Presentation on theme: "COSPAR WORKSHOP ON CAPACITY DEVELOPMENT May 2004 – Beijing, China: SUBSTORMS! Presentation by: Robert L. McPherron Institute of Geophysics and Planetary."— Presentation transcript:
COSPAR WORKSHOP ON CAPACITY DEVELOPMENT May 2004 – Beijing, China: SUBSTORMS! Presentation by: Robert L. McPherron Institute of Geophysics and Planetary Physics and Department of Earth and Space Science University of California Los Angeles May 6, 2004
Importance of Substorm Timing This recent paper concludes reconnection can not be the cause of a substorm! A more appropriate conclusion would have been that the authors believe that the start of the expansion phase is not due to reconnection It is undeniable that reconnection extracts energy from the solar wind and is required to release it from the tail lobes Timing analysis is not simple but the event described here suggests that reconnection was the likely cause of this particular substorm onset Finally, a simple timing analysis of specific substorm onset time features suggests that near-Earth reconnection cannot occur before current disruption and therefore cannot be the cause of substorm.
The Auroral Oval from Space The aurora occurs in a narrow band circling the magnetic pole called the auroral oval The dayside is obvious from UV dayglow on the left side of the figure, midnight is at the east coast of the US Aurora is caused by charged particles hitting the atmosphere and exciting electrons to higher energy levels which decay producing light
DEFINITIONS OF A SUBSTORM A magnetic storm consists of sporadic and intermittent polar disturbances, the lifetimes being usually one or more hours. These I call polar substorms. [Chapman, 1962] The sequence of auroral events over the entire polar region during the passage from auroral quiet through the various active phases to subsequent calm is called an auroral substorm: it coincides with a magnetic (DP) substorm, with which it has some close relationships. [Akasofu, 1964]. To generalize the concept of the auroral substorm to include the worldwide disturbance characteristics and to emphasize the importance of the magnetosphere in auroral zone observations, we have suggested the term, magnetospheric substorm. [Coroniti, McPherron and Parks, 1968] A magnetospheric substorm is a transient process initiated on the night side of the Earth in which a significant amount of energy derived from the solar wind-magnetosphere interaction is deposited in the auroral ionosphere and magnetosphere. [Rostoker et al., 1980]
Magnetic Disturbances during a Polar Magnetic Substorm A stack plot of N-S magnetic field variations from east to west across Canada (midnight from 4 – 8 UT) Positive bays (perturbations) occur from dusk to midnight Negative bays occur from midnight to dawn The disturbances begin gradually and then are suddenly intensified
Ionospheric Currents in a Polar Magnetic Substorm The polar magnetic substorm is caused by two current systems (DP-2 and DP-1) DP-2 consists of two electrojets (east and west) flowing towards midnight DP-1 is a current system centered at local midnight that flows within the region of bright aurora
Satellite Image of Auroral Expansion The aurora during a substorm usually begins as a small spot close to midnight With time the aurora expands westward and poleward The DP-1 current system flows through the region of bright aurora
Akasofu Auroral Substorm Akasofu used all sky camera data to describe the local time development of aurora during a polar substorm He divided the substorm into two phases: expansion and recovery He identified a number of auroral features including: –Sudden brightening –Formation of westward traveling surge –The auroral bulge –Eastward drifting omega bands –Pulsation patches
Why is There a Tail? Viscous Interaction Dynamic pressure produces normal stress on the magnetosphere boundary and creates distorted field lines that are closer to the Earth than those of a dipole The existence of stretched field line requires tangential stress, or a force parallel to the boundary In ordinary fluid flow tangential stress (or drag as sometimes called) is caused by the viscous interaction Particles are scattered from the moving fluid into the stationary fluid adding momentum, i.e. exerting a force on the boundary A flowing boundary layer is produced In a collisionless plasma particle scattering is not very efficient, but surface waves and wave transmission are also effective
MAGNETIC RECONNECTION (Dungey, PRL, 6(2), 47, 1961) Resistivity develops at subsolar point Solar wind field merges with earths field Solar wind transports flux over poles Open field reconnects at distant x-line Magnetic flux returns to day side Open field lines continue in solar wind Solar wind electric field is projected to earths poles Ionospheric plasma drifts in response Pedersen and Hall currents flow in ionosphere E V B
SOLAR WIND TRANSPORT OF DAYSIDE MAGNETIC FLUX (Cowley, Rev. Geophys., 20(3), 531, 1982) Two fundamental processes couple solar wind to magnetosphere Reconnection at subsolar point opens dipole field lines and moves flux over the poles Viscous interaction moves closed field lines along the flanks Reconnection produces stronger convection when IMF is south
Ionospheric Projection of Magnetospheric Convection What happens at the feet of the field lines? They also move in the same direction as the equatorial crossing The insulating atmosphere allows the ionospheric field lines to appear to slip relative to the remainder of the field line in the neutral atmosphere and Earth Thus field lines undergoing reconnection move from the x-line at noon subsolar point, across the polar cap to the nightside x-line where they reconnect, and then return at low latitudes to the dayside Closed field lines in the low latitude boundary layer (llbl) also move backward over the edges of the polar cap and return at higher latitudes
IONOSPHERIC CONVECTION The ionospheric plasma flows along equipotentials of the ionospheric electric field The flow is from noon to midnight across the polar cap then east and west along the auroral oval back to the dayside
