# Yohkoh Thesis Topics

## Doctor Thesis Topics

### Current:

(a) R. Kano (b) M. Weber (c) A. Phillips (d) S. Savy (e) C. Foley (f) K. Yaji (g) M. Sersen (h) M. Takahashi (i) M. Ohyama (j) K. Hori

### History:

(A) Bachtiar Anwar (Completed March 1993) (B) Karen L. Harvey (Completed September 1993) (C) Taro Sakao (Completed March 1994) (D) Satoshi Masuda (Completed March 1994) (E) Mika Inda-Koide (Completed March 1994) (F) Lisa Porter (Completed June 1994) (G) Michal Tomczak (Completed September 1994) (H) Kimerly Dawn Leka (Completed November 1994) (I) Toshifumi Shimizu (Completed March 1995) (J) Leila Belkora (Completed September 1995) (K) Adriana Silva (Completed October 1995) (L) Elizabeth Newton (Completed January 1996) (M) Mike Rilee (Completed January 1996) (N) Hideki Koshiishi (Completed March 1996) (O) Hirohisa Hara (Completed April 1996) (P) S\"am Krucker (September 1996) (Q) David McKenzie (October 1996)

### (a) R. Kano

Temperature distribution and heating function along the coronal loops, and time variability of the 'steady' coronal loops

### R. Kano (Institute of Astronomy, Univ. of Tokyo)

This project is the doctor thesis topics of R. Kano. Thesis adviser: S. Tsuneta (Institute of Astronomy, Univ. of Tokyo)

### Data used: May-July, 1992 SXT PFI

Motivation: The loop structure is the element of the solar corona, as shown by the Skylab X-ray telescope. Thus, in order to reveal the coronal heating mechanism, we study the coronal loops heating. For the most events without flares, the conductive loss dominates the radiative loss. This means that the temperature distribution of the coronal loops reflects the heating function. We, therefore, analyze the temperature distributions of many coronal loops along the loop, and try to obtain the heating function along the loop. We also investigate the time variability of the 'steady' coronal loops.

Data and Method: We will pick up the coronal loops mainly from the SXT images from May 1992 through July 1992 as many as possible. 1. We will obtain the temperature distribution along the coronal

   loops. If we can neglect the radiative loss, the total energy
flux, Ft is proportional to the gradient of T^3.5;
Ft =  2 * 2ko/7*dT^3.5/ds.
From the preliminary results, there seems to be a correlation
between Ft and the X-ray flux, Lx, of the loops.
This and another statistical parameters like time variability of

2. For the good S/N loops, we will try to obtain coronal heating

   function Eh(s) from the curvature of the temperature along the
Eh = 2ko/7*(d/ds)^2 T^3.5,
which will be compared with some heating models.

3. These coronal parameters is compared with the information on the photospheric condition obtained with the LaPalma data.

Progress report: The framework of the item 2 is well developed in the master thesis of R. Kano. The initial result of the item 1. will be presented in the annual ASJ meeting in the autumn of 1994.

### COLLABORATORS: M. Weber, D. Alexander, L. Acton (MSU)

OBJECTIVE: Studies of coronal holes have make it appear that the corona rotates nearly as a solid body. This is a surprising finding because the photosphere, and magnetic structures rooted in the solar surface, are know to rotate differentially. We will use the SXT full disk images to investigate the rotation period of the sun as a function of solar latitude. A new data base, similar to the SXL data base, is being prepared from SFD images which will divide the sun into a number of sectors defined by longitude and latitude. Intensity histograms will be prepared for each image for each of these sectors. Time series analysis of these data will provide new information on the solar coronal rotational period. Other interesting periodicities may also be discovered.

### Update 13-May-95

Previously, this research had only spanned a twelve-week period of the Yohkoh SXT data. Currently, we are looking at a span of a year-and-a-half. This extension of the time range has affected our analysis of the power-spectra of the data, and we have been developing new tools for this analysis. Specifically, we are using the Lomb-Scargle periodogram technique, which is well suited to analysis of unevenly sampled data. This approach also allows for estimation of the "significance" of a particular peak in the power spectrum. We intend to present a paper at the 1995 SPD meeting in Memphis on this topic; the submitted abstract is given below.

### DIFFERENTIAL ROTATION IN THE SOLAR CORONA

Mark Weber, David Alexander, and Loren W. Acton Department of Physics, Montana State University, Bozeman, MT 59717

### ABSTRACT:

The SXT instrument on board the Yohkoh satellite has been observing the solar corona for over three years. This large data base with its concomitant diagnostic information provides an unprecedented look at the global dynamics of the solar corona. It has been shown in previous works that different tracers of solar rotation, each sensitive to a different part of the solar atmosphere, yield varing results for the latitude dependence of the rotation rate; the differential rotation measured using photospheric structures is markedly different from that obtained using coronal tracers.

The long term observations of the solar corona by the SXT make it ideal for the investigation of coronal differential rotation. In this paper we will use the soft X-ray emission of the solar corona to trace out the rotation rate at different latitudes. This is done by dividing the solar disk into a number of latitude strips and carrying out a power-spectrum analysis of the total soft X-ray intensity in each strip (using the Lomb-Scargle algorithm for unevenly-spaced data) over a two year period of the Yohkoh observations. The results of this analysis are compared with the differential rotation rates obtained from other coronal tracers.

