Krishna Prasad Sayamanthula

Slow magnetoacoustic waves in hot coronal loops have remained a topic of considerable interest and debate over the past two decades. The periodic back-and-forth motion of plasma within a coronal loop, often initiated by a flare, is commonly referred to as sloshing oscillation. In the present study, we report unprecedented observations of sloshing oscillations in coronal loops excited by successive M- and C-class flares, using data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. A total of 15 oscillation events were identified within 7 distinct coronal loops, providing the rare opportunity to evaluate the influence of flare strength on the characteristics of the oscillations. Based on the appearance of the oscillations, their properties were extracted mainly from the AIA 131 and 94 Å channels. Additionally, we estimate the deprojected length of each loop by assuming a semicircular geometry. Our results indicate considerable changes in the properties of oscillations from one flare to another, suggesting the role of individual flares in shaping the local physical conditions. The plasma temperature estimated from the loop length and oscillation period ranges from 9 to 31 MK. Additionally, we find that the damping times are not always longer in the colder 94 Å channel as previously observed. By combining the results obtained from all events, we study the interdependences between various parameters, including oscillation period, damping time, loop length, and plasma temperature, and discuss these results in the context of the theory of slow waves.

Recent high spatial and temporal resolution extreme-ultraviolet (EUV) imaging observations have revealed that quasi-periodic pulsations (QPPs), a ubiquitous signature of impulsive energy release in solar and stellar flares, are also present in much smaller-scale coronal events known as EUV brightenings. Whether QPPs observed across such disparate spatial and energetic scales share a common physical origin remains an open question. Here we analyse 2146 EUV brightenings observed with Solar Orbiter/Extreme Ultraviolet Imager and 300 EUV solar flares observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly, identifying 185 brightenings and 89 flares exhibiting statistically significant damped QPPs. We show that the relationship between damping time and oscillation period follows a common power-law scaling for EUV brightenings and EUV solar flares, consistent with previously reported X-ray QPPs spanning both solar and stellar flares. The persistence of this scaling over a wide range of energies and scales suggests that QPPs are governed by a common underlying physical mechanism, potentially associated with standing magnetohydrodynamic waves. Distinguishing between candidate wave modes, however, will require further observational and theoretical investigation.

Magnetohydrodynamic waves play a key role in heating the solar corona and driving the solar wind. Recent observations have shown the presence of slow magnetoacoustic and Alfvénic waves in polar plumes and interplumes. However, a complete understanding of wave dynamics in the polar regions has long been limited by the lack of simultaneous high-resolution observations. In this study, we utilize a high-spatial-resolution (210 km per pixel) and high-cadence (5 s) data set from the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter, acquired on 2021 September 14. Our findings reveal the simultaneous presence of slow magnetoacoustic and Alfvénic waves within the same polar plumes. For the slow magnetoacoustic waves, the amplitudes of the propagating disturbances are 1.4%–3.2% of the background intensity, with periodicities of 9 minutes, and the projected speeds of these disturbances range between 115 and 125 km s⁻¹. The corresponding electron temperatures in plumes range between 0.58 and 0.69 MK. The damping lengths of these propagating disturbances for five plumes are ≈2.4–7.1 Mm. The propagating disturbances are also detected in the fine-scale substructures within the plumes. Alfvénic waves, on the other hand, are detected with average displacement amplitude, periodicity, and velocity amplitude of 165 ± 82 km, 93 ± 39 s, and 12 ± 7 km s⁻¹, respectively. The ranges for the displacement amplitude, period, and velocity amplitude are 50–600 km, 50–250 s, and 3–32 km s⁻¹, respectively. These results mark the first demonstration of the ability of Solar Orbiter/EUI to simultaneously detect both slow magnetoacoustic and Alfvénic wave modes extending up to 20 Mm in polar plumes.

