Coronal Scale-Height Enhancement by Magnetohydrodynamic Waves

Abstract

We discuss the possibility that the transmission of Alfven waves from the photosphere into coronal loops can increase the density scale height in these loops. The mechanism involved is the ponderomotive force of transmitted waves, which opposes the force of gravity. We propose that this effect may account for cool coronal loops observed by the Solar and Heliospheric Observatory(SOHO). We also suggest that it may be an explanation of the emission measure enhancement found at the top of coronal X-ray loops observed by Yohkoh. Subject headings: MHD — waves — gravitation — Sun: corona — Sun: magnetic fields Observations in the O v l629 line emission performed with the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory(SOHO) show the existence of long, cool loops (viz., with temperature »240,000 K and height »50,000 km), at coronal altitudes significantly exceeding their density scale height and persisting over times much longer than the free-fall time. The present paper deals with a possible pro- cess that may lead to the appearance of such loops. A simple way of accounting for the lack of balance between the gravitational force and the thermal pressure in steady state is to postulate the presence of downflows, which in turn would require a constant resupply of plasma at coronal heights. The resupply could be the result of coronal condensations (Foukal 1978), but the occurrence of such condensations is uncertain. Force balance in the coronal loop can also be affected by the presence of magnetohydrodynamic (MHD) waves. Such an effect of electromagnetic waves has been the object of nu- merous investigations in the past, in contexts ranging from acceleration and confinement of laboratory plasmas (Motz & Watson 1967), magnetic field generation (Mora & Pellat 1979), low-frequency stability (Yasaka & Itatani 1984), to the solar wind (e.g., Hollweg 1973) and interstellar gasdynamics (McKee & Zweibel 1995), among others. In solar physics, sound wave pressure has been invoked in the context of planar models of the solar atmosphere (viz., McWhirter, Thoneman, & Wilson 1975; more recently, Woods, Holzer, & MacGregor 1990 and references therein). Such effects have also been con- sidered in the context of dynamic loop structures, in which torsional wave packets were assumed to move material along a loop axis (e.g., Shibata & Uchida 1986). Magnetohydrodynamic waves in coronal loops can be either excited in situ (e.g., by reconnection processes in the corona) or transmitted from the photosphere. Since the Alfven velocity cA in the corona is much larger than the photospheric Alfven velocity cAph, magnetohydrodynamic waves excited in the pho- tosphere tend to reflect in the transition region; significant en- ergy transmission occurs only at resonant frequencies at which standing waves are excited in the coronal portion of the loop (Hollweg 1984). If the loop magnetic field is uniform, this occurs for Alfven waves with parallel wavenumberk 5 res