Storm Main Phase Research Articles

THE CORRELATION OF SHORT-PERIOD WAVE DISTURBANCES IN THE PEAK ELECTRON DENSITY OF THE F2 LAYER AND THE TOTAL ELECTRON CONTENT (TEC) IN THE IONOSPHERE

Using data of the vertical sounding ionosonde and the GNSS receiver operating in Irkutsk, we analyzed the correlation of short-period (periods less than 24 hours) wave disturbances of the peak electron density of the F2 layer (NmF2) and the total electron content (TEC) in the ionosphere. The measurement data for the period 2003–2020, which included almost two solar cycles, were processed. It has been established that the most probable case is a high positive correlation of short-period disturbances of NmF2 and TEC. With the growth of solar and geomagnetic activity, the number of cases of strong positive correlation increases (from 53% to 66%) and the number of cases of both positive and negative weak correlation decreases (from 46% to 33%). During powerful geomagnetic storms, the highest correlation (up to R = 0.89) is observed in the storm main phase and then decreases in the recovery phase up to negative R values. Possible physical mechanisms of the obtained results are considered.

Identification of Penetration and Disturbance Dynamo Electric Fields and Their Effects on the Generation of Equatorial Plasma Bubbles

AbstractA very challenging task in ionospheric studies is to determine the separate contributions of penetration and disturbance dynamo processes in the generation of equatorial plasma drift during magnetic storms. In this study, we analyze the ion drift measured by the Communications/Navigation Outage Forecasting System satellite during the magnetic storm on 15–16 July 2012. A unique feature of this storm is the exceptionally long period of continuous southward interplanetary magnetic field (IMF) for 32 hr. The storm‐induced net change of the meridional/vertical ion drift, the difference drift between the storm time and the quiet time, is derived during the storm main phase and the first 20 hr of the recovery phase with southward IMF. The difference drift during the recovery phase cannot be explained by the disturbance dynamo effect alone. A new method is used to separate the drifts caused by the penetration and disturbance dynamo processes. The penetration drift is represented by an empirical pattern of penetration electric field and depends on the IMF magnitude, and the disturbance dynamo drift is obtained by subtracting the penetration drift from the measured difference drift. The derived disturbance dynamo drift is in good agreement with previous statistical pattern. This is the first effort to identify the separate contributions of the penetration and disturbance dynamo processes to the total drift from observed data. The results have important implications in identifying storm‐time penetration and disturbance dynamo electric fields and their effects on the generation and evolution of plasma bubbles.

Improved Thermosphere Mass Density Recovery During the 5 April 2010 Geomagnetic Storm by Assimilating NO Cooling Rates in a Coupled Thermosphere‐Ionosphere Model

AbstractThe recovery of thermosphere mass density following geomagnetic storms is a result of competing heating and cooling processes. Simulations often underestimate the speed of the recovery. In this study, for the first time, we report that assimilating the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Sounding of the Atmosphere using Broadband Emission Radiometry nitric oxide (NO) cooling rate profiles into a coupled thermosphere‐ionosphere model via the ensemble Kalman filter improves the thermosphere mass density recovery following a geomagnetic storm. This is due to the impact of the assimilation on both the cooling processes and the thermosphere circulation. The dynamical changes due to the assimilation include stronger upwelling and equatorial transport. These lead to an effective increase in NO at all altitudes at mid‐high latitudes, resulting in the improved recovery. The improved representation of cooling processes in the storm's main phase also results in improved >24 hr forecasts of the density recovery.

The variable source of the plasma sheet during a geomagnetic storm

Both solar wind and ionospheric sources contribute to the magnetotail plasma sheet, but how their contribution changes during a geomagnetic storm is an open question. The source is critical because the plasma sheet properties control the enhancement and decay rate of the ring current, the main cause of the geomagnetic field perturbations that define a geomagnetic storm. Here we use the solar wind composition to track the source and show that the plasma sheet source changes from predominantly solar wind to predominantly ionospheric as a storm develops. Additionally, we find that the ionospheric plasma during the storm main phase is initially dominated by singly ionized hydrogen (H+), likely from the polar wind, a low energy outflow from the polar cap, and then transitions to the accelerated outflow from the dayside and nightside auroral regions, identified by singly ionized oxygen (O+). These results reveal how the access to the magnetotail of the different sources can change quickly, impacting the storm development.

