Download X-Ray and Extreme Ultraviolet Emission from Comets

Lisse et al.: X-Ray and Extreme UV Emission from Comets
631
X-Ray and Extreme Ultraviolet Emission from Comets
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 changing 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
Astrophysical X-ray emission is generally found to originate from hot collisional plasmas, such as the million-degree
gas found in the solar corona (e.g., Foukal, 1990), the 100million-degree gas observed in supernova remnants (e.g.,
Cioffi, 1990), or the accretion disks around neutron stars and
black holes. As electromagnetic radiation with wavelength λ
between about 0.01 nm and 100 nm (1 nm = 10 –9 m), extreme ultraviolet (EUV) and X-ray radiation are important
for solar system and astrophysical applications because the
photons are sufficiently energetic and penetrating to ionize
neutral atoms and molecules, and can thus drive chemical
reactions. In fact, current estimates of the X-ray burden per
atom in the young solar system are some 103–10 4 photons
per atom, even as far out as the proto-Kuiper belt, as the
young Sun was much more X-ray active than now (Feigelson, 1982; Dorren et al., 1995).
The Sun is not the only source of X-rays in the solar system (Cravens, 2000a, 2002a). Prior to 1996, X-rays were
found in scattering of solar X-rays from the terrestrial atmosphere and in the terrestrial aurora, as scattered solar X-rays
off the illuminated surface of the Moon, and from the jovian
aurora. Nonetheless, the 1996 discovery, using the Röntgen
Satellite (ROSAT), by Lisse and co-workers (Lisse et al.,
1996) (Fig. 1) of strong X-ray emission from Comet C/
1996 B2 (Hyakutake) was very surprising because cometary
atmospheres are known to be cold and tenuous, with characteristic temperatures between 10 and 1000 K. Compared to
other X-ray sources, comets are moderately weak — the
total X-ray power, or luminosity, of C/Hyakutake was measured to be approximately 109 W. The emission was also
extremely “soft”, or of low characteristic photon energy —
only X-rays of energies less than ~1 keV (or wavelengths
longer than ~1.2 nm) were detected from C/Hyakutake. The
total amount of energy emitted in X-rays from a comet is
approximately 10–4 the energy delivered to a comet from
the Sun due to photon insolation and solar wind impact
(Lisse et al., 2001).
2.
OBSERVED CHARACTERISTICS OF
COMETARY X-RAY EMISSION
Shortly after the initial C/Hyakutake detection, soft X-ray
emission from four other comets was found in the ROSAT
archival database, confirming the discovery (Dennerl et al.,
1997). X-ray emission has now been detected from 18 comets to date (Table 1) using a variety of X-ray sensitive spacecraft — BeppoSAX, ROSAT, the Extreme Ultraviolet Explorer (EUVE), and more recently, Chandra and XMMNewton. All comets within 2 AU of the Sun and brighter
than V = 12 have been detected when observed. We now
recognize that X-ray emission is a characteristic of all active comets.
The observed characteristics of the emission can organized into the following four categories: (1) spatial morphology, (2) total X-ray luminosity, (3) temporal variation,
and (4) energy spectrum. Any physical mechanism that purports to explain cometary X-ray emission must account for
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
Jan 1991
ROSAT PSPC /WFC
0.09–2.0
0.09–2.0
Yes
Yes
[1]
[1]
C/1991 A2 Arai
Nov 1990
ROSAT PSPC/WFC
0.09–2.0
Yes
[1]
C/1990 N1 (Tsuchiya-Kiuchi )
Nov 1990
Jan 1991
ROSAT PSPC/WFC
0.09–2.0
0.09–2.0
Yes
Yes
[1]
[1]
2P/Encke
Nov 1993
Jul 1997
Jul 1997
EUVE DS
EUVE Scanners
ROSAT HRI/WFC
0.02–0.10
0.02–0.18
0.09–2.0
No
Yes
Yes
[8]
[9]
[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
Mar 1996
Mar 1996
Apr 1996
Jun 1996
Jun 1996
Jul 1996
Aug 1996
Sep 1996
ROSAT HRI /WFC
EUVE DS
ALEXIS
XTE PCA
ASCA
ROSAT HRI /WFC
ROSAT HRI /WFC
ROSAT HRI /WFC
ROSAT HRI /WFC
0.09–2.0
0.02–0.10
0.06–0.10
2.0–10.0
0.20–6.0
0.09–2.0
0.09–2.0
0.09–2.0
0.09–2.0
Yes
Yes
No
No
No
Yes
Yes
Yes
Yes
[2]
[3]
Apr 1996
Sep 1996
Sep 1996
Sep 1996
Sep 1996
Mar 1997
Oct 1997
Nov 1997
Nov 1997
Feb 1998
ROSAT HRI/WFC
EUVE Scanners
BeppoSAX
ROSAT HRI/WFC
ASCA
XTE PCA
ROSAT HRI /WFC
ROSAT HRI /WFC
EUVE DS
ROSAT PSPC/WFC
0.09–2.0
0.02–0.10
0.1–200
0.09–2.0
0.20–6.0
2.0–10.0
0.09–2.0
0.09–2.0
0.02–0.10
0.09–2.0
No
Yes
Yes
Yes?
No
No
No
No
Yes
No
Sep 1996
Sep 1996
Oct 1996
ASCA
ROSAT HRI /WFC
ROSAT HRI /WFC
0.20–6.0
0.09–2.0
0.09–2.0
No
Yes
Yes
55P/Temple-Tuttle
Jan 1998
Jan 1998
Jan 1998
Feb 1998
EUVE Scanners
ASCA SIS
ROSAT HRI /WFC
ROSAT HRI /WFC
0.02–0.18
0.20–6.0
0.09–2.0
0.09–2.0
Yes
No
Yes
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
Aug 2000
Chandra ACIS-S
Chandra ACIS-S
0.2–10.0
0.2–10.0
Yes
Yes
[10]
[10]
C/1999 T1 McNaught-Hartley
Jan 2001
Jan 2001
Feb 2001
Chandra ACIS-S
XMM-Newton
FUSE
0.2–10.0
0.2–12.0
0.113–0.117
Yes
Yes
No
[11]
C/1995 O1 (Hale-Bopp)
C/1996 Q1 (Tabur)
[4]
[5]
[6]
[7]
[5]
[8]
[1]
[1]
[12]
633
Lisse et al.: X-Ray and Extreme UV Emission from Comets
TABLE 1.
Comet
C/2001 A2 (LINEAR)
C/2000 WM1 (LINEAR)
C/2002 C1 (Ikeya-Zhang)
(continued).
Time
Instrument
Energy (keV)
Detection
Reference
Jun 2001
Jun 2001
Jul 2001
Chandra HRC/LETG
XMM-Newton
FUSE
0.2–2.0
0.2–12.0
0.113–0.117
No
Yes?
No
[12]
Dec 2001
Dec 2001
Jan 2002
Chandra ACIS/LETG
FUSE
XMM-Newton
0.2–2.0
0.113–0.117
0.2–12.0
Yes
Yes
Yes
[12]
[13]
Apr 2002
May 2002
Chandra ACIS-S
XMM-Newton
0.2–10.0
0.2–12.0
Yes
Yes?
