Field Bz Research Articles

Sunspot magnetic fields: a comparison between the CrAO and SDO/HMI data

We performed a digitization of maximum magnetic field measurements in sunspots. The original data were acquired as drawings at the Crimean Astrophysical Observatory of the Russian Academy of Sciences (CrAO RAS). About 1000 sunspots observed in 2014 have been analyzed. The data were compared to the corresponding measurements from the SDO/HMI instrument (with both the line-of-sight magnetic field Bz(HMI) and the modulus of the magnetic field vector B(HMI)). For the same sunspot, the maximum modulus of the magnetic field derived at CrAO was compared to the corresponding value from HMI. The Crimean data and the space-based data (of both types) were found to be in direct proportion to each other. A linear approximation over the entire range of measurements (1–4) kilogauss (kG) shows a Pearson correlation coefficient of 0.71 (with the 95 % confidence boundaries of 0.68–0.74) and a slope of linear regression of 0.65±0.02 for both types of the space-based data. A linear approximation over the range of strong fields B(CrAO) > 1.8 kG gives a similar correlation, however the slope of linear regression is far closer to unity and constitutes 0.90 for the relationship (Bz(HMI) vs B(CrAO)) and 0.84 for the relationship (B(HMI) vs B(CrAO)). In the range of weak fields B(CrAO) < 1.8 kG, a non-linear deviation (exceeding) of the space-based data is observed. Non-linearity can be explained, in part, by a specific routine of the magnetic field measurements at CrAO, however further investigations are needed to explore sources of possible non-linearity in the HMI data. The Crimean measurements of the maximum magnetic field in sunspots are concluded to be in good agreement with the corresponding SDO/HMI measurements, and therefore they can be used for scientific purposes.

Mathematical Model of the Diffusion Mode of a Short, High-Current Vacuum Arc in the Axial Magnetic Field

In this paper, a theory of a diffuse mode of an axially symmetric short high-current vacuum arc in an external axial magnetic field is developed. A second-order partial differential equation for the lines of discharge current levels (2D model) is obtained. The boundary conditions at the cathode and anode boundaries of the plasma are formulated. The solution of this equation with allowance for the heat-balance equation for electrons and the equation for the plasma concentration makes it possible to determine the dependence of streamlines on the coordinates. With these lines, the obtained analytical formulas can be used to calculate the components of the current density, magnetic field, and the distribution of the electron temperature and plasma concentration in the discharge gap. A (1.5D) mathematical model (jr $$ \ll $$ jz) that significantly simplifies the calculations and has a sufficiently wide range of applicability is proposed and substantiated. The method of successive approximations, which ensures results with the required accuracy, is used in the calculations for this model. The distributions of the components of the magnetic field Br, Bθ, and Bz, the current density (jr, jz, and jθ), the distributions of the electron temperature, and the concentrations of electrons and ions in the plasma of a vacuum arc are calculated.

Random Forest Model of Ultralow‐Frequency Magnetospheric Wave Power

AbstractModels of magnetospheric ultralow‐frequency (ULF) waves can be used to study wave phenomena and to calculate the effect of these waves on the energization and transport of radiation belt electrons. We present a decision tree ensemble (i.e., a random forest) model of ground‐based ULF wave power spectral density driven by solar wind speed vsw, north‐south component of the interplanetary magnetic field Bz, and variance of proton number density var(Np). This model corresponds to four radial locations in the magnetosphere (roughly L ∼ 4.21 to 7.94) and spans 1–15 mHz, with hourly magnetic local time resolution. The decision tree ensembles are easier to use than the previous model generation; they have better coverage, perform better at predicting power, and have reduced error due to intelligently chosen bins in parameter space. We outline the difficulties in extracting physics from parameterized models and demonstrate a hypothesis testing framework to iteratively explore finer driving processes. We confirm a regime change for ULF driving about Bz = 0. We posit that ULF wave power directly driven by magnetopause perturbations corresponds to a latitude‐dependent dawn‐dusk asymmetry, which we see with increasing speed. Model uncertainty identifies where the underlying physics is not fully captured; we find that power due to substorms is less well characterized by Bz > 0, with an effect that is seen globally and not just near midnight. The largest uncertainty is seen for the smallest amounts of solar wind driving, suggesting that internal magnetospheric properties may play a significant role in ULF wave occurrence.

