Daikou Shiota

Self-Consistent Magnetohydrodynamic Modeling of a Coronal Mass Ejection, Coronal Dimming, and a Giant Cusp-shaped Arcade Formation

We performed magnetohydrodynamic simulation of coronal mass ejections (CMEs) and associated giant arcade formations, and the results suggested new interpretations of observations of CMEs. We performed two cases of the simulation: with and without heat conduction. Comparing between the results of the two cases, we found that reconnection rate in the conductive case is a little higher than that in the adiabatic case and the temperature of the loop top is consistent with the theoretical value predicted by the Yokoyama-Shibata scaling law. The dynamical properties such as velocity and magnetic fields are similar in the two cases, whereas thermal properties such as temperature and density are very different. In both cases, slow shocks associated with magnetic reconnection propagate from the reconnection region along the magnetic field lines around the flux rope, and the shock fronts form spiral patterns. Just outside the slow shocks, the plasma density decreased a great deal. The soft X-ray images synthesized from the numerical results are compared with the soft X-ray images of a giant arcade observed with the Soft X-ray Telescope aboard Yohkoh, it is confirmed that the effect of heat conduction is significant for the detailed comparison between simulation and observation. The comparison between synthesized and observed soft X-ray images provides new interpretations of various features associated with CMEs and giant arcades. 1) It is likely that Y-shaped ejecting structure, observed in giant arcade 1992 January 24, corresponds to slow and fast shocks associated with magnetic reconnection. 2) Soft X-ray twin dimming

A study of a tiny two-ribbon flare driven by emerging flux

We present observations of the eruption of a miniature filament that occurred near NOAA Active Region 9537 on 2001 July 14. The eruption was observed by the Hida Observatory Domeless Solar Telescope, in the Hα line center and ±0.4 A wings, the Solar and Heliospheric Observatory EUV Imaging Telescope (EIT) and Michelson Doppler Imager, and the Yohkoh Soft X-Ray Telescope (SXT). The miniature filament began to form and was clearly visible in Hα images by around 06:50 UT. It erupted about 25 minutes later, accompanied by a small two-ribbon subflare (with an area of 61 arcsec2). The two ribbons were also found to approach each other at a speed of 3.33 km s-1. We found that this event was caused by the emergence of new magnetic flux in a quiet region. The emerging flux appeared as a bright region in the EIT and SXT images taken on the previous day. It moved southward into an area of preexisting opposite-polarity flux, where a cancelling magnetic flux region was formed. The miniature filament then appeared, and we suggest that it played some role in inhibiting the release of energy by delaying reconnection between the emerging and preexisting flux, as evidenced by the disappearance of the bright region between opposite polarities in the EUV and soft X-ray images. Consequently, magnetic energy was stored as a result of the slow converging motion of the two opposite-polarity flux regions (0.17 km s-1). Reconnection below the filament provoked the filament eruption, and the two-ribbon flare occurred. Miniature filaments are thought to be small-scale analogs of large-scale filaments. Our observations also suggest some common properties between small-scale and large-scale flares. These results support the view that a unified magnetic reconnection model may be able to explain all scales of flares.

Analysis of the Temperature and Emission Measure of Solar Coronal Arcades and Test of a Scaling Law of Flare/Arcade Loop Length

We analyze 17 arcades to study the relations between solar flares and arcades. Soft X-ray images taken with Yohkohs soft X-ray telescope are used to derive T0, EM0, and temporal variation of Tarc and EMarc, where Tarc and T0 are the temperatures of an arcade and prearcade region and EMarc and EM0 are the volume emission measures of an arcade and prearcade region. It is found that T0 ~ 2 MK and Tarc ~ 4 MK. We also estimate prearcade coronal electron density n0 and arcade electron density narc to find that narc is comparable to n0 (narc ~ n0 ~ 108 cm-3). Using these observed EM, T, and n0, we calculate the theoretical loop length Ltheor based on the scaling law for solar and stellar flares derived by Shibata & Yokoyama and compare it with observed flare/arcade loop length Lobs. The result shows a good correlation between them (Ltheor ~ Lobs) and indicates the need of plasma β for the scaling law (Ltheor ∝ EM3/5T-8/5nβ-6/5). This supports the theory of the scaling law and is indirect evidence that flares and arcades are heated by the same magnetic reconnection mechanism.

Power-law spatial profile in an upstream region of CME-driven interplanetary shock

We study the density decay profile of energetic particles in the upstream region of an interplanetary shock on 14 Dec 2006 observed by the ACE spacecraft at 1 AU. The spatial decay profile of the energetic particle flux does not exhibit an exponential behavior as expected for the standard diffusive shock acceleration process but a power-law behavior in anomalous or superdiffusive transport. The power-law profiles are observed for not only the energetic ions reported in Sugiyama & Shiota (2011) but also heavier ions of He2+, CNO, and Fe. We observe the relation 〈Δx2〉 ∝ tα for α ~ 1.24–1.72, where Δx is the particle displacement within the time scale t, and the bracket denotes an ensemble average. This implies that particle propagation around a near-earth orbit can be intermediate between normal diffusion (α = 1) and ballistic motion (α equals 2).