What Causes the Aurora Borealis ?

The aurora appears as a curtain, an arc or a spiral, usually following the lines of Earth’s magnetic field. Most displays are green, but strong occurrences can be red, violet, and white. For most of human history, the colors were a source of mystery. Northern cultures created legends about the lights, often associating them with life after death. The Inuit believed the spirits of their ancestors were dancing across the sky, and in Norse mythology, the aurora was a bridge of fire connecting the gods to the heavens. But by the 1880s, scientists suspected a connection between the northern lights, as they are also known, and the Sun.

The temperature above the surface of the Sun is millions of degrees Celsius, causing frequent and violent collisions among gas molecules. Electrons and protons thrown free by the collisions hurtle outward from the Sun’s rotation and escape through holes in the magnetic field. Solar wind carries the charged particles, most of which deflect off Earth’s magnetic field. However, near the North and South Pole, the magnetic field is weaker, allowing some particles to enter the atmosphere. When the charged particles from the Sun strike the atoms and molecules in Earth’s atmosphere, they excite those atoms. An excited atom is one whose electrons move to high-energy orbits, and in the process the atom releases a particle of light, or photon. Different gases in the atmosphere give off light of different colors. Oxygen causes a green display and nitrogen produces red or blue colors. We perceive the collisions between solar particles and atmosphere gases as the northern lights. Many tourists trek to the northern and southern poles of Earth to catch a glimpse of the auroras, now considered one of the seven wonders of the natural world. And even though science can explain the oncemysterious phenomenon, the dazzling display of lights still provokes magical thoughts of dancing ancestors and bridges to the world beyond.

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What Causes Volcanic Lightning?

On March 10, 2010, Eyjafjallajökull volcano, a caldera in Iceland covered by an ice cap, erupted. It sent plumes of clouds across most of Europe and the Atlantic Ocean. Photos of the eruption show lightning originating and ending in the cloud of ash that hovered over the volcanic opening. The largest volcanic storms are similar to supercell thunderstorms that spread across the American Midwest. But while those thunderstorms are fairly well understood, volcanic lightning still remains mysterious. The remote location of volcanoes and infrequent eruptions make volcanic lightning difficult to study. In general, lightning occurs through the separation of positively and negatively charged particles. Differences in the aerodynamics of the particles separate the positive and negative. When the difference in charge is great, electrons flow between the positive and negative regions. A lightning bolt is a natural way of correcting the charge distributi...

Will Disease Drive Us All to Extinction ?

Virulent infectious diseases and parasites have long been shown to be a significant cause of decline in biological populations. But can disease lead to the actual extinction of the host species—such as humankind? Scientists attempt to determine the extinction-threatening effects of disease by first studying its role in historical extinctions. But proving that infectious disease is responsible for past extinctions is tricky business. After all, the extinct species is not around for scientific investigation. Even if a pathogen or parasite were discovered in a disappearing population, it would not prove that the pathogen itself was responsible for the decline. However, reasonable evidence exists that historical extinctions and extirpations—local extinctions in which a speciesc eases to exist in the specific geographic area of study—are at least partlya ttributable to infectious disease. Avian malaria and bird pox are believe...

Is the Y Chromosome Doomed?

Humans store their genes in 23 pairs of chromosomes, 22 of which are identically matched. The 23rd is a two-sided biological coin—twin Xs mean you’re female; an X and a Y, male. Chromosome pairs often trade bits of DNA in a process called recombination, the purpose of which is to keep genes functioning properly. Talk of men’s path toward extinction began in the late 1990s, when it was discovered that the human Y chromosome, which is stumpy compared with the X, does not share enough genetic material with the X to practice recombination. Left without a way to renew damaged genes, the Y would continue to degrade and would eventually disappear, geneticists announced. They slapped an expiration date on the male half of the species of sometime in the next 5 to 10 million years. To get a perspective on this prediction, scientists looked to our closest genetic relatives—the chimps. Because humans and chimpanzees shared a common ancestor 6 million years ago, genet...

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