During the month of May, there were 21 large X-class flares and 97 M-class flares [1]. The Earth's magnetic field is affected by such dense and intense solar storms, and there have been many magnetic storm events. The geomagnetic storm on May 11 set a record for the strongest in 20 years, and rare auroras also appeared in the high latitudes of the continent, like fireworks blooming in the sky.
Aurora caused by a geomagnetic storm on May 11 (Image source: CCTV News)
Geomagnetic storms can cause short-term disturbance to the Earth's ionosphere and communication and navigation systems, but have little impact on the human body. If you feel particularly sleepy or weak on a day, it is likely that you have not rested well, and geomagnetic storms do not carry this pot.
The real hard time because of geomagnetic storms is actually a near-Earth man-made object in space. Also in May, a U.S. website dedicated to the orbit of artificial objects reported Space-Track.org the fall of 126 satellites and debris, about four times the average monthly fall in a year of calm solar activity (2020).
Space in low-Earth orbit
As of May 1, 2024, 22,352 man-made objects can be observed in orbit, including 10,707 active or defunct satellites, 953 rocket bodies remaining from launches, and more than 10,000 space debris over 10 centimeters tall. Thanks to the advantages of short communication delay, fast revisit period, and sufficient geographical coverage, many communication, navigation and scientific satellites work in low earth orbit, such as the "Tiangong" space station on the mainland, the "Micius" quantum science experiment satellite, the "Wukong" dark matter particle detection satellite, the advanced space-based solar observatory "Kuafu-1", and the International Space Station (ISS).
The International Space Station in low-Earth orbit (Credit: NASA)
Why is there a relationship between solar geomagnetic bursts and the fall of near-Earth man-made objects? Let's start with an encounter with a Starlink satellite.
Small magnetic storms, big losses
At the beginning of February 2022, a news that a Starlink satellite fell into the atmosphere and burned up rushed to the hot search. In its official statement, SpaceX said that the 49 satellites launched on Feb. 3 were "significantly affected by the Feb. 4 geomagnetic eruption," causing 40 of them to de-orbit and re-enter the Earth's atmosphere.
Post-mortem analysis showed that the Starlink satellites were in a test orbit of 210 km at the time, and on February 3-4, a geomagnetic storm triggered by solar activity increased the density of the atmosphere near the orbit by about 50%, and the atmospheric drag on the satellites increased significantly [2]. Although the Starlink team adjusted the satellites to a sideways attitude in time to reduce the windward area and atmospheric drag, they still failed to maintain and continue to raise the orbital altitude, resulting in 40 satellites crashing into a lower dense atmosphere and burning up in the following days (another version is 38 [3]). Based on the cost of the satellite and the cost of rocket launches, the accident caused about $50 million in damage [4].
在北美洲波多黎各上空由Caribbean Astronomical Society (SAC)的相机拍摄到的星链卫星在陨落过程中解体燃烧的影像(图片来源:euronews)
This is the first time that a large number of satellites have been publicly recorded falling at the same time due to the increase in atmospheric drag caused by geomagnetic storms. The scene of falling stars and fireworks in the deep night sky is very shocking, but what is even more worrying is that geomagnetic storms can have such a serious impact on near-Earth man-made objects!
This incident is a wake-up call for humanity. At present, solar activity is trending towards its peak, and geomagnetic eruptions will inevitably become more frequent and more intense. In the coming period, the operation of near-Earth man-made objects will face a serious threat from solar and geomagnetic eruptions.
Why do geomagnetic storms cause satellites to fall?
As the Starlink satellite fall event revealed, the impact of geomagnetic storms on the orbits of satellites is mainly due to increasing the density of the upper atmosphere.
The upper atmosphere refers to the atmosphere above 80 km above the ground, and the neutral gas components in the thermosphere area below 800 km are mainly nitrogen, oxygen, and atoms of nitrogen, oxygen, and helium. After the occurrence of solar flare events, the solar radiation in the extreme ultraviolet band increases significantly in the short term, the thermosphere is heated and expanded, and the relatively dense atmosphere in the lower layer spreads to the higher layers, resulting in an increase in density. At the same time, geomagnetic storms also trigger a series of complex physical processes, such as Joule heating and ionization, auroral particle deposition, upwelling of the lower atmosphere, and atmospheric circulation, which significantly increase the density of the upper atmosphere.