Magnetosheath Microstructure: Mirror Mode Waves and Jets during Southward IP Magnetic Field

AbstractIn the Earth's magnetosheath plasma waves, nonlinear structures associated with characteristic ion populations can occur. Understanding the interaction of the solar wind with the magnetosphere requires a deeper knowledge of the underlying magnetosheath kinetic microstructure. We study a 45 min interval when the MMS spacecraft observed a southward magnetic field (Bz < 0 nT) in the dayside magnetosheath. Using magnetic field and plasma data, we analyze three transient dynamic pressure enhancements identified as magnetosheath jets. The characteristics of these jets are different, suggesting different origins. While two of them, called J1 and J2, exhibit large increment in velocity and almost no density increment, the third jet (J3) shows large density enhancements with almost no velocity increment. The duration of J3 is ∼7 times longer than those of J1 and J2. J3 occurs at the region where the negative Bz becomes positive. Ion distributions inside the jets are different. J1 and J2 show a secondary field‐aligned beam, which is not present in J3, suggesting that magnetic reconnection at the magnetopause is responsible for their formation. Distributions inside J3 are more isotropic. B field and plasma signatures inside J3 correspond to the crossing of a sector boundary, similar to the heliospheric plasma sheet, suggesting that J3 forms by the evolution and interaction of a solar wind structure with the bow shock and magnetosheath. Fluctuations inside J3 have larger transverse components, although they propagate at large angles to B. In contrast, waves in regions between the jets are compressive and are identified as mirror mode waves.

Theoretical Study of the Effect of Magnetic Field Sweep Rate and Temperature on Shielding Efficiency in a Bulk Tube-Shaped HTS

In this paper, we have studied the shielding properties of a high temperature superconducting (HTS) bulk tube-shaped cylinder in varying magnetic fields. Within this scope, we have simulated the hollow cylinder geometry in axis-symmetric finite element method (FEM) in which the electromagnetic properties of a HTS are described by a nonlinear electric field-current density (E − J) power law including a critical current density depending on magnetic field. In the first stage, we have analyzed temperature dependence of shielding factor (SF), which is described as the ratio of the applied magnetic field to the magnetic field in the center of the hollow cylinder. In the second stage, the effect of magnetic field’s sweep rate (dBapp/dt) on the magnetic shielding was numerically examined. In the final stage, using the certain lines over the superconducting tube, we have worked out the shielding field (Bsz) of the HTS tube by subtracting the z-component of magnetic field (Bz) from applied magnetic field (Bapp). Our numerical investigations indicate that the superconducting tube can provide field and temperature-controlled magnetic shielding.

Interhemispheric Comparisons of Large Nighttime Magnetic Perturbation Events Relevant to GICs

