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Sterling, Alphonse C.

Publications and source records attributed to Sterling, Alphonse C..

At least 109 records · Page 6
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NASA NTRS2006

Origin of the Sheared Magnetic Fields that Erupt in Flares and Coronal Mass Ejections

From a search of the Yohkoh/SXT whole-Sun movie in the years 2000 and 2001, we found 37 flare-arcade events for which there were full-disk magnetograms from SOHO/MDI, coronagraph movies from SOHO/LASCO, and before and after full-disk chromospheric images from SOHOBIT and/or from ground-based observatories. For each event, the observations show or strongly imply that the flare arcade was produced in the usual way by the eruption of sheared core field (as a flux rope) from along the neutral line inside a mature bipolar magnetic arcade. Two-thirds (25) of these arcades had the normal leading-trailing magnetic polarity arrangement of the active regions in the hemisphere of the arcade, but the other third (12) had reversed polarity, their leading flux being the trailing-polarity remnant of one or more old active regions and their trailing flux being the leading-polarity remnant of one or more other old active regions. From these observations, we conclude: (1) The sheared core field in a reversed-polarity arcade must be formed by processes in and above the photosphere, not by the emergence of a twisted flux rope bodily from below the photosphere. (2) The sheared core fields in the normal-polarity arcades were basically the same as those in the reversed-polarity arcades: both showed similar sigmoidal form and produced similar explosions (similar flares and CMEs). (3) Hence, the sheared core fields in normal-polarity mature arcades are likely formed mainly by the same processes as in reversed-polarity arcades. (4) These processes should be discernible in high-resolution magnetogram sequences and movies of the photosphere, chromosphere, and corona such those to come from Solar-B

Moore, Ronald L.↗
NASA NTRS2005

Slow-rise and Fast-rise Phases of an Erupting Solar Filament and Flare Emission Onset

We observe the eruption of an active-region solar filament of 1998 July 11 using high time cadence and high spatial resolution EUV observations from the TRACE satellite, along with soft X-ray images from the soft X-ray telescope (SXT) on the Yohkoh satellite, hard X-ray fluxes from the BATSE instrument on the Compton Gamma Ray Observatory (CGRO) satellite and from the hard X-ray telescope (HXT) on Yohkoh, and ground-based magnetograms. We concentrate on the initiation of the eruption in an effort to understand the eruption mechanism. Prior to eruption the filament undergoes slow upward movement in a "slow rise" phase with an approximately constant velocity of about 15 km/s that lasts about 10 min. It then erupts in a "fast-rise" phase, accelerating to a velocity of about 200 km/s in about 5 min, and then decelerating to approximately 150 km/s over the next 5 min. EUV brightenings begin about concurrent with the start of the filament's slow rise, and remain immediately beneath the rising filament during the slow rise; initial soft X-ray brightenings occur at about the same time and location. Strong hard X-ray emission begins after the onset of the fast rise, and does not peak until the filament has traveled to a substantial altitude (to a height about equal to the initial length of the erupting filament) beyond its initial location. Additional information is available in the original extended abstract.

Sterling, Alphonse C.↗
NASA NTRS2005

Initiation of Coronal Mass Ejections

This paper is a synopsis of the initiation of the strong-field magnetic explosions that produce large, fast coronal mass ejections. Cartoons based on observations are used to describe the inferred basic physical processes and sequences that trigger and drive the explosion. The magnetic field that explodes is a sheared-core bipole that may or may not be embedded in surrounding strong magnetic field, and may or may not contain a flux rope before it starts to explode. We describe three different mechanisms that singly or in combination trigger the explosion: (1) runaway internal tether-cutting reconnection, (2) runaway external tether-cutting reconnection, and (3) ideal MHD instability or loss or equilibrium. For most eruptions, high-resolution, high-cadence magnetograms and chromospheric and coronal movies (such as from TRACE and/or Solar-B) of the pre-eruption region and of the onset of the eruption and flare are needed to tell which one or which combination of these mechanisms is the trigger. Whatever the trigger, it leads to the production of an erupting flux rope. Using a simple model flux rope, we demonstrate that the explosion can be driven by the magnetic pressure of the expanding flux rope, provided the shape of the expansion is "fat" enough.

