Plasma injection at the Earth and Saturn Abi Rymer JHUAPL Misha Sitnov JHUAPL Tom Hill Rice Universi

1
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 Gateways
• Barrow, Alaska
• January, 2008

2
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)
• Saturns 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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The Cassini Spacecraft (Launched October 1997)
Size 7 x 4 m Weight at launch 5574
kg Number of instruments Orbiter 12 Huygens
6 Cost at launch 3.5 billion
CAPS
Photo courtesy of JPL/ NASA
4
Earths Magnetosphere
5
Earth Flyby August 18 1999 introduction to the
data 1
Magnetosheath
Plasmasheet
Plasmasphere
Tail lobe and MP crossings
UT
Closest Approach
RE
20
10
-10
-20
-30
-40
-50
Modulation due to the CAPS actuator
Photoelectrons
6
Photoelectron production
Photoelectrons with Eelectron gt E? escape into
space
Photoelectric effect gives Cassini a positive
potential, ?
photon, Eh?
real electrons are accelerated and are measured
to have energy, Emeasured Ereal electron E?
Photoelectrons with Eelectron lt E? return to the
spacecraft and can be measured in the low
electron sensor
7
then 5 more years in space
8
Saturns Magnetosphere
Aligned spin and dipole axes
Magnetopause
Magnetotail
Cooler and less dense solar wind
Rings
Cusps
Plasma torus due to Titan
9
Saturn Arrival June 2004 introduction to the
data 2
Eclipse
Enter plasmasphere
Evidence of plasmasheet dynamics
Dayside MSph
Two electron populations (both Maxwellian)
Dispersion features observed thoughout
plasmasphere
Ions appear to be slightly faster than corotation
velocity
Radiation belts
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Birkeland current and aurora
In space current closes in the solar wind

• 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
11
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/norwegian
aurorap01chririch "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
Rather than closing in the solar wind as with the
auroral Birkeland currents, current associated
with plasma injection closes in the plasma sheet.

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

- - -
vbubble

Footprint of plasma injection in the planetary
ionosphere
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Pressure crisis
Steady sunward convection consistent with the
adiabatic condition PV5/3constant 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 Saturns
  magnetosphere e.g. Hill et al., 2005

E0
after Pontius and Wolf, 1990
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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Plasma injection at the outer planets
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
For a given energy E and a given L value.
Plasma drift speed in a dipole field scales as
e.g. 1 keV electron at L 7
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Plasma circulation at Saturn
Dipole opposite to the Earth
Gradient and curvature drifts
SUN
North
Corotating plasma
dawn
p
B
Injection at midnight, t0 (say)
Saturn
Saturn
Cassini
e-
magnetopause
dusk
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
Plasma corotation energies
Oxygen
Proton
Electron
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.
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.
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Cassini electron observations at Saturn
Lines of constant first adiabatic invariant, ?
Proton corotation energy
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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Example of Electron and Ion spectra 28 October
2004
Electrons
Ions
Saturns magnetosphere is positively fizzing with
plasma injection events
22
Hill et al., 2005
23
The bubbles are not obviously organised by local
time or planetary longitude
Hill et al., 2005
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Electron pitch angles a powerful diagnostic of
plasma production and transport.
e-
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Evolution of pitch angle distributions
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.
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Observation of a young plasma bubble at Saturn
Rymer et al. 2008
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First butterfly electron observations in the warm
electron component
Interpreted as being due to transport out to L8
from Dione (L6.3)
Interpreted as being due to transport out to L8
from Tethys (L4.9)
Burch et al., Nature 2007.
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Cassini electron observations at Saturn
Dione
Tethys
Electron PADs observed here
Under outward conservative transport these
electron PADs started here.
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.
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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.
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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
The pitch angle distribution of the injected
plasma is consistent with injection from L11
The drift indicates that the injection is 16
minutes old.
Cold plasma formed from Saturns icy moons, rings
and neutral cloud
Rymer et al. 2008
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Speed of injection/BBF at the Earth, Jupiter and
Saturn

• Saturn estimate 1
• Age 16 minutes
• Distance 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 B00.21 G, L7, ly3.4??10-2 Rs and
  Te1keV
• Jupiter estimate
• Thorne et al., 1997 estimate a bubble observed
  near the Io torus had a speed of 100 kms-1

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Link between plasma bubble and the ionosphere
Sergeev et al., 2004
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.
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, J?. Current closure through
the ionosphere leads to collapse of the bubble.
planet

-
Density depleted bubble
Ukhorskiy et al., AGU 2007
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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 Earths 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.

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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
Midnight
Saturns 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.
Dusk
Dawn
5
10
10
5
Gas Cloud
Noon
24 Rs
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END
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Earthward-moving flux ropes
Secondary islands and/or BBF flux ropes Slavin
et al., 2003
M. Shay simulations
in Ohtani et al., 2004
Transient Petschek-type reconnection Semenov et
al., 2005
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
P?V-2/3
Black 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
P?V-5/3
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Plasma injection