Anticipating STEREO: Addressing Issues in Coronal and Heliospheric Physics III Posters
Presiding: N Gopalswamy, NASA Goddard Space Flight Center; J Krall, Plasma Physics Division, Naval Research Laboratory
SH53A-01 1330h
Planning for SECCHI Observations of the Solar Corona and Heliosphere
The Sun-Earth Connections Coronal and Heliospheric Investigation (SECCHI) instrument suite consists of five almost identical telescopes on each of the STEREO spacecraft. Each instrument suite includes an extreme ultraviolet disk imager (EUVI), two coronagraphs (COR1 and COR2) and two heliospheric imagers (HI1 and HI2) that will image the corona and heliosphere to distances beyond the Earth's orbit. SECCHI observations will consist of a synoptic program optimized to provide simultaneous identical images for stereoscopic viewing of coronal and heliospheric structures from both STEREO spacecraft, and special observations that can be tailored to meet specific science objectives. This paper will describe the observational capabilities of the SECCHI instruments, and will discuss the operational resources available and constraints on observations at various stages of the mission.
SH53A-02 1330h
Active region evolution in the build-up to large solar energetic particle events
The SEP signatures of the solar flares occurring on 21 Apr 2002 and 24 Aug 2002 show marked differences in their compositions above 10 MeV/nucleon, yet at the Sun the events themselves display very similar characteristics in the chromospheric (hard X-ray) emission and their coronal (X-ray, EUV) signatures. We consider the prior evolution of the parent active regions to look for possible signatures in the ambient corona, magnetic connectivities (within the active region, large-scale closed field, and open field distributions), and flare/activity history which may differentiate the solar conditions leading to the observed disparate particle signatures at 1 AU.
SH53A-03 1330h
3D Sun Loop Tracer: A Tool for Stereoscopy of Coronal Loops for NASA's STEREO Mission
Stereoscopy and triangulation can be used to determine the three-dimensional geometry of coronal loops seen in simultaneous images from the two STEREO spacecraft. Here we demonstrate a new tool, 3D Sun Loop Tracer (SLT), which uses stereoscopy to determine the 3D structure of a loop that can be identified in both images. SLT proceeds in several stages. First, the user "seeds" the tool by selecting the same loop in the two images of a stereoscopic pair. Next, the tool uses loop tracing algorithms and triangulation techniques to obtain the three-dimensional (x,y,z) coordinates of points on the loop. The tool has been developed and tested using a physics-based synthetic 3D coronal model (K. Schrijver, private communication). The tool has been shown to produce accurate results for loop reconstruction over a wide range of stereoscopic separation angles. The goal is to use this tool to analyze the evolution and stored magnetic energy in loops observed by the EUV instruments on the STEREO spacecraft. Once the loops have been reconstructed in 3D, they can be compared to non-potential magnetic field line extrapolations. In the future, we plan to incorporate a magnetogram-based magnetic field model and implement a procedure for adjusting the parameters of the extrapolation to match the reconstructed loops. In this way, the evolution of the stored magnetic energy can be determined
SH53A-04 1330h
Flux-Rope CME Geometry and its Relation to Observed CME Morphology
Observed flux-rope CME morphology depends on the underlying flux-rope geometry and its observed projection onto the two-dimensional plane of the sky. We use a simple parameterization of a three-dimensional flux rope to determine a "typical model flux-rope geometry" that corresponds to the "average observed flux-rope coronal mass ejection (CME) morphology" as observed at a leading-edge (LE) height of about 5 solar radii (e.g., LASCO/C2). The model flux rope, the curved axis of which is assumed to trace out an ellipse, can be described in terms of eccentricity of the ellipse, the width (minor diameter) of the flux rope at the apex, and the height of the apex above the solar surface. At an LE height of 5 solar radii, the resulting morphology is only a weak function of the foot point separation. Assuming that flux-rope expansion is self-similar, we have only two model-geometry parameters: the eccentricity and the aspect ratio (apex height over apex width). For each given pair of model parameters, we consider an ensemble of possible orientations (latitude, longitude, and rotation about the vertical direction) each with a corresponding synthetic coronagraph image. These images are used to produce statistical measures of the morphology for comparison to statistical measures of observed flux-rope CME morphology. The model parameters that best fit the observations constitute a prediction of the underlying geometry (eccentricity and aspect ratio) of a typical flux-rope CME. When STEREO confirms this prediction, the flux-rope hypothesis will be further validated. Supported by ONR and NASA
SH53A-05 1330h
On the Evolution of CME Mass
Calculating the total mass of a CME is a standard technique that has been used for several decades and which SOHO/LASCO has refined and automated. The total mass of a CME is only that material that arises from below the occulting disk. It doesn't consider any mass that has been swept up from within the field of view and moved, since we always consider a "pre-event" coronal state. In this paper we turn to the question of swept up material and investigate the evolution in height of the volume density of the region at the CME front. Often the post-CME corona is "cleaned out", meaning that it is very dim compared to the pre-event and general background. We thus assume that the material has been swept up by the CME expansion through that volume. Can LASCO detect that swept up mass? To investigate this we have selected only events that have a clearly defined leading and trailing edge to the CME front, and which in an ideal sense are examples of the "three-part" CME. We calculated the mass of those LASCO events, which also gives the column electron density. We convert that electron content to a volume density in a narrow region at the center of the front, by approximating the shape as a small section of a sphere. This enables us to say that the depth along the line of sight is the same as the angular latitudinal span, which we take to be on the order of a few degrees. We find that the volume density in such events decreases with increasing height by a simple power law with an exponent of -3. This is exactly what would be expected for purely radial expansion and no pileup of material in the front. We will discuss why the pileup isn't observed by LASCO and where implications for the height origin of these CMEs. We will also discuss the implications for the upcoming STEREO mission.
