EISCAT Incoherent Scatter Radar Research Articles

Effect of Polar Cap Patches on the High‐Latitude Upper Thermospheric Winds

AbstractThis study focuses on the poorly known effect of polar cap patches (PCPs) on the ion‐neutral coupling in the F‐region. The PCPs were identified by total electron content measurements from the Global Navigation Satellite System (GNSS) and the ionospheric parameters from the Defense Meteorological Satellite Program spacecraft. The EISCAT incoherent scatter radars on Svalbard and at Tromsø, Norway observed that PCPs entered the nightside auroral oval from the polar cap and became plasma blobs. The ionospheric convection further transported the plasma blobs to the duskside. Simultaneously, long‐lasting strong upper thermospheric winds were detected in the duskside auroral oval by a Fabry‐Perot Interferometer (FPI) at Tromsø and in the polar cap by the Gravity Recovery and Climate Experiment satellite. Using EISCAT ion velocities and plasma parameters as well as FPI winds, the ion drag acting on neutrals and the time constant for the ion drag could be estimated. Due to the arrival of PCPs/blobs and the accompanied increase in the F‐region electron densities, the ion drag is enhanced between about 220 and 500 km altitudes. At the F peak altitudes near 300 km, the median ion drag acceleration affecting neutrals more than doubled and the associated median e‐folding time decreased from 4.4 to 2 hr. The strong neutral wind was found to be driven primarily by the ion drag force due to large‐scale ionospheric convection. Our results provide a new insight into ionosphere‐thermosphere coupling in the presence of PCPs/blobs.

Evaluation of the Empirical Scaling Factor of Joule Heating Rates in TIE‐GCM With EISCAT Measurements

AbstractJoule heating is one of the main energy inputs into the thermosphere‐ionosphere system. Precise modeling of this process is essential for any space weather application. Existing thermosphere‐ionosphere models tend to underestimate the actual Joule heating rate quite significantly. The Thermosphere‐Ionosphere‐Electrodynamics General‐Circulation‐Model applies an empirical scaling factor of 1.5 for compensation. We calculate vertical profiles of Joule heating rates from approximately 2,220 hr of measurements with the EISCAT incoherent scatter radar and the corresponding model runs. We investigate model runs with the plasma convection driven by both the Heelis and the Weimer model. The required scaling of the Joule heating profiles is determined with respect to the Kp index, the Kan‐Lee merging electric field EKL, and the magnetic local time. Though the default scaling factor of 1.5 appears to be adequate on average, we find that the required scaling varies strongly with all three parameters ranging from 0.46 to ∼20 at geomagnetically disturbed and quiet times, respectively. Furthermore, the required scaling is significantly different in runs driven by the Heelis and Weimer model. Adjusting the scaling factor with respect to the Kp index, EKL, the magnetic local time, and the choice of convection model would reduce the difference between Joule heating rates calculated from measurement and model plasma parameters.

Large‐Scale Traveling Ionospheric Disturbances Over the European Sector During the Geomagnetic Storm on March 23–24, 2023: Energy Deposition in the Source Regions and the Propagation Characteristics

AbstractMultiple Large‐Scale Traveling Ionospheric Disturbances (LSTIDs) are observed in the European sector in both day‐time and night‐time during the magnetic storm on March 23–24, 2023. The Total Electron Content (TEC) observation from a network of GNSS receivers shows the propagation of LSTIDs with amplitudes between around 0.5 and 1 TECU originating from auroral and polar cusp regions down to southern Europe (35°N) with velocities between around 500 and 1,600 [m/s]. We study the energy deposition to the LSTIDs in the source regions and the resulting horizontal propagation over storm‐time background density by using continuous measurements of EISCAT incoherent scatter radars in northern Norway and Svalbard that allow for estimating the source energy to the thermosphere‐ionosphere system via Joule heating and particle precipitation. Both EISCAT and GNSS TEC data show that the electron density decreased to 50% in the auroral zone after the storm onset. The ionospheric heating caused a nearly 250% increase in the electron temperature above 200 km altitude and the ion temperature above 100 km altitude. We find that Joule Heating acts as a primary energy source for the night‐time LSTIDs triggered in the auroral region, while the day‐time LSTIDs can be also driven by precipitating particles in the polar cusp. We also find that a significant background density decrease over the whole European sector is caused by this storm for the following day, during which almost no clear LSTIDs are observed.

