Auroral asymmetries in the conjugate hemispheres (and wh ere KuaFu B can do better……)

Normal text - click to edit

1August 2011

Auroral asymmetries in the conjugate hemispheres

(and where KuaFu B can do better……)

Nikolai ØstgaardUniversity of Bergen, Norway

Coauthors:B. Krøvel Humberset, K.M. Laundal, A.Aasnes, S. Haaland

H. U. Frey, J. B. Sigwarth, J. Weygand


2August 2011

Outline:

1. Asymmetric auroral intensities

2. Substorm onset location asymmetries

3. What happen to asymmetry during substorm expansion phase

4. Theta Aurora in only one hemisphere

5. Cusp Aurora in both hemispheres


3August 2011

Movie: 2139-2153 UT

1. Very asymmetric aurora –

closed field lines

WIC/FUV - IMAGEVIS Earth Camera - Polar


4August 2011

2. Asymmetric auroral intensities

- Asymmetric intensities- Poleward expansion

Conditions:-Seasons:

-North-summer-South- winter

-Bx dominated IMF-By ~0


5August 2011

Interpretation Laundal and Østgaard, Nature, 2009

X

xA combination of - the more efficient solar wind dynamo- inter-hemispheric currents:

- Persistent spot in south (dynamo + interhemispheric currents)- Transient spot in the north (interhemispheric currents) and the significant increase in reconnection (poleward expansion) can explain its transient character.


6August 2011

2. Asymmetric substorm onset location

Westward bulge: SH ~1.1 MLT duskward of NH

Duskward – BY negative !


7August 2011

Statistical distribution – Polar and IMAGE

• 6600 substorms by IMAGE and Polar• 4671 Northern hemisphere• 1930 Southern hemisphere

Time shifts ACE and Wind:-10 Re +/- 5 min average-20 Re +/- 5 min average-10 Re - 40 min average

294/108

696/264

1121/433

803/372

1117/460

640/293


8August 2011

Statistical distribution


9August 2011

Average onset location – clock angle

Clock angle controls average onset location in each hemispheres

Onset locations are still distributed in a large range of MLTs

Interhemispheric asymmetry might be more robust result

Indirect indication that tail reconnection is significant for Northward IMF

-10 Re +/- 5 min average-20 Re +/- 5 min average-10 Re - 40 min average

17.0)8.4sin(53.0MLT C


10August 2011

Average onset location – By, IMF to -10 Re Reveals saturation towards dusk for - positive By in north - negative By in south Asymmetric

penetration of IMF By in the deep tail (Khurana et al., 1996) extends to

closed field lines

z

y

+

++++++

+

++++++

++++++

++++++

++++++

+++++

++++

++

+

++++++

+

++++++

++++++

++++++

++++++

+++++

++++

++

-------

-------

-------

------

------

-----

----

---

-------

-------

-------

------

------

-----

-------

z

y

By > 0

+ +++++++ ++++++

++++++

++++++

++++++

+++++

++++ ++

++ + + + + +++ + + + + +

+ + + + + +

+ + + + + +

+ + + + + +

+ + + + +

+ + + ++ +

- - - - - - -- - - - - - -- - - - - - -

- - - - - -- - - - - -- - - - -- - - -- - -

------- ------- -------

------ ------ ----- ---- ---

By < 0

20 10 0 -10 -20yGSM

[RE]

-20

-10

0

10

20

z GSM

[R E

]

xGSM

=-18RE xGSM

=-18RE

20 10 0 -10 -20yGSM

[RE]

+1

-1

Østgaard et al, GRL, L08104, 2011.

)3.9nT12

Bsin(88.0MLT Y


11August 2011

3. What happen to asymmetries during expansion phase

Oct 22, 2001:

5 hours of conjugate images

2 substorms

IMAGE

VIS Earth


12August 2011

Method1. Used available DMSP data to ensure pointing accuracy2. Visual Inspection method3. Mapped onto rectangular magnetic grid -0.1 MLT - 1° Maglat resolution 4. 1D correlation (cross correlation)5. 2D correlation (chi square)


13August 2011

Substorm assymmetry

• Onset is asymmetric

• During expansion phase the asymmetry disappears –regardless of clock angle or By.

