The STEREO Encounter with the Tail of Comet Elenin and Expectations for ISON

1. The STEREO Encounter with the Tail of Comet Elenin and Expectations for ISON A.B. Galvin1, H. Kucharek1, B. Klecker1,2, K.D.C. Simunac1, C.J. Farrugia1, R. Wimmer-Schweingruber3, L. Berger3, C. Drews3, A. Vourlidas4 1Space Science Center and Dept. of Physics, University of New Hampshire, Durham, NH 03824, USA2Max Planck Institut fuer Extraterrestrische Physik, Garching, Germany 3Christian-Albrechts-Univeristaet zu Kiel, Kiel Germany 4Space Sciences Division, Naval Research Laboratory, Washington, DC (e-mail: toni.galvin@unh.edu) AGU FALL MEETING 2013 SESSION P31A-1785 The intensity of the observed water-group ions is variable but sustained through mid August. Some of this variability is clearly related to solar wind large-scale structures. Two instances of enhanced rates are observed during the SIR intervals on August 9and August 14 (Fig. 3), and the measured ion energy spectra indicate that the cometary ions were further accelerated above pickup ion energies within these SIR structures. On August 17 (at 3:42 UT), an ICME appeared over the eastern solar limb as seen by the SOHO LASCO coronagraphs and appeared as a halo in the STEREO SECCHI coronagraphs indicating it was directed towards the STEREO B spacecraft. Three days and 16 hours later, between 19:30-19:40 UT on August 20, there is a significant drop (by almost an order of magnitude) in the observed water-group rate at STEREO B. The drop coincides with the beginning of a low beta solar wind structure, a common in-situ signature of an ICME, and indeed the STEREO Level 3 data indicate the passage of an ICME from August 20, 21:51 UT to August 21, 21:20 UT. The magnetic structure of the ICME may have acted as a barrier to penetration by low energy cometary ions. After the ICME local passage at STEREO B, the water-group ions are still present but never return to their previous intensity levels. This overall reduction in intensity is again associated with the ICME discussed above, but in a more direct fashion. The in-situ speed of the ICME at STEREO B is 400 km-s-1. This is consistent with the same ICME structure passing through the solar-radial distance of the comet about a day and a half earlier. An ICME impact with comet Elenin is observed on August 19 by the STEREO SECCHI/HI remote imager, and there is a tail disconnection event observed at 9:29 UT. This constitutes the first time an ICME-comet interaction has been observed remotely and in situ by the same spacecraft. INTRODUCTION Comets are considered a natural space laboratory for the study of dust-plasma and neutral-plasma interactions, and this has prompted both dedicated comet missions as well as a continuous search for coincidental encounters. The longest serendipitous encounter to date of a space probe with a comet occurred in late July through mid August 2011 when the STEREO B (STB) observatory was immersed in the plasma environment of comet C/2010 X1 (Elenin). Closest approach was reached on July 31 at a distance of 7.4 million kilometers, with the nominal radial tail alignment occurring on August 12. Continuous observations of cometary ions are available for over three weeks. These ions were created over a broad range of solar wind conditions, including slow and fast solar wind, stream interaction regions, and an interplanetary coronal mass ejection. In the mass-per-charge range of water-group ions, the STB/PLASTIC instrument found that the major ion constituent was O+, with significant contributions from O+2 and C+. In the mass-per-charge range 23-36 amu/e there was a clear contribution by molecular ions. The composition is tracked on a daily basis and shows variations. There are also temporal variations in the speed distribution profiles. An abrupt decrease in the water-group ion intensity coincided with the passage of an interplanetary coronal mass ejection (ICME). STB/SECCHI imaged this same ICME as it interacted with the comet, observing a comet tail disconnection event. An opportunity for another comet tail passage by STEREO will occur with comet ISON in late November 2013. This comet’s orbital plane is not near the ecliptic, hence this encounter, if it occurs, would be of short duration. LARGE SCALE STRUCTURES INTERACTION WITH COMET WATER GROUP SIGNATURE AT STEREO B Figure 2 gives a time series of ion species spectra measured by PLASTIC covering a seven-month interval (May 1 – November 30, 2011) bracketing the comet encounter. The mass-per-charge (M/Q) spectra are filtered by speed to suppress contributions by solar wind ions and inner source pickup ions. A clear and unusually strong ion signature consistent with water-group ions (O+ and/or OH+, H2O+, H3O+) is observed from late July through mid-August, with less intense signatures extending sporadically into early September. A close up of the encounter period is shown in Figure 3. A detailed analysis of the composition during the August time interval (Figure 4) confirms the ions present are consistent with cometary ions, with O+ the dominant species. The insert to Fig. 4 emphasizes the M/Q range 20 – 40 amu/e. Spectroscopic observations (Korsun et al, 2012; Drahus et al., 2011) for comet Elenin indicate the presence of molecules HCN (mass = 27), CN (mass=26), and C3 (mass=36), but no ion spectral data have been published. Shown within the insert as potential contributions are Gaussian fits for M/Q ~ 23-24, observed previously at Giacobini-Zinner by Ogilvie et al. (1986), suggested as Na+ or C2+. For M/Q between 28-33, the illustrated ions are those suggested by the model of