Transcript LEP and the Magnetosphere
Lightning-induced Electron Precipitation (LEP) events
Prepared by Ben Cotts, Erin Gemelos, and Morris Cohen
Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network
General overview of the LEP process Details of the magnetosphere and trapped particles
Bounce motion Drift motion
Details of the LEP process
Wave-particle interaction Secondary Ionization Types of LEP Events
Satellite observations of LEP
2
The LEP Event
Lightning Induced Electron Precipitation
3
The LEP Event
Lightning strikes somewhere
4
The LEP Event
ELF/VLF bounces between Earth and ionosphere
5
The LEP Event
Energy finds place to cross into magnetosphere
6
The LEP Event
Whistler wave redirects trapped particle
7
The LEP Event
Whistler wave lands, particles precipitate
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Example LEP Events
9
Example LEP Events
10
LEP Overview
LEP combines many areas of ionospheric and magnetospheric physics:
Magnetosphere:
•
Whistler waves
• •
Van-Allen Radiation Belts Wave-particle interaction
Ionosphere
•
Lightning
• •
Subionospheric VLF propagation Remote sensing of ionospheric disturbances
11
Whistler Waves
Right-hand circularly polarized waves Slow propagation 3 types of whistler mode waves
12
Radiation Belts
Comprised of energetic charged particles trapped by the Earth’s magnetic field (from keV to MeV) L shell is the radial location, in R
E
, of the intersection of a magnetic field line with the magnetic equator Three main radiation belt regions:
Inner belt (1.1<L<2.5) Outer belt (3<L<9) Slot region (2.5<L<3.0), intermediate region with depleted fluxes
Cartoon depicting inner and outer radiation belts Source: [
Bortnik, 2004
] 13
Charged Particle Motion
Source: [
Jursa, 1985
] 14
Charged Particle Motion
The electron bounce motion is controlled by the first adiabatic invariant
100 km sin 2
B
constant
where
a tan 1
v
v
||
B increases as the particle moves down the field line, causing
a
to increase, as well:
sin 2
lc
B eq
sin 2
B
100
km
a
lc
sin 1
B eq
/
B
100
km
a
eq
a
lc
a
eq
a
lc
a
eq
a
lc
15
Lightning (whistler) induced electron precipitation
Wave-Particle Interaction
Whistler Wave propagates with Right Hand Circular Polarization (RHCP) Counter-streaming electrons gyrate in same direction In the equatorial region Doppler shifted wave frequency equals the electron gyrofrequency Electron experiences a
constant electric field
ω
ce
= ω
– k ║
v
║
Electrons gain or lose energy
change electron pitch angle
16
LEP Overview
LEP combines many areas of ionospheric and magnetospheric physics:
Magnetosphere:
•
Whistler waves
• •
Van-Allen Radiation Belts Wave-particle interaction
Ionosphere
•
Precipitating electrons
ionospheric disturbances
Subionospheric VLF propagation
Remote sensing of ionospheric disturbances
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Precipitation and Secondary Ionization
18
LEP Observations
19
Sferic
An LEP Event ∆ t – EMP/Whistler propagation time
t d
– Precipitation/secondary ionization
DA/Df
– Change in amplitude and phase
t r
– recovery; return to chemical equilibrium
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VLF Signatures of LEP Events
Theoretical Precipitation Region
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Types of Magnetospheric Interactions leading to LEP events
Ducted Whistler Waves
Propagate along Magnetic Field-aligned “ducts” of enhanced ionization Oblique/Non-Ducted
Propagate at oblique angles to magnetic field lines Magnetospherically Reflecting (MR) Waves
Like Oblique whistler waves, but if conditions are right, they will reflect and continue to propagate within the Magnetosphere
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Types LEP Events
Ducted
~ 100 km lateral extent
Short onset delay Oblique/Non-Ducted
Differential onset delay
~2000 km lateral extent Poleward displaced from causative lightning stroke Magnetospherically Reflecting (MR)
Long onset delay
Multiple oblique reflections – poleward propagating disturbance region ~2000 km
Poleward displaced from causative lightning stroke
* Type of interaction roughly determines size of precipitation region 23
Type I LEP - Ducted Whistlers 1.
2.
3.
4.
5.
6.
Typical onset delay ( ∆t ~ 0.6 s) between causative sferic and LEP event Constant onset delay respect to L-shell with Typical onset duration (t d ~ 1.7 s) of secondary ionization Small ionospheric region of disturbance, typically < 100 km Occur relatively infrequently Recovery typically ~10-100 s
Constant Onset Delay 24
Type II LEP Events - Non-Ducted (Oblique) Whistlers
Differential Onset Delay
1.
2.
3.
4.
5.
6.
Typical onset delay ( ∆t ~ 0.7-1.3 s) between causative sferic and onset Differential onset delay with respect to L-shell (difference of 0.6 s between Las Vegas, and Littleton) Onset duration (t d ~ 2.0 s) of secondary ionization Large ionospheric region of disturbance, ~ 2000 km Occur very frequently Recovery typically ~ 10-100 s
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Type III LEP Events caused by MR Whistlers 1.
2.
3.
4.
Very long onset time with respect to the causative sferic ~4 sec Long onset duration ~2 sec Recovery for this case 30-45 sec
Undetermined occurrence frequency May occur very infrequently – have been measured on satellite, but not conclusively on ground.
