No abstract

Frontside halo coronal mass ejections (CMEs) are generally considered as potential candidates for producing geomagnetic storms, but there was no definite way to predict whether they will hit the Earth or not. Recently Moon et al. suggested that the degree of CME asymmetries, as defined by the ratio of the shortest to the longest distances of the CME front measured from the solar center, be used as a parameter for predicting their geoeffectiveness. They called this quantity a direction parameter, D, as it suggests how much CME propagation is directed to Earth, and examined its forecasting capability using 12 fast halo CMEs. In this paper, we extend this test by using a much larger database (486 frontside halo CMEs from 1997 to 2003) and more robust statistical tools (contingency table and statistical parameters). We compared the forecast capability of this direction parameter to those of other CME parameters, such as location and speed. We found the following results: (1) The CMEs with large direction parameters (D ! 0:4) are highly associated with geomagnetic storms. (2) If the direction parameter increases from 0.4 to 1.0, the geoeffective probability rises from 52% to 84%. (3) All CMEs associated with strong geomagnetic storms ( Dst À200 nT) are found to have large direction parameters (D ! 0:6). (4) CMEs causing strong geomagnetic storms (Dst À100 nT), in spite of their northward magnetic field, have large direction parameters (D ! 0:6). (5) Forecasting capability improves when statistical parameters (e.g., ''probability of detection -yes'' and ''critical success index'') are employed, in comparison with the forecast solely based on the location and speed of CMEs. These results indicate that the CME direction parameter can be an important indicator for forecasting CME geoeffectiveness. Subject headingg s: solar-terrestrial relations -Sun: coronal mass ejections (CMEs)

We present RHESSI and TRACE observations of multiple flare activity that occurred in the active region NOAA 10656 over the period of two hours on 2004 August 18. Out of four successive flares, there were three events of class C while the final event was a major X1.8 solar eruptive flare. The events during the pre-eruption phase, i.e., before the X1.8 flare, are characterized by localized episodes of energy release occurring in the vicinity of an active region filament which produced intense heating along with non-thermal emission. A few minutes before the eruption, the filament undergoes an activation phase during which it slowly rises with a speed of ∼12 km s −1 . The filament eruption is accompanied with an X1.8 flare during which multiple HXR bursts are observed up to 100-300 keV energies. We observe a bright and elongated coronal structure simultaneously in E(UV) and 50-100 keV HXR images underneath the expanding filament during the period of HXR bursts which provides strong evidence for ongoing magnetic reconnection. This phase is accompanied with very high plasma temperatures of ∼31 MK and followed by the detachment of the prominence from the solar source region. From the location, timing, strength, and spectrum of HXR emission, we conclude that the prominence eruption is driven by the distinct events of magnetic reconnection occurring in a current sheet formed below the erupting filament. These multi-wavelength observations also suggest that the localized magnetic reconnections associated with different evolutionary stages of the filament in the pre-eruption phase play a crucial role in destabilizing the filament by a tether-cutting process leading to large-scale eruption and X-class flare.

[1] In this study, we discuss the general behaviors of geomagnetic storm strength associated with observed parameters of coronal mass ejection (CME) such as speed (V) and earthward direction (D) of CMEs as well as the longitude (L) and magnetic field orientation (M) of overlaying potential fields of the CME source region, and we develop an empirical model to predict geomagnetic storm occurrence with its strength (gauged by the Dst index) in terms of these CME parameters. For this we select 66 halo or partial halo CMEs associated with M-class and X-class solar flares, which have clearly identifiable source regions, from 1997 to 2003. After examining how each of these CME parameters correlates with the geoeffectiveness of the CMEs, we find several properties as follows: (1) Parameter D best correlates with storm strength Dst; (2) the majority of geoeffective CMEs have been originated from solar longitude 15°W, and CMEs originated away from this longitude tend to produce weaker storms; (3) correlations between Dst and the CME parameters improve if CMEs are separated into two groups depending on whether their magnetic fields are oriented southward or northward in their source regions. Based on these observations, we present two empirical expressions for Dst in terms of L, V, and D for two groups of CMEs, respectively. This is a new attempt to predict not only the occurrence of geomagnetic storms, but also the storm strength (Dst) solely based on the CME parameters.