Does tiny-scale atomic structure exist in the interstellar medium ?

We report on preliminary results from the recent multi-epoch neutral hydrogen absorption measurements toward three pulsars, B0823+26, B1133+16 and B2016+28, using the Arecibo telescope. We do not find significant variations in optical depth profiles over periods of 0.3 and 9--10 yr, or on spatial scales of 10--20 and 70--85 AU. The large number of non detections of the tiny scale atomic structure suggests that the AU-sized structure is not ubiquitous in the interstellar medium and could be quite a rare phenomenon.