Moore, Ronald L.↗
NASA NTRS2005

A New Variety of CMEs: Streamer Puffs from Compact Ejective Flares

We present SOHO EIT, UVCS and LASCO observations of recurrent (6 --- 8 events per day) narrow (angular widths of about 3 --- 10 degrees) Coronal Mass Ejections (CMEs) which occurred over 2002 November 26--29. The active region where the ejections originate is near the base of a coronal streamer that appears to be unperturbed by the events and keeps stable in time; hence we interpret the observed events as a new class of recursive narrow CMEs that we call "streamer puffs." EIT 304 angstrom (He II) images indicate that the puffs result from compact ejective flares embedded in the streamer, with the ejections from the flares having velocities 100 --- 200 kilometers per second. Most ejections are closely correlated with coronal "jets" seen at 1.7 solar radii in the UVCS data, and a subset of these ejections and jets correspond to streamer puffs observed in LASCO coronagraph images. There are, however, more compact flares and jets than streamer puffs during the observation period, indicating that only a subset of the flare-associated ejections are energetic enough to escape into the heliosphere.

Sterling, Alphonse C.↗
NASA NTRS2004

External and Internal Reconnection in Two Filament-Carrying Magnetic Cavity Solar Eruptions

We observe two near-limb solar filament eruptions. one of 2000 February 26 and the other of 2002 January 4. For both we use 195 A Fe XII images from the Extreme-Ultraviolet Imaging Telescope (EIT) and magnetograms from the Michelson Doppler Imager (MDI). both of which are on the Solar and Heliospheric Observatory. (SOHO). For the earlier event we also use soft X-ray telescope (SXT). hard X-ray telescope (HXT). and Bragg Crystal Spectrometer (BCS) data from the Yohkoh satellite. and hard X-ray data from the BATSE experiment on the Compton Gamma Ray Observatory. (CGRO). Both events occur in quadrupolar magnetic regions. and both have coronal features that we infer belong to the same magnetic cavity structures as the filaments. In both cases. the cavity and filament first rise slowly at approx. 10 km/s prior to eruption and then accelerate to approx. 100 km/s during the eruption. although the slow-rise movement for the higher altitude cavity elements is clearer in the later event. We estimate that both filaments and both cavities contain masses of approx. 10(exp14) - 1 0(exp 15) and approx. l0(exp 15) - l0(exp 16) g. respectively. We consider whether two specific magnetic reconnection-based models for eruption onset. the "tether cutting" and the "breakout" models. are consistent with our observations. In the earlier event, soft X-rays from SXT show an intensity increase during the 12 minute interval over which fast eruption begins. which is consistent with tether- cutting-model predictions. Substantial hard X-ray. however. do not occur until after fast eruption is underway. and so this is a constraint the tether-cutting model must satisfy. During the same 12 minute interval over which fast eruption begins, there are brightenings and topological changes in the corona indicative of high-altitude reconnection early in the eruption. and this is consistent with breakout predictions. In both eruptions. the state of the overlying loops at the time of onset of the fast-rise phase of the corresponding filament can be compared with expectations from the breakout model. thereby setting constraints that the breakout model must meet. Our findings are consistent with both runaway tether-cutting-type reconnection and fast breakout-type reconnection. occurring early in the fast phase of the February eruption and with both types of reconnection being important in unleashing the explosion, but we are not able to say which. if either, type of reconnection actually triggered the fast phase. In any case. we have found specific constraints that either model. or any other model. must satisfy if correct.

