Solar Terrestrial Relations Observatory Ahead Research Articles

Verification of real-time WSA−ENLIL+Cone simulations of CME arrival-time at the CCMC from 2010 to 2016

The Wang-Sheeley-Arge (WSA)−ENLIL+Cone model is used extensively in space weather operations world-wide to model coronal mass ejection (CME) propagation. As such, it is important to assess its performance. We present validation results of the WSA−ENLIL+Cone model installed at the Community Coordinated Modeling Center (CCMC) and executed in real-time by the CCMC space weather team. CCMC uses the WSA−ENLIL+Cone model to predict CME arrivals at NASA missions throughout the inner heliosphere. In this work we compare model predicted CME arrival-times to in situ interplanetary coronal mass ejection leading edge measurements at Solar TErrestrial RElations Observatory-Ahead (STEREO-A), Solar TErrestrial RElations Observatory-Behind (STEREO-B), and Earth (Wind and ACE) for simulations completed between March 2010 and December 2016 (over 1,800 CMEs). We report hit, miss, false alarm, and correct rejection statistics for all three locations. For all predicted CME arrivals, the hit rate is 0.5, and the false alarm rate is 0.1. For the 273 events where the CME was predicted to arrive at Earth, STEREO-A, or STEREO-B, and was actually observed (hit event), the mean absolute arrival-time prediction error was 10.4 ± 0.9 h, with a tendency to early prediction error of −4.0 h. We show the dependence of the arrival-time error on CME input parameters. We also explore the impact of the multi-spacecraft observations used to initialize the model CME inputs by comparing model verification results before and after the STEREO-B communication loss (since September 2014) and STEREO-A sidelobe operations (August 2014–December 2015). There is an increase of 1.7 h in the CME arrival time error during single, or limited two-viewpoint periods, compared to the three-spacecraft viewpoint period. This trend would apply to a future space weather mission at L5 or L4 as another coronagraph viewpoint to reduce CME arrival time errors compared to a single L1 viewpoint.

“Dandelion” Filament Eruption and Coronal Waves Associated with a Solar Flare on 2011 February 16

Abstract Coronal disturbances associated with solar flares, such as Hα Moreton waves, X-ray waves, and extreme ultraviolet (EUV) coronal waves, are discussed herein in relation to magnetohydrodynamic fast-mode waves or shocks in the corona. To understand the mechanism of coronal disturbances, full-disk solar observations with high spatial and temporal resolution over multiple wavelengths are of crucial importance. We observed a filament eruption, whose shape is like a “dandelion,” associated with the M1.6 flare that occurred on 2011 February 16 in Hα images taken by the Flare Monitoring Telescope at Ica University, Peru. We derive the three-dimensional velocity field of the erupting filament. We also identify winking filaments that are located far from the flare site in the Hα images, whereas no Moreton wave is observed. By comparing the temporal evolution of the winking filaments with those of the coronal wave seen in the EUV images data taken by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and by the Extreme Ultraviolet Imager on board the Solar Terrestrial Relations Observatory-Ahead, we confirm that the winking filaments were activated by the EUV coronal wave.

Anisotropy of the He+, C+, N+, O+, and Ne+pickup ion velocity distribution functions

Context. Interstellar and inner-source pickup ions (PUIs) are produced by the ionization of neutral atoms that originate either outside or inside the heliosphere. Just after ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic toroidal features in their velocity distribution functions (VDF). As the plasma parcel moves outwards with the solar wind, the pickup ion VDF gets more and more affected by resonant wave-particle interactions, changing heliospheric conditions, and plasma drifts, which lead to a gradual isotropization of the pickup ion VDF. Past investigations of the pickup ion torus distribution were limited to He+ pickup ions at 1 astronomical unit (AU).Aims. The aim of this study is to quantify the state of anisotropy of the He+ , C+ , N+ , O+ , and Ne+ pickup ion VDF at 1 AU. Changes between the state of anisotropy between PUIs of different mass-per-charges can be used to estimate the significance of resonant wave-particle interactions for the isotropization of their VDF, and to investigate the numerous simplifications that are generally made for the description of the phase-space transport of PUIs.Methods. Pulse height analysis data by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on board the Solar Terrestrial RElations Observatory Ahead (STEREO A) is used to obtain velocity-spectra of He+ , C+ , N+ , O+ , and Ne+ relative to the solar wind, f (w sw ). The w sw -spectra are sorted by two different configurations of the local magnetic field – one in which the torus distribution lies within the instrument’s aperture, φ ⊥ , and one in which the torus distribution lies exclusively outside the instrument’s field of view, φ ∥ . The ratio of the PUI spectra between φ ⊥ and φ ∥ is used to determine the degree of anisotropy of the PUI VDF.Results. The data shows that the formation of a torus distribution at 1 AU is significantly more prominent for O+ (and N+ ) than for He+ (and Ne+ ). This cannot be explained by resonant wave-particle interactions as the sole mechanism for the isotropization of the PUI VDF. The anisotropy of the O+ VDF compared to He+ is highly fluctuating but consistently higher over an observation period of six years and therefore unlikely to be related to either specific heliospheric conditions or solar activity variations. To our surprise, we also found a clear signature of a C+ torus distribution at 1 AU very similar to the one of He+ , although as an inner-source PUI, C+ should have a considerably different spectral and spatial injection pattern than interstellar PUIs.

