Thankfully, all is calm in space on the day that I speak to Bill Murtagh at the US National Oceanic and Atmospheric Administration in Boulder, Colorado. "Last week we saw a moderate storm, and that was about the most interesting event in months," he reassures me. "It's pretty quiet today."
And Murtagh should know - his job is to forecast space weather, which comprises any disturbance in near-Earth space, including the upper reaches of the Earth's atmosphere where satellites roam. Many of the serious events involve disturbances in the charged portion of the atmosphere, known as the ionosphere, which stretches from 80 to 1000 kilometres above sea level. The finger of blame has always been pointed at the sun, which bombards the Earth with a stream of charged particles in the form of the solar wind. During the last three years, though, the sun's cycle of activity has hit a trough, and as Murtagh observes, space weather is temporarily calm.
Yet if the sun really is the only cause, things haven't been quite as quiet as we might have expected. Despite the solar lull, activity in the ionosphere still hots up occasionally - with ghostly tides of charged particles that throw GPS systems out of whack and block radio communication. It seems that as much as one-fifth of this space weather cannot be blamed on the sun after all. If the cause of these charged-particle surges is not above the atmosphere, it must be somewhere below. But where?
Pinning down their exact origin is a tough challenge. Since the 1960s, at least, reports have suggested apparent links with earthquakes. In Taiwan, researchers claim the majority of tremors between 1999 and 2002 appeared to attract or repel the ionosphere from its normal altitude. Exactly how an earthquake could trigger changes high in the atmosphere remains unclear, leaving many sceptical of a connection.
© NASA/GFSElectron disturbances in the upper atmosphere. Terrestrial weather as well as solar activity may be exerting an influence on this region.
Unfortunately, none of these observations has been repeated often enough to convince sceptics that they are anything more than coincidence. Ionospheric behaviour is so complex, and the data sets so small, that there is a risk of seeing connections that simply aren't there. Henry Rishbeth, one of the pioneers of ionospheric physics at the University of Southampton in the UK, once remarked that the search for links between the ionosphere and the lower atmosphere "has a long history and few real outcomes".
Yet history, it seems, is willing to offer the occasional break. In 2007 Larisa Goncharenko, an ionosphere researcher at the Massachusetts Institute of Technology, was delighted to hear that solar activity was fast approaching its lowest ebb since the 1920s.
Excited at the prospect of collecting data without the usual distracting hubbub of solar wind, she decided to focus on a phenomenon called a "stratwarm" - a sudden warming of the stratosphere above the North Pole, which can last for up to a week.
Crucially, a stratwarm can be accurately predicted a few weeks or so in advance. That would give Goncharenko enough time to prepare all 10 of the world's incoherent scatter radars to measure the ionosphere during the event. Such radars bounce signals off of the electrons in the ionosphere. The signal that is scattered back reveals its temperature and the movement of charged particles, among other things.
Such a large-scale experiment spanning the entire globe, she hoped, should quash any doubts that any apparent influence was simply a coincidence - if she was lucky enough to see one at all. And any indication of an interaction between the lower and upper atmosphere would add strength to the possibility that other phenomena, like hurricanes or El Niño, could also have an effect on space weather.
Record stratwarm
With a big stratwarm predicted for January 2008, Goncharenko put the plan in motion. "It turned out to be a record-breaking warming," she says. "We kept asking for the radars to continue operations for as long as they could - pleading, begging and appealing to the inner scientist in every person who had to make the decision."
The first results were clear: during the stratwarm, MIT's own radar station detected a warming at altitudes of around 130 km, and a cooling at higher altitudes, starting at 160 km - both deviating by as much as 80 °C from an average temperature of 230 °C. Unlike sun-driven space weather, which tends to peak around midday, these changes peaked in the morning and afternoon (Geophysical Research Letters, vol 35, p L21103). Somehow, the stratosphere above the North Pole was linked to the ionosphere above Massachusetts, some 100 km higher and 4000 km south.
The MIT results were not alone. In a radar station at the Arecibo Observatory in Puerto Rico, unusual movements were detected in the charged particles in the ionosphere that coincided with the stratwarm event. And at Poker Flat Research Range in Fairbanks, Alaska, the team observed that the wind patterns in the ionosphere above seemed unusually agitated, with vast and rapid changes in the wind speed and direction.
Perhaps the most significant results emerged from the Jicamarca Radio Observatory near Lima, Peru, where its director Jorge Chau observed unusual patterns in the movements of charged particles in the ionosphere. During that same January stratwarm, the ions and electrons rose in the mornings and fell in the afternoons, at speeds of over 60 metres per second. Intrigued, Chau looked back at three previous stratwarm data sets. In each case, he discovered similar patterns (Geophysical Research Letters, vol 36, p L05101).
That drift of particles is enough to reshape the ionosphere. When Goncharenko and her MIT colleague Anthea Coster examined data on the passage of satellite signals during previous stratwarm events for more clues, she found the density of charge in the ionosphere appeared to vary periodically from half to double its original value due to this drift.
So far, all results from Goncharenko's widespread survey point strongly towards a link between the stratosphere and the upper atmosphere. "In a casino, it's called hitting a blackjack," she says. "In research, it's a successful experiment."
