The bottom part of the Earth's ionosphere is the so-called D region, which is typically less dense than the upper regions. Despite the comparably lower electron density, the ionization state of the D region has a significant influence on signal absorption for propagating lower to medium radio frequencies. We present local noon climatologies of electron densities in the upper middle atmosphere (50–90 km) at high latitudes as observed by an active radar experiment. The radar measurements cover 9 years (2014–2022) from the solar maximum of cycle 24 to the beginning of cycle 25. Reliable electron densities are derived by employing signal processing, applying interferometry methods, and applying the Faraday-International Reference Ionosphere (FIRI) model. For all years a consistent spring–fall asymmetry of the electron density pattern with a gradual increase during summer as well as a sharp decrease at the beginning of October was found. These findings are consistent with very low frequency (VLF) studies showing equivalent signatures for nearby propagation paths. It is suggested that the meridional circulation associated with downwelling in winter could cause enhanced electron densities through NO transport. However, this mechanism can not explain the reduction in electron density in early October.

Climatological structure of the quasi-2-day wave (Q2DW) at middle latitudes in temperature and horizontal winds in the mesosphere and lower thermosphere (MLT) was compared between the northern and southern hemispheres. Determination of the Q2DW in temperature was based on observation data by the Microwave Limb Sounder (MLS) onboard NASA's Earth Observing System (EOS) Aura satellite over 17 years from August 2004 to May 2021 and the Q2DW in horizontal winds was derived from Aura/MLS geopotential height data using balance equations. Amplitudes were maximized in summer in the southern hemisphere and in the meridional wind in the northern hemisphere, but in winter in the zonal wind in the northern hemisphere. Summer amplitudes were larger in the meridional wind than the zonal wind in the southern hemisphere, but zonal amplitudes in winter were larger than meridional amplitudes in summer in the northern hemisphere. Westward propagating zonal wavenumber 3 (W3) was largest in both hemispheres, but in addition to well-known W4, W3, W2 and eastward propagating zonal wavenumber 2 (E2), we also found W1, zonally symmetric standing (S0), and E1. EliassenPalm fluxes were derived for each mode. W3, W2, W1, and E2 fluxes were exhibited upward and poleward in January in the southern hemisphere while only W3 fluxes were exhibited clearly upward and poleward in July in the northern hemisphere. The balance winds and radar winds agreed in both amplitude and phase in the southern hemisphere and at lower latitudes in the northern hemisphere in January, and at lower latitudes in both hemispheres in July. Furthermore, the Q2DW is modulation in amplitude and phase from the W3 by accumulating other modes.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Atmospheres. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Interhemispheric Comparisons of Structure and Variability of the Quasi-2-Day Wave at Middle and High LatitudesAuthors Hiroyuki Iimura iD David C. Fritts iD Ruth S. Lieberman Diego Janches Nicholas John Mitchell Werner Singer Steven J. Franke Wayne K. Hocking iDSee all authors Hiroyuki IimuraiDCorresponding AuthorGATS Inc.iDhttps://orcid.org/0000-0001-9174-8396view email addressThe email was not providedcopy email addressDavid C. FrittsiDGATSiDhttps://orcid.org/0000-0002-6402-105Xview email addressThe email was not providedcopy email addressRuth S. LiebermanNASA Goddardview email addressThe email was not providedcopy email addressDiego JanchesNational Aeronautics and Space Administration (NASA)view email addressThe email was not providedcopy email addressNicholas John MitchellUniversity of Bathview email addressThe email was not providedcopy email addressWerner SingerLeibniz-Institute of Atmospheric Physics (LG)view email addressThe email was not providedcopy email addressSteven J. FrankeUniversity of Illinois at Urbana Champaignview email addressThe email was not providedcopy email addressWayne K. HockingiDUniversity of Western OntarioiDhttps://orcid.org/0000-0003-0111-8804view email addressThe email was not providedcopy email address

Leibniz Institute of Atmospheric Physics, University of Rostock, Kuhlungsborn, Germany, Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum, India, Indian Institute of Geomagnetism, Navi Mumbai, India, Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism, Tirunelveli, India, German Aerospace Center Institute of Atmospheric Physics (IPA), Oberpfaffenhofen, Wessling, Germany

