Reply on RC2 (Please see attached PDF - under Reviewer #2)

<strong class="journal-contentHeaderColor">Abstract.</strong> We present dynamic mid-latitude ionospheric changes associated with the Apr 8, 2024 solar eclipse, as observed in the eastern to central United States using wide-field scans from the Millstone Hill incoherent scatter radar. The radar&rsquo;s field of view covers a large portion of the continental US (25&ndash;55&deg; N, 75&ndash;95&deg; W), and is capable of observing altitude profiles from the E-region up to the topside ionosphere. We find that extensive ionospheric changes triggered by the eclipse along the US east coast were detected before the totality shadow entered the continent, and in particular when the eclipse obscuration percentage became greater than 0 %. During the time interval where the obscuration percentage is significant (&gt;75 %) over the radar scanning region, we find maximum electron density (<em>N_e</em>) drops of up to 50 % (2 &times;10¹¹ m^&minus;3) in the F1 region (200&ndash;250 km) to the north of the totality path, with a large <em>N_e</em> decrease lasting for 1 hour, from 19&ndash;20 UT. In the topside ionosphere, a delayed drop in <em>N_e</em> values occurred beginning 30 min&ndash;1 hour later, with electron density decreasing by 40&ndash;50 % (2.5&ndash;3 &times;10¹¹ m^&minus;3) below background levels, followed by a recovery to typical background values only after 22 UT. The electron temperature (<em>T_e</em>) showed a faster response and recovery rate which closely mirrored the eclipse obscuration level, with a visible decrease in <em>T_e</em> when the obscuration rate exceeded 0&ndash;30 % depending on altitude, with a faster response time at higher altitudes. A decrease in <em>T_e</em> of the order of 40 % (850 K) was observed at altitudes from 325&ndash;400 km between 18.75&ndash;19.25 UT. The ion temperature (<em>T_i</em>), similar to <em>T_e</em>, responded rapidly to the obscuration level as it exceeded 30 %, with up to 20 % or 225 K reduction in a narrow geographic area and at two distinct altitude regions: 200&ndash;225 km and 300&ndash;325 km between 19&ndash;20 UT.

The vertical stratification and optical characteristics of aloft aerosol plumes are critical to evaluate their influences on climate radiation and air quality. In this study, we demonstrate the synergistic measurements of aloft aerosol plumes by a ground-based NOAA-CREST lidar network (CLN) along the US East Coast, the AERONET-sun/sky radiometer network at lidar sites, and satellite observations. During the plume intrusion period on March 6, 2012, the CLN and AERONET measurements were consistent in illustrating the onset of dust aerosol plumes. We observed two-layers of aerosol located at 1.0 ~ 8.0 km altitude. The column-average volume size distributions show increasing concentration of both fine- and coarse-modes aerosols, but are dominated by the coarse-mode. Direct lidar inversions illustrate that the aerosol plume layers contributed up to 70% of the total AOD. NOAA-HYSPLIT back-trajectories and CALIPSO observations indicate the trans-Pacific transport of Asian-dust at 3 - 8 km altitude to the US East Coast. Meanwhile, the NOAA-HMS fire and smoke products illustrate the transport and possible mixture of dust with fine-mode smoke particles from the middle and southwestern US. The small Angstrom exponents of MODIS/Aqua in the US East Coast imply the dominance of coarse-mode particles. Accordingly, the upper layer of coarse mode aerosols is most likely transported from the East Asia, while the lower layer at 1-3 km altitude probably consists of continental dust particles from the western US mixed with fine-mode smoke particles. In addition, the transport and vertical structure of aerosol are investigated with the NAAPS global aerosol transport model.

This work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019, which provides a comprehensive SAPS climatology using a significantly larger database of radar observations than was used in seminal earlier works. Key features of SAPS and associated electron density ( N e ), ion temperature ( T i ), and electron temperature ( T e ) are investigated using a superposed epoch analysis method. The characteristics of these parameters are investigated with respect to magnetic local time, season, geomagnetic activity, solar activity, and interplanetary magnetic field (IMF) orientation, respectively. The main results are as follows: (1) Conditions for SAPS are more favorable for dusk than near midnight, for winter compared to summer, for active geomagnetic periods compared to quiet time, for solar minimum compared to solar maximum, and for IMF conditions with negative B y and negative B z . (2) SAPS is usually associated with a midlatitude trough of 15–20% depletion in the background density. The SAPS‐related trough is more pronounced in the postmidnight sector and near the equinoxes. (3) Subauroral ion and electron temperatures exhibit a 3–8% (50–120 K) enhancement in SAPS regions, which tend to have higher percentage enhancement during geomagnetically active periods and at midnight. Ion temperature enhancements are more favored during low solar activity periods, while the electron temperature enhancement remains almost constant as a function of the solar cycle. (4) The electron thermal content, T e × N e , in the SAPS associated region is strongly dependent on 1/ N e , with T e exhibiting a negative correlation with respect to N e .

