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!