SH53A-06 1330h
Three-dimensional velocity field of an accelerating CME
Fast coronal mass ejections (CMEs) are correlated with production of solar energetic particles (SEPs). One mechanism is acceleration by shocks produced by rapidly expanding CMEs, with the resulting SEPs streaming toward the earth along the field lines connecting the sun and the earth. It is therefore important to determine the 3-D velocity field of a CME, which is observed projected onto the 2-D plane of the sky. In this work, the 3-D velocity field of the leading edge of a model flux-rope CME is calculated as a function of the CME source location and flux-rope orientation with respect to the observer. In particular, the velocity component parallel to the Archimedean spiral magnetic field line originating from W60 deg longitude in the solar equator is obtained as a function time for the model CME, expressed in terms of the Alfven Mach number based on an empirical model of magnetic field. A key result that can be tested with STEREO observations is the relationship between the source location and 3-D geometry of a CME near the sun and SEPs detected at the earth. Work supported by ONR and NASA
SH53A-07 1330h
Can flux rope model explain the dynamics of eruptive solar prominences?
Coronal mass ejections usually occur in association with eruptive prominences or flares. The kinematics of flare-associated CMEs are generally characterised by fast speeds with little or no observable acceleration beyond the occulting disk. On the other hand, CMEs associated with eruptive prominences tend to attain lower speeds and have discernible acceleration beyond the occulting disk. It has been suggested that the two classes of CMEs correspond to a common magnetic flux-rope geometry but differ in the amount of magnetic energy that drives the eruption. The acceleration profiles are also critically influenced by the Alfven speed in the erupting flux rope. This paper analyzes a number of eruptive solar prominences in H-alpha which were associated with CMEs in the frame-work of flux-rope model in order to specifically understand the dynamics of erupting prominences. The geometrical properties of prominences viz. the footpoint separation and the apex height are the key parameters of this model which influence the acceleration and the height at which maximum acceleration of the prominence occurs. The comparative study of model and observations using Hα and SECCHI observations will shed new light on the geometrical relationship between the flux rope, the prominence, and the bright rim of the CME during the eruption.
SH53A-08 1330h
Low-Frequency Radio Observations and the Interplanetary Transport of CMEs
There has been much discussion and debate about how CMEs propagate from the solar corona to Earth, including their interactions and the relationship of the driver to the shock. The coronal propagation of CMEs is fairly well determined out to 30 Rs from existing space-based and ground-based coronagraphs. At the present time there are four ways to obtain information on the interplanetary transport of CMEs: IPS and low-frequency radio observations (e.g., various ground-based radio telescopes and space-based Wind/WAVES), white-light all-sky images (SMEI), and a variety of gas-dynamic and MHD models. These various observations and models, all of which have advantages and disadvantages, often give very different results for the kinematics of interplanetary CMEs. The focus of discussion here is a simple quantitative model of the CME kinematics based on the constraints provided by the low-frequency radio observations and the calculated in-situ shock parameters, together with required consistency with the white-light measurements, to deduce the kinematics of interplanetary CMEs. This simple model seems to agree with the white-light SMEI observations in the one case where it has been tested. The above model has been applied to a number of CMEs for which there were good low-frequency radio observations (from Wind/WAVES) and well-defined shock signatures at 1 AU. The analyses of these events provide quantitative insights into how fast CMEs decelerate in the interplanetary medium. The validity of this model will be more directly tested using the STEREO observations. In addition to the white-light and radio observations that are presently available from various interplanetary spacecraft, the STEREO mission will enable some unique measurements such as radio source triangulation, which will provide additional constraints on the determination of CME propagation. These STEREO radio observations, together with the white-light observation from the Heliospheric Imager, may clarify the relationship of the shock to the CME driver during the interplanetary transport of CMEs.