Geomagnetic Storm Associated D‐Region Anomalies Estimated From VLF Observations at a Low‐Latitude Station, Suva, Fiji

AbstractA comprehensive analysis of subionospheric Very Low Frequency (VLF) signals from transmitters with call‐signs NWC, NPM, and NLK monitored at a low‐latitude station, Suva, Fiji, has been carried out to determine the D‐region effects of the intense geomagnetic storms for a period of 7 years (2011–2016 and 2018). The periods of 2011–2016 and 2018 fall under the 24th solar cycle with yearly mean sunspot numbers 80.8–39.8 and 7.0, respectively. Seven out of 12 geomagnetic storms revealed VLF anomalies during the storm main phase day and storm recovery phase days. A pronounced decrease in the VLF amplitude and phase for the storms having main phase onset in the nighttime was found. Out of seven geomagnetic storms, a storm of St Patrick's Day of 17–18 March 2013 is presented as a case study and the same analysis was done for all the storms. The anomalies in the VLF amplitude and phase have been modeled using Long‐Wave Propagation Capability code V2.1 to obtain the changes in Wait parameters (reference height, H′ and sharpness factor, ꞵ) and changes in the D‐region electron density. In most cases of storms, an increase in the H′ and a decrease in the ꞵ during the storm main and recovery phases were obtained which for the 17–18 March 2013 storm increased by 7.4–7.6 km and decreased from 0.03 to 0.07 km−1, respectively. Wavelet analysis of signal anomalies showed clear wave‐like spectra (0.05–0.18 mHz) of atmospheric gravity waves indicating traveling ionospheric disturbances propagating toward low latitudes due to storm‐associated Joule heating at high latitudes.

Contrasting Storm‐Time Radiation Belt Events With and Without Dropouts—The Importance of CME Shocks

AbstractThis study compares three different geomagnetic storms (designated as storms 1, 2, and 3) observed by NASA's Van Allen Probes within the spacecraft's first 50 days in orbit. These storms were Coronal Mass Ejection (CME)‐driven with minimum DST around −138, −100, and −106 nT, respectively. Storms 1 and 2 occurred following the arrival of fast‐forward shocks, which compressed the magnetopause inward to about 6.5 and 7.5 RE, respectively, as a result of the increase in solar wind dynamic pressure and density. The inward magnetopause motion helped contribute to a rapid depletion of MeV electrons across the entire outer belt. For the third storm, however, there was little or no dropout of MeV electrons in the heart of the outer belt during the storm main phase. This third storm was generated by a CME without an associated shock, and the magnetopause actually moved outward at the start of the storm, suppressing loss of electrons through the outer boundary. The study reveals that under certain solar wind driving conditions radiation belt electron dropouts may not occur, even during large geomagnetic storms (Dst_min < −100 nT).

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.

Comparison of Empirical and Theoretical Models of the Thermospheric Density Enhancement During the 3–4 February 2022 Geomagnetic Storm

AbstractOn 3 February 2022, at 18:13 UTC, SpaceX launched and a short time later deployed 49 Starlink satellites at an orbit altitude between 210 and 320 km. The satellites were meant to be further raised to 550 km. However, the deployment took place during the main phase of a moderate geomagnetic storm, and another moderate storm occurred on the next day. The resulting increase in atmospheric drag led to 38 out of the 49 satellites reentering the atmosphere in the following days. In this work, we use both observations and simulations to perform a detailed investigation of the thermospheric conditions during this storm. Observations at higher altitudes, by Swarm‐A (∼438 km, 09/21 Local Time [LT]) and the Gravity Recovery and Climate Experiment Follow‐On (∼505 km, 06/18 LT) missions show that during the main phase of the storms the neutral mass density increased by 110% and 120%, respectively. The storm‐time enhancement extended to middle and low latitudes and was stronger in the northern hemisphere. To further investigate the thermospheric variations, we used six empirical and first‐principle numerical models. We found the models captured the upper and lower thermosphere changes, however, their simulated density enhancements differ by up to 70%. Further, the models showed that at the low orbital altitudes of the Starlink satellites (i.e., 200–300 km) the global averaged storm‐time density enhancement reached up to ∼35%–60%. Although such storm effects are far from the largest, they seem to be responsible for the reentry of the 38 satellites.