### (e) C. Foley

Coronal and Solar Wind Observations of the Sun with the Yohkoh Soft X-Ray Telescope

The observations of the sun with Skylab, revealed that coronal loops form the base building block of the corona. The observations however couldn't resolve the the finer scale structure due to the response of the telescope, much of the corona appearing diffuse. Observations with Yohkoh satellite has revealed the structure in the solar corona even further, there are still regions which appear diffuse with no apparent structure though. It is the the origin of these regions with respect to closed field structures and open field low speed solar wind sources which is first investigated. It is demonstrated that the temperature and emission measure characteristics are consistent with the RTV loop scaling laws, when atmospheric stratification is considered. However comparison of the regions of interest with the MLSO coronagraphs reveal that these diffuse regions often lay at the base of streamers. The frozen in ion temperatures for the slow speed solar wind observed with the swoops instrument aboard Ulysees are consistent with the range of temperatures found for these diffuse regions observed with yohkoh.

Since Kreiger first trace a high speed solar wind stream back to the Sun, coronal holes have been believed to be the source regions of the high speed solar wind. Coronal holes have been the subject of many subsequent observations in an effort to determine the empirical contraints on the physical parameters of the source region of the high speed solar wind. These observations have demonstrated the temperatures in the region of 1-1.5MK and densities a factor of ten less than surrounding quiet regions exist. Yohkoh observations of coronal holes by Hara et al 1995, demonstrated that low latitude coronal holes located at disk center possess tempertures comparable to the surrounding quiet corona. A limb observation with yohkoh is under progress, to investigate the radial temperature and emission meassure evolution with height within coronal holes. Knowledge of these parameters in the source region of the solar wind, are important in understanding the processes which accelerate the high speed solar wind, and in turn fuel the coronal heating debate.

### This is the abstract of the work presented at the Makuhari Conference.

The Diffuse Corona Observed By Yohkoh Radial Temperature Structure.

### Abstract

Yohkoh has observed diffuse emission from different parts of the Corona. These large diffuse regions are observed with Yohkoh to extend up to 0.8 solar radii above the limb. Although partial loop structures can sometimes be discerned, systematic structures such as streamers or active region loops are not observed.

Soft X-ray Telescope, SXT, images recorded through different analysis filters have been used to obtain radially averaged temperature and emission measure profiles above the limb, using the isothermal filter ratio technique. Results for two diffuse region observations are presented. The temperature is observed to increase with increasing heliocentric distance, with a declining temperature gradient. These results are presented as further evidence for the non-isothermal nature of the diffuse corona and the existence of heating at heights up to one solar radii above the limb. The observations when considering the response of the SXT in more detail with the line of sight plasma components is discussed, regarding the true radial temperature dependance in the diffuse corona which is believed to be consistent with static closed structures, as found with EUV data Withbroe, 1988.

### (f) K. Yaji

Behavior of energetic electrons in solar flares deduced by microwave and X-ray imaging observations^*

### *thesis work under the supervision by T. Kosugi (NAOJ)

Abstract: We study spatial as well as other characteristics of solar flares observed in microwaves with the Nobeyama Radioheliograph in view of their relations to those observed in X-rays by YOHKOH. Our goal is to deduce information on the behavior of energetic electrons produced in solar flares.

Motivation: It has long been known that the time profile of solar flares in microwaves resembles that in hard X-rays. Similarly the microwave peak flux is well correlated with that in hard X-rays. These observational facts suggest that a single population of energetic electrons is responsible for the two types of emission, though the emission mechanisms are different from each other. This does not necessarily mean, however, that the two types of emission originate from a common source. Kundu (1984) and Nakajima (1991) have analyzed imaging observations of solar flares simultaneously taken in microwaves and hard X-rays and concluded that the spatial relationship between microwave and hard X-ray sources is quite complicated; microwave emission originates from a site located in between two hard X-ray sources in some cases, while in others a microwave source is coincided at one of the two hard X-ray sources. Sometimes a pair of microwave sources appear coincidentally at the two hard X-ray sources, which are interpreted as footpoints of a flaring loop. With advanced instruments both in microwaves and hard X-rays over those used by Kundu or Nakajima, we expect further progress to be achieved. We use microwave data from the Nobeyama Radioheliograph and X-ray data from the Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT) onboard YOHKOH.

Data and Analysis: About 80 flares were simultaneously observed during the interval from 1992 June through 1994 December. Out of them, 11 events were selected using the criterion of size exceeding 50,000 km. Here the length of an apparent soft X-ray "loop(s)" is used to define the size. With this selection we expect the source structure of these 11 events is to be resolved with the Nobeyama Radioheliograph, the resolution (HPBW) of which being better than 20 arcsec. We have precisely coaligned microwave, hard X-ray, and soft X-ray images for each of the flares listed below.

