Kazunari Shibata

An Emerging Flux Trigger Mechanism for Coronal Mass Ejections

Observations indicate that reconnection-favored emerging flux has a strong correlation with coronal mass ejectons (CMEs). Motivated by this observed correlation and based on the flux rope model, an emerging flux trigger mechanism is proposed for the onset of CMEs, using two-dimensional magnetohydrodynamic (MHD) numerical simulations: when such emerging flux emerges within the filament channel, it cancels the magnetic field below the flux rope, leading to the rise of the flux rope (owing to loss of equilibrium) and the formation of a current sheet below it. Similar global restructuring and a resulting rise motion of the flux rope occur also when reconnection-favored emerging flux appears on the outer edge of the filament channel. In either case, fast magnetic reconnection in the current sheet below the flux rope induces fast ejection of the flux rope (i.e., CME). It is also shown that the nonreconnecting emerging flux, either within the filament channel or on the outer edge of the channel, makes the flux rope move down, i.e., no CMEs can be triggered. Although the present two-dimensional model can not provide many details of the largely unknown three-dimensional processes associated with prominence eruptions, it shows some observational features such as the height-time profile of erupting prominences. Most importantly, our model can well explain the observed correlation between CMEs and the reconnection-favored emerging flux.

Hot-Plasma Ejections Associated with Compact-Loop Solar Flares

Masuda et al. found a hard X-ray source well above a soft X-ray loop in impulsive compact-loop flares near the limb. This indicates that main energy release is going on above the soft X-ray loop, and suggests magnetic reconnection occurring above the loop, similar to the classical model for two ribbon flares. If the reconnection hypothesis is correct, a hot plasma (or plasmoid) ejection is expected to be associated with these flares. Using the images taken by the soft X-ray telescope aboard Yohkoh, we searched for such plasma ejections in eight impulsive compact-loop flares near the limb, which are selected in an unbiased manner and include also the Masuda flare, 1992 January 13 flare. We found that all these flares were associated with X-ray plasma ejections high above the soft X-ray loop and the velocity of ejections is within the range of 50-400 km s-1. This result gives further support for magnetic reconnection hypothesis of these impulsive compact-loop flares.

Coronal Transverse Magnetohydrodynamic Waves in a Solar Prominence

Solar prominences are cool 104 kelvin plasma clouds supported in the surrounding 106 kelvin coronal plasma by as-yet-undetermined mechanisms. Observations from Hinode show fine-scale threadlike structures oscillating in the plane of the sky with periods of several minutes. We suggest that these represent Alfvén waves propagating on coronal magnetic field lines and that these may play a role in heating the corona.

Plasmoid-induced-reconnection and fractal reconnection

As a key to understanding the basic mechanism for fast reconnection in solar flares, plasmoid-induced-reconnection and fractal reconnection are proposed and examined. We first briefly summarize recent solar observations that give us hints on the role of plasmoid (flux rope) ejections in flare energy release. We then discuss the plasmoid-induced-reconnection model, which is an extention of the classical two-ribbon-flare model which we refer to as the CSHKP model. An essential ingredient of the new model is the formation and ejection of a plasmoid which play an essential role in the storage of magnetic energy (by inhibiting reconnection) and the induction of a strong inflow into reconnection region. Using a simple analytical model, we show that the plasmoid ejection and acceleration are closely coupled with the reconnection process, leading to a nonlinear instability for the whole dynamics that determines the macroscopic reconnection rate uniquely. Next we show that the current sheet tends to have a fractal structure via the following process path: tearing ⇒ sheet thinning ⇒ Sweet-Parker sheet ⇒ secondary tearing ⇒ further sheet thinning ⇒ ⋯. These processes occur repeatedly at smaller scales until a microscopic plasma scale (either the ion Larmor radius or the ion inertial length) is reached where anomalous resistivity or collisionless reconnection can occur. The current sheet eventually has a fractal structure with many plasmoids (magnetic islands) of different sizes. When these plasmoids are ejected out of the current sheets, fast reconnection occurs at various different scales in a highly time dependent manner. Finally, a scenario is presented for fast reconnection in the solar corona on the basis of above plasmoid-induced-reconnection in a fractal current sheet.

Chromospheric Anemone Jets as Evidence of Ubiquitous Reconnection

The heating of the solar chromosphere and corona is a long-standing puzzle in solar physics. Hinode observations show the ubiquitous presence of chromospheric anemone jets outside sunspots in active regions. They are typically 3 to 7 arc seconds = 2000 to 5000 kilometers long and 0.2 to 0.4 arc second = 150 to 300 kilometers wide, and their velocity is 10 to 20 kilometers per second. These small jets have an inverted Y-shape, similar to the shape of x-ray anemone jets in the corona. These features imply that magnetic reconnection similar to that in the corona is occurring at a much smaller spatial scale throughout the chromosphere and suggest that the heating of the solar chromosphere and corona may be related to small-scale ubiquitous reconnection.

Superflares on solar-type stars

Solar flares are caused by the sudden release of magnetic energy stored near sunspots. They release 1029 to 1032 ergs of energy on a timescale of hours. Similar flares have been observed on many stars, with larger ‘superflares’ seen on a variety of stars, some of which are rapidly rotating and some of which are of ordinary solar type. The small number of superflares observed on solar-type stars has hitherto precluded a detailed study of them. Here we report observations of 365 superflares, including some from slowly rotating solar-type stars, from about 83,000 stars observed over 120 days. Quasi-periodic brightness modulations observed in the solar-type stars suggest that they have much larger starspots than does the Sun. The maximum energy of the flare is not correlated with the stellar rotation period, but the data suggest that superflares occur more frequently on rapidly rotating stars. It has been proposed that hot Jupiters may be important in the generation of superflares on solar-type stars, but none have been discovered around the stars that we have studied, indicating that hot Jupiters associated with superflares are rare.

