The Solar Wind
The Solar Wind
The space between Sun and Earth is filled with the plasma of the solar wind. This is a flow of charged particles from the Sun, whose existence was first conjectured, in 1908, by the Norwegian physicist Kristian Birkeland (1867–1917) [18]. Birkeland also recognized that this solar wind must comprise both positive ions and negative electrons. Ludwig Biermann, in 1951, inferred [19], that the pressure of the solar radiation on the molecules in the comet tail is by far insufficient to explain why comet tails always point away from the Sun. Rather, a solar corpuscle flow with velocities of the order of 106 ms−1 was necessary to deflect the comet tail. Eugene Parker [20] recognized that the solar magnetic field is “frozen” in the mass flow of the solar wind—an effect from “magneto-hydrodynamics”, a novel concept introduced by Hannes Alfvén (1908–1995). Although the mass flow of the solar wind is radially outward, solar rotation shifts the footpoints of the particle flow azimuthally, which transforms a radial beam into an Archimedian spiral (Fig. 1.5). Experimental evidence of the existence of the solar wind was given by Konstantin Gringauz (1918–1993), who had designed the hemispherical retarding-potential ion detectors aboard the Soviet moon probes Luna 1 and Luna 2, which were both launched in 1959 [21]. The quasistationary solar wind describes plasma streams whose sources on the Sun exist for more than one day, often weeks and even months. There are two types
8 1 Introduction
400 km/s
Fig. 1.5 The sun rotation shapes beams of solar wind, which emerge from distinct spots, into an Archimedian spiral. The motion of the solar wind is radially outward, but the magnetic field is trapped in the spiral arms. The Earth orbit is indicated by the dashed circle
Fig. 1.6 The speed of the solar wind observed by the Solar Wind Observations Over the Poles of the Sun (SWOOPS) instrument aboard the ULYSSES spacecraft during its first passage. (Reprinted from [22] with kind permission from Springer Science+Business Media)
1994 1995
of plasma streams with distinct plasma properties, the slow solar wind with velocities below 450kms−1 originating in the coronal streamer belt at low heliospheric latitudes, and the fast solar wind with velocities between 700 and 800kms−1 flowing out of coronal holes at high heliospheric latitudes. These two types of solar
Table 1.2 Properties of the high-latitude solar wind [24] converted to conditions at 1AU Quantity Fast SW Slow SW vSW 773 ≈(300–500) kms−1 Proton density 2.47×106 (5–15)×106 m−3 Proton temperature 1.86×105 K Electron temperature 0.84×105 K He++ ions/protons 0.044
1.2 The Plasma Environment of Our Earth 9
wind were detected by the ULYSSES4 spacecraft during its first passage of the Sun (Fig. 1.6) [22, 23], when the 11-year solar activity cycle was at its minimum (Table 1.2)
The space between Sun and Earth is filled with the plasma of the solar wind. This is a flow of charged particles from the Sun, whose existence was first conjectured, in 1908, by the Norwegian physicist Kristian Birkeland (1867–1917) [18]. Birkeland also recognized that this solar wind must comprise both positive ions and negative electrons. Ludwig Biermann, in 1951, inferred [19], that the pressure of the solar radiation on the molecules in the comet tail is by far insufficient to explain why comet tails always point away from the Sun. Rather, a solar corpuscle flow with velocities of the order of 106 ms−1 was necessary to deflect the comet tail. Eugene Parker [20] recognized that the solar magnetic field is “frozen” in the mass flow of the solar wind—an effect from “magneto-hydrodynamics”, a novel concept introduced by Hannes Alfvén (1908–1995). Although the mass flow of the solar wind is radially outward, solar rotation shifts the footpoints of the particle flow azimuthally, which transforms a radial beam into an Archimedian spiral (Fig. 1.5). Experimental evidence of the existence of the solar wind was given by Konstantin Gringauz (1918–1993), who had designed the hemispherical retarding-potential ion detectors aboard the Soviet moon probes Luna 1 and Luna 2, which were both launched in 1959 [21]. The quasistationary solar wind describes plasma streams whose sources on the Sun exist for more than one day, often weeks and even months. There are two types
8 1 Introduction
400 km/s
Fig. 1.5 The sun rotation shapes beams of solar wind, which emerge from distinct spots, into an Archimedian spiral. The motion of the solar wind is radially outward, but the magnetic field is trapped in the spiral arms. The Earth orbit is indicated by the dashed circle
Fig. 1.6 The speed of the solar wind observed by the Solar Wind Observations Over the Poles of the Sun (SWOOPS) instrument aboard the ULYSSES spacecraft during its first passage. (Reprinted from [22] with kind permission from Springer Science+Business Media)
1994 1995
of plasma streams with distinct plasma properties, the slow solar wind with velocities below 450kms−1 originating in the coronal streamer belt at low heliospheric latitudes, and the fast solar wind with velocities between 700 and 800kms−1 flowing out of coronal holes at high heliospheric latitudes. These two types of solar
Table 1.2 Properties of the high-latitude solar wind [24] converted to conditions at 1AU Quantity Fast SW Slow SW vSW 773 ≈(300–500) kms−1 Proton density 2.47×106 (5–15)×106 m−3 Proton temperature 1.86×105 K Electron temperature 0.84×105 K He++ ions/protons 0.044
1.2 The Plasma Environment of Our Earth 9
wind were detected by the ULYSSES4 spacecraft during its first passage of the Sun (Fig. 1.6) [22, 23], when the 11-year solar activity cycle was at its minimum (Table 1.2)
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