Sterling, Alphonse C.↗
NASA NTRS2004

External and Internal Reconnection in Two Filament-Carrying Magnetic-Cavity Solar Eruptions

We observe two near-limb solar filament eruptions, one of 2000 February 26 and the other of 2002 January 4. For both we use 195 Angstroms, Fe XII images from the Extreme-Ultraviolet Imaging Telescope (EIT) and magnetograms from the Michelson Doppler Imager (MDI), both of which are on the Solar arid Heliospheric Observatory (SOHO). For the earlier event we also use soft X-ray telescope (SXT), hard X-ray telescope (HXT), and Bragg Crystal Spectrometer (BCS) data from the Yohkoh satellite, and hard X-ray data from the BATSE experiment on the Compton Gamma Ray Observation, (CGRO). Both events occur in quadrupolar magnetic regions, and both have coronal features that we infer belong to the same magnetic cavity structures as the filaments. In both cases, the cavity and filament first rise slowly at approximately 10 kilometers per second prior to eruption and then accelerate to approximately 100 kilometers per second during the eruption, although the slow-rise movement for the higher altitude cavity elements is clearer in the later event. We estimate that both filaments and both cavities contain masses of approximately 10(exp 14)-10(exp 15) and approximately 10(exp 15)-10(exp 16)g, respectively. We consider whether two specific magnetic reconnection-based models for eruption onset, the tether cutting and the breakout models, are consistent with our observations. In the earlier event, soft X-rays from SXT show an intensity increase during the 12 minute interval over which fast eruption begins, which is consistent with tether-cutting-model predictions. Substantial hard X-rays, however, do not occur until after fast eruption is underway, and so this is a constraint the tether-cutting model must satisfy. During the same 12 minute interval over which fast eruption begins, there are brightenings and topological changes in the corona indicative of high-altitude reconnection early in the eruption, and this is consistent with breakout predictions. In both eruptions, the state of the overlying loops at the time of onset of the fast-rise phase of the corresponding filament can be compared with expectations from the breakout model, thereby setting constraints that the breakout model must meet. Our findings are consistent with both runaway tether-cutting-type reconnection and fast breakout-type reconnection, occurring early in the fast phase of the February eruption and with both types of reconnection being important in unleashing the explosion, but we are not able to say which, if either, type of reconnection actually triggered the fast phase. In any case, we have found specific constraints that either model, or any other model, must satisfy if correct.

Sterling, Alphonse C.↗
NASA NTRS2004

External and Internal Reconnection in Two Filament-Carrying Magnetic-Cavity Solar Eruptions

We observe two near-limb solar filament eruptions, one of 2000 February 26 and the other of 2002 January 4. For both we use 195 A Fe XII images from the Extreme-Ultraviolet Imaging Telescope (EIT) and magnetograms from the Michelson Doppler Imager (MDI), both of which are on the Solar and Heliospheric Observatory (SOHO). For the earlier event we also use soft X-ray telescope (SXT), hard X-ray telescope (HXT), and Bragg Crystal Spectrometer (BCS) data from the Yohkoh satellite, and hard X-ray data from the BATSE experiment on the Compton Gamma Ra.v Observatory (CGRO). Both events occur in quadrupolar magnetic regions, and both have coronal features that we infer belong to the same magnetic cavity structures as the filaments. In both cases, the cavity and filament first rise slowly at approx.10 km/s prior to eruption and then accelerate to approx.100 km/s during the eruption, although the slow-rise movement for the higher altitude cavity elements is clearer in the later event. We estimate that both filaments and both cavities contain masses of approx.10(exp 14)-10(exp 15) and approx.10(exp 15)-10(exp 16) g, respectively. We consider whether two specific magnetic reconnection-based models for eruption onset, the "tether cutting" and the "breakout" models, are consistent with our observations. In the earlier event, soft X-rays from SXT show an intensity increase during the 12 minute interval over which fast eruption begins, which is consistent with tether- cutting-model predictions. Substantial hard X-rays, however, do not occur until after fast eruption is underway, and so this is a constraint the tether-cutting model must satisfy. During the same 12 minute interval over which fast eruption begins, there are brightenings and topological changes in the corona indicative of high-altitude reconnection early in the eruption, and this is consistent with breakout predictions. In both eruptions, the state of the overlying loops at the time of onset of the fast-rise phase of the corresponding filament can be compared with expectations from the breakout model, thereby setting constraints that the breakout model must meet. Our findings are consistent with both runaway tether-cutting-type reconnection and fast breakout-type reconnection, occurring early in the fast phase of the February eruption and with both types of reconnection being important in unleashing the explosion, but we are not able to say which, if either, type of reconnection actually triggered the fast phase. In any case, we have found specific constraints that either model, or any other model, must satisfy if correct.

