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ALTITUDE PROFILES OF ELECTRON

DENSITY DURING LEP EVENTS

FROM VLF MONITORINGOF THE LOWER IONOSPHERE

Desanka uli1and Vladimir Srekovi2

1Institute of Physics, Belgrade, Serbia, [email protected],

2Institute of Physics, Belgrade, Serbia, [email protected]

The Sharjah-Stanford AWESOME VLF Workshop

Sharjah, UAE, Feb 22-24, 2010.


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INTRODUCTION

The use of very low frequency (VLF) transmissionspropagating inside the waveguide formed by the Earth

and the lower ionosphere is a well developed technique

for probing conditions within the waveguide.

Measurements of the amplitude and/or phase of VLF

transmissions have provided information on thevariation of the D-region, 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 sufficient

for communications purposes.

The difference in stability reflects short-term variation inthe nighttime D-region and the lack of a dominant energy

source (c.f. the Sun in daytime).

Reflection heights occur at about 8090 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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First step: examination for VLF

signatures of LEP events

Perturbation magnitude DA = -2 [dB]

Perturbation of phase Df = - 160

Onset delay Dt = 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 frequencyprofiles 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, b, in km-1and a reference height, h,in km.

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

'( - ) -3

( ) ( ) 78.57 e [m ]h h

eN h h

b=


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Second step: computer modeling

Computer modeling is purposed to interrupt

quantitatively VLF amplitude and phase changes

in terms of approximate location and size of the

associated ionospheric perturbations along GCP.

We model propagation condition in that way to

obtain: DAnum and Dfnum to be very close withrecorded values of DArecand Dfrec.


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Third step: Gaussian function for vertical

distribution of electron enhancement

Computer modeling yields information about electrondensity at reflection heights for ambient and perturbed

ionospheric 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 .

2 2

0 0/EXP[(h-h ) / ]e eN N =


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Event: 12 May 2009 During night 11-12 May 2009, in duration of six hours, LEP

events were recorded on VLF paths.

Station Arec

[dB]rec

[0]

Anum

[dB]num

[0]

DHO

23.4 kHz

+1.65 -4.6 +1.63 -1.72

GQD

22.1 kHz

+1 -6.2 +1.1 -7.3

1000 10000 100000 1000000 1E7 1E8 1E9

40

50

60

70

80

90

Altitude[km]

Electron density [m-3

]

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

h,=86.8 km

b=0.47 km-1

1000 10000 100000 1000000 1E7 1E8 1E9

40

50

60

70

80

90

Altitude[km]

Electron density [m-3]

Profile of electron density for ambiental plasma

12 May 2009, 00:37:00

h,= 87 km

b=0.km-1

ne=3.14E

7[m

-3]

1000 10000 100000 1000000 1E7 1E8 1E9

40

50

60

70

80

90

Altitude[km]

Electron density [m-3]

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

h,= 86.7 km

b=0.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 kHzBelgrade

1. VLF signal propagates from transmitter to

receiver through disturbed D region

2. Reflection height moved from 87 km to

86.8 km

3. The enhancement of electron density at

86.8 km is 2.7106 [m-3]

GQD/22.1 kHzBelgrade1. VLF signal propagates 600 km from

transmitter to receiver through disturbed

Dregion2. Reflection height moved from 87 km to

86.7 km

3. The enhancement of electron density at

86.7 km is 4106 [m-3]

DHO: distance between transmitter - receiver is

1326 km

GQD: distance between transmitter - receiver is

1948 km


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Summary

VLF data were recorded in 2008 and 2009.

LEP events were typically recorded from 18:00 to 04:00UTwhen the great circle paths between transmitter and receiver are

partially or wholly in the nighttime sector.

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

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


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