ESS 200CSubstormsSubstormsLecture 16• 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.Br• 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 300magnetic 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 200 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 electrojetsand the substorm current wedge.• The sources of the midlatitudestorm 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 u1index (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, ~350with 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 lecture after next.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)–Bnis hourly average of the BzGSMmagnetic field when BzGSM>0.–Bsis hourly average of the BzGSMmagnetic 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 Bnbut 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 phasecorresponds 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