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Recent polar motion data do not show a 6-year beat and indicate the absence of the Chandler wobble (CW), whereas we could observe the 6-year beat even in the 1920-40 s when the CW amplitude was known to be smallest. As a free mode, the CW needs excitation one or more sources that were debated decades ago but are now attributed to the atmosphere, ocean, and possibly land water. Here, we show that the anomaly started in 2015, after which two independent estimates of the atmospheric CW excitation became persistently smaller than before. However, the estimates of the oceanic and land–water contributions are too large, suggesting improved estimates are needed. Taking advantage of the recent CW anomaly, we show that the quality factor of CW is not as high as 100 as previously preferred. Although the CW excitation processes have been assumed random, a termination of near-resonant processes would rather be consistent with the present findings. Graphical Abstract

The circum-arctic permafrost environment is often disturbed by wildfires but could also show resilience to these disturbances. However, the increased frequency and extent of wildfires, coupled with unprecedented hot weather, have introduced greater uncertainties in the post-fire permafrost dynamics. We need to address emerging questions, e.g. How will permafrost respond to the joint effect of hot anomalies and wildfires? To what extent will post-wildfire deformation evolve? How will permafrost resilience to wildfires vary? Utilizing interferometric synthetic aperture radar time series analysis, we investigated the post-wildfire ground deformation around a 2019 fire scar in the lower Mackenzie Valley, Northwest Territories, Canada, where dramatic heat anomalies and severe wildfires have been recorded in recent years. The resilience of permafrost to wildfires appears to be weakened by the continuous and rapid warming after the fire, as evidenced by the year-on-year acceleration in subsidence rates. Such acceleration was never reported by previous findings that typically observed deceleration in subsidence rates four to five years after wildfires. The deformation along the line of sight (LOS) of the satellite demonstrates significant permafrost degradation induced by wildfires and exacerbated by climate warming, and the cumulative subsidence was detected up to 25 cm in the LOS direction in the upland areas and up to 10 cm in the lowland areas four years after the fire. The difference in deformation magnitude could be attributed to local factors, including ground ice, topography, and vegetation. Our study highlights the increasingly severe threat to circum-arctic permafrost due to the combined effects of wildfires and extreme heat anomalies.

The clear ~ 6-year beat in the polar motion data has allowed us to consider the Chandler period ( P ) as ~ 1.2 years, albeit approximately, from solely the observed polar motion data. This has been made possible not only because the period of the beat can be explained by a combination of the periods of both annual wobble (AW) and Chandler wobble (CW), but also because the spectrum of excitation has been assumed to be flat around the resonant frequency. In contrast, substantial uncertainties persist in the assessment of its quality factor ( Q ), given the continuous observation of CW but the uncertainties surrounding its excitation sources. Nonetheless, two recent studies asserted that they could estimate both the P and Q or constrain the upper bound of Q only from the observed polar motion data. Here we point out that both studies have problems in their approaches and argue that we cannot optimize either P or Q without access to geophysical excitation data or their models. Graphical Abstract

Series of earthquakes including three M w  > 6 earthquakes occurred in Kumamoto prefecture in the middle of the Kyushu island, Japan. In order to reveal the associated crustal deformation signals, we applied offset tracking technique to ALOS-2/PALSAR-2 data covering three M w  > 6 earthquakes and derived the 3D displacements around the epicenters. We could identify three NE–SW trending displacement discontinuities in the 3D displacements that were consistent with the surface location of Futagawa and Hinagu fault system. We set three-segment fault model whose positions matched the displacement discontinuities, and estimated the slip distributions on each segment from the observed pixel-offset data. Whereas right-lateral slip was dominant in the shallower depth of the larger segments, normal fault slip was more significant at a greater depth of the other segment. The inferred configuration and slip distribution of each segment suggest that slip partitioning under oblique extension stress regime took place during the 2016 Kumamoto earthquake sequence. Moreover, given the consistent focal mechanisms derived from both the slip distribution model and seismology, the significant non-double couple components in the focal mechanism of the main shock are due to simultaneous ruptures of both strike-slip and normal faulting at the distinct segments. Graphical abstract Using ALOS-2/PALSAR-2 pixel-offset data, Himematsu and Furuya (2016) derived 3D displacements associated with the 2016 Kumamoto earthquake sequence. The 3D displacements revealed three major discontinuities, where we set the top edge of the three segments of our fault source model. The inferred slip distribution indicates that significant strike-slip and normal fault slip occurred separately at distinct segments. Moreover, the non-double couple parameter based on our slip distribution model turns out to be consistent with the seismological estimate, suggesting that dynamic slip partitioning occurred during the main shock.

