Yohkoh Soft X-ray Telescope Research Articles

Restoration and interpretation of images from the Yohkoh Soft X-Ray Telescope

Due to the finite width of the point spread function (PSF) of the Yohkoh Soft X-Ray Telescope (SXT) mirror, a certain amount of blurring effect is inevitable in the observed soft X-ray images. Thus it is necessary to subtract its effect from the observed images for both morphological and photometric purposes. Because of undersampling due to a large pixel size of the CCD camera of SXT, however, the application of general algorithms to image restoration has so far been unsatisfactory for the SXT images. Therefore, we have developed a new method for increasing the sampling density of Yohkoh SXT images under the condition of photon number conservation in each pixel. The correction for the undersampling effect was also necessary to obtain a good estimate of the PSF from the ground experimental data. Based on this best-fit PSF, we apply the maximum entropy method algorithm to the observed SXT images. The restored images succeed in showing fine details in coronal loops, but there still remains the problem of insufficient noise suppression.

Structure and dynamics of the corona surrounding an eruptive prominence

We report on the 1997 December 14 prominence eruption event that was accompanied by eruptive signatures in X-rays, EUV and white light: coronal dimming, X-ray arcade formation, X-ray brightenings, EUV eruption, and a white light CME. The data used were obtained by the Nobeyama Radioheliograph, Yohkoh Soft X-ray Telescope (SXT) and SOHO/LASCO and EIT. We identified various substructures of the eruption and their inter-relationships. We found that the pre-disruption swelling of the equatorial streamer was caused by the outward displacement of the coronal material around the prominence location. The dynamical behaviors of the CME and the accompanying eruptive prominence seem to be very different.

The Sun as an X‐Ray Star. I. Deriving the Emission Measure Distribution versus Temperature of the Whole Solar Corona from theYohkoh/Soft X‐Ray Telescope Data

The scope of this work is to obtain the emission measure distributions versus temperature, EM(T ), of the whole solar corona from Yohkoh Soft X-ray Telescope images. As discussed in Paper II, the EM(T ) is our starting point for studying the Sun as an X-ray star. To this purpose, we need to extract as much information as possible from the Yohkoh/SXT data covering the whole range of the Yohkoh/SXT tem- perature sensitivity, i.e., 5.5 log T (K) 8. In particular at low photon counts and temperatures below 106 K, errors on the temperature and emission measure determination are expected to be large. To this end, we have made an extensive set of simulations to explore the nominal performance of the entire system (instrument and data analysis system) in the determination of the plasma temperature and emis- sion measure at low, intermediate, and high photon counts per pixel. We have shown that low-count data with a number of photons per pixel are aUected by large errors and lead to the deriva- n phot 10 tion of an unrealistic EM(T ) characterized by a steep negative slope. As a result, we have devised an analysis method that minimizes the instrumental and statistical eUects on the determination of EM(T ) and allows us to determine the global coronal EM(T ). As a —rst application to real SXT data, we have derived the EM(T ) of the Sun close to the maximum of the solar cycle, a challenging case. The low- temperature part is in agreement with analogous studies made in the UV band, and it shows a well- de—ned maximum at T D 2 MK. Subject headings: Sun: activitySun: coronaSun: X-rays, gamma rays

Determination of the point spread function of the Yohkoh Soft X-Ray Telescope

We re-evaluated the point spread function (PSF) of the Yohkoh Soft X-Ray Telescope (SXT). The best-fit model to the ground experimental data can be obtained by considering the undersampling effect at the central area of the PSF. Our result shows that the core of the Moffat function which models the PSF begins to be smoothly connected to the scattering wing at the distance of about 15″ from the peak. Although the available experimental data are limited, first-order vector perturbation theory helps us to consider the wavelength dependence of the scattering wing of the PSF. The autocovariance function of the SXT mirror surface roughness is expressed in our study by the product of exponential function and the modified Bessel function. The calculated angular distribution of the scattering component is compared satisfactorily with the result of in-flight data analysis. The results of the wavelength dependence of the PSF are discussed in detail.

