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

TL;DR

This study investigates the density modulation index in the inner heliosphere during solar cycle 23, revealing its stability with distance and a decline over time, which impacts understanding of solar wind turbulence and related phenomena.

Contribution

It provides the first comprehensive analysis of the density modulation index across 0.26 to 0.82 AU during solar cycle 23, using combined IPS and ACE data.

Findings

Density modulation index ranges from 0.001 to 0.02

It remains relatively constant with heliocentric distance

Declines by 8% from 1998 to 2008

Abstract

The ratio of the rms electron density fluctuations to the background density in the solar wind (density modulation index, $\epsilon_{N} \equiv \Delta{N}/N$) is of vital importance in understanding several problems in heliospheric physics related to solar wind turbulence. In this paper, we have investigated the behavior of $\epsilon_{N}$ in the inner-heliosphere from 0.26 to 0.82 AU. The density fluctuations $\Delta{N}$ have been deduced using extensive ground-based observations of interplanetary scintillation (IPS) at 327 MHz, which probe spatial scales of a few hundred km. The background densities ($N$) have been derived using near-Earth observations from the Advanced Composition Explorer ($\it{ACE}$). Our analysis reveals that $0.001 \lesssim \epsilon_{N} \lesssim 0.02$ and does not vary appreciably with heliocentric distance. We also find that $\epsilon_{N}$ declines by 8% from 1998 to 2008. We discuss the impact of these findings on problems ranging from our understanding of Forbush decreases to the behavior of the solar wind dynamic pressure over the recent peculiar solar minimum at the end of cycle 23..