In X-ray literature "prompt event" is defined as an event where the estimated release time of the electrons that are later seen at 1 AU agrees with the hard X-ray flare peak time within the uncertainty of a few minutes. (From the SEP point of view this is called "prompt impulsive". For delayed impulsive see below and next section.)
Also, the spectral shapes of the in situ observed electron spectrum and hard X-ray photo spectrum are statistically correlated. This suggests that a common accelerator produces both the hard X-ray emitting electrons and the escaping electrons (Droege 1996, Krucker et al. 2007). However, the observed correlation does not agree with the prediction of the thick-target model (Brown 1971) that would be expected for footpoint emission. This is currently not understood.
Hard X-ray imaging of the solar source region of prompt events often reveals three sources. Frequently jets are observed in EUV and soft X-rays that appear to emanate from the hard X-ray sources. This geometry is similar to that of interchange reconnection models where closed field lines reconnect with open fields. The jet indicates the direction of open field lines along which electrons escape. Prompt events are well correlated with the occurrence of 3He rich SEP events.
However, arrival time studies of ions seen at 1 AU suggest a delayed release of the ions relative to electrons, at least when assuming scatter-free transport. This is rather puzzling as it suggests a different accelerator for electrons and ions despite the closely connected occurrence. However, electrons and ions could still be released simultaneously with propagation effects explaining the observed delays.
This objective is about exploring if the comparative delay is real or due to propagation effects. It should be studied in different heliocentric distances and hopefully near the perihelion in order to better establish the nature of the flare region (with STIX & EUI).
Complete set of observations is required: STIX, EUI, RPW + ground-based radio observations (good to have). That will allow studying the magnetic structures along which flare-accelerated particles escape into interplanetary space.