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A fundamental property of coronal mass ejections (CMEs) is their radial expansion, which determines the increase in the CME radial size and the decrease in the CME magnetic field strength as the CME propagates. CME radial expansion can be investigated either by using remote observations or by in situ measurements based on multiple spacecraft in radial conjunction. However, there have been only few case studies combining both remote and in situ observations. It is therefore unknown if the radial expansion in the corona estimated remotely is consistent with that estimated locally in the heliosphere. To address this question, we first select 22 CME events between the years 2010 and 2013, which were well observed by coronagraphs and by two or three spacecraft in radial conjunction. We use the graduated cylindrical shell model to estimate the radial size, radial expansion speed, and a measure of the dimensionless expansion parameter of CMEs in the corona. The same parameters and two additional measures of the radial-size increase and magnetic-field-strength decrease with heliocentric distance of CMEs based on in situ measurements are also calculated. For most of the events, the CME radial size estimated by remote observations is inconsistent with the in situ estimates. We further statistically analyze the correlations of these expansion parameters estimated using remote and in situ observations, and discuss the potential reasons for the inconsistencies and their implications for the CME space weather forecasting. [ABSTRACT FROM AUTHOR]

The important role played by magnetic reconnection in the early acceleration of coronal mass ejections (CMEs) has been widely discussed. However, as CMEs may have expansion speeds comparable to their propagation speeds in the corona, it is not clear whether and how reconnection contributes to the true acceleration and expansion separately. To address this question, we analyze the dynamics of a moderately fast CME on 2013 February 27, associated with a continuous acceleration of its front into the high corona, even though its speed had reached ∼700 km sâˆ'1, which is faster than the solar wind. The apparent acceleration of the CME is found to be due to its expansion in the radial direction. The true acceleration of the CME, i.e., the acceleration of its center, is then estimated by taking into account the expected deceleration caused by the drag force of the solar wind acting on a fast CME. It is found that the true acceleration and the radial expansion have similar magnitudes. We find that magnetic reconnection occurs after the eruption of the CME and continues during its propagation in the high corona, which contributes to its dynamic evolution. Comparison between the apparent acceleration related to the expansion and the true acceleration that compensates the drag shows that, for this case, magnetic reconnection contributes almost equally to the expansion and to the acceleration of the CME. The consequences of these measurements for the evolution of CMEs as they transit from the corona to the heliosphere are discussed. [ABSTRACT FROM AUTHOR]

Suprathermal ions in the corona are thought to serve as seed particles for large gradual solar energetic particle (SEP) events associated with fast and wide coronal mass ejections (CMEs). A better understanding of the role of suprathermal particles as seed populations for SEP events can be made by using observations close to the Sun. We study a series of SEP events observed by the Integrated Science Investigation of the Sun (IS⊙IS) suite on board the Parker Solar Probe (PSP) from 2020 May 27 to June 2, during which PSP was at heliocentric distances between ∼0.4 and ∼0.2 au. These events were also observed by the Ahead Solar TErrestrial RElations Observatory (STEREO-A) near 1 au, but the particle intensity magnitude was much lower than that at PSP. We find that the SEPs should have spread in space as their source regions were distant from the nominal magnetic footpoints of both spacecraft and the parent CMEs were slow and narrow. We study the decay phase of the SEP events in the ∼1â€"2 MeV proton energy range at PSP and STEREO-A, and discuss their properties in terms of both continuous injections by successive solar eruptions and the distances where the measurements were made. This study indicates that seed particles can be continuously generated by eruptions associated with slow and narrow CMEs, spread over a large part of the inner heliosphere, and remain there for tens of hours, even if minimal particle intensity enhancements were measured near 1 au. [ABSTRACT FROM AUTHOR]

The Heliospheric Imagers (HI) onboard the two spacecraft of the Solar Terrestrial Relations Observatory (STEREO) provided white‐light images of transients in the solar wind from dual perspectives from 2007 to 2014. In this paper, we develop a new method to identify and locate the transients automatically from simultaneous images from the two inner telescopes, known as HI‐1, based on a correlation analysis. Correlation coefficient (cc) maps along the Sun‐Earth line are constructed for the period from 1 January 2010 to 28 February 2011. From the maps, transients propagating along the Sun‐Earth line are identified, and a 27‐day periodic pattern is revealed, especially for small‐scale transients. Such a periodicity in the transient pattern is consistent with the rotation of the Sun's global magnetic structure and the periodic crossing of the streamer structures and slow solar wind across the Sun‐Earth line, and this substantiates the reliability of our method and the high degree of association between the small‐scale transients of the slow solar wind and the coronal streamers. Besides, it is suggested by the cc map that small‐scale transients along the Sun‐Earth line are more frequent than large‐scale transients by a factor of at least 2, and that they quickly diffused into background solar wind within about 40 Rs in terms of the signal‐to‐noise ratio of white‐light emissions. The method provides a new tool to reconstruct inhomogeneous structures in the heliosphere from multiple perspectives. Key Points: A new method to identify and locate the transients automatically from dual STEREO/HI‐1 images is developed based on correlation analysisA 27‐day periodicity of transients, which originated from coronal streamers propagating as slow solar wind, is revealed by this method [ABSTRACT FROM AUTHOR]