On Heating the Sun’s Corona by Magnetic Explosions: Feasibility in Active Regions and Prospects for Quiet Regions and Coronal Holes

We build a case for the persistent strong coronal heating in active regions and the pervasive quasi-steady heating of the corona in quiet regions and coronal holes being driven in basically the same way as the intense transient heating in solar flares: by explosions of sheared magnetic fields in the cores of initially closed bipoles. We begin by summarizing the observational case for exploding sheared core fields being the drivers of a wide variety of flare events, with and without coronal mass ejections. We conclude that the arrangement of an event's flare heating, whether there is a coronal mass ejection, and the time and place of the ejection relative to the flare heating are all largely determined by four elements of the form and action of the magnetic field: (1) the arrangement of the impacted, interacting bipoles participating in the event, (2) which of these bipoles are active (have sheared core fields that explode) and which are passive (are heated by injection from impacted active bipoles), (3) which core field explodes first, and (4) which core-field explosions are confined within the closed field of their bipoles and which ejectively open their bipoles. We then apply this magnetic-configuration framework for flare heating to the strong coronal heating observed by the Yohkoh Soft X-ray Telescope in an active region with strongly sheared core fields observed by the MSFC vector magnetograph. All of the strong coronal heating is in continually microflaring sheared core fields or in extended loops rooted against the active core fields. Thus, the strong heating occurs in field configurations consistent with the heating being driven by frequent core-field explosions that are smaller but similar to those in confined flares and flaring arches. From analysis of the thermal and magnetic energetics of two selected core-field microflares and a bright extended loop, we find that (1) it is energetically feasible for the sheared core fields to drive all of the coronal heating in the active region via a staccato of magnetic microexplosions, (2) the microflares at the feet of the extended loop behave as the flares at the feet of flaring arches in that more coronal heating is driven within the active bipole than in the extended loop, (3) the filling factor of the X-ray plasma in the core field microflares and in the extended loop is approximately 0.1, and (4) to release enough magnetic energy for a typical microflare (10^27 - 10^28 erg), a microflaring strand of sheared core field need expand and/or untwist by only a few percent at most. Finally, we point out that (1) the field configurations for strong coronal heating in our example active region (i.e., neutral-line core fields, many embedded in the feet of extended loops) are present in abundance in the magnetic network in quiet regions and coronal holes, and (2) it is known that many network bipoles do microflare and that many produce detectable coronal heating. We therefore propose that exploding sheared core fields are the drivers of most of the heating and dynamics of the solar atmosphere, ranging from the largest and most powerful coronal mass ejections and flares, to the vigorous microflaring and coronal heating in active regions, to the multitude of fine-scale explosive events in the magnetic network. The low-lysing exploding core fields in the network drive microflares, spicules, global coronal heating, and ,consequently, the solar wind.

Relation between Flare-associated X-Ray Ejections and Coronal Mass Ejections.

In an attempt to identify the direct signatures of coronal mass ejections (CMEs) in soft X-ray wavelengths, we have searched for plasma ejections in Yohkoh soft X-ray telescope (SXT) images in a total of 17 limb flares, and compared the results with the Solar and Heliospheric Observatory LASCO data. A general correlation exists between the presence/absence of the X-ray ejection and the CME. Although the height versus time relation often indicates (under the assumption of constant speed) that the CME onset coincides with the X-ray ejection, the latter probably does not represent the CME front, because the CME speed must result from acceleration, which would put the estimated onset at an earlier time. In some cases, the estimated CME onset time comes well before the impulsive phase of the associated flare. Although the role of the flare-associated plasma ejection in a CME is still unclear, we propose that its occurrence depends on the presence of open field lines, which can be due to a preceding CME. Lastly, we present a rare example of SXT observations of what appeared to be the three-part structure of a CME, which was seen a few minutes before a major flare started.

