DYNAMICS OF THE EARTH S MAGNETOSPHERE

Transcription

1 DYNAMICS OF THE EARTH S MAGNETOSPHERE PROF JIM WILD With thanks to: Stan Cowley, Rob Fear & Steve Milan

2 OUTLINE So far: Dungey cycle - the stirring of the magnetosphere due to dayside and nightside reconnection Substorms - the explosive reconfiguration of the magnetosphere due to imbalanced dayside and nightside reconnection Co-rotation - the winding-up of the inner magnetosphere due to plasma frozen into the Earth s strong magnetic field Coming up: Substorms vs. steady magnetospheric convection How do the Dungey cycle and Co-rotation combine? The plasmasphere

3 The Dungey cycle Closed Open Closed Sun Magnetic flux is opened Open flux is closed

4 MAGNETOSPHERIC CONVECTION It is not the existence of open flux which generates flows it is the creation/ destruction of open flux (Cowley & Lockwood, 1992) Dayside reconnection adds flux to lobe/polar cap area of polar cap increases Nightside reconnection removes flux from lobe/polar cap area of polar cap decreases Pressure balance acts to restore Magnetosphere Magnetic reconnection reconnection Polar cap polar cap

5 THE EXPANDING CONTRACTING POLAR CAP df dt PC D N

6 THE EXPANDING CONTRACTING POLAR CAP df dt PC D N

7 MAGNETOSPHERIC SUBSTORMS Coupling between the solar wind and the magnetosphere cause energy to be stored in the Earth s magnetic tail. Substorms are the explosive release of this energy. Magnetic reconnection or merging in action. Antiparallel magnetic fieldlines are reconfigured as a result.

8 MAGNETOSPHERIC SUBSTORMS Coupling between the solar wind and the magnetosphere cause energy to be stored in the Earth s magnetic tail. Substorms are the explosive release of this energy. Magnetic reconnection or merging in action. Antiparallel magnetic fieldlines are reconfigured as a result.

9 MAGNETOSPHERIC SUBSTORMS Coupling between the solar wind and the magnetosphere cause energy to be stored in the Earth s magnetic tail. Substorms are the explosive release of this energy. Magnetic reconnection or merging in action. Antiparallel magnetic fieldlines are reconfigured as a result.

10 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

11 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

12 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

13 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

14 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

15 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

16 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

17 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

18 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

19 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

20 THE EVOLUTION OF A SUBSTORM NORTH SUN SOUTH 3 DISTINCT STAGES After Hones, GROWTH PHASE EXPANSION PHASE RECOVERY PHASE

21 MAGNETOSPHERIC SUBSTORMS The auroral substorm

22 MAGNETOSPHERIC SUBSTORMS Substorm Substorm F PC 0.6 GWb 0.3 GWb 0.0 GWb k From Milan et al

23 COMPETING SUBSTORM MODELS Current disruption model Auroral breakup Cross-tail current reduced due to current disruption instability Magnetotail reconnection Order Time Event 1 0s Current Disruption 2 30s Auroral Breakup 3 60s Reconnection Rarefaction wave propagates tailward (induces Earthward flow) Cross-tail current Current wedge created by current disruption Auroral breakup Near-Earth Neutral Line model Cross-tail current reduced due to flux pile-up Magnetotail reconnection Order Time Event 1 0s Reconnection 2 90s Current Disruption 3 120s Auroral Breakup Bursty Bulk Flow as a consequence of Near-Earth reconnection Current wedge created by flow shear breaking Cross-tail current

24 STEADY MAGNETOSPHERIC CONVECTION Dayside reconnection at the magnetopause opens closed magnetic flux Nightside reconnection at a distant location (~100 RE) in the magnetotail closes open magnetic flux At any instant, dayside and nightside reconnection rates do not need to be balanced (although on average they must balance) Substorms are the mechanism by which excess open flux is closed if nightside reconnection cannot keep up with dayside Under steady southward IMF conditions lasting several hours, the terrestrial magnetosphere can remain in a state where no substorms occur at all These periods have been called "steady magnetospheric convection" or SMC events Interesting because many theoretical considerations have indicated that such a steady state is not possible

25 STEADY MAGNETOSPHERIC CONVECTION

26 STEADY MAGNETOSPHERIC CONVECTION

27 STEADY MAGNETOSPHERIC CONVECTION SMC thought to occur from balanced reconnection rates between the dayside and nightside reconnection x-lines Comparisons of average dayside merging electric field and average plasma sheet electric field during SMCs and substorms show that the two flux transport rates are equal during SMC events By comparison, plasma sheet convection is reduced during substorm growth phases and twice as large during substorm expansions, compared to the dayside flux transport rate Pd, npa E mod, mv/m B L 20, nt B z, nt V x, km/s lnβ E y, mv/m E y /E mod 3.0 Solar wind observations (Wind) SMC (32) Substorms (145) Day R-rate Tail observations (Geotail ) Tail R-rate hours from onset From Dmitrieva et al Dynamic pressure Dusk-Dawn E-Field Lobe B-field (20 Re) Tail BZ Earthward flow Plasma beta Convection E-field Ratio of Tail/ Day R-rate

