Ionosphere and VLF radio
In the highest regions of earth’s atmosphere, far above where airplanes fly, solar radiation dislodges electrons from neutral atoms in a process called ionization. This volume of free electrons surrounding earth is known as the ionosphere and has a great impact on the propagation of radio waves over long distances. At the turn of the 20th century, experimenter Guglielmo Marconi received the first radio communications from across the Atlantic—a feat made possible by the refraction of radio waves in the ionosphere. Although the existence of the ionosphere was not proven until 1927 (work which won Edward Appleton the Nobel Prize), the use of radio telegraphy and later telephony grew rapidly and played an important role in globalization.
Today the ionosphere continues to enable shortwave radio communications for military, broadcast, and radio amateur users, but it also influences the signal of global navigation systems such as GPS. Solar storms and energetic particle precipitation can disrupt communication and navigation signals, posing a hazard to users of systems that rely upon radio propagation through the ionosphere. |
Our Work
Ionosphere research in the LAIR focuses on the base of the ionosphere, known as the D-region. The free electron density in the D-region is extremely low, which makes measurements of the region difficult. Dynamics are complicated by cosmic rays and particle precipitation from the radiation belts which influence the ionization levels in addition to solar radiation. Waves in the neutral atmosphere might also imprint themselves on electron density profiles in the D-region.
The primary means of studying the D-region is by remotely sensing the reflection of very low frequency (VLF) radio waves. These waves are kilometers long and are generated naturally by lightning and from manmade communication and navigation transmitters. VLF waves travel for thousands of kilometers in the earth-ionosphere waveguide. We apply a variety of techniques to invert radio measurements of lightning or transmitter signals in order to infer the state of the D-region. In particular, we are developing data assimilation and machine learning techniques to combine observations from arrays of radio receivers to construct spatial maps of the electron density profile in the D-region. This will help provide insight to the terminator D-region and electron loss from the radiation belts into precipitation patches. We design and build our own LF and VLF receivers, and also have CubeSat missions (VPM and CANVAS) for studying VLF wave energy from the top side of the D-region. We also study the propagation of radio waves in the earth-ionosphere waveguide, through the ionosphere, and into the magnetosphere. We have built several propagation models including Finite-Difference Frequency-Domain (FDFD) and Finite-Difference Time-Domain (FDTD) which numerically solve Maxwell’s equations in the waveguide and in the cold, collisional plasma of the ionosphere. We are currently building a full-wave mode theory solver for quickly computing electromagnetic wavefields in the anisotropic, inhomogeneous earth-ionosphere waveguide. These models are used to help invert real VLF measurements and study wave scattering and robust transmission methods. |
Above: Amplitude of the electric field in the earth-ionosphere waveguide from a simulated transmitter source using our finite-difference time-domain (FDTD) propagation model.
Above: Ensemble Kalman filter estimate (right) of a simulated energetic particle precipitation patch (left) in the D-region ionosphere. The top and bottom panels represent h’ and 𝛽, the two parameters of the exponential electron density profile model for the D-region from Wait and Spies (1964).
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References
- Xu, W., R. A. Marshall, A. Kero, E. Turunen, D. Drob, J. Sojka, D. Rice (2019), VLF measurements and modeling of the D-region response to the 2017 total solar eclipse, 57, 7613-7622, doi:10.1109/TGRS.2019.2914920.
- Ramos, D., W. Gordon, A. Sousa, R. A. Marshall, K. Brunetto, J. Ballenthin, R. Kay, J. Patton, S. Quigley, J. Fennelly, M. Starks, T. Willet-Gies, S. Tullino, I. Linscott, and U. Inan (2019), A CubeSat receiver for the study of VLF waves at LEO, SPIE 11131, doi:10.1117/12.2530479.
- Cohen, M. B., N. C. Gross, M. A. Higginson-Rollins, R. A. Marshall, M. Golkowski, W. Liles, D. Rodriguez, and J. Rockway (2018), The lower ionospheric VLF/LF response to the 2017 Great American Solar Eclipse observed across the continent, Geophys. Res. Lett., 45, 3348-3355, doi:10.1002/2018GL077351.
- Marshall, R. A., W. Xu, and A. P. Sousa (2018), Late-time instability in finite difference modeling of very-low-frequency propagation in the earth-ionosphere waveguide, 2018 ACES, doi:10.23919/ROPACES.2018.8364239.
- A. P. Sousa and R. A. Marshall (2018), Spatial distributions of magnetospheric radio energy due to lightning, 2018 ACES, doi:10.23919/ROPACES.2018.8364236.
- Marshall, R. A., T. Wallace, and M. Turbe (2017), Finite-difference modeling of very-low-frequency propagation in the earth-ionosphere waveguide, IEEE Trans. Antennas Propag., 65, 7185-7197, doi:10.1109/TAP.2017.2758392.
- Kabirzadeh, R., R. A. Marshall, U. S. Inan (2017), Early/fast VLF events produced by the quiescent heating of the lower ionosphere by thunderstorms, 122, 6217-6230, doi:10.1002/2017JD026528.
- Graf, K. L., N. G. Lehtinen, M. Spasojevic, M. B. Cohen, R. A. Marshall, and U. S. Inan (2013), Analysis of experimentally validated trans-ionospheric attenuation estimates of VLF signals, J. Geophys. Res. Space Physics, 118, 2708-2720, doi:10.1002/jgra.50228.
- Graf, K. L., M. Spasojevic, R. A. Marshall, N. G. Lehtinen, F. R. Foust, and U. S. Inan (2013), Extended lateral heating of the nighttime ionosphere by ground-based VLF transmitters, J. Geophys. Res. Space Physics, 118, doi:10.1002/2013JA019337.
- Marshall, R. A. (2014), Effect of self-absorption on attenuation of lightning and transmitter signals in the lower ionosphere, J. Geophys. Res. Space Physics, 119, doi:10.1002/2014JA019921.
- Marshall, R. A., and J. B. Snively (2014), Very low frequency subionospheric remote sensing of thunderstorm-driven acoustic waves in the lower ionosphere, J. Geophys. Res. Atmos., 119, doi:10.1002/2014JD021594.
- Marshall, R. A., T. Adachi, R.-R. Hsu, and A. B. Chen (2014), Rare examples of early VLF events observed in association with ISUAL-detected Gigantic Jets, Radio Sci., 49, 36-43, doi:10.1002/2013RS005288.
- Marshall, R. A., R. T. Newsome, N. G. Lehtinen, N. Lavassar, and U. S. Inan (2010), Optical signatures of radiation-belt electron precipitation induced by ground-based VLF transmitters, J. Geophys. Res., 115, A08206, doi:10.1029/2010JA015394.
- Marshall, R. A., and U. S. Inan (2010), Two-dimensional frequency domain modeling of lightning EMP-induced perturbations to VLF transmitter signals, J. Geophys. Res., 115, A00E29, doi:10.1029/2009JA014761.