Download expansion phase

ESS 200C
Substorms
Lecture 14
• Soon after the auroral substorm
was discovered in 1964 the search
began to find the corresponding
changes in the magnetosphere
and the solar wind.
• The auroral activity is associated
with currents in the ionosphere
which create magnetic field
changes.
• Much of the effort in studying
substorms has been to define the
solar-wind coupling parameters
that can be used to predict the
strength of magnetic activity.
– Most approaches have treated the
problem as though the
magnetosphere was a
deterministic system driven by the
solar wind.
– Only about half of the variance can
be accounted for that way.
–The residual is related to discrete
events in which energy stored in the
magnetosphere is suddenly released the magnetospheric substorm.
• Magnetic
coordinates

–
B is the magnetic field vector, F
is the magnitude of the magnetic
field.
– X is northward, Y is eastward,
and Z completes a right handed
system toward the center of the
Earth.
– The magnitude of the vector
projected onto the horizontal
plane is called H. This projection
makes an angle D (for
declination) with positive from
north to the east. The dip angle I
(for inclination) is the angle that
the total field vector makes with
respect to the horizontal plane
and is positive for vectors below
the plane.
• On quiet days, every
midlatitude observatory
records a systematic
variation in each field
component.
– Stations at the same
magnetic latitude but at
different longitudes see the
same pattern delayed by the
Earth’s rotation.
– The pattern is symmetric
with respect to the magnetic
equator and noon
suggesting an ionospheric
current fixed with respect to
the Sun.
• The currents responsible for the
diurnal variation.
– Two cells of current circulate
around foci located at about 300
magnetic latitude.
– At the equator the currents flow
from dawn to dusk.This is called
the equatorial electrojet.
• The SQ currents are caused by
a dynamo in which electric
charges in the ionosphere move
across the Earth’s magnetic
field.
– The motion is driven by winds in
the ionosphere.
– The winds are driven primarily
by solar heating.
• Over the past two centuries over
500 magnetic observatories have
been established.
– Data from so many sources is
difficult to handle.
– Indices have been generated to
organize these observations.
• The primary sources of ground
magnetic disturbances during
substorms are the electrojets
and the substorm current wedge.
• The sources of the midlatitude
storm time variations (Dst) are
the magnetopause current, the
ring current and the partial ring
current.
• Magnetic perturbations in the H component observed by
auroral-zone observatories.
– Positive perturbations are produced by a concentrated current
(called an auroral elecrojet) flowing eastward. They are observed
by stations in the afternoon or evening.
– Negative perturbations are produced by a westward electrojet.
They are observed near and past midnight.
– These currents flow at ~120km altitude and are carried by auroral
particles.
– The positive and negative envelopes give the AU and AL indices.
• There are solar cycle variations
in geomagnetic activity.
– The top panel shows the AA
index, the first difference time
series of daily mean H at
midlatitudes.
– The bottom shows the sunspot
number.
– The pattern holds for both yearly
and monthly averages.
• There is an annual variation of
geomagnetic activity.
– There are two annual peaks in
the u1 index (the difference
between successive daily means
in the H component of a station
normalized so that the index has
the same distribution as the
sunspot number).
– The two peaks are in March and
October.
• Magnetic activity occurs preferentially when the IMF is
southward relative to the dipole axis.
– Activity increases with the size of the southward component.
– Russell and McPherron proposed that the semi-annual variation in
activity is controlled by the projection of the cross flow component
of the IMF onto the cross flow component of the Earth’s dipole
magnetic field.
– At the spring and fall equinoxes the Earth’s dipole axis makes the
largest possible angle, ~350 with respect to the ecliptic normal.
– At these times the IMF at the boundary will have a component
Bz   B sin 350
• Geomagnetic activity also varies with solar rotation. This
variation is closely related to magnetic storms and will be
discussed in the last lecture.
• Correlation analysis between the
auroral-electrojet index AE
(difference between the envelop
of positive -AU- and negative AL- magnetic perturbations at
auroral latitudes) and five solar
wind parameters (u, n, B, Bn, Bs)
– Bn is hourly average of the BzGSM
magnetic field when BzGSM>0.
– Bs is hourly average of the BzGSM
magnetic field when BzGSM<0.
• Activity peaks in Bs for the hour
prior to the hour when the activity
was measured.
• AL/v2 as a function of Bs (Bz<0)
and Bn (Bz>0). No dependence
on Bn but strong dependence on
Bs
• Many phenomena precede the
onset of the expansion phase in
the aurora.
