| Summary: | We present results of three-component VLF measurements of unusual daytime tweek atmospherics, which we\nhave recorded in a favorable electromagnetic environment on the summit of La Grande Montagne (1028 m,\n43.9410N, 5.4836E), Plateau d’Albion, France in January 2015. The observed daytime tweek atmospherics have\na clear frequency dispersion which we are able to analyze above the first ionospheric cutoff. We estimate model\nparameters of the characteristic frequency dispersion as a function of time. Using the obtained parameters we are\nable to evaluate the tweek reflection heights and propagation distances from their source lightning discharges.\nThe three-component measurement allows us to estimate the arrival direction. The source lightning strokes of\nobserved unusual daytime tweeks were found to originate in a sequence of severe winter thunderstorms which hit\nIreland, the UK, Norway, Denmark, Germany, and Poland in January 2015. Based on our analysis we show that a\nthunderstorm occurring at higher latitudes of northern and north-western Europe during winter months is an ideal\ncandidate for the source of tweeks, which could be observed unexpectedly during the day.
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