Other
Scientific paper
Jun 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aipc..471..473y&link_type=abstract
The solar wind nine conference. AIP Conference Proceedings, Volume 471, pp. 473-474 (1999).
Other
Solar Wind Plasma, Sources Of Solar Wind
Scientific paper
It is well known that the solar wind is a magneto-hydrodynamic (MHD) turbulent flow. The MHD turbulence is considered to play an important role in energy transport and acceleration processes in the solar wind (see (1)). Some acceleration models in which the solar wind is primarily driven by the turbulence process caused by the damping of MHD waves have been proposed (e.g., (2)). Study of the turbulence in the solar wind is therefore crucial for understanding the solar wind acceleration mechanism. In this paper, we focused on the radial evolution of the turbulence in the solar wind, in particular the dissipation scale length of density fluctuations, the ``inner scale (si)'' from measurements of the solar wind with the IPS method. Our IPS observations were made at the Kashima Space Research, Communications Research Laboratory at 2 GHz and 8 GHz in 1994. The observations measure the solar wind at solar distances of 1 to 76 solar radii (Rs). To obtain the inner scale from the observations, we applied a spectrum-fitting method, in which a model power spectrum is fit to an observed scintillation power spectrum by adjusting the parameters of solar wind (e.g., (3)). Previous IPS measurements have reported that the size of the inner scale increases with heliocentric distance as si~(r/Rs)1.0+/-0.1 (e.g., (4)). However, our IPS observations carried out in 1994 have revealed that the inner scale inside of 25 Rs deviates from the linear relation and follows the radial profile as si~r+1.8 (see Figure 3 in (5)). We attempted to explain the radial profile of the observed inner scale in comparison with model calculations taking into account the line-of-sight integration effect. For the model calculations, we employed a solar wind acceleration model (6) and the inner scale model si=3/qi=3×VA/ωc=684N-1/2, which Coles and Harmon (7) have proposed suggesting the proton cyclotron damping acts as an effective dissipation mechanism. Here, VA is the Alfvén velocity, ωc is the proton cyclotron frequency and N is the number density. In the case of the slow solar wind, the inner scale calculated with the inner scale model is in good agreement with the observed one, and we concluded that the radial evolution of the inner scale is related to the solar wind acceleration. That is, the acceleration of the slow solar wind which takes place at distances within 25 Rs leads to a steeper density gradient and the deviation of the inner scale from the linear relation. On the other hand, the acceleration and inner scale models are not able to explain the radial dependence of the observed inner scale in the fast solar wind. We propose that the disagreement would be due to the density and magnetic field fluctuations not being correlated in the fast solar wind as shown at greater heliocentric distances by Helios measurements (8), although the functional form of the inner scale model (i.e., si~VA/ωc) requires the correlated behaviors of both fluctuations. To explain the radial profile of the inner scale in the fast solar wind, it is necessary to obtain more detail information on the inner scale in the fast stream in the near-sun region because we have had no observations of the fast solar wind inside of 20 Rs. Detail of this work has been reported in the JGR paper of (5).
Esser Ruth
Kojima Masayoshi
Manoharan Periasamy K.
Tokumaru Munetoshi
Yamauchi Yohei
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