Ionospheric Research Lab MODELING ELF RADIO ATMOSPHERICS GENERATED BY ROCKET TRIGGERED LIGHTNING PRESENTED BY BHARAT KUNDURI CHAIR DR.ROBERT MOORE Ionospheric Research Lab 1

Objectives To model ELF radio atmospherics up to a frequency of 500Hz. To theoretically model the radio atmospheric waveforms generated due to rocket triggered lightning at Camp Blanding, Fl that would be observed at McMurdo Station Antarctica for a propagation path 13760 Km long. To implement LWPC a numerical propagation code to model the radio atmospherics at ELF frequencies. Inhomogeneous ground is considered with realistic conductivity profiles, also a comparison between assuming a homogeneous ground and an inhomogeneous ground is demonstrated. To observe and compare the effects of different components of the current waveform of lightning on sferic waveform 2

Previous Work Previous work (Cummer,1997) modeled the waveforms assuming a homogeneous ground this type of modeling is not applicable to propagation path's where there is a huge change in ground parameters especially when topography changes from land to sea or ice. Previous work (Cummer,1997) assumed the lightning strike to be an ideal return stroke. The maximum distance modeled in previous work (Cummer,1997) was 3000 Km assuming the surface to be land. 3

LIGHTNING 4

Lightning Discharge Thunder Cloud – electric dipole is formed due to a large positively charged and negatively charged layer. Types of lightning flash : 1) Cloud discharges. 2) Cloud to ground discharges. Types of Cloud to Ground discharges : – Negative cloud to ground discharge – Positive cloud to ground discharge – discharges with flow of both negative and positive charges 5

Classical Triggered Lightning Rocket triggered Lightning Classical Triggered Lightning Wire is attached to a rocket with other end connected to ground launcher. Upward positive leader is generated at the tip of the rocket after it reaches an altitude of 200mts. The current of UPL vaporizes the wire and Initial Continuous Current (ICC) follows for some hundreds of milli seconds after which no current flows for a few tens of milliseconds This is followed by generation of a leader return stroke sequences. Classical triggered Lightning (Rakov et.al 1999,JGR) 6

Current Waveform Figure indicates various features associated with negative rocket triggered lightning. The initial stage is characterized by Initial Current Variation (ICV) and Initial Continuous Current (ICC) ICV has a duration that does not exceed 10ms and the ICC flows for approximately hundreds of milliseconds. This is followed by dart leader return stroke sequences. D.Wang et.al (JGR,1999) 7

Current Waveform Employed in this Thesis Courtesy : Dr. Moore 8

RADIO ATMOSPHERICS Radio atmospherics or sferics in short are lightning produced electric and magnetic fields with frequencies ranging from a few Hz to a few Khz. Typically sferics have frequency spectrum in ELF (0-3 Khz) and the VLF (3-30 Khz) range. Propagate in the Earth- Ionosphere waveguide by multiple reflections at the boundaries. Sample sferic Observed at Palmer Station, Antarctica Source: Troy Wood, PhD dissertation,2004 9

EARTH-IONOSPHERE WAVEGUIDE Different from parallel plate waveguide. The boundaries (Earth and Ionosphere) are not perfectly conducting. Earth's conductivity varies from region to region. Curved Nature of Earth- Ionosphere waveguide. 10

Long Wavelength Propagation Code (LWPC) 11

Introduction A collection of FORTRAN programs, developed over many years at Naval Ocean Systems Center (NCCOSC/NRAD). Implements the two dimensional propagation along the great circle path in the Earth Ionosphere waveguide– (ferguson and shellman, 1989). Based on Mode Theory developed by Budden.K.G 12

3 IMPORTANT PARTS PRESEG : FASTMC : MODEFNDR: Segments the propagation path based on the waveguide parameters taken as input. Formats them properly for the next stage. MODEFNDR: Determines the eigen angle solutions over a horizontally homogenous portion of waveguide. Takes the parameters from PRESEG as input and searches for the solutions inside a predefined region Also calculates the excitation factors needed to determine the final field strengths. FASTMC : Determines the strength of the electromagnetic field along the propagation path using the mode solutions for each of the homogeneous slabs. Output – Amplitude in dB (over 1 microvolt/meter), Phase in degrees 13