Event List:

  17-Aug-92 23h(UT) C4.3
16-Dec-92  3h(UT) C4.6
2-Feb-93  3h(UT) C6.3
12-Feb-93  0h(UT) C5.9
18-Feb-93  3h(UT) M4.0
30-May-93  6h(UT) C2.6
7-Oct-93  4h(UT) C2.6
30-Nov-93  6h(UT) C9.2
27-Jan-94  3h(UT) C4.6

Progress Report: We found a microwave source which is located near the middle point of the corresponding soft X-ray loop(s) in 4 out of the 11 events. In these cases the microwave "loop-top" source is accompanied by other two sources which are located at the two ends of the loop(s) and polarized oppositely to each other. The other 7 events show a single source extending along the loop(s), with no "loop-top" source clearly resolved. The microwave "loop-top" source shows the following characteristics in common: i) location near the middle point at the soft X-ray loop(s), ii) brightness temperature ranging from 8x10^4 K to 5x10^6 K, iii) low degree of circular polarization (0-10%), which is clearly different from polarized, "footpoint" sources; and iv) delayed peaking in comparison with the "footpoint" sources by a few to several tens of seconds. This delay can be also perceived from a comparison with the hard X-ray total flux time profile; the microwave "footpoint" sources show a better temporal correlation, i.e., much smaller time lags if any, with the hard X-ray time profile. Although we have much things to do, our tentative interpretation of this "loop-top" source is as follows. The brightness temperature sometimes reaching or exceeding several of millions K is hard to be interpreted as the emission by the thermal Bremsstrahlung. In fact, the temperature and emission measure derived from SXT observations are used to calculate the microwave brightness, yielding much smaller brightness than observed. Also the low degree of circular polarization refuses the interpretation by thermal gyroemission. Such a low degree of circular polarization is most plausibly interpreted by gyrosynchrotron emission at high harmonics, say of the order of 100, from energetic electrons with energies of the order of MeV. The delayed peaking is then suggestive that the MeV electrons accumulate in the loop(s) with some time constant, say 10 s or so. Note that the presence of this "loop-top" source help us understand why the microwave total flux tends to show a smoother (and sometimes slightly delayed) time profile than hard X-ray total flux.

### (A) Bachtiar Anwar

I would like to inform that I wish to use SXT data for my Ph. D thesis. These are the title of topics:

(1) Rapid Sunspot Motions during a Major Solar Flare. Event/Data : Flare of Nov. 15, 1991 (Flare data cube)

(2) Morphological Evolution of Post Flare Loops of June 26, 1992. Event/Data : PFIs of June 26, 1992, some of FFI related images.

Furthermore, I have reached a stage that topic (1) is in near completion for publication.

### Thesis summary:

Magnetic Bipoles on the Sun K. L. Harvey

My doctoral thesis is an observational study of the characteristics of the emergence magnetic flux from the surface of the Sun. The approach to this problem is three fold: (1) an investigation of the properties recently emerged magnetic bipolar regions as a function of region size and of the cycle. Less than 3 of these regions have sunspots. These data, therefore, provide a more complete picture of the emergence of bipolar regions than can be derived from sunspot regions alone, (2) the magnetic field evolution in quiet regions related to short-lived (hours), small scale (< 40 arc-sec) structures observed in the chromosphere and corona; this investigation led to the surprising) result that we were investigating, not only the emergence of magnetic flux, but also its disappearance, and (3) These two studies are brought together within the context of the large-scale, long-term dynamic pattern of magnetic flux emergence and disappearance that defines the solar activity cycle.

On the solar surface, bipolar magnetic regions form in two ways: by the emergence of magnetic fields from the interior and by the approach and encounter of previously magnetically-unconnected opposite-polarity elements of magnetic flux. On time scales ranging from hours to years, the rate at which these processes operate and the balance between them controls the details of the spatial distribution of magnetic flux and the magnetic flux content observed at the solar surface. It is the characteristics of emerging magnetic flux and its spatio-temporal distribution that currently provide the best diagnostics of the source fields in the solar interior and to the processes that give rise to both the emergence and disappearance of magnetic flux on the solar surface.

It is in the second part of my thesis that I use the Yohkoh SXT obser- vations to demonstrate that X-ray Bright Points are predominately associated with sites of magnetic flux disappearance. This result, originally deduced from comparative observations of He I 10830 dark points and the evolution of the photospheric magnetic field, is discussed in the summary of Part II of my thesis.

### II. The Disappearance of Magnetic Flux (Summary)

X-ray bright points are coronal emission structures, generally less than 40 arc-sec in size with an average life-time of around 8 hours, primar- ily observed outside of active regions. The formation of X-ray bright points as a direct result of the emergence of ephemeral regions was suggested in the earlier studies of X-ray observations by Skylab and comparisons with NSO/KP magnetograms. Dark structures, observed in the He I 10830 line, have a similar size scale, lifetime, and in most cases appear to have an X-ray bright point counterpart.

