This high-resolution image shows the Sun in ultraviolet light, revealing its upper atmosphere, the corona. It was taken by the Extreme Ultraviolet Imager instrument on the European Solar Orbiter mission and is a composite of 25 individual exposures taken on 22 March 2023.
Image: 1/4, Credit:
ESA & NASA/Solar Orbiter/EUI Team, Image processing by Emil Kraaikamp (CC BY-SA 3.0 IGO)
Flare and coronal mass ejection
On 25 February 2014, an active region at the edge of the Sun emitted one of the largest flares of the solar cycle as well as a coronal mass ejection.
Image: 2/4, Credit:
Solar Dynamics Observatory/NASA
Elongated coronal hole on the Sun
Coronal holes are regions of open magnetic field on the Sun's surface, from which solar wind particles stream into space. In this image, captured in the extreme-ultraviolet spectrum, a large, dark coronal hole can be seen near the centre and lower part of the Sun. This large, elongated coronal hole rotated across the Sun's surface in early 2017.
Image: 4/4, Credit:
Solar Dynamics Observatory, NASA
This article explores fundamental processes on the Sun's surface that, through various complex mechanisms, ultimately lead to what is commonly known as space weather.
The 'quiet' Sun
Fuelled by nuclear fusion at its core, the Sun's surface has a temperature of approximately 5500 degrees Celsius. Like all objects, it emits thermal radiation. The human body, for instance, radiates in the long-wave infrared range, which is invisible to the eye but can be detected with equipment such as infrared cameras. As temperatures increase, thermal radiation has more energy – and a shorter wavelength – and shifts along the spectrum from the infrared to the visible range. A metal wire, for example, starts to glow when heated. Sunlight is scattered by Earth's atmosphere, which is why the Sun appears yellowish from the surface of our planet.
The Sun's radiation spectrum
The maximum radiation output of the Sun falls within the visible light range, but infrared radiation (on the right, with longer wavelengths than visible light) and ultraviolet radiation (on the left, with shorter wavelengths) are also emitted.
Thermal radiation covers a wide spectrum, and at shorter wavelengths it moves beyond the visible range. This is where the Sun emits its most energetic radiation: primarily ultraviolet (UV) and extreme ultraviolet (EUV) radiation. The Sun's power output is enormous – in the blink of an eye, it radiates the energy demand for all of humanity for around 50,000 years (based on 2021 figures). Only a small fraction of this energy reaches Earth, but it is enough to illuminate our planet with a brightness that we know as daylight and heat it to a temperature suitable for sustaining life.
Only part of the Sun's high-energy radiation reaches Earth's surface as it is partially absorbed by the atmosphere. For example, much of the UV radiation is absorbed by the ozone layer at an altitude of 15 to 60 kilometres, while EUV and even higher-energy radiation are absorbed higher up in the ionosphere, above an altitude of approximately 80 kilometres.
Another component of the Sun's activity is the solar wind – a continuous stream of particles (mainly protons and electrons) ejected by the Sun in all directions. To eject a mass equivalent to that of the Great Pyramid of Giza (around five million metric tonnes), the Sun takes only about two to three seconds. The solar wind travels at an average speed of 400 kilometres per second and hits Earth about 100 hours after leaving the Sun.
If the solar wind were to hit Earth unhindered, it would gradually detach the atmosphere from our planet – just as happened to Mars around four billion years ago. Fortunately for us, Earth has its magnetic field – a protective shield that deflects the majority of solar wind particles. Only near the polar regions, where the magnetic field lines dip into the atmosphere, can some of these particles penetrate.
Heightened solar activity – solar storms
In addition to normal solar activity, extraordinary events can occur in which the Sun suddenly emits high levels of radiation and particles. These solar storms occur in a wide variety of intensities and forms, not all of which are yet understood.
Background info – a simplified standard model of a solar storm
The Sun's magnetic field forms loops on the solar surface due to complex processes at play in the star's interior. The bases of these loops are somewhat cooler than the surrounding area and appear as dark spots (sunspots). These magnetic loops can become compressed in the middle due to the dynamics of the Sun's atmosphere, their upper section becoming a pinched-off bubble and the lower section a taut arch.
