The effect of charged particles on the polar mesopause region, as observed by MST and meteor radars

The polar summer mesopause is the coldest region of Earth's atmosphere, and is most well known for the existence of two widely studied atmospheric phenomena: noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSE). NLCs are the better understood feature, the thin clouds forming as water molecules freeze at the low temperatures. PMSE is less understood despite much progress being made over the past decade. The complexity of the physics in the middle atmosphere along with the inherent difficulty of taking in situ measurements near the mesopause has meant that a full understanding of the theory underlying PMSE still awaits.

PMSE is a radar phenomenon, observed as a dramatic increase in the backscattered power returned from the region slightly below the mesopause, at an altitude of approximately 85 km. Using a Mesosphere Stratosphere Troposphere (MST) radar these echos can be clearly observed with high spatial and temporal resolution. The presence of charged dust and ice particles in the summer mesopause region is thought to be a necessary component of the PMSE environment. If these same charged particles have a noticeable effect on meteor echoes we can move a step closer to better understand the mechanisms responsible for generating and maintaining the PMSE structures.

During the 2005 summer, data were collected remotely from two radars in Northern Sweden. The preliminary analysis of these data yielded promising results and further motivated the proposal for a 2006 summer experimental campaign. For two weeks in June-July, 2006, we visited Kiruna, Northern Sweden, to take measurements using two separate radars. The first radar used was ESRAD, a 52-MHz MST radar, operated by the Swedish Institute of Space Physics (IRF). ESRAD is a sophisticated multiple-receiver radar, capable of observing PMSE. The other radar was a 32.5-MHz meteor radar, located about 1000 m from ESRAD. This All-Sky Interferometric Meteor Radar (SKiYMET) system is capable of measuring a wide range of atmospheric and astronomical parameters. Together these radars were operated in modes that allowed us to study temporal variations in both the PMSE and the meteor signals.

From the 2005 meteor observations, we will present the seasonal variation in the meteor echo decay times. The decay times are of interest since the inferred ambipolar diffusion can be related to the ambient temperature and pressure. As predicted by ambipolar diffusion theory, the meteor echo decay times reduce with height in the mesopause region (85-90 km), however there appears to be a reversal in this trend at lower and upper altitudes. The upper minimum in decay time (at about 96-98 km) has been discussed by others, however no consensus as to a cause for the lower 'kickback' has been reached. There is a distinct seasonal horizontal shift in the average meteor echo decay time that is not accounted for by the vertical shift in the position of the lower kickback. This interesting result suggests a seasonal change in the background microphysical or dynamical environment, allowing slightly slower (faster) diffusion during the summer (winter) months.

It has been recently suggested that charged or neutral particles could cause a reduction in meteor echo decay rates. It is thought that a proportion of the electrons in a meteor trail can be absorbed by the background 'dust' as they diffuse outward. It is important to investigate the magnitude of this absorption effect. If not accounted for, the reduction in the meteor echo power due to this mechanism could be attributed to ambipolar diffusion, which may lead to a significant overestimation of the diffusion. This would in turn impact temperatures that are estimated using the diffusion coefficient.

Since the background dust has a limited electron absorption capability, the relative effect will depend on the initial electron density. Hence, the weaker (stronger) trails should have a greater (smaller) proportion of the initial electron content absorbed by the dust. A systematically faster apparent diffusion for weaker trails compared to stronger trails may be an indication that absorbing dust is present. By comparing the strongest 50% (by maximum backscattered power) of meteor echo trails to the remaining weaker echoes we will present evidence of dust in the PMSE region. It is believed that a reduction in the diffusion coefficient through the presence of charged aerosols can be directly related to PMSE. It is this alteration in the diffusivity of the mesopause region that allows structures in the electron density to persist at observable scales.

Since the nucleation of ice particles is highly dependent on temperature, the low summertime mesopause temperatures probably work together with more complex microphysical processes to enable the generation of PMSE layers. It is likely that the background dust can act as cloud condensation nuclei for the ice particles. Using the MST radar data, a signal-to-noise proxy for the presence of PMSE was established, and each corresponding meteor echo was flagged as being either likely or unlikely within a region of PMSE. We present an analysis of the 2005 summer months, comparing the meteor echo decay times in the presence of PMSE to those in 'clear' conditions.

Finally, after giving some very interesting preliminary results from the 2005 summer data, we will outline some new research questions, many of which are crucial in allowing us to canvass a complete understanding of the mechanisms behind PMSE.