The detection of the auroral electrojet on Jupiter provides a direct link between the region from which precipitating material originates in the magnetosphere and the resultant aurora within the ionosphere. The correlation between the H3+ emission and H3+ column density means that the location of emission is a direct indicator of energy deposition. The energy source for the auroral oval is tied to particle precipitation from the breakdown in corotation, by the formation of the electrojet.
The presence of the electrojet on the auroral oval also provides a direct mapping into the magnetosphere. Although this mapping is not as exact as the footprints of satellites, since the location of corotation breakdown is not well known, it does confirm that the location of the IR (and UV) auroral oval is likely to be located in the range of 12-20RJ, rather than 30RJ.
Given that the energy source of the auroral oval is particle precipitation, it is perhaps a surprising result that the dusk side of the oval appears to be not just generally cooler than the adjacent BPR, but has a definite temperature low directly associated with it. This can be explained in the context of the maintenance or breakdown in QTE, but whatever the cause, it must be directly associated with the auroral oval itself.
Given a breakdown in the assumption of QTE as described in Kim et al. (1992), the lower fundamental (v2 = 1) energy level is overfilled compared with the upper hotband (v2 = 2) level. This is caused by the radiative depopulation exceeding the collisional excitation rate, leading to a breakdown in the thermalisation of upper levels. Thus, for the assumption of QTE to hold in other regions, this suggests that the collisional excitation rate is lower in the dusk auroral oval. One way the collision rate could be reduced is if the particle precipitation occurs at a higher altitude, with a corresponding lower density in the neutral atmosphere.
Under conditions where QTE is maintained, cooling from the surrounding thermosphere has already been implicated as a cause of lower effective H3+ vibrational temperatures. It is possible that the H3+ in the dusk side of the oval is created at lower altitudes, because of higher energy impacting particles. UV images show that the dawn and dusk sides of the auroral oval do appear to be different in basic morphology, and that the dusk side is often associated with diffuse emission, which might have a consequential effect on the depth of H3+ production. The higher line-of-sight column density seen on the dusk side of the oval could be an indicator of a deeper origin, where higher atmospheric pressure allows more H3+ to be produced.
However, an alternative is that the thermospheric source of cooler neutral material is potentially the collisionally accelerated neutral atmosphere moved from cooler regions of Jupiter. The electrojet is not rapid enough to carry neutrals over the limb from the dark side of Jupiter, since the rotational velocity is greater than the velocity variation from the Hall drift (Figure 2-47 shows this clearly). However, the dayside of Jupiter has a temperature gradient across it, hottest at the equator, due to the incident angle with the sun. The electrojet could be transporting neutrals between higher and lower latitudes; the equatorward flow on the dusk side cooling the ions. This does, however, fail to explain why the BPR has a significantly raised temperature over the adjacent dusk oval.
The mechanisms determining conditions in the polar cap regions are relatively uncharacterised in comparison with the main auroral oval. Where the polar cap maps to the magnetosphere is still a matter of great debate, and no real consensus on the matter has been managed. The "Yin-Yang" bright and dark regions in the IR are directly correlated with the polar region of the UV, which appears to be dominated by diffuse polar emission which occurs only on the dusk side.
An important connection made in this thesis is that the DPR is not completely inactive, having a significantly raised emission compared with the non-auroral body of the planet, only a factor of 2-3 times lower than the peak oval values. This follows through with column density, and so the DPR has emission due to particle precipitation. It is thus likely that the UV aurora also has a significant minor component of emission in this dark region, which has been overlooked in the both the UV and IR images.
The presence of some temperature peaks in the middle of the DPR, which appear with no corresponding H3+ emission, are suggestive of a change from the normal conditions in the DPR. The apparent temperature increase could be due to a complete breakdown the assumed QTE, with the hotband being overfilled relative to the fundamental level. The conditions for this are not explained by the breakdown in QTE discussed in Kim et al. (1992), and requires a localised variation in energy level population not seen elsewhere. The reason for such an overpopulation of the upper energy level remains unexplained.
Given the maintenance of the assumption of QTE, the kinetic temperature peak is potentially caused by a collisionally accelerated neutral atmosphere carrying thermal energy with it into a normally cool ionospheric region. This provides additional evidence for the interconnection between the ionosphere and thermosphere through auroral dynamics.