The studies presented in my thesis show that the He dark points (from the NSO/KP He I 10830 spectroheliograms) and X-ray bright points (from the Yohkoh SXT observations) are preferentially associated, both temporally and spatially, with the encounter of previously unrelated network elements of opposite polarity and the resulting cancellation or disappearance of mag- netic flux. He I dark points, for example, occur at the locations of bipoles that, in only one-third of the cases, are associated with ephemeral regions, while two-third of the dark points result from the encounter of pre-existing magnetic network elements of opposite polarity. The probability for such encounters is highest during cycle minimum (because the surface is then cov- ered predominantly by magnetic fields of mixed polarity) and lowest during cycle maximum (when the surface patterns become predominantly unipolar) and accounts for the observed inverse correlation of the occurrence of X-ray bright points and He I dark points with the activity cycle.

The emergence or the cancellation of magnetic flux, however, is neither a necessary, nor a sufficient condition for the formation of a He I dark point, an X-ray bright point, or their radio counterpart observed at a wavelength of 20cm. This is indicated by the observation that these chromo- spheric and coronal structures occur erratically throughout the period of emergence or cancellation of the associated bipole, and sometimes end before these changes in the magnetic field configuration are completed. From this it is inferred that the connection of previously unconnected magnetic fields plays a major role in the formation and persistence of coronal bright points as the local fields adjust to the changes in their configuration with the emergence or disappearance of magnetic flux.

During their emergence and decay, ephemeral regions alter the structure of the existing magnetic network by moving network flux elements around, by merging with adjacent magnetic network or with elements of other ephemeral regions of similar polarity, by colliding with and disappearing together with magnetic elements of opposite polarity, or by simply disappearing. In this way, they may indeed be an important, but not the only, player in the eventual occurrence of small-scale energetic events in the quiet regions of the Sun.

### \chapter*{Extended Abstract of the Thesis}

The {\it Yohkoh} satellite was launched into an orbit around the Earth by the Institute of Space and Astronautical Science (ISAS) on 30 August, 1991. The satellite carries two X-ray imagers as the two major scientific payloads, as well as two types of spectral analysis instruments.

The Hard X-ray Telescope (HXT), one of the two imagers, is an advanced hard X-ray imaging spectrometer for solar flare observations and has the following capability: i) simultaneous imaging in four energy bands (13.9 -- 22.7 -- 32.7 -- 52.7 -- 92.8 keV); ii) angular resolution of $\sim$ 5 arcsec with a wide field of view covering the whole Sun; iii) basic temporal resolution of 0.5 s; and iv) high sensitivity with a total geometrical aperture of $\sim$ 60 cm$^2$.

The Soft X-ray Telescope (SXT), the other imager, uses grazing incidence optics to form soft X-ray images of the Sun on a CCD detector. Both the angular resolution and the pixel size of CCD are $\sim 2.5$ arcsec. In addition to the mirror and the CCD, the optical system includes a filter wheel assembly and a rotating shutter, both controlled by an on-board microprocessor and realizing five different X-ray passbands with an adequate exposure time depending on the solar activity. When a flare occurs, exposure cadence of up to one image per 0.5 s can be achieved by restricting the CCD region to be edited into the telemetry stream to a small region around the flare.

Hence we now have the first opportunity to simultaneously observe solar flares in hard and soft X-rays. Note that HXT and SXT are complementary to each other in that the former observes nonthermal electrons while the latter observes high-temperature plasmas contained by the magnetic field structure in which the electrons are energized.

Making use of this advantage of {\it Yohkoh}, we have tried to clarify the vertical structure of hard X-ray sources in solar flares. The method we adopted is a detailed comparison between hard and soft X-ray images taken simultaneously for flares that occurred near the solar limb. Also we have tried to clarify temporal, spectral, and structural behavior of hard X-ray sources.

### \newpage

The thesis consists of three chapters and two appendices, the essence of which are briefly summarized in the following.

### \subsection*{Chapter 1:}

A summary is given on the {\it Yohkoh} satellite, its mission objectives, and its instruments with an emphasis on the Hard X-ray Telescope (HXT). Also hard X-ray flare observations before {\it Yohkoh} as well as in the very initial period of {\it Yohkoh} are reviewed for specifying the problem areas to be challenged in this thesis work.

\subsection*{Chapter 2:} Accurately coaligned hard and soft X-ray images, taken simultaneously with HXT and SXT, of the impulsive solar flares on 13 January, 1992 (17:29 UT) and 4 October, 1992 (22:21 UT) both occurring near the limb, clearly reveal that, in addition to double footpoint sources, a hard X-ray source exists well above the corresponding soft X-ray loop structure at the peak time of the impulsive phase. This hard X-ray source shows intensity variation similar to the double footpoint sources and a spectrum relatively hard in comparison with the gradual source appearing later in the gradual phase. We believe that this is the first clear evidence that the magnetic reconnection, which is responsible for the primary energy release including the impulsive particle acceleration, is under progress above the soft X-ray flaring loop. Maybe this loop-top'' hard X-ray source represents the reconnection site itself or the site where the downward plasma flow, ejected from the reconnection point far above the hard X-ray source, collides with the underlying closed magnetic loop. Characteristics of this hard X-ray source is quantitatively discussed in the schemes of thermal (T $\gsim 10^8$ K) and nonthermal interpretations of the hard X-ray emission.

