X-Ray and Extreme Ultraviolet Emission from Comets

Home , Comet Hyakutake, Extreme ultraviolet, Local Bubble

Lisse et al.: X-Ray and Extreme UV Emission from Comets 631

C. M. Lisse University of Maryland

T. E. Cravens University of Kansas

K. Dennerl Max-Planck-Institut für Extraterrestrische Physik

The discovery of high energy X-ray emission in 1996 from C/1996 B2 (Hyakutake) has created a surprising new class of X-ray emitting objects. The original discovery (Lisse et al., 1996) and subsequent detection of X-rays from 17 other comets (Table 1) have shown that the very soft (E < 1 keV) emission is due to an interaction between the solar wind and the comet’s atmosphere, and that X-ray emission is a fundamental property of comets. Theoretical and observational work has demonstrated that charge exchange collisions of highly charged solar wind ions with cometary neutral species is the best explanation for the emission. Now a rapidly chang- ing and expanding field, the study of cometary X-ray emission appears to be able to lead us to a better understanding of a number of physical phenomena: the nature of the cometary coma, other sources of X-ray emission in the solar system, the structure of the solar wind in the heliosphere, and the source of the local soft X-ray background.

1. INTRODUCTION total X-ray power, or luminosity, of C/Hyakutake was measured to be approximately 109 W. The emission was also Astrophysical X-ray emission is generally found to origi- extremely “soft”, or of low characteristic photon energy — nate from hot collisional plasmas, such as the million-degree only X-rays of energies less than ~1 keV (or wavelengths gas found in the solar corona (e.g., Foukal, 1990), the 100- longer than ~1.2 nm) were detected from C/Hyakutake. The million-degree gas observed in supernova remnants (e.g., total amount of energy emitted in X-rays from a comet is Cioffi, 1990), or the accretion disks around neutron stars and approximately 10–4 the energy delivered to a comet from black holes. As electromagnetic radiation with wavelength λ the Sun due to photon insolation and solar wind impact between about 0.01 nm and 100 nm (1 nm = 10–9 m), ex- (Lisse et al., 2001). treme ultraviolet (EUV) and X-ray radiation are important for solar system and astrophysical applications because the 2. OBSERVED CHARACTERISTICS OF photons are sufficiently energetic and penetrating to ionize COMETARY X-RAY EMISSION neutral atoms and molecules, and can thus drive chemical reactions. In fact, current estimates of the X-ray burden per Shortly after the initial C/Hyakutake detection, soft X-ray atom in the young solar system are some 103–104 photons emission from four other comets was found in the ROSAT per atom, even as far out as the proto-Kuiper belt, as the archival database, confirming the discovery (Dennerl et al., young Sun was much more X-ray active than now (Feigel- 1997). X-ray emission has now been detected from 18 com- son, 1982; Dorren et al., 1995). ets to date (Table 1) using a variety of X-ray sensitive space- The Sun is not the only source of X-rays in the solar sys- craft — BeppoSAX, ROSAT, the Extreme Ultraviolet Ex- tem (Cravens, 2000a, 2002a). Prior to 1996, X-rays were plorer (EUVE), and more recently, Chandra and XMM- found in scattering of solar X-rays from the terrestrial atmos- Newton. All comets within 2 AU of the Sun and brighter phere and in the terrestrial aurora, as scattered solar X-rays than V = 12 have been detected when observed. We now off the illuminated surface of the Moon, and from the jovian recognize that X-ray emission is a characteristic of all ac- aurora. Nonetheless, the 1996 discovery, using the Röntgen tive comets. Satellite (ROSAT), by Lisse and co-workers (Lisse et al., The observed characteristics of the emission can orga- 1996) (Fig. 1) of strong X-ray emission from Comet C/ nized into the following four categories: (1) spatial mor- 1996 B2 (Hyakutake) was very surprising because cometary phology, (2) total X-ray luminosity, (3) temporal variation, atmospheres are known to be cold and tenuous, with charac- and (4) energy spectrum. Any physical mechanism that pur- teristic temperatures between 10 and 1000 K. Compared to ports to explain cometary X-ray emission must account for other X-ray sources, comets are moderately weak — the all these characteristics.

631 632 Comets II

TABLE 1. Observation times, instruments, and energies* for comets detected† through December 2002 in the X-ray or extreme ultraviolet.

Comet Time Instrument Energy (keV) Detection Reference 45P/Honda-Mrkos-Pajdušáková Jul 1990 ROSAT PSPC/WFC 0.09–2.0 Yes [1]

C/1990 K1 (Levy) Sep 1990 ROSAT PSPC /WFC 0.09–2.0 Yes [1] Jan 1991 0.09–2.0 Yes [1]

C/1991 A2 Arai Nov 1990 ROSAT PSPC/WFC 0.09–2.0 Yes [1]

C/1990 N1 (Tsuchiya-Kiuchi ) Nov 1990 ROSAT PSPC/WFC 0.09–2.0 Yes [1] Jan 1991 0.09–2.0 Yes [1]

2P/Encke Nov 1993 EUVE DS 0.02–0.10 No [8] Jul 1997 EUVE Scanners 0.02–0.18 Yes [9] Jul 1997 ROSAT HRI/WFC 0.09–2.0 Yes [9]

19P/Borrelly Nov 1994 EUVE DS 0.02–0.10 Yes [8]

6P/d’Arrest Sep 1995 EUVE DS 0.02–0.10 Yes [8]

