L1 and Off Sun‐Earth Line Visible‐Light Imaging of Earth‐Directed CMEs: An Analysis of Inconsistent Observations. Issue 4 (14th April 2023) - Harrison, Richard A.
Abstract: The efficacy of coronal mass ejection (CME) observations as a key input to space weather forecasting is explored by comparing on and off Sun‐Earth line observations from the ESA/NASA SOHO and NASA STEREO spacecraft. A comparison is made of CME catalogs based on L1 coronagraph imagery and off Sun‐Earth line coronagraph and heliospheric imager observations, for the year 2011. Analysis reveals inconsistencies in the identification of a number of potentially Earth‐directed CMEs. The catalogs reflect our ability to identify and characterize CMEs, so any discrepancies can impact our prediction of Earth‐directed CMEs. We show that 15 CMEs, which were observed by STEREO, that had estimated directions compatible with Earth‐directed events, had no identified halo/partial halo counterpart listed in the L1 coronagraph CME catalog. In situ data confirms that for 9 of these there was a consistent L1 Interplanetary CME (ICME). The number of such "discrepant" events is significant compared to the number of ICMEs recorded at L1 in 2011, stressing the need to address space weather monitoring capabilities, particularly with the inclusion of off Sun‐Earth line observation. While the study provides evidence that some halo CMEs are simply not visible in near‐Earth coronagraph imagery, there is evidence that some halo CMEs viewed from L1 are compromised by preceding CME remnants or the presence of multiple‐CMEs. This underlines (a) the value of multiple vantage point CME observation, and (b) the benefit of off Sun‐Earth line platform heliospheric imaging, and coronagraph imaging, for the efficient identification and tracking of Earth‐directed events. Plain Language Summary: Coronal Mass Ejections (CMEs) can impact human activities and technological assets and are continuously monitored by dedicated space weather operations centers. This study identifies inconsistencies between the identification of potentially Earth‐directed CMEs in observations made from a near Earth vantage point and those made from off the Sun‐Earth line. Almost a third of a set of potentially Earth‐directed CMEs that were identified in an off Sun‐Earth line heliospheric imager catalog from 2011, which were subsequently evidenced as Interplanetary CMEs (ICMEs) in in situ data near Earth, were not cataloged as halo or partial halo events in near Earth coronagraph imagery. Given the number of such discrepant events, compared to the total number of ICMEs recorded near Earth in 2011, it is important to understand the inconsistencies and to ensure that future space weather monitoring strategies cater for them. Key Points: The identification of Earth‐directed coronal mass ejections (CMEs) is compared for observations from L1 and off the Sun‐Earth line, for 2011 We show a number of CMEs identified as Earth directed, with consistent in situ Interplanetary CME (ICME) data, with no identification from L1 data Given the number of ICMEs recorded at L1 in 2011, the number of inconsistent events is of concern