Aftermath Of Coronal Mass Ejections Research Articles

LASCO White-Light Observations of Eruptive Current Sheets Trailing CMEs

Many models of eruptive flares or coronal mass ejections (CMEs) involve formation of a current sheet connecting the ejecting CME flux rope with a magnetic loop arcade. However, there is very limited observational information on the properties and evolution of these structures, hindering progress in understanding eruptive activity from the Sun. In white-light images, narrow coaxial rays trailing the outward-moving CME have been interpreted as current sheets. Here, we undertake the most comprehensive statistical study of CME-rays to date. We use SOHO/LASCO data, which have a higher cadence, larger field of view, and better sensitivity than any previous coronagraph. We compare our results to a previous study of Solar Maximum Mission (SMM) CMEs, in 1984 – 1989, having candidate magnetic disconnection features at the CME base, about half of which were followed by coaxial bright rays. We examine all LASCO CMEs during two periods of minimum and maximum activity in Solar Cycle 23, resulting in many more events, \(\sim130\) CME-rays, than during SMM. Important results include: The occurrence rate of the rays is \(\sim11~\%\) of all CMEs during solar minimum, but decreases to \(\sim7~\%\) at solar maximum; this is most likely related to the more complex coronal background. The rays appear on average 3 – 4 hours after the CME core, and are typically visible for three-fourths of a day. The mean observed current sheet length over the ray lifetime is \(\sim12~R_{\odot}\), with the longest current sheet of \(18.5~R_{\odot}\). The mean CS growth rates are \(188~\mbox{km}\,\mathrm{s}^{-1}\) at minimum and \(324~\mbox{km}\,\mathrm{s}^{-1}\) at maximum. Outward-moving blobs within several rays, which are indicative of reconnection outflows, have average velocities of \(\sim350~\mbox{km}\,\mathrm{s}^{-1}\) with small positive accelerations. A pre-existing streamer is blown out in most of the CME-ray events, but half of these are observed to reform within \(\sim1\) day. The long lifetime and long lengths of the CME-rays challenge our current understanding of the evolution of the magnetic field in the aftermath of CMEs.

Modeling solar near-relativistic electron events

Context. Solar near-relativistic electrons (>30 keV) are observed as discrete events in the inner heliosphere following different types of solar transient activity. Several mechanisms have been proposed for the production of these electrons. One candidate is related to solar flare activity. Other candidates include shocks driven by fast coronal mass ejections (CMEs) or processes of magnetic reconnection in the aftermath of CMEs. Aims. We study eleven near-relativistic (NR) electron events observed by the Advanced Composition Explorer (ACE) between 1998 and 2005 with the aim of estimating the roles played by solar flares, CME-driven shocks, and processes of magnetic restructuring in the aftermath of the CMEs in the injection of NR electrons. The main goal is to infer the underlying injection profile from particle observations at 1 AU, as well as the interplanetary transport conditions. Methods. We used Monte Carlo simulations to model the transport of particles along the interplanetary magnetic field. By taking the angular response of the LEFS60 telescope of the EPAM instrument onboard ACE into account, we were able to deconvolve the transport effects from the observed intensities, and thus infer the solar injection profile. Results. In this set of events, we have identified two types of injection episodes: short ( 1 h). Short injection episodes seem to be associated with the flare processes and/or the reconnection phenomena in the aftermath of the CME, while time-extended episodes seem to be consistent with injection from CME-driven shocks. Conclusions. We find that there is no single scenario that operates in all the events. The interplanetary propagation of NR electrons can occur both under strong scattering and under almost scatter-free propagation conditions and several injection phases (related to flares and/or CMEs) are possible.

The Post-Coronal Mass Ejection Solar Atmosphere and Radio Noise Storm Activity

We carried out a statistical study of solar radio noise storms whose onset was in the aftermath of coronal mass ejections (CMEs) that occurred during 1997-2004, the first half of present solar cycle 23. The work is an attempt to understand the post-CME corona through observations of noise storms since the latter are considered to be closely related to structural changes there. The radio events were taken as the starting point for our study, and details about start time and location were available for 340 of them. We imposed the following conditions to verify the association between the above two phenomena: (1) the noise storm must have occurred ≤24 hr from the onset of a CME and (2) the central position angle of the CME must be located inside an angular span of ±45° with respect to the noise storm. We found that 196/340 noise storms were associated with CMEs. More interestingly, the time interval between CME liftoff and noise storm onset in all the above cases was ≤13 hr. We suggest that this represents the upper bound of the timescale over which coronal magnetic field reorganization had taken place in the aftermath of the aforementioned 196 noise storm associated CMEs. We also found that for a particular CME, the above temporal cutoff depends on its kinetic energy. Overall, it varies inversely with the logarithm of CME kinetic energy.