Features of Radio Signal Propagation in the VLF Range at High Latitudes during Solar Proton Events

In this paper, we study the amplitude and phase characteristics of VLF signals of an anthropogenicorigin during solar proton events using the methods of a computational experiment. We consider the events ofOctober 30, 2003 and January 23, 2012. Electron density profiles are plotted using data from the VHF EISCATincoherent scatter radar located in Tromsø, Norway. Based on the processed data of computational experiments,that under the conditions of solar proton events, mainly amplitude distortions of VLF signals wereshown to be observed while there is a frequency dependence of the magnitude of distortions of the signals ofthe RSDN-20 long-range navigation radio system. The signal phases of the RSDN-20 system are less affectedby weak solar proton events. The effect of the lower boundary of the Earth-ionosphere waveguide in the casesof propagation of signals from the RSDN-20 system over the surface of land and ocean during a solar protonevent was studied.

Особенности возбуждения искусственной ионосферной турбулентности при О- и Х-нагреве вблизи критической частоты слоя F2

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH<fₒF2. The possible generation mechanisms for the electron density enhancements above the reflection altitude of the powerful HF radio wave of O- and X-polarization are discussed.

Особенности возбуждения искусственной ионосферной турбулентности при О- и Х-нагреве вблизи критической частоты слоя F2

We present experimental results from the studies of large-scale inhomogeneities along the external magnetic field with increased electron density, electron temperature, and excitation of elongated plasma waves (Langmuir and ion-acoustic), induced by the ordinary (O-mode) and extraordinary (X-mode) HF heating near the F2-layer critical frequency, in the high-latitude ionospheric F-region. The experiments have been carried out at the EISCAT/Heating facility (Tromsø, Norway). Powerful HF radio waves radiated towards the magnetic zenith through a step change in the effective radiated power at frequencies fH near and below the F2-layer critical frequency fₒF2. The EISCAT incoherent scatter radar (930 MHz), co-located with the EISCAT/Heating facility, was utilized for diagnostics of ionospheric modification effects. We calculated the electric field of a powerful HF radio wave near the reflection altitude, taking into account the non-deflective absorption along the propagation path. We determined the conditions for electric field generation and its threshold (minimum) values required for electron density enhancements in a wide altitude range, excitation of Langmuir and ion-acoustic plasma waves under fH~fₒF2 and fH<fₒF2. The possible generation mechanisms for the electron density enhancements above the reflection altitude of the powerful HF radio wave of O- and X-polarization are discussed.

GeospaceLAB: Python package for managing and visualizing data in space physics

In the space physics community, processing and combining observational and modeling data from various sources is a demanding task because they often have different formats and use different coordinate systems. The Python package GeospaceLAB has been developed to provide a unified, standardized framework to process data. The package is composed of six core modules, including DataHub as the data manager, Visualization for generating publication quality figures, Express for higher-level interfaces of DataHub and Visualization, SpaceCoordinateSystem for coordinate system transformations, Toolbox for various utilities, and Configuration for preferences. The core modules form a standardized framework for downloading, storing, post-processing and visualizing data in space physics. The object-oriented design makes the core modules of GeospaceLAB easy to modify and extend. So far, GeospaceLAB can process more than twenty kinds of data products from nine databases, and the number will increase in the future. The data sources include, e.g., measurements by EISCAT incoherent scatter radars, DMSP, SWARM, and Grace satellites, OMNI solar wind data, and GITM simulations. In addition, the package provides an interface for the users to add their own data products. Hence, researchers can easily collect, combine, and view multiple kinds of data for their work using GeospaceLAB. Combining data from different sources will lead to a better understanding of the physics of the studied phenomena and may lead to new discoveries. GeospaceLAB is an open source software, which is hosted on GitHub. We welcome everyone in the community to contribute to its future development.