C/1996 B2 (Hyakutake) Mar 1996 ROSAT HRI /WFC 0.09–2.0 Yes [2] Mar 1996 EUVE DS 0.02–0.10 Yes [3] Mar 1996 ALEXIS 0.06–0.10 No Apr 1996 XTE PCA 2.0–10.0 No Jun 1996 ASCA 0.20–6.0 No Jun 1996 ROSAT HRI /WFC 0.09–2.0 Yes [4] Jul 1996 ROSAT HRI /WFC 0.09–2.0 Yes Aug 1996 ROSAT HRI /WFC 0.09–2.0 Yes Sep 1996 ROSAT HRI /WFC 0.09–2.0 Yes

C/1995 O1 (Hale-Bopp) Apr 1996 ROSAT HRI/WFC 0.09–2.0 No Sep 1996 EUVE Scanners 0.02–0.10 Yes [5] Sep 1996 BeppoSAX 0.1–200 Yes [6] Sep 1996 ROSAT HRI/WFC 0.09–2.0 Yes? [7] Sep 1996 ASCA 0.20–6.0 No [5] Mar 1997 XTE PCA 2.0–10.0 No Oct 1997 ROSAT HRI /WFC 0.09–2.0 No Nov 1997 ROSAT HRI /WFC 0.09–2.0 No Nov 1997 EUVE DS 0.02–0.10 Yes [8] Feb 1998 ROSAT PSPC/WFC 0.09–2.0 No

C/1996 Q1 (Tabur) Sep 1996 ASCA 0.20–6.0 No Sep 1996 ROSAT HRI /WFC 0.09–2.0 Yes [1] Oct 1996 ROSAT HRI /WFC 0.09–2.0 Yes [1]

55P/Temple-Tuttle Jan 1998 EUVE Scanners 0.02–0.18 Yes Jan 1998 ASCA SIS 0.20–6.0 No Jan 1998 ROSAT HRI /WFC 0.09–2.0 Yes Feb 1998 ROSAT HRI /WFC 0.09–2.0 Yes

103P/Hartley 2 Feb 1998 ROSAT PSPC/WFC 0.09–2.0 Yes

C/1998 U5 (LINEAR) Dec 1998 ROSAT PSPC/WFC 0.09–2.0 Yes

C/2000 S4 (LINEAR) Jul 2000 Chandra ACIS-S 0.2–10.0 Yes [10] Aug 2000 Chandra ACIS-S 0.2–10.0 Yes [10]

C/1999 T1 McNaught-Hartley Jan 2001 Chandra ACIS-S 0.2–10.0 Yes [11] Jan 2001 XMM-Newton 0.2–12.0 Yes Feb 2001 FUSE 0.113–0.117 No [12] Lisse et al.: X-Ray and Extreme UV Emission from Comets 633

TABLE 1. (continued).

Comet Time Instrument Energy (keV) Detection Reference C/2001 A2 (LINEAR) Jun 2001 Chandra HRC/LETG 0.2–2.0 No Jun 2001 XMM-Newton 0.2–12.0 Yes? Jul 2001 FUSE 0.113–0.117 No [12]

C/2000 WM1 (LINEAR) Dec 2001 Chandra ACIS/LETG 0.2–2.0 Yes Dec 2001 FUSE 0.113–0.117 Yes [12] Jan 2002 XMM-Newton 0.2–12.0 Yes [13]

C/2002 C1 (Ikeya-Zhang) Apr 2002 Chandra ACIS-S 0.2–10.0 Yes [13] May 2002 XMM-Newton 0.2–12.0 Yes? [13] *The full energy range of the observing instrument is given. † This table summarizes published and unpublished (1) dedicated observations, whether successful or not; and (2) successful serendipitous observations. The table is sorted according to time of first observation of the comet.

References: [1] Dennerl et al. (1997); [2] Lisse et al. (1996); [3] Mumma et al. (1997); [4] Lisse et al. (1997a); [5] Krasnopolsky et al. (1997); [6] Owens et al. (1998); [7] Lisse et al. (1997b); [8] Krasnopolsky et al. (2000); [9] Lisse et al. (1999); [10] Lisse et al. (2001); [11] Krasnopolsky et al. (2002); [12] Weaver et al. (2002); [13] K. Dennerl et al. (personal communication, 2003).

2.1. Spatial Morphology al., 2002) at distances that exceed 104 km for weakly active comets, and can exceed 106 km for the most luminous X-ray and EUV images of C/1996 B2 (Hyakutake) made (Dennerl et al., 1997) (Fig. 2). The spatial extent for the by the ROSAT and EUVE satellites look very similar (Lisse most extended comets is independent of the rate of gas emis- et al., 1996; Mumma et al., 1997) (Fig. 1). Except for im- sion from the comet. The region of peak emission is cres- ages of C/1990 N1 (Dennerl et al., 1997) and C/Hale-Bopp cent-shaped with a brightness peak displaced toward the 1995 O1 (Krasnopolsky et al., 1997), all EUV and X-ray Sun from the nucleus (Lisse et al., 1996, 1999). The dis- images of comets have exhibited similar spatial morpholo- tance of this peak from the nucleus appears to increase with gies. The emission is largely confined to the cometary coma increasing values of Q, and for Hyakutake was located at ≈ 4 between the nucleus and the Sun; no emission is found in rpeak 2 × 10 km. the extended dust or plasma tails. The peak X-ray brightness gradually decreases with increasing cometocentric dis- 2.2. Luminosity tance r with a dependence of about r –1 (Krasnopolsky, 1997). The brightness merges with the soft X-ray background The observed X-ray luminosity, Lx, of C/1996 B2 (Hya- emission (McCammon and Sanders, 1990; McCammon et kutake) was 4 × 1015 ergs s–1 (Lisse et al., 1996) for an aper-

