Sun is less active than similar stars

The extent to which solar activity (and thus the number of sunspots and the solar brightness) varies can be reconstructed using various methods - at least for a certain period of time. Since 1610, for example, there have been reliable records of sunspots covering the Sun; the distribution of radioactive varieties of carbon and beryllium in tree rings and ice cores allows us to draw conclusions about the level of solar activity over the past 9000 years. For this period of time, scientists find regularly recurring fluctuations of comparable strength as during recent decades. "However, compared to the entire lifespan of the Sun, 9000 years is like the blink of an eye", says MPS scientist Dr. Timo Reinhold, first author of the new study. After all, our star is almost 4.6 billion years old. "It is conceivable that the Sun has been going through a quiet phase for thousands of years and that we therefore have a distorted picture of our star," he adds. Since there is no way of finding out how active the Sun was in primeval times, scientists can only resort to the stars: Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity. To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "The speed at which a star rotates around its own axis is a crucial variable", explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication.

A star's rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki.

The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are - and thus also how brightly the Sun shines.

A comprehensive catalogue containing the rotation periods of thousands of stars has been available only for the last few years. It is based on measurement data from NASA's Kepler Space Telescope, which recorded the brightness fluctuations of approximately 150000 main sequence stars (i.e. those that are in the middle of their lifetimes) from 2009 to 2013.

The researchers scoured this huge sample and selected those stars that rotate once around their own axis within 20 to 30 days. The Sun needs about 24.5 days for this. The researchers were able to further narrow down this sample by using data from the European Gaia Space Telescope. In the end, 369 stars remained, which also resemble the Sun in other fundamental properties.

The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong.

"We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities".

However, it is not possible to determine the rotation period of all the stars observed by the Kepler telescope. To do this, scientists have to find certain periodically re-appearing dips in the star's lightcurve. These dips can be traced back to starspots that darken the stellar surface, rotate out of the telescope's field of view and then reappear after a fixed period of time.

"For many stars, such periodic darkenings cannot be detected; they are lost in the noise of the measured data and in overlying brightness fluctuations," explains Reinhold. Viewed through the Kepler telescope, even the Sun would not reveal its rotation period.

The researchers therefore also studied more than 2500 Sun-like stars with unknown rotation periods. Their brightness fluctuated much less than that of the other group.

These results allow two interpretations. There could be a still unexplained fundamental difference between stars with known and unknown rotation period. "It is just as conceivable that stars with known and Sun-like rotation periods show us the fundamental fluctuations in activity the Sun is capable of," says Shapiro. This would mean that our star has been unusually feeble over the past 9000 years and that on very large time scales phases with much greater fluctuations are also possible.

There is, however, no cause for concern. For the foreseeable future, there is no indication of such solar "hyperactivity". On the contrary: For the last decade, the Sun has been showing itself to be rather weakly active, even by its own low standards. Predictions of activity for the next eleven years indicate that this will not change soon.

Solar Orbiter ready to depart Europe

ESA's Solar Orbiter mission has completed its test campaign in Europe and is now being packed ready for its journey to Cape Canaveral at the end of this month, ahead of launch in February 2020. The spacecraft was on display today for the final time in Europe, at the IABG test centre near Munich, Germany. It was built at Airbus Stevenage, UK, and has spent the last year at IABG undergoing essential testing such as checking deployment mechanisms, and that it can withstand the vibrations of launch, and the thermal extremes and vacuum of space. It has now been declared ready for shipment to the launch site and will travel an Antonov cargo plane on 31 October. Once launched it will follow an elliptical path around the Sun, at its closest bringing it within the orbit of Mercury, just 42 million kilometres from the Sun. As such, Sun-facing parts of the spacecraft have to withstand temperatures of more than 500 degrees C - due to solar radiation thirteen times more intense than for Earth-orbiting satellites - while other parts remain in shadow at -180 degrees C. The mission is essential to learn more about the Sun-Earth connection. We live inside a giant bubble of plasma generated by the Sun that surrounds the entire solar system, within which we are prey to space weather. Solar Orbiter will provide a deeper understanding as to how activity on the Sun is linked to these solar storms, which can disrupt electrical systems, satellite communications, GPS, and create higher doses of radiation for polar flights and astronauts.

"Solar Orbiter is set for answering some of the biggest scientific questions about our star, and its data will help us to better protect our planet from the global challenges of space weather," says Gunther Hasinger, ESA Director of Science.

"Thanks to the hard work of our teams building and testing this inspiring space mission, we've reached an important milestone today in Europe, and the spacecraft will now be readied for its final phase of pre-launch preparations at Cape Canaveral."

Solar Orbiter will launch on a NASA-provided Atlas V 411. The launch is currently scheduled in the early hours of 6 February (UTC). Once in space, and over the course of several years, it will use the gravity of Venus and Earth to raise its orbit above the poles of the Sun, providing new perspectives on our star, including the first images of the Sun's polar regions.

Its complementary suite of instruments means it will be able to study the plasma environment locally around the spacecraft, and collect data from the Sun from afar, connecting the dots between the Sun's activity, and the space environment in the inner solar system.

Solar Orbiter is an ESA mission with strong NASA participation. The prime contractor is Airbus Defence and Space in Stevenage, UK. It follows in the legacy of missions such as Ulysses (1990-2009) and SOHO (1995-present) and will also provide complementary datasets to NASA's Parker Solar Probe that will allow more science to be distilled from the two missions than either could achieve on their own.

