Download ring current - Laboratory for Atmospheric and Space Physics

Aerospace Environment
ASEN-5335
•
•
•
Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)
Contact info: e-mail: [email protected] (preferred)
phone: 2-3514, or 5-0523, fax: 2-6444,
website: http://lasp.colorado.edu/~lix
Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534; before
and after class Tue and Thu.
TA’s office hours: 3:15-5:15 pm Wed at ECAE 166
•
•
•
Read Chapter 4&5 and class notes
HW4 due 3/13, Thursday
Mid-Term, 3/20, close book.
•
ASEN 5335 Aerospace Environment -- The Magnetosphere
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The Terrestrial Magnetosphere
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•
The terrestrial magnetosphere comprises the region of space where the properties of naturally
occurring ionized gases are controlled by the presence of Earth’s magnetic field.
This broad definition means that the terrestrial magnetosphere extends from the bottom of the
ionosphere to more than ten RE in the sunward direction and to several hundred RE in the antisunward direction.
 The magnetosphere is formed as a result of the interaction of the solar wind with the
intrinsic magnetic field. Other planets’ magnetospheres are formed in the same way.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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As the magnetized solar wind flows past the Earth, the plasma interacts with
Earth’s magnetic field and confines the field to a cavity, the magnetosphere.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Planets in Solar System
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Planet orbits in Solar System
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Properties of Some Solar System Objects
ASEN 5335 Aerospace Environment -- The Magnetosphere
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The Bow Shock and the Magnetopause
 The Earth’s magnetic field can be envisioned as
a stationary object placed into the high-speed
magnetized plasma flow: the solar wind, which is
supersonic and superalfvenic.
 We can expect the development of a standing
shock wave ahead of the Earth.
 Such a shock is necessary to slow down the
solar wind so that the flow can diverted around the
dipole.
 Gas dynamics was used to model the bow
shock in early times.
 For the simplest case, perfect gas, onedimentional and steady-state case, we have the
following:
d(nu)/dx=0
mnu du/dx + dp/dx =0
(3/2)u dp/dx + (5/2)p du/dx =0
Where m is the particle mass and n(x), u(x), and
p(x) represent the particle number density, flow
velocity , and pressure, respectively.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Earth’s Bow Shock
 Since the solar wind is much faster than the Alfvenic speed, the intermediate shock solution is
not relavant, and we need to look at compressive fast mode shock solution.
 In the solar wind both the sonic and Alfvenic Mach numbers are large. Therefore the shock
jump conditions can be greatly simplified. Such as the effects of the upstream thermal pressure
and magnetic field can be neglected altogether. The following shows the jumping conditions:
These jumping conditions do
not give its location. The
location of the bow shock (BS)
is basically determined by the
shape and size of the
magnetospheric “obstacle”, the
magnetopause (MP). The
region between the BS and MP
is called the magnetosheath.
What determines the location of
the MP, BS (or the thickness of
the magnetosheath)?
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Example of Satellite Observations of
the Bow Shock
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Location of MP and BS
 The location of the bow shock is basically
determined by the shape and size of the
magnetospheric “obstacle”, the MP and also
the Mach number. The actual nonlinear
processes involved are still a on going
research.
 The location of the subsolar point of MP is
determined by the pressure balance,
basically between solar wind dynamic
pressure and Earth’s magnetic pressure. For
average solar wind conditions, one gets RMP
~ 10 RE .
 The ratio (RBS – RMP)/RMP has been found
empirically to be 1.1 (SW/2), such that RBS
~ 1.275 RMP ~ 13 RE.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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The neutral points are the only points that connect the earth's surface
to the magnetopause.
The neutral points are
regions of interest since
this is where solar wind
particles (from the
magnetosheath) can enter
the magnetosphere without
having to cross field lines.
There is experimental
evidence that this does
happen. Particles with
energies typical of the
sheath are observed over
some 5° of latitude around
77°, and over 8 hours of
local time around noon.
These regions, being more extended than an idealized point, are
called the polar cusps or polar clefts.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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The Magnetotail & Plasma Sheet
Field lines emerging from the polar regions are swept back, away
from the Sun; some of these would have connected on the dayside
in a dipole field. These field lines constitute the magnetotail.
Magnetosheath
Polar
Cusp
The main distinguishing
feature of the plasma sheet
is that it consists of hot
(keV) particles.
Bow
Shock
Radiation
Belts
The
plasma
contained
between the lobes, mostly
on closed field lines, is
called the plasma sheet.
