What Causes the Aurora Borealis?

The Aurora Borealis (Northern Lights) occurs when charged particles from the solar wind interact with Earth's magnetosphere and atmosphere. This interaction creates the spectacular light displays visible at high latitudes like Iceland.

⚡ Solar Wind and the Bz Component

Solar Wind: A stream of charged particles (mainly protons and electrons) constantly flowing from the Sun at 400-800 km/s.

Interplanetary Magnetic Field (IMF): The solar wind carries magnetic fields from the Sun. The critical component for Aurora activity is the Bz component - the north-south orientation of this magnetic field.

🧲 The Famous "Bz Flip" - Key to Aurora Forecasting

Bz Southern (Negative): When the IMF Bz component points southward (negative values), it can reconnect with Earth's northward-pointing magnetic field. This magnetic reconnection opens a pathway for solar wind particles to enter Earth's magnetosphere.

Bz Northern (Positive): When Bz points northward (positive), it aligns with Earth's field, preventing reconnection and blocking Aurora activity.

Why forecasters watch Bz: A "Bz flip" from positive to negative can trigger Aurora activity within 30-60 minutes. Sustained negative Bz values enhance Aurora probability, with stronger negative values generally increasing activity potential.

🌍 Earth's Magnetosphere and Aurora Formation

When solar wind particles breach Earth's magnetic defenses (during southward Bz), they are funneled along magnetic field lines toward the polar regions. As these high-energy particles collide with atmospheric gases:

  • Oxygen atoms (O): Produce green light at 557.7 nm (most common) and red light at 630.0 nm (higher altitudes)
  • Nitrogen molecules (N₂): Create blue and purple colors
  • Altitude matters: Green Aurora typically occur at 100-300 km altitude, red Aurora above 300 km

🎨 Aurora Colors and Altitude

Color Formation by Atmospheric Gases

Different atmospheric gases produce different colors when excited by high-energy particles:

Green (557.7 nm): Oxygen atoms at 100-300 km - Most common Aurora color
Red (630.0 nm): Oxygen atoms above 300 km - Rare, requires strong activity
Blue/Purple: Nitrogen molecules at lower altitudes
Pink/Magenta: Mix of red oxygen and blue nitrogen - Very rare

🌌 The Aurora Oval

The Aurora doesn't occur randomly across the sky. It follows a predictable pattern called the Aurora oval - an invisible ring around Earth's magnetic poles.

Iceland's Position in the Aurora Oval

Iceland sits at approximately 64°N latitude, placing it directly under the Aurora oval during moderate geomagnetic activity (KP 3-4). During stronger storms, the oval expands southward, making Aurora visible even from Reykjavík.

  • Quiet conditions (KP 0-2): Aurora oval too far north for Iceland visibility
  • Active conditions (KP 3-4): Aurora oval covers northern Iceland
  • Storm conditions (KP 5+): Aurora oval expands to cover all of Iceland

🚀 Space Weather and Aurora Intensity

Coronal Mass Ejections (CMEs)

What are CMEs?

Coronal Mass Ejections are massive bursts of solar plasma and magnetic field released from the Sun's corona. When Earth-directed, they can cause multi-day Aurora storms.

  • Travel Time: 1-3 days from Sun to Earth
  • Impact Duration: Aurora activity can last 12-48 hours
  • Magnetic Field: Strong CMEs often have embedded magnetic fields with rotating Bz components

High-Speed Solar Wind Streams

These originate from coronal holes on the Sun and create recurring Aurora activity patterns, often repeating every 27 days (one solar rotation period).

Solar Cycle and Aurora Activity

The Sun follows an 11-year cycle of activity that directly affects Aurora frequency:

  • Solar Maximum: More sunspots, CMEs, and Aurora activity
  • Solar Minimum: Fewer sunspots, mainly high-speed stream Aurora
  • Current Cycle: Solar Cycle 25 (2019-2030)
  • Peak Activity: Expected around 2024-2026

📡 Measuring Aurora Activity

The KP Index System

Understanding KP Values for Iceland

KP 0-2: Aurora oval too far north - No visibility from Iceland
KP 3-4: Aurora visible from rural Iceland, away from light pollution
KP 5-6: Strong Aurora visible from most locations, including near cities
KP 7-9: Extreme geomagnetic storm - Aurora visible everywhere, even Reykjavík

Advanced Measurement Techniques

Scientists use various instruments to study Aurora:

  • Magnetometers: Measure Earth's magnetic field variations
  • All-Sky Cameras: Capture Aurora movement and structure
  • Riometers: Detect radio wave absorption in the atmosphere
  • Satellites: Monitor solar wind conditions and particle precipitation

🌍 Aurora Around the World

While we focus on Iceland's Aurora Borealis, similar phenomena occur worldwide:

Global Aurora Phenomena

  • Aurora Borealis: Northern hemisphere - Alaska, Canada, Greenland, Scandinavia, Iceland, northern Russia
  • Aurora Australis: Southern hemisphere - Antarctica, southern Chile, southern Australia, New Zealand
  • Rare Equatorial Aurora: During extreme geomagnetic storms, Aurora can be visible from much lower latitudes