– Weak positive and negative
signatures (called bays) are
observed in the H component at
auroral zone stations.
– Gradual increase of the size of
the polar cap.
• McPherron interpreted these
phenomena as the growth
phase of the substorm.
– Energy extracted from the solar
wind is stored in the
magnetosphere.
– The initial interval of slowly
growing AU and AL.
– The growth phase usually lasts
30 minutes to one hour.
– The magnetic perturbations
during the growth phase result
from increased ionospheric
currents.
• The expansion phase
corresponds to the release and
unloading of the stored energy.
• The recovery phase is the
return of the system to its
ground state.
• A new current system forms
during the substorm expansion
phase.
– The field aligned part of the
current has the sense of the
region 1 currents (away from the
Earth on the dusk side and
toward the Earth on the dawn
side).
– In the tail this current flows in the
sense to reduce the cross
magnetosphere current
(sometimes called current
disruption).
– In the ionosphere the current
fiows westward (the auroral
electrojet).
• The magnetic field changes at
midlatitudes corresponding to
this current system are
northward about midnight,
eastward and northward in the
local evening and westward and
northward in the morning.
• The “wedge” is typically about 700
wide.
• That similar changes are seen at
synchronous orbit indicates that
this system closes in the
magnetosphere.
• The best developed model of the
substorm sequence is called the
near-Earth neutral line model
(NENL).
•
(a) The initial (“ground”) state.
– Three types of field lines.
• Open with one end in the solar
wind and one in the Earth. Open
field lines form the polar cap.
• Closed with both ends closing in
the Earth.
• Interplanetary field lines with
both ends closing at the Sun.
– At night the boundary between
open and closed field lines
extends to a preexisting tail xline called the “distant” x-line
(not shown).
– The “gray” region of closed field
lines contains the plasma sheet.
• At the earthward boundary of the
plasma sheet the field rapidly
becomes dipolar.
– The inner edge of the plasma
sheet is roughly at 10RE
depending on the electric field.
– The plasmasphere and radiation
belts are earthward of the inner
edge of the plasma sheet.
•
(b) A southward turning of the IMF
initiates or increases dayside
reconnection.
– Magnetic flux from the Earth
connects to the IMF and is
transported over the polar caps
into the lobes.
– This causes increased
convection in the plasma sheet
but this is limited by the finite
conductivity of the ionosphere at
the foot of the field lines.
– The return flow in the
magnetosphere is unable to
return flux to the dayside as fast
as it is removed. The dayside
magnetopause is eroded.
• Dayside erosion increases
magnetopause flaring and
increases the pressure on the
boundary.
– The magnetosphere is
compressed until the increased
magnetic pressure in the lobes
balances the new pressure.
– This increases the pressure on the
plasma sheet and increased flow
toward the Earth causes the
plasma sheet to thin.
• Increased drag on the magnetotail
caused by newly opened field lines
is balanced by the tail current
moving earthward. This causes
even more thinning of the plasma
sheet.
• The energy input from the solar
wind compared with the Joule
heating rate in the ionosphere.
– The energy input rate is -uxBz of
the solar wind times the assumed
7RE width of the reconnection
region on the magnetopause.
– The energy output rate is the sum
of the ring current energy injection
rate, the Joule heating rate and
auroral particle energy flux.
• The Joule heating rate starts to
increase gradually during the
growth phase but increases
sharply about one hour later during
the expansion phase.
• Some time during the late growth phase the vertical component
of the magnetic field across the plasma sheet becomes very
small and reconnection begins on closed field lines in the nearEarth plasma sheet.
– The reconnection is slow at first.
– As closed field lines are cut they reconnect to form a magnetic O
region called a plasmoid (technically a magnetic flux rope).
– This stage of the substorm continues until the last closed field line
is severed by the reconnection process.
– The reconnection rate increases during the late growth phase.
• When the last closed field line is severed the reconnection rate
becomes explosive. This is the onset of the expansion phase of
the substorm.
– The current “wedge” is thought to occur at this time.
– The energy dissipation increases.
– 20%-30% of the open magnetic flux stored in the tail lobes is
rapidly reconnected.
– This is the principal energy conversion process during substorms.
• The severed plasmoid leaves the magnetotail.
• If the reconnection fails to reach the lobe field lines the
disturbance is quenched. This is called a pseudobreakup.
• The reconnection of open field lines also forms closed field lines
earthward of the X-line.
• Eventually the balance of forces in the plasma sheet changes and the
X-line begins to move tailward.
