Effect of magnetic storms in variations in the atmospheric electric field at midlatitudes

The observations of the variations in the vertical component of the atmospheric electric field (E _z ) at Swider midlatitude Poland observatory (geomagnetic latitude 47.8°) under the conditions of fair weather during 14 magnetic storms have been analyzed. The effect of the magnetic storm main phase in the daytime midlatitude variations in E _z in the absence of local geomagnetic disturbances has been detected for the first time. Considerable (～100–300 V m^?1) decreases in the electric field strength (E _z ) at Swider observatory were observed in daytime simultaneously with the substorm onset in the nighttime sector of auroral latitudes (College observatory). The detected effects indicate that an intensification of the interplanetary electric field during the magnetic storm main phase, the development of magnetospheric substorms, and precipitation of energetic electrons into the nighttime auroral ionosphere can result in considerable disturbances in the midlatitude atmospheric electric field.     

Modelling the peak of the ionospheric E-layer

Calculations with a full time-varying model are used to study changes in the height and density of the E-layer peak, caused by known changes in the neutral atmosphere. Agreement with mean observed values of N_mE requires an increase of 10% in calculated ion densities, and an increase of 33% in the solar-maximum EUV model at λ<150 Å. At a fixed site, changes with the solar zenith angle χ agree well with the simple Chapman theory during most of the daylight hours. Simple modifications to the Chapman equations give improved accuracy near sunrise and sunset. When corrected for changes in χ, model results for summer and equinox show a decrease in the peak density N_mE at increasing latitudes. The overall change agrees well with experimental data, as summarised in the IRI model. Known changes in the neutral atmosphere also reproduce the increase in N_mE in winter, at latitudes up to 30°. The continuing increase at higher winter latitudes, in the IRI model, requires a major reduction in NO densities in winter. A suitable compromise is suggested. Equations fitted to the model results then provide a simpler and better behaved replacement for the IRI equations. Calculations at night show that known sources of ionisation, largely from starlight, can produce observed peak densities using current chemistry. There is an appreciable change with latitude, as starlight production increases in the southern hemisphere. The improbably large solar cycle change built into the IRI model, at night, cannot be reproduced and is not found in recent data. A new, simpler model is suggested. Changes in zenith angle and atmospheric composition cause the peak height (h_mE) to vary between 105 and 120 km, as a function of time, latitude, season and solar flux. These changes are approximated by simple equations that should be definitely preferable over the single, fixed height used in the IRI models.     

Morning polar substorms and variations in the atmospheric electric field

The effects of morning magnetospheric substorms in the variations in near-Earth atmospheric electricity according to the observations of the electric field vertical component (E _z ), at Hornsund polar observatory (Spitsbergen). The E _z, data, obtained under the conditions of fair weather (i.e., in the absence of a strong wind, precipitation, and fog), are analyzed. An analysis of the observations indicated that the development of a magnetospheric substorm in the Earth’s morning sector is as a rule accompanied by positive deviations in E _z, independently of the Hornsund location: in the polar cap or at its boundary. In all considered events, Hornsund was located near the center of the morning convection vortex. In the evening sector, when Hornsund fell in the region of evening convection vortex, the development of a geomagnetic substorm was accompanied by negative deviations in E _z., It has been concluded that the variations in the atmospheric electric field E _z), at polar latitudes, observed during the development of magnetospheric substorms, result from the penetration of electric fields of polar ionospheric convection (which are intensified during a substorm) to the Earth’s surface.     

A Note on the Estimation of Eddy Diffusivity and Dissipation Length in Low Winds over a Tropical Urban Terrain

— Urban terrain poses a challenge for modeling air pollutant diffusion. In tropics, because of the dominant low wind speed environment, the importance of understanding the turbulence diffusion is even more critical, and uncertain. The objective of this study is to estimate the vertical eddy diffusivity of an urban, tropical atmosphere in low–wind speeds. Turbulence measurements at 1 Hz were made at 4-m level over an urban terrain with a roughness length of 0.78 m during winter months. Eddy diffusivity is estimated from spectral quantities of the turbulence data involving turbulent kinetic energy (E) and its dissipation rate (?). The spectral information of the vertical velocity fluctuations is used to estimate the vertical length scale which provides information on the eddy diffusivity. In addition, the product of friction velocity and the vertical length scale has been used to non-dimensionalize the eddy diffusivity, which is shown to increase with increasing instability. Using the eddy diffusivity (K) estimates from the E – ? approach, a relation is suggested for the mixing length based eddy diffusivity models of the form: K = c _w .[2.5 ? 0.5(z/L)], where z is the measurement height, L is the Obukhov length, and c _w has an average value close to 1 for unstable and near 0.5 for stable conditions for the urban terrains.     

