What Is a Solar Flare?

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Solar Flare Definition
A solar flare is an intense burst of electromagnetic energy from the Sun that is associated with sunspots. (photo: NASA/SDO)

A solar flare is a dazzling burst of electromagnetic energy from the Sun. Flares play a central role in space weather, sometimes disrupt our technological infrastructure, and offer a fascinating glimpse into the dynamic processes at work in stellar atmospheres.

  • A solar flare is a burst of electromagnetic energy from the Sun.
  • Most solar flares are associated with sunspots. Both sunspots and flares are more common near the maximum of the 11-year solar cycle.
  • Solar flares do not harm people on Earth, but they can disrupt communication and cause issues for satellites and space stations.
  • However, some solar flares are associated with coronal mass ejections, which are potentially more dangerous if they are directed toward Earth.

What Is a Solar Flare?

A solar flare is a sudden and intense burst of energy and electromagnetic radiation emanating from the Sun’s surface and its outer atmosphere. Essentially, it’s akin to an enormous explosion in the Sun’s atmosphere. Flares result from the release of magnetic energy stored in the Sun’s atmosphere due to the complex interactions between magnetic fields. When these events happen on stars beside the Sun, they are called stellar flares.

How a Solar Flare Works

Solar flares are a manifestation of the Sun’s magnetic activity. The Sun’s outer layer or photosphere consists of a magnetized plasma, where currents generate magnetic fields. When these magnetic fields become twisted and distorted—often because of the Sun’s differential rotation—they store vast amounts of energy. When these fields reconfigure to a lower energy state, the stored energy gets released as light, X-rays, and other forms of radiation. The magnetic field lines act sort of like a stretched rubber band snapping back. Plasma reaches incredible hot temperatures greater than 107 K, while particles like protons, electrons, and ions accelerate to nearly the speed of light. The result is a solar flare.

Relationship Between Solar Flares and Sunspots

Solar flares often occur in or around active sunspot regions. Sunspots are dark, cooler areas on the Sun’s surface caused by intense magnetic activity. These magnetic fields involve the photosphere, corona, and solar interior. Sometimes the magnetic field lines get twisted or disrupted. When the lines reconnect quickly, a helix of the magnetic field gets left out and unconnected to the arcade. The helical magnetic field and the matter within it violently expands outward. In essence, sunspots are precursors or potential sites for solar flares.

Solar Flares and Coronal Mass Ejections (CMEs)

Solar flares and CMEs are closely related but distinct solar phenomena. While a solar flare is a sudden release of energy and radiation, a CME is a massive burst of solar wind and magnetic fields rising above the solar corona or being released into space.

Flares and CMEs often occur together, especially during larger events. A solar flare can be the trigger for a CME, but not all flares produce CMEs, and not all CMEs are preceded by flares.

Is a Solar Flare Visible?

Of course, looking at the Sun is dangerous. But, even viewing it safely through a solar filter, you might not see a solar flare. The reason is that a flare releases energy across the entire electromagnetic spectrum. Visible light is only a small portion of that spectrum.

Frequency and Duration

Solar flares occur with varying frequencies depending on the current solar cycle. The solar cycle is an approximately 11-year period during which the Sun’s magnetic activity waxes and wanes. When the Sun is at solar maximum, the peak of its cycle, flares can occur several times a day. Conversely, during solar minimum, they may only happen once a week.

Most solar flares last for several minutes to several hours, although the precursors and aftermath can extend over days.

How Long Does It Take for a Solar Flare to Reach Earth?

The electromagnetic radiation from a solar flare, including visible light and X-rays, travels at the speed of light, so it takes approximately 8 minutes and 20 seconds to reach Earth. However, if the flare is associated with a CME, which involves actual particles being thrown outwards, those particles typically take 1 to 3 days to reach Earth, depending on their speed.