AbstractNearly all studies of impulsive magnetic perturbation events (MPEs) with large magnetic field variability (dB/dt) that can produce dangerous geomagnetically induced currents (GICs) have used data from the Northern Hemisphere. Here we present details of four large‐amplitude MPE events (|ΔBx| > 900 nT and |dB/dt| > 10 nT/s in at least one component) observed between 2015 and 2018 in conjugate high‐latitude regions (65–80° corrected geomagnetic latitude), using magnetometer data from (1) Pangnirtung and Iqaluit in eastern Arctic Canada and the magnetically conjugate South Pole Station in Antarctica and (2) the Greenland West Coast Chain and two magnetically conjugate chains in Antarctica, AAL‐PIP and BAS LPM. From one to three different isolated MPEs localized in corrected geomagnetic latitude were observed during three premidnight events; many were simultaneous within 3 min in both hemispheres. Their conjugate latitudinal amplitude profiles, however, matched qualitatively at best. During an extended postmidnight interval, which we associate with an interval of omega bands, multiple highly localized MPEs occurred independently in time at each station in both hemispheres. These nighttime MPEs occurred under a wide range of geomagnetic conditions, but common to each was a negative interplanetary magnetic field Bz that exhibited at least a modest increase at or near the time of the event. A comparison of perturbation amplitudes to modeled ionospheric conductances in conjugate hemispheres clearly favored a current generator model over a voltage generator model for three of the four events; neither model provided a good fit for the premidnight event that occurred near vernal equinox.

Polar substorm on 7 December 2015: preonset phenomena and features of auroral breakup

Abstract. Comprehensive analysis of a moderate 600 nT substorm was performed using simultaneous optical observations inside the auroral oval and in the polar cap, combined with data from satellites, radars, and ground magnetometers. The onset took place near the poleward boundary of the auroral oval that is not typical for classical substorms. The substorm onset was preceded by two negative excursions of the interplanetary magnetic field (IMF) Bz component, with a 1 min interval between them, two enhancements of the antisunward convection in the polar cap with the same time interval, and 15 min oscillations in the geomagnetic H component in the auroral zone. The distribution of the pulsation intensity along meridian has two local maxima, namely at the equatorial and poleward boundaries of the auroral oval, where pulsations occurred in the out-of-phase mode resembling the field line resonance. At the initial stage, the auroral breakup developed as the auroral torch stretched and expanded poleward along the meridian. Later it took the form of the large-scale coiling structure that also distinguishes the considered substorm from the classical one. Magnetic, radar, and the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite data show that, before the collapse, the coiling structure was located between two field-aligned currents, namely downward at the poleward boundary of structure and upward at the equatorial boundary. The set of GEOTAIL satellites and ground data fit to the near-tail current disruption scenario of the substorm onset. We suggest that the 15 min oscillations might play a role in the substorm initiation.

Geomagnetic Storm‐Induced Plasma Density Enhancements in the Southern Polar Ionospheric Region: A Comparative Study Using St. Patrick's Day Storms of 2013 and 2015

AbstractThe occurrence of St. Patrick's Day (17 March) geomagnetic storms during two different years (2013 and 2015) with similar solar flux levels but varying storm intensity provided an opportunity to compare and contrast the responses of the ionosphere‐thermosphere (IT) system to different levels of geomagnetic activity. The evolution of positive ionospheric storms at the southern polar stations Bharati (76.6°S MLAT) and Davis (76.2°S MLAT) and its causative connection to the solar wind driving mechanisms during these storms has been investigated in this paper. During the main phase of both the storms, significant enhancements in TEC and phase scintillation were observed in the magnetic noon/ midnight period at Bharati and Davis. The TEC in the midnight sector on 17 March 2015 was significantly higher compared to that on 17 March 2013, in line with the storm intensity. The TEC enhancements during both the storm events are associated with the formation of the storm‐enhanced densities (SEDs)/tongue of ionization (TOI). The strong and sustained magnetopause erosion led to the prevalence of stronger storm time electric fields (prompt penetration electric field (PPEF)/subauroral polarization streams (SAPS)) for long duration on 17 March 2015. This combined with the action of neutral winds at midlatitudes favored the formation of higher plasma densities in the regions of SED formation on this day. The same was weaker during the 17 March 2013 storm due to the fast fluctuating nature of interplanetary magnetic field (IMF) Bz. This study shows that the duration and extent of magnetopause erosion play an important role in the spatiotemporal evolution of the plasma density distribution in the high‐midlatitude ionosphere.