Sterling, Alphonse C.↗
NASA NTRS2004

Evidence for Gradual External Reconnection Before Explosive Eruption of a Solar Filament

We observe a slowly evolving quiet-region solar eruption of 1999 April 18, using extreme-ultraviolet (EUV) images from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) and soft X-ray images from the Soft X-ray Telescope (SXT) on Yohkoh. Using difference images, in which an early image is subtracted from later images, we examine dimmings and brightenings in the region for evidence of the eruption mechanism. A filament rose slowly at about 1 km/s for 6 hours before being rapidly ejected at about 16 km/s leaving flare brightenings and postflare loops in its wake. Magnetograms from the Michelson Doppler Imager (MDI) on SOHO show that the eruption occurred in a large quadrupolar magnetic region with the filament located on the neutral line of the quadrupole s central inner lobe between the inner two of the four polarity domains. In step with the slow rise, subtle EIT dimmings commence and gradually increase over the two polarity domains on one side of the filament, i.e., in some of the loops of one of the two sidelobes of the quadrupole. Concurrently, soft X-ray brightenings gradually increase in both sidelobes. Both of these effects suggest heating in the sidelobe magnetic arcades. which gradually increase over several hours before the fast eruption. Also, during the slow pre- eruption phase, SXT dimmings gradually increase in the feet and legs of the central lobe, indicating expansion of the central-lobe magnetic arcade enveloping the filament. During the rapid ejection. these dimmings rapidly grow in darkness and in area, especially in the ends of the sigmoid field that erupts with the filament. and flare brightenings begin underneath the fast-moving but still low-altitude filament. We consider two models for explaining the eruption: "breakout. which says that reconnection occurs high above the filament prior to eruption, and tether cutting, which says that the eruption is unleashed by reconnection beneath the filament. The pre-eruption evolution is consistent with gradual breakout that led to (and perhaps caused) the fast eruption. Tether-cutting reconnection below the filament begins early in the rapid ejection. but our data are not complete enough to determine whether this reconnection began early enough to be the cause of the fast-phase onset. Thus, our observations are consistent with gradual breakout reconnection causing the long slow rise of the filament, but allow the cause of the sudden onset of the explosive fast phase to be either a jump in the breakout reconnection rate or the onset of runaway tether-cutting reconnection. or both.

Sterling, Alphonse C.↗
NASA NTRS2004

External and Internal Reconnection in Two Filament-Carrying Magnetic-Cavity Solar Eruptions