Fig. 1. Images of C/Hyakutake 1996 B2 on 26–28 March 1996 UT: (a) ROSAT HRI 0.1–2.0 keV X-ray; (b) ROSAT WFC 0.09– 0.2 keV extreme ultraviolet; and (c) visible light, showing a coma and tail, with the X-ray emission contours superimposed. The Sun is toward the right, “+” marks the position of the nucleus, and the orbital motion of the comet is toward the lower right in each image. From Lisse et al. (1996). 634 Comets II

Fig. 2. Spatial extent of the X-ray emission vs. the comet’s out- Fig. 3. X-ray vs. optical luminosity plot for the eight detected gassing rate. Plot of the gas production rate Qgas vs. radial dis- ROSAT comets and the Chandra comets C/1999 S4 (LINEAR) tance from the comet nucleus required to encircle 95% of the total and C/1999 T1 (McNaught-Hartley) observed at 1–3 AU. Groups observed cometary X-ray flux (triangles). Upper curve: Broken of equal emitted dust mass to emitted gas mass ratio (D/G), as power law with radial extent ~Q1.00 up to Q ~ 1029 mol s–1 and measured in the optical by the ratio Afρ/Q , are also shown. gas gas H2O ~106 km for higher values fits the imaging data well. Lower curve: For Encke and other “gassy”, optically faint comets, the resulting 19 –1 Estimated radius of the bow shock for each observation, allowing slope Lx/Lopt is roughly constant. Above Lopt ~ few × 10 erg s , 16 –1 for variable cometary outgassing activity and heliocentric distance however, Lx appears to reach an asymptote of ~5 × 10 erg s . (boxes). X-ray emission has been found outside the bow shock for A possible explanation is that the coma is collisionally thick to all comets except C/1996 B2 (Hyakutake) in March 1996. the solar wind within the neutral coma radius of ~106 km at 1 AU (Lisse et al., 2001). It is also possible that the relatively large amounts of dust in these comets, as noted from their increasing D/G ratio, may be somehow inhibiting the CXE process. Follow- ing Dennerl et al. (1997); copyright journal Science (1997). ture radius at the comet of 1.2 × 105 km. [Note that the photometric luminosity depends on the energy bandpass and on the observational aperture at the comet. The quoted value ≈ 25 –1 assumes a ROSAT photon emission rate of PX 10 s (0.1–0.6 keV), in comparison to Krasnopolsky et al.’s (2000) impulsive spikes of a few hours’ duration, and maximum ≈ 25 –1 EUVE estimate of PEUV 7.5 × 10 s (0.07–0.18 keV and amplitude 3 to 4 times that of the baseline emission level 120,000 km aperture.] A positive correlation between opti- (Lisse et al., 1996, 1999, 2001). Figure 4 demonstrates the cal and X-ray luminosities was demonstrated using obser- strong correlation found between the time histories of the vations of several comets having similar gas (Q ) to dust solar wind proton flux (a proxy for the solar wind minor H2O [Afρ, following A’Hearn et al. (1984)] emission rate ratios ion flux), the solar wind magnetic field intensity, and a com- (Fig. 3) (Lisse et al., 1997b, 1999, 2001; Dennerl et al., et’s X-ray emission for the case of Comet 2P/Encke 1997 1997; Mumma et al., 1997; Krasnopolsky et al., 2000). Lx (Lisse et al., 1999). Neugebauer et al. (2000) compared the correlates more strongly with the gas production rate Qgas ROSAT and EUVE luminosity of C/1996 B2 (Hyakutake) ρ than it does with Lopt ~ Qdust ~ Af (Figs. 2 and 3). Particu- with time histories of the solar wind proton flux, oxygen larly dusty comets, like Hale-Bopp, appear to have less X-ray ion flux, and solar X-ray flux, as measured by spacecraft emission than would be expected from their overall optical residing in the solar wind. They found the strongest corre- luminosity Lopt. The peak X-ray surface brightness decreases lation between the cometary emission and the solar wind with increasing heliocentric distance r, independent of Q oxygen ion flux, a good correlation between the comet’s (Dennerl et al., 1997), although the total luminosity appears emission and the solar wind proton flux, but no correlation roughly independent of r. The maximum soft X-ray lumi- between the cometary emission and the solar X-ray flux. nosity observed for a comet to date is ~2 × 1016 erg s–1 for For the four comets for which extended X-ray light- C/Levy at 0.2–0.5 keV (Dennerl et al., 1997) (Fig. 3). curves were obtained during quiet Sun conditions, the time delay between the solar wind proton flux and the comet’s 2.3. Temporal Variation X-ray impulse (Table 2) was well predicted by assuming a simple latitude-independent solar wind flow, a quadrupole Photometric lightcurves of the X-ray and EUV emission solar magnetic field, and propagation of the sector bound- typically show a long-term baseline level with superimposed aries radially at the speed of the solar wind and azimuth- Lisse et al.: X-Ray and Extreme UV Emission from Comets 635

