Watching movies with my wife Olga can be a challenging experience. After many years in the radio and TV business, she works as free-lance writer of serials and short movies, so that her professional attitude provides the chance for long discussions. She can often predict the whole plot after ten minutes, and catch script details that escape the absent-minded me. For example, watching a romantic comedy like You’ve Got Mail we ended up discussing why in the whole second half of the movie, all the side characters disappear, and we are left for one good hour with just Tom Hanks chasing a wishing-to-be-your-prey Meg Ryan; after Eyes Wide Shut, the endless discussion revolved around Christmas trees, why the hell Kubrick has been putting so many yolkas (it’s Russian for that tree) just in every other scene? Therefore, no wonder that, when during the past holidays we watched the semi-old, sci-fi catastrophe movie Knowing, a highly professional debate ensued between the movie-geek and the physicist-nerd.
Admittedly, the movie’s plot is quite preposterous (watch out, spoilers ahead). A time capsule buried in a schoolyard that contains perfect predictions of the dire events in the following 50 years, the hero-scientist Nicholas Cage races to avert disaster, two kids hear whispers in their ears, a soft love story seems to blossom but without hope, there are sensational special effects, mysterious figures loom in the woods, and at the end the kids are taken to another planet while Earth is incinerated by a humongous solar flare. Plus a cerebral debate at MIT about whether the universe is deterministic or random.
Critics were hard on this movie, but Roger Egbert loved it, so it makes at least three of us. Then, after harshly criticising the role of the kids and the aliens/angels, the big question left on the table was: would such a humongous solar event be even remotely possible? Here comes the physics. Two further coincidences made our discussion livelier. Just a few days earlier, the Parker solar probe from NASA had officially “touched” the Sun surface, plunging for the first time a man-made object into the corona, the tenuous, chaotic bubble of gas extending around the Sun up to lengths about 20 times its radius. The corona is 10-12 times less dense, and thousands of times hotter than the Sun surface (photosphere), a still unexplained physical condition. Second, around the same time I had found on the internet a beautiful amateur astrophoto of a neat solar flare, included here: below the two withish solar spots on the center-right, you can see a kind of arched moustache in the lower-left part, extending about 17 times the Earth’s diameter. That is the solar flare, an event that is all but rare: solar flares can accompany explosive coronal mass ejections (CME). CMEs are large expulsions of plasma from the Sun outer layers. They can eject billions of tons of coronal material traveling at speeds of millions km/h, and carry an embedded magnetic field that is stronger than any other surface magnetic emission from the star. The fastest Earth-directed CMEs can reach our planet in as little as 15-18 hours, slower CMEs can take several days to arrive. They expand in size as they propagate away from the Sun, a large CME can reach a size comprising nearly a quarter of an astronomical unit (equal to the Sun-Earth distance) by the time it reaches our planet. The most intense, explosive CMEs also result in the sudden release of electromagnetic energy in the form of a solar flare, which typically accompanies the explosive acceleration of plasma away from the Sun. Summing up the flare and the plasma particulate ejections, overall energies in the range of 1026 J can be emitted.
What happens when (a small fraction of) this much energy hits the Earth surface? The spectrum of a solar flare covers all wavelengths from microwaves to high-energy gamma rays, and the CME emission includes beams of electrons in the 100s MeV, in loose coincidence with x-ray peaks, and ions (mostly protons) that can attain GeV kinetic energy. While astronauts traveling to the Moon or to Mars would be dramatically endangered by the high energy radiation, solar storms are generally no problem for us on Earth’s surface. The Earth’s atmosphere and magnetosphere largely protect living beings from the effects of solar events.
However, solar storms can be dangerous to our technologies. When a CME strikes the Earth’s atmosphere, it causes a temporary disturbance of the Earth’s magnetic field, that is a geomagnetic storm. Such a tsunami of high-energy charged particles slamming into our atmosphere may perturb satellites in orbit and even cause them to fail, and bathe high-flying airplanes with penetrating radiation. This exceptional cascade of cosmic rays can disrupt telecommunications and navigation systems, it has the potential to affect power grids, and in the past has provoked black out to entire cities, even entire regions. On March 31, 1989, a large CME caused a power failure in Québec, as well as across parts of the northeastern U.S. In this event, the electrical supply was cut off to over 6 million people for nine hours.
The largest solar flare and CME ever recorded on Earth is the so-called “Carrington event” occurred on 1-2 September, 1859, and it was identified independently by two British amateur astronomers, Richard Carrington and Richard Hodgson (for the sake of justice, it could have been called “the two-Richards’ event”). Telegraph poles sparkled light, while transmission lines failed. Fantastic auroras extended with bright colors as far south as the Caribbean. Some gold miners in the Rocky Mountains came out of their barracks and started preparing breakfast, believing it was already morning. According to a study by the National Academy of Sciences, If a solar event of the size of Carrington were to occur today, the total economic impact could exceed 20 times the costs of the hurricane Katrina in 2005. Most of the internet would suddenly fail, some of the largest power transformers, containing tons of magnetic iron, if damaged might take years to repair, not to think of the many casualties that would occur in hospitals because of loss of power and dead accumulators, or in aviation because of faults in global navigation systems.
