Why are non-radial solar eruptions less frequent than radial ones?

Abstract

Coronal mass ejections from the Sun are not always initiated along a radial trajectory; such non-radial eruptions are well known to be caused by the asymmetry of the pre-eruption magnetic configuration, which is primarily determined by the uneven distribution of magnetic flux at the photosphere. Therefore, it is naturally expected that the non-radial eruptions should be rather common, at least as frequent as radial ones, given the typically asymmetrical nature of photospheric magnetic flux. However, statistical studies have shown that only a small fraction of eruptions display non-radial behavior. Here we aim to shed light on this counterintuitive fact, based on a series of numerical simulations of eruption initiation in bipolar fields with different asymmetric flux distributions. As the asymmetry of the flux distribution increases, the eruption direction tends to deviate further away from the radial path, accompanied by a decrease in eruption intensity. In case of too strong asymmetry, no eruption is triggered, indicating that excessively inclined eruptions cannot occur. Therefore, our simulations suggest that asymmetry plays a negative role in producing eruption, potentially explaining the lesser frequency of non-radial solar eruptions compared to radial ones. With increasing asymmetry, the degree of non-potentiality the field can attain is reduced. Consequently, the intensity of the pre-eruption current sheet decreases, and reconnection becomes less efficient, resulting in weaker eruptions.