We present a statistical survey of the latitudinal structure of the fast magnetosonic wave mode detected by the Van Allen Probes spanning the time interval of 21 September 2012 to 1 August 2014. We show that statistically, the latitudinal occurrence of the wave frequency (f) normalized by the local proton cyclotron frequency (fcP) has a distinct funnel-shaped appearance in latitude about the magnetic equator similar to that found in case studies. By comparing the observed E/B ratios with the model E/B ratio, using the observed plasma density and background magnetic field magnitude as input to the model E/B ratio, we show that this mode is consistent with the extra-ordinary (whistler) mode at wave normal angles (θk) near 90°. Performing polarization analysis on synthetic waveforms composed from a superposition of extra-ordinary mode plane waves with θk randomly chosen between 87 and 90°, we show that the uncertainty in the derived wave normal is substantially broadened, with a tail extending down to θk of 60°, suggesting that another approach is necessary to estimate the true distribution of θk. We find that the histograms of the synthetically derived ellipticities and θk are consistent with the observations of ellipticities and θk derived using polarization analysis. We make estimates of the median equatorial θk by comparing observed and model ray tracing frequency-dependent probability occurrence with latitude and give preliminary frequency dependent estimates of the equatorial θk distribution around noon and 4 RE, with the median of ~4 to 7° from 90° at f/fcP = 2 and dropping to ~0.5° from 90° at f/fcP = 30. The occurrence of waves in this mode peaks around noon near the equator at all radial distances, and we find that the overall intensity of these waves increases with AE∗, similar to findings of other studies.
Bibliographical noteFunding Information:
We thank Dr. K. R?nnmark at Ume? University in Sweden for providing us with the warm plasma instability code WHAMP. The EMFISIS data can be obtained from the electronic archive at the Space Physics Data Facility (http://spdf.gsfc.nasa.gov/). We acknowledge use of NASA/GSFCs Space Physics Data Facilitys OMNIWeb (or CDAWeb or ftp) service and OMNI data. The EFW-derived densities, frompotentials, are available at http://www.space.umn.edu/rbspefwdata. The shadow times of the antisunward short antenna are available at http://emfisis.physics.uiowa.edu/events/rbsp-a/SP5antinshadow/ and http://emfisis.physics.uiowa.edu/events/rbspb/SP5antinshadow/. The EMFISIS-WFR noise levels are available at http://emfisis.physics.uiowa.edu/events/rbsp-a/backgrounds/and http://emfisis.physics.uiowa.edu/events/rbsp-b/backgrounds/. At Goddard this study was supported by NASA prime contract NAS5-01072. The research at The University of Iowa was supported by JHU/APL contract 921647 under NASA Prime contract NAS5-01072.
© 2016. American Geophysical Union. All Rights Reserved.
- fast magnetosonic waves
- latitudinal distribution
- statistical study
- wave normal angle