The National Center for Atmospheric Research (NCAR) thermospheric general circulation model (TGCM) is used to calculate the time‐dependent thermospheric response to the May 30, 1984, annular solar eclipse. The path of maximum obscurity begins at sunrise in the Pacific Ocean near 2°N and 135°W. It moves northeastward, passing across central Mexico, the eastern United States, and then the Atlantic before ending near 28°N and 4°E in Algeria. The area of the partial shadow is relatively large, and the total solar flux incident on the dayside of the earth is decreased by about 6% during the eclipse. The TGCM calculates the time‐dependent response of the winds, temperature, and the mass mixing ratios of the major constituents throughout the thermosphere. Perturbations follow the path of the annular eclipse, with maximum deviations occurring near 1700 UT at about 300 km for the temperature and at higher altitudes for the winds and composition. The perturbation winds converge from all directions toward the shadow at speeds reaching 75 m s−1 in the upper thermosphere. The maximum temperature anomaly (−55 K) and vertical wind anomaly (−7 m s−1) occur near the center of the shadow. At a constant altitude of 300 km, both the N2 density and the O density decrease by about 10% and 6%, respectively. The path of maximum obscuration passes within 3° of latitude of the incoherent scatter radar at Millstone Hill, Massachusetts (42.6°N, 71.5°W). The station experiences a maximum solar obscuration of 86% at 1700 UT (1200 LT). A time‐dependent one‐dimensional numerical model of the ionosphere that uses the TGCM‐calculated winds, temperature, and composition at Millstone Hill is used to calculate the electron and ion densities and temperatures and the densities of the odd‐nitrogen species NO, N(4S), and N(²D) during the eclipse. The calculated electron density decreases by about factors of 2 in the F region, 4 in the F1 region, and 3 in the E region compared to a similar control run without an eclipse. The F1 region emerges during the eclipse with an increase in the NO+/O+ ratio. The calculated electron temperature decreases by 460 K during the eclipse but then increases 200 K following the eclipse because of the intense solar heating in a region of reduced electron densities. The calculated ion temperature generally follows the changes in neutral temperature. The calculated thermospheric and ionospheric responses agree well with measurements made by the Millstone Hill incoherent scatter radar.

Multiple observational studies have demonstrated large ionospheric variations during the daytime associated with sudden stratospheric warming (SSW) events, but only limited evidence of ionospheric disturbances during the nighttime has been reported up to now. We focus on the American longitudinal sector with its extensive observational network of Global Navigation Satellite System receivers, four Digisondes located at low and middle latitudes, and the Arecibo and Millstone Hill incoherent scatter radars. The study focuses on a major SSW event of January 2013 to investigate large‐scale disturbances in the nighttime ionosphere. We report a deep decrease in total electron content that reaches a factor of 2–5 as compared to the background level and is observed between local midnight and local sunrise (6–12 UT). This decrease is observed for several consecutive days in the range of latitudes from ~55°S to ~45°N. It is accompanied by a strong downward plasma motion and a significant decrease in ion temperature, as observed by both Arecibo and Millstone Hill radars. These results demonstrate that SSW events cause changes in the nighttime ionosphere that are even larger than in the daytime ionosphere. We discuss variations in electric field and F‐region dynamics as possible drivers of this behavior and suggest that thermospheric winds play a much larger role than previously thought.

Climatological study of the ion temperature in the ionosphere as recorded by Millstone Hill incoherent scatter radar and comparison with the IRI model

