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ALTITUDE PROFILES OF ELECTRONDENSITY DURING LEP EVENTS

FROM VLF MONITORING

OF THE LOWER IONOSPHERE

Desanka Šulić1

and Vladimir Srećković2

1 Institute of Physics, Belgrade, Serbia, dsulic@phy.bg.ac.yu,

2 Institute of Physics, Belgrade, Serbia, vlada@phy.bg.ac.yu

The Sharjah-Stanford AWESOME VLF Workshop

Sharjah, UAE, Feb 22-24, 2010.

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INTRODUCTION

•• The use of very low frequency (VLF) transmissionsThe use of very low frequency (VLF) transmissionspropagating inside the waveguide formed by the Earthpropagating inside the waveguide formed by the Earthand the lower ionosphere is a well developed techniqueand the lower ionosphere is a well developed techniquefor probing conditions within the waveguide.for probing conditions within the waveguide.

•• Measurements of the amplitude and/or phase of VLFMeasurements of the amplitude and/or phase of VLFtransmissions have provided information on thetransmissions have provided information on thevariation of the Dvariation of the D--region, both spatially and temporallyregion, both spatially and temporally

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Nighttime variations in subionospheric

propagation

• Nighttime propagation at VLF frequencies is less stableand predictable than for daytime paths, although sufficientfor communications purposes.

• The difference in stability reflects short-term variation inthe nighttime D-region and the lack of a dominant energysource (c.f. the Sun in daytime).

• Reflection heights occur at about 80–90 km altitude..

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Perturbations on VLF transmissions

Adopted from Lanben et al., 2001

Lightning discharges indirectly produce localized ionosphericdisturbances through lightning induced bursts of precipitation

of energetic radiation belt electrons.

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Belgrade

NWC

NSC

ICV

HWU

NAA GQD DHO

NRK

DESCRIPTION OF EXPERIMENT

19.80 kHzAustraliaNWC

45.90 kHzItalyNSC

20.27 kHzItalyICV

18.30 kHzFranceHWU

24.00 kHzUSANAA

22.10 kHzUKGQD

23.40 kHzGermanyDHO37.50 kHzIcelandNRK

AWESOME SYSTEM was installed at the Institute of PhysicsBelgrade (44.50N 20.23E) in June 2008.

••The transmitterThe transmitter––receiver distance ranges from 950 to 6600 km.receiver distance ranges from 950 to 6600 km.

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First step: examination for VLF

signatures of LEP events

Perturbation magnitude ∆A = -2 [dB]Perturbation of phase ∆φ = − 160

Onset delay ∆t = 1.3 [s]

Event duration td = 0.5 [s]

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Storm over Europe

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Second step: computer modeling• The ionospheric electron density and collision frequency

profiles are given by a standard nighttime ionosphericmodel.

• The collision frequency profile is given by:

• The unperturbed electron density profile is given by:

• The model of the ionosphere used in LWPC2.1 producesan exponential increase in conductivity with height by aslope, β , in km-1 and a reference height, h’, in km.

11 -0.15 -1( ) 1.86 10 e [s ]hhν  ⋅= ⋅ ⋅

'( - ) -3( ) ( ) 78.57 e [m ]h h

e N h hβ ν = ⋅ ⋅

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Second step: computer modelingComputer modeling is purposed to interrupt

quantitatively VLF amplitude and phase changesin terms of approximate location and size of theassociated ionospheric perturbations along GCP.

We model propagation condition in that way to

obtain: ∆Anum and ∆fnum to be very close withrecorded values of ∆Arec and ∆frec.

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Third step: Gaussian function for vertical

distribution of electron enhancement• Computer modeling yields information about electron

density at reflection heights for ambient and perturbedionospheric D region as a pointer for further modeling.

• The altitude dependence of the electron densityperturbation is assumed to be Gaussian, centered at h0.

with a variance σ.

2 20 0 /EXP[(h-h ) / ]e e N N δ δ σ =

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Event: 12 May 2009•• During night 11During night 11--12 May 2009, in duration of six hours, LEP12 May 2009, in duration of six hours, LEPevents were recorded on VLF paths.events were recorded on VLF paths.

-7.3+1.1-6.2+1GQD

22.1 kHz

-1.72+1.63-4.6+1.65DHO

23.4 kHz

DfDfDfDfDfDfDfDfnum

[0]

DDDDDDDDAnum

[dB]

DfDfDfDfrec

[0]

DDDDArec

[dB]

Station

1000 10000 100000 1000000 1E7 1E8 1E9

40

50

60

70

80

90

   A   l   t   i   t  u   d  e   [   k  m   ]

Electron density [m-3

]

DHO/23.4 kHz - Belgrade 12. May 2009, 00:37:00 UT

h,=86.8 km

β=0.47 km

-1

1000 10000 100000 1000000 1E7 1E8 1E9

40

50

60

70

80

90

   A   l   t   i   t  u   d  e   [

   k  m   ]

Electron density [m-3]

Profile of electron density for ambiental plasma12 May 2009, 00:37:00

h,= 87 km

β=0.43 km-1

ne=3.14E7 [m-3]

1000 10000 100000 1000000 1E7 1E8 1E9

40

50

60

70

80

90

   A   l   t   i   t  u   d  e   [   k  m   ]

Electron density [m-3]

GQD/22.1 - Belgrade, 12 May 2009, 00:37:00

h,= 86.7 km

β=0.44 km-1

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Event: 12 May 2009

-5 0 5 10 15 20

44

46

48

50

52

54GQD DHO

BELGRADE

DHO/23.4 kHz –Belgrade

1. VLF signal propagates from transmitter toreceiver through disturbed D region

2. Reflection height moved from 87 km to86.8 km

3. The enhancement of electron density at

86.8 km is 2.7·106

[m-3

]

GQD/22.1 kHz –Belgrade1. VLF signal propagates 600 km from

transmitter to receiver through disturbedD– region2. Reflection height moved from 87 km to

86.7 km3. The enhancement of electron density at

86.7 km is 4·106 [m-3]

DHO: distance between transmitter - receiver is

1326 km

GQD: distance between transmitter - receiver is1948 km

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Summary•• VLF data were recorded in 2008 and 2009.VLF data were recorded in 2008 and 2009.

•• LEP events were typically recorded from 18:00 to 04:00UTLEP events were typically recorded from 18:00 to 04:00UTwhen the great circle paths between transmitter and receiver arewhen the great circle paths between transmitter and receiver arepartially or wholly in the nighttime sector.partially or wholly in the nighttime sector.

•• The recorded signals from transmitters in Europe are good baseThe recorded signals from transmitters in Europe are good basefor studying localized ionization enhancements in the nighttimefor studying localized ionization enhancements in the nighttimeD regionD region

•• By comparing simulated effects of LEP produced ionosphericBy comparing simulated effects of LEP produced ionosphericdisturbances on VLF signal with experimental data we weredisturbances on VLF signal with experimental data we wereable to access the ionospheric electron density profiles mostable to access the ionospheric electron density profiles mostlikely to have been in effect during the observed events.likely to have been in effect during the observed events.

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