### \subsection*{Chapter 3:}

>From a detailed analysis of ten selected flares that occurred near the solar limb and were observed with HXT, it is found that three distinct types of hard X-ray sources coexist in a single flare, {\it i.e.}, (double) footpoint source(s)'', loop-top impulsive source'', and loop-top gradual source''. Hard X-ray images, taken with HXT in its four energy bands as well as in time series, clearly reveal the following characteristics of the individual types of sources.

{\bf (Double) Footpoint Source(s)}: Sources of this type usually dominate over the hard X-ray emission in the impulsive phase, especially in the higher X-ray energy range $\gsim 30$ keV. They tend to appear in pairs at the two ends (or footpoints) of a soft X-ray flaring loop; the double-source footpoint structure is seen in eight events out of the ten. The impulsive time variation of the hard X-ray emission seen in the total flux record is mainly due to footpoint sources. Their spectrum is relatively hard, and is approximated by a single power-law with a spectral index $\gamma$ of about 2 -- 4, with some softening towards higher-energy hard X-rays. It is most plausible that the emission originates from nonthermal electrons accelerated around the top of the loop and precipitating down along the loop into the chromosphere.

{\bf Loop-top Impulsive Source}: Sources of this type appear as a single source in six events out of the ten. The sources are seen up to the HXT M2-band (33 -- 53 keV). They are compact and single-source shaped, and are located well above the apex of the corresponding soft X-ray flaring loop (three events out of the six) or at or near the apex (the remaining three events). The temporal and spectral characteristics are similar to those of the footpoint source. We believe that sources of this type represent the primary energy release site of solar flares. Maybe the downward outflow, ejected from the X-type reconnection point far above the hard X-ray source, collides here with the underlying closed magnetic loop and forms a shock, resulting in plasma heating up to $\gsim 200$ MK as well as production of energetic electrons, which then precipitate towards footpoint sources.

{\bf Loop-top Gradual Source}: Sources of this type appear at the apex portion of the corresponding soft X-ray flaring loop as a single source in all of the ten flares analyzed. Loop-top sources of this type are diffuse and seem to trace the loop seen in soft X-rays. Although the sources begin to brighten in the impulsive phase, they are most clearly seen in the gradual phase in the X-ray energy range $\lsim 30$ keV. They show smooth temporal variation and a soft spectrum, suggestive of a thermal plasma emission. The temperature and emission measure, derived from the count ratio between the L- and M1-bands, are typically $\sim 30$ MK and $\sim 10^{47}$ -- $\sim 10^{48}$ cm$^{-3}$, respectively. \subsection*{Appendix A:}

To achieve our science goal, it is required that HXT images are overlaid precisely on the corresponding SXT images taken simultaneously. Since the angular resolutions of the two imagers are $\sim 5$ and $\sim 2.5$ arcsec, respectively, we need to establish coalignment accuracy of better than 1 arcsec for detailed comparison between the two sets of images.

For attaining this high accuracy, in-orbit calibration has been conducted to derive several parameters which were not determined accurately enough before launch on the ground. These parameters include the scales of the HXT and SXT coordinates, the relative shift of the HXT axis with respect to the SXT axis, and the relative rotation of the HXT coordinates around its axis with respect to those of SXT. Some of these parameters have been determined with the aid of aspect determination systems of HXT and SXT. Remaining terms have been solved by comparing HXT L-band images taken for twelve flares with the corresponding SXT images. Most of the parameters thus obtained are in agreement with their prelaunch values, with one exception of the relative rotation that had not been given at all from prelaunch calibration.

With the rotation corrected, the twelve flares show good coincidence between HXT and SXT images within $\sim 1$ arcsec accuracy. This good coincidence holds for many other flares. Thus we are confident that all the parameters have been successfully fixed which determine the relation between the HXT and SXT coordinates.

### \subsection*{Appendix B:}

In order to obtain high-quality HXT images, uniformity of the gain among the 64 detectors is necessary to be maintained within accuracy of $\sim$ 1\%. We have been maintaining this uniformity since launch by calibrating and readjusting the gain approximately once every one and a half month. The history of the in-orbit calibration and the fine gain readjustment is summarized. We have confirmed high stability of the whole detector system including scintillation counters, photomultiplier tubes, and electronic circuits; the gain on the average has decreased by only $\sim$ 4 \% during the two-year period. No detectors which show irregular variations are found; even the worst among the 64 detectors shows a long-term decrease in gain by $\sim$ 10 \% . Thus we conclude that all parts have been operating as designed.

### (E) M. Inda-Koide (See fl133.inda03 and fl161.inda04 for detail)

Title: Systematic Study of Temporal and Spectral Variation of Solar Flare Hard X-ray Source Morphology

### Abstract:

We made a systematic study of correlated morphological, temporal, and spectral behavior of solar flares in hard X-rays, using the data obtained with the HXT aboard the Yohkoh satellite.