[13]
[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
X-ray and EUV images of C/1996 B2 (Hyakutake) made
by the ROSAT and EUVE satellites look very similar (Lisse
et al., 1996; Mumma et al., 1997) (Fig. 1). Except for images of C/1990 N1 (Dennerl et al., 1997) and C/Hale-Bopp
1995 O1 (Krasnopolsky et al., 1997), all EUV and X-ray
images of comets have exhibited similar spatial morphologies. The emission is largely confined to the cometary coma
between the nucleus and the Sun; no emission is found in
the extended dust or plasma tails. The peak X-ray brightness gradually decreases with increasing cometocentric distance r with a dependence of about r –1 (Krasnopolsky, 1997).
The brightness merges with the soft X-ray background
emission (McCammon and Sanders, 1990; McCammon et
al., 2002) at distances that exceed 104 km for weakly active comets, and can exceed 106 km for the most luminous
(Dennerl et al., 1997) (Fig. 2). The spatial extent for the
most extended comets is independent of the rate of gas emission from the comet. The region of peak emission is crescent-shaped with a brightness peak displaced toward the
Sun from the nucleus (Lisse et al., 1996, 1999). The distance of this peak from the nucleus appears to increase with
increasing values of Q, and for Hyakutake was located at
rpeak ≈ 2 × 10 4 km.
2.2.
Luminosity
The observed X-ray luminosity, Lx, of C/1996 B2 (Hyakutake) 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 outgassing rate. Plot of the gas production rate Qgas vs. radial distance from the comet nucleus required to encircle 95% of the total
observed cometary X-ray flux (triangles). Upper curve: Broken
29
–1
power law with radial extent ~Q1.00
gas up to Qgas ~ 10 mol s and
~106 km for higher values fits the imaging data well. Lower curve:
Estimated radius of the bow shock for each observation, allowing
for variable cometary outgassing activity and heliocentric distance
(boxes). X-ray emission has been found outside the bow shock for
all comets except C/1996 B2 (Hyakutake) in March 1996.
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
assumes a ROSAT photon emission rate of PX ≈ 1025 s–1
(0.1–0.6 keV), in comparison to Krasnopolsky et al.’s (2000)
EUVE estimate of PEUV ≈ 7.5 × 1025 s–1 (0.07–0.18 keV and
120,000 km aperture.] A positive correlation between optical and X-ray luminosities was demonstrated using observations of several comets having similar gas (QH2O) to dust
[Afρ, following A’Hearn et al. (1984)] emission rate ratios
(Fig. 3) (Lisse et al., 1997b, 1999, 2001; Dennerl et al.,
1997; Mumma et al., 1997; Krasnopolsky et al., 2000). Lx
correlates more strongly with the gas production rate Qgas
than it does with Lopt ~ Qdust ~ Afρ (Figs. 2 and 3). Particularly dusty comets, like Hale-Bopp, appear to have less X-ray
emission than would be expected from their overall optical
luminosity Lopt. The peak X-ray surface brightness decreases
with increasing heliocentric distance r, independent of Q
(Dennerl et al., 1997), although the total luminosity appears
roughly independent of r. The maximum soft X-ray luminosity observed for a comet to date is ~2 × 1016 erg s–1 for
C/Levy at 0.2–0.5 keV (Dennerl et al., 1997) (Fig. 3).
2.3.
Temporal Variation
Photometric lightcurves of the X-ray and EUV emission
typically show a long-term baseline level with superimposed
Fig. 3. X-ray vs. optical luminosity plot for the eight detected
ROSAT comets and the Chandra comets C/1999 S4 (LINEAR)
and C/1999 T1 (McNaught-Hartley) observed at 1–3 AU. Groups
of equal emitted dust mass to emitted gas mass ratio (D/G), as
measured in the optical by the ratio Afρ/QH2O, are also shown.
For Encke and other “gassy”, optically faint comets, the resulting
slope Lx/Lopt is roughly constant. Above Lopt ~ few × 1019 erg s–1,
however, Lx appears to reach an asymptote of ~5 × 1016 erg s–1.
A possible explanation is that the coma is collisionally thick to
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. Following Dennerl et al. (1997); copyright journal Science (1997).
impulsive spikes of a few hours’ duration, and maximum
amplitude 3 to 4 times that of the baseline emission level
(Lisse et al., 1996, 1999, 2001). Figure 4 demonstrates the
strong correlation found between the time histories of the
solar wind proton flux (a proxy for the solar wind minor
ion flux), the solar wind magnetic field intensity, and a comet’s X-ray emission for the case of Comet 2P/Encke 1997
(Lisse et al., 1999). Neugebauer et al. (2000) compared the
ROSAT and EUVE luminosity of C/1996 B2 (Hyakutake)
with time histories of the solar wind proton flux, oxygen
ion flux, and solar X-ray flux, as measured by spacecraft
residing in the solar wind. They found the strongest correlation between the cometary emission and the solar wind
oxygen ion flux, a good correlation between the comet’s
emission and the solar wind proton flux, but no correlation
between the cometary emission and the solar X-ray flux.
For the four comets for which extended X-ray lightcurves were obtained during quiet Sun conditions, the time
delay between the solar wind proton flux and the comet’s
X-ray impulse (Table 2) was well predicted by assuming a
simple latitude-independent solar wind flow, a quadrupole
solar magnetic field, and propagation of the sector boundaries radially at the speed of the solar wind and azimuth-
Lisse et al.: X-Ray and Extreme UV Emission from Comets
Fig. 4. Temporal trends for Comet 2P/Encke 1997 on 4–9 July
1997 UT. = ROSAT HRI lightcurve, 4–8 July 1997. = EUVE
scanner Lexan B lightcurve 6–8 July 1997 UT, taken contemporaneously with the HRI observations, and scaled by a factor of 1.2.
All error bars are ±1σ. Also plotted are the WIND total magnetic
field Btotal ( ), the SOHO CELIAS/SEM 1.0–500-Å solar X-ray
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.
After Lisse et al. (1997a).
ally with period one-half the solar rotation period of 28 d
(Lisse et al., 1997b, 1999; Neugebauer et al., 2000)
∆t total = ∆t Carrington rotation + ∆tradial =
longitudecomet – longitudeEarth
(rcomet – rEarth)
+
14.7˚/d
400 km/s ⋅ 86400 s/d
2.4.
Spectrum
Until 2001, all published cometary X-ray spectra had
very low spectral energy resolution (∆E/E ~ 1 at 300–600 eV),
and the best spectra were those obtained by ROSAT for C/
1990 K1 (Levy) (Dennerl et al., 1997) and by BeppoSAX
Fig. 5. Chandra ACIS-S medium resolution CCD X-ray spectrum for Comet C/1999 S4 (LINEAR). Soft X-ray spectrum of
C/1999 S4 (LINEAR) obtained by the Chandra X-ray Observatory (crosses) and a six-line best-fit “model” spectrum (solid line).
The positions of several possible atomic lines are noted. Adapted
from Lisse et al. (2001).
for Comet C/1995 O1 (Hale-Bopp) (Owens et al., 1998).
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,
and established upper limits to the contribution of the flux
from K-shell resonance fluorescence of carbon at 0.28 keV
and oxygen at 0.53 keV. However, even in these “best” spectra, continuum emission could not be distinguished from a
multiline spectrum. Nondetections of Comets C/Hyakutake,
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 spectrum (Lisse et al., 1996, 1997b).
Higher-resolution spectra of cometary X-ray emission
have just appeared in the literature. The Chandra X-ray Observatory (CXO) detected soft X-ray spectra from Comet
C/1999 S4 (LINEAR) (Lisse et al., 2001) over an energy
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.
