Masumi Shimojo

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.

Evidence for Alfvén Waves in Solar X-ray Jets

Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed (∼800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 × 103 to 2 × 104 kilometers wide and 1 × 105 kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.

Prominence Eruptions and Coronal Mass Ejection: A Statistical Study Using Microwave Observations

We present the results of a statistical study of a large number of solar prominence events (PEs) observed by the Nobeyama Radioheliograph. We studied the association rate, relative timing, and spatial correspondence between PEs and coronal mass ejections (CMEs). We classified the PEs as radial and transverse, depending on whether the prominence moved predominantly in the radial or horizontal direction. The radial events were faster and attained a larger height above the solar surface than the transverse events. Out of the 186 events studied, 152 (82%) were radial events, while only 34 (18%) were transverse events. Comparison with white-light CME data revealed that 134 (72%) PEs were clearly associated with CMEs. We compare our results with those of other studies involving PEs and white-light CMEs in order to address the controversy in the rate of association between CMEs and prominence eruptions. We also studied the temporal and spatial relationship between prominence and CME events. The CMEs and PEs seem to start roughly at the same time. There was no solar cycle dependence of the temporal relationship. The spatial relationship was, however, solar cycle dependent. During the solar minimum, the central position angle of the CMEs had a tendency to be offset closer to the equator as compared to that of the PE, while no such effect was seen during solar maximum.

Continuous plasma outflows from the edge of a solar active region as a possible source of solar wind

The Sun continuously expels a huge amount of ionized material into interplanetary space as the solar wind. Despite its influence on the heliospheric environment, the origin of the solar wind has yet to be well identified. In this paper, we report Hinode X-ray Telescope observations of a solar active region. At the edge of the active region, located adjacent to a coronal hole, a pattern of continuous outflow of soft-x-ray–emitting plasmas was identified emanating along apparently open magnetic field lines and into the upper corona. Estimates of temperature and density for the outflowing plasmas suggest a mass loss rate that amounts to ∼1/4 of the total mass loss rate of the solar wind. These outflows may be indicative of one of the solar wind sources at the Sun.

The Magnetic Landscape of the Sun's Polar Region

We present observations of the magnetic landscape of the polar region of the Sun that are unprecedented in terms of spatial resolution, field of view, and polarimetric precision. They were carried out with the Solar Optical Telescope aboard Hinode. Using a Milne-Eddington inversion, we find many vertically oriented magnetic flux tubes with field strengths as strong as 1 kG scattered in latitude between 70° and 90°. They all have the same polarity, consistent with the global polarity of the polar region. The field vectors are observed to diverge from the centers of the flux elements, consistent with a view of magnetic fields that are expanding and fanning out with height. The polar region is also found to have ubiquitous horizontal fields. The polar regions are the source of the fast solar wind, which is channeled along unipolar coronal magnetic fields whose photospheric source is evidently rooted in the strong-field, vertical patches of flux. We conjecture that vertical flux tubes with large expansion around the photospheric-coronal boundary serve as efficient chimneys for Alfven waves that accelerate the solar wind.

Physical Parameters of Solar X-Ray Jets

We derived the physical parameters of X-ray jets and associated flares using the high-resolution data taken with the soft X-ray telescope aboard Yohkoh. We analyzed 16 X-ray jets and found the following properties of the jets and the footpoint flares: (1) the temperatures and density of the jets, respectively, are 3-8 MK (average: 5.6 MK) and 0.7-4.0 × 109 cm-3 (average: 1.7 × 109 cm-3), (2) the temperatures of the jets are similar to those of the footpoint flares, (3) the thermal energies of the jets are 1027-1029 ergs, which is to of those of the footpoint flares, (4) the apparent velocity of the jets is usually slower than the sound speed, and (5) there is a correlation between the temperatures of the jets and the sizes (square root of area) of the footpoint flares. On the basis of these results, we find that the temperatures of a jet and a footpoint flare are determined by the balance between heating flux and conductive flux and that the mass of a jet is comparable to the theoretical value based on the balance between conductive flux and enthalpy flux carried by the evaporation flow. These results suggest that X-ray jets are evaporation flows produced by the reconnection heating.

