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We present the first reconstruction of the three-dimensional shape of magnetic loops in an active region from two different vantage points based on simultaneously recorded images. The images were taken by the two EUVI telescopes of the SECCHI instrument onboard the recently launched STEREO spacecraft when the heliocentric separation of the two space probes was 12 degrees. We demostrate that these data allow to obtain a reliable three-dimensional reconstruction of sufficiently bright loops. The result is compared with field lines derived from a coronal magnetic field model extrapolated from a photospheric magnetogram recorded nearly simultaneously by SOHO/MDI. We attribute discrepancies between reconstructed loops and extrapolated field lines to the inadequacy of the linear force-free field model used for the extrapolation.

[1] The breakdown of a high-frequency quasi-monochromatic gravity wave into smallscale ripples in OH airglow was observed on the night of 28 October 2003 at Maui, Hawaii (20.7°N, 156.3°W). The ripples lasted $20 min. The phase fronts of the ripples were parallel to the phase fronts of the breaking wave. The mechanism for the ripple generation is investigated using simultaneous wind and temperature measurements made by a sodium (Na) lidar. The observations suggest that the wave breaking and the subsequent appearance of ripples were related to dynamical (or Kelvin-Helmholtz) instabilities. The characteristics of the ripples, including the alignment of the phase fronts with respect to the wind shear, the motion of the ripples, and the horizontal separation of the ripple fronts were consistent with their attribution to Kelvin-Helmholtz billows. It is likely that the dynamical instability was initiated by the superposition of the background wind shear and the shear induced by the wave. The wind shear, the mean wind acceleration, and the propagation of the breaking wave were found to be in the same direction, suggesting that wave-mean flow interactions contributed significantly to the generation of the strong (>40 m/s/km) wind shear and instability.

A sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to observe a large fraction of the northern sky to hunt for γ-ray sources at energies above 10 TeV. Even though the detector construction is still underway, half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the KM2A data analysis pipeline and the first observation of the Crab Nebula, a standard candle in very high energy γ-ray astronomy. We detect γ-ray signals from the Crab Nebula in both energy ranges of 10 100 TeV and 100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance, including angular resolution, pointing accuracy and cosmic-ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE = (1.13 0.05 0.08 ) 10 (E/20 TeV) cm s TeV . It is consistent with previous measurements by other experiments. This opens a new window of γ-ray astronomy above 0.1 PeV through which new ultrahigh-energy γ-ray phenomena, such as cosmic PeVatrons, might be discovered.