A Search for Extreme‐Ultraviolet Emission from Comets with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)

We have obtained EUV spectra between 90 and 255 \AA of the cometsC/2002 T7(LINEAR), C/2001 Q4 (NEAT), and C/2004 Q2 (Machholz) near their perihelionpassages in 2004 with the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS).We obtained contemporaneous data on Comet NEAT Q4 with the $Chandra$ X-rayObservatory ACIS instrument, marking the first simultaneous EUV and X-rayspectral observations of a comet. The total CHIPS/EUV observing times were 337ks for Q4, 234 ks for T7, and 483 ks for Machholz and for both CHIPS and$Chandra$ we calculate we have captured all the comet flux in the instrumentfield of view. We set upper limits on solar wind charge exchange emission linesof O, C, N, Ne and Fe occurring in the spectral bandpass of CHIPS. The spectrumof Q4 obtained with $Chandra$ can be reproduced by modeling emission lines ofC, N O, Mg, Fe, Si, S, and Ne solar wind ions. The measured X-ray emission lineintensities are consistent with our predictions from a solar wind chargeexchange model. The model predictions for the EUV emission line intensities aredetermined from the intensity ratios of the cascading X-ray and EUV photonsarising in the charge exchange processes. They are compatible with the measuredlimits on the intensities of the EUV lines. For comet Q4, we measured a totalX-ray flux of 3.7$\times 10^{-12}$ ergs cm$^{-2}$ s$^{-1}$, and derive frommodel predictions a total EUV flux of 1.5$\times 10^{-12}$ erg cm$^{-2}$s$^{-1}$. The CHIPS observations occurred predominantly while the satellite wason the dayside of Earth. For much of the observing time, CHIPS performedobservations at smaller solar angles than it was designed for and EUV emissionfrom the Sun scattered into the instrument limited the sensitivity of the EUVmeasurements.Comment: 28 pages total, 4 tables, 7 figures. Accepted by The Astrophysical Journa