The solar wind is a flow of charged particles from the Sun’s corona, carrying the energy that causes the aurora.
The solar wind is a flow of charged particles or plasma, mostly electrons but also protons, from the Sun’s outer atmosphere, the solar corona. The solar wind travels at different speeds, fast and slow. The solar wind is what carries the energy that causes the aurora. Faster solar wind means that the particles are more energetic when they hit Earth which often leads to brighter and more dynamic aurora displays. Higher density means more vibrant colours.
Near the Earth, solar wind speeds of 300-400 km/s is considered slow while speeds of more than 400 km/s is considered fast. Faster solar winds means that the solar particles have more energy to cause larger impact on Earth, not only bright aurorae, but occasionally powerful geomagnetic storms.
The faster the solar wind, the wider the auroral oval becomes and moves further southward (or north in the southern hemisphere). For comparison, average solar wind is almost 4000 thousand times faster than the highest wind speed recorded on Earth.
Interested in seeing current conditions of the solar wind? Take a look at the real time data below from the DSCOVR satellite.
Solar wind arrival time chart
The fast solar wind originates in coronal holes, regions where the Sun’s magnetic field lines are open, extending far into interplanetary space. From coronal holes, the solar wind can reach speeds upwards of 800 km/s. However, when explosions of solar particles occur called coronal mass ejections, near active sunspot regions, the solar wind speeds can reach up to 3000 km/s near the Earth.
Earth’s magnetic field protects us from these energetic solar particles forming a large bubble or cavity around it called the magnetosphere. On the daytime side of the Earth that faces the Sun, the magnetosphere is compressed by the solar wind. On the opposite side, the nighttime side, it stretches out into a magnetotail.
The solar wind has embedded magnetic field orientation called Bz. Like a magnet, the solar wind has regions of positive and negative polarity, pointing either north and south.
When the solar wind hits our magnetosphere, it is deflected back into space if the orientation is north. If on the other hand the orientation is south (negative) — opposite the direction of the Earth's northward-pointing magnetic field — the two opposing fields reconnect. Then, a crack opens up in Earth’s magnetic field and the solar wind comes pouring in.
This magnetic reconnection happens first on the sunward side of Earth and a second time on the night side, in the magnetotail. When that happens, electrons are accelerated, spiralling along Earth’s magnetic field lines towards the polar regions. There, the electrons collide with atoms and molecules of Earth’s upper atmosphere, energetic enough excites nitrogen and oxygen and cause it to glow colourfully. This is how auroras form. This is also why the charged solar particles that cause the Northern Lights do not come directly from the Sun, but from the magnetotail. And why the Northern Lights are on average most commonly seen around midnight.
Fast solar winds are often energetic enough to cause geomagnetic storms. The Kp-index is used to quantify the magnitude of geomagnetic disturbance caused by the solar wind on a scale from 0-9. A Kp index of 5-9 means geomagnetic storms are occurring, with 5 being minor and 9 extreme. Minor storms are very frequent, especially under the auroral oval.
Extreme geomagnetic storms (Kp 9, G5) are very rare, fortunately. During the most powerful storms, currents are induced which can cause damage.
Geomagnetic storms and Northern Lights are most common in the spring and autumn, in the months closest to the equinoxes.
The solar wind radiates out all over the solar system, more than a million tons of material every second. Its pressure keeps all the planets within a vast Sun-centred protective magnetic bubble called the heliosphere. The heliosphere protects Earth and the other planets from energetic cosmic rays.
When the solar wind speed is low, the heliosphere is small. As the solar wind speed increases during the solar cycle, the heliosphere expands.
The Sun rotates once every 27 days or so. Because of that, the magnetic field embedded in the solar wind forms a spiral called the Parker spiral. Named after solar physicist Eugene Parker, this spiral is in many ways like a ballerina skirt with hills and valleys.
