Ionospheric Conductivity Research Articles

Calculating the High‐Latitude Ionospheric Electrodynamics Using a Machine Learning‐Based Field‐Aligned Current Model

AbstractWe introduce a new framework called Machine Learning (ML) based Auroral Ionospheric electrodynamics Model (ML‐AIM). ML‐AIM solves a current continuity equation by utilizing the ML model of Field Aligned Currents of Kunduri et al. (2020, https://doi.org/10.1029/2020JA027908), the FAC‐derived auroral conductance model of Robinson et al. (2020, https://doi.org/10.1029/2020JA028008), and the solar irradiance conductance model of Moen and Brekke (1993, https://doi.org/10.1029/92gl02109). The ML‐AIM inputs are 60‐min time histories of solar wind plasma, interplanetary magnetic fields (IMF), and geomagnetic indices, and its outputs are ionospheric electric potential, electric fields, Pedersen/Hall currents, and Joule Heating. We conduct two ML‐AIM simulations for a weak geomagnetic activity interval on 14 May 2013 and a geomagnetic storm on 7–8 September 2017. ML‐AIM produces physically accurate ionospheric potential patterns such as the two‐cell convection pattern and the enhancement of electric potentials during active times. The cross polar cap potentials (ΦPC) from ML‐AIM, the Weimer (2005, https://doi.org/10.1029/2004ja010884) model, and the Super Dual Auroral Radar Network (SuperDARN) data‐assimilated potentials, are compared to the ones from 3204 polar crossings of the Defense Meteorological Satellite Program F17 satellite, showing better performance of ML‐AIM than others. ML‐AIM is unique and innovative because it predicts ionospheric responses to the time‐varying solar wind and geomagnetic conditions, while the other traditional empirical models like Weimer (2005, https://doi.org/10.1029/2004ja010884) designed to provide a quasi‐static ionospheric condition under quasi‐steady solar wind/IMF conditions. Plans are underway to improve ML‐AIM performance by including a fully ML network of models of aurora precipitation and ionospheric conductance, targeting its characterization of geomagnetically active times.

Statistical Features of Polar Cap North and South Indices in Response to Interplanetary and Terrestrial Conditions: A Revisit

AbstractIn this study, we investigate statistical features of polar cap north (PCN) and south (PCS) indices in response to various interplanetary conditions (interplanetary magnetic field [IMF] orientation in three‐dimensions) and terrestrial conditions (seasonal and magnetic local time [MLT] locations of the index stations). The concurrent PCN‐PCS pairs for 1998–2002 and 2004–2018 are divided based on their sign type (positive‐positive, negative‐negative, negative‐positive, and positive‐negative PCN‐PCS pairs) and time coverage (the times when both index stations are in the dawn/dusk MLT sector during northern summer/winter). Analyzing the IMF orientation dependence on the occurrence probabilities of concurrent indices and on the differences between the indices in various sign types for each time coverage reveals that the statistical features in PCN‐PCS pairs obtained in the dawn MLT sector can be largely explained by the effects of the three‐component IMF (related to the polar cap convection patterns) combined with season (related to the hemispheric asymmetry in solar illumination‐induced ionospheric conductance). However, those obtained in the dusk MLT sector are controlled dominantly by seasonal effects rather than IMF orientation effects. Our findings indicate that PCN‐PCS pair data provide local views about the solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling system with different control efficiencies of IMF orientation and season depending on the MLT location of the stations. Therefore, introducing polar cap indices recorded simultaneously at various locations in both hemispheres and analyzing them are strongly required to infer global views of the coupled SW‐M‐I system in the open field regions with higher confidence.

