Electrojet poleward boundary variations with IMF By and season

<p>By utilising measurements from twenty ground magnetometer stations in Fennoscandia, divergence-free ionospheric currents above this region are modelled using spherical elementary currents (SECS). New modelling techniques are implemented that coerce the model to find a solution that resembles the resolvable ionospheric currents. The divergence-free currents are evaluated along the 105<sup>o</sup> magnetic meridian covering a period of almost 20 years with a resolution of 1 minute, as a result of the magnetometers chosen. From these sheet current density latitude profiles, the boundaries of the auroral electrojet are identified. After performing a large statistical analysis it is found that there is a significant IMF B<sub>y</sub> effect on the poleward boundary of the electrojets during the Summer but not during the Winter. We suggest that this seasonal effect can be attributed to the effects of lobe reconnection on the extent of currents in the auroral electrojets. Further work is done to compare the SECS derived electrojet boundaries with particle precipitation data from low orbit satellites.</p>

Even moderate geomagnetic pulsations can cause fluctuations of foF2 frequency of the auroral ionosphere

Using data of the ionosonde in Sodankyla, (SOD, 67° N, 27° E, Finland), variations of the critical frequency of o-mode radiowave reflected from ionospheric F2 layer (foF2) in 1–5 mHz frequency range and their possible association with long period (Pc5/Pi3) geomagnetic pulsations are studied. For that, a technique of automatic detection of the foF2 critical frequency from an ionogram is developed and applied to daytime Pc5/Pi3 geomagnetic pulsations and foF2 fluctuations during several months of years 2014–2015 near the maximum of 24-th Solar cycle. The variations of foF2 are compared with the Pc5/Pi3 geomagnetic pulsations at SOD station, and the influence of pulsations' spatial scale is analyzed with the data of a station pair located at the same magnetic meridian but separated in latitude. The variations of foF2 are in the majority of cases decoupled from the geomagnetic pulsations on the ground. Meanwhile, the analysis of geomagnetic and foF2 variations show intervals with noticeable coherence for both horizontal components. These coherent pulsations are predominantly registered in the afternoon sector of the magnetic local time (MLT). Statistically, their spectral content, polarization and spatial distribution differ from averaged parameters of post-noon Pc5 pulsations. The pulsations, coherent to foF2 fluctuations, demonstrate features typical for Alfven field-line resonance. The analysis of space weather conditions favorable for the occurrence of coherent geomagnetic/foF2 pulsations show that these pulsations are registered mostly under moderately disturbed conditions. Comparison of space weather parameters for all the intervals analyzed and the intervals of high geomagnetic/foF2 coherence show that the latter correspond mostly to intermediate values of indexes of geomagnetic (Dst) and auroral (AE) activity, solar wind speed and dynamic pressure fluctuations.

ULF waves registered with the Ekaterinburg radar: Statistical analysis and case studies

<p>A midlatitude coherent decameter radar installed near Ekaterinburg, Russia (EKB radar) has been operating since 2012. It is aimed at monitoring dynamics of the ionosphere–magnetosphere system in Eastern Siberia. A special operation mode is used at the radar to study ULF wave activity: three adjacent beams oriented approximately along the magnetic meridian are surveyed with time resolution of 18 s each. A number of wave observation events registered with the radar was analyzed and discussed in several papers. An overview of the main results from these studies is presented here.</p><p>A statistical study of more than 30 waves observed in the nightside ionosphere revealed that in the majority of the cases their frequencies are considerably lower than those of field line resonance (FLR) for appropriate magnetic shells and longitudinal sectors (FLR fundamental frequencies for each case were estimated based on spacecraft data). Thus, these waves cannot be associated with the Alfvén mode. It was assumed that at least part of them should be identified with the drift compressional mode. Indeed, in individual cases oscillations exhibited signatures of this mode: in one of the events a linear dependence of frequency on azimuthal wave number <em>m</em> at a fixed magnetic shell was found. Only the drift compressional mode can feature such dependence in the inner magnetosphere. For two other cases merging of drift compressional and Alfvén modes at some critical value of <em>m</em> was shown. A case of simultaneous spacecraft and radar wave observation accompanied by increases in energetic particle fluxes was shown. A modulation with the frequency of this wave was found for flux intensity of those energetic protons, whose phase velocity is close to that of the wave. This implies that the source of the wave was a drift resonance with the substorm injected protons.</p>

