Observables Indicating Two Major Coronal Mass Ejections During the WHI     

N. V. Nitta 《Solar physics》2011,274(1-2):219-232

Two of the five fast (v?900 km?s^?1) coronal mass ejections (CMEs) between January 2007 and December 2009 were observed during the Whole Heliosphere Interval (WHI: 20 March??C?16 April 2008). The main purpose of this article is to discuss possible observational signatures that could have been used to predict these CMEs. During the WHI, there were three active regions aligned almost East?CWest in a longitudinal span of about 60°. They were NOAA active region (AR) 10987, 10988, and 10989. In terms of the sunspot area, AR 10988 was the largest. However, the fast CMEs were launched from AR 10989 on 25 March and from AR 10987 on 5 April. One explanation for this may be that AR 10988, unlike the other two regions, emerged underneath a predominantly closed magnetic-field environment, as shown by global magnetic-field extrapolations. Around the times of these CMEs, however, magnetic-field observations of the source regions were essentially missing, because they were close to, or behind, the limb as viewed from Earth. Therefore, we explore an extended view in longitude of the regions from the Solar Terrestrial Relations Observatory (STEREO). The two STEREO spacecraft were located ???24° East and West of the Sun?CEarth line during the period of interest. We study the frequency of microflares in the three regions and changes in large-scale structures including streamers, but the CMEs do not seem to be correlated with either of them. Instead, activation of filaments or prominences may directly signal subsequent eruptions.  相似文献   

STEREO/Extreme Ultraviolet Imager (EUVI) Event Catalog 2006 – 2012     

Markus J. Aschwanden  Jean-Pierre Wülser  Nariaki V. Nitta  James R. Lemen  Sam Freeland  William T. Thompson 《Solar physics》2014,289(3):919-938

We generated an event catalog with an automated detection algorithm based on the entire EUVI image database observed with the two Solar Terrestrial Relations Observatory (STEREO)-A and -B spacecraft over the first six years of the mission (2006?–?2012). The event catalog includes the heliographic positions of some 20?000 EUV events, transformed from spacecraft coordinates to Earth-based coordinates, and information on associated GOES flare events (down to the level of GOES A5-class flares). The 304 Å wavelength turns out to be the most efficient channel for flare detection (79?% of all EUVI event detections), while the 171 Å (4?%), 195 Å (10?%), and the 284 Å channel (7?%) retrieve substantially fewer flare events, partially due to the suppressing effect of EUV dimming, and partially due to the lower cadence in the later years of the mission. Due to the Sun-circling orbits of STEREO-A and -B, a large number of flares have been detected on the farside of the Sun, invisible from Earth, or seen as partially occulted events. The statistical size distributions of EUV peak fluxes (with a power-law slope of α _P =2.5±0.2) and event durations (with a power-law slope of α _T =2.4±0.3) are found to be consistent with the fractal-diffusive self-organized criticality model. The EUVI event catalog is available on-line at secchi.lmsal.com/EUVI/euvi_autodetection/euvi_events.txt and may serve as a comprehensive tool to identify stereoscopically observed flare events for 3D reconstruction and to study occulted flare events.  相似文献   

Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from Small (6°) to Large (170°) Spacecraft Separation Angles     

Markus J. Aschwanden  Jean-Pierre Wülser  Nariaki Nitta  James Lemen 《Solar physics》2012,281(1):101-119

We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles (?? _sep??6^°,43^°,89^°,127^°,and 170^°). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of ?? _sep??22^°??C?125^°, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of ?? _sep??6^°??C?127^° we find a mean 3D misalignment angle of ?? _PF??21^°??C?39^° of stereoscopically triangulated loops with magnetic potential-field models, and ?? _FFF??15^°??C?21^° for a force-free field model, which is partly caused by stereoscopic uncertainties ?? _SE??9^°. We predict optimum conditions for solar stereoscopy during the time intervals of 2012??C?2014, 2016??C?2017, and 2021??C?2023.  相似文献   

Earth-Affecting Coronal Mass Ejections Without Obvious Low Coronal Signatures     

Nariaki V. Nitta  Tamitha Mulligan 《Solar physics》2017,292(9):125

We present a study of the origin of coronal mass ejections (CMEs) that were not accompanied by obvious low coronal signatures (LCSs) and yet were responsible for appreciable disturbances at 1 AU. These CMEs characteristically start slowly. In several examples, extreme ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory reveal coronal dimming and a post-eruption arcade when we make difference images with long enough temporal separations, which are commensurate with the slow initial development of the CME. Data from the EUV imager and COR coronagraphs of the Sun Earth Connection Coronal and Heliospheric Investigation onboard the Solar Terrestrial Relations Observatory, which provide limb views of Earth-bound CMEs, greatly help us limit the time interval in which the CME forms and undergoes initial acceleration. For other CMEs, we find similar dimming, although only with lower confidence as to its link to the CME. It is noted that even these unclear events result in unambiguous flux rope signatures in in situ data at 1 AU. There is a tendency that the CME source regions are located near coronal holes or open field regions. This may have implications for both the initiation of the stealthy CME in the corona and its outcome in the heliosphere.  相似文献   

Vertical structure of hard X-ray flare     

S. Tsuneta  T. Takakura  N. Nitta  K. Ohki  K. Makishima  T. Murakami  M. Oda  Y. Ogawara 《Solar physics》1983,86(1-2):313-321

This paper presents studies of the vertical structure of hard X-ray flares for two contrasting examples. The 1981 May 13 flare contained a coronal hard X-ray source which was located above 50000 km above the photosphere. On the other hand, the 1981 July 20 flare had a chromospheric double source structure in the initial phase. Electrons in this case were able to stream freely from the corona to the chromosphere.  相似文献   