Relation between the atmospheric characteristics in the antarctic regions and the space weather factors

Considerable variations in the cloud cover level and air temperature, related to the variations in GCRs and IMF, have been revealed based on an analysis of the meteorological and aerological data obtained at Vostok station from 1974 to 1994. It has been found out that the cloud cover at Vostok decreased, on average, by 35% a day after powerful Forbush decreases in GCRs following a considerable increase in the southward IMF component. In the years of solar activity minimum, when the variations in SCRs and GCRs are insignificant, the cloudiness and surface temperature increase on a day of B _z minimum and decrease on a day of maximum as compared to the average level. On days of B _z minimum, the air temperature rises at altitudes of h = 3.5–7 km, remains almost unchanged at an altitude of h = 8 km, and slightly decreases at higher altitudes. An increase in cloudiness at altitudes below 8 km causes warming, probably due to the greenhouse effect, because the temperature of the Earth’s surface decreases.     

Geophysical effects of the interplanetary magnetic cloud on October 18–19, 1995, as deduced from observations at Irkutsk

During an interaction of the Earth’s magnetosphere with the interplanetary magnetic cloud on October 18–19, 1995, a great magnetic storm took place. Extremely intense disturbances of the geomagnetic field and ionosphere were recorded at the midlatitude observatory at Irkutsk (Φ′≈45°, Λ′≈177°, L≈2) in the course of the storm. The most important storm features in the ionosphere and magnetic field are: a significant decrease in the geomagnetic field Z component during the storm main phase; unusually large amplitudes of geomagnetic pulsations in the Pi1 frequency band; extremely low values of critical frequencies of the ionospheric F2-layer; an appearance of intense E_s-layers similar to auroral sporadic layers at the end of the recovery phase. These magnetic storm manifestations are typical for auroral and subauroral latitudes but are extremely rare in middle latitudes. We analyze the storm-time midlatitude phenomena and attempt to explore the magnetospheric storm processes using the data of ground observations of geomagnetic pulsations. It is concluded that the dominant mechanism responsible for the development of the October 18–19, 1995 storm is the quasi-stationary transport of plasma sheet particles up to L≈2 shells rather than multiple substorm injections of plasma clouds into the inner magnetosphere.     

Estimation of storm-time level of day-side wave geomagnetic activity using a new <Emphasis Type="Italic">ULF</Emphasis> index

The level of wave geomagnetic activity in the morning and daytime sectors of auroral latitudes during strong magnetic storms with Dst _min varying from ?100 to ?150 nT in 1995–2002 have been studied using a new ULF index of wave activity proposed in [Kozyreva et al., 2007]. It has been detected that daytime Pc5 pulsations (2–6 mHz) are most intense during the main phase of a magnetic storm rather than during the recovery phase as was considered previously. It has been indicated that morning geomagnetic pulsations during the substorm recovery phase mainly contribute to daytime wave activity. The appearance of individual intervals with the southward IMF B _z component during the magnetic storm recovery phase results in increases in the ULF index values.     

Comparison of the Hill–Siscoe polar cap potential theory with the Weimer and AMIE models

The magnetic storm on November 2004 was characterized by a high solar wind pressure and thus offers a unique opportunity to test the Hill–Siscoe formula (H–S) for the polar cap potential (PCP). To estimate the polar cap potential, we use the Weimer Statistical Convection Model (WCM), and the Assimilative Mapping of Ionospheric Electrodynamics Model (AMIE), based on ingestion of a number of data sets. H–S is in excellent agreement with WCM, and with AMIE during times when DMSP is used in the latter. The implication is that the AMIE conductivity model yields conductivities that are too high by a factor of 2–3. Both H–S and WCM display saturation effects, although WCM is more severe. The two methods track well until an IEF of about 20 mV/m occurs, where H–S continues to increase while WCM levels off. Even at high electric field values, the pressure increases the denominator of the H–S formula by 60%, keeping the potential lower than its saturation value. There are several H–S points above 250 kV, even up to 400 kV, that are not found in WCM and occur right after a rapid transition from B_z north to south. For B_z north, we find evidence for a saturation effect on the PCP at large IEF, little effect as a function of solar wind velocity, and an increase of the PCP with increasing pressure. This seems to rule out viscous interaction but may involve geometric changes in the high-altitude polar cusp that affect recombination there for B_z north.     

