Role Of Coronal Mass Ejections Research Articles

Coronal mass ejections: a driver of severe space weather

Coronal mass ejections: a driver of severe space weather

Angular momentum evolution of low‐mass pre‐main sequence stars via extreme coronal mass ejections

AbstractThe angular momentum evolution of cool stars during the pre‐main sequence phase of stellar evolution remains a major outstanding problem. Multiple processes are likely involved in the transfer of mass and angular momentum within and out of the star+disk system. The role of coronal mass ejections (CMEs), energetic events which shed mass and magnetic flux in the Sun, has yet to be fully explored in the context of pre‐main sequence stars. It is well established that young, solar‐type stars exhibit X‐ray activity levels up to four orders of magnitude higher than the present‐day Sun, suggesting that CMEs associated with these extreme X‐ray flares could be an important process for expelling mass and angular momentum. We present a novel approach to modeling the CMEs of low‐mass pre‐main sequence stars that uses a solar‐calibrated CME model and observed X‐ray flare rates for young stars. We derive mass loss rates via stellar CMEs and calculate their attendant angular momentum losses during the pre‐main sequence phase. While we find the mass loss rates to be modest, ∼10 % of steady‐state stellar wind values from the literature, the angular momentum losses can be substantial, potentially counteracting the effects of initial stellar spin‐up due to contraction in tens of Myr. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CORONAL MASS EJECTIONS AND GLOBAL CORONAL MAGNETIC FIELD RECONFIGURATION

We investigate the role of coronal mass ejections (CMEs) in the global coronal magnetic field reconfiguration, a debate that has lasted for about two decades. Key evidence of the coronal field restructuring during the 2007 December 31 CME is provided by combining imaging observations from widely separated spacecraft with the potential-field source-surface (PFSS) model, thanks to the extraordinarily quiet Sun at the present solar minimum. The helmet streamer, previously disrupted by the CME, re-forms but is displaced southward permanently; the preexisting heliospheric plasma sheet (HPS) is also disrupted as evidenced by the concave-outward shape of the CME. The south polar coronal hole shrinks considerably. Plasma blobs moving outward along the newly formed HPS suggest the occurrence of magnetic reconnection between the fields blown open by the CME and the ambient adjacent open fields. A streamer-like structure is also observed in the wake of the CME and interpreted as a plasma sheet where the thin post-CME current sheet is embedded. These results are important for understanding the coronal field evolution over a solar cycle as well as the complete picture of CME initiation and propagation.

Combining remote and in situ observations of coronal mass ejections to better constrain magnetic cloud reconstruction

Determination of the nonradial extent of magnetic clouds (MCs) is vital for two key reasons. First, it affects the amount of “drag” a fast MC experiences and therefore controls the travel time from the Sun to 1‐AU, a critical parameter for space‐weather prediction. Second, it is vital to estimating the flux content of MCs, which in turn is important for understanding both the formation and eruption of the magnetic flux rope and for determining the role of coronal mass ejections in the heliospheric flux budget and the evolution of heliospheric flux over the solar cycle. In this study, it is demonstrated that the cross‐sectional elongation of MCs is poorly constrained by in situ observations of the magnetic field alone. A method for combining remote and in situ observations of ejecta to better determine MC cross‐sectional elongation is then outlined and applied to a previously studied event which occurred during the SOHO‐Ulysses quadrature of late 1996. The new technique reveals an axial magnetic flux content ∼4 times higher than that inferred by a force‐free flux rope model fit to the same in situ observations of the magnetic cloud. This event also shows evidence of axial distortion by the structured ambient solar wind.

