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Cosmic Rays and the Earth's Atmosphere

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Cosmic Rays and the Earth's Atmosphere COSMIC RAYS AND THE EARTH’S ATMOSPHERE A.D. Erlykin1,2 and A.W. Wolfendale2 1 P.N. Lebedev Physics Institute, Leninsky Prospekt, Moscow, Russia. 2 Department of Physics, University of Durham, South Road, Durham, DH1 3LE, U.K. Abstract Avery brief summary is given of aspects of cosmic ray physics which have rel- evance to the possible effects of cosmic rays on ‘climate’. It is concluded that a more detailed look at the effect of fast ionizing particles on the atmosphere from the standpoint of cloud production would be advantageous. 1. INTRODUCTION There have been many claims for a correlation between solar properties (e.g. sunspot number) and climate but all have suffered from the absence of a reasonable physical cause, the point being that the energy changes in the solar variations are deemed to be too small to account for the necessary climate forcing. This is not to say that there are no well-documented effects on very long time scale; there are. Those due to variations in the sun-earth distance and the inclination of the earth’s axis (Milankovic effects), which relate to 103 − 106 y periods, are generally agreed. What is not (yet) agreed is that the 11-year solar cycle has a significant correlation with climate. The best evidence favouring a specific cause for a sunspot (SS) — climate connection relates to the apparent role of cosmic rays which are, themselves, modulated by solar activity. The observation is that by the Danish group in which there is a correlation of cloud cover and CR intensity over the oceans. The likelihood of this being genuine comes from the fact that CR are the major source of ionization away from land and CR, of course, provide ionization. Insofar as the cloud cover/CR intensity results are considered in detail elsewhere, more discussion will not be given here; rather, we will concentrate on the CR aspects. 2. COSMIC RAY INTENSITY AS A FUNCTION OF ATMOSPHERIC DEPTH It is relevant to consider the manner in which the vertical intensity of cosmic rays varies with height in the atmosphere. Considering the three major components: protons, electrons and muons, the values of −2 −1 −1 IV , the vertical intensity in cm s sr ,atheights above sea level of 2, 5 and 10 km, respectively, are: − − − p :2× 10 4, 1.6 × 10 3 and 2 × 10 2 − − − e :5× 10 3, 3 × 10 2 and 2 × 10 1 − − − µ :1× 10 2, 2 × 10 2 and 5.5 × 10 2. Some comments can be made, as follows: (i) The peak of the ionization (for µ and e)isinthe 10 km region, much higher than the common cloud level. Such an observation is not ‘the kiss of death’ to the correlation idea, because of uncertainty in transport phenomena for the products of CR ionization between the 10 km level and much lower levels. (ii) Although the ionization produced by protons is lower than that produced by e it is important to point out that the rare ‘stars’, produced by proton interactions in the atmosphere, contain very highly ionizing nuclear fragments. It is not inconceivable that subtle effects leading to cloud droplets are associated with these highly ionizing fragments. 3. KNOWN METEOROLOGICAL EFFECTS Concerning the CR intensity at ground level there are three major ‘meteorological variations’: (i) The pressure coefficient, due simply to absorption of the secondaries, of −2% per cm Hg. (ii) A correlation with the height of the 100 mb level, amounting to ∼−5% per km. The reason is to do with µ − e decay. (iii) The mean air temperature between the 100 and 200 mb levels. The dependence is +0.1% per ◦K and is due to π − µ decay. All the effects are well understood by the cosmic ray fraternity. However, they should be borne in mind by the wider community when correlations are sought. 4. COSMIC RAY ORIGIN — AND EFFECTS 4.1 Galactic particles Below the ‘knee’ in the energy spectrum, at ∼ 3 × 1015 eV, it is probable that most CR come from supernova remnants (SNR) by way of shock acceleration. It is these particles, mainly — specifically, below about 1012 eV — that are modulated by the solar wind which has, itself, an 11-year cycle. It is inevitable that there should be intensity variations due to the stochastic nature of SNR but these should be rare. There has been a claim for a 2-fold increase in intensity some 35 thousand years ago but it seems likely (Beer, private communication) that the increase is due not to an SNR but to a variation of the earth’s magnetic field and/or solar variability. 