An aurora borealis, or northern lights, twists across the heavens above Ersfjord (map), Norway, in the first hour of September 15.
Three days earlier NASA's Solar Dynamics Observatory, a sun-monitoring satellite, witnessed a magnetic eruption on the sun, which unleashed a gigantic cloud of charged particles into space. The southern part of the particle cloud, or solar wind, grazed Earth's magnetic field on the 14th and 15th, resulting in a particularly good night for aurorae.
During the Northern Hemisphere's autumn and spring, solar magnetic fields are oriented in just the right way to cause "rips" in Earth's magnetic field. The resulting increase in solar wind on Earth encourages auroral sky shows but can also damage satellite technology and electrical grids on the ground.
"Changes in the magnetic field for both the sun and Earth really determine what [particles] get launched into space and hit the Earth," said John Manuel, a research scientist with the Canadian Space Agency.
"The orientation and variability in the magnetic fields are really what end up making it more or less favorable for auroras here on Earth."
Sparked by charged particles that had been ejected by the sun three days earlier, an aurora borealis streaks into view in the wee hours of September 15 over Ersfjord, Norway. The same night, similar shows enlivened skies over northern Canada and elsewhere in Europe.
When a charged-particle cloud enters the upper atmosphere of Earth, it smashes into and breaks up gas molecules, creating the northern lights (or in the Southern Hemisphere, the southern lights).
"Like gas inside a neon sign, as the atoms smash together they begin to glow—producing a great light show," Manuel said.
The colors a sky-watcher sees depends on what type of gas is being hit and how high it is. For example, the green aurora pictured was the result of oxygen-atom collisions about 60 to 120 miles (100 to 200 kilometers) up.
An aurora borealis appears to touch down on an illuminated bridge in Sommarøya in northern Norway on September 8.
In addition to the September 11 coronal mass ejection (CME)—a giant bubble of charged solar particles—multiple bursts of solar magnetic activity shot CMEs toward Earth in the first few days of September.
The Solar Dynamics Observatory caught sight of the initial explosive flare emanating from a giant group of sunspots. The flare sent a CME racing toward Earth at 250 miles (400 kilometers) per second. When the particles smacked Earth's magnetic field on the eighth, auroral sparks flew.