SH22A-01 INVITED
Difference Between Magnetic Clouds and Non-cloud Ejecta in the Interplanetary Medium
Solar cycle 23 has witnessed the accumulation of data on an unprecedented number of coronal mass ejections (CMEs) at the Sun and in the interplanetary (IP) medium, thanks to the large array of spaceborne observatories such as SOHO, Wind, and ACE. These observations have helped us make significant progress on the structure and evolution of CMEs in the inner heliosphere. One important question is whether the magnetic cloud (MC) and non-cloud ejecta have any difference in their solar origin. The ubiquitous nature of post-eruption arcades suggests that there should not be any difference. However, CMEs associated with MCs all originate from very close to the solar disk center (both in latitude and longitude). To zeroth order, the non-cloud ejecta seem to originate at larger central meridian distances (CMDs). In the extreme case of shocks without discernible ejecta, the corresponding CMEs have their solar sources near the limb. These observations suggest that whether one observes a flux rope (MC) or not depends mainly on the location of the observer with respect to the Sun-Earth line. Observations from solar cycle 23 indicate that there are significant deviations from the zeroth order picture, especially for non-cloud ejecta and the "driverless shocks": their solar sources near the disk center. The question is whether these ejecta do not have flux-rope structure by birth or they somehow got deflected away from the Sun-Earth line by other large- scale structures in the IP medium. The latter seems to be true for at least a subset of events, which seems to be affected by coronal holes located between the eruption center and the Sun-Earth line. This needs to be checked for all the events that deviate from the zeroth order picture. Charge-state signatures of MCs and non-cloud ejecta also support such a picture: the solar sources of IP CMEs with high charge states seem to originate close to the disk center, similar to the MC-associated CMEs. Another piece of evidence comes from the high correspondence between halo CMEs and MCs, both of which are highly geoeffective. Halo CMEs originating at larger CMDs produce geomagnetic storms via their sheath fields, again pointing to the importance of geometry. Contrary to importance of internal structure of CMEs for geoeffectiveness, the production of gradual solar energetic particle (SEP) should not depend on the internal structure of CMEs if the particles are accelerated by CME-driven shocks. In fact, there is a significant difference in the source distribution of SEP-producing CMEs (western sources) and MC CMEs (close to disk center). This paper illustrates these results using coronal and IP data.
SH22A-02
Disentangling the Magnetic Flux Rope Topology From Coronal Mass Ejections
It is not always straightforward to associate the measured plasma variables inside a magnetic cloud with a
magnetic flux-rope topology. Therefore, to explain non flux-rope topology, recent theoretical and numerical
work has focused on two scenarios based on the classical flux-rope theory. In one case, the complex
structure is simply due to the off-axis detection of a magnetic cloud; in the other one, interaction of multiple
CMEs on their way to Earth destroys the regularity of the magnetic field.
Here, we present recent simulation results which show that the magnetic cloud paradigm itself may need to
be revised. Using a realistic initiation mechanism for a Coronal Mass Ejection, we are able to simulate an
interplanetary CME, which possesses at 1AU the characteristics of a magnetic cloud, like the smooth rotation
of the magnetic field vector. However, no flux-rope structure is present in the simulated CME. In our model,
reconnection following a shearing phase results in the appearance of erupting magnetic field lines showing
writhe and not twist. By presenting synthetic white-light images and satellite data at 1AU, we argue that this
type of ejections could not be distinguished from the classical picture of a flux-rope. We also present
evidence that this initiation mechanism can help explaining some complex SEP observations such as those
during the August 24, 2002 CME.
