Chapter 5
The Plasma Environment
5.1 Initial Considerations and the Solar Wind
Ultimately, all gas molecules leaving the nucleus become ionized by solar radiation
or electron impact and, after acceleration, assume the velocity of the solar wind. This
process leads to the production of a plasma tail. Historically, cometary plasma tails
(Fig. 5.1) have played an important role in the study of the interplanetary space
environment. It was Ludwig Biermann in 1951 who deduced the existence of an
outflow of material from the Sun based upon the motion of cometary ions with
respect to the (then assumed) nucleus. When it became clear that Coulomb collisions
between the cometary ions and ionized particles in this “solar wind” were not
sufficiently effective at providing the required momentum transfer to the cometary
ions, Alfvén proposed the presence of an interplanetary magnetic field to produce the
necessary acceleration. It subsequently became apparent that the solar wind was
exhibiting the properties of a conducting fluid in a magnetic field.
The relative motion of a conducting fluid with respect to a magnetic field produces an electromotive force and electrical currents arise. These currents generate a
second, induced magnetic field which adds to the original field. The combination of
these two field components interact with the induced current density to produce a
J Â B (Lorentz) force that acts on the conducting fluid and, usually, inhibits the
relative motion of the fluid with respect to the field. The relative motion of the fluid
with respect to the magnetic field is therefore reduced. One can imagine this in two
ways. Magnetic fields can pull on the fluid. Alternatively, the conducting fluid can
drag the magnetic field lines. This is where the concept of a “frozen-in” magnetic
field arises when discussing the fields associated with the outflowing plasma of the
solar wind.
The fundamental properties of the solar wind were deduced through comprehension of the dynamical properties of cometary ions and from understanding of the
physical effects of relative motion between a plasma and a magnetic field through
application of Faraday’s and Ampere’s laws. The basic properties of the solar wind
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_5
399
The Plasma Environment
5.1 Initial Considerations and the Solar Wind
Ultimately, all gas molecules leaving the nucleus become ionized by solar radiation
or electron impact and, after acceleration, assume the velocity of the solar wind. This
process leads to the production of a plasma tail. Historically, cometary plasma tails
(Fig. 5.1) have played an important role in the study of the interplanetary space
environment. It was Ludwig Biermann in 1951 who deduced the existence of an
outflow of material from the Sun based upon the motion of cometary ions with
respect to the (then assumed) nucleus. When it became clear that Coulomb collisions
between the cometary ions and ionized particles in this “solar wind” were not
sufficiently effective at providing the required momentum transfer to the cometary
ions, Alfvén proposed the presence of an interplanetary magnetic field to produce the
necessary acceleration. It subsequently became apparent that the solar wind was
exhibiting the properties of a conducting fluid in a magnetic field.
The relative motion of a conducting fluid with respect to a magnetic field produces an electromotive force and electrical currents arise. These currents generate a
second, induced magnetic field which adds to the original field. The combination of
these two field components interact with the induced current density to produce a
J Â B (Lorentz) force that acts on the conducting fluid and, usually, inhibits the
relative motion of the fluid with respect to the field. The relative motion of the fluid
with respect to the magnetic field is therefore reduced. One can imagine this in two
ways. Magnetic fields can pull on the fluid. Alternatively, the conducting fluid can
drag the magnetic field lines. This is where the concept of a “frozen-in” magnetic
field arises when discussing the fields associated with the outflowing plasma of the
solar wind.
The fundamental properties of the solar wind were deduced through comprehension of the dynamical properties of cometary ions and from understanding of the
physical effects of relative motion between a plasma and a magnetic field through
application of Faraday’s and Ampere’s laws. The basic properties of the solar wind
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_5
399