28 STEADY MAGNETOSPHERIC CONVECTION 206 KISSINGER ET AL.: SMC FLUX DIVERSION DUE TO HIGH PRESSURE A05 Figure 4. Average fast Earthward flow vectors during (left) substorm expansions and (right) SMCs. Flows were averaged into 3 3R E bins and are plotted in the GSM x-y plane. The dashed semicircle represents geosynchronous orbit (6.6 R ) and the solid semicircle represents the apogee of the THEMIS D and From Kissenger et al E

29 STEADY MAGNETOSPHERIC CONVECTION A05206 KISSINGER ET AL.: SMC FLUX DIVERSION DUE TO HIGH PRESSURE A05206 Figure 12. Average plasma parameters along the midnight meridian (within Y <5R E ): (top left) density, (top right) total pressure, (bottom left) temperature, and (bottom right) Earthward magnetic From Kissenger et al

30 STEADY MAGNETOSPHERIC CONVECTION 9. Schematic comparison of (left) magnetospheric substorms and (right) steady conve From Pulkkinen et al

31 OUTSTANDING QUESTIONS Solar wind-magnetosphere coupling leads to the occurrence of substorms, but... What triggers substorm onset? How does the dayside reconnection rate influence the rate and size of substorms? Why does the auroral oval move to very low latitudes during disturbed conditions? Why does the magnetosphere allow itself to accumulate more open flux prior to some substorms than others? Why does the magnetosphere sometime load/unload open flux (substorms) and sometimes respond with steady convection?

32 Corotation The rotation of the planet also imparts momentum to the magnetospheric plasma Ionospheric plasma is frictionally coupled to the neutral atmosphere The magnetic field lines, frozen to this plasma, attempt to rotate with the planet In turn, the magnetospheric plasma is frozen to the corotating magnetic field

33 MAGNETOSPHERIC FLOW Planetary co-rotation Solar wind-driven convections The Dungey Cycle Flow streamlines Magnetic field

34 MAGNETOSPHERIC FLOW A worked example Derive expressions for the potential, electric field and flow associated with solar-wind driven convection and corotation flow. 2. By combining these expressions, derive an equation for the radial position of the stagnation point on the dusk meridian. 3. Using the data below, comment on whether convective or corotational flow dominate at Earth and Jupiter. Earth: ωp= rad s -1, Rp=6,400 km, Beq=31,000 nt, E0= V m -1 Jupiter: ωp= rad s -1, Rp=71,400 km, Beq=500,000 nt, E0= V m -1

35 MAGNETOSPHERIC FLOW 1. Derive expressions for the potential, electric field and flow associated with solar-wind driven convection and corotation flow. Solar wind driven convection Dawn To Sun B B E E Noon x E E Midnight V V V V E E B B View in noon-midnight meridian Equipotentials and streamlines y Dusk View in equatorial plane Flow in equatorial plane is towards to Sun, associated with E in y direction (from dawn to dusk). Assume that E is constant = E 0 in equatorial plane

36 MAGNETOSPHERIC FLOW In the equatorial plane, the electric field is: From this, the electric potential is: So the velocity in the equatorial plane is: The magnetic field can be expressed as: Field at surface of planet, R P is planet s radius The convection velocity is therefore...

37 MAGNETOSPHERIC FLOW Corotation driven convection Here it is easier to specify v corot directly, then calculate E corot and ɸ corot. If the plasma and field rotates exactly at the angular velocity of the planet ω P, then: and everywhere in eq. plane Equipotentials and streamlines To Sun Noon B φ E Dawn E B Midnight r E E B and integrating Dusk

38 MAGNETOSPHERIC FLOW 1. Derive expressions for the potential, electric field and flow associated with solar-wind driven convection and corotation flow. Solar wind driven convection Corotation driven convection

39 MAGNETOSPHERIC FLOW 2. By combining these expressions, derive an equation for the radial position of the stagnation point on the dusk meridian. Consider the total flow as the sum of convection and corotation components. Note that V corot r while V conv r 3 so corotation dominates at small r, and convection dominates at large r. v v corot v conv r Dawn Consider the flow directions, they are in the same direction on the dawn merdian (-y axis) and in opposite directions on the dusk meridian (+y axis). A stagnation point (where v=0) forms on the dusk meridian at R sp. The flow is in the sense of rotation for r < R sp and in the sense of sw-driven convection for r > R sp Equipotentials and streamlines Noon x Stagnation point y Dusk Midnight

40 MAGNETOSPHERIC FLOW 2. By combining these expressions, derive an equation for the radial position of the stagnation point on the dusk meridian. The radius of the stagnation point on the dusk meridian is given by putting v conv = v corot i.e. The streamlines of the overall flow are given by: So in the equatorial plane: =y The stagnation streamline is the streamline that passes through the stagnation point r = R SP and ɸ=!/2 (ɸ is measured from the noon-meridian (x-axis)) and this divides the flow into corotation and solar wind driven convection dominated regions.

