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What is Solar Wind?

solar wind
This is an artist's concept of the Earth's global magnetic field, with the bow shock. Earth is in the middle of the image, surrounded by its magnetic field, represented by purple lines. The bow shock is the blue crescent on the right. Many energetic particles in the solar wind, represented in gold, are deflected by Earth's magnetic "shield".
Credit: Walt Feimer (HTSI)/NASA/Goddard Space Flight Center Conceptual Image Lab
solar wind
This is an artist's concept of the Earth's global magnetic field, with the bow shock. Earth is in the middle of the image, surrounded by its magnetic field, represented by purple lines. The bow shock is the blue crescent on the right. Many energetic particles in the solar wind, represented in gold, are deflected by Earth's magnetic "shield".
Credit: Walt Feimer (HTSI)/NASA/Goddard Space Flight Center Conceptual Image Lab

The solar wind streams plasma and particles from the sun out into space. Though the wind is constant, its properties aren't. What causes this stream, and how does it affect the Earth?

Windy star

The corona, the sun's outer layer, reaches temperatures of up to 2 million degrees Fahrenheit (1.1 million Celsius). At this level, the sun's gravity can't hold on to the rapidly moving particles, and it streams away from the star.

The sun's activity shifts over the course of its 11-year cycle, with sun spot numbers, radiation levels, and ejected material changing over time. These alterations affect the properties of the solar wind, including its magnetic field properties, velocity, temperature and density. The wind also differs based on where on the sun it comes from and how quickly that portion is rotating.

The velocity of the solar wind is higher over coronal holes, reaching speeds of up to 500 miles (800 kilometers) per second. The temperature and density over coronal holes are low, and the magnetic field is weak, so the field lines are open to space. These holes occur at the poles and low latitudes, and reach their largest when activity on the sun is at its minimum. Temperatures in the fast wind can reach up to 1 million degrees F (800,000 C).

At the coronal streamer belt around the equator, the solar wind travels more slowly, at around 200 miles (300 km) per second. Temperatures in the slow wind reach up to 2.9 million F (1.6 million C).

Affecting Earth

As the wind travels off the sun, it carries charged particles and magnetic clouds. Emitted in all directions, some of the solar wind is constantly buffeting our planet, with interesting effects.

If the material carried by the solar wind reached a planet's surface, its radiation would do severe damage to any life that might exist. Earth's magnetic field serves as a shield, redirecting the material around the planet so that it streams beyond it. The force of the wind stretches out the magnetic field so that it is smooshed inward on the sun-side and stretched out on the night side.

Sometimes the sun spits out large bursts of plasma known as coronal mass ejections (CMEs), or solar storms. More common during the active period of the cycle known as the solar maximum, CMEs have a stronger effect than the standard solar wind. [Photos: Stunning Photos of Solar Flares & Solar Storms]

When the solar wind carries CMEs and other powerful bursts of radiation into a planet's magnetic field, it can cause the magnetic field on the back side to press together, a process known as magnetic reconnection. Charged particles then stream back toward the planet's magnetic poles, causing beautiful displays known as the aurora borealis in the upper atmosphere. [Photos: Amazing Auroras of 2012

Though some bodies are shielded by a magnetic field, others lack their protection. Earth's moon has nothing to protect it, so takes the full brunt. Mercury, the closest planet, has a magnetic field that shields it from the regular standard wind, but it takes the full force of more powerful outbursts such as CMEs.

When the high- and low-speed streams interact with one another, they create dense regions known as co-rotating interaction regions (CIRs) that trigger geomagnetic storms when they interact with Earth's atmosphere.

Studying the solar wind

NASA's Ulysses mission launched on Oct. 6, 1990, and studied the sun at various latitudes. It measured the various properties of the solar wind over the course of more than a dozen years.

The Advanced Composition Explorer (ACE) satellite orbits at one of the special points between Earth and the sun known as the Lagrange point. In this area, gravity from the sun and the planet pull equally, keeping the satellite in a stable orbit. Launched in 1997, ACE measures the solar wind and provides real-time measurements of the constant flow of particles.

Further reading:

Solar wind conditions for the last 24 hours (NOAA/Space Weather Prediction Center)

Solar wind conditions for the last 7 days (NOAA/Space Weather Prediction Center)

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