Magnetic Clouds: Solar Cycle Dependence, Sources, and Geomagnetic Impacts

TL;DR

This study analyzes magnetic clouds over 42 years, revealing their solar cycle dependence, sources, and impact on geomagnetic storms, with implications for space weather forecasting.

Contribution

It provides a comprehensive analysis of magnetic clouds, their solar cycle dependence, sources, and their role in geomagnetic disturbances from 1995 to 2017.

Findings

85% of MCs contain south Bz up to 50 nT

80% of large Dst storms caused by MCs

Bipolar MCs' polarity depends on the solar cycle

Abstract

Magnetic clouds (MCs) are transient magnetic structures giving the strongest southward magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry southward magnetic field. Southward magnetic field is responsible for causing space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions during 1995 to 2017. Eighty-five percent of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. The MCs of strongest magnetic magnitude and Bz south occur in the declining phase of the solar cycle. The bipolar MCs have solar-cycle dependence in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency but not in their polarity. MCs with the highest speeds, largest total B magnitudes and sheath Bz south are from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. The combinations of south Bz in the sheath and the south-first MCs in close succession have given the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017, spanning 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.