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). They are thought to be significantly geoeffective.

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 (often within 5 solar radii (Re))，heavy-ion (atomic number >2) 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. High/low ion charge states witnessed at interplanetary usually implies a high/low electron temperature at the corresponding region. The ion charge states help us understand the heating process of MCs in their solar source regions.

To extract the information carried by the ion charge states, researchers need to explore the spatial distribution of the ion charge states inside the MCs. Generally, they do it by single event study or statistical analysis by time series. Recently, HUANG Jin and LIU Yu from Yunnan Observatories employed a method that related the measurements to their position within the magnetic flux rope structure. HUANG Jin and his cooperators made a comprehensive survey of 124 MCs events using Advanced Composition Explorer (ACE) spacecraft data.

They 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. Followed 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.

In this study, the researchers divided MC into two groups, fast and slow MCs, they found that 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.

They also 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.

This result is 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 research enables us to further understand the ion distributions inside MCs. It also 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. The paper was recently published in The Astrophysical Journal.

Contact:

HUANG Jin, YNAO, CAS
huang_heliophysics@163.com