Unlike in most other regions, Karakoram glaciers are either stable or advancing, a phenomenon known as the Karakoram anomaly. Despite studies of glacier surges and the derivation of surface velocity maps, the spatiotemporal variability of glacier dynamics still remains poorly understood, particularly in the Eastern Karakoram Range. We use Advanced Land Observing Satellite/the Phased Array type L-band Synthetic Aperture Radar (ALOS/PALSAR)-1/2 data from 2007 to 2011 and 2014 to 2015 to examine detailed surface velocity patterns of the Siachen, Baltoro, Kundos, Singkhu and Gasherbrum Glaciers. The first three glaciers show considerable velocity variability (20–350 m a−1), with clear seasonal patterns. Although all glaciers, except for Baltoro, flow slowest in 2015, the velocity structures are individual and vary in space and time. In Gasherbrum Glacier, peak surge-phase velocities are seasonally modulated, with maxima in summers 2006 and 2007, suggesting surface melt plays an important role in maintaining the active phase. Given the relatively close proximity of these glaciers, we assume that surface melt timing and rates are comparable. We therefore argue that the observed spatiotemporal and interannual velocity patterns are determined by local and internal mechanisms, including englacial and subglacial hydrology, thermal processes and tributary configuration of each individual glacier.

Combining the total electron content (TEC) data from two nationwide Global Navigation Satellite System (GNSS) networks in Japan with the L -band synthetic aperture radar (SAR) data, we reveal the fine spatial and temporal structure of a daytime sporadic- E (Es) episode in Shikoku, Japan. The snapshot of the Es is derived not only from interferometric SAR (InSAR) but also from multiple aperture interferometry (MAI), the latter of which performs better in isolating the fine spatial structure. The GNSS TEC maps indicate that the Es episode is accompanied by a primary east–west elongated (up to ~ 180 km) southward migrating TEC striation with a speed of ~ 90 m/s and ~ 10–20 km widths in the north–south direction. As previously suggested by the GNSS TEC time series, the present InSAR and MAI data independently confirm that electron density in the primary striation gradually increases from the frontal leading edge but abruptly drops in the trailing edge. MAI-based TEC map confirms multiple TEC striations as previously suggested in GNSS TEC time-series, which are reminiscent of the quasi-periodic (QP) echoes in nighttime Es detected by the middle and upper atmosphere (MU) radar, but the periodicity is not as clear as that observed by MU radar. The Kelvin–Helmholtz (KH) instabilities around the wind shear of neutral winds could be responsible for the QP TEC striations. Graphical abstract

