The first observation of later arrival of more energetic particles during solar eruptions observed by the Solar Orbiter mission

Velocity dispersion (VD) is a common feature of solar energetic particle (SEP) events, in which particles with higher kinetic energies (and thus higher speeds) arrive earlier than those with lower energies. However, recent observations by the Solar Orbiter (SolO) mission have identified a series of SEP events with a previously not reported feature in which particles with higher energies arrive later, apparently exhibiting inverse velocity dispersion (IVD). We analyse several such SEP events and suggest two explanations for this effect: 1) changes in magnetic connectivity between the observer and the outward-propagating shock which continuously accelerates particles; 2) the acceleration time for higher-energy particles is longer during the diffusive shock acceleration process so that they are released later. These observations provided a first opportunity to quantify the energy-dependent release process which greatly advances our understanding of the particle acceleration process.