Statistics of Traveling Ionospheric Disturbances at High Latitudes Using a Rapid‐Run Ionosonde

AbstractThe potential of deep learning for the investigation of medium scale traveling ionospheric disturbances (MSTIDs) has been exploited through the Sodankylä rapid‐run ionosonde in this statistical study. The complementing observations of the Sodankylä ionosonde with those of the Sodankylä meteor radar reveals the diurnal and seasonal occurrence rate of high‐latitude MSTIDs in the recent low solar activity period, 2018–2020. In our results, the daytime, nighttime and dusk MSTIDs are predominantly identified during winter, summer, and equinoctial months, respectively. The winter daytime higher (lower) occurrence rate is well correlated with the lower (higher) altitude of the height of the F2‐layer peak (hmF2), and the low occurrence rate of the summer daytime is well correlated with the mesosphere‐lower‐thermosphere wind shear and higher gradient of temperature. Relatively high occurrence rate (>0.4) of summer nighttime MSTIDs has a general—but not one‐to‐one agreement—with post‐noon to evening IU (eastward auroral current index) inferred ionospheric conductivity. Rather, we see a one‐to‐one relationship between the summer nighttime MSTIDs and zonal wind shear suggesting that the wind shear‐induced electrodynamic processes could play significant roles for higher occurrence rate of MSTIDs. Furthermore, significant MSTIDs with ∼0.4 occurrence rate are so far revealed during spring and autumn transition periods. The enhanced nighttime MSTID amplitudes during the equinox are observed to be well correlated with IL index (westward auroral current indicator) suggesting that the particle precipitation during substorms could be the primary cause.

Exploring Localized Geomagnetic Disturbances in Global MHD: Physics and Numerics

AbstractOne of the prominent effects of space weather is the formation of rapid geomagnetic field variations on Earth's surface driven by the magnetosphere‐ionosphere system. These geomagnetic disturbances (GMDs) cause geomagnetically induced currents to run through ground conducting systems. In particular, localized GMDs (LGMDs) can be high amplitude and can have an effect on scale sizes less than 100 km, making them hazardous to power grids and difficult to predict. In this study, we examine the ability of the Space Weather Modeling Framework (SWMF) to reproduce LGMDs in the 7 September 2017 event using both existing and new metrics to quantify the success of the model against observation. We show that the high‐resolution SWMF can reproduce LGMDs driven by ionospheric sources, but struggles to reproduce LGMDs driven by substorm effects. We calculate the global maxima of the magnetic fluctuations to show instances when the SWMF captures LGMDs at the correct times but not the correct locations. To remedy these shortcomings we suggest model developments that will directly impact the ability of the SWMF to reproduce LGMDs, most importantly updating the ionospheric conductance calculation from empirical to physics‐based.

A Combined Effect of the Earth's Magnetic Dipole Tilt and IMF By in Controlling Auroral Electron Precipitation

AbstractAuroral particle precipitation is usually assumed to be equally strong for both signs of the By component of the interplanetary magnetic field (IMF). This is also the case in most currently used precipitation models, which parameterize solar wind driving by empirical coupling functions. However, recent studies have showed that geomagnetic activity is significantly modulated by the signs and amplitudes of IMF By and the Earth's dipole tilt angle Ψ. This so called explicit By dependence is not yet included in any current precipitation models. In this paper, we quantify this By dependence for auroral electron precipitation for the first time. We use precipitation measurements of the Defense Meteorological Satellite Program (DMSP) Special Sensor J instruments from years 1995–2022. We show that the dawnside electron precipitation at energies 13.9–30 keV is greater at auroral latitudes for opposite signs of By and Ψ in both hemispheres, while the dusk sector is mostly unaffected by By and Ψ. For energies below 6.5 keV the By dependence is strong poleward of the auroral oval in the summer hemisphere, also exhibiting a strong dawn‐dusk asymmetry. We also show that By dependence of precipitation modulates ionospheric conductance.

Ionospheric Conductances Due To Electron and Ion Precipitations: A Comparison Between EISCAT and DMSP Estimates

