. Polar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is effectively determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition for monostatic radars; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5–50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the neutral gas turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e., 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like neutral temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes.

/pdf/polar-mesosphere-summer-echoes-pmse-review-of-observations-3mmz8vo62v.pdf

Polar mesosphere summer echoes (PMSE): Review of observations and current understanding

Digisonde ionospheric sounders installed at 80+ locations in the world have gradually evolved their generally independent existence into a Global Ionospheric Radio Observatory (GIRO) portal. Today GIRO provides public access to 30+ million records of ionospheric measurements collected at 64 locations, of which 42 provide realtime feeds, publishing their measurement data within several minutes from their completion. GIRO databases holding ionogram and Doppler skymap records of high-frequency ionospheric soundings have registered connections from 123 organizations in 33 countries. Easy access to the global state of the ionospheric plasma distribution given in accurate and fine detail by the ionosonde measurements has inspired a number of studies of the ionospheric response to space weather events. Availability of GIRO data with minimal latency allows for the assimilation of the ionogram-derived data in real-time models such as the real-time extension planned for the International Reference Ionosphere.

/pdf/global-ionospheric-radio-observatory-giro-3ytx3uz55g.pdf

Global Ionospheric Radio Observatory (GIRO)

A record of the geomagnetic field on the ground sometimes shows smooth daily variations on the order of a few tens of nano teslas. These daily variations, commonly known as Sq, are caused by electric currents of several $\upmu \mbox{A}/\mbox{m}^{2}$
 flowing on the sunlit side of the E-region ionosphere at about 90–150 km heights. We review advances in our understanding of the geomagnetic daily variation and its source ionospheric currents during the past 75 years. Observations and existing theories are first outlined as background knowledge for the non-specialist. Data analysis methods, such as spherical harmonic analysis, are then described in detail. Various aspects of the geomagnetic daily variation are discussed and interpreted using these results. Finally, remaining issues are highlighted to provide possible directions for future work.

/pdf/sq-and-eej-a-review-on-the-daily-variation-of-the-2vlnpg7jur.pdf

Sq and EEJ—A review on the daily variation of the geomagnetic field caused by ionospheric dynamo currents

The subject of this review is the atmospheric chemistry of the metals which ablate from meteoroids in the Earth’s upper atmosphere. The major meteoric species are Fe, Mg, Si, and Na, against which two minor species, Ca and K, offer surprising contrasts. These metals exist as layers of atoms between about 80 and 105 km and atomic ions at higher altitudes. Below 85 km they form compounds—oxides, hydroxides, and carbonates—which polymerize into nanometer-sized meteoric smoke particles (MSPs). These particles probably act as condensation nuclei for clouds in the mesosphere and stratosphere and eventually after about 4 years are deposited at the Earth’s surface. The subject of meteoric metal chemistry was reviewed in 19911 and 2003,2 and there were also more focused reviews on laboratory studies of metal reactions in 19943 and 20024 and the atmospheric modeling of metals in 2002.5

The present review will therefore concentrate on the many developments that have taken place in the past decade. On the observational side, these developments include the near-global measurement of the Na, K, Mg, and Mg+ layers from satellite-borne spectrometers and lidar observations of Na and Fe from several Antarctic observatories, the discovery that metal atoms are removed in the vicinity of noctilucent (or polar mesospheric) clouds, the surprising observation of metal atoms up to around 180 km in the thermosphere, the unexpected finding that the ratio of the Na d lines in the terrestrial nightglow is variable, the first observations of the molecular bands of FeO and NiO in the nightglow, the first measurements of the vertical flux of Na atoms in the upper mesosphere, the measurement of MSPs from rockets, incoherent scatter radars, satellites, and aircraft, and measurements of the depositional flux of meteoric smoke in polar ice cores.

Laboratory measurements (including the application of quantum chemistry calculations) have addressed several issues: the ion and neutral gas-phase chemistries of compounds containing Fe, Ca, Mg, Si, and K, leading to the first chemically closed reaction schemes for these metals, the uptake of metal atoms on low-temperature ice surfaces and the resulting photoelectric emission, understanding the variable Na d line ratio observations, and the formation of a variety of iron oxide and Fe–Mg–silicate nanoparticles as analogues of meteoric smoke.

