Observations of counter-propagating Alfvénic and compressive fluctuations in the chromosphere

Recent observations have found that chromospheric spicules behave like Alfvénic fluctuations. Low-frequency Alfvén waves are predicted to partially reflect in the transition region that has a gradient in the Alfvén speed, thereby producing inward Alfvén waves, which may interact nonlinearly with outward Alfvén waves to generate Alfvénic turbulence. However, the signature of Alfvénic turbulence in the chromosphere has not yet been quantitatively analyzed with observations. Here we analyze some characteristics related to Alfvénic turbulence with the observations from Hinode/SOT. We decompose the height-time diagram of the transverse oscillations to separate the outward and inward propagating Alfvénic-like signals. The counter-propagating waves are found to have similar amplitude, period and phase speed, suggesting a state having an approximate balance in bi-directional energy fluxes. Counter-propagation of intensity oscillation with lower propagation speed is also presented, probably indicating the presence of slow mode waves. Moreover, we attempt to estimate the Elsässer spectra of the chromospheric turbulence for the first time. The relative fluctuations in the magnetic field may be measured as the local slope of wave-like shapes in spicules. The resulting low-frequency Elsässer power spectra look similar to each other without showing a dominant population, which confirms these counter-propagating low-frequency Alfvénic waves are in a state of balanced flux. These observational results are believed to help us better understand the nature of chromospheric turbulence as well as chromospheric heating.