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The Cluster Epic: unravelling the consequences of the electromagnetic forces at the planetary scale


Cluster is one of the few space missions that is older than 22% of the Belgian population! Celebrating its 20th birthday this year, it has been gathering data about the Earth’s magnetosphere ever since. This continuous monitoring is of particular interest to researchers, as it allows for the study of long-term processes in the Earth’s magnetosphere, and how these processes depend on the Sun’s variability. Some of our scientists at BIRA-IASB have made good use of the data to study plasma turbulence around the Earth, to compare Earth’s magnetic properties with that of other planets, to develop computer models of planetary magnetospheres and even to study special types of auroras.

Initially, the Cluster mission was only meant to last for two years, but its lifetime keeps being extended thanks to excellent engineering and highly automated spacecraft operations. The mission end is currently set for 2022, but might still be extended further if the satellites keep the stream of high-quality data going and if there is enough fuel and battery lifetime left. As of today, more than 3000 scientific publications have benefited from Cluster data to reveal ever more information about the shape and structure of Earth’s magnetosphere, as well as how it interacts with the charged particles from the solar wind.

In this article, we’ll take you on a tour of a few things that our scientists have learned using data from the Cluster mission :

  1. Theta aurora’s, when polar lights show up in unexpected places
  2. The relationship between the plasmasphere and the Van Allen radiation belts
  3. Comparing Earth’s magnetosphere to that of other planets and bodies in the solar system
  4. Solar system plasma turbulence

But first…


Earth’s magnetic field and magnetosphere

Earth’s magnetic field is the result of its internal structure: the spinning liquid metal core acts as a dynamo that generates a magnetic field. The magnetosphere is the region of space around the Earth in which the motion of electrically charged particles is controlled by this magnetic field. Most notably, particles streaming towards Earth from the Sun (known as “the solar wind”) are blocked, trapped, deviated or strongly accelerated by the Earth’s magnetic field, the most well-known effect of this acceleration being the aurora. The magnetic field also traps many of the “cosmic rays”, charged particles with much higher energies that come from the Sun, from the galaxy, or even beyond.

Earth mangetosphere structure
Figure 3 : The structure of Earth’s magnetosphere. Credit: ESA/C.T. Russell.

Earth’s internally generated magnetic field thus acts as a protective shield against particles with high energy, which could otherwise inflict a lot of damage to life and electronics on Earth. The magnetosphere is the protected region of space near the Earth. It has a complex structure (see figure 3), with a bow shock where the solar wind first encounters the terrestrial obstacle, the magnetosheath where particles are significantly slowed down by the bow shock, and the magnetopause, the outer frontier of the magnetosphere which only few solar wind particles can cross. The interactions that ensue in each of these regions are very complex, and the structure itself is highly dynamic. Despite Cluster’s twenty candles, there is still a lot of research to be done to fully understand the behaviour of Earth’s magnetosphere.

This begs the question: what have we learned from Cluster’s observations? Several physicists at BIRA-IASB have been involved in research projects using Cluster data, to unravel a few of the consequences of the electromagnetic forces at the planetary scale.


The advantage of four vs. one

The peculiarity of the Cluster mission is that it actually consists of four identical satellites with the following playful names: Rumba, Salsa, Samba and Tango. They were launched with Russian Soyuz rockets from the launch complex in Baikonur, Kazakhstan, in two pairs: the first two on the 16th July 2000, and the second two a few weeks later on the 9th August. These quadruplets all carry an identical set of eleven instruments that measure charged particles and electric and magnetic fields. The spacecraft fly in a nearly tetrahedral formation (a pyramid shape) along a polar elliptical orbit, coming as close to the Earth as 19 000 km and going as far away as 119 000 km.

Being able to manipulate four separate entities and the distances between them is a clear advantage, for it means that operators can gather data that would be impossible to get with a single satellite. However, the techniques needed to take full advantage of the power of four did not come easily.

Johan De Keyser, space physicist at BIRA-IASB, explains :

It took time for scientists to understand the full benefits of having measurements from four separate points in space. Digital techniques and software had to be developed to extract the maximum amount of information. At the Royal Belgian Institute for Space Aeronomy, we have developed two techniques: one is very general and allows us to calculate the gradient of a quantity measured by the four satellites (least-squares gradient calculation); another one is very specific and allows us to plot the position of the magnetopause continuously for several hours (empirical reconstruction of the magnetopause and boundary layer).



