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At least 289 records · Page 16
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NASA NTRS1991

X-ray observations of filament eruption in the 1980 May 21 flare

X-ray and H-alpha observations of an erupting filament and other observations of the associated flare on May 21, 1980 suggest that an erupting filament played a major role in the X-ray flare. While Antonucci et al. (1985) analyzed the May 21 flare as one of the best cases of chromospheric evaporation, the possible contribution from X-ray emitting erupting plasma has been ignored. It is that preheated plasma existed and may have contributed part of the blue-shifted X-ray emission observed in the Ca XIX line, which was formerly attributed solely to chromospheric evaporation. Thus it remains an open question - in two-ribbon flares in particular - just how important chromospheric evaporation is in flare dynamics.

Batchelor, D. A.↗
NASA NTRS1992

Martens-Kuin models of normal and inverse polarity filament eruptions and coronal mass ejections

An analysis is made of the Martens-Kuin filament eruption model in relation to observations of coronal mass ejections (CMEs). The field lines of this model are plotted in the vacuum or infinite resistivity approximation with two background fields. The first is the dipole background field of the model and the second is the potential streamer model of Low. The Martens-Kuin model predicts that, as the filament erupts, the overlying coronal magnetic field lines rise in a manner inconsistent with observations of CMEs associated with eruptive filaments. This model and, by generalization the whole class of so-called Kuperus-Raadu configurations in which a neutral point occurs below the filament, are of questionable utility for CME modeling. An alternate case is considered in which the directions of currents in the Martens-Kuin model are reversed resulting in a so-called normal polarity configuration of the filament magnetic field. The background field lines now distort to support the filament and help eject it. While the vacuum field results make this configuration appear very promising, a full two- or more-dimensional MHD simulations is required to properly analyze the dynamics resulting from this configuration.

Smith, D. F.↗
NASA NTRS1992

Volcanic winter and accelerated glaciation following the Toba super-eruption

Model calculations that investigate the possible climatic effects of the Toba volcanic cloud are presented. The increase in atmospheric opacity might have produced a 'volcanic winter', followed by a few years with maximum estimated annual hemispheric surface-temperature decreases of 3-5 C. The eruption occurred during the stage 5a-4 transition of the oxygen isotope record, a time of rapid ice growth and falling sea level. It is suggested that the Toba eruption may have greatly accelerated the shift to glacial conditions that was already under way, by inducing perennial snow cover and increased sea-ice extent at sensitive northern latitudes. As the onset of climate change may have helped to trigger the eruption itself, it is proposed that the Toba event may exemplify a more general climate-volcano feedback mechanism.

Rampino, Michael R.↗
NASA NTRS1993

Tropical ozone loss following the eruption of Mt. Pinatubo

Total Ozone Mapping Spectrometer (TOMS) measurements of equatorial total ozone following the eruption of Mt. Pinatubo show a decrease of up to 6 percent over climatology. Ozone losses begin approximately a month following the eruption, consistent with the time required for the SO2 to convert to sulfuric acid aerosol. The thick aerosol layer interferes with the TOMS retrieval, but this interference is small and easily accounted for in the retrieval. Ozone values remain below climatology until December, 1991. Ozonesonde data from Natal, Brazil taken before and two months after the eruption support TOMS observations of ozone loss. These sondes show that the ozone loss region is confined to a 2-3 km thick layer between 24 and 28 km.

Schoeberl, Mark R.↗
NASA NTRS1993

Multiple wavelength observations of an off-limb eruptive solar flare

The eruptive prominence and limb flare which occurred at 1454 UT on June 20, 1989 is described and analyzed. This event was observed by many different instruments providing an unusual amount and variety of data: images at 1.4 GHz, 37 GHz, and H-alpha, and spectra in hard X-ray, soft X-ray, and radio frequencies. This array of data makes it possible to explore the relationships between flare and eruptive prominence emissions at different wavelengths. VLA images at 1.4 GHz show changing sources in a set of high (about 10 exp 10 cm) coronal loops associated with the erupting prominence. We use a full gyrosynchrotron code to model a 1.4 GHz source early in the flare as a large coronal loop. The model results lead us to conclude that the initial acceleration occurs in smaller, denser loops which also produce the flare's hard X-ray emission. We also present evidence that a source at 1.4 GHz later in the event is due to second-harmonic plasma emission. This source is adjacent to a leg of the prominence and comes from a dense column of material in the magnetic structure supporting the prominence.