SH22A-03 INVITED
Dynamics of Flux-Rope CMEs: SECCHI Observation and Theory
The physical mechanism responsible for the acceleration and propagation of CMEs through the corona and interplanetary medium has been an important open question of solar-heliospheric physics. Often associated with CMEs are SEPs and flares. The physical connections between CMEs and these phenomena are also major questions. The new SECCHI observations represent unprecedented opportunities to test and establish new understanding of CME physics both closer to and farther away from the Sun than was previously possible. The LASCO observations have established that most, if not all, CMEs can be understood as expanding magnetic flux ropes. Much work has been done to understand flux-rope dynamics based on this hypothesis. In particular, the erupting flux-rope model has been extensively tested against LASCO data with good agreement. In this talk, I will discuss new results from recent applications of this theoretical model to SECCHI EUVI, COR-1, COR-2, and HI-1 observation. Several CMEs and their dynamics observed to about 100 R☉ (projected) have been studied. Forces acting on these CMEs are discussed for both the inner-corona and heliospheric regions. It is shown that the erupting flux-rope model is able to fit the observed CME trajectories throughout the EUVI-COR1-COR2-HI1 field of view, indicating that the model correctly captures the basic physics, i.e., forces and magnetic geometry, of acceleration and propagation of CMEs. It is found that significantly larger values of the drag coefficient in the model than previously used are required to fit both the COR-1/COR-2 data and HI-1 data. Thus, the new expanded field of view of SECCHI imposes stronger constraints on model parameters than does the previous LASCO data set. It is also shown that the duration of the poloidal flux injection functions chosen to fit the CME trajectory closely match the duration of the observed GOES X-ray light curves for both short-duration and long-duration flares. The velocity components of the 3D flux rope parallel and perpendicular to the Archimedean spiral magnetic field are calculated along the CME trajectory. Possible physical connections of CMEs to flare energy release and shock acceleration of SEPs are discussed, suggesting a unified way to understand these related phenomena. Work supported by ONR and NASA
SH22A-04
Observation of a solar energetic particle event behind previous coronal mass ejection
On 2001 October 19--21 the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on the Solar and Heliospheric Observatory (SOHO) observed two gradual solar energetic particle (SEP) events separated by 15 hours, in association with two X1.6/2B solar flares and halo coronal mass ejections (CMEs). The observation data suggests that the second acceleration of ~10--100 MeV protons occurred behind the first CME and the previous CME was not an obstacle for the new particles to directly access 1~AU. This observation calls into question the view that in all gradual events the high-energy particles are continuously produced at a CME bow shock as it travels from near Sun to beyond 1~AU. The data support an idea that the >10~MeV protons of the 2001 October 19 event were accelerated mainly within ~0.1~AU from the Sun and then temporarily confined in structured solar wind.
SH22A-05 INVITED
Solar Energetic Particle Acceleration at Coronal Mass Ejection-driven shocks
Coronal Mass Ejection- or CME-driven shocks are presently believed to be the primary cause of the so-called large gradual solar energetic particle (SEP) events and the energetic storm particle (ESP) events observed at Earth. However, despite recent observational and theoretical advances, many important questions regarding such CME-associated particle events remain unanswered. This is because the SEP observations near Earth orbit are smeared by a confluence of numerous poorly understood physical effects all of whose contributions can vary with time and location. These effects include: the origin, structure, and evolution of the CME shocks, the nature of wave-particle interactions and the type of turbulence that is present near the shocks, the distribution and composition of the seed populations, the type of injection and acceleration processes involved and the manner in which the interplanetary medium affects their transport to Earth. In this paper we review SEP measurements from solar cycle 23 in terms of their relationship with CMEs and of our current understanding of the origin, energization, and interplanetary transport of the accelerated particles. We also discuss new opportunities that will be provided by missions such as STEREO in the upcoming solar cycle that will enable us to quantitatively model and predict key properties such as event durations and energy spectra of the accelerated ions.