AbstractEnergetic particle precipitation is the major source of electron production that controls the ionospheric Pedersen and Hall conductances at high latitudes. Many studies use empirical formulas to estimate conductances. The particle precipitation spectra measured by the Defense Meteorological Satellite Program (DMSP) Special Sensor J are often used as the input to the empirical formulas. In this study, we evaluate the empirical formulas of ionospheric conductances during four different types of auroral precipitation conditions based on 63 conjugate events observed by DMSP and EISCAT. The conductances calculated from the DMSP data with the empirical formulas are compared with those based on EISCAT measurements with the standard equations. The best correlation between these two is found when the empirical Robinson formulas (Robinson et al., 1987, https://doi.org/10.1029/ja092ia03p02565) are used in the presence of diffuse electron precipitation without ions. In the presence of ion precipitation, the correlation coefficients are smaller, but the correlation improves when the Galand formulas (Galand & Richmond, 2001, https://doi.org/10.1029/1999ja002001) are used to estimate the contribution of ion precipitation to the conductances. We also found that pure ion precipitation can cause the increase of conductances up to 2–7 S for Pedersen and 2.5–10 S for Hall conductances, which is positively correlated with the auroral electrojet index. Overall, the empirical formulas applied to the DMSP particle spectra underestimate the ionospheric conductances.

Schumann resonance anomalies associated with two (M~7) Tohoku offshore earthquakes in the spring of 2021

ABSTRACT. We describe the Schumann resonance (SR) anomalies associated with two earthquakes (EQs) observed in Japan in the spring of 2021. SR is the global electromagnetic phenomenon observed in the ELF (extremely low frequency) band, and its resonance peaks are observed in the power spectra on natural radio noise at frequencies of 8, 14, 20, Hz, etc. The natural source of ELF radiation is the global lightning activity occurring in the Earth-ionosphere cavity. The anomalies were observed for the first time in Japan for the EQs in Taiwan when the distance between the observatory and the EQ epicenter was a few Mm (1 Mm = 1000 km). Recently, a new SR anomaly was addressed, related to nearby (a few hundred km) EQs (Hayakawa et al., 2019, 2020a,b). This paper presents the SR anomalies observed in the vicinity of Nagoya-city for two relatively close (~500 km) successive EQs with magnitude around 7 that occurred offshore the Tohoku area in Japan. The anomaly is characterized by the noticeable simultaneous increase or decrease in the amplitudes of three SR modes. This SR unusual behavior was observed prior to and after each of the two EQs that occurred in February and March, 2021. Observational data were interpreted in the model of seismogenic perturbations of the lower ionospheric conductivity profile. Model computations imply the full-wave solution of the ELF electromagnetic problem in the form of the Riccati equation and the 2D (two dimensional) telegraph equations. We show that observed disturbances in the SR power spectra might be attributed to two types of seismogenic modifications in the lower ionospheric profile: the compression or the expansion of the vertical profiles of mesospheric conductivity over the EQ epicenter.

Substorms and Solar Eclipses: A Mutual Information Based Study

AbstractSolar eclipses present a rare glimpse into the impact of ionospheric electrodynamics on the magnetosphere independent of other well studied seasonal influences. Despite decades of study, we still do not have a complete description of the conditions for geomagnetic substorm onset. We present herein a mutual information based study of previously published substorm onsets and the past two decades of eclipses which indicates the likelihood of co‐occurrence is greater than random chance. A plausible interpretation for this relation suggests that the abrupt fluctuations in ionospheric conductivity during an eclipse may influence the magnetospheric preconditions of substorm initiation. While the mechanism remains unclear, this study presents strong evidence of a link between substorm onset and solar eclipses.

Three-dimensional ionospheric conductivity associated with pulsating auroral patches: reconstruction from ground-based optical observations

Abstract. Pulsating auroras (PsAs) appear over a wide area within the aurora oval in the midnight–morning–noon sector. In previous studies, observations by magnetometers on board satellites have reported the presence of field-aligned currents (FACs) near the edges and interiors of pulsating aurora patches. PsAs are thus a key research target for understanding the magnetosphere–ionosphere coupling process. However, the three-dimensional (3-D) structure of the electric currents has yet to be clarified, since each satellite observation is limited to a single dimension along its orbit. This study's aim was a reconstruction of the 3-D structure of ionospheric conductivity, which is necessary to elucidate the 3-D ionospheric current. Tomographic analysis was used to estimate the 3-D ionospheric conductivity for rapidly changing auroral phenomena such as PsAs. The reconstructed Hall conductivity reached its maximum value of 1.4 × 10−3 S m−1 at 94 km altitude, while the Pedersen conductivity reached its maximum value of 2.6 × 10−4 S m−1 at 116 km altitude. A secondary peak in the Pedersen conductivity, due to electron motion, at 9.9 × 10−5 S m−1 appears at 86 km altitude. The electron Pedersen conductivity maximum value in the D region was approximately 38 % of the ion Pedersen conductivity maximum value in the E region. The FAC, derived under the assumption of a uniform ionospheric electric field, was approximately 70 µA m−2 near the edge of the PsA patch. This FAC value was approximately 10 times that observed by satellites in previous studies. If the conductivity around the patch is underestimated or the assumption of a uniform field distribution is incorrect, the FAC could be overestimated. By contrast, due to sharper boundary structures, the FAC could actually have had such a large FAC.