Kugera, T. A.↗
NASA NTRS1993

Eruption history of the Tharsis shield volcanoes, Mars

The Tharsis Montes volcanoes and Olympus Mons are giant shield volcanoes. Although estimates of their average surface age have been made using crater counts, the length of time required to build the shields has not been considered. Crater counts for the volcanoes indicate the constructs are young; average ages are Amazonian to Hesperian. In relative terms; Arsia Mons is the oldest, Pavonis Mons intermediate, and Ascreaus Mons the youngest of the Tharsis Montes shield; Olympus Mons is the youngest of the group. Depending upon the calibration, absolute ages range from 730 Ma to 3100 Ma for Arsia Mons and 25 Ma to 100 Ma for Olympus Mons. These absolute chronologies are highly model dependent, and indicate only the time surficial volcanism ceased, not the time over which the volcano was built. The problem of estimating the time necessary to build the volcanoes can be attacked in two ways. First, eruption rates from terrestrial and extraterrestrial examples can be used to calculate the required period of time to build the shields. Second, some relation of eruptive activity between the volcanoes can be assumed, such as they all began at a speficic time or they were active sequentially, and calculate the eruptive rate. Volumes of the shield volcanoes were derived from topographic/volume data.

Plescia, J. B.↗
NASA NTRS1994

Helicity charging and eruption of magnetic flux from the Sun

The ejection of helical toroidal fields from the solar atmosphere and their detection in interplanetary space are described. The discovery that solar magnetic fields are twisted and that they are segregated by hemisphere according to their chirality has important implications for the escape process. The roles played by erupting prominences, coronal mass ejections (CME's) and active region (AR) loops in expressing the escape of magnetic flux and helicity are discussed. Sporadic flux escape associated with filament eruptions accounts for less than one-tenth the flux loss. Azimuthal flux loss by CME's could account for more, but the major contributor to flux escape may be AR loop expansion. It is shown how the transfer of magnetic helicity from the sun's interior into emerged loops ('helicity charging') could be the effective driver of solar eruptions and of flux loss from the sun.

Rust, David M.↗
NASA NTRS1993

Properties and decay of stratospheric aerosols in the Arctic following the 1991 eruptions of Mount Pinatubo

Sunphotometer observations made from an aircraft several months after the June 1991 eruptions of Mount Pinatubo are used to quantify the spectral opacity of the Arctic stratosphere. Ancillary surface-based measurements are presented in support of the aircraft data that show large increases in stratospheric optical depth attributed to the presence of volcanic aerosols. Visible optical depths greater than 0.2 were observed during flight segments flown above the tropopause. An inversion algorithm and the optical depth data are used to infer effective aerosol size distributions. The distributions tend to be bimodal, having a large-partical mode radius of about 0.50 micron and a small-particle mode of higher concentration with radii less than 0.18 micron. Surface measurements made during spring 1992 and 1993 are also used to estimate a time constant (e-folding time) of about 13.5 months assuming that the Arctic stratosphere's opacity decays exponentially; this estimate is larger than decay times observed following other major volcanic eruptions. Our results suggest that any climate perturbations in the Arctic caused by eruptions of Pinatubo may be significant and will very likely persist longer than any volcanically-induced changes observed there during the past century.

Stone, Robert S.↗
NASA NTRS1994

On current sheet approximations in models of eruptive flares

We consider an approximation sometimes used for current sheets in flux-rope models of eruptive flares. This approximation is based on a linear expansion of the background field in the vicinity of the current sheet, and it is valid when the length of the current sheet is small compared to the scale length of the coronal magnetic field. However, we find that flux-rope models which use this approximation predict the occurrence of an eruption due to a loss of ideal-MHD equilibrium even when the corresponding exact solution shows that no such eruption occurs. Determination of whether a loss of equilibrium exists can only be obtained by including higher order terms in the expansion of the field or by using the exact solution.