High‐Speed Solar Wind Imprints on the Ionosphere During the Recovery Phase of the August 2018 Geomagnetic Storm

AbstractThe low‐latitude ionospheric TEC observed by the Beidou geostationary satellite showed large enhancement during 27–30 August 2018 of the storm recovery phase. The cause of the positive ionospheric storm during the recovery phase has yet to be resolved. In this study, multiple observations, including aurora, high‐latitude convection, potential, and the TEC maps, were used to study the contributions from the high‐speed solar wind to the ionosphere during the recovery phase of the storm. It was found that the high‐speed solar wind was effective in modulating the intensity and the size of the auroral oval, high‐latitude convection, and potential pattern. The Thermosphere Ionosphere Electrodynamics General Circulation Model generally reproduced the observed evolution of the ionosphere at high latitudes during the recovery phase of the storm, and it was used to quantitatively investigate the effects of the high‐speed solar wind on the recovery phase ionosphere. The results suggested that the high‐speed solar wind caused increase of TEC at auroral oval was about 2 TECU. The high‐speed solar wind, combined with oscillating interplanetary magnetic field Bz, led to the enhancement of the low‐latitude prompt penetrating electric fields and increased the low‐latitude TEC of about 2 TECU. Therefore, the high‐speed solar wind was a possible driver to the ionospheric positive storm during the recovery phase, but the causes for the more than 10‐TECU enhancement at low and middle latitudes during the recovery phase of this event are unknown.

Ionospheric Plasma Irregularities Based on In Situ Measurements From the Swarm Satellites

AbstractIn this study, we present global climatological distributions of ionospheric plasma irregularities based on measurements by the Swarm satellites. These first global statistics obtained by direct, in situ measurements of plasma variations with Swarm confirm the presence of three main regions of strong ionospheric irregularities: the magnetic equator extending from postsunset to early morning, in the auroral ovals (from dayside cusp to nightside), and inside the polar caps. At equatorial latitudes, ionospheric irregularities form two bands of enhanced plasma fluctuations centered around ±10° magnetic latitude. Due to different plasma processes, ionospheric irregularities at high and low latitudes show different distributions. Though the averaged intensity of plasma irregularities is weaker at equatorial latitudes than at high latitudes, the occurrence rate of significant plasma fluctuations (corresponding to extreme events) is much higher at the equator than that at high latitudes. Equatorial irregularities display clear seasonal and longitudinal variations; that is, they are most prominent over South America during the December solstice and are located over Africa during the June solstice. The magnitude of ionospheric irregularities at all latitudes is strongly controlled by the solar activity. Ionospheric irregularities become significantly weaker after 2016 during the current declining phase of solar activity. The interplanetary magnetic field Bz modulates the occurrence of ionospheric irregularities at both high and low latitudes.

Global MHD Simulation of a Prolonged Steady Weak Southward Interplanetary Magnetic Field Condition

We performed high-resolution three-dimensional global magnetohydrodynamic (MHD) simulations to study the interaction between the Earth’s magnetosphere and a prolonged steady southward interplanetary magnetic field (IMF) (Bz = –2nT) and slow solar wind. The simulation results show that dayside magnetic reconnection continuously occurs at the subsolar region where the magnetosheath magnetic field is antiparallel to the geomagnetic field. The plasmoid developed on closed plasma sheet field lines. We found that the vortex was generated at the magnetic equator such as (X, Y) = (7.6, 8.9) RE due to the viscous-like interaction, which was strengthened by dayside reconnection. The magnetic field and plasma properties clearly showed quasiperiodic variations with a period of 8–10 min across the vortex. Additionally, double twin parallel vorticity in the polar region was clearly seen. The peak value of the cross-polar cap potential fluctuated between 17 and 20 kV during the tail reconnection.

