Outreach
Below you can find a popular description of the topics that we address in our research projects.Project participants also give presentations at schools and to student groups.
Meteoroid Material & Dust in the Mesosphere and D-region Ionosphere
Mesosphere is the region of atmosphere typically between 50 and 100 km altitudes. The temperature in this region decreases with altitude and reaches the coldest levels near 95 km, which is called the mesopause (Figure 1). The temperatures at the mesopause altitudes reach values below 180 K. Interestingly, in the sunlit summer polar region, the mesopause occurs around 85 km altitudes itself with values plummeting below 140 K. This is clearly counterintuitive as it defies the radiative equilibrium in which the sunlit portion is expected to be warmer.
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Figure 1. Temperature structure of atmosphere with structure of ionosphere (Figure from the sources Schunk and Nagy, 2009 and Tascione, 1988. Artistic effects by Margaretha Myrvang)
Such unique thermal structure is made possible due to the atmospheric gravity waves and associated meridional circulation. As a result of large reduction in densities, the atmospheric gravity waves (example in Figure 2) and tides grow in their amplitudes and break in the upper mesospheric altitudes depositing their energy and momentum and generating turbulence. This makes the dynamics an integral component in almost all phenomena occurring in the mesosphere.
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Figure 2. Examples of gravity waves propagating in the upper mesospheric heights around 85 km seen from OH airglow imaging. The arrows represent direction of propagation of the waves (Narayanan and Gurubaran, J. Atmos. Sol-Terr. Phys., 2013)
In simpler terms, the summer polar mesosphere undergoes an adiabatic expansion while winter polar mesosphere undergoes a compression as a result of atmospheric circulation dominated by gravity waves and this creates cooler summer region and warmer winter region (Figure 3).
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Figure 3. The latitudinal structure of mesopause altitude and temperature from summer to winter hemisphere (modified from Xu et al., J. Geophys. Research, 2007)
Mesosphere is the region where the ionosphere starts to form. The ionosphere contains D, E, F and topside regions. Mesosphere coexists with D-region and bottom most portion of the E-region ionosphere. The D and E regions contain molecular ions. Particularly interesting and complicated is the D-region ionosphere that coexists with the mesosphere. It contains not only positive ions but also negative ions resulting from electron attachment processes. Therefore, charge balance requires inevitable consideration of multiple species, in contrast to the higher levels of ionosphere where one to three ion species and electron density is sufficient to study the charge balance. Because the electron attachment also happens with large clusters of molecules, the D-region ionosphere behaves as dusty plasma and offers unique opportunity to study the dusty plasma processes in the natural laboratory within the earth environment.
In addition, other interesting aspects occurring in the mesosphere are formation of aurorae in the upper mesospheric altitudes, occurrence of lighting in the upper atmosphere known as red sprites that are associated with intense thunderstorm activities in troposphere. The upper mesosphere region also hosts permanent light emitting layers known as airglow layers that are often used to study the dynamics, chemistry and physical properties of the region. The emitted light is faint and hence they are usually not visible to naked eye. The airglow of green light emitted by atomic oxygen at the top of the upper mesosphere is often captured as a beautiful layer surrounding the Earth from low earth orbiting platforms (Figure 4). It is the same green color that dominates in the auroral emission. There are also other airglow layers like yellow coloured Na airglow and near infrared emissions of OH from the mesosphere, and red coloured and ultraviolet emission layers of atomic oxygen from thermosphere altitudes.
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Figure 4. Photograph taken from International Space Station (ISS) showing aurora and airglow layers. Courtesy NASA.
Further, most of the meteoroids entering the atmosphere experience sufficient frictional force in the altitudes of upper mesosphere and lower thermosphere (80 to 120 km). As a result, the upper mesosphere hosts metallic layers consisting of Na, Fe, Mg, Ca, Si and K resulting from the ablation of meteoroids entering the earth's atmosphere. In addition, the mesosphere contains active minor species such as CO 2 , CH 4 , H 2 O, OH, O 3 and NO. Though the pressure decreases exponentially between 1 mb and 0.001 mb in this region (1/1000 thto 1 millionth of pressure at sea level), enough collision occurs between the atoms and molecules so that they often form cluster molecules and cluster ions in the mesosphere. The ablated meteoric material again coagulates into the so-called meteoric smoke particles (MSPs) with sizes ranging from 0.2 to 2 nm in the upper mesospheric heights (Figure 5). The detection of these particles and conclusively identifying their global distribution, size distribution and understanding their role in mesospheric and D region chemistry remains a challenge.
