on timescales of weeks, presumably in response to instabilities in the relativistic wind from the pulsar (Hester et al. 1995). Greiveldinger and Aschenbach (1999) have also reported changes in the X-ray surface brightness of the Crab Nebula on larger scales and somewhat longer timescales. Thus, we have examined our two images of the Vela nebula carefully in a search for surface brightness fluctuations.

We examined the count rate in the kidney-bean region bracketed by the outer-arc and an inner circle with $r = 1\farcs32$ centered on the pulsar. No significant change was observed between the two observations. Similarly, no measurable change in the count rate associated with the jet-like feature was found. Comparisons of other regions defined in Table 1 also showed no change, with the notable exception of the outer arc itself which appeared to increase in brightness by $\sim 5\%$ in the second observation.

To investigate this further, we examined regions congruent with the morphology of the nebula by constructing radial bins which are elliptical in shape; the ratio of the semi-major to semi-minor axes of the ellipse is 1.76, and the elliptical annuli are oriented at a position angle of $50^{\circ}$ (east of north). As Figure 6 