Magnetospheric Time History in Storm‐Time Magnetic Flux Dynamics

AbstractMagnetospheric magnetic flux dynamics is quantified in 29 geomagnetic storms between 2015 and 2019, using near‐equatorial Van Allen Probe, GOES, and Magnetospheric Multiscale satellites. For the first time, concurrent, multi‐probe observations are utilized to preserve magnetospheric time history, defined as the state of the magnetosphere leading up to an observation. It is revealed that, relative to pre‐storm conditions, (a) during the storm sudden commencement (SSC), magnetic flux uniformly increases ΔΨ = +15% throughout the magnetosphere, except in the nightside inner magnetosphere where ΔΨ = −30%, and (b) during storm main and recovery phases, ΔΨ = −30% and −15%, respectively, in the nightside magnetosphere, at radial distances 5 ≤ r [RE] < 8. It is found that a symmetric ring current is likely formed in the nightside, early in the storm process (localized during SSC), which then broadens during the main phase, before weakening during the recovery phase. The current system on the dayside shows a distinct dawn‐dusk asymmetry.

Significant Mid‐ and Low‐Latitude Ionospheric Disturbances Characterized by Dynamic EIA, EPBs, and SED Variations During the 13–14 March 2022 Geomagnetic Storm

AbstractThis work investigates mid‐ and low‐latitude ionospheric disturbances over the American sector during a moderate but geo‐effective geomagnetic storm on 13–14 March 2022 (π‐Day storm), using ground‐based Global Navigation Satellite System total electron content data, ionosonde observations, and space‐borne measurements from the Global‐scale Observations of Limb and Disk (GOLD), Swarm, the Defense Meteorological Satellite Program (DMSP), and the Ionospheric Connection Explorer (ICON) satellites. Our results show that this modest but geo‐effective storm created a number of large ionospheric disturbances, especially the dynamic multi‐scale electron density gradient features in the storm main phase as follows: (a) The low‐latitude equatorial ionization anomaly (EIA) exhibited a dramatic storm‐time deformation and reformation, where the EIA crests evolved into a bright equatorial band for 1–2 hr and then quickly separated back into the typical double‐crest structure with a broad crest width and deep equatorial trough. (b) Strong equatorial plasma bubbles (EPBs) occurred with an abnormally high latitude/altitude extension, reaching the geomagnetic latitude of ∼30°, corresponding to an Apex height of 2,600 km above the dip equator. (c) The midlatitude ionosphere experienced a conspicuous storm‐enhanced density (SED) plume structure associated with the subauroral polarization stream (SAPS). This SED/SAPS feature showed an unusual temporal variation that intensified and diminished twice. These distinct mid‐ and low‐latitude ionospheric disturbances could be attributed to the storm‐time electrodynamic effect of electric field perturbation, along with contributions from neutral dynamics and thermospheric composition change.

Comments on “New Insights From the 2003 Halloween Storm Into the Colaba 1600 nT Magnetic Depression During the 1859 Carrington Storm” by S. Ohtani (2022)

