GRIPS-HI, a novel spectral imager for ground based measurements of mesopause temperatures

The NDMC (Network for the Detection of Mesospheric Change) is a global network of measurement sites dedicated to the surveillance of the mesopause region. One main objective of the network is the early identification of climate signals. A key parameter is the mesopause temperature which can be derived from the emission spectrum of a layer of vibrationally excited hydroxyl (OH) at an altitude of approximately 87 km 87\hspace{0.1667em}\text{km} . Foremost, emission lines in the SWIR regime between 1520 nm 1520\hspace{0.1667em}\text{nm} and 1550 nm 1550\hspace{0.1667em}\text{nm} are of interest for remote temperature sensing. This report deals with the development of a new generation of GRIPS instruments, which are commonly employed for the observation of mesopause temperatures. The new prototype demonstrates how the application of so called Spatial Heterodyne Interferometers (SHI) can overcome the limitations of currently used grating spectrometers, in terms of spectral resolution and optical throughput. The presented prototype proposes improvements in optical throughput and spectral resolution of about one order of magnitude, significantly reducing the uncertainties of the measured mesopause temperatures. Furthermore, an SHI can be built in monolithic configurations which are aligned and characterized once during assembly without the need of realignment at the measurement site. This makes SHI based instruments ideal for mobile applications.

What caused the frequent and widespread occurrences of noctilucent clouds at middle latitudes in 2020?

The 2020 summer season has revealed frequent occurrences of noctilucent clouds (NLCs) around the Northern hemisphere at middle latitudes (45–55° N), with the lowest latitude at which NLCs were seen being 34.1° N. In order to investigate a reason for this NLC extraordinary summer season, we have analyzed long-term Aura/MLS satellite data for all available summer periods from 2005 to 2020. Both Aura/MLS summer temperature and water vapor in the upper mesosphere and the mesopause region, between 74 and 89 km altitude, have been considered. We have found that there has been a moderate decrease in the upper mesosphere temperature between 2016 and 2020 and no dramatic changes have been observed in temperature in the summer of 2020 at the middle latitude mesopause. At the same time, water vapor concentration has significantly increased (by about 12–15 %) in the zonal mean H2O value in the 2020 summer compared to 2017, meaning that the summer mesopause at middle latitudes has become more wet. At the same time, no increase in water vapor has been detected at the high latitude high altitude mesopause. A combination of lower mesopause temperature and water vapor concentration maximum at middle latitudes is the main reason for frequent and widespread occurrences of NLCs seen around the globe at middle latitudes in the summer of 2020. The 24th solar cycle minimum cannot explain the H2O maximum in 2020 since the correlation between Lyman-α flux and the amount of water vapor is low. The increase in volcanic activity from 2013 to 2015 (and its recent maximum occurred in 2015) explains the increased amount of water vapor in the upper mesosphere for the past years and its maximum in 2020 due to volcanic water vapor being injected into the atmosphere and transported into the upper mesosphere. The 5-year delay between volcanic activity and water vapor maximum is well explained by a general meridional-vertical atmospheric circulation.

Temporal Evolutions of N2+ Meinel (1,2) Band Near 1.5 μm Associated with Aurora Breakup and their Effects on Mesopause Temperature Estimations from OH Meinel (3,1) Band