We observe two near-limb solar filament eruptions, one of 2000 February 26 and the other of 2002 January 4. For both we use 195 Angstroms Fe XII images from the Extreme-Ultraviolet (EUV) Imaging Telescope (EIT) and magnetograms from the Michelson Doppler Imager (MDI), both on the Solar and Heliospheric Observatory (SOHO) satellite. For the earlier event we also use soft X-ray telescope (SXT), hard X-ray telescope (HXT), and Bragg Crystal Spectrometer (BCS) data from the Yohkoh satellite, and hard X-ray data from the BATSE experiment on the Compton Gamma Ray Observatory (CGRO). Both events occur in quadrupolar magnetic regions, and both have coronal features that we infer belong to the same magnetic-cavity structures as the filaments. In both cases the cavity and filament first rise slowly at approximately 10 kilometers per second prior to eruption, and then accelerate to approximately 100 kilometers per second during the eruption, although the slow-rise movement for the higher-altitude cavity elements is clearer in the later event. We estimate both filaments and both cavities to contain masses of approximately 10(exp 14-15) g and approximately 10(exp 15-16) g respectively. We consider whether two specific magnetic-reconnection-based models for eruption onset, the tether cutting and the breakout models, are consistent with our observations. In the earlier event soft X-rays from SXT show an intensity increase during the 12-minute interval over whch fast eruption begins, which is consistent with tether-cutting-model predictions. Substantial hard X-rays, however, do not occur until after fast eruption is underway, and so this is a constraint the tether-cutting model must satisfy. During the same 12-minute interval over which fast eruption begins, there are brightenings and topological changes in the corona indicative of high-altitude reconnection early in the eruption, and this is consistent with breakout predictions. In both eruptions, however, the onset of the filament fast-rise phase occurs while overlying cavity-related coronal loops are still evolving from a "closed" state to an "open" state, and our observations of the time evolution of these loops compared to the eruption state in both events are constraints the breakout model must meet. Therefore our findings are consistent with runaway-tether-cutting-type reconnection and fast breakout-type reconnection both occurring early in the fast phase of the February eruption and with both types of reconnection being important in unleashing the explosion, but we are not able to say which, if either, type of reconnection actually triggered the fast phase. We have also found specific constraints that either model, or any other model, must satisfy if correct.

Sterling, Alphonse C.↗
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 NTRS2003

Tether-Cutting Energetics of a Solar Quiet Region Prominence Eruption

We study the morphology and energetics of a slowly evolving quiet-region solar prominence eruption occurring on 1999 February 8-9 in the solar north polar crown region, using soft X-ray data from the soft X-ray telescope (SXT) on Yohkoh and Fexv EUV 284 Angstrom data from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). After rising at approximately equal to l kilometer per second for about six hours, the prominence accelerates to a velocity of approximately equal to 10 kilometers per second, leaving behind EUV and soft X-ray loop arcades of a weak flare in its source region. Intensity dimmings occur in the eruption region cospatially in EUV and soft X-rays, indicating that the dimmings result from a depletion of material. Over the first two hours of the prominences rapid rise, flare-like brightenings occur beneath the rising prominence that might correspond to tether-cutting magnetic reconnection. These brightenings have heating requirements of up to approximately 10(exp 28)-10(exp 29) ergs, and this is comparable to the mechanical energy required for the rising prominence over the same time period. If the ratio of mechanical energy to heating energy remains constant through the early phase of the eruption, then we infer that coronal signatures for the tether cutting may not be apparent at or shortly after the start of the fast phase in this or similar low-energy eruptions, since the plasma-heating energy levels would not exceed that of the background corona.

Sterling, Alphonse C.↗
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 NTRS2003

Evidence for Gradual External Reconnection Before Explosive Eruption of a Solar Filament

We observe a slowly-evolving quiet region solar eruption of 1999 April 18, using EUV images from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), and soft X-ray images from the Soft X-ray Telescope (SXT) on Yohkoh. Using difference images, where an early image is subtracted from later images, we examine dimmings and brightenings in the region for evidence of the eruption mechanism. A filament rose slowly at about 1 kilometer per second for six hours before being rapidly ejected at about 16 kilometers per second, leaving flare brightenings and post-flare loops in its wake. Magnetograms from the Michelson Doppler Imager (MDI) on SOHO show that the eruption occurred in a large quadrupo1ar magnetic region, with the filament located on the neutral line of the quadrupole's central inner lobe, between the inner two of the four polarity domains. In step with the slow rise, subtle EIT dimmings commence and gradually increase over the two polarity domains on one side of the filament, i.e. in some of the loops of one of the two side lobes of the quadrupole. Concurrently, soft X-ray brightenings gradually increase in both side lobes. Both of these effects suggest heating in the side-lobe magnetic arcades, which gradually increase over several hours before the fast eruption. Also during the slow pre-eruption phase, SXT dimmings gradually increase in the feet and legs of the central lobe, indicating expansion of the central-lobe magnetic arcade enveloping the filament. During the rapid ejection, these dimmings rapidly grow in darkness and in area, especially in the ends of the sigmoid field that erupts with the filament, and flare brightenings begin underneath the fast-moving but still low-altitude filament. We consider two models for explaining the eruption: "breakout," which says that reconnection occurs high above the filament prior to eruption, and tether cutting, which says that the eruption is unleashed by reconnection beneath the filament. The pre-eruption evolution is consistent with gradual breakout that led to (and perhaps caused) the fast eruption. Tether-cutting reconnection below the filament begins early in the rapid ejection, but our data are not complete enough to determine whether this reconnection began early enough to be the cause of the fast-phase onset. Thus, our observations are consistent with gradual breakout reconnection causing the long slow rise of the filament, but allow the cause of the sudden onset of the explosive fast phase to be either a jump in the breakout reconnection rate or the onset of runaway tether-cutting reconnection, or both.