Comet
Hyakutake
Hale-Bopp
Encke
Temple-Tuttle
635
Time of Impulse
(00:00H UT)
∆tlong
(d)
∆tradial
(d)
∆ttotal
(d)
∆tobserved
(d)
27 Mar 1996
11 Sep 1996
7 Jul 1997
29 Jan 1998
–0.23
–4.60
–0.26
–2.31
0.032
5.9
0.093
0.37
–0.20
1.30
–0.17
–1.94
–0.24
+1.4
–0.1
–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).
The spectrum is dominated by line emission, not by continuum. Using the CXO, a new spectrum of Comet C/1999
T1 (McNaught-Hartley) (Krasnopolsky et al., 2002) shows
similar line-emission features. Line emission is also found
in XMM-Newton spectra of Comet C/1999 T1 (McNaught
Hartley) and, more recently, in CXO spectra of C/2001 WM1
(LINEAR) and C/2002 Ikeya-Zhang (K. Dennerl et al. and
C. M. Lisse et al., personal communication, 2003). An
XMM-Newton spectrum of C/2001 WM1 (LINEAR) shows
characteristic CXE X-ray signatures in unprecedented detail
(K. Dennerl et al., personal communication, 2003). A re-
Lisse et al.: X-Ray and Extreme UV Emission from Comets
analysis of archival EUVE Deep Survey spectrometer spectra (Krasnopolsky and Mumma, 2001) suggests EUV lineemission features from Comet C/1996 B2 (Hyakutake)
(Figs. 6a,b). Recent FUSE observations (Weaver et al.,
2002) also indicate the presence of possible O VI 1032-Å
emission lines in far-UV spectra of C/2001 WM1 (LINEAR) (Fig. 6c).
3.
PROPOSED X-RAY MECHANISMS
A large number of explanations for cometary X-rays
were suggested following the discovery paper in 1996.
These included thermal bremsstrahlung (German for “braking radiation”) emission due to solar wind electron collisions with neutral gas and dust in the coma (Bingham et al.,
1997; Dawson et al., 1997; Northrop, 1997; Northrop et al.,
1997; Uchida et al., 1998; Shapiro et al., 1999), microdust
collisions (Ibadov, 1990; Ip and Chow, 1997), K-shell ionization of neutrals by electron impact (Krasnopolsky, 1997),
scattering or fluorescence of solar X-rays by cometary gas or
by small dust grains (Lisse et al., 1996; Wickramasinghe and
Hoyle, 1996; Owens et al., 1998), and by charge exchange
between highly ionized solar wind ions and neutral species
in the cometary coma (CXE) (Cravens, 1997a; Häberli et
al., 1997; Wegmann et al., 1998; Kharchenko and Dalgarno,
2000; Kharchenko et al., 2003; Schwadron and Cravens,
2000). In the thermal bremsstrahlung mechanism, fast electrons are deflected in collisions with charged targets, such
as the nuclei of atoms, and emit continuum radiation. Electron energies in excess of 100 eV (T > 106 K) are needed for
the production of X-ray photons. In the K-shell mechanism,
a fast electron collision removes an orbital electron from
an inner shell of the target atom. Early evaluation of these
various mechanisms (Dennerl et al., 1997; Krasnopolsky,
1997; Lisse et al., 1999) favored only three of them: the
CXE mechanism, thermal bremsstrahlung, and scattering of
solar radiation from very small (i.e., attogram; 1 attogram =
10 –19 g) dust grains.
A significant problem with mechanisms involving solar
wind electrons (i.e., bremsstrahlung or K-shell ionization)
is that the predicted emission luminosities are too small by
factors of 100–1000 compared to observations. The flux of
high-energy solar wind electrons near comets is too low
(Krasnopolsky, 1997; Cravens, 2000b, 2002a). Furthermore,
X-ray emission has been observed out to great distances
from the nucleus, beyond the bow shock (Fig. 2), and the
thermal energy of unshocked solar wind electrons at these
distances is about 10 eV. No emission has ever been found
to be associated with the plasma tail of a comet, which has
similar plasma densities and temperatures. Finally, the new,
high-resolution spectra demonstrating multiple atomic lines
are inconsistent with a continuum-type mechanism or a
mechanism producing only a couple of K-shell lines as the
primary source of cometary X-rays. Lisse et al. (2001) tried
several thermal bremsstrahlung continuum model fits to the
C/1999 S4 spectrum, and Krasnopolsky and Mumma (2001)
tried the same for the C/1996 B2 (Hyakutake) spectrum, but
neither was successful.
637
Mechanisms based on dust grains also have a number
of problems. It has been known since 1996 that Rayleigh
scattering of solar X-ray radiation from ordinary cometary
dust grains (i.e., about 1 µm in size) cannot produce the
observed luminosities — the cross section for this process
is too small (Lisse et al., 1996). A potential solution to this
problem is to invoke a population of very small, attogram
(10 –19 g) grains with radii on the order of the wavelengths
of the observed X-ray radiation, 10–100 Å, which can resonantly scatter the incident X-ray radiation. The abundance
of such attogram dust grains is not well understood in comets, as they are undetectable by remote optical observations;
however, there were reports from the VEGA Halley flyby
of a detection of an attogram dust component using the
PUMA dust monitor (Vaisberg et al., 1987; Sagdeev et al.,
1990). However, the statistical studies of the properties of
several comets (Figs. 2 and 3) demonstrate that X-ray emission varies with a comet’s gas production rate and not the
dust production rate (Dennerl et al., 1997; Lisse et al., 1999,
2001). Furthermore, the cometary X-ray lightcurves (Lisse
et al., 1996, 1999, 2001; Neugebauer et al., 2000) correlate with the solar wind ion flux and not with solar X-ray
intensity. Finally, dust-scattering mechanisms cannot account for the pronounced lines seen in the new high-resolution spectra — emission resulting from dust scattering of
solar X-rays should mimic the Sun’s X-ray spectral continuum, similar to what is observed in the terrestrial atmosphere for Rayleigh scattering of sunlight (Krasnopolsky,
1997).
The CXE mechanism requires that the observed X-ray
emission is driven by the solar wind flux and that the bulk
of the observed X-ray emission be in lines. Localization of
the emission to the sunward half of the coma, a solar wind
flux-like time dependence, and a line-emission-dominated
spectral signature of the observed emission all strongly
point to the solar wind charge exchange mechanism as
being responsible for cometary X-rays.
4.
SOLAR WIND CHARGE EXCHANGE
X-RAY MECHANISM
The solar wind is a highly ionized but tenuous gas (i.e.,
a plasma) (Cravens, 1997b). At its source in the solar corona, the million-degree gas is relatively dense and in collisional equilibrium, but its density drops within a few solar
radii into a freeflow regime wherein collisions are infrequent. Both the solar wind and corona have “solar” composition — 92% hydrogen by volume, 8% helium, and 0.1%
heavier elements. The heavier, “minor ion” species are
highly charged (e.g., oxygen in the form of hydrogen-like
O7+ or helium-like O6+ ions, N6+/N5+, C5+/C4+, Ne8+, Si9+,
Fe12+, etc.) due to the high coronal temperatures (Bame,
1972; Bocshler, 1987; Neugebauer et al., 2000).