X-ray imaging of a solar limb flare on 1982 January 22     

T. Takakura  K. Tanaka  N. Nitta  K. Kai  K. Ohki 《Solar physics》1986,107(1):109-121

Simultaneous X-ray images in hard (20–40 keV) and softer (6.5–15 keV) energy ranges were obtained with the hard X-ray telescope aboard the Hinotori spacecraft of an impulsive solar X-ray burst associated with a flare near the solar west limb.The burst was composed of an impulsive component with a hard spectrum and a thermal component with a peak temperature of 2.8 × 10⁷ K. For about one minute, the impulsive component was predominant even in the softer energy range.The hard X-ray image for the impulsive component is an extended single source elongated along the solar limb, rather steady and extends from the two-ribbon H flare up to 10⁴ km above the limb. The centroid of this source image is located about 10 (7 × 10³ km) ± 5 above the neutral line. The corresponding image observed at the softer X-rays is compact and located near the centroid of the hard X-ray image.The source for the thermal component observed in the later phase at the softer X-rays is a compact single source, and it shows a gradual rising motion towards the later phase.  相似文献   

Flare loop geometry     

Nitta  N.  Van Driel-Gesztelyi  L.  Harra-Murnion  L.K. 《Solar physics》1999,189(1):181-198

In selected flares that occurred in AR 7260, we have studied the geometry of the brightest soft X-ray loop by tracing it on an image. Even under the assumption that the loop is contained in a plane, it is clear that a single image does not permit us to determine the full geometry. It only provides possible loop shapes as a function of the inclination angle of the loop plane with respect to the vertical. However, all the loops that reproduce the observed appearance give the same direction of increasing height as projected on to the image plane. This direction is compared with two relevant observations. Based on 2-D reconnection models that involve a cusp configuration, it is expected that the soft X-ray loop top source moves upward with time and that a higher temperature region exists above the loop top. Several flares are found to contradict these predictions, presumably implying the inadequacy of the models. Lastly we discuss a possibility of constraining the inclination angle (and hence the loop shape) with spatially-unresolved soft X-ray line spectra which are Doppler-shifted due to plasma upflows.  相似文献   

Soft X-ray Fluxes of Major Flares Far Behind the Limb as Estimated Using STEREO EUV Images     

N. V. Nitta  M. J. Aschwanden  P. F. Boerner  S. L. Freeland  J. R. Lemen  J.-P. Wuelser 《Solar physics》2013,288(1):241-254

With increasing solar activity since 2010, many flares from the backside of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes. Because of the presence of the Fe xxiv line at 192 Å, the response of the EUVI 195 Å channel has a secondary broad peak around 15 MK, and its fluxes closely trace X-ray fluxes during the rise phase of flares. If the flare plasma is isothermal, the EUVI flux should be directly proportional to the GOES flux. In reality, the multithermal nature of the flare and other factors complicate the estimation of the X-ray fluxes from EUVI observations. We discuss the uncertainties, by comparing GOES fluxes with the high cadence EUV data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We conclude that the EUVI 195 Å data can provide estimates of the X-ray peak fluxes of intense flares (e.g., above M4 in the GOES scale) to small uncertainties. Lastly we show examples of intense flares from regions far behind the limb, some of which show eruptive signatures in AIA images.  相似文献   

Heliospheric Imaging of 3D Density Structures During the Multiple Coronal Mass Ejections of Late July to Early August 2010     

D. F. Webb  C. Möstl  B. V. Jackson  M. M. Bisi  T. A. Howard  T. Mulligan  E. A. Jensen  L. K. Jian  J. A. Davies  C. A. de Koning  Y. Liu  M. Temmer  J. M. Clover  C. J. Farrugia  R. A. Harrison  N. Nitta  D. Odstrcil  S. J. Tappin  H.-S. Yu 《Solar physics》2013,285(1-2):317-348

It is usually difficult to gain a consistent global understanding of a coronal mass ejection (CME) eruption and its propagation when only near-Sun imagery and the local measurements derived from single-spacecraft observations are available. Three-dimensional (3D) density reconstructions based on heliospheric imaging allow us to “fill in” the temporal and spatial gaps between the near-Sun and in situ data to provide a truly global picture of the propagation and interactions of the CME as it moves through the inner heliosphere. In recent years the heliospheric propagation of dense structures has been observed and measured by the heliospheric imagers of the Solar Mass Ejection Imager (SMEI) and on the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft. We describe the use of several 3D reconstruction techniques based on these heliospheric imaging data sets to distinguish and track the propagation of multiple CMEs in the inner heliosphere during the very active period of solar activity in late July?–?early August 2010. We employ 3D reconstruction techniques used at the University of California, San Diego (UCSD) based on a kinematic solar wind model, and also the empirical Tappin–Howard model. We compare our results with those from other studies of this active period, in particular the heliospheric simulations made with the ENLIL model by Odstrcil et al. (J. Geophys. Res., 2013) and the in situ results from multiple spacecraft provided by Möstl et al. (Astrophys. J. 758, 10?–?28, 2012). We find that the SMEI results in particular provide an overall context for the multiple-density flows associated with these CMEs. For the first time we are able to intercompare the 3D reconstructed densities with the timing and magnitude of in situ density structures at five spacecraft spread over 150° in ecliptic longitude and from 0.4 to 1 AU in radial distance. We also model the magnetic flux-rope structures at three spacecraft using both force-free and non-force-free modelling, and compare their timing and spatial structure with the reconstructed density flows.  相似文献   

Soft X-ray observation of a large-scale coronal wave and its exciter     

Hudson  Hugh S.  Khan  Josef I.  Lemen  James R.  Nitta  Nariaki V.  Uchida  Yutaka 《Solar physics》2003,212(1):121-149

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfvén speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022904125479 deceased  相似文献