Sterling, Alphonse C.↗
NASA NTRS2003

Tether-Cutting Energetics of a Solar Quiet Region Prominence Eruption

We study the morphology and energetics of a slowly-evolving quiet region solar prominence eruption occurring on 1999 February 8-9 in the solar north polar crown region, using soft X-ray data from the soft X-ray telescope (SXT) on Yohkoh and Fe xv EUV 284 A data from the EUV Imaging Telescope (EIT) on SOHO. After rising at approx. 1 km/s for about six hours, the prominence accelerates to a velocity of approx. 10 km/s, leaving behind EUV and soft X-ray loop arcades of a weak flare in its source region. Intensity dimmings occur in the eruption region cospatially in EUV and soft X-rays, indicating that the dimmings result from a depletion of material. Over the first two hours of the prominence s rapid rise, flare-like brightenings occur beneath the rising prominence which may correspond to "tether cutting" magnetic reconnection. These brightenings have heating requirements of up to approx. 10(exp 28)-10(exp 29) ergs, and this is comparable to the mechanical energy required for the rising prominence over the same time period. If the ratio of mechanical energy to heating energy remains constant through the early phase of the eruption, then we infer that coronal signatures for the tether cutting may not be apparent at or shortly after the start of the fast phase in this or similar low-energy eruptions, since the plasma-heating energy levels would not exceed that of the background corona.

Sterling, Alphonse C.↗
NASA NTRS2002

Initiation of Coronal Mass Ejections by Tether-Cutting Reconnection

We present and interpret examples of the eruptive motion and flare brightening observed in the onset of magnetic explosions that produce coronal mass ejections. The observations are photospheric magnetograms and sequences of coronal and/or chromospheric images. In our examples, the explosion is apparently driven by the ejective eruption of a sigmoidal sheared-field flux rope from the core of an initially closed bipole. This eruption is initiated (triggered and unleashed) by reconnection located either (1) internally, low in the sheared core field, or (2) externally, at a magnetic null above the closed bipole. The internal reconnection is commonly called 'tether-cutting" reconnection, and the external reconnection is commonly called "break-out' reconnection. We point out that break-out reconnection amounts to external tether cutting. In one example, the eruptive motion of the sheared core field starts several minutes prior to any detectable brightening in the coronal images. We suggest that in this case the eruption is triggered by internal tether-cutting reconnection that at first is too slow and/or too localized to produce detectable heating in the coronal images. This work is supported by NASA's Office of Space Science through its Solar & Heliospheric Physics Supporting Research & Technology program and its Sun-Earth Connection Guest Investigator program.

Moore, Ronald L.↗
NASA NTRS2001

Material Outflows from Coronal Intensity "Dimming Regions" during Coronal Mass Ejection Onset

