Incoherent Scatter Radar Studies of Daytime Plasma Lines

Frank T. Djuth,Liwei D. Zhang,Herbert C. Carlson

doi:10.1007/s11038-018-9513-5

Frank T. Djuth, Liwei D. Zhang + Show 1 more

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https://doi.org/10.1007/s11038-018-9513-5

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Abstract

First results from wideband (electron phase energies of 5–51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory are presented. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys. These and other structures occur within an enhancement zone extending from electron phase energies of 14–27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14–27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas.

Highlights

In the past, Langmuir waves (LWs) measured with incoherent scatter radars at fo ± fr, where fo is the radar transmission frequency and fr is the Langmuir wave frequency in the ionosphere (e.g. Evans 1969), have yielded a great deal of information about the ionosphere and thermosphere
The photoelectron peaks in the bottomside F region and the topside peaks and structures smeared out by Coulomb losses were clearly evident in the AE-E data (e.g. Lee et al 1980a, b) It is apparent from Fig. 8 that the plasma line peaks are very weak at a zenith angle of 1.10°
We have demonstrated that the solar EUV line generated PL peaks and structures obtained from radar observations of Langmuir waves have a dependence on θ, the pitch angle, where B is determined from the IGRF model

Summary

Introduction

Langmuir waves (LWs) measured with incoherent scatter radars at fo ± fr, where fo is the radar transmission frequency and fr is the Langmuir wave frequency in the ionosphere (e.g. Evans 1969), have yielded a great deal of information about the ionosphere and thermosphere. High resolution techniques developed in the past to study PEPLs include the cutoff technique of Showen (1979), the chirp technique of Hagfors et al (1984), the application of the coded long-pulse (CLP) technique of Sulzer (1986a) to PEPLs (Djuth et al 1994) (CLPPL), and high resolution measurements of photoelectron enhanced plasma lines using the alternating code technique (32 bits code) with the EISCAT VHF radar (Guio and Kofman 1996) Such techniques have been used to monitor gravity waves (Vidal-Madjar 1978; Djuth et al 1994, 1997, 2004, 2010; Livneh et al 2007, 2009; Nicolls et al 2014), to develop strategies leading to the assessment of ion composition in the lower thermosphere (Bjørnå and Kirkwood 1988; Fredriksen 1990; Nicolls et al 2006; and Aponte et al 2007), and to investigate PL asymmetries at low altitudes (Oran et al 1978) and the impact of collisions between electrons and neutrals on the PL (Newman and Oran 1981; Bjørnå 1989). Carlson et al (1977) included PEPLs in an electron thermal balance study and concluded that the resolution of the factor of two difference between the solar EUV flux and the electron heat balance calculations should be sought in more accurate effective heating rates rather than by increasing the EUV flux

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