ACCELERATION RESULTS

1. GOALS 1.0 1.0 1.0 The two goals of this study are to examine the acceleration of CMEs in the low corona and to determine the most accurate CME start times in order to better understand the relationship between CMEs and other forms of solar activity, such as flares. 0.8 0.8 0.8 0.6 0.6 0.6 Method 0.4 0.4 0.4 0.2 0.2 0.2 0.0 0.0 0.0 CMEs are identified in the low corona using white light observations from the Mauna Loa Solar Observatory (MLSO) MK3 K-Coronameter (1.15 to 2.45 solar radii, 1980 to 1999) and the MK4 K-Coronameter (1.12 to 2.7 solar radii, 1998 to present). Observations of the upper corona, from SMM (1.8 to 5 solar radii, 1980 and 1984 to 1989) and LASCO (2 to 32 solar radii, 1996 to present), are used to determine if the MLSO observations have recorded the outer front of the CME or part of the CME core. Some CMEs are in progress when MLSO observations begin at ~1700 UT and in some cases the outer portion of the CME is not observed in MLSO images. CMEs originating away from the solar limb are much less visible in the MLSO coronameters, which record the corona in white light polarization brightness. The height of the CME front is measured in MLSO and in SMM or LASCO white light intensity images and the data are fit with a linear, constant velocity fit and a second order, constant acceleration fit. CME acceleration is determined for those CMEs whose constant velocity fits do not intersect all of the measured heights of the CME front vs. time and where the constant acceleration fit produces a better (smaller) reduced chi-square residual than the constant velocity fit. The CME height is fit for each instrument separately, to determine the acceleration, if any, in that instrument’s field-of-view and then the MLSO low coronal measurements are combined with either the SMM or LASCO measurements to determine the acceleration over many coronal scale heights. CONCLUSIONS The largest CME acceleration occurs in the low corona despite the strong force of gravity in this region. Few CMEs are seen to be decelerating at or below the SMM field-of-view (~5 solar radii). There is a strong correlation between CME kinetic energy and the peak emission of the associated X-ray flare, for above background intensity flares. Low coronal observations are crucial to determining accurate and precise CME start times. Inclusion of data from the outermost corona can complicate the trajectory by including forces on the CME other than the driving force. Fast CMEs and associated X-ray emission appear to occur simultaneously, within the errors of the observations. This is consistent with previous results (Nitta et al. 1999, J. Zhang et al. 2001, M. Zhang et al. 2002) and with some models of CMEs. In some models simultaneity of CME and flare onset is a necessary condition for fast CMEs (e.g. Low and Zhang 2002). ACCELERATION RESULTS CME START TIMES To meet the goal of determining accurate CME start times, we examine the start times for 20 CMEs between Jan 1996 and August 2001. CME start times are determined by projecting the constant velocity and constant acceleration fits back to 1.0 solar radii, or for some constant acceleration fits, to the height in the corona where the velocity is zero. Onset times are computed in 3 ways: 1) using low coronal trajectories only, 2) using LASCO observations only and 3) combining low and upper coronal data to form a trajectory over a wide range of heights. 61% of the CME start times produced from the LASCO observations alone are LATER than the start time from the trajectory combining MK3/4 and LASCO while only 28% of the start times from the MK3/4 trajectories produce a LATER start time than the combined trajectory. The later LASCO start times can be caused by undetected CME acceleration in the low corona. But do combined trajectories really provide more accurate start times? CMEs interact with coronal and solar wind structures as they propagate outward and these interactions alter the CME shape, course and speed. These forces may be quite independent from the physical processes that produce the CME. Good observations of CMEs at very low heights, where CME-wind interactions are negligible, should provide the most accurate determination of CME start times. Detecting the CME as it forms is an ideal way to determine accurate start times. Most of the CMEs seen in the MK3/4 coronameters appear to form below the apparent height of the occulting disk (1.12 solar radii). While lower coronal observations are needed, the MK3/4 provide essential information in determining the CME start time. We examined CMEs with measurable acceleration in each of the 3 datasets. We are able to find 28 CMEs that had measurable accelerations in the MLSO observations, 120 in SMM , and 11 in LASCO. The LASCO events are chosen due to the visibility of the CME in the MK3/4 