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.