Storm Initial Phase Research Articles

Differences in Ionospheric O+ and H+ Outflow During Storms With and Without Sawtooth Oscillations

AbstractPrevious simulations have suggested that O+ outflow plays a role in driving the sawtooth oscillations. This study investigates the role of O+ by identifying the differences in ionospheric outflow between sawtooth and non‐sawtooth storms using 11 years of FAST/Time of flight Energy Angle Mass Spectrograph (TEAMS) ion composition data from 1996 through 2007 during storms driven by coronal mass ejections. We find that the storm's initial phase shows larger O+ outflow during non‐sawtooth storms, and the main and recovery phases revealed differences in the location of ionospheric outflow. On the pre‐midnight sector, a larger O+ outflow was observed during the main phase of sawtooth storms, while non‐sawtooth storms exhibited stronger O+ outflow during the recovery phase. On the dayside, the peak outflow shifts significantly toward dawn during sawtooth storms. This strong dawnside sector outflow during sawtooth storms warrants consideration.

Low-latitude ionospheric responses and positioning performance of ground GNSS associated with the geomagnetic storm on March 13–14, 2022

Interplanetary coronal mass ejections (ICMEs) and the driven geomagnetic storms have a profound influence on the ionosphere, potentially leading to a degradation in positioning performance. In this study, we made a comprehensive analysis of the entire process of the impact of a typical ICME and its driven geomagnetic storm on the low-latitude ionosphere during March 13–14, 2022 (π-day storm) and the positioning performance of Global Navigation Satellite System (GNSS). During the passage of the ICME event, significant ionospheric scintillation, and TEC (total electron content) disturbances were observed in the low-latitude Hong Kong region. The ICME sheath region intensively compressed the magnetosphere via solar wind dynamic pressure enhancement and subsequently drove the storm main phase. It is found that both the magnetospheric compression that formed the storm initial phase and the storm main phase caused ionospheric scintillation. In comparison, the intensity of the ionospheric scintillation caused by the intense magnetospheric compression just before the storm main phase is even more pronounced. We also analyzed the impact of storms on standard point positioning (SPP), precise point positioning (PPP) and real-time kinematic (RTK) techniques. The positioning accuracies of single-frequency SPP and PPP experienced the most severe decline, and there was a noticeable increase in the initialization time for dual-frequency static PPP and RTK during the event. RTK demonstrated a shorter convergence time and higher accuracy during this event, but it was limited to short-baseline RTK (<30 km).

Investigation of Interhemispheric Asymmetry of the Thermospheric Composition Observed by GOLD During the First Strong Geomagnetic Storm in Solar‐Cycle 25, 1: IMF By Effects

AbstractAn interhemispheric asymmetry of thermospheric Oxygen (O) to Nitrogen (N2) column density ratio (∑O/N2) variations was observed by the Global‐scale Observations of the Limb and Disk (GOLD) during the 3–5 November 2021 geomagnetic storm. ∑O/N2 depletion is more equatorward in the local morning than in the local afternoon in the northern hemisphere (NH), while more equatorward in the local afternoon than the local morning in the southern hemisphere (SH). The Thermosphere Ionosphere Electrodynamics General Circulation Model simulations reproduced the observed ∑O/N2 asymmetry. The impacts of interplanetary magnetic field (IMF) east‐west component (By) on the interhemispheric asymmetry of ∑O/N2 depletion was investigated. Comparisons of simulation results with and without real By show that the dominant positive IMF By results in the Joule heating maximum occurring in the local afternoon of NH and local morning in the SH during the storm initial phase, which generates the corresponding ∑O/N2 changes. Additionally, the IMF By during this storm are more crucial in the SH than in the NH. Without By, equatorward penetration of the ∑O/N2 depletion in GOLD Field of View are almost same in the SH. However, in the NH, there is still more equatorward depletion in the local morning than the local afternoon without By.