The solar wind flow starts out slowly in the corona but
becomes supersonic at a distance of few solar radii (Parker,
1963; Cravens, 1997b). The gas cools as it expands, falling from T ≈ 106 K down to about 105 K at 1 AU. The average 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,
magnetic field strength ≈5 nT, and Mach number ≈8 (Hundhausen et al., 1968). However, the composition and charge
state distribution far from the Sun are “frozen in” at coronal values due to the low collision frequency outside the
corona. The solar wind contains structure, such as slow
(400 km s–1) and fast (700 km s–1) streams, which can be
mapped back to the Sun. The solar wind “terminates” in a
shock called the heliopause, where the ram pressure of the
streaming solar wind has fallen to that of the instellar material (ISM) gas (Suess, 1990). The region of space that contains 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;
the bulk of the wind interacts with neutral ISM gas in the
heliosphere and neutral and ionized ISM material at the
heliopause.
As the solar wind streams into a comet’s atmosphere,
cometary ion species produced from solar UV photoionization of neutral coma gas species are added to the flow as
“pick-up ions.” The resulting mass addition slows down the
solar wind due to momentum conservation and a bow shock
forms upwind of the comet (Galeev, 1991; Szegö et al.,
2000) (Fig. 7). The flow changes from supersonic to subsonic across the shock, and the magnetic field strength increases by a factor of ~5. Closer to the nucleus, where the
cometary gas density is higher and collisions more frequent,
the flow almost completely stagnates (Flammer, 1991). The
outer boundary of this stagnation region is often called the
cometopause (cf. review by Cravens, 1991). The observed
X-ray brightness peak resides within this boundary. Mag-
where A denotes the solar wind projectile ion (e.g., O, C,
Si . . . ), q is the projectile charge (e.g., q = 5, 6, 7) and B
denotes the neutral target species (e.g., H2O, OH, CO, O,
H . . . for cometary comae) (Fig. 8). The cross section for
this process is large, on the order of 10 –15 cm2, about 1 order of magnitude larger than the hardsphere collisional cross
section.
The product ion deexcites by emitting one or more photons (A(q – 1)+* → A(q – 1)+* + hν, where hν represents a photon). It is the characteristic radiation of the product ion that
is measured with astronomical X-ray instrumentation, and
so one labels the radiation detected by the charge state of
the final ion. The deexcitation usually takes place via a cascade through intermediate states rather than in one step to
the ground state. For large enough values of q, the deexcitation transitions lead to the emission of X-ray photons. For
species and charge states relevant to comets, the principal
quantum number of the ion A(q – 1)+ is about n = 4 or 5
Fig. 7. Spatial schematic of the solar wind-comet interaction.
The relative locations of the bow shock, the magnetic barrier, and
the tail are shown (not to scale). The Sun is toward the left. Also
represented is a charge transfer collision between a heavy solar
wind ion and a cometary neutral water molecule, followed by the
emission of an X-ray photon. After Cravens (2002b); copyright
journal Science (2002).
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
reaction involving a projectile ion Be4+. The internuclear distance
chosen is 10 Bohr radii or 5.29 × 10 –10 m, the curve-crossing
distance for the n = 3 ion final state. The target energy level (and
binding energy Eb) and product ion (Be3+) energy levels are shown
in units of hartrees (1 hartree = 27.2 eV). A possible cascading
pathway for the deexcitation by photon emission is shown. After
Cravens (2002b); copyright journal Science (2002).
netic field lines pile up into a “magnetic barrier” in this stagnation region and drape around the head of the comet forming
the plasma tail in the downwind direction (Brandt, 1982).
From experimental and theoretical work in atomic and
molecular physics it is found that solar wind minor ions
readily undergo charge transfer (or exchange) reactions
(Phaneuf et al., 1982; Dijkkamp et al., 1985; Gilbody, 1986;
Janev et al., 1988; Wu et al., 1988) when they are within
~1 nm of a neutral atomic species
Aq+ + B → A(q – 1)+* + B+
(1)
Lisse et al.: X-Ray and Extreme UV Emission from Comets
(Ryufuku et al., 1980; Mann et al., 1981). The cross section
for charge exchange between a high charge state solar wind
ion and an ionized coma gas species is negligible in comparison, due to the effects of Coulomb repulsion between
the two reactants. Once a coma neutral atom is ionized,
either by CXE processes or solar UV flux, the CXE mechanism is no longer an important energy transfer process.
4.1.
4.2. Charge Exchange Luminosity
and Temporal Variation
To first order, the CXE local X-ray power density Px can
be estimated assuming only one CXE collision per solar
wind ion per coma passage. This approximation yields the
expression
Px = αnswuswnn
Charge Exchange Morphology
Numerical simulations of the solar wind interaction with
Hyakutake including CXE have been used to generate X-ray
images. A global magnetohydrodynamic (MHD) model
(Häberli et al., 1997) and a hydrodynamic model (Wegmann
et al., 1998) were used to predict solar wind speeds and
densities and the X-ray emission around a comet. The simulated X-ray images are similar to the observed images,
which is relatively unsurprising, as any dissipative solar
wind–coma process with total optical depth near unity
would create the observed morphology (Fig. 1). The emission is found to lie in the sunward hemisphere of the neutral coma, varying from collisionally thin to collisionally
thick as the solar wind approaches the nucleus. Emission
is predicted to be in the soft X-ray, UV, and optical wavelengths, with the softest photons emitted closest to the nucleus. However, the spatial models to date have included
only highly simplified models of the CXE deexcitation cascade, as compared to the detailed spectral models of the global behavior discussed below.
As an example of the potential of studying the behavior
of the solar wind inside the coma using CXE reactions, we
consider the dissimilar morphologies of the extended Lyman α comae and the X-ray emitting regions of comets (cf.
Keller, 1973; Festou et al., 1979; Combi et al., 2000). The
CXE mechanism should not only transfer electrons from
cometary neutrals to solar wind minor ions, but to the solar wind majority ions H+ and He2+ as well. These ions are
roughly 1000 times more abundant than the X-ray active
highly ionized minor ions. The prompt photon-emission
energy produced from CXE by He2+ ions produces at least
three times the energy released by all the minor ions combined (D. Shemansky, personal communication, 2003).
Further, the neutral atoms produced are capable of scattering emission from the Sun. At luminosities of ~1016 erg s–1,
and production rates of ~1027 s–1, the HI created by CXE
should be detectable in Lyman α comet images. The fact
that it is not is puzzling. A possible solution is that the neutral hydrogen atoms produced by CXE retain relatively large
velocities with respect to the Sun, i.e., the solar wind is not
appreciably slowed at the cometary bow shock. This large
remnant velocity redshifts the CXE-produced neutral hydrogen with respect to the peak of the solar Lyman α emission,
so that fluorescence from these atoms is greatly reduced in
efficiency vs. H atoms produced from dissolution of cometary water group species. Recently Raymond et al. (2002)
have reported the effects of highly redshifted neutral hydrogen created by CXE emission in SOHO UVCS measurements of 2P/Encke during its 1997 apparition.
