Magnetic Local Time Research Articles

Survey and Analysis of Whistler‐ and Z‐Mode Emission in the Juno Extended Mission

AbstractWhistler‐mode and Z‐mode waves may have significant impact on electron scattering and acceleration at Jupiter as recent stochastic models indicate. New observations of these waves in the Juno extended mission expand the orbital coverage from previous surveys, extending especially to smaller radial distances near the magnetic equator, and to extended ranges of dusk‐side magnetic local time. M‐shells transiting the Io torus and the orbit of Europa are recognized as a significant source region for whistler‐mode lower and upper band chorus emission with intensity peaks near 9 RJ. Ganymede is a significant source of intense whistler‐mode chorus confined to the local moon environment. New Z‐mode observations now extend from a few kHz to over 80 kHz. Inferred source regions for Z‐mode are consistent with the outer edge of the Io torus and with auroral field lines that may also support broadband kilometric or decametric emission.

A Comparative Assessment of the Distribution of Joule Heating in Altitude as Estimated in TIE‐GCM and EISCAT Over One Solar Cycle

AbstractDuring geomagnetically active times, Joule (or frictional) heating in the Lower Thermosphere‐Ionosphere is a significant source of thermal energy, greatly affecting density, temperature, composition and circulation. At the same time, Joule heating and the associated Pedersen conductivity are amongst the least known parameters in the upper atmosphere in terms of their quantification and spatial distribution, and their parameterization by geomagnetic parameters shows large discrepancies between estimation methodologies, primarily due to a lack of comprehensive measurements in the region where they maximize. In this work we perform a long‐term statistical comparison of Joule heating as calculated by the NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM) and as obtained through radar measurements by the European Incoherent Scatter Scientific Association (EISCAT). Statistical estimates of Joule heating and Pedersen conductivity are obtained from a simulation run over the 11 year period spanning from 2009 until 2019 and from radar measurements over the same period, during times of radar measurements. The results are statistically compared in different Magnetic Local Time sectors and Kp level ranges in terms of median values and percentiles of altitude profiles. It is found that Joule heating and Pedersen conductivity are higher on average in TIE‐GCM than in EISCAT for low Kp and are lower than EISCAT for high Kp. It is also found that neutral winds cannot account for the discrepancies between TIE‐GCM and EISCAT. Comparisons point toward the need for a Kp‐dependent parameterization of Joule heating in TIE‐GCM to account for the contribution of small scale effects.

A Numerical Investigation on the Formation of the Stalling of the Cross‐Polar Jet in Midnight Thermospheric Winds

AbstractThermospheric winds usually flow from noon to the midnight inside the polar cap and spill further equatorward around midnight, due to the combined effects of solar radiation‐driven pressure gradient and ion drag. Recent observations from the Scanning Doppler Imager at Poker Flat indicated a distinct feature that the cross‐polar jet stalls over short horizontal distances around the magnetic midnight. However, the mechanisms responsible for the occurrence of stalling have not been well understood. In this study, we investigated the physical mechanisms of the stalling of the cross‐polar jet based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations. The model reproduced the general stalling features at midnight over a short horizontal distance. The occurrence of stalling is mainly associated with Joule heating, which enhances the neutral temperature and then turns winds poleward by the pressure gradient force, and ion‐neutral coupling, which drags neutral winds westward in the pre magnetic local time midnight sector and contributes to the poleward turning of the meridional winds by the poleward Coriolis force. Meanwhile, the stalling region has a large latitudinal range and depends on the interplanetary magnetic field (IMF). Under southward IMF Bz, thermospheric winds are likely to stall at lower latitudes associated with strong ionospheric convection. The stalling of the cross‐polar jet in the midnight thermospheric winds has UT/longitude dependence, which could be associated with the orientation of the alignment of geomagnetic and geographic poles with respect to ionospheric convection.

The Repetition Period of MeV Electron Microbursts as Measured by SAMPEX/HILT

AbstractHere we examine properties of MeV electron microbursts to better understand their generation mechanisms. Using 15 years of data from Solar, Anomalous, Magnetospheric Particles Explorer/Heavy Ion Large Telescope, >1 MeV microburst repetition periods (time spacing between bursts) are examined and clear dependencies on Auroral Electrojet (AE), L shell, and magnetic local time (MLT) are discovered. Microburst repetition periods are shortest around 0–6 hr MLT and 4–5 L shell, and grow longer toward the day and afternoon sectors and larger L shells. Shorter repetition periods (<1 s) are also found to be more common during higher AE, while longer periods (>10 s) more common during quiet times. The microburst repetition period distributions are compared directly to those of rising tone chorus wave elements and found to be similar in the night, dawn and day MLT sectors, suggesting chorus wave repetition periods are likely directly controlling those of microburst precipitation. However, dusk‐side distributions differ, indicating that the dusk‐side microbursts properties may be controlled by other processes.

