Engineering topics

Simnett, G. M.

Publications and source records attributed to Simnett, G. M..

At least 19 records

NASA NTRS1999

Bright Points and Subflares in Ultraviolet Lines and X-Rays

We have analyzed an active region which was observed in H.alpha (Multichannel Subtractive Double Pass Spectrograph), in UV lines (SMM/UVSP), and in X-rays (SMM/HXIS). In this active region there were only a few subflares and many small bright points visible in UV and in X-rays. Using an extrapolation based on the Fourier transform, we have computed magnetic field lines connecting different photospheric magnetic polarities from ground-based magnetograms. Along the magnetic inversion lines we find two different zones: (1) a high-shear region (> 70 deg) where subflares occur, and (2) a low-shear region along the magnetic inversion line where UV bright points are observed. In these latter regions the magnetic topology is complex with a mixture of polarities. According to the velocity field observed in the Si IV lamda.1402 line and the extrapolation of the magnetic field, we notice that each UV bright point is consistent with emission from low-rising loops with downflows at both ends. We notice some hard X-ray emissions above the bright-point regions with temperatures up to 8 x 10(exp 6) K, which suggests some induced reconnection due to continuous emergence of new flux. This reconnection is also enhanced by neighboring subflares.

Rovira, M.↗

NASA NTRS1998

Bright Points and Subflares in UV Lines and in X-Rays

We have analysed an active region which was observed in Halpha (MSDP), UV lines (SMM/UVSP), and in X rays (SMM/HXIS). In this active region there were only a few subflares and many small bright points visible in UV and in X rays. Using an extrapolation based on the Fourier transform we have computed magnetic field lines connecting different photospheric magnetic polarities from ground-based magnetograms. Along the magnetic inversion lines we find 2 different zones: 1. a high shear region (less than 70 degrees) where subflares occur 2. a low shear region along the magnetic inversion line where UV bright points are observed.

Rovira, M.↗

NASA NTRS1998

Ulysses-UVCS Coordinated Observations

We present results from coordinated observations in which instruments on SOHO and Ulysses were used to measure the density and flow speed of plasma at the Sun and to again measure the same properties of essentially the same plasma in the solar wind. Plasma was sampled by Ultraviolet Coronagraph Spectrometer (UVCS) at 3.5 and 4.5 solar radii and by Ulysses at 5 AU. Data were acquired during a nearly 2 week period in May-June 1997 at a latitude of 9-10 degrees north of the equator, on the east limb and, hence, in the streamer belt region and the source location of slow wind. Density and outflow plasma speed are compared, in order to check for preservation of the near Sun characteristics in the interplanetary medium. By chance, Ulysses was at the very northern edge of the visible streamer belt. Nevertheless, no evidence of fast wind, or mixing with fast wind coming from the northern polar coronal hole was evident at Ulysses. The morphology of the streamer belt was the same at the beginning and end of the observation period, but changed markedly during the middle of the period. A corresponding change in density (but not flow speed) was noted at Ulysses.

Suess, Steven T.↗

NASA NTRS1997

White-light coronal mass ejections: A new perspective from LASCO

More than 275 coronal mass ejections (CMEs) were recorded by the large angle spectroscopic coronagraph (LASCO) from January 1996 through August 1997. Some of the characteristics of 65 of these CMEs, occurring during a three month period (May to July 1997) were quantified. During this time the rate of CME detection was about 0.7 CMEs per day; the distribution of apparent latitudes was clustererd near the equator with an average latitude of 3 deg N; the average width of the CMEs was 49 deg; and the average speed was 324 km/s. The statistical measures and the distributions for these CMEs agree with the existing literature. One new result was the high fraction (plus or minus 35 deg) of CMEs with at least one concave-outward morphological feature, which was considered a possible signature of magnetic disconnection. A new small-scale phenomenon that appears to be the white light counterpart of the extreme ultraviolet microjets detected in the polar coronal holes is described.

