Moderate Geomagnetic Storm Research Articles

A case study of isolated auroral spots based on DMSP data

This study employed ultraviolet images and particle data to investigate isolated auroral spots away from the Earth's auroral oval. Data from SSUSI (Special Sensor Ultraviolet Spectrographic Imager) and SSJ (Special Sensor J) mounted on the DMSP (Defence Meteorological Satellite Program) spacecraft were examined. The isolated auroral spots were observed by DMSP F16/SSUSI and F17/SSUSI on 29 May 2010 during the recovery phase of a moderate geomagnetic storm with a minimum SYM-H index of −70 nT. The auroral spots were observed between 18:00–21:00 MLT and corotated with the Earth, but stayed almost at the same magnetic latitude (MLAT) of −60°. It is found that the isolated auroral spots were produced mainly by energetic ring current ions at energies above ~10 keV. The enhancement in the electron flux with energy below ~200 eV was also observed for the isolated auroral spots. The MLAT of the electron flux was nearly 2°higher than that for the precipitating ions.

Earth's Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era

AbstractDiscovery of the Earth's Van Allen radiation belts by instruments flown on Explorer 1 in 1958 was the first major discovery of the Space Age. The observation of distinct inner and outer zones of trapped megaelectron volt (MeV) particles, primarily protons at low altitude and electrons at high altitude, led to early models for source and loss mechanisms including Cosmic Ray Albedo Neutron Decay for inner zone protons, radial diffusion for outer zone electrons and loss to the atmosphere due to pitch angle scattering. This scattering lowers the mirror altitude for particles in their bounce motion parallel to the Earth's magnetic field until they suffer collisional loss. A view of the belts as quasi‐static inner and outer zones of energetic particles with different sources was modified by observations made during the Solar Cycle 22 maximum in solar activity over 1989–1991. The dynamic variability of outer zone electrons was measured by the Combined Radiation Release and Effects Satellite launched in July 1990. This variability is caused by distinct types of heliospheric structure that vary with the solar cycle. The launch of the twin Van Allen Probes in August 2012 has provided much longer and more comprehensive measurements during the declining phase of Solar Cycle 24. Roughly half of moderate geomagnetic storms, determined by intensity of the ring current carried mostly by protons at hundreds of kiloelectron volts, produce an increase in trapped relativistic electron flux in the outer zone. Mechanisms for accelerating electrons of hundreds of electron volts stored in the tail region of the magnetosphere to MeVenergies in the trapping region are described in this review: prompt and diffusive radial transport and local acceleration driven by magnetospheric waves. Such waves also produce pitch angle scattering loss, as does outward radial transport, enhanced when the magnetosphere is compressed. While quasilinear simulations have been used to successfully reproduce many essential features of the radiation belt particle dynamics, nonlinear wave‐particle interactions are found to be potentially important for causing more rapid particle acceleration or precipitation. The findings on the fundamental physics of the Van Allen radiation belts potentially provide insights into understanding energetic particle dynamics at other magnetized planets in the solar system, exoplanets throughout the universe, and in astrophysical and laboratory plasmas. Computational radiation belt models have improved dramatically, particularly in the Van Allen Probes era, and assimilative forecasting of the state of the radiation belts has become more feasible. Moreover, machine learning techniques have been developed to specify and predict the state of the Van Allen radiation belts. Given the potential Space Weather impact of radiation belt variability on technological systems, these new radiation belt models are expected to play a critical role in our technological society in the future as much as meteorological models do today.

Equatorial and low-latitude positive ionospheric phases due to moderate geomagnetic storm during high solar activity in January 2013

The day-to-day variability of the equatorial and low-latitude ionosphere during quiet and disturbed periods is one of the ionospheric highlighted Space Weather research topics, particularly the ionospheric electrodynamics during geomagnetic storms. This study investigates the response of ionospheric F-region from the equatorial region to beyond the Equatorial Ionization Anomaly (EIA) crest during moderate geomagnetic storm (minimum Dst = −53 nT) that took place on January 17 to 18, 2013, during the high solar activity period of solar cycle 24. The Total Electron Content (TEC) obtained through a network of 82 dual frequency GPS receivers, spanning over an area of 30° × 30° in latitude and longitude are used. Also the F-layer virtual height (h’F) and critical frequency (foF2) observations from 3 ionosondes, in the South American sector are used. Specifically, these GPS-TEC receivers and ionosondes are used to investigate how the F-layer was disturbed by two positive ionospheric phases occurred during the aforementioned disturbed period. The first positive ionospheric phase was probably due to a travelling ionospheric disturbance (TID). When this TID reached the Brazilian coast at low-latitude, the EIA crest was in the growth phase and makes it challenging to separate the spatial-temporal evolution of both phenomena. The second positive ionospheric phase was caused by an anomalous nighttime equatorial positive ionospheric cloud travelling from the east sector towards the west sector. In addition, how the EIA was disturbed by these two positive ionospheric phases in the eastern and western Brazilian sectors is also investigated.

