Cycles of the Galactic Plane

How often does the Sun pass through a spiral arm in the Milky Way?

Knowing that our solar system does not just stay in one place as it revolves around the center of the galaxy, how long will it take for our solar system to move out of the spiral arm it is currently in (the Orion arm) and into a different one? Also, will it stay roughly the same distance away from the center of the galaxy or will it move back (towards) and forth (away) from the center?

The solar motion on top of its circular orbit about the centre of the Galaxy (which has a period of about 230 million years) can be described by how fast it is going in three different directions:

U = 10 km/s (radially inwards)
V = 5 km/s (in the direction of Galactic rotation)
W = 7 km/s (northwards out of the plane of the Galaxy)

Of course the Sun won’t keep on going in this direction forever. In fact we approximate its motion by an ‘epicycle’ on top of the mean motion around the Galaxy. The period of oscillation in and out of the plane of the galaxy (up and down) is about 70 million years. This means that we pass through the Galactic midplane about every 35 million years which some people have compared with the period between mass extinctions on Earth to come up with yet another doomsday theory. In fact it is true that the number of cosmic rays which hit the Earth will increase during the (about a) hundred thousand years we are closer to the Galactic plane. There have also been some plausible theories about the overall temperature of the Earth increasing (with the relevent climatic changes that implies).









In the plane of the galaxy the Sun is located in the small spiral arm we call the Orion arm (or local arm) which is really just connection between the two nearest major spiral arms (the Sagitarius and Perseus arms). There is a neat page on these structures: SEDS Milky Way Spiral Structure page. We pass through a major spiral arm about every 100 million years, taking about 10 million years to go through. During the transit, there would be a higher rate of ‘nearby’ supernova and possibly other so called ‘environmental stresses’ which could alter the climate of the Earth.








There is an interesting review of this (and other external influences on the climate of the Earth with reference to possible causes of the extinction of the dinosaurs) from which I get most of my figures. It’s Russell, 1978 in the Annual Review of Earth and Planetary Science.

Karen Masters
Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

The Milky Way Galaxy’s Spiral Arms and Ice-Age Epochs and the Cosmic Ray Connection

1. Ice Age Epochs and Milky Way Spiral Arm Passages:

The link between solar activity cosmic rays and climate on Earth

Figure 1 – The cosmic ray link between solar activity and the terrestrial climate. The changing solar activity is responsible for a varying solar wind strength. A stronger wind will reduce the flux of cosmic ray reaching Earth, since a larger amount of energy is lost as they propagate up the solar wind. The cosmic rays themselves come from outside the solar system. Since cosmic rays dominate the troposphere ionization, an increased solar activity will translate into a reduced ionization, and empirically, also to a reduced low altitude cloud cover. Since low altitude clouds have a net cooling effect (their “whiteness” is more important than their “blanket” effect), increased solar activity implies a warmer climate. Intrinsic cosmic ray flux variations will have a similar effect, one however, which is unrelated to solar activity variations.

Different empirical evidence convincingly support the existence of a link between solar activity and the terrestrial climate. In particular, various climate indices appear to correlate with solar activity proxies on time scales ranging from years to many millennia. For example, small but statistically significant temperature variations (of about 0.1°C) exist in the global temperature, following the 11 year solar cycle. On longer time scales, the climate system has enough time to adjust, and larger temperature variations arise from the secular variations in the solar activity.

One mechanism which can give rise to a notable solar/climate link was suggested by the late Edward Ney of the U. of Minnesota, in 1959. He suggested that any climatic sensitivity to the density of tropospheric ions would immediately link solar activity to climate. This is because the solar wind modulates the flux of high-energy particles coming from outside the solar system. These particles, the cosmic rays, are the dominant source of ionization in the troposphere. Thus, a more active sun which accelerates a stronger solar wind, would imply that as cosmic rays diffuse from the outskirts of the solar system to its center, they lose more energy. Consequently, a lower tropospheric ionization rate results. Over the 11-yr solar cycle and the long term variations in solar activity, these variations amount to typically a 10% change in this ionization rate. Moreover, it now appears that there is a climatic variable sensitive to the amount of tropospheric ionization – clouds. Thus, the emerging picture is as described in figure 1.

The Milky Way's spiral arms

Figure 2 – An artist rendition of the spiral structure of the Milky Way’s spiral structure. Illustration Credit: R. Hurt (SSC), JPL-Caltech, NASA.

If this is true, then one should expect climatic variations while we roam the galaxy. This is because the density of cosmic ray sources in the galaxy is not uniform. In fact, it is concentrated in the galactic spiral arms (it arises from supernovae, which in our galaxy are predominantly the end product of massive stars, which in turn form and die primarily in spiral arms). Thus, each time we cross a galactic arm, we should expect a colder climate. Current data for the spiral arm passages gives a crossing once every 135 ± 25 Million years. (See fig. 2 on the left. Note also that the spiral arms are density waves which propagate at a different speed than the stars, that is, nothing moves at their rotation speed).

A record of the long term variations of the galactic cosmic ray flux can be extracted from Iron meteorites. It was found in the present work that the cosmic ray flux varied periodically (with flux variations greater than a factor of 2.5) with an average period of 143 ± 10 Million years. This is consistent with the expected spiral arm crossing period and with the picture that the cosmic ray flux should be variable. The agreement is also with the correct phase. But this is not all.

The Sikhote Alin iron meteorite

Figure 3 – An Iron meteorite, a large sample of which can be used to reconstruct the past cosmic ray flux variations. The reconstructed signal reveals a 145 Myr periodicity shown below. This particular one is part of the Sikhote Alin meteorite that fell over Siberia in the middle of the 20th century, it broke off its parent body about 300 Million years ago.

