Archive for the ‘geomagnetism’ Category

Earth's magnetic field too weak to modulate galactic cosmic rays?

07/24/2009

“Kenneth Chang in the New York Times reports that some observations seem inconsistent with the solar magnetic field–cosmic ray–cloud formation hypothesis. He wrote (CCNet 113/2009 — 21 July 2009, item 3):

Terry Sloan, a cosmic ray expert at the University of Lancaster in England, said if the idea were true, one would expect the cloud-generation effect to be greatest in the polar regions where the Earth’s magnetic field tends to funnel cosmic rays.

“You’d expect clouds to be modulated in the same way,” Dr. Sloan said. “We can’t find any such behavior.” Still, “I would think there could well be some effect,” he said, but he thought the effect was probably small. Dr. Sloan’s findings indicate that the cosmic rays could at most account for 20 percent of the warming of recent years. [sic — he clearly means the *reduction* in cosmic ray influx to the Earth in recent decades of the more active Sun — SA]

I am skeptical about Dr Sloan’s claim. The reason is as follows.

A few years ago I read a suggestion that an interstellar space probe might be able to do a flyby of the star Sirius, and use its gravity to redirect itself to a subsequent flyby of Procyon, in the same way that Pioneer, Voyager and other probes have used the gravity of Jupiter to redirect themselves to Saturn and beyond. I have a formula for the change in direction caused by a flyby of a massive body, so I was able to check this idea numerically.

It turned out that if the interstellar probe was travelling at a speed that was a significant fraction of the speed of light, say 0.1c — which it would have to if it was to reach Sirius in only a few decades flight time — then the deflection of its trajectory even on a flyby which grazed the star’s atmosphere was only in the region of one degree, totally insufficient to redirect it to Procyon.

The lesson was that the gravitational fields of planets and even stars (Sirius is more massive than our Sun) are almost imperceptible to a vehicle if it is travelling at such a high speed.

Cosmic ray particles come into the Solar System at a significant fraction of the speed of light. I would therefore expect them to be largely immune to our local gravitational and magnetic fields. I would not expect Earth’s magnetic field to funnel them towards the poles, as it does with the lower-energy solar particle flux. (Presumably someone has already checked this numerically?)

It would seem that Svensmark’s cosmic ray–cloud formation hypothesis depends on the difference in strength between the Sun’s and the Earth’s magnetic fields: the Sun being strong enough to modulate the cosmic ray flux in the inner Solar System over its longer-term cycles of activity, while the Earth is too weak to redistribute incoming particles geographically during their last second or so of flight before hitting the atmosphere.

Best wishes,

Stephen Ashworth
23 July 2009

Stephen Ashworth, Oxford, U.K.
http://www.astronist.demon.co.uk/ ” CCNet today

Earth’s magnetic field too weak to modulate galactic cosmic rays?

07/24/2009

“Kenneth Chang in the New York Times reports that some observations seem inconsistent with the solar magnetic field–cosmic ray–cloud formation hypothesis. He wrote (CCNet 113/2009 — 21 July 2009, item 3):

Terry Sloan, a cosmic ray expert at the University of Lancaster in England, said if the idea were true, one would expect the cloud-generation effect to be greatest in the polar regions where the Earth’s magnetic field tends to funnel cosmic rays.

“You’d expect clouds to be modulated in the same way,” Dr. Sloan said. “We can’t find any such behavior.” Still, “I would think there could well be some effect,” he said, but he thought the effect was probably small. Dr. Sloan’s findings indicate that the cosmic rays could at most account for 20 percent of the warming of recent years. [sic — he clearly means the *reduction* in cosmic ray influx to the Earth in recent decades of the more active Sun — SA]

I am skeptical about Dr Sloan’s claim. The reason is as follows.

A few years ago I read a suggestion that an interstellar space probe might be able to do a flyby of the star Sirius, and use its gravity to redirect itself to a subsequent flyby of Procyon, in the same way that Pioneer, Voyager and other probes have used the gravity of Jupiter to redirect themselves to Saturn and beyond. I have a formula for the change in direction caused by a flyby of a massive body, so I was able to check this idea numerically.

It turned out that if the interstellar probe was travelling at a speed that was a significant fraction of the speed of light, say 0.1c — which it would have to if it was to reach Sirius in only a few decades flight time — then the deflection of its trajectory even on a flyby which grazed the star’s atmosphere was only in the region of one degree, totally insufficient to redirect it to Procyon.

The lesson was that the gravitational fields of planets and even stars (Sirius is more massive than our Sun) are almost imperceptible to a vehicle if it is travelling at such a high speed.

Cosmic ray particles come into the Solar System at a significant fraction of the speed of light. I would therefore expect them to be largely immune to our local gravitational and magnetic fields. I would not expect Earth’s magnetic field to funnel them towards the poles, as it does with the lower-energy solar particle flux. (Presumably someone has already checked this numerically?)

It would seem that Svensmark’s cosmic ray–cloud formation hypothesis depends on the difference in strength between the Sun’s and the Earth’s magnetic fields: the Sun being strong enough to modulate the cosmic ray flux in the inner Solar System over its longer-term cycles of activity, while the Earth is too weak to redistribute incoming particles geographically during their last second or so of flight before hitting the atmosphere.

Best wishes,

Stephen Ashworth
23 July 2009

Stephen Ashworth, Oxford, U.K.
http://www.astronist.demon.co.uk/ ” CCNet today

Ocean flow and secular varation of geomagnetism — connection?

