A study of density modulation index in the inner heliospheric solar wind during solar cycle 23

An understanding of variations of density modulation index e N = ΔN/N, the ratio of the electron density fluctuation (ΔN) to the absolute solar wind density (N), in the inner heliosphere is of vital importance for understanding turbulent dissipation and consequent local heating of solar wind. In addition, the density modulation index plays crucial role in understanding the propagation of energetic electrons, through the heliosphere, produced by solar flares and other explosive solar surface phenomena. We have made a detailed study of e N in the inner heliosphere spanning the distance range from 0.2 to 0.8 AU, for the period 1998–2008, covering solar cycle 23. The rms electron density fluctuations (ΔN) have been deduced using ground-based interplanetary scintillation (IPS) observations at 327 MHz from the multi-station IPS observatory, at STEL, Japan. Before deriving ΔN, we have appropriately normalized scintillation measurements to remove the effect of finite source size. The absolute solar wind density (N), on the other hand, has been obtained from the space-borne Advanced Composition Explorer (ACE) mission. However, ACE density measurements are effectively at a distance of 1 AU at the Largangian point L1. Thus, for estimation of density at the location of the relevant scintillating sources, spreading over distances of 0.2–0.8 AU, the measured ACE densities at 1 AU are extrapolated in the sunward direction using an electron density model. Our analysis shows that e N does not vary with heliocentric distances r and the typical value of e N ranges from 1% to 10% which is is consistent with the earlier findings. A systematic decline in the solar wind electron density turbulence levels has been reported earlier for the period 1995 to 2008. Our investigation of the long-term temporal variations of e N over the distance range 0.2–0.8 AU have also shown a similar decline during the period 1998–2008. It therefore appears reasonable, from the linear relationship between the density fluctuations and magnetic field fluctuations, to conclude that the decrease in e N is connected to the unusual solar magnetic activity during the long and deep solar minimum at the end of the solar cycle 23.