Slow magnetoacoustic waves with a 3 minute period are upward-propagating waves traveling through the density-stratified umbral atmosphere. The decreasing density causes their amplitude to increase, developing into nonlinear waves through steepening and eventually forming shocks. To investigate the vertical evolution of this wave nonlinearity, we utilized multiwavelength data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory, covering from the photosphere to the lower corona across 20 active regions. The steepening of the wave profile leads to the generation of higher harmonics. We quantify this using a nonlinearity index (NI), defined as the ratio of the amplitude of the second harmonic to the fundamental obtained using wavelet analysis. We find a characteristic pattern: nonlinearity increases from the photosphere through the lower chromosphere, peaking near the AIA 1700 Å formation height, and decreases at higher altitudes, notably in the AIA 304 Å channel. This trend indicates progressive wave steepening and subsequent energy dissipation before reaching the formation of AIA 304 Å, consistent with shock formation in the lower atmosphere. An additional rise in NI is observed at the AIA 131 Å channel, followed by a decline in AIA 171 Å, suggesting a second phase of wave nonlinearity evolution in the lower corona. Based on the NI profile and the formation heights of these channels, we conjecture that nonlinear wave processes are most prominent between the AIA 1700 Å and AIA 304 Å formation layers and again between AIA 131 Å and AIA 171 Å.

Plasma-β is an important fundamental physical quantity in solar plasma physics, which determines the dominating process in the solar atmosphere, i.e., magnetic or thermodynamic processes. Here, for the first time, we provide variations of magnetic field and plasma-β along magnetically structured loops from the photosphere to the corona. We have selected several fan loops rooted in sunspot umbra observed simultaneously by the Interface Region Imaging Spectrograph and Solar Dynamics Observatory. The 3 minute slow waves enabled us to trace and analyze several fan loops with cross-sectional areas in the lower atmosphere and locate their footpoints at the photosphere. We find the rms magnetic field strengths in the range 1596–2269 G at the photospheric footpoints of the fan loops, which decrease rapidly to 158–236 G at the coronal footpoints. We estimated the plasma-β at the photospheric and coronal footpoints in the range 0.2–0.5 and 0.0001–0.001, respectively. We found plasma-β < 1 along the whole loop, whereas the plasma-β ≈ 1 layer is found to be at subphotospheric heights. We compared our findings for isolated individual fan loops with a previously established model for active regions and found an almost similar pattern in variations with height, but with different plasma-β values. Our results demonstrate the seismological potential of 3 minute slow waves omnipresent in the umbral sunspot atmosphere to probe and map isolated loops and determine magnetic field and plasma-β along these loops. The obtained parameters provide crucial ingredients for the theoretical modeling of the umbral atmosphere and wave dynamics along loops.

Slow magnetoacoustic waves with a 3 minute period are upward-propagating waves traveling through the density-stratified umbral atmosphere. The decreasing density causes their amplitude to increase, developing into nonlinear waves through steepening and eventually forming shocks. To investigate the vertical evolution of this wave nonlinearity, we utilized multi-wavelength data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), covering from the photosphere to the lower corona across 20 active regions. The steepening of the wave profile leads to the generation of higher harmonics. We quantify this using a nonlinearity index (NI), defined as the ratio of the amplitude of 2nd harmonic to the fundamental obtained using wavelet analysis. We find a characteristic pattern: nonlinearity increases from the photosphere through the lower chromosphere, peaking near the AIA 1700 Å formation height, and decreases at higher altitudes, notably in the AIA 304 Å channel. This trend indicates progressive wave steepening and subsequent energy dissipation before reaching the formation of AIA 304 Å, consistent with shock formation in the lower atmosphere. An additional rise in NI is observed at the AIA 131 Å channel, followed by a decline in AIA 171 Å, suggesting a 2nd phase of wave nonlinearity evolution in the lower corona. Based on the NI profile and the formation heights of these channels, we conjecture that nonlinear wave processes are most prominent between the AIA 1700 Å and AIA 304 Å formation layers and again between AIA 131 Å and AIA 171 Å.

