ALTITUDE PROFILES OF ELECTRON DENSITY DURING LEP EVENTS
FROM VLF MONITORING OF THE LOWER IONOSPHERE
Desanka Šulić1 and Vladimir Srećković2
1Institute of Physics, Belgrade, Serbia, [email protected],2Institute of Physics, Belgrade, Serbia, [email protected]
The Sharjah-Stanford AWESOME VLF WorkshopSharjah, UAE, Feb 22-24, 2010.
INTRODUCTION
• The use of very low frequency (VLF) transmissions The use of very low frequency (VLF) transmissions propagating inside the waveguide formed by the Earth propagating inside the waveguide formed by the Earth and the lower ionosphere is a well developed technique and the lower ionosphere is a well developed technique for probing conditions within the waveguide. for probing conditions within the waveguide.
• Measurements of the amplitude and/or phase of VLF Measurements of the amplitude and/or phase of VLF transmissions have provided information on the transmissions have provided information on the variation of the D-region, both spatially and temporally variation of the D-region, both spatially and temporally
Nighttime variations in subionospheric
propagation
• Nighttime propagation at VLF frequencies is less stable and predictable than for daytime paths, although sufficient for communications purposes.
• The difference in stability reflects short-term variation in the nighttime D-region and the lack of a dominant energy source (c.f. the Sun in daytime).
• Reflection heights occur at about 80–90 km altitude..
Perturbations on VLF transmissions
Adopted from Lanben et al., 2001
Lightning discharges indirectly produce localized ionospheric disturbances through lightning induced bursts of precipitation of energetic radiation belt electrons.
Belgrade
NWC
NSC
ICV
HWU
NAA GQD DHO
NRK
DESCRIPTION OF EXPERIMENT
NRK Iceland 37.50 kHz
DHO Germany 23.40 kHz
GQD UK 22.10 kHz
NAA USA 24.00 kHz
HWU France 18.30 kHz
ICV Italy 20.27 kHz
NSC Italy 45.90 kHz
NWC Australia 19.80 kHz
AWESOME SYSTEM was installed at the Institute of Physics Belgrade (44.50N 20.23E) in June 2008.
•The transmitter–receiver distance ranges from 950 to 6600 km. The transmitter–receiver distance ranges from 950 to 6600 km.
First step: examination for VLF signatures of LEP events
Perturbation magnitude A = -2 [dB]
Perturbation of phase
Onset delay t = 1.3 [s]
Event duration td = 0.5 [s]
Storm over Europe
Second step: computer modeling
• The ionospheric electron density and collision frequency profiles are given by a standard nighttime ionospheric model.
• The collision frequency profile is given by:
• The unperturbed electron density profile is given by:
• The model of the ionosphere used in LWPC2.1 produces
an exponential increase in conductivity with height by a slope, , in km-1 and a reference height, h’, in km.
11 -0.15 -1( ) 1.86 10 e [s ]hh
'( - ) -3( ) ( ) 78.57 e [m ]h heN h h
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: Anum and fnumto be very close with recorded values of Arec and frec
Third step: Gaussian function for vertical distribution of electron enhancement
• Computer modeling yields information about electron density at reflection heights for ambient and perturbed ionospheric D region as a pointer for further modeling.
• The altitude dependence of the electron density perturbation is assumed to be Gaussian, centered at h0.
with a variance σ.
2 20 0/EXP[(h-h ) / ]e eN N
Event: 12 May 2009• During night 11-12 May 2009, in duration of six hours, LEP During night 11-12 May 2009, in duration of six hours, LEP
events were recorded on VLF paths.events were recorded on VLF paths.
Station DArec
[dB]
Dfrec
[0]
DDAnum
[dB]
DfDfnum
[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 1E940
50
60
70
80
90
Alt
itud
e [k
m]
Electron density [m-3]
DHO/23.4 kHz - Belgrade 12. May 2009, 00:37:00 UT
h,=86.8 km
=0.47 km-1
1000 10000 100000 1000000 1E7 1E8 1E940
50
60
70
80
90
Alt
itud
e [k
m]
Electron density [m-3]
Profile of electron density for ambiental plasma 12 May 2009, 00:37:00
h,= 87 km
km-1
ne=3.14E7 [m-3]
1000 10000 100000 1000000 1E7 1E8 1E940
50
60
70
80
90
Alt
itud
e [k
m]
Electron density [m-3]
GQD/22.1 - Belgrade, 12 May 2009, 00:37:00
h,= 86.7 km
km-1
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 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 2.7·106 [m-3]
GQD/22.1 kHz –Belgrade 1. VLF signal propagates 600 km from
transmitter to receiver through disturbed D– region
2. Reflection height moved from 87 km to 86.7 km
3. 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 is
1948 km
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 04:00UT LEP events were typically recorded from 18:00 to 04:00UT when the great circle paths between transmitter and receiver are when the great circle paths between transmitter and receiver are partially or wholly in the nighttime sector. partially or wholly in the nighttime sector.
• The recorded signals from transmitters in Europe are good base The recorded signals from transmitters in Europe are good base for studying localized ionization enhancements in the nighttime for studying localized ionization enhancements in the nighttime D regionD region
• By comparing simulated effects of LEP produced ionospheric By comparing simulated effects of LEP produced ionospheric disturbances on VLF signal with experimental data we were disturbances on VLF signal with experimental data we were able to access the ionospheric electron density profiles most able to access the ionospheric electron density profiles most likely to have been in effect during the observed events.likely to have been in effect during the observed events.