Dependence of Electron Flux Dropouts in the Earth's Outer Radiation Belt on Energy and Driving Parameters During Geomagnetic Storms

AbstractUsing 5‐year of measurements from Van Allen Probes, we present a survey of the statistical dependence of the Earth's outer radiation belt electron flux dropouts during geomagnetic storms on electron energy and various driving parameters including interplanetary magnetic field Bz, PSW, SYM‐H, and AE. By systematically investigating the dropouts over energies of 1 keV–10 MeV at L‐shells spanning 4.0–6.5, we find that the dropouts are naturally divided into three regions. The dropouts show much higher occurrence rates at energies below ∼100 keV and above ∼1 MeV compared to much smaller occurrence rate at intermediate energies around hundreds of keV. The flux decays more dramatically at energies above ∼1 MeV compared to the energies below ∼100 keV. The flux dropouts of electrons below ∼100 keV strongly depend on magnetic local time (MLT), which demonstrate high occurrence rates on the nightside (18–06 MLT), with the highest occurrence rate associated with northward Bz, strong PSW and SYM‐H, and weak AE conditions. The strongest flux decay of these dropouts is found on the nightside, which strongly depends on PSW and SYM‐H. However, there is no clear MLT dependence of the occurrence rate of relativistic electron flux dropouts above ∼1 MeV, but the flux decay of these dropouts is more significant on the dayside, with stronger decay associated with southward IMF Bz, strong PSW, SYM‐H, and AE conditions. Our statistical results are crucial for understanding of the fundamental physical mechanisms that control the outer belt electron dynamics and developing future potential radiation belt forecasting capability.

Statistical Characteristics of the Electron Isotropy Boundary

AbstractUtilizing observations from the Electron Losses and Fields Investigation satellites, we present a statistical study of ∼2,000 events in 2019–2020 characterizing the occurrence in magnetic local time (MLT) and latitude of ≥50 keV electron isotropy boundaries (IBs) and associated electron precipitation. The isotropy boundary of an electron of a given energy is the magnetic latitude poleward of which persistent isotropized pitch angle distributions (Jprec/Jperp ∼ 1) are first observed to occur, interpreted as resulting from magnetic field‐line curvature scattering in the equatorial magnetosphere. We find that energetic electron IBs can be well‐recognized on the nightside from dusk until dawn, under all geomagnetic activity conditions, with a peak occurrence rate of almost 90% near ∼22 hr in MLT, remaining above 80% from 21 to 01 MLT. The observed IBs span International Geophysical Reference Field (IGRF) magnetic latitudes of 60°–74° with a maximum occurrence between 66° and 71° (L of 6–8), trending toward lower latitudes and premidnight local times with activity. The precipitating energy flux of ≥50 keV electrons averaged over the IB‐associated latitudes varies over four orders of magnitude, up to 1 erg/cm2‐s, and often includes wide‐energy electron spectra exceeding 1 MeV. The IB‐associated energies and precipitating fluxes also exhibit peak values near midnight for low activity, shifting toward premidnight for elevated activity. The average total precipitating power deposited over the high‐latitude nightside atmosphere (55°–80°; IGRF L ≥ 3) attributed to IBs is 10%–20%, or 10 MW, but at times can approach 100% of the total ≥50 keV electron energy deposition over the entire subauroral and auroral zone region, exceeding 1 GW.

Cluster Observation on the Latitudinal Distribution of Magnetic Pc5 Pulsations in the Inner Magnetosphere

AbstractUltralow frequency (ULF) waves in the Pc5 band are ubiquitous in the inner magnetosphere and can impact radiation belt dynamics by interacting with electrons through drift or drift‐bounce resonance. In this paper, based on ∼ 19 years of Cluster measurements, we perform a comprehensive study of the three‐dimensional distribution of poloidal, toroidal, and compressional ULF waves from L = 4 to 10, for magnetic latitudes (MLAT) up to Ultralow frequency ±50°, and in all magnetic local times (MLT). The distribution of the Pc5 ULF wave power is found to vary greatly as a function of L and MLT. For all L and MLT sectors, wave power of the poloidal and toroidal modes of the magnetic field increase with increasing |MLAT|, while the compressional mode decreases with increasing |MLAT|. The dawn–dusk asymmetries of wave power in poloidal and toroidal modes are more pronounced at higher |MLAT|. Furthermore, the wave power for Kp > 2 is approximately 2.93, 3.21, and 3.42 times greater than the wave power for Kp ≤ 2, respectively for compressional, poloidal and toroidal components. The information on the latitudinal distributions of ULF waves presented in this paper is important for future investigations on the radial diffusion process of radiation belt electrons with non‐90° pitch angles while they bounce away from the magnetic equator.

