Ionospheric Fireworks Illuminate Auroral Science – GWC Mag

Last March, reindeer out for an evening snack on the Scandinavian tundra, as well as other nighttime observers, saw an unusual display of bright purple and green clouds intersecting an auroral display in an otherwise clear sky. Nature provided the aurora, but the clouds were courtesy of a sounding rocket that released barium into the atmosphere to help reveal the electrodynamics near the aurora. The electrodynamics of the auroral ionosphere give us important information about space weather that can affect the operation of satellites, power lines, and more.

The Barium Release Optical and Radio rocket (BROR) was launched at 19:23 local time (18:23 UTC) on 23 March 2023 from the Esrange Space Center in northern Sweden. The glowing clouds were visible from central Finland and Sweden to northern Norway. The clouds took various shapes at different altitudes, elongating along vertically oriented geomagnetic field lines, and remained for a few tens of minutes after release.

A few minutes after the first barium release, a natural auroral arc appeared, intersecting the region where the clouds were. The interaction between the auroral arc and clouds created a stunning purple and green auroral curtain with fine-scale striations, wowing casual skywatchers and exciting scientists who anticipated the data that these stunning visuals heralded.

Reviving an Old Approach to Answer New Questions

The BROR project involved a sounding rocket (a suborbital research rocket) carrying eight canisters of a barium-thermite mixture. The rocket released neutral barium (Ba) as a tracer at different altitudes to study small-scale electrodynamics in the auroral ionosphere.

The ionosphere was long regarded as a “passive load” in magnetosphere-ionosphere electrodynamics. That is, it was assumed to dissipate electrical power without amplifying or otherwise altering the electrical current. However, recent studies have demonstrated that small-scale structures in the ionosphere can significantly contribute to magnetosphere-ionosphere coupling, causing the formation of intense parallel electric fields, generating Alfvén waves, and producing strong electron and ion heating.

The barium chemical tracer technique is the most efficient way to measure dynamical behavior in the high-latitude ionosphere.

For years, many space physics researchers have studied cross-scale energy transfer to improve our fundamental understanding of the overall energy balance in the ionized gas (plasma) making up the auroral ionosphere. This cross-scale transfer is most clearly seen during multiscale auroral displays in which structures such as auroral arcs (with a scale of ~100 kilometers) and ray auroral structures (with a scale of ~100 meters) coexist and interact dynamically with each other. The barium chemical tracer technique is the most efficient way to measure such dynamical behavior in the high-latitude ionosphere, offering advantages over other methods such as radar.

Experiments using sounding rockets for chemical releases into the upper atmosphere were initially proposed in 1950 and were often conducted in the 1960–1970s, including with rockets launched from Esrange. In the 1980s and, particularly, the 1990s, the popularity of sounding rocket experiments decreased because of drastic improvement in ground-based and satellite remote sensing techniques.

Over the past decade, however, interest in performing rocket chemical release experiments has resumed. New insights into the roles that the ionosphere plays in magnetosphere-ionosphere coupling have generated new science questions, such as what the role of small- and medium-scale (from a few to tens of kilometers) ionospheric structures is in the magnetosphere-ionosphere interaction and what the mechanism of interscale energy transfer is. And rocket-based chemical release experiments are seen as a means of providing answers.

Significant recent advances in flight control, chemical release technology, and, especially, ground-based optical and radio instrumentation have also promoted the return of such experiments. For example, the international Grand Challenge Initiative (GCI) rocket program GCI-CUSP has conducted three experiments with chemical release into the auroral ionosphere over the past 5 years. The BROR experiment is the first Swedish-led chemical release aurora experiment in 36 years.

A Delayed Start, Then Success

The idea for BROR was proposed in 2020 and scheduled for the spring or autumn of 2022, in cooperation with the Swedish Space Corporation. However, a fire at Esrange Space Center on 26 August 2021 delayed the experiment to spring 2023.

The rocket launch required a strict set of conditions, including the proper solar zenith angle, the presence of geomagnetic activity, clear skies, and low or no wind. Thus, the launch window was limited to only about 2 weeks around the vernal and autumnal equinoxes. The spring 2023 launch window spanned 12 to 22 March, but bad weather persisted and caused us to wait one more day. Finally, conditions were ideal on 23 March, and we conducted the experiment successfully.

During the rocket’s ascent, five barium releases occurred in 2 minutes at altitudes between 130 and 245 kilometers; three additional releases occurred in a 1-minute span at altitudes between 230 and 180 kilometers during the descent. (In total, about 64 kilograms of barium were released over a roughly 3,000-square-kilometer area, a much smaller amount—spread over a much larger area—than is typical of holiday fireworks displays.)

For all releases, sunlight illuminating the ionosphere (but not the ground) ionized a portion of the neutral Ba, allowing us to trace the wind and ion drift by tracking neutral Ba and ionized Ba⁺, respectively. We chose to release barium in the experiment instead of other materials (e.g., strontium or europium) because solar ultraviolet light ionizes neutral Ba very quickly and both Ba and Ba⁺ produce strong resonance emissions in the visible light range, making them easy for ground-based instruments to observe.