Emission Measure Distribution for an Active Region Using Coordinated SERTS andYohkohSXT Observations

Often the derived temperature of an active region re—ects the method and the nature of the instrument used in its measurement. The emission measure (i.e., the amount of emitting material) derived from spec- troscopic observations usually depends on assumptions about the absolute elemental abundances and ionization fractions of the emitting ions. Yet establishing the distribution of emission measure with tem- perature is the —rst step needed to proceed with most of the interesting physics of active regions¨ including heating processes, cooling timescales, and loop stability. Accurately characterizing the thermal distribution of the coronal plasma requires data which can resolve multithermal features and constrain both low- and high-temperature emission. To model the temperature distribution of NOAA Active Region 7563, we have combined broadband —lter data from the Yohkoh Soft X-Ray Telescope (SXT) with simultaneous spectral line data from the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) taken during its —ight on 1993 August 17. We have used a forward-folding technique to deter- mine the emission measure distribution of the active region loops. We have found that (1) the SXT response functions are sensitive to both the elemental abundances and the ionization fractions assumed to compute the solar spectrum that is folded through the instrument eUective area; (2) the relative cali- bration between the SERTS and the SXT instruments must be adjusted by a factor of 2 (a value consis- tent with the absolute measurement uncertainty of the 1993 SERTS —ight) no matter which abundances or iron ionization fractions are used; (3) the two-peaked diUerential emission measure previously deter- mined using SERTS data alone is not consistent with the SXT data: including the SXT data as a high- temperature constraint in the analysis requires that the emission above about 3 MK drop oU steeply rather than extending out to 6 MK. The sensitivity of the SXT —lter response functions to elemental abundance and iron ionization fraction could have a major impact on many routine analyses of SXT data. The emission measures can be greatly aUected (up to a factor of 7) and temperatures derived from —lter ratios can be signi—cantly altered (up to at least 40%) by adopting diUerent sets of commonly used elemental and ionic abundances. The results of our multithermal analysis imply that using broadband SXT data or a comparable high-temperature constraint in conjunction with high-resolution spectra covering a wide lower temperature range to study solar active regions can signi—cantly improve the information derived from either data set alone. In this study, the revised multithermal distribution reduces the thermal energy content of the region by about a factor of 2 and the required heating by about a factor of 5, which in turn relaxes some constraints on possible heating models. Subject headings: Sun: activitySun: X-rays, gamma rays

Radio and X‐Ray Imaging Observations of a Continuum Burst

We study a metric continuum burst observed on 1993 February 18, and its X-ray signatures from imaging observations in radio and X-rays using the Nancay radioheliograph and the Yohkoh Soft X-Ray Telescope (SXT). The event in question was associated with weak type III bursts; these were detected at only one frequency (164 MHz), except for one burst (at 10:58:05 UT), which was observed over a broad frequency range (164-435 MHz). We believe that the early metric continuum burst is an extension of the microwave continuum which was observed at frequencies as high as 5 GHz, and its onset at ~10:50 UT is associated with the development of an X-ray-emitting diffuse loop system which appears to advance with a speed of ~50-100 km s-1. The observed type III bursts seem to correspond to the repeated occurrence/appearance of a collimated jet emanating from the loop system that is responsible for the continuum burst. A few minutes prior to the main continuum onset there is a soft X-ray ejection from the main flare region. The main continuum has a brightness temperature greater than 108 K; it is unpolarized, and it shows dispersion in position with frequency and moves with speeds of ~50 km s-1 at 236-410 MHz. The SXT images reveal that this initially ejected soft X-ray-emitting hot plasma seems to gradually fill up the loop system with hot material. This hot plasma must contain enough energetic electrons of energy greater than several tens of keV, which are responsible for producing the metric continuum burst by plasma radiation mechanism.

