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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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