Grey diamonds: all images < 1 min overlap

Black diamonds: - time overlap (< 21 sec)- corr.coeff > 0.4 (1D)- chi square < 0.4 (2D)


14August 2011

i) Asymmetry is created in the mid-tail

ii) I inverted-V structures are formed which decouple magnetosphere from the ionosphere,–

iii) and magnetic stress is released (Haerendel, 2007 rectifying the asymmetric field lines

Rectifying the field lines during expansion phase

b) Twist on field lines create net electric field pointing from north to south

c) Faradays loop on closed field lines

0 ABlBuE ddt

dd 0BvE

BL

lEuu ||

ns

∆

Østgaard et al, GRL, L03101,2011


15August 2011

Non-conjugate theta (transpolar arcs) consistent with IMF Bx but more observations to see if

this holds statistically

Østgaard et al., Geophys. Res. Lett., 2003

Also the cusp aurora in bothhemispheresIMF By: longitudinal shiftTilt angle: latitudinal shift

Østgaard et al., Geophys. Res. Lett., 2005

3. Theta 4. Cusp aurora


16August 2011

Summary:

1. Asymmetric auroral intensities• Indication of interhemispheric currents

2. Substorm onset asymmetries• Usually asymmetric locations at substorm onset• Strong statistical correlation with clock angle and By

3. Substorm expansion phase• Field lines are rectified during expansion phase

4. Theta Aurora• Maybe a IMF-Bx control of theta aurora?

5. Cusp Aurora• IMF control of longitudinal shifts and maybe a tilt angle of the

latitudinal shift ?

KuaFu can address these questions and many more ….


17August 2011

Thank you!


18August 2011

Tilt angle effects

More intense nightside FACs were found by Ohtani et al., 2005 in the winter hemisphere

Needs more data to confirm!

Onset in south is downwardof north onset for positive tilt,i.e., south=winter


19August 2011

Summary and challenges:

Cusp and reconnection – temporal behavior1. We have shown the asymmetry of cusp location2. Challenge: To determine the relative reconnection rate

1. Optical measurements: Relative intensity of the cusp spot2. Even better: Reconnection rate with radars (Pinnock et al., 1999) and

optical data. More than one radar, get the relative reconnection rate. Critical: OC boundary position, tilt and speed.

3. Relative size of the polar caps, optics and radars to get the opening and closing of magnetic flux in the two hemispheres

Transpolar arcs and theta aurora1. We have shown that theta can be non-conjugate, but to determine what

controls it; more observations are needed: ground based optics, GUVI and SSUSI, low altitude satellites.

Subtsorm onset location1. The IMF clock angle control is well documented. The global asymmetry

suggests that the tilt effect and difference in FAC may be insignificant.Dynamic features of the expansion phase of substorms1. The potential mismatch??


20August 2011

The penetration of the IMF field

Has been directly observed in the outer magnetosphere by Sibeck et al, 1985 and the inner magnetosphere Wing et al., 1995

How IMF affect the magnetospheric configuration

noon

midnight

dawn

Wing et al, 1995


21August 2011

Reconnection rate - Method – Vasyliunas [1984]

m

i

E

E

E

E

R

R

i

m

cos

40

iEiBuvmE iiim

ABlBuE ddt

dd

0BvE (>300 km)

Faraday’s law

0E ||

Assumptions


22August 2011

Is it real?1) Relative variations in countsNorth-WIC: red South-VIS: black

2) Electron Energies and different cameras:Dawn-spot: 10 kR (WIC)

~5 kR (VIS)High electron energies will give higher intensity in WIC than in VIS, If 25 keV mean energy -> 40 mW/m2 and should give about 10 kR in VIS, but we see only 5 kR.Energy cannot explain the dusk differences.

3) N2/O2 ratio increases due to heating WIC would be slightly brighter – this effect is very small; intensity differences would be underestimated at dusk and exagerated at dawn.


23August 2011

Challenge: Dayside reconnection rate with radars and imaging

Pinnock et al., 1999Two critical points: 1) get the OC boundary and its speed correct Imaging is needed2) More than one radar: Differences in E_rec strength are interpreted as moving away from the throat region (i.e. anti-parallel site)

North

South

IMF Bz<0 Variable By

Vasyliunas [1984]

Auroral asymmetries in the conjugate hemispheres (and wh ere KuaFu B can do better……)

Documents

Transcript of Auroral asymmetries in the conjugate hemispheres (and wh ere KuaFu B can do better……)