Occurrence Rate of Microflares in an X‐Ray–bright Point within an Active Region

We examine the occurrence rate of microflares (transient brightenings) in an X-ray bright point (XBP) during the lifetime of the XBP observed with the Yohkoh soft X-ray telescope (SXT). The XBP with a size of ~30'' appeared near a preceding spot of NOAA 7270 on 1992 September 1 and disappeared on 1992 September 4. The XBP produced 92 microflares during the observation time of the SXT. We found that the occurrence rate (number of events per hour) did not change much during the lifetime of the XBP, although the magnetic flux of the XBP changed. We also found that the frequency distribution of microflares as a function of the soft X-ray peak intensity shows a power law with index 1.7 ± 0.4, which is consistent with the previously known index for flares and microflares based on the data for the whole Sun or a whole active region. This result suggests that the power-law distribution of flares holds not only for larger areas but also for smaller areas.

The Solar Flare Soft X‐Ray Differential Emission Measure and the Neupert Effect at Different Temperatures

Data from the Yohkoh Soft X-ray Telescope (SXT) and Bragg Crystal Spectrometer (BCS) are used to obtain the differential emission measure (DEM) of solar flare soft X-ray plasma. The maximum entropy method is used to obtain the DEM for 80 flares observed by SXT and BCS from 1991 October to 1992 November. We find that it is often difficult to describe the DEM using a monotonically decreasing function; functions with a hump at high temperature are occasionally more useful. From the time history of the DEM, we show that higher temperature plasma peaks earlier and cools faster. We also show that high-temperature plasma (>16.5 MK) is more likely than low-temperature plasma to exhibit the so-called Neupert effect, in which the time derivative of the soft X-ray emission measure is similar to the light curve of the impulsive hard X-ray emission for the flare.

Soft X-Ray Coronal Structure Surrounding High-Latitude Dark Filaments Observed by Yohkoh

Abstract The faint coronal structure existing prior to high-latitude X-ray arcade formation oberved by the Yohkoh Soft X-ray Telescope has turned out to be “overlapped dual arcades” whose inside legs cross with one another, rather than being a simple bipolar arcade whose top part is sagging due to loading of the dark filament material, as considered in previous models of dark filament suspension. The photospheric magnetic field under the crossed legs is found to be a belt of mixed polarity, extending tens of thousand kilometers, rather than a clean border of the two extended unipolar regions. The magnetic field distribution, as well as the coronal structure above it, are very different from the previously assumed situation for dark filaments based on the assumption of a bipolar photospheric magnetic field distribution. The coronal structure found in the region, the “overlapped dual arcades” structure, is most likely to be the “separatrix surfaces” in the quadruple photospheric magnetic source model for the magnetic configuration surrounding a dark-filament, as proposed by one of the present authors in his previous work (Uchida 1980, AAA 28.073.129).

FexviiSoft X‐Ray Lines: Theory and Data Comparisons

Fe XVII soft X-ray spectral emission lines are examined using data from the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission satellite. Results are compared with theoretical calculations and with other recent observational results. Disparate findings from different studies on the inferred opacity of the bright resonance line at 15.01 A and on its center-to-limb behavior are reviewed. Present limitations on the use of resonance scattering to infer coronal plasma densities and absolute elemental abundances are discussed. An analysis is made of the temperature-insensitive ratio of the 15.01 A line of Fe XVII to the optically thin 16.78 A line. This analysis shows that approximately half of the photons expected in the 15.01 A line are missing from the bright emission cores of quiescent active regions on the solar disk; the missing fraction increases at most by 50% near the solar limb. If the missing flux has been resonantly scattered out of the line of sight, then the equivalent optical depth at line center of the 15.01 A line is τ0~2 on the disk, based on a simple escape probability treatment for a slab geometry. This suggests that the effects of resonance scattering for other FCS lines, with the possible exception of the O VIII doublet at 18.97 A, should be negligible for quiescent active region conditions. This is consistent with the lack of systematic center-to-limb dependence found previously for FCS lines other than Fe XVII at 15.01 A. Yohkoh Soft X-ray Telescope measurements of the expected lines of sight for active regions as a function of location on the solar disk, and resonance scattering results from other soft X-ray active region data sets all support a trend of increased opacity at the limb compared to disk center.

