Nandita Srivastava

Solar and interplanetary sources of major geomagnetic storms during 1996–2002

[1] During the 7-year period of the current solar cycle, 64 geoeffective coronal mass ejections (CMEs) were found to produce major geomagnetic storms (DST < � 100 nT) at the Earth. In this paper we examine solar and interplanetary properties of these geoeffective coronal mass ejections (CMEs). The observations reveal that full-halo CMEs are potential sources of intense geomagnetic activity at the Earth. However, not all fullhalo CMEs give rise to major geomagnetic storms, which complicates the task of space weather forecasting. We examine solar origins of the geoeffective CMEs and their interplanetary effects, namely, solar wind speed, interplanetary shocks, and the southward component of the interplanetary magnetic field, in order to investigate the relationship between the solar and interplanetary parameters. In particular, the present study aims at ascertaining solar parameters that govern important interplanetary parameters responsible for producing major geomagnetic storms. Our investigation shows that fast full-halo CMEs associated with strong flares and originating from a favorable location, i.e., close to the central meridian and low and middle latitudes, are the most potential candidates for producing strong ram pressure at the Earth’s magnetosphere and hence intense geomagnetic storms. The results also show that the intensity of geomagnetic storms depends most strongly on the southward component of the interplanetary magnetic field, followed by the initial speed of the CME and the ram pressure. INDEX TERMS: 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass ejections; 2784 Magnetospheric Physics: Solar wind/ magnetosphere interactions; 2788 Magnetospheric Physics: Storms and substorms; 2139 Interplanetary Physics: Interplanetary shocks; KEYWORDS: CME, halo CMEs, IP shocks, solar wind, geomagnetic storms, DST index

ESTIMATING THE ARRIVAL TIME OF EARTH-DIRECTED CORONAL MASS EJECTIONS AT IN SITU SPACECRAFT USING COR AND HI OBSERVATIONS FROM STEREO

Predicting the arrival time and transit speed of coronal mass ejections (CMEs) near the Earth is critical to understanding the solar-terrestrial relationship. Even though STEREO observations now provide multiple views of CMEs in the heliosphere, the true speeds derived from stereoscopic reconstruction of SECCHI coronagraph data are not quite sufficient for accurate forecasting of the arrival time at Earth of a majority of CMEs. This uncertainty is due to many factors that change CME kinematics, such as the interaction of two or more CMEs or the interaction of CMEs with the pervading solar wind. In order to understand the propagation of CMEs, we have used the three-dimensional triangulation method on SECCHI coronagraph (COR2) images and geometric triangulation on the J-maps constructed from Heliospheric Imagers HI1 and HI2 data for eight Earth-directed CMEs observed during 2008-2010. Based on the reconstruction, and implementing the drag-based model for the distance where the CMEs could not be tracked unambiguously in the interplanetary (IP) medium, the arrival time of these CMEs have been estimated. These arrival times have also been compared with the actual arrival times as observed by in situ instruments. The analysis reveals the importance of heliospheric imaging for improved forecasting of the arrival time and direction of propagation of CMEs in the IP medium.

Source region of the 18 November 2003 coronal mass ejection that led to the strongest magnetic storm of cycle 23

[1] The superstorm of 20 November 2003 was associated with a high-speed coronal mass ejection (CME) which originated in the NOAA AR 10501 on 18 November. This coronal mass ejection had severe terrestrial consequences leading to a geomagnetic storm with Dst index of -472 nT, the strongest of the current solar cycle. In this paper, we attempt to understand the factors that led to the coronal mass ejection on 18 November. We have also studied the evolution of the photospheric magnetic field of NOAA AR 10501, the source region of this coronal mass ejection. For this purpose, the Michelson Doppler Imager line-of-sight magnetograms and vector magnetograms from Solar Flare Telescope, Mitaka, obtained during 17-19 November 2003 were analyzed. In particular, quantitative estimates of the temporal variation in magnetic flux, energy, and magnetic field gradient were estimated for the source active region. The evolution of these quantities was studied for the 3-day period with an objective to understand the preflare configuration leading up to the moderate flare which was associated with the geoeffective coronal mass ejection. We also examined the chromospheric images recorded in H α from Udaipur Solar Observatory to compare the flare location with regions of different magnetic field and energy. Our observations provide evidence that the flare associated with the CME occurred at a location marked by high magnetic field gradient which led to release of free energy stored in the active region.

