Geophysical Research Letters

A comparison tool has been developed by mapping the global GPS total electron content (TEC) and large coverage of ionospheric scintillations together on the geomagnetic latitude/magnetic local time coordinates. Using this tool, a comparison between large‐scale ionospheric irregularities and scintillations is pursued during a geomagnetic storm. Irregularities, such as storm enhanced density, middle‐latitude trough, and polar cap patches, are clearly identified from the TEC maps. At the edges of these irregularities, clear scintillations appeared but their behaviors were different. Phase scintillations (σ_φ) were almost always larger than amplitude scintillations (S₄) at the edges of these irregularities, associated with bursty flows or flow reversals with large density gradients. An unexpected scintillation feature appeared inside the modeled auroral oval where S₄ were much larger than σ_φ, most likely caused by particle precipitations around the exiting polar cap patches.

Global overturn of a hot, gravitationally unstable lunar mantle immediately following the solidification of a magma ocean [and essentially complete by 4.4 Ga] explains several characteristics of lunar petrology. Lunar mare basalt sources are inferred to be depleted in europium and alumina. These depletions are consensually attributed to complementary plagioclase floating from a magma ocean. However, the connection cannot be so simple and direct: in contrast to the mare basalt source parent magma, the ferroan anorthosite parent magma was more evolved by virtue of its lower Mg/Fe ratio and Ni abundances, although less evolved in its poverty of clinopyroxene constituents, flat rare earth pattern, and lower incompatible element abundances. The europium anomaly in mare sources is inferred to be present at 400 km depth, too deep to have been directly influenced by plagioclase crystallization. Massive overturning of the post‐magma ocean mantle would have carried down clinopyroxene, ilmenite, and phases containing fractionated rare earths, europium anomalies, and some heat‐producing radionuclides. These phases contributed to deep mare basalt sources. Upward‐moving phases would have been magnesian mafic minerals; their immediate melting on pressure release contributed the magnesian suite of plutonic norites and troctolites that post‐date the anorthosites in the highlands crust.

The sensitivity of the Greenland ice sheet to climate forcing is of key importance in assessing its contribution to past and future sea level rise. Surface mass loss occurs during summer, and accounting for temperature seasonality is critical in simulating ice sheet evolution and in interpreting glacial landforms and chronologies. Ice core records constrain the timing and magnitude of climate change but are largely limited to annual mean estimates from the ice sheet interior. Here we merge ice core reconstructions with transient climate model simulations to generate Greenland‐wide and seasonally resolved surface air temperature fields during the last deglaciation. Greenland summer temperatures peak in the early Holocene, consistent with records of ice core melt layers. We perform deglacial Greenland ice sheet model simulations to demonstrate that accounting for realistic temperature seasonality decreases simulated glacial ice volume, expedites the deglacial margin retreat, mutes the impact of abrupt climate warming, and gives rise to a clear Holocene ice volume minimum.

Water system operations require subannual streamflow data—e.g., monthly or weekly—that are not readily achievable with conventional streamflow reconstructions from annual tree rings. This mismatch is particularly relevant to highly seasonal rivers such as Thailand's Chao Phraya. Here, we combine tree ring width and stable oxygen isotope ratios (δ¹⁸O) from Southeast Asia to produce 254‐year, monthly‐resolved reconstructions for all four major tributaries of the Chao Phraya. From the reconstructions, we derive subannual streamflow indices to examine past hydrological droughts and pluvials, and find coherence and heterogeneity in their histories. The monthly resolution reveals the spatiotemporal variability in wet season timing, caused by interactions between early summer typhoons, monsoon rains, catchment location, and topography. Monthly‐resolved reconstructions, like the ones presented here, not only broaden our understanding of past hydroclimatic variability, but also provide data that are functional to water management and climate‐risk analyses, a significant improvement over annual reconstructions.

Increased flood risks have been projected, but with large uncertainties, in the Kabul River Basin (Afghanistan and Pakistan). To place future changes in a long‐term perspective, we produce a 382‐year precipitation reconstruction for the basin using seven tree‐ring chronologies of old‐growth conifers from the Hindu Kush Mountains, a monsoon‐shadow area. The reconstruction proves robust over rigorous cross‐validations (R² = 0.60, RE = 0.60, CE = 0.53). The full reconstruction (1637–2018) reveals a steady decline in the low end of the precipitation distribution, implying increasing drought risks. We show that droughts are getting more severe, shorter, and more frequent, interspersed with more frequent pluvials in the past century. Drought risks, compounded with projected flood intensification, pose significant threats for this transboundary river. Therefore, future water management needs to account for both flood and drought risks and be informed by long‐term hydroclimatic variability.