The split-spectrum method (SSM) can largely isolate and correct for the ionospheric contribution in the L-band interferometric synthetic aperture radar (InSAR). The standard SSM is performed on the assumption of only the first-order ionospheric dispersive effect, which is proportional to the total electron content (TEC). It is also known that during extreme atmospheric events, either originated from the ionosphere or in the troposphere, other dispersive effects do exist and potentially provide new insights into the dynamics of the atmosphere, but there have been few detection reports of such signals by InSAR. We apply L-band InSAR into heavy rain cases and examine the applicability and limitation of the standard SSM. Since no events such as earthquakes to cause surface deformation took place, the non-dispersive component is apparently attributable to the large amount of water vapor associated with heavy rain, whereas there are spotty anomalies in the dispersive component that are closely correlated with the heavy rain area. The ionosonde and Global Navigation Satellite System (GNSS) rate of total electron content index (ROTI) map both show little anomalies during the heavy rain, which suggests few ionospheric disturbances. Therefore, we interpret that the spotty anomalies in the dispersive component of the standard SSM during heavy rain are originated in the troposphere. While we can consider two physical mechanisms, one is runaway electron avalanche and the other is the dispersive effect due to rain, comparison with the observations from the ground-based lightning detection network and rain gauge data, we conclude that the rain dispersive effect is spatiotemporally favorable. We further propose a formulation to examine if another dispersive phase than the first-order TEC effect is present and apply it to the heavy rain cases as well as two extreme ionospheric sporadic-E events. Our formulation successfully isolates the presence of another dispersive phase during heavy rain that is in positive correlation with the local rain rate. In comparison with other dispersive phases during Sporadic-E episodes, the dispersive heavy rain phases seem to have the same order of magnitude with the ionospheric higher order effects.

Permafrost, a critical global cryospheric component, supports subarctic boreal forests but is frequently disturbed by wildfires, an important driver of permafrost degradation. Wildfires reduce vegetation, organic layers, and surface albedo, leading to active layer thickening and ground subsidence. Recent studies using interferometric synthetic aperture radar (InSAR) have confirmed the rapid and extensive post‐fire permafrost degradation, and have largely focused on short‐term impacts. However, the longer‐term post‐fire permafrost deformation, potentially persisting for decades, remains poorly understood due to limited data. Here, we applied InSAR in North Yukon to detect deformation signals across multiple fire scars in the past five decades. Using a chronosequence (space‐for‐time substitution) approach, we summarize a continuous trajectory of post‐fire permafrost evolution: (a) an initial degradation stage, characterized by abrupt subsidence up to 50 mm/year and gradually slowing over the first decade, with cumulative subsidence exceeding 100 mm locally; (b) an aggradation stage from approximately 15 to 30 years after fire, marked by ground uplift reaching 25 mm/year before gradually declining, compensating for the earlier subsidence; and (c) a stabilization stage beyond three to four decades, where permafrost nearly recovers to pre‐fire conditions with indistinguishable deformation between burned and unburned areas. Notably, the rarely‐reported uplift phase appears closely related to vegetation regeneration and fire‐greening feedback that provide thermal protection, suggesting a critical mechanism of permafrost recovery. These findings provide new insights into the resilience of boreal‐permafrost systems to wildfires and also underscore the importance of long‐term InSAR monitoring in understanding permafrost responses to wildfires under climate change. Plain Language Summary Permafrost plays an important role in global climate systems and is closely associated with the circum‐Arctic boreal forest environments. While wildfires are common disturbances of sub‐arctic boreal forests. Wildfires remove insulating vegetation and organic layers, allowing more heat to reach the ground, leading to permafrost thaw and ground subsidence. Currently, many studies have shown how permafrost degrades soon after wildfires, but little is known about what happens over the following decades. In this study, we used satellite radar technology to monitor ground deformation in North Yukon, Canada, across areas that burned recently to nearly five decades ago. We found that permafrost typically follows three stages after wildfire: initial subsidence over the first decade, uplift over the next 15–30 years as permafrost recovers, and finally a recovered phase when burned areas behave similarly to undisturbed ground. A key discovery is that the ground can recover to pre‐fire condition two to three decades after a fire, owing to vegetation regrowth that helps the ground stay frozen again. These findings suggest that, in cold sub‐arctic boreal forest regions like North Yukon, permafrost can recover after wildfires, showing resilience, but long‐term monitoring is essential as climate change and wildfires continue to intensify. Key Points Interferometric synthetic aperture radar is applied to study long‐term post‐fire permafrost deformation in North Yukon by chronosequence Wildfires cause permafrost degradation and ground subsidence, but recovery with ground uplift can occur over decades Permafrost may recover to pre‐fire conditions in three decades due to the vegetation regeneration that helps ground ice aggradation