Statistically, the cool loop’s footpoints are diagnosed using Si iv resonance lines observations provided by Interface Region Imaging Spectrograph (IRIS). The intensity and full width at half-maximum (FWHM) of the loop’s footpoints in β–γ active regions (ARs) are higher than the corresponding parameters of footpoints in β ARs. However, the Doppler velocity of footpoints in both ARs are almost similar to each other. The intensities of footpoints from β–γ AR is found to be around nine times that of β AR when both ARs are observed nearly at the same time. The same intensity difference reduces nearly to half (four times) when considering all ARs observed over 9 yr. Hence, the instrument degradation affects comparative intensity analysis. We find that Doppler velocity and FWHM are well correlated while peak intensity is neither correlated with Doppler velocity nor FWHM. The loop’s footpoints in β–γ ARs have around four times more complex Si iv spectral profiles than that of β ARs. The intensity ratios (Si iv 1393.78 Å/1402.77 Å) of the significant locations of footpoints differ, marginally, (i.e. either less than 1.9 or greater than 2.10) from the theoretical ratio of 2, i.e. 52 per cent (55 per cent) locations in β (β–γ) ARs significantly deviate from 2. Hence, we say that more than half of the footpoint locations are either affected by the opacity or resonance scattering. We conclude that the nature and attributes of the footpoints of the cool loops in β–γ ARs are significantly different from those in β ARs.

Waves are an integral part of the solar atmosphere, and their characteristics (e.g., dominant period, range of periods, power, and phase angle) change on a diverse spatiotemporal scale. It is well established observationally that the dominant periods of solar oscillations are 5 and 3 minutes in the photosphere and chromosphere, respectively. This shows that the wave spectra and their dominant periods evolve between these two layers. We present observational results that demonstrate variations of the dominant period with heights in the photosphere and chromosphere. Six photospheric absorption lines and one chromospheric line are analyzed by using the Interface Region Imaging Spectrograph data, and the Doppler velocity time series at seven different atmospheric heights are determined. The wavelet analysis is applied to these time series, and the resulting spectrum of wave periods and its dominant period are deduced at these heights, which gives height variations of the dominant period. The obtained data show that the dominant period decreases with height and that there are also changes in the range of wave periods within the spectrum. Numerical simulations of filtered wave spectra through the solar atmosphere are also performed, and the obtained results match the observational data.

We have studied a B-class solar flare and an associated filament eruption through multiwavelength observations. The flare is triggered at 16:24 UT on 2017 June 7 from Active Region(AR) 12661, and it maximizes at 16:54 UT. Magnetic flux cancellation occurs near the polarity inversion line preceding the flare, and ultraviolet (UV) brightenings occur in the preflare phase at the flux cancellation sites, suggesting reconnection occurs in the lower atmosphere, initially. An S-shaped sigmoid forms through successive steps in the corona, i.e., small-scale brightenings, helical/twisted field lines, bright patches, and finally, a developed sigmoid. This observation justifies that runaway reconnection within the coronal arcades forms the sigmoid within the filament. Differential emission measure analysis reveals the existence of plasma at a temperature of more than 10 MK within the sigmoid. The initial magnetic reconnection reorganizes the field overlying the filament as per the tether cutting model. Therefore, it enables the filament to rise slowly, and around 16:41 UT, the eruption phase of the filament begins. The filament eruption removes the overlying coronal field, including the sigmoid. During the eruption phase, we have found the intersecting/crossing of coronal loops and jet-like structures far away from the sigmoid–filament system. In conclusion, all the observational findings (e.g., magnetic flux convergence, cancellation, UV brightenings, and spatial and temporal correlation between formation/evolution of the sigmoid and rise/eruption of the filament) suggest that the formation of a solar flare and the eruption of the filament are consistent with the tether cutting model of solar eruptions.

We have studied a B-class solar flare and an associated filament eruption through multi-wavelength observations. The flare triggers at 16:24~UT on June 7$^{th}$, 2017 from an active region (AR) 12661, and it maximizes at 16:54~UT. The magnetic flux cancellation occurs near the polarity inversion line (PIL) preceding the flare, and ultraviolet (UV) brightenings occur in the pre-flare phase at the flux cancellation sites, suggesting the reconnection occurs in the lower atmosphere, initially. The S-shaped sigmoid forms through successive steps in corona, i.e., small-scale brightenings, helical/twisted field lines, bright patches, and finally, a developed sigmoid. It justifies that runaway reconnection within the coronal arcades forms the sigmoid within the filament. The differential emission-measure (DEM) analysis reveals the existence of the plasma at a temperature of more than 10 MK within the sigmoid. The initial magnetic reconnection reorganizes the field overlying the filament as per the tether-cutting model. Therefore, it enables the filament to rise slowly, and around~16:41~UT, the eruption phase of the filament begins. The filament eruption removes the overlying coronal field, including the sigmoid. During the eruption phase, we have found intersecting/crossing of coronal loops and jet-like structures far away from the sigmoid-filament system. In conclusion, all the observational findings (e.g., magnetic flux convergence, cancellation, UV brightenings, and spatial and temporal correlation between formation/evolution of the sigmoid and rise/eruption of the filament) suggest that the formation of a solar flare and the eruption of the filament are consistent with the tether-cutting model of solar eruption.