No Evidence Supporting Flare Driven High-Frequency Global Oscillations

The underlying physics that generates the excitations in the global low-frequency, < 5.3 mHz, solar acoustic power spectrum is a well known process that is attributed to solar convection; However, a definitive explanation as to what causes excitations in the high-frequency regime, > 5.3 mHz, has yet to be found. Karoff and Kjeldsen (Astrophys. J. 678, 73-76, 2008) concluded that there is a correlation between solar flares and the global high-frequency solar acoustic waves. We have used the Global Oscillations Network Group (GONG) helioseismic data in an attempt to verify Karoff and Kjeldsen (2008) results as well as compare the post-flare acoustic power spectrum to the pre-flare acoustic power spectrum for 31 solar flares. Among the 31 flares analyzed, we observe that a decrease in acoustic power after the solar flare is just as likely as an increase. Furthermore, while we do observe variations in acoustic power that are most likely associated with the usual p-modes associated with solar convection, these variations do not show any significant temporal association with flares. We find no evidence that consistently supports flare driven high-frequency waves.Comment: 20 pages, 9 figures, Accepted for publication in Solar Physic

Lifetimes of High-Degree p Modes in the Quiet and Active Sun

We study variations of the lifetimes of high-degree solar p-modes in the quiet and active Sun with the solar activity cycle. The lifetimes in the degree range 300 - 600 and frequency 2.5 - 4.5 mHz were computed from SOHO/MDI data in an area including active regions and quiet Sun using the time-distance technique. We applied our analysis to the data in four different phases of solar activity: in 1996 (at minimum), 1998 (rising phase), 2000 (at maximum) and 2003 (declining phase). The results from the area with active regions show that the lifetime decreases as activity increases. The maximal lifetime variations are between solar minimum in 1996 and maximum in 2000; the relative variation averaged over all mode degree values and frequencies is a decrease of about 13%. The lifetime reductions relative to 1996 are about 7% in 1998 and about 10% in 2003. The lifetime computed in the quiet region still decreases with solar activity although the decrease is smaller. On average, relative to 1996, the lifetime decrease is about 4% in 1998, 10% in 2000 and 8% in 2003. Thus, measured lifetime increases when regions of high magnetic activity are avoided. Moreover, the lifetime computed in quiet regions also shows variations with activity cycle.Comment: 13 pages, 5 figures; Accepted for publication in Solar Physic

Fabry-Perot filter based solar video magnetograph

A tunable Lithium Niobate (LiNbO3) Fabry-Perot filter (FP) (passband 165 mÅat 6122 Å) based video magnetograph has been designed and fabricated. This instrument is capable of providing near simultaneous observations of photospheric longitudinal magnetic field, chromospheric Hα , and photospheric CaI pictures using the same telescope and back-end set-up. The magnetic field measurements are made by using the polarization properties of the Zeeman components of the photospheric CaI line at 6122 Å(Landé g factor of 1.75). The CaI line has been chosen due to its low temperature sensitivity and no blend with other solar or atmospheric lines. A variable electro-optic quarter wave retarder, KD*P (Potassium di-Deuterium Phosphate) along with a linear polarizer is used for analyzing the circular polarization of the Zeeman components. The filter tuned at 140 mÅaway from the line center in the blue wing is found to give the best linear response for the field strength up to 1500 Gauss. A field of view (FOV) of $\sim4 \times 3$ arcmin on the solar disk is imaged using a $699 \times 288$ pixel Cohu CCD camera in synchronous with the KD*P modulation. The $\lambda/4$ modulation is achieved by applying ± 2100 volts to the KD*P to obtain alternate frames of oppositely circular polarized images. These images are stored in separate frame buffers of an image acquisition system. To achieve high signal to noise ratio, a large number of images (maximum 256) are added in the respective frame buffers and then the difference between the left and the right circularly polarized images is obtained. This difference is related to the magnetic field strength. On comparing the video magnetograms (VMG) obtained at Udaipur Solar Observatory (USO) on 09 April 1997 at 09:32 UT with those taken by SOHO/MDI at 09:41 UT, it was found that all the magnetic features matched very well in both the magnetograms. In this paper we present the details of the instrument and examples of observations