Space Weather Disturbances in Non‐Stormy Times: Occurrence of dB/dt Spikes During Three Solar Cycles

AbstractSpatio‐temporal variations of ionospheric currents cause rapid magnetic field variations at ground level and Geomagnetically Induced Currents (GICs) that can be harmful for human infrastructure. The risk for large excursions in the magnetic field time derivative, “dB/dt spikes”, is known to be high during geomagnetic storms and substorms. However, less is known about the occurrence of spikes during non‐stormy times. We use data from ground‐based globally covering magnetometers (SuperMAG database) from the years 1985–2021. We investigate the spike occurrence (|dB/dt| > 100 nT/min) as a function of magnetic local time (MLT), magnetic latitude (Mlat), and the solar cycle phases during non‐stormy times (−15 nT ≤ SYM‐H < 0). We sort our data into substorm (AL < 200 nT) intervals (“SUB”) and less active intervals between consecutive substorms (“nonSUB”). We find that spikes commonly occur in both SUBs and nonSUBs during non‐stormy times (3–23 spikes/day), covering 18–12 MLT and 65°–80° Mlat. This also implies a risk for infrastructure damage during non‐stormy times, especially when several spikes occur nearby in space and time, possibly causing infrastructure weathering. We find that spikes are more common in the declining phase of the solar cycle, and that the occurrence of SUB spikes propagates from one midnight to one morning hotspot with ∼10 min in MLT for each minute in universal time (UTC). Finally, we discuss causes for the spikes in terms of spatio‐temporal variations of ionospheric currents.

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.

Global ULF Waves Excited by Solar Wind Dynamic Pressure Impulses: 2. The Spatial Distribution Asymmetry

AbstractAsymmetry is a prevalent characteristic in numerous space physics phenomena. In this study, we investigate the statistical properties and spatial variations of ultra‐low frequency power globally after positive dynamic pressure pulses, utilizing high‐resolution magnetic field data from SuperMAG between 2012 and 2019. Specifically, we focus on the dawn‐dusk and north‐south asymmetries of Pc2‐5 fluctuations. Our analysis reveals that the power enhancement in the Pc2 band at approximately 30° magnetic latitude (MLAT) in the southern hemisphere is attributable to the South Atlantic Anomaly region. At MLAT ≈ 15°, the power of Pc3‐5 waves in both hemispheres exhibits a local minimum, which is associated with the strong coupling of compressional and Alfvén waves. Moreover, around MLAT = 60°, the dawnside Pc5 wave power exceeds that on the duskside when the interplanetary magnetic field (IMF) is westward, and the result is reversed when the IMF is eastward. Notably, Pc3‐5 wave power from MLAT = 30° to MLAT = 75° in the northern hemisphere is generally higher than that in the southern hemisphere. In regions with MLAT > 75°, which corresponds to the polar cap between 0 and 15 magnetic local time, the power of Pc3 pulsations is higher during summer in the northern hemisphere and higher during winter in the southern hemisphere. These findings underscore the significant role of the solar wind and the IMF in controlling geomagnetic pulsations and further deepen our understanding of the coupling between fluctuations in the ionosphere and the magnetosphere.

Plasma Pressure Distribution of Ions and Electrons in the Inner Magnetosphere During CIR Driven Storms Observed During Arase Era

AbstractUsing Arase observations of the inner magnetosphere during 26 CIR‐driven geomagnetic storms with minimum Sym‐H between −33 and −86 nT, we investigated ring current pressure development of ions (H+, He+, O+) and electron during prestorm, main, early recovery and late recovery phases as a function of L‐shell and magnetic local time. It is found that during the main and early recovery phase of the storms the ion pressure is asymmetric in the inner magnetosphere, leading to a strong partial ring current. The ion pressure becomes symmetric during the late recovery phase. H+ ions with energies of ∼20–50 keV and ∼50–100 keV contribute more to the ring current pressure during the main phase and early/late recovery phase, respectively. O+ ions with energies of ∼10–20 keV contribute significantly during main and early recovery phase. These are consistent with previous studies. The electron pressure was found to be asymmetric during the main, early recovery and late recovery phase. The electron pressure peaks from midnight to the dawn sector. Electrons with energy of <50 keV contribute to the ring current pressure during the main and early recovery phase of the storms. Overall, the electron contribution to the total ring current is found to be ∼11% during the main and early recovery phases. However, the electron contribution is found to be significant (∼22%) in the 03–09 MLT sector during the main and early recovery phase. The results indicate an important role of electrons in the ring current build up.