Two Techniques for Determining F‐Region Ion Velocities at Meso‐Scales: Differences and Impacts on Joule Heating

AbstractWe have investigated the difference between two standard techniques for deriving the ionospheric ion velocity using data taken with the EISCAT incoherent scatter radar between 1987 and 2007. For large‐scale convection flows, there is little difference between the tristatic and monostatic techniques, though the biggest relative difference occurs during periods when the interplanetary magnetic field (IMF) is strongly northward. At small scales the difference between the two techniques is correlated with a measure of the variability of the tristatic measurement. This suggests that small‐scale flow bursts, such as those associated with enhanced auroral arcs, could explain the local time variation in the velocity difference distributions. The difference in velocities obtained from the monostatic and tristatic techniques can make a significant difference in the estimate of the magnitude of Joule heating in the thermosphere. Considering only the electric field dominated component of Joule heating, Q, the difference in the two techniques can be as much as 52% of the tristatic measurement (Qm = 0.48Qt) in the morning sector (0–6 MLT), during a moderate to large geomagnetic storm. This reduces to a difference of 36% at non‐storm times in the same MLT period. Careful averaging of the velocity field with the future EISCAT_3D radar system will allow us to establish the impact of both spatial and temporal scales on the magnitude of the observations.

Excitation of Langmuir and Ion–Acoustic Turbulence in the High-Latitude Ionosphere by a High-Power HF Radio Wave Simultaneously Below and Above the F2-Layer Maximum

We present the results of experimental studies of the generation peculiarities and features of the Langmuir and ion–acoustic turbulence in the F region of the high-latitude ionosphere excited by high-power HF O-mode radio waves emitted by the EISCAT/Heating facility (Tromso, Norway) in the direction of the Earth’s magnetic field. The experiment was carried out at frequencies fH close to the fourth gyroharmonic, fH < 4fce, and the cutoff frequency of the F2 layer, foF2 , fH < foF2 < fH+fce/2, where fce is the electron gyrofrequency. By using the EISCAT incoherent scatter radar (930 MHz), a joint analysis of the plasma and ion line spectra simultaneously below and above the F2-layer maximum was performed. The excitation of the HF-induced plasma lines outshifted by 0.35–0.45 MHz from the pump-wave frequency and HF-enhanced ion lines simultaneously below and above the F2-layer maximum was found for the first time. The mechanisms of the pump-wave propagation to altitudes above the F2-layer maximum and a plausible mechanism for the excitation of the instability responsible for the generation of HF-enhanced ion lines and HF-induced plasma lines above the F2-layer maximum are discussed.

Outshifted Plasma Lines Observed in Heating Experiments in the High-Latitude Ionosphere at Pump Frequencies Near Electron Gyroharmonics

We present experimental results concerning the features and origin of the outshifted plasma lines in the high-latitude ionospheric F region, which are excited by high-power HF O-mode radio waves radiated by the EISCAT/Heating facility at pump-wave frequencies near the fourth and fifth gyroharmonics. HF pump waves were transmitted towards the magnetic zenith during long-lasting (2–20 min) heater-on cycles. The studies were carried out using the measurement data obtained by the EISCAT incoherent scatter radar in Tromso (the radar operating frequency 930 MHz). The analysis of the radar data has demonstrated the simultaneous excitation of several plasma lines at frequencies close to the pump frequency and at frequencies upshifted by 0.15–0.45 MHz on long-lasting (longer than 30 s) time intervals. The spectral width of the outshifted plasma lines was 0.10–0.15 MHz. The features of their occurrence were analyzed as functions of the relation between the pump-wave frequency and gyroharmonics and the proximity of the pump-wave frequency to the cutoff frequency of the ionospheric F2 layer. Plausible excitation mechanisms of the outshifted plasma lines are discussed. Comparison between the experimental data and calculation results concerning the features of the outshifted plasma lines for different pump-wave frequencies was made.