Fig. 4. Temporal trends for Comet 2P/Encke 1997 on 4–9 July Fig. 5. Chandra ACIS-S medium resolution CCD X-ray spec- 1997 UT. = ROSAT HRI lightcurve, 4–8 July 1997. = EUVE trum for Comet C/1999 S4 (LINEAR). Soft X-ray spectrum of scanner Lexan B lightcurve 6–8 July 1997 UT, taken contempo- C/1999 S4 (LINEAR) obtained by the Chandra X-ray Observa- raneously with the HRI observations, and scaled by a factor of 1.2. tory (crosses) and a six-line best-fit “model” spectrum (solid line). All error bars are ±1σ. Also plotted are the WIND total magnetic The positions of several possible atomic lines are noted. Adapted field Btotal ( ), the SOHO CELIAS/SEM 1.0–500-Å solar X-ray from Lisse et al. (2001). flux ( ), and the SOHO CELIAS solar wind proton flux ( ). There is a strong correlation between the solar wind magnetic field/density and the comet’s emission. There is no direct correlation between outbursts of solar X-rays and the comet’s outbursts. for Comet C/1995 O1 (Hale-Bopp) (Owens et al., 1998). After Lisse et al. (1997a). These observations were capable of showing that the spectrum was very soft (characteristic thermal bremsstrahlung temperature kT ~ 0.23 ± 0.04 keV) with intensity increasing toward lower energy in the 0.01–0.60 keV energy range, ally with period one-half the solar rotation period of 28 d and established upper limits to the contribution of the flux (Lisse et al., 1997b, 1999; Neugebauer et al., 2000) from K-shell resonance fluorescence of carbon at 0.28 keV and oxygen at 0.53 keV. However, even in these “best” spec- ∆t = ∆t + ∆t = total Carrington rotation radial tra, continuum emission could not be distinguished from a longitudecomet – longitudeEarth + (rcomet – rEarth) multiline spectrum. Nondetections of Comets C/Hyakutake, 14.7˚/d 400 km/s ⋅ 86400 s/d C/Tabur, C/Hale-Bopp, and 55P/Temple-Tuttle using the XTE PCA (2–30 keV) and ASCA SIS (0.6–4 keV) imaging spectrometers were consistent with an extremely soft spec- 2.4. Spectrum trum (Lisse et al., 1996, 1997b). Higher-resolution spectra of cometary X-ray emission Until 2001, all published cometary X-ray spectra had have just appeared in the literature. The Chandra X-ray Ob- very low spectral energy resolution (∆E/E ~ 1 at 300–600 eV), servatory (CXO) detected soft X-ray spectra from Comet and the best spectra were those obtained by ROSAT for C/ C/1999 S4 (LINEAR) (Lisse et al., 2001) over an energy 1990 K1 (Levy) (Dennerl et al., 1997) and by BeppoSAX range of 0.2–0.8 keV, using an energy resolution with a full-

TABLE 2. Predicted and observed lightcurve phase shifts using the latitude-independent model.

∆ ∆ ∆ ∆ Time of Impulse tlong tradial ttotal tobserved Comet (00:00H UT) (d) (d) (d) (d) Hyakutake 27 Mar 1996 –0.23 0.032 –0.20 –0.24 Hale-Bopp 11 Sep 1996 –4.60 5.9 1.30 +1.4 Encke 7 Jul 1997 –0.26 0.093 –0.17 –0.1 Temple-Tuttle 29 Jan 1998 –2.31 0.37 –1.94 –2.5 Time shifts assume solar wind velocity as measured near-Earth; positive time shifts = impulse happens at Earth first, comet next; negative time shifts = boundary hits comet first, Earth next. 636 Comets II

Fig. 6. EUVE observations of line emission from C/1996 B2 (Hyakutake), following Krasnopolsky and Mumma (2001). (a) MW (middle wavelength) 0.034–0.073 keV spectrum on March 23, 1996. (b) LW (long-wavelength) 0.018–0.04 keV. The extreme ultraviolet spectra are clearly dominated by line emission. The best agreement with CXE model predictions are for the O4+, C4+, and Ne7+ lines. (c) FUSE observations of three comets, with a marginal detection of the CXE OVI line in C/2001 WM1 (LINEAR) at 1032 Å (following Weaver et al., 2002). The nondetections in Comets C/2001 A2 (LINEAR) and C/1999 T1 (McNaught-Hartley) and the marginal detection in C/2001 WM1 (LINEAR) are consistent with CXE predictions for the luminosity of these lines.

width half-maximum (FWHM) of ∆E = 0.11 keV (Fig. 5). Hartley) and, more recently, in CXO spectra of C/2001 WM1 The spectrum is dominated by line emission, not by con- (LINEAR) and C/2002 Ikeya-Zhang (K. Dennerl et al. and tinuum. Using the CXO, a new spectrum of Comet C/1999 C. M. Lisse et al., personal communication, 2003). An T1 (McNaught-Hartley) (Krasnopolsky et al., 2002) shows XMM-Newton spectrum of C/2001 WM1 (LINEAR) shows similar line-emission features. Line emission is also found characteristic CXE X-ray signatures in unprecedented detail in XMM-Newton spectra of Comet C/1999 T1 (McNaught (K. Dennerl et al., personal communication, 2003). A re- Lisse et al.: X-Ray and Extreme UV Emission from Comets 637

