Unusual properties of large amplitude VLF waves in STEREO

burst data A. Breneman, C. Cattell, J. Wygant, K.

Kersten, L.B. Wilson III, L. Dai, C. Colpitts, P.J. Kellogg, K. Goetz, A.

Paradise RBSP SWG Baltimore, 2012

What we observe • On a significant % of STEREO burst

waveforms: – Different wave modulation on

different antennas – (apparent) reversals of polarization

caused by beating of two waves • Burst waveforms associated with:

– multiple wave interaction – decay into short wavelength waves – density cavities

• VLF examples of this: – NPM transmitter (L=1.2) – lightning whistler (L=2) – whistler/Zmode (L=3.2)

…show movie Breneman et al., 2011

NPM transmitter wave

Why observed on STEREO? • STEREO has:

– Short antennas (3D): antennas don’t strongly couple to waves with

– high time-resolution waveform capture capability

λ < d

Geodynamic importance

• Interaction with density cavities – Loss mechanism for VLF energy - localized heating, acceleration, etc… – Change wave propagation characteristics

• Excitation of short wavelength waves – Exchange energy between e- and ions – Excited lower hybrid waves linked to explosive spread F above

thunderstorms (Baker, S.D. et al. [2000], Labno et al. [2007])

• Unusual wave modulations – Multiple waves present and possibly interacting – Modulation not necessarily intrinsic to source

• Wave/particle interactions – Can fall out of Doppler-shifted cyclotron resonance with MeV e- – Large amplitude whistler mode waves rarely field-aligned – Trapped particles (Cattell et al., 2008; Kellogg et al., 2011)

Lightning whistler example

• STB whistlers “normal”, STA whistlers not

• Huge FA component on STA!!

• Indicates dynamic and/or spatially varying (on small scale) inner radiation belt

Whistler and/or Z-mode • STA in plasmasphere at L=3.2, ilat=56, lat=30, MLT=17

• When LH wave grows, RH wave tends to disappear.

• Not just a simple mixed state (which would manifest as a linear or highly elliptical polarization)

• Possibilities: – Small-scale density fluctuations

causing Z-mode to change from LH to RH polarization

– Mix of whistler (RH) and Z-mode (LH) with energy exchange

n=3 -> f=10 kHz, fce=68; flh=1.6, fpe=17, fz=4 n=1.5 -> fpe~10 kHz (10% fluctuation in density can cause 1 kHz change in fpe 131037.508 STA

Nov17

Proposed mechanism

• Reversals occur when wave at 21.4 kHz splits into sidebands at 21.4 kHz +/- 200 Hz

Proposed mechanism - 4-wave decay

Summary

– Large amplitude waves not just scaled-up versions of smaller amplitude waves. They can have fundamentally different properties, suggesting interesting physics is occurring.

– Possible associations with short wavelength waves and density variations.

– RBSP will provide Ew and Bw waveform and continuous survey data to

further study waves

Acknowledgements

• We thank D. Shklyar, S. Bale and T. Bell for discussion of relevant topics. This research was supported by NASA grants NNX07AF23G and NAS5-01072.

Extra slides

STEREO perigee passes • Four perigee passes for STA and STB in 2006

Beat interference of two waves

Low lat transmitter whistlers • Eight burst captures on STA

on Nov06, 2006 NPM • 09:06:53 to 09:07:03 • Inner radiation belt/upper

ionosphere: – alt=550 km – mlat=15 deg – nightside, – L=1.16

• Two types of waves appear – Main wave at 21.4 kHz – Waves at 21.2 and 21.6 kHz