On the necessity of using foμEs instead of foEs in estimating the intensity and variability of sporadic E layers

https://doi.org/10.1016/j.jastp.2020.105327Get rights and content

Highlights

  • The ionosonde parameter foEs is biased, leading to overestimates; instead, a new parameter named foμEs is proposed.

  • The necessity of replacing foEs by foμEs in studying sporadic E properties and variability is assessed.

  • foEs needs to be abandoned and instead, foμEs must be adopted by the ionospheric community in ionosonde sporadic E studies.

Abstract

The ordinary wave critical frequency foEs is measured routinely by ionosondes and used widely in estimating the intensity and occurrence of sporadic E (Es) metal ion layers. Recently, however, it was realized that foEs overestimates Es intensities because it corresponds to the sum of both the layer metal ion plasma and the regular E region plasma densities. To account for this, a new parameter, foμEs, was introduced that corresponds to the Es layer metal plasma density only, and a method was proposed for its calculation. In the present work, an ionosonde data set is analyzed to assess the importance of replacing foEs by foμEs in Es studies. To this objective, the diurnal and seasonal variability of sporadic E is re-examined. It turns out that the Es mean diurnal and seasonal variations estimated by means of using foμEs, differ substantially from those based on foEs, implying that the use of foEs leads to biased results and wrong physical inferences. It is therefore urged that foμEs is adopted and used instead of foEs, in future ionosonde sporadic E studies. The same applies for the case of satellite radio occultation (RO) sporadic E investigations as well, where foμEs should be used, instead of foEs, for calibration and validation purposes. All this is equally valid in the case of the blanketing sporadic E frequency fbEs and the corresponding new parameter fbμEs that needs also to be adopted and used.

Introduction

The generic term sporadic E (Es) refers to long-living metal ion layers that exist mostly in the midlatitude E region ionosphere, being strongest between about 100 and 120 km. Although the Es phenomenon has been investigated extensively for decades, it remains still a challenging research topic. For more on the observed properties and interpretation mechanisms of sporadic E, see a sequence of review papers by Whitehead (1989), Mathews (1998), Haldoupis (2011, 2012), and references cited therein.

Most of the sporadic E studies were made with ionosondes, which have been kept carrying out routine ionospheric measurements for many years in numerous locations around the globe. The key ionosonde parameter used to quantify Es intensities is the ordinary wave critical frequency foEs, which often is taken to be approximately equal to the extraordinary wave critical frequency fxEs because in the lower E region below Es the incident ionosonde signals undergo little magnetoionic splitting (Rishbeth and Garriot, 1969). With regards to Es, and in accord with the magnetoionic theory, the ionosonde incident radio wave reflects back at a critical frequency foEs that equals a sporadic E plasma frequency fpEs, relating to the maximum electron density in Es through the equationfoEs=fpEs=(80.6NmEs)1/2,where foEs and NmEs are in Hz and electrons per cubic meter (m−3), respectively. The notation NmEs refers to the maximum plasma density somewhere inside the layer, which has been adopted as being a measure of Es layer intensity. In the long going ionosonde sporadic E research, foEs is measured routinely and used extensively in many works and publications in the investigation of sporadic E properties and in correlation studies with other physical phenomena, e.g., see Whitehead (1989).

Given that sporadic E plasma layers are due to long-living metal ions, it was recognized recently by Haldoupis (2019), in a paper hereafter referred to as paper A, that foEs is an incorrect estimate of Es intensity. This is simply because foEs corresponds to the total plasma (electron or ion) density inside the layer and not the metal ion plasma density only, that is the proper measure of Es layer strength. As mentioned in paper A, the use of foEs is expected to overestimate Es layer intensities in daytime, especially in the case of weak layers, because the regular E region plasma contribution to foEs is enhanced by solar photoionization. Apparently, this simple fact was overlooked and ignored for decades, which must have led to incorrect and possibly misleading results.

To deal with this deficiency, paper A proposed a simple algorithm, which combines the ionosonde-measured foEs and virtual height h'Es, with International Reference Ionosphere (IRI) model predictions of regular E region electron densities, in order to estimate the maximum metal plasma density in Es, NmμEs. Next, this is introduced in Equation (1) to derive the critical frequency of Es metal plasma, named foμEs, where the letter μ is borrowed here from the Greek word μέταλλον (= metal) because m had been used for maximum. Note that foμEs is a quantity that cannot be measured by the ionosonde.

The present work comes as a continuation of paper A. The idea was to use a reliable set of ionosonde observations, to examine if the new parameter foμEs has a significant effect on Es intensities and other known results, obtained in past studies by means of using foEs. If this were the case, the intention was to promote the foμEs concept in the research community as an improved parameter to be used, instead of foEs, in future ionosonde-based sporadic E studies. To attain these objectives, the work here re-examines the Es layer morphology that refers to midlatitude sporadic E diurnal and seasonal variability and uses this opportunity to update its physical understanding.

Before presenting and discussing the results, the methodology for computing foμEs, proposed in paper A, will be outlined.

Section snippets

The foμEs algorithm

The methodology for computing foμEs at a given ionosonde location, date and time, relies on the measured foEs and h'Es parameters, and the estimate of regular E region electron density at the Es layer height, predicted by the International Reference Ionosphere (IRI) model (e.g., see Bilitza et al., 2017). The algorithm proposed in paper A is based on the following steps:

  • (a)

    computes NmEs from Equation (1) using foEs,

  • (b)

    takes the Es height, hEs, to be nearly equal to the virtual height h'Es, this is,

Results and discussion

The ionosonde foEs and h'Es parameters used in the present analysis were obtained from sequential ionograms measured with the Digisonde DPS-4D system operating at midlatitudes near Nicosia, Cyprus (35.1° N, 33.2° E). The ionograms were recorded every 15 min with 3 km height resolution. The Es parameters were estimated manually, for the purpose of increasing their accuracy that can be undermined in automatic scaling. The lowest detectable foEs frequency is 1.5 MHz, which, according to Equation

Summary and concluding comments

The present paper, which comes as a continuation of paper A (Haldoupis, 2019), aims in examining the severity of biases in Es intensities caused by the use of foEs, relative to the results obtained when using foμEs for the same purpose. To carry out this task, the well-known mean diurnal and seasonal variability of midlatitude sporadic E was re-examined by analyzing a data set measured by the Nicosia Digisonde in Cyprus, located at latitude 35.1° N and longitude 33.2° E. A key objective was to

Acknowledgements

The International Reference Ionosphere model IRI-2016 used in this work is provided by the Community Coordinated Modelling Centre at https://ccmc.gsfc.nasa.gov/. The first author (C.H.) dedicates this paper to his grandson Christos, born on September 20, 2019.

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