Hard X-ray Pulsations in Flares

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Revision as of 09:38, 15 May 2009

Introduction

For several decades astronomers have observed curious oscillations and pulsations in the emission from solar flares. Collectively these are usually known as "quasi-periodic pulsations" or QPP, although their properties vary considerably. The periods of oscillation, for example, can vary from fractions of a second up to several minutes. Understandably then, fully explaining these pulsations remains an active study in solar physics. Fortunately, recent advances in instrument technology, including the launch of the RHESSI satellite, have allowed us to probe these phenomena in more detail.

The emphasis in recent years has been on spatially resolved study of QPP events, however in many cases this is simply not possible. A prime example of this is the study of stellar oscillations, however even in the solar context it is rare to achieve good spatial resolution of QPP events. As we will show in this nugget, this should not be taken as an insurmountable barrier to studying interesting QPP events.

Multiple periods in the 3rd July 2002 flare

On 3rd July 2002 an X1.5-class flare exhibiting QPP occured and was observed at a number of wavelengths, including by the Nobeyama Radioheliograph in the microwave band and by RHESSI in X-rays. The pulsations in this event were investigated in detail by Inglis and Nakariakov 2009.

Time series data of the 3rd July 2002 flare at 17 GHz.

Time series data of the 3rd July 2002 flare from the RHESSI satellite.

To study the periods of oscillation, the Scargle periodogram and the Morlet wavelet were utilised. The periodogram method is only capable of giving us global information about any periods present in observational data, but more positively it features a robust method for calculating the confidence level of spectral peaks.

The periodogram was applied to the RHESSI time series data between 02:11 and 02:13 UT. To obtain a good compromise between spectral resolution and the signal-to-noise ratio, the RHESSI counts were binned in 2s intervals, while a bandpass filter was employed to remove the effects of RHESSI spacecraft rotation. From this data we found two significant periods, at 28 s and 18 s.

Periodogram of X-ray data showing the periods of 28s and 18s.

These two periods were also found in radio data from the Nobeyama Radioheliograph, as was a third period of 11s. From the periodogram alone though, it is not possible to establish whether these periods are present throughout the event, or indeed to test how stable they are as a function of time. Fortunately, a convenient tool for such tests may be found in the form of wavelets.

Morlet wavelet showing the persistence of all three periods throughout the flare.

The original wavelet of the flaring data shows that there are significant periods consistent with the periodogram. As a further test we take wavelets in three frequency bands corresponding to the detected periods. In this way we can show that all three periods exist for the duration of the QPP event.

Period ratios

Using the RHESSI satellite in tandem with radio data, we were able to detect three significant periods in an X-class flare from 3rd July 2002. These were 28s, 18s and 12s respectively.

Crucially, the ratios of these periods provides us with information about the driving mechanism for the QPP. We can quickly see that P2/P1 = 0.64 and P3/P1 = 0.43. For a simple, non-dispersive resonator we would expect ratios of ~ 0.5 and ~0.33, so why our the measured values so different?

The answer is dispersion. MHD wave modes are known to deviate significantly from the ideal PN/P1 pattern, in particular the sausage and kink modes. Conversely, it is difficult to see why other pulsation mechanisms (i.e. magnetic reconnection) would result in this deviation from the ideal ratio. Due to the lack of thermal modulation during this flare, a kink mode - which is incommpressible - would seem the likeliest candidate.

Conclusions

The RHESSI satellite is an important tool in the study of flaring QPP, as it allows us to study the X-ray band with excellent time resolution. In this nugget, we showed that it is invaluable in not only detecting multiple periods, but also in interpreting the result. For the flare of 3rd July 2002, the lack of thermal oscillation leads us to the conclusion that an MHD kink mode is responsible for the observed oscillations.