The sun sneezes, and the spacecraft is caught in the crossfire

The increasing frequency of human space activities means that there are more and more space weather hazard challenges to deal with.

The faint aurora is a unique natural landscape. Generally speaking, only people in the high latitudes of the North and South Poles can have the opportunity to see its beauty. The appearance of the aurora requires three conditions: atmosphere, magnetic field and high-energy charged particles. The sun continuously ejects charged particles with huge energy into space. When these particles come near the earth, they are disturbed by the earth's strong magnetic field and fly to the north and south poles. After friction and collision with atmospheric molecules, the aurora phenomenon is produced. In addition to the colorful aurora, there are still some undercurrents that are invisible to the naked eye. The high-energy charged particles from the sun interact with the earth's magnetic field, giving rise to various changes in the earth's ionosphere, magnetosphere and middle and upper atmosphere. People refer to this phenomenon of changes in the Sun-Earth space that are closely related to solar activity as space weather.

The Aurora Borealis, photographed by the Space Shuttle Discovery in May 1991

In November 1994, the United States defined "space weather" as conditions near the sun and in the solar wind, Earth's magnetosphere, ionosphere and thermosphere that affect the operation and reliability of space and ground-based technical systems and endanger human health and life. The formation mechanism of space weather is complex. Excessively intense solar activities such as solar wind can cause geomagnetic storms, thermospheric storms, expansion of the outer Van Allen radiation belts, changes in ionosphere properties and other hazards, endangering the normal life of humans on the surface, interfering with the normal operation of near-Earth space satellites, and even posing a threat to the lives of astronauts in outer space.

The Sun: The Furious Mother of All Creatures

Traditional weather changes in the atmosphere are ultimately due to changes in the sun. Summer is when the sun is close and the sunshine is long, while winter is when the sun is far away and the sunshine is short. As the saying goes, "winter comes and summer goes, autumn harvests and winter stores", "clouds rise to bring rain, dew turns to frost", and even natural disasters such as floods, droughts, and locust plagues are closely related to solar activity.

The emerging concept of space weather is directly related to solar activity. The sun is continuously carrying out nuclear fusion reactions and continuously ejecting high-temperature ionized gases. In a quiet state, the phenomenon of the sun constantly ejecting matter into interstellar space is called solar wind. The solar wind roars at a speed of 1 million to 10 million kilometers per hour, bringing 1 million tons of matter into space every second. When the solar wind encounters the earth's magnetic field, the earth's magnetic field is "blown" and deformed to form a teardrop-shaped magnetosphere facing the sun. It is precisely because of the existence of this magnetosphere that the solar wind is shut out and cannot cause substantial interference to the earth's atmosphere, preventing the earth's surface atmosphere and moisture from being eroded and blown away by the solar wind. It can be said that the magnetosphere is the earth's protective umbrella, which is one of the key conditions for the earth to nurture life.

Schematic diagram of the Earth's magnetosphere under the influence of solar wind. The size and distance of celestial bodies are not in proportion.

However, the sun does not always remain quiet, and it often "sneezes". Solar activity can be divided into two types: gradual and explosive. Abnormal structures in the sun, such as sunspots and coronal holes, belong to gradual solar activity, while solar flares and coronal mass ejections belong to explosive solar activity.

The energy released by a typical solar activity is equivalent to the energy produced by the explosion of hundreds of billions of Hiroshima atomic bombs. When a solar storm occurs, the interaction between the solar wind and the Earth's magnetic field is enhanced, the balance is broken, and the Earth's magnetosphere changes dramatically. The magnetic field on the ground and in near-Earth space is violently disturbed, causing ground circuits, communication equipment and various satellites to be damaged or even fail. This is the geomagnetic storm phenomenon.

The strongest geomagnetic storm ever: the Carrington event

The Carrington Event on September 1, 1859 was the strongest geomagnetic storm ever recorded. It was the first time that humans observed a solar flare, and it was also the strongest solar flare ever. This event caused nearly 200,000 kilometers of telegraph lines in the United States and Europe to fail. The auroras at the poles are so dazzling that you can even read books and newspapers directly under the auroras at night. Even mid- and low-latitude regions such as Hawaii and the Caribbean Sea have seen unprecedented auroras. However, 163 years ago, humans did not have satellite technology or radio communication technology, so the space weather changes caused by such a violent magnetic storm did not have much impact on people's lives.