Magnetopause
ASEN 5335 Aerospace Environment -- The Magnetosphere
Plasma sheet is a mixture of
particles originating in the
solar
wind
(H+)
and
ionosphere (O+).
15
The boundary of the plasma sheet is determined by a balance between
the magnetic pressure of the tail lobes and the kinetic pressure of the
plasma sheet plasma:
BT2
nkT ~
2 o
where BT = tail field
intensity outside
the plasma sheet.
The tail region is highly dynamic, and is only reasonably represented
by the above description during quiescent periods.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Magnetic Connection
In the correct geometry, magnetic field lines can interconnect.
In general, the field lines must be anti-parallel and there must be
an electric field as shown.
The electric field causes
plasma and field lines to
drift into the merging
region (black arrows
pointing along Z)
After merging, the B
field lines have a
component directed
along Z, causing a drift
perpendicular to the
new B direction and to
the E field. The new
flow is along X as shown.
Magnetic connection occurs in the
magnetotail and at the dayside
magnetopause.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Magnetospheric Circulation
P
Sq
Historically, the
current system (as inferred from ground
magnetometers) led to the first speculations about circulation
patterns in the magnetosphere, and theories about solar wind magnetosphere interactions.
Given that e- are
bound to field lines
in
the
E-region
(where ionospheric
current flows) and
ions are stationary
by comparison, the
following motions of
field lines (electrons,
opposite
to
conventional current
flow) are inferred
from the P current
S
patterns. q
Motion of
“feet” of field lines
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Magnetospheric Circulation
The generally accepted explanation involves magnetic connection or
merging (between the terrestrial magnetic field and the IMF) on the
dayside and reconnection on the night side.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Recall from earlier notes that in a plasma an applied E-field results in
the plasma drift
E B
VP 
2
B
E and Vp are equivalent in highly conducting plasmas (frozen-in)
such as the magnetosphere and solar wind.
E
B
Vp
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Convection Electric Field
 The ultimate driver of magnetospheric
circulation is the momentum of the solar wind.
 Applying the frozen-in-flux concept, an observer
on Earth detects an electric field of E = -VP x B.
 If the solar wind cannot penetrate the
magnetopause, the convection electric field is
also excluded from the magnetosphere.
 The convection electric field in the solar wind is
about 2 mV/m (assuming u=400km/s and B=5
nT), corresponding to about a 640 kV potential
drop across the geomagnetic tail (assuming a tail
diameter of 50 RE). The observed electric
potential drop across the region of open field
lines in the ionosphere is about 10 times smaller.
This implies that about 10% of the interplanetary
magnetic flux that impacts the geometric cross
section of the magnetosphere reconnects with
the geomagnetic field.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Magnetospheric Circulation - Equatorial Plane View
+ +
+Dawn+
+
+
E
E  
- -
Dusk
-
-
-
-
The cross-cap potential is often used as a measure of intensity of solarwind/magnetosphere interaction, and can reach values of order 200 kV during intense
magnetic
disturbances.
ASEN
5335 Aerospace
Environment -- The Magnetosphere
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ASEN 5335 Aerospace Environment -- The Magnetosphere
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Polar Cap Electric Potential - Southward IMF
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Polar Cap Electric Potential - Northward IMF
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Some of the sunward-convecting particles precipitate into the
upper atmosphere and produce the aurora.
recon
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Aerospace Environment
ASEN-5335
•
•
•
Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)
Contact info: e-mail: [email protected] (preferred)
phone: 2-3514, or 5-0523, fax: 2-6444,
website: http://lasp.colorado.edu/~lix
Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534; before
and after class Tue and Thu.
TA’s office hours: 3:15-5:15 pm Wed at ECAE 166
•
•
•
•
Read Chapter 5 and class notes
HW4 due today
Quiz-5, 3/18 (Tue), close book.
Mid-Term, 3/20 (Thu), close book.
•
ASEN 5335 Aerospace Environment -- The Magnetosphere
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PARTICLES IN THE MAGNETOSPHERE
The main particle populations are:
-- plasmasphere
-- ring current
-- radiation belts
-- plasma sheet
-- boundary layers (magnetosheath, mantle)
We have discussed the radiation belts extensively, and the
plasma sheet to some extent. We will return to the plasma sheet
when discussing magnetic storms.
The plasmasphere represents the relatively cold
ionospheric plasma (~ .3 eV or T ~ 2000 K) which is co-rotating with
the earth (frictional coupling).