Solar wind dependence of electric conductances and currents in the auroral zone

Based on 20 years-long data base of EISCAT incoherent scatter radar and IMAGE magnetometer observations in Scandinavia, we investigate statistically the ionospheric conductance variations in the dark nightside auroral zone. We focus on the relationship of precipitation-caused conductances with the variations of local equivalent current and global AL index, as well as on their dependence on the solar wind (SW) parameters. In terms of paired correlation, the main SW drivers for AL index and for the Pedersen and Hall conductances (ΣP and ΣH) are the SW merging electric field (characterized, e.g., with the Kan-Lee function, Ekl) and the solar wind velocity Vsw. The relative importance of these SW drivers varies. Whereas Ekl is the main driver of AL index, the role of Vsw increases for the conductances so that it outruns the Ekl as the main driver for the Hall conductance. Quantitatively this dependence is represented as ΣH=(7.7*V+1.75*V2)+(5.7*E−0.86*E2)−6.1 Siemens, where E and V are Ekl and Vsw normalized with <Ekl>=0.79 mV/m and <Vsw>=429 km/s. The strongest influence of Vsw is, however, observed for the Hall-to-Pedersen conductance ratio RHP=ΣH/ΣP, indicating solar wind velocity control of the electron acceleration. Physically the energization is a major factor which contributes to the large conductance values. On the nightside, local equivalent currents are significantly controlled by the local Hall conductance (CC = 0.78) and most of the equivalent current increase during active periods is due to the conductivity change. In that sense the AL index variations during active times are controlled by the Hall conductance variations which, to a large extent, are controlled by the processes of magnetospheric electron acceleration.

Modification of the High-Latitude Ionospheric F Region By High-Power HF Radio Waves at Frequencies Near the fifth and Sixth Electron Gyroharmonics

We study the modification effects of the high-latitude ionospheric F region induced by a highpower O-mode HF radio wave injected towards the magnetic zenith, at frequencies near the fifth and sixth electron gyroharmonics using the EISCAT/Heating facility. Multi-instrument diagnostics with the EISCAT incoherent scatter radar (930 MHz) at Tromso, Norway, the CUTLASS coherent radar at Hankasalmi, Finland, and stimulated electromagnetic emission (SEE) receiver at Tromso, has been used for analysis of the observed phenomena. The behavior of the ionospheric plasma parameters (electron’s density and temperature), small-scale artificial field-aligned irregularities, plasma and ion-line spectra, and ionospheric SEE are analyzed in detail. Modification effects near the fifth and sixth electron gyroharmonics have been compared. The coexistence of the thermal (resonance) parametric instability, parametric decay (striction) instability, and/or oscillating two-stream instability was found at these frequencies. The excitation of instabilities occurred at altitudes close to the reflection altitude of the HF pump wave and at the altitudes of the upper-hybrid resonance.

TID characterised using joint effort of incoherent scatter radar and GPS

Abstract. Travelling Ionospheric Disturbances (TIDs), which are caused by Atmospheric Gravity Waves (AGWs), are detected and characterised by a joint analysis of the results of two measurement techniques: incoherent scatter radar and multiple-receiver GPS measurements. Both techniques to measure TIDs are already well known, but are developed further in this study, and the strengths of the two are combined, in order to obtain semi-automatic tools for objective TID detection. The incoherent scatter radar provides a good vertical range and resolution and the GPS measurements provide a good horizontal range and resolution, while both have a good temporal resolution. Using the combination of the methods, the following parameters of the TID can be determined: the time of day when the TID occurs at one location, the period length (or frequency), the vertical phase velocity, the amplitude spectral density, the vertical wavelength, the azimuth angle of horizontal orientation, the horizontal wavelength, and the horizontal phase velocity. This technique will allow a systematic characterisation of AGW-TIDs, which can be useful, among other things, for statistical analyses. The presented technique is demonstrated on data of 20 January 2010 using data from the EISCAT incoherent scatter radar in Tromsø and from the SWEPOS GPS network in Sweden. On this day around 07:00–12:00 UT, a medium-scale TID was observed from both data sets simultaneously. The TID had a period length of around 2 h, and its wave propagated southeastward with a horizontal phase velocity of about 67 m s−1 and a wavelength of about 500 km. The TID had its maximum amplitude in Tromsø at 10:00 UT. The period length detected from the GPS results was twice the main period length detected from the radar, indicating a different harmonic of the same wave. The horizontal wavelength and phase velocity are also estimated from the radar results using Hines' theory, using the WKB approximation to account for inhomogeneity of the atmosphere. The results of this estimate are higher than those detected from the GPS data. The most likely explanation for this is that Hines' theory overestimated the values, because the atmosphere was too inhomogeneous even for the WKB approximation to be valid.