Latitudinal variation of Pc3-Pc5 geomagnetic pulsation amplitude across the dip equator in central South America

In order to clarify the equatorial electrojet effects on ground magnetic pulsations in central South America, we statistically analyzed the amplitude structure of Pc3 and Pc5 pulsations recorded during quiet to moderately disturbed days at multiple equatorial stations nearly aligned along the 10° magnetic meridian. It was observed that Pc3 amplitudes are attenuated around noon at the dip equator for periods shorter than ~ 35 s. It is proposed that daytime Pc3s are related to MHD compressional waves incident vertically on the ionosphere, with the screening effect induced by enhanced conductivity in the dip equator causing wave attenuation. Daytime Pc5s showed amplitude enhancement at all equatorial stations, which can be explained by the model of waves excited at higher latitudes and propagating equatorward in an Earth-ionosphere waveguide. However, a slight depression in Pc5 amplitude compared to neighboring equatorial stations and a phase lag in relation to an off-equatorial station were detected at the dip equator. This result cannot be explained by the ionospheric waveguide model alone and we propose that an alternative propagation model that allows ULF waves to penetrate directly from the magnetosphere to low latitudes could be operating simultaneously to produce these features at the dip equator. Significant effects of the sunrise terminator on Pc3 pulsations were also observed at the stations closest to the dip equator. Contrary to what is reported at other longitudes, in central South America the sunrise effect increases the H-to-D amplitude ratio. We suggest that these differences may arise from the unique characteristics of this sector, with a strong longitudinal variation in the magnetic declination and precipitation of energetic particles due to the presence of the South Atlantic Magnetic Anomaly. The H-component amplification can be explained by enhancements of the zonal electric field near the magnetic equator driven by F-region neutral winds during sunrise.

Numerical evaluation of magnetic absolute measurements with arbitrarily distributed DI-fluxgate theodolite orientations

At geomagnetic observatories the absolute measurements are needed to determine the calibration parameters of the continuously recording vector magnetometer (variometer). Absolute measurements are indispensable for determining the vector of the geomagnetic field over long periods of time. A standard DI (declination, inclination) measuring scheme for absolute measurements establishes routines in magnetic observatories. The traditional measuring schema uses a fixed number of eight orientations (Jankowski et al., 1996). We present a numerical method, allowing for the evaluation of an arbitrary number (minimum of five as there are five independent parameters) of telescope orientations. Our method provides D, I and Z base values and calculated error bars of them. A general approach has significant advantages. Additional measurements may be seamlessly incorporated for higher accuracy. Individual erroneous readings are identified and can be discarded without invalidating the entire data set. A priori information can be incorporated. We expect the general method to also ease requirements for automated DI-flux measurements. The method can reveal certain properties of the DI theodolite which are not captured by the conventional method. Based on the alternative evaluation method, a new faster and less error-prone measuring schema is presented. It avoids needing to calculate the magnetic meridian prior to the inclination measurements. Measurements in the vicinity of the magnetic equator are possible with theodolites and without a zenith ocular. The implementation of the method in MATLAB is available as source code at the GFZ Data Center (Brunke, 2017).

Monitoring and investigating space weather effects with meridional chain of instruments in Yakutia: a brief overview

The Yakutsk Meridional Chain (YMC) of IKFIA SB RAS, located along the 190° magnetic meridian, is equipped with geophysical and radiophysical instruments for monitoring space weather in northeast-ern Russia. YMC includes four basic stations in Yakutsk, Tixie, Zhigansk, and Maymaga, and six additional observation sites in Neryungri, Zyryanka, Kystatyam, Dzhardzhan, Chokurdakh, and Kotelny Island. It provides continuous monitoring of near-Earth space in order to obtain data on magnetic field variations, cosmic radio noise, VLF radiation, and ionospheric parameters in the complex upper atmosphere – ionosphere –magnetosphere system. In addition, long-term experimental research into space weather effects on human health is conducted at Tixie and Yakutsk. The report describes the meridional chain of instruments at subauroral and auroral latitudes and gives a brief overview of scientific results of monitoring and investigation into space weather effects in Yakutia. It also observes participation of IKFIA SB RAS in international projects (Intermagnet, MAGDAS, GIRO).