analysis of archival EUVE Deep Survey spectrometer spec- Mechanisms based on dust grains also have a number tra (Krasnopolsky and Mumma, 2001) suggests EUV line- of problems. It has been known since 1996 that Rayleigh emission features from Comet C/1996 B2 (Hyakutake) scattering of solar X-ray radiation from ordinary cometary (Figs. 6a,b). Recent FUSE observations (Weaver et al., dust grains (i.e., about 1 µm in size) cannot produce the 2002) also indicate the presence of possible O VI 1032-Å observed luminosities — the cross section for this process emission lines in far-UV spectra of C/2001 WM1 (LIN- is too small (Lisse et al., 1996). A potential solution to this EAR) (Fig. 6c). problem is to invoke a population of very small, attogram (10–19 g) grains with radii on the order of the wavelengths 3. PROPOSED X-RAY MECHANISMS of the observed X-ray radiation, 10–100 Å, which can reso- nantly scatter the incident X-ray radiation. The abundance A large number of explanations for cometary X-rays of such attogram dust grains is not well understood in com- were suggested following the discovery paper in 1996. ets, as they are undetectable by remote optical observations; These included thermal bremsstrahlung (German for “brak- however, there were reports from the VEGA Halley flyby ing radiation”) emission due to solar wind electron colli- of a detection of an attogram dust component using the sions with neutral gas and dust in the coma (Bingham et al., PUMA dust monitor (Vaisberg et al., 1987; Sagdeev et al., 1997; Dawson et al., 1997; Northrop, 1997; Northrop et al., 1990). However, the statistical studies of the properties of 1997; Uchida et al., 1998; Shapiro et al., 1999), microdust several comets (Figs. 2 and 3) demonstrate that X-ray emis- collisions (Ibadov, 1990; Ip and Chow, 1997), K-shell ion- sion varies with a comet’s gas production rate and not the ization of neutrals by electron impact (Krasnopolsky, 1997), dust production rate (Dennerl et al., 1997; Lisse et al., 1999, scattering or fluorescence of solar X-rays by cometary gas or 2001). Furthermore, the cometary X-ray lightcurves (Lisse by small dust grains (Lisse et al., 1996; Wickramasinghe and et al., 1996, 1999, 2001; Neugebauer et al., 2000) corre- Hoyle, 1996; Owens et al., 1998), and by charge exchange late with the solar wind ion flux and not with solar X-ray between highly ionized solar wind ions and neutral species intensity. Finally, dust-scattering mechanisms cannot ac- in the cometary coma (CXE) (Cravens, 1997a; Häberli et count for the pronounced lines seen in the new high-reso- al., 1997; Wegmann et al., 1998; Kharchenko and Dalgarno, lution spectra — emission resulting from dust scattering of 2000; Kharchenko et al., 2003; Schwadron and Cravens, solar X-rays should mimic the Sun’s X-ray spectral con- 2000). In the thermal bremsstrahlung mechanism, fast elec- tinuum, similar to what is observed in the terrestrial atmos- trons are deflected in collisions with charged targets, such phere for Rayleigh scattering of sunlight (Krasnopolsky, as the nuclei of atoms, and emit continuum radiation. Elec- 1997). tron energies in excess of 100 eV (T > 106 K) are needed for The CXE mechanism requires that the observed X-ray the production of X-ray photons. In the K-shell mechanism, emission is driven by the solar wind flux and that the bulk a fast electron collision removes an orbital electron from of the observed X-ray emission be in lines. Localization of an inner shell of the target atom. Early evaluation of these the emission to the sunward half of the coma, a solar wind various mechanisms (Dennerl et al., 1997; Krasnopolsky, flux-like time dependence, and a line-emission-dominated 1997; Lisse et al., 1999) favored only three of them: the spectral signature of the observed emission all strongly CXE mechanism, thermal bremsstrahlung, and scattering of point to the solar wind charge exchange mechanism as solar radiation from very small (i.e., attogram; 1 attogram = being responsible for cometary X-rays. 10–19 g) dust grains. A significant problem with mechanisms involving solar 4. SOLAR WIND CHARGE EXCHANGE wind electrons (i.e., bremsstrahlung or K-shell ionization) X-RAY MECHANISM is that the predicted emission luminosities are too small by factors of 100–1000 compared to observations. The flux of The solar wind is a highly ionized but tenuous gas (i.e., high-energy solar wind electrons near comets is too low a plasma) (Cravens, 1997b). At its source in the solar co- (Krasnopolsky, 1997; Cravens, 2000b, 2002a). Furthermore, rona, the million-degree gas is relatively dense and in colli- X-ray emission has been observed out to great distances sional equilibrium, but its density drops within a few solar from the nucleus, beyond the bow shock (Fig. 2), and the radii into a freeflow regime wherein collisions are infre- thermal energy of unshocked solar wind electrons at these quent. Both the solar wind and corona have “solar” com- distances is about 10 eV. No emission has ever been found position — 92% hydrogen by volume, 8% helium, and 0.1% to be associated with the plasma tail of a comet, which has heavier elements. The heavier, “minor ion” species are similar plasma densities and temperatures. Finally, the new, highly charged (e.g., oxygen in the form of hydrogen-like high-resolution spectra demonstrating multiple atomic lines O7+ or helium-like O6+ ions, N6+/N5+, C5+/C4+, Ne8+, Si9+, are inconsistent with a continuum-type mechanism or a Fe12+, etc.) due to the high coronal temperatures (Bame, mechanism producing only a couple of K-shell lines as the 1972; Bocshler, 1987; Neugebauer et al., 2000). primary source of cometary X-rays. Lisse et al. (2001) tried The solar wind flow starts out slowly in the corona but several thermal bremsstrahlung continuum model fits to the becomes supersonic at a distance of few solar radii (Parker, C/1999 S4 spectrum, and Krasnopolsky and Mumma (2001) 1963; Cravens, 1997b). The gas cools as it expands, fall- tried the same for the C/1996 B2 (Hyakutake) spectrum, but ing from T ≈ 106 K down to about 105 K at 1 AU. The aver- neither was successful. age properties of the solar wind at 1 AU are proton number 638 Comets II density ≈7 cm–3, speed ≈450 km s–1, temperature ≈105 K, netic field lines pile up into a “magnetic barrier” in this stag- magnetic field strength ≈5 nT, and Mach number ≈8 (Hund- nation region and drape around the head of the comet forming hausen et al., 1968). However, the composition and charge the plasma tail in the downwind direction (Brandt, 1982). state distribution far from the Sun are “frozen in” at coro- From experimental and theoretical work in atomic and nal values due to the low collision frequency outside the molecular physics it is found that solar wind minor ions corona. The solar wind contains structure, such as slow readily undergo charge transfer (or exchange) reactions (400 km s–1) and fast (700 km s–1) streams, which can be (Phaneuf et al., 1982; Dijkkamp et al., 1985; Gilbody, 1986; mapped back to the Sun. The solar wind “terminates” in a Janev et al., 1988; Wu et al., 1988) when they are within shock called the heliopause, where the ram pressure of the ~1 nm of a neutral atomic species streaming solar wind has fallen to that of the instellar material (ISM) gas (Suess, 1990). The region of space that con- Aq+ + B → A(q – 1)+* + B+ (1) tains plasma of solar origin, from the corona to the heliopause at ~100 AU, is called the heliosphere. A very small part of the solar wind interacts with the planets and comets; where A denotes the solar wind projectile ion (e.g., O, C, the bulk of the wind interacts with neutral ISM gas in the Si . . . ), q is the projectile charge (e.g., q = 5, 6, 7) and B heliosphere and neutral and ionized ISM material at the denotes the neutral target species (e.g., H2O, OH, CO, O, heliopause. H . . . for cometary comae) (Fig. 8). The cross section for As the solar wind streams into a comet’s atmosphere, this process is large, on the order of 10–15 cm2, about 1 or- cometary ion species produced from solar UV photoioniza- der of magnitude larger than the hardsphere collisional cross tion of neutral coma gas species are added to the flow as section. “pick-up ions.” The resulting mass addition slows down the The product ion deexcites by emitting one or more pho- solar wind due to momentum conservation and a bow shock tons (A(q – 1)+* → A(q – 1)+* + hν, where hν represents a pho- forms upwind of the comet (Galeev, 1991; Szegö et al., ton). It is the characteristic radiation of the product ion that 2000) (Fig. 7). The flow changes from supersonic to sub- is measured with astronomical X-ray instrumentation, and sonic across the shock, and the magnetic field strength in- so one labels the radiation detected by the charge state of creases by a factor of ~5. Closer to the nucleus, where the the final ion. The deexcitation usually takes place via a cas- cometary gas density is higher and collisions more frequent, cade through intermediate states rather than in one step to the flow almost completely stagnates (Flammer, 1991). The the ground state. For large enough values of q, the deexci- outer boundary of this stagnation region is often called the tation transitions lead to the emission of X-ray photons. For cometopause (cf. review by Cravens, 1991). The observed species and charge states relevant to comets, the principal X-ray brightness peak resides within this boundary. Mag- quantum number of the ion A(q – 1)+ is about n = 4 or 5

