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The NASA JPL YouTube video Sounds of Saturn: Hear Radio Emissions of the Planet and Its Moon Enceladus (below, also here) provides and audio and spectral representation of plasma waves recored by Cassini as it passed between Saturn and it's moon Enceladus shortly before Cassini's end of mission.

News associated with this recent release also links to two paywalled papers:

The abstract of the first paper reads as follows:

Cassini's Radio and Plasma Wave Science (RPWS) instrument detected intense auroral hiss emissions during one of its perikrone passes of the Grand Finale orbits. The emissions were detected when Cassini traversed a flux tube connected to Enceladus' orbit (L‐shell = 4) and at a time when both the spacecraft and the icy moon were in similar longitudes. Previous observations of auroral hiss related to Enceladus were made only during close flybys and here we present the first observation of such emissions close to Saturn. Further, ray‐tracing analysis shows the source location at a latitude of 63°, in excellent agreement with earlier UVIS observations of Enceladus' auroral footprint by Pryor et al. (2011). The detection has been afforded exclusively by the Grand Finale phase which enabled sampling of Enceladus' high‐latitude flux tube near Saturn. This result provides new insight into the spatial extent of the electrodynamic interaction between Saturn and Enceladus.

Question: What is the nature of a "flux tube" between Saturn and Enceladus? Does this refer to magnetic lines flux, or flux of particles, or something else. Also, what does "L‐shell=4" mean?

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  • $\begingroup$ I'm curious too. It seems to me like you might have more luck finding a subject matter expert on Astronomy, though. $\endgroup$ – Bear Jul 10 '18 at 14:42
  • $\begingroup$ @Bear that's a good point. While the observation was made by a NASA spacecraft and some of the authors are at NASA/Goddard, and is definitely an aspect of Space Exploration the work is mostly done by folks in the Department of Physics and Astronomy, University of Iowa. Then again, the work is published in Geophysics Research Letters. I think if I were to do it again, I would post this in Astronomy SE instead. I'll leave a blurb in The Observatory for now. $\endgroup$ – uhoh Jul 10 '18 at 23:32
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A "flux tube" is a magnetically-confined conduit that allows charged particles to flow between one place in a planet's magnetosphere and another.

An "L-shell" is a subset of a planet's magnetic field lines that cross the planet's equator (for most planets, anyway) at the specified number of planetary radii from the planet's center.

In a uniform magnetic field, a charged particle traveling along field lines experiences no net force from the field. One traveling perpendicular to the field lines experience the Lorentz force, F = q (V X B), where q is the particle's charge, V is the velocity vector, and B is the field vector. Since that force is always perpendicular to the particle's velocity vector, this leaves the particle running around in circles! If the V vector is askew of the field vector, i.e. it has a component parallel to the magnetic field vector and a component perpendicular to the field vector, its velocity along the field lines remains unchanged, while the component perpendicular to the lines does its circular thing: the particle spirals along the field lines!

Net result: charged particles have an easy time of traveling along magnetic field lines, but traveling significant distances perpendicular to the field is nearly impossible. The flux tube is like a conductor surrounded by insulating material.

If there is a potential difference between the planet and a moon traveling in the magnetic field, charged particles can easily flow along the flux tube from the moon to the planet, or the other way. That flux tube will follow the usual toroidal geometry of a dynamo field, so for most moons the tube connects to the planet somewhere near the poles.

An L-shell is a toroidal surface within a dynamo field. If you imagine all the field lines that pass through the equatorial plane at a specified radius, say at 4 planetary radii from the center, and follow them all to the poles, you get a toroidal surface. Anything within that surface is said to be "at L = 4" or "at an L-shell of 4". L = 2 would intersect the equator at 2 planetary radii, and so on. So a flux tube within the L = 4 L-shell would be said to be "at L = 4".

The image below described further here, from Cassini shows (at ultraviolet wavelengths) two things: 1) the "normal" aurora at very high L-shells (thus closer to the pole), arising from charged-particle currents generated at many Saturn radii, where the solar wind and Saturn's magnetic field interact; and 2) the much smaller spot where the Enceladus flux tube, carrying the charged-particle currents, intersects Saturn's atmosphere and makes its own little aurora (in the white boxes).