Triangulating the height of cosmic noise absorption: A method for estimating the characteristic energy of precipitating electrons

Energetic electrons (tens to hundreds of keV) deposit significant energy into the D layer of the ionosphere. Riometers provide a means of monitoring this electron precipitation by measuring the associated cosmic noise absorption (CNA), but individually they are incapable of resolving the associated energy. However, the combination of two imaging riometers with overlapping beams allows an estimate of the height of peak CNA and so the associated energy to be made. We examine two methods for estimating the height of CNA using data from two imaging riometers in northern Fennoscandia; a 3‐D reconstruction of CNA using Occam's inversion and a technique based upon the triangulation of discrete absorption structures are developed. We compare these two methods with the results from a previously published technique. It is found that for the case studies and test phantoms the height triangulation and 3‐D reconstruction offer improvement over previous methods. These techniques are tested by comparison with data from the EISCAT incoherent scatter radar. Observations show good correlation between the estimates of peak height of CNA from EISCAT and from the triangulation and 3‐D reconstruction methods for this case. Three case studies are examined in detail, a slowly varying absorption, afternoon spike, and evening absorption spike event. Estimates of the characteristic energy are made. The substorm event had a characteristic energy of ∼5 keV, whereas the characteristic energy for the morning event was 17–20 keV. Analyses indicate the afternoon spike event having characteristic energy greater than 100 keV.

Effect of neutral particle density in the upper atmosphere on the generation of artificial magnetic pulsations in the Pc1 range

A series of experiments on modification of the ionosphere by a powerful ground HF transmitter was performed using the EISCAT heating facility in order to generate artificial magnetic pulsations in the frequency range 0.1–3 Hz. In several cases, the ionospheric electric field and the electron density vertical profile were measured with the EISCAT incoherent scatter radar. The measurement of the background values of the ionospheric parameters made it possible to verify the numerical model for generating artificial emissions. The calculated amplitudes of magnetic pulsations correspond to the values measured on the Earth’s surface. However, the model cannot explain the sporadic nature of artificial signals, which indicates that this model is incomplete. Disturbances of the neutral particle density in the upper atmosphere are one of the possible causes explaining a difference between the calculations and the experimental values. The numerical simulation indicated that the amplitude variations caused by such disturbances can be 20%. For artificial emissions whose intensity is comparable with the intensity of artificial noise, variations in the neutral components can result in the disappearance of an artificial signal on the spectrogram.

A comparison of EISCAT and SuperDARN F‐region measurements with consideration of the refractive index in the scattering volume

Gillies et al. (2009) proposed the use of interferometric measurements of the angle of arrival as a proxy for the scattering region refractive index ns needed to estimate the line‐of‐sight Doppler velocity of the ionospheric plasma from HF [Super Dual Auroral Radar Network (SuperDARN)] radar observations. This study continues this work by comparing measurements of line‐of‐sight velocities by SuperDARN with tristatic velocity measurements by the EISCAT incoherent scatter radar from 1995 to 1999. From a statistical viewpoint, velocities measured by SuperDARN were lower than velocities measured by EISCAT. This can, at least partially, be explained by the neglect in the SuperDARN analysis of the lower‐than‐unity refractive index of the scattering structures. The elevation angle measured by SuperDARN was used as a proxy estimate of ns and this improved the comparison, but the velocities measured by SuperDARN were still lower. Other estimates of ns using electron densities Ne based on both EISCAT measurements and International Reference Ionosphere model values did not increase the SuperDARN velocities enough to attain the EISCAT values. It is proposed that dense structures that were of comparable size to the SuperDARN scattering volume partially help resolve the low‐velocity issue. These dense, localized structures would provide the Ne gradients required for generation of the coherent irregularities from which the SuperDARN radar waves scatter, whereas EISCAT incoherent radar measurements provide only the background Ne and not the density of the small‐scale structures. The low‐velocity SuperDARN results suggest that small‐scale dense structures with refractive indices well below unity must exist within the SuperDARN scattering volume and may contribute greatly to the scattering process.