Photospheric and Coronal Observations of Abrupt Magnetic Restructuring in Two Flaring Active Regions

For two major X-class flares observed by the Solar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory Ahead (STEREO-A) spacecraft when they were close to quadrature, we compare major, abrupt changes in the photospheric magnetic vector field to changes in the observed coronal magnetic structure during the two flares. The Lorentz force changes in strong photospheric fields within active regions are estimated from time series of SDO Helioseismic and Magnetic Imager (HMI) vector magnetograms. These show that the major changes occurred in each case near the main neutral line of the region and in two neighboring twisted opposite-polarity sunspots. In each case the horizontal parallel field strengthened significantly near the neutral line while the azimuthal field in the sunspots decreased, suggesting that a flux rope joining the two sunspots collapsed across the neutral line with reduced magnetic pressure because of a reduced field twist component. At the same time, the coronal extreme ultraviolet (EUV) loop structure was observed by the Atmospheric Imaging Assembly (AIA) onboard SDO and the Extreme Ultraviolet Imager (EUVI) on STEREO-A to decrease significantly in height during each eruption, discontinuous changes signifying ejection of magnetized plasma, and outward-propagating continuous but abrupt changes consistent with loop contraction. An asymmetry in the observed EUV loop changes during one of the flares matches an asymmetry in the photospheric magnetic changes associated with that flare. The observations are discussed in terms of the well-known tether-cutting and breakout flare initiation models.

TEMPERATURE AND ELECTRON DENSITY DIAGNOSTICS OF A CANDLE-FLAME-SHAPED FLARE

Candle-flame-shaped flares are archetypical structures that provide indirect evidence of magnetic reconnection. A flare resembling Tsuneta's famous 1992 candle-flame flare occurred on 2011 January 28; we present its temperature and electron density diagnostics. This flare was observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), Hinode/X-Ray Telescope (XRT), and Solar Terrestrial Relations Observatory Ahead (STEREO-A)/Extreme Ultraviolet Imager, resulting in high-resolution, broad temperature coverage, and stereoscopic views of this iconic structure. The high-temperature images reveal a brightening that grows in size to form a tower-like structure at the top of the posteruption flare arcade, a feature that has been observed in other long-duration events. Despite the extensive work on the standard reconnection scenario, there is no complete agreement among models regarding the nature of this high-intensity elongated structure. Electron density maps reveal that reconnected loops that are successively connected at their tops to the tower develop a density asymmetry of about a factor of two between the two legs, giving the appearance of half-loops. We calculate average temperatures with a new fast differential emission measure (DEM) method that uses SDO/AIA data and analyze the heating and cooling of salient features of the flare. Using STEREO observations, we show that the tower and the half-loop brightenings are not a line-of-sight projection effect of the type studied by Forbes & Acton. This conclusion opens the door for physics-based explanations of these puzzling, recurrent solar flare features, previously attributed to projection effects. We corroborate the results of our DEM analysis by comparing them with temperature analyses from Hinode/XRT.

Simulation of the 23 July 2012 extreme space weather event: What if this extremely rare CME was Earth directed?

