The Science Behind the Aurora Borealis

Last night’s aurora brought rare and beautiful views to parts of the U.S. – and while the solar storm is still ongoing, tonight (Wednesday, November 12) will be much more cloud-limited across New England.

We reached G4 (Severe) levels yesterday with a Kp index of 7.  What does that mean? Well the Kp index is a numerical scale (0-9) that measures the strength of global geomagnetic activity, while the G-scale (G1-G5) is a descriptive, alert-based system used by NOAA to communicate the potential effects and severity of a geomagnetic storm to the public. A G4 storm is explicitly defined by its corresponding Kp index level. Tonight, it’s expected to remain elevated but slightly lower, generally hovering around Kp 6 – still strong enough for potential aurora visibility here in New England if skies are clear.

What Is the Aurora Borealis?

This celestial light show is fundamentally a solar phenomenon. Here’s the science breakdown:

1. Solar Wind: It begins with the solar wind, a continuous stream of charged particles (plasma) emitted from the Sun’s atmosphere. These particles, mainly electrons and protons, travel across space at speeds of hundreds of kilometers per second.
2. Magnetic Deflection: When the solar wind reaches Earth, the planet’s magnetic field (magnetosphere) acts as a shield, deflecting most of these particles.
3. Polar Entry: However, near the North and South Poles, the magnetic field lines converge, creating funnels that allow a fraction of the charged particles to slip through and enter the upper atmosphere.
4. Atmospheric Collisions: These high-speed particles collide with atoms and molecules of gases in the Earth’s atmosphere, primarily oxygen and nitrogen, at altitudes between 100 to 300 kilometers.
5. Light Emission: The collisions excite the atmospheric gases, causing their electrons to jump to a higher energy state. When these electrons drop back down to their normal, lower energy state, they release the excess energy in the form of a photon of light—this is the aurora.

Colors of the Lights

The specific colors we see depend on the type of gas being hit and the altitude of the collision:

• Green: The most common color, produced by the excitation of oxygen atoms, usually at lower altitudes (around 100-250 km).
• Red: Less frequent, produced by oxygen at higher altitudes (above 250 km).
• Blue or Violet: Produced by nitrogen molecules, usually at lower altitudes.
  

  Athol, MA 11/11/25
  Courtesy Tom Williams

Extended Visibility

While typically confined to the polar regions (the auroral oval), the aurora can be seen in lower-latitude locations, such as New England, during particularly intense geomagnetic storms. When a Coronal Mass Ejection (CME) from the Sun is strong enough (reaching high levels on the Kp-index scale), the auroral oval temporarily expands toward the equator, making the magnificent display visible far south of its usual home.

The Dynamic Nature of the Aurora

Seeing the lights is often fickle and requires patience. The aurora is a dynamic phenomenon—it can pulse, flutter, or change intensity within minutes. Because the display is directly tied to the highly variable solar wind and geomagnetic conditions, auroral forecasts are extremely short-term, often only predicting activity with reliability a few hours in advance, or less!

Why New England May Miss Out Tonight:

Unfortunately, cloud cover is more extensive tonight than it was during Tuesday’s display. While brief clearing may occur in spots, most of New England will remain mostly to completely cloudy through peak viewing hours.

📸 If you’re still trying: Use a long-exposure camera, get away from city lights, and look north – but understand clouds may fully block the view.

Ofcourse, if sky conditions change or if we see reports of the Aurora coming in, we’ll be sure to let you know on social & notify you on our FREE 1DegreeOutside weather app!