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Project: Accelerating waves in polar coronal holes

Results:
We present the EIS-SUMER Imaging and spectrometer observations of propagating disturbances as detected in coronal lines in inter-plume region of a polar coronal hole which has its origin in the on-disk bright region of the coronal hole. The disturbances show the signature of acceleration while propagating from the on-disk region to the offlimb region. The observation was carried out on 13th November 2007 as a JOP196/HOP045 programme with SUMER and EIS instruments on board SoHO and HINODE respectively. The SUMER spectroscopic observation gives the information about the fluctuation in intensity as well as in velocity whereas EIS slot image gives fluctuations only in intensity but maximizes the probability of overlapp between two instruments. We detect the presence of propagating waves in a polar inter-plume region having period of 15-20 minute and speed of 130-330 km/s. These waves can also be traced to originate from a bright region of the on-disk part of the coronal hole where the speed is in range of 25-55 km/s with the same periodicity. These on-disk bright regions can be visualized as the base of the coronal funnels. The adjacent plume in polar region also shows the presence propagating disturbance of same range of period but speeds in the range 130-165 km/s. On comparison between the time-distance radiance map of both the regions, it indicates that the disturbance is getting dissipated in the far offlimb region of plume whereas this is not the case in inter-plume region. This may lead to the conclusion that inter-plumes are preferred channel for acceleration of the fast solar wind. These oscillations are been observed in many spectroscopic lines from EIS-SUMER instrument having different formation temperature but not seen clearly in the time distance radiance map of all those lines. The correlation analysis is been done to find out the time delay between the oscillations at several height in offlimb region. Our result provides spectroscopic evidence of acceleration of propagating disturbances in the polar region close to the sun, which provides clues to the understanding of the origin of these waves.
 

Figure: (Left) A figure indicates the time taken by the disturbance to propagate at several heights in solar atmosphere with respect to solar-Y=1000. The time taken in propagating the same distance of height is different for different heights and is decreases with height indicating the acceleration of propagation. (Right) Variation of the velocity of propagating disturbance at several heights in solar atmosphere. The propagation speed increases with height indicating the acceleration of propagation. The asterisk symbols indicate the velocity points obtained from the time delay measurement. The continuous line is obtained after taking the derivative of fitted line in left Figure and dotted line is linear fit applied on to asterisk marked velocity points.

Reference: Gupta, et. al., 2010, ApJ, 718, 11



Project: Statistical detection of propagating waves in the Solar Atmosphere

Background:
Evidence for waves in the solar atmosphere comes from meaurements of intensities in a wide range of electromagnetic spectrum including visible, ultraviolet, X-ray, and radio in addition to Doppler oscillations in visible  and ultraviolet light given off by different solar structures at chromospheic, transition region and coronal temperatures.

Results:
Using the temopral series data from the SUMER/SoHO, we studied the oscillation found in transition region (N IV 765) and lower coronal (Ne VIII 770) lines in on-disk both polar regions. We have identified the compressional waves in network and internetwork regions of both the pCHs. Measurements of phase delay between those two line pairs reveals that the measured phases tend to line up along diagonal lines corresponding to fixed time delays. From the orientation of the slops of these diagonal lines, we can conclude the direction of propagation of these waves. Here in this case, we have reported the upward progating waves in network region and in internetwork region we have reported the both upward and downword propagation waves refer fig.1. From the estimate of the formation heights of the lines, and using the measured time delays, we had estimated the propagation speed for the line pair. The estimated speed indicates that the waves producing these observed phase delays are slow magneto-acoustic waves propagating at speed very less than the sound speed. By comparing with the previous studies, here we have hypothesise that as waves propagates upward it's speed increases with height in the solar atmosphere.



Figure 1: Phase delays measured between the oscillations in the spectroscopic line pair for the dark locations of pCH. The phases in radiant flux oscillations are shown in the grey circle symbols while that in LOS velocities are shown as the black circle symbols. Overplotted on each figure are black parallel lines, corresponding to fixed time delays. The inclined parallel straight lines indicate the presence of both upwardly and downwardly propagating waves. Representative errors are indicated by the error bars.


Reference: Gupta, et. al., 2009, A&A, 251-257