AbstractThe Colaba, India ∼−1,600 nT magnetic spike caused by an interplanetary sheath magnetic field inducing a “dayside R1‐field aligned current wedge” during the Carrington magnetic storm proposed by Ohtani (2022), https://doi.org/10.1029/2022JA030596 seems highly improbable. Normal interplanetary magnetic field intensities of ∼5 nT have previously been shown to be sufficient to explain the ∼+120 nT sudden impulse (SI+) observed at Colaba during the storm (Tsurutani et al., 2018, https://doi.org/10.1002/2017JA024779). Magnetohydrodynamic theory (Kennel et al., 1985, https://doi.org/10.1029/GM034p0001) predicts a maximum of 4× magnetic field compression by a fast shock, giving an interplanetary sheath field of ∼20 nT, a value too low to support the Ohtani (2022), https://doi.org/10.1029/2022JA030596 hypothesis. The Ohtani (2022), https://doi.org/10.1029/2022JA030596 (and G. Siscoe et al., 2006, https://doi.org/10.1016/j.asr.2005.02.102) claim of a further 10× amplification of the interplanetary sheath fields has not been verified in near‐Earth interplanetary sheaths. The original (Tsurutani et al., 2003, https://doi.org/10.1029/2002JA009504) hypothesis that an interplanetary coronal mass ejection magnetic cloud (MC) having southward magnetic fields of ∼90 nT caused the Carrington magnetic storm main phase of peak SYM‐H/Dst = −1,760 nT seems more likely. The short time between the SI+ and the storm main phase onset implies a foreshortened interplanetary sheath. The extremely rapid recovery of the magnetic storm was hypothesized by Tsurutani et al. (2018), https://doi.org/10.1002/2017JA024779 as being due to nonlinear ring current losses. We point out that the Hydro‐Quebec 1989 storm was caused by multiple shock‐sheaths and MCs (Lakhina & Tsurutani, 2016, https://doi.org/10.1186/s40562‐016‐0037‐4) unlike the interplanetary causes of the Carrington storm. The Hydro‐Quebec event was a “stealth” magnetic storm.

The March 1940 Superstorm: Geoelectromagnetic Hazards and Impacts on American Communication and Power Systems

AbstractAn analysis is made of geophysical records of the 24 March 1940, magnetic storm and related reports of interference on long‐line communication and power systems across the contiguous United States and, to a lesser extent, Canada. Most long‐line system interference occurred during local daytime, after the second of two storm sudden commencements and during the early part of the storm's main phase. The high degree of system interference experienced during this storm is inferred to have been due to unusually large‐amplitude and unusually rapid geomagnetic field variation, possibly driven by interacting interplanetary coronal‐mass ejections. Geomagnetic field variation, in turn, induced geoelectric fields in the electrically conducting solid Earth, establishing large potential differences (voltages) between grounding points at communication depots and transformer substations connected by long transmission lines. It is shown that March 1940 storm‐time communication‐ and power‐system interference was primarily experienced over regions of high electromagnetic surface impedance, mainly in the upper Midwest and eastern United States. Potential differences measured on several grounded long lines during the storm exceeded 1‐min resolution voltages that would have been induced by the March 1989 storm. In some places, voltages exceeded American electric‐power‐industry benchmarks. It is concluded that the March 1940 magnetic storm was unusually effective at inducing geoelectric fields. Although modern communication systems are now much less dependent on long electrically conducting transmission lines, modern electric‐power‐transmission systems are more dependent on such lines, and they, thus, might experience interference with the future occurrence of a storm as effective as that of March 1940.

Disturbance electric fields and their effect on ionospheric TEC and scintillations over south China

This study investigated both the disturbance electric fields processes including the Interplanetary Electric Field (IEFy) to low-latitude, known as prompt penetration electric field (PPEF) and the disturbance dynamo electric field (DDEF), and their effects on ionospheric TEC and scintillations over south China during four storms of 17 March 2015, 15 July 2012, 7 October 2015 and 20 December 2015. GPS ionospheric and magnetometer data were derived from receiver stations at Guangzhou and Shenzhen, and at Guangdong Zhaoqin (GZH) and equatorial station Dalat, respectively. Results presented that cases of the eastward PPEF on dayside with durations of 3 and 8 h were identified during storm main phases of 17 March 2015 and of 20 December 2015 respectively. DDEFs were found to dominate on nightside during the three main storm phases other than storm of 15 July 2012, and on dayside during the recovery storm phase of 21 December 2015. The responses of positive TEC deviation to the eastward PPEF on dayside were identified during the storm main phase of 20 December 2015 rather than during the storm of 17 March 2015, which showed longitudinal, latitudinal and seasonal dependences as compared to the previous studies. During the storm of 15 July 2012, positive TEC deviations with no response to the eastward PPEF on dayside were identified. Negative TEC deviation responses to the DDEF on dayside were seen during all four storm recovery phases. GPS ionospheric scintillations (S4 ≥ 0.4) dominated in the equinox months before the occurrences of four storms, but were inhibited during the storm of 17 March 2015 over south China, which was attributed to the inhibitions of irregularities caused scintillations.