Gravity waves (GW) are important for the coupling between the different regions of the middle atmosphere. They are normally generated in the troposphere, are filtered by the wind field in the stratosphere and lower mesosphere and dissipate at least partly in upper mesosphere and lower thermosphere (MLT). The activity of gravity waves, their filtering by the mean circulation, and the variation of GW activity with solar activity have been studied using long-term wind measurements with Medium Frequency (MF) radars and meteor radars at high and middle northern latitudes. The GW activity is characterized by a semi-annual variation with a stronger maximum in winter and a weaker in summer consistent with the selective filtering of westward and eastward propagating GWs by the mean zonal wind. The latitudinal variation of GW activity shows the largest values in summer at mid-latitudes between 65 km and 85 km accompanied with an upward shift of the height of wind reversal towards the pole. Long-term observations of the MLT winds at mid latitudes indicate a stable increase of westward directed winds below about 85 km and an increase of eastward directed winds above 85 km especially during summer. The observed long-term trend of zonal wind at about 75 km goes along with an enhanced activity of GWs with periods of 3 to 6 hours at altitudes between 80 km and 88 km. In addition, the mesosphere responds to severe solar proton events (SPE) with increased eastward directed winds above about 85 km. The vertical coupling from the troposphere up to the lower thermosphere due to gravity waves and planetary waves is discussed for major sudden stratospheric warmings (SSW) for the winters 2006 and 2009.

The ALOMAR eARI Hotel Payload 2 (HotPay 2) rocket campaign took place at Andoya Rocket Range, Norway, in January 2008. The rocket was launched on January 31, 2008 at 19:14 UT, when auroral activity appeared after a long geomagnetically quiet period. In this paper we present an overview of the HotPay 2 measurements of upper mesospheric and lower thermospheric (UMLT) electron, atomic oxygen (O) and nitric oxide (NO) densities. [O] and [NO] were retrieved from a set of three photometers, Night-Time Emissions from the Mesosphere and Ionosphere (NEMI). Faraday rotation receivers on the rocket and the EISCAT UHF incoherent scatter radar provided simultaneous electron density profiles, whereas the ALOMAR Na lidar and meteor radar measured the temperature profile and wind. The aurora was also observed with ground-based imagers.The retrieved oxygen number density profile has a maximum at 89 km, some 10 km lower than expected from earlier measurements and modelled profiles based on climatological averages (such as the MSIS model), and the retrieved NO densities are also lower than the expected. Satellite measurements indicate that subsidence over the winter pole controlled the densities. Quantitative chemistry model results based on climatological average atmospheric density and temperature profiles were, therefore, not in good agreement with the measured profiles. The Hotel Payload 2 measurements thus confirm the importance of downward transport from the thermosphere into the winter polar vortex. (C) 2011 Elsevier Ltd. All rights reserved.

The seasonal variation of the wave activity in the mesosphere/lower thermosphere is investigated using wind measurements with meteor and MF radars at Juliusruh (55°N, 13°E) and Andenes (69°N, 16°E), as well as on the basis of the simulated annual cycle using a gravity-wave resolving mechanistic general circulation model. For the observations, proxies for the activity of gravity waves (GWs) and waves with longer periods are computed from wind variances for defined bandwidths. Our corresponding proxy for the simulated GWs is the non-rotational kinetic energy due to the resolved mesoscales. Both observational and computational results show the strongest GW energy during winter and a secondary maximum during summer. Additional observational analysis of short-period GWs yields a more pronounced summer maximum. The semi-annual variation is consistent with the selective filtering of westward and eastward GWs by the mean zonal wind. The latitudinal dependence during summer is characterized by stronger GW energy between 65 and 85km at middle latitudes than at polar latitudes, and a corresponding upward shift of the wind reversal towards the pole which is also reflected by the simulated GW drag. Also the observed oscillations with periods from 2 to 4 days show a latitudinal dependence and a clear seasonal cycle which is related to the mean zonal wind shear.

For the analysis of gravity waves the method presented by Hocking (2005) is used, which enables us to derive wind variances and gravity wave momentum fluxes in the mesosphere and lower thermosphere from all-sky interferometric meteor radar wind measurements considering waves and variances with periods less than 2h. A sensitivity study for the applicability of this method has been performed for the first time using a mechanistic general circulation model with high spatial resolution and explicit description of gravity waves. Wind variances and momentum fluxes have been determined from the model directly and by Hocking’s method. Results of both methods are in good agreement except for vertical wind variances in case of weak vertical winds, which in the model are of the order of 1m/s, whereas short period gravity waves estimated by meteor radar lead to larger vertical winds with a smaller ratio between horizontal and vertical wind fluctuations. A latitudinal comparison of mean annual variations of wind variances and momentum fluxes has been performed using meteor radar measurements at the high latitude site Andenes (69.3°N, 16.0°E) and the midlatitude site Juliusruh (54.6°N, 13.4°E). A semi-annual variation of the activity of short period gravity waves has been found having stronger magnitudes at high latitudes. The mean zonal winds show the typical summer wind reversal that shifts to higher altitudes from middle to high latitudes. Finally, the coupling between gravity waves and the mean background circulation is investigated based on long-term measurements at Andenes and the midlatitude site Collm (51.3°N, 13.0°E) during a period from 2004 to 2009.

