I saw a flash of green above the horizon at sunset. Was it real? What caused it? Will I be able to see it again?
Congratulations! The green flash is real but it is rarely seen, since it requires special conditions to be observed. The green flash is usually a band or vertical ray of green light just above the setting or rising sun, and can be green, violet, or blue. To see it, you need a clear, flat horizon and a haze free sky. An ocean works well - so do deserts.
The green flash is caused by rays of sunlight refracting (bending) in the atmosphere. Because refraction depends on the wavelength (color) of the light, blue, violet and green light are refracted more than yellow, orange, and red light. So at sunset, when the light has the most atmosphere to be bent by, the sun is surrounded by "shadows" of different colors, with the blue/violet/green shadows farther out. The red, orange, and yellow shadows are absorbed by the atmosphere, and the blue and violet shadows are scattered by the atmosphere, so the strongest shadow left is usually the green one. This effect is only strong enough to see for a few seconds during sunrise and sunset, hence the "green flash."
To see the green flash again, you'll probably need to watch a lot of sunsets on clear days over the ocean. It really is mostly just a matter of being in the right place at the right time.
Briony is an Assistant Professor at Purdue University, and uses orbital remote sensing of Mars and the Moon supported by laboratory and field work to investigate planetary surface processes. Her primary tool is spectroscopy, including both visible/near-infrared and mid-infrared. Briony earned her B.S. in Physics from Oregon State University in 2005 and her Ph.D. in Astronomy and Space Sciences from Cornell University in 2010. Her thesis advisor was Prof. Jim Bell (now at ASU). Her thesis was titled "Wind, water, and the sands of Mars", and focused on using spectral and morphologic characteristics of sediments in the northern lowlands of Mars to reveal past and ongoing interactions with liquid water. After her PhD, Briony became an Exploration Postdoctoral Fellow in the School of Earth and Space Exploration at Arizona State University, working primarily in the Mars Space Flight Facility with Phil Christensen. There she investigated the composition, spectral properties, and terrestrial field analogs of soils and sediments on Mars. The results of these studies will aid in constraining the habitability of ancient surface environments on Mars, and may have implications for our understanding of the early Earth.