Recovery Phase Of Geomagnetic Storm Research Articles

Ring Current Decay During Geomagnetic Storm Recovery Phase: Comparison Between RBSP Observations and Theoretical Modeling

Ring Current Decay During Geomagnetic Storm Recovery Phase: Comparison Between RBSP Observations and Theoretical Modeling

Ionospheric Pc1 waves during a storm recovery phase observed by the China Seismo-Electromagnetic Satellite

Abstract. During the storm recovery phase on 27 August 2018, the China Seismo-Electromagnetic Satellite (CSES) detected Pc1 wave activities in both the Northern Hemisphere and Southern Hemisphere in the high-latitude, post-midnight ionosphere with a central frequency of about 2 Hz. Meanwhile, the typical Pc1 waves were simultaneously observed for several hours by the Sodankylä Geophysical Observatory (SGO) stations on the ground. In this paper, we study the propagation characteristics and possible source regions of those waves. Firstly, we find that the Pc1 waves observed by the satellites exhibited mixed polarisation, and the wave normal is almost parallel with the background magnetic field. The field-aligned Poynting fluxes point downwards in both hemispheres, implying that the satellites are close to the wave injection regions in the ionosphere at about L=3. Furthermore, we also find that the estimated position of the plasmapause calculated by models is almost at L=3. Therefore, we suggest that the possible sources of waves are near the plasmapause, which is consistent with previous studies in that the outward expansion of the plasmasphere into the ring current during the recovery phase of geomagnetic storms may generate electromagnetic ion cyclotron (EMIC) waves, and these EMIC waves propagate northwards and southwards along the background magnetic field to the ionosphere at about L=3. Additionally, the ground station data show that Pc1 wave power attenuates with increasing distance from L=3, supporting the idea that the CSES observes the wave activities near the injection region. The observations are unique in that the Pc1 waves are observed in the ionosphere in nearly conjugate regions where transverse Alfvén waves propagate down into the ionosphere.

Quantifying the Contribution of Microbursts to Global Electron Loss in the Radiation Belts

AbstractWe use the Solar, Anomalous, and Magnetospheric Particle Explorer to explore the relationship between microbursts and global flux decay of electrons from the outer Van Allen belt during the recovery phase of geomagnetic storms. We investigate the correlation between microbursts and global electron loss in each of the quasi‐trapped (drift loss cone), stably trapped, and untrapped electron (bounce loss cone) populations. For the quasi‐trapped electrons, we separately classify the storms as driven by coronal mass ejections or corotating interaction regions and explore their connection to microburst loss. We find that the decay lifetime of electron fluxes, that is, e‐folding times of macroscopic fluxes in the recovery phase is correlated with strong microburst activity. That is, when the microburst activity is high, global flux decay times are short, and vice versa, suggesting a cross‐scale coupling between microloss and macroloss phenomena. Furthermore, we find that the microburst to global loss coupling is predominant in the quasi‐trapped population of radiation belt electrons while having negligible influence on the untrapped and stably trapped populations. We find that microburst activity during storms driven by coronal mass ejections is coupled more strongly with global flux decay as compared with corotating interaction regions. In addition, we find that distance from the plasmapause is likely a better indicator of microburst location than L‐shell, with most microbursts occurring ~0.5–2.0 L from the model plasmapause location.

Processes in auroral oval and outer electron radiation belt

We have analyzed the role of auroral processes in the formation of the outer radiation belt, considering that the main part of the auroral oval maps to the outer part of the ring current, instead of the plasma sheet as is commonly postulated. In this approach, the outer ring current is the region where transverse magnetospheric currents close inside the magnetosphere. Specifically, we analyzed the role of magnetospheric substorms in the appearance of relativistic electrons in the outer radiation belt. We present experimental evidence that the presence of substorms during a geomagnetic storm recovery phase is, in fact, very important for the appearance of a new radiation belt during this phase. We discuss the possible role of adiabatic acceleration of relativistic electrons during storm recovery phase and show that this mechanism may accelerate the relativistic electrons by more than one order of magnitude.