Mathematical analysis of the 09 March 2012 intense storm

The in-depth investigation of the parameters of the solar wind, the zonal geomagnetic indices and the interplanetary magnetic field (IMF) is the key to furthering space weather researches. The investigations got meaning through modeling various interplanetary parameters and geomagnetic indices. These models, built on the basis of the correlations between solar wind parameters and zonal geomagnetic indices, and that must strictly obey physical principles, will not only reveal the properties of geomagnetic activity but also allow estimates of their formations. This study focuses on the 2012 March 09 phenomenon, the first intense geomagnetic storm of the 24th solar cycle. The deeper understanding of the storm depends on a correct interpretation and visualization of the relationship between solar wind plasma parameters (solar wind dynamic pressure (P), flow speed (v), electric field (E), magnetic field (Bz), proton density (N), temperature (T)) and zonal geomagnetic indices (AE, ap, Kp, Dst). The study investigates, visualizes and interprets the solar wind plasma parameters and zonal geomagnetic indices themselves. The storm variables set included of observations of ten variables from NASA are described. By this investigation a covariance matrix indicates the strength of bivariate relationships, a dendrogram indicates hierarchical clustering into groups, and factor analysis indicates three distinct eigenvectors. In addition, linear and nonlinear models about the problem are introduced to readers. The author discuses mathematically what happened in the storm on 2012 March 09 at the beginning of the 24th solar cycle. The aim of this work is to help the reader understand the storm by providing the models that the representatives of parameters and indices coexist.

Quantifying the Effect of Plasmaspheric Hiss on the Electron Loss From the Slot Region

AbstractWe present global statistical models of both wave amplitude and wave normal angle (WNA) of plasmaspheric hiss using Van Allen Probe‐A observations. They utilize the time history of solar wind parameters, that is, interplanetary magnetic field BZ and solar wind speed, and the AE index for each measurement of hiss waves as inputs. The solar wind parameter‐based model generally results in higher performance than using only the AE index as an input. Both observations and model results reveal a clear dependence of hiss wave distribution on the magnetic local time (MLT): Higher amplitudes with field‐aligned (<30o) WNAs occur more frequently on the dayside than on the nightside. Such a tendency does not depend on magnetic latitude (MLAT), but slightly larger WNAs with a relatively low amplitude frequently appear for larger MLAT (>10o). We also examine how significantly the electron loss rates in the slot region can be changed by incorporating the model output of hiss waves into a diffusive transport simulation. Simulation results show that during a typical timescale (roughly a couple of days) of a corotating interaction region‐driven storm, the nightside hiss waves with larger WNA (>30o) do not contribute to the electron loss in the slot region due to their low amplitude and large WNA, while dayside hiss with WNAs less than 30o and comparatively higher amplitudes leads to a fast drop in flux, especially for electrons of a few hundred keV.

Quantifying Event‐Specific Radial Diffusion Coefficients of Radiation Belt Electrons With the PPMLR‐MHD Simulation

AbstractUsing the global Lagrangian version of the piecewise parabolic method‐magnetohydrodynamic (PPMLR‐MHD) model, we simulate two consecutive storms in December 2015, a moderate storm on 14–15 December and a strong storm on 19–22 December, and calculate the radial diffusion coefficients (DLL) from the simulated ultralow frequency waves. We find that even though the strong storm leads to more enhanced Bz and Eφ power than the moderate storm, the two storms share in common a lot of features on the azimuthal mode structure and power spectrum of ultralow frequency waves. For both storms, the total Bz and Eφ power is better correlated with the solar wind dynamic pressure in the storm initial phase and more correlated with AE index in the recovery phase. Bz wave power is shown to be mostly distributed in low mode numbers, while Eφ power spreads over a wider range of modes. Furthermore, the Bz and Eφ power spectral densities are found to be higher at higher L regions, with a stronger L dependence in the Bz spectra. The estimated DLL based on MHD fields shows that inside the magnetopause, the contribution from electric fields is larger than or comparable to that from magnetic fields, and our event‐specific MHD‐based DLL can be smaller than some previous empirical DLL estimations by more than an order of magnitude. At last, by validating against in situ observations from Magnetospheric Multiscale spacecraft, our MHD results are found to generally well reproduce the total Bz fields and wave power for both storms, while the Eφ power is underestimated in the MHD simulations.

