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.