A global numerical weather prediction system is extended to the mesosphere and lower thermosphere (MLT) and used to assimilate high-altitude satellite measurements of temperature, water vapor and ozone from MLS and SABER during May–July 2007. Assimilated temperature and humidity from 100 to 0.001hPa show minimal biases compared to satellite data and existing analysis fields. Saturation ratios derived diagnostically from these assimilated temperature and water vapor fields at PMC altitudes and latitudes compare well with seasonal variations in PMC frequency measured from the aeronomy of ice in the mesosphere (AIM) satellite. Synoptic maps of these diagnostic saturation ratios correlate geographically with three independent transient mesospheric cloud events observed at midlatitudes by SHIMMER on STPSat-1 and by ground observers during June 2007. Assimilated temperatures and winds reveal broadly realistic amplitudes of the quasi 5-day wave and migrating tides as a function of latitude and height. For example, analyzed winds capture the dominant semidiurnal MLT wind patterns at 55°N in June 2007 measured independently by a meteor radar. The 5-day wave and migrating diurnal tide also modulate water vapor mixing ratios in the polar summer MLT. Possible origins of this variability are discussed.

The Leibniz-Institute of Atmospheric Physics in Kuhlungsborn, Germany (IAP) is installing a new powerful VHF radar on the North-Norwegian island Andoya (69.30 degrees N, 16.04 degrees E) in 2009. The new radar replaces the existing ALWIN radar which has been operated continuously on Andoya for more than 10 years. The new system will be operated again at 53.5 MHz. The phased-array antenna is designed by IAP and consists of 433 Yagi antennas. The 3-element Yagi antennas are arranged in an equilateral triangle grid forming a circular aperture of approximately 6300m(2). Each individual antenna is connected to its own transceiver with independent phase control and a scalable output up to 2kW. This arrangement allows very high flexibility of beam forming and beam steering with a symmetric radar beam of a minimum half power beam width of 3.6 degrees, a maximum directive gain of 33.5 dB and a total transmitted peak power of approximately 800kW. The IF signals of each 7 transceivers connected to each 7 antennas arranged in a hexagon are combined to 61 receiving channels. Selected channels or several combination of IF signals are sent to a 16-channel data acquisition system with 25 meter sampling resolution and 16-bit digitisation specified which will be upgraded to 64 channels in the final stage. The high flexibility of the new system allows classical Doppler beam swinging as well as experiments with simultaneously formed multiple beams and the use of modern interferometric applications for improved studies of the Arctic atmosphere from the troposphere up to the lower thermosphere with high spatiotemporal resolution.

Radar measurements with a high temporal and vertical resolution were conducted at Andenes, North Norway, during the ECOMA rocket campaign in 2008. The results obtained from these measurements show a distinct double layer structure of PMSE for the period of the ECOMA-04 rocket flight at altitudes between 80 and 90 km. The structure of these radar echoes at 50 MHz are investigated in more detail, including the aspect sensitivity, turbulence, and the simultaneously measured background wind field. It turns out that the radar echoes during this rocket flight are highly aspect sensitive with different aspect ratios across the observed altitude range. The in-situ turbulence observation using rocket-borne instruments allows a more detailed discussion of the fine structure observed in the radar sounding. The results indicate strong anisotropic scattering structures, especially at bounderies between strong turbulent and less turbulent altitudes.

VHF Radar echoes in the summer mesosphere at mid- and polar latitudes ([P]MSE—[polar] mesosphere summer echoes) are connected with very cold temperatures where ice particles can exist. Temperature variations can cause conditions for the generation and evaporation of ice particles and affect the [P]MSE occurrence. The impact of temperature and meridional wind oscillations on [P]MSE is described. Generally at mid-latitudes, strong mesosphere summer echoes are strongly affected by meridional wind variations if the mean temperature is near the frost point of water vapor. In contrast, at polar latitudes there is mostly no significant impact of the meridional wind on radar echoes. A mean temperature well below the frost point and a weaker meridional temperature gradient than at mid-latitudes are reasons for this reduced influence. Due to higher temperatures in 2002, long period temperature and meridional wind variations impact the PMSE more than during the other years.

An inter-hemispheric asymmetry is found in the characteristics of polar mesosphere summer echoes (PMSE) and upper mesosphere temperatures at conjugate latitudes (∼69°) above Antarctica and the Arctic. The second complete mesosphere–stratosphere–troposphere (MST) radar summer observation season at Davis (68.6°S) revealed that PMSE occur less frequently, with lower strength and on average 1km higher compared with their northern counterparts at Andenes (69.3°N). We consider the thermodynamic state of the mesosphere for conjoining hemispheric summers based on satellite and ground-based radar measurements, and show the mesopause region near ∼80–87km of the Southern Hemisphere (SH) to be up to 7.5K warmer than its Northern Hemisphere (NH) counterpart. We show that this is consistent with our observation of asymmetries in the characteristics of PMSE and demonstrate how the mesosphere meridional wind field influences the existence and strength of the echoes in both hemispheres.