STEREO IMPACT News #1 - February 22, 2007

STEREO IMPACT Detects its First SEP and ICME Events

Time series from several IMPACT instruments (from the top: LET, STE-D, SWEA and MAG) on STEREO B from the period December 5-16, 2006. Similar observations were obtained on STEREO A. Vertical dashed lines mark the occurrence of X-class flares, described in the text below. The dotted vertical lines bracket the time of STEREO B's passage through the magnetosphere. The first energetic particle event seen in the LET, HET (not shown), STE-D and SWEA data was associated with a far eastern limb event and so was not accompanied by an ICME. The second major onset on December 13 occurred when the active region had rotated to the western central disk. An ICME preceded by a shock can be seen in the MAG data a few days later, together with the peak particle flux for that event.

(Sunspot 930 from SOHO MDI on Dec. 13, EIT image, Movie from the SOHO LASCO website at NRL)

STEREO IMPACT, with its commissioning nearly completed following the STEREO launch on October 25, 2006, detected its first major Solar Energetic Particle (SEP) and Coronal Mass Ejection (CME) events in the weeks following December 5 -while both spacecraft were near their Lunar orbit apogee in their geocentric phasing orbits. The detected particles and interplanetary field disturbances were associated with multiple eruptions involving an active region including Sunspot 930 that rotated onto the visible disk on December 5, 2006. (see archive for December 7, 8, and 9, 2006).This event was somewhat surprising because of the current quiet state of the Sun, near the minimum phase of its activity cycle. The IMPACT sensors that detected the SEP event included LET, the Low Energy Telescope, developed by Caltech, Goddard, and JPL and designed to measure SEP composition from ~3 to 30 MeV/nucleon, the Suprathermal Electron Detector (STE), built by the University of California Berkeley Space Sciences Lab, the Solar Wind Electron Analyzer (SWEA) from CESR, and the Magnetometer, supplied by Goddard Space Flight Center. (The HET sensor also detected this event but the data are not shown here. Several other IMPACT SEP sensors were not yet fully operational.)

The related solar events, also detected at ACE (see the ACE News item at and WIND spacecraft at the L1 upstream libration point, started following an X9 solar flare from longitude E71 near the far eastern limb seen by the imagers on the TRACE satellite. (Also see the SolarSoft "latest" page display for December 10). The energetic particles associated with this flare and a probably associated CME were detected, apparently with better magnetic connectivity to the flare and CME site at Venus and Mars, by the Venus Express and Mars Express Spacecraft. A subsequent X6.5 flare, occuring when the active region reached a more central disk location (at E57 longitude) produced additional SEPs, but in neither case did a significant interplanetary plasma or field disturbance follow- presumably due to the eastern locations of these flares and inferred coronal eruptions. Unfortunately, SOHO imagers were undergoing some routine maintenance checks and missed the opportunity to observe the presumed X9 flare-associated CME near the Sun. However, TRACE images of the flare were obtained, as were ground based images including a remarkable sequence of Moreton wave images in the H-alpha line from the National Solar Observatory (see Finally the active region rotated west to W23 (as seen in the images above) where another X-class flare (X3.4) and fast coronal mass ejection (CME) occurred on December 13, initiating another large SEP event followed by a smaller event late on December 14. (See the SolarSoft "latest" page display for December 17).

The time history of these SEP events at STEREO-B is shown above. This time the SOHO imagers caught the event, which was seen in both the LASCO coronagraph and EIT images, also shown above. The first SEP event has a gradual onset, a characteristic of magnetic field connection to an eastern event. The second SEP event, which follows the more central CME as seen from the Earth and detected by SOHO's LASCO coronagraph as a near-halo event, has a rapid rise and a second peak in its flux versus time profile when the CME-driven interplanetary shock arrives at the Earth. The magnetic field observations also show the shock and the arrival of the driver "ejecta" fields that the CME produced. In the case of the first (eastern disk) event the CME shock barely grazed the Earth and the ejecta fields were missed entirely at the Earth. When the STEREO spacecraft become more widely separated they will be better able to characterize the different appearances of SEP profiles and interplanetary CME (ICME) signatures in relation to the solar events and images.

The STEREO SECCHI imaging system has produced its first light images and will be operating in a STEREOscopic imaging mode after both STEREO spacecraft are in heliocentric orbit, following the final January 21 Lunar gravity assist ejection of STEREO-B. Information about the current STEREO location can be obtained from the STEREO Science Center at the URL

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Posted 22 February 2007