Richard A. Harrison

The Coronal Diagnostic Spectrometer for the Solar and Heliospheric Observatory

The Coronal Diagnostic Spectrometer is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet wavelength range 150–800 A. By observing the intensities of selected lines and line profiles we may derive temperature, density, flow and abundance information for the plasmas in the solar atmosphere. Spatial and temporal resolutions of down to a few arcseconds and seconds, respectively, allow such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of large wavelength bands provides the capability for simultaneously observing the properties of plasmas across the wide temperature ranges of the solar atmosphere.

First imaging of corotating interaction regions using the STEREO spacecraft

Plasma parcels are observed propagating from the Sun out to the large coronal heights monitored by the Heliospheric Imagers (HI) instruments onboard the NASA STEREO spacecraft during September 2007. The source region of these out-flowing parcels is found to corotate with the Sun and to be rooted near the western boundary of an equatorial coronal hole. These plasma enhancements evolve during their propagation through the HI cameras fields of view and only becoming fully developed in the outer camera field of view. We provide evidence that HI is observing the formation of a Corotating Interaction Region (CIR) where fast solar wind from the equatorial coronal hole is interacting with the slow solar wind of the streamer belt located on the western edge of that coronal hole. A dense plasma parcel is also observed near the footpoint of the observed CIR at a distance less than 0.1AU from the Sun where fast wind would have not had time to catch up slow wind. We suggest that this low-lying plasma enhancement is a plasma parcel which has been disconnected from a helmet streamer and subsequently becomes embedded inside the corotating interaction region.

A synoptic view of solar transient evolution in the inner heliosphere using the Heliospheric Imagers on STEREO

[1]xa0By exploiting data from the STEREO/heliospheric imagers (HI) we extend a well-established technique developed for coronal analysis by producing time-elongation plots that reveal the nature of solar transient activity over a far more extensive region of the heliosphere than previously possible from coronagraph images. Despite the simplicity of these plots, their power in demonstrating how the plethora of ascending coronal features observed near the Sun evolve as they move antisunward is obvious. The time-elongation profile of a transient tracked by HI can, moreover, be used to establish its angle out of the plane-of-the-sky; an illustration of such analysis reveals coronal mass ejection material that can be clearly observed propagating out to distances beyond 1AU. This work confirms the value of the time-elongation format in identifying/characterising transient activity in the inner heliosphere, whilst also validating the ability of HI to continuously monitor solar ejecta out to and beyond 1AU.

CHARACTERISTICS OF KINEMATICS OF A CORONAL MASS EJECTION DURING THE 2010 AUGUST 1 CME–CME INTERACTION EVENT

We study the interaction of two successive coronal mass ejections (CMEs) during the 2010 August 1 events using STEREO/SECCHI COR and heliospheric imager (HI) data. We obtain the direction of motion for both CMEs by applying several independent reconstruction methods and find that the CMEs head in similar directions. This provides evidence that a full interaction takes place between the two CMEs that can be observed in the HI1 field of view. The full de-projected kinematics of the faster CME from Sun to Earth is derived by combining remote observations with in situ measurements of the CME at 1 AU. The speed profile of the faster CME (CME2; similar to 1200 km s(-1)) shows a strong deceleration over the distance range at which it reaches the slower, preceding CME (CME1; similar to 700 km s(-1)). By applying a drag-based model we are able to reproduce the kinematical profile of CME2, suggesting that CME1 represents a magnetohydrodynamic obstacle for CME2 and that, after the interaction, the merged entity propagates as a single structure in an ambient flow of speed and density typical for quiet solar wind conditions. Observational facts show that magnetic forces may contribute to the enhanced deceleration of CME2. We speculate that the increase in magnetic tension and pressure, when CME2 bends and compresses the magnetic field lines of CME1, increases the efficiency of drag.

Coronal dimming and the coronal mass ejection onset

A set of five observations of extreme-ultraviolet (EUV) coronal dimming associated with coronal mass ejection (CME) activity is examined. Using spectroscopic data, plasma characteristics across a broad range of temperatures from 20 000 K to 2 million K are determined. The dimming events are found to coincide in time, and to coincide spatially, with the projected onset times and locations of the associated CMEs. The spectral data confirm that the dimming is due to mass- loss, and not temperature variations. The actual mass-loss calculated from the degree of dimming, using two dierent methods, shows that the extracted mass in each case, is of the same order as the mass of the associated CME. In some cases, the EUV observations are limited to relatively small regions under the CME events and it is expected that we do not witness the mass- loss associated with the entire event, for these. However, we believe that this analysis has provided a method for locating the source region of the trigger for a CME eruption, and that the dimming characteristics can be used to distinguish between onset processes of the CME. In particular, the gradual nature of the dimming process, which takes place over several hours, suggests that either the CME has a continuous driver rather than a sudden impulsive onset, or the low coronal response to a CME extends over a long period.

