Solar Heliosphere: History
Apart from the many discussions of solar ‘corpuscular’ radiation in the 1800’s to account for aurora, in 1903 Kristian Birkeland in Norway explained aurora as some kind of medium consisting of a stream of electrons that travels from sun to earth. Sidney Chapman again raised the idea of solar electron streams in a 1918 paper on magnetic storms. Frederick Lindemann, Oxford professor of physics, pointed out that the negative charge accumulated on the Earth would disrupt the process. Lindemann then suggested that any cloud or stream expelled from the Sun would have to be electrically neutral, containing equal charge from ions and electrons.
The real turning point for the solar wind concept came 25 years later in 1943 when astronomer Cuno Hoffmeister in Germany provided the crucial observations of a gas tail aberration of about 6 degrees, i.e. the angle between the observed tail and the anti-solar direction.
Ludwig Biermann (1951) at the University of Gottingen correctly interpreted this deflection in terms of the interaction between the cometary ions in the tail and the solar wind. The tails should always point directly away from the sun if the only thing acting on them was the pressure from sunlight. Comet tails, like million-mile-long windsocks, pointed in the direction that the solar wind was blowing near them. Biermann showed that the pressure from Sunlight was not enough, and that the force must be provided by a stream of particles travelling away from the Sun at speeds of hundreds of kilometers per second.
In 1955, Sydney Chapmen (Britain) concluded that because the corona was so hot (million degrees) that it must exist beyond the orbit of the sun. A few years later, Eugene Parker showed mathematically that an expanding, supersonic, hot corona has to produce a solar wind that accounts for comet tail deflections. Despite the work by Hoffmeister, Biermann and Chapman the concept of a solar wind was still considered controvercial by many researchers in the 1950’s. This wasn’t settled until space probes were flown that were able to record this stream of material high above the Earth's atmosphere, proving its existence.
Much of the early discussions about the heliosphere were the result of cosmic ray studies. In 1956, studies of cosmic ray energies by Philip Morrison (1915-2005) at Cornell (Phys Rev 101, p. 139 see abstract below) led to the realization that Earth had to be immersed in a region of tangled interplanetary magnetic field of solar origin. Leverett Davis (1955, Phys Rev `100, p. 144; see abstract below) at CalTech, and Meyer (1956, Phys rev 104, p. 768 abstract below) at the University of Chicago concluded from their studies that a good fit to the data would obtain if the cavity were about 200 AU in diameter. At the outer boundary, cosmic rays from solar flares would be scattered back into the inner solar system and detected at earth. Hannes Alfven (1957) later introduced the notion of an interplanetary magnetic field which is carried along with the solar wind.
Meanwhile, Konstantin Gringauz in 1959 flew "ion traps" on the Soviet Lunik 2 and 3 missions, instruments measuring the total electric charge of arriving ions. He reported that the signal fluctuated as the spacecraft spun around its axis, suggesting an ion flow was entering the instrument whenever it faced the Sun. But a more careful analysis of the data failed to find the necessary signal. In 1961 Herbert Bridge with Bruno Rossi and the MIT team obtained more detailed observations with an elaborate ion trap on NASA's Explorer 10, but the data were still not convincing to many because the probe was designed to study the magnetotail,which confused the analysis.
Then in 1962, Mariner II (built on a rush 11-month schedule at JPL) flew towards Venus. It not only detected a continuously flowing solar wind, but also observed in it fast and slow streams, approximately repeating at 27 day intervals, suggesting that their sources rotated with the Sun. The discovery of the solar wind is almost universally credited to the Mariner 2 probe which flew past Venus on December 14, 1962.
From "Interplanetary Magnetic Fields and Cosmic Rays" by Leverett Davis, Jr. 1955, Phys Rev 100, p. 144:
The existence in the region around the sun of a field-free cavity in the galactic magnetic field seems indicated by the low-energy cosmic rays that reach the earth from the sun. Such a cavity would be produced by the solar corpuscular emission. A mean radius of the order of 200 times the distance from the sun to the earth may be estimated for this cavity by balancing the flux of momentum against the lateral pressure exerted by a field of 10-5 gauss. Such a cavity would trap cosmic rays of energy less than 100 Bev for periods long compared to a sunspot cycle, but does not seem to make possible a solar origin of cosmic rays. Expected fluctuations in cavity size would explain the 4% fluctuation in cosmic-ray intensity observed by Forbush. A simple model of the cavity is considered in some detail, rates of escape from and entry to the cavity, acceleration by the Fermi mechanism, and change in energy density being estimated. More complicated models involving a solar magnetic field are considered briefly.
From: "Solar Cosmic Rays of February, 1956 and Their Propagation through Interplanetary Space" P. Meyer, E. N. Parker, and J. A. Simpson Phys. Rev. 104, p. 768:
The data from six neutron-intensity monitors distributed over a wide range of geomagnetic latitudes have been used to study the large and temporary increase of cosmic-ray intensity which occurred on February 23, 1956, in association with a solar flare. During the period of enhanced intensity a balloon-borne neutron detector measured the absorption mean free path and intensity of the flare particles at high altitudes. From these experiments the primary particle intensity spectrum as a function of particle rigidity, over the range <2 to> 15-30 Bv rigidity, has been deduced for different times during the period of enhanced intensity. It is shown that the region between the sun and the earth should be free of magnetic fields greater than ∼10-6 gauss and that the incoming radiation was practically isotropic for more than 16 hours following maximum flare particle intensity. The decline of particle intensity as a function of time t depends upon the power law t-3/2, except for high-energy particles and late times, where the time dependence approaches an exponential. The experiments lead to a model for the inner solar system which requires a field-free cavity of radius greater than the sun-earth distance enclosed by a continuous barrier region of irregular magnetic fields [B(rms)≈10-5 gauss] through which the cosmic-ray particles must diffuse to reach interstellar space. This barrier is also invoked to scatter flare particles back into the field-free cavity and to determine the rate of declining intensity observed at the earth. The diffusion mechanism is strongly supported by the fact that the time dependence t-3/2 represents a special solution of the diffusion equation under initial and boundary conditions required by experimental evidence. The coefficient of diffusion, the magnitude of the magnetic field regions, the dimensions of the barrier and cavity, and the total kinetic energy of the high-energy solar injected particles have been estimated for this model. Recent studies of interplanetary space indicate that the conditions suggested by the experiments may be established from time to time in the solar system. The extension of the model to the explanation of earlier cosmic-ray flare observations appears to be satisfactory.
- Hoffmeister, C., Physikalisch Untersuchungen auf Kometen, I, Die Beziehungen des primaren Schweifstrahl zum Radiusvektor, Z. Astrophys., 22, 265-285, 1943.
- Hoffmeister, C., Physikalisch Untersuchungen auf Kometen, ll, Die Bewegung der Schweifmaterie und die Repulsivkraft der Sonne beim Kometen, Z. Astrophys., 23, 1-18, 1944.
- Ludwig Biermann and R. Luest, “The Tails of Comets,” Scientific American, October, 1958.
- Neugebauer, M., Snyder, C., 1962, “Solar Plasma Experiment”, Science, 138, 1095-1097
- Sonett, C. P. "A summary review of the scientific findings of the Mariner Venus Mission." Space Science Review, 2, No. 6, 751-777, December 1963.
- Sound recording of an interview with Biermann discussing his solar wind work. American Institute of Physics. Center for History of Physics. Niels Bohr Library. One Physics Ellipse, College Park, MD 20740, USA.
"The heliosphere, in which the Sun and planets reside, is a large bubble inflated from the inside by the high-speed solar wind blowing out from the Sun." - NASA, Solar System Exploration.