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

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

Investigating the Development of Localized Neutral Density Increases During the 24 August 2005 Geomagnetic Storm

AbstractWe investigate localized thermospheric neutral density increases (or density spikes) developed during the 24 August 2005 geomagnetic storm and utilize a multi‐instrument database for monitoring the underlying thermospheric, ionospheric, magnetospheric, and solar wind conditions. We study five scenarios occurring under eastward/westward BY dominations and demonstrating the development of density spikes during various events. These scenarios show (1) a northern density spike associated with enhanced antisunward polar flows driven by dayside merging along old‐open field lines during the initial phase, (2) some southern density spikes associated with enhanced sunward auroral flows caused by auroral brightening events during the storm initial phase and main phase, (3) two pairs of southern dayside‐nightside density spike configuration associated with enhanced antisunward polar flows caused by dayside merging and nightside magnetotail reconnections along old‐open field lines during the underlying substorm activity, and (4) a series of enhanced polar sunward flows developed in the polar cap driven by different processes during the recovery phase. We provide a detailed analysis of these events including the specification of underlying field aligned current systems, flow channel types, and auroral forms. Observational results demonstrate the various density spikes and their associated auroral forms and/or flow channels and thus provide a better understanding of their development during this intense geomagnetic storm.

Daytime geomagnetic disturbances at high latitudes during a strong magnetic storm of June 21–23, 2015: The storm initial phase

The high-latitude geomagnetic effects of an unusually long initial phase of the largest magnetic storm (SymH ~–220 nT) in cycle 24 of the solar activity are considered. Three interplanetary shocks characterized by considerable solar wind density jumps (up to 50–60 cm–3) at a low solar wind velocity (350–400 km/s) approached the Earth’s magnetosphere during the storm initial phase. The first two dynamic impacts did not result in the development of a magnetic storm, since the IMF Bz remained positive for a long time after these shocks, but they caused daytime polar substorms (magnetic bays) near the boundary between the closed and open magnetosphere. The magnetic field vector diagrams at high latitudes and the behaviour of high-latitude long-period geomagnetic pulsations (ipcl and vlp) made it possible to specify the dynamics of this boundary position. The spatiotemporal features of daytime polar substorms (the dayside polar electrojet, PE) caused by sudden changes in the solar wind dynamic pressure are discussed in detail, and the singularities of ionospheric convection in the polar cap are considered. It has been shown that the main phase of this two-stage storm started rapidly developing only when the third most intense shock approached the Earth against a background of large negative IMF Bz values (to–39 nT). It was concluded that the dynamics of convective vortices and the related restructing of the field-aligned currents can result in spatiotemporal fluctuations in the closing ionospheric currents that are registered on the Earth’s surface as bay-like magnetic disturbances.

Cluster observations of unusually high concentration of energetic O+ carried by flux ropes in the nightside high‐latitude magnetosheath during a storm initial phase

AbstractWe present measurements from Cluster spacecraft to investigate the energetic singly charged oxygen ions, O+, within the flux ropes in the nightside high‐latitude magnetosheath during the initial phase of an intense storm on 24 October 2011. Three magnetic flux ropes were identified by Cluster 4 in the intervals from 20:10 UT to 20:20 UT. Unusually, large number density of energetic O+ ions at energy of tens of keV was detected within these flux ropes. The number density of O+ ions was above 0.1 cm−3 and the maximum value was about 0.25 cm−3, 1 order of magnitude larger than the ambient value (~0.01 cm−3) in the magnetosheath. The O+/H+ ratio is as large as ~0.08 within the flux ropes. Enhanced convection electric fields Ey (~10 mV/m) are associated with the flux rope and the high concentrations of energetic O+. The flux ropes, which are presumably produced by magnetic reconnection at the dayside magnetopause or cusp, are convected at a larger velocity than the tailward velocity of ambient flows in the magnetosheath. These observations together show that abundant energetic O+ ions are carried by the flux ropes toward tail in the nightside magnetosheath. Our observations present new evidence for a chain linking the dayside to the nightside in the global O+ transport process.

Large magnetic storms as viewed by TWINS: A study of the differences in the medium energy ENA composition

AbstractDuring large geomagnetic storms (Dst ≤−100 nT), oxygen can become a significant component of the energetic particles of the inner magnetosphere. Until recently, there were no available global observations of the medium energy (<50 keV) oxygen populations. Using observations from the Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) Energetic Neutral Atom (ENA) imagers we present a study of nine large storms of solar cycle 24 as a function of storm phase. For these storms we observe that the H and O ENA fluxes and their temperatures increase in tandem during the storm's initial phase. However, there is no increase in the O+/H+ ratio in the inner magnetosphere until the storm main phase. Also seen during the main phase is an energy dispersion with higher‐energy (32 keV) H ENAs seen before the arrival of O ENAs of the same energy. The O ENAs take longer to return to prestorm levels during the recovery phases. This longer recovery time is likely because of the large difference between the storm time and prestorm O populations compared to H (i.e., there is always some prestorm H in the inner magnetosphere, but effectively no O prestorm). These results imply that medium‐energy O ENAs evolve over long time scales (hours to days) as opposed to the shorter substorm time scales of the higher‐energy (>52 keV) O ENAs.