The ionospheric response to the cme event of 6–11January 1997

An active aurora was observed at Eureka, Canada (88.9° N magnetic) following the arrivalat the magnetopause of the shock front resulting from the solar Coronal Mass Ejection of 6January 1997. This onset at 02:20 UT on 10 January marked the beginning of an aurora whichcontinued until at least 15:00 UT on 11 January, as viewed from both Eureka and the CANOPUSsite at Ft Smith (67.8° N magnetic). There were enhanced OI 630 nm polar F-region emissionsthroughout this period, with the IMF B_z controlling their form. When B_z was positive, there were continuous polar arcs; when B_zturned negative there were F-layer patches on open field lines. While the strong H_β observed over Ft Smith (240 R) in addition to the extended 630 nm emissions and theirpersistence over two days suggested a red aurora of global proportions, it apparently did notextend to latitudes below 60°. There was a moderate magnetic storm associated with the aurora,but the D_st index reached only −81 nT on 10 January.     

Form induced stresses over rough gravel-beds

This paper presents an analysis of spatial flow heterogeneity over rough gravel beds for shallow flows in terms of form induced stresses. Data from experiments specifically designed with the intention to analyze the flow data with the double-averaging methodology are used to investigate the nature of form induced stresses. It is shown that spatial flow heterogeneity is small at greater distances to the roughness tops (z ₁₀₀), increases slightly towards z ₁₀₀, and increases significantly below z ₁₀₀. Form induced stresses determined over the same bed and with the same slope are found to be independent of discharge. The influence of the number of measuring verticals on the magnitude of form induced stresses is discussed. The distributions of form induced stresses ?<?~w> are used to define the geodetic level of the roughness crest for rough, irregular beds from hydraulic data.     

Dynamics of the ionospheric electric currents and auroral luminosity boundaries during strong magnetic storms

The regularities in the southward drift of the ionospheric current centers and luminosity boundaries during strong magnetic storms of November 2003 and 2004 (with Dst ≈ ?400 and ?470 nT, respectively) are studied based on the global geomagnetic observations and TV measurements of auroras. It has been indicated that the eastward and westward electrojets in the dayside and nightside sectors simultaneously shift equatorward to minimal latitudes of Φ _min ^° ～53°–55°. It has been obtained that the Φ _min ^° latitude decreases with increasing negative values of Dst, IMF B _z component, and westward electric field strength in the solar wind. The dependence of the electrojet equatorward shift velocity (V _av) on the rate of IMF B _z variations (ΔB _z /Δt) has been determined. It is assumed that the electrojet dynamics along the meridian is caused by a change in the structure of the magnetosphere and electric fields in the solar wind and the Earth’s magnetosphere.     

Reaction of the high-latitude lower ionosphere to solar proton events from observations in the ELF range

The reaction of the lower ionosphere to the solar proton events that occurred in 2011–2012 is studied in this paper based on the results of measurements of the propagation velocity and the E _z /H _τ ratio of the low-frequency electromagnetic pulses (atmospherics) in the ELF range at the high-latitude observatories Lovozero and Barentsburg. With numerical modeling methods, it is shown that horizontal local irregularities of the lower ionosphere conductivity profile could be a cause of the splashes in the E _z /H _τ ratio observed in the experiment during the solar proton event of March 7, 2012, which was a unique event in both the proton flux value and energy.     

