August 11th saw the launch of NASA’s Parker solar probe. A probe with a unique mission; to beam back a record of the plasma that comprises the Sun’s corona and the magnetic fields which shape it. The probe will fly closer to the Sun than any probe has before, facing brutal heat and radiation, even flying through the Sun’s corona, the outermost part of the star’s atmosphere. The research team behind the mission hope that this data will help solve several mysteries surrounding the Sun, such as what heats this plasma to 200 times temperature of the sun’s surface?
Despite its exceptional temperature and the fact that it extends millions of miles of the surface of the Sun, the corona is only visible from Earth during a solar eclipse when the moon blocks out the visible light from the much more luminous photosphere. Solar winds, which carry particles from the corona to Earth take enough time to reach us that they have time to interact with other particles and space dust and debris. These characteristics mean that the study of the corona is extremely difficult without travelling directly to the source.
In fact, the corona is so mysterious that scientists are as yet unsure what form coronal plasma actually takes, whether it is thin and wispy, much like cirrus clouds. Or thick and pipe-like radiating from the Sun.
The Parker solar probe, designed by Betsy Congdon and her team at John Hopkins Applied Physics Laboratory (APL), only a little larger than a king-size mattress, will pass through the sun’s corona, a tenuous atmosphere of hot charged particles, or plasma, on the first of two dozen flybys between now and 2024 to facilitate this study.
Its closest approach will be approximately 0.04AU, or roughly 6 million kilometres. Whilst this may not sound particularly close to the Sun, it has to be considered in comparison the roughly 148 million km between the Earth and the Sun.
Obviously, such a close approach will expose the probe to harsh temperatures of up to 1370ºC and dangerous solar radiation, requiring that the engineers behind the Parker probe had to develop a remarkable new heat shield to protect the probe’s electronics. These temperatures do not come from the corona itself, which is too nebulous to transfer much heat, but from the powerful glare of the Sun.
This heat shield comprises of a thick filling of carbon foam, an airy mesh of carbon molecules, sitting between thin sheets of carbon-carbon, a material woven from carbon fibres that gets stronger, not weaker when heated to a few thousand degrees. Inside this carbon foam, is the real heat shield, painted white to reflect as much heat as possible.
The shield must remain permanently in position between the Parker probe and the Sun. Hence systems within the probe are designed to rotate the heat shield, which weighs no more than the average human being, back into place should it move from between Parker and the Sun, even when it passes to the far side of the Sun and loses contact with mission control. To prevent severe damage the heat shield must be able to reorientate within minutes of its displacement.
Ironically, the heat shield is highly flammable in Earth’s atmosphere due to the presence of oxygen. In the solar corona, oxygen is rare, and what oxygen does exist there is ionised by the violent conditions and thus less reactive. In fact, part of the mission will assess just why the conditions in the corona are so violent.
What is so mysterious about the Sun’s corona?
One of the biggest unanswered questions in astrophysics is just how the Sun’s corona manages to maintain its heat of 1,000,000 Kelvins when the surface of the Sun, the photosphere itself is only around 6,000 Kelvins. Logic would suggest that the further out an element of the Sun is from the centre of the star, the cooler it should be. Clearly, the fact that the corona is the furthest element of the Sun and yet it is hotter than the surface is extremely curious.
“The corona is incredibly hot, hundreds of times hotter than the layers below,” Bernhard Fleck, a European Space Agency project scientist for NASA’s Solar and Heliospheric Observatory (SOHO) said in a statement. “Since the sun’s source of energy is at the centre, on a simple level, we would expect the corona — the outermost layer — to be the coolest.”
Physicists have long disputed the cause of this apparent disparity, some theories suggest that the violent convective motion of cells plasma throughout the Sun generate magnetic fields to which the plasma in the corona responds. When the fields become tangled this transfers massive amounts of heat to the plasma that comprises the corona.
One theory suggests that the magnetic waves produced by this motion are of a certain frequency, Alfvén waves, which send charged particles spinning and heat the atmosphere, a bit like how ocean waves push and accelerate surfers toward the shore.
Another theory suggests that small explosions on the Sun’s surface, known as nanoflares send heat into the Sun’s atmosphere. The nanoflares are caused by a process known as magnetic reconnection. This is also the cause of coronal mass ejections (CMEs) when massive amounts of solar-material is ejected from the Sun. Reconnection occurs when field lines become tangled, break and reconnect in different formations.
To complicate matters these two theories and others are not mutually exclusive. It is possible that the mechanisms of heating mentioned above, for example, work in conjunction to heat the corona. Magnetic reconnection could trigger nanoflares and then Alfvén waves, both of which drive the temperature of the atmospheric plasma.
It is hoped that these are the kind of processes that the Parker probe can investigate as well as uncovering just how frequently these events occur.
How the Parker probe will investigate the Sun
Whilst the Parker probe carries a variety of instruments, it is two in particular that will be of most use in investigating coronal heating. The FIELDS experiment, led by the University of California, Berkeley, will directly measure the Sun’s electric and magnetic fields, in order to understand the shocks, waves and magnetic reconnection events that heat the solar wind.
Whilst simultaneously, SWEAP conducted by the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Massachusetts will gather data on the plasma that comprises the corona, by counting the most abundant particles in the solar wind, electrons, protons and helium ions, and by measuring their temperature, velocity and direction.
Each of the forms of heating mentioned above should give its own particular signature, thus allowing heliophysicists to build a satisfactory model of coronal heating. Together, both instruments are precise enough that they should give a complete picture of the electromagnetic fields and particle interactions which drive the temperature of the corona to such extremes.
“Even though magnetic reconnection events take place lower down near the Sun’s surface, the spacecraft will see the plasma right after they occur,” said Goddard solar scientist Nicholeen Viall. “We have a chance to stick our thermometer right in the corona and watch the temperature rise. Compare that to studying plasma that was heated four days ago from Earth, where a lot of the 3D structures and time-sensitive information are washed out.”
Whatever the case, the results will take some time to resolve. Not only is there a considerable amount of data from multiple sources to be considered, but the Sun’s surface is unsmooth and variable and thus requires multiple fly-bys to collect data.
“I’m pretty sure when we get that first round of data back, we’ll see the solar wind at lower altitudes near the Sun is spiky and impulsive,” said Stuart Bale, University of California, Berkeley, astrophysicist and FIELDS principal investigator. “I’d lay my money on the data being much more exciting than what we see near Earth.”
Lighting the way to future discoveries
Within six weeks the Parker solar probe will have reached Venus, using the planet’s gravity it will engage in a further six-week journey towards the Sun. In doing so the probe will fulfil a mission that has been an aim of NASA since it was first conceived. The name ‘Parker’ alludes to the work of Eugene Parker in describing solar-winds, which dates back to 1958.
The hope is that by understanding the corona better, physicists will be better positioned to understand space-weather in the solar system and in the vicinity of Earth. In addition to this, we may be able to piece together a better understanding of other stars with Sun-like heating. In doing so we will begin to build a picture of environments in other solar-systems without having to travel there.
It is likely that data gathered from the Parker solar probe will open a whole new era of solar physics.