Abnormal Sub‐Auroral Ion Drifts (ASAID) Developed in the Postmidnight (1–4) Magnetic Local Time Sector in 2013

AbstractWe investigate the Abnormal Sub‐Auroral Ion Drifts (ASAID) developed in the postmidnight (1–4) magnetic local time sector in 2013. Shown by the Defense Meteorological Satellite Program F15 horizontal ion velocity measurements, we specified 21 antisunward (eastward) ASAID flow channels (FCs) based on their respective underlying polar convections. Overall, the polar convection formed a two‐cell pattern with its convection axis tilted in the morning‐evening direction because of the underlying BY < 0 conditions. Its dawn cell intruded into the duskside from the dawnside where the eastward ASAID flows were streaming antisunward, opposite to the dawn cell sunward convection flows. Mostly, the ASAID FC appeared in the coinciding ring‐current–related ionospheric trough and flow‐stagnation–related main ionospheric trough in the South American sector because of the favorable conditions created by the South Atlantic Magnetic Anomaly promoting also the development of Stable Auroral Red (SAR) arc. Rarely observed, the ASAID FC was associated with the SAR arc detected over Rothera. Detected in the Mid Pacific longitude sector, a pair of northern‐southern ASAID FCs demonstrated northern‐southern conjugacy. With correlated observations, demonstrating magnetosphere‐ionosphere conjugacy, the inward (earthward) ASAID electric (E) field could also be investigated. These reveal the ASAID E field's fast‐time development by short‐circuiting when the hot‐ring‐current—cold‐plasmasphere interface developed and became impacted by finite Larmor radius effects because of the earthward intrusion of hot ring current protons. The ASAID E field's inner‐magnetosphere environment was characterized by the hot zone where heating was generated by low‐frequency waves and auroral kilometric radiations.

Antisunward Streaming Westward Sub‐Auroral Ion Drifts (SAID) Developed in the Postmidnight (1–4) Magnetic Local Time Sector During 2013

AbstractIn this study, we have surveyed the Defense Meteorological Satellite Program F15 plots of horizontal ion velocity measurements for the calendar year of 2013 and specified 15 antisunward (westward) sub‐auroral ion drifts (SAID) detections taken in the 1–4 magnetic local time (MLT) sector based on their respective underlying polar convections. We find that during these 15 SAID events, the strong duskside westward polar plasma convection extended across the midnight meridian into the postmidnight 1–4 MLT sector making there the westward SAID flows streaming antisunward. These antisunward (westward) SAID flows were observed only once as a northern‐southern conjugate pair over the Mid Pacific and repeatedly in the South American longitude sector's wider region. We could correlate six antisunward (westward) SAID flows with their respective nearby outward SAID E fields observed by one of the Van Allen Probe (VAP) satellites in the inner magnetosphere. These provided two strong correlations depicting inner‐magnetosphere—topside‐ionosphere conjugate observations and four weak correlations. The VAP observations illustrate (a) the newly‐formed outward SAID E field and (b) its plasma environment still showing (c) the signatures of short‐circuiting across (d) the newly‐formed plasmapause appearing with (e) the hot zone on the earthward/tailward side and sometimes with (f) a localized heat source. Implying different underlying heating mechanisms, the hot zone and the heat source were associated with energetic plasma particles accelerated by plasma turbulence, low‐frequency waves including electromagnetic ion cyclotron waves, and/or auroral kilometric radiation bursts.