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Figure 5. Schematic showing formation of meteoric smoke particles (MSPs) due to ablation of meteoroids (Tim Dunker, ArXiv: 1801.05223v2, 2018)
These elusive MSPs play an active role by getting charged and acting as condensation nuclei for formation of ice crystals at altitudes close to 85 km in the summer polar region.Since the ice particles are larger and the structures in the charged ice particle distributions produce large scattering of radio waves in the HF and VHF frequencies, they are relatively easier to detect and study during summer times. Such radar echoes received from scattering of ice particles are referred to as Polar Mesospheric Summer Echoes (PMSEs). The ice particles often grow into larger size and form clouds that become visible during twilight hours in the mid to high latitude regions. Such clouds are known as Polar Mesospheric Clouds (PMCs) when viewed from space or Noctilucent Clouds (NLCs) when viewed from ground. Figure 6 shows examples of PMCs, NLCs and PMSEs. There are other non-summer phenomena like Polar Mesospheric Winter Echoes (PMWEs) that occur at lower mesospheric altitudes typically within 55 and 80 km, which are not fully understood. It is speculated that such phenomena may also have a role of MSPs combined with mesospheric dynamics as their cause.
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Figure 6. Top left: Observations of PMCs over Antarctica from NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission, Top right: PMC seen from ISS (provided by NASA), Bottom left: Ground photograph of NLCs (Dubietis et al., Appl. Opt. 2011), Bottom right: EISCAT VHF radar observations of PMSEs (Mann et al., J. Geophys. Res. Space Phys. 2016)
In addition to the meteoroids that ablate, there exist a population of micrometeoroids with lesser surface area that do not ablate. There are also large clusters of molecules with minor species and metal species that are sometimes referred to as mesospheric aerosols. All such MSPs, ice crystals, mesospheric aerosols and non-ablated micrometeoroids existing in the mesosphere can be collectively referred to as ‘Mesospheric Dust’. They can also be referred to as ‘Nanodust’ as their sizes range from 0.2 to 200 nm. The aim of this project is to understand the mesospheric dust, how it is affected by the upper mesospheric environment and its effects on the mesosphere and D-region.
Some of the important questions focused herein are:
How to measure the mesospheric dust particles using incoherent scatter radar spectra in a regular basis and how reliable will it be?
How the charging of MSPs occurs and what is the role of charged particles in generation of the ice crystals?
How different geophysical conditions affect the processes associated with mesospheric dust?
How the dynamics affect the ice crystal formations and the NLCs, PMSEs and metal layers?
What can we learn about the dynamics and winds using the layered phenomena like PMSEs?
We intend to address the above questions using both experimental observations and model calculations. A suite of experimental facilities located in the Northern Scandinavia such as EISCAT VHF and UHF incoherent scatter radars, meteor radars, MF radars, airglow instruments, Metal fluorescence lidars, ionosondes and NLC photogrammetry observations are to be used. Rocket experiments will also be carried out to make in-situ measurements of mesospheric dust particles. In addition, there are satellite missions such as Aura Earth Observation mission, SABER / TIMED mission, AIM mission that are capable of providing valuable complementary datasets. In the modeling efforts, we plan to use the general circulation model Whole Atmosphere Community Climate Model (WACCM) along with the Community Aerosol and Radiation Model for Atmospheres (CARMA) and Sodankylä Ion Chemistry (SIC) model to study the interconnections between the nanodust, ice crystals and their redistribution and effects in the D-region ionosphere. In addition to gaining deeper understanding on the mesospheric dust and its effects, this project may support essential preparatory steps to design new experiments and data analysis methods towards effective use of technological advancements in our observational capabilities such as the upcoming EISCAT_3D facility.