Plasma injection at the Earth and SaturnAbi Rymer (JHU-APL)

Misha Sitnov (JHU-APL) Tom Hill (Rice University)

Sasha Ukhorskhiy (JHU-APL)Barry Mauk (JHU-APL)

Andrew Coates (MSSL-UCL)and

Duane Pontius (Birmingham-Southern College)

Polar GatewaysBarrow, AlaskaJanuary, 2008

Abi Rymer

Introduction

It is thought that small scale plasma injection might explain 80% of the mass, energy and momentum transport at the Earth, the small scale injections are commonly referred to as “bursty bulk flows” (BBFs)

Saturn’s magnetosphere has a large scale cold outflowing plasma component with small scale plasma injections superposed.

Our presentation will meander toward discussion of if BBFs at the Earth and plasma injection at Saturn are the same and how observations at Saturn might help to inform plasma processes at the Earth.

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Size: ~7 x 4 m

Weight at launch: 5574 kg

Number of instruments:Orbiter: 12Huygens 6

Cost at launch:~$3.5 billion

The Cassini Spacecraft(Launched October 1997)

Photo courtesy of JPL/ NASA

CAPS

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Earth’s Magnetosphere

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Magnetosheath

RE 20 10 -10 -20Closest Approach

UT

-30 -40 -50

Plasmasphere Plasmasheet Tail lobe and MP crossings

Earth Flyby August 18 1999 – introduction to the data #1

PhotoelectronsModulation due to the CAPS actuator

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photon, Eh

Photoelectrons with Eelectron > E escape into

space

Photoelectrons with Eelectron < E return to the spacecraft and can be measured in

the low electron sensor

Photoelectric effect gives

Cassini a positive potential,

‘real’ electrons are accelerated and are measured to have

energy, Emeasured = E’real’ electron+ E

Photoelectron production

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…then 5 more years in space…

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Saturn’s Magnetosphere

Magnetotail

Magnetopause

Cusps

Plasma torus due to Titan

Rings

Cooler and less dense solar wind

Aligned spin and dipole axes

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Saturn Arrival June 2004 – introduction to the data #2

Eclipse

Dayside MSphEnter plasmasphere

Dispersion features observed thoughout plasmasphere

Radiation belts

Evidence of plasmasheet

dynamics

Ions appear to be slightly faster than corotation velocity

Two electron populations (both ~ Maxwellian)

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Birkeland current and aurora

aka: Field aligned current Magnetic flux rope Auroral electrojet

Downward currents on morning side of the aurora

Upward currents on the evening side

Currents closes through the ionosphere

In space current closes in the solar wind

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Birkeland currents were predicted by Kristian Birkeland based on three polar expeditions between 1896 and 1903. His 1908 book detailing their results and adventures has been made available online by the American Library.

It is available at:www.archive.org/details/norwegianaurorap01chririch

"The [first] expedition has not been described before, because it was such a sad adventure; but now that time has drawn a veil of melancholy oblivion over the misfortune that befell us, I will briefly relate some of our experiences." Birkeland, 1908

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“No one who has not tried it can imagine what it is to be out in such weather. Knudsen, for instance, once had one hand frost-bitten in the few minutes he was out to take a reading”Birkeland, 1908

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First direct measurements of Birkeland currents

The first direct measurements of Birkeland currents were made ~60 years after Birkelands predictions by an APL weather satellite (1968 3c).

The satellite used a bar magnet to maintain its course. It was observed that the magnet began to oscillate at some

locations. These locations were eventually logged and collated. It was recognised that the locations coincided with typical

auroral locations – and so in situ measurements of the aurora by in-situ satellites began.

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Birkeland currents and plasma injection

Plasma injection, aka:• Bubble• Transient fast flow• Solitary electromagnetic pulse• Bursty bulk flow• Travelling compression region• Flux transfer event• Magnetic flux rope

Rather than closing in the solar wind as with the auroral Birkeland currents, current associated with plasma injection closes in the plasma sheet.

Footprint of plasma injection in the planetary ionosphere

+++

- - -vbubble

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Pressure crisis

Steady sunward convection consistent with the adiabatic condition PV5/3=constant is not possible in the tail-like magnetic configuration of the Earth.Yet overwhelming evidence exists that largescale sunward convection exists.