NASA postpones for 24 hours launch of historic spaceship to Sun

NASA postponed until Sunday the launch of the first ever spacecraft to fly directly toward the Sun on a mission to plunge into our star's sizzling atmosphere and unlock its mysteries. The reason for the delay was not immediately clear, but was called for after a gaseous helium alarm was sounded in the last moments before liftoff, officials said. Engineers are taking utmost caution with the $1.5 billion Parker Solar Probe, which Thomas Zurbuchen, head of NASA's science mission directorate, described as one of the agency's most "strategically important missions." The next launch window opens at 3:31 am (0731 GMT) on Sunday, when weather conditions are 60 percent favorable for launch, NASA said. By coming closer to the Sun than any spacecraft in history, the unmanned probe's main goal is to unveil the secrets of the corona, the unusual atmosphere around the Sun. Not only is the corona about 300 times hotter than the Sun's surface, but it also hurls powerful plasma and energetic particles that can unleash geomagnetic space storms, wreaking havoc on Earth by disrupting the power grid. These solar outbursts are poorly understood, but pack the potential to wipe out power to millions of people.

- 'Full of mysteries' -

The probe is protected by an ultra-powerful heat shield that is 4.5 inches (11.43 centimeters) thick.

The shield should enable the spacecraft to survive its close shave with the fiery star, coming within 3.83 million miles (6.16 million kilometers) of the Sun's surface.

The heat shield is built to withstand radiation equivalent to up to about 500 times the Sun's radiation on Earth.

Even in a region where temperatures can reach more than a million degrees Fahrenheit, the sunlight is expected to heat the shield to just around 2,500 degrees Fahrenheit (1,371 degrees Celsius).

If all works as planned, the inside of the spacecraft should stay at just 85 degrees Fahrenheit.

"The sun is full of mysteries," said Nicky Fox, project scientist at the Johns Hopkins University Applied Physics Lab.

- 91-year-old namesake -

The tools on board will measure the expanding corona and continually flowing atmosphere known as the solar wind, which solar physicist Eugene Parker first described in 1958.

Parker, now 91, recalled that at first some people did not believe in his theory.

But then, the launch of NASA's Mariner 2 spacecraft in 1962 -- becoming the first robotic spacecraft to make a successful planetary encounter -- proved them wrong.

"It was just a matter of sitting out the deniers for four years until the Venus Mariner 2 spacecraft showed that, by golly, there was a solar wind," Parker said earlier this week.

Parker said he was "impressed" by the Parker Solar Probe, calling it "a very complex machine."

According to Zurbuchen, Parker is an "incredible hero of our scientific community."

"Life is all about these big arcs. Sometimes you just see, like how over a lifetime, things just come together and create these amazing stories, these leaps going forward."

Giant solar tornadoes put researchers in a spin

Despite their appearance solar tornadoes are not rotating after all, according to a European team of scientists. A new analysis of these gigantic structures, each one several times the size of the Earth, indicates that they may have been misnamed because scientists have so far only been able to observe them using 2-dimensional images. Dr Nicolas Labrosse will present the work, carried out by researchers at the University of Glasgow, Paris Observatory, University of Toulouse, and Czech Academy of Sciences, at the European Week of Astronomy and Space Science (EWASS) in Liverpool on Friday 6 April. Solar tornadoes were first observed in the early 20th century, and the term was re-popularised a few years ago when scientists looked at movies obtained by the AIA instrument on the NASA Solar Dynamics Observatory (SDO). These show hot plasma in extreme ultraviolet light apparently rotating to form a giant structure taking the shape of a tornado (as we know them on Earth). Now, using the Doppler effect to add a third dimension to their data, the scientists have been able to measure the speed of the moving plasma, as well as its direction, temperature and density. Using several years' worth of observations, they were able to build up a more complete picture of the magnetic field structure that supports the plasma, in structures known as prominences.

Dr Nicolas Labrosse, lead scientist in the study, explains: "We found that despite how prominences and tornadoes appear in images, the magnetic field is not vertical, and the plasma mostly moves horizontally along magnetic field lines. However we see tornado-like shapes in the images because of projection effects, where the line of sight information is compressed onto the plane of the sky."

Dr Arturo Lopez Ariste, another member of the team, adds: "The overall effect is similar to the trail of an aeroplane in our skies: the aeroplane travels horizontally at a fixed height, but we see that the trail starts above our heads and ends up on the horizon. This doesn't mean that it has crashed!"

Giant solar tornadoes - formally called tornado prominences - have been observed on the Sun for around a hundred years. They are so called because of their striking shape and apparent resemblance to tornadoes on Earth, but that is where the comparison ends.

Whereas terrestrial tornadoes are formed from intense winds and are very mobile, solar tornadoes are instead magnetized gas. They seem to be rooted somewhere further down the solar surface, and so stay fixed in place.

"They are associated with the legs of solar prominences - these are beautiful concentrations of cool plasma in the very hot solar corona that can easily be seen as pink structures during total solar eclipses," adds Labrosse.

"Perhaps for once the reality is less complicated than what we see!" comments Dr Brigitte Schmieder, another scientist involved in the work.

She continues: "Solar tornadoes sound scary but in fact they normally have no noticeable consequences for us. However, when a tornado prominence erupts, it can cause what's known as space weather, potentially damaging power, satellite and communication networks on Earth."