impact communications and GPS satellites that we depend on daily.”

The observations also have implications beyond protecting Earth. By sending spacecraft to observe plasma here, scientists can take advantage of the only area of the universe where we can study such plasma movement directly -- and thus apply the research to information about stars across the galaxy as well. For example, astrophysicists would like to better understand what causes cosmic ray acceleration -- particles that are generally much faster than the field aligned ion beams, but accelerated in similar manners, says Wilson. One theory is that a magnetic mirror of some kind causes the particles to bounce back and forth and gain more speed and energy as the mirrors move closer together. Near the front of the magnetosphere, the SLAMS might be doing just that.

For more information about NASA’s Wind mission, please visit:  http://wind.nasa.gov/

Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Md.

NASA Deciphering the Mysterious Math of the Solar Wind

A constant stream of particles and electromagnetic waves streams from the Sun toward Earth, which is surrounded by a protective bubble called the magnetosphere. A scientist at NASA Goddard has recently devised, for the first time, a set of equations that can help describe waves in the solar wind known as Alfven waves. Credit: European Space Agency (ESA). › View larger

Many areas of scientific research -- Earth's weather, ocean currents, the outpouring of magnetic energy from the Sun -- require mapping out the large scale features of a complex system and its intricate details simultaneously.

Describing such systems accurately, relies on numerous kinds of input, beginning with observations of the system, incorporating mathematical equations to approximate those observations, running computer simulations to attempt to replicate observations, and cycling back through all the steps to refine and improve the models until they jibe with what's seen. Ultimately, the models successfully help scientists describe, and even predict, how the system works.

Understanding the Sun and how the material and energy it sends out affects the solar system is crucial, since it creates a dynamic space weather system that can disrupt human technology in space such as communications and global positioning system (GPS) satellites.

However, the Sun and its prodigious stream of solar particles, called the solar wind, can be particularly tricky to model since as the material streams to the outer reaches of the solar system it carries along its own magnetic fields. The magnetic forces add an extra set of laws to incorporate when trying to determine what's governing the movement. Indeed, until now, equations for certain aspects of the solar wind have never been successfully devised to correlate to the observations seen by instruments in space. Now, for the first time, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., has created a set of the necessary equations, published in Physical Review Letters on Dec. 4, 2012.

"Since the 1970s, scientists have known that movement in the solar wind often has the characteristics of a kind of wave called an Alfvén wave," says Aaron Roberts, a space scientist at Goddard. "Imagine you have a jump rope and you wiggle one end so that it sends waves down the rope. Alfvén waves are similar, but the moving rope is a magnetic field line itself."

The Alfvén waves in this case tended to have great consistency in height -- or amplitude, which is the common term when talking about waves -- but they are random in direction. You might think of it like a jump rope twirling, always the same distance from center, but nonetheless able to be in many places in space. Another way scientists have envisioned the waves is as a "random walk on a sphere." Again, always the same distance from a given center, but with a variable placement.

Such metaphorical descriptions are based on what instruments in space have, in fact, observed when they see magnetic waves go by in the solar wind. But it turns out that the equations to describe this kind of movement -- equations necessary to advance scientific models of the entire system -- were not easily found.

"The puzzle has been to figure out why the amplitude is so constant," says Roberts. "But it's been very difficult to find equations that satisfy all the characteristics of the magnetic field."

Similar waves are, in fact, seen in light, known as polarized waves. But magnetic fields have additional constraints on what shapes and configurations are even possible. Roberts found a way to overlap numerous waves of different wavelengths in such a way that they ultimately made the variation in amplitude as small as possible.

To his surprise, the equations Roberts devised matched what was observed more closely than he'd expected. Not only did the equations show waves of constant amplitude, but they also showed occasional random jumps and sharp changes -- an unexplained feature seen in the observations themselves.

"Overlapping the waves in this way gives us a way of writing down equations that we didn't have before," says Roberts. "It also has this nice consequence that it is more realistic than we expected, since it shows discontinuities we actually see in the wind. This is important for simulations and models where we want to start with initial conditions that are as close to the observed solar wind as we can get."

Of course, having an equation doesn't yet tell us the reason why the waves in the solar wind are shaped in this way. Nonetheless, equations that describe how the waves move open the door to increasingly accurate simulations that may well help explain such causes. By alternately improving models and improving observations, scientists continue the cyclic nature of such research, until just what physical action on the sun causes these curiously-shaped Alfvén waves someday becomes clear.

 

Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Md.

High-speed plasma jets: origin uncovered

For more than a decade, mysterious, high-speed plasma jets have been observed in space, downstream of the Earth's bow shock. The underlying formation mechanism for these jets has now been unveiled, thanks to data collected by the four ESA Cluster satellites. This study also suggests that such mechanisms may be relevant to other astrophysical shocks.

Image 1 Orbit of the Cluster satellites on 17 March 2007. Credit: ESA

The four Cluster satellites orbit the Earth in a pyramidal configuration along a nominal polar orbit of 4 × 19.6 Earth radii (1 Earth radius = 6380 km). The image above depicts the configuration on 17 March 2007.

In the summer of 2000 the four Cluster satellites embarked on the first three-dimensional mapping of the Earth's magnetosphere. During the second extension of the mission (2005-2009), Cluster explored regions of the magnetosphere not originally targeted, thanks to the natural evolution of their nominal polar orbit.

On 17 March 2007, the four satellites were travelling along an orbit that crossed the magnetospheric region closest to the Sun, known as the subsolar magnetopause. Shortly after 17:00 UT, they exited the terrestrial magnetosphere. Five and a half hours later, they crossed the Earth's bow shock at a distance of close to 100 000 km from Earth. The bow shock is the boundary where the permanent flow of solar particles - the solar wind - is decelerated, typically from 500 km/s to 100-300 km/s.

