Some of the more interesting experiments exploring cosmic electromagnetism were done by Kristian Birkeland in the early 1900s. Birkeland documented these experiments and his other ideas in his tome The Norwegian Aurora Polaris Expedition (which I will reference as NAPE) which was published in two sections. Section 1 was published in 1908 and deals largely with the aurora expeditions and observations as well as Birkeland's initial terella experiments. In Section 2, published in 1913, Birkeland attempts to expand the scope of the interpretation of his experiments.
As a consequence of Birkeland's work with the aurora and the laboratory terella (which I will explore in a future post), Birkeland explored a number of ideas about the nature of the Sun.
Birkeland speculated that sun was powered by the decay of radium (NAPE, pg 314, 670). It was not a new idea and it did not originate with him, as the question of the energy source of the Sun had been a long-standing problem. With the discovery of radioactivity, the question of the heat source within the Earth was thought to be solved (1907JRASC...1..145R). The term 'transmutation' was apparently first used (with some trepidation) in 1901 by Rutherford and Soddy in describing nuclear decay of thorium to radium (Wikipedia). Beyond nuclear decay, other nuclear reactions such as fission or fusion were not yet known. The first artificially-induced nuclear reaction would not be discovered until 1919, by Rutherford, and after the death of Birkeland.
Birkeland mentions Rutherford's ideas that in the solar interior, that ordinary matter may become radioactive (NAPE, pg 315). However, in reading NAPE cover-to-cover, I found no use of the term 'transmutation' at all, much less in context of nuclear reactions.
These speculations on the solar energy source may have been the motivation of spectroscopic searches for radium in the Sun common around this time. However, a number of other elements such as iron, titanium, and lanthanum had spectral lines very close to those of radium so the status remained unclear for some time (1912AN....192..265M, 1912Obs....35..360E, 1913PA.....21..321M). Eventually the spectral measurements become sufficiently accurate to conclude there was no significant amount of radium in the Sun (1929ApJ....70..160S).
At the bottom of page 665 in NAPE, Birkeland proposed THREE possible solar electrical configurations:
1) Photosphere is cathode & anode located below.
"In the first place, it might be imagined that the interior of the sun formed the positive pole for enormous electric currents, while perhaps the faculae, in particular, round the spots, formed the negative poles."2) Photosphere is cathode & anode located above in corona
"Or it might be imagined that the positive poles for the discharges were to be found outside the photosphere, for instance in the sun's corona, the primary cause of the discharge being the driving away of negative ions from the outermost layers of the sun's atmosphere in some way or other for instance, as ARRHENIUS has assumed, by light-pressure after condensation of matter round them."Note that Birkeland introduces this model with 'or' to label it as an alternative to model 1.
3) Photosphere is cathode & anode located above in interplanetary space
"Finally, it might be assumed and this, according to the experimental analogies, seems the most probable assumption that the sun, in relation to space, has an enormous negative electric tension of about 600 million volts."Models 1 & 2 are clearly different, placing the anode on opposite sides of the photosphere. It might be tempting to consider 2 & 3 as the same model, with the anode just further away in model 3, but model 3 places Earth IN the space of the anode, which has stronger implications for measurements near Earth not possible in model 2.
So Birkeland describes three different cathode-anode configurations for an electromagnetic solar model. All the models keep the cathode at or on the solar photosphere, perhaps at sunspots. The anode is proposed at three possible locations: inside the sun, just above the photosphere in the corona, and further out in interplanetary space. The final configuration is apparently favored by Birkeland, probably for its similarity to his terella configuration.
Of course, since these configurations all have a common cathode, it might also be possible to consider combinations of the anode positions: 1+2, 2+3, 1+3, 1+2+3, analogous to the multi-grid electron tubes (Wikipedia) used in the first half of the 20th century, providing up to seven possible configurations. Each one of these configurations would require a different analysis as the each has different implications for where and how we can make measurements.
It's difficult to find more specific information on these models with only very limited information in NAPE (pg 665, 716). Apparently there are more details in the French publication:
K. Birkeland. Sur la source de 1’eleclricite des etoiles. Comptes Rendus Hebdomadaires des Seances de l’Academie des Sciences, T. 155:1467–1470, December 1912.but the description in Science Abstracts seems to be mostly based on some of Birkeland's experimental configurations.
However, even in NAPE, it appears that Birkeland recognized the model had serious problems that would require more than Maxwell's equations to resolve (NAPE pg 720):
"According to our manner of looking at the matter, every star in the universe would be the seat and field of activity of electric forces of a strength that no one could imagine.Each one of these models above have the same problem: How is the electric potential maintained? In model 2, Birkeland suggests Arrehenus' idea of electrons driven out by radiation pressure might help maintain such a voltage. However, other researchers, such as Milne, Rosseland, Panneokeok and others explored the voltages possible driven by particle speeds, but the predicted voltages turned out to be FAR lower than Birkeland needed. Rosseland also explored mechanisms for generating currents in sun for solar magnetic field (1925CMWCI.302....1R).
We have no certain opinion as to how the assumed enormous electric currents with enormous tension are produced, but it is certainly not in accordance with the principles we employ in technics on the earth at the present time. One may well believe, however, that a knowledge in the future of electrotechnics of the heavens would be of great practical value to our electrical engineers." [italics mine]
In part II of this post, I will go over some of the other problems associated with Birkeland's solar models, some of which were apparently recognized by Birkeland.
A Note for those Wishing to Comment on this Topic (under this or other posts):
No comments supporting the Birkeland solar model will be posted to these comment streams unless the commenter can provide correct numerical answers to at least half of these questions. Actually, if they are competent enough to answer half of these questions, which are at the level of high-school physics, they should be able to answer all of them.
Birkeland Solar Models vs. the Solar Wind
- If it starts from rest, what is the final speed of an electron accelerated across a potential difference of 600 million volts?
- If it starts from rest, what is the final speed of an proton accelerated across a potential difference of 600 million volts?
- Given a cathode at the photosphere and an anode in distant space, which way do the electrons go in this potential? Which way do the protons go?
- What is the speed of the solar wind? How do the speeds found above compare to the solar wind speed?
- What voltage is needed to accelerate protons to the speed of the solar wind? What voltage is needed to accelerate electrons to the speed of the solar wind?
- How do these voltages compare to the 600 million volts of Birkeland's model? Is the Birkeland model consistent with these measurements?
- How much speed does an electron obtain in this potential difference?
- How much speed does a proton obtain in this potential difference?
- What is the mean thermal speed of electrons and protons due to temperature in lab (say 60F?). Does the electric field overpower the thermal motion?
- What is the air pressure in the terella? The atomic density of the air in the terella?
- What is the mean-free-path for atoms in the terella at operating pressure?
- K.R. Birkeland. The Norwegian Aurora Polaris Expedition. 1902-1903. Section 1, volume 1. 1908.
- K.R. Birkeland. The Norwegian Aurora Polaris Expedition. 1902-1903. Section 2., volume 1. 1913.
- A. Egeland and W. J. Burke, editors. Kristian Birkeland, The First Space Scientist, volume 325 of Astrophysics and Space Science Library, May 2005.