Loredana Perrone

Long-term trends of the critical frequency of the F2 layer at northern and southern high latitude regions

Abstract The long-term behaviour of the plasma frequency of the F2 layer, foF2, is investigated to contribute to the discussion about the natural or anthropogenic origin of the ionospheric long-term changes. In this paper, long time series of hourly foF2 data from two high latitude ionospheric observatories of both the hemispheres are analysed. A decreasing trend of foF2 is found confirming a previous result obtained by using the foF2 monthly medians from Antarctic stations. This negative trend does not show a clear dependence on the long-term behaviour of the geomagnetic activity impact on the ionosphere.

Geomagnetic control of the midlatitude foF1 and foF2 long‐term variations: Recent observations in Europe

A new very simple method, allowing an easy control, has been applied to extract long-term (11 year) δfoF2 11y and δfoF1 11y variations from June foF2 and foF1 monthly median observations at European Slough/Chilton and Juliusruh stations, including recent data until 2015. The aim of the analysis was to check the validity of the geomagnetic control of foF2 and foF1 long-term variations in the 21st century with the main accent on the period including the last deep solar minimum in 2008–2009. The geomagnetic control was shown to be valid. Moreover, the dependence on geomagnetic activity has become more pronounced and explicit after 1990. A simultaneous analysis of foF2 and foF1 long-term variations improves the reliability of the obtained conclusions and helps understand the physical mechanism of these variations. Due to common neutral composition and the similarity of photochemical processes noontime foF2 and foF1 demonstrate similar long-term variations: the correlation coefficient between δfoF2 11y and δfoF1 11y is 0.834 at Slough/Chilton and 0.884 at Juliusruh with the 99% confidence level according to Fisher’s F criterion. Midnight long-term δfoF2 11y variations alsomanifest a pronounced dependence on Ap11y whichmay be interpreted in the framework of the geomagnetic control concept.

Geomagnetic control of the midlatitude daytime foF1 and foF2 long-term variations: Physical interpretation using European observations

Morphological analysis of Slough/Chilton and Juliusruh foF2 and foF1 long-term variations for the period including recent observations made in the previous paper (PM) has shown that the geomagnetic control is valid in the 21st century, moreover the dependence on geomagnetic activity has become more pronounced and explicit after 1990. A new method to retrieve thermospheric neutral composition (O, O2, N2), exospheric temperature Tex, and the total solar EUV flux with λ < 1050 A from routine foF1 ionosonde observations has been developed to understand the mechanism of this geomagnetic control. The method was tested using CHAMP/STAR neutral gas density measurements. The retrieved for the first time thermospheric parameters at Slough/Chilton and Juliusruh over the period of ~ 5 solar cycles were used to analyze the mechanism of foF1 and foF2 long-term variations in the light of the geomagnetic control concept. It was shown that the control was provided via two channels: [O] and [O]/[N2] variations. Geomagnetic activity presented by 11-year running mean weighted index Ap11y controls the (O/N2 )11y ratio variations, while solar activity presented by (F10.7)11y controls atomic oxygen [O]11y variations. Atomic oxygen, the main aeronomic parameter controlling daytime foF1 and foF2 variations, manifests solar cycle and long-term (for some solar cycles) variations with the rising phase in (1965-1985) and the falling phase in (1985-2008). These long-term [O] variations are reflected in foF2 and foF1 long-term variations. The origin of these long-term variations is in the Sun. The empirical thermospheric model MSIS-86 driven by Ap and F10.7 indices manifests [O]11y and (O/N2 )11y variations similar the retrieved ones including the period of deep solar minimum with a very low atomic oxygen concentration in 2008. This confirms the basic idea of the geomagnetic control concept that ionospheric long-term variations have a natural (not anthropogenic) origin related to long-term variations in solar and geomagnetic activity.

Long term trend in the high latitude ionosphere

Abstract The long-term variability of the critical frequency of the F2 ionospheric layer, f0F2, has been investigated as a possible indicator of the increasing greenhouse effect. The preliminary results obtained by analysing 34 years of data from high latitude ionospheric stations in the Southern Hemisphere have shown a decreasing of the plasma frequency according to what predicted by theoretical model calculations.

A correlation study between time-weighted magnetic indices and the high latitude ionosphere

Abstract Time weighted accumulations of the ap, AE and PC magnetic indices have been here analysed together with the hourly values of the critical frequency of the F2 ionospheric layer, foF2, coming from several ionospheric stations located in the European longitudinal sector at geographic latitudes ranged between 60°N and 68°N. The preliminary results obtained for different seasons and for different solar activity conditions indicate that the response time of the high latitude ionosphere to the magnetic activity is of the order of about 15 and/or 20 hours depending on the magnetic index considered.