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Principal Plasma Populations in Earth’s Magnetosphere
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Main Zones of Magnetospheric Plasma Precipitation
Boundary between
Open field lines at
High latitudes and
Closed field lines at
Low latitudes
ASEN 5335 Aerospace Environment -- The Magnetosphere
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The Plasmapause
The outer boundary of the plasmasphere, at about 4 RE, is where
the plasma density undergoes a sudden drop.
This is the
plasmapause.
Ring Current
However, the plasmapause
boundary is very dynamic,
and varies between about
3 to 6 RE, sometimes
getting as low as 2 RE.
Although not depicted as
such in the previous
figure, note that the
plasmasphere
overlaps
with a considerable part of
the radiation belt region.
However, these represent
two
entirely
different
particle
(energy)
populations.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Plasmapause Boundary
The co-rotating plasmasphere sets up a "co-rotation" electric field:
E   R   B
Outside the plasmapause the plasma is not co-rotating, and the
circulation there is determined by the cross-tail potential.
Essentially, the plasmapause represents the boundary where
these two electric fields are of the same order:
BE
E T ~ 3 LR E 
L
where BE = equatorial magnetic flux density at the surface, L =
distance in RE, and RE = radius of earth.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Putting in numbers,
14.4
ET ~ 2
L
mVm-1
~ 1 mVm-1 at 4 RE
Put another way, the plasmapause represents the boundary
between co-rotating plasmas and plasmas strongly influenced by the
solar wind interaction (see following figure):
Viewed this way, one expects intensification of the outer
magnetospheric circulation to lead to a contraction of the
plasmasphere (inward movement of the plasmapause). This indeed
happens (see subsequent figures).
In fact, it is thought that the intensified outer circulation
leads to a peeling off of outer layers of the plasmasphere, which are
then lost as detached plasma chunks in the magnetotail and solar
wind.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Schematic view of the plasmapause in relation
to plasma convection in the equatorial plane
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Satellite observations of ion density, showing
the plasmapause at several Kp levels
L
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Flow patterns for cross-tail fields of 0.2
and 0.6 mV/m
For 0.6 mVm-1, the
magnetosphere
circulation “intrudes”
upon the plasmasphere.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Detaching of plasma due to changing flow
patterns during a magnetic storm
ASEN 5335 Aerospace Environment -- The Magnetosphere
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“Filling In” of Plasmasphere
With the decay of magnetic activity, the magnetospheric
circulation and electric fields return to their previous state but
now the outer tubes of magnetic flux are devoid of plasma.
These gradually refill from the ionosphere over a period of days.
The rate of filling is determined by the diffusion speed of
protons (formed in the upper ionosphere by charge exchange
between hydrogen atoms and oxygen ions) coming up along the
field, and by the volume of the flux tube which varies as L4(?). It
therefore takes much longer to refill tubes originating at higher
latitude.
Observations of the filling are shown in the following figure.
Since active periods may occur every few days there will be times
when the outer tubes are never full and the plasmasphere has
some degree of depletion.


O H H O
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“Filling In” of Plasmasphere
Refilling of plasmasphere after a storm, 18-19 June 1965
ASEN 5335 Aerospace Environment -- The Magnetosphere
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BOUNDARY LAYERS AND PARTICLE TRANSFER
TO PLASMA SHEET
Solar wind particles find their way from the magnetosheath
into the cusp region. There is experimental evidence for this entry,
in that particles with characteristic "magnetosheath energy" (i.e.,
less than 1 keV) have been observed over a limited region centered
around 77° magnetic latitude and noon (see following figure).
Such particles on newly-merged field lines flow down
towards the earth, mirror there, and then return to find themselves
on a field line sweeping back towards the tail. These particles form a
particle population known as the "plasma mantle" (see following
figures).
At many (~100) RE, these particles are swept into the plasma
sheet. Another closer (~ 50 RE) source of plasma sheet particles is
the polar wind emanating from the ionosphere at high latitudes.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Magnetosheath
Entry
Layer
Details of the
Cusp Region
Low-Latitude Boundary Layer
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Note: since ~1028-1029 particles/s impact the dayside
magnetopause, and ~ 1026 particles/s are estimated to enter the
plasma sheet, only 1% efficiency of this process is required.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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1. The AE-8 and AP-8 radiation belt models are used by satellite designers to
estimate mission lifetime particle fluxes. These models can be run on-line at
http://nssdc.gsfc.nasa.gov/space/model/models/trap.html
The input parameters are energy range, L-value and B/Bo where Bo is the magnetic
field strength at the magnetic equator (minimum value).
a. Estimate the flux of electrons in particles/cm2/MeV between energies of 1 and 2
MeV that would be experienced by a geostationary satellite over the period of one
day during solar maximum conditions. Do this for two cases: B/Bo={1, 2}.
b. Assuming isotropy, estimate the flux of electrons in units of particles/cm2/s/sr/keV
between energies of 200 and 300 keV that would be experienced by a satellite in
geostationary orbit during solar maximum conditions.
