Response of the low ionosphere to X-ray and Lyman-[alpha] solar flare emissions

Using soft X-ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high-cadence Lyman-[alpha] observations from the Large Yield Radiometer (LYRA) onboard the Project for On-Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium-size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub-ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman-[alpha] radiation excesses during the flares. Indeed, Lyman-[alpha] excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman-[alpha] emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman-[alpha] variations along the solar cycle, they cannot be used to scale explosive Lyman-[alpha] emission during flares. Key Points Excesses of flare Ly-a emissions are below the VLF technique sensitivity Quiescent Ly-a can be tracked using VLF waves, transient flare excesses cannot Observed disturbances are in the expected range of solar activity conditions