Two aspects of the interactions between the Gulf Stream (GS) and the bottom topography are investigated: 1. the spatial variations associated with the north-south tilt of mean sea level along the US East Coast and 2. the high-frequency temporal variations of coastal sea level (CSL) that are related to Gulf Stream dynamics. A regional ocean circulation model is used to assess the role of topography; this is done by conducting numerical simulations of the GS with two different topographies–one case with a realistic topography and another case with an idealized smooth topography that neglects the details of the coastline and the very deep ocean. High-frequency oscillations (with a 5-day period) in the zonal wind and in the GS transport are imposed on the model; the source of the GS variability is either the Florida Current (FC) in the south or the Slope Current (SC) in the north. The results demonstrate that the abrupt change of topography at Cape Hatteras, near the point where the GS separates from the coast, amplifies the northward downward mean sea level tilt along the coast there. The results suggest that idealized or coarse resolution models that do not resolve the details of the coastline may underestimate the difference between the higher mean sea level in the South Atlantic Bight (SAB) and the lower mean sea level in the Mid-Atlantic Bight (MAB). Imposed variations in the model’s GS transport can generate coherent sea level variability along the coast, similar to the observations. However, when the bottom topography in the model is modified (or not well resolved), the shape of the coastline and the continental shelf influence the propagation of coastal-trapped waves and impact the CSL variability. The results can explain the different characteristics of sea level variability in the SAB and in the MAB and help understand unexpected water level anomalies and flooding related to remote influence of the GS.

Surveys were conducted in selected coastal bays from Delaware to New York to describe sandbar shark (Carcharhinus plumbeus) essential habitat, assess abundance, growth during the nursery season, and length-at-birth in Delaware Bay, and to delimit the current northern limit of pupping grounds along the US east coast. Additionally, reproductive tracts from sandbar sharks were sampled to describe development, length-at-maturity, litter size and gonadal cycle. Neonate and juveniles (n = 943) ranging from 40 to 120 cm fork length (FL) were captured in Delaware Bay from June to October between 1995 and 1997. Results from gillnet and longline sampling and tag-recaptures show the main pupping area is located along the southwest coast of Delaware Bay. Mean length-at-birth is 50 cm FL in Delaware Bay. No sandbar sharks were caught in the historical pupping grounds of Great South Bay, New York. Seventeen sandbar shark neonates ranging in length from 42 to 52 cm FL were captured by gillnet in Great Bay, New Jersey. Results support that Great Bay, New Jersey is the current northern boundary of sandbar shark pupping grounds along the US east coast. In males and females, marked increases in reproductive tract anatomy occurs at about 140 cm fork length (FL), indicating the transition between juvenile and subadult stages. The smallest mature female was 148 cm FL and the largest immature was 175 cm FL. The smallest mature male was 139 cm FL and the largest immature was 153 cm FL. Probit analysis was used to produce comprehensive maturity schedules for males and females. Length-at-maturity in both female and male sandbar sharks are consistent with reports in the literature, but the maturity schedules produced here describe the range in length-at-maturity.

Extratropical cyclones exert a large socioeconomic impact. It is therefore important to assess their interannual variability. We generate cyclone tracks from the National Center for Environmental Prediction’s Reanalysis I and the European Centre for Medium Range Prediction ERA-40 reanalysis datasets. To investigate the interannual variability of cyclone tracks, we compare the effects of El Niño, the North Atlantic Oscillation (NAO), the Indian Ocean Dipole (IOD), and the Pacific North American Pattern (PNA) on cyclone tracks. Composite analysis shows similar results for the impacts of El Niño, NAO, and the PNA on NH storm tracks. Although it is encouraging, we also found regional differences when comparing reanalysis datasets. The results for the IOD suggested a wave-like alteration of cyclone frequency across the northern US/Canada possibly related to Rossby wave propagation. Partial correlation demonstrates that although El Niño affects cyclone frequency in the North Pacific and along the US east coast, its impact on the North Pacific is accomplished via the PNA. Similarly, the PNA’s impact on US east coast storms is modulated via El Niño. In contrast, the impacts of the NAO extend as far west as the North Pacific and are not influenced by either the PNA or El Niño.

Abstract. The impact of large-scale circulation on urban gaseous elemental mercury (GEM) was investigated through analysis of 2008–2015 measurement data from an urban site in New York City (NYC), New York, USA. Distinct annual cycles were observed in 2009–2010 with mixing ratios in warm seasons (i.e., spring–summer) 10–20 ppqv ( ∼ 10–25 %) higher than in cool seasons (i.e., fall–winter). This annual cycle was disrupted in 2011 by an anomalously strong influence of the US East Coast trough in that warm season and was reproduced in 2014 associated with a particularly strong Bermuda High. The US East Coast trough axis index (TAI) and intensity index (TII) were used to characterize the effect of the US East Coast trough on NYC GEM, especially in winter and summer. The intensity and position of the Bermuda High appeared to have a significant impact on GEM in warm seasons. Regional influence on NYC GEM was supported by the GEM–carbon monoxide (CO) correlation with r of 0.17–0.69 (p ∼ 0) in most seasons. Simulated regional and local anthropogenic contributions to wintertime NYC anthropogenically induced GEM concentrations were averaged at ∼ 75 % and 25 %, with interannual variation ranging over 67 %–83 % and 17 %–33 %, respectively. Results from this study suggest the possibility that the increasingly strong Bermuda High over the past decades could dominate over anthropogenic mercury emission control in affecting ambient concentrations of mercury via regional buildup and possibly enhancing natural and legacy emissions.