Note: The differential flux gives the flux between the energy values given. Integral
flux gives the total flux greater than the each energy value input.
ASEN-5335 Aerospace Environment Homework Assignment #5: Radiation Belts
Printable Version
1. The AE-8 and AP-8 radiation belt models are used by satellite designers to
estimate mission lifetime particle fluxes. These models can be run on-line at
http://nssdc.gsfc.nasa.gov/space/model/models/trap.html
The input parameters are energy range, L-value and B/Bo where Bo is the magnetic
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field strength at the magnetic equator (minimum value).
ASEN 5335 Aerospace Environment -- The Magnetosphere
Particle Flow in the Merging - Reconnection - Convection Process
“Dipolarization” of the B-field
During the return
flow, the particles are
energized by the
down-dusk electric
field and their first
adiabatic remains as
an invariant,
2
1 mv 
= const As particles convect towards the earth, B increases, therefore

2 B
the particle energies increase. The energy comes from the E-field.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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The Phases of the Aurora
Quiet
Expansion
Growth
Maximum Area
ASEN 5335 Aerospace Environment -- The Magnetosphere
Onset
49
Recovery
Early Auroral Studies
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Producing the Aurora
Auroras are produced by electrons and
protons striking Earth’s atmosphere.
When oxygen and nitrogen atoms are
hit by these energetic particles, they
become excited. As they relax to their
original state, they emit light of a
characteristic color
Green = Oxygen
Red = Oxygen (lower energy electrons)
Blue = Nitrogen
Also emitted in UV and X-ray
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Magnetospheric Current
Systems
 Currents are produced by moving electrically
charged particles. In turn, current systems
generate magnetic fields. In the magnetosphere
there are five principal current systems that play
an important role in the formation of the
magnetosphere. (1) the magnetopause current,
(2) the tail current , (3) the ring current, (4) fieldaligned (Birkeland) currents and (5) ionospheric
currents.
 (1) The magnetopause current generates a magnetic field that “prevents” the terrestrial dipole
field from penetrating into the solar wind (vice versa). An enhanced MP current gives rise to an
increase of the Dst index.
 (2) The magnetic tail is formed by the highly stretched (“open”) magnetic field lines originating
from the polar ionosphere (called the polar cap). The tail magnetic field is naturally generated by a
current system, based on the Ampere’s law: xB=0 j.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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(Temerin and Li , 2002)
Prediction efficiency=91%
Linear correlation coeff.=0.95
ASEN 5335 Aerospace Environment -- The Magnetosphere
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MAGNETOSPHERIC CURRENTS
The combination of plasma and electric fields in the
magnetosphere allows electric current to flow.
Several current
systems have been identified:
• magnetopause current (A)
• Birkeland (field-aligned) currents
• tail current (B)
• ring current (C)
The strong deviations of the magnetosphere from a dipole shape are in fact due to the
first three (A, B, C) of the above current systems. These are now discussed in turn.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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MAGNETOPAUSE CURRENT
If we ignore any magnetic or electric field in the solar wind, the
origin of the magnetopause current and the corresponding
modifications of the magnetic field can be grossly understood as
follows:
Consider a small section of the dayside magnetopause with the
solar wind normal to it (see following figure). The ions are deflected
one way and the electrons the other, causing a current to flow
(consisting mostly of ions due to their greater penetration depth). The
current flow at the magnetopause is such that its magnetic field
cancels the geomagnetic field outside the boundary.
Similarly, earthward of the boundary the field strength is
doubled -- this is essentially the "compression" of the dayside
magnetosphere that we have alluded to before.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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Currents and Fields at the Magnetopause Boundary
ASEN 5335 Aerospace Environment -- The Magnetosphere
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It turns out that the force produced by this current (the so-called
Lorentz or J X B force) balances the momentum force of the solar
wind, which is another way of stating the "dynamic pressure" vs.
"magnetic pressure" balance we discussed before.
