Engineering topics

Lemen, J. R.

Publications and source records attributed to Lemen, J. R..

At least 19 records

NASA NTRS2000

Onset of the Magnetic Explosion in Solar Flares and Coronal Mass Ejections

We present observations of the magnetic field configuration and its transformation in six solar eruptive events that show good agreement with the Hirayama-Shibata model for eruptive flares. The observations arc X-ray images from the Yohkoh Soft X-ray Telescope (SXT) and magnetograms from Kitt Peak National Solar Observatory, interpreted together with the 1-8 Angstrom X-ray flux observed by GOES. The observations show: 1. Each event is a magnetic explosion that occurs in an initially closed magnetic bipole in which the core field is sheared and twisted in the shape of a sigmoid, having an oppositely curved elbow on each end. The arms of the opposite elbows are sheared past each other so that they overlap and are crossed low above the neutral line in the middle of the bipole. 2. Although four of the explosions arc ejective (blow open the bipole) and two are confined (arc arrested within the closed bipole), all six begin the same way. In the SXT iniages, the explosion begins with brightening and expansion of the two elbows together with the appearance of short bright sheared loops low over the neutral line under the crossed arms and, rising up from the crossed arms, long strands connecting the far ends of the elbows. 3. All six events arc single-bipole events in that during the onset and early development of the explosion they show no evidence for reconnection between the exploding bipole and any surrounding magnetic fields. We conclude that in each of our events the magnetic explosion was unleashed by runaway tether-cutting via implosive/explosive rcconnection in the middle of the sigmoid, as in the Hirayama-Shibata model. The similarity of the onsets of the two confined explosions to the onsets of the four ejective explosions and their agreement with the model indicate that runaway reconnection inside a sheared core field can begin whether or not the overlying fields (the envelope field and contiguous fields that press down on the envelope) allow the explosion to be ejective. We consider the implications ofthis for the unleashing of magnetic explosions in field configurations composed of to or more impacted bipoles.

Moore, R. L.↗

NASA NTRS1997

Extreme Ultraviolet Imaging Telescope (EIT)

Efforts concentrated on development and implementation of the SolarSoft (SSW) data analysis system. From an EIT analysis perspective, this system was designed to facilitate efficient reuse and conversion of software developed for Yohkoh/SXT and to take advantage of a large existing body of software developed by the SDAC, Yohkoh, and SOHO instrument teams. Another strong motivation for this system was to provide an EIT analysis environment which permits coordinated analysis of EIT data in conjunction with data from important supporting instruments, including Yohkoh/SXT and the other SOHO coronal instruments; CDS, SUMER, and LASCO. In addition, the SSW system will support coordinated EIT/TRACE analysis (by design) when TRACE data is available; TRACE launch is currently planned for March 1998. Working with Jeff Newmark, the Chianti software package (K.P. Dere et al) and UV /EUV data base was fully integrated into the SSW system to facilitate EIT temperature and emission analysis.

Lemen, J. R.↗

NASA NTRS1997

The dynamic quiet solar corona: 4 days of joint observing with MDI and EIT

The analysis of a sequence of joint extreme ultraviolet imaging telescope (EIT) Fe XII and Michelson Doppler imager (MDI) magnetogram observations of the quiet sun near disk center is presented. It was found that: all the emerging flux above the threshold of approximately 10(sup 17) Mx is associated with enhanced coronal emissions; loop systems between the polarities in ephemeral regions remain visible up to separations of 10000 up to 30000 km; brightenings between approaching opposite polarity network concentrations form when the concentrations are between 5000 and 25000 km apart, and that faint connections up to 40000 km in length form as sets of concentrations of the same polarity coagulate. The coronal emission over patches of the quiet sun depends on the total flux in connected concentrations, on their distance and on the positions and strengths of neighboring concentrations.