41 MAGNETOSPHERIC FLOW 3. Using the data below, comment on whether convective or corotational flow dominate at Earth and Jupiter. Earth: ωp= rad s -1, Rp=6,400 km, Beq=31,000 nt, E0= V m -1 Jupiter: ωp= rad s -1, Rp=71,400 km, Beq=500,000 nt, E0= V m -1 Earth R sp = 8.5 Rp (R mp ~ 10 Rp) Jupiter R sp = 390 Rp (R mp ~ 40 Rp) Jupiter s magnetosphere is dominated by plasma corotating with the planet! Unlike Earth, where convection due to solar wind coupling is important

42 COROTATION IN THE MAGNETOSPHERE Interplanetary Magnetic Field Tail Current Plasma Mantle Polar Cusp Plasmasphere Magnetic Tail Plasma Sheet Northern Lobe Neutral Sheet Current Ring Current Field-Aligned Current Solar Wind Magnetopause Current Magnetopause The plasmasphere represents the relatively cold ionospheric plasma (~.3 ev or T ~ 2000 K) which is co-rotating with the Earth.

43 THE PLASMASPHERE DAWN DAWN SUN 12 SUN DUSK DUSK A typical convective flow diagram for the magnetosphere. This diagram shows the plasmapause(solid closed line) as the boundary between flux tubes that approximately corotate with the earth (dotted lines) and always remain closed and flux tubes that are convected(dashed lines) to the magnetopause and lose their plasma. A super position f the convective flow patterns for two different values of the convective electric field. The dark grey region A and the dashed lines represent the corotation and convection flow directions for an active magnetic-activity period. The entire grey area (regions A+B) and the dotted lines represent the corotation and convection flow directions for a quiet magnetic-activity level. From Chappell et al

44 GEOMAGNETIC STORMS Impacts of solar wind driving can be seen throughout the entire couple magnetosphere-ionosphere system Substorms are a fundamental response mode of the system that typically occur over hours and happen multiple times per day Storms contain substorms and last for days during periods with strong solar wind driving Prolonged IMF Bsouth (valve kept open) Large Psw and/or Vsw (lots of energy to couple in) Region (volume) of magnetosphere dominated by solar wind dynamics expands comes closer to Earth Storms are defined by a large enhancement of the Ring Current (as measured by DST index) Magnetic Storms are the principal terrestrial space weather event Large storms (DST <-200nT) are all caused by fast CME s hitting earth. Medium and Small storms (DST < -50nT) can be caused by either CME s or fast solar wind streams (from equatorial coronal holes)

45 THE PLASMASPHERE a. b. c. d. From Moldwin et al

46 THE PLASMASPHERE From Sandel et al

47 THE PLASMASPHERE Midnight Noon From Borovsky & Denton, Figure 3

48 THE PLASMASPHERE When geomagnetic activity declines, 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 L 4. It therefore takes much longer to refill tubes originating at higher latitude. O + + H H + + O Since active periods may recur every few days there will be times when the outer tubes are never full and the plasmasphere has some degree of depletion.

49 THE PLASMASPHERE X [R E ] From Walsh et al., Y [R E ] Near-simultaneous measurements from two THEMIS spacecraft at the dayside magnetopause when one spacecraft can observe a high-density plasmaspheric plume while the other does not. Both spacecraft observe signatures of magnetic reconnection, providing a test for the changes to reconnection in local time along the magnetopause as well as the impact of high densities on the reconnection process. In the localised region where the plume contact the magnetopause, the high-density plume may impede the solar wind-magnetosphere coupling by mass loading the reconnection site. A survey of plasmaspheric plumes during the first 3.5 days of high-speed-stream-driven storms found that it is common for plasmaspheric plumes to last 3.5 days Plumes lasting this long (and longer - up to 7 days) raise questions about how long it takes to completely drain the outer plasmasphere and whether these long-lived plumes have the same properties as younger plumes. Where does the plasma in the long-lived plumes comes from? Drainage of plasmaspheric plasma that has been residing in the magnetosphere? Fresh ionospheric outflow? From Borovsky & Denton, 2008.

50 SUMMARY Dungey cycle - the stirring of the magnetosphere due to dayside and nightside reconnection A good description of the long-term convection pattern, but does probably not describe the loading/unloading of open flux in the magnetosphere that occurs most of the time Substorms - the explosive reconfiguration of the magnetosphere due to imbalanced dayside and nightside reconnection What, where and how are these triggered? Why does the magnetosphere go into SMC sometimes? Co-rotation - the winding-up of the inner magnetosphere due to plasma frozen into the Earth s strong magnetic field How does this region fill and empty during geomagnetic storms? What is the impact of the drainage plume on dayside reconnection?