Transcript Document 7488768

Dissipation of the sectored heliospheric magnetic
field: a mechanism for the generation of ACRS
J. F. Drake
University of Maryland
M. Swisdak
M. Opher
University of Maryland
George Mason University
Anomalous Cosmic Rays (ACRs)
• 10-100MeV/nucleon particles
• Local interstellar medium neutrals
are ionized and picked-up deep in
the heliosphere and carried back
out to the heliospheric termination
shock (TS) where they are
accelerated
– The Voyager 1 & 2 spacecraft
observations revealed that the local
TS was not the source of the ACRs.
– Produced by the TS at the flanks of
the heliosphere (McComas and
Schwadron 2006)?
MHD model of the heliosphere
• Supersonic solar wind
becomes subsonic at the
termination shock
• The heliospheric toroidal
field B changes sign
across the heliospheric
current sheet
• The tilt of the solar
magnetic field with respect
to the rotation axis
generates a sectored
magnetic field
– Latitudinal extent depends
on solar cycle ~ 30
degrees
– Sectors are compressed
across the TS and as the
flow slows as it
approaches the heliopause
Burlaga et al 2005
Profiles along the stagnation line
MHD model
• Squeezing of plasma near the HP
causes  to drop
– Widely observed at the Earth’s
magnetospause
 min ≈ 0.5
– Magnetic energy dominates near the
heliopause
Collisionless reconnection of the sectored
heliospheric field
• The sectored field is stable
to reconnection upstream
of the TS because the
width of the current sheet
is much wider than c/ pi.
– Collisionless reconnection
is very weak
• The current layers
compress on their
approach to the heliopause
– Inevitably have the onset of
collisionless reconnection
– Dissipation of nearly all of
the magnetic energy
Jez
Proton temperature
• Within islands
Ti|| > Ti
• In exhaust regions
Ti|| < Ti
• Violate marginal
firehose condition
within the islands
– Self-consistency is
crucial
• Energetic electron
pressure in flares can
approach the
magnetic pressure
(Krucker et al 2009)
Ti||
Ti
Mirror and firehose
conditions
mirror
firehose
• Data from PIC simulations
– Each point corresponds to a
grid point in the simulation

• As reconnection strongly
onsets both the firehose
and mirror stability
boundaries are violated
• At late time the firehose
and mirror conditions act
as constraints
||
Wind data on solar wind turbulence
mirror
• Solar wind turbulence
bumps against the the
firehose and mirror
stability boundaries
• Very similar to the
reconnection
simulations
T/T||
firehose
– What are the
implications?
Bale et al 2009
||
Mirror and firehose
conditions
•
Within islands bump
against the firehose
condition
– This condition limits
island contraction
– Controls particle
spectra
•
•
In current layers and
along separatrices
bump against mirror
mode limit
Self-consistency is
crucial in exploring
particle acceleration
firehose
mirror
Electron and ion energy spectra
ions
• Both ions and electrons gain
energy
• Include 5% population of pickup
particles to simulate the
production of ACRs
• A key feature is that the rate of
energy gain of particles
increases with energy
d

dt
 first order Fermi

electrons
acrs
Distribution of most energetic ions
• The most enegetic ions are
located in regions of island
merging which leads to
contraction of a very large
island
• The particles circulate in the
contracting islands and gain
energy as they reflect from
the ends of the island
– Energetic ions are
accelerated through the
contracting island
mechanism
– A first order Fermi process
d P
dt
~ 2 P
cA
Lx
CAx
1-D Model equations
•
Rate of energy gain: first order Fermi
dv 1  4p 
vÝ

1 2
v
dt  h  B 
1/ 2
cA
h 
Lw

•

1
Model equation for the omnidirectional distribution function
F(v,t) = 4 v2f (v,t)
F 
1
 vÝF   F  F0 (v)
t v
L
•
cA
L 
L
Above the source energy this is an equidimensional equation
 powerlaw solutions
1
Distributions and spectral indices
• Exact steady state solutions for F(v)
v
F(v)   1v   dss 1F0 (s)
0
• Spectral index
1/ 2

 4p0   1 
(  1)1 2


B   3
h

L
• Heliopause limit h << L
  3  0

 spectral index controlled by marginal firehose condition
Implications for ACRs
• Squeezing of plasma near the HP
causes  to drop
 0 ≈ 0.5
• For ACRs
F ~ -1.75
• The minor species have the same
form when written on a per nucleon
basis
• Background plasma also acts like a
minor species
f ~ v -5.5
• Is the approach to marginal firehose
the source of the Fisk/Gloeckler v-5
distributions?
MHD model
Conclusions
• The sectored heliospheric magnetic field compresses and
increases in strength as it approaches the heliopause
–  falls below unity
– Collisionless reconnection inevitably onsets and dissipates the
sectored field energy
• large reservoir of energy
• Preferential heating of pickup particles
• Efficient heating of interstellar pickup ions through a first
order Fermi process during the contraction of reconnecting
magnetic islands
– Most of the magnetic energy goes into the ACRs
– Balance of contraction drive and convective loss yields powerlaw
solutions
– Spectral indices are controlled by the approach to firehose stability
• Minority ions have similar spectra to the main He and H
• Background protons are also strongly heated and have
spectra close to the v-5 spectrum seen by Fisk/Gloeckler
Universal super-Alfvenic ion spectrum in the
quiet solar wind
103
6
10
10-1
quiet time
tails
Fisk and Gloeckler,
2006
4.23 AU
94 AU
10-3
10
H+
SWICS
1
3
Phase Space Density (s /km )
Core pickup protons
f(w) = fow -5
-5
*FWtail
*tail+SW
SW distribution
1FW
FW
<FW>mean
FWnet
FWbkg
sum core+tail quiet
FWPI up
FW -26day to TS LECP
FW -20day to TS LECP
FW -26day to TS LECP
FW -20day to TS LECP
FW(Vr broadened)
Tail with cutoff
(in solar wind frame)
10-7
Solar wind protons
1 AU
10-9
10
ULEIS
-11
1
10
W
100
Ion Speed/Solar W ind Speed
<R> = 4.86 AU
• Proton spectra of the form f  v-5 are observed throughout the heliosphere
tail retail 2:40:15 PM 1/22/06