Radial transport of radiation belt electrons in kinetic field‐line resonances

AbstractA representative case study from the Van Allen Probes during a geomagnetic storm recovery phase reveals enhanced electron fluxes at intermediate pitch angles over energies from ~100 keV to 5 MeV coincident with broadband low‐frequency electromagnetic waves. The statistical properties of these waves are used to build a model for radial diffusion via drift‐bounce resonances in kinetic Alfvén eigenmodes/kinetic field‐line resonances. Estimated diffusion coefficients indicate timescales for radial transport on the order of hours in storm time events at energies from <100 keV to MeVs over equatorial pitch angles from the edge of the loss cone to nearly perpendicular to the geomagnetic field. The correlation of kinetic resonances with electron depletions and enhancements during storm main phase and recovery, and the rapid diffusion these waves drive, suggests that they may modulate the outer radiation belt.

Ion temperature intensification in southern convection flow channels during the 1 October 2001 geomagnetic storm recovery phase

AbstractIn this study, we investigate Southern Hemisphere flow channel (FC) events and their underlying thermal and drift variations during the 1 October 2001 storm recovery phase. We adopt FC classification introduced by previous studies for specifying FCs, ranging from FC‐0 to FC‐4, according to the stages of convection cycle they are related to. Our investigation includes also the subauroral FC known as the subauroral polarization stream (SAPS) and the localized FC underlying plasma density increases crossing the polar cap. For tracking FCs, we utilize multi‐instrument data from the Defence Meteorological Satellite Program (DMSP). Since our focus is on the region of magnetic South Pole, we utilize DMSP passes that crossed the magnetic pole. We present various scenarios with polar cross sections, constructed with ion density (Ni), electron and ion temperature (Te; Ti), and zonal and vertical drift (VY; VZ) data, where the location of magnetic pole is marked. Our results show (1) the occurrence of FC‐2 in the central polar cap, (2) the propagation of localized FC from the dayside to the nightside across the polar cap implying dayside‐nightside coupling across the polar cap, and (3) the structuring of SAPS FC. These scenarios reveal the local intensification of Ti and/or VZ in FCs (a) ranging from FC‐0 to FC‐3 and (b) specified as SAPS FC and localized FC passing over the magnetic pole. We conclude that strong upward drift, reaching sometimes ~1000 m/s, could enhance localized thermospheric impact caused by elevated Ti in FCs.

Radio signal anomalies detected with MEXART in 2012 during the recovery phase of geomagnetic storms

In this work we present MEXART observations in 2012 from 17 radio sources in which we detected anomalies in the radio signal of these sources occurring during the recovery phase of some geomagnetic storms. We performed FFT and wavelet analysis of the radio signals during these periods and found that rather than IPS the anomalies seem to originate in the ionosphere, especially because of the frequencies at which they are observed. We discuss this results under the view that the source of the geomagnetic storm is no longer in the interplanetary medium.

Analysis of the effectiveness of ground‐based VLF wave observations for predicting or nowcasting relativistic electron flux at geostationary orbit

AbstractPoststorm relativistic electron flux enhancement at geosynchronous orbit has shown correlation with very low frequency (VLF) waves measured by satellite in situ. However, our previous study found little correlation between electron flux and VLF measured by a ground‐based instrument at Halley, Antarctica. Here we explore several possible explanations for this low correlation. Using 220 storms (1992–2002), our previous work developed a predictive model of the poststorm flux at geosynchronous orbit based on explanatory variables measured a day or two before the flux increase. In a nowcast model, we use averages of variables from the time period when flux is rising during the recovery phase of geomagnetic storms and limit the VLF (1.0 kHz) measure to the dawn period at Halley (09:00–12:00 UT). This improves the simple correlation of VLF wave intensity with flux, although the VLF effect in an overall multiple regression is still much less than that of other factors. When analyses are performed separately for season and interplanetary magnetic field (IMF) Bz orientation, VLF outweighs the influence of other factors only during winter months when IMF Bz is in an average northward orientation.