The Global Dynamics of the High-Temperature Corona

This paper surveys coronal motions detected by the Yohkoh soft X-ray telescope SXT, emphasizing ‘global restructuring’. Large-scale structures in the solar corona can persist for time scales much longer than those of the supergranulation, and may have larger spatial scales. Flares and coronal mass ejections (CMEs) can disrupt these structures. A grazing-incidence telescope such as SXT provides a view of the corona biased in temperature towards the hotter components, but in a thick ‘observing slice’ (spatial contribution function). This helps in seeing excitation (energy release) but may conceal some of the structural changes. The observations of restructuring largely appear to consist of expanding or outwards magnetic motions, which are endoergic. This suggests a conjecture regarding the existence of magnetic implosions on scales not yet detected, as a source of free energy.

Heliospheric tomography using interplanetary scintillation observations: 1. Combined Nagoya and Cambridge data

We have produced a computer assisted tomography program that optimizes a three‐dimensional model to fit observational data. We have used this program with interplanetary scintillation data from Nagoya, Japan, and Cambridge, England. The program iterates to a least squares solution fit of observed data using solar rotation and solar wind motion to provide perspective views of each point in space accessible to the observations. We plot the optimized model as Carrington maps in velocity V and density Ne for the two data sets with resolutions of 10° in heliographic longitude and latitude. We map the model to 1 AU and compare this to in situ observations from the IMP spacecraft. From this comparison we find ΔNe ∝ Ne0.3. We plot Carrington maps extrapolated to the solar surface to compare with Yohkoh Soft X ray Telescope (SXT), Sacramento Peak green line, and Mark III K‐coronameter observations. High velocities modeled at the solar surface for individual rotations trace coronal holes (including polar ones) observed in SXT data. Regions of high density modeled from the Cambridge scintillation level data generally show a high correlation with regions of high solar activity observed as bright in Yohkoh SXT and green line observations. There is also a general correspondence of the regions of high density and the areas which are bright in K‐coronameter observations.

Axial Soft X-ray Brightening Associated with Hα Filaments

AbstractWe have studied coronal soft X-ray (SXR) arcades around filament channels, as observed with the Yohkoh Soft X-ray Telescope (SXT). We have found about a dozen events (1991–1993). Comparisons with Hα filaments have revealed an association between some SXR structures and filaments. These bright thread-like SXR structures (often) appear to be parallel to and co-spatial with Hα filaments. The SXR structures are occasionally maintained for days, although transient brightening of axial structures is more common.

Evidence of Magnetic Field Reconnection in the Hα Eruptive Prominence on 18 september 1995

In this paper we present a detailed study of a violent evolution of the 18 September 1995 eruptive prominence observed by the Hα telescope and the Multichannel Optical Flare Spectrograph in Ondřejov. The fast changes of the prominence structure started immediately after a weak radio burst at 3 GHz. This circumstance shows the presence of non-thermal processes. In the later phase of the prominence evolution a comparison of the Hα filtergrams with the Yohkoh Soft X-ray Telescope pictures was made. For a search of fine structures in the Hα images, an image processing technique was used. A detailed analysis of observations indicates magnetic field line reconnection, mainly in space below the rising Hα prominence. These reconnection processes are manifested not only by structural changes of the Hα prominence and X-ray loops but also by the character of Doppler velocities. Evidence of splitting and rotation was found in the Hα spectrum formed close to the reconnection space, and the typical velocities of such plasma movement were evaluated. We estimated amplitudes of rotational velocities, giving evidence about the rearrangement of helical structures during the process of the eruptive prominence activation. In the conclusion we discuss some implications of our results.

