The mesospheric sodium layer as a remotely, optically pumped magnetometer for investigation of Birkeland currents

In this project, we plan to study the aurora by making the first long-term in situ measurements of the geomagnetic field in the actual auroral region. Geomagnetic measurements are also made to study Earth's core and crust, as well as to investigate processes related to aurora and their connection to the sun through the solar wind and Earth's magnetic field. These magnetic measurements are typically performed from the ground or from satellites. Thus, apart from very short-lived rocket experiments, we hardly have any magnetic data between the ground and 300 km. The upper mesosphere at around 90 km height is a region of great interest, due to its proximity to the aurora and its associated processes. By coincidence, there exists a sodium layer at this height. By illuminating this layer with laser light at a specific wavelength, it will scatter light, also back to the ground. A sodium lidar uses this mechanism to measure temperatures and wind velocities between 80 and 100 km. In this project we have established a similar instrument at ALOMAR in Northern Norway and, for the first time, made continuous mesospheric magnetic field measurements in the auroral region. By pulsing the laser light and varying the frequency of the pulses, it is possible to influence the intensity of the scattered light. The pulse frequency causing the strongest scatter is related directly to the strength of the magnetic field in the sodium layer. Hence it is possible to remotely measure the magnetic field in the mesosphere from ground. We have applied this technique, which has been applied earlier at low or mid latitudes, and opened up for a completely new domain of measurements of magnetic fields generated by currents in the magnetosphere-ionosphere system. It is anticipated that small, horizontal current structures, whose signature is not resolvable by other means, will be resolved when magnetic measurements are performed in the mesosphere. In the project, we have had two master students who have successfully finished their theseses with the titles "Laser diagnostics of the mesospheric magnetic field - Understanding remote laser magnetometry in Northern Norway" and "An Investigation of Magnetic Field Disturbances on the Ground and in the Mesosphere". In the former work the student has gone in depth to understand the physics behind our proposed technique for measuring the magnetic field in the mesosphere, and done modelling on what we can expect from such measurements at high latitudes. The study also investigates what hardware is necessary for successful measurements. In the latter work the student applied different model approaches to evaluate what sort of geomagnetic variations we may expect to see in the mesosphere during our measurements. The laser systemt that we have used has been borrowed from the University of Arizona. During the autumn of 2018 we went to the Kuiper Observatory at Mt. Biggelow outside Tucson, Arizona, where we packed and crated the laser system. It was thereafter shipped to Andøya and arrived there in November. In December the system was unpacked and established in the laboratory at ALOMAR. Collaborators from the US have been at ALOMAR for four 2-3 week periods in 2019 and one 3 week period in 2020, to aid in mounting, adjusting, improving and preparing the laser system as well as performing the magnetic field measurements. The laser had not been in use since 2016, and it was therefore in need of a great deal of cleaning, adjusting, aligning and calibration. However, we did manage to get the laser operational in the lab and close to ready for magnetic field measurements toward the end of March 2019. In September-October 2019 the system became entirely ready and the laser beam was pointed at the sky. The telescopes used for detecting the light returning from the mesosphere have a very narrow field of view and the laser beam needs to be accurately directed into it. This task is challenging and time-consuming, but in December 2019 we mastered this and made the first magnetic field measurements. Measurements were continued in January-February 2020. We have thereby succeeded in the first mesospheric magnetic field measurements in the auroral region ever. The winter of 2019/20 has been under average when it comes to cloud cover and clear nights at Andøya, this has therefore resulted in only 13 nights of measurements and only 3 hours of these with entirely clear skies. The data collected are therefore somewhat limited and we have not had the opportunity to investigate different measuring approaches and optimalizations to the desired extent. Nevertheless, we have shown that the magnetic field measurements are possible, and we are currently working to process and analyze the results aiming for publication iin a scientific journal. Our results already show that the project has been a scientific success and justifies a continuation of such measurements in the future.

Although not generating a wealth of geophysical data to make large advances in the understanding of the upper atmosphere, the project has contributed significantly towards this aim. We have proved that it is possible to make measurements of the magnetic field in the mesosphere remotely in the auroral zone, and made great steps towards doing this with the necessary high temporal resolution. Therefore, we are now among three groups, worldwide, with expertice and experience of doing remote mesospheric magnetometry. We are already communicating with one of the other groups on how we can join efforts to continue mesospheric magnetometry and establish routine measurements in the auroral zone. We conclude that our project and acquired knowhow lays the foundation for establishing an observation program including a more advanced and autonomous system to obtain a large data-set, which will be to the benefit of science and the general public through improved understanding of ionospheric dynamics.

By means of optical pumping, it is possible to use the naturally occurring sodium layer in the mesosphere to measure Earth's scalar magnetic field at ~90 km above ground. This is an altitude not accessible by other means than rockets which will provide point measurements of very short time scales. We are proposing to modify the ALOMAR sodium lidar to be able, for the first time, to measure and monitor the magnetic field in the high latitude mesosphere over longer time scales. In addition to proving a new method for monitoring Earth's magnetic field, which will attract great interest from the geomagnetic community, we will combine the efforts from different communities (astronomy, mesosphere/atmosphere, optics and geomagnetism) into a common science goal. Furthermore, new science applications will be added to lidar measurements. The technique, which has been proposed earlier for measurements at low or mid latitudes for studies of Earth's internal magnetic field, will in our project be applied to high latitudes in the auroral zone. This opens for a completely new domain of measurements of externally generated geomagnetic variations related to currents in the magnetosphere-ionosphere system. In particular, we aim to measure the magnetic field variations in close vicinity to Birkeland currents associated with particle precipitation events penetrating to altitudes below 90 km and small-scale, discrete auroral arcs. It is, furthermore, anticipated that small, horizontal current structures, whose signature is not apparent from ground based measurements, will be easier discernible when measurements are performed closer. During the project we plan to run 4 campaigns, and we will take advantage of the rich scientific infrastructure located in northern Norway, including EISCAT and the Tromsø Geophysical Observatory magnetometer network. Furthermore, during the campaigns, if possible we aim to make measurements in conjunction with overpasses of one or more SWARM satellites