The question remained: how exactly might weather on Earth drive these space-weather events. No process known to atmospheric physics would allow a specific local phenomenon like the stratwarm to propagate all the way from the stratosphere above the North Pole to the ionosphere above the equator.
So maybe the stratwarm and the non-solar space weather are just symptoms of a deeper disruption in the atmosphere. Since the 1970s, we have known that a stratwarm is the result of a natural oscillation in the air currents that usually flow in the stratosphere. During a stratwarm, this oscillation, known as a "planetary wave", temporarily disrupts the air circulation above the North Pole, which leads the air to become compressed and heated.
Perhaps it's the planetary wave, and not the stratwarm itself, that triggers the anomalous space weather events. That's the conclusion of Hanli Liu at the US National Center for Atmospheric Research in Boulder, Colorado. Liu has designed computer simulations that he claims reveal the chain of events in the lower and upper atmosphere that link terrestrial weather to space weather.
© New Scientist
As Goncharenko and Coster's observations suggested, this drift can shape how much charged material the ionosphere holds. If the particles move upwards, they reach higher altitudes where the air is less dense, making it less likely that the electrons and positive ions recombine to form neutral particles. The result would be an increase in the amount of charged material in the ionosphere. But if they are forced downwards to lower altitudes, where the air density is greater, recombination becomes more likely and the ionosphere will lose charged material and shrink.
Broken signals
Either result could have serious implications for communications. One of the few ways of transmitting signals across long distances is to bounce short-wave radio waves off the ionosphere, enabling them to reach over the horizon. It's what allows you to pick up the BBC's World Service almost anywhere, or to tune in to ham radio channels from across a continent. If the ionosphere is lurching up or down at 60 metres per second, say, these signals could be disrupted.
More concerning is the possible impact on global positioning systems. GPS receivers make their measurements by timing the arrival of radio signals sent from several satellites in known orbits. From this, a receiver calculates its distance from each satellite, and discerns its own position. But if the ionosphere swells, its refractive index increases, which slows the arrival of any signals that passed through the bulge and could throw off the receiver's measurements by as much as 30 m.
For ramblers orienting themselves on a hillside, such an error might be slightly irritating, not a disaster. But GPS also steers ships along rivers, lands planes and guides cruise missiles and smart bombs. In these cases, an error of 30 m could be catastrophic.
There are systems in place to avoid mishaps. The size of the satellite-signal delay also depends on the frequency of the signal, so by comparing two signals of different frequency, some military receivers can estimate the ionosphere's depth and correct its readings.
Few civilian receivers have the capability to estimate the depth of the ionosphere, though. Aerospace companies, for example, typically only deploy this advanced technique at their base stations. If they detect any anomalous activity that might throw off GPS, they issue warnings to planes. That's fine if the ionosphere is relatively level, but errors could still creep in if the ionosphere is bumpy - if there's a bulge directly above the plane but not above the base station, for example. "You can foresee a problem happening in the future," says Coster. The only solution, she says, will be to build better forecasts that account for the impact of stratwarms on space weather.
The movement of particles in the upper atmosphere caused by the planetary wave could also affect the drag on satellites by changing its speed, that could alter a satellite's orbit or throw it off course during a manoeuvre - to avoid space junk, for example. Better forecasts of space weather would add greater precision to these movements.
These matters will be all the more urgent when solar activity revives from its lull. That's because the solar wind feeds the ionosphere by ionising neutral atoms. It's relatively thin at the moment, but it will bloat out again during the next solar cycle - potentially amplifying all space weather events, including those linked to stratwarms.
Climate change
There may be more to worry about than escalating solar activity, though. In the past few years, stratwarms have been at their strongest and longest-lasting since regular records began three decades ago. During a stratwarm in January this year, for example, the air above the North Pole heated by 70 °C above its winter average of -50 °C - exceeding typical summer temperatures. Some speculate that this trend is a product of climate change, and warn that if it is a reflection of a change to the underlying planetary waves, it could have a big impact on space weather. "This is an open question: the climate implication requires long-term monitoring and modelling studies," Liu notes.
Not everyone, of course, is convinced that the link between stratwarms and space weather is sufficiently robust to revise space weather forecasts just yet. True, Goncharenko has recorded an effect all over the globe and covered many different aspects of space weather, and her observations seem to be supported by previous records of space weather during stratwarm events. But some researchers, like Tim Fuller-Rowell at the University of Colorado at Boulder, want to see the same results repeated many more times before they are convinced. "The question is, how reproducible is it? Is it just another source of variability along with all the other day-to-day changes, or is this a much more predictable, consistent response?"
As reliance on satellite technology continues to grow, an answer to these questions cannot come soon enough. Goncharenko has been combing through more sets of past data to verify her results, and so far she hasn't found anything that has shaken her conviction. "One researcher said he remembered several strange cases of ionospheric variations, and asked us to check if there was anything happening in the stratosphere at that time," she recalls. "For every case of odd behaviour in the ionosphere, there was a stratospheric warming - and in 15 minutes he turned from sceptic to believer."
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