Fig. 8. Energy level diagram for a CXE process. Electron potential energy (atomic units, a.u.) vs. distance from the target atom nucleus (assumed here to be an H atom) for a charge transfer Fig. 7. Spatial schematic of the solar wind-comet interaction. reaction involving a projectile ion Be4+. The internuclear distance The relative locations of the bow shock, the magnetic barrier, and chosen is 10 Bohr radii or 5.29 × 10–10 m, the curve-crossing the tail are shown (not to scale). The Sun is toward the left. Also distance for the n = 3 ion final state. The target energy level (and 3+ represented is a charge transfer collision between a heavy solar binding energy Eb) and product ion (Be ) energy levels are shown wind ion and a cometary neutral water molecule, followed by the in units of hartrees (1 hartree = 27.2 eV). A possible cascading emission of an X-ray photon. After Cravens (2002b); copyright pathway for the deexcitation by photon emission is shown. After journal Science (2002). Cravens (2002b); copyright journal Science (2002). Lisse et al.: X-Ray and Extreme UV Emission from Comets 639

(Ryufuku et al., 1980; Mann et al., 1981). The cross section 4.2. Charge Exchange Luminosity for charge exchange between a high charge state solar wind and Temporal Variation ion and an ionized coma gas species is negligible in comparison, due to the effects of Coulomb repulsion between To first order, the CXE local X-ray power density Px can the two reactants. Once a coma neutral atom is ionized, be estimated assuming only one CXE collision per solar either by CXE processes or solar UV flux, the CXE mecha- wind ion per coma passage. This approximation yields the nism is no longer an important energy transfer process. expression

α 4.1. Charge Exchange Morphology Px = nswuswnn (2)