Evolution and Consequences of Interacting CMEs of 9 – 10 November 2012 Using STEREO/SECCHI and In Situ Observations

Understanding the kinematic evolution of coronal mass ejections (CMEs) in the heliosphere is important to estimate their arrival time at Earth. The kinematics of CMEs can change when they interact or collide with each other as they propagate in the heliosphere. In this article, we analyze the collision and post-interaction characteristics of two Earth-directed CMEs that were launched successively on 9 and 10 November 2012. To do this, we used white-light imaging observations from STEREO/SECCHI and in situ observations taken from the Wind spacecraft. We tracked two density-enhancement features associated with the leading and trailing edge of the 9 November CME and one density enhanced feature associated with the leading edges of the 10 November CME by constructing J-maps. We found that the leading edge of the 10 November CME interacted with the trailing edge of the 9 November CME. We also estimated the kinematics of these features of the CMEs and found a significant change in their dynamics after interaction. In in situ observations, we identified distinct structures associated with interacting CMEs and also observed heating and compression as signatures of their interaction. Our analysis shows an improvement in the arrival-time prediction of CMEs when their post-collision dynamics are used instead of the pre-collision dynamics. By estimating the true masses and speeds of these colliding CMEs, we investigated the nature of the observed collision, which is found to be almost perfectly inelastic. The investigation also places in perspective the geomagnetic consequences of the two CMEs and their interaction in terms of occurrence of geomagnetic storms and triggering of magnetospheric substorms.

KINEMATICS OF TWO ERUPTIVE PROMINENCES OBSERVED BY EUVI/STEREO

Two large northern polar crown prominences that erupted on 2010 April 13 and 2010 August 1 were analyzed using images obtained from the Extreme UltraViolet Imager on the twin Solar Terrestrial Relations Observatory spacecraft. Several features along the prominence legs were reconstructed using a stereoscopic reconstruction technique developed by us. The three-dimensional changes exhibited by the prominences can be explained as an interplay between two different motions, namely helical twist in the prominence spine, and overall non-radial equatorward motion of the entire prominence structure. The sense of twist in both the prominences is determined from the changes in latitudes and longitudes of the reconstructed features. The prominences are observed starting from a few hours before the eruption. Increase in height before and during the eruption allowed us to study the kinematics of the prominences in the two phases of eruption, the slow-rise and the fast-eruptive phase. A constant value of acceleration was found for each reconstructed feature in each phase, but it showed a significant change from one leg to the other in both the prominences. The magnitude of acceleration during the eruptive phase is found to be commensurate with the net effect of the two motions stated above.

ACCELERATION OF CORONAL MASS EJECTIONS FROM THREE-DIMENSIONAL RECONSTRUCTION OF STEREO IMAGES

We employ a three-dimensional (3D) reconstruction technique for the first time to study the kinematics of six coronal mass ejections (CMEs), using images obtained from the COR1 and COR2 coronagraphs on board the twin STEREO spacecraft, and also the eruptive prominences (EPs) associated with three of them using images from the Extreme UltraViolet Imager. A feature in the EPs and leading edges (LEs) of all the CMEs was identified and tracked in images from the two spacecraft, and a stereoscopic reconstruction technique was used to determine the 3D coordinates of these features. True velocity and acceleration were determined from the temporal evolution of the true height of the CME features. Our study of the kinematics of the CMEs in 3D reveals that the CME LE undergoes maximum acceleration typically below 2 R ☉. The acceleration profiles of CMEs associated with flares and prominences exhibit different behaviors. While the CMEs not associated with prominences show a bimodal acceleration profile, those associated with prominences do not. Two of the three associated prominences in the study show a high and increasing value of acceleration up to a distance of almost 4 R ☉, but acceleration of the corresponding CME LE does not show the same behavior, suggesting that the two may not be always driven by the same mechanism. One of the CMEs, although associated with a C-class flare, showed unusually high acceleration of over 1500 m s–2. Our results therefore suggest that only the flare-associated CMEs undergo residual acceleration, which indicates that the flux injection theoretical model holds well for the flare-associated CMEs, but a different mechanism should be considered for EP-associated CMEs.

