To make progress in this area, we now need to explore the internal structures of radio jets rather than their global properties. As we describe below, combinations of analytic and numerical models of relativistic jets can predict internal structural aspects of the jets that will be accessible to the enhanced VLA but which can only be glimpsed in a few special (bright, nearby) cases with the present instrument.
In low-power radio galaxies without hot spots (Fanaroff-Riley Class I structures-FR I), the jets usually symmetrize on scales of a few kiloparsecs or less, suggesting that bulk relativistic motion in these jets is confined to the basal regions. It is becoming clear, however, that the basal regions that can be resolved with the VLA share the brightness asymmetries of the first few parsecs that can be imaged by VLBI, i.e., that an asymmetry which begins on parsec scales persists to kiloparsec scales. It is therefore important to establish if, and if so how and where, such flows decelerate between the parsec and kiloparsec scales. The jet dynamics are thought to be dominated by turbulent entrainment of material from the galactic atmosphere in a boundary layer that (a) decelerates and (b) spreads to fill the jet volume. Models have been developed that predict explicit relationships between the internal brightness and polarization structures of the jets including the effects of such boundary layers. Variations in flow velocity both across and along the jets strongly affect Doppler favoritism and relativistic aberration (which can determine the perceived polarization state). The models can therefore predict relationships between brightness asymmetries, limb-brightening and apparent polarization states across and along the jets and their counterjets, if the jets are intrinsically similar on both sides of the AGN (as suggested directly by their large-scale properties). Specifically, if counterjet emission can be detected in the ``gaps" close to the AGN in current VLA images, it should be found to have longitudinal magnetic fields with a higher degree of linear polarization than the main jet at similar distances. The counterjet emission should be limb-brightened nearest the AGN, but at all distances the transverse intensity profiles of the main jet should be more centrally peaked than those in the counterjet. In contrast, the outer envelopes of both jets should be similar in brightness and in their collimation properties at all distances from the AGN. The current VLA provides tantalizing evidence that the scale on which such effects become apparent is indeed accessible in nearby radio galaxies, but its sensitivity and angular resolution are barely adequate to begin testing these predictions. The sensitivity and resolution of the enhanced VLA, particularly using wide IF bandwidths and the A+ configuration at 6-8 GHz, would transform our ability to test such models by imaging the transverse structures, brightness and polarization symmetries of the inner parts of both the jets and the counterjets in nearby low-power radio galaxies. (Sensitivity is critical because the best diagnostics require accurate measures of the degree of polarization in the fainter, counterjet, emission).
A second way to test the decelerating-jet picture for nearby galaxies is to look for evidence for the entrainment in the distribution of Faraday depth along the jet, i.e., as a function of distance from the AGN. (The internal Faraday depth in an entraining jet may be almost independent of distance from the AGN, whereas that in a jet that is not entraining declines rapidly as it spreads.) We know from observations with the current VLA that the Faraday-thick regime in most such jets is below 2 GHz, and in many it must be below 1.5 GHz. Sensitive polarimetry with enough resolution () to distinguish jets clearly from their surroundings below 1.5 GHz is needed to explore this possibility. The 0.6 GHz system at the highest possible angular resolution (A+ configuration) would be the best hope for adding this diagnostic to the repertoire of probes of jet dynamics in galactic atmospheres.
In radio-loud quasars, one jet is usually much brighter than the other until the terminal hot spots (tens or hundreds of kiloparsecs from the AGN). The kiloparsec-scale jets are not as prominent as those on the parsec scales however, suggesting that the emission is dominated by flows with relatively low Lorentz factors (). Some parameters of quasar hot spots (compactness, location) also depend on whether they are on the side of the brighter jet. This is remarkable because jet deceleration should be mediated by a sequence of oblique shocks in such powerful sources. Candidates for such shocks are indeed present in the regular patterns of bright knots along many quasar jets. One way to preserve bulk relativistic motions as far as the hot spots is for part of the flow to have very high Lorentz factors. This suggests that jets in Fanaroff-Riley Class II (FR II) sources may also contain a range of Lorentz factors at any distance, in this case a high-speed spine being essentially invisible until the end of the jet while most of the observed jet emission comes from a lower-speed boundary layer, or sheath. Once again, we must resolve the internal structures of the jets to test this hypothesis. This is difficult because the jets in FR II quasars and radio galaxies are much better collimated than those in weaker sources, those in the FR II radio galaxies also being relatively faint. Resolution increases of at least a factor of two to three beyond those in the A configuration are necessary, even for the nearest FR II sources. These must be complemented by matching sensitivity increases (preserving surface brightness sensitivity), especially for polarimetry. It is particularly important to learn whether the radio jets are truly center-brightened, or whether we are observing only the outer, slower parts of the flow. These issues require transverse resolution of the jets in FR II sources with a representative range of source powers, and will best be approached with resolutions of or better in the A+ configuration at 8 GHz.
Are particles being accelerated in the jets, and if so how? To answer these questions, we need to obtain the radio spectra of jet substructures such as knots and hot spots. This requires multi-frequency imaging with scaled arrays at resolutions beyond what is now possible for most FRII's. Scaled-array observations, which are essential for reliable spectral imaging, are limited to about resolution with the current VLA. This is inadequate to resolve shocks and boundary layer features in any but a few nearby jets. We need to push the scaled-array capability to higher resolution without losing sensitivity at the higher frequencies. This requires the A+ configuration and the sensitivity improvements expected from the broader bandwidth (used in a spectral line mode to minimize bandwidth smearing for wide-field observing).
The mechanisms for stabilizing and confining radio jets are unclear. The jets contain magnetic fields that are often well-ordered but it is unclear whether these fields actively influence jet dynamics. Progress in understanding jet stability depends on knowing what aspects of the flows are visualized through their synchrotron radiation. Once the previously-mentioned rôles of relativity and particle acceleration in controlling the appearance of the flows are determined, the ability of an expanded VLA to measure the spreading rates of jets in powerful (FRII) radio galaxies and quasars will be an important factor in understanding jet stability and dynamics. There have also been hints of rotation measure gradients across some jets (e.g., that in NGC6251) in regions where their collimation properties change. Such gradients-a possible signature of magnetically-assisted collimation-are difficult to detect with the limited resolution and sensitivity of the current VLA. This arena of high-resolution polarimetry will be transformed by the combination of the A+ configuration and wide bandwidths, and will also benefit greatly from improved sampling in the domain provided by a new 2.4 GHz system.