Schrijver, C. J.↗

NASA NTRS1995

Yohkoh/SXT soft x-ray observations of sudden mass loss from the solar corona

Direct X-ray observations allow us to estimate the hot coronal mass before and after a flare or other disturbance of the type leading to a coronal mass ejection. The sudden disappearance of a large coronal structure (scale greater than 105 km) gives evidence that an ejection has occurred, if the time scales are much shorter than the conductive or radiative cooling times for such structures. A flare also typically adds large amounts of new material to the corona via evaporation resulting from the coronal energy release. This provides a competing mechanism that makes the estimation of the total mass loss somewhat difficult. We note that the X-ray observations have the advantage of covering the entire corona rather than the limb regions unlike the coronagraph observations. We have identified two examples of coronal mass disappearances. before and during long duration flare events on 21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In latter case the total mass amounted to some 4 x 10(exp 14) g with a density of 3 x 10(exp 8)cm(exp -3) and a temperature of 2.8 MK before its disappearance. This corresponds to a radiative cooling time of some 104 S. much longer than the observed time of disappearance. We therefore suggest that these sudden mass disappearances correspond with coronal mass ejections (CMEs), and suggest that further data analysis will be able to confirm this by comparison with optical observations of specific CMEs.

Hudson, H. S.↗

NASA NTRS1995

The solar origins of two high-latitude interplanetary disturbances

Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming were detected by Ulysses in 1994. Ground-based and Yohkoh/SXT observations show two strikingly different solar events that could be associated with them: an LDE flare on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April 1994. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. The sharply contrasting nature of these solar events argues against an energetic causal relationship between them and the bidirectional streaming events observed by Ulysses during its S polar passage. We suggest instead that for each pair of events. a common solar trigger may have caused independent instabilities leading to the solar and interplanetary phenomena.

Hudson, H. S.↗

NASA NTRS1995

Comparison of Yohkoh x-ray coronal events with Ulysses interplanetary events

The Yohkoh soft X-ray telescope (SXT) has observed several largescale eruptive events per year for the first three years of observations (Aug. 1991 - Nov. 1994) Such events are most prominent at high latitudes, but resemble long-duration flare events seen in active regions. Some of the high-latitude events have now been identified in the Ulysses/SWICS data base during the Ulysses south polar passage. There are puzzling examples of solar events with no interplanetary counterparts. A comparison of coronal and interplanetary events can lead to better models for mapping interplanetary disturbances back to their source location, especially by combining Yohkoh morphology with three-dimensional representations of the coronal magnetic field. In this paper we describe the parameters of the hot plasma seen by SXT. There is clear evidence for non radial motion in specific events. We present comparisons between the ionization temperature of the interplanetary plasma with that observed at the Sun in cases where this is possible.

Lemen, J. R.↗

NASA NTRS1995

Yohkoh/SXT x-ray synoptic maps of coronal brightness and temperature

The Yohkoh soft X-ray telescope (SXT) records on the order of 50 solar images per day in two different color filters. These provide material for the generation of synoptic maps, which compress the 3-dimensional data cube into two dimensions. We are creating synoptic maps from strips of data both at disk center and at different heights, including limb maps that are analogous to those produced by ground-based coronagraphs. The ratios of intensities in images taken in two filters provide estimates of the electron temperature in the range 1 - 3 x 10(exp 6) K. These are broad-band temperature maps; rather than maps created with discrete sampling as in the case of the coronal green and red lines. We discuss the properties of these maps and their application to the study of energy release in the corona.

Slater, G. L.↗

NASA NTRS1994

Transient microwave brightenings in solar active regions: Comparison between VLA and Yohkoh observations

We report observations of transient microwave (2 cm) brightenings and their relationship with brightenings in soft X-rays. The peak flux of the microwave brightenings observed by the Very Large Array (VLA) is smaller than the previously reported fluxes by two orders of magnitude. The microwave sources were highly polarized (up to 100%) and were situated on the periphery of a sunspot umbra. Among the many transients observed in X-rays by Yohkoh, two were observed simultaneously in microwaves. The microwave sources were found to be closer to the umbra of the sunspot than were the X-ray loops. It seems that the microwave sources are located at the footpoints of the looplike X-ray transients. Using the combined VLA, Yohkoh, and Mees data set, we determine the physical parameters of the loop in which the brightenings occur. We find that an increase in emission measure accompanied by small-scale heating can account for the X-ray brightening. The microwave emission can be interpreted as thermal gyroresonance or nonthermal gyrosynchrotron processes during the X-ray brightening. The magnetic field in the microwave-source region is found to be 1200-1800 G. The observations also provide evidence for temperature gradient in the coronal loops.