The wind system detected within the polar region has not been previously predicted. It thus provides a vital new source of information about the effects of magnetospheric/ionospheric coupling in the polar region. Interpreting the wind system detected is difficult, because so little is currently known about coupling from the ionosphere out to the magnetosphere. The origins of the bright and dark regions are not necessarily the same, and the inability to distinguish these two sources adds to the difficulty of interpretation.
The DPR has, until now, had very little variation detected in it other than sporadic "dawn storms", and so discussion over its origin is scarce. On the other hand, the BPR with the analogous UV diffuse emission, and the "dawn storms" themselves, have been discussed more extensively. Interpretation of the velocity system depends greatly on the assumed origin, so the two main theories of the magnetosphere origin will be discussed in terms of the possible resultant interpretation of the dynamics presented within this thesis.
The two main theories for the origin of the BPR result from calculating the location on the planet, and extrapolating this out to the corresponding location in the magnetosphere. The first explanation places the origin far out in the magnetotail, through particles carried in the convective system driven by the solar wind, similar to the magnetosphere of Earth. The second suggested origin is the Jovian middle magnetosphere, beyond the point where corotation has broken down (reflecting fast rotational dynamics, connected to the edge of the current sheet).
The Jovian magnetosphere is substantially dominated by the Io fed current sheet to distances as far out as 125RJ (Kane et al., 1995). But the solar wind must have effects at some point within the magnetosphere, in the tail where the influence of the corotating plasma is much less. Return flows in the tail have been deduced from in-situ measurements (Cowley et al., 1993, 1996; Hawkins et al., 1998). However, the Ulysses spacecraft failed to detect any clear field-aligned current signatures in the polar region (Dougherty et al., 1993). The UV dawn storms have been interpreted as being analogous to "substorms" on Earth, the manifestations of which include the down-tail ejection of a significant portion of the plasmasheet (see 1.1.3 The outer magnetosphere).
Figure 5-1: The Earth's magnetospheric tail convection, and resultant ionospheric dynamics, based on diagram in Hill and Dessler (1991)
To understand how the features we have noted in the Jovian polar regions are formed, it is useful to refer briefly to the Terrestrial situation. The effects of the solar wind dominate the Earth’s magnetosphere (see Figure 5-1). One of the effects of this domination, first described in the Dungey model (1961), is a convection pattern in the magnetotail driven by the viscous interaction at the magnetopause, in turn powered by magnetic tension developed along "open" magnetic field lines that link the solar wind directly to the Earth’s polar caps.
If this were the case for Jupiter, then the entire polar cap of the Jovian aurora would emulate the situation across Earth’s pole (Figure 5-2). The solar wind, connecting with open field lines over the pole, stream past the planet at high speed, pulling the field lines with them. This causes the plasma on the flux tube to sense an electric field (through the motion of the magnetic field line relative to the plasma, E = u ´ B), which in turn drives an antisunward motion of plasma in the high-latitude ionosphere. This is compensated by a sunward return flow at lower latitudes, associated with the reconnection of the field lines in the magnetotail.
Figure 5-2: The suggested whole polar cap convection based on the observations presented here, in context with the Earth’s ionospheric pole cap (Figure 5-1), overlaid on a representation of the H3+ emission model of Satoh and Connerney (1999)
As in the case of the Earth, this process would depend on conditions in the solar wind, and these are by no means constant. For the convection to occur at all, it needs the interplanetary magnetic field to have a northward component (the reverse direction to that required by Earth), so that the "open" field lines can connect to the interplanetary field. In addition, the return flow (the Earth’s "substorms"), just above the auroral oval, may be intrinsically sporadic even if the flow imposed on the dayside is constant (Erikson and Wolf, 1980).
The sporadic nature does explain the velocity differences seen between the dark polar region, containing a strong and constant antisunward flow, and the bright polar region, with a less determinate return flow. The major problem with this polar configuration is that there appears to be only a dusk side sunward flow, with very little evidence of IR "dawn storms". Additionally, the antisunward flow on the dark polar cap does not vary a great deal, and yet reversals in the interplanetary magnetic field can occur on a short timescale in comparison with the period of observations, when fast solar wind and slow solar wind meet, although the slow wind reversals do occur over a timescale long enough to theoretically not affect the observations taken here.
In this configuration, the solar wind has a significant effect upon the Jovian ionosphere and magnetosphere that has not previously been expected, and the DPR interconnects directly with the solar wind. This potentially explains the occurrence of high speed ionospheric winds, without any apparent particle precipitation.
