[Paper Review] Evidence of the Solar EUV hot channel as a magnetic flux rope from remote-sensing and in-situ observations

Song, Hongqiang, Chen, Yao+6 more|arXiv (Cornell University)|Jun 30, 2015

Solar and Space Plasma Dynamics37 citations

TL;DR

This study provides strong observational evidence that solar EUV hot channels (HCs) in the lower corona are magnetic flux ropes (MFRs) by linking the pre-eruption HC to its interplanetary counterpart—a magnetic cloud (MC)—detected at ACE. Using multi-point remote-sensing (SDO/AIA, STEREO/EUVI, COR) and in-situ (ACE) observations, the authors show that the MC exhibits a low-ionization-state center and high-ionization-state shell, consistent with the HC’s pre-eruption high-temperature structure and growth via magnetic reconnection, confirming the HC as an MFR.

ABSTRACT

Hot channels (HCs), high temperature erupting structures in the lower corona of the Sun, have been proposed as a proxy of magnetic flux ropes (MFRs) since their initial discovery. However, it is difficult to make definitive proof given the fact that there is no direct measurement of magnetic field in the corona. An alternative way is to use the magnetic field measurement in the solar wind from in-situ instruments. On 2012 July 12, an HC was observed prior to and during a coronal mass ejection (CME) by the AIA high-temperature images. The HC is invisible in the EUVI low-temperature images, which only show the cooler leading front (LF). However, both the LF and an ejecta can be observed in the coronagraphic images. These are consistent with the high temperature and high density of the HC and support that the ejecta is the erupted HC. In the meanwhile, the associated CME shock was identified ahead of the ejecta and the sheath through the COR2 images, and the corresponding ICME was detected by extit{ACE}, showing the shock, sheath and magnetic cloud (MC) sequentially, which agrees with the coronagraphic observations. Further, the MC contained a low-ionization-state center and a high-ionization-state shell, consistent with the pre-existing HC observation and its growth through magnetic reconnection. All of these observations support that the MC detected near the Earth is the counterpart of the erupted HC in the corona for this event. Therefore, our study provides strong observational evidence of the HC as an MFR.

Motivation & Objective

• To establish observational evidence that hot channels (HCs) in the lower corona are magnetic flux ropes (MFRs), given the lack of direct coronal magnetic field measurements.
• To investigate the interplanetary evolution of an erupting HC by identifying its counterpart in the interplanetary medium.
• To determine whether the magnetic cloud (MC) detected at ACE corresponds to the erupted HC, using multi-instrument observations.
• To analyze the ionization state of iron in the MC to infer thermal history and confirm the HC-MFR connection.

Proposed method

• Utilized high-cadence EUV images from SDO/AIA (131 Å, 171 Å) to identify the hot channel (HC) and its thermal evolution prior to eruption.
• Combined remote-sensing observations from STEREO/A and B (EUVI and COR) to track the HC eruption and associated CME from the Sun to 15 R⊙.
• Analyzed in-situ data from ACE (MAG, SWEPAM, SWICS) to identify the shock, sheath, and magnetic cloud (MC) structure in the interplanetary medium.
• Used differential emission measure (DEM) modeling to estimate the HC’s initial temperature (~5 MK) and track its thermal evolution.
• Examined the iron ion charge states in the MC to infer temperature stratification, with low-ionization-state center (Fe ~10.5+) and high-ionization-state shell (beyond +12).
• Correlated the spatial and temporal evolution of the HC with the CME shock and sheath structure observed in COR2 images to confirm the ICME's origin.

Experimental results

Research questions

• RQ1 Is the hot channel (HC) in the lower corona a magnetic flux rope (MFR), given the absence of direct coronal magnetic field measurements?
• RQ2 Does the interplanetary magnetic cloud (MC) detected at ACE correspond to the erupted HC observed in the corona?
• RQ3 Can the thermal and ionization state structure of the MC be explained by the pre-eruption HC's properties and reconnection-driven heating?
• RQ4 What evidence supports that the MC's low-ionization-state center originated from the pre-existing HC?
• RQ5 How does the evolution of the HC into the MC support the MFR nature of the HC?

Key findings

• The hot channel (HC) was observed in AIA 131 Å images prior to eruption, with a temperature of ~5 MK, indicating a high-temperature structure in the lower corona.
• The HC was invisible in cooler EUVI 171 Å images, which instead revealed a cooler leading front (LF) ahead of the ejecta, consistent with the HC's high temperature and density.
• Coronagraphic (COR) images showed both the LF and the ejecta, confirming the ejecta as the erupted HC, which was also associated with a CME shock and sheath.
• The ICME detected at ACE exhibited a magnetic cloud (MC) with a low-ionization-state center (Fe ~10.5+) and a high-ionization-state shell (beyond +12), matching the pre-eruption HC's thermal structure.
• The observed ionization state distribution in the MC supports the scenario that the pre-existing HC grew via magnetic reconnection, adding hot plasma layers like an 'onion', and evolved into the MC.
• The absence of a corotating interaction region (CIR) ahead of the ICME allowed free expansion and cooling, enabling the formation of a distinct MC, confirming the HC-MFR link.