Moderate Geomagnetic Storm Condition, WAAS Alerts and real GPS Positioning Quality

The most significant part of the Wade Area Augmentation System (WAAS) integrity data consists of the User Differential Range Error (UDRE) and the Grid Ionospheric Vertical Error (GIVE). WAAS solutions are not completely appropriate to determine the GIVE term within the entire wade area coverage zone taking in account real irregular structure of the ionosphere. It leads to the larger confidence bounding terms and lower expected positioning availability in comparison to the reality under geomagnetic storm conditions and system outages. Thus a question arises: is the basic WAAS concept appropriate to provide the same efficiency of the integrity monitoring for both “global differential correction (i.e. clock, ephemeris etc)” and “local differential correction (i.e. ionoshrere, troposhpere and multipath)”? The aim of this paper is to compare official WAAS integrity monitoring reports and real positioning quality in US coverage zone (CONUS) and Canada area under geomagnetic storm conditions and system outages. In this research we are interested in comparison between real GPS positioning quality based on one-frequency C/A ranging mode and HAL (VAL) values which correspond to the LP, LPV and LPV200 requirements. Significant mismatch of the information between WAAS integrity data and real positioning quality was unfolded as a result of this comparison under geomagnetic storms and system outages on February, 2011 and June 22, 2015. Based on this result we think that in order to achieve high confidence of WAAS positioning availability alerts real ionospheric measurements within the wide area coverage zone must be involved instead of the WAAS GIVE values. The better way to realize this idea is to combine WAAS solutions to derive “global differential corrections” and LAAS solutions to derive “local differential corrections”.

Distribution of intense, moderate and weak geomagnetic storms over the solar cycle

The geomagnetic storms observed during different phases of a solar cycle have distinct primary solar sources. Therefore, we have statistically examined, in the present study, how the geomagnetic storms of different intensities are distributed over a solar cycle. We have considered previous four cycles, viz. cycle 21, 22, 23 and 24. We selected all the storms which were observed during these cycles and classified them into three categories as: intense (Dst ≤ − 100 nT), moderate (− 100 ≤ Dst ≤ − 50) and weak (− 50 ≤ Dst ≤ − 30). We found that geomagnetic storms are distributed in such a way that maximum number of intense storms occur around peak phase of a solar cycle, while maximum number of weak storms occur during the starting and ending phases of a cycle. About 73.20% of intense storms, 58.07% and 47.93% of moderate and weak storms occurred around the peak of a cycle. Therefore, it was concluded that peak phase is dominated by intense and very intense storms due to increase in frequency of occurrence of coronal mass ejections during such periods. At the same time, the ending phase of solar cycle is dominated by the occurrence of weak storms due to the frequent occurrences of fast solar wind streams during such periods. Moreover, the total number of storms occurring during the descending phase is greater than those observed during the ascending phase. The storms with peak Dst ≤ − 200 nT were classified as very intense and designated as outstanding Sun–Earth connections. It was found that 90% of such events occur around the peak phase of solar cycle. Therefore, it was concluded that the high-risk events occur during the peak phase of a solar cycle. Therefore, extreme care must be taken while launching new space missions and operating the existing ones, during this period.