Ionospheric irregularities observed during the geomagnetic storm of March 2015 and June 2015 over Ethiopia

In this paper we studied the occurrence of ionospheric irregularities over Ethiopia as the consequence of the intense geomagnetic storm of March 2015 and June 2015. Total Electron Content (TEC) data obtained from six Ethiopian GPS reciever stations and those data were used to derive the ionospheric irregularities proxy indices, e.g.,rate of change of TEC (ROT) and ROT index (ROTI). These indices were used to characterize the intensity of the fluctuation of total electron content. ROTI was used as a criterion of ionospheric irregularities that took place during the storm. These indices were characterized along side with the disturbance storm time (Dst),the Interplanetary Electric Field (IEFy) and the SYM-H index (high resolution version of Dst) from ground-based magnetometers to see the effect of geomagnetic storm on horizontal component of geomagnetic field. Irregularities manifested in the form of fluctuations in TEC. Prompt penetration of electric field (PPEF) modulated the behaviour of irregularities during the initial and recovery phases of the geomagnetic storms. As a result,it was observed that the irregularities in the ionosphere of Ethiopia was in the range from weak to moderate. These irregularities generally occurred during the initial and recovery phases of the storm at all selected stations. The effect of electric field is found to depend on the local time at which the IMF Bz turned into southward. Therefore,east - west fluctuating Ey in March storm main phase is associated with suppression of irregularities in all stations and eastward Ey in the main phase of June 2015 is associated with small scale irregularity formation. The generation (inhibition) of irregularities is related to the effect of eastward (westward) storm time electric field disturbance dynamo electric fields and prompt penetration electric fields that created favourable (unfavourable) conditions for the generation of irregularities by uplifting (lowering) the F region.

Analysis of ionospheric storm-time effects over the East African sector during the 17 March 2013 and 2015 geomagnetic storms

An analysis of the mechanisms that caused the storm-time effects during two geomagnetic storms that occurred on 17 March 2013 and 2015 is presented. We used Global Navigation Satellite System (GNSS) derived Total Electron Content (TEC) data over the trough (Addis Ababa, ADIS, 38.8^circE geographic longitude, 0.18^circN geomagnetic latitude) and near the crest (Mbarara, MBAR, 30.7^circE geographic longitude, 10.22^circS geomagnetic latitude) regions of East African sector. Magnetometer data over Addis Ababa (AAE) and Mbour (MBO) were also used to derive the disturbance in ionospheric currents during the two storm periods. The monthly median TEC values for a month within which the storm under consideration occurred were used as a measure of background variability to analyse the response of the ionosphere to the storms. The response of the ionosphere to a geomagnetic storm is considered to be significant when the magnitude of TEC deviation (|Delta TEC|) is ge 45%. During the storm main phase, the ionosphere over the East African trough responded positively to the 17 March 2015 geomagnetic storm at 1800 UT, whilst at the crest regions, there was no significant response to the two St. Patrick’s day geomagnetic storms. However, during the storm recovery phase of 17 March 2013 and 2015 storms, both the stations over the trough and crest regions of East Africa showed a positive response. We checked thermospheric [O]/[text {N}_2] changes as a result of the two storms. There were no appreciable changes in [O]/[text {N}_2] over this sector between 16 and 18 March 2013. We observed an appreciable change in [O]/[text {N}_2] between 16 and 18 March 2015. The [O]/[text {N}_2] increase was more pronounced/obvious on 18 March 2015. The positive ionospheric responses during the recovery phases of the two geomagnetic storms could not be attributed to changes in thermospheric [O]/[text {N}_2] because the responses were nighttime features. The southward turning of the z component of Interplanetary magnetic field (IMF Bz) led to enhanced eastward equatorial electric field (EEF) during 0725 UT (1025 LT) and 0645 UT (0945 LT) for 17 March 2013 and 17 March 2015 storms, respectively. We note that when the IMF Bz turned northward, the EEF turned westward. During the southward turnings of IMF Bz that took place at about 1435 UT (1735 LT) on 17 March 2013, eastward prompt penetration electric field (PPEF) occurred in the post-sunset period starting at about 1600 UT (1900 LT) and enhanced the Prereversal enhancement (PRE). The presence of westward PPEF at around 1500 UT (1800 LT) acted to suppress the PRE on 17 March 2015. The positive storm effects during the recovery phases of the two storms may be attributed to strong disturbed dynamo electric field (DDEF), which was eastward during the night. We may thus surmise that the ionospheric responses to geomagnetic storms of St. Patrick’s day over the equatorial and low-latitude region of Africa were as a result of the combined effect of equatorward neutral wind, PPEF and DDEF.Graphical

