From the Upper Atmosphere to the Ocean Depths: A Personal Exploration of Wave Physics

The terrestrial ionosphere is one of the most fascinating natural plasma laboratories available to us. Stretching from roughly 60 km to more than 1000 km above Earth’s surface, it is sculpted by solar radiation, geomagnetic fields, and atmospheric chemistry. This region reflects and refracts radio waves, enabling long‑distance communication long before satellites existed. Its layered structure—D, E, and F regions—responds dynamically to solar activity, making it both a challenge and a delight for physicists and communication engineers.

I began my research career at the Space Physics Laboratory in the Department of Physics, Andhra University, Visakhapatnam, as a UGC‑NET scholar. Those early years gave me the rare opportunity to work hands‑on with a suite of ionospheric measurement systems:

• Radio-wave absorption instruments

• HF Doppler radar systems that we designed and fabricated to study F-region ionospheric irregularities and dynamics,

• VLF (Very Low Frequency) receivers for probing the lower ionosphere, particularly the D‑region

• Digital Ionosonde - a radar‑like system that transmits swept‑frequency pulses vertically into the ionosphere to retrieve electron density profiles, layer heights, and the dynamic behavior of the E and F regions.

My PhD work focused on simulating the terrestrial ionospheric F‑region anomaly, a topic I plan to write about in detail in a future post. I’ve been thinking about revisiting that work using modern compilers—perhaps even CUDA‑Fortran—to explore how far today’s computational tools can push those models.

After completing my PhD, I transitioned to the Signal Processing Group at a Naval Science laboratory, where my focus shifted from the upper atmosphere to the ocean. Here, the physics changes dramatically. Unlike in free space, electromagnetic waves barely penetrate seawater. Only the very lowest frequency band

• ELF (Extremely Low Frequency): below 30 Hz, and

• VLF (Very Low Frequency): 3–30 kHz

can travel any meaningful distance underwater. These bands have enormous wavelengths and are capable of penetrating seawater to depths unreachable by ordinary radio waves.

Because of this, submarines historically had to rise at least to periscope depth to receive VLF transmissions, while ELF systems were reserved for one‑way, low‑bit‑rate messaging to deeply submerged vessels. Both ELF and VLF signals propagate through the Earth–ionosphere waveguide, a natural channel formed between the planet’s surface and the lower ionosphere. Long before satellites and GPS, this was the backbone of global naval communication.

In air, radio waves attenuate roughly with the inverse square of frequency, and the lower ionosphere acts as a reflective boundary for signals below about 20 MHz. But once you enter the ocean, the rules flip. Radio waves die out quickly, and sound becomes the primary carrier of information. Underwater communication and sensing rely on sound waves - mechanical waves—compressions and rarefactions—rather than electromagnetic ones.

Water behaves almost like the inverse of air. Where we use radar in the atmosphere, we use SONAR underwater. Sound propagation in the ocean is shaped by temperature gradients, pressure variations, salinity, and the layered structure of the water column. Unlike air, which is effectively a semi‑infinite medium, the ocean is bounded above and below, creating complex propagation paths. Sound waves almost get total internally reflected at water air interface.

One striking feature of underwater acoustics is how sound from a surface ship’s engines can travel remarkably long distances, especially when it couples into favorable channels such as the deep sound channel (SOFAR). These low‑frequency engine signatures can propagate far beyond visual range—sometimes hundreds of kilometers—depending on environmental conditions. Yet, despite this long‑range propagation, detecting a quiet, well‑designed submarine remains extremely challenging because it can exploit thermal layers, pressure gradients, and complex bathymetry to mask its acoustic footprint.

These constraints make the detection of submersible vessels a demanding problem. Recent reports about an Iranian vessel that was sunk by a torpedo fired from a submarine have raised the possibility that it may not have detected the approaching torpedo in time—an example that highlights how environmental conditions, acoustic propagation, and platform capabilities can converge to create blind spots in underwater sensing.

All these factors make the physics of underwater propagation—and the engineering built on top of it—an endlessly intriguing domain.