Flux Rope Coronal Mass Ejections Research Articles

Numerical simulations of fast and slow coronal mass ejections

AbstractSolar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux‐rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force‐free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energetics of solar coronal mass ejections

Aims: We investigate whether solar coronal mass ejections are driven mainly by coupling to the ambient solar wind or through the release of internal magnetic energy. Methods: We examine the energetics of 39 flux-rope like coronal mass ejections (CMEs) from the Sun using data in the distance range $\sim$ 2--20 $R_{\odot}$ from the Large Angle Spectroscopic Coronograph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). This comprises a complete sample of the best examples of flux-rope CMEs observed by LASCO in 1996-2001. Results: We find that 69% of the CMEs in our sample experience a clearly identifiable driving power in the LASCO field of view. For the CMEs that are driven, we examine if they might be deriving most of their driving power by coupling to the solar wind. We do not find conclusive evidence in favor of this hypothesis. On the other hand, we find that their internal magnetic energy is a viable source of the required driving power. We have estimated upper and lower limits on the power that can possibly be provided by the internal magnetic field of a CME. We find that, on average, the lower limit to the available magnetic power is around 74% of what is required to drive the CMEs, while the upper limit can be as much as an order of magnitude larger.

Remote sensing of the solar site of interchange reconnection associated with the May 1997 magnetic cloud

The direction of suprathermal electron flux on open magnetic field lines in the 15 May 1997 magnetic cloud is used to predict the solar location of the interchange reconnection that released one end of what presumably were doubly connected field lines in the coronal mass ejection (CME) of origin on 12 May. A search for an X‐ray signature of the interchange reconnection in the predicted location reveals a long‐lasting arched structure stretching from high above the CME site to the northern polar coronal hole. At the edge of the coronal hole, coincident with the X‐ray feature, are previously reported extreme ultraviolet brightenings (“crinkles”). The observations are consistent with a CME flux rope forming in a near‐quadrupolar configuration while overhead open field lines reconnect with the rising, closed, rope fields to open one leg of the rope loop. The pattern is similar to the breakout model except there are no closed overhead field lines through which the rising flux rope must break out. The near‐quadrupolar source appears to be responsible for the mismatch between the polarity of the flux rope observed at 1 AU and the sector in which it was embedded. Spacecraft interception of the leg rather than the apex of the flux rope loop may be responsible for the mismatch between the low inclination of the cloud axis and the high inclination predicted from the preexisting filament and magnetic configuration at the source.

Fitting flux ropes to a global MHD solution: a comparison of techniques

Flux rope fitting (FRF) techniques are an invaluable tool for extracting information about the properties of a sub-class of coronal mass ejections (CMEs) in the solar wind. However, it has proven difficult to assess their accuracy since the underlying global structure of the CME cannot be independently determined from the data. In contrast, large-scale MHD simulations of CME evolution can provide both a global view as well as localized time series at specific points in space. In this study we apply five different fitting techniques to two hypothetical time series derived from MHD simulation results. Independent teams performed the analysis of the events in “blind tests”, for which no information, other than the time series, was provided. From the results, we infer the following: (1) Accuracy decreases markedly with increasingly glancing encounters; (2) Correct identification of the boundaries of the flux rope can be a significant limiter; and (3) Results from techniques that infer global morphology must be viewed with caution. In spite of these limitations, FRF techniques remain a useful tool for describing in situ observations of flux rope CMEs.

The Role of Magnetic Reconnection in the Observable Features of Solar Eruptions

There are two competing classes of models for coronal mass ejections (CMEs): those that assume a preexisting magnetic flux rope and those that can make a flux rope during the eruption by magnetic reconnection. The present work is based on the model with a preexisting flux rope. We investigate the evolution of morphological features of the magnetic configuration in a CME according to a catastrophe model of flux rope CMEs developed previously. For the parameters chosen for the present work, roughly half of the total mass and magnetic flux are contained in the initial flux rope, while the remaining plasma and poloidal magnetic flux are brought by magnetic reconnection from the corona into the current sheet and from there into the CME bubble. These features and the corresponding physical processes are identical to those described by the non-flux rope models. Thus, the flux rope and non-flux rope models are less distinct than is generally assumed. The reconnected magnetic flux can account for the rapid expansion of the ejecta, and the plasma flowing out of the current sheet fills the outer shell of the ejecta. We tentatively identify the outer shell, the expanded bubble, and the flux rope with the leading edge, void, and core of the three-component CME structure, respectively. Thus, the final mass, speed, and magnetic energy—the quantities that determine the geoeffectiveness of the CME—are determined not in the initial eruption but during the CME expansion, at heights of a few solar radii. The aspects of this explanation that need improvement are also discussed.

Internal structure of magnetic clouds: Plasma and composition

A comprehensive analysis of magnetic clouds observed by the Advanced Composition Explorer (ACE) spacecraft from February 1998 to July 2001 is presented. The magnetic field data from the MAG instrument is fit with the cylindrically symmetric, linear force‐free model and the fit parameter distributions are examined. This magnetic field model enables us to map plasma data from the SWEPAM and SWICS instruments to a position within the model cylinder. A superposed epoch analysis of all our magnetic cloud events is used to construct diameter cuts through an “average” cloud profile in any desired plasma, elemental composition, or charge state quantity. These diameter cuts are found to have nontrivial structure and there appears to be significant composition and structural differences between clouds of different speeds. The slow magnetic clouds (〈Vrad〉 < 500 km/s) have an almost constant proton density profile whereas the fast magnetic cloud (〈Vrad〉 ≥ 500 km/s) profile is depleted throughout with symmetric dips and a local maximum at the cloud center. The fast magnetic clouds have a slightly higher Nα/Np ratio than the slow clouds. Both the fast and slow events have enhanced oxygen and iron charge states compared to the slow solar wind. The fast events have a slightly increased O7+/O6+ average profile and a much stronger Fe≥16+/Fetotal profile than the slow events. We briefly discuss the implications for physical conditions at the Sun, the role these coronal mass ejections (CMEs) may play in transporting magnetic flux, and the application of our structure results to the current flux rope CME modeling effort.

The Hydromagnetic Origin of the Two Dynamical Types of Solar Coronal Mass Ejections

We propose a qualitative theory relating flux rope coronal mass ejections (CMEs) to initial states, represented by inverse or normal quiescent prominences corresponding to whether the flux rope magnetic field circulates in the same or opposite sense relative to the surrounding coronal magnetic field. Our theory explains the observed dual character of CME speed-height profiles in terms of the interplays between flux rope expulsion and magnetic reconnection in topologically different magnetic environments and relates CMEs, prominences, and flares to coronal magnetic field reversal during activity maximum.