Flux Balance in Convection It is important to consider the reservoirs of magnetic flux that exist in a dipole field These include the dayside magnetosphere (closed field lines), the tail lobes (open field lines), and the nightside plasma sheet (closed field lines) In general it is unlikely that the rate of change of flux in all regions are balanced These rates depend on the merging rate M, the transport rate T, the reconnection rate R, and the return rate R A steady state tail occurs when all rates are equal
Field Aligned Currents Related to Magnetospheric Convection The low latitude boundary layer moves tailward just inside the magnetopause The electric field within the dawn side layer points outward Just inside the boundary layer the plasma flows Sunward so the electric field is inward Magnetic field lines map the magnetospheric electric fields to the ionosphere On the dawn side the E field point poleward across the polar cap and equatorward The electric fields drive currents in the ionosphere that diverge at the point where the inner edge of the boundary layer maps The current divergence is fed by a field-aligned current called the Region 1 current
WHAT CAUSES A SUBSTORM? IMF - Bz and RECONNECTION! Use 30 years of hourly AE and IMF Bz For each 1 nT bin of Bz create the cumulative probability distribution for corresponding AE index Display contour map of the probability of observing AE exceeding a given value as a function of Bz The probability of observing large values of AE decreases very rapidly as Bz changes to positive values The median value of AE for Bz > 0 is less than 100, the background level of AE from Sq variations in measurements
WHAT ARE THE PHASES OF A SUBSTORM? Start of Substorm - Southward turning of the IMF at dayside magnetopause Growth Phase - Enhanced solar wind coupling and energy storage in tail with ionospheric manifestations of enhanced convection Pseudo Breakup - Release of small amount of stored energy Substorm Onset - Beginning of expansion phase Expansion Phase -Release of stored energy into radiation belts (ring current), particle precipitation (aurora), Joule heating (bay activity), plasmoids (magnetic bubble) Intensification - Renewal of substorm expansion that increases disturbed region and strength of disturbances Recovery Phase - Reestablishment of quiet conditions and regular structure after IMF has turned northward End of Substorm - Electrojet currents fade into background
A Keogram Showing Growth and Expansion This figure shows a keogram plotted from a meridian scanning photometer on the meridian of auroral breakup. The four Pi 2 onsets are shown by the vertical dashed lines. From the beginning of the data until 0408 Ut the region of auroral luminosity is moving southward consistent with an expansion of the polar cap related to the storage of flux in the tail lobes. During this expansion there were several intensifications of the aurora visible on the Churchill meridian. The 0321, 0339 and 0352 UT events suggest pseudo breakups that did not penetrate the poleward boundary of the auroral oval. The local expansion onset agrees with the Image auroral images; the main expansion follows the 0408 UT Pi 2 burst. Note also that the breakup arc is not at the equatorward edge of the aurora, and appears to be close to the poleward edge. The main onset and the expansion phase begin at 04:06.26 UT
Cause of the DP-1 Magnetic Pertubations At expansion onset a portion of the tail current is diverted along field lines to the ionosphere The current is down post midnight, westward across the auroral bulge, and upward out of the westward traveling surge The ionospheric segment causes the sudden increase in negative X perturbations The field-aligned segments produce effects at midlatitudes as indicated by diagram at bottom This 3-D system is called the substorm current wedge
Lobe Pressure, Convective Flow, B z A lobe pressure enhancement between 02-04 UT The onset of pressure decrease was associated with strong, convective tailward flow and southward Bz The tailward flow precedes the major expansion onset at 0407 UT
Dipolarization at Goes-8 The inclination and Z component of B field Pi 2 power in Florida Dashed vertical lines show Pi 2 onsets Two minor Pi2 onsets during growth phase Major dipolarization after 04:08:40 UT Simultaneous signatures within ½ Pi 2 cycle
WHAT CAUSES A SUBSTORM? A substorm is caused by a southward turning of the interplanetary magnetic field which initiates reconnection at the dayside magnetopause Dipole field lines connect with interplanetary magnetic field lines allowing a fraction of the interplanetary electric field to penetrate the magnetosphere and energy to be extracted from the solar wind This coupling drives an internal convection system and associated electrical currents which link the outer magnetosphere to the ionosphere. Time lags in flux and plasma transport lead to distortion of the magnetotail and the creation of a thin current sheet close to the Earth that becomes progressively more unstable The unstable current sheet becomes susceptible to triggered release of energy An instability begins releasing energy and allowing flux and plasma to return to the dayside Energy in converted to flux ropes, injection into the ring current and outer radiation belts, Joule heating of the ionosphere by steady currents, waves, and particle precipitation
THE NEAR-EARTH NEUTRAL LINE MODEL FOR PLASMA AND FIELD
SUBSTORM GROWTH PHASE Magnetopause Open Field Line Dayside Reconnection Transpolar Flux Transport Erosion and Flairing Creation of thin current sheet Thinning of plasma sheet Plasma Sheet Thinning Sunward Flow
FORMATION OF NENL EXPANSION ONSET Substorm onset begins with the formation of a localized pair of X- and O-lines on closed field lines at center of near-Earth plasma sheet Reconnection creates a flux rope centered around the O-line. The flux rope moves tailward due to momentum provided by forces in X-line Earthward flow is decelerated as it runs into rigid dipole field. Magnetic flux piles up creating a turbulent region that maps to the auroral breakup region. The substorm current wedge is created by the inertial current of the flow deceleration.
