The solar wind is approximately 95% protons (H
), 4% alpha
particles (He
) and 1% minor ions, of which carbon,
nitrogen, oxygen, neon, magnesium, silicon and iron are the most
abundant. Solar wind velocity when measured in the ecliptic
plane is normally in the range from 300 to 600 km/s, but under
some conditions can exceed 1000 km/s. The energy of solar wind
ions is from 0.5 to 2.0 keV/nucleon. The density of the solar
wind is normally between 1 and 10 particles/cm
, and the
temperature associated with the random motion of the particles is
in the range from 10
to 10
K. A second temperature,
referred to as the coronal temperature, is used to characterize
the solar wind, it is derived from the relative charge state
distributions of the wind elements and is typically of the order
of the temperature of the solar corona, 10
K (Hundhausen,
1972).
The first space-based composition measurements were of the
proton to alpha particle ratio. These measurements used simple
electrostatic energy analyzers and relied on the fact that the
mean energy per charge of the alpha particles is twice that of
the protons (Neugebauer and Snyder 1966). During periods of high
density, low velocity and low temperature, minor ion composition
can be measured with high resolution electrostatic energy
analyzers (Bame et al., 1968), but composition over a wide range
of solar wind conditions requires more complex instruments.
These instruments are generally of two types, the combined
velocity filter-electrostatic energy analyzer and the
time-of-flight mass spectrometer. The first instrument measures
ion velocity and energy per charge separately. From the two
measurements, mass per charge is derived (Coplan et al. 1978).
One limitation of this instrument is the need to scan over the
full ranges of both velocity and energy per charge. This results
in a low duty cycle; a disadvantage that can only be partially
overcome with adaptive scanning schemes. A second limitation is
overlapping values of mass per charge. For example, alpha
particles and C
have the same mass per charge and cannot be
separated with the instrument. As a result, the abundance of
C
can only be inferred from measurements of the other charge
states of carbon. This is done by assigning a value of coronal
temperature to the solar wind. For cases where the most abundant
charge state of an element is obscured, a large uncertainty in
the abundance of the element is introduced.
For time-of-flight spectrometers, the energy per charge, velocity and total energy of each ion are determined with a triple coincidence scheme (Gloeckler et al. 1992). With this information, the mass and charge state of each ion are determined. These instruments can cover a wide range of solar wind velocities and have a very low background. The duty cycle for these instruments is determined by the number of energy per charge steps needed to cover the full energy per charge range.
More recently, isochronous time-of-flight instruments have been developed to measure ion mass directly (Möbius et al. 1990, Hamilton et al. 1990). These instruments use electric deflection fields shaped in such a way that the residence time of an ion in the field is inversely proportional to the square root of the mass of the ion, independent of the energy of the ion. For isotope abundance measurements it is necessary to be able to distinguish ions differing in mass by 1 amu for masses as large as 100 amu. This is now possible with these instruments.