YohkohSoft X‐Ray Determination of Plasma Parameters in a Polar Coronal Hole

The Yohkoh Soft X-Ray Telescope (SXT) has been used to study the emission from a coronal hole surrounding the north pole of the Sun. Stronger emission from closed coronal structures in the line of sight can interfere with attempts to measure properties of coronal hole plasma. SXT observations indicate that the north polar region was free of such contamination on 1992 October 3. Measured X-ray intensities, corrected for background and scattered X-rays, are compared with a theoretical coronal hole model. They are found to be broadly consistent with the model predictions for variation of intensity with height and for limb brightening, although the electron density is lower than would be appropriate for model predictions based on solar maximum densities. Electron temperatures estimated by the filter ratio method are also consistent with the model and with an in situ estimate of the maximum electron temperature in the solar wind by the Ulysses ionic charge composition experiment.

Deep Survey of Solar Nanoflares withYohkoh

The Yohkoh Soft X-Ray Telescope has observed numerous transient brightenings of compact active-region coronal loops (microflares). Shimizu (1995) found that the distribution function of the total energy of transient brightenings is a decreasing power-law with an index (1.5 ~ 1.6) in the energy range greater than 1027 erg, suggesting that the large number of small brightenings is energetically less important than the few large ones. However, the distribution function for smaller brightenings (< 1027 erg) is still unknown. This paper performs a deep search for smaller brightenings (termed nanoflares, Parker 1988) using the soft X-ray position-dependent time profiles. For details, please refer to the full-length paper published in the 1997 September issue of the Astrophysical Journal with the same title (Shimizu and Tsuneta 1997).

Moving Plasmoid and Formation of the Neutral Sheet in a Solar Flare

A spectacular erupting feature with a plasmoid-like structure is observed before and during the solar flare that occurred on the limb on 1991 December 2 with the Yohkoh soft X-ray telescope. The rise of a loop structure starts ~10 min before the flare, evolving to a plasmoid-like structure in the impulsive phase of the flare. The speed of the rising loop (plasmoid) is almost constant (~96 km s-1) throughout the observation. A clear X-shaped structure is formed underneath the rising plasmoid, and a bright soft X-ray loop is formed below the X-point. The X-shaped structure indicates a magnetic neutral point with a large-scale magnetic separatrix structure. Inverse-V-shaped high-temperature ridges are located above the soft X-ray loop and below the X-point. We interpret these as reconnected loops heated by slow shocks. A moving high-temperature (15 MK) source is found, coincident in position with the rising structure above the X-point. A hard X-ray source (33-53 keV) is located at the top of the soft X-ray flare loop. These two compact high-temperature sources located above and below the X-point would be formed by fast shocks due to the symmetric reconnection outflows both upward and downward from the X-point.

SEARCH FOR PERIODICITY IN IMAGE SEQUENCES FROM THE YOKOH SOFT X-RAY TELESCOPE

ABSTRACT The process by which the Sun's corona is heated is not well known. A leading model is the resonant absorption of Alfvénic surface waves. In order to be efficient as a heating mechanism, this process depends on the surface waves being well matched ot the length of the coronal loop through the condition of global-mode resonance: vAlfv\\acute{en} = 2L/period (Davila, 1987). The Yohkoh Soft X-Ray Telescope (SXT) provides a means of recording the behavior and evolution of structures in the solar corona. Relatively fast-cadence sequences of CCD images become movies depicting the changing conditions in the X-ray emitting regions. By using software provided by the Yohkoh science team to measure the brightness of a selected subset of pixels in all the images of a movie, we can generate an X-ray light curve of a small portion of a given coronal structure. For the present dissertation, a number of SXT image sequences were selected from the Yohkoh archives. Light curves were generated in the manner outlined above, and these light curves were then examined for periodic modulations. It is shown that periodic modulations of X-ray brightness were detected with amplitudes of nearly 1%, and that the speeds inferred from interpreting these oscillations as global-mode resonance are in the neighborhood of 1800 -- 14000 km/s, consistent with Alfvén wave speeds but much higher than sound speeds implied by the temperatures measured from the images. The wave speeds and the modulation amplitudes imply plasma beta values in the range βp \\approx 10-3 -- 10-2.