St.Cyr, O. C.↗

NASA NTRS1997

The evolution of a rapidly-expanding active region loop into a trans-equatorial coronal mass ejection

On 23 February 1997, a coronal mass ejection erupted off the NE limb of the sun from a coronal loop system which had earlier been visible soft X-rays and Fe XIV. The ejection coincided with the onset of a small soft X-ray event, and it left the corona at a position angle of around 60 deg at around 880 km s(exp -1). This ejection then merged with a much larger event which spanned the equator and became indistinguishable, in projection, with the primary event. The soft X-ray images indicate that the highest temperature plasma was associated with the loop system near the original erupting loop. A large loop system became visible south of the equator as the coronal mass ejection developed. It appears that there are high closed coronal magnetic loops linking the northern region to that in the south.

Simnett, G. M.↗

NASA NTRS1997

Prominence activity related to CME observed by SOHO, Yohkoh and ground-based observatories

Examples of destabilization of prominences and their associated coronal mass ejections (CMEs) are presented. During the 1996 campaigns of multi-wavelength observations with the Solar and Heliospheric Observatory (SOHO), the Yohkoh satellite's soft X-ray telescope (SXT) and the Meudon (France) H alpha spectroheliograph eruptive solar filaments and prominences associated with the CMEs were observed. Two of the observed events showed that CMEs and 'brusques disparitions' (BDs) seem to be consequences of global magnetic field instability.

Schmieder, B.↗

NASA NTRS1995

Simultaneous ultraviolet and X-ray observations of solar microflares

We present a comparison of observations of a solar active region obtained with the Ultraviolet Spectrometer and Polarimeter (UVSP) and the Hard X-ray Imaging Spectrometer (HXIS) on board the Solar Maximum Mission (SMM). The data show many subflares and smaller events during a 9 hr period in which there were no major flares. This activity is associated with areas of complex, evolving magnetic fields. Events substantially smaller than subflares, identified in C IV 1548 A emission (T = 10(exp 5) K), are found to have impulsive counterparts in 3.5-5.5 keV X-ray emission characteristic of T approximately 10(exp 7) K. These 'microflares' are therefore true members of the flare family, reaching temperatures greater than that of the ambient solar corona. Plots of X-ray versus UV emission show that the emission from the smaller microflares is softer than that from the larger microflares and subflares. Distributions of solar flares with energy that are based on hard X-ray fluxes can be corrected using this X-ray/UV relationship. The corrected distributions suggest that microflares play a larger role in coronal heating than was indicated by analysis of the original distributions.

Porter, J. G.↗

NASA NTRS1994

Corotating particle enhancements out of the ecliptic plane

We have studied greater than 40 keV ion and electron enhancements seen at Ulysses from a series of corotating interaction regions (CIR), from mid-1992 until the end of 1993. The event intensity increased up to latitude 20 deg S; after that the ion flux maxima decreased by approximately 23%/deg while the electron maxima decreased more erratically, and more slowly, during the last half of 1993. After April, 1993, the electron increases lagged the ions by up to four days. The electron energy spectrum hardened both towards the end of the events and as the latitude went above approximately 35 deg. The electron/ion delays and the electron spectral hardening are related to the three-dimensional structure of the reverse shocks at high latitudes. The H/He ratio at approximately 0.4-1.2 MeV/nucleon decreased towards the end of each event; the mean minimum value was 3.5 +/- 0.3, lower than observed in earlier studies of CIR-associated events near the ecliptic plane. The He increases have recurred with a period of 26.0 days. There have been two phase changes: +4.3 days (February 1993) and +2.4 days (August 1993), related to changes in the coronal structure of the source regions for the high speed solar wind streams.