Statistical relations between the probability of occurrence of Pc3-4 pulsations at high latitudes in the antarctic regions and the solar wind and IMF parameters

Geomagnetic pulsation in the Pc3-4 bands have been studied at high Antarctic latitudes during the local summer. The statistical relation between the occurrence probability of Pc3 and Pc4 pulsations and the solar wind (SW) and IMF parameters has been revealed by verifying the hypothesis that an indication is identical in two distributions. Different dependences of the occurrence probability of high-latitude Pc3 and Pc4 pulsations on the IMF value and orientation and SW density and velocity have been found out. It has been indicated that these dependences remain unchanged in the range of geomagnetic latitudes from 66° to 87°. It has been established that the Pc3 observation probability at small (20°–50°) IMF cone angles (θ = cos^?1(B _x/|B|)) is a factor of 1.5 higher than the average statistical probability and depends on the IMF value, which confirms the hypothesis that the Pc3 source is the turbulent region upstream of the magnetospheric quasiparallel low shock. On the contrary, the probability of occurrence of Pc4 weakly depends on the IMF cone angle and is maximal at θ ～ 0° and ～90°. With increasing negative B _z values, the generation probability increases in the Pc4 band and tends to decrease in the Pc3 band. It has been found out for the first time that the dependence of the Pc4 occurrence probability on the IMF clock angle (? = tan^?2 (B/B _z) is identical in the regions of projections of closed and open field lines, whereas this dependence is different for Pc3. In the region of projections of closed field lines, the Pc3 occurrence probability increases at B _z < 0 and B _y > 0 (the condition under which the cusp shifts on the dawn side) and at B _y < 0 and B _z > 0 (which is typical of the formation of the low-latitude boundary plasma sheet). In the region of projections of open field lines such a probability increases at B _y < 0 and B _z < 0 (which results in the formation of the high-latitude boundary plasma sheet). Based on the discovered regularities, the conclusion has been made that the sources of generation of high-latitude Pc3 and Pc4 pulsations are different.     

A Monte Carlo simulation of the effect of ion self-collisions on the ion velocity distribution function in the high-latitude F-region

Non-Maxwellian ion velocity distribution functions have been theoretically predicted and confirmed by observations, to occur at high latitudes. These distributions deviate from Maxwellian due to the combined effect of the E×B drift and ion-neutral collisions. The majority of previous literature, in which the effect of ion self-collisions was neglected, established a clear picture for the ion distribution under a wide range of conditions. At high altitudes and/or for solar maximum conditions, the ion-to-neutral density ratio increases and, hence, the role of ion self-collisions becomes appreciable. A Monte Carlo simulation was used to investigate the behavior of O⁺ ions that are E×B-drifting through a background of neutral O, with the effect of O⁺ (Coulomb) self-collisions included. Wide ranges of the ion-to-neutral density ratio n _i /n _n and the electrostatic field E were considered in order to investigate the change of ion behavior with solar cycle and with altitude. For low altitudes and/or solar minimum (n _i /n _n \leq10^?5), the effect of self-collisions is negligible. For higher values of n _i /n _n , the effect of self-collisions becomes significant and, hence, the non-Maxwellian features of the O⁺ distribution are reduced. For example, the parallel temperature T _i\Vert increases, the perpendicular temperature T _i⊥ decreases, the temperature anisotropy approaches unity and the toroidal features of the ion distribution function become less pronounced. Also, as E increases, the ion-neutral collision rate increases, while the ion-ion collision rate decreases. Therefore, the effect of ion self-collisions is reduced. Finally, the Monte Carlo results were compared to those that used simplified collision models in order to assess their validity. In general, the simple collision models tend to be more accurate for low E and for high n _i /n _n .     

Polar patches observed by ESR and their possible origin in the cusp region

Observations by the EISCAT Svalbard radar in summer have revealed electron density enhancements in the magnetic noon sector under conditions of IMF B_z southward. The features were identified as possible candidates for polar-cap patches drifting anti-Sunward with the plasma flow. Supporting measurements by the EISCAT mainland radar, the CUTLASS radar and DMSP satellites, in a multi-instrument study, suggested that the origin of the structures lay upstream at lower latitudes, with the modulation in density being attributed to variability in soft-particle precipitation in the cusp region. It is proposed that the variations in precipitation may be linked to changes in the location of the reconnection site at the magnetopause, which in turn results in changes in the energy distribution of the precipitating particles.     

Multi-scale entropy analysis of Mississippi River flow

Multi-scale entropy (MSE) analysis was applied to the long-term (131 years) daily flow rates (Q) of the Mississippi River (MR) to investigate possible change in the complexity of the MR system due to human activities since 1940s. Unlike traditional entropy-based method that calculates entropy at only one single scale, the MSE analysis provided entropies over multiple time scales and thus accounts for multi-scale structures embedded in time series. It is found that the sample entropy (S _E) for Q of the MR and its two components, overland flow (OF) and base flow (BF), generally increase as time scale increases. More importantly, it is found that there have been entropy decreases in Q, OF, and BF over large time scales. In other words, the MR may have been losing its complexity since 1940s. We explain that the possible loss in the complexity of the MR system may be due to the major changes in land use and land cover and soil conservation practices in the MR basin since 1940s.     

