Lockheed Martin Research Laboratory, Palo Alto, CA 94304, U.S.A.

In order to interpret hard X-ray imaging data, it is important to understand 3-d geometry of the flare loop. But the appearance of the loop in a single soft X-ray image does not strongly constrain the geometry even under simplifying assumptions. In other words, the true loop shape can be very different from what we intuitively anticipate. Here we consider the possibility of using spatially integrated soft X-ray spectra, presently available, to constrain the loop geometry. These high resolution spectra often show that the resonant lines are shifted in wavelength due to upward motions of heated plasma (chromospheric evaporation). The magnitude of line shift should depend on how the loop is oriented with respect to the line of sight. It is concluded that observations are generally consistent with the somewhat asymmetric loop standing at a large angle from the vertical. We also discuss possible extensions of this work.