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

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.

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.

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…

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 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.

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.

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.

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.