Evidence of EIT and Moreton Waves in Numerical Simulations

Solar coronal mass ejections (CMEs) are associated with many dynamical phenomena, among which EIT waves have always been a puzzle. In this Letter MHD processes of CME-induced wave phenomena are numerically simulated. It is shown that as the flux rope rises, a piston-driven shock is formed along the envelope of the expanding CME, which sweeps the solar surface as it propagates. We propose that the legs of the shock produce Moreton waves. Simultaneously, a slower moving wavelike structure, with an enhanced plasma region ahead, is discerned, which we propose corresponds to the observed EIT waves. The mechanism for EIT waves is therefore suggested, and their relation with Moreton waves and radio bursts is discussed.

X-Ray Plasma Ejection Associated with an Impulsive Flare on 1992 October 5: Physical Conditions of X-Ray Plasma Ejection

The 1992 October 5 flare was associated with an X-ray plasma ejection. Although the ejected plasma looks like a blob (or plasmoid) in short-exposure images, in long-exposure images it appears to be penetrated by or connected to the top of a large-scale loop. The ejecta had started to rise with a speed of ~250 km s-1 before the main peak of the hard X-ray emission and was accelerated during the impulsive phase (to ~500 km s-1). We derived the physical parameters of the ejected plasma and obtained the following results: (1) The temperature of the ejected plasma was 10.6 ± 3.6 MK. (2) Its density was (8-16) × 109 cm-3 and was an order of magnitude larger than that of the typical active-region corona. (3) The mass of the ejected plasma was (2-4) × 1013 g. (4) The kinetic energy of the ejecta was smaller than the thermal energy content of the flare loop. The overall features and evolution of the hot plasma ejection and flare are in rough agreement with those expected from the reconnection model, and the reconnection rate (MA = Vin/VA) is estimated to be ~0.02, where Vin is the speed of the inflow into the reconnection region, and VA is the Alfven speed. Result 4, however, is not consistent with the assumption in some reconnection models that an ejected plasma stretches the overlying magnetic fields to form a current sheet and hence leads to magnetic reconnection. Instead, our results suggest that both X-ray plasma ejection and reconnection are a consequence of a common dynamical process such as the global MHD instability.

Alfvén Wave Model of Spicules and Coronal Heating

Magnetohydrodynamic simulations are performed for torsional Alfven waves propagating along an open magnetic flux tube in the solar atmosphere. It is shown that, if the root mean square of the perturbation is greater than ~1 km s-1 in the photosphere, (1) the transition region is lifted up to more than ~5000 km (i.e., the spicule is produced), (2) the energy flux enough for heating the quiet corona (~3.0×105 ergs s-1 cm−2) is transported into the corona, and (3) nonthermal broadening of emission lines in the corona is expected to be ~20 km s-1. We assumed that the Alfven waves are generated by random motions in the photosphere. As the Alfven waves propagate upward in the solar atmosphere, longitudinal motions are excited by the nonlinear couplings. The longitudinal motions propagate upward as slow or fast waves and lift up the transition region (i.e., the spicule is produced). A part of the Alfven waves are reflected in the transition region, but the remaining waves propagate upward to the corona and contribute both to the heating of the corona and the nonthermal broadening of emission lines. The result of our simulation would suggest that the quiet hot corona, nonthermal broadening of lines, and spicules are caused by Alfven waves that are generated in the photosphere.

Magnetohydrodynamic Simulation of a Solar Flare with Chromospheric Evaporation Effect Based on the Magnetic Reconnection Model

Two-dimensional magnetohydrodynamic (MHD) simulation of a solar flare including the effect of anisotropic heat conduction and chromospheric evaporation based on the magnetic reconnection model is performed. In the simulation model, the coronal magnetic energy is converted to the thermal energy of plasma by magnetic reconnection. This energy is transported to the chromosphere by heat conduction along magnetic field lines and causes an increase in temperature and pressure of the chromospheric plasma. The pressure gradient force drives upward motion of the plasma toward the corona, i.e., chromospheric evaporation. This enhances the density of the coronal reconnected flare loops, and such evaporated plasma is considered to be the source of the observed soft X-ray emission of a flare. The results show that the temperature distribution is similar to the cusp-shaped structure of long-duration-event (LDE) flares observed by the soft X-ray telescope aboard the Yohkoh satellite. The simulation results are understood by a simple scaling law for the flare temperature described as where Ttop, B, ?, and ?0 are the temperature at the flare loop top, coronal magnetic field strength, coronal density, and heat conduction coefficient, respectively. This formula is confirmed by the extensive parameter survey about B, ?0, and L in the simulation. The energy release rate is found to be described as a linearly increasing function of time: |dEm/dt| ? B2/(4?)VinCAt ? B2/(4?)0.1Ct, where Em is the magnetic energy, Vin is the inflow velocity, and CA is the Alfv?n velocity. Thus, the second time derivative is found to be |d2Em/dt2| B4. We also find that the major feature of the reconnection inflow region is the expansion wave propagating outward from the magnetic neutral point. This expanded plasma has very low emission measure, which is 4 orders of magnitude smaller than that of the brightest feature in a flare. This explains the dimming phenomena associated with flares.