Transcript Document

The Physics of Space Plasmas
Aurorae and High-Latitude Electrodynamics
William J. Burke
24 October 2012
University of Massachusetts, Lowell
Aurorae and High-Latitude Electrodynamics
Lecture 6
• These notes deal primarily with the electrodynamic characteristics of the
high-latitude ionosphere.
• We briefly review the expected signatures of electric potential and fieldaligned current distributions during southward IMF BZ intervals.
• Examine ground- and space-based observations near the dayside cusp,
emphasizing responsiveness to changes in IMF orientations.
• Examine in detail E and B variations associated with the infinite
current sheet approximation and apply them to a interpreting acase
frequently encountered by satellites near the open-closed boundary
in the evening MLT sector.
Aurorae and High-Latitude Electrodynamics
Heppner and Maynard, JGR, 1987
Iijima and Potemra, JGR, 1978
• Pattern-recognition, steady-state guides that should not be taken too literally
• No E-fields represented at magnetic latitudes < 60. They are measureable
with reversed polarities during main and early recovery of magnetic storms.
• There exist extended regions in which   E  0  quasi neutrality preserved.
• How do we join them?
Aurorae and High-Latitude Electrodynamics
BY j||1 B
Y
j||1
j||2
BY
• Schematic for interpreting EX and
BY measurements from a s/c in
polar-orbit along dawn dusk meridian.
j||2
• Satellite-based coordinates
X positive along s/c trajectory
Y positive in anti-sunward direction
Z positive toward nadir
EX
j||2
BY
j||2
BY
j||1
BY
j||1
BY
Aurorae and High-Latitude Electrodynamics
Aurorae and High-Latitude Electrodynamics
Heppner-Maynard, JGR, 1987
Northern Hemisphere:
BY > 0, BZ < 0
Model BC
Southern Hemisphere:
BY < 0, BZ < 0
Northern Hemisphere :
BY < 0, BZ < 0
Model DE
Southern Hemisphere:
BY > 0, BZ < 0
Aurorae and High-Latitude Electrodynamics
Dayside Precipitation Pattern
Newell and Meng, GRL, 1992
Dayside FAC System
Erlandson et al., JGR, 1988
Heppner - Maynard Convection Patterns (JGR, 1987)
Aurorae and High-Latitude Electrodynamics
Nopper and Carovillano, GRL 699, 1978
Region 1 = 106 A
Region 2 = 0 A
Region 1 = 106 A
Region 2 = 3105 A
Wolf, R. A., Effects of Ionospheric Conductivity on Convective Flow of Plasma
in the Magnetosphere, JGR, 75, 4677, 1970.
Aurorae and Dayside Cusp
Aurorae and Dayside Cusp
Sandholt et al. , JGR 1998
Aurorae and Dayside Cusp
Sandholt et al., JGR 1993
Aurorae and Dayside Cusp
Aurorae and Dayside Cusp
5577 Å emissions monitored by all-sky imager at Ny Ålesund after
09:00 UT on 19 December 2001. The colored lines are placed at
constant positions as guide to the eye for discerning optical changes.
Inverted Vs
Aurorae and High-Latitude Electrodynamics
Polar Rain
F15 / F13 crossed local noon MLT at ~ 09:22 an 09:36 UT
Aurorae and High-Latitude Electrodynamics
Top right: 6300 Å emissions mapped to 220 km. All 5577 Å emissions mapped to
an altitude of 190 km. Middle traces indicate that 5577 Å variations are responses
to changes in the IMF clock angle.
Aurorae and High-Latitude Electrodynamics
Fridman, M., and J. Lemaire, JGR, 664, 1980.
Kan and Lee, JGR, 788, 1979.
Discrete Aurorae: Current –Voltage Relation
• Consider a trapped electron population
with an isotropic, Maxwellian distribution
function whose mean thermal energy = Eth
Lyons, JGR, 17, 1980
j|| - V|| Relationship
• Assume that there is a field-aligned
potential drop V|| that begins at a height
where the magnetic field strength is BV||.
 E B
j||  en  th  i
 2 me  BV||
• Knight (PSS, 741, 1973) showed that
j||
carried by precipitating electrons is
given by the top equation, where Bi is the
magnetic field strength at the ionosphere.
Let
8
B / B = 10
i
V
6
B /B =5
V
4
||
j / j
||0
i
B /B =2
i
0
0
2
4
6
eV / E
||
th
8
10
 eV|| / Eth 

