source and solar wind conditions. This is now being addressed with more sophisticated time-dependent models (e.g. Nemeth 2020).
In the case of very weak outgassing (10
25
–10
26 molecule s
À1 ), the coma can be
penetrated by the solar wind particles through to the nucleus surface itself. Volwerk
et al. (2018) showed that the interaction is very different in cases similar to these and
does not show the bilobed induced magnetotail structure found at higher production
rates as one would qualitatively expect from consideration of the pressure balance.
One of the key issues that arises in the case of weak outgassing is that the gyroradii of newly picked-up ions become much larger than the interaction region and the
MHD approach to modelling used above is no longer tenable. Hybrid models
describe the plasma environment in a semi-kinetic manner. In these models, electrons are assumed to be a massless charge-neutralizing fluid and are modeled using
typical fluid methods (conservation equations). The ions, which have much larger
gyroradii, are treated as particles. A hybrid code therefore solves individual equations of motion to construct local densities (Hansen et al. 2007). This method
resolves physical structures down to the required length scales and also allows
non-Maxwellian distribution functions. Crudely, one can imagine this as the kinetic
counterpart to the fluid approach we also saw in the description of the gas dynamics.
Delamere (2006) used a hybrid code to simulate the solar wind interaction with
19P/Borrelly at the time of the Deep Space 1 encounter. The DS1/PEPE experiment
had shown a north-south asymmetry in the plasma boundaries which were initially
unexpected because the neutral coma was observed to be symmetric. Delamere
showed that this asymmetry was a natural consequence of the large pickup ion
gyroradius (Fig. 5.8) compared to the size of the obstacle. Test particle methods have
also been used to study effects upstream of the bow shock for more active comets
(e.g. McKenzie et al. 1994). Koenders et al. (2013, 2015) provided pre-Rosetta
models of 67P using a hybrid code.
5.5 Spectroscopy and Imaging of Plasma Tails
5.5.1 Main Ion Species
As can be seen in Fig. 5.1, plasma tails can be imaged by ground-based observers out
to several tens of millions kilometres from the nucleus. Both broad-band and narrowband imaging as well as spectroscopy have been used to study the composition and
dynamics of tails. A compilation of large-scale images for 1P/Halley has been
produced by Brandt et al. (1993) and various phenomena associated with plasma
tails can be seen therein. The plasma tails are bright as a result of emission from CO
+
(4260 Å and 5077 Å) and H 2 O
+ (typically 6148 Å and 6198 Å). Contributions from
OH
+ and CO 2
+ are other important species for remote observation (Jockers 1991).
Wyckoff et al. (1999) looked at the fine structure of some of these emissions in C/
1996 B2 (Hyakutake).
422
5 The Plasma Environment
In the case of very weak outgassing (10
25
–10
26 molecule s
À1 ), the coma can be
penetrated by the solar wind particles through to the nucleus surface itself. Volwerk
et al. (2018) showed that the interaction is very different in cases similar to these and
does not show the bilobed induced magnetotail structure found at higher production
rates as one would qualitatively expect from consideration of the pressure balance.
One of the key issues that arises in the case of weak outgassing is that the gyroradii of newly picked-up ions become much larger than the interaction region and the
MHD approach to modelling used above is no longer tenable. Hybrid models
describe the plasma environment in a semi-kinetic manner. In these models, electrons are assumed to be a massless charge-neutralizing fluid and are modeled using
typical fluid methods (conservation equations). The ions, which have much larger
gyroradii, are treated as particles. A hybrid code therefore solves individual equations of motion to construct local densities (Hansen et al. 2007). This method
resolves physical structures down to the required length scales and also allows
non-Maxwellian distribution functions. Crudely, one can imagine this as the kinetic
counterpart to the fluid approach we also saw in the description of the gas dynamics.
Delamere (2006) used a hybrid code to simulate the solar wind interaction with
19P/Borrelly at the time of the Deep Space 1 encounter. The DS1/PEPE experiment
had shown a north-south asymmetry in the plasma boundaries which were initially
unexpected because the neutral coma was observed to be symmetric. Delamere
showed that this asymmetry was a natural consequence of the large pickup ion
gyroradius (Fig. 5.8) compared to the size of the obstacle. Test particle methods have
also been used to study effects upstream of the bow shock for more active comets
(e.g. McKenzie et al. 1994). Koenders et al. (2013, 2015) provided pre-Rosetta
models of 67P using a hybrid code.
5.5 Spectroscopy and Imaging of Plasma Tails
5.5.1 Main Ion Species
As can be seen in Fig. 5.1, plasma tails can be imaged by ground-based observers out
to several tens of millions kilometres from the nucleus. Both broad-band and narrowband imaging as well as spectroscopy have been used to study the composition and
dynamics of tails. A compilation of large-scale images for 1P/Halley has been
produced by Brandt et al. (1993) and various phenomena associated with plasma
tails can be seen therein. The plasma tails are bright as a result of emission from CO
+
(4260 Å and 5077 Å) and H 2 O
+ (typically 6148 Å and 6198 Å). Contributions from
OH
+ and CO 2
+ are other important species for remote observation (Jockers 1991).
Wyckoff et al. (1999) looked at the fine structure of some of these emissions in C/
1996 B2 (Hyakutake).
422
5 The Plasma Environment