Bungey, T. N.↗
NASA NTRS2001

EIT Crinkles as Evidence for the Breakout Model of Solar Eruptions

We present observations of two homologous flares in NOAA active region 8210 occurring on 1998 May 1 and May 2, using EUV data from the Extreme Ultraviolet Radiation Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), high-resolution and high-time cadence images from the soft X-ray telescope (SXT) on Yohkoh, images or fluxes from the hard X-ray telescope (HXT) on Yohkoh and the BATSE experiment on the Compton Gamma Ray Observatory (CGRO), and Ca xix soft X-ray spectra from the Bragg crystal spectrometer (BCS) on Yohkoh. Magnetograms indicate that the flares occurred in a complex magnetic topology, consisting of an emerging flux region (EFR) sandwiched between a sunspot to the west and a coronal hole to the east. In an earlier study we found that in EIT images, both flaring episodes showed the formation of a crinkle-like pattern of emission ("EIT crinkles") occurring in the coronal hole vicinity, well away from a central "core field" area near the EFR-sunspot boundary. With our expanded data set, here we find that most of the energetic activity occurs in the core region in both events, with some portions of the core brightening shortly after the onset of the EIT crinkles, and other regions of the core brightening several minutes later, coincident with a burst of hard X-rays: there are no obvious core brightenings prior to the onset of the EIT crinkles. These timings are consistent with the "breakout model" of solar eruptions, whereby the emerging flux is initially constrained by a system of overlying magnetic field lines, and is able to erupt only after an opening develops in the overlying fields as a consequence of magnetic reconnection at a magnetic null point. In our case, the EIT crinkles would be a signature of this pre-impulsive-phase magnetic reconnection, and brightening of the core only occurs after the core fields begin to escape through the newly-created opening in the overlying fields. Morphology in soft X-ray images and properties in hard X-rays differ between the two events, with complexities that preclude a simple determination of the dynamics in the core at the times of eruption. From the BCS spectra, we find that the core region expends energy at a rate of approx. 10(exp 26) erg per second during the time of the growth of the EIT crinkles; this rate is an upper limit to energy expended in the reconnections opening the overlying fields. Energy losses occur at an order-of-magnitude higher rate near the time of the peak of the events. There is little evidence of asymmetry in the spectra, consistent with the majority of the mass flows occurring normal to the line-of-sight. Both events have similar electron temperature dependencies on time.

Sterling, Alphonse C.↗
NASA NTRS2001

EIT Crinkles as Evidence for the Breakout Model of Solar Eruptions

We present observations of two homologous flares in NOAA Active Region 8210 occurring on 1998 May 1 and 2, using EUV data from the EUV Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory, high-resolution and high-time cadence images from the soft X-ray telescope on Yohkoh, images or fluxes from the hard X-ray telescope on Yohkoh and the BATSE experiment on board the Compton Gamma Ray Observatory, and Ca(XIX) soft X-ray spectra from the Bragg crystal spectrometer (BCS) on Yohkoh. Magnetograms indicate that the flares occurred in a complex magnetic topology, consisting of an emerging flux region (EFR) sandwiched between a sunspot to the west and a coronal hole to the east. In an earlier study we found that in EIT images, both flaring episodes showed the formation of a crinkle-like pattern of emission (EIT crinkles) occurring in the coronal hole vicinity, well away from a central 'core field' area near the EFR-sunspot boundary. With our expanded data set, here we find that most of the energetic activity occurs in the core region in both events, with some portions of the core brightening shortly after the onset of the EIT crinkles, and other regions of the core brightening several minutes later, coincident with a burst of hard X-rays; there are no obvious core brightenings prior to the onset of the EIT crinkles. These timings are consistent with the 'breakout model' of solar eruptions, whereby the emerging flux is initially constrained by a system of overlying magnetic field lines, and is able to erupt only after an opening develops in the overlying fields as a consequence of magnetic reconnection at a magnetic null point. In our case, the EIT crinkles would be a signature of this pre-impulsive phase magnetic reconnection, and brightening of the core only occurs after the core fields begin to escape through the newly created opening in the overlying fields. Morphology in soft X-ray images and properties in hard X-rays differ between the two events, with complexities that preclude a simple determination of the dynamics in the core at the times of eruption. From the BCS spectra, we find that the core region expends energy at a rate of approximately 10(exp 26) ergs/s during the time of the growth of the EIT crinkles; this rate is an upper limit to energy expended in the reconnections opening the overlying fields. Energy losses occur at an order of magnitude higher rate near the time of the peak of the events. There is little evidence of asymmetry in the spectra, consistent with the majority of the mass flows occurring normal to the line of sight. Both events have similar electron temperature dependencies on time.