There have also been significant developments in modeling: a chemical ablation model to predict the evaporation rates of individual elements from a meteoroid, coupling this ablation model with an astronomical model of dust input to generate the meteoric input function (MIF), the inclusion of the MIF together with metal chemistry in a whole atmosphere chemistry climate model to create the first global models of the Na, Fe, Mg, and K layers, an explanation for the 50 year old puzzle of why the Na and K layers exhibit such different seasonal behavior, modeling the growth and transport of MSPs through the mesosphere and stratosphere, the paleoclimate implications of an enhanced cosmic dust input, and the climate implications of the deposition of meteoric Fe into the Southern Ocean.

The present review is divided into five sections following this Introduction. Section 2 is a general review of the mesosphere and lower thermosphere from the perspective of understanding the metal layers and the sensitivity of this atmospheric region to solar activity and longer term anthropogenic changes. Section 3 describes the atmospheric chemistry of the meteoric metals and then reviews observations of the metal layers and MSPs. Section 4 deals with laboratory and theoretical studies of gas-phase metal reactions and particle formation under mesospheric conditions. Section 5 is concerned with the development of global models of metal chemistry which describe the input and ablation of cosmic dust, the gas-phase chemistry of metallic species, the formation of MSPs, and transport to the Earth’s surface. Section 6 is then a summary with a discussion of future directions for the field.

The mesosphere and metals: chemistry and changes.

The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some char- acteristics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the lowest overall temperature and has the unique characteristic that the temperature is much lower in summer than in winter. The summer-to-winter-tem- perature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynamics of the MLT and the inter- actions of dynamics with chemistry and radiation. This review describes recent observa- tions and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmosphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Radiometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and dis- crepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system.

/pdf/global-dynamics-of-the-mlt-27clum6d5c.pdf

Global Dynamics of the MLT

Long-term trends in foF2: A comparison of various methods

Solar cycle dependence and long-term trends in the wind field of the mesosphere/lower thermosphere

Meteor radar temperatures at multiple sites derived with SKiYMET radars and compared to OH, rocket and lidar measurements

Ground based ionosonde measurements are the most essential source of information about long-term varia- tions in the ionospheric E and F1 regions. Data of such ob- servations have been derived at many different ionospheric stations all over the world some for more than 50 years. The standard parameters foE, h'E, and foF1 are used for trend analyses in this paper. Two main problems have to be con- sidered in these analyses. Firstly, the data series have to be homogeneous, i.e. the observations should not be disturbed by artificial steps due to technical reasons or changes in the evaluation algorithm. Secondly, the strong solar and geo- magnetic influences upon the ionospheric data have carefully to be removed by an appropriate regression analysis. Other- wise the small trends in the different ionospheric parameters cannot be detected. The trends derived at individual stations differ markedly, however their dependence on geographic or geomagnetic lat- itude is only small. Nevertheless, the mean global trends es- timated from the trends at the different stations show some general behaviour (positive trends in foE and foF1, negative trend in h'E ) which can at least qualitatively be explained by an increasing atmospheric greenhouse effect (increase of CO2 content and other greenhouse gases) and decreasing ozone values. The positive foE trend is also in qualitative agreement with rocket mass spectrometer observations of ion densities in the E region. First indications could be found that the changing ozone trend at mid-latitudes (before about 1979, between 1979 until 1995, and after about 1995) modi- fies the estimated mean foE trend.

/pdf/long-term-trends-in-the-ionospheric-e-and-f1-regions-3fpjlr20sa.pdf

J. Bremer

Papers

Long-term trends in foF2: A comparison of various methods

Solar cycle dependence and long-term trends in the wind field of the mesosphere/lower thermosphere

Meteor radar temperatures at multiple sites derived with SKiYMET radars and compared to OH, rocket and lidar measurements

Long-term trends in the ionospheric E and F1 regions

Mean characteristics of mesosphere winter echoes at mid- and high-latitudes