Evidence of wave harmonics in a brightened magnetic network observed from Hinode/EIS

Abstract

We study intensity oscillations in the upper chromosphere to search acoustic wave harmonics in a lowatmospheric solar cavity associated with a brightened magnetic network near the south pole of the Sun. We use a wavelet tool to produce power spectrum of He II 256.32 Å intensity oscillation as observed by EUV Imaging Spectrometer (EIS) onboard Hinode spacecraft. We detect a periodicity of ~14.33 min with a probability of 99%–100%. Using wavelet filtering and reconstruction techniques, we detect other statistically significant periodicities of ~8.51 min and ~5.51 min with the probabilities 98% and 97% respectively. These observed periodicities provide the first possible signature of acoustic wave harmonics in a low-atmospheric cavity-canopy system. The ratio of the period of first harmonic (P₁ = 14.33 min) to the period of second harmonic (P₂ = 8.51 min) is 1.68. The difference of this period ratio from 2.0 may be the first signature of a density stratification in low-atmospheric solar cavities, complementing such period ratio differences of acoustic wave harmonics have been reported previously in the solar loops as a diagnostic of their density stratification. We seismologically estimate the density scale height as ~0.57 Mm in the cavity–canopy loop interface. This scale height is low compared to the hydrostatic scale heights estimated at wide range of temperatures that correspond to the considerable formation of He II ion in the lower solar atmosphere. The departure of density scale height estimated by the principle of MHD seismology from the values of hydrostatic scale heights estimated at wide temperature range (4.2 < Log Te < 5.2), probably indicates non-hydrostatic conditions above the cavity–canopy interface.