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 !