Saturn’s 10.8 hour periodicity—relationship between cold, sub-corotating plasma and hot ring current particles Don Mitchell Pontus Brandt Abi Rymer Jim Carbary

Hydrogen ENA Mapped to ionosphere SOI to 222, 2007 Not this talk: ENA emissions show the same local time dependence and radial displacement as that caused by the convection pattern suggested by Roussos et al., and Thomsen. And, they map to the lower bounds of the auroral zone. Carbary, J. F., D. G. Mitchell, P. Brandt, E. C. Roelof, and S. M. Krimigis (2008), Statistical morphology of ENA emissions at Saturn, J. Geophys. Res.

The reference frame: Saturn’s cold plasma sub-corotates

Gurnett et al., 2007 Science BUT! Cold plasma density (here in the 3 – 5 Rs range) is ordered by the SKR corotation period, not the plasma rotation period.

Burch, J. L., A. D. DeJong, J. Goldstein, and D. T. Young (2009), Periodicity in Saturn's magnetosphere: Plasma cam, GRL. And the cold plasma appears to describe a spiral form in the SLS3 coordinate system. (SKR peaks as 100° passes noon).

Hydrogen ENA Protons SOI to 222, 2007 Furthermore, ENA emission as well as in situ energetic protons are also organized in SLS3. Carbary, J. F., D. G. Mitchell, P. Brandt, E. C. Roelof, and S. M. Krimigis (2008), Statistical morphology of ENA emissions at Saturn, J. Geophys. Res.

Average 6 to 12 Rs The SLS3 ordering of energetic ions depends on energy (gradient and curvature drift effects). Yet even for those ions with significant azimuthal drifts, ordering in SLS3 remains.

Average 6 to 12 Rs

In periodic sequences, ENA emission “jumps” forward as a new event replaces the previous one. Saturn SKR Titan’s orbit

Concept based on observations. Cold plasma (light and dark blue) propagates outward, driven by centrifugal acceleration. Plasma is released on the night side when the current sheet becomes too loaded--Vasyliunas reconnection. Slightly asymmetric system Spontaneous plasmoid release (Vasyliunas)

If plasma loading was very fast, such that in one rotation a segment that had just spawned a plasmoid was again unstable and ready to spawn another, then the steady state pattern might look something like this: But then plasmoid release would be continuous, and the sense of the spiral is the reverse of what is observed

But if the plasma loading is slower, it looks like this

(Or this…) It doesn’t matter whether the cold plasma leaves in a block—ie, a plasmoid—or as “drizzle”. The important thing is that it only leaves when it is sufficiently loaded, and that turns on and off because any particular longitudinal sector does not load to instability in only one rotation.

For simplicity, I’ll stick to the big plasmoid picture. The difference between the pictures may depend on solar wind pressure, and/or may bear on the behavior of the ring current (big plasmoid--strong more impulsive ring current injection; drizzle--weaker, less impulsive ring current injection).

So add a partial ring current to the picture. We assume this occurs during large plasmoid release. (The difference between this and the Kivelson/Jia picture is the role of the ring current. Otherwise, they are effectively similar.)

Through gradient and curvature drift, the ring current enhancement has period closer to the SKR period. The plasma period is 13 to 15 hours, energetic particles 8 to 12 hours (energy dependent).

Note that the ring current, through gradient and curvature drift, moves prograde relative to the cold plasma, and therefore under the oldest, most loaded sector.

The previous ring current enhancement facilitates the next plasmoid (or it could equally be continuous plasma) release

Vasyliunas, 1983 This conforms pretty closely with the picture drawn by Vasyliunas in 1983.

Release 1/4 rotation 1/2 rotation 3/4 rotation What about the SKR? The field aligned current system associated with the partial ring current generated as the field dipolarizes following plasmoid release would be in about the right place. SKR

Titan’s orbit, 20 Rs 8.5 Rs 6 Rs A FAC structure set up by the plasmoid/flux rope release rotates with the ionosphere. The early stages of this can be seen in the INCA/UVIS/SKR movie from day 129, Clearly there is enhanced conductance in the ionosphere, and that extends down to ~70° latitude. It appears to rotate near rigid corotation. Role of ionosphere and coupling

A spiral pattern (quasi-wave) is a natural quasi-stable end state to a rotating, mass loaded, triggered-release system. Do we see the plasmoids? Jackman et al., 2011 does, and so does INCA (under special conditions…). A B C D Mass loading (generation + transport) is constant and radial. The plasma rotation period is ~14 hours; A full cycle (plasmoid to plasmoid) is significantly less than the plasma rotation period.

ENA Movie of Tail Reconnection Event During Saturn Eclipse

Reconnection begins…

Plasmoid release…

Plasmoid moves tailward, dawnward; current sheet rotates

Plasmoid further tailward, current sheet rotating…

Plasmoid disappearing tailward and dawnward…

Plasmoid gone from FOV; current sheet intensifying, rotating.

Conceptual model with ionospheric feedback One driver: the natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

As the lower stability sector beneath the ionospheric enhancement reaches the night side, the absence of the confining force of the magnetopause allows the unstable plasma to break free The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system). As the lower stability sector beneath the ionospheric enhancement reaches the night side, the absence of the confining force of the magnetopause allows the unstable plasma to break free

The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system). New reconnection and current sheet disruption driven by the plasma release reinforces the parallel currents and regenerates the high ionospheric conductance in the same sector

The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system). New reconnection and current sheet disruption driven by the plasma release reinforces the parallel currents and regenerates the high ionospheric conductance in the same sector

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Yellow represents the partial ring current, which is assumed to couple with the ionosphere through FACs. The red contour represents a stability limit. When the blue exceeds the red boundary, it breaks off back to its inner boundary (plasmoid release). This can happen in a block (large plasmoid), or a dribble (more like dusk tailward cold plasma flow). Blue represents plasma loading in the outer magnetosphere. Think of it more as mass content than a boundary. Cold plasma (blue) rotates at 14 hour period. As the ring current moves forward (10.8 hour period), it “pushes out” the cold plasma (representing the destabilizing effect of the current system).