Characteristics of storm time ion composition in the near-Earth plasma sheet using Geotail and RBSP measurements

Solar wind particles and ionospheric O+ ions influence the near-Earth plasma sheet and inner magnetospheric composition. We studied the behavior of H+, O+ and He+ ions (9–210 keV) for intense and moderate geomagnetic storms of solar cycle 23 and 24. An average energy density < ε > of ions over a given interval and flux enhancement is estimated using observations from satellites at different L values, namely STICS sensor on-board Geotail spacecraft and HOPE spectrometer on-board Radiation Belt Storm Probes. It provides a comprehensive understanding of the energy density variation of H+, O+ and He+ ions with the strength of IMF Bz, Psw, intensity of storms and L value. Statistically, we observed that (1) In the plasma sheet region, during main phase of the intense geomagnetic storm, < εO+/H+ > and < εHe+/H+ > enhances, (2) < εO+/H+ > is well correlated with Psw (CC = 0.86) and IMF Bz (CC = 0.85), (3) < εO+/H+ > shows higher correlation (CC = 0.73) with Kp than < εHe+/H+ > (CC = 0.65), indicating a fairly good dependence on the strength of geomagnetic activity, (4) < εO+/H+ > and < εHe+/H+ > dependence on L value indicates that O+/H+ and He+/H+ is more pronounced near L = 3. It is a cumulative extension of the previous studies on ion composition change which is in accordance with the existing picture of the plasma sheet and inner magnetosphere.

Improvements and validation of the IRI UP method under moderate, strong, and severe geomagnetic storms

This work describes several important improvements made to the International Reference Ionosphere UPdate (IRI UP) method, and a careful validation of its performances under disturbed conditions. The IRI UP method has been improved developing an algorithm capable to properly filter wrongly autoscaled ionosonde data to be assimilated, avoiding the use of these in the assimilation process. Furthermore, the preliminary quality check used to choose the variogram model in the Universal Kriging method has been replaced with a new quality check routine (NQCR), based on statistical tests carried out using the variables Q1, Q2, and cR, built on variogram’s residuals. NQCR objectively identifies the best variogram model from which to get more reliable effective indices maps to be ingested in the IRI model to obtain updated foF2 and hmF2 maps. IRI UP has been applied on 30 different time intervals, between January 1, 2004, and December 31, 2016, characterized by moderate, strong, and severe geomagnetic conditions, over the European region. A statistical comparison between IRI UP and IRI at the truth sites located at Fairford (51.7°N, 1.5°W, UK) and San Vito (40.6°N, 17.8°E, Italy), for foF2 and hmF2, has been performed. From the statistical validation clearly emerges how IRI UP, for foF2, performs significantly better than IRI, for each of the 30 geomagnetic storms considered. Regarding hmF2, IRI UP performances are lower than those for foF2, although still better than IRI ones. In the light of the results achieved in this investigation, the IRI UP method represents an interesting approach to Space Weather forecast in the ionospheric domain.

Unusual Generation of Localized EPB in the Dawn Sector Triggered by a Moderate Geomagnetic Storm

AbstractAn interesting piece of observation related to the equatorial plasma bubble (EPB) that is triggered by a geomagnetic storm at dawn sector on 4–5 February 2011 is presented. The storm activity was initiated at 17 UT when interplanetary magnetic field (IMF) Bz turned southward and the Dst index started decreasing (accompanied by AE intensifications) attaining its minimum at −70 nT. The IMF Bz presented large fluctuations marked by southward/northward transitions at slow and rapid rates and eventually returning to normal in the morning of 5 February. Observations from the very high frequency radar located at Gadanki, an off‐equatorial station; an all‐sky imager located at Kolhapur, a low‐latitude station; and an ionosonde at Tirunelveli, an equatorial station, are analyzed to study the impact of this storm on the postmidnight to presunrise equatorial spread F. The results demonstrate that EPB irregularities can be generated by penetration electric field during what can be characterized as a moderate storm event. In contrast to prevailing belief, the present results show that EPB/equatorial spread F can be generated by undershielding electric field in the dawn sector, caused by an unusually delayed polarity reversal of the penetration electric field. The results further reveal oscillatory behavior in the height of F layer topside irregularities in association with IMF Bz oscillations. Observation of radar echoes from nighttime E layer irregularity structures and their Doppler velocities representing the penetration electric field polarity and intensity are also a notable finding of this paper.

Long-lasting poloidal ULF waves observed by multiple satellites and high-latitude SuperDARN radars.