Classification of Solar Flares

The classification of solar flares depends on their X-ray brightness in the wavelength range of 1 to 8 Angstroms. They are classified into three main categories (C, M, X), but there are five categories in all:

  1. A-Class: An A-class flare emits soft x-rays with a peak flux range of less than 10-7 W/m2. There are no noticeable effects on Earth.
  2. B-Class: A B-class flare emits soft x-rays with a peak flux range between 10-7 to 10-6 W/m2. There are no noticeable effects on Earth.
  3. C-class flares: These are small flares with few noticeable consequences on Earth.
  4. M-class flares: These are medium-sized flares, which cause brief radio blackouts on the sunlit side of the Earth.
  5. X-class flares: These are the largest and most powerful flares. An X-class flare can lead to significant disruptions on Earth, affecting satellites, power grids, and radio communication.

Each class has a tenfold increase in energy output compared to the preceding one. Each class (except X) has a nine-point scale. So, the next class up from a C9 flare is an M1 flare. Because there is no numerical limit on X-class flares, there can be an X-11 or higher-level flare. Informally, an M-class flare is “moderate,” while an X-class flare is “extreme.”

Predicting Solar Flares

Forecasting solar flares remains a challenging task. While scientists have made progress in identifying regions on the Sun (often sunspots) that are likely to produce flares, predicting their exact timing, intensity, and potential Earth impact is still a developing science. Current forecasts are based on observing the magnetic complexity of sunspots and understanding the history of a given active region.

Effects on Earth and Space

Solar flares influence Earth in numerous ways:

  1. Radio Communication: Flares can cause high-frequency radio blackouts, especially on the sunlit side of the planet.
  2. Satellites: The increased radiation from a flare can interfere with satellite electronics and can also expand Earth’s atmosphere, increasing drag on low-Earth orbit satellites.
  3. Auroras: Flares can enhance the auroras (Northern and Southern Lights), causing them to be more vivid and seen at lower latitudes than usual.
  4. Power Grids: Intense flares, especially if accompanied by a coronal mass ejection (CME), can induce electric currents in power lines, potentially damaging transformers and other infrastructure.

Examples of Strong Solar Flares

One of the most famous solar flares occurred in 1859 and is known as the Carrington Event. The Carrington Event likely included both a solar flare and a CME. This event caused auroras to be seen as far south as the Caribbean and disrupted telegraph systems, even shocking some telegraph operators.

The November 2003 solar flare was around X28. No one knows for certain because it overloaded the sensors monitoring it. This storm occurred two or three years past the solar maximum. It caused brief power outages and affects satellites and communications. People reported seeing the aurora as far south as Texas and Florida.

Risks to Astronauts in Low Earth Orbit (LEO)

Solar flares, especially intense ones, can pose a risk to astronauts in space, including those in LEO. The concern is mainly due to the increased radiation from the flare. While Earth’s magnetic field and atmosphere protect those on the surface, astronauts outside this protective shield are exposed to radiation. In anticipation of significant solar events, astronauts on the International Space Station (ISS) or other platforms often take shelter in more shielded parts of their spacecraft.

Observation of Solar Flares

Scientists observe solar flares using a variety of instruments:

  1. Space-Based Observatories: Instruments like the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO) provide detailed images and data of the Sun in multiple wavelengths, helping scientists detect and analyze solar flares.
  2. Radiospectrographs: These detect the radio waves produced during a flare.
  3. X-ray detectors: Solar flares emit X-rays, which can be detected and analyzed to understand the flare’s intensity and classification.

References

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  • Reep, Jeffrey W.; Knizhnik, Kalman J. (2019). “What Determines the X-Ray Intensity and Duration of a Solar Flare?”. The Astrophysical Journal. 874 (2): 157. doi:10.3847/1538-4357/ab0ae7
  • Reep, Jeffrey W.; Barnes, Will T. (2021). “Forecasting the Remaining Duration of an Ongoing Solar Flare”. Space Weather. 19 (10). doi:10.1029/2021SW002754
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