When the solar wind intensifies, the magnetopause current is
increased:
--
This further "compresses" the dayside magnetosphere;
--
The ground magnetic signature of this sudden current
increase associated with the compression is called a
"sudden impulse" (SI), or if it is connected with the
beginning of a storm, it is called a "sudden storm
commencement" (SSC).
ASEN 5335 Aerospace Environment -- The Magnetosphere
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TAIL CURRENT
The down-wind extension of the magnetosphere into a tail
indicates the presence of a current system as follows:
View from Earth
ASEN 5335 Aerospace Environment -- The Magnetosphere
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This is what keeps the dark-side magnetic field from assuming its
dipolar form. The energy to do this comes from the solar wind. The
magnetic flux in a current loop is
BT  o iT
Since BT ~ 20 nT, then iT ~ .016 A/m. Assming something reasonable
for the tail length, iTail ~ 108 A. For a cross-tail potential of 60 kV, the
power extracted from the solar wind ~ 6,000 GW !!
1
j=

0
3
The neutral sheet separates the northern
lobe from the southern lobe.
 B
2
B
 j =  B =  2 iˆ
o1
x 1
3
1
•
j
•
•
•
•
Current out of page
•
•
"neutral sheet" here refers to the magnetic
field, and the region where currents flow so
that reconnection is inhibited, similar to the
heliospheric current sheet.
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Energy is Imparted to the Plasma During Reconnection
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Ring Current and
Field-Aligned Current
 The ring current is primarily composed of
geomagnetically trapped or quasi-trapped 10 to
200 keV particles (H+ and O+) drifting around
the Earth, due to the gradient-curvature drift in
conjunction with ExB drift. The electric field is
directed from dawn from dusk (as we have
discussed earlier).
 The gradient-curvature drift is ~L2 while the
ExB drift is ~L3.
 The ring current is the main contributor to the
Dst index.
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RING CURRENT
Under magnetic storm conditions the magnetic field of the
earth at low latitudes may be depressed ~ 1-2% for a day or two (main
phase). This is mainly due to a westward ring current which we have
already discussed in relation to particle drift (gradient-curvature drift)
on curved field lines with the magnitude of B increasing towards the
earth.
Recall that the gradient drift depends on the particle
"magnetic dipole moment"
BB
vg  
2
eB
2
mv
1 

2 B
-- hence gradient drift is not important for "cold"
particles like those populating the ionosphere
and plasmasphere; these particles co-rotate.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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However, it is also true that it is not the energetic Van Allen particles
that are the main contributors to the ring current. The fluxes of
these particles are too small. the main ring current particles are H+
and O+ of 20 - 100 keV (see following figure).
The ring current is located between 3 and 6 RE, close to the
inner edge of the plasma sheet and and outer edge of the slot
region.
Where do the ring current particles come from?
----- magnetospheric convection, after reconnection,
charged particles are transported inward by the
dawn to dusk electric field. As they gain energies, their
gradient-curvature B drift will start to compete with the ExB drift.
Since E varies with time, some of the charged particle will be
trapped.
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Energy and Number of Protons in the Ring Current
at L = 4 during a Magnetic Storm
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Aerospace Environment
ASEN-5335
•
•
•
Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)
Contact info: e-mail: [email protected] (preferred)
phone: 2-3514, or 5-0523, fax: 2-6444,
website: http://lasp.colorado.edu/~lix
Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534; before
and after class Tue and Thu.
TA’s office hours: 3:15-5:15 pm Wed at ECAE 166
•
•
•
•
Read Chapters 1-5 and class notes.
Quiz-5, today, close book.
Mid-Term, 3/20 (Thu), close book.
HW5 due 4/3 (Thu).
•
ASEN 5335 Aerospace Environment -- The Magnetosphere
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1. The AE-8 and AP-8 radiation belt models are used by satellite designers to
estimate mission lifetime particle fluxes. These models can be run on-line at
http://nssdc.gsfc.nasa.gov/space/model/models/trap.html
The input parameters are energy range, L-value and B/Bo where Bo is the
magnetic field strength at the magnetic equator (minimum value).
a. Estimate the flux of electrons in particles/cm2/MeV between energies of 1 and 2
MeV that would be experienced by a geostationary satellite over the period of one
day during solar maximum conditions. Do this for two cases: B/Bo={1, 2}.
b.
Assuming isotropy, estimate the flux of electrons in units of
particles/cm2/s/sr/keV between energies of 200 and 300 keV that would be
experienced by a satellite in geostationary orbit during solar maximum
conditions.