639
(2)
where nsw usw, and nn are the solar wind proton density, solar
wind speed, and neutral target density respectively (Cravens, 1997a, 2002a). All the “atomic and molecular details”
as well as the solar wind heavy ion fraction fh are combined
into the parameter α, given by α ≈ fh〈sct〉Eave, where 〈sct〉 is an
average CXE cross section for all species and charge states,
and Eave an average photon energy. A simple spherically
symmetric approximation to the neutral density in the coma
is given by nn = Q/[4πunr2], for r less than the ionization
scale length R = unt, where t ≈ 106 s is the ionization lifetime [for 1 AU (Schleicher and A’Hearn, 1988)] and un ≈
1 km s–1 the neutral gas outflow speed. Integration of Px
over the volume of the neutral coma yields an X-ray luminosity typically within a factor of 2–3 of the observed luminosity (Cravens, 1997a, 2000a; Lisse et al., 2001). The
observed luminosity is a function of both the solar wind
flux density and the com-etary neutral gas production rate
up to the limit of 100% charge exchange efficiency of all
solar wind minor ions within an ionization scale length of
106 km. The maximum expected X-ray luminosity at 1 AU
and 0.2–0.5 keV is ~1016 erg s–1 (Fig. 3). Temporal variations of the solar wind flux directly translate into time variations of the X-ray emission (Fig. 4).
4.3.
Charge Exchange Spectra
Model CXE spectra are in good agreement with the lowresolution X-ray spectra of cometary X-ray emission, and
the line centers of the high-resolution spectra have been
successfully predicted using CXE theory (Figs. 5 and 6)
(Lisse et al., 1999, 2001; Krasnopolsky and Mumma, 2001;
Weaver et al., 2002). The application of the CXE model to
comets has entailed a number of approaches to date. Some
work has included only a few solar wind species but used a
careful cascading scheme (Häberli et al., 1997). Other approaches have used a simple cascading scheme and simple
collision cross sections, but included a large number of
solar wind ions and charge states (Wegmann et al., 1998;
Schwadron and Cravens, 2000). Kharchenko and colleagues
(Kharchenko and Dalgarno, 2000; Kharchenko et al., 2003)
have treated the atomic cascading process more carefully
than other modelers, although the spatial structure of the
solar wind–cometary neutral interaction in their model was
highly simplified. They predicted the existence of a large
number of atomic lines, including O5+ (1s25d → 1s22p) at
106.5 eV, C4+ (1s2s → 1s2) at 298.9 eV, C5+ (2p → 1s) at
367.3 eV, C5+ (4p → 1s) at 459.2 eV, and O6+ (1s2p → 1s2)
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
C/1999 S4 (LINEAR) and C/1999 T1 (McNaught-Hartley).
The peak measured near 0.56 keV is certainly the combination of three closely spaced helium-like O6+ (1s2p and
1s2s → 1s2) transitions (which comes from CXE of solar
wind O7+) and the line located at 0.32 keV is due to heliumlike C4+ (1s2p and 1s2s → 1s2). Similar identifications have
been made for the EUVE spectrum of C/1996 B2 (Hyakutake) (Krasnopolsky and Mumma, 2001). The spectral problem is still far from totally solved, however. Careful comparisons and calculations needed to interpret the subtleties
of the high-resolution spectral observations, including the
role of collisions after charge transfer, solar wind ion–dust
interactions, and the exact species present at each point in
the coma, are only now starting to be done (Krasnopolsky et
al., 2002; Kellett et al., 2003).
5.
THE FUTURE OF COMETARY
X-RAY EMISSION STUDIES
The discovery that comets are X-ray sources can now
be explained by charge exchange reactions of highly charged
solar wind ions with neutral atoms and molecules residing
in the cometary coma. The energy required to power this
emission originates in the hot solar corona and is stored as
potential energy in highly stripped solar wind ions (Cravens, 2000a, 2002b; Dennerl, 1999). Charge exchange reactions with neutral species, like cometary coma neutral atoms
and molecules, release this potential energy in the form of
X-ray and UV radiation. This simple model can explain the
gross features of the observed crescent-shaped emission
with its sunward displaced peak, the maximum spatial extent of the emission of ~106 km (Figs. 1 and 2), the maximum observed luminosity of ~1016 erg s–1 (Fig. 3), and spectra dominated by line emission (Fig. 4).
5.1. Plasma-Neutral Interactions in the
Cometary Coma
However, a more careful treatment of the CXE mechanism is needed to fully understand the phenomenon of cometary X-ray emission. One complication is that the multiple
CXE collisions take place in regions close to the nucleus
where the target density is high (the so-called “collisionally
thick” case). The charge state for the initially hydrogen-like
or helium-like solar wind minor ion is reduced by one during each CXE collision, ultimately leading to its conversion
to a neutral atom. When the ion’s charge state becomes too
low, X-ray photons are no longer emitted. When the number
of neutrals in a volume of space becomes too low, due to
coma expansion and ionization of coma gas molecules by
solar UV radiation, X-ray photons are no longer detectable.
Another complication is that spectral differences are expected for slow and fast solar wind streams, with the slow
solar wind, with its higher coronal freeze-in temperature,
producing a harder spectrum than does the fast solar wind
(Schwadron and Cravens, 2000).
Further theoretical progress will require the integration
of several ingredients into a single model: (1) accurate solar
wind composition for a range of solar wind types; (2) a
suitable MHD model of the solar wind interaction with the
coma, in order to accurately predict the densities of ions
and neutrals in equation (2); and (3) a more detailed understanding of the atomic processes, in order to improve our
understanding of the parameter α in equation (2). Success
for the first point requires new and improved measurements
of the solar wind throughout the heliosphere, and/or large
number statistical studies of cometary X-ray emission
throughout the heliosphere; the second point requires improved MHD codes on modern supercomputers; and to
achieve the third point, additional laboratory measurements
of state-specific CXE cross sections will be required. The
first and second points are being actively pursued by astronomers and modelers in the field. New laboratory work
is now being undertaken (Beiersdorfer et al., 2000, 2001;
Greenwood et al., 2000; Hasan et al., 2001) to measure
CXE cross sections for cometary target species such as H2O
at collision energies relevant to the solar wind (a few keV/
amu). Recent measurements have indicated, for example,
that multiple as well as single-electron CXE makes a contribution to the X-ray emission (Hasan et al., 2001; Greenwood et al., 2001; Gao and Kwong, 2002). Given detailed
MHD models of the solar wind passing through the coma
and accurate cross sections for the CXE process, we will
be able to map out the density of solar wind minor ions in
the coma.
5.2.
Remote Sensing of the Solar Wind
Driven by the solar wind, cometary X-rays provide an
observable link between the solar corona, where the solar
wind originates, and the solar wind where the comet resides.
Once we have understood the CXE mechanism’s behavior
in cometary comae in sufficient detail, we will be able to
use comets as probes to measure the solar wind throughout the heliosphere. This will be especially useful in monitoring the solar wind in places hard to reach with spacecraft — over the solar poles, at large distances above and
below the ecliptic plane, and at heliocentric distances greater
than a few AU (Lisse et al., 1996, 2001; Krasnopolsky et
al., 2000). For example, approximately one-third of the observed soft X-ray emission is found in the 530–700-eV
oxygen O7+ and O6+ lines; observing photons of this energy
allows studies of the oxygen ion charge ratio of the solar
wind, which is predicted to vary significantly between the
slow and fast solar winds (Neugebauer et al., 2000; Schwadron and Cravens, 2000; Kharchenko and Dalgarno, 2001).
5.3.