As a Ph.D. student I was in a research group with people that worked with these all the time. Not only could the flux tubes guide charged particles, they can guide radio waves, so these folks would inject powerful radio signals into specific regions and see how the magnetosphere responded.

enter image description here

Enceladus 'Footprint' on Saturn

NASA's Cassini spacecraft has spotted a glowing patch of ultraviolet light near Saturn's north pole that marks the presence of an electrical circuit that connects Saturn with its moon Enceladus. This newly discovered patch occurs at the "footprint" of the magnetic connection between Saturn and Enceladus and indicates electrons and ions accelerating along magnetic field lines. White boxes indicate the location of this footprint, which scientists have long predicted but never before seen.

The patch glows because of the same phenomenon that makes Saturn's well-known north and south polar auroras glow: energetic electrons diving into the planet's atmosphere. However, the footprint is not connected to the rings of auroras around Saturn's poles.

The two images shown here were obtained by Cassini's ultraviolet imaging spectrograph on Aug. 26, 2008, separated by 80 minutes. The footprint moved according to changes in the position of Enceladus. In the image, the colors represent how bright the extreme ultraviolet emissions are. The lowest emission areas (one to two extreme ultraviolet counts per pixel) are in black/blue. The brightest emission areas (500 to 1,000 extreme ultraviolet counts per pixel) are in yellow/white.

The footprint appeared at about 65 degrees north latitude. It measured about 1,200 kilometers (750 miles) in the longitude direction and less than 400 kilometers (250 miles) in latitude, covering an area comparable to that of California or Sweden.

In the brightest image the footprint shone with an ultraviolet light intensity of about 1.6 kilorayleighs, far less than the Saturnian polar auroral rings. This is comparable to the faintest aurora visible at Earth without a telescope in the visible light spectrum.

The sun was illuminating Saturn's north pole from the left and the footprint is on the day side of the planet. The night side of the planet was to the right of the hashed line.

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  • $\begingroup$ Okay so Enceladus' semimajor axis is roughly 240,000 km and Saturn's radius is roughly 60,000 km, so that's the 4, got it. As far as the nature of the flux tube between Enceladus and Saturn, is it possible to describe something about it? Is the "tube" just a collection of a subset drawn field lines that happen to geometrically pass through Enceladus' instantaneous location, as the moon and lines pass through each other, or is the tube some kind of connected set of lines? Considering that Enceladus' orbit around Saturn is synchronous... $\endgroup$ – uhoh Jul 11 '18 at 3:21
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    $\begingroup$ @uhoh, Yes, the flux tube is the set of magnetic field lines that, at any given time, pass through Enceladus. This isn't quite straightforward, because the "atmosphere" from the geysers gets partially ionized and winds up impeding passage of the field lines through it. (Enceladus' orbit is not synchronous with Saturn rotation: orbit period is ~33 hrs, rotation period is ~10.6 hrs; the Enceladus flux tube "footprint" isn't fixed to a particular Saturn longitude) Since Enceladus is essentially spherical, the flux tube cross-section is essentially circular, getting smaller closer to Saturn... $\endgroup$ – Tom Spilker Jul 11 '18 at 4:03
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    $\begingroup$ ...This is how Cassini first found the geysers: the magnetic field lines, impeded by the partially ionized gas around the south pole, "draped" around the moon, but only around the southern hemisphere. The mag team was puzzled. "How can a little moon have half an atmosphere??" They concluded that it could only be the result of gas being continuously replenished, and from that concluded that gas must be erupting from the surface at the south pole. They talked the imaging team into getting a high-phase image, and that very clearly showed the dust carried along with the gas. $\endgroup$ – Tom Spilker Jul 11 '18 at 4:09
  • $\begingroup$ Oh, I read the Wikipedia article too quickly. Yes, an orbital period of 10 hours sounds awfully fast. So there really isn't much of a "tube" connecting the two bodies, even conceptually. I'm really confused by this whole thing. Do trapped charged particles and their associated field lines rotate around Saturn's axis with the 10.6 hour period of the planet as well? Or are these lines roughy fixed in inertial space? $\endgroup$ – uhoh Jul 11 '18 at 4:10
  • $\begingroup$ They rotate with the planet and its magnetic field. They are within the "co-rotation zone" where the mag field is sufficiently strong to sweep the usual concentration of charged particles along with it. Actually, there is a conceptual tube: the equatorial cross-section of Enceladus, projected along mag field lines all the way to Saturn's ionosphere. All the electric currents between Enceladus and Saturn, with charged particles as the current carriers, flow in that tube. $\endgroup$ – Tom Spilker Jul 11 '18 at 4:16

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