We have carried out ground-based NIRAS (Near-InfraRed Aurora and airglow Spectrograph) observations at Syowa station, Antarctic (69.0°S, 39.6°E) and Kiruna (67.8°N, 20.4°E), Sweden for continuous measurements of hydroxyl (OH) rotational temperatures and a precise evaluation of aurora contaminations to OH Meinel (3,1) band. A total of 368-nights observations succeeded for two winter seasons, and three cases in which N+2 Meinel (1,2) band around 1.5 μm was significant were identified. Focusing on two specific cases, detailed spectral characteristics with high temporal resolutions of 30 seconds are presented. Intensities of N+2 band were estimated to be 228 kR and 217 kR just at the moment of the aurora breakup and arc intensifications during pseudo breakup, respectively. At a wavelength of P1(2) line (∼ 1523 nm), N+2 emissions were almost equal to or greater than the OH line intensity. On the other hand, at a wavelength of P1(4) line (∼ 1542 nm), the OH line was not seriously contaminated and still dominant to N+2 emissions. Furthermore, we evaluated N+2 (1,2) band effects on OH rotational temperature estimations quantitatively for the first time. Aurora contaminations from N+2 (1,2) band basically lead negative bias in OH rotational temperature estimated by line-pair-ratio method with P1(2) and P1(4) lines in OH (3,1) band. They possibly cause underestimations of OH rotational temperatures up to 40 K. In addition, N+2 (1,2) band contaminations were temporally limited to a moment around aurora breakup. This is consistent with proceeding studies reporting that enhancements of N+2 (1,2) band were observed associated with International Brightness Coefficient 2-3 auroras. It is also suggested that the contaminations would be neglected in polar cap and sub-aurora zone, where strong aurora intensifications are less observed. Further spectroscopic investigations at this wavelength are needed especially for more precise evaluations of to N+2 (1,2) band contaminations. For example, simultaneous 2-D imaging observation and spectroscopic measurement with high spectral resolutions for airglow in OH (3,1) band will make great advances in more robust temperature estimations.

Correlation between wave activities in different layers of the atmosphere

<p>We study the correlation between wave activities in different layers of the atmosphere. The variability of the measured characteristic in the range of internal gravity wave periods is used as a proxy of wave activity. In the case of ground-based measurements, we consider temporal variations with periods less than ~ 6 hours; while in the case of satellite measurements we take into account spatial variations with periods less than ~ 1000 km. The wave activity is calculated as the standard deviation of variations in the indicated period range with averaging over one day. The aim of the study is to detect a correlation between day-to-day variations of wave activity in different layers of the atmosphere. Correlation coefficients are calculated for various intervals from one month to one year. Correlation analysis reveals the potential relationship between wave phenomena in the stratosphere, mesosphere and ionosphere. The study uses the following characteristics. The ionospheric characteristics are the peak electron density from the Irkutsk ionosonde (52.3 N, 104.3 E) and the total electron content from the Irkutsk GPS receiver. The characteristic of the mesosphere is the mesopause temperature from spectrometric measurements of the OH emission (834.0 nm, band (6-2)) near Irkutsk (51.8 N, 103.1 E, Tory). The stratospheric characteristic is the vertical gas velocity at 1 hPa from the ERA-Interim reanalysis (apps.ecmwf.int/datasets/data/).</p><p>This study was supported by the Grant of the Russian Science Foundation (Project N 18-17-00042). The observational results were obtained using the equipment of Center for Common Use «Angara» http: //ckp-rf.ru/ckp/3056/ within budgetary funding of Basic Research program II.12.</p>

GRIPS-HI - a Novel Interferometer for Measuring Two-Dimensional Temperature Distributions at the Mesopause

<p>The NDMC (<em>Network for the Detection of Mesopause Change</em>) is a global network of ground based observatories with the objective of monitoring key parameters of the mesopause region. For temperature monitoring GRound-based Infrared P-branch Spectrometers (GRIPS) are widely deployed. These spectrometers allow for the retrieval of the mesopause temperature from the OH* P-band emission lines around 1530 nm. A common technology for GRIPS instruments are spectrometers based on diffraction gratings. To overcome the limitations of conventional grating spectrometers, a new type of spectrometer is being developed within the project <em>Metrology for Earth Observation and Climate - 3</em> (MetEOC-3) which is coordinated by the <em>European Metrology Project for Innovation and Research</em> (EMPIR). The new spectrometer shall improve the quality and traceability of the atmospheric data obtained by the NDMC. It is intended to serve as a reference instrument with significantly smaller measurement uncertainties. It is also designed to identify temperature trends of 1K/decade. A Spatial Heterodyne Interferometer (SHI) was chosen as the most promising technology, offering several advantages. Compared to conventional grating spectrometers, the throughput and resolution of the interferometer is one order of magnitude larger. The use of a two-dimensional detector array in combination with an imaging optics enables the detection of spatial temperature distributions in the mesopause region, as caused by dynamical processes like gravity waves. The talk gives an introduction to the technology of spatial heterodyne interferometry, and the new instrument design and calibration results are presented.</p>