The global statistical median behavior of the electron temperature (Te) in the topside ionosphere was investigated through in-situ data collected by Langmuir Probes on-board the European Space Agency Swarm satellites constellation from the beginning of 2014 to the end of 2020. This is the first time that such an analysis, based on such a large time window, has been carried out globally, encompassing more than half a solar cycle, from the activity peak of 2014 to the minimum of 2020. The results show that Swarm data can help in understanding the main features of Te in the topside ionosphere in a way never achieved before. Te data measured by Swarm satellites were also compared to data modeled by the empirical climatological International Reference Ionosphere (IRI) model and data measured by Jicamarca (12.0°S, 76.8°W), Arecibo (18.2°N, 66.4°W), and Millstone Hill (42.6°N, 71.5°W) Incoherent Scatter Radars (ISRs). Moreover, the correction of Swarm Te data recently proposed by Lomidze was applied and evaluated. These analyses were performed for two main reasons: (1) to understand how the IRI model deviates from the measurements; and (2) to test the reliability of the Swarm dataset as a new possible dataset to be included in the underlying empirical dataset layer of the IRI model. The results show that the application of the Lomidze correction improved the agreement with ISR data above all at mid latitudes and during daytime, and it was effective in reducing the mismatch between Swarm and IRI Te values. This suggests that future developments of the IRI Te model should include the Swarm dataset with the Lomidze correction. However, the existence of a quasi-linear relation between measured and modeled Te values was well verified only below about 2200 K, while for higher values it was completely lost. This is an important result that IRI Te model developers should properly consider when using the Swarm dataset.

Observations of a positive storm phase on September 10, 2005

O2+ Temperature Profiles Measured in the Martian Ionosphere

ABSTRACTExtratropical cyclone (ETC) tracks across eastern Canada are examined by applying a Lagrangian tracking algorithm to the lower‐tropospheric relative vorticity field of reanalysis data. Both the seasonal cycle and the interannual variability of ETCs are quantified in terms of overall cyclone frequency, intensity, and regions of development and decay. We find that ETCs travelling to eastern Canada tend to develop over the Rockies, the Great Lakes and the US East Coast. The ETCs are most intense over Newfoundland and the North Atlantic Ocean, confirming previous findings. While ETCs at cities along the Atlantic coastline (e.g. St. John's) are dominated by East Coast cyclones (which are intense in winter), those inland (e.g. Toronto) track primarily from the Great Lakes. ETCs that develop over the Gulf of Mexico affect eastern Canada infrequently, but those that do tend to be intense. The interannual variability of the wintertime ETCs is influenced by the El Niño‐Southern Oscillation (ENSO). Significant ENSO‐related variability is found over most regions of southern Canada, except on the east coast. Although ETCs at Toronto are significantly modulated by ENSO, no visible changes are found at St. John's. These ENSO‐related ETC changes are mostly due to the shifts in ETC development regions, with minor changes in the travelling direction of ETCs.

The center path of the 21 June 2020 solar eclipse, which passed through Guam (13.62°N, 144.86°E, 94.6% obscuration), United States, at the end of its journey, provides a peculiar opportunity to study the ionospheric changes as the moon shadow moves into the nightside. In this study, remote-sensing observations of the ionosphere taken from the ionosonde, and total electron content from the International GNSS Service over Guam were analyzed to examine the related ionospheric changes. Independent in situ observations of electron density (Ne), electron (Te) and ion temperatures (Ti) from DMSP-F17 at ~850 km, and Ne and Te from Swarm-B at ~540 km were also studied. With a significant enhancement of the critical frequency and downward movement of F2-layer, the ionosphere was compressed as the moon shadow swept over Guam near its sunset hours. Neutral wind observations from the Ionospheric Connection Explorer (ICON) showed the westward reversed wind occurring in the F-region near sunset. The westward wind disturbance and downward press over the Western Pacific suggest changes in the electrodynamics in the ionosphere and thermosphere near sunset at the end of the solar eclipse, which further contributes to ~85% decrease of Ne and 157% enhancement of Te at ~540 km near midnight.