Numerical simulations of the solar wind interaction with where nsw usw, and nn are the solar wind proton density, solar Hyakutake including CXE have been used to generate X-ray wind speed, and neutral target density respectively (Cra- images. A global magnetohydrodynamic (MHD) model vens, 1997a, 2002a). All the “atomic and molecular details” (Häberli et al., 1997) and a hydrodynamic model (Wegmann as well as the solar wind heavy ion fraction fh are combined α α ≈ 〈 〉 〈 〉 et al., 1998) were used to predict solar wind speeds and into the parameter , given by fh sct Eave, where sct is an densities and the X-ray emission around a comet. The simu- average CXE cross section for all species and charge states, lated X-ray images are similar to the observed images, and Eave an average photon energy. A simple spherically which is relatively unsurprising, as any dissipative solar symmetric approximation to the neutral density in the coma π 2 wind–coma process with total optical depth near unity is given by nn = Q/[4 unr ], for r less than the ionization ≈ 6 would create the observed morphology (Fig. 1). The emis- scale length R = unt, where t 10 s is the ionization life- ≈ sion is found to lie in the sunward hemisphere of the neu- time [for 1 AU (Schleicher and A’Hearn, 1988)] and un –1 tral coma, varying from collisionally thin to collisionally 1 km s the neutral gas outflow speed. Integration of Px thick as the solar wind approaches the nucleus. Emission over the volume of the neutral coma yields an X-ray lumi- is predicted to be in the soft X-ray, UV, and optical wave- nosity typically within a factor of 2–3 of the observed lu- lengths, with the softest photons emitted closest to the nu- minosity (Cravens, 1997a, 2000a; Lisse et al., 2001). The cleus. However, the spatial models to date have included observed luminosity is a function of both the solar wind only highly simplified models of the CXE deexcitation cas- flux density and the com-etary neutral gas production rate cade, as compared to the detailed spectral models of the glo- up to the limit of 100% charge exchange efficiency of all bal behavior discussed below. solar wind minor ions within an ionization scale length of As an example of the potential of studying the behavior 106 km. The maximum expected X-ray luminosity at 1 AU of the solar wind inside the coma using CXE reactions, we and 0.2–0.5 keV is ~1016 erg s–1 (Fig. 3). Temporal varia- consider the dissimilar morphologies of the extended Ly- tions of the solar wind flux directly translate into time varia- man α comae and the X-ray emitting regions of comets (cf. tions of the X-ray emission (Fig. 4). Keller, 1973; Festou et al., 1979; Combi et al., 2000). The CXE mechanism should not only transfer electrons from 4.3. Charge Exchange Spectra cometary neutrals to solar wind minor ions, but to the solar wind majority ions H+ and He2+ as well. These ions are Model CXE spectra are in good agreement with the low- roughly 1000 times more abundant than the X-ray active resolution X-ray spectra of cometary X-ray emission, and highly ionized minor ions. The prompt photon-emission the line centers of the high-resolution spectra have been energy produced from CXE by He2+ ions produces at least successfully predicted using CXE theory (Figs. 5 and 6) three times the energy released by all the minor ions com- (Lisse et al., 1999, 2001; Krasnopolsky and Mumma, 2001; bined (D. Shemansky, personal communication, 2003). Weaver et al., 2002). The application of the CXE model to Further, the neutral atoms produced are capable of scatter- comets has entailed a number of approaches to date. Some ing emission from the Sun. At luminosities of ~1016 erg s–1, work has included only a few solar wind species but used a and production rates of ~1027 s–1, the HI created by CXE careful cascading scheme (Häberli et al., 1997). Other ap- should be detectable in Lyman α comet images. The fact proaches have used a simple cascading scheme and simple that it is not is puzzling. A possible solution is that the neu- collision cross sections, but included a large number of tral hydrogen atoms produced by CXE retain relatively large solar wind ions and charge states (Wegmann et al., 1998; velocities with respect to the Sun, i.e., the solar wind is not Schwadron and Cravens, 2000). Kharchenko and colleagues appreciably slowed at the cometary bow shock. This large (Kharchenko and Dalgarno, 2000; Kharchenko et al., 2003) remnant velocity redshifts the CXE-produced neutral hydro- have treated the atomic cascading process more carefully gen with respect to the peak of the solar Lyman α emission, than other modelers, although the spatial structure of the so that fluorescence from these atoms is greatly reduced in solar wind–cometary neutral interaction in their model was efficiency vs. H atoms produced from dissolution of comet- highly simplified. They predicted the existence of a large ary water group species. Recently Raymond et al. (2002) number of atomic lines, including O5+ (1s25d → 1s22p) at have reported the effects of highly redshifted neutral hydro- 106.5 eV, C4+ (1s2s → 1s2) at 298.9 eV, C5+ (2p → 1s) at gen created by CXE emission in SOHO UVCS measure- 367.3 eV, C5+ (4p → 1s) at 459.2 eV, and O6+ (1s2p → 1s2) ments of 2P/Encke during its 1997 apparition. at 568.4 eV. At least some of these lines appear in the best 640 Comets II cometary X-ray spectra to date, the CXO ACIS-S spectra of Further theoretical progress will require the integration C/1999 S4 (LINEAR) and C/1999 T1 (McNaught-Hartley). of several ingredients into a single model: (1) accurate solar The peak measured near 0.56 keV is certainly the combi- wind composition for a range of solar wind types; (2) a nation of three closely spaced helium-like O6+ (1s2p and suitable MHD model of the solar wind interaction with the 1s2s → 1s2) transitions (which comes from CXE of solar coma, in order to accurately predict the densities of ions wind O7+) and the line located at 0.32 keV is due to helium- and neutrals in equation (2); and (3) a more detailed under- like C4+ (1s2p and 1s2s → 1s2). Similar identifications