Top three mysteries Smile will solve

Specifically, Smile will improve our understanding of space weather and solar storms. The magnetospheric bubble that surrounds Earth is invisible to our eyes, but with its UV and X-ray cameras, Smile will reveal this shield that protects us from the Sun. Its findings will help protect space-based technology and the lives of any humans in orbit around Earth, as well as infrastructure on Earth’s surface.

What makes Smile unique is its ability to collect data on the entire Sun-facing side of Earth in just a few minutes. Thanks to this special skill, the mission will help answer three fundamental questions.

1. What happens where the solar wind meets Earth’s magnetic shield?

The Sun is essentially a huge chaotic magnet with a north and a south pole, producing a messy magnetic field. This magnetic field is dragged outwards from the Sun by the solar wind. Earth is a smaller and more stable magnet, with a magnetic field that we call the magnetosphere. Life on Earth depends upon this protective bubble that shields us from the solar wind.

On the side of Earth that faces the Sun, the solar wind meets and squashes the edge of the magnetosphere. Around 50% of the time, the Sun's magnetic field lines point in the opposite direction to Earth’s lines. In this opposing state, during moments when the Sun is particularly active, Earth’s magnetic field lines can ‘reconnect’ with the Sun’s, before snapping like overstretched elastic bands.

Sometimes the reconnection and snapping of the field lines can be a steady process, and sometimes it can be quick and ‘bursty’. Smile will help us uncover why each version occurs.

This ‘magnetic reconnection’ is a violent process that flings particles into the magnetosphere at superfast speeds. Some of the particles shoot along the snapped field line towards Earth’s poles. There, they collide with atoms in the atmosphere, causing the sky to light up with the northern and southern lights. But they may also cause electric currents that put a strain on our Earth-based power systems (see infographic at the beginning of this article).

Smile will pay a lot of attention to Earth’s polar regions. Because most magnetic field lines start and end at the poles, their behaviour there gives information about the magnetosphere more generally. Sometimes we hear on the news that the northern or southern lights are visible from unusually low or high latitudes; the reason for this is that the particles reaching Earth form a ring around each pole that can become bigger and smaller. We know that this movement is related to the strength of the interaction between the solar wind and the magnetosphere, but we are not exactly sure how.

By measuring steady and unsteady changes in the solar wind, the steady and unsteady movement of Earth’s magnetosphere on the Sun-facing side, and changes in brightness and location of auroras, Smile will help us understand exactly what exactly happens where the solar wind meets Earth’s magnetic shield.

The reconnection of magnetic field lines also plays a big role elsewhere in space. For example, it is involved in heating the Sun’s mysteriously hot outer atmosphere and accelerating the solar wind. What scientists learn about magnetic reconnection by studying the relatively easy-to-examine Earth can be applied to processes further away.

2. What causes magnetic glitches on the dark side of Earth?

Perhaps surprisingly, energy from the solar wind can also enter the magnetosphere through the side of Earth that faces away from the Sun. On Earth’s ‘dark side’, or nightside, the magnetosphere is dragged out like a windsock, with teardrop-shaped magnetic field lines. Sometimes, the solar wind squashes these magnetic field lines, forcing them to pinch together and reconnect closer to Earth (see video below).

Just like when the field lines reconnect on Earth’s dayside, scientists believe that each reconnection on the nightside sends a burst of energetic particles shooting along the affected field line towards Earth’s north and south poles (see video below). We call these brief disturbances in Earth’s magnetosphere ‘substorms’. As with the dayside reconnections, substorms can light up the sky with the northern and southern lights, but they may also cause electric currents that put a strain on spacecraft and Earth-based power systems.

 

Substorms are not rare, in fact they occur most days. Nor are they in general particularly dramatic. But this makes the controversies surrounding them even more mysterious. Fundamental questions about what actually triggers the magnetic field lines on the dark side of Earth to pinch together remain unanswered. Could it be to do with the direction of the Sun’s magnetic field? Or is it more related to the amount of pressure that the solar wind exerts on the magnetosphere? Maybe Earth itself is partially to blame, with conditions in the upper atmosphere also playing a role.

By monitoring the edge of the magnetosphere on the dayside of Earth, the northern and southern lights, how the auroras vary in brightness in response to a substorm, and the characteristics of the solar wind, Smile will shed light on substorms. The mission will seek to work out how often they occur, how strong they are, and how much they change Earth’s magnetic field. Ultimately, Smile’s observations will help us predict substorms in advance, ensuring that we are better prepared for potential power failures (and can better plan opportunities to view the auroras!)

3. How can we predict the most dangerous space weather threats in advance?

In 1859 an astronomer named Carrington was observing the Sun when he was dazzled by a sudden flare of light that lasted about five minutes. Over the following days, Earth’s telegraph systems went haywire and auroras lit up the sky even in the tropics.

The cause? The Sun had blasted a huge, fast-moving burst of material in Earth’s direction. When the Sun is at its most active, it can send these coronal mass ejections (CMEs) out into space several times a week. If one comes towards Earth, it rages against the magnetosphere, causing a disturbance known as a ‘geomagnetic storm’, which is much more violent than a substorm. Extreme geomagnetic storms are rare, but they can be a threat to life on Earth and the impact of a large one on our present-day technology-based lifestyle would be enormous.

Smile will explore how and why geomagnetic storms develop in response to CMEs. It will investigate whether geomagnetic storms are related to substorms, aiming to determine whether the two are always separate phenomena, or whether geomagnetic storms can be considered to be a group of multiple substorms. And – importantly – it will explore how a geomagnetic storm ends, knowledge that is vital for giving the ‘all clear’ to safely switch power grids back on.

With existing spacecraft, we can predict geomagnetic storms up to an hour in advance. By improving our models of the Sun-Earth system, Smile’s observations will help us predict them a few days in advance, enabling us to take special measures to reduce the potential impact of a CME on life on Earth.