FORMATION OF NENL EARLY EXPANSION PHASE Continued reconnection adds to flux rope moving its center further tailward. Reconnection also adds flux to the Earthward pile up region creating a compression front moving tailward. The front maps to the poleward edge of the expanding auroral bulge Closed field lines are present between poleward edge of aurora and X-line Closed field lines are also present poleward of projected X- line The last closed field lines moves equatorward at the speed of the convective flow (no reconnection of open field lines)
FORMATION OF NENL LATE EXPANSION PHASE Reconnection moves into open field lines of lobe The x-line moves tailward in discrete jumps Earthward flows are slowed and diverted around the Earth The plasma sheet expands close to Earth A thin current sheet forms tailward of X-line Open field lines help propel a macroscopic plasmoid downtail Note compression front may still be on previously closed field lines Both old and new closed field lines are still poleward of poleward edge of auroral bulge
A SCHEMATIC SUMMARY OF THE Pi 2 GENERATION MECHANISM (Shiokawa et al., 1998)
CURRENT DIVERSION INTEGRATED OVER Z (Birn et al., Preprint, 1999)
DO SUBSTORMS HAVE MULTIPLE ONSETS OR ARE THEIR MULTIPLE SUBSTORMS? Most disturbed intervals contain multiple occurrences of the phenomena characteristic of onset This substructure is quasi-periodic with a period of 15-20 minutes Some researchers call each such event a substorm although they do not include all the characteristics originally used to define a substorm The question is whether these events are separate and unrelated, or they are organized by a larger phenomenon (the original substorm) If they are part of something larger, which member of the sequence is most important and can be used as a time reference (main onset)?
Plasma Flows Near Substorm Onset We have shown above that the major expansion occurred about 04:07:34 UT These Cluster plasma data indicate that a tailward flow characteristic of a NENL began six minutes earlier It appears that this auroral expansion was caused by formation of a NENL
Relation of Field, Flow and Current Four intervals defined in previous figure are shown Blue vectors are the field, red are the current, black are the flow The anti parallel current and field are tilted at 45 to X axis The flow is first orthogonal to the direction of current and flow It then becomes steadily tailward It returns to the initial configuration Finally, the flow rotates to Earthward as the field returns to normal orientation
Pi 2 Power on US East Coast The East Coast is close to midnight and near statistical center of substorms A sequence of progressively stronger Pi 2 bursts was observed The strongest at 04:07:58 corresponds to the poleward expansion in aurora What is the change at about 0400?
Pseudo Breakups on August 27, 2001 Two auroral intensifications accompanied by Pi 2 and localized bays were seen at 03:39 and 03:53 prior to the main onset These last only 1-2 images and cause no major changes in auroral oval or the tail magnetic field
04:00:07 Pseudo Breakup or Main Onset? Auroral expansion onsets are often preceded by pseudo breakups Is the expansion onset the lingering effect of the pseudo breakup or a new event? If a tail event occurs between the pseudo breakup and the main onset which is cause and which is effect? Pseudo breakup 0400
Canada East-West Magnetometer Chain August 27, 2001 03:0003:1003:2003:3003:4003:5004:0004:1004:2004:3004:4004:5005:00 0 100 200 300 400 500 600 700 800 900 1000 1100 stj ott pbq gil rab smi sim daw mea vic Universal Time Y Disturbance (nT) 03:39:1203:52:5504:06:5104:16:22 Is this the onset?Or is this it?
Discussion Reconnection began Earthward of 20 Re six minutes before a major substorm expansion The x-line quickly moved outward beyond 20 Re Earthward flow probably persisted from Cluster onset at 0401 until at least 0433 Image aurora, Gillam keogram, Florida Pi 2 – all show a major expansion onset at 04:08 UT Dipolarization at premidnight Goes-8 was delayed from auroral onset by ~1 minute Dipolarization at post midnight Polar was delayed by ~12 minutes The main onset was preceded by three Pi 2 and accompanying pseudo breakups There were additional short-lived auroral intensifications not evident in distinct Pi 2
Questions for You! Are the pseudo breakups and weak auroral intensifications the cause of this substorm? or Is the major reconfiguration of the tail, high speed flows, and loss of energy from the tail lobe the cause? or Did the weak auroral events cause the reconnection event?