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Whistler Observations on DEMETER Compared with Full Electromagnetic Wave Simulations Sferic Earth Ionosphere ( 60 km – 80 km) 0 + Whistler Lightning
Andrew J. Compston, Morris B. Cohen, Nikolai G. Lehtinen, Umran S. Inam, Ryan K. Said, Ivan R. Linscott, Michel Parrot
To cite this version:
Andrew J. Compston, Morris B. Cohen, Nikolai G. Lehtinen, Umran S. Inam, Ryan K. Said, et al.. Whistler Observations on DEMETER Compared with Full Electromagnetic Wave Simulations Sferic Earth Ionosphere ( 60 km – 80 km) 0 + Whistler Lightning. American Geophysical Union, Fall Meeting 2014, Dec 2014, San Francisco, United States. pp.abstract #AE31B-3415. �insu-01387095�
AE31B-3415: Whistler Observations on DEMETER Compared with
Full Electromagnetic Wave Simulations
Sferic Earth Ionosphere (~60 km – 80 km) 0+ Whistler Lightning
Data: DEMETER and NLDN
DEMETER: Satellite to study ionosphere and EM spectrum Orbit: Sun-synchronous, circular, 660 km
Instrument: Horizontal E and B at 40 kHz (below, right). NLDN: Lightning detection network over USA
Introduction
Goal: Compare the field strengths of lightning-induced 0+
whistlers predicted by a numerical model of trans-ionospheric VLF wave propagation with measurements made by a satellite.
Satellite (665 km)
Energy injected into the magnetosphere by whistlers from
lightning plays an important role in Earth’s inner radiation belt, so accurate models are needed
Model: Full Wave Method
0 125 250 Number of Whistlers Night 0 5 10 15 0 5 10 15 0 5 10 15 0 400 800 1200 0 5 10 15 0 75 150 Day DEMETER 0 5 10 15 FWM 0 5 10 15 DEMETER 0 5 10 15 FWM 0 400 800 1200 0 5 10 15 E Field B Field Frequency (kHz) Median Amplitude (dB µ V/m/kA/Hz) −70 −50 −30 −10 Frequency (kHz)
Distance from Magnetic Field Footprint (km)
Median Amplitude (dBpT/kA/Hz) −100 −80 −60 −40 Night −1000 0
1000 Day Night Day DEMETER
−1000 0 1000 FWM −1000 0 1000 −1000 0 1000 −1000 0 1000 −1000 0 1000 −1000 0 1000
E−Field Energy (dBµV/m/kA/Hz) −20 0 20 40
B−Field Energy (dBpT/kA/Hz) −40 −20 0 20 FWM / DEMETER (dB) −15 −10 −5 0 5 10 15
Distance in km North (+) or South (−)
Distance in km East (+) or West (−)
Median Amplitude over
Horizontal Distance
Conclusions
Andrew J. Compston1 (drewc@stanford.edu), Morris B. Cohen2, Nikolai G. Lehtinen1, Umran S. Inan1,3, Ryan K. Said4, Ivan R. Linscott1, Michel Parrot5
1 VLF Group, Department of Electrical Engineering, Stanford University, USA
2 School of Electrical and Computer Engineering, Georgia Institute of Technology, USA 3 Koç University, Istanbul, Turkey
4 Vaisala, Inc., Boulder, Colorado, USA
5 LPC2E, Centre National de la Recherché Scientifique, Orléans, France
Frequency (kHz) 0 5 10 15 20 28 30 32 34 36 38 −60 −30 0 30
NLDN Peak Current (kA)
110° W 90° W 30° N 40° N 50° N DEMETER NLDN 0 60 120 180 269 dB µ V/m/ √ Hz −10 10 30 CG IC
Seconds after 27−Jul−2009 04:02:31 UTC (21:32:29 LST)
Ionosphere is split into
horizontally stratified layers
each with different (a function of electron density Ne, collision frequency νe, and Earth’s
magnetic field, which we get from the IRI and IGRF). The
lightning stroke is modeled as a point source Bruce and Golde
current moment.
We identified >20,000 whistlers in 14 night and 7 day passes.
3 6 9 12 0 330 660 Night Day log 10(Ne) (m −3 ) Height (km) −3 0 3 6 8 Night Day log 10(νe) (Hz)
We sorted the whistlers by the distance from their parent lightning stroke to the magnetic footprint of the DEMETER satellite and
grouped them into 10 km-spaced bins. Then, we took the median field amplitude at each frequency for all the bins. The results are to the right.
The streaks going up in frequency with
increasing distance are the same as the “V-shaped” streaks observed in DEMETER survey mode data after the satellite passed over a lightning storm. They are caused by a mapping of the Earth-ionosphere waveguide interference pattern to the satellite altitude. The simulation reproduces the streaks well.
To the left, we compare the total whistler energy between 2 kHz and 20 kHz. That is:
d
where is either the simulated or measured electric or magnetic field
normalized by the parent lightning peak
current. The lightning stroke source is placed at the origin.
The peak in whistler energy occurs slightly south of the lightning stroke, which is in the direction of Earth’s magnetic field in this
hemisphere. 0 0.08 0.16 Electric Field 0 0.08 0.16 Night Magnetic Field −30 −150 0 15 30 0.08 0.16 −30 −150 0 15 30 0.08 0.16 Day Frequency Range (kHz) 0 − 2 2 − 4 4 − 6 6 − 8 8 − 10 10 − 12 12 − 14 14 − 16 16 − 18 18 − 20 ∫ f 1 f 2 |X sim(f)| 2df / ∫ f 1 f 2 |X meas(f)| 2 df (dB) Relative Occurence 0 0.1 0.2 Electric Field 0 0.1 0.2 Night Magnetic Field −30 −150 0 15 30 0.1 0.2 −30 −150 0 15 30 0.1 0.2 Day Horizontal Distance (km) 0 − 150 150 − 300 300 − 450 450 − 600 600 − 750 750 − 900 900 − 1050 1050 − 1200 ∫ 2 kHz 20 kHz |X sim(f)| 2df / ∫ 2 kHz 20 kHz |X meas(f)| 2 df (dB) Relative Occurence
Histograms of All
Observed Whistlers
Our simulations underestimate the field amplitudes measured by DEMETER by as much as 6 dB. Likely, the simulation is overestimating loss in the propagation through the ionosphere. The predicted field amplitude is closest to matching the satellite measurements for close lightning strokes, which could be due to modification of the ionosphere by lightning. Finally, the
electric fields predicted by the simulation show a relative increase with increasing frequency that is not present in the magnetic field, which possibly indicates conversion to quasi-electrostatic waves.
Whistler Energy over
Horizontal Displacement
Two trends in the results are worth highlighting:
1. Below, we compute the energy in various 2 kHz wide frequency rages and histogram the ratio between the simulation and measurements for all the whistlers:
2. Next, we compute the full whistler energy but group the
whistlers according to the distance from the parent lightning stroke to the satellite’s magnetic footprint:
