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12 Appendix A: Some Characteristic Solar Wind Parameters

Although solar wind is a highly variable medium, it is possible to identify some characteristic values for its most common parameters. Since the wind is an expanding medium, we ought to choose one heliocentric distance to refer to and, usually, this distance is 1 AU. In the following we will provide different tables referring to several solar wind parameters, velocities, characteristic times, and lengths.

As it can be seen, the solar wind is a super-Alfvénic, collisionless plasma, and MHD turbulence can be investigated for frequencies smaller than -1 ~ 10 Hz.


Table 6: Typical values of several solar wind parameters as measured by Helios 2 at 1 AU.



Wind Parameter Slow wind Fast wind



number density ~ 15 cm -3 ~ 4 cm - 3
bulk velocity ~ 350 km s- 1 ~ 600 km s- 1
proton temperature ~ 5 .104 K ~ 2 .105 K
electron temperature 5 ~ 2 .10 K 5 ~ 1 .10 K
a-particles temperature ~ 2 .105 K ~ 8 .105 K
magnetic field ~ 6 nT ~ 6 nT





Table 7: Typical values of different speeds obtained at 1 AU. The Alfvén speed has been measured, while all the others have been obtained from the parameters reported in Table 6.



Speed Slow wind Fast wind



Alfvén ~ 30 km s- 1 ~ 60 km s- 1
ion sound - 1 ~ 60 km s - 1 ~ 60 km s
proton thermal - 1 ~ 35 km s - 1 ~ 70 km s
electron thermal ~ 3000 km s-1 ~ 2000 km s-1





Table 8: Typical values of different frequencies at 1 AU. These values have been obtained from the parameters reported in Table 6.



Frequency Slow wind Fast wind



proton cyclotron ~ 0.1 Hz ~ 0.1 Hz
electron cyclotron ~ 2 .102 Hz ~ 2 .102 Hz
plasma 5 ~ 2 .10 Hz 5 ~ 1 .10 Hz
proton-proton collision ~ 2 .10 -6 Hz ~ 1 .10-7 Hz





Table 9: Typical values of different lengths at 1 AU plus the distance traveled by a proton before colliding with another proton. These values have been obtained from the parameters reported in Table 6.



Length Slow wind Fast wind



Debye ~ 4 m ~ 15 m
proton gyroradius ~ 130 km ~ 260 km
electron gyroradius ~ 2 km ~ 1.3 km
distance between 2 proton collisions ~ 1.2 AU ~ 40 AU





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