Sterling, Alphonse C.↗
NASA NTRS2001

Did the Eruption of the Mt. Pinatubo Volcano Affect Cirrus Properties?

Some observations suggest that the volcanic aerosols produced by the Mt. Pinatubo eruption may have altered cirrus properties. We look for evidence that such modification of cirrus is extensive enough to be climatically significant by comparing three satellite-based cirrus datasets produced by the ISCCP (International Satellite Cloud Climatology Project) analysis, the 'split-window' analysis, and 3I analysis. Since the former two have not been compared in detail before, we conduct such a comparison here. When applied to AVHRR (Advanced Very High Resolution Radiometer) data, both the ISCCP and split-window analyses identify about 0.2 - 0.3 cirrus cloud amount in tropical latitudes; however, there are detailed differences of classification for about half of these clouds. The discrepancies are attributed to the simplified assumptions made by both methods. The latter two datasets are derived from infrared radiances, so they are much less sensitive to volcanic aerosols than the ISCCP analysis. After the Mt. Pinatubo eruption, the ISCCP results indicate a dramatic decrease of thin cirrus (cloud top pressure less than 440 mb and visible optical thickness less than 1.3) over ocean, accompanied by a comparable increase of altocumulus and cumulus clouds; over land, there are no significant changes. In contrast, results from the split-window and 3I analyses show little change in thin cirrus amount over either ocean or land that is associated with the volcanic eruption. The ISCCP results can, therefore, be understood as a misclassification of thin cirrus because the additional reflected sunlight by the volcanic aerosol makes the cirrus clouds appear to be optically thicker. Examination of the split-window signature and the infrared emissivities from 3I show no significant change in infrared emissivity (or optical thickness). These results indicate that the Mt. Pinatubo volcanic aerosol did not have a significant systematic effect on tropical cirrus properties, but rather produced only temporary, local effects. Hence, these results indicate that there was no significant climate feedback produced by aerosol-cirrus-radiative interactions.

Luo, Zhengzhao↗
NASA NTRS2002

Trajectories of Microwave Prominence Eruptions

On the basis of 17 GHz microwave images from the Nobeyama Radioheliograph near solar maximum (1999-2000), we examined trajectories of 50 prominence eruptions in order to address how prominence motions affect or reflect the surrounding coronal structures. We marked the heliocentric latitude of the top of the moving prominences on the white-light synoptic maps, which were constructed at 2.5-4.5 solar radii from the LASCO C2 coronagraph on the SOHO spacecraft. We found that i) 92% of the prominence eruptions were associated with coronal mass ejections (CMEs) and that the remaining 8% show weak mass motions confined to nearby streamers, and ii) coronal mass motions involving prominence eruptions and CMEs are not random but are organized by bundles of streamers. Our findings support the idea of multiple plasma sheets emanating from active regions, arcades, trans-equatorial interconnecting loops, and polar crown filaments, through which coronal mass is transported toward interplanetary space. This study suggests that microwave observations can provide useful information on the activity at the base of such "coronal mass corridors".

Hori, K.↗
NASA NTRS2003

Hard X-Ray, Soft X-Ray, and EUV Studies of Solar Eruptions

Document study the hard X-ray (HXR), soft X-ray (SXR) ,EUV, and magnetic nature of solar eruptions, with the objective of elucidating the physics of the eruption process. In particular, it was examine the viability of two specific eruption mechanisms, detailed in our proposal. These mechanisms are the "breakout model", and the "tether cutting model". During the second year, it was a significant progress in the goals to Data Sets Utilized. In the publications during this second year of the grant period, the data was used from the E W Imaging Telescope (EIT) and the Michelson Doppler Imager (MDI) instruments on SOHO, and from the Soft X-ray Telescope (SXT), Hard X-ray Telescope (HXT), and the Bragg Crystal Spectrometer (BCS) on Yooh.

