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 9 and 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.

At STEREO B, there is an increase (> 20 counts/hour) observed in the water-group ions in mid September. At this time, STEREO B is separated from Elenin by 132 million km, with an angular STB-Sun-Elenin separation of 55o. This increase is likely due to a different comet. On September 19, Swan C/2011 Q4 crossed the ecliptic plane downstream from STB. At 23 million km, this is beyond the typical extent seen for water-group ions seen in-situ upstream of comets. Another candidate under consideration is the fragmented comet 73P/Schassmann-Wachmann.

An Extended Spacecraft Encounter with the Tail of Comet Elenin: Plasma and Suprathermal Ion Observations by STEREO-B

1Space Science Center and Dept. of Physics, University of New Hampshire, Durham, NH 03824, USA 2St Petersburg College, Tarpon Springs, FL 36689 (e-mail: toni.galvin@unh.edu)

A.B. Galvin1, K.D.C. Simunac1.2

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 overview of the ion

observations are presented.

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).

Drahus, M., B. Yang, and J. Hoge, Comet C/2010 X1 (Elenin), CBET 2781, 1 (2011).

Galvin, A. B., et al., Space Sci. Rev., 136, 437–486, doi:10.1007/s11214- 007-9296-x, 2008.

Geiss, J., et al., Astron. Astrophys., 247, 226-234, 1991.

Howard, R.A. Moses, A. Vourlidas, J.S. Newmark, et al., 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, Science, 232, 374-377, 1986

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.

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.

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.

WATER GROUP IN-SITU

SIGNATURE AT STEREO B

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+.

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.

Acknowledgements

ReferencesFig. 3. Close-up of encounter period, July 20 through September 20. Top panel: Solar wind conditions measured by STB/PLASTIC, including indicators for stream interaction regions (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.

LARGE SCALE SW STRUCTURES INTERACTION WITH COMET

SECOND COMET?

IAU General Assembly August 2015 Poster P2.160