07/01/2009

“[A] controversial new paper published in New Journal of Physics (co-owned by the Institute of Physics and the German Physical Society), ‘Secular variation of the Earth’s magnetic field: induced by the ocean flow?’, will deflect geophysicists’ attention from postulated motion of conducting fluids in the Earth’s core, the twentieth century’s answer to the mysteries of geomagnetism and magnetosphere.

Professor Gregory Ryskin from the School of Engineering and Applied Science at Northwestern University in Illinois, US, has defied the long-standing convention by applying equations from magnetohydrodynamics to our oceans’ salt water (which conducts electricity) and found that the long-term changes (the secular variation) in the Earth’s main magnetic field are possibly induced by our oceans’ circulation.

With calculations thus confirming Ryskin’s suspicions, there were also time and space correlations – specific indications of the integral relationship between the oceans and our magnetospheric buffer. For example, researchers had recorded changes in the intensity of current circulation in the North Atlantic; Ryskin shows that these appear strongly correlated with sharp changes in the rate of geomagnetic secular variation (“geomagnetic jerks”).

Tim Smith, senior publisher of the New Journal of Physics, said, “This article is controversial and will no doubt cause vigorous debate, and possibly strong opposition, from some parts of the geomagnetism community. As the author acknowledges, the results by no means constitute a proof but they do suggest the need for further research into the possibility of a direct connection between ocean flow and the secular variation of the geomagnetic field.” …

In the early 1920s, Einstein highlighted the large challenge that understanding our Magnetosphere poses. It was later suggested that the Earth’s magnetic field could be a result of the flow of electrically-conducting fluid deep inside the Earth acting as a dynamo.

In the second half of the twentieth century, the dynamo theory, describing the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field, was used to explain how hot iron in the outer core of the Earth creates a magnetosphere. …

Familiar text book images that illustrate a flow of hot and highly electrically-conducting fluid at the core of the Earth are based on conjecture and could now be rendered invalid. As the flow of fluids at the Earth’s core cannot be measured or observed, theories about changes in the magnetosphere have been used, inversely, to infer the existence of such flow at the core of the Earth.

While Ryskin’s research looks only at long-term changes in the Earth’s magnetic field, he points out that, “If secular variation is caused by the ocean flow, the entire concept of the dynamo operating in the Earth’s core is called into question: there exists no other evidence of hydrodynamic flow in the core.” …

Dr Raymond Shaw, professor of atmospheric physics at Michigan Technological University, said, “It should be kept in mind that the idea Professor Ryskin is proposing in his paper, if valid, has the potential to deem irrelevant the ruling paradigm of geomagnetism, so it will be no surprise to find individuals who are strongly opposed or critical.”” “The Earth’s magnetic field remains a charged mystery

Ocean flow and secular varation of geomagnetism — connection?

07/01/2009

“[A] controversial new paper published in New Journal of Physics (co-owned by the Institute of Physics and the German Physical Society), ‘Secular variation of the Earth’s magnetic field: induced by the ocean flow?’, will deflect geophysicists’ attention from postulated motion of conducting fluids in the Earth’s core, the twentieth century’s answer to the mysteries of geomagnetism and magnetosphere.

Professor Gregory Ryskin from the School of Engineering and Applied Science at Northwestern University in Illinois, US, has defied the long-standing convention by applying equations from magnetohydrodynamics to our oceans’ salt water (which conducts electricity) and found that the long-term changes (the secular variation) in the Earth’s main magnetic field are possibly induced by our oceans’ circulation.

With calculations thus confirming Ryskin’s suspicions, there were also time and space correlations – specific indications of the integral relationship between the oceans and our magnetospheric buffer. For example, researchers had recorded changes in the intensity of current circulation in the North Atlantic; Ryskin shows that these appear strongly correlated with sharp changes in the rate of geomagnetic secular variation (“geomagnetic jerks”).

Tim Smith, senior publisher of the New Journal of Physics, said, “This article is controversial and will no doubt cause vigorous debate, and possibly strong opposition, from some parts of the geomagnetism community. As the author acknowledges, the results by no means constitute a proof but they do suggest the need for further research into the possibility of a direct connection between ocean flow and the secular variation of the geomagnetic field.” …

In the early 1920s, Einstein highlighted the large challenge that understanding our Magnetosphere poses. It was later suggested that the Earth’s magnetic field could be a result of the flow of electrically-conducting fluid deep inside the Earth acting as a dynamo.

In the second half of the twentieth century, the dynamo theory, describing the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field, was used to explain how hot iron in the outer core of the Earth creates a magnetosphere. …

Familiar text book images that illustrate a flow of hot and highly electrically-conducting fluid at the core of the Earth are based on conjecture and could now be rendered invalid. As the flow of fluids at the Earth’s core cannot be measured or observed, theories about changes in the magnetosphere have been used, inversely, to infer the existence of such flow at the core of the Earth.

While Ryskin’s research looks only at long-term changes in the Earth’s magnetic field, he points out that, “If secular variation is caused by the ocean flow, the entire concept of the dynamo operating in the Earth’s core is called into question: there exists no other evidence of hydrodynamic flow in the core.” …

Dr Raymond Shaw, professor of atmospheric physics at Michigan Technological University, said, “It should be kept in mind that the idea Professor Ryskin is proposing in his paper, if valid, has the potential to deem irrelevant the ruling paradigm of geomagnetism, so it will be no surprise to find individuals who are strongly opposed or critical.”” “The Earth’s magnetic field remains a charged mystery