Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun

We search for persistent and quasi-periodic release events of streamer blobs during 2007 with the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory and assess the velocity of the slow solar wind along the plasma sheet above the corresponding streamer by measuring the dynamic parameters of blobs. We find ten quasi-periodic release events of streamer blobs lasting for three to four days. In each day of these events, we observe three – five blobs. The results are in line with previous studies using data observed near the last solar minimum. Using the measured blob velocity as a proxy for that of the mean flow, we suggest that the velocity of the background slow solar wind near the Sun can vary significantly within a few hours. This provides an observational manifestation of the large velocity variability of the slow solar wind near the Sun.     

Multiscale Edge Detection in the Corona

Coronal Mass Ejections (CMEs) are challenging objects to detect using automated techniques, due to their high velocity and diffuse, irregular morphology. A necessary step to automating the detection process is to first remove the subjectivity introduced by the observer used in the current, standard, CME detection and tracking method. Here we describe and demonstrate a multiscale edge detection technique that addresses this step and could serve as one part of an automated CME detection system. This method provides a way to objectively define a CME front with associated error estimates. These fronts can then be used to extract CME morphology and kinematics. We apply this technique to a CME observed on 18 April 2000 by the Large Angle Solar COronagraph experiment (LASCO) C2/C3 and a CME observed on 21 April 2002 by LASCO C2/C3 and the Transition Region and Coronal Explorer (TRACE). For the two examples in this work, the heights determined by the standard manual method are larger than those determined with the multiscale method by ≈10% using LASCO data and ≈20% using TRACE data.     

Latitudinal and Solar-Cycle Variations of the White-Light Corona from SOHO/LASCO Observations

SOHO/LASCO data were used to obtain the latitudinal and radial distributions of the brightness of the K- and F-corona in the period of 1996 – 2007, and their solar-cycle variations were studied. Then an inversion method was employed to obtain the radial distributions of the electron density N _e(R,θ) for various latitude values on the coronal images. Our values of N _e(R,θ) are in good agreement with the findings of other authors. We found that in an edge-on streamer belt the electron density, like the K-corona brightness, varies with distance more slowly in the near-equatorial rays than in near-polar regions. We have developed a method for assessing the maximum values of the electron density at the center of the face-on streamer belt in its bright rays and depressions between them. Not all bright rays observed in the face-on streamer belt are found to be associated with an increased electron density in them. Mechanisms for forming such rays have been suggested.     

Deflection of Coronal Rays by Remote CMEs: Shock Wave or Magnetic Pressure?

We analyze five events of the interaction of coronal mass ejections (CMEs) with the remote coronal rays located up to 90° away from the CME as observed by the SOHO/LASCO C2 coronagraph. Using sequences of SOHO/LASCO C2 images, we estimate the kink propagation in the coronal rays during their interaction with the corresponding CMEs ranging from 180 to 920 km s⁻¹ within the interval of radial distances from 3 R _⊙ to 6 R _⊙. We conclude that all studied events do not correspond to the expected pattern of shock wave propagation in the corona. Coronal ray deflection can be interpreted as the influence of the magnetic field of a moving flux rope within the CME. The motion of a large-scale flux rope away from the Sun creates changes in the structure of surrounding field lines, which are similar to the kink propagation along coronal rays. The retardation of the potential should be taken into account since the flux rope moves at a high speed, comparable with the Alfvén speed.     

The Radiometric and Pointing Calibration of SECCHI COR1 on STEREO

COR1 is the innermost coronagraph of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite aboard the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. The paired COR1 telescopes observe the white-light K-corona from 1.4 to 4 solar radii in a waveband 22.5 nm wide centered on the Hα line at 656 nm. An internal polarizer allows the measurement of both total and polarized brightness. The co-alignment of the two COR1 telescopes is derived from the star λ Aquarii for the Ahead spacecraft, and from an occultation of the Sun by the Moon for Behind. Observations of the planet Jupiter are used to establish absolute photometric calibrations for each telescope. The intercalibration of the two COR1 telescopes are compared using coronal mass ejection observations made early in the mission, when the spacecraft were close together. Comparisons are also made with the Solar and Heliospheric Observatory (SOHO) Large Angle and Spectrometric Coronagraph (LASCO) C2 and Mauna Loa Solar Observatory Mk4 coronagraphs.     