Determination of the Effective Collision Frequency of Electrons in the E and D Regions of the High-Latitude Ionosphere from Analysis of Radio Occultation Measurements

Collisions between electrons and neutral molecules are of special interest for the physics of the Earth’s ionosphere, in particular, for determining the ionospheric conductivity and current systems in the lower ionosphere of the planet, as well as elucidating the role they play in attenuating radio waves propagating inside the D and E regions of the ionosphere. The effective collision frequency of electrons can be estimated from laboratory studies of electron mobility in atmospheric gases in combination with rocket measurements of temperature and particle density in the Earth’s upper atmosphere, or it can be determined independently from analysis of radio occultation data. We have developed a method for reconstructing the vertical profiles of the absorption coefficient of decimeter (wavelength ~19 cm) radio waves by solving the inverse problem of signal absorption in the D and E regions of the Earth’s ionosphere. Based on the analysis of radio occultation data from the FORMOSAT-3/COSMIC satellites, the altitude profiles of the absorption coefficient of decimeter (DM) radio waves in the planet’s ionosphere during the geomagnetic storm on June 22–23, 2015, were determined. It is known that the absorption coefficient at a given fixed frequency is directly proportional to both the electron density and the collision frequency of electrons with ions and neutrals. Using the data on the vertical profiles of the absorption coefficient of DM radio waves and the electron density reconstructed from the analysis of FORMOSAT-3/COSMIC radio occultations, we estimated the effective collision frequency of electrons in the D and E regions of the Earth’s high-latitude ionosphere. The practical significance of studying the frequency of electron collisions and the effects of radio wave absorption in the D and E regions of the planet’s ionosphere is associated with maintaining the uninterrupted operation of space radio communication and navigation systems.

External and Internal Causes of the Stormtime Intensification of the Dawnside Westward Auroral Electrojet

AbstractThe dawn‐dusk asymmetry of magnetic depression is a characteristic feature of the storm main phase. Recently Ohtani (2021, https://doi.org/10.1029/2021JA029643) reported that its magnitude is correlated with the dawnside westward auroral electrojet (AEJ) intensity, and suggested that the dawnside AEJ intensification is a fundamental process of the stormtime magnetosphere‐ionosphere coupling. In this study we observationally address the cause of the dawnside AEJ intensification in terms of four scenarios. That is, the dawnside AEJ intensifies because (a) the external driving of global convection strengthens, (b) solar wind compression enhances energetic electron precipitation, and therefore, ionospheric conductance, through wave‐particle interaction, (c) the substorm current wedge forms in the dawn sector, and (d) energetic electrons injected by nightside substorms drift dawnward, and the subsequent precipitation enhances ionospheric conductance. We find an event that fits each scenario, and therefore, none of these scenarios can be precluded. However, the result of a superposed epoch analysis shows that some causes are more prevalent than others. More specifically, (a) although the enhancement of external driving may precondition the dawnside AEJ intensification, it is rarely the direct cause; (b) external compression probably explains only a small fraction of the events; (c) prior to the dawnside AEJ intensification, the westward AEJ tends to intensify in the midnight sector along with mid‐latitude positive bays, which suggests that the substorm injection of energetic electrons is the most prevalent cause. This last result may also be explained by the dawnside expansion of the substorm current wedge, which, however, is arguably far less common.