Another movie that we truly enjoyed during the holidays, despite the somewhat negative reviews (or, especially because of that), was the newest Don’t look up, on Netflix. With a bitterly satirical look hinting at how climate scientists are ignored by politicians, in this dark comedy a team of scientists desperately tries to warn the World about the impending impact of a comet (a proxy for climatic dangers) that will erase all life on Earth, but they remain ignored and even mocked by people, press and politics, until it’s too late. Singularly, a somewhat similar situation happened about ten years ago. A huge solar storm nearly-missed the Earth, by hitting the Earth’s orbit just one week behind its passage [ DN Baker et al., A major solar eruptive event in July 2012, Space Weather 11, 581 (2013) ], but the media and press largely ignored the event, most people being focussed on the fabulous show of the Summer Olympics in London. An energy flux of more than 50 GeV cm-2s-1 was recorded by the satellite STEREO-A; peaks of the magnetic field about ±90 nT were measured, about 100 times larger than normal, yet 10 times smaller than the Carrington event estimates; previous severe events such as the Halloween-2003 attained close to -400 nT. Computer simulations of the July-23 CME demonstrate that if such a storm would have directly hit the Earth, it would have been rather disruptive, and similar to the Carrington despite the lower magnetic strenght, because of the peculiar conditions. (In fact, geomagnetic storms are very complex events, depending on the combination of the solar driver and the magnetosphere response, and merely the absolute field and flux values are not enough to characterise their intensity and impact.)
Space weather is a rapidly growing discipline, which describes the way in which the Sun, and conditions in space more generally, impact human activity and technology, both in space and on the ground. I learned about the existence of this research area, and the larger field of aeronomy, with our recently closed “Mars weather project” concerning the simulation of space radiation effects, which was actually led by scientists of the Aeronomy Institute in Bruxelles. It is now well understood that space weather represents a significant threat to infrastructure resilience, and is a sizeable source of risk, wide-ranging in its impact and the pathways by which this impact may occur.
From a more mathematical point of view, the intensities of all relevant space weather phenomena, such as flares, CMEs, and geomagnetic storms, typically follow power-law distributions. Solar flare statistics in particular have been interpreted in terms of self-organized criticality. In such models, persistent small changes in the evolving magnetic field of the Sun’s corona are thought to trigger periodic energy release events, whose size follows a power-law distribution, analogous to avalanches occurring on the surface of a sand-pile upon adding grains. Some think that the distribution of flare energies may hold the key to the coronal heating enigma – the already cited fact that the lower corona is ~3 orders of magnitude hotter than the photosphere. One of the proposed solutions to this problem is that ubiquitous, small-scale, unresolved flaring events, known as nanoflares, may liberate enough energy from the stressed coronal magnetic field, to produce the observed high coronal temperatures. If the flare energy distribution follows a power law over all scales, then a critical value of the power-law exponent can be determined. Older studies [ Hudson, Solar Phys. 133, 357 (1991) ] found that with a scaling exponent larger than 2 there could be enough energy in the nanoflares to sustain the temperature of the corona. However, such findings are today challenged by more elaborate models, and the question is still unsolved.
Given the rarity of very large solar flares, parallel analysis of Sun-like stars using spacecrafts such as NASA’s Kepler, suggests that superflares on the scale of about 10x the Carrington event may occur on millennial timescales, albeit this is still controversial. On this basis, the probability of a large flare in the next 30 years, with strength broadly exceeding all the recent ones, should be about 10%. Notably, about 4 large solar storms per Sun 11-years cycle are expected, but reliable statistics in this domain are still too young to provide better forecasts. However, the Sun is a very stable, boring, average star in the middle of its lifespan, which is one of the reasons that life has survived here for so long. This should make us safe enough from electromagnetic burn-out.
To cheer up the hearts of catastrophe-movie lovers, it it generally thought more likely that some small asteroid could do some localized damage, but over a time scale of a few centuries, while something of the extent of the rock that wiped out dinosaurs is estimated to be in the (more comfortable) time scale of hundreds millions of years. As of today, the trajectories of about 90% of the visible astronomic objects around us with size of at least 0,5 km, have been measured and checked for potential danger. Yet, statistical predictions are depending on too many little-known parameters. Indeed, less than 20 years ago all the astronomers of the World could admire in awe the impact of the Shoemaker-Levi 9 comet on the surface of Jupiter (see the NASA video https://www.youtube.com/watch?v=gbsqWozEBBw), in a splash hit that, if occurring on Earth, could have been on a similar scale of the late-Cretaceous, dino-killer event… Starting to be scared…?