ALTITUDE PROFILES OF ELECTRON DENSITY DURING LEP EVENTS FROM VLF MONITORING

1. The Sharjah-Stanford AWESOME VLF Workshop Sharjah, UAE, Feb 22-24, 2010. 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, dsulic@phy.bg.ac.yu, 2Institute of Physics, Belgrade, Serbia, vlada@phy.bg.ac.yu

2. INTRODUCTION • The use of very low frequency (VLF) transmissions propagating 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 the variation of the D-region, both spatially and temporally

3. 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.

4. Perturbations on VLF transmissions Lightning discharges indirectly produce localized ionospheric disturbances through lightning induced bursts of precipitation of energetic radiation belt electrons. Adopted from Lanben et al., 2001

5. NRK NAA GQD DHO HWU Belgrade NWC ICV NSC DESCRIPTION OF EXPERIMENT 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.

6. 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]

7. Storm over Europe

8. 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, b, in km-1 and a reference height, h’, in km.

9. 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 Dfnumto be very close with recorded values of DArec and Dfrec.

10. 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.

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

12. GQD DHO BELGRADE Event: 12 May 2009 DHO/23.4 kHz –Belgrade • VLF signal propagates from transmitter to receiver through disturbed D region • Reflection height moved from 87 km to 86.8 km • The enhancement of electron density at 86.8 km is 2.7·106 [m-3] • GQD/22.1 kHz –Belgrade • VLF signal propagates 600 km from transmitter to receiver through disturbed D– region • Reflection height moved from 87 km to 86.7 km • 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

13. Summary • VLF data were recorded in 2008 and 2009. • LEP events were typically recorded from 18:00 to 04:00UT when 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 base for studying localized ionization enhancements in the nighttime D region • By comparing simulated effects of LEP produced ionospheric disturbances on VLF signal with experimental data we were able to access the ionospheric electron density profiles most likely to have been in effect during the observed events.