Kaori Nagashima

Systematic Center-to-limb Variation in Measured Helioseismic Travel Times and its Effect on Inferences of Solar Interior Meridional Flows

We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m s{sup -1} slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior and needs to be better understood.

Observations of Sunspot Oscillations in G Band and Ca II H Line with Solar Optical Telescope on Hinode

Exploiting high-resolution observations made by the Solar Optical Telescope onboard Hinode, we investigate the spatial distribution of power spectral density of oscillatory signal in and around NOAA active region 10935. The G-band data show that in the umbra the oscillatory power is suppressed in all frequency ranges. On the other hand, in Ca II H intensity maps oscillations in the umbra, so-called umbral flashes, are clearly seen with the power peaking around 5.5 mHz. The Ca II H power distribution shows the enhanced elements with the spatial scale of the umbral flashes over most of the umbra but there is a region with suppressed power at the center of the umbra. The origin and property of this node-like feature remain unexplained.

Triggering Mechanism for the Filament Eruption on 2005 September 13 in NOAA Active Region 10808

On 2005 September 13 a filament eruption accompanied by a halo coronal mass ejection (CME) occurred in the most flare-productive active region, NOAA 10808, in solar cycle 23. Using multiwavelength observations before the filament eruption on September 13, we investigate the processes leading to the catastrophic eruption. We find that the filament slowly ascended at a speed of 0.1 km s-1 over 2 days before the eruption. During slow ascension, many small flares were observed close to the footpoints of the filament, where new magnetic elements were emerging. On the basis of the observational facts, we discuss the triggering mechanism leading to the filament eruption. We suggest that the process toward the eruption is as follows. First, a series of small flares played a role in changing the topology of the loops overlying the filament. Second, the small flares gradually changed the equilibrium state of the filament and caused the filament to ascend slowly over 2 days. Finally, a C2.9 flare that occurred when the filament was close to the critical point for loss of equilibrium directly led to the catastrophic filament eruption right after it.

Statistical study of the reconnection rate in solar flares observed with Yohkoh SXT

We report a statistical study of flares observed with the Soft X-Ray Telescope (SXT) on board Yohkoh in the year 2000. We measure physical parameters of 77 flares, such as the temporal scale, size, and magnetic flux density, and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationships among these parameters and find weak correlation between the temporal and spatial scales of the flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using the observed values. The inflow velocities are distributed from a few km s-1 to several tens of km s-1, and the Alfven velocities in the corona are in the range from 103 to 104 km s-1. Hence, the reconnection rate is 10-3 to 10-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within 1 order of magnitude of the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.

DETECTION OF SUPERGRANULATION ALIGNMENT IN POLAR REGIONS OF THE SUN BY HELIOSEISMOLOGY

We report on a new phenomenon of “alignment” of supergranulation cells in the polar regions of the Sun. Recent high-resolution data sets obtained by the Solar Optical Telescope on board the Hinode satellite enabled us to investigate supergranular structures in high-latitude regions of the Sun. We have carried out a local helioseismology time–distance analysis of the data and detected acoustic travel-time variations due to the supergranular flows. The supergranulation cells in both the north and south polar regions show systematic alignment patterns in the north–south direction. The south-pole data sets obtained in a month-long Hinode campaign indicate that the supergranulation alignment property may be quite common in the polar regions. We also discuss the latitudinal dependence of the supergranulation cell sizes; the data show that the east–west cell size decreases toward higher latitudes.

Helioseismic Signature of Chromospheric Downflows in Acoustic Travel-Time Measurements from Hinode

We report on a signature of chromospheric downflows in two emerging flux regions detected by time-distance helioseismology analysis. We use both chromospheric intensity oscillation data in the Ca II H line and photospheric Dopplergrams in the Fe I 557.6 nm line obtained by Hinode/SOT for our analyses. By cross-correlating the Ca II oscillation signals, we have detected a travel-time anomaly in the plage regions; outward travel times are shorter than inward travel times by 0.5-1 minute. However, such an anomaly is absent in the Fe I data. These results can be interpreted as evidence of downflows in the lower chromosphere. The downflow speed is estimated to be below 10 km s-1. This result demonstrates a new possibility of studying chromospheric flows by time-distance analysis.

The amplitude of the cross-covariance function of solar oscillations as a diagnostic tool for wave attenuation and geometrical spreading

Context. In time-distance helioseismology, wave travel times are measured from the two-point cross-covariance function of solar oscillations and are used to image the solar convection zone in three dimensions. There is, however, also information in the amplitude of the cross-covariance function, for example about seismic wave attenuation. Aims. Here we develop a convenient procedure to measure the amplitude of the cross-covariance function of solar oscillations. Methods. In this procedure, the amplitude of the cross-covariance function is linearly related to the cross-covariance function and can be measured even for high levels of noise. Results. As an example application, we measure the amplitude perturbations of the seismic waves that propagate through the sunspot in active region NOAA 9787. We can recover the amplitude variations due to the scattering and attenuation of the waves by the sunspot and associated finite-wavelength effects. Conclusions. The proposed definition of cross-covariance amplitude is robust to noise, can be used to relate measured amplitudes to 3D perturbations in the solar interior under the Born approximation, and will provide independent information from the travel times.

Local Helioseismology Analyses with Hinode/SOT Datasets

The solar internal structure and dynamics have been probed by examining the oscillations of the Sun, or by “listening to the Sun”; helioseismology provides us with unique tools for these analyses. Starting from a brief introduction on local helioseismology, in this article I will describe what were our goals and what has been achieved, in learning from the local helioseismology analyses using observations by the Solar Optical Telescope aboard Hinode. This includes the detection of helioseismic signatures of flows in the solar atmosphere, study of supergranular cell structures in the solar polar regions, and sunspot seismology.

Probing sunspots with two-skip time–distance helioseismology

Previous helioseismology of sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure. We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated. We study waves reflected almost vertically from the underside of a sunspot. Time-distance helioseismology was used to measure travel times for the waves. Ray theory and a detailed sunspot model were used to calculate travel times for comparison. It is shown that these large distance waves are insensitive to the magnetic field in the sunspot. The largest travel time differences for any solar phenomena are observed. With sufficient modeling effort, these should lead to better understanding of sunspot structure.

Statistics of the two-point cross-covariance function of solar oscillations

Context: The cross-covariance of solar oscillations observed at pairs of points on the solar surface is a fundamental ingredient in time-distance helioseismology. Wave travel times are extracted from the cross-covariance function and are used to infer the physical conditions in the solar interior. Aims: Understanding the statistics of the two-point cross-covariance function is a necessary step towards optimizing the measurement of travel times. Methods: By modeling stochastic solar oscillations, we evaluate the variance of the cross-covariance function as function of time-lag and distance between the two points. Results: We show that the variance of the cross-covariance is independent of both time-lag and distance in the far field, i.e., when they are large compared to the coherence scales of the solar oscillations. Conclusions: The constant noise level for the cross-covariance means that the signal-to-noise ratio for the cross-covariance is proportional to the amplitude of the expectation value of the cross-covariance. This observation is important for planning data analysis efforts.