Emission from Other Planetary Systems
The CXE mechanism operates wherever the solar wind
(or any highly ionized plasma) interacts with a substantial
quantity of neutral gas. Motivated in part by the discovery
of the new class of cometary X-ray emitters, further meas-
Lisse et al.: X-Ray and Extreme UV Emission from Comets
urements of potential solar system sources (Holmström et al.,
2001; Cravens and Maurellis, 2001) of X-ray emission have
been undertaken recently. X-rays have now been observed
from Venus as solar X-rays fluorescently scattered by the
thick (compared to comets) venusian atmosphere (Dennerl
et al., 2002), and from Mars as ~90% fluorescently scattered solar X-rays from the thick martian atmosphere and
~10% CXE-derived X-rays (Dennerl, 2002). The solar wind
is known to approach very closely to these planets due to
their weak intrinsic magnetic fields, which cannot act as effective obstacles to the external flow, allowing for CXE processes to occur. The small spatial extent and high density
of the gravitationally bound planetary atmospheres provides
a higher X-ray luminosity due to scattering of solar X-rays
than from CXE. X-rays have now been detected at Io and
Europa and in the Io flux torus using Chandra, although
the exact mechanism of emission is still unknown (Elsner
et al., 2002). Another neutral gas system from which X-ray
emission has been predicted is the terrestrial hydrogen geocorona (Cox, 1998; Cravens, 2000a; Dennerl, 1999; Freyberg, 1998; Robertson and Cravens, 2003), although no detection has yet been made.
X-ray emission from the heliosphere is also expected
from the interaction of the solar wind with the interstellar
neutral gas (mainly HI and HeI) that streams into the solar
system (Cox, 1998; Cravens, 2000a,b). Cravens (2000b)
demonstrated that roughly one-half of the observed 0.25keV X-ray diffuse background can be due to this process.
Solar and Heliospheric Observatory (SOHO) observations
of neutral hydrogen Lyman α emission show a clear asymmetry in the ISM flow direction, with a clear deficit of
neutral hydrogen in the downstream direction of the incoming neutral ISM gas, most likely created by CXE ionization
of the ISM as it transits the heliosphere. Cravens et al.
(2001) have shown a strong correlation between the solar
wind flux density and the ROSAT “long-term enhancements,” systematic variations in the soft X-ray background
of the ROSAT X-ray detectors. Photometric imaging observations of the lunar nightside by Chandra made in September 2001 do not show any lunar nightside emission above
a CXE background. The soft X-ray emission detected from
the darkside of the Moon, using ROSAT, would appear to
be due not to electrons spiraling from the sunward to the
dark hemisphere, as proposed by Schmitt et al. (1991), but
instead be due to CXE in the column of solar wind between
Earth and the Moon. The analogous process applied to other
stars has been suggested as a means of detecting stellar
winds (Lisse, 2002a,b; Wargelin and Drake, 2001).
5.4.
Soft X-Ray Background
Heliospheric and geocoronal X-ray emission have been
suggested (Cravens, 2000b, 2002b; Dennerl, 1999; Lisse et
al., 2001) to make significant contributions to the observed
soft X-ray background [previously attributed entirely to hot
interstellar gas (Snowden et al., 1990, 1998; McCammon et
al., 2002)]. This supposition is supported by the positive cor-
641
relations that have been found between measured solar wind
fluxes and measured X-ray background intensities (Cravens
et al., 2001; Robertson et al., 2001). Recent large angular
scale measurements of the diffuse soft X-ray background in
a 100-s rocket flight by McCammon et al. (2002) and in the
Chandra background toward MBM12 (R. Edgar et al., personal communication, 2001) show a clear peak at 560 eV, as
expected for CXE-driven emission (Fig. 4). A significant fraction of the line emission observed in the soft X-ray background, e.g., the 560-eV emission, is probably due to CXE
emission in our solar system and around other stars.
REFERENCES
A’Hearn M. F., Schleicher D. G., Millis R. L., Feldman P. D., and
Thompson D. T. (1984) Comet Bowell 1980b. Astron. J., 89,
579–591.
Bame S. J. (1972) Spacecraft observations of the solar wind composition. In Solar Wind (C. P. Sonnett et al., eds.), p. 529.
NASA SP-308, Washington, DC.
Beiersdorfer P., Olson R. E., Brown G. V., Chen H., Harris C. L.,
Neill P. A., Schweikhard L., Utter S. B., and Widmann K.
(2000) X-ray emission following low-energy charge exchange
collisions of highly charged ion. Phys. Rev. Lett., 85, 5090–
5093.
Beiersdorfer P., Chen H., Lisse C. M., Olson R., and Utter S. B.
(2001) Laboratory simulation of cometary x-ray emission.
Astrophys. J. Lett., 549, L147.
Bingham R., Dawson J. M., Shapiro V. D., Mendis D. A., and
Kellett B. J. (1997) Generation of X-rays from C/Hyakutake
1996B2. Science, 275, 49–51.
Bochsler P. (1987) Solar wind ion composition. Phys. Scripta,
T18, 55–60.
Brandt J. C. (1982) Observations and dynamics of plasma tails.
In Comets (L. L. Wilkening, ed.), pp. 519–537. Univ. of Arizona, Tucson.
Cioffi D. F. (1990) Supernova remnants as probes of the interstellar medium. In Physical Processes in Hot Cosmic Plasma
(W. Brinkmann et al., eds.), p. 17. Kluwer, Dordrecht.
Combi M. R., Reinard A. A., Bertaux J.-L., Quemerais E., and
Makinen T. (2000) SOHO/SWAN observations of the structure
and evolution of the hydrogen Lyman-α coma of Comet HaleBopp (1995 O1). Icarus, 144, 191–202.
Cox D. P. (1998) Modeling the local bubble. In Lecture Notes in
Physics, Vol. 506, pp. 121–131. Springer-Verlag, Berlin.
Cravens T. E. (1991) Plasma processes in the inner coma. In
Comets in the Post-Halley Era (R. L. Newburn Jr. et al., eds.),
pp. 1211–1255. Kluwer, Dordrecht.
Cravens T. E. (1997a) Comet Hyakutake X-ray source: Charge
transfer of solar wind heavy ions. Geophys Res. Lett., 24, 105–
109.
Cravens T. E. (1997b) Physics of Solar System Plasmas, p. 477.
Cambridge Univ., Cambridge.
Cravens T. E. (2000a) X-ray emission from comets and planets.
Adv. Space Res., 26, 1443–1451.
Cravens T. E. (2000b) Heliospheric X-ray emission associated with
charge transfer of the solar wind with interstellar neutrals.
Astrophys. J. Lett., 532, L153–L156.
Cravens T. E. (2002a) X-ray emission in the solar system. In
Atomic Processes in Plasmas: 13th APS Topical Conference
on Atomic Processes in Plasmas (D. R. Schultz et al., eds.),
642
Comets II
pp. 173–181. AIP Conference Proceedings 635, American Institute of Physics, Melville, New York.
Cravens T. E. (2002b) X-ray emission from comets. Science, 296,
1042–1046.
Cravens T. E. and Maurellis A. N. (2001) X-ray emission from
scattering and fluorescence of solar X-rays at Venus and Mars.
Geophys. Res. Lett., 28, 3043.
Cravens T. E., Robertson I. P., and Snowden S. L. (2001) Temporal variations of geocoronal and heliospheric x-ray emission
associated with the solar wind interaction with neutrals. J. Geophys. Res., 106, 24883–24892.
Dawson J. M., Bingham R., and Shapiro V. D. (1997) X-rays from
Comet Hyakutake. Plasma Phys. Control. Fusion, 39, A185–
A193.
Dennerl K. (1999) X-ray emission from comets. In Atomic Physics
Vol. 16: Sixteenth International Conference on Atomic Physics
(W. E. Baylis and G. W. F. Drake, eds.), pp. 361–376. AIP
Conference Proceedings 477, Springer-Verlag, Berlin.