Cosmic ray cutoff rigidity governing by solar wind and magnetosphere parameters during the 2017 Sep 6–9 solar-terrestrial event

For the 2017 Sep 6–9 geomagnetic disturbance, we calculated (within the Tsyganenko magnetospheric magnetic field model Ts01) the cosmic ray (CR) geomagnetic cutoff threshold rigidities. We also compared those values with the calculation results obtained from ground-based observations at the global network of neutron monitors (GNNM) through the spectrographic global survey (SGS). Shown is a satisfactory correlation between the geomagnetic cutoff threshold rigidity variations obtained based on the Ts01 model and those obtained from the GNNM ground-based observations. An intercorrelation of the CR geomagnetic threshold variations with the parameters for geomagnetic activity, for solar wind, and for the interplanetary magnetic field (IMF) was established to be different at different stages of the magnetospheric-geomagnetic disturbance. This is because such an intercorrelation is caused by the dynamics of current systems, whose intensifying and damping occur differently over different time intervals. During the storm main phase, the geomagnetic cutoff rigidity threshold variation is shown to be governed, mainly, by the Dst and by the solar wind velocity, as well as by the IMF southern component and by the electric field azimuth component.

Testing the key processes that accelerate outer radiation belt relativistic electrons during geomagnetic storms

Since the discovery of the Earth’s radiation belts in 1958, it has always been a challenge to determine the dominant physical mechanisms, whether local acceleration by chorus or inward radial diffusion, that leads to outer radiation belt relativistic electron flux enhancements. In this study, we test a chain of processes with several potential successive steps that is believed to accelerate outer belt relativistic electrons. By performing correlation analysis of different part of this chain, including the geomagnetic condition, evolution of source and seed electron fluxes, chorus wave activity, and maximum fluxes (jmax) of relativistic electrons, we aim to identify the critical steps that lead to acceleration of MeV electrons. Based on 5-years of Van Allen Probes observations, our results confirm the repeatable response of both source and seed electrons to the storms, showing a significant flux enhancement during the main phase of storms, followed by either a gradual decay or flux persistence at a stable level. However, it is the intense and prolonged occurrence of substorms that contributes to the long-lasting existence of both source and seed electrons, which is also strongly associated with the jmax of relativistic electrons. The significant correlation (Correlation Coefficient, CC∼0.8) between the seed electron fluxes and jmax reveal that the prolonged and pronounced seed electrons are the prerequisite for the significant flux enhancement of relativistic electrons regardless of the acceleration mechanism. The slightly smaller CC (∼0.5–0.7) between source electron fluxes and jmax of relativistic electrons indicates that while local acceleration by chorus wave plays an important role to accelerate relativistic electrons to jmax, other mechanisms such as inward radial diffusion are still needed in this process. The CC between the source electrons and the chorus wave amplitude increases with increasing levels of substorms, showing (CC)max of ∼0.8, which further supports the crucial role of chorus waves in accelerating the relativistic electrons during intense substroms.

