SH52B-01
Interplanetary Dust Clouds Near 1 AU Detected by STEREO
For a 21 day interval beginning on Jan 28, 2007, the radio and plasma wave instruments (S/WAVES) on the twin STEREO spacecraft detected intense impulse-like emissions in both the time-domain receivers and the low end of the frequency-domain receivers. These emissions were much more intense on the STEREO Ahead instrument, but were detected by both. During that interval, the two STEREO spacecraft were only separated by about 0.1° in heliocentric longitude. All attempts to understand the emissions in terms of electrical disturbances onboard the spacecraft were unsuccessful and, since the impulsive events essentially ceased on Feb 18, 2007, the entire study was put on lower priority. However, beginning abruptly on July 20, 2007, a very similar and more intense episode of impulses was detected exclusively by S/WAVES on the STEREO Ahead spacecraft, now separated from Behind by more than 20° of heliocentric longitude. This new episode lasted nearly as long as the Jan-Feb episode, some 20 days, ending on Aug. 9. On Aug. 14, 2007, the STEREO Behind spacecraft arrived at the same heliocentric longitude as the Ahead spacecraft was at on July 20 and the S/WAVES instrument abruptly started detecting impulsive events. We noted that the individual waveform of the detected impulses was very similar to signals recorded by the Voyager plasma wave (PWS) and radio astronomy (PRA) instruments and more recently by the Cassini radio and plasma wave instrument when those spacecraft plunged through Saturn's rings. Those Saturnian signals are attributed to micron-sized dust particle impacts on the spacecraft which creates a temporary plasma cloud which, in turn, induces an electrical signal on the antennas. We reanalyzed the STEREO episodes along the lines of the Saturnian ring plane analyses and have concluded that the STEREO impulses are very likely due to similar dust impacts with bands or clouds of dust of cometary or, more generally, zodiacal light material near 1 AU. The distribution of these clouds of small dust particles seems to be significantly different than theoretical models of interplanetary dust distribution.
SH52B-02
The Spatial Distribution of Upstream Ion Events Measured by ACE, Wind, and STEREO Near the Earth's Bow Shock
Abrupt enhancements in the intensities of ions in the energy range of a few 10s of keV to 100s of keV upstream of the Earth's Bow Shock –- upstream ion events –- are characterized by short durations (~1-2 hours), steeply falling spectra, large (>100:1) field-aligned sunward anisotropies, and positive correlations with the solar wind speed and geomagnetic indices. Despite the wealth of information available, however, it is still not clear whether these ions are accelerated at the bow shock or somewhere inside the Earth's magnetosphere. Furthermore, such events are also often observed simultaneously at two or more spacecraft, perhaps indicating that a large source region covering the entire size of the bow shock fills large spatial structures in the upstream region. In this paper we use simultaneous measurements of >40 keV upstream ions observed at ACE, Wind, and STEREO-A between 2007, day 1 through 2007, day 181 to calculate the occurrence probability of upstream events as a function of lateral and radial separation between L1 and STEREO-A. During the end of this ~6- month period, Wind (or ACE) and STEREO-A were separated by ~1750 RE in the radial direction and laterally in YGSE by ~3800 RE. Despite this large separation, STEREO-A continued to observe upstream events right up until the end of our survey period. More surprisingly, we found that the occurrence probability for measuring simultaneous upstream events at Wind or ACE at L1 and STEREO-A was ~20- 30%, i.e., far greater than that expected from accidental coincidences. We discuss the implications of these results for the size of the source region, the conditions under which upstream events occur, and the size and nature of the spatial structures in which these ions populate and propagate in the upstream region.
SH52B-03
Multispacecraft Observations of "Dropouts" in Impulsive Events
We will present multispacecraft observations and analysis of intensity "dropouts" in solar-energetic particle (SEP) events. When seen by ACE/ULEIS and Wind/STEP, flare-related energetic ion events display a velocity dispersion, with the higher velocity particles arriving earlier than the lower velocity particles. In 2000, Mazur et al. first reported "dropouts", consisting of sudden decreases in intensity occuring simultaneously across all energies and lasting on the order of a few hours. In that paper and the accompanying one by Giacalone et al., they propose that these dropouts occur at the passage of flux tubes unconnected to the flare source that have been mixed in with full flux tubes due to field line random walk. We have continued this analysis by considering the entire time period when both of these spacecraft are observing, creating a list of observed impulsive flare-related dispersions with and without dropouts. We will present several additional examples of these events and relate them to the relative spacecraft orientations, as spacecraft which are separated by more than a correlation scale in the direction perpendicular to the Earth-sun line should see different flux tubes. We will also discuss the frequency of these dropouts and how it relates to the amount of field line mixing.