We have first established the imaging and spectral responses of the HXT through detailed calibration works both on the ground before launch and in orbit after launch. This has enabled the HXT to achieve the full instrumental capability as an imaging spectrometer, i.e., four energy-band (14-23-33-53-93 keV) simultaneous imaging with the angular resolution as high as $\sim 5$ arcsec. It is to be noted that the HXT can provide rather good spectral information even though it has only four energy bands.

The HXT is a Fourier-synthesis type telescope with 64 subcollimators. HXT images need be synthesized using a sophisticated algorithm such as the maximum entropy method (MEM), which is used in practice. Although MEM works fairly well, it has shortcomings such that it is CPU time consuming so that not suitable for dealing with a large quantity of data, and also that it sometimes yields an image with many spuriouses for data with poor photon counting statistics.

Thus as a complementary method to MEM we have introduced a new measure named Configurational Difference Measure" (CDMt or CDMe). This is a measure which characterizes a configurational change between two brightness distributions in the time domain (CDMt) or in the spectrum domain (CDMe). Since this measure can be directly related to the photon counts measured with the HXT 64 subcollimators, we can briefly estimate configurational difference between two brightness distributions quantitatively without relying upon synthesized MEM images. The advantages of using CDMs are summarized as follows:

1. The CDM gives a reliable quantitative measure to estimate the configurational difference between two brightness distributions.

     The CDM represents the morphological change by one scalar value
which is obtainable with a simple calculation directly from observed HXT data, thus we can quickly examine a large quantity of data. 3. The CDM provides a relatively stable value for data with poor photon counting statistics; thus this method has an wide applicability than MEM.

From a number of solar flares detected with the HXT, we selected the following 14 flares for our systematic study: 1991 October 24, 27, 29, 30, and 31; 1991 November 10 and 15; 1991 December 3 and 16; 1992 July 16; 1992 October 28; 1992 December 9; 1993 February 11; and 1993 August 4. We evaluated time-resolved X-ray spectra for these events in the 14--93 keV range, employing the results of calibration of the spectral response. We synthesized hard X-ray images of these flares in time series and in the four energy bands, using MEM with the calibration information on the angular response. Using new measures mentioned above (CDMt and CDMe), we also examined temporal changes and spectral dependences of hard X-ray configurations of these events quantitatively. Finally we compared spectral, temporal, and morphological behaviors to each other, and obtained the following couclusions:

1. In the impulsive phase where a flare exhibits spiky time

   variations, the energy spectrum is well represented
by a single power-law or a broken power-law model.
The power-law photon index ranges from about 2 to 5.
The morphology tends to be double-peaked or multiple-peaked,
suggesting that hard X-rays are produced at footpoints of
magnetic loops.
Probably electrons are accelerated at higher locations in
the loops and stream down along the magnetic fields towards the
footpoints (electron bombardment'').
Sometimes the hard X-ray morphology changes significantly
between successive spikes in the impulsive phase,
indicative of successive flaring of neighboring loops.
Using $CDM_e$ we confirmed quantitatively that
in some cases the morphology differs significantly between
the L-band (14-23 keV) and the other higher energy bands
(23-33, 33-53, and 53-93 keV).
A possible interpretation of this difference is that thermal
bremsstrahlung from high-temperature (higher than 2 x 10^7 K)
plasmas is admixed with bremsstrahlung emission
from nonthermal electrons only in the L-band,
and also that the thermal and nonthermal
hard X-rays are emitted from different sites in a flaring region.

2. In the gradual phase, the energy spectrum is more likely to be

### Update 27-May-95

Image sequences were selected from SXT archives resident at the Solar Data Analysis Center, at Goddard Space Flight Center. Light curves have been generated from some of these image sequences, and the search for periodic fluctuations in X-ray brightness has begun. Work is under way now to characterize quasiperiodic modulations caused by instrumental variations, and to assign statistical confidences to periods detected. (A poster will be displayed by D. McKenzie at the June SPD meeting in Memphis.)

## Master Thesis Topics

### Current:

(a) T. Yoshida (b) M. Shimojo (c) S. Yashiro

### History:

(A) R. Kano (Completed, March 1994) (B) M. Takahashi (Completed, March 1994) (C) J. Sato (Completed, March 1994) (D) M. Ohyama (Completed, March 1993) (E) T. Takahashi (Completed, March 1993) (F) K. Yaji (Completed, March 1993) (G) S. Dimascio (Completed, August 1995)

### T. Yoshida ( Institute of Astronomy, Univ. of Tokyo )

Master thesis topics Thesis adviser: S. Tsuneta

### Date used: May-July, 1992 SXT PFI

The purpose of this analysis is to get the time scales of the loops' time variations and to see the global temperature structures. Here we sum up the images taken in the intervals of one Yohkoh orbit revvolution (100min) and get the good S/N images. Surprisingly, the Yohkoh-SXT structures don't always coincide with the temperature structures, and sometimes these look quite different. For example, very hot (8MK) regions are in some no-structural diffuse corona above the bright magnetic loops. In the events that whole loops are brighten up after the brightenings occure at the footpoints of the loops, not only those bright loop but also the diffusive loops around the bright loops are heated up to very high temperatures (6MK). The temperatures of the compact steady loops (4MK) are generally higher than the temperatures of the diffusive loops (3MK), and there are low temperature layers (5000km 3MK) around the limbs. Then, the temperatures of the footpoints of loops may be lower than those of the tops. Thus, we can get the new information about the coronal heating mechanism from the temperature maps and their time variations.

