Cosmic rays surprise us again

The Alpha Magnetic Spectrometer (AMS-02)being tested at CERN by being exposed to a beam of positrons. (Picture from Wikipedia)

[Text and figures updated on April 9th 2013 due to insight from Ryan Nichol: Thanks]

The team running the Alpha Magnetic Spectrometer (AMS-02) have produced their first set of results. And as expected, they are full of surprises.

AMS-02 is an awesomely complex device – too power-hungry, heavy and complex to be placed on its own space platform, it was attached to the International Space Station 18 months ago on the last space shuttle mission. I wrote about this here.

It has with 650 separate microprocessors, 1118 temperature sensors and 298 active thermostatically-controlled heaters. It is basically a general-purpose particle detector like those found at CERN, and represents the culmination of nearly one hundred years of ‘fishing for particles’ in the high atmosphere.

• First we flew balloons and found that ‘radiation levels’ increased as we went higher.
• Then we discovered a ‘zoo’ of particles not yet observed on Earth – positrons, muons, pions, and anti-protons.
• Then we discovered that ‘cosmic rays’ were not ‘rays’ but particles. And we realised that at the Earth’s surface we only observed the debris of collisions of ‘cosmic ray’ particles with the atoms in the upper atmosphere.

Where did these primary cosmic ray particle from?  What physical process accelerated them? Why did they have the range of energies that we observed? What were they? Protons? Electrons? Positrons? We just didn’t know. The AMS-02 was sent up to answer these questions.

I have found much of the comment on the results incomprehensible (BBC Example) with the discussion being exclusively focussed on ‘dark matter’.  So I thought I would try to summarise the results as I see them based on reading the original paper.

Over the last 18 months (roughly 50 million seconds) AMS-02 has observed 25 billion ‘events’  (roughly 600 per second). However, the results they report concern only a tiny fraction of these events – around 6.8 million observations of positrons or electrons believed to be ‘primary’ – coming straight from outer space.

• They found that – as is usual for cosmic rays – there were fewer and fewer particles with high energies (Figure 1 below)
• Looking at just the electrons and positrons (i.e. ignoring the protons and other particles they observed) there were only about 10% the number of positrons compared with electrons, but that the exact fraction changed with energy (See Figure 2 below)
• They found that there were no ‘special’ energies – the spectrum was smooth.
• They observed that the particles came uniformly from all directions  – the distribution was uniform with variations of greater 4% very unlikely.
• The electron and positron fluxes followed nearly the same ‘power law’ i.e. the number of particles observed with a given energy changes in nearly the same way – indicating that they probably have the same source.

They conclude very modestly that the detailed observation of this positron ‘spectrum’ demonstrates…

“…the existence of new physical phenomena, whether from a particle physics of astrophysical origin.”

I like this experiment because it represents a new way to observe the Universe – and our observations of the Universe have always surprised us. Observations have the power to puncture the vast bubbles of speculation and fantasy that constitute much of cosmology. I am sure that over the 20 year lifetime of this experiment, AMS-02 will surprise us again and again.

Figures

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

Figure 2: Graph of the fraction of positrons compared with electrons as a function of energy in billions of electron volts (GeV). The ‘error’ bars show the uncertainty in the fraction due to the small number of events detected.

Cosmic Rays and Climate Change

Clouds - condensed water vapour - formed around tiny particles emitted from jet engines. Do cosmic rays give rise to similar 'contrails' that initiate the growth of clouds? Click for larger version

Cosmic rays are the particles (probably protons and not ‘rays’ at all) that are ejected from extreme events throughout our galaxy and beyond, that bombard the Earth from all directions. I discussed the basic phenomenology and the fantastic satellite recently launched to study them here. This article is about recent stories reporting a link between cosmic ray flux and the formation of water droplets in the atmosphere -clouds. Various articles describing the research can be found here:

• BBC Coverage – actually not bad – and check out the last time the story was dug up in 2008
• Physics World coverage. Rather detailed but readable.
• The Register – used to disingenuously undermine the relevance of human involvement in climate change

Water molecules in the atmosphere have quite different effects depending on whether they are present as droplets – i.e. in a cloud – or as isolated molecules i.e. water vapour. In either form they have roughly similar effects on infra red light emitted by the Earth, but as we all know, clouds block visible light. The process by which droplets form has been the object of extensive study for more than a century – one of the major effects affecting the stability of droplets is called the Kelvin effect – and yet still we do not collectively understand how water vapour condenses in the atmosphere to form cloud droplets.