Geiss et al. (1991): M/Q=28 (H2CN+, CO+, C2H4+), M/Q=30 (NO+, H2CO+, C2H6+), M/Q=31 (H3CO+), M/Q=32 (CH3OH+, S+) and M/Q=33 (CH3OH2+, SH+). The Elenin data also show a trace contribution near M/Q ~ 36. There were temporal variations in the presence of these molecules as observed in-situ at Elenin. ISON Fig 2. Species (M/Q) spectral time series for the M/Q range 3-20 amu/e covering the time period May 1, 2011 through November 30, 2011. A period in late July to mid August stands out for an exceptional contribution by ions with M/Q consistent with water-group ions. Comets Encke and ISON had perihelions on November 21, 2013 and November 28, 2013, respectively. On December 2, 2013, STEREO A encountered a shock with a subsequent signature of suprathermal O+ ions. This is likely an stream interaction region, although not all data are yet available for a full identification. COMET ELENIN AND STEREO B ORBITS Comet C/2010 X1 (Elenin) was discovered in December 2010 and achieved perihelion on September 10, 2011. The brightness of the comet reached 8.m1 on August 17, 2011 (Korsun et al., 2012). Then, by the end of August and weeks prior to perihelion, its brightness decreased to 12m, and the comet coma became elongated. It has been suggested that the comet’s reduced brightness may have been related to an ICME interaction. During the time interval prior to and encompassing the comet’s unexpected decline, its trajectory brought it into close proximity with NASA’s Solar Terrestrial Relations Observatory Behind (STEREO B). The STEREO B observatory is in a heliocentric, in-ecliptic orbit at 1.05 AU, and in late July 2011 it was at a longitudinal separation from Earth of about 93o. Elenin’s trajectory was particularly favorable for both remote and in-situ observations by STEREO because the comet’s orbital plane was close to the ecliptic plane and the latitudinal difference from STEREO B was always less than 2o (Fig. 1). The in-situ observations we report here are from the Plasma and Suprathermal Ion Composition (PLASTIC) experiment (Galvin et al., 2008), with remote imaging provided by the SECCHI/HI instrument (Howard et al., 2008). The presence of O+ is not conclusive of a comet origin, as interstellar O+ is routinely observed as accelerated in SIRs. Further reduction of the data is in progress to determine if distinguishing comet-like ions (O+2, C+) are also present. Acknowledgements • We thank Eric Christian, who suggested that the STEREO in-situ teams look at the Elenin encounter period for a potential tail passage. We thank the STEREO IMPACT team for Level 2 and Level 3 databases. We thank the Jet Propulsion Laboratory HORIZONS on-line ephemeris computation service. Margaret Chutter assisted in the data reduction. This work is supported by NASA Grant NNX13AP52G. Fig. 1. Top: The relative positions of the STEREO B observatory and the comet Elenin projected onto the ecliptic (X-Y) plane. The radial from the Sun to the comet is extended anti-sunward to illustrate the nominal direction expected for an ion tail (neglecting aberration angles, typically 2o to 9o). STB/PLASTIC began observing water-group ions on July 30, 2011. The intensity decreased dramatically after August 20, 2011. Bottom: During the entire encounter period, the difference in latitude between comet Elenin and STEREO B never exceeded 2o. Both STB and the comet were located slightly below the ecliptic plane. References • Drahus, M., B. Yang, and J. Hoge, Comet C/2010 X1 (Elenin), CBET 2781, 1 (2011). • Galvin, A. B., et al., The Plasma and Suprathermal Ion Composition (PLASTIC) Investigation on the STEREO Observatories, Space Sci. Rev., 136, 437–486, doi:10.1007/s11214- 007-9296-x, 2008. • Geiss, J., et al., Interpretation of the Ion Mass Spectra in the Mass Per Charge Range 25-35 amu/e Obtained in the Inner Coma of Halley’s Comet by the HIS-Sensor of the Giotto IMS Experiment, Astron. Astrophys., 247, 226-234, 1991. • Howard, R.A. Moses, A. Vourlidas, J.S. Newmark, et al., Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), Space Sci. Rev., 136, 67-115, doi:10.1007/s11214-008-9341-4, 2008. • Korsun, P.P., I.V. Kulyk, A.V. Moiseev, and V.L. Afanasiev, Comet C/2010 X1 (Elenin). Unrealized Expectations, Astrophysical Bulletin, 67, 414-424, 2012. • Ogilvie, K.W., M.A. Coplan, P. Bochsler, and J. Geiss, Ion Composition Results During the International Cometary Explorer Encounter with Giacobini-Zinner, Science, 232, 374-377, 1986 Fig. 4. M/Q PHA (pulse height analysis) spectra measured by STB/PLASTIC accumulated over several days, binned by 0.1 amu/e. Ion events are filtered for ion speeds ≥ 1.5*solar wind speed. Gaussian fits are illustrated for the resolved ions O+, C+, O+2 and the unresolved but expected ions N+, Ne+ (trace), OH+, H2O+, and H3O+. The profile of the sum of the Gaussians is shown as the shaded broad solid line. Given the instrument resolution, the abundances of the trace constituents shown in the figure should not be considered a unique solution. Also some M/Q values may have multiple species contributing. For example, M/Q = 14 may have contributions from CH2+ and N+. Fig. 3. Close-up of encounter period, July 20 through September 20. Top panel: Solar wind conditions measured by STB/PLASTIC, including indicators for SIRs (green arrow heads), shocks (yellow arrow heads), and ICME (red bar) event times provided in STEREO Level 3 data. The middle panel provides basic trajectory information on the distances (in millions of kilometers) of the comet to the Sun and the comet to STEREO B. The bottom panel provides the STB/PLASTIC count rate (1-hr) for water group ions.