Preliminary result will be presented in the annual ASJ meeting in the autumn of 1994.

### M. Shimojo, K. Shibata, ...

motivation: The Yohkoh SXT has revealed that the solar corona is full of jet-like features (hereafter, simply called X-ray jets). According to preliminary study by us, the number of X-ray jets is more than 20 per month during Nov. 1991 - March 1992 in full frame images (FFI). These jets are one of new discoveries by Yohkoh. The purpose of this study is to study statistical properties of the X-ray jets using FFI data.

how to do it: We will make a catalogue of X-ray jets observed in FFI data. This calalogue include (1) date, time (begin, max, end), (2) place (heliocentric coordinate), (3) length, (4) (apparent) translational velocity, (5) association with XBP, EFR, or AR (with NOAA number), (6) (apparent) shape (7) other comments if available. On the basis of this calalogue, we will study statistical properties of jets, such as velocity, length, occurrence place (lattitude), classification of jets, and so on.

note: This study will be done as undergraduate thesis (1993) and master thesis (1994-1995) of M. Shimojo at Tokai University.

### Thesis adviser: K. Shibata (collaborator: M. Shimojo)

The purpose of this study is to clarify the evolution of active regions, using the soft X-ray images taken with the Soft X-ray Telescope aboard Yohkoh. At first, we derive the time variation of projected area and total soft X-ray intensity of active regions (emerging flux regions or EFRs) using SFD file. We will confirm the results by Ishido et al. (1992) and Su et al. (1992) and examine many cases. (Note that they found that the expansion velocity of EFRs is 2-5 km/s in a few cases, though these studies were published only in the proceeding of the meeting in Japan.) As a preliminary result, we found that the expansion velocity of some EFRs is 1-3 km/s. We will then compare the SXT images with the Kitt Peak magnetograms and study the role of magnetic field in the evolution of active regions.

### Scaling Law and Heating Function of Coronal Loops obtained by Yohkoh SXT

Ryouhei Kano Institute of Astronomy, University of Tokyo

### ABSTRACT

Yohkoh soft X-ray images clearly demonstrate that the solar corona consists of the loop structures. Thus, it is important to study the heating mechanism of those individual loops for the investigation of the coronal heating mechanism. If we can observationally know which part of the coronal loops is heated, we will be able to constrain the heating models such as the DC current heating and the wave heating models.

As a first step toward this attempt, I derive the temperature T, pressure p and length L of 43 steady soft X-ray loops in the active region NOAA 7150, observed by Yohkoh Soft X-ray Telescope (SXT) on 1992 May 2. The transient brightening (micro flares) are excluded in the analysis: only loops which do not show appreciable changes over 30 minutes are used in the analysis. I find that the data set is very much consistent with the scaling law T = 1.4e3 (p L)^{1/3} derived by Rosner, Tucker, Vaiana (1978) within statistical and systematic errors.

Since the scaling law is essentially insensitive to the distribution of the heat input (the heating function E_H(s)) along the loops, we can not obtain E_H(s) itself only from the scaling law. An attempt is then made to obtain the heating function E_H(s) directly from the observed temperature gradients with the energy equation, because the energy equation underlying the scaling law is verified by the above analysis.

Since the heating function is sensitive to the noise in the derived temperatures, I use the maximum entropy method (MEM), which allows us to obtain

the smoothest solution (heating function) among the solutions consistent with
the data. The accuracy of the heating function derived by MEM is checked by
the numerical simulation for various heat input patterns with noise.

The heating function thus obtained for a loop shows that the heat input is concentrated near the loop top. Both the DC current heating and the wave heating with constant damping length do not appear consistent with the heat input distribution. More loops, however, need to be analyzed for definite conclusion. The coronal heating rate is derived for the single loop for the first time: Total heat flux obtained from integration of the heating function is found to be 1.2e7 [erg/cm^2/s].

The major parts of this thesis with some additions will be submitted to the Publications of the Astronomical Society of Japan (PASJ).

### A flare of 1992 Aug 17 23:58 UT

M.Takahashi(Tokai Univ.) J.Sakai(Toyama Univ.) Te.Watanabe T.Sakao T.Kosugi T.Sakurai S.Enome(NAOJ) S.Tsuneta(Univ. of Tokyo) H.Hudson(Univ. of Hawaii) N.Nitta(LPARL) S.Hashimoto(Tokai Univ.)