Simnett, G. M.↗

NASA NTRS1994

Time evolution of a miniflare as seen in H-alpha, UV lines, and X-rays

A miniflare that occurred in active region Hale 16896 on 1980 June 15 was observed in H-alpha, UV lines, and soft X-rays. These data allow us to analyze the flare emission and derive the energetics and dynamics of the chromospheric plasma involved. Our results indicate that the energy released by the miniflare was about 10(exp 28) ergs, about four orders of magnitude smaller than that of a large flare. However, hard X-rays (5.5-8.0 keV) were observed which indicated a plasma with temperature as high as 27 x 10(exp 6) K. The H-alpha wings, C IV, and X-rays all showed a short-lived and compact (3 sec x 3 sec) brightening in a location near the leading sunspot. At this location, small-scale changes in the magnetic field were observed from about 2 hr before to about 6 hr after the miniflare. Only very small velocities seem to have been associated with most of the event in H-alpha. The data are consistent with short and dense loops at temperatures above 10(exp 7) K which lost a large fraction of their energy via downward conduction through regions at the footpoints. Several secondary events appear to have been triggered by the miniflare along an arch filament which itself was not greatly affected. A number of much less energetic (10(exp 25)-10(exp 26) erg) events preceded and followed the mini-flare. One of these occurred 11 minutes after the mini-flare and displayed a highly Doppler-shifted signature from the foot-point of a C IV arch. This arch delineates the connection that existed between the location of the mini-flare and the location of secondary events triggered by the flare.

Fontenla, J.↗

NASA NTRS1994

The correlation of solar flare hard X-ray bursts with Doppler blueshifted soft X-ray flare emission

We have investigated the temporal correlation between hard X-ray bursts and the intensity of Doppler blueshifted soft X-ray spectral line emission. We find a strong correlation for many events that have intense blueshifted spectral signatures and some correlation in events with modest blueshifts. The onset of hard X-rays frequently coincides to within a few seconds with the onset of blueshifted emission. The peak intensity of blueshifted emission is frequently close in time to the peak of the hard X-ray emission. Decay rates of the blueshifted and hard X-ray emission are similar, with the decay of the blueshifted emission tending to lag behind the hard X-ray emission in some cases. There are, however, exceptions to these conclusions, and, therefore, the results should not be generalized to all flares. Most of the data for this work were obtained from instruments flown on the Japanese Yohkoh solar spacecraft.

Bentley, R. D.↗

NASA NTRS1992

The hot plasma environment at Jupiter - Ulysses results

Initial results obtained from measurements made by the HI-SCALE (heliosphere instrument for spectra, composition, and anisotropy at low energies) experiment are reported. Data revealed that the Jovian magnetosphere is very extended, with the day-side magnetopause located at about 105 Jupiter radii. The relative abundances of sulfur, oxygen, and sodium to helium decreased with the decreasing radial distance from the planet on the day-side, which suggests that the abundances of Jupiter-derived species are dependent on latitude. Intense fluxes of counter-streaming ions and electrons were discovered in the dusk-side, high-latitude region from the edge of the plasma sheet to the dusk-side magnetopause. These beams of ions and electrons appeared to be very tightly aligned with the magnetic field and to be superimposed on a time- and space variable isotropic hot plasma background. The current carried by measured hot plasma particles are about 1.6 x 10 exp -4 microamps per sq m.

Lanzerotti, L. J.↗

NASA NTRS1992

Low-energy solar electrons and ions observed at Ulysses February-April, 1991 - The inner heliosphere as a particle reservoir

Ulysses observations at 2.5 AU of 38-315 keV electrons and 61-4752 keV ions during February-April 1991 suggest in several ways that, during periods of sustained high solar activity, the inner heliosphere serves as a 'reservoir' for low-energy solar particles. Particle increases were not associated one-to-one with large X-ray flares because of their poor magnetic connection, yet intensities in March-April remained well above their February levels. The rise phase of the particle event associated with the great flare of 2245UT March 22 lasted most of two days, while throughout the one-week decay phase, the lowest-energy ion fluxes were nearly equal at Ulysses and earth (IMP-8).

Roelof, E. C.↗

NASA NTRS1992

The effect of the shock of 15:43 UT March 23, 1991 on 50 keV to 5 MeV ions at Ulysses

The intensities, angular distributions, and energy spectra of 50 keV to 5 MeV ions are examined in the several hour time interval when an interplanetary shock passed the Ulysses spacecraft. The increase in intensity, evolution of pitch angle distribution from upstream flow away from the shock to a post-shock 'pancake' distribution, the earlier arrival of more energetic particles, and the presence of non-gyrotropic effects at the time of shock passage all can be understood in terms of kinematical behavior of the particles crossing the shock.

