Waiting for the big one

Many stars produce flares thousands of times more powerful than any we have seen on our own...so far. Which raises the question – could one of these potentially devastating 'superflares' occur on the Sun?

Solar flares are huge bursts of light, and are the result of the complex magnetic field of the Sun. Rather than being a simple dipole field with a single north and south pole like the Earth’s, the Sun’s magnetic field changes with time, and has many different regions of north/south polarity.

Regions with a particularly strong magnetic field are called active regions, and these are visible on the solar surface in the form of sunspots. The most widely accepted explanation of how solar flares occur begins with a cool, dense plasma structure, known as a filament or plasmoid, suspended by the magnetic field in the solar atmosphere, or corona, above an active region.

In the convective zone of the Sun, hot plasma bubbles to the surface, cools, and sinks back down, and this turbulent convective motion perturbs the magnetic field, causing the magnetic field lines to move around.

The magnetic field can become complicated and twisted, building up free magnetic energy. This free magnetic energy is defined as the energy a magnetic field has stored in addition to the energy of the simplest possible structure the magnetic field could have, termed a potential field. Eventually the magnetic field can reach a non-equilibrium state, where the filament/plasmoid rises upwards without being pulled back down, and drags the magnetic field with it.

Below the distorted magnetic field lines reconnect into a lower energy configuration, in a process referred to as magnetic reconnection, which releases the free magnetic energy. This energy causes charged particles to be accelerated downwards, along the magnetic field lines.

The charged particles reach relativistic speeds, close to the speed of light, and relativistic charged particles in a magnetic field release microwaves via the gyrosynchrotron mechanism. When the particles reach the more dense plasma near the surface of the Sun, they interact with the plasma and emit X-rays via Bremsstrahlung (electromagnetic radiation produced by the deceleration of a charged particle). This solar flare model explains why flares emit light predominantly at microwave and X-ray wavelengths.

A powerful flare from the Sun, even one that is not a superflare, can result in a large scale loss of electricity, serious damage to satellites, and the disruption of radio communications and GPS. The most powerful solar flare on record occurred in 1859, and is referred to as the Carrington flare. Although electricity was not in widespread use at this time, the resulting solar storm caused an extensive failure of telegram systems.

In 1989 a smaller solar storm caused a power blackout in Quebec which lasted 9 hours. A 2008 report from the US National Research Council concluded that the cost of damage in the US if a Carrington-like solar storm occurred today could be between $1 trillion and $2 trillion in the first year alone, with full recovery taking an estimated 4 to 10 years.

Superflares are 10-1000 times more powerful than the Carrington flare, however if the Sun were to produce a superflare, we would only be affected if it was directed towards the Earth; if one happened on the side of the Sun facing away from the Earth we might not even notice.

One question to ask, to help deduce whether one of these superflares could occur on the Sun, is whether stellar superflares are the result of the same processes as solar flares, or something completely different which could not happen on the Sun. We have numerous instruments taking high-resolution images of the Sun, so we can study the regions of the Sun that produce flares. For other stars, however, we cannot resolve them beyond a point source, meaning that the only information we have is the light received from the star as a whole.

Fortunately there are certain features of solar and stellar flares that can be directly compared, without the need for spatially resolved data.

Solar flares are commonly observed to consist of a series of regularly occurring pulses, known as quasi-periodic pulsations (QPPs). Often these pulsations resemble waves, with a wavelength that relates to various properties of the region of the Sun that is producing the flare, such as the size of the magnetic structure and the magnetic field strength.

Occasionally plots of the brightness variation with time of solar flares contain multiple waves superimposed on top of one another, which hold additional information about the cause of the QPPs and the plasma conditions in the flaring region.

[caption id="attachment_51880" align="alignnone" width="620"] Fig. 1: Left: artist’s impression of the 'quiet' Sun, with no solar flares. Right: what the Sun might look like if it were to produce a superflare. A large flaring coronal loop structure is shown towering over a solar active region. Image credit: Ronald Warmington.[/caption]

 

 

QPPs can be caused by either ‘magnetic dripping' mechanisms or magnetohydrodynamic (MHD) oscillations. The magnetic dripping mechanisms are based on the idea that a continuous supply of magnetic energy could cause magnetic reconnection to repetitively occur each time a threshold energy is surpassed, therefore resulting in pulsations of the flare.

On the other hand, MHD oscillations, where the flaring active region itself moves with a wave motion, could cause the plasma parameters (such as magnetic field strength and density) or the charged particle acceleration to vary periodically, resulting in the variation in brightness of the light emitted by the flare.

Now evidence for multiple waves, or multiple periodicities, has been found in a stellar superflare observed by NASA’s Kepler space telescope, and the properties of these waves are consistent with those that occur in solar flares. Kepler measured how the light of around 150,000 stars varied with time over the course of 4 years. The flare studied occurred on KIC 9655129, a binary star slightly cooler than the Sun.

When a flare occurs on a star we typically see a rapid increase in the intensity of light emitted, followed by a gradual decline. Usually the decline phase is relatively smooth but occasionally there are noticeable bumps, and these are the QPPs. Two different techniques, known as wavelet analysis and Monte Carlo modelling, were used in order to assess the periodicity and statistical significance of these QPPs, which revealed not one but two significant periodicities, with less than a 1% probability that these pulsations would be observed by chance.

The simultaneous presence of multiple periodicities in the light curve is difficult to explain with the magnetic dripping mechanisms, and suggests that MHD oscillations were the cause of the QPPs in this flare. The properties of the periodicities, such as their decay times and periods, imply that the two periodicities are spatial harmonics of an MHD wave, or two separate MHD waves.

The QPPs in this flare are similar to those observed in some solar flares, and this is some of the best evidence yet that the underlying physics in solar flares and stellar superflares is similar.

In terms of the effects on Earth that an extreme flare might have, harmful ultraviolet (UV) and X-ray radiation emitted by flares is filtered out by the Earth's atmosphere, so would be unlikely to reach the ground.

Powerful flares also release energetic charged particle radiation which can penetrate the Earth's magnetic field, known as a solar proton event. Some research suggests that this radiation could damage the ozone layer (which filters out UV light), so there could temporarily be increased UV reaching the Earth. There would also be a danger to any astronauts in space, who would be outside the protection of the Earth's magnetic field and atmosphere, and to air passengers and crew travelling at high latitudes, as the radiation is more likely to enter the atmosphere near the magnetic poles.

The more powerful the solar storm, the closer to the equator the radiation can reach.

Another solar phenomenon associated with powerful flares is a coronal mass ejection (CME), where a huge amount of plasma is hurled outwards from the Sun. CMEs carry a magnetic field with them, and if it is orientated in the opposite direction to the Earth’s (there is a 50% chance of this), then the CME distorts the Earth’s magnetic field and injects charged particles into the atmosphere near the poles.

The resultant geomagnetic storm can induce immense electrical currents in the ground, which are able to bring power grids down if they reach them. Fortunately there are some precautions that can be taken to mitigate damage. For example, flights can be diverted if there is enough warning, and power grids can be designed in such a way that if one line is damaged, an alternative route can be used.

Fortunately the conditions needed for a superflare are unlikely to occur on the Sun, based on previous observations of solar activity. To produce a superflare, the free magnetic energy would need to build up for a long time before being released, and it is much more likely that the energy would be released in one or more smaller flares before the Sun got to this stage.

This does not exclude the possibility, however, and so governments should have a strategy to deal with such an event.

Author:

Chloe Pugh is currently working towards a PhD in Astrophysics at the University of Warwick,  supported by the European Research Council under the SeismoSun Research Project.