Of course we don’t know nothing, but even though the process of droplet formation is ubiquitous and important, the process is complex. The most significant fact is that even when there is more than enough water vapour in the air to form liquid droplets (so-called super-saturated air) they just don’t form by themselves. The chance of the water molecules clumping together by chance is infinitesimal. In practice, they need a ‘seed’ of some kind which allows water molecules to stick to it and which forms the ‘nucleus’ of a droplet which can grow.

CERN's Illustration of the process of droplet formation. Click for larger version. Courtesy CERN

The research from CERN (who can generate proton beams very easily) evaluated the effect of cosmic rays (i.e. fast protons) on the formation of the smallest droplets under different simulated atmospheric conditions and in the presence of different impurities. The results were complex, but can be divided into two parts:

• When simulating the atmosphere at an altitude of 1 kilometer (3000 feet) where the temperature is approximately -10 °C (prime cloud forming temperature) , they were surprised to find that the rate of droplet formation was only one thousandth of  that observed in the real atmosphere, with or without the ‘cosmic ray’ bombardment.
• When simulating the atmosphere at an altitude of 5 kilometers (16000 feet) where the temperature is approximately -25 °C – they found that ‘cosmic ray’ bombardment enhanced the rate of tiny droplet formation by a factor 10.

So the results indicate that droplet formation is even more complex than had been previously considered. But as many reports were at pains to point out, this is not really news because nobody ever thought they understood the process in the first place! And the droplets formed in the experiment were still too small to grow into cloud droplets and scatter light. Small droplets – perhaps 10 nanometers (50 atoms) do not necessarily grow to be large 1 micron size droplets typically found in clouds. Small droplets tend to evaporate faster than larger droplets and so when there is a mix of droplet sizes, small droplets tend to shrink and larger ones tend to grow – that is a manifestation of the Kelvin effect I mentioned above. However, no doubt we will eventually figure out how the process works.

However, I would like to single out the disingenuous Andrew Orlowski who writes for the Register for special castigation. Mr. Orlowski is an iconoclast who enjoys mocking the achievements of others. From reading his articles it is cleat that Mr. Orlowski objects to the idea that anthropogenic carbon dioxide emissions could conceivably be affecting the climate. So Mr Orlowski loves the idea that Cosmic Rays could be affecting the climate because they are ‘not our fault’ and we can just ignore the ‘liberal whingers’ calling for controls on energy usage. I don’t know why he so persistently rejects the idea that carbon dioxide emissions could be affecting the climate, because its a pretty sound idea with quite a lot of evidential and theoretical support. But every report he writes on the subject focuses on the things which people can’t explain and implies that the whole concept is thought up by a liberal/authortarian elite who – unlike the free-thinking Mr Orlowski – are unwilling to accept new data. However he never has the honesty to state what he actually thinks. So, for example,  he ends his article with a quote implying that the lead author thinks that previous climate studies are bunkum.

When Dr Kirkby first described the theory in 1998, he suggested cosmic rays “will probably be able to account for somewhere between a half and the whole of the increase in the Earth’s temperature that we have seen in the last century.”

But in fact the actual results of Dr. Kirkby’s work are completely inconclusive – telling us only what we knew before – that we don’t understand the basic process of cloud formation.

As new research fills in the gaps in our knowledge of the many complex factors that affect our climate, many media sources invite us to view the work in an essentially confrontational light. The question they ask is whether this report strengthens the views of climate change ‘skeptics’ or climate change ‘supportors'(!). Frequently one voice from each camp will be quoted to further this sense of antagonism. But in fact there will always remain many areas of uncertainty and we – you and I and scientists and governments – have to cope with this uncertainty. We have to make our Climate Models as best we can even though we don’t understand all the elements: We have to make decisions about energy usage and generation (Wind turbines, Electricity pylons, banned light bulbs) in the face of this uncertainty. These decisions  are difficult enough in themselves and we would all do better without this kind of tribal response to each new piece of information.