The polar
This paper reports our simulations of the volume emission rate of the O(1
- NSF-PAR ID:
- 10450828
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 124
- Issue:
- 11
- ISSN:
- 2169-9380
- Page Range / eLocation ID:
- p. 9348-9363
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract F region ionosphere frequently exhibits sporadic variability (e.g., Meek, 1949,https://doi.org/10.1029/JZ054i004p00339 ; Hill, 1963,https://doi.org/10.1175/1520‐0469(1963)020<0492:SEOLII>2.0.CO;2 ). Recent satellite data analysis (Noja et al., 2013,https://doi.org/10.1002/rds.20033 ; Chartier et al., 2018,https://doi.org/10.1002/2017JA024811 ) showed that the high‐latitudeF region ionosphere exhibits sporadic enhancements more frequently in January than in July in both the northern and southern hemispheres. The same pattern has been seen in statistics of the degradation and total loss of GPS service onboard low‐Earth orbit satellites (Xiong et al. 2018,https://doi.org/10.5194/angeo‐36‐679‐2018 ). Here, we confirm the existence of this annual pattern using ground GPS‐based images of TEC from the MIDAS algorithm. Images covering January and July 2014 confirm that the high‐latitude (>70 MLAT)F region exhibits a substantially larger range of values in January than in July in both the northern and southern hemispheres. The range of TEC values observed in the polar caps is 38–57 TECU (north‐south) in January versus 25–37 TECU in July. First‐principle modeling using SAMI3 reproduces this pattern, and indicates that it is caused by an asymmetry in plasma levels (30% higher in January than in July across both polar caps), as well as 17% longer O+plasma lifetimes in northern hemisphere winter, compared to southern hemisphere winter. -
more » « less
Abstract The D‐region ionosphere (60
90 km) plays an important role in long‐range communication and response to solar and space weather; however, it is difficult to directly measure with currently available technology. Very low frequency (VLF) radio remote sensing is one of the more promising approaches, using the efficient reflection of VLF waves from the D‐region. A number of VLF beacons can therefore be turned into diagnostic tools. VLF remote sensing techniques are useful and can provide global coverage, but in practice have been applied to a limited area and often on only a small number of days. In this work, we expand the use of a recently introduced machine learning based approach (Gross & Cohen, 2020, https://doi.org/10.1029/2019JA027135 ) to observe and model the D‐region electron density using VLF transmitting beacons and receivers. We have extended the model to cover nighttime in addition to daytime, and have applied it to track D‐region waveguide parameters, h’ and, over 400 daytimes and 150 nighttimes on up to 21 transmitter‐receiver paths across the continental US. Using an exponential fit, h’ represents the height of the ionosphere and represents the slope of the electron density. Using this data set, we quantify diurnal, daily and seasonal variations of the D‐region ionosphere for both daytime and nighttime D‐region ionosphere. We show that our model identifies expected variations, as well as producing results in line with other previous studies. Additionally, we show that our daytime predictions exhibit a larger autocorrelation at higher time lags than our nighttime predictions, indicating a model with persistence may perform better. -
more » « less
Abstract Recent work has indicated the presence of a nitric oxide (NO) product channel in the reaction between the higher vibrational levels of the first electronically excited state of molecular nitrogen, N2(A
