First observations of Polar Mesospheric Echoes at both 31 MHz and 53.5 MHz over Svalbard (78.2°N 15.1°E)

Abstract

During summer 2020, observations of the mesosphere using a 53.5 MHz radar on Svalbard, at 78.2°N 15.1°E, revealed the well-known Polar Mesospheric Summer Echoes (PMSE). At the same time, a co-located meteor detection radar, operating at 31 MHz detected corresponding echoes very distinct from those associated with meteor trails. Comparing as many days as possible during 2020, incontestable evidence arose to demonstrate that the meteor detection radar was capable of observing PMSE, although not in the optimised fashion of the 53.5 MHz system. We present examples of results from both systems, supplementing the earlier findings of Swarnalingam et al. (2009), and simultaneously show very first results from this particular geographical location.

Introduction

Strong coherent echoes occur when structures defined by discontinuities in refractive index are present with scales that match half the radar wavelength (Bragg condition, https://en.wikipedia.org/wiki/Bragg%27s_law). Such structures, simplistically envisaged as created by turbulent eddies, are present in the mesosphere and visible where the refractive index is governed by ionisation. The distribution of eddy sizes is determined by the inertial subrange of the Kolmogorov spectrum (Obukhov. 1941, and more accessibly, e.g., Batchelor, 1953 and references therein) and this may be modified by charged dust and/or proton hydrates – a consequence of the particularly low temperatures at the high latitude mesopause – causing echoes at frequencies into the UHF (in the framework of these observations: Hall, 1993). Presence of echoes at frequencies lower than ~50 MHz, i.e. wavelengths greater than ~6 m, thus corresponding to eddies larger than ~3 m is surprisingly little investigated, one notable study being Swarnalingam et al. (2009). Here we have the opportunity to observe structures of ~5 m. We merely report the phenomenon, but otherwise the reader is referred to Batchelor (1953) for appreciation of the result in the framework of the fluid dynamics of the upper mesosphere.

The first observations of such particularly strong echoes – known as Polar Mesospheric Summer Echoes (PMSE) – are attributed to Czechowsky et al. (1979) and Ecklund and Balsley (1981), and thereafter, at 78.2°N 15.1°E by the 53.5 MHz “SOUSY” radar (Hall et al. 2009). A meteor wind radar operating at 31 MHz is co-located with SOUSY, and hereafter referred to as “NSMR” (Hall et al. 2002). The systems were modernised in 2019 and therefore producing simultaneous observation of the upper mesosphere during the summer of 2020. PMSE are a known feature of the high-latitude mesosphere, normally observed by systems such as SOUSY, but here we present first results of PMSE detected by the meteor radar NSMR, demonstrating the echoes to be a feature at 31 MHz, supplementing the report by Swarnalingam et al. (2009), and furthermore over Svalbard.

Objective

To demonstrate that Polar Mesospheric Summer Echoes can be observed by a 31 MHz meteor detection radar, as well as by a 53.5 MHz radar optimised to such observations, and at 78.2°N 15.1°E.

Methods

The most up-to-date operating specifications of the two radars are fully described by Hall et al. (2002, 2009). The antenna patterns of the two systems are somewhat different and are fully described (including lucid illustrations) by Swarnalingam et al. (2009). Range resolutions are 1 km for SOUSY and 2 km for NSMR. These factors explain the marked superior definition of echoes from the (optimal) SOUSY system as compared to NSMR (optimised to meteor trail detection).

Results

As shown in Figure 1, we have compared mesospheric echo occurrences from two co-located radars operating at 53.5 and 31 MHz. When examining the figure, it must be reiterated that here the echo powers are not directly comparable. While Swarnalingam et al. (2009) determined consistent backscattered power from each instrument, this was outside the scope of our preliminary study. Since SOUSY is optimised to such observations, PMSE will be observed over longer intervals than by NSMR. Times and heights of peaks in occurrence can, however be compared with a good degree of confidence. The 31 MHz system is optimised to meteor trail observations, and these have a peak at around 90 km. Thus we focus on echoes near 85 km – the peak occurrence for PMSE and approximate location of the high latitude summer mesopause; a more advanced analysis and portrayal could hopefully filter out meteor trail signals. The effect is seen in the day-average height profiles: for the meteor radar, although the trail echoes begin to dominate immediately above the PMSE peak, whereas for 53.5 MHz (optimised to coherent echoes from abrupt changes in refraction coefficient i.e. Bragg scattering, explained earlier) there is a well-defined peak. The data presented here are, however, only an example of over two months of such observations, and the presence of echoes at 31 MHz corresponding to the PMSE observations at 53.5 MHz is undeniable.

Figure 1. Echo power as a function of height and time, and as mean power for the two radars from 31st May 2020. Top row: the 53.5 MHz SOUSY radar; bottom row: the 31 MHz NSMR system; left column: power vs. height and time; right column: corresponding mean power vs. height for the day.

Conclusions

We establish that a 31 MHz radar optimised to observing echoes from meteor trails occurring primarily between 85 and 95 km altitude, and over a wide field of view, is capable of detecting polar mesospheric summer echoes (PMSE) independently detected by a co-located 53.5 MHz system, specifically designed for PMSE observations. Although not the first such observation, it is in fact the first at 78.2°N 15.1°E over Svalbard, and that turbulent scales exist to at least 5 m.