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How the Sun’s “seasons” affect our planet’s atmosphere

Pierre Henriquet
Pierre Henriquet
Doctor in Nuclear Physics and Columnist at Polytechnique Insights
Key takeaways
  • The Sun has an impact on its immediate environment and on more distant planets, but also on the entire region of space surrounding our Solar System, which is called the heliosphere.
  • There are various consequences of the interaction of this flow of solar particles with the Earth's magnetosphere; one of the most beautiful being the Northern Lights.
  • A negative consequence of this permanent bombardment is it that it can prematurely age satellites and even lead to them failing because they are highly exposed to solar wind particles in orbit.
  • The Sun's extremely high temperature allows for the formation of bubbles that 'burst' its atoms into a soup of nuclei and free electrons. This 'plasma' is very sensitive to electric and magnetic fields.
  • Solar activity evolves periodically in a long cycle of about 11.5 years, passing through a maximum of activity and a minimum. These can be referred to as the Sun's 'seasons'.

Every 11.5 years, the Sun goes through a peri­od of par­tic­u­lar­ly intense activ­i­ty. It is char­ac­terised by strong solar wind, lead­ing to more fre­quent satel­lite fail­ures due to a bom­bard­ment of par­ti­cles from the Sun. But whether solar activ­i­ty is high or low, life is tough for tech­nolo­gies in orbit. The Sun has an impact on its imme­di­ate envi­ron­ment, on the more dis­tant plan­ets, but also on the whole region of space sur­round­ing our Solar Sys­tem, which is called the heliosphere.

What are the consequences for space activities?

The con­se­quences of the inter­ac­tion of this flow of solar par­ti­cles with the Earth­’s mag­ne­tos­phere – our plan­et’s mag­net­ic field – are numer­ous and var­ied. One of the most beau­ti­ful is, of course, the auro­ras in the north­ern and south­ern hemi­spheres (auro­ra bore­alis and aus­tralis respec­tive­ly). At the poles, a small frac­tion of the solar wind can cross the mag­ne­tos­phere (see image of the Earth­’s mag­net­ic field below) and vio­lent­ly inter­act with the Earth­’s atmos­phere. This trans­fer of ener­gy excites the rar­efied atoms and mol­e­cules at the top of the atmos­phere, which re-emits it in the form of mag­nif­i­cent drapes of light.

Mag­net­ic field lines sur­round­ing the Earth. In colour: solar wind den­si­ty. (The Sun is on the left of the image) © NASA

Oth­er con­se­quences are more prob­lem­at­ic. Satel­lites in orbit around our plan­et are high­ly exposed to solar wind par­ti­cles. This per­ma­nent bom­bard­ment is one of the main rea­sons for the age­ing or pos­si­ble fail­ure of these satel­lites. Like the irra­di­a­tion process­es used in indus­try on Earth, the mechan­i­cal, elec­tri­cal and/or opti­cal prop­er­ties of satel­lite com­po­nents is grad­u­al­ly altered. Tran­sient or per­ma­nent fail­ures of the on-board elec­tron­ics may also occur (more fre­quent­ly, of course, dur­ing solar max­i­mum peri­ods). There­fore, they must often be shield­ed, and their elec­tron­ics rein­forced to bet­ter with­stand solar radiation.

Ground tech­nolo­gies can also be affect­ed by the solar cycle. While the Sun’s tem­per­a­ture and light inten­si­ty do not change dur­ing the solar cycle (it does not get hot­ter every 11.5 years), the geo­mag­net­ic storms caused by a coro­nal mass ejec­tion enter­ing the Earth­’s envi­ron­ment can be so intense that huge induced elec­tri­cal cur­rents appear in the Earth­’s pow­er grid. In 1989, for exam­ple, a wide­spread pow­er fail­ure brought Cana­da to a stand­still for nine hours. The entire net­work col­lapsed in just 25 sec­onds. This is prob­lem­at­ic as our civil­i­sa­tion is becom­ing ever more depen­dent on electricity…

The Sun makes bubbles

Astronomers have long sought to char­ac­terise the so-called helios­phere, which lim­its two zones of space: the inte­ri­or, dom­i­nat­ed by the solar wind, and the exte­ri­or, made up of par­ti­cles in the inter­stel­lar medi­um. Since the Sun emits solar wind par­ti­cles in a glob­al­ly isotrop­ic man­ner, its shape was ini­tial­ly imag­ined as an elon­gat­ed « bub­ble » sur­round­ing the solar sys­tem. How­ev­er, a recent paper1 sug­gests, based on exten­sive numer­i­cal sim­u­la­tions and sky obser­va­tions, that the helios­phere of our Solar Sys­tem is shaped like… a crois­sant! This is due to the com­plex inter­ac­tions between the hydro­gen atoms in the inter­stel­lar medi­um and the charged par­ti­cles emit­ted by the Sun.