May be masked by stronger upgoing 0+/1- (Type II) non-ducted components
Different disposition of paths may be better suited for observation of MR whistler driven precipitation than previously
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Ionospheric Modification due to Electron Precipitation
precipitating electrons of varying energy
Amplitude of LEP event
Location of precipitation.
Amount of secondary ionization
•
Electron flux & energy
Flux of electrons
Mostly determined by trapped population (Kp)
Also a function Whistler strength Energy of electrons
Resonant condition: function of location (L Shell)
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Source: [
Peter, 2007
]
LEP Occurrence Rate Dependence on Geomagnetic Conditions
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Lightning-induced precipitation detection aboard low earth orbit satellite DEMETER
Sferics on the ground Whistlers on a satellite Resulting electron
precipitation
(increase in flux) observed on the same satellite at 700 km altitude.
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Future Work
Does geomagnetic latitude or longitude affect the LEP signature or occurrence rate?
Are occurrence rates or characteristics different in your part of the world?
Drift loss cone
|B| lowest over South Atlantic Anomaly (SAA)
Particle can drift into region of low |B| and precipitate.
a
eq
a
lc
30
R efer ences [Bortnik et al.(2003)Bortnik, I nan, and Bell ] Bort nik, J., U. S. Inan, and T . F. Bell (2003), Frequency-t ime spect ra of magnet ospherically re 10.1029/ 2002JA009387.
° ect ing whist lers in t he plasmasphere, (Space Physics), 108, 19{ 1 { 19{ 12, doi: [Burgess and I nan(1993)] Burgess, W. C., and U. S. Inan (1993), T he role of duct ed whist lers in t he precipit at ion loss and equilibrium ° ux of radiation belt elect rons, , 98, 15,643{ + .
[I nan et al.(1989)I nan, Walt, Voss, and I mhof ] Inan, U. S., M. Walt , H. D.
Voss, and W. L. Imhof (1989), Energy spect ra and pit ch angle dist ribut ions of light ning-induced elect ron precipit at ion - analysis of an event observed on t he s81-1 (seep) sat ellit e, , 94, 1379{ 1401.
[Johnson et al.(1999)Johnson, I nan, and Lauben] Johnson, M. P., U. S. Inan, and D. S. Lauben (1999), Subionospheric vlf signat ures of oblique (nonduct ed) whist ler-induced precipit at ion, , 26, 3569{ 3572, doi: 10.1029/ 1999GL010706.
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R efer ences [Lauben et al.(1999)Lauben, I nan, and Bell ] Lauben, D. S., U. S. Inan, and T . F. Bell (1999), Poleward-displaced elect ron precipit at ion from light ning generat ed oblique whist lers, , 26, 2633{ 2636, doi:10.1029/ 1999GL900374.
[Lev-Tov et al.(1995)Lev-Tov, I nan, and Bell ] Lev-Tov, S. J., U. S. Inan, and T . F. Bell (1995), Alt it ude pro ¯les of localized d region density disturbances produced in light ning-induced elect ron precipit at ion event s, , 100, 21,375{ 21,384, doi:10.1029/ 95JA01615.
[Peter and I nan(2004)] Pet er, W. B., and U. S. Inan (2004), On t he occurrence and spat ial ext ent of elect ron precipit at ion induced by oblique nonduct ed whist ler waves, , 109(A12215), 12,215{ + , doi:10.1029/ 2004JA010412.
[Voss et al.(1998)Voss, Walt, I mhof, Mobilia, and I nan] Voss, H. D., M. Walt , W. L. Imhof, J. Mobilia, and U. S. Inan (1998), Sat ellit e observa t ions of light ning-induced elect ron precipit at ion, , 103, 11,725{ 11,744, doi: 10.1029/ 97JA02878.
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References
Abel, B. and R. M. Thorne, “Electron scattering loss in Earth's inner magnetosphere,1: Dominant physical processes,”
J. Geophys. Res.
, 103, 2385-2396, 1998a.
Abel, B., and R. M. Thorne, “Electron scattering loss in Earth’s inner magnetosphere 2. Sensitivity to model parameters,”
J. Geophys. Res.
, 103(A2), 2397 –2408, 1998b.
Bortnik, J., “Precipitation of Radiation Belt Electrons by Lightning-Generated Magnetospherically Reflecting Whistler Waves,”
Stanford University Thesis
, Stanford University, Palo Alto, CA USA, 2004. Inan, U. S., D. C. Shafer, W. Y. Yip, and R. E. Orville, “Subionospheric VLF signatures of nighttime D region perturbations in the vicinity of lightning discharges,”
J
.
Geophys. Res., 93,
(A10), 11455 - 11427, 1988.
Inan, U. S., D. Piddyachiy, W. B. Peter, J. A. Sauvaud, and M. Parrot (2007), “DEMETER satellite observations of lightning-induced electron precipitation,” Geophys. Res. Lett., 34, L07103, doi:10.1029/2006GL029238 Jursa, A. S., “Handbook of Geophysics and the Space Environment,”
United States Airforce
, 1985.
Peter, W.B., “Quantitative Measurement of Lightning-Induced Electron Precipitation Using VLF Remote Sensing,”
Stanford University Thesis
, Stanford University, Palo Alto, CA USA 2007.
Voss, H.D. M. Walt., W. L. Imhof, J. Mobilia, and U.S. Inan, “Satellite Observations of Lightning Induced Electron Precipitation,”
J. Geophys. Res.
, 103, (A6), 11,725-11,744, 1998.
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