The study of the preparation phase of large earthquakes is essential to understand the physical processes involved, and potentially useful also to develop a future reliable short-term warning system. ...Here we analyse electron density and magnetic field data measured by Swarm three-satellite constellation for 4.7 years, to look for possible in-situ ionospheric precursors of large earthquakes to study the interactions between the lithosphere and the above atmosphere and ionosphere, in what is called the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC). We define these anomalies statistically in the whole space-time interval of interest and use a Worldwide Statistical Correlation (WSC) analysis through a superposed epoch approach to study the possible relation with the earthquakes. We find some clear concentrations of electron density and magnetic anomalies from more than two months to some days before the earthquake occurrences. Such anomaly clustering is, in general, statistically significant with respect to homogeneous random simulations, supporting a LAIC during the preparation phase of earthquakes. By investigating different earthquake magnitude ranges, not only do we confirm the well-known Rikitake empirical law between ionospheric anomaly precursor time and earthquake magnitude, but we also give more reliability to the seismic source origin for many of the identified anomalies.
Recently, our paper “Poststorm Thermospheric NO Overcooling?” has been published in JGR. Lei with colleagues (2012) who have proposed the “NO overcooling” concept have written Comments on this paper. ...Below is given our reply. Everywhere MP20 means the reference to the paper by Mikhailov and Perrone (2020). In the beginning to avoid misunderstanding, it is necessary to stress that in MP20 we did not touch on the well‐documented process of the thermosphere NO cooling (e.g., Gordiets et al., 1982; Maeda et al., 1989; Mlynczak et al., 2018; Prölss, 2004, 2011; Roble, 1995; Weimer et al., 2011) which mainly takes place in the lower thermosphere. We only explained a decrease of neutral gas density at F2‐layer heights during the recovery storm phase. The effect manifests seasonal dependence which is not explained by the “NO overcooling” mechanism.
Key Points
The “NO overcooling concept” is unable to explain the post‐storm seasonal variations of neutral gas density at F2‐layer heights
Contemporary F2‐layer storm mechanism adequately explains the observed seasonal differences in neutral gas density variations during the magnetic storm recovery phase
The observed post‐storm variations of neutral gas density at F2‐layer heights cannot be related to the process of NO cooling in the thermosphere
The effect of montmorillonite clay (MMT) and/or chain extender (CE) on rheological, morphological and biodegradation properties of PLA/PBAT blend was investigated. The biodegradation behavior was ...evaluated by CO2 evolution in soil burial. CE incorporation resulted in an increase in the complex viscosity of PLA/PBAT blends, an increase in PLA crystallinity and a decrease in the dispersed phase diameter. MMT incorporation resulted in an increase in the complex viscosity, more pronounced shear-thinning behavior and a decrease in the dispersed phase diameter. CE incorporation resulted in a slight effect in the rheological properties of PLA/PBAT blend in the presence of MMT. Unfilled PLA/PBAT blend presented the highest amount of evolved CO2, and the micrographs indicated that degradation tends to occur on the surface. MMT delayed biodegradation of PLA/PBAT blends even although their surfaces presented some cracks and holes in a few localized regions. PLA/PBAT + CE blend presented the lowest amount of evolved CO2.
The majority of Zhang et al. critical comments concern our method to retrieve aeronomic parameters from ionosonde foF1 observations. The main idea of the method was described by Mikhailov and Perrone ...(2016, https://doi.org/10.1002/2016JA022716), but it is seen that an additional clarification is required. The method has been slightly changed to meet numerous comments of referees, and its description is given here. The comments by Zhang et al. are analyzed and answered.
Key Points
An improved method to extract a self‐consistent set of seven aeronomic parameters responsible for the midlatitude F1 region formation is presented
Mean ion mass inferred from foF1 observations manifests negative trends that should correspond to Tn negative trend contrary to ISR results
Routine ISR method based on a fixed model of ion composition (crucial for the method) should inevitably result in wrong Tn long‐term trends
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, and N2), exospheric temperature Tex, and the total solar EUV flux with λ < 1050 Å 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 O11y 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 Mass Spectrometer Incoherent Scatter‐86 driven by Ap and F10.7 indices manifests O11y and (O/N2 )11y variations similar to 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.
Key Points
A new method to retrieve thermospheric parameters from routine foF1 ionosonde observations
Atomic oxygen demonstrates solar cycle
The negative foF2 trend commonly discussed in literature was due to a very long‐term decrease in the atomic oxygen abundance
June noontime monthly median foF1 ionosonde observations at Sodankylä (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 incoherent scatter 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.
