First imaging spectroscopy observations of solar drift pair bursts

Alexey Kuznetsov, Institute of Solar-Terrestrial Physics

Drift pairs are an unusual type of fine structures observed sometimes in dynamic spectra of metric solar radio emission. They appear as two identical short narrowband drifting bursts separated in time; both positive and negative frequency drifts are observed. Currently, there are no satisfactory explanation of this phenomenon. Observations with spatial resolution can be crucial to identify the formation mechanism of drift pair bursts. On 12 July 2017, the Low Frequency Array (LOFAR) observed a cluster of drift pair bursts in the frequency range of 30-70 MHz. Spectral imaging capabilities of this instrument for the first time have allowed us to study the emission sources of drift pair bursts in broad frequency range and to track their motion. Sources of two components of a drift pair propagate in the same direction along nearly the same trajectories; motion of the second burst source is delayed with respect to that of the first one. In general, negative or positive frequency drifts correspond to upward or downward motion, respectively, although the source trajectories can be complicated and non-radial. The drift pair bursts with positive and negative frequency drifts, as well as the associated broadband type III-like bursts, are produced in the same regions. The visible source velocities are of order of a few tens thousand km/s. The source sizes are not resolved with LOFAR, which indicates very compact emission sources. Our observations rule out all existing models of drift pair bursts; we discuss the key issues that need to be addressed. The broadband bursts observed simultaneously with the drift pairs differ in some aspects from common type III bursts and may represent a separate type of emission.