The main result of this research, is that the variations of the flux, as predicted from the galactic model and as observed from the Iron meteorites is in sync with the occurrence of ice-age epochs on Earth. The agreement is both in period and in phase: (1) The observed period of the occurrence of ice-age epochs on Earth is 145 ± 7 Myr (compared with 143 ± 10 Myrs for the Cosmic ray flux variations), (2) The mid-point of the ice-age epochs is predicted to lag by 31 ± 8 Myr and observed to lag by 33 ± 20 Myr. This can be seen in the first figure.

A second agreement is in the long-term activity: On one hand there were no ice-age epochs observed on Earth between 1 and 2 billion years ago. On the other hand, it appears that the star formation rate in the Milky way was about 1/2 of its average between 1 billion and 2 billion year ago, while it was higher in the past 1 billion years, and between 2 to 3 billion years ago.

Another point worth mentioning is that, unlike some articles which misquote me (or copy from a misquoting article), I don’t think we wont have an ice age coming in the coming few tens of millions of years. If this galactic-climate picture is correct (and you should judge yourself from the evidence, in particular by the paper in New Astronomy), it implies that we are at the end of a several 10 million year-long “icehouse” epoch during which we have ice-ages come and go, and gradually over the next few millions of years, the severity of ice-ages should diminish, until they will disappear altogether. I wouldn’t buy real estate in Northern Canada just yet.

Correlation between cosmic rays and climate over geological time scales

Figure 4 – The top panel describes our passages through galactic spiral arms. The second panel describes the predicted cosmic ray flux and the predicted occurrence of ice-age epochs. The third panel describes the actual occurrence of ice-age epochs. The fourth panel indirectly describes the variable cosmic ray flux. Due to the fact that the cosmic ray flux is the “clock” used to exposure date meteorites, the meteoric ages are predicted to cluster around periods when the “clock” ticks slower, which is when the cosmic ray flux was lowest, as is seen in the data.

2. Cosmic Rays vs. CO2 as a climate driver over geological time scales:

By comparing cosmic ray flux variations to a quantitative record of climate history, more conclusions can be drawn. This was done together with Jan Veizer, whose group reconstructed the temperature on Earth over the past 550 million years by looking at 18O to 16O isotope ratios in fossils formed in tropical oceans. The following astonishing results were found once the reconstructed temperature was compared with the reconstructed cosmic ray flux variations:

Correlation between cosmic ray flux reconstruction and climate reconstruction using geochemical isotope measurements

Figure 5: Comparison between the reconstructed cosmic ray flux and the quantitative temperature reconstruction over the Phanerozoic: The top panel describes the reconstructed Cosmic Ray Flux variations over the past 500 Million years using the exposure ages Iron Meteorites. The bottom panel depicts in black, the reconstructed tropical ocean temperature variations using isotope data from fossils. The red line is the fit to the temperature using the cosmic ray flux variations. The notable fit implies that most of the temperature variations can be explained using the cosmic ray flux, and not a lot is left to be explained by other climate factors, including CO2. This implies that cosmic rays are the dominant (tropical) climate driver over the many million year time scale.
  1. Cosmic Ray Flux variations explain more than 2/3’s of the variance in the reconstructed temperature. Namely, Cosmic Ray Flux variability is the most dominant climate driver over geological time scales.
  2. An upper limit can be placed on the relative role of CO2 as a climate driver.
  3. Using point #2, an upper limit can be place on the global “radiative forcing” sensitivity – the ratio between changes to the radiation budget and ensuing temperature increase. The upper limit obtained is lower than often stated value. This implies that a large fraction of the global warming witnessed over the past century is not due to CO2. Instead, it should be attributable to the increased solar activity which diminished the cosmic ray flux reaching Earth (It has nothing to do with spiral arms as some people misquote me!).

Note however:

  • Some of the global warming is still because of us humans (probably about 1/3 to 1/2 of the warming)
  • There are many good reasons why we should strive towards using less fossil fuels and more clean alternatives, even though global warming is not the main reason.
  • A more recent analysis, which includes: (a) Corrections to the temperature reconstruction due to ocean pH variations, and (b) more empirical comparisons between actual temperature variations and changes in the radiative budget further constrain the global sensitivity to about 1-1.5°C change for CO2 doubling (as compared with the 1.5-4.5°C with the “commonly accepted range” of the IPCC, obtained from global circulation models).

3. Cosmic Rays and the Faint Sun Paradox:

The sun, like other stars of its type, is slowly increasing its energy output as it converts its Hydrogen into Helium. 4.5 Billion years ago, the sun was 30% fainter than it is today and Earth should have been frozen solid, but it wasn’t. This problem was coined as the “Faint Sun Paradox” by Carl Sagan.

If the Cosmic Ray Flux climate link is real, it significantly extenuates this discrepancy. This is because the young sun, which was rotating much faster, necessarily had a much stronger solar wind. This implies that less cosmic rays from the galaxy could have reached Earth because cosmic rays lose energy in the solar wind as they propagate from the interstellar medium to Earth. Since less cosmic rays implies a higher temperature, this effect will tend to compensate for the fainter sun.

Plugging in the numbers reveals that about 2/3’s of the temperature increase required to warm the young Earth to above today’s temperature, can be explained with this effect. The remaining 1/3 or so, can be explained with moderate amounts of greenhouse gases, such as 0.01 bar of CO2 (amounts which are consistent with geological constraints), or some NH3 or CH4

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