PMSE strength during enhanced D region electron densities: Faraday rotation and absorption effects at VHF frequencies

Highlights

• PMSE strength during enhanced D region electron densities.

• Faraday rotation and absorption effects on PMSE strength.

• D and E region electron density measurements from coherent echoes at high latitudes.

Abstract

In this paper we study the effects of absorption and Faraday rotation on measurements of polar mesosphere summer echoes (PMSE). We found that such effects can produce significant reduction of signal-to-noise ratio (SNR) when the D region electron densities (N_e) are enhanced, and VHF radar systems with linearly polarized antennas are used. In particular we study the expected effects during the strong solar proton event (SPE) of July 2000, also known as the Bastille day flare event. During this event, a strong anti-correlation between the PMSE SNR and the D-region N_e was found over three VHF radar sites at high latitudes: Andøya, Kiruna, and Svalbard. This anti-correlation has been explained (a) in terms of transport effects due to strong electric fields associated to the SPE and (b) due to a limited amount of aerosol particles as compared to the amount of D-region electrons. Our calculations using the N_e profiles used by previous researchers explain most, if not all, of the observed SNR reduction in both time (around the SPE peak) and altitude. This systematic effect, particularly the Faraday rotation, should be recognized and tested, and possibly avoided (e.g., using circular polarization), in future observations during the incoming solar maximum period, to contribute to the understanding of PMSE during enhanced D region N_e.

Introduction

Polar mesosphere summer echoes (PMSE) are extremely strong radar echoes that were first observed with a VHF radar in Poker Flat, Alaska in the late 1970s (e.g., Ecklund and Balsley, 1981). Previously similar echoes, but occurring at mid latitudes, were observed over Lindau, Germany by Czechowsky et al. (1979). Later, Kelley et al. (1987) pointed out that the VHF observations were anomalous. Since then, many different groups have studied these echoes with radars operating in the VHF and UHF frequencies from different locations in the northern and southern polar regions. These radar observations, in combination with in situ rocket measurements, heating experiments, numerical simulations and microphysical modeling of aerosols, have helped to develop a consistent explanation on the generation of these radar echoes. A complete review of the current understanding on PMSE is given by Rapp and Lübken (2004).

Although most of the PMSE features are understood, the anti-correlation between PMSE signal-to-noise ratio (SNR) and the D-region electron density (N_e) during events of strongly enhanced D-region N_e still needs a proper explanation. Such anti-correlation has been observed during the strong solar proton event (SPE) of July 2000, also known as the Bastille day flare event, using three independent VHF radars in the northern polar regions: Andøya (e.g., Rapp et al., 2002), Svalbard (Kubo et al., 2003), and Kiruna (Barabash et al., 2004). Initially it was proposed that Joule heating was the cause for such anti-correlation, but independent observations and calculations have ruled out that explanation (Kubo et al., 2003, Barabash et al., 2004). Furthermore, it was suggested by Rapp et al. (2002) that a limited amount of charged aerosols as compared to the background electron density was the cause for the observed signal reduction. This reasoning rested on results of Cho et al. (1992) showing a strong dependence of the electron diffusivity on the ratio between charged particle densities and ambient electrons. This result has, however, in the meantime been shown to be an artifact caused by unphysical initial conditions used in Cho et al.'s (1992) treatment of electron diffusion in the presence of charged aerosol particles (e.g., Rapp and Lübken, 2003, Appendix B). Hence, at the present stage there is only one remaining hypothesis which is trying to explain the anti-correlation, namely transport effects due to the strong electric fields associated to the SPE (Barabash et al., 2004). In this work, we will not discuss any of these hypotheses, since more observations, including hardware improvements, are needed to support them.

The focus of this work is on the effects that absorption and Faraday rotation could produce on the PMSE measurements performed with the aforementioned VHF radars. All of these three radars operated close to 50-MHz, and all three used linearly polarized antennas for both transmission and reception.

We first summarize the general concepts of absorption and Faraday rotation, with a special emphasis on the effects on VHF radars located at high latitudes. It is important to note that the possible effects of Faraday rotation on high-latitude VHF radars have been already introduced by Kirkwood and Röttger (1995), but we have further extended such analysis to the July 2000 SPE. Then we present the expected effects during normal summer conditions as well as the conditions of the July 2000 SPE. Finally we compared our results with the previous observations and discussed them.