Dennerl K. (2002) Discovery of X-rays from Mars with Chandra.
Astron. Astrophys., 394, 1119–1128.
Dennerl K., Englhauser J., and Trümper J. (1997) X-ray emissions
from comets detected in the Röntgen X-ray satellite all-sky
survey. Science, 277, 1625–1629.
Dennerl K., Burwitz V., Englhauser J., Lisse C. M., and Wolk S.
(2002) Discovery of X-rays from Venus with Chandra. Astron.
Astrophys., 386, 319–330.
Dijkkamp D., Gordeev Y. S., Brazuk A., Drentje A. G., and
de Heer F. J. (1985) Selective single-electron capture into (n, l)
subshells in slow collisions of C6+, N6+, O6+ and Ne6+ with He,
H2 and Ar. J. Phys. B. Atom. Mol. Phys., 18, 737–756.
Dorren J. D., Guedel M., and Guinan E. F. (1995) X-ray emission
from the Sun in its youth and old age. Astrophys. J., 448, 431–436.
Elsner R. F. and 15 colleagues (2002) Discovery of soft X-ray
emission from Io, Europa, and the Io plasma torus. Astrophys.
J., 572, 1077–1082.
Feigelson E. D. (1982) X-ray emission from young stars and implications for the early solar system. Icarus, 51, 155–163.
Festou M., Jenkins E. B., Barker E. S., Upson W. L., Drake J. F.,
Keller H. U., and Bertaux J. L. (1979) Lyman-alpha observations of comet Kobayashi-Berger-Milon (1975 IX) with Copernicus. Astrophys. J., 232, 318–328.
Flammer K. R. (1991) The global interaction of comets with the
solar wind. In Comets in the Post-Halley Era (R. L. Newburn
Jr. et al., eds.), pp. 1125–1144. Kluwer, Dordecht.
Foukal P. (1990) Solar Astrophysics. Wiley, New York. 475 pp.
Freyberg M. J. (1998) On the zero-level of the soft X-ray background. In The Local Bubble and Beyond (D. Breitschwerdt
et al., eds.), pp. 113–116. Lecture Notes in Physics, Vol. 506,
Springer-Verlag, Berlin.
Galeev A. A. (1991) Plasma processes in the outer coma. In Comets in the Post-Halley Era (R. L. Newburn Jr. et al., eds.),
pp. 1145–1169. Kluwer, Dordrecht.
Gao H. and Kwong V. H. S. (2002) Charge transfer of O+5 and
O+4 with CO at keV energies. Astrophys. J., 567, 1272–1275.
Gilbody H. B. (1986) Measurements of charge transfer and ionization in collisions involving hydrogen atoms. Adv. Atom. Mol.
Phys., 22, 143–195.
Greenwood J., Williams I. D., Smith S. J., and Chutjian A. (2000)
Measurement of charge exchange and X-ray emission cross
sections for solar wind-comet interactions. Astrophys. J. Lett.,
533, L175–L178.
Greenwood J. B., Williams I. D., Smith S. J., and Chutjian A.
(2001) Experimental investigation of the processes determining X-ray emission intensities from charge-exchange collisions.
Phys. Rev. A, 63, 627071–627079.
Häberli R. M., Gombosi T. I., deZeeuw D. L., Combi M. R., and
Powell K. G. (1997) Modeling of cometary X-rays caused by
solar wind minor ions. Science, 276, 939–942.
Hasan A. A., Eissa F., Ali R., Schultz D. R., and Stancil P. C.
(2001) State-selective charge transfer studies relevant to solar
wind-comet interactions. Astrophys. J. Lett., 560, L201–L205.
Holmström M. S., Barabash S., and Kallio E. (2001) X-ray imaging of the solar wind-Mars interaction. Geophys. Res. Lett., 28,
1287.
Hundhausen A. J., Gilbert H. E., and Bame S. J. (1968) Ionization state of the interplanetary plasma. J. Geophys. Res., 73,
5485–5493.
Ibadov S. (1990) On the efficiency of X-ray generation in impacts
of cometary and zodiacal dust particles. Icarus, 86, 283–288.
Ip W.-H. and Chow V. W. (1997) NOTE: On hypervelocity impact
phenomena of microdust and nano X-ray flares in cometary
comae. Icarus, 130, 217–221.
Janev R. K., Phaneuf R. A., and Hunter H. T. (1988) Recommended cross sections for electron capture and ionization in
collisions of Cq+ and Oq+ ions with H, He and H2. Atomic Data
and Nuclear Data Tables, 40, 249.
Keller H. U. (1973) Lyman-alpha radiation in the hydrogen atmospheres of comets: A model with multiple scattering. Astron.
Astrophys., 23, 269–280.
Kellett B. J., Bingham R., Lisse C. M., Torney M., Summers H. P.,
and Shapiro V. D. (2003) A detailed model of the X-ray emission from comets. In Plasma Physics: 11th International Congress on Plasma Physics (ICPP2002) (I. S. Falconer et al., eds.),
pp. 704–707. AIP Conference Proceedings 669, American Institute of Physics, Melville, New York.
Kharchenko V. and Dalgarno A. (2000) Spectra of cometary X rays
induced by solar wind ions. J. Geophys. Res., 105, 18351–18360.
Kharchenko V. and Dalgarno A. (2001) Variability of cometary
X-ray emission induced by solar wind ions. Astrophys. J. Lett.,
554, L99–L102.
Kharchenko V., Rigazio M., Dalgarno A., and Krasnopolsky V. A.
(2003) Charge abundances of the solar wind ions inferred from
cometary X-ray spectra. Astrophys. J. Lett., 585, L73–L75.
Krasnopolsky V. A. (1997) On the nature of soft X-ray radiation
in comets. Icarus, 128, 368–385.
Krasnopolsky V. A. and Mumma M. J. (2001) Spectroscopy of
Comet Hyakutake at 80–700 Å: First detection of solar wind
charge transfer emissions. Astrophys. J., 549, 629–634.
Krasnopolsky V. A., Mumma M. J., Abbott M., Flynn B. C.,
Meech K. J., Yeomans D. K., Feldman P. D., and Cosmovici
C. B. (1997) Discovery of soft X-rays from Comet Hale-Bopp
using EUVE. Science, 277, 1488–1491.
Krasnopolsky V. A., Mumma M. J., and Abbott M. J. (2000) EUVE
search for X-rays from Comets Encke, Mueller (C/1993 A1),
Borrelly, and postperihelion Hale-Bopp. Icarus, 146, 152–160.
Krasnopolsky V. A., Christian D. J., Kharchenko V., Dalgarno A.,
Wolk S. J., Lisse C. M., and Stern S. A. (2002) X-ray emission
from Comet McNaught-Hartley (C/1999 T1). Icarus, 160, 437–
447.
Lisse C. M. (2002a) Cometary X-ray emission — the view after
the first Chandra observation of a comet. In The High Energy
Universe at Sharp Focus: Chandra Science (E. M. Schlegel
and S. B. Vrtilek, eds.), pp. 3–17. ASP Conference Series 262.
Lisse C. M. (2002b) Cometary X-rays — the EUVE photometric
Lisse et al.: X-Ray and Extreme UV Emission from Comets
legacy. In Continuing the Challenge of EUV Astronomy:
Current Analysis and Prospects for the Future (S. B. Howell
et al., eds.), pp. 254–267. ASP Conference Series 264.
