6. Conclusions and outlook
Perhaps the most interesting outcome of the calculations which we presented in this paper is the fact not anticipated in earlier theoretical representations that pick-up ions substantially spread out from the plasma volume comoving with the solar wind into which they were initially injected. Most of the papers in the past dealing with pick-up ion motions in configuration and velocity space are based on the assumption that these ions are sticking to the solar wind plasma box into which they were injected (see e.g. Fahr & Rucinski, (1989); Lee & Ip, (1987); Isenberg, (1997); Bogdan et al., (1991); Rucinski et al., (1993); Chalov et al., (1995), (1997); Chalov & Fahr, (1996); Rucinski et al., (1998). This means that due to spatial diffusion of pick-up ions any spatial pattern of the pick-up ion injection rapidly and systematically loses its stigmatic imprint on the subsequently evolving pick-up ion distribution in space. In Figs. 1-5 one can see that when the bulk of pick-up ions injected at 1 AU has reached a distance of 3 AU this sample of pick-up ions is already spread out over a distance range of 1.5 AU.
This phenomenon especially touches the problem of He+-pick up ions emanating from the downwind cone of neutral interplanetary helium. These He+-pick-up ions had first been detected by Möbius et al. (1985, 1988) with the SULEICA plasmaanalyzer onboard of the AMPTE satellite. In later publications these authors have interpreted their He+-flux measurements in terms of interstellar helium gas parameters, like the LISM He-density and He-temperature (see Möbius, 1990, Möbius et al., 1995, Möbius, 1996, Möbius et al., 1996). While in the earlier of the just mentioned publications it was assumed that He+-pick up ions are strictly convected outwards with the solar wind, in the later publications it was realized that pick-up ions undergo pitch angle scattering processes and thereby are diffusing out of their co-moving injection volumes thus spreading out from the cone (see Möbius et al., 1996). Considering a 3-d random walk process connected with a pitch-angle scattering mean free path these authors could argue that their earlier LISM helium temperature derivations of between 12000 to 18000 K can then be reduced to temperature ranges of between 6500 to 8500 K derived by Witte et al. (1993) from neutral helium gas detections for scattering mean free paths of the order of 1 AU.
With the results of this paper here we shed interesting new light onto these derivations by showing both that spatial diffusion of pick-up ions in fact is important and, even more important, that it takes place in an anisotropic form. In the solar rest frame spatial diffusion predominantly takes place in the plane defined by the solar wind velocity and the frozen-in magnetic field , whereas the drifts perpendicular to this plane are of very minor importance (see Rucinski et al., 1993). As we have shown here seen from the reference frame co-moving with the solar wind more particles are diffusing in the hemisphere of negative pitch-angles () compared to the hemisphere with positive pitch angles (). This means that the cone-injected particles may preferentially diffuse to the side left of the cone axis as seen from the sun. Looking to He+-fluxes reaching the earth orbit this allows to conclude that left of the cone axis one may find a milder flux decrease compared to the decrease right of the cone axis which should be steeper. In fact this seems to be evidently appearing in the data shown by Möbius (1990) where the flux increase from November towards December is steeper than the decrease from December towards January. Thus neither should the flux maximum be identified with the cut of the helium cone axis and the earth orbit, nor should the left or right flux decrease profile be taken as direct indication for the LISM helium temperature. More theoretical work has to be done on He+ pick-up ion phase space diffusion before safe conclusions with respect to LISM parameters can be drawn.
Pick-up ions may also be observationally accessible by means of their resonance emissions. Especially interesting is this access for the case of He+-pick-up ions which are resonantly excited by the solar 304 line emission. As already discussed in the paper by Paresce et al.(1983) the resulting resonance intensity strongly depends on the prevailing He velocity distribution function and practically is determined by the fraction of pick-up ions within a velocity range of less than 100 km/s relative to the sun. The results in this paper show that pick-up ions can be expected with predominance in the sunward hemisphere of the velocity space and thus with this predominance have effective outward velocities smaller than the solar wind velocity . Therefore they are good candidates to become resonantly excited. A measurement of the existing interplanetary He+-pick-up ion resonance glow intensity (from the earth depending on the solar offset angle one could expect intensities between 101 and 10-1 Rayleighs!) thus could be an effective means to observationally study phase space diffusion of He+ pick-up ions (see Fahr et al. (1998)).
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998