Magnetic Longitudinal and Local Time Variations of Polar Electrojet and Field‐Aligned Currents

AbstractIn this work, prominent longitudinal variation of polar electrojet (PEJ) and field‐aligned currents (FACs) are reported for the first time in different local times and hemispheres. Larger longitudinal differences exist mostly in the south than in the north with nearly opposite phase except for the nighttime. The most prominent longitudinal variation occurs around noontime. In the nighttime, the longitudinal changes of PEJ and upward FACs in both hemispheres are similar. The larger northern summer FACs and PEJ at noon occur more frequently around 0°–60° and 300°–360° magnetic longitude (MLon), while the larger southern summer currents occur around 120°–300° MLon. The summer‐winter hemispheric differences in PEJ and upward FACs disappear in the midnight sector. The northern FACs dominance exists around 0°–120° and 270°–360° MLon, except for winter when the northern dominance of upward FACs at night exist in nearly all longitudes. In local summer, PEJ shows northern dominance around 0°–60° and 270°–360° MLon, and southern dominance around 120°–240° MLon. In local winter, PEJ shows northern dominance around 180° MLon near midnight, while in most other local times, PEJ shows northern dominance around 270° MLon. These longitudinal, seasonal and hemispheric differences are discussed in terms of mixing effects of ionospheric conductance and merging electric field in different local time sectors.

A Conjunction of Pc5 ULF Waves From Spaceborne and Ground‐Based Observations

AbstractLarge scale (low wavenumber m) ultra‐low frequency (ULF) waves observed in the inner magnetosphere can propagate into the ionosphere and to the ground, which are recognized as magnetic pulsations by ground‐based magnetometers. We present the conjunctive observations of Pc5 ULF waves from Van Allen Probes (at ∼6MLT, earlier), GOES13 (at ∼1MLT, later), SuperDARN radar (at ∼6MLT), and ground magnetometers. The phase shifts of the waves observed by satellites at different MLTs indicate westward propagation from the noon sector. The traveling Alfven waves are thought to propagate along the field line and reflect mostly at the ionosphere, while a few wave energy leak from the ionosphere to the ground. Thus, we suppose that standing waves are observed by the spacecraft while traveling signals are observed by the ground‐based magnetometers in the form of electromagnetic waves. By mapping the ground magnetic pulsations to the magnetic equatorial plane (which depends on the total ionosphere conductivity from IRI and MSIS model), the expected ULF waves match well with the satellite observations. Besides, it suggests that the ionosphere conductivity can be estimated from the conjunctive ULF waves observed on the ground and in the magnetosphere. The calculated total conductivity is 15–30% higher than the model.

Solar-Cycle Variations of Forbidden Energetic Electrons Enhancements

Intense fluxes of electrons from the Earth’s radiation belt (ERB) with energies of tens and hundreds of keV can penetrate to low altitudes at low latitudes outside the South Atlantic Anomaly. This region is known as a forbidden zone of quasi-trapped energetic particles. Flux enhancements of energetic electrons in the forbidden zone, so-called forbidden energetic electrons (FEE), produce significant ionization effects in the upper atmosphere at low latitudes. In this work, solar-cycle variations of the FEE enhancements with energy > 30 keV were analyzed over a 25-year period using a database of low-orbit satellites of the NOAA/POES and MetOp series. We found the highest correlations of the annual occurrence of FEE with the F10.7 solar activity index (−0.87) and the Alfven Mach number of the upstream solar wind (0.76). Using multiparameter regression analysis, a power expression was obtained with those parameters as well as with plasma beta and the interplanetary magnetic field strength with a total correlation coefficient of 0.94. The role of the conductivity of the high-latitude ionosphere in the mechanism of the penetration of ERB electrons into the forbidden zone is discussed.

Quantifying the ability of magnetohydrodynamic models to reproduce observed Birkeland current and auroral electrojet magnitudes

Although global magnetohydrodynamic (MHD) models have increased in sophistication and are now at the forefront of modeling Space Weather, there is still no clear understanding of how well these models replicate the observed ionospheric current systems. Without a full understanding and treatment of the ionospheric current systems, global models will have significant shortcomings that will limit their use. In this study we focus on reproducing observed seasonal interhemispheric asymmetry in ionospheric currents using the Space Weather Modeling Framework (SWMF). We find that SWMF does reproduce the linear relationship between the electrojets and the FACs, despite the underestimation of the currents’ magnitudes. Quantitatively, we find that at best SWMF is only capturing approximately 60% of the observed current. We also investigate how varying F10.7 effects the ionospheric potential and currents during the summer and winter. We find that simulations ran with higher F10.7 result in lower ionospheric potentials. Additionally, we find that the models do not always replicate the expected behavior of the currents with varying F10.7. This work points to a needed improvement in ionospheric conductance models.

