Isolated lower mesospheric echoes seen by medium frequency radar at 70° N, 19° E

Abstract

Abstract.We have noted sporadic instances of strong isolated reflections of medium frequency (MF) radar waves from the mesosphere from as low as 50 km altitude and have devised a set of criteria for isolating these apparently anomalous echoes from those normally occurring from progressive partial reflections in the D-region. The object of this study is to map the occurrences of such echoes facilitating comparisons with other observations. For example, the similarity and simultaneity of the echo structure for the 20 January 2005 with VHF radar results presented by Lübken et al. (2006) are particularly striking. In presenting a number of such echo events since 2001 selected from the MF radar dataset (which spans 1997 to present), we find that virtually all echo occurrences coincide with enhanced solar proton fluxes suggesting that substantial ionisation of the mesosphere is a necessary condition. Strong partial reflections of the radio wave in the lower mesosphere combined with seasonally varying total absorption higher up, thus giving false impressions of lower mesospheric layers preferentially in winter, constitute a scenario consistent with our observations.

First observations of Southern Hemisphere polar mesosphere winter echoes including conjugate occurrences at ∼69°S latitude

“…[13] Our findings provide experimental evidence that PMWE are common at Davis (68.6°S) during SPE and their intensity (SNR) is less compared with their NH counterpart during winter. Interestingly, PMWE above Davis were observed coincident with ILME [Hall et al, 2006]. Indeed the terminology 'polar mesosphere winter echoes' (PMWE) is a misnomer since they can occur at low latitudes [Czechowsky et al, 1979], in seasons other than winter (this paper), and they are distinct from PMSE.…”

“…[10] Unlike PMSE layers which only occur during the respective summer seasons of each hemisphere, we find that PMWE sometimes occur simultaneously in both hemispheres. Since PMWE observed in the NH winter throughout the SPE of 8-16 November 2004 were reported in the literature [Kirkwood et al, 2006b;Hall et al, 2006;Kirkwood, 2007;Lübken et al, 2007], we examined the Davis MST radar record and found that PMWE also occurred, albeit constrained to discrete short duration episodic bursts on 10 and 11 November 2004 in the SH summer. For this interval Kirkwood et al [2006b] reported PMWE intensity to peak at ESRAD (MST radar at Kiruna, 67.9°N; 21.2°E) on the evening of 10 November, although PMWE occurrence was patchy throughout the day in the altitude range 64-78 km (see their Figure 1).…”

“…However, successive daily occurrences of the PMWE suggest that similar mesosphere environment conditions prevailed throughout the SPE. Following Hall et al [2006] we show the coincident hourly averaged SNR backscattered echoes observed on the co-located Davis MF radar in Figure 1e. Interestingly, ILME [Hall et al, 2006] occur simultaneously, albeit at lower heights, with PMWE during the winter months, as illustrated in Figures 1d and 1e.…”

“…Based on observations between ∼54°N to 78°N latitude, the general characteristics of PMWE can be summarised as follows: duration minutes to tens of hours; vertical thickness from 300 m to 23 km; height range 50-80 km; diurnal occurrence peaking around local solar noon (daylight) throughout the boreal winter [Czechowsky et al, 1979;Zeller et al, 2006]. PMWE have also been observed using incoherent scatter radar (ISR) [Kirkwood et al, 2006a] and appear closely related to isolated lower mesospheric echoes (ILME) on MF radar [Hall et al, 2006]. PMWE occurrence is correlated with enhancements in electron density primarily following SPEs and auroral substorms [Kirkwood et al, 2006b].…”

MF and HF radar techniques for investigating the dynamics and structure of the 50 to 110 km height region: a review

“…For example, Dieminger (1952), Gregory (1956Gregory ( , 1961, Titheridge (1962), Reid (1990), Jones et al (2004), and Hall et al (2006) discuss this for MF and HF, Bailey et al (1955) and Pineo (1956) for very high frequency (VHF; 30 to 300 MHz) forward scatter results, Bowles et al (1964) for both vertical and forward scatter VHF results, and Flock and Balsley (1967) for vertical incidence VHF results. At MF/HF, such preferred heights are subject to seasonal and annual variations, but persistent echoes occur from mean heights of about 65-68, 74-75, 83-85, and 92 km.…”