"Between 17:00 and 20:00 UT, instruments on the Cluster satellites observed several high-speed jets of plasma behind the bow shock with a speed close to 500 km/s," says Heli Hietala, a PhD student at the University of Helsinki, Finland, and lead author of the study that was published in the 11 December 2009 issue of Physical Review Letters.

"The speed of these jets was very close to the incoming solar wind speed (~530 km/s) as if, locally, the bow shock was not able to slow down the solar wind as it normally would," adds Hietala.

In a detailed analysis, Hietala and co-authors convincingly argue that these jets are due to passing ripples that travel along the Earth's bow shock. Such a shock ripple produces two main effects (see animated illustration):

Image 2 Producing high-speed plasma jets. Credit: ESA

(A larger version of this animation is available here.)

Image 3 Cluster detects high-speed jets. Credit: ESA

Top panel: total plasma speed measured by one of the Cluster satellites (C1) on 17 March 2007 from 18:13 to 18:17 UT. The plasma regions crossed by the satellite on its way out of the magnetosphere are colour coded as follows: yellow (magnetosphere), green (magnetosheath), pink (2nd shock) and purple (jet). The lower panel displays the angle (α) between the upstream plasma velocity and the normal to the bow shock, located upstream of the second shock. The calculation is not expected to be valid at the edges of the jet where the shock is weak and hence α is shown for the centre only.

First, an indentation – caused by the ripple - induces a higher-density region downstream of the shock. Second, it locally changes the angle between the normal of the shock and the velocity direction of the solar wind. When this angle is large, for example on the borders of the ripple, the solar wind goes through the bow shock experiencing only minor effects. In particular, it is hardly slowed down and continues at high-speed through the magnetosheath. This induces the creation of a second (local) shock downstream of the nominal shock – this was observed by the Cluster satellites just before 18:15 UT (see Image 3). The higher density together with the high speed leads to a jet with very high dynamic pressure.

"The mechanism that we have proposed to generate such high-speed jets is not only in agreement with these Cluster measurements but with all those reported so far," notes Tiera Laitinen, a co-author of the study and post-doctoral researcher at the Swedish Institute of Space Physics, Uppsala, Sweden, at the time of the study.

Uncovering this mechanism was possible because the Cluster satellites were in the right place at the right time to observe these transient jets. In addition, having several satellites operating together provided additional clues. For instance, the distance between two of the satellites was only 950 km. Since their observations were very similar, this immediately provides a lower limit to the spatial scale of the jet, of approximately 1000 km. The three-dimensional structure of the ripple analysed in this paper is the subject of an on-going study.

Image 4 Features of the heliosphere. Credit: NASA

The Solar System is immersed in a protective bubble, known as the heliosphere. This is caused by the solar wind, the expanding plasma of charged atoms and electrons emitted by the Sun, which excludes the local interstellar medium from the area encompassing the Sun and the planets.

The termination shock marks the region where the solar wind is quickly decelerated to subsonic speeds.

The heliosphere is bordered by a layer called the heliopause. Here, the pressure from the solar wind balances that of the interstellar medium.

The bow shock marks the region where the interstellar medium slows down as it impacts the heliosphere.

Moreover, this result may apply not only to the Earth's bow shock but to other astrophysical shocks. As Voyager 1 and 2 crossed the heliospheric termination shock (Image 4), in 2004 and 2007 respectively, their observations also revealed a rippled shock even though high-speed jets have not been observed. In an astrophysical context, such jets can act as seeds for magnetic field amplification and particle acceleration on the downstream side of supernova blast waves. Some of these blast waves are triggered by the death of massive stars, which end their lifetime as neutron stars.

"Explaining a phenomenon that has been observed for more than 10 years but whose origin was unknown is exciting. This result is an unexpected science nugget from this mission," says Philippe Escoubet, ESA Cluster Mission Manager.

Related publication

Hietala, H., T. V. Laitinen, K. Andréeová, R. Vainio, A. Vaivads, M. Palmroth, T. Pulkkinen, H. Koskinen, E. A. Lucek, and H. Rème, Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock, Physical Review Letters, 103, 245001, 2009.

DOI: 10.1103/PhysRevLett.103.245001

Notes for editors

Cluster is the first space mission able to study, in three dimensions, the natural physical processes occurring within and in the near vicinity of the Earth's magnetosphere. It is a project of international collaboration between ESA and NASA. Launched in 2000, Cluster is composed of four identical spacecraft orbiting the Earth in a pyramidal configuration, along a nominal polar orbit of 4 × 19.6 Earth radii (1 Earth radius = 6380 km). Cluster's payload consists of state-of-the-art plasma instrumentation to measure electric and magnetic fields over a wide frequency range, and key physical parameters characterizing electrons and ions from energies of nearly 0 eV to a few MeV. The science operations are coordinated by the Joint Science Operations Centre (JSOC), at the Rutherford Appleton Laboratory, United Kingdom, and implemented by ESA's European Space Operations Centre (ESOC), in Darmstadt, Germany.

Contact

Heli Hietala
Division of Geophysics and Astronomy, Department of Physics, University of Helsinki, Finland
Email: heli.hietalahelsinki.fi

Web story author and co-editor

Arnaud Masson
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email: Arnaud.Massonesa.int
Phone: +31-71-565-5634

Web story co-editors

Philippe Escoubet
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email: Philippe.Escoubetesa.int
Phone: +31-71-565-4564

Matt Taylor
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Email: Matthew.Tayloresa.int
Phone: +31-71-565-8009