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Lake Notasha, near the crest of the Oregon Cascade mountain range, is the most dilute lake known. The measured conductivity during two visits was 1.3 and 1.6 µS cm-1 with a sum of base cations of 9 and 18 µeq L-1; bicarbonate was the dominant anion. Most of the cations in the lake can be accounted for by evapoconcentration of precipitation, although input of weathering products cannot be excluded as a source. The topographic watershed has a mixed coniferous forest, but the physical setting of the lake apparently minimizes hydrologic and ionic contributions from the watershed. This feature makes lakes such as Notasha appropriate receptors for monitoring atmospheric contaminants.

Near solar maximum, hard X-ray microflares with peak 20 keV fluxes of ?10-2 (cm2 s keV)-1, more than ten times smaller than for typical flares and subflares, can occur at the rate of about once every five minutes. We report here on a search for hard X-ray microflares made on a long duration balloon flight in February 1987 near solar minimum, at a time when no active regions were on the Sun. No microflares were observed over a total observing time of 16.5 hours spread over three days, implying a statistical upper limit to their rate of occurrence about a factor often lower than observed near solar maximum. Thus hard X-ray microflaring appears to be an active region phenomenon, and apparently not associated with flaring of soft X-ray bright points.

A quiescent X-ray source detected with the Einstein X-ray Observatory in a location consistent with that of an intense γ-ray burst1–3is shown here to be also consistent with the location of the 1928 optical transient4, the likely optical counterpart of the γ-ray burst source GBS0117 – 29. The probable maximum X-ray luminosity is only ∼1031erg s−1and supports the hypothesis that either γ bursts are produced by accretion instabilities5,6or, if due to published models for a thermonuclear flash7, they recur with intervals \[gsims]50 yr. Such long recurrence times may be inconsistent with limits inferred from both the γ burst observations8and optical data4.