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By J K Olesen, Senior Scientist, Dag Hammarskjolds Alle 21, DK 2100 Copenhagen, Denmark
Abstract
While the source and characteristics of the E-F Lacuna ionograms have been reasonably well established as an effect of an E-region plasma instability causing the so-called Slant E Condition (SEC), it seems that the F2 lacuna concept as defined in the Dumont d'Urville ionograms is still on a more debatable level. The present note emphasises the gain-sensitive character of lacuna and demonstrates how this fact - in combination with the ionogram interpretation practice used - make the Dumont d'Urville F2 lacuna occurrences a mixture of several unrelated factors. This might make it desirable to reconsider the F2 lacuna concept and possibly split it up into groupings with more homogeneous sources.
Reference is made to 2 previous LETTERS to the Editor in ANNALES GEOPHYSICAE on the subject: Polar lacuna on ionograms, i.e. Cartron and Vila (1994), and Stauning (1995) including a responding Note: by P. Vila. The main theme in the discussions concerns the nature and relationships of various types of lacuna on ionograms. In the present note, the main theme shall be on an F2-region lacuna (lack of F2-echoes only) contra what here shall be called on the E-F lacuna (lack of echoes from the upper E-region through part or all of the F-region) - called E- region lacuna by Stauning, and L1 or L3 lacuna respectively, by Cartron-Vila. In my notes below I present various assumed conditions for the definition of Dumont d'Urville F2 lacuna and its observation as the necessary preconditions for its existence.
It is a special pleasure for me to comment on this subject, which I have been involved in for more than 40 years of experimental and theoretical activities, and - of special importance in the present context - including 7 years as operator of ionosondes at one-man operated ionosonde stations at various latitudes from the auroral Zone to the geomagnetic polar regions.
I agree on the statements in Stauning's Letter. They are along the lines of previous studies, that I have been involved in on the subject - all the way from the very first identification of the E-F lacuna, (Olesen, 1957, Olesen and Rybner, 1958) then called the "height interval of missing reflections" or "the E-F frequency gap", and described as an "appendix" to the associated slant Es phenomenon, since that was the only phenomenon of the two identified before that time. In particular I agree on Stauning's recommendation on a very strict discrimination between cases of F-region lacuna, with or without E-F lacuna, respectively.
As to the Cartron-Vila Letter and the Vila Note; I refer to the discussions below on the various aspects.
In their hope of reaching an agreement on our different viewpoints on the lacuna, and in particular the F2 lacuna concept, I shall below discuss the main features related to the Dumont d'Urville F2 lacuna concept, as I have understood them. My goal is to reach a common basis for interpretation of lacuna ionograms, or at least to be aware where our standpoints differ. Unfortunately original Dumont d'Urville F2 lacuna ionograms, so important for the discussions, are rather seldom shown, but a few have been found as referred to below.
The occurrence of F2 lacuna at Dumont d'Urville is tabulated not only when the F2-echoes are completely missing at maximum system sensitivity, but also when the F2 echoes are weak or absent at the existing gain setting and overall system sensitivity - even if one or both of them might be low. This impression is stressed by using the term "F2 quasi lacuna".
Documentation/discussion on these statements may be found e.g. In the following reports: Cartron et al. (1971, fig 4 and 6), Cartron et al. (1972,PL24), Olesen (1980), Sylvain (1972), Sylvain (1979, fig.1), Sylvain et al. (1978). The effect of gin setting is illustrated e.g. in figure12.2 in Piggott (1975). See also Rodger (1989), whose figure 1 and table 1 illustrate a unified model for E-F lacuna and F2 lacuna. My main reservation is, that if there is not a G-condition (see below), then in reality the F2-echoes do not disappear as it might be indicated in Rodger's figure, they just weaken. At sufficiently high system sensitivity they show up, maybe somewhat disordered with spreading and obliques, but echoes from the F2-region are there. But naturally, if the French definition of lacuna described above is accepted, we may call it an F2 lacuna under these circumstances.
My further comments: If the above conditions, stated for Dumont d'Urville F2 lacuna, are correct and accepted, I agree on its existence. My reservation is that this will imply the mixture of several unrelated conditions under the lacuna concept. Such as low F2-reflectivity for some reason: tilts, blobs, spreading and other irregularities, and in addition: absorption of the F1 deviative type, when foF2 is little above foF1, probable during SEC events, when some R-region absorption might also add to F2-echo attenuation. Also D-region absorption might contribute, and maybe worst of all; low system sensitivity might be misinterpreted and tabulated as F2 lacuna. See further remarks in next paragraph on system sensitivity.
G-condition (F2 electron density less than underlying layer densities) is only tabulated at Dumont d'Urville when the succession of ionograms clearly demonstrates a gradual decrease of foF2 towards underlying layer densities. Also, it is presumed that a substantial change of foF2, in less than one minute, is not possible. So when an F2-echo disappears that fast, a G-condition interpretation is excluded. All cases of F2-echo weakening or disappearance that do not satisfy these requirements for G-condition qualification are tabulated as F2 lacunae. It is emphasised that the above criteria are evaluated on the basis of ionograms recorded with the system sensitivity that happens to be there at the time in question - even if that system sensitivity might be low at the relevant frequencies.
