Coronal mass ejections (CMEs) are the most severe explosive phenomena in the heliosphere. Many CMEs relate to a magnetically organized geometry of flux rope. The occurrence of such magnetic flux ropes in interplanetary space is referred to as magnetic clouds (MCs).

On CME/MC transits’ way to the interplanetary, the coronal electron density decreases continually, causing the timescales of ionic ionization/recombination to keep increasing. After they are larger than solar wind expansion timescale at a certain position, heavy-ion charge states and elemental composition do not change anymore. Therefore, the ion charge states of plasma in MCs retain information of electron temperature above the solar surface.

Recently, HUANG Jin and LIU Yu from Yunnan Observatories of the Chinese Academy of Sciences employed a method that related the measurements to their position within the magnetic flux rope structure. They made a comprehensive survey of 124 MCs events using Advanced Composition Explorer (ACE) spacecraft data. The study was published in The Astrophysical Journal.

The researchers fitted the data with a cylindrically symmetric, linear force-free flux rope model. With the fitting parameters, the in-situ measurements can be associated with a radial distance. Following that, they divided the normalized positions into 11 bins and calculated the mean value in each bin, thereby the measurement (plasma, composition, and charge state) spatial distributions can be extracted.

They then divided MC into two groups, fast and slow MCs. Fast MCs have enhanced mean charge states of heavy-ion compared with the slow MCs. For ionic species in fast MCs, a higher atomic number represents a greater enhancement of mean charge states than slow MCs.

Besides, they found that both the fast and slow MCs display clear bimodal structure distributions in the heavy-ion (especially Fe) mean charge states. Considering during CME eruption from the Sun, reconnection heats the plasma, which injects into the flux rope along magnetic field lines around the flux rope. The bimodal charge state distribution inside MCs suggests that the existence of flux rope before the eruption is common. Furthermore, some heavy-ion (e.g. Fe) mean charge state distributions inside fast MCs have the feature that the posterior peak is higher than the anterior one.

The results were consistent with the “standard model” for CME/flare, by which magnetic reconnection occurs beneath the flux rope. The reconnection ionizes the ions of the posterior part of the flux rope sufficiently, by high-energy electron collisions or by direct heating in the reconnection region.

This study enables people to further understand the ion distributions inside MCs, and has important implications for testing the formation and eruption models of CME magnetic flux ropes. With the statistical parameter values in this research, the space weather forecasting capability will hopefully be improved.