Observations on geomagnetic auroral infrasound waves 2003—2013

Persistent, high trace velocity infrasound activity, associated with auroral events, has been routinely observed from the CTBT/IMS I53US infrasound station in Fairbanks. Comparisons of the infrasound data with data from the Geophysical Institute Magnetometer Array, the Poker Digital All-Sky Camera, and historic data from the Poker Flat Imaging Riometer show that the observed infrasound is correlated with periods of heightened geomagnetic activity and is produced in the lower Ionosphere. With the infrasound array operating near-continuously now for roughly one full period of the solar cycle, we have systematically isolated all such geomagnetic auroral infrasound events to form a data set suitable for statistical analysis. We note a relationship between the occurrence of geomagnetic infrasound waves (GAIW) and the recovery phase of geomagnetic storms when the geomagnetic H component has a peak-to-peak amplitude of ~1500 gamma during the local time period from 5 to 10 h at the CIGO Magnetic Observatory in Fairbanks. During this time interval I53US is under the auroral oval when there are large pulsating aurora events that produce infrasonic waves. These observations restrict the apparent source geometry and generating phenomena of the infrasound, as well as providing a basis for comparison with idealized models of GAIW generation.

TEC evidence for near‐equatorial energy deposition by 30 keV electrons in the topside ionosphere

Observations of energetic electrons (10 – 300 keV) by NOAA/POES and DMSP satellites at heights <1000 km during the period from 1999 to 2010 allowed finding abnormal intense fluxes of ~106 – 107 cm−2 s−1 sr−1 for quasi‐trapped electrons appearing within the forbidden zone of low latitudes over the African, Indo‐China, and Pacific regions. Extreme fluxes appeared often in the early morning and persisted for several hours during the maximum and recovery phase of geomagnetic storms. We analyzed nine storm time events when extreme electron fluxes first appeared in the Eastern Hemisphere, then drifted further eastward toward the South‐Atlantic Anomaly. Using the electron spectra, we estimated the possible ionization effect produced by quasi‐trapped electrons in the topside ionosphere. The estimated ionization was found to be large enough to satisfy observed storm time increases in the ionospheric total electron content (TEC) determined for the same spatial and temporal ranges from global ionospheric maps. Additionally, extreme fluxes of quasi‐trapped electrons were accompanied by the significant elevation of the low‐latitude F‐layer obtained from COSMIC/FORMOSAT‐3 radio occultation measurements. We suggest that the storm time ExB drift of energetic electrons from the inner radiation belt is an important driver of positive ionospheric storms within low‐latitude and equatorial regions.

Physical mechanisms of the ionospheric storms at equatorial and higher latitudes during the recovery phase of geomagnetic storms

AbstractThe paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes, which are generally opposite both during the main phase (MP) and recovery phase (RP) of geomagnetic storms. The mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable. The paper first illustrates the recent developments in the understanding of the mechanisms during daytime MPs when generally negative ionospheric storms (in Nmax and TEC) develop at equatorial latitudes and positive storms occur at higher latitudes, including why the storms are severe only in some cases. The paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes (within ±15°) during daytime RPs when negative storms occur at higher latitudes using CHAMP Ne and GPS‐TEC data and Sheffield University Plasmasphere Ionosphere Model. The results indicate that the mechanical effect of the storm‐time equatorward neutral winds that causes plasma convergence at equatorial F region could be a major source for the positive storms, with the downwelling effect of the winds and zero or westward electric field, if present, acting as minor sources.