(first recognised by Erickson and Wolf [1980] and referred to as a “mild dilemma” and “the pressure balance inconsistency” it has since been known as the “pressure crisis” or even “pressure catastrophe”

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Possible resolution: injection of plasma bubbles

First proposed by Pontius and Hill [1989] to explain Voyager observations at Jupiter.

Introduced as a mechanism applicable to the Earth by Pontius and Wolf [1990].

Observed to be a prolific feature of Saturn’s magnetosphere [e.g. Hill et al., 2005]

after Pontius and Wolf, 1990

E0

Angelopoulos et al., 1992 and Baumjohann et al. 1990 showed that at the Earth the apparent steady sunward convection of the plasma sheet could, in reality, be a superposition of bursty high speed flows with intermittent intervals of near stagnant plasma and that small bubbles could accomplish earthward mass, energy and flux transport comparable with that expected from “stead state” convection.

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Small scale plasma injection is a vital aspect of large scale magnetospheric flow, but it is relatively difficult to observe at the Earth.The ratio of rotation speed to drift speed at Saturn make it an ideal place to observe plasma injection as explained by Tom Hill soon after Cassini arrival at Saturn:

2

dp p

ELv

B R

Earth Jupiter Saturn

Corotation velocity, kms-1 4 41 35

Gradient drift velocity, kms-1 6 0.045 1

e.g. 1 keV electron at L = 7

For a given energy E and a

given L value.

Plasma injection at the outer planets

Plasma drift speed in a dipole field scales as:

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e-

p+

magnetopausedusk

SUNdawn

Injection at midnight, t=0 (say)

Saturn Saturn

North

Gradient and curvature drifts Dipole opposite to the Earth

Corotating plasma

Plasma circulation at Saturn

Cassini

B

As drifted plasma corotates over Cassini, Cassini will measure first the hottest protons (which drift with corotation) then the coolest protons (which have drifted the least far) then the coolest electrons followed finally by the hottest electrons which have drifted the furthest in the direction opposite to corotation.

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Plasma corotation energies

If we assume that charge exchange/photo-ionisation results in the production of one ion and one electron with zero energy each then they will experience the planetary field and accelerate to the local ion and electron speed respectively.

Plasma corotation energies

If we assume that charge exchange/photo-ionisation results in the production of one ion and one electron with zero energy each then they will experience the planetary field and accelerate to the local ion and electron speed respectively.

Oxygen

Proton

Electron

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penetrating particles

Lines of constant first adiabatic invariant,

Proton corotation energy

Cassini electron observations at Saturn

Rymer et al., 2007

We estimate it would take ~150 hours (15 Saturn rotations) for the electrons to equilibrate to the proton corotation energy. We therefore assume that the outflow of plasma is slow and that magnetic flux is returned via plasma injection - as proposed by Pontius and Hill [1989] for Jupiter.

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Electrons

Ions

Example of Electron and Ion spectra: 28 October 2004

Saturn’s magnetosphere is positively fizzing with plasma injection events…

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Hill et al., 2005

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Hill et al., 2005

The bubbles are not obviously organised by local time or planetary longitude

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e-

Electron pitch angles – a powerful diagnostic of plasma production and transport.

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Inward transport of an isotropic distribution leads to a pancake distribution.

Outward transport of an isotropic distribution goes field aligned

Outward transport of a pancake distribution can go butterfly: depends on distance travelled and steepness of original distribution.

Evolution of pitch angle distributions

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Observation of a young plasma bubble at Saturn

Rymer et al. [2008]

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Burch et al., Nature 2007.

Interpreted as being due to transport out to L=8 from Dione (L=6.3)

Interpreted as being due to transport out to L=8 from Tethys (L=4.9)

First butterfly electron observations in the warm electron component

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penetrating particles

Cassini electron observations at Saturn

Rymer et al., 2007 showed that the PSD at Dione and Tethys is insufficient for the butterfly PADs observed at ~8 Rs to originate there.

TethysDione

Electron PADs observed here

Under outward conservative transport these electron PADs started here.

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Fit to butterfly pitch angle distribution for loss free transport from Enceladus L-shell

Can vary the values of m and n in to optimise the fit.sin

( , )m

nf E

E

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Plasma production injection and drift and circulation at Saturn.

Rymer et al., 2008

Rymer et al., 2008 proposed an alternative explanation wherein the butterlfy PADs evolve from magnetospheric circulation.