Studying ionospheric responses to magnetic storms depending on the local time of the storm main phase onset

A morphological analysis of vertical sounding data obtained in Irkutsk from 2003 to 2008 has been performed. The AE index was used to determine the geomagnetic activity level, and the storm main phase onset was registered based on the D st index. The ionospheric response to a magnetic storm was estimated based on the relative deviation of the critical frequency and altitude of the ionospheric F2 region from the median values. Superstrong magnetic storms and storms without positive initial phases were not considered when the data were selected. We found that positive ionospheric disturbances, which were accompanied by an increase in the F2 region maximum altitude, predominated between the storm initial phase and main phases during all considered magnetic storms. Between these storm phases, negative disturbances were only registered at night. Predominance of positive ionospheric disturbances over negative ones can be related to the selection of storms for studies.

High-latitude geomagnetic disturbances during the initial phase of a recurrent magnetic storm (from February 27 to March 2, 2008)

A complex of geophysical phenomena (geomagnetic pulsations in different frequency ranges, VLF emissions, riometer absorption, and auroras) during the initial phase of a small recurrent magnetic storm that occurred on February 27–March 2, 2008, at a solar activity minimum has been analyzed. The difference between this storm and other typical magnetic storms consisted in that its initial phase developed under a prolonged period of negative IMF Bz values, and the most intense wave-like disturbances during the storm initial phase were observed in the dusk and nighttime magnetospheric sectors rather than in the daytime sector as is observed in the majority of cases. The passage of a dense transient (with Np reaching 30 cm−3) in the solar wind under the southward IMF in the sheath region of the high-speed solar wind stream responsible for the discussed storm caused a great (the AE index is ∼1250 nT) magnetospheric substorm. The appearance of VLF chorus, accompanied by riometer absorption bursts and Pc5 pulsations, in a very long longitudinal interval of auroral latitudes (L ∼ 5) from premidnight to dawn MLT hours has been detected. It has been concluded that a sharp increase in the solar wind dynamic pressure under prolonged negative values of IMF Bz resulted in the global (in longitude) development of electron cyclotron instability in the Earth’s magnetosphere.

The properties of two solar wind high speed streams and related geomagnetic activity during the declining phase of solar cycle 23

Two high speed stream (HSS) solar wind intervals (days 283–294 and 314–318, 2003, hereafter called Events 1 and 2) during the declining phase of solar cycle 23 have been examined in detail for their interplanetary characteristics and their resultant geomagnetic activity. Event 1 had an associated storm initial phase with peak Dst=+9 nT. This was caused by a high plasma density heliospheric plasma sheet (HPS) which impacted the magnetosphere. The southward component of IMF Bz fluctuations in the corotating interaction regions (CIRs) of both Events 1 and 2 led to peak storm main phases of Dst=−85 and −62 nT, respectively. The extended storm “recovery” phases were associated with Δ B/ B o ∼1–2 Alfvénic fluctuations in the HSS proper. High-intensity, long-duration, continuous AE (HILDCAA) intervals were present, presumably due to the southward component of the Alfvén waves. The IMF Bx–Vx 4-h cross-correlation values were >0.8 in Event 2, and lower, >0.6, in Event 1. The difference in Alfvénicity between the two HSS events is not understood at present. The IMF Bz 10-min to 3-h variances ( σ z 2 ) and are highest during the CIRs. The normalized variances ( σ z 2 / B o 2 ) during the HSS proper are approximately the same as those for the CIRs. For Event 1, the 1-h IMF σ z 2 and σ z 2 / B o 2 are 5.0 nT 2 and 1.1×10 −1, respectively. The IMF Bz-AE cross-correlation (c.c.) coefficients during the storm main phase of Event 1 and for 24-h of the HSS of Event 2 give similar results. For the Event 1, a peak c.c. of −0.4 occurred with a lag of 103 min, and for Event 2 a peak c.c of −0.38 with a lag of 67 min was obtained. Both c.c. results were sharply peaked. The decay-portion of a HSS prior to Event 1 was characterized by low Np, low B o and low Alfvén wave amplitudes. The 1-h IMF σ z 2 and σ z 2 / B o 2 were 0.84 nT 2 and 2.9×10 −2, respectively. This quiet interplanetary interval led to a quiet geomagnetic activity period (AE<100 nT, Dst ∼+5 nT). On the other hand, what is quite surprising is that this region was the most purely “Alfvénic” interval studied (c.c. of Bx-Vx=0.95). The ε parameter was calculated using both GSE and GSM coordinates. It was found that ε is ∼30% larger for GSM coordinates. Thus, the major cause of geomagnetic activity during HSSs is the large amplitude Alfvén waves but not coordinate transformations. Sector polarities (IMF By values) may be a secondary factor. However, other models, like the tilted solar dipole, should be considered as well.