With its multiple chemical releases during both ascent and descent, the Barium Release Optical and Radio rocket (BROR) experiment offered the novel possibility of studying altitudinal differences in the motions of neutral and ionized particles.

Once released, neutral Ba moves without being affected by the ionospheric electric field, emitting pale green light with a wavelength of 553.5 nanometers, whereas ionized Ba⁺ follows the electric field, emitting purple light at 493.4 nanometers. The barium-thermite release technique was used intensively in the past to measure high-latitude ionospheric electric fields, and such studies greatly contributed to the understanding of ionospheric convection and magnetosphere-ionosphere coupling.

With its multiple chemical releases during both ascent and descent, the BROR experiment offered the novel possibility of studying altitudinal differences in the motions of neutral and ionized particles. The experiment also allowed us to obtain neutral wind and ion convection profiles simultaneously and to estimate directly the frequency of neutral ion collisions, which is crucially needed for computer modeling and is difficult to get in laboratory experiments.

Not Just a Pretty Light Show

A preliminary analysis of the BROR data has shown that the observations clearly captured distinct behaviors of the barium clouds at different altitudes. For example, in the two lower ion clouds, at 130- and 160-kilometer altitude, significant vertical motion of ions was observed together with normal horizontal convection. The velocities of this vertical motion at these low altitudes were unexpected, and we are working to determine whether this result indicates the existence of an unexpected electric field aligned with the magnetic field in the lower ionosphere.

Information about auroral ionosphere dynamics helps us understand fundamental atmospheric science, but it also has broader social benefits. For example, the requirements of our increasingly information-based society for high-speed communications urgently increase the necessity to comprehend space weather phenomena that can affect our communication networks and electrical transmission systems. Information about upper atmospheric dynamics contributes significantly to these activities.

Data collection during the BROR experiment was supported by a high-quality ground-based instrument network at the Swedish Institute of Space Physics’ (IRF) Kiruna Atmospheric and Geophysical Observatory (KAGO). Among the KAGO instruments, the most remarkable is an optical tomography system called the Auroral Large Imaging System 4D (ALIS_4D), the only one of its kind in the world. This system enables application of tomography-like reconstruction techniques to recover the 3D structure of ionospheric plasma from observations of individual optical emissions from neutral and ionized barium clouds.

The temporally and spatially high-resolution observations by KAGO will provide important insights into small- to mesoscale structures of the electric field and their roles in magnetosphere-ionosphere coupling. We also expect to learn more about cross-scale energy transfer. Furthermore, observations of the background ionosphere collected with the European Incoherent Scatter (EISCAT) radar system (1 of only 10 in the world) during BROR will provide the parameters of the ambient ionosphere, such as electron density and electron and ion temperatures.

Crowdsourcing Complementary Science

The BROR experiment demonstrated the potential of public participation for providing supplemental monitoring of ionospheric phenomena.

Beyond the formal data collection supported by KAGO’s instruments, the BROR experiment demonstrated the potential of public participation for providing supplemental monitoring of ionospheric phenomena. We advertised the experiment beforehand and encouraged the public to join us to take photos and videos, and more than 20 semiprofessional photographers attended the rocket launch from a much broader region than our observation network covered.

As a result of the experiment’s public visibility, which also included a real-time webcast of the launch and sky from Esrange, many lay people watched and took photos. Some shared their photos and videos, taken from various locations across Scandinavia, which complemented our observations greatly and helped us get a wider picture of the phenomena. During the experiment, a strong auroral arc appeared and intersected the region of the barium-triggered clouds. Photos and videos from members of the public captured high-quality views of both the natural aurora and the artificial airglow at the same time.

The ALIS_4D network used filters to limit the range of wavelengths it recorded, so it could focus on detecting either barium or auroral emissions. The photos and videos from the public captured both the natural aurora and the barium clouds simultaneously and thus helped us better comprehend the interaction between barium cloud development and the natural aurora.

As commercially available digital cameras continue to evolve, we anticipate the public will contribute even more to auroral science in the near future. Finally, the BROR experiment created a beautiful fireworks display for the reindeer, who have not seen it for the past 30 years.

Acknowledgments

The BROR experiment was led by T.S. and financed by the Swedish National Space Agency. The project was made possible and successful with the cooperation and assistance of IRF, the Department of Physics and Astronomy of Clemson University, the Swedish Space Corporation (SSC), and the Mobile Rocket Base of the German Aerospace Center. We express special gratitude to Krister Sjölander of SSC, who played a crucial role in the design and manufacture of the payload, and to Miguel Larsen of Clemson University for sharing his experience in conducting rocket experiments for chemical release in the upper atmosphere.

Author Information

Tima Sergienko, Yoshihiro Yokoyama (yokoyama@irf.se), Urban Brändström, and Masatoshi Yamauchi, Swedish Institute of Space Physics, Kiruna; and Anders Tjulin, EISCAT Scientific Association, Kiruna, Sweden

Citation: Sergienko, T., Y. Yokoyama, U. Brändström, M. Yamauchi, and A. Tjulin (2024), Ionospheric fireworks illuminate auroral science, Eos, 105, https://doi.org/10.1029/2024EO240083. Published on 28 February 2024.

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