Armstrong, T. P.↗

NASA NTRS1992

Heliosphere Instrument for Spectra, Composition and Anisotropy at Low Energies

The Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI-SCALE) is designed to make measurements of interplanetary ions and electrons throughout the entire Ulysses mission. The ions (E(i) greater than about 50 keV) and electrons (E(e) greater than about 30 keV) are identified uniquely and detected by five separate solid-state detector telescopes that are oriented to give nearly complete pitch-angle coverage from the spinning spacecraft. Ion elemental abundances are determined by Delta E vs E telescope using a thin (5 microns) front solid state detector element in a three-element telescope. Experimental operation is controlled by a microprocessor-based data system. Inflight calibration is provided by radioactive sources mounted on telescope covers which can be closed for calibration purposes and for radiation protection during the course of the mission. Ion and electron spectral information is determined using both broad-energy-range rate channels and a 32 channel pulse-height analyzer for more detailed spectra. Some initial in-ecliptic measurements are presented which demonstrate the features of the instrument.

Lanzerotti, L. J.↗

NASA NTRS1991

Accelerated particles in solar flares

The variety of accelerated particle phenomena in solar flares is reviewed, including particle observation in interplanetary space and gamma-ray, neutron, hard X-ray, and radio emissions. It is emphasized that a significant or perhaps even large fraction of the total solar-flare energy is in accelerated particles. It is pointed out that the majority of the particles that produce impulsive flare phenomena remain trapped at the sun, probably due to confinement in the flaring magnetic loops. Gamma-ray observations are used to calculate the current rate of irradiation of the sun by MeV protons accelerated in solar flares. The current number of escaping protons is compared with irradiation rates of meteorites implied by observations of isotopic anomalies, and it is found that, except for the Ne-21 anomaly, enhanced proton irradiations from the ancient sun could not have been responsible for the observed isotopic anomalies.

Ramaty, R.↗

NASA NTRS1990

Evaporation in the transition region during the gradual phase of flares

Previous observations have revealed that small, but sustained, H-alpha blueshifts occur in flare ribbons during the gradual, or late, phase of flares. These blueshifts suggest that there is a gentle evaporation of chromospheric material throughout the late phase of flares, but ambiguities in the interpretation of H-alpha leave open the possibility that these blueshifts are caused by downflowing, rather than upflowing, material. Using both C IV and soft (3.5-8 keV) X-ray data from SMM observations, evidence is found which supports the interpretation of the H-alpha blueshifts as upflows in the range from 4-12 km/s. The blueshifts are interpreted in terms of the reconnection model proposed by Carmichael (1964). The model produces a sustained energy release through-out the late phase which accounts for the prolonged soft X-ray emission after a flare. This energy release comes from the reconnecting magnetic field above the flare site, and some of the energy is transported along field lines mapping to the chromosphere where it drives chromospheric evaporation.

Schmieder, B.↗

NASA NTRS1988

An example of the association of X-ray and UV emission with H-alpha surges

From H-alpha UV, and X-ray data, the nature of a well-observed surge on November 12, 1980 has been studied to try to understand the relationships between the mechanical motions and the high-temperature emissions. The cool (H-alpha) and the hot (O v) parts of the surge plasma both show velocities in the range of 100-120 km/s. The kinetic and potential energies of the surge are two orders of magnitude higher than the radiative energy. The observations suggest that the surge occurs in open structures parallel to one leg of a large scale coronal magnetic loop. The energy is released principally in the open structure (surge) and only a small amount heats the coronal loop (X-ray signature). This energy division should probably not be treated as a general characteristic of surge events. Different events are expected to exhibit a different energy balance, and this can account for the lack of a consensus in the previous literature regarding surge/X-ray associations.

Schmieder, B.↗

NASA NTRS1987

Electroformed grazing incidence X-ray mirrors for a mirror array telescope

Grazing incidence Wolter type I mirrors for higher-energy X-rays have been replicated from two superpolished mandrels by electroforming. Single mirrors and a nested pair were tested with 1.5- and 6.4-keV X-rays, and their subminute of arc resolution and reflectivity close to the theoretical values are confirmed. The design of the mandrels, the mirror mounting scheme, and results of the X-ray test are presented. The microroughnesses of the mirrors measured using an optical profilometer were compared with the X-ray test results.

Ulmer, Melville P.↗