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.