Acceleration of radiation belts electrons by oblique chorus waves

The redistribution of energy during the recovery phase of geomagnetic storms related to the acceleration of electrons in the Earth's outer radiation belt by cyclotron‐resonant chorus waves is an important and challenging topic of magnetospheric plasma physics. An approximate analytical formulation of energy diffusion coefficients is derived in this paper, on the basis of a quasi‐linear formalism valid for large enough bandwidths or for successive random scatter by uncorrelated waves of different frequencies and moderate average amplitudes. We make use of chorus wave parameterizations derived from CLUSTER measurements to show that oblique whistler waves can significantly increase the energy diffusion rate of small pitch angle electrons on the dayside. On the other hand, the energization rate of the more numerous high pitch angle electrons is typically reduced by a factor of 2 on the dayside, while it remains nearly unchanged on the nightside where high‐intensity waves are less oblique. Besides, lifetimes are strongly reduced on the dayside, which could also impact the long‐term time‐integrated acceleration rates of injected electrons. Comparison between the analytical formulas and full numerical results demonstrates a good agreement and provides new scaling laws as a function of whistler mean frequency, plasma density and particle energy. It is also suggested that the enhancement of energy diffusion of low energy electrons (<100 keV) at small pitch angles with oblique waves could result in an intensification of wave growth at latitudes higher than 15°. This might contribute to explain high chorus intensities measured by CLUSTER on the dayside at high latitudes.

Intense dayside Joule heating during the 5 April 2010 geomagnetic storm recovery phase observed by AMIE and AMPERE

When the interplanetary magnetic field (IMF) is northward, dawnward, or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp of the magnetosphere. While these periods are usually considered “quiet,” they can lead to intense, but highly localized, energy deposition into the dayside ionosphere. We identify such an occurrence during a series of two geomagnetic storms on 5 April 2010. Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) for the first time as an input to the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) algorithm, we show that during the recovery phase of the first storm there is intense ionospheric Joule heating in the dayside polar regions. This is associated with an intense field‐aligned current pair near the noon meridian that is associated with northward IMF and a strong IMF By component. AMIE outputs are used to drive the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model to demonstrate that the intense levels of Joule heating can lead to anomalous thermospheric density enhancements and traveling disturbances.

Global-scale observations of ionospheric convection during geomagnetic storms

[1] The global effects on the ionosphere during periods of intense geomagnetic activity associated with geomagnetic storms are investigated using the Super Dual Auroral Radar Network (SuperDARN). The influence of the main and recovery phases of geomagnetic storms on ionospheric properties such as backscatter occurrence rates, velocity distributions, and convection patterns are presented. The evolution of magnetosphere and ionosphere parameters during the storms did not depend on the origin of the storm (e.g., a coronal mass ejection or a corotating interaction region). Instead, there was a continuum of response to the intensity of the driver. For example, we found a clear relationship between the most negative value of the southward component of the interplanetary magnetic field (IMF Bz) and the most negative value of the Sym-H index, which marks the end of the main phase of a storm. This is one of the first superposed epoch studies that analyzes the sunward/antisunward line-of-sight velocity as a function of magnetic local time for geomagnetic storms of various intensities. In the noon sector, before and during the main phase of the storms, the SuperDARN radars recorded faster antisunward ionospheric plasma drifts together with a significant increase in the number of ionospheric echoes. This is consistent with the expected increase in soft particle precipitation in the noon sector and with the reconnection electric field that occurs when the IMF Bz is strongly negative, as is the case during the main phase of storms. The SuperDARN echo occurrence in the noon sector returned to prestorm values early in the recovery phase. The overall response was similar in the midnight sector, except that the peak echo occurrence for the most intense storms was limited to a narrower time interval centered on the end of the main phase. There were reductions in the strong antisunward flows near local midnight observed during the main phase and early in the recovery phase, particularly for the intense storm class. Strong electric fields are applied in the nightside ionosphere during storms, and the decameter structures from which SuperDARN scatter are more easily produced. However, in regions of energetic auroral precipitation and after a long exposure to strong electric fields, there is often a reduction in SuperDARN echoes due to absorption or changes in radio wave propagation.