The increase in density means that the number of gas particles in the same volume increases, and the total mass of the gas increases, which produces greater resistance to the surface of the artificial celestial body, slows down the movement speed, consumes flight kinetic energy, reduces the orbital altitude, and causes the artificial celestial body to encounter a denser atmospheric environment.
Schematic diagram of the heights of the different spheres of the Earth's atmosphere, with a thermosphere of neutral gas between 80 km and 800 km, and an ionosphere in a partially ionized state (Image source: NOAA)
Taking the Wukong Dark Matter Particle Detection Satellite (DAMPE) on the mainland and the MINOTAUR 4 debris of the United States as examples, both orbits were around 500km in 2016, and the next six years coincided with the low year of solar activity, the radiation intensity was relatively low, the geomagnetic activity was generally calm, the atmospheric density remained at a low level, and the orbital altitude attenuation was relatively slow. After 2022, the solar radiation continued to increase and accompanied by multiple short-term bursts, and the resistance increased due to the increase of atmospheric density, and the semi-long axis of the orbit of the two targets showed a trend of accelerated attenuation, among which the size of the MINOTAUR 4 fragment was 1 order of magnitude larger than that of DAMPE (represented by the ballistic coefficient Bc in the figure below), and the orbital attenuation was more dramatic, and the altitude had decreased by 100 km in more than two years.
Long-term attenuation of the semi-major axis of the orbit of DAMPE and MINOTAUR 4 fragments and its relationship with solar radiation intensity (Image data source: Space-Track)
During a megamagnetic storm on May 11, a piece of space debris with NORAD (North American Air Defense Command Directory Serial Number) number 57453 fell. Tracing its historical orbit revealed that it was still at an altitude of 400 km on April 6 and should not have re-entered the atmosphere so quickly according to conventional calculations. However, in just one month, dozens of solar flares occurred one after another, triggering four geomagnetic storms of moderate intensity or above, which increased the density of the upper atmosphere and accelerated the fall of debris.
The solid brown line in the figure below shows that the orbital attenuation of this space debris has undergone three significant accelerations, the first occurred around April 18, and the decay rate suddenly increased from the original 683 m per day to 3.48 km per day, and the second and third occurred in the continuous eruption phase of the sun from May 5 to 11, especially in the event of a massive magnetic storm that lasted for 40 hours from the 10th, and the attenuation speed finally increased to 83 km per day, which directly led to the fall of the debris.
3 Accelerated Attenuation of Orbital Altitude of Debris No. 57453 and Correspondence with Solar and Geomagnetic Burst Events (2024-04-05 to 05-17).
Thankfully, man-made objects of the meter size or smaller usually burn up and disintegrate in the atmosphere when they fall, with little impact on human safety and the surface environment.
Solar activity is at its peak
The Sun ejects a large amount of coronal material outward during the eruption, and these clouds of charged energetic particles can cause a geomagnetic storm if they hit the Earth and couple with the Earth's magnetosphere. Therefore, the protection of artificial objects requires special attention to the monitoring of solar activity.
Since 1947, ground-based telescopes have been used to continuously measure the flow of radio radiation at the sun's wavelength of 10.7 cm to characterize the intensity of solar activity. As can be seen in Figure A below, solar activity has a fundamental cycle of about 11 years, and the period of change in geomagnetic disturbances is highly synchronized with this cycle.
The Kp index or ap index is commonly used to indicate the degree of activity of the Earth's magnetic field (the level of disturbance), and if Kp≥4 or the maximum daily ap≥56 is used as the threshold for more significant disturbances, the number of geomagnetic disturbance days per year will fluctuate with the intensity of solar activity (Figure b, c below).
Since 1970, (a) solar radio band radiant flux F10.7, (b) daily maximum geomagnetic index ap, (c) days of geomagnetic disturbance at Kp≥4 or daily maximum ap≥56 on an annual basis (Image source: CelesTrak)
The 24th solar cycle ended in December 2019, which is a solar cycle with low amplitude and a correspondingly low number of days when the geomagnetic field is disturbed. Starting in 2020, the Sun entered the 25th week of its rising phase, with rising radiation levels and increasing days of geomagnetic disturbance every year. In particular, solar activity has become more frequent and intense since 2024 [5], and studies estimate that the Sun may enter the maximum period of the 25th active week later this year, with the largest geomagnetic storms generally occurring months to years after this peak [6]. For a period of time to come, the space environment around the earth will be like waves crashing on the shore, with ups and downs.