SH22A-06
Geometry of ICMEs with and without magnetic flux-rope structure from a Cosmic Ray perspective
We have determined ICME (Interplanetary coronal mass ejection) geometry from galactic cosmic ray data recorded by the ground-based muon detector network. The cosmic ray density depression inside the ICME, which is the cause of a Forbush decrease, is represented by an expanding cylinder that is based on a theoretical model of cosmic ray particle diffusion. In 8 ICME events from March 2001 to May 2005, ICME geometry and orientation are deduced from observed time variations of cosmic ray density and density gradient, and are compared with that deduced from a magnetic flux rope model. In 3 of these 8 events, clear signatures of flux rope structure were seen, and the ICME geometry and orientation deduced from the two methods (flux rope modeling and cosmic ray modeling) were very similar. Flux rope modeling was also applied to 2 other events with weaker flux rope signatures, but here the agreement with the cosmic ray method was not as good, presumably because the small magnetic field rotations reduce the efficacy of the flux rope method. In the final 3 events, magnetic fields are distorted because of multiple CME interactions, so the flux rope analysis was not applied. The cosmic ray method could nonetheless be applied and yielded physically reasonable results compatible with results obtained for the flux-rope events.
SH22A-07 INVITED
Cornonal Mass Ejections and Their Role in Solar Energetic Particle Acceleration and Transport
Solar energetic particle (SEP) events have traditionally been classified as either gradual or impulsive.
Particles in gradual events are accelerated by shocks driven by large, fast coronal mass ejections (CMEs)
and can be observed over a broad range of heliographic longitudes and latitudes, while energetic particles
from impulsive solar flares are only detected when the observer is magnetically connected to the flare site.
Any CMEs observed in association with impulsive events are typically much smaller than those responsible
for gradual events.
Observations of SEPs indirectly probe the effects of the transit of CMEs through the inner heliosphere.
Timing studies have shown that the highest energy SEPs are accelerated within a few solar radii of the Sun,
while at lower energies CME-driven shocks accelerate particles at 1 AU and beyond. Recent studies have
shown that a substantial fraction (~10%) of the CME kinetic energy goes into the acceleration of SEPs,
making SEPs a significant part of the CME phenomenon. The geometry of the CME, both in terms of the
solar longitude of its source and the orientation of its shock relative to the ambient field, plays a critical role in
determining the time profiles and possibly the composition of the resulting SEPs. Transport of SEPs can be
affected by CMEs, as occasionally an interplanetary CME magnetic loop rooted at the Sun may serve as a
conduit for SEPs if a flare injects particles near its footpoint.
Recent observational studies relating to SEP acceleration and transport and the role of CMEs in these
processes are reviewed, and new observations that will be possible from STEREO and future missions are
discussed.
This work was supported by NASA under grant NNX08AI11G.
SH22A-08
MHD Simulation of Non-Flux-Rope CMEs Associated with Impulsive SEP Events
Observations at 1 AU indicate that only 1/3 of coronal mass ejections (CMEs) have a flux-rope structure, while other 2/3 of CMEs have a complex disordered magnetic field. The initiation of the flux-rope CMEs has been extensively simulated with various magnetohydrodynamic (MHD) models. However, the initiation of the non-flux-rope CMEs is rarely done in numerical modeling. Recently, we have numerically simulated the physical origin and characteristics of the non-flux-rope CMEs by using a three-dimensional axisymmetric time- dependent self-consistent MHD model. The results show that the emergence of counter-clockwise flux rope from the photosphere at the open field region, near the closed magnetic field lines, can initiate a magnetic configuration which produces the magnetic reconnection at the coronal base. Physically, the magnetic field of the emerged counter-clockwise flux rope has anti-polarity with the background open field and the location of the magnetic flux emergence is at the edge of a coronal hole. This magnetic reconnection between closed and open field lines at the coronal base generates an Alfvenic jet-like plasma outflow, which can be developed into a non-flux-rope CME, if enough magnetic energy deposits, with strong disturbances of the magnetic fields in the solar corona and interplanetary space. Solar energetic particle (SEP) events in association with the non-flux-rope CMEs are usually impulsive. In this presentation, we will present our MHD simulation results of non-flux-rope CMEs associated with the impulsive SEP events.