Extreme space weather events are known to cause adverse impacts on critical modern day technological infrastructure such as high‐voltage electric power transmission grids. On 23 July 2012, NASA's Solar Terrestrial Relations Observatory‐Ahead (STEREO‐A) spacecraft observed in situ an extremely fast coronal mass ejection (CME) that traveled 0.96 astronomical units (∼1 AU) in about 19 h. Here we use the Space Weather Modeling Framework (SWMF) to perform a simulation of this rare CME. We consider STEREO‐A in situ observations to represent the upstream L1 solar wind boundary conditions. The goal of this study is to examine what would have happened if this Rare‐type CME was Earth‐bound. Global SWMF‐generated ground geomagnetic field perturbations are used to compute the simulated induced geoelectric field at specific ground‐based active INTERMAGNET magnetometer sites. Simulation results show that while modeled global SYM‐H index, a high‐resolution equivalent of the Dst index, was comparable to previously observed severe geomagnetic storms such as the Halloween 2003 storm, the 23 July CME would have produced some of the largest geomagnetically induced electric fields, making it very geoeffective. These results have important practical applications for risk management of electrical power grids.

Eruption of a Bifurcated Solar Filament

We study the partial eruption of a solar filament observed by the Solar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) spacecraft on 9 May 2012. This filament was located in Active Region NOAA 11475 and consisted of two distinct branches, separated in height above the active region’s primary polarity-inversion line. For two days prior to the filament eruption, several threads of filament material were observed to connect the lower branch to the upper branch with evidence of a transfer of mass along them. The eruption commenced as a slow rise of the upper branch that began at 9 May 2012 23:40 UT, with the main eruption occurring half an hour later, producing a coronal mass ejection (CME). During the eruption, the upper branch was observed to rotate approximately 120 degrees in a counter-clockwise direction. We suggest that the mass transfer events also comprised a transfer of magnetic flux that led the upper branch of the filament to lose equilibrium as a result of a helical kink instability or torus instability.

Twist in a polar blowout jet

It is well known that some coronal jets exhibit helical structures and untwisting. We attempt to inspect the origin of twist in a blowout jet. By means of multi-wavelength and multi-angle observations from Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory-Ahead (STEREO-A), we firstly report a polar untwisting jet that is a blowout jet which leads to a jet-like coronal mass ejection. From the viewpoint of SDO, the jet shows clear untwisting behavior and two jet-spires. However, from the viewpoint of STEREO-A the jet actually comes from the whiplike prominence eruption and is followed by a white-light jet. Our observations indicate that twist in blowout jets may result from the erupting mini-prominences/mini-filaments in the jet base.

ERUPTION OF A SOLAR FILAMENT CONSISTING OF TWO THREADS

The trigger and driving mechanism for the eruption of a filament consisting of two dark threads was studied with unprecedented high cadence and resolution of He II 304 angstrom observations made by the Atmospheric Imagining Assembly (AIA) on board the Solar Dynamics Observatory (SDO) and the observations made by the Solar Magnetic Activity Research Telescope and the Extreme Ultraviolet Imager (EUVI) telescope on board the Solar Terrestrial Relations Observatory Ahead (STEREO-A). The filament was located at the periphery of the active region NOAA 11228 and erupted on 2011 June 6. At the onset of the eruption, a turbulent filament thread was found to be heated and to elongate in stride over a second one. After it rose slowly, most interestingly, the elongating thread was driven to contact and interact with the second one, and it then erupted with its southern leg being wrapped by a newly formed thread produced by the magnetic reconnection between fields carried by the two threads. Combining the observations from STEREO-A/EUVI and SDO/AIA 304 angstrom images, the three-dimensional shape of the axis of the filament was obtained and it was found that only the southern leg of the eruptive filament underwent rotation. We suggest that the eruption was triggered by the reconnection of the turbulent filament thread and the surrounding magnetic field, and that it was mainly driven by the kink instability of the southern leg of the eruptive filament that possessed a more twisted field introduced by the reconnection-produced thread.

