Flare Pulsation and the Heliosphere

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Nugget
Number: 411
1st Author: Brendan CLARKE
2nd Author: et al.
Published: 5 July 2021
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Contents

Introduction

Solar flares are the largest explosions in our solar system, being capable of releasing up to 1033 ergs of energy over the course of mere minutes. Despite significant progress being made towards our understanding of the energy release and transport mechanisms in flares, they remain a contemporary topic of solar physics research. This is due to the many aspects of the phenomenon that remain unknown, preventing us from building a complete model of flares.

An important observational feature of solar flare emission is the presence of quasi-periodic pulsations (QPPs). QPPs are defined as intensity modulations in the flare electromagnetic radiation as a function of time. These modulations have been found to have characteristic periodicities that range from < 1 s up to several minutes.

QPPs were first associated with the impulsive phase of flares and observed in the hard X-ray (HXR) and radio wavebands (e.g. Ref. [1]). They have more recently been observed within the thermal component of flares and have been identified to persist into the decay phase (see RHESSI Nuggets No. 347 and No. 262. Hence studies that explore signatures of QPPs across the entire electromagnetic spectrum are important to help us build a picture of their underlying driver. Recent statistical studies have determined that at least half of the X-class flares in the last solar cycle contained QPPs (Ref. [2]). Despite this near-ubiquity of QPPs in flares, the underlying mechanism that produces them remains unknown.

Several models have been proposed as explanations for the presence of QPPs in solar and stellar flares, which are typically categorized as oscillatory or self-oscillatory processes (see Nugget No. 379, for example).

QPPs from the Chromosphere into interplanetary space

In Ref. [3] we have investigated the GOES class M3.7 flare SOL2015-11-04. This flare exhibited pronounced QPPs across a very broad band of emission wavelengths that we analysed using imaging and time-series analysis. We identify QPPs in the X-ray, low-frequency radio, and extreme ultraviolet (EUV) emission from the flare, determine their periodicities, and locate the region of the flare site from which the QPPs originate. Note that these signatures range over essentially the whole volume of the heliosphere

The QPPs in this event appear to be consequence of intermittent particle acceleration, likely due to bursty magnetic reconnection. At the energy release site of the flare, accelerated electrons precipitate toward the chromosphere to produce the X-ray and EUV pulsations. The electrons accelerated towards interplanetary space escape along nearby open magnetic field lines, resulting in low-frequency radio pulses in the form of type III radio bursts.

Figure 1 illustrates the multi-wavelength nature of the QPPs identified and analysed in this work. Figure 1(a) shows the dynamic spectrum of radio emission containing a sequence of type III radio bursts, and Figure 1(b) shows the EUV, soft X-ray (SXR), and HXR light curves in which we identify seven distinct QPPs. The EUV light curves were extracted from the QPP source region we identified via imaging analysis as shown in Figure 2, and the evident match in timing solidly links the low-coronal emissions to fast (0.3 c) electrons at some 16 solar radii.

Figure 1: Multi-wavelength views of QPPs from SOL2015-11-04.

Locating the QPP source

The RHESSI imaging observations directly identify the energy-release sites in the chromosphere for the first two pulsations, as shown in Figure 2. Integration of the EUV images from [AIA 171Å] yields a best match with kernel K1, consistent with energy injection by non-thermal electrons producing both bremsstrahlung hard X-rays and footpoint heating, according to the well-explored "thick target" picture. Here the QPP cycles reflect a quasi-periodic variation in the acceleration of the energy-transporting electrons.

Figure 2: Spatial analysis reveals the QPP origins: left, an AIA 171Å image with RHESSI 35-70 keV image contours in red. The light curves on the right compare hard X-rays (full Sun) with EUV from two of the kernels marked in the image, showing K1 to be the main source region.

A Natural Escape Route

The direct association of electrons into the deep heliosphere, as shown by the type III radio bursts (Figure 1), makes this QPP event most interesting. How are these closely synchronized populations of energetic electrons related?

We find the QPP source region, kernel K1, to be associated with open magnetic field lines, as identified in the potential field source surface (PFSS) extrapolation shown in Figure 3. This magnetic-field geometry could provide a path for the escape of some of the same electrons responsible for the hard X-ray bremsstrahlung; these then form the stream producing the radio emission.

Figure 3: PFSS extrapolation of the photospheric magnetic field, showing the geometry of the magnetic field lines of the flaring region overlaid on the AIA 171 Å image.

Interpretation and Conclusions

We interpret the QPPs identified in this flare in terms of pulsed electron acceleration caused by time-dependent intermittent reconnection.

In Figure 4 we show a cartoon scenario of the flare site to illustrate how the QPP sources are related to the magnetic field configuration. Following each burst of electron acceleration, those that escape upward along the open magnetic field lines result in the type III QPPs, and those that travel along closed lines precipitate in the chromosphere to cause the QPPs we observe in hard X-ray and EUV.

This work provides new evidence that oscillatory or time-dependent reconnection can naturally generate QPPs, providing an explanation for their presence across the entire spatial range of flaring emission. It also shines light onto the nature of energy release in flares and provides new insight into how QPPs may be localized to specific regions within flare sites. Future work to investigate the details and conditions necessary for the triggering of magnetic reconnection in this bursty fashion is required.

Figure 4: Cartoon of the flaring region illustrating the likely mechanism through which we observe the episodic particle acceleration resulting in QPPs in EUV, radio, SXR, and HXR.

Co-authors

Laura A. Hayes, Peter T. Gallagher, Shane A. Maloney & Eoin P. Carley.

References

[1] "Sixteen-Second Periodic Pulsations Observed in the Correlated Microwave and Energetic X-Ray Emission from a Solar Flare"

[2] "Statistical Study of GOES X-Ray Quasi-periodic Pulsations in Solar Flares"

[3] "Quasi-periodic Particle Acceleration in a Solar Flare"

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