Solar & Space Weather

Co-rotating Interaction Regions

A co-rotating interaction region (CIR) is a compressed zone that forms where fast solar wind from a coronal hole runs into slower wind ahead of it.

Because coronal holes can last for many months, a CIR can sweep past Earth again about every 27 days as the Sun rotates.

Observe
A rise in density and magnetic field strength followed by a jump in wind speed at spacecraft near Earth.
Measure
Solar wind speed, density, temperature and magnetic field at the L1 point, about 1.5 million kilometers sunward of Earth.
Model
Models of solar wind flow that follow fast and slow streams outward along the spiral magnetic field.
Explain
A coronal hole sent out fast wind that overtook slower wind and compressed it.

What is it?

The solar wind does not flow out evenly. Coronal holes, where the Sun's magnetic field opens into space, release faster wind, while other regions release slower wind. Since the Sun rotates about once every 27 days as seen from Earth, streams of fast and slow wind are twisted into a spiral pattern.

Where a fast stream catches up with slower wind ahead of it, the plasma is squeezed into a denser, hotter region with a stronger magnetic field. That compressed region, which rotates with the Sun, is a co-rotating interaction region. Behind it comes the high-speed stream itself.

How does it happen?

Stage 1
A coronal hole releases fast wind
Stage 2
Fast wind overtakes slower wind
The two streams collide
Stage 3
A compressed interaction region forms
Stage 4
The region and the fast stream reach Earth
Often again about 27 days later

What do we actually observe?

Compression first, speed after: Density and magnetic field rise in the compressed region, then wind speed climbs and stays high as the fast stream arrives.
Repeating pattern: When the same coronal hole persists, the disturbance can recur roughly every 27 days, which makes it easier to anticipate than many other events.
Moderate geomagnetic activity: CIRs and their high-speed streams commonly cause minor to moderate geomagnetic storms, and stronger ones are less common. Activity can last for days.
Radiation-belt electrons: High-speed streams are associated with increased energetic electrons in Earth's outer radiation belt in the days that follow.

How do scientists detect it?

Solar wind monitors at L1. Spacecraft such as DSCOVR, ACE and Wind measure the wind before it reaches Earth, giving roughly an hour of warning.
Extreme-ultraviolet solar imaging. Coronal holes appear dark in extreme-ultraviolet images, so imagers such as those on SDO show where fast wind may come from days ahead.
Multi-spacecraft views. Missions positioned at other points around the Sun help track how streams and their interaction regions evolve as they travel outward.

How we know

Spacecraft at L1 record the characteristic signature directly: density and field strength rise first, then speed. Matching it to a dark coronal hole seen about three to four days earlier links the disturbance to its source.

NOAA Space Weather Prediction Center

Why does it matter?

CIRs are a leading cause of recurring, moderate geomagnetic activity, especially in the declining phase of a solar cycle.
Their repeating nature makes them a useful test of space weather forecasts.
The electron increases linked to high-speed streams matter for satellites, because energetic electrons can build up charge on spacecraft surfaces.

The SpaceWeatherTracker connection

SpaceWeatherTracker already shows coronal holes, live solar wind speed and density, and the Kp index. A CIR is the story that connects them: a dark hole on the Sun, a squeeze in the wind, and a rise in geomagnetic activity at Earth.

Primary family: Solar & Space Weather