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ALF radio bursts: early signatures of plasma waves in solar flares

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Solar flares and coronal mass ejections occurring in the solar atmosphere have a profound impact on the space weather near the Earth. While observing solar radio emissions with the large Ukrainian radio telescope URAN-2, BIRA-IASB scientists—in collaboration with Ukrainian colleagues—have identified a new type of solar radio burst associated with solar flares: ALF bursts. Observations of ALF bursts provide unique information on the early stages of solar flares. Theoretical modelling of ALF bursts allowed us to predict kinetic Alfven waves and their amplitudes in the flaring solar atmosphere.
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ALF radio bursts: observations

Solar radio observations provide valuable information on the energetic phenomena in the solar atmosphere and are used for their early diagnostics. In particular, solar type III and II radio bursts manifest solar flares and coronal mass ejections, respectively. These energetic events affect the radiation state of the near-Earth space and can cause severe geomagnetic storms.

Using large radio telescope URAN-2 (Ukraine), we study ALF radio bursts – faint solar radio bursts in the frequency range 20-30 MHz. ALF bursts were not visible before because of lower sensitivity and resolution of previous radio telescopes. Similar to type III burst, the ALF bursts occur in close association with solar flares.

In Figure 2 we show the radio spectrogram of a flare-related event recorded 18 April 2014, in which several ALF burst are identified (they are encircled by dashed lines). Measuring polarization of radio emission (bottom panel of Fig. 1), URAN-2 allows to identify very weak ALF bursts not clearly visible in the emission power (upper panel of Fig. 1).

The measured ALF parameters (frequency drift rates δf/δt ∼ -0.1 MHz/s, relative frequency bandwidths δf/f ≳ 0.01, and burst durations δt ∼ 3 s) clearly set ALF bursts apart from the previously known burst types.

ALF radio bursts: theory

The fact that the emission sources of ALF bursts propagate with near-Alfvén velocities suggests a generation mechanism for them involving Alfvén waves. To make the ALF bursts clearly visible, the responsible Alfvén waves have to perturb plasma density, which implies kinetic effects operating throughout. These, and other properties of ALF bursts deduced from radio spectrograms, receive natural explanation in the model incorporating intermittent kinetic Alfvén waves (KAWs) propagating upward in the solar corona at heliocentric distances ∼ 2 solar radii. Such KAWs can be generated by magnetic reconnection in solar flares.

The key element of the proposed generation mechanism is that the KAWs can sweep and accumulate Langmuir waves (these Langmuir waves were excited earlier by the electron beams that are faster than KAWs). The main steps of the proposed generation scenario are presented and explained schematically in Figure 3.  

Our theory explains not only frequency drift rates of ALF bursts, but also their instantaneous frequency bandwidths, durations, and life times. On the other hand, it predicts properties of KAWs in the flaring solar atmosphere.

All the theoretically predicted KAW parameters, magnetic amplitudes Bk/B0∼ 0.02 , angular frequencies ωk ≲ 1 rad s⁻¹, and perpendicular wavenumbers k⊥ρi ∼ 1 (B0 is the background coronal magnetic field, ρi is the ion gyro radius) are reasonable and compatible with observations. These KAWs are not merely sources for ALF radio bursts, but also contribute to the processes of energy release and transport in the solar atmosphere.

The theoretically constrained wave and plasma parameters in the solar corona above active regions can be used in modeling coronal mass ejections and solar energetic particles.

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Figure 2 Flare-related event with multiple ALF bursts (group of ALF bursts is encircled by the dashed lines). Top panel: power of radio emission. Bottom panel: polarization of radio emission. Some ALFs are better visible in the polarization spectrogram.
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Figure 3 caption (legend)
Figure 3 Generation scenario for ALF bursts: (i) magnetic reconnection in flares accelerates electron beams and generates KAWs; (ii) electron beams excite Langmuir waves; (iii) KAWs capture and amplify Langmuir waves; (iv) clusters of amplified Langmuir waves generate ALF radio bursts by the plasma emission mechanism.
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