Ultra-high Energy Air Showers Observed by ANITA-IV

ANITA Collaboration, P. W. Gorham, A. Ludwig, C. Deaconu, P. Cao, P. Allison, O. Banerjee, L. Batten, D. Bhattacharya, J. J. Beatty, K. Belov, W. R. Binns, V. Bugaev, C. H. Chen, P. Chen, Y. Chen, J. M. Clem, A. Connolly, L. Cremonesi, B. Dailey, P. F. Dowkontt, B. D. Fox, J. W. H. Gordon, C. Hast, B. Hill, S. Y. Hsu, J. J. Huang, K. Hughes, R. Hupe, M. H. Israel, T. C. Liu, L. Macchiarulo, S. Matsuno, K. McBride, C. Miki, J. Nam, C. J. Naudet, R. J. Nichol, A. Novikov, E. Oberla, M. Olmedo, R. Prechelt, S. Prohira, B. F. Rauch, J. M. Roberts, A. Romero-Wolf, B. Rotter, J. W. Russell, D. Saltzberg, D. Seckel, H. Schoorlemmer, J. Shiao, S. Stafford, J. Stockham, M. Stockham, B. Strutt, M. S. Sutherland, G. S. Varner, A. G. Vieregg, S. H. Wang, S. A. Wissel

Published 2020-08-13Version 1

ANITA's fourth long-duration balloon flight in late 2016 detected 29 cosmic-ray (CR)-like events on a background of $0.37^{+0.27}_{-0.17}$ anthropogenic events. CRs are mainly seen in reflection off the Antarctic ice sheets, creating a characteristic phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous non-inverted polarity, a $\sim 3.2\sigma$ fluctuation if due to background. All anomalous events are from locations near the horizon; ANITA-IV observed no steeply-upcoming anomalous events similar to the two such events seen in prior flights.