Richard Carrington's sunspots during the Carrington Event on September 1, 1859

However, if another Carrington event were to occur today, the consequences would be disastrous. Geomagnetic storms would generate induced currents in the power supply network, causing transformers to overload or even burn out, and the global power grid would be completely paralyzed. All satellites would be scrapped, communications would be completely interrupted, navigation systems would not work properly, and ships and aircraft would become headless flies. All the direct or indirect losses would take about 4 to 10 years to recover. According to the prediction of the Key Laboratory of Near-Earth Space Environment of the Chinese Academy of Sciences, the total global losses would reach about 2 trillion US dollars.

Although people entering the satellite age have not yet experienced a space weather disaster on the level of the Carrington Event, several events with lesser power still make people feel uneasy.

The Quebec incident in Canada in March 1989 caused a nine-hour power outage in the entire city, leaving six million residents without electricity in a cold, dark night, and causing direct economic losses of $500 million. All military radar stations in Quebec failed, and the North American Aerospace Defense Command even believed that Quebec had been hit by a nuclear attack.

The Bastille storm in 2000 caused sensor failures on the US weather satellites GOES-8 and GOES-10, GPS navigation satellites failed for several hours, the solar wind velocity detector on the space station's Advanced Composition Explorer satellite (ACE) failed, and the International Space Station's orbit was lowered by 15 kilometers. The most serious damage was to the Japanese astronomical observation satellite ASCA, whose solar panels could not work properly. After two months of efforts to save it, the staff still declared a failure and the satellite eventually lost contact.

The influence of thermospheric storm on satellite orbit

Thermosphere storms are a subsidiary phenomenon of geomagnetic storms, which will have a direct impact on the orbits of near-Earth satellites. The thermosphere is a thin atmospheric layer with a high temperature above 100 kilometers from the ground. When a thermosphere storm occurs, the atmospheric temperature rises and expands, causing changes in atmospheric circulation and a significant increase in the density of the upper atmosphere. The atmospheric drag experienced by spacecraft in lower orbits will increase exponentially, which will have a fatal impact on spacecraft that cannot raise their orbits in time. Excessive atmospheric drag will cause the spacecraft's speed to decay faster, the orbital altitude to decrease faster, and it will re-enter the atmosphere earlier, ending its life.

The most famous spacecraft affected by this is the U.S. Skylab space station. Skylab was launched on May 14, 1973 by a Saturn V rocket. This 76-ton behemoth was highly anticipated by NASA. After three manned missions, Skylab entered a mission window, waiting for the successful launch of the space shuttle to refuel and raise its orbit. At that time, Skylab was in an orbit of about 435 kilometers, and engineers expected it to continue operating in space until 1983, waiting for the arrival of the space shuttle. However, before the successful development of the space shuttle in 1981, sunspot activity became more intense than expected, and the upper atmosphere of the earth was heated and expanded. The greater resistance meant that Skylab could only last until 1979. On July 11, 1979, Skylab re-entered the atmosphere with regret and burned up.

Skylab

Skylab debris falling into Australia

The influence of the Van Allen radiation belts on satellite electronic systems

In addition to the increase in atmospheric density causing the satellite orbit to decay faster, severe space weather can also have a negative impact on the satellite's electronic system. There are a large number of high-energy charged particles bound by the Earth's magnetic field in the altitude range of 1,000 to 60,000 kilometers on Earth, which gather into the Van Allen radiation belts surrounding the Earth. When a solar storm occurs, the Van Allen radiation belts will expand outward and expand in range, threatening spacecraft originally outside the radiation belts.

Schematic diagram of the Van Allen radiation belts. The inner belt (red) is dominated by protons, and the outer belt (blue) is dominated by electrons.

The sudden increase in charged particles will charge the surface of the satellite, with a voltage of up to tens of thousands of volts. When the charge accumulates to a certain level, it will discharge at certain tips and gaps, instantly breaking through the satellite's protective layer. Some high-energy particles can also directly penetrate the satellite, bombarding the satellite's circuits and chips, affecting their performance, and in severe cases directly causing the satellite to fail. According to incomplete statistics, nearly 90% of satellite circuit system failures are related to abnormal solar activity.