EMIC waves observed at geosynchronous orbit during solar minimum: Statistics and excitation

[1] We identified 1875 wave events in magnetic field data from geosynchronous orbit. Most of these events were transverse with respect to the background magnetic field, left-hand polarized, and were observed in the post-noon magnetic local time sector at frequencies just below the helium gyrofrequency. Combined, these observations strongly suggest that most of these events are Electromagnetic Ion Cyclotron (EMIC) waves. Average wave amplitudes are presented, binned by frequency, geomagnetic activity and magnetic local time. The amplitude increases with increasing geomagnetic activity; increased activity also narrows the local time sector in which the waves are observed. A superposed epoch analysis of solar wind parameters and geomagnetic activity indices shows that 12 hours before wave onset the AE and Kp index increased, indicating storm and substorm activity that injects hot ion populations needed to drive the EMIC instability and providing ample time for those populations to drift into the post-noon local time sector. Just before wave onset a sudden enhancement in the AE index and the solar wind dynamic pressure are observed, indicating that a final perturbation of the magnetosphere is needed to excite EMIC wave growth. EMIC waves are thought to cause loss of relativistic particles in the radiation belt. Large solar wind densities have been associated with low flux of relativistic particles during the recovery phase of geomagnetic storms. We show that EMIC waves are preferentially generated during intervals of large solar wind density, indicating that such conditions drive EMIC waves which in turn cause enhanced loss of relativistic particles.

ULF wave characteristics at geosynchronous orbit during the recovery phase of geomagnetic storms associated with strong electron acceleration

[1] We identified 17 geomagnetic storms between January 2007 and December 2008. Using particle measurements, each storm is classified as a high-flux event or a low-flux event. High-flux events are those storms associated with enhanced flux of relativistic electrons at geosynchronous orbit, and low-flux events show no such enhancement. We study the characteristics of ultralow frequency waves between 0 and 80 mHz during the recovery phase of the high-flux storms using magnetic field data. By determining the wave propagation direction and magnetic perturbation direction, we are able to calculate how the power is distributed between toroidal, poloidal, and compressional wave modes. We find that on the nightside most wave power is compressional at all frequencies below 80 mHz. On the dayside compressional wave power dominates the lowest frequencies below 20 mHz; toroidal waves carry most energy between 20 and 50 mHz, and poloidal waves are associated with higher power levels between 50 and 80 mHz. This suggests that on the nightside electrons are predominantly transported and energized by mechanisms involving the compressional wave mode; on the dayside, electron energization is achieved by acceleration mechanisms involving toroidal waves and mechanisms dependent on poloidal and compressional waves.

Temporal variations and spatial extent of the electron density enhancements in the polar magnetosphere during geomagnetic storms

In order to understand temporal variations and spatial distributions of plasma density enhancements in the polar magnetosphere during geomagnetic storms, nearly simultaneous observations of storm time electron densities in the polar magnetosphere by the Akebono satellite and ion upflows in the polar ionosphere by the DMSP satellites were investigated. Akebono observations show that the electron densities were highest (>100 cm−3 at ∼9000 km altitude) from the main to early recovery phases of geomagnetic storms. The regions of enhanced electron density were not localized but widely spread in the polar magnetosphere. Coordinated observations by the DMSP satellites detected ion upflows with large fluxes (∼1010/cm2/s) in and near the cusp, when the electron density enhancements were observed by the Akebono satellite. This result indicates that the storm time electron density enhancements are caused by cleft ion fountain mechanisms from the polar ionosphere. Very low‐energy component (<13 eV) of the cleft ion fountain drifted deep into the polar cap, and increased the plasma densities in a wide region of the polar cap and auroral zone. A large amount of the very low‐energy plasma may flow out through the polar magnetosphere during the main phase of geomagnetic storms.

