The sporadic sodium layer: a possible tracer for the conjunction between the upper and lower atmospheres

In this research, we reveal the inter-connection between lightning strokes, reversal of the electric field, ionospheric disturbances, and a sodium layer (NaS), based on the joint observations by a temperature/wind (T/W, where the slash means “and”) lidar, an ionosonde, an atmospheric electric mill, a fluxgate magnetometer, and the World Wide Lightning Location Network (WWLLN). Our results suggest that lightning strokes could trigger or amplify the formation of an NaS layer in a descending sporadic E layer (ES), through a mechanism that involves the overturning of the electric field. A conjunction between the lower and upper atmospheres could be established as follows by these inter-connected phenomena, and the key processes could be suggested to be: lightning strokes → overturning of the electric field → ES generating NaS.

Formation of an additional density peak in the bottom side of the sodium layer associated with the passage of multiple mesospheric frontal systems

We present a detailed investigation of the formation of an additional sodium density peak at altitudes of 79–85 km below the main peak of the sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway, on the night of 19 December 2014. The airglow imager observations of OH emissions revealed four passing frontal systems that resembled mesospheric bores, which typically occur in ducting regions of the upper mesosphere. For about 1.5 h, the lower-altitude sodium peak had densities similar to that of the main peak of the layer around 90 km. The lower-altitude sodium peak weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column-integrated sodium densities increased gradually during the formation of the lower-altitude sodium peak. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in the OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km, wherein the temperature was also higher. Both factors would have liberated sodium atoms from the reservoir species and suppressed the reconversion of atomic sodium into reservoir species so that the lower-altitude sodium peak could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

Sporadic sodium layer: A possible tracer for the conjunction between the upper and lower atmospheres

In this research, we reveal the inter-connection between lightning strokes, reversal of the electric field, ionospheric disturbances, and a trigger of sporadic sodium layer event (NaS), based on the joint observations by three lidars, an ionosonde, an atmospheric electric mill, a fluxgate magnetometer, and World Wide Lightning Location Network (WWLLN). Our results suggest that lightning strokes would probably have an influence on the ionosphere and thus give rise to the occurrence of NaS, with the overturning of electric field playing an important role. Statistical results reveal that the sporadic E layers (ES) could hardly be formed or maintained when the atmospheric electric field turns upward. A conjunction between the lower and upper atmospheres could be established by these inter-connected phenomena, and the key processes could be suggested as follows: lightning strokes→overturning of electric field→different collisional frequencies for ions and electrons→depletion of ES/generation of NaS.

Formation of a bottomside secondary sodium layer associated with the passage of multiple mesospheric frontal systems

We present a detailed investigation of the formation of a secondary sodium layer at altitudes of 79–85 km below the main sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway on the night of 19 December 2014. The airglow imager observations of OH emission revealed four passing frontal systems that resembled mesospheric bores which typically occur in ducting regions of the upper mesosphere. For about 1.5 hours, the lower altitude sodium layer had densities similar to that of the main layer with a peak around 90 km. The lower altitude sodium layer weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column integrated sodium densities increased gradually during formation of the lower altitude sodium layer. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km wherein the temperatures were also relatively high. This would have liberated sodium atoms from the reservoir species and suppressed the re-conversion of atomic sodium into reservoir species so that the lower altitude sodium layer could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

First measurements of the Mesospheric Magnetic Field in the Auroral Zone by means of laser

<p>In December 2019, for the first time, we were able to remotely measure the magnetic field in the mesospheric sodium layer, in the auroral zone.</p><p>By means of laser optical pumping and Larmor-resonance detection, it is possible to use the naturally occurring sodium layer in the mesosphere to measure Earth’s magnetic field magnitude at 90 km above ground. This is an altitude otherwise only accessible by rockets, which only will provide point measurements of very short time scales.</p><p>During the winter of 2019-20 we have applied a cw sum-frequency fasor/laser for probing the sodium-atom Larmor resonance at the Artic Lidar Observatory for Mesospheric Research (ALOMAR) at Andøya in northern Norway in order to measure and monitor the magnetic field in situ in the high latitude mesosphere over longer time scales.</p><p>The technique, which has been proved earlier at mid-latitudes, has now been confirmed and applied to high latitudes in the auroral zone during disturbed auroral and geomagnetic conditions. The magnetic field in the auroral zone is close to vertical making our measurements a notable achievement since the beam is closer to parallel with the magnetic field, contary to earlier measurements being closer to perpendicular as shown as best by theory.</p><p>This opens up for a completely new domain of measurements of externally generated geomagnetic variations related to currents in the magnetosphere-ionosphere system.</p><p>Here we report on the instrumental setup, and discuss our measurements of the mesospheric magnetic field.</p>

Mesospheric fronts in airglow images and the variation of the bottomside sodium layer densities measured by a sodium lidar at Tromsø, Norway

<p>Mesospheric frontal systems are waves extending to hundreds of kilometers along their phase fronts and appear like a boundary. They are observed in the upper mesospheric airglow imaging observations of OH, sodium and OI greenline nightglow emissions. It is believed that the fronts result from gravity wave dynamics associated with favorable background conditions like thermal ducting. Many of the frontal systems are identified as mesospheric bores when they are accompanied with sudden airglow intensity changes across the frontal boundary. Most of the frontal systems propagate with phase locked undulations following the leading front, while some induce turbulence behind the front. Though the existence of the frontal systems in the mesosphere is known for more than two decades, their role and importance is not understood properly. In this work, we use airglow data from an all-sky imager located at Tromsø to identify the frontal systems, particularly using OH images. Collocated five-beam sodium lidar measurements are used to identify the structuring in sodium densities around time of passage of the frontal systems. The sodium lidar at Tromsø is a versatile system capable of measuring sodium densities, temperatures and winds in the upper mesospshere region. Hence, we obtain the wind and temperature information to study the background conditions during passage of the intense frontal systems. Though, mostly we focus on OH airglow images as they are observed with broad pass band resulting in higher signal strength, we also utilize images from other emissions like OI greenline and sodium whenever they are available and free from auroral features. Interestingly, we find formation of some unusual structuring in the bottomside sodium layer around the passage of the frontal systems. We show different cases during winter months of the years 2013-14 and 2014-15 and investigate the relationship between unusual bottomside structuring in the sodium layer and passage of the frontal systems.</p>