A flare of GOES X-ray class C4.3 was observed in NOAA 7260 on 1992 Aug. 17 at 23:58 UT. All the instruments on board YOHKOH made high time and spatial resolution observations. The soft X-ray time profile in the 3 - 15 keV energy range obtained by Soft X-ray Spectrometer (SXS) shows double peaks around Aug. 18, 00:00 and 00:05 UT. Images of Soft X-ray Telescope (SXT) show that four discrete points brighten in a straight line from northeast to southwest around Aug. 17 23:58 UT, which is also confirmed in Hard X-ray Telescope (HXT) images. We name these four points in SXT images as Points 1, 2, 3 and 4 from northeast of the sun. Four points observed by SXT can be considered as the foot-points of magnetic flux tubes. Moreover HXT contour images show that the fifth point exists between Point 3 and Point 4. The logarithmic scale SXT images indicate the existence of two loops connecting Point 1 with Point 4, and Point 2 with Point 3. Overlaid SXT images on the vector magnetogram obtained by the Solar Flare Telescope (National Astronomical Observatory of Japan) reveal that a magnetic neutral line runs between the second and third points. Obtaining the electron temperature and emission measure by the SXT filter ratio method, the RTV scaling law (Rosner, Tucker, and Viana 1978) implies the connections of Point 1 with Point 4 and of Point 2 with Point 3 in the flare decay phase. The time profile of electron temperature derived from the Bragg Crystal Spectrometer (BCS) also has double peaks. Various information can be obtained from the high resolution soft X-ray line spectra.

### (D) M. Ohyama

Coronal Disturbances Associated with Solar-Filament Eruptions (a planned dissertation for the degree of the Master of Science to be submitted to Nagoya University in early 1993) M. Ohyama (Solar-Terrestrial Environment Laboratory, Nagoya Univ.) SXT images showing coronal disturbances associated with filament disappearances (or eruptive prominences) will be analyzed to understand relevant physical processes. Since the phenomena showed a variety of appearances, it is required to study as possible as many events to obtain statistically significant results. (Supervisor: Takashi Watanabe)

Point 4 and of Point 2 with Point 3 in the flare decay phase. The time profile of electron temperature derived from the Bragg Crystal Spectrometer (BCS) also has double peaks. Various information can be obtained from the high resolution soft X-ray line spectra.

### (E) T. Takahashi

X-ray synoptic maps and comparison with magnetic synoptic structure

(a planned dissertation for the degree of the Master of Science to be submitted to Tokyo University in early 1993)

T. Takahashi (Department of Astronomy, University of Tokyo) X-ray synoptic maps are created, and its structures are compared with Kitt Peak and WSO magnetic synoptic sturucture. Long term evolution of solar magnetic structure will become evident. The rigidly rotating components including dark channel and differentially rotating components including active regions will be studied. The location of coronal holes predicted by potential field calcuration will be compared with X-ray holes.

### Hard X-ray Frares Associated with Evolution of Active Region NOAA 7270

(a planned dissertation for degree of the Master of Arts and Sciences to be submitted to Tokyo University in early 1993)

### K. Yaji (College of Arts and Sciences, University of Tokyo)

In early September, 30 flares (more than C5.0 class by GOES) occurred at NOAA 7270. 16 hard X-ray flares of them were observed with YOHKOH, and some were simultaneously with heliograph in Nobeyama. Each of the flares is defferent in the HXTtime profiles and the HXT images. What does determin such caracteristics of the flares? We hope that a set of the flares at NOAA 7270 give answers for this question. First, we compare the connection between the character(impulsive or gradual,etc...) of the HXT time profiles and the location(for the sunspots) of flares. And, we study the HXT images, the HXT spectrum , the SXT images, the heliograph data and so on.

### Flare events to analyse

All Flares(mainly more than C5.0 class) that occurred in NOAA 7270 from Sep.4 to Sep.9

### (G) S. Dimascio

An analysis of the bright knots at the tops of solar flare loops using soft X-ray data from the Yohkoh mission.

### Simone Dimascio (University College London)

The purpose of this study was to carry out a computer analysis on soft X-ray images of solar flares obtained by the Soft X-ray Telescope on board the Yohkoh spacecraft. This instrument has commonly revealed the presence of bright knots at the top of many flare loops, so this investigation was undertaken to study these features.

An intensive search was made through the Yohkoh data archive to identify images that would be suitable for analysis. At the same time, familiarisation had to be gained with the software and techniques required to display X-ray images, manipulate them and analyse them. Eight flares were eventually short-listed for study. Initial analysis included studying the shape and size of the loop top source and how this varied over the lifetime of the flares. The results seem to indicate that this loop top region remained approximately constant in area well into the decay phase of the flares. An initial attempt to simplify the problem by reducing the flare loops to one-dimensional structures failed and so this technique was abandoned.

A method of analysis that has not been done before was attempted. A program was written to measure the average intensity per pixel contained within contours of increasing areas concentric around the loop top. Though it was uncertain whether this would yield any useful information, it did in fact clearly demonstrate the differing degrees of compactness of the loop top source observed in different events. The results were looked at in different ways to study the features of the data, but more importantly, a measure of the compactness of the loop top regions was obtained by comparing the results with those expected for normal flares with no loop-top enhancement. Finally a brief investigation of the effects of instrumental scattering was undertaken. The results from this suggested that this could be an important effect that needs consideration in any future work of this kind.

### (Supervisor: Joseph I. Khan)

Updated: 23-Jun-2002