First imaging spectroscopy observations of solar drift pair bursts

Abstract

Drift pairs are an unusual and puzzling type of fine structure sometimes observed in dynamic spectra of solar radio emission. They appear as two identical short narrowband drifting stripes separated in time; both positive and negative frequency drifts are observed. Currently, due to the lack of imaging observations, there is no satisfactory explanation for this phenomenon. Using the Low Frequency Array (LOFAR), we report unique observations of a cluster of drift pair bursts in the frequency range of 30 − 70 MHz made on 12 July 2017. Spectral imaging capabilities of the instrument have allowed us for the first time to resolve the temporal and frequency evolution of the source locations and sizes at a fixed frequency and along the drifting pair components. Sources of two components of a drift pair have been imaged and found to propagate in the same direction along nearly the same trajectories. Motion of the second component source is seen to be delayed in time with respect to that of the first one. The source trajectories can be complicated and non-radial; positive and negative frequency drifts correspond to opposite propagation directions. 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 variable from zero to a few 10 4 (up to ∼ 10 5 ) km s −1 , which often exceeds the velocities inferred from the drift rate (∼ 10 4 km s −1 ). The visible source sizes are of about 10 ′ − 18 ′ ; they are more compact than typical type III sources at the same frequencies. The existing models of drift pair bursts cannot adequately explain the observed features. We discuss the key issues that need to be addressed, and in particular the anisotropic scattering of the radio waves. 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.

First Frequency-time-resolved Imaging Spectroscopy Observations of Solar Radio Spikes

“…However, the duration of spikes near 30 MHz are shorter up to a factor of ∼20. The observed spike areas decrease with increasing frequency from 297 to 122 arcmin 2 between 30 and 45 MHz (Figure 4(f)) in a similar manner to drift-pair bursts (Kuznetsov & Kontar 2019), and is approximated with a power law A ∼ f −γ where γ = 2.3 and 1.9 for spikes and striae, respectively. The large uncertainties are due to their low intensities.…”

Solar Radio Spikes and Type IIIb Striae Manifestations of Subsecond Electron Acceleration Triggered by a Coronal Mass Ejection

“…The simulations in Kuznetsov et al (2020) show that sources observed away from the disk center can present superluminal centroid velocities, as was observed for striae (Zhang et al 2020) and spikes (Clarkson et al 2021). If the source motion is nonradial, then sources closer to the disk center can also present speeds near c, as observed by drift-pair bursts (Kuznetsov & Kontar 2019).…”

First Frequency-Time-Resolved Imaging Spectroscopy Observations of Solar Radio Spikes

“…In comparison, the bulk Type IIIb structures shown in Figure 2(b,e) have drift rates of −3.14±0.21 and −2.31±0.26 MHz/s, respectively, measured by a linear fit to the peak flux position at the central frequency of each striae.The observed spike FWHM area at the peak of the central frequency is 202.19 ± 16.3 arcmin 2 at 34.5 MHz (Figure3(d)). The observed spike areas decrease with increasing frequency from 297 to 122 arcmin 2 between 30 to 45 MHz (Figure4(f)) in a similar manner to driftpair bursts(Kuznetsov & Kontar 2019), and is approximated with a power law A ∼ f −γ where γ = 2.3 and 1.9 for spikes and striae, respectively. The large uncertainties are due to their low intensities.…”