Do cosmic rays cause lightning? —B. Whiteside, Woodridge, III.

Joseph R. Dwyer, a professor of physics and space sciences at the Florida Institute of Technology, has wondered the same thing:

Although some researchers have proposed that cosmic rays instigate lightning, others, including me, have voiced doubts about this theory. At present, the debate remains unsettled.

Decades of measurements inside thunderstorms have failed to find electric fields larg e enough to spontaneously spark lightning. A mechanism proposed in 1992 by physicist Alex V. Gurevich of the Lebedev Physical Institute in Moscow and his collaborators suggests that the movement of large showers of energetic particles produced by high-energy cosmic rays—which originate from exploding stars—might trigger lightning's massive energy discharge. For Gurevich's mechanism to work, many charged particles must pass through the storm at once. Because cosmic-ray showers alone do not produce enough such particles, Gurevich postulated that a thunderstorm gives the cosmic-ray shower a boost by increasing the number of energetic particles through a process called runaway breakdown.

Runaway breakdown occurs when a cosmic-ray particle hits air molecules in the atmosphere, knocking loose high-energy electrons. As these ejected electrons collide with other air molecules, they generate more runaway electrons as well as x-rays and gamma rays, resulting in an avalanche of energetic particles that tears through the cloud. According to the Gurevich model, this cascade is the catalyst that sparks a lightning bolt.

We know that runaway breakdown does work for the low-level electric fields inside thunderstorms. From observing big bursts of x-rays and gamma rays shooting out of thunderstorms, we also know that it sometimes happens right before lightning strikes. But skepticism still surrounds the cosmic-ray proposal. (Some theories, in fact, involve runaway breakdown spurred by other sources.) The main stumbling block arises because lightning must form a conductive channel to propagate. This channel, extremely hot and just a few centimeters in width, acts like a metal wire, allowing tremendous electric currents to flow through. It remains unclear how a large, diffuse discharge produced by cosmic-ray-induced runaway breakdown would result in such a narrow, hot channel.

How do three tiny bones amplify sound into the inner ear? —P. Madsen, Brooklyn, N.Y.

Douglas E. Vetter, assistant professor of neuroscience at the Tufts University Sackler School of Graduate Biomedical Sciences, replies:

The hammer, anvil and stirrup bones of the middle ear—also known as the malleus, incus and stapes, respectively, and as ossicles, collectively—are arranged in a lever system. Their leveraging capabilities, combined with the concentration of vibration energies from the larger eardrum to the much smaller stirrup, efficiently transmit the forces that allow us to hear.

The middle-ear ossicles lie between the eardrum and the cochlea (the spiral-shaped conduit whose hair cells transmit sound to the inner ear). The inner ear is filled with fluid, so our hearing system must transmit airborne sound vibrations to that fluid. Without these ossicles, only about 0.1 percent of sound energy would make it into the inner ear—the rest would reflect off the surface much like voices on land do when a listener is underwater.

When vibrated by sound, the eardrum sets the middle-ear ossicles into motion. One end of the hammer is attached to the eardrum, and the other end forms a hinge with the anvil. The opposite end of the anvil is fused to the stirrup. The footplate of the stirrup—the flat part that resembles the footrest in an actual stirrup—is loosely attached to an opening in the cochlea known as the oval window, and it moves in and out like a piston. This motion transfers the amplified vibrations to the fluid-filled inner ear, thereby signaling the brain of a sound event.

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