Simultaneous appearance of isolated auroral arcs and Pc 1 geomagnetic pulsations at subauroral latitudes

We have been conducting observations of aurora and geomagnetic pulsations at Athabasca, Canada, located at a subauroral latitude (magnetic latitude: 62°, L ∼ 4.6), using an all‐sky imager and an induction magnetometer. Isolated auroral arcs at wavelengths of 557.7 nm, 630.0 nm, and 486.1 nm (Hβ) were often observed at latitudes separated equatorward from the main auroral oval. From a 1‐year observation (4 September 2005 to 3 September 2006), we found 13 isolated arc events. All these isolated arcs occurred coincidentally with Pc 1 geomagnetic pulsations, although there were nine other Pc 1 events without isolated arcs in the field of view of the imager. The arcs were observed in both pre‐ and post‐midnight sectors and tended to appear during the late recovery phase of geomagnetic storms. The isolated arcs had limited latitudinal and longitudinal widths of less than 230 km and 250–800 km, respectively. We found that as isolated arcs moved equatorward (poleward), the frequencies of the simultaneous Pc 1 pulsations increased (decreased). Using the Tsyganenko‐02 magnetic field model, the observed Pc 1 frequencies were almost the same as the frequencies of He+ electromagnetic ion cyclotron (EMIC) waves at the equatorial plane connected to observed isolated arcs. These results indicate that interactions of spatially localized EMIC waves with ring current ions cause high‐energy ion precipitation and associated isolated auroras at subauroral latitudes. These results also imply that the dynamics and instabilities in the inner magnetosphere can be monitored as low‐latitude auroral emissions away from the ordinary auroral oval.

Characterization of relativistic electron flux rise times during the recovery phase of geomagnetic storms as measured by the NS41 GPS satellite

Intense relativistic electron enhancements in the Earth's radiation belts are observed during periods of enhanced geomagnetic activity. Different physical processes—identified as being responsible for these enhancements—would lead to different characteristic rise times of the electron fluxes. Here we present for the first time MeV electron flux rise times near the equator as estimated from 5 1 2 years of data from the NS41 Global Positioning System (GPS) satellite, in an effort to relate measured electron flux rise timescales with those predicted by theory. The GPS orbit crosses the heart of the radiation belts, covering the L>4 region and measuring equatorial fluxes at L∼4.2. We have calculated L* values using the Tsyganenko 2001 storm magnetic field model and have limited our study to the equator by imposing L*=4–4.5. Forty events, for which fluxes rise by a factor of 5 or more after the storm main phase, were selected from the analysis of the >1.22 MeV electron channel. The main results of our study are as follows: (1) the electron flux rise time distribution is very large indicating that there are a large variety of events observed at GPS orbit, similar to that observed at GEO, (2) fluxes rise in two phases, an initial fast rise is observed where most of the flux increase takes place, followed by a slower increase to the maximum flux, (3) fluxes gain 1–2 orders of magnitude on a timescale of 1–2 days, on average, in good agreement with timescales predicted by electron-chorus resonant interaction in quasi-linear theory using average wave characteristics for AE>500 nT, and (4) the direction of the IMF Bz could be an important parameter in determining the behavior of the flux of relativistic electrons during the recovery phase.

Magnetosphere inflation during the recovery phase of geomagnetic storms as an excellent magnetic confinement of killer electrons

Extreme flux enhancement of outer radiation belt electrons was observed at geosynchronous orbit during the recovery phase of a large magnetic storm on 27–30 July 2004. The storm main phase is driven by a very fast magnetic cloud of ∼1000 km/s associated with a coronal mass ejection (CME). The high‐speed stream of ∼600 km/s originated from a coronal hole follows the magnetic cloud and the coronal hole stream (CHS) is rarefied due to the speed difference between the CME and CHS. During the storm recovery phase, the magnetosphere is surrounded by the very low‐density CHS, causing an inflation of the magnetosphere. It is found that such a combination of the CME and CHS can be one of the most dangerous solar wind structures for the outer radiation belt, and the associated very low dynamic pressure can cause the magnetosphere inflation during the storm recovery phase as an excellent magnetic confinement of killer electrons.