Theta auroras, when polar lights show up in unexpected places

Theta aurora satellite image
Figure 4: A theta aurora as observed by the
ASA IMAGE FUV camera on September 15,
2005 and anlayzed using Cluster data in the
paper by Fear et al. (2014).
Credit: NASA/R. Fear et al. (2014)

Auroras usually occur along oval shapes that encircle the polar regions. Though not completely understood, this phenomenon has been extensively studied. Simplified, the solar wind particles induce electric fields in the magnetosphere that accelerate magnetospheric particles down along the magnetic field lines until they bombard the Earth’s atmosphere around those ovals. Atoms of the atmosphere are excited by these collisions and start to glow in undulating curtains that can be very bright in the night sky.

Occasionally, something different happens with the aurora. Instead of a simple oval being formed, an additional “transpolar arc” crosses the middle in a shape resembling the Greek letter “theta” (see figure 4), where this type of aurora gets its name from. It was first observed in the 80’s, and scientists have been trying to figure out what causes this shape ever since.

Using ESA’s Cluster spacecraft and NASA’s IMAGE spacecraft at the same time, a team of researchers coordinated by BIRA-IASB physicist Romain Maggiolo and led by Rob Fear (University of Leicester) were able to find evidence for an explanation. In their paper published in Science in 2014, they proposed the following answer: the formation of a transpolar arc crossing the pole - making the aurora visible in places where it shouldn’t - is linked to the presence of hot charged particles in the night side magnetosphere (see figure 3) where we normally find cold material.

To follow this story, we need to catch you up on a few things about magnetic fields :

  1. Closed magnetic field lines are connected to the Earth at both ends, forming a closed loop. Hot charged particles can remain trapped on such field lines for a long time. The auroral oval is connected to closed magnetic field lines.
  2. Open magnetic field lines have one end connected to the polar regions of the Earth and the other one to interplanetary space. Particles on open field lines therefore leak out into space.
  3. Open field lines in the magnetosphere have a tendency to reconnect to form closed field lines. If left alone, more and more closed field lines would form and the auroral ovals would extend to higher latitudes while the polar regions connected to open field lines would shrink.
  4. Two magnetic fields can reconnect if they are oriented in opposite direction.

Now that you know some of the basic rules of the game, let’s begin!

The Interplanetary Magnetic Field (IMF) is the magnetic field carried by the solar wind (because the particles are electrically charged, and electricity always goes hand in hand with magnetism). As it is embedded in the solar wind, it comes into contact with Earth’s magnetosphere on its dayside part (the region of the magnetosphere located between the Sun and the Earth) where the magnetic field lines from the Earth point northward.

A normal, oval-shaped aurora forms when the IMF points southward (see figure 5). Since opposite magnetic fields can reconnect (rule 4), field lines from Earth will “break” and open to reconnect with the IMF. This opening of field lines is what counteracts the tendency of open field lines to close themselves (rule 3). The closed lines cannot pile up and move to higher latitudes. The aurora is thus only visible at around 60° to 75° latitude.

Southward IMF
Figure 5 :
1) IMF reconnects with magnetospheric field lines
2) and 3) It creates open field lines that are transported to the nightside (here from the 'front' to the 'back')
4) Open field lines reconnect and close
5) A new closed field line is formed
Credit : Dylan Leivers

Romain explains why theta auroras sometimes form :

Theta auroras are formed by the trapping of hot charged particles in the magnetospheric lobes. Open magnetic field lines from the magnetospheric lobes (one from the northern hemisphere, one from the southern hemisphere) reconnect to form a closed field line where hot particles are trapped (rule 1). This happens all the time in the magnetospheric tail (the nightside part of the magnetosphere).
For southward IMF, new open field lines are created at the same time, so there is some kind of equilibrium between the creation and destruction of closed field lines and the auroral oval keeps its shape. For northward IMF, no open field lines are created. The amount of closed field lines increases and the region of closed field lines (the auroral oval) extends toward higher latitudes. Actually, the field lines close preferentially at the night side part of the oval. A channel of closed field lines then protrudes from the night side part of the auroral oval toward the poles, until it reaches the dayside part. That's how it forms a shape looking like a theta.