4.2 Solar particles With the advent of space vehicles, the study of the (mainly) low energy solar CR has become a ‘growth industry’. The range-energy relation is such that most protons, or their progeny, do not reach ground level; even at 10 GeV the particles only reach a height of about 10 km. Nevertheless, solar CR are important, particularly in the polar regions where effects on the ozone layer have been claimed, and their presence is eminently reasonable. Finally, we can remark on the possibility of very rare solar flares having serious effects on climate and, indeed, mankind itself. An extrapolation of the log N − log S curve for energy deposited on earth above S would indicate very serious effects every million years, or so. However, this interval is surely too short (otherwise we would not have survived for so long!). What can be said is that significant effects might be expected every 1000 years, or so. Effects on climate might occur if the atmosphere happened to be in an unstable phase at the time. 5. CONCLUSIONS Cosmic ray effects offer the possibility of being relevant to climate change. Although it is premature to be dogmatic, the likelihood of significant climatic effects is high enough for a detailed analysis of the physics — and meteorology — of CR-air interactions to be not just desirable, but vital. ACKNOWLEDGEMENTS The authors are grateful to Jasper Kirkby for re-kindling our interest in this topic. ICE CORE DATA ON CLIMATE AND COSMIC RAY CHANGES J. Beer Federal Institute of Environmental Science and Technology, EAWAG, CH-8600 Dübendorf, Switzerland, tel: +41 1 823 51 11 / fax: +41 1 823 52 10, email: [email protected] Abstract Ice cores represent archives which contain unique information about a large variety of environmental parameters. Climatic information is stored in the form of stable isotopes, greenhouse gases and various chemical substances. The content of cosmogenic nuclides such as 10Be and 36Cl provide long-term records of the intensity of the cosmic ray flux and its modulation by solar activity and the geomagnetic dipole field. Cosmogenic nuclides are produced by the interaction of cosmic ray particles with the atmosphere. After production, these nuclides are transported and distributed within the environment, depending on their geochemical properties. Some of them are removed from the atmosphere by snow and incorporated into ice sheets and glaciers. The analysis of the Greenland ice cores GRIP and GISP2 are discussed in terms of climate and cosmic ray changes during the past 50’000 years. 1. ARCHIVE ICE Polar ice sheets are formed from snow. The snowflakes grow together to grains which slowly increase in size. Due to the pressure of the overlying new snow layers, the grains become more and more compacted and finally turn into ice. The consequence of this formation process is that the ice not only preserves all the atmospheric constituents such as aerosols and dust, it also contains air bubbles that enable to determine the atmospheric composition and in particular the reconstruction of greenhouse gases in the past. This unique property makes ice the only archive that virtually stores all the climate forcing factors (greenhouse gases, aerosols and volcanic dust, solar irradiance) except internal variability. Ice cores also contain information on the corresponding climate response (temperature, precipitation rate, wind speed, atmospheric circulation). Another important property of ice is that it flows. This can be seen in Fig. 1, which schematically depicts an ice-sheet. The ice slowly flows towards the margin of the ice sheet, where it partly melts and partly breaks up as icebergs. Under steady-state conditions, the ice lost in the ablation area is replaced by snow falling on the accumulation area where new layers are formed continuously. As a consequence of the horizontal movement of the ice, the annual layers become thinner with increasing depth, as indicated in Fig. 1. This leads to another special property of the archive ice. The depth–age relationship is non- linear, which has the advantage that the uppermost part of the core is well resolved and the total time period covered is long (of the order of 105 years for polar ice cores). The disadvantage of this non-linear time-scale is, however, that dating ice is difficult and relies strongly on correct modeling of the ice-flow. The main ice sheets are situated in polar regions (Greenland, with a maximum thickness of approx. 3 km and Antarctica, with a thickness of up to 4 km). Smaller ice sheets at lower latitudes can only be found at high altitudes (Andes, Himalayas, Alps) [1]. There is a steadily growing number of parameters which can be measured in ice cores. It is beyond the scope of this paper to discuss all these parameters. In Table 1, a small selection of those related to climate forcing and climate response is given. ❄❄❄❄❄❄❄❄ ❄ ❄ ❄❄❄ ❄ ❄❄❄ ACCUMULATION ABLATION ABLATION BEDROCK Figure 1: Formation of an ice sheet. The snow falling in the accumulation region turns into ice that slowly flows towards the ablation area where it breaks up into ice-bergs or melts.
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