Model for artificial ionospheric duct formation due to HF heating

Strong electron heating by the injection of highly powerful HF waves can lead to the formation of ionospheric plasma density perturbations that stretch along the magnetic field lines. Those density perturbations can serve as ducts for guiding natural and artificial ELF/VLF waves. This paper presents a theoretical model of duct formation due to HF heating of the ionosphere. The model is based on the modified SAMI2 code, and is validated by comparison with two well documented experiments. One experiment, conducted at the SURA heating facility, used the low orbit satellite DEMETER as a diagnostic tool to measure the electron and ion temperature and density along the overflying satellite orbit close to the magnetic zenith of the HF‐heater. The second experiment, conducted at the EISCAT HF facility and diagnosed by the EISCAT Incoherent Scatter Radar, measured the vertical profiles of the electron and ion temperature between 150–600 km. The model agrees well with the observations, and provides a new understanding of the processes during ionospheric modification.

Nowcasting, forecasting and warning for ionospheric propagation: supporting databases

The use of data is essential in the context of nowcasting, forecasting and warning of ionospheric propagation conditions, with roles to play in the development, evaluation and operation of models and services. Descriptions are given of three databases that have been established in the course of the COST 271 Action: a database of prompt ionospheric soundings, an extension to a database generat- ed by the EISCAT incoherent scatter radars, and a database intended to facilitate evaluation of TEC estimation methods. Each database includes some background information, a description of the con- tents and interface, and instructions as to how to gain access to it.

Phase-coded pulse aperiodic transmitter coding

Abstract. Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately overspread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC), whereas the weather radar users have adopted the term Simultaneous Multiple Pulse-Repetition Frequency (SMPRF). When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thousand kilometers – about seven milliseconds – whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of microseconds in the F-region. If one is interested in estimating the scattering autocorrelation function (ACF) at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR) conditions using lag profile inversion. In addition, a qualitative evaluation under high-SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F-region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromsø VHF (224 MHz) radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper.

Statistical properties of Joule heating rate, electric field and conductances at high latitudes

Abstract. Statistical properties of Joule heating rate, electric field and conductances in the high latitude ionosphere are studied by a unique one-month measurement made by the EISCAT incoherent scatter radar in Tromsø (66.6 cgmlat) from 6 March to 6 April 2006. The data are from the same season (close to vernal equinox) and from similar sunspot conditions (about 1.5 years before the sunspot minimum) providing an excellent set of data to study the MLT and Kp dependence of parameters with high temporal and spatial resolution. All the parameters show a clear MLT variation, which is different for low and high Kp conditions. Our results indicate that the response of morning sector conductances and conductance ratios to increased magnetic activity is stronger than that of the evening sector. The co-location of Pedersen conductance maximum and electric field maximum in the morning sector produces the largest Joule heating rates 03–05 MLT for Kp≥3. In the evening sector, a smaller maximum occurs at 18 MLT. Minimum Joule heating rates in the nightside are statistically observed at 23 MLT, which is the location of the electric Harang discontinuity. An important outcome of the paper are the fitted functions for the Joule heating rate as a function of electric field magnitude, separately for four MLT sectors and two activity levels (Kp&lt;3 and Kp≥3). In addition to the squared electric field, the fit includes a linear term to study the possible anticorrelation or correlation between electric field and conductance. In the midday sector, positive correlation is found as well as in the morning sector for the high activity case. In the midnight and evening sectors, anticorrelation between electric field and conductance is obtained, i.e. high electric fields are associated with low conductances. This is expected to occur in the return current regions adjacent to auroral arcs as a result of ionosphere-magnetosphere coupling, as discussed by Aikio et al. (2004) In addition, a part of the anticorrelation may come from polarization effects inside high-conductance regions, e.g. auroral arcs. These observations confirm the speculated effect of small scale electrodynamics, which is not included in most of the global modeling efforts of Joule heating rate.