An Attempt to Detect Coronal Mass Ejections in Lyman-α Using SOHO Swan

In this study, the possibility that coronal mass ejections (CMEs) may be observed in neutral Lyman-α emission was investigated. An observing campaign was initiated for SWAN (Solar Wind ANisotropies), a Lyman-α scanning photometer on board the Solar and Heliospheric Observatory (SOHO) dedicated to monitoring the latitude distribution of the solar wind from its imprints on the interstellar sky background. This was part of SOHO Joint Observing Program (JOP) 159 and was an exploratory investigation as it was not known how, or even if, CMEs interact with the solar wind and interstellar neutral hydrogen at this distance (≈60 and 120 R _S). The study addresses the lack of methods for tracking CMEs beyond the field-of-view of current coronagraphs (30 R _S). In our first method we used LASCO, white-light coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope also on SOHO, to identify CME candidates which, subject to certain criteria, should have been observable in SWAN. The criteria included SWAN observation time and location, CME position angle, and extrapolated speed. None of the CME candidates that we discuss were identified in the SWAN data. For our second method we analyzed all of the SWAN data for 184 runs of the observing campaign, and this has yielded one candidate CME detection. The candidate CME appears as a dimming of the background Lyman-α intensity representing ≈10% of the original intensity, moving radially away from the Sun. Multiple candidate CMEs observed by LASCO and EIT were found which may have caused this dimming. Here we discuss the campaign, data analysis technique and statistics, and the results.     

Unusual Zebra Patterns in the Decimeter Wave Band

An analysis of new observations showing fine structures consisting of narrowband fiber bursts as substructures of large-scale zebra-pattern stripes is carried out. We study four events using spectral observations taken with a newly built spectrometer located at the Huairou station, China, in the frequency range of 1.1 – 2.0 GHz with extremely high frequency and time resolutions (5 MHz and 1.25 ms). All the radio events were analyzed by using the available satellite data (SOHO LASCO, EIT, and MDI, TRACE, and RHESSI). Small-scale fibers always drift to lower frequencies. They may belong to a family of ropelike fibers and can also be regarded as fine structures of type III bursts and broadband pulsations. The radio emission was moderately or strongly polarized in the ordinary wave mode. In three main events fiber structure appeared as a forerunner of the entire event. All four events were small decimeter bursts. We assume that for small-scale fiber bursts the usual mechanism of coalescence of whistler waves with plasma waves can be applied, and the large-scale zebra pattern can be explained in the conventional double plasma resonance (DPR) model. The appearance of an uncommon fine structure is connected with the following special features of the plasma wave excitation in the radio source: Both whistler and plasma wave instabilities are too weak at the very beginning of the events (i.e., the continuum was absent), and the fine structure is almost invisible. Then, whistlers generated directly at DPR levels “highlight” the radio emission only from these levels owing to their interaction with plasma waves.     

Background Subtraction for the SECCHI/COR1 Telescope Aboard STEREO

COR1 is an internally occulted Lyot coronagraph, part of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite aboard the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. Because the front objective lens is subjected to a full solar flux, the images are dominated by instrumental scattered light which has to be removed to uncover the underlying K corona data. We describe a procedure for removing the instrumental background from COR1 images. F coronal emission is subtracted at the same time. The resulting images are compared with simultaneous data from the Mauna Loa Solar Observatory Mk4 coronagraph. We find that the background subtraction technique is successful in coronal streamers, while the baseline emission in coronal holes (i.e. between plumes) is suppressed. This is an expected behavior of the background subtraction technique. The COR1 radiometric calibration is found to be either 10 – 15% lower, or 5 – 10% higher than that of the Mk4, depending on what value is used for the Mk4 plate scale, while an earlier study found the COR1 radiometric response to be ∼ 20% higher than that of the Large Angle Spectroscopic Coronagraph (LASCO) C2 telescope. Thus, the COR1 calibration is solidly within the range of other operating coronagraphs. The background levels in both COR1 telescopes have been quite steady in time, with the exception of a single contamination event on 30 January 2009. Barring too many additional events of this kind, there is every reason to believe that both COR1 telescopes will maintain usable levels of scattered light for the remainder of the STEREO mission.     