Sterling, Alphonse C.↗
NASA NTRS2004

Topographic Evidence for Eruptive Style Changes and Magma Evolution of Small Plains-style Volcanoes on Earth and Mars

Topographic profiles and surface characteristics of small (5 - 25 km diameter) plains-style shield volcanoes on the eastern Snake River Plain (ESRP) provide a method to evaluate eruptive processes and magmatic evolution on Martian volcanic plains. The ESRP is an ideal place to observe Mars-like volcanic features where hundreds of small monogenetic basaltic shields dominate the volcanic-sedimentary depositional sequence, and numerous planetary analogues are evident: coalescent mafic shields, hydromagmatic explosive eruptions, the interaction of lava flows with surficial water and glacial ice, and abundant eolian sand and loess. Single flows cannot be correlated over great distances, and are spatially restricted. These relations are useful for planetary exploration when inferring volcanic evolutionary patterns in lava plains represented by numerous eruptive vents. High spatial resolution imagery and digital topographic data for Mars from MOC, MOLA, and THEMIS is allowing for improvements in the level of detail of stratigraphic mapping of fields of small (< 25 km in diameter) volcanoes as well as studies of the morphological characteristics of individual volcanoes. In order to compare Mars and Earth volcanic features, elevation data from U.S.G.S. 10-meter digital elevation models (DEMs) and high-precision GPS field measurements are used in this study to generate approx. 20m spacing topographic profiles from which slope and surface morphology can be extracted. Average ESRP flank and crater slopes are calculated using 100 - 200 m spacing for optimum comparison to MOLA data, and to reduce the effects of surface irregularities.

Hughes, S. S.↗
NASA NTRS2004

Evidence of "Tether-Cutting" Reconnection in the Onset of a Quadrupolar Solar Magnetic Eruption

Extensive study of the near-limb solar filament eruption event on 2000 February 26, involving coronal images from YOHKOH, SOHO EIT and photospheric magnetogram from MID have shown that that both "runaway-tether-cutting-type reconnection" and "fast breakout-type reconnection" may have occurred early in the fast phase of the eruption and may have played an important role in unleashing the explosion (Sterling & Moore 2004). That study did not identify which or if either of these types of reconnection actually triggered the fast phase. Here, together with a magnetogram and He1 10830 A filtergram from NSO/KP, we present Halpha filtergrams from Big Bear Solar Observatory, that show evidence of "tether-cutting-type reconnection" before and during the eruption of the southern filament, situated at one of the neutral lines of the quadrupole magnetic structure.

Choudhary, Debi Prasad↗
NASA NTRS2004

Post Eruption Hazards at Mt. Pinatubo, Philippines

Our project focused on the investigation of the post-eruption hazards at Mt. Pinatubo (Philippines) using remote sensing data, and field observations of the 1991 eruption deposits. Through the use of multiple satellite images, field work, and the 1996/2000 PacRim data sets, we conducted studies of the co- and post-eruption hazards of the volcano due to erosion and re-deposition of the extensive pyroclastic flow deposits. A major part of this project was the assembly and analysis of a database of over 50 high resolution (1 - 50 m/pixel) images that will facilitate this study. We collected Ikonos, SPOT, SIR-C/X-SAR, Landsat, ERS, RADARSAT, and ASTER images of the area around Mt. Pinatubo. An example of the changes that could be seen in these data is shown. Our investigation focused on a retrospective analysis of the erosion, redeposition, and re-vegetation of the 1991 pyroclastic flow deposits of Mt. Pinatubo. The primary geologic goal of our work was the analysis of the spatial distribution and volume change of the sources and sinks of materials associated with mudflow ('lahar') events. This included the measurement of river valley gradients and cross-sections using TOPSAR digital elevation data, as we are participating in the PacRim 2000 deployment to the Philippines specifically so that we can collect a second set of TOPSAR data that can then be used to create a topographic difference image of the volcano. The main results from this multi-sensor study have been published as Torres et al.. A discussion of the methodology that we used to assemble an appropriate database was included in Mouginis-Mark and Domergue-Schmidt. As part of an educational outreach effort, we also helped the Philippine Institute of Volcanology and Seismology (PHIVOLCS) in the Philippines to use NASA data to study Mt. Pinatubo and other Filipino volcanoes.

Mouginis-Mark, Peter J.↗