• Earthward of the X-line the plasma sheet thickens and strong
earthward flows are observed.
• As the X-line moves toward its distant location, the currents and aurora
begin to die at the lower edge of the auroral bulge. This is the
beginning of the recovery phase.
• In time all the disturbances die away, the substorm is over, and the
magnetosphere returns to its ground state.
• All of the changes in the
magnetospheric configuration are
coupled.
– In recent years global
magnetohydrodynamic
simulations of the solar wind,
magnetosphere, ionosphere
system have been used to selfconsistently model the substorm
process.
• Magnetic field lines crossing the
equator following a southward
turning of the IMF.
– a.) At 60 minutes after the
southward turning the plasma
sheet is thinning.
– b.) At ~75 minutes reconnection
has started on closed plasma
sheet field lines.
– c.) At ~90 minutes reconnection
has started on lobe field lines
– d.) At ~105 minutes new IMF field
lines drape over the plasmoid.
The combination of magnetic
tension and pressure gradients
move the plasmoid tailward
• The NENL model has been quite
successful in reproducing the
observations from space.
– There is considerable evidence
for dayside reconnection and the
enhancement of the magnetic
field in the tail lobes.
– The earthward and tailward
flows from the reconnection
have been observed.
 However rather than being a
global phenomenon these
frequently show evidence of
being localized in space and
time.
 The most probable location of
the NENL is -20RE>x>-30RE
 The mean time of start of
earthward and tailward flow is
the substorm onset.
• The plasmoids usually in the
form of magnetic flux ropes have
been observed moving tailward.
– Flux rope plasmoids are formed
when reconnection occurs in the
presence of an IMF BY.
– Currents flow along the axis of
the flux rope. In some cases
they
be force free
 may

( J  B  0 ).
– A tailward moving flux rope will
have a characteristic bipolar
signature in Bz (an increase
followed by a decrease)
– The fine dashed line shows the
results from a theoretical flux
rope moving tailward.
– The heavy dashed line shows
the results from a flux rope
simulation.
• There is evidence that the substorm begins near the Earth (7RE 8RE) rather than in the near-Earth plasma sheet.
– The expansion phase begins with brightening of the equator-most
auroral arc.
– In both empirical models of the Earth’s magnetic field and MHD
simulations the aurora map earthward of the near-Earth plasma
sheet.
• Some studies using data from spacecraft orbiting earthward of
the plasma sheet seem to indicate that expansion phase changes
occur first there then move down the tail.
– Near 8.8RE oscillations with periods of about 13s were found in
components of the magnetic field near substorm onset.
– Ion fluxes (100 keV to 1 MeV) increased by an order of magnitude.
– Within 4 min of the fluctuations the magnetic field became dipolar.
– Either the neutral line had to be very close to the site of the
observations or the substorm was starting earthward of the NENL.
• This led to the development of an alternate model – the current
disruption model.
• In both models during the growth phase a thin current sheet
develops in the inner magnetosphere.
• As the current sheet thins, the ions become non-adiabatic and
begin to stream across the current sheet in serpentine orbits.
• The streaming ions interact with adiabatic electrons drifting in
the opposite direction producing plasma waves (called lower
hybrid waves).
• At the same time a density gradient on the boundary of the
plasma sheet drives the lower-hybrid-drift instability. The two
types of waves cause the plasma sheet to act resistively
thereby disrupting the cross tail current.
• This gives the “current wedge” which launches a rarefaction
wave that propagates down the tail. This causes reconnection
deeper in the tail.
• Supporters of the near-Earth neutral line model have modified it
to account for the current distruption occurring near the Earth
and the near-Earth mapping of the aurora.
– The near-Earth neutral line causes high speed flows to move
toward the Earth.
– When these encounter the strong field region near the Earth they
slow down.
– Parallel currents in the region where the x-component of flow
decreases form the current “wedge”.
– The initial brightening of the aurora occurs at the upward current
associated with the current wedge.
• In both the current sheet disruption model and the near-Earth
neutral line model the thin current sheet becomes unstable.
• Recently Lyons has suggested that the onset of substorms is
triggered by changes in the interplanetary planetary magnetic
field.
– Northward turnings of the IMF are thought to reduce
magnetospheric convection.
– Many substorm onsets occur during reductions in the earthward
convection.
– The northward turning does not have to be complete (ie. the IMF
does not have to become northward).
– One study indicates that approximately 50% of all substorms may
be triggered.
• Auroral substorm in the ultraviolet – VIS
camera on the Polar Satellite