Author: Samuel Grant & David Jess at Queen’s University Belfast

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Introduction

The complex processes that allow for the upper solar atmosphere to exhibit extra-ordinary temperatures remain a mystery. The majority of research into atmospheric heating has centred on the corona, which is super-heated to temperatures exceeding 1MK. However, a greater mystery lies in how the chromosphere can maintain a temperature of up to 10,000K, as its greater plasma density demands a far larger energy input than is necessary in the corona [1]. In the past, it was difficult to observe the dynamic chromosphere, thus prohibiting detailed studies of the region. However, with the advent of modern imaging and spectroscopic instruments (Hinode/ROSA/IBIS), the intricacies of the chromosphere and its fine-scale magnetic structures can now be revealed.

Magnetic regions in the solar atmosphere have long been proposed as conduits for the energy carried by magnetohydrodynamic (MHD) waves, which may be responsible for atmospheric heating. Recent studies have shown an abundance of energetic wave activity in the chromosphere [2], indicating that they may play a role in providing the necessary energy flux for heating. In particular, sausage mode waves exhibit high energy flux at chromospheric heights [3]. What remains to be observed are energetic wave trains travelling from their formation height into the chromosphere. Here we present the first observations of MHD sausage mode waves propagating from the solar surface into the chromosphere, with the energy flux of the waves decreasing with height and potentially providing energy to the heating processes of the chromosphere.

Observations and Analysis

Figure 1 displays an image of a magnetic pore in the 4 wavelength bands utilised by the ROSA imaging system, spanning from the photosphere to the chromosphere. The high spatial and temporal resolution of ROSA provides the optimal conditions to resolve the periodic intensity and area fluctuations associated with sausage modes, and a magnetic pore has already been observed as a viable conduit for sausage mode waves in the photosphere [5]. Using the large sample of pore pixels present in the time series, we found high power intensity oscillations within the pore at all wavelengths. Further to this, coherent wave power was seen to exist in adjacent wavebands, with phase differences between the wave signals indicating that these intensity oscillations were predominantly propagating upwards from the photosphere into the chromosphere. These intensity perturbations can be a direct signature of magneto-acoustic wave modes, thus the evolution of pore area in each bandpass was investigated to identify complementary signatures of sausage modes. The observed similarities in period, wave power and the phase differences of the intensity and area oscillations between bandpasses provides conclusive evidence that these coherent wave trains are signatures of upwardly propagating sausage mode waves. The waves could further be classified as slow modes by identifying in-phase fluctuations between the intensities and area perturbations in the same bandpasses, as theory predicts [6]. A full description of this work is presented in [6].

The energetics of these waves as they propagate can be derived for the first time thanks to a new technique [7]. By combining the oscillation amplitudes and wavelength characteristics for isolated periodicities with scaled plasma parameters (i.e density, temperature and magnetic field) taken from models for the pore and surrounding quiet Sun, the energy flux in each bandpass can be estimated. The amplitudes are used to define the phase speed of the waves at each bandpass height, which is then used in conjunction with the derived parameters to average the energy over a uniform flux tube with respect to the particular periodicities. Figure 2 shows the energetics of two periodicities and the clear wave damping experienced during their upward propagation, from up to 50,000 W/m² in the photosphere to just 100 W/m² in the high chromosphere.

Conclusions

The significance of these results in terms of the nature of the heated chromosphere is two-fold. Firstly, magnetic pores have been identified as a viable conduit for the propagation of energetic MHD waves that proceed to damp as they progress from the solar surface into the chromosphere. Secondly, only a fraction of the estimated energy loss of these waves needs to dissipate into heating in order to maintain the local plasma temperatures. Multiple dissipation methods are suggested in [6], such as mode conversion into energetically leaky modes, that would allow these observed waves to play a key part in providing a solution for the chromospheric heating problem.

With the advent of DKIST, high-resolution imaging will become more abundant, making the techniques used here more accurate in determining the viability of sausage modes as a means of transporting and dissipating energy as chromospheric heating. However, we also show that these waves will produce an analogous oscillation in the magnetic field. This allows for further study of sausage modes using spectro-polarimetric data and can provide a method for interpreting complex measurements and oscillations already seen in Stokes profiles.

References

• [1] – Withbroe and Noyes, 1977, ARA&A, 15, 363
• [2] – Jess et al, 2015, Space Sci. Rev., 14
• [3] – Morton et al, 2012, Nature Comms, 3, 1315
• [4] – Morton et al, 2011, ApJL, 729, L18
• [5] – Moreels, Goossens & Van Doorsselaere, 2013, A&A, 555, A75
• [6] – Grant et al, 2015, ApJ, 806, 132
• [7] – Moreels et al, 2015, A&A, 578, A60