observations. Histograms of the log of the CME acceleration measured in each dataset are shown at left. The largest accelerations are observed in the low corona and no decelerating CMEs are seen here, despite the strong force of gravity in this region. Further, the average CME acceleration decreases rapidly with height and a greater percentage of decelerating events are measured in the LASCO images. This is likely due to CME-solar wind interactions, which can be significant at the great heights observed by LASCO. The LASCO average acceleration of -1 m/sec2 is derived from a small set of events and not from a systematic study of LASCO CMEs. Additional events need to be included to provide a more accurate measure of the ‘true’ average CME acceleration in the LASCO field-of-view. MK3/4 1.12 to 2.8 solar radii Avg. Accel.= 454 m/s2 SMM FRACTION OF TOTAL 1.8 to ~5 solar radii Avg. Accel.= 68 m/s2 LASCO C2/C3 2 to 32 solar radii Avg. Accel.= -1 m/s2 LOG of ACCELERATION [m/s2] Histograms of CME accelerations; accelerating CMEs are on the right hand side of the vertical bar and decelerating CMEs are on the left hand side of the vertical bar. The upper panel illustrates the CME accelerations derived using only low coronal data from MK3 and MK4. The middle panel displays the CME accelerations derived from SMM observations only and the lowermost panel are CME accelerations in the outermost corona using only LASCO C2 and C3 observations. CME accelerations are largest in the lower corona, despite the strong gravitational force in that region. In order to better understand the relationship between the onset of CMEs and X-ray emission, we examine the start times of the 12 fastest CMEs (speed > 800 km/sec) observed with the MK3/4 coronameters. Of those 12 events, 11 have an associated X-ray flare, which include 1 C-, 7 M- and 3 X-Class GOES flares. This is consistent with previous studies which indicate that fast CMEs are associated with high intensity flares (Gosling et al. 1976, MacQueen and Fisher 1983, St.Cyr et al. 1999). Of these 11 flares, 7 are located within 30 degrees of the solar limb, where projection errors in the CME start times are minimized. Is it possible to accurately determine which phenomenon occurs first? In 5 of the 7 cases, the flare onset precedes the CME and in 2 cases the CME precedes the flare. The differences between the CME and flare onset times are: FLARE PRECEDES CME BY: 12, 7, 4, 1, and 1minutes. CME PRECEDES FLARE BY: 3 minutes and 5 minutes. These timing differences are within the margin of error in determining the CME and flare start times. Nevertheless, only a tiny fraction of the X-ray emission occurs prior to CME onset, consistent with some CME models. High time coronal images are needed to further reduce the errors in the calculated start times. To further illustrate the changes in CME accelerations with height we plot the CME ‘final’ speed vs. its acceleration. The CME final speed is defined as the speed of the CME at the uppermost height in the instrument field-of-view, for a constant acceleration fit. The CME final speed vs. acceleration is plotted at right for MK3/4 (red), SMM (green) and LASCO (blue) observations. The CME accelerations decrease with increasing height of the instrument field-of-view, as stated above. The CME final speed and its acceleration are correlated in the lower and middle corona, as seen in the MK3/4 and SMM results, but there is no strong correlation seen in the LASCO results. This may be due to CME-solar wind interactions which can be significant in the outer corona and further complicate the CME dynamics. Flare Intensity vs. CME Kinetic Energy CME Speeds and X-ray Emission CMEs are associated with X-ray emission (Gosling et al. 1974, Sheeley et al. 1975, Rust and Webb 1977, Kahler 1992) and correlations have been noted between higher intensity X-ray flares and CME speeds. A recent study of projection effects on white light observations (Burkepile et al. 2003) shows a stronger correlation, than previously reported, between CME kinetic energies and the peak X-ray emission of the associated X-ray flare. At right is a plot of the kinetic energies of 57 SMM CMEs, believed to be located within 30o of the solar limb (where projection errors are small), vs. the peak X-ray emission of the associated GOES X-ray flare. Log (CME Kinetic Energy) [ergs] A sequence of 3 composite images of a CME on August 2, 2001. The inner corona is recorded by the MK4 K-Coronameter, and the outer corona is recorded by the LASCO C2 Coronagraph at 19:24 UT. This CME is accelerating at 165 m/s2 in the MK4 field-of-view GOES CLASS: A B C M X Log (Flare Intensity) [w/m2] The linear correlation coefficient = 0.78. CMEs associated with below background X-ray emission have been systematically excluded and may lower the correlation at or below C-Class X-ray intensity levels.