Mass spectrometer measurements of the positive ion composition in the D- and E-regions of the ionosphere

Abstract

On 14 December 1971, during the maximum of the Geminid Meteor Shower, the positive ion composition was measured in the D- and E-regions above Sardinia. The payload was launched at 12:11 UT, and measurements were made between 68.5 and 152 km altitude. A magnetic sector type mass spectrometer with dual collector and a liquid helium cryopump was used. The instrument covered the mass range from 11 to 73 AMU and had a resolution at the 1 % level of M ΔM = 60 . In the E-region two distinct metal ion layers were observed, centred at 95 and 119 km, respectively. In the lower layer Fe + and Mg + were the most abundant metal ions, and in the upper layer Si + was dominant. Si + ions were conspicuously absent in the lower layer ( Si + Mg + < 2 × 10 −3 ). This particular behaviour of Si could be due to the inability of atomic oxygen to reduce SiO, whereas in the upper layer Si +ions might be formed directly by the charge rearrangement reaction SiO + O + → Si + + O 2. In addition, Na +, Al +, K +, Ca +, Ti +, Cr +, Ni + and Co + were also identified. The metal oxide ions AlO + and SiO + were detected, and probably also MgO + and SiOH +. The concentrations of NO + and O 2 + show a deep minimum at the maximum of the lower metal ion layer. A very high neutral metal density of 6 × 10 7 cm −3 would be required to explain this feature as resulting from charge transfer reactions between the molecular and metal ions Such a high metal density is in contradiction to direct measurements and to cosmic dust influx rates. The isotopic ratios of Mg +, Si +, and of the major isotopes of Fe + and Ni + were measured, some of them with an accuracy of a few per cent ( 25 Mg + 24 Mg + = 0.124 ± 0.006 ; 26 Mg + 24 Mg + = 0.139 ± 0.008 ; 29 Si + 28 Si + = 0.050 ± 0.004 ; 54 Fe + 56 Fe + = 0.069 ± 0.005 ; 57 Fe + 56 Fe + = 0.029 ± 0.004 ; 60 Ni + 58 Ni + = 0.31 ± 0.12 ). The isotopic ratios agree within the experimental errors with the corresponding terrestrial ratios, thus giving evidence that these elements have the same isotopic composition in the Geminid meteors as in the Earth's crust, in chrondrites, and in lunar material. In the D-region the ions Na +H 2O, Na +(H 2O) 2, NaO + and NaOH + were tentatively identified. Below 95 km altitude the relative abundances of the ions 32 +, 33 + and 34 + deviate from the values expected for molecular oxygen isotopes. Their abundances can not be explained by the presence of S-ions only, and we conclude that HO 2 + and H 2O 2 + are present. The ion density profiles of the major D-region constituents show some remarkable deviations from typical D-region conditions. These deviations are related to the winter anomaly in ionospheric absorption observed over Spain during the launch day, and our data represent the first ion composition measurements during such conditions. In particular, H +(H 2O) 2 is the major ion only up to 77 km, and at 80 km altitude the NO + concentration exceeds the total water cluster ion density by almost two orders of magnitude. An increase of the mesospheric NO, O 3 and O concentrations as well as of the O H 2 O ratio could explain the observed ion profiles. The low NO + O 2 + ratios of approximately unity measured in the E-region are in agreement with a strong downward transport of NO and/or O into the mesosphere during the launch day. A simple four-ion model was used to interpret our D-region data. The calculated neutral NO concentration increases from about 2 × 10 7 cm −3 at 85 km to 5 × 10 7 cm −3 at 80 km. In addition, evidence for an increased O 2 + production rate above 83 km was found, probably due to an enhanced O 3 concentration. We conclude that our data strongly support vertical transport of minor neutral consituents as cause of the winter anomaly.