A miniaturized scintillator ("microscintillator"), was flown on a meteorological radiosonde to observe energetic particles in the lower atmosphere. A PiN diode was used to measure the light from the microscintillator to count rates and energies of ionizing radiation from the ground up to the stratosphere (32 km). The flight, over the southern UK on August 27, 2018, occurred during a geomagnetic storm. Low‐energy (50–150 keV) particles in the stratosphere were detected in addition to the usual signal from higher‐energy cosmic rays. Unusually for these miniaturized radiosonde systems, which are designed to be disposable, the payload was retrieved. This allowed for re‐calibration of the microscintillator, which confirmed the low‐energy particle detection. Laboratory tests excluded thermal effects on the microscintillator instrument as the origin of the signal. Data from the NOAA POES spacecraft offer the explanation that the microscintillator detected bremstrahhlung X‐rays from energetic electron precipitation (EEP). EEP events may affect weather and climate through a range of physical mechanisms, and this midlatitude observation, well away from the auroral oval, extends the region over which meteorological effects of EEPs need to be assessed. Our findings underline the value of balloon measurements in providing rapid response to space weather events. The energy‐discriminating and altitude‐sensitive capability of the microscintillator augments spacecraft observations from below. Plain Language Summary: We describe results from flying a miniaturized radiation detector up to 32 km altitude on a weather balloon, in a region of the atmosphere called the stratosphere. The radiation detector was able to measure the energy of the particles it detects in addition to counting them. As well as the counts we expected from high‐energy particles entering the atmosphere from space, we also unexpectedly measured some lower‐energy particles. Normally the weather balloons are not retrieved, but for this flight we were able to retrieve it and test it again in the lab, confirming that the low‐energy particle measurements were real and not due to a problem with the equipment. These low‐energy particles are likely to be X‐rays which can sometimes enter the atmosphere during periods of enhanced space weather activity. The electrons that are known to produce these X rays were present in enhanced concentrations in near‐Earth space, as measured from above by the NOAA POES spacecraft: our measurements are from below. We believe this is the first detection of stratospheric X‐rays well away from the polar regions. X‐rays affect atmospheric chemistry, which, in turn, to a lesser extent, affects the weather. Key Points: Energetic particles in the atmosphere have been observed with a "microscintillator" detector flown on a weather balloonLow‐energy ionizing radiation was unexpectedly observed in the stratosphere. Instrument retrieval and recalibration confirmed the findingsDetection of Bremsstrahlung X rays at unusually low latitudes from a geomagnetic storm is corroborated by spacecraft observations [ABSTRACT FROM AUTHOR]