Northward IMF
Figure 6 :
1) Reconnection at the dayside is impossible, no open field lines are created
2) But reconnection on the nightside still occurs
3) Closed field lines are formed, they accumulate and fill the high-latitude regions
Credit : Dylan Leivers
THeta aurora plot
Figure 7 : The plot shows the geometry of a layer of closed magnetic field lines extending toward high latitudes, associated with a theta aurora as determined from conjugated observations by the Cluster and TIMED satellites. The black sphere represents the Earth, the grey perimeter line the magnetopause, and the green line Cluster’s orbit, while the X-axis is oriented towards the Sun. The orange surface is the projection of a theta aurora imaged by the TIMED satellite while the orange line corresponds to the region where Cluster crossed a region of trapped energetic plasma at high latitude.
(Image source: Maggiolo et al. 2012.)



The relationship between the plasmasphere and the Van Allen radiation belts

Van Allen radiation belts
Figure 8: The plasmasphere (in blue) and the two
Van Allen radiation belts (in red) overlap in different
places in different situations. The plasmasphere can
extend beyond the outer Van Allen belt in times of
low geomagnetic activity (top panel), and shrinks
inside the outer belt in times of high geomagnetic
activity (bottom panel).
Credit: ESA - C. Carreau

There are two more constituents of the magnetosphere that we haven’t mentioned yet: the plasmasphere and the Van Allen radiation belts. In 2013, BIRA-IASB physicist Fabien Darrouzet led a team of scientists - including Viviane Pierrard and Johan De Keyser (BIRA-IASB) - to publish a paper in the Journal of Geophysical Research on the relation between these two entities, using Cluster data.

The plasmasphere is the innermost part of the magnetosphere. It is a doughnut-shaped region surrounding Earth’s equator, composed of low energy charged particles (or cold plasma) which mostly co-rotate with the Earth. The Van Allen radiation belts are two separate regions where high energy particles originating from the solar wind are trapped. However, these are not static regions. In the paper, their boundaries are shown to fluctuate strongly under the influence of geomagnetic activity, caused by solar activity.

The Sun operates under an 11-year cycle, with solar minima and maxima as a result. During the solar minimum, the Sun emits fewer particles and the Earth’s magnetic field is less disturbed, less dynamic. This is a period of low geomagnetic activity, seemingly allowing for more particles from the Earth’s upper atmospheric layer (the ionosphere) to leak into the plasmasphere, making it expand, even beyond the outer Van Allen belt.

The Van Allen belts also appear to be influenced by the geomagnetic activity. The “slot region” - the space between the two main belts (see figure 6) – was found to become wider during periods of low geomagnetic activity, because of particle loss in the atmosphere.
Knowing how these different parts of the magnetosphere behave under different circumstances is important if we want to safely send electronics and living organisms, like humans, into space.



Comparing Earth to other planets and bodies in the solar system

In 2011, BIRA-IASB physicist Marius Echim led a team - including Johan De Keyser and Romain Maggiolo - to publish a paper in Planetary and Space Science, in which they compared Earth’s magnetopause (the outer boundary of the magnetosphere – see figure 3) with that of Venus by using data gathered almost simultaneously on the 27th June 2006, from two space missions 88 million kilometres apart: Venus Express and Cluster.

Marius explains :

We chose that day because we knew that the spacecraft were crossing the planets' magnetopauses at the same time and the conditions were comparable. We were able to compare the two magnetopauses, including their thickness and the density of the electric current in them.

Despite being very similar to our planet, Venus has quite different electromagnetic properties: its rotation speed is much slower (and it rotates in the opposite direction than all other planets, though this does not have a direct influence). The rotation speed is insufficient to generate an intrinsic magnetic field, and so Venus only has an induced magnetosphere, created from the interaction between the solar wind and the planet’s own atmosphere.