 exp 

(
B
/
B

1)

 i V||

 E 
j||0  en  th 
 2 me 
then
Bi   BV||
1  1 
j||  j||0
BV||  
Bi

If
V||  0
or
BV||  Bi
V
2
  BV
1  1  ||
Bi
 
j||  j||0
 eV|| / Eth 

 exp 

(
B
/
B

1)
 i V||


Infinite Current Sheet Approximation
Normal Incidence
E  0
 E  e /  0
(   B)||  0 j||
  B  0
 j||
   I 
s
Oblique Incidence
I   PE   H E  bˆ
j||     P E   H E  bˆ      P E  E  bˆ  H
(  B)||      P E  E  bˆ  H 
0
Infinite Current Sheet Approximation
If ( = 0), then
Normal Incidence
  Y
j|| = (1/  0) [Y BZ] = (1/ Vsat  0) [t  BZ]
Vsat  7.5 km/s  1 A/m2  9.4 nT/s
J|| = ∫ j|| dY  J|| = (1/  0) [  BZ]
1 A/m    BZ = 1256 nT
Oblique Incidence
In DMSP-centered coordinates:
j|| = Y [P EY -  H EZ] = (1/  0) [Y BZ]
Y [ BZ -  0 ( P EY -  H EZ)] = 0
Where  BZ and EY vary in the same way
 P ≈ (1/  0) [  BZ /  EY].
Infinite Current Sheet Approximation
Normal Incidence
0
 AX '  1
 A   0 Cos
 Y'  
 AZ '  0 Sin
0   AX 
 Sin   AY 
Cos   AZ 
EY '  EY Cos  ET Sin
Oblique Incidence
EZ '  EY Sin  ET Cos  EY 'Tan  ET / Cos
EY  EY ' Cos  EZ ' Sin
ET   EY ' Sin  EZ ' Cos
 BY '   BZ Sin
 BZ '   BZ Cos
 BZ   BY2 '   BZ2 '
  Tan 1 ( BY ' /  BZ ' )
Infinite Current Sheet Approximation
Normal Incidence
Let
Y  (VS Cos )-1 d/dt,
and assume:
H =   P
then
d   BZ '
 0P

dt  Cos

Sin




E
' Sin  E ' Cos    0


Z
Y
Z

E Cos  E
Y
'
'

EZ '  EY 'Tan  ET / Cos
Oblique Incidence
d 
 BZ '  0  P EY '  ET Sin   Cos   0

dt
In regions where EY’ and BZ’ are highly correlated
P 
1  BZ '
0 EY '
and
H  1.5 SaCos
Infinite Current Sheet Approximation
Normal Incidence
 P , H    P , H ds  
P 
H 
Oblique Incidence
i / in
1  i / in 
2
1
1  i / in 
2
 P,H
CosI
dh
ne
B
ne
B
Since in = 4.2 ×10-10 nn and


H

  H ds   in ds
P
P
i
weak transverse gradients in  are expected.
Electron Precipitation
Robinson et al., JGR,
92, 2565, 1987.
P  [40E0 /(16  E02 )]I 0.5
E0 = mean thermal energy in keV
I = energy flux in ergs/cm2-s
Solar EUV
P  0.88 SaCos
H  1.5 SaCos
Infinite Current Sheet Approximation
VY > 0  eastward flow
Spike always occurred at the
poleward boundary of auroral
electron precipitation
Dynamics Explorer 2 saw similar features:
- Double probe
- Triaxial fluxgate magnetometer
- Drift meter and RPA
Burke et al., ,J. Geophys. Res., 99, 2489, 1994.
Infinite Current Sheet Approximation
• EX peak coincided with auroral electron
boundary, but not with the BZ peak
 P,H gradients.
• Convection reversal inside of R1 FAC
R0 R1
R2
• BZ decreased by 220 nT between
07:38 and 07:39 UT  j|| = 0.44 A/m2
• EX and BZ variations in R0 consistent
with P  2 mho
• Auroral electrons underwent field-aligned
accelerations of ~ 7 keV
• BX and BZ variations in R1 indicate
an attack angle  of ~ 18
• From RPA: Etang  10 mV/m
Infinite Current Sheet Approximation
• Assumed P  H and set X0 at 07:37 UT
X = X0 + Vsat  t  Bmax = 275 nT
• Integrated numerically