Solar Orbiter "will open a window into the solar corona"

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Solar Orbiter "will open a window into the solar corona"
Solar Orbiter "will open a window into the solar corona"

The sun, the most observable object in the solar system since the appearance of man, is of immense complexity, in connection with which we are still very far from fully understanding its influence on the earth's environment and the effect on human activity in orbit. The European mission to the Sun will answer many questions.

After several decades of waiting, the Solar Orbiter will embark on an unprecedented voyage to the Sun. The project was proposed in 2000 and approved in 2011, and the start will take place on February 10 this year. The device carries ten instruments that will allow it to better understand the behavior of the sun. We are discussing it with Milan Maksimovich, an astrophysicist and the staff of the National Center for Scientific Research, who took part in the project.

The Solar Orbiter is scheduled to launch on the night of February 9-10 from Cape Canaveral. The Airbus-built probe will head towards the Sun. After two years of travel, he will enter an elliptical orbit around the star and will work there for at least seven years.

The sun, which has attracted the most attention to itself as an object of the solar system since the appearance of man, is of immense complexity, in connection with which we are still very far from fully understanding its work, from its influence on the earth's environment to the effect on human activity in orbit. Despite a number of scientific programs, experts still cannot predict his behavior. As Milan Maksimovic notes, they realized that in order to improve their knowledge of “space meteorology and predict the effects of solar variability and activity”, they needed to “look closer” to the Sun, that is, to get as close to it as technology allows.

In August 2018, the Solar Parker Probe was launched to explore the corona, the little-known atmosphere from which the solar wind blows out. Now the European Space Agency and NASA are preparing to launch the Solar Orbiter, which should establish "the connection between what happens in the sun and what the solar wind blows", as well as "deepen our knowledge of the sun and the inner heliosphere."

After launch, the Solar Orbiter probe will move in an elliptical trajectory around the Sun, "approaching it up to 42 million kilometers." The device will be located at a greater distance from the luminary than the Solar Parker Probe, but they have different goals. The Solar Parker Probe is to survey the outer solar corona right up to the beginning of the heliosphere on-site, and the Solar Orbiter will take "ultraviolet images of the solar corona with a record cosmic resolution (70 km / pixel)." In addition, the Solar Orbiter will conduct "solar wind research on an ongoing basis, particularly when approaching the Solar Parker Probe." This will open for astronomers a window into the solar corona, from where the solar wind blows, which washes the entire solar system, and whose interaction with our planet determines cosmic meteorology.

The probe will approach the Sun as close as possible once every five months.

Like the Solar Parker Probe, the Solar Orbiter will not always be as close to the Sun as possible. The satellite will make a rendezvous with it every five months. At this moment, it will be located only 42 million kilometers from our star, that is, closer to Mercury. During the period of maximum approach, when the device will move at the highest speed, it will be for several days over approximately one region of the atmosphere while the Sun rotates around its axis. In other words, for a while, the Solar Orbiter will hover over the sun, as geostationary meteorological and telecommunications satellites do in earth orbit. As a result, he will be able to monitor the formation of storms in the Sun's atmosphere. This will create an opportunity for unprecedented observation of magnetic activity, which is concentrated in the atmosphere and causes storms and flares.

Among the questions that haunt scientists, it is worth noting the mysterious heating of the solar corona, which contradicts the usual rules of physics: with distance from the surface of a star or planet, the temperature should decrease. At the Sun, it rises. And significantly. The surface temperature of the luminary is about 5,500 ° C, but reaches "10,000 ° C in the chromosphere and more than a million in the corona, even 2 million in some areas." Experts believe that "the necessary transfer of energy can be the result of fluctuations in the magnetic field and many small emissions that are invisible from the Earth." Be that as it may, so far everything remains at the level of hypotheses, since "measurements of the Sun from the Earth and its orbit do not allow us to dispel the uncertainty."

As for the Solar Orbiter, scientists are convinced that “if the data do not allow explaining the heating mechanism, they should still help to discard some theories” and leave only a small number of them, in particular, the one that relies on “the presence of Alfvén waves in the solar corona, which represent a significant energy transfer mechanism”. The issue of Alfveno waves is also of interest to Solar Parker Probe specialists, who intend to observe from an even closer distance when the probe approaches the Sun at 9 million kilometers. The more distant Solar Orbiter, which has long-range corona diagnostics and solar wind measurement tools, will be able to study the same solar wind flux that will reach its position several tens of hours later.

The closest approach to the Sun will allow observing the solar wind in its "young state", which should help to understand why the Sun ejects such an amount of matter (about 70,000 tons per second), and what mechanisms lead to the acceleration of the solar wind. "It appears in two forms, slow, at 300-400 km / s, and fast, up to 600-800 km / s."

In addition, the probe should help us better understand the mechanisms of solar flares, “why they occur, the processes that cause them, as well as the phenomena associated with them and the consequences they cause,” notes astrophysicist Etienne Paria. The Solar Orbiter should see the origin of flares, which "will allow us to draw a connection between what is happening in this region of the Sun, and the consequences in the heliosphere and the interplanetary medium." In terms of corona emissions, "which can have the greatest impact on the earth's environment," the Solar Orbiter should provide a better "understanding of how they form and travel through the solar system."

Poles of the Sun

After the first phase of observations in an elliptical orbit, which will take about 4 years, Solar Orbiter "uses the gravitational field of Venus and the Earth to exit this plane and begin observations of the Sun and solar wind at high latitudes." The device should take pictures of the polar regions of the Sun for the first time. In addition, important data are expected on the poorly studied magnetic environment of these zones, which play a key role in the 11-year solar cycle and the regular occurrence of solar storms. This part of the mission will be devoted to "local helioseismological measurements and observations of polar holes in the corona, which are the source of the fast solar wind."

Finally, the information accumulated by the Solar Orbiter will be useful in the future for other disciplines such as exobiology. A better understanding of the solar wind should help to understand how the winds of other stars interact with and affect the planets around them, to the extent of changing the atmosphere and the potential emergence of life.

Solar Orbiter carries 10 instruments, which are divided into two categories. Some are designed for spectral analysis and creating images of the heliosphere and corona, while others are used for measuring plasma. One of the probe's unique instruments is to analyze the time-swept electromagnetic field using a set of sensors and antennas to characterize electromagnetic and electrostatic waves in the solar wind.

Key questions that the mission should help answer:

- How is the magnetic field formed and what is its effect on the solar atmosphere?

- What are the mechanisms of formation of the solar corona and wind?

- What physical processes explain solar emissions?

- Why can the temperature of the corona reach more than a million degrees Kelvin, while the temperature of the visible surface is only 6,000?

- How does the plasma of the solar wind accelerate to supersonic speeds of almost 1,000 km / s?

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