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NASA's Parker Solar Probe passed into and through the solar corona, the Sun's upper atmosphere, on April 28, 2021. Not only did it survive – showing the effectiveness of Parker's high-tech heat shielding – but it also took in situ measurements, providing us with a trove of never-before-seen data on our Solar System's core.

"The Parker Solar Probe 'touching the Sun' is a watershed moment for solar science and a really extraordinary achievement," said astrophysicist Thomas Zurbuchen, associate administrator for NASA Headquarters' Science Mission Directorate.

"Not only does this achievement provide us a better understanding of our Sun's evolution and its effects on our Solar System, but everything we learn about our own star tells us more about stars throughout the Universe."

The Parker Solar Probe was launched in 2018 with the primary goal of studying the solar corona. It should make a total of 26 close approaches, or perihelions, to the Sun over the course of its seven-year mission, employing a total of seven gravity assist manoeuvres from Venus to bring it closer. The perihelion in April was the seventh and first to penetrate the corona.

Parker recorded variations in the Sun's magnetic field and sampled particles during his nearly five-hour stay inside the solar atmosphere. Previously, we depended on external data to estimate these qualities.

"Flying so near to the Sun, the Parker Solar Probe now detects circumstances in the magnetically dominated layer of the solar atmosphere - the corona – that we've never been able to detect before," said astronomer Nour Raouafi of the Johns Hopkins Applied Physics Laboratory.

"Magnetic field measurements, solar wind data, and photos all provide evidence of being in the corona. The spacecraft can be seen travelling through coronal structures that can be seen during a total solar eclipse."

Above: Coronal streamers, which can only be viewed from Earth during an eclipse, are the brilliant features shown in these images. The Parker probe captured these images during the ninth perihelion in August of this year.

There is no solid surface on the Sun. Instead, the Alfvén critical surface, where gravity and the Sun's magnetic fields are too weak to hold the solar plasma, defines its border.

Above this point, the solar wind appears, sweeping powerfully through the Solar System and breaking away from the Sun in waves. The photosphere, which is made up of churning convection cells and plasma, is much below what we call the Sun's'surface.'

Parker's purpose was to learn more about the Alfvén critical surface, such as where it is and what its topography is like, because we didn't know anything about it. The Alfvén critical surface was estimated to be between 10 and 20 solar radii from the Sun's centre. Parker reached the corona at a distance of 19.7 solar radii and sank as low as 18.4 solar radii throughout its corona journey.

Surprisingly, the probe only encountered the corona's magnetic conditions on a sporadic basis, implying that the Alfvén critical surface is wrinkled. Parker came across a magnetic structure known as a pseudostreamer at a lower depth, which we can see arcing out from the Sun during solar eclipses. Parker's findings show that these structures are to blame for the Alfvén critical surface's deformation, albeit we don't know why.

Conditions were quieter inside the pseudostreamer than in the surrounding solar environment. Particles were no longer as chaotically buffeting the spaceship, and the magnetic field was more ordered.

Parker also looked on the occurrence of solar switchbacks. These are Z-shaped kinks in the solar wind's magnetic field, and it's unclear where or how they develop. Switchbacks have been around since the 1990s, but it wasn't until Parker examined them in 2019 that we discovered how ubiquitous they are. The data from the probe's sixth flyover revealed that switchbacks are caused by patches.

Parker has now discovered them within the solar atmosphere, indicating that at least some of the switchbacks originate in the lower corona.

"The structure of the switchback zones lines up with a small magnetic funnel structure at the base of the corona," astronomer Stuart Bale of the University of California, Berkeley, stated. "This is what some theories predict, and it identifies a source for the solar wind itself."

We still don't know how these strange structures came to be, but with dozens more perihelions on the way, some as close as 9.86 solar radii from the Sun's centre, we're sure to find out.

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We've been studying the Sun and its corona for decades, and we know there's some fascinating physics at work to heat and accelerate the solar wind plasma. We still don't know exactly what that physics is "Raouafi explained.

"With the Parker Solar Probe now travelling towards the magnetically dominated corona, we will finally gain some answers about how this mysterious region works."

When children draw pictures of the Sun, they often show rays radiating outwards – similar to the image below.

The Sun

At this stage of our Sun’s life cycle, hydrogen atoms are fused to form helium atoms. This nuclear reaction produces very large amounts of energy.

These light rays travel in a straight line at nearly 300,000 kilometres per second. Sunlight that travels towards the Earth takes just over 8 minutes to reach us. When the rays reach Earth, they hit whatever is in their path. If the object they hit is opaque, the light cannot pass through, and a shadow forms.

Simply speaking, a shadow is an absence of light. If light cannot get through an object, the surface on the other side of that object (for example, the ground or a wall) will have less light reaching it.

A shadow is not a reflection, even though it is often the same shape as the object.

Light sources and shadows

There are many sources of light – stars like our Sun, candle flames, light bulbs, glow-worms and computer screens produce light. All of this light travels in a straight line until it hits something. Sometimes, it travels a short distance – like when we switch on the lamp. Other times, light travels thousands of years – like the light from stars we see in the Milky Way.

It is easy to see our shadows when we are outdoors in the sunshine on a clear, bright sunny day, but do shadows form when an object blocks light from other sources? The answer is yes, but they may be difficult to see if the light source is not very bright (has a low light intensity). Shadows are also more definite (sharper) where there is contrast between the shadow and the lit surface, for example, a shadow on a white wall will be more easily seen.

The size of the light source can sharpen or blur the shadow. A small spotlight like a cellphone torch forms a more distinct shadow than an overhead room light, but the sharpness of the shadow changes when the torch moves away from the object.

Long penguin shadow

The Sun is low on the horizon so the penguin’s shadow is long. An object is always between a light source and the surface on which its shadow forms.

Changing shapes and sizes

A shape of an object always determines the shape of its shadow. However, the size and shape of the shadow can change. These changes are caused by the position of the light source.

When we are outside on a sunny day, we can see how our shadows change throughout the day. The Sun’s position in the sky affects the length of the shadow. When the Sun is low on the horizon, the shadows are long. When the Sun is high in the sky, the shadows are much shorter. We can create the same effects indoors by changing the position of a torch as it shines on an object.

Although the shadow effects are the same, the reasons for the moving light source are very different. When we use a torch to make long and short shadows indoors, it is the light source that moves. When the Sun makes long and short shadows outdoors, it is the Earth, not the light source (Sun), that moves.

The Sun appearing in the east

As the Earth’s axial rotation spins our planet towards the light of the Sun, we see the Sun appear in the east. Due to the Earth’s rotation, our view of the Sun changes throughout the day.

The spinning Earth

From our vantage point on Earth, it appears that the Sun moves across the sky during the day. We see the Sun appear to rise in the east and set in the west. Actually, the Earth is spinning (rotating on its axis) so it is our view of the Sun in the sky that changes during each 24-hour cycle of light and dark.

We see the sunrise when our location on Earth spins towards the light of the Sun. As the Earth continues to spin, we see the Sun higher in the sky. As the Earth spins away from the light, we see the sunset. The Earth continues to spin until we are in a shadow – our place on Earth is dark because the Sun’s light is blocked by the magnitude of our planet! We have several hours of night with our side of the Earth in darkness, and then as the Earth spins towards the Sun’s light, we see a sunrise. When New Zealand is in darkness during the night, the opposite side of the world is in sunlight.

Shadows change with the seasons

The tilt of the Earth’s axis affects the length of our shadows. During the summer, our location is tilted towards the Sun, so our midday shadows are very short. During the winter, our location is tilted away from the Sun, so our midday shadows are longer.