Implementation in LWPC Output 2 from PRESEG TO FASTMC Output from MODEFNDR to FASTMC Output 1 from PRESEG to MODEFNDR Model Input File Script Controlling the execution of programs and their inputs PRESEG MODEFNDR FINAL OUTPUT - Fields along the path FASTMC Set of parameters that need to be input to set the LWPC running. These are entered through a model file. A script is written to run PRESEG, MODEFNDR and FASTMC in proper order and give them appropriate inputs. 14

PARAMETERS FOR INPUT Ionospheric electron density profiles Taken from International Reference Ionosphere (IRI) upto a height of 300Km. Current Waveform of Lightning strike Taken from Rocket Triggered Lightning at ICLRT 15

Calculations LWPC calculates the electric field (Amplitude in dB over 1 µV/m and phase in degrees) for a dipole oriented in any direction. In this thesis a Vertical Dipole is assumed. The results of LWPC are convolved with the Current Moment waveform of the lightning strike to obtain the sferic waveform radiated due to a lightning discharge. 16

MODELING ELF RADIO ATMOSPHERICS 17

Modeling Approach Homogeneous ground Earth (the lower boundary of the waveguide) is assumed to be uniform throughout the propagation path. Conductivity of ground 10^-2 S/m Relative Permittivity of ground 15. Inhomogeneous ground This model employs a more realistic approach with variations in the ground according to the region. Conductivity of ground: Land – 10^-2 to 3*10^-3 S/m Sea – 4 S/m Ice – 10^-4 S/m, Relative Permittivity – 10 to 15 18

DR.CUMMER'S RESULTS – HOMOGENEOUS GROUND AND IONOSPHERE Sferic Spectrum in linear scale Sferic spectrum in dB scale S.A.cummer and U.S.Inan,2000, radio sci. 19

Sferic Spectrum in linear scale Sferic spectrum in dB scale Comparison of Dr. Cummer's results with inhomogeneous Ground (conductivity varying from 10-2 to 10-3 S/m, Relative Permittivity – 15, Distance - 2000km) Sferic Spectrum in linear scale Sferic spectrum in dB scale 20

Sferic propagation from Camp Blanding, FL to McMurdo Station, Antarctica Camp Blanding,Florida (29.94 N and −82.03 W) McMurdo Station, Antarctica (−77.88 N and 166.73 W). Propagation Distance – 13740 Km Frequency band of sferic spectra 45-500 Hz. Ground conductivity varies from 10-4 S/m to 4 S/m. Homogeneous Ionosphere assumed throughout the propagation path. 21

Night Time Ionosphere with valley Output of LWPC convolved with lightning current Electron Density profiles of ionosphere Output of LWPC 22

Night Time Ionosphere with Valley Sferic spectrum in linear scale Sferic waveform 23

Night Time Ionosphere without a valley Output of LWPC convolved with lightning current Electron Density profiles of ionosphere Output of LWPC 24

Night Time Ionosphere without a Valley Sferic spectrum in linear scale Sferic waveform 25

Day Time Ionosphere type 1 Output of LWPC convolved with lightning current Electron Density profiles of ionosphere Output of LWPC 26

Day Time ionosphere type 1 Sferic spectrum in linear scale Sferic waveform 27

Daytime Ionosphere type 2 Output of LWPC convolved with lightning current Electron Density profiles of ionosphere Output of LWPC 28

Day Time Ionosphere type 2 Sferic spectrum in linear scale Sferic waveform 29

Comparison of Results 30

Effects of Different Components of Current on Sferic Waveform 31

Current Waveform and its Components Current Waveform employed in this work Components of Current waveform 32

Components in the sferic waveform Nighttime Ionosphere with a valley 33

Components in the sferic waveform Nighttime Ionosphere without a valley 34

Components in the sferic waveform Daytime Ionosphere type 1 35

Components in the sferic waveform Daytime Ionosphere type 2 36

Challenges and Problems The lowest frequency for which the LWPC could give a result was 45 Hz, this could be brought down. There were sudden jumps in amplitude whenever there was a change in the conductivity of ground, In this thesis these jumps were manually corrected to obtain a smooth plot. 37

Acknowledgements Dr. Robert Moore Dr. Martin Uman and Dr. Vladimir Rakov All my lab mates – Shuji , Tong, Michael, Divya, Ryan, Sandeep, Neal and Eric. My Parents and friends. 38

Photos: Dr Robert Moore 39