In a process known as magnetic reconnection, the two parts of the magnetic field loop can separate from each other. The upper bubble is released and hurled away from the Sun by the magnetic tension now released, taking with it the trapped plasma within. This is a coronal mass ejection, named after the Sun's corona, its outermost atmospheric layer which, during a solar eclipse, is visible to the human eye and looks like a stellar crown.
The lower part of the magnetic field loop contracts due to the released magnetic tension, and catapults plasma back onto the Sun's surface. When these plasma particles hit the solar surface, they are suddenly slowed down, releasing a huge stream of high-energy X-ray radiation known as a solar flare. Medical X-ray machines and computed tomography (CT) scanners are based on the same principle.
This image depicts the magnetic reconnection process, and while highly simplified it serves as a good standard model for illustrative purposes. In practice, processes can vary greatly: coronal mass ejections and solar flares occur in very different forms and intensities, and not necessarily together.
Standard model of a solar storm
A magnetic field loop on the surface of the Sun is split by magnetic reconnection. The upper part is expelled along with the enclosed plasma (a coronal mass ejection), while the lower part collapses back onto the Sun. When it strikes, it produces X-ray radiation (known as solar flares).
Given that the Earth, as seen from the Sun, is only about the size of an apple at a distance of one kilometre, most solar storms miss our planet. Only a small proportion hit near-Earth space, triggering a geomagnetic storm. A solar flare, which travels at the speed of light, takes about eight minutes to travel from the Sun to Earth, while a coronal mass ejection takes an average of two to three days. In exceptional cases, it may arrive in less than 24 hours.
Since major solar storms are often accompanied by flares, the arrival of the solar plasma on Earth can be predicted, with very little lead time. Particularly strong geomagnetic storms are often the result of several solar storms hitting Earth in quick succession.
Noteworthy space weather events
The strongest directly measured geomagnetic storm, the Carrington Event, was probably the result of two coronal mass ejections – the first storm effectively clearing the way for the second, which then hit Earth particularly hard. The Carrington Event occurred in September 1859, at a time when telegraph lines were the only widespread electrical infrastructure, limiting its impact on society.
A Carrington-class event would cause enormous damage in today's highly electrified world. This became clear when a significantly weaker geomagnetic storm caused widespread power outages in Quebec, Canada, in March 1989. Safety standards for power grids were increased worldwide as a result. The Starlink Event in February 2022 – so called because the Starlink project lost around 40 of its satellites as a result – was a geomagnetic storm categorised as 'moderate'. Its severe consequences were therefore all the more surprising.
Historical solar storms have been inferred through records of extraordinary aurora sightings and radioactive isotopes in the rings of trees. It is believed that two geomagnetic storms occurred in 774/775 and 993/994 AD and were ten times stronger than the Carrington Event.
Overview of noteworthy space weather events
Event
Date
Impact
Carrington Event
September 1859
Strongest geomagnetic storm ever recorded; damage to telegraph lines worldwide
New York Railroad Storm
May 1921
Almost as strong as the Carrington Event; southernmost aurora observations; fire in New York’s Grand Central Station caused by overvoltage in cables
August 1972
Fastest coronal mass ejection ever recorded (14.6 hours from the Sun to Earth); storm occurred between the Apollo 16 and 17 Moon missions and would have likely posed serious health risks to astronauts
March 1989
Power outages in Quebec, Canada
Halloween Storm
October and November 2003
Series of solar storms; first major event to be extensively measured with satellites
St. Patrick’s Day Storms
March 2013 and 2015
Two solar storms during the otherwise very quiet Solar Cycle 24; detailed investigation with modern measuring satellites
Starlink Event
February 2022
Moderate geomagnetic storm; caused the loss of around 40 Starlink satellites from their intermediate transfer orbit
Mother's Day Storm
May 2024
First extreme event of the current Solar Cycle 25; isolated power outages in New Zealand; media reports of disruptions or failure of GPS-controlled agricultural equipment and drones; auroras visible across Germany, including from the Zugspitze (the country’s highest mountain)
October 2024
Aurora sightings throughout Germany; heightened interest in the topic of space weather
