Soft X-ray observations of large-scale coronal active region brightenings

Abstract

One-hundred fifty-six large-scale enhancements of X-ray emission from solar active regions were studied on full-disk filterheliograms to determine characteristic morphology and expansion rates for heated coronal plasma. The X-ray photographs were compared with Hα observations of flares, sudden filament disappearances, sprays and loop prominence systems (LPS). Eighty-one percent of the X-ray events were correlated with Hα filament activity, but only forty-four percent were correlated with reported Hα flares. The X-ray enhancements took the form of loops or arcades of loops ranging in length from 60 000 km to 520 000 km and averaging 15 000 km in width. Lifetimes ranged from ≥3 hr to >24 hr. Event frequency was ∼1.4 per day. X-ray loop arcades evolved from sharp-edged clouds in cavities vacated by rising Hα filaments. Expansion velocities of the loops were ∼50 km s^-1 immediately after excitation and 1–10 km s^-1 several hours later. These long-lived loop arcades are identified with LPS, and it is suggested that the loops outlined magnetic fields which were reconnecting after filament eruptions. Another class of X-ray enhanced loops stretched outside active regions and accompanied sprays or lateral filament ejections. Hα brightenings occurred where these loops intersected the chromosphere. Inferred excitation velocities along the loops ranged between ∼300 and 1200 km s^-1. It is suggested that these loops outlined closed magnetic fields guiding slow mode shocks from flares and filament eruptions.