Direct Observation of Low‐Energy Electron Precipitation in the Subauroral Region

AbstractThis study used data collected by the Defense Meteorological Satellite Program (DMSP) and the Radiation Belt Storm Probes (RBSP) to identify a unique subauroral arc in the duskside, which is associated with precipitations of both ions and electrons. The auroral arc was found to be separated from the auroral oval, extending from 14 to 19 magnetic local time hours. Subauroral arcs are usually generated by energetic protons with energies above approximately 10 keV, while enhanced fluxes of low‐energy (<200 eV) electrons were revealed to occur and last for a few hours in this arc region from in situ observations by the DMSP satellite. The observations from RBSP‐B indicate that the low‐energy electron fluxes, with pitch angles near the loss cone, were accompanied by intensified electromagnetic ion cyclotron (EMIC) waves. It is suggested that the enhanced fluxes of cold electrons were heated by the Landau damping of the observed H+ band EMIC waves, which was in the frequency range below the local H+ gyrofrequency and above the local He+ gyrofrequency. Additionally, both electron density and temperature in the ionosphere increased significantly. This study, using conjugate observations of magnetospheric and ionospheric spacecraft, providing evidence of precipitation of low‐energy electrons within the subauroral arc region, thereby offering new insight into understanding the ionospheric effects of cold electron heating by EMIC waves.

Drift phase resolved diffusive radiation belt model: 2. implementation in a case of random electric potential fluctuations

In the first part of this work, we highlighted a drift-diffusion equation capable of resolving the magnetic local time dimension when describing the effects of trapped particle transport on radiation belt intensity. Here, we implement these general considerations in a special case. Specifically, we determine the various transport and diffusion coefficients required to solve the drift-diffusion equation for equatorial electrons drifting in a dipole magnetic field in the presence of a specific model of time-varying electric fields. Random electric potential fluctuations, described as white noise, drive fluctuations of trapped particle drift motion. We also run a numerical experiment that consists of tracking trapped particles’ drift motion. We use the results to illustrate the validity of the drift-diffusion equation by showing agreement in the solutions. Our findings depict how a structure initially localized in magnetic local time generates drift-periodic signatures that progressively dampen with time due to the combined effects of radial and azimuthal diffusions. In other words, we model the transition from a drift-dominated regime, to a diffusion-dominated regime. We also demonstrate that the drift-diffusion equation is equivalent to a standard radial diffusion equation once the distribution function is phase-mixed. The drift-diffusion equation will allow for radiation belt modeling with a better spatiotemporal resolution than radial diffusion models once realistic inputs, including localized transport and diffusion coefficients, are determined.

Understanding Quiet and Storm Time EMIC Waves—Van Allen Probes Results

AbstractGeomagnetic indices have been used as a proxy for studying electromagnetic ion cyclotron (EMIC) wave occurrences under different geomagnetic conditions. However, the drivers of EMIC waves are different during non‐storm, storm time and during individual storm phases. Using ∼7 years of data from the twin Van Allen Probes, we demonstrate that the occurrence probability of EMIC waves are not well captured by a specific geomagnetic activity index alone, but is rather well manifested by considering individual storm phases. We show EMIC wave occurrence statistics during different storm phases (pre‐onset, main and recovery) for geomagnetic activity indices Sym‐H, AE, and Kp and solar wind dynamic pressure Pdyn, illustrating that the occurrence rates vary significantly during different storm phases even for a given geomagnetic index. We also utilize this large database to show EMIC wave occurrence distribution, and how various wave and plasma parameters behave under different geomagnetic conditions. EMIC waves occur 2.9 times more often during geomagnetic storms than during non‐storm times. The majority (72%) of storm time EMIC waves occur during the recovery phase due to long recovering time, while the highest occurrence rates are in the pre‐onset phase, followed by main and recovery phases. EMIC waves in the main phase have occurrence peaks in the dusk to pre‐midnight sectors while recovery phase events spread to more Magnetic Local Time (MLT) sectors with peaks in the morning sector. Wave amplitudes are found to be evenly distributed across different MLT sectors during all geomagnetic conditions.

Modeling TEC Irregularities in the Arctic Ionosphere Using Empirical Orthogonal Function Method

AbstractWe develop a climatological model for the Arctic ionosphere based on more than one solar cycle of data (2010–2021) of the rate of change of the total electron content index (ROTI) maps from the International GNSS Service (IGS). The IGS ROTI maps are daily averaged in magnetic latitude and local time coordinates. To develop a climatological model, the ROTI maps are decomposed into base functions and coefficients using the empirical orthogonal function (EOF) method. The EOF method converges very quickly, and the first four EOFs reflect the majority (96%) of the total data variability. Furthermore, different EOF components can reflect different drivers of ionospheric irregularities. The first EOF reflects the averaged ROTI activity and the impact of the solar radiation and geomagnetic activity; the second EOF reflects the impact of the interplanetary magnetic field (IMF) Bz and electric field; the third and fourth EOFs reflect the dawn‐dusk asymmetry around the auroral oval and polar cap, and they can be related to the IMF By. To build an empirical model, we fit the EOF coefficients using helio‐geophysical indices from four different categories (solar activity; geomagnetic indices; IMF; the solar wind coupling function). The final EOF model is dependent on seven selected indices (F10.7P, Kp, Dst, Bt, By, Bz, and EKL). The statistical data‐model comparisons show satisfactory results with a good correlation coefficient. However, the model cannot capture the significant expansion of the dayside ROTI activity during strong geomagnetic storms. Future effort is needed to provide corrections to the model during severe storms.