Gopalswamy, N.↗

NASA NTRS1992

Fluid flow in a jet and the Ca XIX line profiles observed during solar flares

The present study considers two types of fluid models as methods for reproducing the blueshifts and line broadening observed in soft X-ray lines during the rise of solar flares. The fluid models representing the laminar flow of material in a jet and through a pipe were used to derive the velocity at each location in the flow. The optimal values for the free parameters in the jet model were within the ranges expected for solar flare conditions. The jet was much smaller than typical flare volumes estimated from spatial images, but the resulting densities were consistent with the upper limit of densities derived using density-sensitive line ratios. This supports previous reports of small filling factors and is interpreted as evidence for a chromospheric origin for the flow. Stability analysis indicates that, although the laminar jet can reproduce the observations, instabilities may cause the flow to become turbulent, and that this laminar treatment is not strictly valid.

Bornmann, P. L.↗

NASA NTRS1991

Solar-A reformatted data files and observing log

An overview is presented of the Solar-A telemetry data files which are to be created and the format and organization which the files are to use. The organization chosen is to be efficient in space, to facilitate access to the data, and to allow the data to be transportable to different machines. An observing log file is to be created automatically, using the reformatted data files as the input. It will be possible to perform searches with the observing log to list cases where instruments are in certain modes and/or seeing certain signal levels. A user will be able to search the observing log and obtain a list of all cases where a given set of conditions are satisfied. An event log will be created listing the times when the instrument or spacecraft modes change.

Morrison, M. D.↗

NASA NTRS1991

Interrelation of soft and hard X-ray emissions during solar flares. I - Observations

The interrelation between the acceleration and heating of electrons and ions during impulsive solar flares is determined on the basis of simulataneous observations of hard and soft X-ray emission from the Solar Maximum Mission at high time resolution (6 s). For all the flares, the hard X-rays are found to have a power-law spectrum which breaks down during the rise phase and beginning of the decay phase. After that, the spectrum changes to either a single power law or a power law that breaks up at high energies. The characteristics of the soft X-ray are found to depend on the flare position. It is suggested that small-scale quasi-static electric fields are important for determining the acceleration of the X-ray-producing electrons and the outflowing chromospheric ions.

Winglee, R. M.↗

NASA NTRS1991

A statistical study of high coronal densities from X-ray line-ratios of Mg XI

An X-ray line-ratio density diagnostic was applied to 50 Mg XI spectra of flaring active regions on the sun recorded by the Flat Crystal Spectrometer on the SMM. The plasma density is derived from R, the flux ratio of the forbidden to intercombination lines of the He-like ion, Mg XI. The R ratio for Mg XI is only density sensitive when the electron density exceeds a critical value (about 10 to the 12th/cu cm), the low-density limit (LDL). This theoretical value of the low-density limit is uncertain as it depends on complex atomic theory. Reported coronal densities above 10 to the 12th/cu cm are uncommon. In this study, the distribution of R ratio values about the LDL is estimated and the empirical values are derived for the 1st and 2nd moments of this distribution from 50 Mg XI spectra. From these derived parameters, the percentage of observations is derived which indicated densities above this limit.

Linford, G. A.↗

NASA NTRS1991

Abundance variations in solar active regions

The diversity in the published values of coronal abundances is unsettling, especially as the range of results seems to be beyond the quoted uncertainties. Measurements of the relative abundance of iron and neon derived from soft X-ray spectra of active regions are presented. From a data base of over 200 spectra taken by the Solar Maximum Mission Flat Crystal Spectrometer, it is found that the relative abundance can vary by as much as a factor of about 7 and can change on timescales of less than 1 h.