Poloidal ultra-low frequency (ULF) waves between 5-10 mHz were observed by multiple satellites and three high-latitude Super Dual Auroral Radar Network (SuperDARN) radars during the recovery phase of a moderate geomagnetic storm on Jan 24-27, 2016. The long-lasting ULF waves were observed in the magnetic field and energetic particle flux perturbations during three successive passes by two Geostationary Operational Environmental Satellites (GOES) through the dayside magnetosphere, during which plasmasphere expansion and refilling were observed by two Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes. The radial magnetic field oscillation was in phase (~ 180° out of phase) with the northward (southward) moving proton flux oscillation at 95 keV, consistent with high-energy drift-bounce resonance signatures of protons with second harmonic poloidal standing Alfvén waves. The longitudinal extent of the waves approached 10 hours in local time on the dayside and gradually decreased with time. High-time resolution (~ 6 s) data from three high-latitude SuperDARN radars show that the wave intensification region was localized in latitude with a radial extent of ~ 135-225 km in the subauroral ionosphere. No signature of these waves were observed by ground-based magnetometers colocated with the GOES satellites suggesting that the poloidal waves were high-m mode and thus screened by the ionosphere. During this interval one of the THEMIS probes observed a bump-on-tail ion distribution at 1-3 keV which we suggest is the source of the long-lasting second harmonic poloidal ULF waves.

Localized Increment and Decrement in the Total Electron Content of the Ionosphere as a Response to the April 20, 2018, Geomagnetic Storm

A moderate geomagnetic storm occurred on April 20, 2018. Using vertical total electron content (VTEC) maps provided by the Center for Orbit Determination in Europe, ionospheric responses to the geomagnetic storm could be identified in generated two-dimensional differential VTEC maps. During the day of the storm the enhancement of the equatorial ionization anomaly (EIA), product of the super-fountain effect was identified. A localized TEC enhancement (LTE) was also observed to the south of the EIA on April 20, 2018. It was also possible to visualize this LTE in a longitudinal section of the EIA as a third crest. The maximum increment of VTEC for the LTE was 204%. This LTE is quite unique because it happened during the expected solar cycle 24 and 25 minimum, and according to a previous study no LTE observation could be done for the last solar two-cycle minimum. The origin of the observed LTE is suggested to be partly product of the super-fountain effect. Finally, a localized TEC decrement (LTD) was observed towards the end of the day, April 20, 2018. Because this LTD consisted in the disappearance of the northern and southern crests of the EIA and this occurred during the recovery phase of the geomagnetic storm, it can be suggested that the LTD origin is due to the westward disturbance electric field. This mechanism was put forward by a past study that also analyzed the responses to a geomagnetic storm (the 2015 St. Patrick’s day storm), being one of the responses the inhibition of both crests of the EIA.

Determining the Mode, Frequency, and Azimuthal Wave Number of ULF Waves During a HSS and Moderate Geomagnetic Storm.

Ultralow frequency (ULF) waves play a fundamental role in the dynamics of the inner magnetosphere and outer radiation belt during geomagnetic storms. Broadband ULF wave power can transport energetic electrons via radial diffusion, and discrete ULF wave power can energize electrons through a resonant interaction. Using observations from the Magnetospheric Multiscale mission, we characterize the evolution of ULF waves during a high‐speed solar wind stream (HSS) and moderate geomagnetic storm while there is an enhancement of the outer radiation belt. The Automated Flare Inference of Oscillations code is used to distinguish discrete ULF wave power from broadband wave power during the HSS. During periods of discrete wave power and utilizing the close separation of the Magnetospheric Multiscale spacecraft, we estimate the toroidal mode ULF azimuthal wave number throughout the geomagnetic storm. We concentrate on the toroidal mode as the HSS compresses the dayside magnetosphere resulting in an asymmetric magnetic field topology where toroidal mode waves can interact with energetic electrons. Analysis of the mode structure and wave numbers demonstrates that the generation of the observed ULF waves is a combination of externally driven waves, via the Kelvin‐Helmholtz instability, and internally driven waves, via unstable ion distributions. Further analysis of the periods and toroidal azimuthal wave numbers suggests that these waves can couple with the core electron radiation belt population via the drift resonance during the storm. The azimuthal wave number and structure of ULF wave power (broadband or discrete) have important implications for the inner magnetospheric and radiation belt dynamics.

