Propagating Waves and Magnetohydrodynamic Mode Coupling in the Quiet‐Sun Network

Mullard Space Science Laboratory ,Dorking, Surrey RH5 6NT, England, UKAbstractHigh-cadence, multiwavelength optical observations,taken during two separate observing runs at the NationalSolar Observatory/Sacramento Peak, are presented here.A total of fteen network bright points have been stud-ied at diering atmospheric heights, using images takenin narrow wavebands. Wavelet analysis was used to studywavepackets,andidentifytravelingmagnetohydrodynamic(MHD) waves. Wave speeds were estimated through tem-poral cross-correlation of signals, in selected frequencybands of wavelet power, in each wavelength. Seven mode-coupling cases were identied, one in each of seven of theNBPs. MHD mode coupling is a viable mechanism fortransport of energy from the photosphere into the chro-mosphere, where subsequent deposition can contribute toatmospheric heating.Key words: Sun: oscillations – Sun: chromosphere – Sun:photosphere – Sun: magnetic elds1. Background theoryIt has been known for a long time that the Sun exhibits areversal in its temperature prole above the photosphere,which then increases throughout the chromosphere. How-ever, it is not known what mechanism heats this regionof the atmosphere. Any complete theory attempting toaddress this matter must manage to explain a number ofprocesses: (1) the generation of non-radiative energy; (2)the transportation from the source region to that which isto be heated; and (3) the deposition of this energy.One theory to explain the transportation of energyfrom the underlying photosphere into the chromospherehas been proposed by Kalkofen (1997). This states thatnon-dissipative, transverse-mode, magnetohydrodynamic(MHD) waves travel up magnetic ux tubes located atthe supergranular cell boundaries (network bright points– NBPs) into less dense regions. Consequently the wave ve-locity amplitude increases and enters a non-linear regime,enabling ecient transfer of transverse-mode energy tolongitudinal-mode waves at twice the transverse-mode fre-quency. These longitudinal waves may then heat the sur-rounding plasma either through shocks, since they arecompressible (Zhugzhda et al. 1995), or by driving dissi-pative Pedersen currents (Goodman 2000, 2004). As such,the concept of mode coupling may provide an explana-tion for point (2) mentioned above, and lead to suitablesituations to describe point (3).To address the generation of MHD waves in the un-derlying photosphere, Hasan & Kalkofen (1999) modelledthe generation of both transverse- and longitudinal-modeMHD waves in a thin ux tube through external granu-lar bueting. They found the energy ux of the transversemode to be an order of magnitude greater than that of thelongitudinal mode for typical NBP eld strengths. This in-dicates preferential transverse-mode generation from thephotosphere, providing a mechanism which satises point(1) above, and leads directly into the requirements formode coupling.2. ObservationsAll the data presented here were obtained with the DunnSolarTelescopeattheNationalSolarObservatory