SH52B-04
STEREO observations of CIR-accelerated particle events during 2007
Coronal holes are the origin of fast solar wind streams which interact with the preceding slow wind leading to the formation of corotating interaction regions (CIRs). Beyond 1 AU, CIRs are bounded by forward-reverse shock pairs that become the dominant source of ion increases observed at 1 AU in the energy range ~20 keV/n to ~ 20MeV/n during solar quiet time periods. Recurrent CIR-accelerated ion events have been regularly observed during January-September 2007 by the multiple sensors of the Solar Electron and Proton Telescope (SEPT) onboard the twin STEREO spacecraft. The increasing angular separation between STEREO-A and STEREO-B and the additional observations available from near-Earth spacecraft provide an excellent opportunity for the multi-spacecraft analysis of the CIR-associated particle events in the inner heliosphere during an extended period of low solar activity. Time delays and the differences in the temporal profiles observed during these events are presented and compared with the expected behaviour assuming an idealized co-rotation scenario.
SH52B-05
Energetic Particles from Corotating Interaction Regions as Observed by STEREO and ACE
Since the two STEREO spacecraft entered heliocentric orbit early in 2007 there have been no large solar energetic particle events, however significant particle enhancements due to corotating interaction regions (CIRs) have been regularly observed at 1 AU. The Low Energy Telescope (LET) on each STEREO spacecraft measures elemental composition and spectra for particles from H to Ni from ~2 to ~50 MeV/nucleon, depending on species, and provides anisotropy information for 6 different species or element groups. The Suprathermal Ion Telescope (SIT), also on STEREO, extends ion composition measurements down to lower energies of ~0.05 to 5 MeV/nucleon. As the two STEREO spacecraft move farther apart, the differences in CIR time profiles observed on each become ever greater. Delays on the order of a day are now common between the Behind and Ahead spacecraft, with different features present in the time profiles on each. Using STEREO data along with measurements from ULEIS and SIS on ACE, we present observations of H and He spectra, anisotropies, and time profiles for the CIR events of 2007. This work was supported by NASA under grants NAS5-03131 and NAG5-12929.
SH52B-06
A Coronal Mass Ejection at Venus observed with STEREO HI and Venus Express
Observations of a CME interacting with comet Encke were made by the STEREO HI cameras in April 2007. The position of the comet allowed the direction of the CME to be determined even though it had only been observed with one STEREO spacecraft. An examination of the planetary positions indicated that the CME would intersect with Venus. Data from the Venus Express spacecraft show the passage of a CME front at the time predicted by the HI data. http://www.stereo.rl.ac.uk
SH52B-07
Solar Wind Charge State Composition Results from PLASTIC
The PLASTIC instrument on the STEREO spacecraft provides solar wind proton moments and heavy ion composition. Using an electrostatic analyzer with a time of flight and residual energy measurement, it can supply mass and ionic charge state for solar wind heavy ions. Preliminary results for iron will be shown for selected events, including the possible flux rope passage on May 21-22, 2007, and a near-magnetotail passage in February, 2007.
SH52B-08
Multi-Spacecraft Observations of ICMEs and Magnetic Clouds: Ulysses and STEREO
The Ulysses spacecraft, in a six year period polar orbit of the Sun, is presently making its third fast latitude scan through heliolatitudes from 80°S in February 2007 to 80°N in January 2008. At perihelion in August 2007 the spacecraft is at a distance of 1.4 AU from the Sun and fortuitously is almost radially aligned with the Earth in heliolongitude. Thus the orbit is also bisecting the locations of the two STEREO spacecraft as they separate in longitude between 10 and 15° either side of the Earth. This period therefore provides a unique opportunity to study the solar wind and its embedded magnetic field from vantage points separated in both longitude and latitude. In this paper we will focus on what we can learn about the structure and propagation of the interplanetary manifestations of coronal mass ejections (ICMEs) from observations by this combination of spacecraft. At the time of writing, data from the prime August period is not yet fully available. However, a period of probable coronal mass ejection activity is apparent in the Ulysses data over about 8 days at the end of June, into July, culminating in a very clear magnetic cloud on the 4th and 5th of July. On a first look, a similarly clear signature is not readily apparent in browse data from either of the STEREO spacecraft or ACE but at this time Ulysses is still about 40° in longitude west (ahead) of STEREO-A and about 30° south in latitude. Nonetheless, this event provides a first interesting case study on which we can build.