Observation of quasi-periodic solar radio bursts associated

with propagating fast-mode waves

by C. R. Goddard et al.*

2017-01-03Solar Radio Science Highlights

Flaring activity on the Sun triggers waves and oscillations in the solar corona. The study of these waves and oscillations allows comparisons to magnetohydrodynamic (MHD) theory and modelling to be made, and seismological inversions based on this comparison allow local plasma parameters to be measured indirectly (e.g. De Moortel & Nakariakov 2012).

Simultaneous EUV imaging and radio observations make it possible to study how these waves and oscillations can produce or modify radio emission from the coronal plasma. One of the most intensively studied examples of this are type II radio bursts, which are observed during coronal mass ejections (CMEs) (e.g. Pick & Vilmer 2008). Coronal type II radio bursts are usually seen as two locally-parallel emission lanes on solar radio spectrograms with an instant frequency ratio of approximately 2, drifting from high to low frequencies over time. It is generally accepted that this radio emission is a result of plasma wave excitation at fronts of MHD shock waves propagating upwards through the corona. The lower and higher frequency lanes are thought to be emission at the fundamental and second harmonic of local plasma frequency, respectively

We analysed an individual flaring event, SOL2014-11-03T22:15, the subsequent CME and wave activity and the associated type II burst. We found evidence that a series of quasi-periodic ‘sparks’ in the radio spectra are linked to disturbances seen in the low corona in the EUV band. This is the first observation which links quasi-periodic fast waves of the EUV intensity to quasi-periodic features in radio spectra.

Figure

1. A schematic synopsis of the event. A flare occurs which is followed

by a CME comprising of the leading edge or EUV wave (green) and the main

CME plasmoid (blue). A funnel structure (red) within the active region

is seen to host a series of rapidly propagating quasi-periodic waves. A

brightening is observed at the base of this structure and is interpreted

as a reconnection site where the waves are impulsively excited. After a

certain delay periodic radio sparks are observed, which occur at an

estimated height consistent with the leading feature of the CME, and a

periodicity consistent with the fast wave period.

Periodic signatures in radio and EUV emission

Figure 2. Left: Learmonth (top) and

BIRS (bottom) radio spectra in the ranges 25-170 MHz and 5-50 MHz

respectively. Four regions of enhanced emission are indicated by R1-R4.

Right: A zoomed region of the Learmonth spectra highlighting the radio

sparks as well as the three fundamental and harmonic emission lanes

of the type II burst.

A series of periodic ‘radio sparks’

(finite-bandwidth, short-duration isolated radio features) were detected

in both the Learmonth and BIRS dynamic spectra. They preceded a

split-band type II burst consisting of three fundamental lanes and their

second harmonic counterparts (Figure 2). Four radio sparks were

detected with a period of 1.78±0.04 min, with the first spark occurring

at 22:33 UTC. Their central frequencies drift from high to low

frequencies in the same manner as the type II lanes.

EUV imaging from SDO/AIA identified a

series of quasi-periodic rapidly propagating enhancements (Figure 3),

which we interpret as a fast wave train. These propagate upwards into

the corona along a guiding plasma structure. At least five enhancements

were detected, with a period of 1.7 ±0.2 min, starting at 22:28 UTC.

These waves occur after part of the CME plasmoid has interacted with the

base of the plasma funnel, potentially resulting in magnetic

reconnection in that region.

Figure

3. Left: Two running-difference snapshots of the periodic fast waves

detected in the EUV images. The vertical blue lines indicate the

position of the wave front. Right: An intensity time series extracted at

one position along the path of the fast waves. The periodic

enhancements are labelled as E1-E5. Below is the wavelet spectra of the

time-series, showing the main periodicity of just under 2 mins.

Unfortunately, no spatially-resolved

observations of the radio sources are available for this event,

therefore our analysis was restricted to the radio spectra. From the

central frequency of the radio sparks the local electron density at the

emission location is calculated for each. The electron density

calculated by an empirical formula, was used to obtain the height above

the surface at which the emission occurs, and the propagation speed of

the radio emission location. The emission heights derived coincide

roughly with the forward-projected location of the leading edge of the

CME. The inferred propagation speed of 630 km s⁻¹, is comparable to the measured speed of the CME leading edge, 500 km s⁻¹, and the speeds derived from the drifting of the type II lanes.

Physical Scenario

Due to the matching periods, we consider

the radio sparks to be caused by the periodic fast waves observed in

the EUV band. This is supported by the time offset between the

observations, due to the time taken for the fast waves to reach the

height at which the radio emission is generated. The height of the radio

source and its speed approximately match the height and velocity of the

leading CME feature.

We interpret our observations with the

following physical scenario (Figure 1). A series of fast waves are

produced in the active region during a flare, during an observed

magnetic reconnection event triggered by the CME. The waves propagate

upwards along a funnel plasma structure, and interact with the CME

leading edge, or some associated disturbance that propagates slower than

the fast wave train. This results in the acceleration of electrons, the

bump-on-tail instability, and emission of radio waves with the

frequency corresponding to the local electron plasma frequency,

appearing as quasi-periodic sparks in the radio spectrograph. However,

proper theoretical modelling of the process is needed.

Summary

The period of a series of radio sparks

which precede a type II burst, 1.78±0.04 min, matches the period of a

fast wave train observed at 171 Å, 1.7 ±0.2 min. The inferred speed of

the emission location of the radio sparks, 630 km s⁻¹, is comparable to the measured speed of the CME leading edge, 500 km s⁻¹,

and the speeds derived from the drifting of the type II lanes. The

calculated height of the radio emission matches the observed location of

the CME leading edge. From the above evidence we propose that the radio

sparks are caused by the quasi-periodic fast waves, and the emission is

generated as they catch up and interact with the leading edge of the

CME (Figure 1).

Based on the paper:

Goddard,

C., Nisticò, G., Nakariakov, V., Zimovets, I., & White, S. (2016).

Observation of quasi-periodic solar radio bursts associated with

propagating fast-mode wavesAstronomy & Astrophysics, 594DOI:10.1051/0004-6361/201628478

References

[1] C. R. Goddard, G. Nisticò, V. M. Nakariakov, I. V. Zimovets and S. M. White,2016, A&A, 594, A96

[2] De Moortel, I. & Nakariakov, V. M.2012, Royal Society of London Philosophical Transactions Series A, 370, 3193

[3] Pick, M. & Vilmer, N.2008, A&A Rev., 16, 1

*Complete list of authors:C. R. Goddard, G. Nistico, V. M. Nakariakov, I. V. Zimovets, and S.M. White