The ‘crois­sant’ pro­duced by the lat­est numer­i­cal sim­u­la­tions of the helios­phere. © NASA

The shape of this helios­phere will obvi­ous­ly depend on the com­po­si­tion, den­si­ty, and speed of the flow of par­ti­cles com­ing from the Sun. Every sec­ond, the Sun los­es about 1 mil­lion tonnes of elec­trons, hydro­gen nuclei and heli­um in a “wind” that spreads around it, bathing the plan­ets and oth­er bod­ies as far as the edges of the Solar Sys­tem. It is, for exam­ple, this solar wind that shapes one of the two types of tails of comets by car­ry­ing the dust and gas par­ti­cles eject­ed from the comets’ nuclei away from the Sun.

Comet C/2021 A1 (Leonard) imaged by Dan Bartlett at the begin­ning of Decem­ber 2021

This solar wind also inter­acts with the plan­ets of the Solar Sys­tem, more or less direct­ly, depend­ing on the pres­ence or absence of a mag­net­ic field around them. The very high tem­per­a­ture of the Sun “explodes” its atoms into a soup of nuclei and free elec­trons, called a “plas­ma”, which is very sen­si­tive to elec­tric and mag­net­ic fields. So, when these par­ti­cles arrive near Earth, its mag­net­ic field will divert a sig­nif­i­cant por­tion of them and deflect the par­ti­cle show­er before it can irra­di­ate and ster­ilise the Earth­’s surface.

Solar cycles and space weather

What are the prop­er­ties of the Sun’s ‘wind’ of par­ti­cles? Is the wind con­stant? Reg­u­lar? Or do “solar wind storms” exist? Also, are there, as on Earth, “sea­sons” when the solar wind is more intense? The answer to both of these ques­tions is yes.

Just as the sea­sons on Earth reap­pear reg­u­lar­ly each year, solar activ­i­ty (which includes all the phe­nom­e­na that occur on the sur­face of the Sun and around it) also evolves peri­od­i­cal­ly in a long cycle of about 11.5 years.

Evo­lu­tion (observed et sim­u­lat­ed) of the solar cycle between 1995 et 2035. © NASA

Dur­ing this cycle, the Sun goes through a max­i­mum of activ­i­ty (the solar wind will be on aver­age denser and faster) and a min­i­mum. This vari­a­tion in activ­i­ty is not lim­it­ed to the amount of solar wind par­ti­cles, how­ev­er. The Sun also reg­u­lar­ly pro­duces erup­tive phe­nom­e­na called coro­nal mass ejec­tions. These very intense bursts of solar par­ti­cles are pro­duced in very local regions on the sur­face of our star and are also eject­ed radi­al­ly, at great dis­tances from the Sun.

When the Earth finds itself in this direc­tion, a par­tic­u­lar­ly dense “wave” of par­ti­cles arrives two to three days after the start of the ejec­tion. This wave of par­ti­cles joins the clas­sic solar wind for sev­er­al hours. It is obvi­ous­ly dur­ing the years of max­i­mum solar activ­i­ty that the prob­a­bil­i­ty of these pow­er­ful erup­tive phe­nom­e­na pro­duc­ing “geo­mag­net­ic storms” in the Earth­’s envi­ron­ment is greatest.

It would be wrong, how­ev­er, to think that space activ­i­ties are qui­eter dur­ing solar min­i­mum peri­ods. The solar wind is less dense and there are few­er solar flares, but oth­er phe­nom­e­na take cen­tre stage. For exam­ple, the Earth­’s atmos­phere also varies accord­ing to the “pres­sure” of the solar wind. At solar min­i­mum, the pres­sure is low­er and the atmos­phere extends fur­ther into space. This puts aero­dy­nam­ic stress­es on low-orbit­ing satel­lites and sig­nif­i­cant­ly reduces their life span.

To con­clude, our star sig­nif­i­cant­ly influ­ences the Earth­’s envi­ron­ment. Its plas­ma fin­gers caress us night and day, and it is through under­stand­ing the com­plex inter­ac­tions between the Sun and the Earth that human­i­ty will be able to con­tin­ue to under­stand space.

For more information, you can watch this ARTE video with Tahar Amari from the Center for Theoretical Physics at Ecole Polytechnique:

Find out more

1« A Tur­bu­lent Heliosheath Dri­ven by the Rayleigh–Taylor Insta­bil­i­ty » —The Astro­phys­i­cal Jour­nal, Vol­ume 922, Num­ber 2 – https://doi.org/10.3847/1538–4357/ac2d2e

Contributors

Pierre Henriquet

Pierre Henriquet

Doctor in Nuclear Physics and Columnist at Polytechnique Insights

After a doctorate in Nuclear Physics applied to Medicine and a university degree in Astronomy/Astrophysics, Pierre Henriquet worked for 10 years at the Planetarium of the city of Vaulx-en-Velin where he perfected his talents as a science communicator with multiple audiences, both novices and specialists. Today, he is a freelance writer and mediator of science.