A Study of Chromospheric Oscillations Using the SOHO and TRACE Spacecraft

Abstract

We analyze line and continuum time-series data of the solar atmosphere, with between 10 and 60 s cadence, using the MDI and SUMER instruments on the SOHO spacecraft and the UV bandpasses on the T RACE satellite. The co-aligned data sets sample spectral features formed from photosphere to the middle transition region, spanning Ðve decades in pressure, under quiet-Sun and plage conditions. We discuss power, phase di †erence, and coherence spectra, and examine data in the time domain. The observed photospheric and chromospheric oscillations are strongly coupled for frequencies between 2 and 8 mHz. Phase coherences decrease with increasing height, with only occasional periods and locations of observable coherence up to heights where transition region emission lines are formed. The middle chromosphere (in the SUMER continua) oscillates in several megameter (Mm) diameter coherent patches with power predominantly in the 5È7 mHz range. The T RACE data, formed in the upper photo-sphere, show smaller patterns superimposed on these large-scale oscillations, resulting (at least in part) from granulation. At the observed spatial scales, all the observed properties point to p-modes, especially the "" pseudomodes ÏÏ just above the acoustic cuto † frequency, as the dominant mode of the chromo-spheric dynamics. Smaller scale "" acoustic event ÏÏ drivers, associated with granular dynamics, appear to be less important. The predominant internetwork chromospheric oscillations arise from regions much larger horizontally than vertically. If propagating largely vertically, this can naturally explain why the one-dimensional simulations of Carlsson & Stein might be more successful than expected. The chromo-spheric response to the p-mode driving is, however, intermittent in space and time. Some of the inter-mittency appears to result from the interaction of the upward-propagating waves with magnetic Ðelds. Evidence for this includes suppressed intensities and oscillations near quiet-Sun network elements (which we dub "" magnetic shadows ÏÏ), absence of oscillations in internetwork regions neighboring plage magnetic Ðelds, and a change in character of the quiet-Sun internetwork oscillations between the 119 and 104 nm continua formed at 1 and 1.2 Mm. The latter might be caused by canopy Ðelds that form between these heights under typical quiet-Sun conditions. A SUMER-only data set reported by et al. has a WikstÔl factor of 3 more oscillatory power in the 104 nm continuum than the data analyzed here, with stronger coherences extending into the solar transition region. Together, these data support the general picture that the chromosphere oscillates primarily in response to forcing by the p-modes, they are therefore large-scale (several Mm across) waves, and they are often strongly inÑuenced by magnetic e †ects (internetwork Ðelds, or the overlying canopy), before the oscillations even reach the transition region.