Origin of Coronal Shocks without Mass Ejections

We present an analysis of all the events (around 400) of coronal shocks for which the shock-associated metric type IIs were observed by many spectrographs during the period April 1997– December 2000. The main objective of this analysis is to give evidence for the type IIs related to only flare-blast waves, and thus to find out whether there are any type II-associated coronal shocks without mass ejections. By carefully analyzing the data from multi-wavelength observations (Radio, GOES X-ray, Hα, SOHO/LASCO and SOHO/EIT-EUV data), we have identified only 30 events for which there were actually no reports of CMEs. Then from the analysis of the LASCO and EIT running difference images, we found that there are some shocks (nearly 40%, 12/30) which might be associated with weak and narrow mass ejections. These weak and narrow ejections were not reported earlier. For the remaining 60% events (18/30), there are no mass ejections seen in SOHO/LASCO. But all of them are associated with flares and EIT brightenings. Pre-assuming that these type IIs are related to the flares, and from those flare locations of these 18 cases, 16 events are found to occur within the central region of the solar disk (longitude ≤45^∘). In this case, the weak CMEs originating from this region are unlikely to be detected by SOHO/LASCO due to low scattering. The remaining two events occurred beyond this longitudinal limit for which any mass ejections would have been detected if they were present. For both these events, though there are weak eruption features (EIT dimming and loop displacement) in the EIT images, no mass ejection was seen in LASCO for one event, and a CME appeared very late for the other event. While these two cases may imply that the coronal shocks can be produced without any mass ejections, we cannot deny the strong relationship between type IIs and CMEs.     

Cyclical Behavior of Coronal Mass Ejections

With the use of coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO) from January 1996 through December 2005, it is found that, for the cyclical activity of CMEs, there is surprisingly no equatorward drift at low latitudes (thus, no “butterfly diagram”) and no poleward drift at high latitudes, and no antiphase relationship between CME activity at low and high latitudes. The cyclical behaviors of CMEs differ in a significant way from that of the small-scale solar photospherical and chromospherical phenomena. Thus, our analysis leads to results that are inconsistent with a close, physical relationship with small-scale aspects of solar activity, and it is suggested that there is possibly a single so-called large-scale activity cycle in CMEs.     

Solar cycle variation of interplanetary coronal mass ejection latitudes

With the use of interplanetary coronal mass ejections (ICMEs) compiled by Richardson and Cane from 1996 to 2007 and the associated coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO), we investigate the solar cycle variation of real ICME-associated CME latitudes during solar cycle 23 using Song et al.’s method. The results show the following:

Bridging EUV and White-Light Observations to Inspect the Initiation Phase of a “Two-Stage” Solar Eruptive Event

The initiation phase of coronal mass ejections (CMEs) is a very important aspect of solar physics, as these phenomena ultimately drive space weather in the heliosphere. This phase is known to occur between the photosphere and low corona, where many models introduce an instability and/or magnetic reconnection that triggers a CME, often with associated flaring activity. To this end, it is important to obtain a variety of observations of the low corona to build as clear a picture as possible of the dynamics that occur therein. Here, we combine the EUV imagery of the Sun Watcher using Active Pixel System Detector and Image Processing (SWAP) instrument onboard the Project for Onboard Autonomy (PROBA2) with the white-light imagery of the ground-based Mark-IV K-coronameter (Mk4) at Mauna Loa Solar Observatory (MLSO) to bridge the observational gap that exists between the disk imagery of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and the coronal imagery of the Large Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). Methods of multiscale image analysis were applied to the observations to better reveal the coronal signal while suppressing noise and other features. This allowed an investigation into the initiation phase of a CME that was driven by a rising flux-rope structure from a “two-stage” flaring event underlying an extended helmet streamer. It was found that the initial outward motion of the erupting loop system in the EUV observations coincided with the first X-ray flare peak and led to a plasma pile-up of the white-light CME core material. The characterized CME core then underwent a strong jerk in its motion, as the early acceleration increased abruptly, simultaneously with the second X-ray flare peak. The overall system expanded into the helmet streamer to become the larger CME structure observed in the LASCO coronagraph images, which later became concave-outward in shape. Theoretical models for the event are discussed in light of these unique observations, and it is concluded that the formation of either a kink-unstable or torus-unstable flux rope may be the likeliest scenario.     