Propagating slow magnetoacoustic waves are commonly observed in different coronal structures but are most prominent in active region fan loops. Their rapid damping with damping lengths of the order of a wavelength has been investigated in the past by several authors. Although different physical mechanisms have been proposed, significant discrepancies between the theory and observations remain. Recent high-resolution observations captured simultaneously by two different instruments reveal distinct damping lengths for slow magnetoacoustic waves, although their passbands are similar. These results suggest a possible contribution of instrumental characteristics to the measurement of damping lengths. Here, we analyze the behavior of slow waves using a different pair of instruments in order to check the prevalence of such results. In particular, the cotemporal observations of active region NOAA AR 12712 by the High-resolution Coronal Imager (Hi-C 2.1; Hi-C) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) are utilised. The estimated oscillation periods of slow magnetoacoustic waves identified from these data are 2.7 ± 0.2 minutes from SDO/AIA and 2.8 ± 1.2 minutes from Hi-C 2.1. The corresponding propagation speeds are found to be 46.0 ± 1.7 and 48.1 ± 0.6 km s⁻¹, respectively. Damping lengths were calculated by two different methods, the phase tracking method (PTM) and the Amplitude Tracking Method (ATM). The obtained values from the PTM are 4.0 ± 2.1 and 4.1 ± 0.3 Mm, while those from the ATM are 3.4 ± 1.0 and 3.7 ± 0.1 Mm, respectively, for the AIA and Hi-C data. Our results do not indicate any notable difference in damping lengths between these instruments.

Slow magnetoacoustic waves (SMAWs) have been considered a possible candidate for chromospheric heating in the past. This study analyzed 20 active regions observed between 2012 and 2016 to examine the amplitude and energy flux variation of SMAWs in the umbral atmosphere. Six different wavelength channels from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, covering regions from the photosphere to the low corona, were utilized for this purpose. The wave amplitude estimations show a gradual increase in 3 minute oscillation amplitude, peaking between 700 and 900 km, followed by a steady decrease; further, at altitudes greater than 1800 km, it appears to increase and decrease again. The corresponding energy flux, on the other hand, displays a steady and monotonous decrease with a significant reduction in value from approximately 3.32 ± 0.50 kW m⁻² near the photosphere to about 6.47 ± 3.16 ×10⁻⁴ W m⁻² at an altitude of 2585 km. This decay may be attributed to radiative damping and shock dissipation in the lower altitudes and thermal conduction and viscosity in the higher altitudes. The missing flux is a factor of 3–15 lower than that required to counterbalance the chromospheric radiative losses.

The multiwavelength data from the Solar Dynamics Observatory is extensively used in studying the physics of the Sun and its atmosphere. In this study, we estimate the formation heights of low-corona and chromospheric channels of the Atmospheric Imaging Assembly (AIA) over the atmospheres of sunspot umbrae during the quiet condition period within 20 different active regions. The upward propagating slow magnetoacoustic waves of a 3 minute period, which are perpetually present in sunspots, are utilized for this purpose. Employing a cross-correlation technique, the most frequent time lag between different channel pairs is measured. By combining this information with the local sound speed obtained from the characteristic formation temperatures of individual channels, we estimate the respective formation heights. The median values of formation heights obtained across all active regions in our sample are 356, 368, 858, 1180, and 1470 km, respectively, for the AIA 1600, 1700, 304, 131, and 171 Å channels. The corresponding ranges in the formation heights are 247–453, 260–468, 575–1155, 709–1937, and 909–2585 km, respectively. These values are measured with respect to the Helioseismic Magnetic Imager continuum. We find the formation height of UV channels is quite stable (between 250 and 500 km) and displays only a marginal difference between the AIA 1600 and 1700 Å channels during quiet conditions. On the other hand, the formation height of coronal channels is quite variable.

We study the propagation properties of slow magnetoacoustic waves in a multithermal coronal loop using a 3D MHD model, for the first time. A bundle of 33 vertical cylinders, each of 100 km radius, randomly distributed over a circular region of radius 1 Mm, is considered to represent the coronal loop. The slow waves are driven by perturbing the vertical velocity (v _z ) at the base of the loop. We apply forward modeling to the simulation results to generate synthetic images in the coronal channels of the Solar Dynamics Observatory/Atmospheric Imaging Assembly. Furthermore, we add appropriate data noise to enable direct comparison with the real observations. It is found that the synthetic images at the instrument resolution show noncospatial features in different temperature channels in agreement with previous observations. Time–distance maps are constructed from the synthetic data to study the propagation properties. The results indicate that the oscillations are only visible in specific channels, depending on the temperature range of the plasma existing within the loop. Additionally, the propagation speed of slow waves is also found to be sensitive to the available temperature range. Overall, we propose that the cross-field thermal properties of coronal structures can be inferred using a combination of numerical simulations and observations of slow magnetoacoustic waves.