Rapid Enhancements of Relativistic Electrons in the Earth's Outer Radiation Belt Caused by the Intense Substorms: A Statistical Study

AbstractUsing the data from Van Allen Probe A and B, we investigate rapid enhancements of relativistic electrons in the Earth's outer radiation belt caused by the intense substorms (AEmax > ∼900 nT) for 29 events from January 2013 to April 2015. These intense substorms may occur during the storm main phase or recovery phase. Based on the different substorm evolution characteristics, the intense substorms are divided into non‐continuous intense substorm activities and continuous intense substorm activities. In this study, we set a criterion for rapid enhancements when the electron phase space densities for μ = 1,096, 2,290, and 3,311 MeV/G increased by more than two times in 9 hr. In the time interval of 9 hr, the local acceleration by chorus waves is the dominant process for accelerating the seed populations (hundred kiloelectron volts) up to MeV energies. Our statistical results show that enhanced chorus waves and seed electrons during the intense substorms are observed in the outer radiation belt. Continuous intense substorm activities can more rapidly (<9 hr) and efficiently accelerate relativistic electrons in the outer radiation belt than non‐continuous intense substorm activities. During the intense substorms, the electric field acceleration could contribute to rapid enhancements of relativistic electrons in the outer radiation belt. Our statistical study suggests that the intense substorms during geomagnetic storms have a significant effect on the rapid variations of relativistic electron dynamics.

Ionospheric Oscillations With a Quasi‐Period of ∼6 hr During Intense Magnetic Storms

AbstractWe study the variations of the topside ionospheric ion density measured by Defense Meteorological Satellite Program satellites during the intense magnetic storm on 7–10 November 2004. It is found for the first time that quasi‐periodic enhancements in the ion density with a period of ∼6 hr occur nearly simultaneously at 0630, 0830, and 0930 local time in the dawn sector during the storm main phase with southward interplanetary magnetic field (IMF). The quasi‐periodic density enhancements extend from the southern subauroral latitudes to the northern subauroral latitudes. In the dusk sector, the topside ion density during the storm main phase is increased at middle latitudes for ∼12 hr but shows decrease or relatively small increase over the magnetic equator, indicating that penetration electric fields dominate the ion density redistribution. Similar quasi‐periodic enhancements in the topside ion density are also observed in the dawn sector during other intense magnetic storms. The solar wind and IMF do not have quasi‐periodic variations in this storm case. Periodic processes in geospace, such as periodic substorms in the magnetosphere, waves and tides in the atmosphere, and traveling ionospheric disturbances, cannot explain the observed periodic enhancements of the ionospheric ion density. We suggest that the magnetosphere‐ionospheric‐thermospheric system may have an intrinsic period of ∼6 hr and that oscillations of the magnetosphere‐ionospheric‐thermospheric system with this period can be excited during intense magnetic storms, although the mechanisms for the generation of the long‐periodic oscillations are not understood.

First Observations of Equatorial Ionospheric Electric Fields Driven by Storm‐Time Rapidly Recurrent Magnetospheric Substorms

AbstractGeomagnetic storm ionospheric electrodynamics at the magnetic equator are examined in detail using upgraded, higher time resolution, Jicamarca incoherent scatter radar drift observations, SuperMAG geomagnetic field, and solar wind data to study equatorial ionospheric electric fields during the 8 September 2017 storm main and early recovery phases. We show that during a period of mostly large and southward Interplanetary Magnetic Field (IMF) Bz there were numerous daytime prompt penetration vertical and zonal plasma drifts. These prompt penetration events were closely associated with rapidly recurring (quasi‐periods ∼30 − 60 min) magnetospheric substorms with expansion phases corresponding to upward and westward drift (eastward and downward electric field) perturbations, and recovery phases to opposite polarity changes. The magnitudes of the zonal prompt penetration drifts were about twice larger than those of the vertical drifts. Our study suggests that magnetospheric substorms are the main drivers of prompt penetration electric fields during extended periods (over ∼2 hr) of nearly steady southward IMF Bz. We also find that in general substorms are major driving factors for disturbance electric fields during geomagnetic active times. Our data highlight the powerful capabilities of the Jicamarca incoherent scatter radar as a tool for detailed studies of short‐lived solar wind‐magnetosphere‐ionosphere coupling processes.