A wavelet based method to remove the long term periodicities of geophysical time series

In this work it is shown how to remove the long-term periodicities present in geophysical time series using the Daubechies (db4) wavelet functions. This methodology consists in extraction of part of the energy from large scales (long periods) determined through of a threshold chosen from the variance analysis performed by scale. The Morlet wavelet transform was used in order to show that this procedure removes the energy from long-term periodicities while preserving all original frequencies. This method is applied to a case study of magnetotail response to interplanetary magnetic field (IMF) Bz fluctuations during a high-intensity long-duration continuous AE activity (HILDCAA) event.

RELATIONSHIP OF THE ASY-H INDEX WITH INTERPLANETARY MEDIUM PARAMETERS AND AURORAL ACTIVITY IN MAGNETIC STORM MAIN PHASES DURING CIR AND ICME EVENTS

In this study, we examine the relationship of the ASY-H index characterizing the partial ring current intensity with interplanetary medium parameters and auroral activity during the main phase of magnetic storms, induced by the solar wind (SW) of different types. Over the period 1979–2017, 107 magnetic storms driven by CIR and ICME (MC + Ejecta) events have been selected. We consider magnetic storms with Dstmin≤ – 50 nT. The average ASY-H index (ASYaver) during the magnetic storm main phase is shown to increase with increasing SW electric field and southward IMF Bz regardless of SW type. There is no relationship between ASYaver and SW velocity. For the CIR and ICME events, the average AE (AEaver) and Kp (Kp aver) indices have been found to correlate with ASYaver. The highest correlation coefficient between AEaver and ASYaver (r = 0.74) is observed for the magnetic storms generated by CIR events. A closer relationship between Kp aver and ASYaver (r = 0.64) is observed for the magnetic storms induced by ICME events. The ASYaver variations correlate with Dstmin. The relationship between ASYaver and the rate of storm development is weak.

Solar Rotational Oscillation and Its Subharmonics in Solar Wind Plasma Field, Geomagnetic, and Cosmic Ray Intensity Indicator in the Solar Cycle 24/25 Minimum

AbstractTo study synodic period and its subharmonics, we have utilized hourly average data of solar wind plasma field, geomagnetic index, and galactic cosmic ray intensity along with 5 min average data of selected solar wind field parameters during the recent minimum of Solar Cycle 24. Wavelet analyses of hourly average data reveal 28.1, 13.1, and 6.8 day of sigma in interplanetary magnetic field; 27.1, 13.1, and 9.0 day of plasma speed; 27.1, 13.6, 3.9, and 2.2 day of interplanetary electric field; 27.1, 13.6, 3.8, and 2.2 day of southward component of magnetic field; 26.2, 13.6, 8.6, and 3.0 day of geomagnetic Ap index; and 28.1, 11.0, 9.3, 7.0, and 1.6 day periods of galactic cosmic ray intensity. In addition to these prominent short‐term periods, we have significantly observed 6.6, 3.3, and 1.7 day periods of sigma in interplanetary magnetic field (σB); 5.6, 4.2, and 2.2 day of electric field (Ey) and 5.6, 3.3, and 2.4 day of southward component of magnetic field (Bz) in the 5 min average data. During the recent solar minimum, we have observed one solar rotational period and its second, third, fourth, fifth, sixth, seventh, eighth, tenth, eleventh, and twelfth subharmonics. The observed subharmonics of solar rotational period make the current solar minimum very special and noteworthy to study.