Observations on the variability and screening effect of Sporadic-E

Sporadic-E is a problem in high latitude HF communications because of its irregular and as yet unpredictable behaviour. This paper characterises the change in E-layer critical frequency (f_oE) that occurs in the four hours following a storm sudden commencement (SSC) event which is a precursor to magnetospheric storms which adversely affect ionospheric communications.Following checks for any seasonal or solar activity dependency in the data, further analysis determines the occurrence of full blanketing (screening) Sporadic-E layer formation at high latitudes as seen on vertical ionosondes in Northern Finland (67° Latitude). An appropriate threshold value of f_oE is proposed that could be used following the commencement of ionospheric storms, indicating that there is a high probability that F-layer screening will occur. As far as the authors are aware this is the first statistical analysis of the onset of Sporadic-E following SSC.     

The delayed occurrence of equatorial ionospheric F2 layer post-sunset height decreases following auroral-zone substorm onsets

Nighttime F₂ layer height decreases have been examined for post-sunset intervals at the equatorial station, Huancayo. The analyses involved mainly R_z max years, (1957–1960), although R_z min years (1974–1977) were also used. The data were obtained from tabulations of the ionogram parameters h'F, f_○F₂ and f_○E_s. The height reductions are delayed by more than 7 h following geomagnetic substorm onsets for locations at longitudes to the east. The reduced occurrence of spread-F and f_○E_s enhancements is found to be associated. These enhancements are also recorded at the European station, Dourbes about 4.5 h before the Huancayo enhancements. It is proposed that an LS-TID which propagates in the 80 km sound channel may be involved. Also, the experimental evidence suggests that the westward propagation of the LS-TIDs allows equatorial disturbances to occur at local times (before midnight) which are similar to the times when the LS-TIDs are generated.     

Theoretical grain size limits for single-domain,pseudo-single-domain and multi-domain behavior in titanomagnetite (x = 0.6) as a function of low-temperature oxidation

A theoretical model of grain size variation of domain transitions in titanomagnetite (x = 0.6) as a function of oxidation (z) is presented. The superparamagnetic (SP) to single-domain (SD) transition d_s, the SD to two-domain (TD) transition d₀, the TD to three-domain (3D) transition and the pseudo-single domain (PSD) to multi-domain (MD) transition are calculated as a function of z. It is shown that all the transition grain sizes increase with z, except for the PSD-MD transition for z > 0.6. The calculations predict that d_s increases from 0.044 to 0.197 μm, d₀ increases from 0.54 to 13 μm, the TD-3D transition increases from 1.6 to 49 μm as z varies from 0 to 0.8. The PSD-MD transition increases from 42 μm at z = 0 to 150 μm at z = 0.6, whereas between z = 0.6 to z = 0.8, the PSD-MD transition decreases to 49 μm. Qualitatively, the model explains some of the trends in magnetic properties of submarine basalts with low-temperature oxidation. Quantitatively, the model does give reasonable estimates of the PSD-MD boundary and d₀, which are close to the experimental values for x = 0.6 and z = 0. Furthermore, the model predicts that psarks or two-domain grains could be the major contributors to the remanence of oxidized submarine pillow basalts.     

Effective energy loss per electron-ion pair in proton aurora

Effective energy loss per electron-ion pair produced, <xi>(E ₀), as a function of a particle’s initial energy has been obtained for proton transport in the atmosphere. The influence of some transport parameters on the shape of <xi>(E ₀) has been studied. Comparisons with the case of electron transport and with other results were made. It has been shown that: 1. for E ₀>1 keV, <xi>(E ₀) varies within the range 30–36 eV; 2. as E ₀ increases the value of <xi>(E ₀) tries to attain an asymptotic value that is the same as for electrons (\approx35 eV); 3. <xi>(E ₀) strongly depends on the average energy of secondary electrons, but the energy distribution of secondary electrons is not as important. The range of possible changes in <xi>(E ₀) associated with discrepancies in cross sections has been obtained.