SH53A-09 1330h
Improved Empirical CME Arrival Time Model Via Cone Model
In this study, we compare the results obtained from two cone models and carry out the statistical study of the distribution of the actual size and space speed of Coronal Mass Ejections (CMEs). We improved the existing empirical CME arrival (ECA) model, based on previously developed empirical models and provided the prediction of CME transit time from the Sun to the Earth. The previous ECA model was in good agreement with the observations for high-speed CMEs. However, the agreement was not as good for low-speed events. One of possible reasons may be due to errors caused by the significant scatter of CME projection speeds in low projected-speed events. Using the cone models we reduced the errors and improved the accuracy of the ECA model by applying the cone models to halo CMEs erupted from near disk center of the Sun (within < 30 deg.) to determine the actual speed. We found that both cone models provide similar improved accuracy for the arrival time.
SH53A-10 1330h
Sun-Earth Propagation Time of CMEs Originated at different Helio Longitudes
We present a study of the transport of coronal mass ejections (CMEs) in the interplanetary medium and the probability that they, or the associated shocks, reach the Earth surroundings when they are ejected in different Helio-longitudes. To reach this goal we choose the CME events associated with the active region 0486 which crosses the solar disk during October - November 2003 and produced several CMEs during its crossing from East to West limb. We measured and analyzed the speed profile of each event, we found that the speed profile of halos and partial-halo CMEs are very symmetric and an elliptical model seems to fit the profiles very well. Using a cone model we determine the space direction of CMEs and then, the most probable speed in the Sun-Earth direction. Using these speeds, we applied the CME travel time empirical model to determine the near Earth arrival times of both interplanetary CME and related shock. We found that the difference between the predicted and observed arrival times increase with the Helio-longitude of the CME. To help in the identification of CME - 1 AU shocks and to validate the empirical model, we use 2D numerical simulations of the events.
SH53A-11 1330h
Identifying and Characterizing ICMEs Using Total Perpendicular Pressure
It is conventional wisdom that only about one-third of Interplanetary Coronal Mass Ejections are found to contain a magnetic cloud. However, one of the most secure identifying features of an ICME is the presence of a cloud and is used by many ICME researchers to classify ICMEs. In order to determine how many ICME observations do or do not include entry into a cloud, we must have a reliable method of defining an ICME encounter that does not depend on the presence of a rotating magnetic field. Total perpendicular pressure can be used to distinguish ICMEs from other solar wind disturbances such as stream interactions without examining the direction of the magnetic field on its temporal behavior. We have compared our identifications with those of other groups and conclude that this identifier is quite effective. We find that many of the ICMEs that do not exhibit rotating magnetic fields are encountered far to the side of the center of the ICME, thus missing the cloud. Nevertheless there are ICMEs with strong field enhancements that do not exhibit rotations. There may be magnetic clouds but do not appear to be nice ropes.
SH53A-12 1330h
Investigating Coronal Origin of the Solar Wind, a Joint SOHO/UVCS and ACE/SWICS Analysis
The solar wind ion composition is generally 'frozen-in' within 5 solar radii of the Sun. Many characteristics in the elemental abundances measured in the solar wind are believed to be set in the chromospheric and low coronal levels. Therefore solar wind ion and elemental composition data combined with spectroscopic observations of the inner corona such as those from SOHO/UVCS, are ideal for investigating the coronal origin of the solar wind. We present such joint analysis using SOHO/UVCS and ACE/SWICS data along with a 3-D MHD traceback model. In October 1999, UVCS observed the west limb for 7 consecutive days with the passing of an equatorial coronal hole followed by an active region. This corresponds to a rarefaction transition from fast to slow wind seen by ACE. We present a correlation study of the electron temperature and elemental abundances between the corona and the solar wind from these two datasets. The solar wind ion and elemental composition measured by PLASTIC onboard STEREO would be valuable in conducting such analysis for investigating the formation of the solar wind.
SH53A-13 1330h
Velocities of Density Fluctuations in the Solar Wind
Density fluctuations can be measured using the potential difference between the current-biased probes and a spacecraft body. With the Cluster satellites, I have attempted to use time differences between similar fluctuation features seen with the EFW experiment on the four spacecraft to determine their velocities. It turns out that high accuracy in this determination is not possible, at least with the sampling rate which has been used to the present, as the time differences are small compared with the sampling interval when the spacecraft are close, and similar features are difficult to find when they are far apart. The 2001 period in the solar wind (Jan-Mar), when typical spacecraft separation is about 1000 km, allows some determinations. As expected, the velocities, relative to the plasma, are of the order of the ion acoustic and Alfven speeds. As these are usually similar at 1 AU, it has not yet been possible to distinguish between these possibilities.