Documentation: Sylvain (1979, figure 3). See also Rodger (1989) on foF2 decrease tendency during lacuna, and Sylvain (179) on foF2 decreases as a general tendency during magnetic storms and thus during SEC.
My comments: I do not agree with the 1-minute reservations about foF2 changes indicated by Dr. Sylvain. Changes of F2 electron density during SEC are not necessarily of the normal rather slow recombination type, cf. e.g. Rodger (1989). Also the appearance of an F2-trace, when foF2 decreases towards foF1, requires an extraordinary height system sensitivity at the frequencies in question (due to foF1 deviative absorption - and maybe E-region absorption), cf, the Handbook recommendation on the use of high-gain recording for reliable G-evaluation (Piggott and Rawer, 1972). High sensitivity was not present in the Sylvain (1979 figure 1 and 3) illustration, cf. the lower noise levels at 2.25-6.75 MHz revealing lower system sensitivity through that band. The "Magnetic A/B, 1005W" ionosonde used has a special weakness in that the receiver has 3 bands, which are individually tuned, and there are difficulties at attaining a flat sensitivity curve over the frequency bands. Especially band 2 covering the - in the present context very important-frequency band 2.25 to 6.75 MHz is difficult, which is evident in the referred figures. That problem was a main reason why we - after many years of extra care and troubles - exchanged our "Magnetic" ionosonde at Godhavn with a "C4", which has a broadband receiver without that complication. This sensitivity problem might - in addition to antenna characteristics - contribute to the surprising scarcity of slant Es cases reported at Dumont d'Urville during E-F lacuna occurrences, cf. King (1971).
The argument for the F2 lacuna existence, that its daily occurrence distribution is similar to that for E-F lacunae (ref. their figure 3), is not very weighty. The similarity would be there, if the F2 lacuna class was created partly by misinterpreted G-conditions, partly by cases, where the E-F lacuna of F1 deviative absorption might be the last "drop" of attenuation needed to reduce the F2-echo strength sufficiently for the authors to tabulate an F2 lacuna. This might be with the existing - maybe low - system sensitivity through the frequency band in question, in a situation, when foF2 is likely to be little above foF1, so that F1 deviative absorption might be considerable. Cases with other causes for low F2 echo signal strength might be those, that widens the curve for L2-occurrence beyond MLT-hours for L1 and L3 as seen in figure 3.
With reference to some critical remarks in his Note, I can assure Dr Vila and all other users of the High Latitude Supplement (Piggott, 1975), that they can be confident on its description of the main features of SEC, the E region phenomenon responsible for the slant Es trace as well as the E-F lacuna. It is stressed, that when the term SEC is used to describe the configurations in the ionogram, it covers both of them-or any one of the two, depending on which of them is absent for some reason. Linguistic formulations sometimes seen where "SEC" is co-ordinated with "lacuna" is thus irrelevant and confusing and should be avoided. The combination in the SEC concept of both slant Es and lacuna is very important due to the "opposite" gain-sensitive character of the two (E-F lacuna increases and slant Es decreases at low sensitivity, ref. e.g. figure 12.2 in Piggott, 1975). The SEC concept thus gives a "double" check on the evaluation of the ionogram. Another important consequence of slant Es-lacuna coupling as done in the SEC concept is, that it gives the only chance to reveal E-region heating events in ionograms, when there is no F-layer to reveal a lacuna; just a slant Es trace attached to a sporadic layer - often of the totally blanketing auroral type (Olesen, 1972). These events may occur at night and in winter, primarily at auroral zone stations like Tromso and Narsarssuak - and in very rare cases, at high latitude stations like Godhavn (and Dumont d'Urville?), when auroral activity happens to move up there, see Piggott (1975, fig.12.5 and 12.6) and Olesen (1989, fig. 2 and 3).
In summary I agree on the existence of an F2 lacuna phenomenon, if the various preconditions above an accepted, especially "weak F2 traces" and "low equipment sensitivity".
I personally would have preferred another designation for the F2-echo weakening. In particular, a more reliable evaluation of G-conditions, when no F2-echoes are there, e.g. by acceptance of the Handbook recommendation on high gain records for G-evaluation. Also, I would have preferred weak F2-echoes characterised e.g. by letters like R (attenuation in vicinity of critical frequency) or C (interpretation impossible for non-ionospheric reason, or equipment failure), so that the Y designation might have been reserved for the daylight E-F lacuna, where a clear unavoidable gap occurs even at absolute maximum system sensitivity due to the excessive amount of absorption present. Based on incoherent scatter measurements during an E-F lacuna event the attenuation of lacuna frequencies between 3 and 3.8 has been calculated to be 22 to 61 dB (Stauning, 1984, Stauning et al., 1985, Olesen et al., 1986).
In fact, the above complications might be reduced if the recommendation on the use of height gain records was prescribed and obeyed, not only for G-evaluation, but for all gain-sensitive phenomena including the lacuna phenomenon; at least, for data used in research.
I appreciate very much the continued studies of lacuna and associated problems. I hope my comments above may help in our efforts to unveil the final full story on the lacuna and related phenomena.
References
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