Abstract
We have obtained EUV spectra between 90 and 255 8 of the comets C/2002 T7 (LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelion passages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS). We obtained contemporaneous data on NEAT with the Chandra ACIS instrument, marking the firstsimultaneousEUVandX-rayspectralobservationsofacomet.ThetotalCHIPS/EUVobservingtimeswere337ks for NEAT, 234 ks for LINEAR, and 483 ks for Machholz, and for both CHIPS and Chandra we calculate we have capturedall the comet flux in the instrumentfield of view. We set upper limits on solar wind charge-exchange emission lines of O, C, N, Ne, and Fe occurring in the spectral bandpass of CHIPS. The spectrum of NEAT obtained with ChandracanbereproducedbymodelingemissionlinesofC,N,O,Mg,Fe,Si,S,andNesolarwindions.Themeasured X-rayemission-lineintensitiesareconsistentwithourpredictionsfromasolarwindcharge-exchangemodel.Themodel predictions for the EUVemission-line intensities are determined from the intensity ratios of the cascading X-ray and EUVphotonsarisinginthecharge-exchangeprocesses.Theyarecompatiblewiththemeasuredlimitsontheintensities of the EUV lines. For NEAT, we measured a total X-ray flux of 3:7 ; 10 � 12 ergs cm � 2 s � 1 and derive from model predictions a total EUV flux of 1:5 ; 10 � 12 ergs cm � 2 s� 1. The CHIPS observations occurred predominantly while the satellite was on the dayside of Earth. For much of the observing time, CHIPS performed observations at smaller solar angles than it was designed for, and EUVemission from the Sun scattered into the instrument limited the sensitivity of the EUV measurements. Subject headingg comets: general — comets: individual (C/2002 T7 (LINEAR), C/2001 Q4 (NEAT), C/2004 Q2 (Machholz)) — ultraviolet: solar system
Highlights
Soft X-ray emission has been detected from at least 20 comets since 1996
The solar wind charge-exchange (SWCX) process occurs in the heliosphere, where it contributes to the soft X-ray background (Cox 1988; Cravens 2000; Robertson & Cravens 2003; Pepino et al 2004; Lallement 2004) and may be responsible for some fraction of the short and- long-term enhancements seen at 250 eV (Snowden et al 1995, 2004)
This background is very faint compared with the interstellar medium emission lines from Fe ix–Fe xii that Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) was designed to observe, we have studied it in depth, because it is a significant background to these measurements
Summary
Soft X-ray emission has been detected from at least 20 comets since 1996. After the consideration and rejection of several possible emission mechanisms, a consensus has been reached that the primary mechanism is charge-exchange collisions between highly charged solar wind minor ions and neutral atoms and molecules of the cometary atmospheres, as suggested by Cravens (1997). The solar wind charge-exchange (SWCX) process occurs in the heliosphere, where it contributes to the soft X-ray background (Cox 1988; Cravens 2000; Robertson & Cravens 2003; Pepino et al 2004; Lallement 2004) and may be responsible for some fraction of the short and- long-term enhancements seen at 250 eV (Snowden et al 1995, 2004). Krasnopolsky & Mumma (2001) observed comet C/1996 B2 ( Hyakutake) at low resolution at EUV wavelengths and claimed detection of lines from O, C, He, and Ne. Kharchenko & Dalgarno (2001) predict that, at lower energies, emission lines from the O vi doublet at 1032 and 1038 8 should be the brightest far-ultraviolet (FUV) SWCX lines. We discuss the observational data from CHIPS and Chandra, describe the procedures by which we measure the line strengths, and discuss the implications for the cometary charge-exchange process
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