Why the Shock-ICME Complex Structure Is Important: Learning from the Early 2017 September CMEs

Abstract In the early days of 2017 September, an exceptionally energetic solar active region AR 12673 aroused great interest in the solar physics community. It produced four X class flares, more than 20 coronal mass ejections (CMEs), and an intense geomagnetic storm, for which the peak value of the Dst index reached up to −142 nT at 2017 September 8 02:00 UT. In this work, we check the interplanetary and solar source of this intense geomagnetic storm. We find that this geomagnetic storm was mainly caused by a shock-interplanetary CME (ICME) complex structure, which was formed by a shock driven by the 2017 September 6 CME propagating into a previous ICME, which was the interplanetary counterpart of the 2017 September 4 CME. To better understand the role of this structure, we conduct a quantitative analysis on the enhancement of ICME’s geoeffectiveness induced by the shock compression. The analysis shows that the shock compression enhanced the intensity of this geomagnetic storm by a factor of two. Without shock compression, there would have been only a moderate geomagnetic storm with a peak Dst value of ∼−79 nT. In addition, the analysis of the proton flux signature inside the shock-ICME complex structure shows that this structure also enhanced the solar energetic particle intensity by a factor of approximately five. These findings illustrate that the shock-ICME complex structure is a very important factor in solar physics study and space weather forecast.

The Acceleration of Ultrarelativistic Electrons During a Small to Moderate Storm of 21 April 2017

AbstractThe ultrarelativistic electrons (E &gt; ~3 MeV) in the outer radiation belt received limited attention in the past due to sparse measurements. Nowadays, the Van Allen Probes measurements of ultrarelativistic electrons with high energy resolution provide an unprecedented opportunity to study the dynamics of this population. In this study, using data from the Van Allen Probes, we report significant flux enhancements of ultrarelativistic electrons with energies up to 7.7 MeV during a small to moderate geomagnetic storm. The underlying physical mechanisms are investigated by analyzing and simulating the evolution of electron phase space density. The results suggest that during this storm, the acceleration mechanism for ultrarelativistic electrons in the outer belt is energy‐dependent: local acceleration plays the most important role in the flux enhancements of ~3–5 MeV electrons, while inward radial diffusion is the main acceleration mechanism for ~7 MeV electrons at the center of the outer radiation belt.

Variations in the Geomagnetic Cutoff Rigidity during the Magnetic Storm in March 2015

Variations in the planetary system of the geomagnetic cutoff rigidity during the moderate geomagnetic storm in March 2015 are calculated on the basis of data from cosmic-ray measurements at the world network of stations. The distance to the subsolar point and the radius of the ring current are calculated on the basis of the axisymmetric restricted Earth’smagnetosphere model that takes into account currents at the magnetopause and the ring current. The contribution of the ring current to variations in the geomagnetic cutoff rigidity and to the disturbance storm time (Dst) index is also determined within this model.

The Detached Auroras Induced by the Solar Wind Pressure Enhancement in Both Hemispheres From Imaging and In Situ Particle Observations

AbstractThis paper presents simultaneous detached proton auroras that appeared in both hemispheres at 11:06 UT, 08 March 2012, just 2 min after a sudden solar wind pressure enhancement (~11:04 UT) hit the Earth. They were observed under northward interplanetary magnetic field Bz condition and during the recovery phase of a moderate geomagnetic storm. In the Northern Hemisphere, Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observed that the detached arc occurred within 60°–65° magnetic latitude and covered a few magnetic local time (MLT) hours ranging from 0530 to 0830 MLT with a possible extension toward noon. At the same time (11:06 UT), Polar Orbiting Environment Satellites 19 detected a detached proton aurora around 1300 MLT in the Southern Hemisphere, centering ~62° magnetic latitude, which was at the same latitudes as the northern detached arc. This southern aurora was most probably a part of a dayside detached arc that was conjugate to the northern one. In situ particle observations indicated that the detached auroras were dominated by protons/ions with energies ranging from around 20 keV to several hundreds of keV, without obvious electron precipitations. These detached arcs persisted for less than 6 min, consistent with the impact from pressure enhancement and the observed electromagnetic ion cyclotron (EMIC) waves. It is suggested that the increasing solar wind pressure pushed the hot ions in the ring current closer to Earth where the steep gradient of cold plasma favored EMIC wave growth. By losing energy to EMIC waves the energetic protons (&gt;20 keV) were scattered into the loss cone and produced the observed detached proton auroras.

