Northern lights visible in 18 states tonight. See where.

Key Points:

• The northern lights (aurora borealis) are expected to be most intense on April 16 and 17, with visibility in parts of 18 U.S. states including Alaska, Idaho, Michigan, and New York, according to NOAA's Space Weather Prediction Center.
• Clear weather conditions are necessary to view the aurora, and NOAA provides a daily "viewline" forecast and a three-day aurora forecast with a Kp index to help determine how far south the lights may be visible.
• The auroras occur when charged particles from solar flares collide with atoms in Earth's magnetic field, causing them to emit light in colors such as green, red, blue, and purple.
• The Kp index, ranging from 0 to 9, measures fluctuations in Earth’s magnetic field and helps predict auroral activity; extreme events can make the lights visible much farther south, as happened in 1958 when they were seen as far south as Mexico City.
• Earth's magnetic field acts as a shield against solar winds, but interactions between these particles and atmospheric gases during strong solar activity create the stunning auroral displays.