The team had developed a mathematical model of the structure of the magnetopause, and had the opportunity to apply it to the data from Venus Express for the first time. The results from these measurements seemed to agree with the expectations. Venus’ magnetopause layer is thinner than Earth’s, and is primarily filled with particles of solar wind origin and few of its own atmosphere, whereas Earth’s magnetopause is dominated by its own planetary plasma and magnetic field.

Marius :

The comparative study of planetary plasma environments contributes to a better understanding of the general principles that govern magnetospheric configuration and dynamics, in particular the role of the solar wind state and of the planetary plasma environment.



Solar system plasma turbulence

Plasma turbulence
Figure 9: Simulation of turbulence in solar wind plasma.
Credit: ESA

The space environment is constantly varying. For instance, the solar wind speed always fluctuates – tiny variations of the speed by just a few km/s occur in a few seconds time, changes with more than 100 km/s often take place over hours or days, but sometimes such large changes come about in a matter of seconds too. Yet, these fluctuations are not completely random. They reflect the underlying physical processes.

One particular way to try to understand complicated phenomena in fluids or gases or plasmas is precisely by studying these fluctuations in an overall manner, rather than trying to follow all the individual particles in the plasma. Let us look at one example that everyone is familiar with: your cup of coffee, in the middle of which you pour just a few drops of milk. Stir the mixture and you see that first big eddies form, which progressively break down into ever smaller vortices, and ultimately the milk mixes into your coffee at the microscopic scale. This is precisely what happens in space plasmas too. For instance, as the shock of a big solar eruption propagates as a large-scale phenomenon through the solar wind, it gradually becomes weaker and more diffuse as the energy represented in the large-scale structure is transmitted through the creation of structures on increasingly smaller scales.

Measuring how many and how big and how often structures of each scale size are found in the medium is called “turbulence analysis”. Such an analysis can provide information regarding the energy redistribution in the plasma, the mechanisms by which the energy is redistributed to smaller scales, and the scales at which the behaviour of the plasma changes (in plasmas there are multiple scales that play a role, as opposed to the coffee cup mentioned above).

During the STORM project (Solar system plasma Turbulence: Observations, inteRmittency and Multifractals), running from 2013 to 2015 and coordinated by Marius Echim, vast amounts of data from many different space missions (Giotto, Ulysses, Rosetta, Cluster and Venus Express, Cassini, Mars Global Surveyor and THEMIS) were processed to characterize plasma turbulence in different regions in space.

Marius Echim about the STORM project :

In this project we gave Cluster a new dimension by integrating the mission into a virtual solar system observatory, consisting of spacecraft distributed within the entire system: Ulysses in the solar wind, Venus Express at Venus, Cassini close to Saturn…
Data from Cluster specifically helped us to discover a turbulent solar wind in the vicinity of the Earth and how this turbulence “shakes” the Earth’s environment. In STORM we made a survey of tens of thousands of hours of data and produced thousands of scientific data analyses, grouped into a database curated by our institute.
We even went a step further and created the tools to analyse the Cluster data, and shared these tools with the entire space science community. A key finding of STORM revealed by the analysis of Cluster data was that the terrestrial plasma is much more variable than originally thought, and has different properties compared to the other planets in the solar system.  We also discovered that this variability is so complex that it escapes the efforts to model and understand it.

Cluster’s insights

Our knowledge of the Earth’s magnetosphere and that of other bodies has greatly expanded, largely thanks to the information that the four Cluster spacecraft, Rumba, Salsa, Samba and Tango, have been gathering continuously for the past 20 years. We have learned more about the structure and dynamics of the magnetosphere, its interactions with the atmosphere on the inside and the solar wind on the outside, and the behaviour of space plasmas in general.

This knowledge, besides being of a fascinating complexity, is crucial to our modern lifestyles, with our dependency on satellites for our technology, and our thirst for more space exploration. We need to be able to predict how the magnetosphere and all its constituents behave if we want to keep satellites and astronauts orbiting around Earth or other planets safe from the harmful effects of the solar wind, solar flares, coronal mass ejections and cosmic radiation.


Sources and further reading

News image 1
News image legend 1
Figure 1: Artistic rendering of the Earth’s magnetosphere.
Credit: NASA
News image 2
News image legend 2
Figure 2: Integration of the four Cluster spacecraft.