Dynasonde Observations of Ionospheric Polar Holes Under Quiet Geomagnetic Conditions at Jang Bogo Station, Antarctica

AbstractNoticeable F‐region electron density (NmF2) depletions were observed in the winter‐nighttime polar cap ionosphere during solar minimum from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) with Dynasonde analysis at Jang Bogo Station (JBS) in Antarctica. We focus on the F‐region density depletion events (known as polar holes) following a steady quiet condition that is defined with Kp values ≤ 1+ during 6 hr. Forty‐five polar holes were identified by JBS VIPIR/Dynasonde (JVD) in 2019. All of the events started over a wide range of nightside magnetic local time (22‐05 MLT) with a peak occurrence at 01–03 MLT. JVD measured exponential NmF2 decrease in the nightside MLT (∼19–2.5 hr) zone with e‐fold decay times distributed in the range of ∼0.5–∼3.5 hr before the onset of a polar hole. The e‐folding times decrease along the longitude from dusk toward midnight. The horizontal ion drift velocity (Vhor) estimated from JVD monotonically goes down from ∼190 m/s at 18 MLT to ∼100 m/s near magnetic midnight, and the NmF2 is depleted as Vhor decreases prior to the polar hole formation. The observations of the exponential NmF2 decrease and the positive correlation between NmF2 and Vhor prior to the polar holes are discussed in light of possible formation mechanisms of polar holes, including temporal variations and spatial structure of the polar ionosphere.

Drift phase resolved diffusive radiation belt model: 1. Theoretical framework

Most physics-based models provide a coarse three-dimensional representation of radiation belt dynamics at low time resolution, of the order of a few drift periods. The description of the effect of trapped particle transport on radiation belt intensity is based on the random phase approximation, and it is in one dimension only: the third adiabatic invariant coordinate, akin to a phase-averaged radial distance. This means that these radiation belt models do not resolve the drift phase or, equivalently, the magnetic local time. Yet, in situ measurements suggest that radiation belt intensity frequently depends on magnetic local time, at least transiently, such as during active times. To include processes generating azimuthal variations in trapped particle fluxes and to quantify their relative importance in radiation belt energization, an improvement in the spatiotemporal resolution of the radiation belt models is required. The objective of this study is to pave the way for a new generation of diffusive radiation belt models capable of retaining drift phase information. Specifically, we highlight a two-dimensional equation for the effects of trapped particle transport on radiation belt intensity. With a theoretical framework that goes beyond the radial diffusion paradigm, the effects of trapped particle bulk motion, as well as diffusion, are quantified in terms of Euler potentials, α,β, quantities akin to the radial and azimuthal directions. This work provides the theoretical foundations underlying the drift phase resolved transport equation for radiation belt dynamics. It also brings forward the concept of azimuthal diffusion as a phase-mixing agent.

Statistical Characteristics of Mid‐Latitude Ionospheric F‐Region Backscatter Observed by the SuperDARN Jiamusi Radar

AbstractThe Jiamusi (JME) radar is the first high‐frequency coherent scatter radar independently developed in China. In this study, we investigate the statistical characteristics of the Jiamusi radar scattering occurrence rate from the F‐region ionosphere between 40°N and 65°N geomagnetic latitude (MLAT) from March 2018 to November 2019. Then, the diurnal and seasonal variations in scattering echoes and their dependence on geomagnetic conditions are statistically investigated. It is shown that the local time of the peak scattering occurrence rate varies depending on the seasons, that is, approximately 20–22.5 magnetic local time (MLT) in summer, 17.5–20.5 MLT in equinox, and 16–17.5 MLT in winter, which is closely associated with the time of sunset. The occurrence rate also increases with the enhancement of the Kp index. To further understand the mechanism of these features, we simulate the distribution of the gradient drift instability (GDI) indicator by using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). The analysis results indicate that the GDI may be one of the factors that contribute to these characteristic features.