Comparing magnetopause predictions from two MHD models during a geomagnetic storm and a quiet period

Magnetopause location is an important prediction of numerical simulations of the magnetosphere, yet the models can err, either under-predicting or over-predicting the motion of the boundary. This study compares results from two of the most widely used magnetohydrodynamic (MHD) models, the Lyon–Fedder–Mobarry (LFM) model and the Space Weather Modeling Framework (SWMF), to data from the GOES 13 and 15 satellites during the geomagnetic storm on 22 June 2015, and to THEMIS A, D, and E during a quiet period on 31 January 2013. The models not only reproduce the magnetopause crossings of the spacecraft during the storm, but they also predict spurious magnetopause motion after the crossings seen in the GOES data. We investigate the possible causes of the over-predictions during the storm and find the following. First, using different ionospheric conductance models does not significantly alter predictions of the magnetopause location. Second, coupling the Rice Convection Model (RCM) to the MHD codes improves the SWMF magnetopause predictions more than it does for the LFM predictions. Third, the SWMF produces a stronger ring current than LFM, both with and without the RCM and regardless of the LFM spatial resolution. During the non-storm event, LFM predicts the THEMIS magnetopause crossings due to the southward interplanetary magnetic field better than the SWMF. Additionally, increasing the LFM spatial grid resolution improves the THEMIS predictions, while increasing the SWMF grid resolutions does not.

Evidence of CME‐Magnetospheric Shock Disturbance of the D‐Region Observed in the VLF Signal

AbstractThe observation of the D‐region response to the interplanetary shock (IS) during the storm of 17 March 2015 is carried out using two very low frequency (VLF) transmitter signals (NRK and GQD) recorded at Algiers and Tunis. Data from THEMIS‐E and RBSP are used to correlate between the ground and satellite observations. The important finding is the wavy structure of the perturbations observed on the two VLF signals tens of seconds after the space detection of the shock, in concordance with the RBSP electric field measurements. The measured VLF signal amplitude and phase perturbations were: −0.65 dB, 5.81° for NRK and −0.12 dB, −126° for GQD measured at Algiers. For Tunis receiver, the perturbations were: −0.58 dB, 4.7° for NRK and 0.15 dB, −1.19° for GQD. In addition to the observations, simulations of the NRK signal perturbations were done using the long wavelength propagating capability code to determine the electron density modification that lead to the measured perturbations. The simulation results showed that the modified Wait's parameters above the NRK transmitter were: 80.391 km for h′ and 0.43 km−1 for β in the case of NRK‐Tunis path. Concerning the NRK‐Algiers path, the simulation gave 80.316 km for h′ and 0.431 km−1 for β. From these parameters values, the modified electron density was not important to explain the observed perturbations. Therefore, the IS effect could be explained by induced heating of the ionosphere due to the penetration of the electric field which leads to the changes of the ionospheric conductivity.

The Significance of Magnetospheric Precipitation for the Coupling of Magnetosphere-Ionosphere-Thermosphere Systems: Effects on Ionospheric Conductance

The Significance of Magnetospheric Precipitation for the Coupling of Magnetosphere-Ionosphere-Thermosphere Systems: Effects on Ionospheric Conductance

Imaging Ionospheric Electric Fields and Conductances at Small-Scales Globally: Advancing Research on Cross-Scale Coupling in the M-I-T System

Imaging Ionospheric Electric Fields and Conductances at Small-Scales Globally: Advancing Research on Cross-Scale Coupling in the M-I-T System

Networked HF Sounding to Determine the High-Latitude Ionospheric Conductivity

Networked HF Sounding to Determine the High-Latitude Ionospheric Conductivity