Monitoring and investigating space weather effects with meridional chain of instruments in Yakutia: a brief overview

The Yakutsk Meridional Chain (YMC) of IKFIA SB RAS, located along the 190° magnetic meridian, is equipped with geophysical and radiophysical instruments for monitoring space weather in northeast-ern Russia. YMC includes four basic stations in Yakutsk, Tixie, Zhigansk, and Maymaga, and six additional observation sites in Neryungri, Zyryanka, Kystatyam, Dzhardzhan, Chokurdakh, and Kotelny Island. It provides continuous monitoring of near-Earth space in order to obtain data on magnetic field variations, cosmic radio noise, VLF radiation, and ionospheric parameters in the complex upper atmosphere – ionosphere –magnetosphere system. In addition, long-term experimental research into space weather effects on human health is conducted at Tixie and Yakutsk. The report describes the meridional chain of instruments at subauroral and auroral latitudes and gives a brief overview of scientific results of monitoring and investigation into space weather effects in Yakutia. It also observes participation of IKFIA SB RAS in international projects (Intermagnet, MAGDAS, GIRO).

Numerical Evaluation of magnetic absolute Measurements with arbitrary distributed DI-Fluxgate Theodolite Positions

At geomagnetic observatories so called absolute measurements are used to determine the calibration parameters of the main three-axis-magnetometer and their long term drift. This allows to get the vector of the geomagnetic field in an absolute geographic reference frame over long periods of time in order to study the secular variation of the Earth's magnetic field. Absolute measurements of the magnetic declination D and inclination I are done by means of a nonmagnetic theodolite with a fluxgate sensor mounted on its telescope parallel to the optical axis. A fluxgate measures the magnetic field component along its sensor axis. The reading S of this magnetometer vanishes if it points towards a direction perpendicular to the field. For absolute measurements standard measuring scheme using six to eight such theodolite positions are established routines in magnetic observatories. These standard DI schemes allow for a simple numeric evaluation and cancel out the influence of instrument parameters like sensor offset and misalignment angles between fluxgate sensor and telescope. We present a numerical method that allows to evaluate measurements of an arbitrary number (minimum 5 as there are 5 independent parameters) and of arbitrary theodolite positions and exploit it to this end. We implement an instrument model to calculate the fluxgate reading S in dependence of field, instrument parameters and telescope direction (herein after referred to as theodolite position). Inserting actual measured values gives one nonlinear equation for each theodolite position. Eventually this is resulting in an overdetermined system of nonlinear equations. This system is solved in the sense of a least square solution using the Gauss-Newton-method generalized to an overdetermined system. The accuracy of the resulting D, I and base values is given in terms of estimated variances. The accuracy of the resulting D and I values depends on both, the choice of used theodolite directions and on the accuracy of the measurements. The quality of each individual measurement can be assessed by means of calculated residuals. A general approach has significant advantages. The method allows to seamlessly incorporate additional measurements for higher accuracy. Individual erroneous readings are identified and discarded without invalidating the entire data set. We show how a-priory information can be incorporated and how that allows to even evaluate a very reduced data set. We expect the general method to ease requirements for both manual and automated DI-flux measurements. It can reveal certain properties of the DI-theodolite which are not captured by the conventional method. Based on the new method, a new measuring schema is presented. It avoids the need to calculate the magnetic meridian prior to the inclination measurements. Adjustment is always done with the same fine adjustment wheel, the one for the horizontal circle. Leveling of the telescope is not necessary and thus leveling errors are avoided. All these makes the measurements faster and less prone to errors. The option of using measurements off the normal DI positions makes measurements in the vicinity of the magnetic equator possible for theodolites without zenith ocular.