Press Release
Cassini Captures Best View Yet of Saturn's Ring Currents
Particle Population in Saturn's Magnetosphere
This is an artist's concept of the Saturnian plasma sheet based on data from the Cassini Magnetospheric Imaging Instrument. It shows Saturn's embedded "ring current," an invisible ring of energetic ions trapped in the planet's magnetic field.
Saturn is at the center, with the red "donut" representing the distribution of dense neutral gas outside Saturn's icy rings. Beyond this region, energetic ions populate the plasma sheet to the dayside magnetopause filling the faintly sketched magnetic flux tubes to higher latitudes and contributing to the ring current. The plasma sheet thins gradually toward the nightside. The view is from above Saturn's equatorial plane, which is represented by grid lines. The moon Titan's location is shown for scale. The location of the bow shock is marked, as is the flow of the deflected solar wind in the magnetosheath.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Magnetospheric Imaging Instrument was designed, built and is operated by an international team led by The Johns Hopkins University Applied Physics Laboratory, Laurel, Md. The 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. The Cassini orbiter was designed, developed and assembled at JPL.
Credit: NASA/Jet Propulsion Laboratory/Johns Hopkins University Applied Physics Laboratory (NASA/JPL/JHUAPL)
Wed, 12/12/2007 - 09:10
Scientists have gotten their best "look" ever at the invisible ring of energetic ions trapped in Saturn's giant magnetic field, finding that it is asymmetric and dynamic, unlike similar rings that appear around Earth.
Using the Magnetospheric Imaging Instrument on NASA's Cassini spacecraft, a team led by Dr. Stamatios Krimigis of The Johns Hopkins University Applied Physics Laboratory (APL) discovered that Saturn's ring of energetic ions — called a "ring current" — is a warped disc that is deflected by the solar wind out of the equatorial plane on the planet's night side and thickens dramatically on the day side. The images were obtained by a unique camera that Krimigis says "visualizes the invisible" and show the plasma and radiation belts in Saturn's environment.
In the Dec. 13 issue of the journal Nature, Krimigis' team describes how Saturn's ring current changes over time; it's a dynamic system, doughnut shaped but sometimes appearing like someone took a bite out of it. They also found that Saturn's ring current is persistently asymmetric — unlike Earth's — and it rotates closely in-step with Saturn itself. Ring currents form when hot ionized gas (known as plasma) becomes trapped on a planet's magnetic field lines. The main source of the plasma that forms Saturn's ring current is material from the gas vented by geysers on the moon Enceladus.
Saturn's 'Ring Current'
Like Earth, Saturn has an invisible ring of energetic ions trapped in its magnetic field. This feature is known as a "ring current." This ring current has been imaged with a special, APL-designed camera on Cassini sensitive to energetic neutral atoms.
This is a false color map of the intensity of the energetic neutral atoms emitted from the ring current through a processed called charge exchange. In this process a trapped energetic ion steals and electron from cold gas atoms and becomes neutral and escapes the magnetic field.
The Cassini Magnetospheric Imaging Instrument's ion and neutral camera records the intensity of the escaping particles, which provides a map of the ring current. In this image, the colors represent the intensity of the neutral emission, which is a reflection of the trapped ions. This "ring" is much farther from Saturn (roughly five times farther) then Saturn's famous icy rings. Red in the image represents the higher intensity of the particles, while blue is less intense.
Saturn's ring current had not been mapped before on a global scale, only "snippets" or areas were mapped previously but not in this detail. This instrument allows scientists to produce movies that show how this ring changes over time. These movies reveal a dynamic system, which is usually not as uniform as depicted in this image. The ring current is doughnut shaped but in some instances appears as if someone took a bite out of it.
This image was obtained on March 19, 2007, at a latitude of about 54.5 degrees and radial distance of 1.5 million kilometers (920,000 miles). Saturn is at the center, and the dotted circles represent the orbits of the moons Rhea and Titan. The Z axis points parallel to Saturn's spin axis, the X axis points roughly sunward in the sun—spin axis plane, and the Y axis completes the system, pointing roughly toward dusk. The ion and neutral camera's field of view is marked by the white line and accounts for the cutoff of the image on the left. The image is an average of the activity over a (roughly) 3-hour period.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Magnetospheric Imaging Instrument was designed, built and is operated by an international team lead by The Johns Hopkins University Applied Physics Laboratory, Laurel, Md. The 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. The Cassini orbiter was designed, developed and assembled at JPL.
