The existence of the solar wind was first put forward by Ludwig Biermann in 1951, who suggested that comet tails are always directed away from the Sun because they are pushed by a continuous flow of charged particles streaming out from the Sun. It was already known that the Sun's atmosphere, or corona, is very hot - a million degrees or more - and in 1957 Sydney Chapman showed that it should extend far out into space. One year later, Eugene Parker, the "father" of solar wind science, proposed that this hot corona not only extends far into space, but escapes the Sun's gravity as a "supersonic" wind, dragging the solar magnetic field with it as it expands. His ideas were greeted with scepticism initially, but supporting evidence soon followed from space probes like Mariner 2.
This stream of outflowing particles, however, is far from uniform or constant: it comes in two varieties, "fast" (about 750 kms-1) and "slow" (350 to 400 kms-1), which vary with solar latitude and level of solar activity. During periods of low activity, the fast wind emanates from the poles and the slow wind from near the equator. Ulysses found that they meet at a clearly defined boundary. Somewhat counter-intuitively, the fast wind comes from relatively cool gaps in the corona, called coronal "holes" and the slow wind from hotter coronal "streamers". During periods of low activity, holes tend to develop around the poles and streamers around the equator. But during high activity, the pattern of holes and streamers becomes more jumbled.
Solar Wind speed measured by
Ulysses near solar minimum
(left panel), and solar maximum
(right panel), plotted as a function
of solar latitude (D. McComas).
Four instruments on Ulysses measure the magnetized solar wind:
- SWOOPS (Solar Wind Observations Over the Poles of the Sun)
- SWICS (Solar Wind Ion Composition Spectrometer)
- URAP (Unified Radio and Plasma Wave experiment)
- MAG (Vector Helium and Flux Gate Magnetometers)
SWOOPS (Solar Wind Observations Over the Poles of the Sun)
SWOOPS measures the basic characteristics of the solar wind plasma with two sensors, one for ions (mainly protons and helium ions), and one for electrons. These sensors provide data on the flow speed and direction, the density, and the temperature of the solar wind.
SWICS (Solar Wind Ion Composition Spectrometer)
SWICS measures the elemental and ionic-charge composition, temperature, and mean speed of all major solar wind ions. The abundance ratios of the different ionization states of solar wind ions like oxygen (e.g., O6+/O7+) are being used as a highly effective tool to infer the temperature in the corona from which a given solar wind stream originates.
URAP (Unified Radio and Plasma Wave experiment)
URAP detects a wide variety of natural radio sources originating at the Sun, the heliosphere, and in the magnetospheres of Jupiter and Saturn. The characteristics of the radio signals generated in the vicinity of the spacecraft are being used to infer the solar wind electron density and temperature from URAP data. URAP is also able to track solar wind disturbances (coronal mass ejections, or CMEs) as they move through interplanetary space.
MAG (Vector Helium and Flux Gate MAGnetometers)
The two MAG sensors measure the strength and direction of the heliospheric magnetic field that is carried away from the Sun by the solar wind.