One signature of expulsion of coronal mass ejections (CMEs) from the solar corona is the appearance of transient intensity dimmings in coronal images. These dimmings have generally been assumed to be due to discharge of CME material from the corona, and thus the 'dimming regions' are thought of as an important signature of the sources of CMEs. We present spectral observations of two dimming regions at the time of expulsion of CMEs, using the Coronal Diagnostic Spectrometer (CDS) on the SOHO satellite. One of the dimming regions is at the solar limb and associated with a CME traveling in the plane of the sky, while the other region is on the solar disk and associated with an Earth-directed 'halo' CME. From the limb event, we see Doppler signatures of approximately 30 km/s in coronal (Fe XVI and Mg IX) emission lines, where the enhanced velocities coincide with the locations of coronal dimming. This provides direct evidence that the dimmings are associated with outflowing material. We also see larger (approximately 100 km/s) Doppler velocities in transition region (O V and He I) emission lines, which are likely to be associated with motions of a prominence and loops at transition region temperatures. An 'EIT wave' accompanies the disk event, and a dimming region behind the wave shows strong blueshifted Doppler signatures of approximately 100 km/s in O V, suggesting that material from the dimming regions behind the wave may be feeding the CME.

Harra, Louise K.↗
NASA NTRS2001

H(alpha) Proxies for EIT Crinkles: Further Evidence for Preflare "Breakout"-Type Activity in an Ejective Solar Eruption

We present H(alpha) observations from Big Bear Solar Observatory of an eruptive flare in NOAA Active Region 8210, occurring near 22:30 UT on 1998 May 1. Previously, using the Extreme Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) spacecraft, we found that a pattern of transient, localized brightenings, which we call 'EIT crinkles,' appears in the neighborhood of the eruption near the time of flare onset. These EIT crinkles occur at a location in the active region well separated from the sheared core magnetic fields, which is where the most intense features of the eruption are concentrated. We also previously found that high-cadence images from the Soft X-ray Telescope (SXT) on Yohkoh indicate that soft X-ray intensity enhancements in the core begin after the start of the EIT crinkles. With the H(alpha) data, we find remote flare brightening counterparts to the EIT crinkles. Light curves as functions of time of various areas of the active region show that several of the remote flare brightenings undergo intensity increases prior to the onset of principal brightenings in the core region, consistent with our earlier findings from EIT and SXT data. These timing relationships are consistent with the eruption onset mechanism known as the breakout model, introduced by Antiochos and colleagues, which proposes that eruptions begin with reconnection at a magnetic null high above the core region. Our observations are also consistent with other proposed mechanisms that do not involve early reconnection in the core region. As a corollary, our observations are not consistent with the so-called tether-cutting models, which say that the eruption begins with reconnection in the core. The H(alpha) data further show that a filament in the core region becomes activated near the time of EIT crinkle onset, but little if any of the filament actually erupts, despite the presence of a halo coronal mass ejection (CME) associated with this event.

Sterling, Alphonse C.↗
NASA NTRS2001

EIT And SXT Observations of a Quiet-Region Filament Ejection: First Eruption, Then Reconnection

We observe a slow-onset quiet-region filament eruption with the Extreme Ultraviolet Imaging Telescope (EIT) on the Solar Heliospheric Observatory (SOHO) and the Soft X-ray Telescope (SXT) on Yohkoh. This event occurred on 1999 April 18 and was likely the origin of a coronal mass ejection detected by SOHO at 08:30 UT on that day. In the EIT observation, one-half of the filament shows two stages of evolution: stage 1 is a slow, roughly constant upward movement at approximately 1 km/s lasting approximately 0.5 hr, and stage 2 is a rapid upward eruption at approximately 16 km/s occurring just before the filament disappears into interplanetary space. The other half of the filament shows little motion along the line of sight during the time of stage 1 but erupts along with the rest of the filament during stage 2. There is no obvious emission from the filament in the SXT observation until stage 2; at that time, an arcade of EUV and soft X-ray loops forms first at the central location of the filament and then expands outward along the length of the filament channel. A plot of EUV intensity versus time of the central portion of the filament (where the postflare loops initially form) shows a flat profile during stage 1 and a rapid upturn after the start of stage 2. This light curve is delayed from what would be expected if 'tether-cutting' reconnection in the core of the erupting region were responsible for the initiation of the eruption. Rather, these observations suggest that a loss of stability of the magnetic field holding the filament initiates the eruption, with reconnection in the core region occurring only as a by-product.

Sterling, Alphonse C.↗