Interacting CMEs and their associated flare and SEP activities

We have analyzed a set of 25 interacting events which are associated with the DH type II bursts. These events are selected from the Coronal Mass Ejections (CMEs) observed during the period 1997–2010 in SOHO/LASCO and DH type IIs observed in Wind/WAVES. Their pre and primary CMEs from nearby active regions are identified using SOHO/LASCO and EIT images and their height–time diagrams. Their interacting time and height are obtained, and their associated activities, such as, flares and Solar Energetic Particles (>10 pfu) are also investigated. Results from the analysis are: primary CMEs are much faster than the pre-CMEs, their X-ray flares are also stronger (X- and M-class) compared to the flares (C- and M-class) of pre-CMEs. Most of the events (22/25) occurred during the period 2000–2006. From the observed width and speed of pre and primary CMEs, it is found that the pre-CMEs are found to be less energetic than the primary CMEs. While the primary CMEs are tracked up to the end of LASCO field of view (30 R_s), most of the pre-CMEs can be tracked up to <26 R_s. The SEP intensity is found to be related with the integrated flux of X-ray flares associated with the primary CMEs for nine events originating from the western region.     

Mhd Interpretation of LASCO Observations of a Coronal Mass Ejection as a Disconnected Magnetic Structure

We present a qualitative and quantitative comparison of a single coronal mass ejection (CME) as observed by LASCO (July 28–29, 1996) with the results of a three-dimensional axisymmetric time-dependent magnetohydrodynamic model of a flux rope interacting with a helmet streamer. The particular CME considered was selected based on the appearance of a distinct ‘tear-drop’ shape visible in animations generated from both the data and the model. The CME event begins with the brightening of a pre-existing coronal streamer which evolves into a ‘tear-drop’ shaped loop followed by a Y-shaped structure. The brightening moves slowly outward with significant acceleration reaching velocities of ∼450 km s^-1 at 30 R⊙. The observed CME characteristics are compared with the model results. On the basis of this comparison, we suggested that the observed features were caused by the evacuation of a flux rope in the closed field region of the helmet streamer (i.e., helmet dome). The flux rope manifests itself as the cavity of the quasi-static helmet streamer and the whole system becomes unstable when the flux rope reaches a threshold strength. The observed ‘tear-drop’ structure is due to the deformed flux rope. The leading edge of the flux rope interacts with the helmet dome to form the typical loop-like CME. The trailing edge of this flux rope interacts with the local bi-polar field to form the observed Y-shaped structure. The model results for the evolution of the magnetic-field configurations, velocity, and polarization brightness are directly compared with observations. Animations have been generated from both the actual data and the model to illustrate the good agreement between the observation and the model. These animations can be found on the CD-ROM which accompanies this volume. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004923016322     

Width of Radio-Loud and Radio-Quiet CMEs

In the present paper we report on the difference in angular sizes between radio-loud and radio-quiet CMEs. For this purpose we compiled these two samples of events using Wind/WAVES and SOHO/LASCO observations obtained during 1996 – 2005. We show that the radio-loud CMEs are almost twice as wide as the radio-quiet CMEs (considering expanding parts of CMEs). Furthermore, we show that the radio-quiet CMEs have a narrow expanding bright part with a large extended diffusive structure. These results were obtained by measuring the CME widths in three different ways.     

Numerical Magnetohydrodynamic Experiments for Testing the Physical Mechanisms of Coronal Mass Ejections Acceleration

Analysis of observations from both space-borne (LASCO/SOHO, Skylab and Solar Maximum Mission) and ground-based (Mauna Loa Observatory) instruments show that there are two types of coronal mass ejections (CMEs), fast CMEs and slow CMEs. Fast CMEs start with a high initial speed, which remains more or less constant, while slow CMEs start with a low initial speed, but show a gradual acceleration. To explain the difference between the two types of CMEs, Low and Zhang (2002) proposed that it resulted from a difference in the initial topology of the magnetic fields associated with the underlying quiescent prominences, i.e., a normal prominence configuration will lead to a fast CME, while an inverse quiescent prominence results in a slow CME. In this paper we explore a different scenario to explain the existence of fast and slow CMEs. Postulating only an inverse topology for the quiescent prominences, we show that fast and slow CMEs result from different physical processes responsible for the destabilization of the coronal magnetic field and for the initiation and launching of the CME. We use a 2.5-D, time-dependent streamer and flux-rope magnetohydrodynamic (MHD) model (Wu and Guo, 1997) and investigate three initiation processes, viz. (1) injecting of magnetic flux into the flux-rope, thereby causing an additional Lorentz force that will destabilize the streamer and launch a CME (Wu et al., 1997, 1999); (2) draining of plasma from the flux-rope and triggering a magnetic buoyancy force that causes the flux-rope to lift and launch a CME; and (3) introducing additional heating into the flux-rope, thereby simulating an active-region flux-rope accompanied by a flare to launch a CME. We present 12 numerical tests using these three driving mechanisms either alone or in various combinations. The results show that both fast and slow CMEs can be obtained from an inverse prominence configuration subjected to one or more of these three different initiation processes.     