A COMPARISON OF RECONSTRUCTION METHODS FOR THE ESTIMATION OF CORONAL MASS EJECTIONS KINEMATICS BASED ON SECCHI/HI OBSERVATIONS

A study of the kinematics and arrival times of coronal mass ejections (CMEs) at Earth, derived from time-elongation maps (J-maps) constructed from STEREO/heliospheric imager (HI) observations, provides an opportunity to understand the heliospheric evolution of CMEs in general. We implement various reconstruction techniques, based on the use of time-elongation profiles of propagating CMEs viewed from single or multiple vantage points, to estimate the dynamics of three geo-effective CMEs. We use the kinematic properties, derived from analysis of the elongation profiles, as inputs to the Drag Based Model for the distance beyond which the CMEs cannot be tracked unambiguously in the J-maps. The ambient solar wind into which these CMEs, which travel with different speeds, are launched, is different. Therefore, these CMEs will evolve differently throughout their journey from the Sun to 1 AU. We associate the CMEs, identified and tracked in the J-maps, with signatures observed in situ near 1 AU by the WIND spacecraft. By deriving the kinematic properties of each CME, using a variety of existing methods, we assess the relative performance of each method for the purpose of space weather forecasting. We discuss the limitations of each method, and identify the major constraints in predicting the arrival time of CMEs near 1 AU using HI observations.

A possible explanation of reversed magnetic field features observed in NOAA AR 7321

Observations of reversed-polarity features in the chromosphere as well as in the photosphere in the form of magnetic gulfs or islands of opposite polarity have been reported recently. In this paper, we present a possible explanation for the appearance of reversed-polarity features observed in the chromospheric magnetograms of the NOAA AR 7321 observed during October 25–27, 1992. It is suggested that the large-scale reversed-polarity features may occur due to the twisting of the smaller-scale magnetic flux tubes in the layer between the photosphere and the chromosphere.

Automated Detection of Filaments and Their Disappearance Using Full-Disc Hα Images

A new algorithm is presented that automatically detects filaments on the Sun in full-disc Hα images. Pre-processing of Hα images includes corrections for limb darkening and foreshortening. Further, by applying suitable intensity and size thresholds, filaments are extracted, while other solar features, e.g. sunspots and plages, are removed. Filament attributes such as their position on the solar disc, total area, length, and number of fragments are determined. In addition, every filament is also labelled with a unique number for identification. The algorithm is capable of following a particular filament through successive images, which allows us to detect their changes and disappearance. We have analysed ten cases of filament eruption from different observatories, and the results obtained are presented. The algorithm will eventually be integrated with an upcoming telescope at the Udaipur Solar Observatory for real-time monitoring of activated/eruptive filaments. This aspect should prove to be of particular importance in studies pertaining to space weather.

Evolution of helically twisted prominence structures of March 11, 1979

Helical structures are generally associated with many eruptive solar prominences. Thus, study of their evolution in the solar atmosphere assumes importance. We present a study of a flare-associated erupting prominence of March 11, 1979, with conspicuous helically twisted structure, observed in Hα line center. We have attempted to understand the role played by twisted force-free magnetic fields in this event. In the analysis, we have assumed that the helical structures visible in Hα outline the field lines in which prominence tubes are embedded. Untwisting of observed prominence tubes and later, formation of open prominence structures provide evidence of restructuring of the magnetic field configuration over the active region during the course of prominence eruption. Temporal evolution of the force-free parameter α is obtained for two main prominence tubes observed to be intertwined in a rope-like structure. Axial electric currents associated with the prominence tubes are estimated to be of the order of 1011 A which decreased with time. Correspondingly, it is estimated that the rate of energy release was ≈ 1028 erg s−1 during the prominence eruption.