Key Points
A positive trend in the O+/Ne ratio (which corresponds to a negative trend in mean ion mass) and negative trend in Ti takes place at 175 km
Routine incoherent scatter radar data cannot be used in long term trend analyses
The revealed small Tex trends strongly contradict the results obtained from ISR observations
A new method to retrieve hmF2 and vertical plasma drift W from foF1 and foF2 ionosonde observations has been proposed and applied at five European stations for June noontime conditions over the ...(1958–2017) period. Linear long‐term trends were estimated using the retrieved hmF2 and W variations. It has been shown that negative and statistically significant hmF2 trends manifest a pronounced latitudinal dependence: the largest (~2.7 km per decade) is at lower‐latitude station Rome, while the smallest (< 1 km per decade) is at high‐latitude station Sodankylä. It was shown that hmF2 trends are due to vertical plasma drifts that manifest similar latitudinal dependence: the largest (~1 m/s per decade) is at Rome and the smallest (~0.3 m/s per decade) is at Sodankylä. Vertical plasma drift converted to the meridional northward thermospheric wind manifests an average increase of (11.6 ± 3.0) m/s over the period of ~60 years. This may be related to a decrease in ion drag due to negative trends in the ionospheric F1 and F2 regions. An additional factor is a decrease in auroral heating related to long‐term decrease in solar and geomagnetic activity observed for June months over the analyzed period.
Key Points
A new method to retrieve hmF2 from foF1 and foF2 ionosonde observations has been proposed
Negative hmF2 trends manifest a latitudinal dependence reflecting the corresponding dependence in vertical plasma drifts
Long‐term increase of the Northward thermospheric wind reflects a long‐term decrease in auroral heating
A new method to extract neutral composition (O, O2, N2), exospheric temperature Tex, vertical plasma drift, W, and the total solar Extreme Ultraviolet flux with λ ≤ 1050 Å from routine ionosonde ...bottom‐side electron density, Ne(h), observations has been proposed. The method can be used around noontime hours for all months of the year at middle latitudes where the ionospheric F‐layer is formed by solar Extreme Ultraviolet radiation. The uncertainty of the retrieved neutral gas density coincides with the announced Mean Relative Deviation ±(10‐15%) of CHAMP/STAR neutral gas density observations. The method also provides statistically significant better results in a comparison with modern Mass‐Spectrometer‐Incoherent‐Scatter, Jacchia‐Bowman 2008, and Drag Temperature Model 2013 empirical models. The thermospheric parameters retrieved for the St. Patrick Day magnetic storm and two so‐called Q‐disturbance periods are given as an example of the method application. The retrieved neutral gas densities for the St. Patrick Day storm are compared to Swarm‐B accelerometer observations. The proposed method may be considered as a useful tool for analyses of the state of the upper atmosphere under various geophysical conditions.
Key Points
A new method to extract a self‐consistent set of aeronomic parameters (Tex, O, O2, N2, W, and EUV flux) from ionosonde observations has been proposed
The method is applied for all seasons at middle and lower latitudes where the ionospheric F‐layer is formed by solar EUV radiation
Testing of the method on CHAMP/STAR neutral gas density has shown better accuracy than modern empirical models MSISE00, JB2008, DTM2013
A sunrise F2‐layer short‐term (1–24) h foF2 prediction method has been developed to forecast foF2 variations at a given ionosonde station during magnetically quiet and disturbed periods. The proposed ...method efficiently describes both positive and negative quiet time F2‐layer disturbances under daily Ap <30 nT and this was done for the first time. A comparison with modern global empirical models demonstrates a statistically significant advantage over them under various seasons and levels of solar activity.
Plain Language Summary
A new method to predict hourly foF2 with a lead time of (1–24) h at an ionosonde station has been proposed. Input information to run the derived method includes: hourly real‐time foF2 observations, a monthly Australian T‐index to specify the median foF2 background level, and a predicted level of geomagnetic activity with two gradations: quiet (daily Ap < 30 nT) and disturbed (daily Ap ≥ 30 nT). The method is efficient to predict both negative and positive quiet time F2‐layer disturbances and this is done for the first time. The method is also efficient under magnetically disturbed conditions but with shorter lead times. The prediction accuracy is significantly better than global empirical IRI (STORM) and GDMF2 models provide. This is valid for any season and level of solar activity. All these open wide possibilities for the method's application in practice.
Key Points
A prediction method for short‐term (1–24) h foF2
The method is efficient under low magnetic activity to distinguish negative and positive Q‐disturbances
The method is also efficient under magnetically disturbed conditions with shorter lead times
PDF