Credit: NASA/Jet Propulsion Laboratory/Johns Hopkins University Applied Physics Laboratory (NASA/JPL/JHUAPL)
At Earth, ring currents form during large solar wind-driven magnetic storms, although they fade quickly as the driving solar wind disturbance recedes into deep space. At Saturn, the Magnetospheric Imaging Instrument (MIMI) observed that the ring current's intensity seemed only weakly related to solar activity.
"We might get a more intense reading when a solar wind pressure spike passes by," says Dr. Donald Mitchell, a MIMI co-investigator from APL. "But the surprise is that Saturn's ring current didn't become symmetric or dissipate as it does at Earth. It stayed lumpy and rotated around the planet several times. We don't know exactly why that happens, but we have seen it exhibit this behavior repeatedly."
The presence of a ring current around Saturn was first suggested in the early 1980s from magnetic anomalies observed by NASA's Pioneer 11 and Voyager 1 and 2 spacecraft. But Saturn's ring current had never been mapped on a global scale; only small areas were mapped previously, and not in this detail. MIMI was designed for just this purpose; developed by an APL-led international team, MIMI has three distinct sensors, one of which contributed the images for this work.
'Ring Current' Rotation
This series of Magnetospheric Imaging Instrument images shows the energetic neutral atom emission from Saturn's ring current. The sun is to lower left (X axis), and the orbits of the moons Titan, Rhea and Dione, and Saturn's rings, are shown. The pronounced asymmetry (bright emission in the upper quadrant, located between midnight and dawn) rotates with the planet, and the bright spot rotates through 360 degrees over one Saturn rotation (about 10 hours and 40 minutes).
Credit: NASA/Jet Propulsion Laboratory/Johns Hopkins University Applied Physics Laboratory (NASA/JPL/JHUAPL)
False-color images accompanying the Nature article were taken by MIMI's ion and neutral camera and show the intensity of the energetic neutral atoms emitted from the ring current through a process called charge exchange. This happens when a trapped energetic ion steals an electron from a cold gas atom, becomes neutral and escapes the magnetic field. Scientists are using these images to create a map of the invisible ring current, which is roughly five times farther from Saturn than its famous icy rings.
MIMI gathered the images for the Nature paper in March 2007 as Cassini looped nearly 1.5 million kilometers (920,000 miles) over Saturn's poles, giving the instrument a bird's eye view of the magnetic activity swirling around the planet.
Movie of Saturn's 'Ring Current'
This movie is made up of more than 600 consecutive images taken March 16-18, 2007, with the Cassini Magnetospheric Imaging instrument's ion and neutral camera. The movie clip shows Saturn's dynamic "ring current," which is an invisible ring of energetic ions trapped in the magnetic field of the planet.
The ion and neutral camera allows scientists to produce movies that show how this ring changes over time, and these movies reveal a dynamic system. The ring current is doughnut shaped but, in some instances, appears like someone took a bite out of it.
The ion and neutral camera records the intensity of the escaping particles, which provides a map of the ring current. In this movie, the colors represent the intensity of the neutral emission, which is a reflection of the trapped ions. This "ring" is much farther from Saturn (roughly five times farther) then Saturn's famous icy rings. Red represents the higher intensity of the particles, while blue is less intense.
Saturn's ring current had not been mapped before on a global scale, only "snippets" or areas were mapped previously but not in this detail.
The spacecraft coordinates are shown with each frame. Saturn is at the center and the dotted circles represent the orbits of the moon's Rhea and Titan. The Z axis points parallel with Saturn's spin axis, the X axis points roughly sunward in the sun—spin axis plane, and Y completes the system, roughly toward dusk. The magenta-colored axes represent a longitude system rotating with the planet, developed by the radio and plasma wave instrument team.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Magnetospheric Imaging Instrument was designed, built and is operated by an international team lead by The Johns Hopkins University Applied Physics Laboratory, Laurel, Md. The 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. The Cassini orbiter was designed, developed and assembled at JPL.
Credit: NASA/Jet Propulsion Laboratory/Johns Hopkins University Applied Physics Laboratory (NASA/JPL/JHUAPL)
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The 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. APL's Krimigis is the principal investigator for MIMI, which was designed and built and is operated by an Applied Physics Laboratory-led team. The University of Maryland and the Max Planck Institute for Solar Physics in Germany contributed two of the three sensors. Part of the analyses for this work was performed as a collaborative effort with the Academy of Athens in Greece.
For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the MIMI instrument team's home page, http://sd-www.jhuapl.edu/CASSINI/index.html.