Strong, K. T.↗

NASA NTRS1991

Imaging capabilities of SXT for Solar-A

The present study characterizes the Soft X-ray Telescope (SXT), a grazing-incidence solar X-ray telescope which will be flown on the Japanese Solar-A satellite. X-ray tests showed that the SXT mirror meets or exceeds design requirements. SXT advantages over Skylab are: improved time resolution (not less than 0.5 s), greatly decreased mirror scatter, coaligned aspect telescope to intersperse images of sunspots and magnetic plage, and digital data readout.

Lemen, J. R.↗

NASA NTRS1991

Plasma diagnostic capabilities of the Soft X-ray Telescope on Solar-A

The predicted response of the Solar-A Soft X-ray Telescope (SXT) to various solar targets is presented. It is found that the SXT is very sensitive and should be able to observe active regions and flares at its nominal cadence with exposure times ranging from 0.0001 to 1 s. The SXT temperature diagnostic capabilities are presented for various combinations of the X-ray filters.

Strong, K. T.↗

NASA NTRS1989

Turbulent and directed plasma motions in solar flares

An improved method for fitting asymmetric soft X-ray line profiles from solar flares is presented. A two-component model is used where one component represents the total emission from directed upflow plasma and the other the emission from the plasma at rest. Unlike previous methods, the width of the moving component is independent from that of the stationary component. Time variations of flare plasma characteristics (i.e., temperature, emission measure of moving and stationary plasma, upflow and turbulent velocities) are derived from the Ca XIX and Fe XXV spectra recorded by the Bent Crystal Spectrometer on the Solar Maximum Mission. The fitting technique provides a statistical estimation for the uncertainties in the fitting parameters. The relationship between the directed and turbulent motions has been studied, and a correlation of the random and directed motions has been found in some flares with intensive plasma upflows. Mean temperatures of the upflowing and stationary plasmas are compared for the first time from ratios of calcium to iron X-ray line intensities. Finally, evidence for turbulent motions and the possibility of plasma upflow late into the decay phase is presented and discussed.

Fludra, A.↗

NASA NTRS1989

A comparison of theoretical and solar-flare intensity ratios for the Fe XIX X-ray lines

Atomic data including energy levels, gf-values, and wavelengths are given for the Fe XIX transitions that give rise to lines in solar-flare and active-region X-ray spectra. Collision strengths and theoretical intensity ratios are presented for lines which occur in the 13.2-14.3-A range. Observed spectra are found to be consistent with those derived from the present Fe XIX atomic data. For the case of spectra in which the Fe XIX lines are very strong, such as those at the maxima of hot flares, two observed line features due to Fe XIX are shown to have larger intensities than calculated. The calculated Fe XIX and Ne IX line spectra are used to determine electron densities from Ne IX line ratios.

Bhatia, A. K.↗

NASA NTRS1989

Coronal activity in F-, G-, and K-type stars. III - The coronal differential emission measure distribution of Capella, Sigma-squared CrB, and Procyon

EXOSAT soft X-ray spectra of three binary systems of cool stars are analyzed: Capella (G6 III + F9 III), Sigma-squared CrB (F8 V + G1 V), and Procyon (F5 IV-V + DF). The EXOSAT transmission grating spectrometer permits the study of individual spectral lines and line complexes between 10 and 200 A with approximately 3 A resolution. First it is demonstrated that the spectra can be described reasonably well by a two-temperature model corona. Then the assumption that only two temperatures exist in the stellar coronas is relaxed and differential emission measure distributions are derived from the three spectra. The results from the multithermal modeling are consistent with those of the two-temperature models: emission from the coronas of each of the three stars is dominated by plasma in two relative narrow temperature intervals. These intervals are centered on 5 MK and 25 MK in the cases of Capella and Sigma-squared CrB, and 0.6 MK and 3 MK in the case of Procyon. The implications of the results for the structure of stellar coronas are briefly discussed.

Lemen, J. R.↗