ESS 200C

SubstormsSubstorms

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– 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.

B

• 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

• Geomagnetic activity also varies with solar rotation. This variation is closely related to magnetic storms and will be discussed in the last lecture.

035sinBBz

• 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 x-line 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 near-Earth 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 self-consistently 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 may be force free ( ).

– 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.

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• 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