Lisse C. M. and 11 colleagues (1996) Discovery of X-ray and extreme ultraviolet emission from Comet Hyakutake C/1996 B2.
Science, 274, 205–209.
Lisse C. M., Mumma M. J., Petre R., Dennerl K., Englhauser J.,
Schmitt J., and Truemper J. (1997a) Comet C/1996 B2 (Hyakutake). IAU Circular No. 6433.
Lisse C. M., Dennerl K., Englhauser J., Trümper J., Marshall F. E.,
Petre R., Valina A., Kellett B. J., and Bingham R. (1997b) Xray emission from Comet Hale-Bopp. Earth Moon Planets, 77,
283–291.
Lisse C. M., Christian D., Dennerl K., Englhauser J., Trümper J.,
Desch M., Marshall F. E., Petre R., Snowden S. (1999) X-ray
and extreme ultraviolet emission from Comet P/Encke 1997.
Icarus, 141, 316–330.
Lisse C. M., Christian D. J., Dennerl K., Meech K. J., Petre R.,
Weaver H. A., and Wolk S. J. (2001) Charge exchange-induced
X-ray emission from Comet C/1999 S4 (LINEAR). Science,
292, 1343–1348.
Mann R., Folkmann F., and Beyer H. F. (1981) Selective electron
capture into highly stripped Ne and N target atoms after heavyion impact. J. Phys. B.: Atom. Molec Phys., 14, 1161–1181.
McCammon D. and Sanders W. T. (1990) The soft X-ray background and its origins. Annu. Rev. Astron. Astrophys., 28, 657–
688.
McCammon D. and 19 colleagues (2002) High spectral resolution
observation of the soft X-ray diffuse background with thermal
detectors. Astrophys. J., 576, 188–203.
Mumma M. J., Krasnopolsky V. A., and Abbott M. J. (1997) Soft
X-rays from four comets observed by EUVE. Astrophys. J.
Lett., 491, L125–L128.
Neugebauer M., Cravens T. E., Lisse C. M., Ipavich F. M., Christian D., von Steiger R., Bochsler P., Shah P. D., Armstrong T. P.
(2000) The relation of temporal variations of soft X-ray emission from Comet Hyakutake to variations of ion fluxes in the
solar wind. J. Geophys. Res., 105, 20949–20956.
Northrop T. G. (1997) The spectrum of X-rays from Comet Hyakutake. Icarus, 128, 480–482.
Northrop T. G., Lisse C. M., Mumma M. J., and Desch M. D.
(1997) A possible source of the X-rays from Comet Hyakutake.
Icarus, 127, 246–250.
Owens A., Parmar A. N., Oostrbroek T., Orr A., Antonelli L. A.,
Fiore F., Schultz R., Tozzi G. P., Macarone M. C., and Piro L.
(1998) Evidence for dust-related X-ray emission from Comet
C/1995 O1 (Hale-Bopp). Astrophys J., 493, 47–51.
Parker E. N. (1963) Interplanetary Dynamical Processes. Wiley,
New York. 272 pp.
Phaneuf R. A., Alvarez I., Meyer F. W., and Crandall D. H. (1982)
Electron capture in low-energy collisions of Cq+ and Oq+ with
H and H2. Phys. Rev. A, 26, 1892–1906.
Raymond J. C., Uzzo M., Ko Y.-K., Mancuso S., Wu R., Gardner
L., Kohl J. L., Marsden B., and Smith P. L. (2002) Far-ultraviolet observations of Comet 2P/Encke at perihelion. Astrophys. J., 564, 1054–1060.
Robertson I. P. and Cravens T. E. (2003) X-ray emission from the
terrestrial magnetosheath. Geophys. Res. Lett., 30, 1439–1442.
Robertson I. P., Cravens T. E., Snowden S., and Linde T. (2001)
Temporal and spatial variations of heliospheric X-ray emissions
associated with charge transfer of the solar wind with interstellar neutrals. Space Sci. Rev., 97, 401–405.
643
Ryufuku H., Sasaki K., and Watanabe T. (1980) Oscillatory behavior of charge transfer cross sections as a function of the
charge of projectiles in low-energy collisions. Phys. Rev. A,
21, 745–750.
Sagdeev R. Z., Evlanov E. N., Zubkov B. V., Prilutskii O. F., and
Fomenkova M. N. (1990) Detection of very fine dust particles
near the nucleus of Comet Halley. Sov. Astron. Lett., 16, 315–
318.
Schleicher D. G. and A’Hearn M. F. (1988) The fluorescence of
cometary OH. Astrophys. J., 331, 1058–1077.
Schmitt J. H. M. M., Snowden S. L., Aschenbach B., Hasinger
G., Pfeffermann E., Predehl P., and Trumper J. (1991) A soft
X-ray image of the Moon. Nature, 349, 583–587.
Schwadron N. A. and Cravens T. E. (2000) Implications of solar
wind composition for cometary X-rays. Astrophys. J., 544,
558–566.
Shapiro V. D., Bingham R., Dawson J. M., Dobe Z., Kellett B. J.,
and Mendis D. A. (1999) Energetic electrons produced by
lower hybrid waves in the cometary environment and soft X ray
emission: Bremsstrahlung and K shell radiation. J. Geophys.
Res., 104, 2537–2554.
Snowden S. L., Cox D. P., McCammon D., and Sanders W. T.
(1990) A model for the distribution of material generating the
soft X-ray background. Astrophys. J., 354, 211–219.
Snowden S. L., Egger R., Finkbeiner D. P., Freyberg M. J., and
Plucinsky P. P. (1998) Progress on establishing the spatial distribution of material responsible for the 1/4 keV soft X-ray diffuse
background local and halo components. Astrophys. J., 493, 715.
Suess S. T. (1990) The heliopause. Rev. Geophys., 28, 97–115.
Szegö K. and 20 colleagues (2000) Physics of mass loaded plasmas. Space Sci. Rev., 94, 429–671.
Uchida M., Morikawa M., Kubotani H., and Mouri H. (1998) Xray spectra of comets. Astrophys. J., 498, 863–870.
Vaisberg O. L., Smirnov V., Omel’Chenko A., Gorn L., and Iovlev
M. (1987) Spatial and mass distribution of low-mass dust particles (m < 10–10 g) in Comet P/Halley’s coma. Astron. Astrophys., 187, 753–760.
Wargelin B. J. and Drake J. J. (2001) Observability of stellar winds
from late-type dwarfs via charge exchange X-ray emission.
Astrophys. J. Lett., 546, L57–L60.
Weaver H. A., Feldman P. D., Combi M. R., Krasnopolsky V.,
Lisse C. M., and Shemansky D. E. (2002) A search for argon
and O VI in three comets using the Far Ultraviolet Spectroscopic Explorer. Astrophys. J. Lett., 576, L95–L98.
Wegmann R., Schmidt H. U., Lisse C. M., Dennerl K., and
Englhauser J. (1998) X-rays from comets generated by energetic solar wind particles. Planet. Space Sci. 46, 603–612.
Wickramsinghe N. C. and Hoyle F. (1996) Very small dust particles (VSDP’s) in Comet C/1996 B2 (Hyakutake). Astrophys.
Space Sci., 239, 121–123.
Wu W. K., Huber B. A., and Wiesemann K. (1988) Cross sections
for electron capture by neutral and charged particles in collisions with He. Atomic Data and Nuclear Data Tables, 40, 57.
644
Comets II