Note: The differential flux gives the flux between the energy values given. Integral
flux gives the total flux greater than the each energy value input.
ASEN 5335 Aerospace Environment -- The Magnetosphere
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2. Daily plots of actual measured charged particle fluxes in geostationary orbit can
be obtained from the Los Alamos National Laboratory at
http://leadbelly.lanl.gov/lanl_ep_data/lanl_ep.html
Under the selection "Browse the Summary Plot Database", one can select a date
and request a plot of proton or electron flux from the Los Alamos satellites in
geostationary orbit on a particular day.
a.
Generate plots of the low-energy (30-300 KeV) electron fluxes
(particles/cm2/s/sr/KeV) that were measured in geostationary orbit on March 22,
1979 (this is the same magnetically disturbed day considered in HW #4).
b. What LANL satellites were available to provide these data? (international
satellite designations are of the form 1985-032).
c. What LANL instrument provided these data?
d. Comment on the behavior of the electron fluxes exhibited in the plots. What
features are noteworthy?
e. How do the measurements of the 200-300 KeV fluxes compare with those which
you estimated in question #1b?
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BIRKELAND (FIELD-ALIGNED) CURRENTS
Magnetometers carried on board satellites have detected persistent
perturbations of the earth's magnetic fields over the auroral zones which can
only be interpreted as resulting from currents flowing into and out of the
ionosphere.
The following figure shows the average locations of these currents for two
levels of magnetic disturbance as determined from measurements on the TRIAD
satellite.
The solar wind/magnetosphere interaction provides energy and momentum to
the magnetosphere system; the magnetospheric circulation is determined by
redistributing its plasma and fields in a way that allows for dissipation of this
energy. This dissipation occurs in the form of:
• energizing particles which give up their • dispelling blobs of plasma out the
energy to the neutral atmosphere;
magnetotail;
• developing a current system capable of
dissipating energy through ohmic losses;
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• transferring momentum
to the neutral atmosphere.
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Region 1 and Region 2 Current Systems
R2
R2
R2
R1
R1
R2
Quiet
Active
Current flow is also consistent with the requirement for dissipation of the energy
deposited into the magnetosphere by the solar wind; ohmic dissipation of currents in
the ionosphere is one way of doing this.
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Region 1 Current System
The magnetic field lines are highly conducting, and so it is natural that
the magnetosphere seeks some closure of current through an
ionospheric route. In fact, the so-called Region 1 currents are
necessary if we are to require the polar ionosphere to convect with the
magnetic field lines:
Sun
B
Dusk
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Dawn
JE  0
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The origin of the Region 2 currents is connected with a longitudinal
asymmetry in the ring current, also referred to as partial ring current.
The partial ring current must be closed, it must therefore discharge
current (into the conducting ionosphere) on the dusk sector and draw
current from the ionosphere in the dawn sector, as shown in the
following figure.
Connection
between
Ionospheric and
Magnetospheric
Currents
A variety of plasma processes also occur that induce electric
fields || to B; these fields accelerate energetic particles into and out
of the auroral region.
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Magnetic Storms and Substorms
Recall that coupling between the magnetosphere and solar wind is
considerably more efficient during southward IMF (Bz < 0) than
northward IMF (Bz > 0), simply because of the way that magnetic
merging occurs in the dayside magnetosphere. A reminder of this
process for Bz < 0 is shown on the following page.
During extended periods of northward IMF, the magnetosphere is
usually in a quiescent state, unless discontinuities in the solar wind
impulsively impact the magnetosphere. In this case there is sure to be
an impulsive response of some significance; but the exact scenario
through which this occurs is not very well established for Bz > 0. We
will concentrate on those "storm" events for which Bz < 0.
At a southward turning of the IMF, merging on the dayside, day/night
transfer of magnetic flux, and nightside reconnection are increased.
The system is no longer in equilibrium. Energy in the form of
magnetic flux builds up in the tail.
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Summary of a Magnetic Substorm Scenario
A substorm is a time period of enhanced energy input from the solar wind into the magnetosphere and its
subsequent dissipation in the magnetosphere-ionosphere system.
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Some of the sunward-convecting particles precipitate into the
upper atmosphere and produce the aurora.
recon
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A thinning and pinching of the plasma sheet occurs at the
beginning of a substorm
This neutral point in the tail
now occurs much closer to
earth. This buildup occurs
over about 1/2 - 1 hour, and is
called the initial or growth
phase of the substorm.
The substorm onset phase occurs
when the tail energy is released, and
marks the beginning of the substorm
expansion phase.