Interactions between Coronal Mass Ejections Viewed in Coordinated Imaging and In Situ Observations

The successive coronal mass ejections (CMEs) from 2010 July 30 to August 1 present us the first opportunity to study CME-CME interactions with unprecedented heliospheric imaging and in situ observations from multiple vantage points. We describe two cases of CME interactions: merging of two CMEs launched close in time and overtaking of a preceding CME by a shock wave. The first two CMEs on August 1 interact close to the Sun and form a merged front, which then overtakes the July 30 CME near 1 AU, as revealed by wide-angle imaging observations. Connections between imaging observations and in situ signatures at 1 AU suggest that the merged front is a shock wave, followed by two ejecta observed at Wind which seem to have already merged. In situ measurements show that the CME from July 30 is being overtaken by the shock at 1 AU and is significantly compressed, accelerated, and heated. The interaction between the preceding ejecta and shock also results in variations in the shock strength and structure on a global scale, as shown by widely separated in situ measurements from Wind and STEREO B. These results indicate important implications of CME-CME interactions for shock propagation, particle acceleration, and space weather forecasting.

EUV Blinkers: The Significance of Variations in the Extreme Ultraviolet Quiet Sun

A search for microflare activity in the extreme ultraviolet (EUV) quiet Sun using the Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft has not resulted in the identification of microflare activity, but has resulted in the identification of a hitherto unknown phenomenon: enhancements of a factor of 2–3 in the flux of transition region lines at network junctions. A total of some 6 hours of observation of 5 different target areas showed this ‘blinker’ activity at each area, with durations ranging from 1 to 30 min and averaging 13 min, and thermal energy content of order 10-6 that of a ‘standard’ flare. Assuming that the observations are of typical quiet Sun, and projecting these data to predict a distribution of these events over the entire Sun, the total thermal energy content of these ‘blinkers’ is insignificant when compared to the energy required to heat the corona. The nature of these events and their significance are discussed in this paper.

SECCHI observations of the Sun-s Garden-Hose Density spiral

The SECCHI HI2 white-light imagers on the STEREO A and B spacecraft show systematically different proper motions of material moving outward from the Sun in front of high-speed solar wind streams from coronal holes. As a group of ejections enters the eastern (A) field of view, the elements at the rear of the group appear to overrun the elements at the front. (This is a projection effect and does not mean that the different elements actually merge.) The opposite is true in the western (B) field; the elements at the front of the group appear to run away from the elements at the rear. Elongation/time maps show this effect as a characteristic grouping of the tracks of motion into convergent patterns in the east and divergent patterns in the west, consistent with ejections from a single longitude on the rotating Sun. Evidently, we are observing segments of the garden-hose spiral made visible when fast wind from a low-latitude coronal hole compresses blobs of streamer material being shed at the leading edge of the hole.

First Direct Observation of the Interaction between a Comet and a Coronal Mass Ejection Leading to a Complete Plasma Tail Disconnection

This a discovery report of the first direct imaging of the interaction a comet with a coronal mass ejection (CME) in the inner heliosphere with high temporal and spatial resolution. The observations were obtained by the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) Heliospheric Imager-1 (HI-1) aboard the STEREO mission. They reveal the extent of the plasma tail of comet 2P/Encke to unprecedented lengths and allow us to examine the mechanism behind a spectacular tail disconnection event. Our preliminary analysis suggests that the disconnection is driven by magnetic reconnection between the magnetic field entrained in the CME and the interplanetary field draped around the comet and not by pressure effects. Further analysis is required before we can conclude whether the reconnection occurs on the day side or on the tail side of the comet. However, the observations offer strong support to the idea that large-scale tail disconnections are magnetic in origin. The online movie reveals a wealth of interactions between solar wind structures and the plasma tail beyond the collision with the CME. Future analyses of this data set should provide critical insights on the structure of the inner heliosphere.

Discovery of the atomic iron tail of comet McNaught using the Heliospheric Imager on STEREO

In 2007 January, at the heliocentric distance r < 0.3 AU, comet McNaught 2006P1 became the brightest comet since C/Ikeya-Seki 1965S1 and was continuously monitored by space-based solar observatories. We provide strong evidence that an archlike tail observed by the Heliospheric Imager aboard the STEREO spacecraft is the first ever detected tail composed of neutral Fe atoms. We obtain an Fe lifetime τ = (4.1 ± 0.4) × 104 s at r = 0.25 AU, in agreement with theoretical predictions of the photoionization lifetime. The expected dust temperature is inconsistent with iron sublimation, suggesting that Fe atoms are coming from troilite evaporation.