Variations in the ULF index of geomagnetic pulsations during strong magnetic storms

The level of wave geomagnetic activity in the morning, afternoon, and nighttime sectors during strong magnetic storms with Dst varying from -100 to -150 nT has been statistically studied based on a new ULF wave index. It has been found out that the intensity of geomagnetic pulsations at frequencies of 2- 7 mHz during the magnetic storm initial phase is maximal in the morning and nighttime sectors at polar and auroral latitudes, respectively. During the magnetic storm main phase, wave activity is maximal in the morn� ing sector of the auroral zone, and the pulsation intensity in the nighttime sector is twice as low as in the morning sector. It has been indicated that geomagnetic pulsations excited after substorms mainly contribute to a morning wave disturbance during the magnetic storm main phase. During the storm recovery phase, wave activity develops in the morning and nighttime sectors of the auroral zone; in this case nighttime activity is also observed in the subauroral zone.

Auroral zone dayside precipitation during magnetic storm initial phases

Significant charged-particle precipitation occurs in the dayside auroral zone during and after interplanetary shock impingements on the Earth's magnetosphere. The precipitation intensities and spatial and temporal evolution are discussed. Although the post-shock energy flux (10– 20 erg cm −2 s −1 ) is lower than that of substorms, the total energy deposition rate may be considerably greater (∼ an order of magnitude) than nightside energy rates due to the greater area of the dayside portion of the auroral oval (defined as extending from 03 MLT through noon to 21 MLT). This dayside precipitation represents direct solar wind energy input into the magnetosphere/ionosphere system. The exact mechanisms for particle energization and precipitation into the ionosphere are not known at this time. Different mechanisms are probably occurring during different portions of the storm initial phase. Immediately after shock compression of the magnetosphere, possible precipitation-related mechanisms are: (1) betatron compression of preexisting outer zone magnetospheric particles. The anisotropic plasma is unstable to loss-cone instabilities, leading to plasma wave growth, resonant particle pitch-angle scattering and electron and proton losses into the upper ionosphere. (2) The compression of the magnetosphere can also lead to enhanced field-aligned currents and the formation of dayside double-layers. Finally (3) in the latter stages of the storm initial phase, there is evidence for a long-lasting viscous-like interaction occurring on the flanks of the magnetopause. Ground-based observations identifying the types of dayside auroral forms would be extremely useful in identifying the specific solar wind energy transfer mechanisms.

Auroral electrojets during geomagnetic storms

On the basis of digital magnetometers from the International monitor auroral geomagnetic effects (IMAGE) and European incoherent scatter (EISCAT) meridional chains in Scandinavia dynamics of the eastward and westward electrojets during the main phase of magnetic storms are considered. For the intense magnetic storm on May 10–11, 1992, with Dst = −300 nT, magnetograms of subauroral and midlatitudinal stations Leningrad, Borok, and Moscow were examined. It is found that the eastward electrojet center during the storm main phase shifts equatorward as |Dst| increases. The electrojet center is located at the corrected geomagnetic latitude Φ ∼ 59°–60° when Dst ∼ −100 nT and at Φ ∼ 54°–55° when Dst ∼ −300 nT. Data from meridional chains of magnetometers support earlier results pertaining to the relationship between the westward electrojet center position and the ring current intensity for intervals between substorms. During substorms expansive phases the westward electrojet expands poleward covering auroral latitudes Φ ∼ 65°. The electrojets location during the storm main phase and their dynamics in connection with substorms allow for interpretations of effects described in the literature: the AE indices saturation during the main phase of magnetic storms; approximately equal values of AU and AL indices during the storm initial phase and AL ≫ AU during the storm main phase.