Direct Observations of a Polar Cap Patch Formation Associated With Dayside Reconnection Driven Fast Flow

AbstractDayside solar‐produced concentrated F region plasma can be transported from the midlatitude region into the polar cap during geomagnetically disturbed period, creating plasma density irregularities like polar cap patches, which can cause scintillation and degrade performance of satellite communication and navigation at polar latitudes. In this paper, we observed and investigated a dynamic formation process of a polar cap patch during the 13 October 2016 intense geomagnetic storm. During the storm main phase, storm‐enhanced density (SED) was formed within an extended period of strong southward interplanetary magnetic field (IMF) Bz condition. Total electron content (TEC) map shows that a polar cap patch was segmented from the SED plume. The Sondrestrom Incoherent Scatter Radar (ISR) was right underneath the segmentation region and captured the dynamic process. It shows that the patch segmentation was related with a sudden northeastward flow enhancement reaching ~2 km/s near the dayside cusp inflow region. The flow surge was observed along with abrupt E region electron temperature increase, F region ion temperature increase, and density decrease. The upstream solar wind and IMF observations suggest that the flow enhancement was associated with dayside magnetic reconnection triggered by a sudden and short period of IMF By negative excursion. Quantitative estimation suggests that plasma density loss due to enhanced frictional heating was insufficient for the patch segmentation because the elevated F region density peaking at ~500 km made dissociative recombination inefficient. Instead, the patch was segmented from the SED by low‐density plasma transported by the fast flow channel from earlier local time.

Связь индекса ASY-H с параметрами межпланетной среды и авроральной активностью на главных фазах магнитных бурь во время событий CIR и ICME

In this study, we examine the relationship of the ASY-H index characterizing the partial ring current intensity with interplanetary medium parameters and auroral activity during the main phase of magnetic storms, induced by the solar wind (SW) of different types. Over the period 1979–2017, 107 magnetic storms driven by CIR and ICME (MC + Ejecta) events have been selected. We consider magnetic storms with Dstmin≤ – 50 nT. The average ASY-H index (ASYaver) during the magnetic storm main phase is shown to increase with increasing SW electric field and southward IMF Bz regardless of SW type. There is no relationship between ASYaver and SW velocity. For the CIR and ICME events, the average AE (AEaver) and Kp (Kp aver) indices have been found to correlate with ASYaver. The highest correlation coefficient between AEaver and ASYaver (r = 0.74) is observed for the magnetic storms generated by CIR events. A closer relationship between Kp aver and ASYaver (r = 0.64) is observed for the magnetic storms induced by ICME events. The ASYaver variations correlate with Dstmin. The relationship between ASYaver and the rate of storm development is weak.

Unprecedented Hemispheric Asymmetries During a Surprise Ionospheric Storm: A Game of Drivers

AbstractThe geomagnetic storm occurred on 25–26 August 2018 as a surprise to forecasters. The arrival of a weak coronal mass ejection did not show a sudden impulse in the magnetic data; however, when the Interplanetary magnetic field Bz turned southward, it intensified and further remained unchangeably negative for the next 9 hr, causing a major storm with the minimum SYM‐H excursion of −205 nT. In this work, we study the thermospheric, ionospheric, and electrodynamic behavior during this storm. We use a set of space‐borne (the Swarm constellation, GUVI/TIMED) and ground‐based (GPS receivers, magnetometers, SuperDARN) instruments. We particularly focus on storm effects in the American and East Pacific sectors, where unprecedented hemispheric asymmetries occurred in the thermosphere and ionosphere during the main and the recovery phases of the storm. At the beginning of the storm, a strong positive ionospheric storm was observed in the Northern Hemisphere, while in the Southern Hemisphere, surprisingly, no storm effect occurred. During the recovery phase, the thermospheric composition ratio O/N2 showed an extreme expansion of the bulge into the opposite hemisphere. Our analysis shows that in each case the asymmetry was produced by a unique combination of drivers that acted at particular moment of time and in particular place. The seasonal asymmetry in the high‐latitude plasma and neutral mass density distributions along with the asymmetries in the geomagnetic field and the timing of these impacts played the decisive role.