How do solar winds impact Earth?

A new gen­er­a­tion of space mis­sions is under way. Their aim is to meas­ure the elec­tric and mag­net­ic fields of space plas­mas as well as particles (elec­trons, pro­tons and heavy ions). A bet­ter under­stand­ing of these fields allows research­ers to study phe­nom­ena like tur­bu­lence in the sol­ar wind and how it inter­acts with plan­et­ary magnetospheres.

The res­ults of these mis­sions are of great import­ance, not only for under­stand­ing these effects, but also for bet­ter char­ac­ter­ising large-scale struc­tures. For example, coron­al mass ejec­tions in the sol­ar wind gen­er­ate so-called ‘sol­ar storms’ that impact the Earth’s mag­net­ised envir­on­ment¹. These can dam­age elec­tri­city and com­mu­nic­a­tion net­works and satel­lites if they are strong enough.

Sol­ar wind is an ion­ised gas, known as a plasma, com­posed mostly of elec­trons and pro­tons. It is con­tinu­ously ejec­ted from the Sun’s upper atmo­sphere in all dir­ec­tions into inter­plan­et­ary space, along the mag­net­ic field lines eman­at­ing from the Sun. It has two com­pon­ents: a ‘fast’ wind mov­ing at about 500–800 km/s from coron­al holes at the poles of our star and a ‘slow’ wind trav­el­ling at about 200–400 km/s emit­ted mainly at the equat­ori­al plane of the Sun. When the sol­ar wind col­lides with particles in the Earth’s atmo­sphere, many photons are emit­ted in the same amount of time, cre­at­ing the beau­ti­ful polar auror­as that can be seen at high lat­it­udes in the north­ern and south­ern hemi­spheres. These auror­as are known as aurora boreal­is and aurora aus­tral­is, respectively.

Sol­ar wind is very tur­bu­lent. Among oth­er things, my work involves study­ing the prop­er­ties of the tur­bu­lence of this wind around Earth, but also around oth­er plan­ets, like Sat­urn and Mer­cury. The Earth is one astro­nom­ic­al unit (AU = 150 000 000 km) from the Sun, where­as Sat­urn is much fur­ther away, at 10 AUs. By ana­lys­ing ‘in situ’ wave and particle data (ion dens­ity and tem­per­at­ure, elec­trons, mag­net­ic and elec­tric fields) from instru­ments on board the European Space Agency’s (ESA) Cluster probe orbit­ing Earth and the US space agency’s (NASA) Cas­sini probe around Sat­urn, my col­leagues and I were able to study and com­pare the prop­er­ties of the sol­ar wind’s tur­bu­lence at these dif­fer­ent distances.

The Plasma Phys­ics Labor­at­ory is involved in two oth­er recent sol­ar mis­sionsLe labor­atoire de physique des plas­mas (LPP)est impli­qué dans deux autres mis­sions sol­aires récen­tes]. The first, NAS­A’s Park­er Sol­ar Probe (PSP), was launched in 2018 and made meas­ure­ments of the Sun at an extremely close dis­tance of just 24 mil­lion km. PSP con­tin­ues to edge closer to the Sun and, as its orbit shrinks, it will even­tu­ally reach a peri­he­li­on dis­tance of just 6.16 mil­lion km in 2024–25 (a few hun­dredths of the dis­tance from the Earth to the Sun), where it will exper­i­ence tem­per­at­ures of nearly 1400°C. It will be the first space mis­sion to pen­et­rate the Sun­’s atmo­sphere. The second mis­sion is ESA’s Sol­ar Orbit­er, launched in 2020. This probe will get as close as 42 mil­lion km to the Sun and explore its off-eclipt­ic regions (30° lat­it­ude) that have nev­er been observed before and meas­ure its elec­tro­mag­net­ic environment.

The first res­ults from PSP show bizarre mag­net­ic field reversals in the radi­al dir­ec­tion, called ‘switch­backs’, accom­pan­ied by, a yet unex­plained, spon­tan­eous increase in the sol­ar wind speed. How particles are accel­er­ated in the sol­ar wind and what role the heat­ing of the sol­ar corona (the Sun­’s mil­lion-degree hot upper atmo­sphere) plays remain great mys­ter­ies and PSP will cer­tainly help to shed more light on these. Tur­bu­lence is one of the phys­ic­al pro­cesses that could explain the heat­ing of the sol­ar corona and the sol­ar wind.

In addi­tion to study­ing the prop­er­ties of tur­bu­lence in the sol­ar wind and plan­et­ary ‘mag­neto­gaines’ (the inter­fa­cial zones between the sol­ar wind and the mag­neto­sphere), we were involved in the final phase of NAS­A’s Cas­sini mis­sion, when the satel­lite crossed the space between the plan­et Sat­urn and its rings². Cas­sini made 23 orbits across the plane of the rings, passing inside the inner­most D ring for the first time. Each week, we received data from the Lang­muir probe on board the satel­lite that allowed us to determ­ine the elec­tron dens­ity of Sat­urn’s iono­sphere (the out­er lay­er of its atmo­sphere). Not only were we able to char­ac­ter­ise this iono­sphere in detail for the first time, we also meas­ured dust grains as well as organ­ic mat­ter that had fallen dir­ectly from the D ring into the plan­et’s atmo­sphere³.

We are also involved in the Bepi­Colombo mis­sion, a joint ESA and JAXA (Japan­ese Space Agency) mis­sion, launched in 2018 to explore the ion­ic com­pos­i­tion, atmo­sphere and mag­neto­sphere, as well as the his­tory of Mer­cury and its geo­phys­ics. Bepi­Colombo is a coupled sys­tem: a plan­et­ary orbit­er (the Mer­cury Plan­et­ary Orbit­er provided by ESA); and a mag­neto­spher­ic orbit­er (the Mer­cury Mag­neto­spher­ic Orbit­er, provided by JAXA).

LPP provided two instru­ments for this mis­sion: a mass spec­tro­met­er (MSA), which meas­ures the ion­ic com­pos­i­tion in Mer­cury’s mag­neto­sphere, and a Dual Band Induc­tion Mag­ne­to­met­er (DB-SC), or Search Coil, which meas­ures the high-fre­quency (1Hz-640kHz) mag­net­ic field of the plan­et. The two satel­lites of Bepi­Colombo, cur­rently in cruise phase, will enter final orbit around Mer­cury in Decem­ber 2025.

On 10 August 2021 Bepi­Colombo flew for the second and last time over the plan­et Venus, and on 1 Octo­ber 2021 for the first time over the plan­et Mer­cury, to bene­fit from a grav­it­a­tion­al assist that bent its tra­ject­ory towards the interi­or of the Sol­ar Sys­tem. Many of the instru­ments onboard were act­ive dur­ing these flybys, provid­ing unique data on the envir­on­ment of Venus and Mer­cury, includ­ing their inter­ac­tion with the sol­ar wind. In par­tic­u­lar, the MSA ion spec­tro­met­er and the DBSC mag­ne­to­met­ers at LPP col­lec­ted sci­entif­ic data in space for the very first time. We are cur­rently ana­lys­ing these data. On the oth­er hand, in addi­tion to the meas­ure­ments made dur­ing the plan­et­ary flybys, some of the instru­ments on board Bepi­Colombo will be oper­a­tion­al in the sol­ar wind. Indeed, I coordin­ated a work­ing group to define strategies for multi-point obser­va­tions, that is, joint obser­va­tions between Bepi­Colombo, Sol­ar Orbit­er and Park­er Sol­ar Probe satel­lites when the three are radi­ally or mag­net­ic­ally aligned in the sol­ar wind⁴. A first!