The SWILM approach for regional long-term modelling of middle/high latitude ionosphere

Abstract The Space - Weighted Ionospheric Local Model, SWILM, is an alternative approach developed for the regional long-term mapping and modelling of the plasma frequency of the F2 layer, foF2, and the M(3000)F2 propagation factor. This model is based on the past monthly medians data from several vertical sounding stations in the region of interest and on the R12 index as indicator of the solar activity. In order to allow users to switch to a planetary scale, a good merge with the global ITU-R (Radiocommunication Sector of the International Telecommunication Union) model is also provided by SWILM with acceptable gradients at the borders of the region. In this paper the basic formulation of the model will be given and the results will be shown as obtained by comparing SWILM and ITU-R performances for mapping and modelling the fbF2 and the M(3000)F2 over a geographic area ranged between 35°–70°N and 10°W–60°E.

Thermospheric parameters long-term variations retrieved from ionospheric observations in Europe

Recently developed method to retrieve thermospheric parameters (Tex, O, O2, and N2) and the total solar EUV flux with λ< 1050Å from routine foF1 ionosonde observations has been applied to monthly median foF1 data on Rome, Slough/Chilton, and Juliusruh stations to analyze long-term trends in the thermospheric parameters. For the first time exospheric temperature and neutral composition were obtained for June noontime conditions over the period of ~ 5 solar cycles. The retrieved parameter manifested solar cycle and long-term (some solar cycles) variations with a rising phase in 1965–1985 and falling phase in 1985–2008. The retrieved thermospheric parameters were shown to be close to the MSIS-86 model ones exhibiting very small (<1% per decade) and statistically insignificant linear trends estimated either over all 56 years or only over the years of solar minimum. No peculiarities in long-term variations in relation with the last deep solar minimum have been revealed. The source of the thermospheric parameter long-term trends is the Sun, i.e., they have a natural (not anthropogenic) origin and are mainly controlled by long-term variations of solar and geomagnetic activity.

Spectral and fractal analyses of geomagnetic and riometric antarctic observations and a multidimensional index of activity

Abstract The Istituto Nazionale di Geofisica under the P.N.R.A. (National Program of Research in Antarctica) has the responsibility of acquiring geophysical observations at the Italian Antarctic Base of Terra Nova Bay. Among others, geomagnetic and riometric data can provide some new insights into local and global activity of the magnetosphere-ionosphere coupling. This article investigates some properties of these kinds of data by means of spectral and fractal analyses. In addition, a multidimensional index is derived from this single-point dataset to represent not only the local but also the global state of the magnetospheric activity.

Long‐term variations of exospheric temperature inferred from foF1 observations: A comparison to ISR Ti trend estimates

June noontime monthly median foF1 ionosonde observations at Sodankyla (auroral zone), Juliusruh and Rome (middle latitudes) were used to retrieve exospheric temperature, Tex long-term variations over the (1958-2015) period. After removing solar activity effects the residual linear trends were found to be small (0.05-0.6)% per decade and statistically insignificant at middle latitudes. Therefore, the revealed Tex long-term variations are mainly due to long-term variations of solar activity i.e. they have a natural (not anthropogenic) origin. Large trends in ion temperature, Ti inferred from incoherent scatter radar (ISR) observations which the researchers identify with trends in neutral temperature, Tn may be related to the IS method routinely based on a fixed model of ion composition (O+/Ne ratio and mean ion mass, correspondingly) under varying geophysical conditions. Mean ion mass number manifests a negative trend at 175 km which should correspond to a negative trend in Ti contrary the results obtained below 200 km with ISRs. Therefore, routine ISR observations based on a fixed model of ion composition may be not appropriate for long-term trend analyses.

The time-weighted magnetic indices ap(τ), PC(τ), AE(τ) and their correlation to the southern high latitude ionosphere

Abstract In this investigation different data transformations of the ionospheric characteristic foF2 from high latitude stations in the Southern Hemisphere are considered and correlated with time weighted accumulation of the geomagnetic indices ap, AE and PC. The analysis, performed during periods of severe magnetic activity, shows a decrease in foF2 as a consequence of geomagnetic perturbations and a characteristic response recovery time t ranged between 15–20 hours. The best correlation coefficients are obtained by comparing the time weighted accumulation of the auroral index AE and a data transformation based on the ratio NmF2(t)/NmF2 M where NmF2(t) is the hourly maximum electron density at the F2 peak layer and NmF2 M is its monthly median value.