According to statistics from the U.S. National Space Weather Strategic Plan, the United States suffers tens of millions of dollars in losses each year due to space weather. In 1998, the U.S. Galaxy 4 synchronous orbit communications satellite failed due to a solar storm, and 80% of the U.S. fax and paging services were paralyzed. my country also suffered losses due to space weather changes when its satellite technology was still immature. The Fengyun-1A meteorological satellite launched in September 1988 only worked normally for 39 days, and the Fengyun-1B meteorological satellite launched in November 1990 only worked normally for 165 days. Both of them were damaged by the onboard computers due to bad space weather and did not reach the one-year design life. Later, my country deeply summed up the lessons of satellite failure, overcame difficulties, and greatly improved the radiation resistance of my country's satellites. The Fengyun-1C satellite launched in May 1999 had a design life of 2 years, but it served normally for nearly 5 years, making a beautiful comeback.

Fengyun-1 satellite

Ionospheric storms affect radio wave propagation

Solar activity can also cause disturbances in the Earth's ionosphere, resulting in ionospheric storms. The ionosphere is an atmospheric region with an altitude of 60 to 1,000 kilometers that contains charged particles. As the altitude changes, the ionosphere can reflect, refract and absorb electromagnetic waves of different frequency bands, just like a mirror in the sky. Humans rely on this mirror in the sky for radio communications.

When an ionospheric storm occurs, the density of charged particles, electric field distribution, and magnetic field distribution in the ionosphere will fluctuate suddenly in milliseconds, and the fluctuation range can even reach more than 80%, causing rapid changes in radio wave reflection, refraction, absorption, dispersion, and Doppler shift, which can cause damage or even complete loss of information from communication satellites, navigation satellites, and remote sensing satellites. Charged particles can also bombard satellite solar panels, which is the culprit for the degradation of satellite solar panel performance. In severe cases, it can lead to insufficient power supply to the satellite or even power failure of the entire satellite.

When the density of small-scale charged particles in the ionosphere is uneven, the intensity and phase of the radio signal will fluctuate. This phenomenon is called "ionospheric scintillation". In addition, the uneven density of larger-scale charged particles will cause the refraction path of the radio signal to bend, causing the ground tracking satellite to deviate when receiving the signal. The accumulation of various small-scale and large-scale ionospheric storm errors will cause the signal-to-noise ratio of the satellite signal to decrease, the packet loss rate to increase, the navigation satellite error to increase, the resolution of remote sensing satellite images to decrease, and even cause the satellite signal to be completely interrupted. For example, during the Halloween solar storm in late October 2003, the US GPS navigation satellite was forced to be suspended for 30 hours due to the ionospheric storm phenomenon. In September 2017, a strong solar flare event occurred, and the errors of the US GPS and my country's Beidou and other satellite positioning systems were several times larger than normal. Low-latitude areas such as the South China Sea in my country are prone to ionospheric scintillation. In the era when maritime satellite technology was immature, it was common for ships to lose maritime satellite signals.

An X-class solar flare captured by NASA's SDO observatory on March 6, 2012

The harm of space radiation to astronauts

Astronauts in space are also constantly facing the threat of space weather disasters.

Astronauts face potential radiation threats in space

High-energy charged particles can penetrate into human cell tissues, ionize molecules in the body, and even damage genetic materials such as DNA. The normal function of cells is destroyed. When the amount of radiation is small, it will cause symptoms such as endocrine disorders. In severe cases, it will cause DNA mutations and even cell cancer. Researchers pointed out that if astronauts in space experience a solar flare event of the same level as the Carrington Event, they are likely to suffer from acute radiation sickness and may even die. Astronauts who have performed extravehicular activities have reported that their eyes have been constantly flashing abnormally, and closing their eyes does not help. According to ground analysis, this is caused by their retinas being bombarded by high-energy particles.

Therefore, the outer walls of the space station need special materials to minimize the damage of space radiation to astronauts, and the time astronauts spend on their mission should not be too long.

In summary, the causes of space weather phenomena are complex. The sun, which is out of human control, may sneeze at the earth at any time, causing drastic changes in the earth's magnetosphere, ionosphere, thermosphere and various physical properties of the ground, posing a threat to the smooth operation and healthy development of human society. At present, the research on space weather is still in-depth. I believe that one day we will be able to fully understand the mechanism and changing laws of space weather, and we will be more comfortable and calm in the face of space weather events.