We present measurements of the latitudinal variation of nighttime O₃ and H₂O in the mesosphere and (for O₃) lower thermosphere obtained with the Millimeter‐wave Atmospheric Sounder (MAS) instrument during the ATLAS 2 mission (8–15 April 1993). These are the first such measurements that have ever been reported. They indicate an O₃ mixing ratio minimum at mid‐latitudes in the upper mesosphere, with maxima in the tropics and at high latitudes. The H₂O retrievals indicate H₂O mixing ratios decreasing toward the poles in both hemispheres in the upper mesosphere. We also present measurements of the diurnal variation of O₃ at southern mid‐latitudes, at higher vertical resolution than has ever been reported previously. The results are generally consistent with previous measurements and modeling studies.

The vertical profile of CCl₂F₂ has been retrieved in the altitude range 15 to 22 km from solar occultation spectra recorded with a balloon‐borne Fourier transform spectrometer (0.013 cm⁻¹ resolution) launched on 22 March 1995 from Kiruna (Sweden, 67°N, 22°E). The choices of the grid of retrieval altitudes, of the spectral range, and of the absorption cross‐section data used for the retrievals are studied carefully. The vertical distribution obtained is characteristic of conditions pertaining to the late winter Arctic vortex. The measured mixing ratio profile of CCl₂F₂ is discussed in connection with the N₂O profile also retrieved with the same instrument. These remote sensing measurements of CCl₂F₂ are compared with in situ results obtained from the same location and during the same period with balloon‐borne grab or cryosamplers. These data are analyzed in terms of diabatic descent in the polar vortex.

Observations of the balloon‐borne LPMA/DOAS remote sensing instruments performed in the Arctic stratosphere in February 1999 are used to constrain a photochemical model. Measurements of all relevant nitrogen, chlorine, and bromine species indicate that moderate heterogeneous chlorine activation occurred in a filament of the Arctic vortex. Model‐measurement comparisons for OClO serve as an indicator of how well various scenarios of the involved reaction kinetics, in particular of the ClO‐BrO and the ClO‐ClO cycles, reproduce the observations. Recent suggestions for the photolysis rate of the ClO dimer, the equilibrium constant between ClO dimer and monomer, the rate of the ClO‐ClO association reaction, and the branching ratio of the ClO‐BrO reaction are consistent with the observations. Formation of an unstable isomer of ClONO₂ cannot be reconciled with the observations. Modeled odd oxygen loss rates can be larger by 10% to 20% for the updated reaction kinetics compared to standard recommendations.

Seismic ambient noise cross correlation is increasingly used to monitor volcanic activity. However, this method is usually limited to volcanoes equipped with large and dense networks of broadband stations. The single‐station approach may provide a powerful and reliable alternative to the classical “cross‐station” approach when measuring variation of seismic velocities. We implemented it on the Piton de la Fournaise in Reunion Island, a very active volcano with a remarkable multidisciplinary continuous monitoring. Over the past decade, this volcano has been increasingly studied using the traditional cross‐correlation technique and therefore represents a unique laboratory to validate our approach. Our results, tested on stations located up to 3.5 km from the eruptive site, performed as well as the classical approach to detect the volcanic eruption in the 1–2 Hz frequency band. This opens new perspectives to successfully forecast volcanic activity at volcanoes equipped with a single three‐component seismometer.

Regeneration of the magnetic field by convection in the core places demands on heat flow into the base of the mantle. If the heat flow is too low, thermal convection is shut off and the rate of generation of compositional buoyancy by the solidification of the core becomes too low to sustain the geodynamo. Conversely, a large heat flow causes rapid growth of the inner core, so that convection prior to the appearance of the inner core must be sustained by thermal buoyancy alone. The attendant requirements on primordial heat become more severe as the age of the inner core decreases. We show that estimates of the present‐day heat flow satisfy the power requirements for the geodynamo. However, the resulting thermal history is incompatible with estimates of mantle temperatures prior to 3 Ga. This discrepancy can be resolved by accumulating radioactive isotopes in D″ or adding heat sources to the core.