Dynamics of Energetic Particles and Neutrals in Saturn’s Variable Magnetosphere

by S.M. Krimigis (1), D.G. Mitchell (1), D.C. Hamilton (2), N. Krupp (3), S. Livi (1), E. C. Roelof (1), Dandouras (4), B. H. Mauk (1), J. P. Brandt (1), C. Paranicas (1) , J. Saur (1), T. P. A rmstrong (5), S. Bolton (6), A. F. Cheng (1), G. Gloeckler (2), K. C. Hsieh ( 7), W. -H. Ip (8), A. Lagg (3), L. J. Lanzerotti (9), R. W . McEntire (1), D. J. Williams (1), W. Kurth (10), and M. Dougherty (11) (1) Applied Physics Laboratory, Johns Hopkins Uni versity, Laurel, Maryland, (2) University of Maryland, College Park, Maryland, (3) Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany (4) Centre D'Etude Spatiale Des Rayonnements, Toulouse, France, (5) Fundamental Technologies, Inc., Lawrence, Kansas, (6) Jet Propulsion Laboratory, Pasadena, CA, (7) Univer sity of Arizona, Tucson, Arizona, (8) National Central University, Taiwan, R.O.C., (9) Bell Laboratories, Murray Hill, New Jersey, (10) Univers ity of Iowa, Iowa City IA (11) Imperial College, London, UK

ENA generation mechnism

Krimigis et al, 2004

Some Questions about the Interaction between Trapped

Particles and Neutrals

What is the source of trapped particles?

How are they accelerated to high energies?

What is the principal loss mechanism?

How do particles respond in the rotating magnetic field of Saturn?

Are particle collisions with ring and icy moon surfaces important?

Rev 0

1

10

100

1 10 100

Aug. 18, 1999

H+

He+

O++

O+

He++

N+

High ChargeC,N,O

1 10 100

July 1, 2004

H+

He+

O++

O+ & water group

He++

H2

+

O2

+

C+

1 10 100

H+

He++

He+

S+

O+

O++O

3+

S++

S3+

S4+

Jan. 10, 2001

ma

ss (

am

u)

Earth Jupiter Saturn

Charge Energy Mass Spectrometer (CHEMS) on Cassini records “fingerprints” of ion composition at Earth, Jupiter, and Saturn

Plotted: 2/25/05

mass per charge (amu/e)

Some Questions about the Interaction between Trapped

Particles and Neutrals

What is the source of trapped particles?

How are they accelerated to high energies?

What is the principal loss mechanism?

How do particles respond in the rotating magnetic field of Saturn?

Are particle collisions with ring and icy moon surfaces important?

ENA generation mechnism

Energetic Neutral Atom (ENA) ImagingO+

fast + H Ofast + H+

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Grid outlines INCA FOV

Hydrogen

Oxygen

ENA emission modulated at Saturn’s rotation period

ENA PERIODICITIES

With what else does this ENA oscillation correlate?

Energetic (25-200keV) e-

B-field

SKR

Oxygen ENA modulation is time-dispersed in energy--high energies lead

low, due to grad-B drifts

At what radial distance is the corotating ENA emission concentrated?

7 Rs

Highest ENA production

The density of OH in the 6-10 Rs region is comparable to H density in the Geocorona. O is probably about the same as OH. O+ lifetime in the Earth’s ring current is ~ 8h. O+ lifetime in the inner Saturn magnetosphere could be much shorter, depending on its radial placement (O, OH charge-exchange cross-section is larger than H)

Hydrogen Geocorona Rairden et al. (1986)

Earth RingCurrent

Devoid of energetic ions

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

(Mauk et al, GRL, June 2005)

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

Modulation persistence and phase lock suggest an “active” longitude (could be a quadrant) coupled with a preferred local time for ion injection

Neutral Gas?

Ion Dispersion

Event Gas

HotH+, O+

Saturn

Cassini

How do these events look from an off-equatorial vantage point?

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Same event, higher energy H

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Watch the rotating blob…

•Brightest ENA emission from interface between dense gas cloud in vicinity of E-Ring, and energetic ions just outside that region.

•Large scale ion injections are commonly seen as corotating brightenings in ENA.

•Ion injections are well correlated with SKR, including when they are seen repeating at Saturn’s rotational period.

•Oxygen ENA emission shows more pronounced, and more regular, rotation modulation than hydrogen.

•Energetic oxygen lifetime in the inner magnetosphere should be no longer than hours, given the density of the water-product cloud in the vicinity of the E-ring. The repeated, un-damped rotational modulation therefore requires nearly continual replenishment of the energetic oxygen ions.

•The synchronicity of the modulated ENA emission, as well as the unchanging energy dispersion, requires repeated injections preferentially at the same Saturn IAU longitude --possibly the same ‘clock’ that drives the SKR.

Summary of Observations on Periodicities