Downflow Motions Associated with Impulsive Nonthermal Emissions Observed in the 2002 July 23 Solar Flare

We present a detailed examination of downflow motions above flare loops observed in the 2002 July 23 flare. The extreme-ultraviolet images obtained with the Transition Region and Coronal Explorer show dark downflow motions (sunward motions) above the postflare loops, not only in the decay phase but also in the impulsive and main phases. We also found that the times when the downflow motions start to be seen correspond to the times when bursts of nonthermal emissions in hard X-rays and microwaves are emitted. This result implies that the downflow motions occurred when strong magnetic energy was released and that they are, or are correlated with, reconnection outflows.

Periodic Acceleration of Electrons in the 1998 November 10 Solar Flare

We present an examination of the multiwavelength observation of a C7.9 flare that occurred on 1998 November 10. This is the first imaging observation of the quasi-periodic pulsations (QPPs). Four bursts were observed with the hard X-ray telescope aboard Yohkoh and the Nobeyama Radioheliograph during the impulsive phase of the flare. In the second burst, the hard X-ray and microwave time profiles clearly showed a QPP. We estimated the Alfv?n transit time along the flare loop using the images of the soft X-ray telescope aboard Yohkoh and the photospheric magnetograms and found that the transit time was almost equal to the period of the QPP. We therefore suggest, based on a shock acceleration model, that variations of macroscopic magnetic structures, such as oscillations of coronal loops, affect the efficiency of particle injection/acceleration.

Flare Ribbon Expansion and Energy Release Rate

We have examined the relation between the evolution of the Hα flare ribbons and the released magnetic energy in a solar flare that occurred on 2001 April 10. Based on the magnetic reconnection model, the released energy was quantitatively calculated by using the photospheric magnetic field strengths and separation speeds of the fronts of the Hα flare ribbons. We compared the variation of the released energy with the temporal and spatial fluctuations in the nonthermal radiation observed in hard X-rays and microwaves. These nonthermal radiation sources indicate when and where large energy releases occur. We also estimated the magnetic energy released during the flare. The estimated energy release rates in the Hα kernels associated with the hard X-ray sources are locally large enough to explain the difference between the spatial distributions of the Hα kernels and the hard X-ray sources. We also reconstructed the peaks in the nonthermal emission by using the estimated energy release rates.

RECONNECTION RATE IN THE DECAY PHASE OF A LONG DURATION EVENT FLARE ON 1997 MAY 12

Recent analyses of long duration event (LDE) flares indicate successive occurrences of magnetic reconnection and resultant energy release in the decay phase. However, quantitative studies of the energy release rate and the reconnection rate have not yet been made. In this paper we focus on the decay phase of an LDE flare on 1997 May 12 and derive the energy release rate H and the reconnection rate MA = vin/vA, where vin is the inflow velocity and vA is the Alfven velocity. For this purpose, we utilize a method to determine vin and the coronal magnetic field Bcorona indirectly, using the following relations: where Ar, Bfoot, and vfoot are the area of the reconnection region, the magnetic field strength at the footpoints, and the separation velocity of the footpoints, respectively. Since H, Ar, vfoot, and Bfoot are obtained from the Yohkoh Soft X-Ray Telescope data and a photospheric magnetogram, vin and Bcorona can be determined from these equations. The results are as follows: H is ~1027 ergs s-1 in the decay phase. This is greater than 1/10th of the value found in the rise phase. MA is 0.001-0.01, which is about 1 order of magnitude smaller than found in previous studies. However, it can be made consistent with the previous studies under the reasonable assumption of a nonunity filling factor. Bcorona is found to be in the range of 5-9 G, which is consistent with both the potential extrapolation and microwave polarization observed with the Nobeyama Radioheliograph.