Conceptual model with ionospheric feedback The natural development of a plasma spiral. Much later…

Conceptual model with ionospheric feedback And now, the movie version

Conceptual model with ionospheric feedback, 2 periods Two ionospheric enhancements (N and S)

Response to changes in mass source rate If the mass source rate increases, the outer magnetosphere will reach a critical loading rate faster. This may be compensated for by having a larger longitudinal sector break off in the tail on each rotation. The additional mass loading may slow down the plasma corotation, but the longer segment breaking off can compensate with a larger phase jump and stay in synch with the ionosphere. A lower mass loading rate would result in smaller chunks breaking off, and perhaps faster plasma rotation.

With a lower plasma loading rate, however, the sense of the spiral will be as observed, and the plasmoid release will not be continuous.

3. How is the SKR period expressed in the inner, close-field magnetosphere?

In spite of the much longer cold plasma rotation period, Saturn ring current activity shows well defined periodicity at about the SLS3 period (Carbary et al., 2008)

Fresh Injection No dispersion, NO ambient cold plasma Older injections, dispersion, new ionospheric electrons, higher plasma density Older injections, dispersion, new ionospheric electrons, higher plasma density DispersedUn-dispersed Ionization Cross- sections

3. Mechanism for inner magnetosphere modulation. As mass is unloaded in longitudinal chunks at the perimeter, the gradient between loaded and unloaded flux tubes is steepened, and the remaining trapped plasma is heated (in that longitudinal sector). This results in a hotter electron population being transported inward through flux tube interchange, and the interchange tubes may also penetrate to lower radii (because they are more buoyant, or lighter, than those in other sectors). This ionizing electron population keeps jumping ahead of the plasma rotation speed, as the steep gradient jumps ahead by the mechanism previously described. This transport enhancement may be communicated all the way back to 3-5 Rs, where Gurnett shows the density enhancement, in synch with SKR, not in synch with plasma rotation. Tom Hill (Mauk, Saturn Book, 2010)

Conclusions Concept reconciles slow plasma rotation rate with shorter modulation period (we believe this to be the ionospheric period, slipping relative to neutral winds, slowed by magnetospheric drag). Model provides framework for modulation of the inner magnetosphere via flux tube inter- change of hotter plasma in the “active” sector. Model CAN accommodate multiple periods simultaneously since it is clocked from the ionosphere (ionospheric anomalies in both hemispheres-rotation periods different in north and south; no reason they should be identical).

1.Outer Magnetosphere: unloading via Vasyliunas reconnection (average period shorter than plasma sub- corotation period—e.g., see Jackman et al., 2011) 2.Regulation of unloading by coupling from the ionosphere (both north and south) 3. Communication from outer to inner magnetosphere by interchange radial transport of hot ionizing electrons In light of substantial sub- corotation, how does Saturn’s magnetosphere modulate so many phenomena at so many latitudes/radial distances with the same modulation period? (SKR, magnetic field, current sheet, magnetopause, energetic particles, plasma density, etc.) Thomsen et al., 2010, JGR Model Features:

Release 1/4 rotation 1/2 rotation 3/4 rotation A plasmoid release will leave hot, low density flux tubes planetward of the release. This will snap back planetward, and accelerate to super-rotation through conservation of angular momentum. Field aligned current associated with the hot, high pressure partial ring current will provide important coupling to the ionosphere. An ionospheric conductance anomaly can continue to support field aligned currents that preferentially destabilize the release of cold plasma. In a sub-corotating magnetosphere, such a region rotates into the oldest, most heavily loaded cold plasma sector on every ionospheric rotation period.

How Long Is the Day on Saturn? Agustín Sánchez-Lavega SCIENCE VOL FEBRUARY 2005 Discovery of a north-south asymmetry in Saturn’s radio rotation period, Gurnett et al., GRL Vol. 36, 2009 Does the ionosphere have the right period? Yes.

How Long Is the Day on Saturn? Agustín Sánchez-Lavega SCIENCE VOL FEBRUARY 2005 Saturn’s probable interior period S. Auroral Zone N. Auroral Zone SKR Period 2. The Role of the Ionosphere as Driver

FACs intensification on the dawn side Map to a region of super-corotating plasma in the magnetosphere A layer of depleted flux tubes containing hot plasma return from tail reconnection sites.

Depleted flux tubes Plasma loss on flanks Convection and distribution of flux-tube content (during northward IMF)

3. Mechanism for inner magnetosphere modulation. Hot electron population transported by interchange ionizes neutral gas, enhances plasma density in the inner magnetosphere

Tom Hill (Mauk, Saturn Book, 2010) Flux tube interchange (simulation) Flux tube interchange: the dominant process through which plasma transport is thought to proceed in Saturn’s middle magnetosphere.

From Burch et al., 2008 (GRL) Current sheet encounters every 10.8 hours Plasmoids? Probably, beyond ~30 Rs

McAndrews et al., 2009 PSS Kane et al. JGR % SLS3 75% SLS3

Burch, J. L., A. D. DeJong, J. Goldstein, and D. T. Young (2009), Periodicity in Saturn's magnetosphere: Plasma cam, GRL.