In the future, the space environment around the earth will be turbulent (Image source: National Space Science Center)
In order to extend the in-orbit time and working life of spacecraft, the level of solar activity in the next few years (or mission duration) should be accurately predicted during the mission design phase, so that sufficient fuel can be stored for the necessary orbit maintenance. Taking the International Space Station (ISS) as an example, in the years of calm solar activity (2019 and 2020), it actively raised its orbit 17 times in two years (including 5 maneuvers to avoid collision risks); In years with strong solar activity (July 2022 to the present), in order to maintain the orbital altitude, the ISS has actively lifted its orbit as many as 32 times, which is about 1 times more than that of the solar calm period.
Comparison of the number of orbital maneuvers (active lifting) of the ISS at different levels of solar activity shows (a) a period of calm solar activity and (b) a period of strong solar activity. (Image data source: Space-Track)
The impact of geomagnetic storms on the upper atmosphere is elusive
At present, it is difficult for countries around the world to accurately predict the changes in the density of the upper atmosphere caused by geomagnetic storms. Taking the large magnetic storm from November 19 to 23, 2003 as an example (ap maximum value of 300, Dst minimum value -422, the blue dot in the figure below represents the Dst value), the gravity satellite CHAMP and GRACE-A satellite recorded a short-term sharp rise in atmospheric density at the same time (the indigo blue line in the figure below): the CHAMP satellite was located at an altitude of 400 km at that time, and the atmospheric density increased by nearly 5 times only one day after the geomagnetic explosion; The GRACE-A satellite recorded a nearly seven-fold increase in density at 500 km. Three of the latest atmospheric models (DTM2020, JB2008 and MSIS2.0) were used to simulate the changes of atmospheric density in this event (red, violet and brown lines in the figure).
More challengingly, there is currently no optimal model on a global scale that can accurately predict the density distribution and changes in the upper atmosphere. For a long time, it has been one of the bottlenecks that limit the accuracy of the orbit prediction of near-Earth artificial objects, so the prediction of the entire atmosphere has also been included in the "25 Techniques for 2023-2048" report by the US Naval Laboratory (NRL).
Hourly Dst index during the Great Magnetic Storm from 19 to 23 November 2003, atmospheric density from CHAMP and GRACE-A satellite observations and calculated values from empirical atmospheric models. The DST index is obtained by weighting the horizontal component of the magnetic field strength measured by the low-latitude geomagnetic stations near the equator, which reflects the strength of the magnetic field disturbance, and the Dst≤-200 is a very large magnetic storm. (Image source: https://sol.spacenvironment.net/JB2008/)
2024~2026 is considered to be the peak year of the 25th week of solar activity, which is likely to lead to more frequent and intense geomagnetic storm events. The orbits of near-Earth artificial objects will experience greater drag and accelerate decay; The launch and operation control of the giant constellation represented by Starlink will withstand more storms; Manned space stations, low-orbit science and applied satellites also require more frequent orbital maneuvers and attitude control. Therefore, it is of great significance to closely monitor solar activities, accurately predict the impact of solar and geomagnetic bursts on the Earth's upper atmosphere from the aspects of physical mechanisms and statistical models, and improve the prediction accuracy of the orbits of near-Earth artificial objects.
Bibliography:
[1] National Space Science Center, Chinese Academy of Sciences, http://www.sepc.ac.cn
[2] Luo Bingxian et al., Space Environment Analysis of the "Starlink" Satellite Destroyed in Geomagnetic Storm Event, Space International, 2022-05
[3] Berger T.E., et.al, The Thermosphere is a Drag: The 2022 Starlink Incident and the Threat of Geomagnetic Storms to Low Earth Orbit Space Operations, Space Weather, 21,e2022SW003330, 2023
[4] SpaceX starlink satellite’s flaming re-entry caught on camera after geomagnetic storm, www.euronews.com, 2022-02-11
[5] Kuafu Flash丨9 days, 11 X-class flares, Purple Mountain Astronomical Observatory, Chinese Academy of Sciences, 2024-05-12
[6] Dazzling auroras are just a warm-up as more solar storms are likely, scientists say, Nature, News Explainer, https://doi.org/10.1038/d41586-024-01432-7, 2024-05-13
About the Author
Hongbo Wang, associate researcher at the Purple Mountain Astronomical Observatory of the Chinese Academy of Sciences, research interests: upper atmosphere density model research, high-precision prediction methods for near-Earth artificial object orbits.