On the Nature Of Propagating MHD Waves In The Solar Atmosphere

Abstract

One of the most persistent problem in solar physics is the identification of the mechanism that heats the solar corona and accelerates the fast solar wind. Magneto-hydrodynamic (MHD)waves play a crucial role in heating of the solar corona and acceleration of the solar wind. Different types of oscillations have been now observed by various instruments. These are interpreted as due to ubiquitous presence of MHD waves. The magnetic field plays a fundamental role in the propagation and properties of these MHD waves. The topology (structure)of the magnetic fields are different in different regions of the solar atmosphere viz., active regions (high-lying closed magnetic fields), quiet Sun (low-lying closed magnetic fields) and coronal holes (open magnetic fields). The purpose of this dissertation is to study the nature of these propagating MHD waves in different regions of the solar atmosphere. It is believed that polar coronal holes which connects the inner corona and the solar wind, are the source regions of the fast solar wind. The on-disk part of a polar coronal hole can be divided into network and internetwork regions. Long time series(sit-and-stare)data have been obtained from the SUMER/SoHO spectrometer in N iv 765Å and Ne viii 770Å spectral lines to search for the presence of waves in these two different regions from a statistical approach. The network bright regions indicate the presence of compressional waves with a dominant period of ≈ 25 min in both the lines. Moreover, we found that there is a difference in the nature of the wave propagation in the bright (‘network’), as opposed to the dark (‘internetwork’) regions, with the latter sometimes showing evidence of downwardly propagating waves that are not seen in the former. This is consistent with the magnetic topology, as open field lines are rooted in network regions whereas internetwork region has low lying closed field lines. From a measurement of propagation speeds, we found all waves are subsonic, indicating that the majority of them are slow magneto-acoustic in nature. The off-limb part of coronal holes can be divided into plume and inter-plume regions. The simultaneous observations were performed with EIS/Hinode and SUMER/SoHO spectrometer in Fe xii 195Å and Ne viii 770Å spectral lines respectively. We detected the presence of accelerating waves in a polar inter-plume region with a period of 15 min to 20 min in both the spectral lines and a propagation speed increasing from 130 ± 14 km s−1 just above the limb, to 330 ± 140 kms s−1 around 160” above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole which can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbance with the same range of periodicity but with propagation speeds in the range of 135 ± 18 kms s−1 to 165 ± 43 kms s−1 only. We found that the waves within the plumes are not observable (may be getting dissipated) far off-limb whereas this is not the case in the inter-plume region. We suggested that the waves are likely either Alfv´enic or fast magneto-acoustic in the inter-plume regions and slow magneto-acoustic in the plume regions. These results support the view that the inter-plume regions area preferred channel for the acceleration of the fast solar wind. The quiet Sun can be further divided into bright magnetic (network), bright non-magnetic and dark non-magnetic (internetwork) regions. Simultaneous observations were performed in Ca ii filtergram from SOT/Hinode, TRACE 1550Åpassband and with SUMER/SoHO spectrometer in N iv 765ÅandNe viii 770Åspectral lines to study the oscillations in these different regions. We detected the presence of long period oscillations with periods between 15 min to 30 min in bright magnetic regions. The oscillations were detected from chromospheric height to low coronal heights. Power maps showed that low period powers are mainly concentrated in dark regions whereas long period powers are concentrated in bright magnetic regions. We proposed that these 15 min and above periods can propagate up to the coronal heights through ‘magneto¬acoustic portals’. However in this case only with the spectral imaging data, it was not possible to identify the mode of wave propagation. To detect the presence of waves in active regions, we have analysed the imaging and spec¬troscopic data acquired during the total solar eclipse of 2006 and 2009 respectively. We found the oscillations of periods 27 s and 20 s in imaging data obtained in green (Fe xiv 5303Å) and red (Fe x 6374Å) coronal emission lines respectively. Significant oscillations with high proba¬bility estimates were detected at boundary of active region and in the neighbourhood, rather than within the loops itself. We also reported the detection of oscillations in intensity, velocity and line width having periods in the range of 25 s to 50 s with spectroscopic data again obtained in green and red coronal emission lines. These high frequency oscillations were interpreted in terms of presence of fast magneto-acoustic waves or torsional Alfv´en waves. These detected propagating MHD waves may carry sufficient energy to heat the corona and provide enough momenta to accelerate the fast solar wind. In addition, these waves may also provide input for the measurement of coronal magnetic field using the technique of ‘coronal seismology’.