The Disappearing Solar Filament of 2013 September 29 and Its Large Associated Proton Event: Implications for Particle Acceleration at the Sun

We present observations of a notable example of a disappearing solar filament (DSF) on 2013 September 29 that was associated with a large solar proton event (SPE) and discuss this event in the context of four recent studies that compare flare and SPE size parameters. The DSF-associated flare was characterized by weak radio and soft X-ray emissions and a low reconnection flux. It was accompanied by a fast coronal mass ejection (CME) and a decametric-hectometric type II burst. We assembled a list of eight such events that are outliers in plots of SPE versus flare size parameters. These events were characterized by weak magnetic field source regions (predominantly DSFs but including one case of a transequatorial loop and another of a decaying active region), fast CMEs, type II bursts with low starting frequencies, high proton yields (ratio of proton intensity to 1 MHz radio fluence), and low high-energy Fe/O ratios. The last of these attributes suggests quasi-parallel shock acceleration. The relationship between SPE and flare size parameters in large (gradual), well-connected proton events can be illustrated by a schematic diagram with three principal regions: (1) a DSF zone of weak flares and large SPEs, (2) a big flare syndrome main sequence of loosely con elated flare and SPE parameters, and (3) a zone of moderate to large flares with no SPEs. The existence of regions 1 and 3 argues against a significant role for flares in large proton events: region 1 implies that flares are not necessary for such SPEs, and region 3 indicates that they are not sufficient.