Role of coronal mass ejections in the heliospheric Hale cycle

The 11‐year solar cycle variation in the heliospheric magnetic field strength can be explained by the temporary buildup of closed flux released by coronal mass ejections (CMEs). If this explanation is correct, and the total open magnetic flux is conserved, then the interplanetary‐CME closed flux must eventually open via reconnection with open flux close to the Sun. In this case each CME will move the reconnected open flux by at least the CME footpoint separation distance. Since the polarity of CME footpoints tends to follow a pattern similar to the Hale cycle of sunspot polarity, repeated CME eruption and subsequent reconnection will naturally result in latitudinal transport of open solar flux. We demonstrate how this process can reverse the coronal and heliospheric fields, and we calculate that the amount of flux involved is sufficient to accomplish the reversal within the 11 years of the solar cycle.

Magnetic flux emergence and geomagnetic activity, a close correlation

We analyse data of magnetic flux emergence for solar cycles 21 and 22, Helios 1 interplanetary shocks for cycle 21, and sudden storm commencements (SSCs) for cycles 11–22. A dominant variation of 3-year periodicity was found for all three phenomena during cycles 21 and 22. This indicates a correlation and a possible influence of the rate of solar magnetic flux emergence to produce the interplanetary phenomena studied in this work; in particular, the suggested role of coronal mass ejections as a means by which magnetic flux and stresses are taken out of the corona seems to be plausible. When taking cycles 11–22 in SSCs, the main periodicity changes to around 4 years; this may be an indication of flux emergence rate variations over the cycles.

CME-driven shocks as a possible source of the electron component in SEP-events

Contrary to the case of protons and ions, our understanding of the role of coronal mass ejections (CME) and shocks initiated by CMEs in the generation of electrons in solar energetic particle (SEP) events is far from complete. We have considered 65 SEP-events for which it was possible to study relations between the flux profile parameters of E > 0.3 MeV electron enhancements and the velocity of CMEs, as well as characteristics of associated flares. We found that the time to electron peak flux correlates strongly with CME velocity (correlation coefficient is ∼0.7–0.9) both after impulsive and long duration (LDE) flares. The best correlation was obtained in the case of electron enhancements related to flares that were accompanied by hard X-ray bursts with spectrum containing photons with enery <200 keV only.

The role of coronal mass ejections and interplanetary shocks in interplanetary magnetic field statistics and solar magnetic flux ejection

We examine the role of coronal mass ejections (CMEs) and interplanetary shocks in modifying the large‐scale winding of the interplanetary magnetic field (IMF) by extracting CME and shock observations from the ISEE 3 data set and analyzing periods of the disturbed and undisturbed solar wind separately. We use the full ISEE 3 data set representing the entire L1 mission (1978 – 1982). We conclude that CMEs, the shocks upstream of CMEs, and other interplanetary shocks are responsible for the apparent overwinding of the IMF spiral relative to the Parker prediction. The IMF winding angle asymmetry is preserved following the removal of the interplanetary disturbances. We also examine the IMF components, the IMF magnitude and the solar wind speed, and the dependence of those averages and asymmetries on CMEs and shock disturbances. An estimate is obtained for the anomalous azimuthal field contained within CMEs which apparently results from the closed‐field topology. We provide new evidence for a nonzero field component crossing the heliospheric current sheet. Last, we examine the role of CMEs and shocks in the measurement of solar magnetic flux ejection. We provide estimates for the average amount of flux transported by CMEs and the error in flux transport analyses that include shock data, and we examine the possible north‐south asymmetry of the flux.

Coronal transients: a summary

Observations with orbiting coronagraphs have illuminated the role of coronal mass ejections in solar activity, and raised a number of questions concerning their origin, the nature of the forces driving the coronal material, and their signature in interplanetary space. Current models of the ejection process - including propagation of loops as a result of azimuthal field gradients, ring currents or a build-up of magnetic pressure from below - are summarized, as are magnetohydrodynamic codes intended to stimulate transient conditions. Metric radio observations, can, in principle, distinguish the relative role of the magnetic field in the ejection process; observations to date are surveyed. It is concluded that at present, no compelling evidence is available to distinguish between transient driving mechanisms, but future observations of the corona and interplanetary medium may resolve the present ambiguity.