), and atomic oxygen. A steady‐state model for the N2(A) vibrational distribution in the terrestrial thermosphere is here described and validated by comparison with N2A‐X, Vegard‐Kaplan dayglow spectra from the Ionospheric Spectroscopy and Atmospheric Chemistry spectrograph. A computationally cheaper method is needed for implementation of the N2(A) chemistry into time‐dependent thermospheric models. It is shown that by scaling the photoelectron impact production of ionized N2by a Gaussian centered near 100 km, the level‐specific N2(A) production rates between 100 and 200 km can be reproduced to within an average of 5%. This scaling, and thus the N2electron impact ionization/excitation ratio, is nearly independent of existing uncertainties in the 2–20 nm solar soft X‐ray irradiance. To investigate this independence, the N2electron‐impact excitation cross sections in the GLOW photoelectron model are replaced with the results of Johnson et al. (2005, https://doi.org/10.1029/2005JA011295 ) and the multipart work of Malone et al. (2009https://doi.org/10.1103/PhysRevA.79.032704 ) (Malone, Johnson, Young, et al., 2009,https://doi.org/10.1088/0953-4075/42/22/225202 ; Malone, Johnson, Kanik, et al., 2009,https://doi.org/10.1103/PhysRevA.79.032705 ; Malone et al., 2009,https://doi.org/10.1103/PhysRevA.79.032704 ), together denotedJ 05M 09. Upon updating these cross sections it is found that (1) the total N2triplet excitation rate remains nearly constant; (2) the steady state N2(A) vibrational distribution is shifted to higher levels; (3) the total N2singlet excitation rate responsible for the Lyman‐Birge‐Hopfield emission is reduced by 33%. It is argued that adopting theJ 05M 09 cross sections supports (1) the larger X‐ray fluxes measured by the Student Nitric Oxide Explorer (SNOE) and (2) a temperature‐independent N2(A)+O reaction rate coefficient. -
more » « less
Abstract Atomic oxygen (O) in the mesosphere and lower thermosphere (MLT) results from a balance between production via photo‐dissociation in the lower thermosphere and chemical loss by recombination in the upper mesosphere. The transport of O downward from the lower thermosphere into the mesosphere is preferentially driven by the eddy diffusion process that results from dissipating gravity waves and instabilities. The motivation here is to probe the intra‐annual variability of the eddy diffusion coefficient (k
zz ) and eddy velocity in the MLT based on the climatology of the region, initially accomplished by Garcia and Solomon (1985,https://doi.org/10.1029/JD090iD02p03850 ). In the current study, the intra‐annual cycle was divided into 26 two‐week periods for each of three zones: the northern hemisphere (NH), southern hemisphere (SH), and equatorial (EQ). Both 16 years of SABER (2002–2018) and 10 years of SCIAMACHY (2002–2012) O density measurements, along with NRLMSIS®2.0 were used for calculation of atomic oxygen eddy diffusion velocities and fluxes. Our prominent findings include a dominant annual oscillation below 87 km in the NH and SH zones, with a factor of 3–4 variation between winter and summer at 83 km, and a dominant semiannual oscillation at all altitudes in the EQ zone. The measured global average kzz at 96 km lacks the intra‐annual variability of upper atmosphere density data deduced by Qian et al. (2009,https://doi.org/10.1029/2008JA013643 ). The very large seasonal (and hemispherical) variations in kzz and O densities are important to separate and isolate in satellite analysis and to incorporate in MLT models. -
more » « less
Abstract Estimates of ice volume over the last 120 ka, from marine isotope Stage (MIS) 5d (∼110 ka) through MIS 3 (60–26 ka) are uncertain. Weiss et al. (2022,
https://doi.org/10.1029/2021PA004361 ) offer an innovative new constraint on past sea level using the oxygen isotopes (δ 18O) of planktic (surface and thermocline dwelling) foraminifers to infer the salinity of the Sulu Sea in the Indo‐Pacific Ocean and assess flow through the Karimata Strait (Indonesia) over the last glaciation. Based on the timing of Karimata Strait flooding, the study concludes that local relative sea level in the Karimata Strait was >−86 m during MIS 5c (∼100 ka) and >−12 6 m during MIS 5a (∼80 ka), relative to present. For MIS 3, a maximum possible relative sea level of −16 6 m is determined. Here, these results are placed into the context of current knowledge of last glacial sea‐level change and the implications for climate forcings and feedbacks (e.g., global average surface temperature and greenhouse gases) and ice sheet growth are discussed. By tracing past ocean circulation patterns that are modulated by the depth of shallow straits such as the Karimata Strait, Weiss et al. (2022, https://doi.org/10.1029/2021PA004361 ) provide independent constraints on local sea level, which are essential for improving global mean sea level reconstructions on late Pleistocene glacial‐interglacial cycles.