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After magnetic merging near ~100 RE then another merging
at 20-30 RE , a plasmoid is ejected out the magnetotail
Neutral line location under quiescent conditions ~1000 RE
Formation of
second
neutral
line and
accumulation
of magnetic
flux.
expel plasmoid
Return to pre-substorm conditions
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Particles are energized and injected into the
ring current, as reflected in the Dst index
Region 1 currents flow
between the plasma sheet
and the ionosphere
(100’s keV)
The flow of current through the ionospheric circuit is
consistent with the collapse of the tail at the neutral
point; the latter can only occur if the cross-tail current
is substantially reduced, as it is when being diverted
along the field lines into the ionosphere.
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• Concurrent with the flow of Region-1 currents into the ionosphere at
the onset of a substorm, currents and electric fields spread throughout
the conducting ionosphere; even at equatorial latitudes the signatures
of “penetration” electric fields are seen.
• After ~1 hour, the Region-2
currents close in the ionosphere
and set up a counter-potential
field, shielding the lower latitudes
from electro-dynamic coupling
from high latitudes.
• The magnetosphere can be
viewed as a voltage generator
imparting the same potential
across N. and S. polar regions;
the currents that flow depend
upon the conductivity of the
ionosphere.
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Auroral Potential Structure
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Electron Precipitation & the Aurora
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Major Magnetic Storms
A major magnetic storm generally follows an extended period of southward IMF.
However, although severe and nearly continuous substorms always accompany
major magnetic storms, a magnetic storm is not just a collection of magnetic
substorms.
The main distinguishing feature is the build up of an enhanced radiation environment
in the inner magnetosphere.
Consequences of Major Magnetic Storms
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Consequences of Major Magnetic Storms
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Consequences of Major Magnetic Storms
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Consequences of Major Magnetic Storms
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COMPARATIVE MAGNETOSPHERES
The outer planets -- the "gas giants" (Jupiter, Saturn, Uranus, Neptune) provide
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interesting
comparisons with the earth's magnetosphere.
The magnetospheres of the outer planets have been explored by the
Pioneer, Voyager, and Voyager 2 missions.
V2
V2
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Some of the factors accounting for differences between the
magnetospheres of the outer planets and that of earth include:
• Properties of the solar wind change as we move outward,
affecting the coupling of energy flux from the solar wind to
the magnetospheres.
• Magnetic fields of the outer planets are generally much
larger than earth's.
• Rapid rotation of the outer planets provide centrifugal
forces large compared to those of earth.
• Jupiter's moon Io represents a dominant source of plasma
to the Jovian magnetosphere.
• The outer planets have rings capable of absorbing trapped
radiation (especially in the case of Saturn).
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VARIATION IN SOLAR WIND PROPERTIES
The solar wind number density and the radial component of
the IMF decrease (inverse square) as distance from the sun increases.
(electron and ion temperatures of the solar wind plasma also decrease
with distance). The solar wind velocity, on the other hand, increases
(slightly) with distance.
The increased Mach numbers consistent with
the above imply stronger shock fronts at the
outer planets.
The weaker dynamic pressure suggests larger
magnetospheres for comparable B-values.
Weaker IMF suggests merging and reconnection
not so important.
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MAGNETOSPHERIC SIZE
The size of a magnetosphere is determined by the relative
importance of the planetary magnetic field strength and the solar
wind dynamic pressure.
The following table provides data on the relative sizes of the
magnetospheres (in terms of subsolar magnetopause distances) for
earth and the four outer planets:
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Relative magnetospheric sizes based on pressure balance
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URANUS
Axis tilted about 59 degrees and
offset from center of planet by
30% of its radius, placing magnetic
poles nearer the equator.
NEPTUNE
Field highly tilted: 47 degrees from
rotational axis and offset at least
0.55 radius from physical center
of planet.
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Jupiter and Io
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For Jupiter, the actual subsolar magnetopause is found to be over
100 RJ , as opposed to the value of 45 RJ expected from pressure
balance arguments. This is due to the effects of the plasma
spewing from its moon, Io.
Io
represents
a source of
particles in
the inner
magnetosphere
of Jupiter.
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Jupiter and Io
The largest entity in the Jovian system is the invisible force of the planet's magnetic field. Planetary
magnetic fields are created by the motion of fluid interiors. Fifteen thousand miles deep within
Jupiter's interior, hydrogen undergoes a dramatic change. At a pressure three million times that at
Earth's surface, and at temperatures exceeding 19,000 degrees Fahrenheit, the hydrogen changes
from molecular liquid to a state called liquid metallic hydrogen, an excellent electrical conductor.
