Comparison of counterstreaming suprathermal electron signatures of ICMEs with and without magnetic cloud: are all ICMEs flux ropes? (1911.06526v1)

Abstract: Magnetic clouds (MCs), as large-scale interplanetary magnetic flux ropes, are usually still connected to the sun at both ends near 1 AU. Many researchers believe that all non-MC interplanetary coronal mass ejections (ICMEs) also have magnetic flux rope structures, which are inconspicuous because the observing spacecraft crosses the flanks of the rope structures. If so, the field lines of non-MC ICMEs should also be usually connected to the Sun on both ends. Then we want to know whether the field lines of most non-MC ICMEs are still connected to the sun at both ends or not. This study examined the counterstreaming suprathermal electron (CSE) signatures of 266 ICMEs observed by the \emph{Advanced Composition Explorer} (\emph{ACE}) spacecraft from 1998 to 2008 and compared the CSE signatures of MCs and non-MC ICMEs. Results show that only 10 of the 101 MC events ($9.9\%$ ) and 75 of the 171 non-MC events ($43.9\%$) have no CSEs. Moreover, 21 of the non-MC ICMEs have high CSE percentages (more than $70\%$) and show relatively stable magnetic field components with slight rotations, which are in line with the expectations that spacecraft passes through the flank of magnetic flux ropes. So the 21 events may be magnetic flux ropes but the \emph{ACE} spacecraft passes through their flanks of magnetic flux ropes. Considering that most other non-MC events have disordered magnetic fields, we suggest that some non-MC ICMEs inherently have disordered magnetic fields, namely have no magnetic flux rope structures.

• The paper demonstrates that most magnetic cloud ICMEs exhibit counterstreaming electron signatures with a 57% mean occurrence, supporting magnetic connectivity at 1 AU.
• The study employs ACE spacecraft data and suprathermal electron pitch-angle distributions to differentiate between MC and non-MC events using robust analysis methods.
• The results reveal that many non-MC ICMEs lack typical CSE signatures, challenging the assumption that all ICMEs inherently display magnetic flux rope structures.

Analyzing Counterstreaming Suprathermal Electron Signatures in ICMEs: Insights into Magnetic Flux Rope Structures

This paper investigates whether all interplanetary coronal mass ejections (ICMEs), including those not exhibiting magnetic cloud (MC) properties, possess inherent magnetic flux rope structures. Previous assumptions have posited that all ICMEs, when observed correctly, inherently have magnetic flux rope formations. This paper provides a comparative analysis of MC and non-MC ICMEs with regard to counterstreaming suprathermal electron (CSE) signatures, which may help verify such hypotheses.

Methodology

The research conducted utilized data from the Advanced Composition Explorer (ACE) spacecraft, specifically focusing on 272 eV suprathermal electron pitch-angle distributions (PADs) measured between 1998 and 2008. This involved a robust methodological framework:

• ICME Identification: A total of 272 ICMEs were identified, drawing from existing databases and filtering out events with exceptionally short durations. Events were classified either as MCs or non-MC based on magnetic field characteristics.
• CSE Analysis: The CSE signature detection relied on PADs analysis, where only significant differences in phase space densities determined valid CSE intervals, discounting depletion CSEs.

Results

The results provided intriguing insights into the percentages of CSE within MCs and non-MC ICMEs, challenging previously held beliefs about the universal presence of magnetic flux ropes:

• MC ICMEs: Of the 101 MC events, only 9.9% displayed no CSE signatures. The majority exhibited CSEs, with an average occurrence of 57%. This supports the assumption that most MCs maintain connectivity to the solar magnetic field at 1 AU.
• Non-MC ICMEs: Out of 171 non-MC events, a substantial 43.9% did not demonstrate CSEs. The mean CSE occurrence in non-MC events was notably lower at 45%. However, 21 non-MC ICMEs displayed a CSE occurrence exceeding 70%, with magnetic field characteristics that may imply missed magnetic flux rope detection due to observational geometry.

Implications and Future Considerations

The results indicate that not all ICMEs unequivocally conform to the magnetic flux rope structure hypothesis, particularly among non-MC events exhibiting disordered magnetic fields. The paper suggests that a portion of non-MC events may either possess inherently disordered fields or complex structures developed through interactions in interplanetary space. The evidence does not conclusively support that all Earth-directed ICMEs retain their original flux rope configurations by the time they reach 1 AU, challenging the existing paradigm.

Given these findings, future research could benefit from the utilization of multi-spacecraft observations and advancements in heliophysics simulations to further dissect ICME structural integrity and evolutions over various spatial trajectories. Understanding the nuanced operation of flux ropes within solar-terrestrial interactions can significantly enhance space weather forecasting, contributing to better preparation and mitigation strategies against geomagnetic disturbances.

In conclusion, while the existence of flux ropes in ICMEs is partially corroborated, this paper highlights the complexity and diversity inherent in interplanetary solar structures, calling for refined investigative approaches in understanding solar ejecta and their interplanetary journey.