“Bursty” Reconnection Following Solar Eruptions: MHD Simulations and Comparison with Observations

Posteruptive arcades are frequently seen in the aftermath of coronal mass ejections (CMEs). The formation of these loops at successively higher altitudes, coupled with the classic two-ribbon flare seen in H-alpha, are interpreted as reconnection of the coronal magnetic field that has been dragged outward by the CME. White-light observations of rays, which have been interpreted as being coincident with the current sheet at the reconnection site underneath the erupting CME, also provide evidence for its occurrence. Blobs occasionally seen within these rays suggest an even richer level of structure. In this report, we present numerical simulations that reproduce both the observed rays and the formation and evolution of the blobs. We compare their properties with SOHO/LASCO observations of similar structures, and relate their formation to standard theories of reconnection,

The basic characteristics of EUV post-eruptive arcades and their role as tracers of coronal mass ejection source regions

The Extreme ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) space- craft provides unique observations of dynamic processes in the low corona. The EIT 195 A data taken from 1997 to the end of 2002 were investigated to study the basic physical properties of post-eruptive arcades (PEAs) and their relationship with coronal mass ejections (CMEs) as detected by SOHO/LASCO (Large Angle Spectrometric Coronagraph). Over the investigated time period, 236 PEA events have been identified in total. For each PEA, its EUV lifetime as derived from the emission time at 195 A, its heliographic position and length, and its corresponding photospheric source region inferred from SOHO/MDI (Michelson Doppler Imager) data has been studied, as well as the variation of these parameters over the investigated phase of solar cycle 23. An almost one to one correspondence is found between EUV PEAs and white-light CMEs. Based on this finding, PEAs can be considered as reliable tracers of CME events even without simultaneous coronagraph observations. A detailed comparison of the white-light, soft X-ray and EUV observation for some of the events shows, that PEAs form in the aftermath of CMEs likely in the course of the magnetic restructurings taking place at the coronal source sites. The average EUV emission life-time for the selected events ranged from 2 to 20 h, with an average of 7 h. The heliographic length of the PEAs was in the range of 2 to 40 degrees, with an average of 15 degrees. The length increased by a factor of 3 to 4 in the latitude range of 20 to 40 degrees in the northern and southern hemispheres, with longer PEAs being observed preferentially at higher latitudes. The PEAs were located mainly in the activity belts in both hemispheres, with the southern hemispheric ones being shifted by about 15 degree in latitude further away from the solar equator during 1997−2002. The decrease in latitude of the PEA positions was 10 to 15 degrees in the northern and southern hemispheres over this period. The axes of the PEAs were overlying magnetic polarity inversion lines when traced back to the MDI synoptic charts of the photospheric field. The magnetic polarities on both sides of the inversion lines agreed with the dominant magnetic pattern expected in cycle 23, i.e. being preferentially positive to the West of the PEA axes in the North and negative in the South. One third (31%) of the PEA events showed reversed polarities. The origin of PEAs is found not just in single bipolar regions (BPRs), but also in between pairs of neighboring BPRs.

Observations of Core Fallback during Coronal Mass Ejections

White-light observations made with the Large Angle and Spectrometric Coronagraph (LASCO) during the present solar maximum have revealed a multitude of faint, inward-moving features at heliocentric distances of r ~ 2-6 R☉. Most of these structures appear to originate above r ~ 3 R☉ and may be signatures of the closing-down of magnetic flux at the boundaries of coronal holes or in the aftermath of coronal mass ejections (CMEs). Here, we present observations of a different type of inflow, in which material within the bright core of a CME collapses back toward the Sun after rising to heights of r ~ 2.5-6 R☉. We have identified roughly 20 such fallback events during 1998-2001. The core structures, which have the form of loops or concave-outward flux ropes, ascend into the coronagraph field of view beyond 2 R☉ with speeds of ~100-400 km s-1 but return with speeds of only ~50-200 km s-1. The initial deceleration rates of ~20-100 m s-2 are comparable to the local gravitational deceleration GM☉/r2 but continually decrease with time. The associated CMEs tend to be impulsive but relatively slow, with the leading front moving outward at ~250-450 km s-1 and often showing some deceleration. It is thus not surprising that some fraction of the core material fails to reach escape speeds, remaining bound to the Sun by gravitational and magnetic tension forces. We suggest that the dynamical behavior of the core may be determined in part by momentum exchanges with the background medium, which consists of ongoing outflows of CME material, ambient solar wind, and inflow streams. In particular, we attribute the asymmetry of the up-down trajectories to the action of such drag forces, whose direction changes from inward to outward as the core decelerates.