Streamer Belt and Chains as the Main Sources of Quasi-Stationary Slow Solar Wind

Synoptic maps of white-light coronal brightness from SOHO/LASCO C2 and distributions of solar wind velocity obtained from interplanetary scintillation are studied. Regions with velocity V≈300 – 450 km s⁻¹ and increased density N>10 cm⁻³, typical of the “slow” solar wind originating from the belt and chains of streamers, are shown to exist at Earth’s orbit, between the fast solar wind flows (with a maximum velocity V _max≈450 – 800 km s⁻¹). The belt and chains of streamers are the main sources of the “slow” solar wind. As the sources of “slow” solar wind, the contribution from the chains of streamers may be comparable to that from the streamer belt.     

The Relationship of Green-Line Transients to White-Light Coronal Mass Ejections

We report observations by the Large Angle Spectrometric Coronagraph (LASCO) on the SOHO spacecraft of three coronal green-line transients that could be clearly associated with coronal mass ejections (CMEs) detected in Thomson-scattered white light. Two of these events, with speeds >25 km s^-1, may be classified as ‘whip-like’ transients. They are associated with the core of the white-light CMEs, identified with erupting prominence material, rather than with the leading edge of the CMEs. The third green-line transient has a markedly different appearance and is more gradual than the other two, with a projected outward speed <10 km s^-1. This event corresponds to the leading edge of a ‘streamer blowout’ type of CME. A dark void is left behind in the emission-line corona following each of the fast eruptions. Both fast emission-line transients start off as a loop structure rising up from close to the solar surface. We suggest that the driving mechanism for these events may be the emergence of new bipolar magnetic regions on the surface of the Sun, which destabilize the ambient corona and cause an eruption. The possible relationship of these events to recent X-ray observations of CMEs is briefly discussed. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004981125702     

Investigation on spectral behavior of Solar transients and their interrelationship

We probe the spectral hardening of solar flares emission in view of associated solar proton events (SEPs) at earth and coronal mass ejection (CME) acceleration as a consequence. In this investigation we undertake 60 SEPs of the Solar Cycle 23 along with associated Solar Flares and CMEs. We employ the X-ray emission in Solar flares observed by Reuven Ramaty Higly Energy Solar Spectroscopic Imager (RHESSI) in order to estimate flare plasma parameters. Further, we employ the observations from Geo-stationary Operational Environmental Satellites (GOES) and Large Angle and Spectrometric Coronagraph (LASCO), for SEPs and CMEs parameter estimation respectively. We report a good association of soft-hard-harder (SHH) spectral behavior of Flares with occurrence of Solar Proton Events for 16 Events (observed by RHESSI associated with protons). In addition, we have found a good correlation (R=0.71) in SEPs spectral hardening and CME velocity. We conclude that the Protons as well as CMEs gets accelerated at the Flare site and travel all the way in interplanetary space and then by re-acceleration in interplanetary space CMEs produce Geomagnetic Storms in geospace. This seems to be a statistically significant mechanism of the SEPs and initial CME acceleration in addition to the standard scenario of SEP acceleration at the shock front of CMEs.     

Prediction of Space Weather Using an Asymmetric Cone Model for Halo CMEs

Halo coronal mass ejections (HCMEs) are responsible of the most severe geomagnetic storms. A prediction of their geoeffectiveness and travel time to Earth’s vicinity is crucial to forecast space weather. Unfortunately, coronagraphic observations are subjected to projection effects and do not provide true characteristics of CMEs. Recently, Michalek (Solar Phys. 237, 101, 2006) developed an asymmetric cone model to obtain the space speed, width, and source location of HCMEs. We applied this technique to obtain the parameters of all front-sided HCMEs observed by the SOHO/LASCO experiment during a period from the beginning of 2001 until the end of 2002 (solar cycle 23). These parameters were applied for space weather forecasting. Our study finds that the space speeds are strongly correlated with the travel times of HCMEs to Earth’s vicinity and with the magnitudes related to geomagnetic disturbances.