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Observation of a young plasma bubble at Saturn

Rymer et al. [2008]

Cold plasma formed from Saturn’s icy moons, rings and neutral cloud

The pitch angle distribution of the injected plasma is consistent with injection from L~11

The drift indicates that the injection is ~16 minutes old.

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Speed of injection/BBF at the Earth, Jupiter and Saturn

Saturn estimate 1:

Age ~ 16 minutesDistance travelled ~ 4 Rs

Speed ~ 260 kms-1

Saturn estimate 2:

Ukhorskiy et al., [2007] estimate a maximum floating speed of the bubble ~ 200 kms-1

where: B0=0.21 G, L=7, ly=3.410-2 Rs and Te=1keV

Jupiter estimate:

Thorne et al., [1997] estimate a bubble observed near the Io torus had a speed of ~100 kms -1

Earth measurements:

Earthward flows 200-600 km/s [e.g. Zesta et al., 2004]

km/s2002

0

3max

y

e

z

yx elB

LcTB

Ecv

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Sergeev et al., 2004

Link between plasma bubble and the ionosphere

At the Earth it is believed that the plasma bubbles are elongated structures with footprints that map to the auroral zone.

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The bubble moves due to a relative build up of charge causing planetward ExB drift.

E+-

||Jz

yx B

Ecv

yl

Ukhorskiy et al., AGU 2007

planet

Density depleted bubble

Consider a bubble depleted in plasma compared to its surroundings.

Protons drift onto one side of the bubble and electrons drift onto the other side.

This creates an electric field, E, across the bubble and the bubble ExB drifts planetward.

The electric field across the bubble is strong enough to generate field-aligned Birkeland currents, Jװ.

Current closure through the ionosphere leads to collapse of the bubble.

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What we know about bubbles• They are generally 1-2 planetary radii in azimuthal extent

• They propagate quickly (a few hundred km/s)

• They contain reduced density compared to surroundings

• Contained plasma is hotter than the surroundings (especially at the outer planets)

• Contained magnetic field is more dipolar.

• It is estimated that bubbles could be responsible for as much as 80% of the mass, energy and momentum transport in Earth’s plasma sheet.

• The fast drift speed and slow corotation speed at the Earth make it difficult to unambiguously link the ion and electron drift from a single injection.

• At the outer planets, especially at Saturn, the fundamental plasma timescales and abundance of plasma injection make the outer planets an ideal laboratory for studying this phenomenon.

Abi Rymer

Summary and musings

We are increasingly confident that plasma injection at Saturn and BBFs or bubbles at the Earth are related phenomena.

Both return dipole field to the inner magnetosphere and apparently play an important role in largescale plasma convection.

There exist some key differences and mysteries which should be resolved. • Injections at Saturn happen closer (5-12 Rs) to the planet than those at the Earth – they therefore map to lower latitudes in the ionosphere – how does the conductivity of the ionosphere affect the progress of the bubble?

• At Saturn we observe the pressure inside the injection to be reduced, models usually presume the BBFs at the Earth to be pressure depletions.

• What is the origin of the plasma depleted region?

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Summary, musings and future work

Dawn Dusk

Midnight

Noon

5 510 10

Gas Cloud

~ 24 Rs

Saturn’s magnetosphere is positively fizzing with plasma injections.

It seems likely that these injections play a significant part in plasma transport at Saturn.

Very high energy injections observed by MIMI are apparently superposed on this fizzy regime – the role of the very high energy injections is as yet poorly understood.

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END

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Transient Petschek-type reconnection [Semenov et al., 2005]

Secondary islands and/or BBF flux ropes [Slavin et al., 2003]

M. Shay simulations in [Ohtani et al., 2004]

Geotail fast flow statistics [Ohtani et al., 2004]

Earthward (818 events)

Tailward (290 events)

Earthward-moving flux ropesEarthward-moving flux ropes

Consistent with original BBF observations [Angelopoulos et al., 1992] and distinct from reconnection, the plasma sheet retains its integrity

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Roussos et al., 2007

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Pressure crisis EARTH

SATURN

PV-5/3

PV-2/3Black diamonds:Saturn pressure derived from thermal and energetic particle measurements

Solid black line:Saturn pressure derived from wave measurements of electron density and energetic particle measurements

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Plasma injection