Formation of a White-Light Jet Within a Quadrupolar Magnetic Configuration

We analyze multi-wavelength and multi-viewpoint observations of a large-scale event viewed on 7 April 2011 originating from an active region complex. The activity leads to a white-light jet being formed in the outer corona. The topology and evolution of the coronal structures were imaged in high resolution using the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). In addition, large field-of-view images of the corona were obtained using the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) telescope onboard the PRoject for Onboard Autonomy (PROBA2) microsatellite, providing evidence for the connectivity of the coronal structures with outer coronal features that were imaged with the Large Angle Spectrometric Coronagraph (LASCO) C2 on Solar and Heliospheric Observatory (SOHO). The data-sets reveal an Eiffel-tower type jet configuration extending into a narrow jet in the outer corona. The event starts from the growth of a dark area in the central part of the structure. The darkening was also observed in projection on the disk by the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) spacecraft from a different point of view. We assume that the dark volume in the corona descends from a coronal cavity of a flux rope that moved up higher in the corona but still failed to erupt. The quadrupolar magnetic configuration corresponds to a saddle-like shape of the dark volume and provides a possibility for the plasma to escape along the open field lines into the outer corona, forming the white-light jet.

3D Reconstruction of a Rotating Erupting Prominence

A bright prominence associated with a coronal mass ejection (CME) was seen erupting from the Sun on 9 April 2008. This prominence was tracked by both the Solar Terrestrial Relations Observatory (STEREO) EUVI and COR1 telescopes, and was seen to rotate about the line of sight as it erupted; therefore, the event has been nicknamed the "Cartwheel CME." The threads of the prominence in the core of the CME quite clearly indicate the structure of a weakly to moderately twisted flux rope throughout the field of view, up to heliocentric heights of 4 solar radii. Although the STEREO separation was 48 degrees, it was possible to match some sharp features in the later part of the eruption as seen in the 304 {\AA} line in EUVI and in the H\alpha-sensitive bandpass of COR1 by both STEREO Ahead and Behind. These features could then be traced out in three-dimensional space, and reprojected into a view in which the eruption is directed towards the observer. The reconstructed view shows that the alignment of the prominence to the vertical axis rotates as it rises up to a leading-edge height of \approx 2.5 solar radii, and then remains approximately constant. The alignment at 2.5 solar radii differs by about 115 degrees from the original filament orientation inferred from H{\alpha} and EUV data, and the height profile of the rotation, obtained here for the first time, shows that two thirds of the total rotation is reached within \approx 0.5 solar radii above the photosphere. These features are well reproduced by numerical simulations of an unstable moderately twisted flux rope embedded in external flux with a relatively strong shear field component.

STEREO SECCHI COR1-A/B Intercalibration at 180° Separation

The twin Solar Terrestrial Relations Observatory (STEREO) spacecraft reached a separation angle of 180 degrees on 6 February 2011. This provided a unique opportunity to test the intercalibration between the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) telescopes on both spacecraft for areas above the limb. So long as the corona is optically thin, at 180 degree separation each spacecraft sees the same corona from opposite directions. Thus, the data should appear as mirror images of each other. We report here on the results of the comparison of the images taken by the inner coronagraph (COR1) on the STEREO Ahead and Behind spacecraft in the hours when the separation was close to 180 degrees. We find that the intensity values seen by the two telescopes agree with each other to a high degree of accuracy. This validates both the radiometric intercalibration between the COR1 telescopes, and the method used to remove instrumental background from the images. The relative error between COR1-A and COR1-B is found to be less than 10(exp -9) B/B solar over most of the field-of-view, growing to a few x 10(exp -9) B/B solar for the brighter pixels near the edge of the occulter. The primary source of error is the background determination. We also report on the analysis of star observations which show that the absolute radiometric calibration of either COR1 telescope has not changed significantly since launch.

FIRST DETERMINATION OF THE TRUE MASS OF CORONAL MASS EJECTIONS: A NOVEL APPROACH TO USING THE TWOSTEREOVIEWPOINTS

The twin Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) COR2 coronagraphs of the Solar Terrestrial Relations Observatory (STEREO) provide images of the solar corona from two view points in the solar system. Since their launch in late 2006, the STEREO Ahead (A) and Behind (B) spacecrafts have been slowly separating from Earth at a rate of 22.5 degrees per year. By the end of 2007, the two spacecraft were separated by more than 40 degrees from each other. At this time, we began to see large-scale differences in the morphology and total intensity between coronal mass ejections (CMEs) observed with SECCHI-COR2 on STEREO-A and B. Due to the effects of the Thomson scattering geometry, the intensity of an observed CME is dependent on the angle it makes with the observed plane of the sky. From the intensity images, we can calculate the integrated line of sight electron density and mass. We demonstrate that is is possible to simultaneously derive the direction and true total mass of the CME if we make the simple assumption that the same mass should be observed in COR2-A and B.