have standing of the atomic processes, in order to improve our been made for the EUVE spectrum of C/1996 B2 (Hyaku- understanding of the parameter α in equation (2). Success take) (Krasnopolsky and Mumma, 2001). The spectral prob- for the first point requires new and improved measurements lem is still far from totally solved, however. Careful com- of the solar wind throughout the heliosphere, and/or large parisons and calculations needed to interpret the subtleties number statistical studies of cometary X-ray emission of the high-resolution spectral observations, including the throughout the heliosphere; the second point requires im- role of collisions after charge transfer, solar wind ion–dust proved MHD codes on modern supercomputers; and to interactions, and the exact species present at each point in achieve the third point, additional laboratory measurements the coma, are only now starting to be done (Krasnopolsky et of state-specific CXE cross sections will be required. The al., 2002; Kellett et al., 2003). first and second points are being actively pursued by as- tronomers and modelers in the field. New laboratory work 5. THE FUTURE OF COMETARY is now being undertaken (Beiersdorfer et al., 2000, 2001; X-RAY EMISSION STUDIES Greenwood et al., 2000; Hasan et al., 2001) to measure CXE cross sections for cometary target species such as H2O The discovery that comets are X-ray sources can now at collision energies relevant to the solar wind (a few keV/ be explained by charge exchange reactions of highly charged amu). Recent measurements have indicated, for example, solar wind ions with neutral atoms and molecules residing that multiple as well as single-electron CXE makes a con- in the cometary coma. The energy required to power this tribution to the X-ray emission (Hasan et al., 2001; Green- emission originates in the hot solar corona and is stored as wood et al., 2001; Gao and Kwong, 2002). Given detailed potential energy in highly stripped solar wind ions (Cra- MHD models of the solar wind passing through the coma vens, 2000a, 2002b; Dennerl, 1999). Charge exchange reac- and accurate cross sections for the CXE process, we will tions with neutral species, like cometary coma neutral atoms be able to map out the density of solar wind minor ions in and molecules, release this potential energy in the form of the coma. X-ray and UV radiation. This simple model can explain the gross features of the observed crescent-shaped emission 5.2. Remote Sensing of the Solar Wind with its sunward displaced peak, the maximum spatial extent of the emission of ~106 km (Figs. 1 and 2), the maxi- Driven by the solar wind, cometary X-rays provide an mum observed luminosity of ~1016 erg s–1 (Fig. 3), and spec- observable link between the solar corona, where the solar tra dominated by line emission (Fig. 4). wind originates, and the solar wind where the comet resides. Once we have understood the CXE mechanism’s behavior 5.1. Plasma-Neutral Interactions in the in cometary comae in sufficient detail, we will be able to Cometary Coma use comets as probes to measure the solar wind throughout the heliosphere. This will be especially useful in moni- However, a more careful treatment of the CXE mecha- toring the solar wind in places hard to reach with space- nism is needed to fully understand the phenomenon of com- craft — over the solar poles, at large distances above and etary X-ray emission. One complication is that the multiple below the ecliptic plane, and at heliocentric distances greater CXE collisions take place in regions close to the nucleus than a few AU (Lisse et al., 1996, 2001; Krasnopolsky et where the target density is high (the so-called “collisionally al., 2000). For example, approximately one-third of the ob- thick” case). The charge state for the initially hydrogen-like served soft X-ray emission is found in the 530–700-eV or helium-like solar wind minor ion is reduced by one dur- oxygen O7+ and O6+ lines; observing photons of this energy ing each CXE collision, ultimately leading to its conversion allows studies of the oxygen ion charge ratio of the solar to a neutral atom. When the ion’s charge state becomes too wind, which is predicted to vary significantly between the low, X-ray photons are no longer emitted. When the number slow and fast solar winds (Neugebauer et al., 2000; Schwad- of neutrals in a volume of space becomes too low, due to ron and Cravens, 2000; Kharchenko and Dalgarno, 2001). coma expansion and ionization of coma gas molecules by solar UV radiation, X-ray photons are no longer detectable. 5.3. Emission from Other Planetary Systems Another complication is that spectral differences are expected for slow and fast solar wind streams, with the slow The CXE mechanism operates wherever the solar wind solar wind, with its higher coronal freeze-in temperature, (or any highly ionized plasma) interacts with a substantial producing a harder spectrum than does the fast solar wind quantity of neutral gas. Motivated in part by the discovery (Schwadron and Cravens, 2000). of the new class of cometary X-ray emitters, further meas- Lisse et al.: X-Ray and Extreme UV Emission from Comets 641

urements of potential solar system sources (Holmström et al., relations that have been found between measured solar wind 2001; Cravens and Maurellis, 2001) of X-ray emission have fluxes and measured X-ray background intensities (Cravens been undertaken recently. X-rays have now been observed et al., 2001; Robertson et al., 2001). Recent large angular from Venus as solar X-rays fluorescently scattered by the scale measurements of the diffuse soft X-ray background in thick (compared to comets) venusian atmosphere (Dennerl a 100-s rocket flight by McCammon et al. (2002) and in the et al., 2002), and from Mars as ~90% fluorescently scat- Chandra background toward MBM12 (R. Edgar et al., per- tered solar X-rays from the thick martian atmosphere and sonal communication, 2001) show a clear peak at 560 eV, as ~10% CXE-derived X-rays (Dennerl, 2002). The solar wind expected for CXE-driven emission (Fig. 4). 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