Properties and Geoeffectiveness of Magnetic Clouds during Solar Cycles 23 and 24

TL;DR

This study compares magnetic cloud properties during solar cycles 23 and 24, revealing that despite similar numbers, their geoeffectiveness was lower in cycle 24 due to reduced magnetic field strength and speed, affecting geomagnetic storm intensity.

Contribution

It provides a detailed analysis of magnetic cloud characteristics and their geoeffectiveness differences between the two cycles, highlighting the role of magnetic field and speed reductions.

Findings

Number of MCs remained constant in cycle 24.

Geoeffectiveness, measured by Dst index, was significantly lower in cycle 24.

Reduction in MC strength linked to decreased VBz factor.

Abstract

We report on a study that compares the properties of magnetic clouds (MCs) during the first 73 months of solar cycles 23 and 24 in order to understand the weak geomagnetic activity in cycle 24. We find that the number of MCs did not decline in cycle 24, although the average sunspot number is known to have declined by ~40%. Despite the large number of MCs, their geoeffectiveness in cycle 24 was very low. The average Dst index in the sheath and cloud portions in cycle 24 was -33 nT and -23 nT, compared to -66 nT and -55 nT, respectively in cycle 23. One of the key outcomes of this investigation is that the reduction in the strength of geomagnetic storms as measured by the Dst index is a direct consequence of the reduction in the factor VBz (the product of the MC speed and the out-of-the-ecliptic component of the MC magnetic field). The reduction in MC-to-ambient total pressure in cycle 24 is compensated for by the reduction in the mean MC speed, resulting in the constancy of the dimensionless expansion rate at 1 AU. However, the MC size in cycle 24 was significantly smaller, which can be traced to the anomalous expansion of coronal mass ejections near the Sun reported by Gopalswamy et al. (2014a). One of the consequences of the anomalous expansion seems to be the larger heliocentric distance where the pressure balance between the CME flux ropes and the ambient medium occurs in cycle 24.