The liquid metallic hydrogen and the planet's rapid rotation (9 hours 55 minutes) generate electric
currents that create Jupiter's magnetic field, which is more than 10 times stronger than that of Earth.
Jupiter's ring and moons are embedded in an intense radiation belt of electrons and ions trapped in
the magnetic field. The Jovian magnetosphere, which comprises these particles and fields, balloons
two or three million miles towards the Sun and tapers into a wind sock-shaped tail extending at least
465 million miles behind Jupiter as far as Saturn's orbit.
The relationship between the magnetic field and Io is unique. As the magnetosphere rotates with
Jupiter, it sweeps past Io, stripping away about a ton of matter per second and forming a torus—a
doughnut-shaped ring around Jupiter predominantly composed of electrified oxygen and sulfur
glowing in the ultraviolet.
As these heavy ions migrate outward, their pressure inflates the magnetosphere to more than twice its
expected size. Some of the more energetic ions fall into the atmosphere along the magnetic field to
create Jupiter's auroras.
As Io moves through Jupiter's magnetic field, it acts as an electrical generator, developing 400,000
volts across its diameter and generating an electrical current of three million amperes. The current
flows along the magnetic field to Jupiter's ionosphere.
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Jupiter’s Io
 The reason for the larger value of Juipter’s subsolar magnetopause is that
Io provides a significant source of mass to the Jovian magnetosphere,
which in turn accelerates this mass to high velocities because of the rapid
rotation of the planet. The resulting centrifugal force of the plasma pushes
out against the solar wind leading to a more distant standoff distance.
 The weak atmosphere of Io is maintained by continual volcanic eruptions.
Radiation belt particles collide with atoms/molecules in Io’s atmosphere,
knocking off atoms in a process called “sputtering”, so that atoms escape
from Io and form a cloud in orbit around the planet. Neutral atoms in the
cloud become ionized, and form a co-rotating torus about the planet
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Saturn's moon Titan is
also an important source
of particles, but much of
this mass is added near
Saturn's magnetopause,
and subsequently much
of it is lost to the solar
wind.
Saturn
The rings of Saturn are efficient absorbers of radiation-belt particles
and limit the buildup of intense fluxes of energetic particles in the
inner radiation zone of Saturn.
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The moons of the gas giants also provide sinks for
energetic (radiation belt) particles (absorption).
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HST Images of Aurora
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Jupiter Aurora
NASA and Prof. John Clarke (Boston University)
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HST-STIS
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Mercury
Mercury has a significant magnetic field, but no atmosphere;
interaction of the solar wind with Mercury forms a magnetosphere
that shares many gross characteristics with that of earth.
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Some Existing
Concepts
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Schematic of Mercurian Magnetosphere and
Anticipated Processes
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VENUS AND MARS
Venus and Mars have very weak magnetic fields, and the interaction
of the solar wind with these planets is governed by different
processes.
The interaction of a
solar wind with a
weakly-magnetized
planet is determined
by the planetary
ionosphere.
without an IMF, the flow
around the planet is
similar to that of fluid
flow around a sphere.
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With an IMF that is not steady (as in the solar wind), the interaction of
the changing IMF with the conducting ionosphere generates currents
that keep the field from penetrating through the body by generating a
canceling field (Faraday's law -- time changing magnetic field induces
currents in a conductor).
A steady IMF would
eventually diffuse
into the body (time
constant depends
on ionosphercic
conductivity)
A bow shock is expected, since the supersonic solar wind plasma is
diverted around the body.
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For Venus, pressure
balance considerations
similar to those for
a magnetosphere
apply, except with
the ionospheric
pressure replacing
the magnetic pressure.
An ionopause is formed
when the solar wind
pressure balances the
ionospheric pressure.
The subsolar ionopause
of Venus is quite low -about 350 km, as
compared with about
1,000 km at the terminator.
The bow shock is at about
2,000 km.
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Ionospheric ions can gyrate around the flowing IMF lines, and thus
be "picked up" by the solar wind.
Ion
pickup
Integrated over the 4.5 billion year lifetime of the planet, this
scavenging by the solar wind has probably had an impact on the
evolution of the atmospheres of Mars and Venus.
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As shown below, this ion removal process becomes evident in
comparisons of measured ionospheric profiles with those obtained
from models which do not take into account this process.
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