Response of equatorial ionosphere to the transit of interplanetary magnetic cloud of January 13–15, 1967. Transient disturbance in F region

Quarter-hourly ionograms of Kodaikanal (10°4′N, 77°29′E, dip 3°N) are analysed to evaluate the response of equatorial ionosphere to the severe geomagnetic storm (| Dst|max, 176 nT) associated with the transit of an interplanetary magnetic cloud at Earth during January 13–15, 1967. It is shown that during the early stage of the storm initial phase on January 13, which corresponded to the local dusk-midnight sector, the entire bottomside F region over Kodaikanal experienced a sudden and spectacular downward drift for a short-duration of ∼2 h (18:45 – 20:30 L.T.). The decrease in height of F layer peak ( hm) by 223 km over the interval 18:45–20:30 L.T. implied a gross apparent downward drift of ∼35.4 m s −1. The height disturbance occured during the recovery phase of an auroral substorm (bay disturbance) that developed at the commencement of the geomagnetic storm. It also displayed an apparent temporal association with a prominent northward swing in IMF Bz and decrease in polar cap potential drop, φ, estimated from IMF parameters. The anomalous and rapid post-sunset descent of F layer was accompanied by an increase in electron density at and below the F layer peak. The evident perturbation in F region height near the dip Equator is interpreted as the signature of a transient westward electric field disturbance generated by the decrease in magnetospheric convection (decrease in polar cap potential) during the recovery phase of the substorm. The values of vertical downward drift derived from the time rate of change of h′F and corrected for chemical loss effects indicated the maximum amplitude of the westward electric fields to be ∼1.9 mV m −1. The increase in Nm is shown to be the outcome of the ionization convergence rate induced by the abnormally large downward drift exceeding the chemical loss rate ( βNe) at altitudes ⩾250 km.

Ionospheric storm of 4–5 August 1972 in the Asia-Australia-Pacific sector

The ionospheric storm of 4–5 August 1972 is analyzed using data from 35 middle and low latitude ionospheric stations and 7 magnetic stations in the Asia-Australia-Pacific longitude sector. The largest ionospheric and geomagnetic disturbances were both observed in the mid-Pacific. Strong upward electrodynamic drifts occurred in the western Pacific during the initial phase of the storm. True-height profiles of electron density in the Pacific area show that the ionosphere was raised during the first several hours after the geomagnetic storm sudden commencement, that large variations later occurred in the mid-Pacific, possibly caused by wave-like thermospheric winds, and that the F-layer was reduced in density and raised in height during the main phase of the storm on the following day. Synoptic pictures of the variations in foF2 are presented, showing that electron density enhancements occurred during the storm initial phase in the western Pacific at middle latitudes, peaking in the 20°–45° geomagnetic latitude range, and becoming density deficits during the storm main phase. The observed variations are interpreted in terms of possible physical mechanisms.

High-latitude proton precipitation and light ion density profiles during the magnetic storm initial phase

Measurements of precipitating protons and light ion densities by experiments on Ogo 4 indicate that widespread proton precipitation occurs in predawn hours during the magnetic storm initial phase from the latitude of the high-latitude ion trough, or plasmapause, up to Λ > 75°. A softening of the proton spectrum is apparent as the plasmapause is approached. The separation of the low-latitude precipitation boundaries for 7.3-kev and 23.8-kev protons is ≲1°, compared with a 3.6° separation that has been computed by using the formulas of Gendrin and Eather and Carovillano. Consideration of probable proton drift morphology leads to the conclusion that protons are injected in predawn hours, widespread precipitation occurring in the region outside the plasmapause. Protons less energetic than ∼7 kev drift eastward, whereas the more energetic protons drift westward, producing the observed dawn-dusk asymmetry for the lower-energy protons.