There is significant evidence within this thesis that the bright polar emission and dark polar emission have occurred from different sources; they contain very different temperature, column density and total emission environments, as well as an imbalance in the associated velocities. Observations by Ulysses have suggested that the bright polar region is connected with regions in the magnetosphere just beyond the breakdown in corotation, or even as the breakdown occurs (Prangé, private communication), and notably that the BPR is separate from the DPR, which does not connect to these regions. In UV observations, the appearance of "dawn storm" emission along the reference oval has been implicated as evidence that the origin of the storm is the middle magnetosphere (Clarke et al., 1998).
If the BPR is connected with the plasmasheet beyond the breakdown in corotation, then the processes involved in linking the ionosphere with the magnetosphere must explain the lower velocities seen in the profiles within this thesis. The temperature and column density of the BPR are similar to those of the dawn auroral oval; temperatures in the BPR are often higher than in the dusk oval. This does suggest that their origin within the magnetosphere may be similar to that of the auroral oval itself, with a significant influx of particles for the ionisation process.
A continuance of the sequential breakdown in corotation could be involved as a potential driver of the H3+ velocity, which is similar to or less than that seen on the oval itself. Since the breakdown in corotation is not thought to be a smooth decay, sudden regions of slowing can be expected beyond the point of initial breakdown. The location where this occurs is, unlike the initial breakdown, determined by the chaotic processes within the partially unstable disk. This scenario might also account for the arc-like features often seen in UV emission in the BPR (Clarke et al., 1996; Ballester et al., 1996; Prangé et al., 1998), features that appear in our data as intensity peaks inside the main auroral oval.
The DPR has to be considered separately, since the processes in control of this region appear to be very different from the rest of the aurora. The generally low temperatures and low column densities are suggestive of a lower energy regime, and yet the associated velocities are the largest seen within the aurora.
It is possible that the solar wind control described above is limited to the DPR alone. This would have an appearance somewhat distorted in comparison with the Earth’s antisunward polar wind, having to move around the BPR which is connected to the middle magnetosphere; a representation of what this could appear as is given in Figure 5-3.
The same problem with a lack of evidence for a return of flow is self evident, but the location of UV "dawn storms" in the proximity of the dark polar region is perhaps evidence that sudden surges of input can occur in an otherwise quiescent region.
Figure 5-3: The sectional polar cap, with solar wind controlled dark polar region, and middle magnetosphere controlled bright polar region, overlaid on a representation of the H3+ emission model of Satoh and Connerney (1999)
If a solar wind origin for the antisunward flow is ruled out, and without the large scale energy inputs that would cause H3+ production, then non-magnetospheric origin has to be considered. The transpolar winds on Earth are in part caused by the day/night temperature differential. However, if the magnetic field in this region is closed, there would be a significant magnetic friction to the flow of ions across the pole, which in turn requires a thermal wind significantly larger than 3 km/s in order to collisionally accelerate the H3+ ions to the velocities measured. We therefore consider this scenario to be unlikely.
While the observations made in this thesis centred on the aurora, rather than lower latitudes in the ionosphere, the detection of a continuous auroral electrojet has important implications for these lower latitudes. Examples of possible evidence of the underlying thermosphere being collisionally accelerated by the H3+ ions has already been shown in this chapter, and the electrojet has previously been implicated as a significant energy redistribution method in the ionosphere, providing lower latitudes with auroral energy (see 1.3.8 Indirect evidence of the electrojet).
By using the electrojet velocities taken from this thesis in the Jovian Ionosphere Model (JIM, Achilleos et al., 1998), it is possible to model the effect that the acceleration process would have on the lower latitudes. Achilleos et al. (submitted) suggests that for an electrojet that produces velocities in excess of 1 km/s - beyond the current velocity limit of the model - there would be significant transferral of momentum to the thermosphere. In Figure 5-4, the electrojet is clearly defined as a strong antirotational velocity, and its effects on the thermosphere can be seen in Figure 5-5. This clearly shows that not only does the motion of H3+ produce a significant degree of collisional acceleration, but also that this results in the neutral atmosphere flowing at lower latitudes.
Figure 5-4: JIM calculated H3+ velocities at 0.1mm, in a rotational frame; antirotational motion in blue, and superrotational motion in red, ranging on a linear scale between -124.3 to 97.6 m/s
Figure 5-5: JIM calculated neutral atmosphere velocities at 0.1mm; antirotational motion in blue, and superrotational motion in red, ranging on a linear scale between -100 and 100 m/s. Note that the velocity of the neutrals is a significant percentage (>40%) of the electrojet velocity in Figure 5-4
