5.2.2 Charge-Exchange
Charge-exchange reactions are of greatest significance near the nucleus. Their
importance derives from the fact that while the old ion (now transformed to a neutral)
leaves the system as a fast neutral, the new ion has almost zero velocity with respect
to the solar wind and therefore adds to the mass loading in the system.
The proton-hydrogen atom charge exchange cross-section has been investigated
by many authors. Freeman and Jones (1974) provide a simple formula for the crosssection for charge exchange between protons and hydrogen atoms that, according to
Gerhardt (2004), fits most of the data reasonably well. It has the form
σ cx,H ¼
0:6937 10
À14 1 À 0:155 log 10 E
ð
Þ
2
1 þ 0:1112 10
À14 E
3:3
ð5:9Þ
and is shown in Fig. 5.4. Proton energies in the solar wind are typically around
1 keV. Figure 5.4 also shows the charge exchange cross-sections for proton-H 2 O and
proton-CO 2 reactions.
The main point here is that the cross-sections are relatively large. Combi et al.
(2004) summarized this by stating the proton-H 2 O charge exchange reaction at 1 AU
removes neutral H 2 O as fast as photoionization for quiet Sun conditions. It is less
competitive for active Sun conditions (see also Huebner et al. 1992) but this
indicates that charge exchange is a notable process in any solar wind interaction
with either the neutral parents or the hydrogen coma.
It may also play a significant role in the generation of cometary X-ray emissions
(Cravens 2000 and refs therein). In March 1996, X-ray (0.1–2 keV) and extreme
ultraviolet (0.09–0.2 keV) emission was observed from C/1996 B2 (Hyakutake) by
instruments on ROSAT (Lisse et al. 1996) and EUVE (Mumma et al. 1997).
Subsequently several other detections of X-ray emissions have been reported
(e.g. from C/2007 N3 (Lulin) by Carter et al. 2012, from 103P/Hartley 2 by Lisse
Fig. 5.4 The chargeexchange cross-sections for
three reactions of cometary
significance. Solid line:
Calculation of the protonhydrogen neutral crosssection according to
Eq. (5.9). Triangles:
Measurements of protonCO 2 charge exchange by
Greenwood et al. (2000).
Diamonds: Measurements
of proton-H 2 O charge
exchange by Greenwood
et al. (2000)
406
5 The Plasma Environment
Charge-exchange reactions are of greatest significance near the nucleus. Their
importance derives from the fact that while the old ion (now transformed to a neutral)
leaves the system as a fast neutral, the new ion has almost zero velocity with respect
to the solar wind and therefore adds to the mass loading in the system.
The proton-hydrogen atom charge exchange cross-section has been investigated
by many authors. Freeman and Jones (1974) provide a simple formula for the crosssection for charge exchange between protons and hydrogen atoms that, according to
Gerhardt (2004), fits most of the data reasonably well. It has the form
σ cx,H ¼
0:6937 10
À14 1 À 0:155 log 10 E
ð
Þ
2
1 þ 0:1112 10
À14 E
3:3
ð5:9Þ
and is shown in Fig. 5.4. Proton energies in the solar wind are typically around
1 keV. Figure 5.4 also shows the charge exchange cross-sections for proton-H 2 O and
proton-CO 2 reactions.
The main point here is that the cross-sections are relatively large. Combi et al.
(2004) summarized this by stating the proton-H 2 O charge exchange reaction at 1 AU
removes neutral H 2 O as fast as photoionization for quiet Sun conditions. It is less
competitive for active Sun conditions (see also Huebner et al. 1992) but this
indicates that charge exchange is a notable process in any solar wind interaction
with either the neutral parents or the hydrogen coma.
It may also play a significant role in the generation of cometary X-ray emissions
(Cravens 2000 and refs therein). In March 1996, X-ray (0.1–2 keV) and extreme
ultraviolet (0.09–0.2 keV) emission was observed from C/1996 B2 (Hyakutake) by
instruments on ROSAT (Lisse et al. 1996) and EUVE (Mumma et al. 1997).
Subsequently several other detections of X-ray emissions have been reported
(e.g. from C/2007 N3 (Lulin) by Carter et al. 2012, from 103P/Hartley 2 by Lisse
Fig. 5.4 The chargeexchange cross-sections for
three reactions of cometary
significance. Solid line:
Calculation of the protonhydrogen neutral crosssection according to
Eq. (5.9). Triangles:
Measurements of protonCO 2 charge exchange by
Greenwood et al. (2000).
Diamonds: Measurements
of proton-H 2 O charge
exchange by Greenwood
et al. (2000)
406
5 The Plasma Environment