Fe/O ratio behavior as an indicator of solar plasma state at different solar activity manifestations and in periods of their absence

We report the results of the investigation into plasma physical characteristics at various solar activity manifestations and in periods of their absence. These results have been obtained from quantitative estimates of the relative abundance of Fe/O ions in different energy ranges. Maximum values of the Fe/O ratio is shown to correspond to particle fluxes from impulsive flares for ions with energies &lt;2 MeV/n (the most significant manifestation of the FIP effect). In particle fluxes from gradual flares, the Fe/O value decreases smoothly with ion energy and is noticeably inferior to values of fluxes in impulsive events. We have established that the properties of flares of solar cosmic rays indicate their belonging to a separate subclass in the total population of gradual events. Relying on variations in the abundance of Fe/O ions, we propose an xplanation of the solar plasma behavior during the development of flares of both classes. Magnetic clouds (a separate type of coronal mass ejections (CME)), which have regions of turbulent compression and are sources of strong geomagnetic storms, exhibit a relative composition of Fe ions comparable to the abundance of Fe in ion fluxes from gradual flares. We have found out that the Fe/O value can be used to detect penetration of energetic flare plasma into the CME body at the initial phase of their joint development and to estimate its relative contribution. During solar minimum with complete absence of sunspots, the Fe/O ratio during periods of “quiet” solar wind show absolutely low values of Fe/O=0.004–0.010 in the energy range from 2–5 to 30 MeV/n. This is associated with the manifestation of the cosmic ray anomalous component, which causes an increase in the intensity of ion fluxes with a high first ionization potential, including oxygen (O), and elements with a low first ionization potential (Fe) demonstrate weakening of the fluxes. As for particles with higher energies (Ek&gt;30 MeV/n), the Fe/O increase is due to the decisive influence of galactic cosmic rays on the composition of impurity elements in the solar wind under solar minimum conditions. The relative content of heavy elements in galactic cosmic rays 30–500 MeV/n is similar to values in fluxes from gradual flares during high solar activity. During solar minimum without sunspots, the behavior of Fe/O for different ion energy ranges in plasma flows from coronal holes (CH) and in the solar wind exhibits only minor deviations. At the same time, plasma flows associated with the disturbed frontal CH region can be sources of moderate geomagnetic storms.

Поведение отношения Fe/O как показателя состояния солнечной плазмы при различных проявлениях активности и в периоды ее отсутствия

We report the results of the investigation into plasma physical characteristics at various solar activity manifestations and in periods of their absence. These results have been obtained from quantitative estimates of the relative abundance of Fe/O ions in different energy ranges. Maximum values of the Fe/O ratio is shown to correspond to particle fluxes from impulsive flares for ions with energies &lt;2 MeV/n (the most significant manifestation of the FIP effect). In particle fluxes from gradual flares, the Fe/O value decreases smoothly with ion energy and is noticeably inferior to values of fluxes in impulsive events. We have established that the properties of flares of solar cosmic rays indicate their belonging to a separate subclass in the total population of gradual events. Relying on variations in the abundance of Fe/O ions, we propose an xplanation of the solar plasma behavior during the development of flares of both classes. Magnetic clouds (a separate type of coronal mass ejections (CME)), which have regions of turbulent compression and are sources of strong geomagnetic storms, exhibit a relative composition of Fe ions comparable to the abundance of Fe in ion fluxes from gradual flares. We have found out that the Fe/O value can be used to detect penetration of energetic flare plasma into the CME body at the initial phase of their joint development and to estimate its relative contribution. During solar minimum with complete absence of sunspots, the Fe/O ratio during periods of “quiet” solar wind show absolutely low values of Fe/O=0.004–0.010 in the energy range from 2–5 to 30 MeV/n. This is associated with the manifestation of the cosmic ray anomalous component, which causes an increase in the intensity of ion fluxes with a high first ionization potential, including oxygen (O), and elements with a low first ionization potential (Fe) demonstrate weakening of the fluxes. As for particles with higher energies (Ek&gt;30 MeV/n), the Fe/O increase is due to the decisive influence of galactic cosmic rays on the composition of impurity elements in the solar wind under solar minimum conditions. The relative content of heavy elements in galactic cosmic rays 30–500 MeV/n is similar to values in fluxes from gradual flares during high solar activity. During solar minimum without sunspots, the behavior of Fe/O for different ion energy ranges in plasma flows from coronal holes (CH) and in the solar wind exhibits only minor deviations. At the same time, plasma flows associated with the disturbed frontal CH region can be sources of moderate geomagnetic storms.

The Role of Storm Time Electrodynamics in Suppressing the Equatorial Plasma Bubble Development in the Recovery Phase of a Geomagnetic Storm

AbstractWe investigate the role of storm time electrodynamics in suppressing the equatorial plasma bubble (EPB) development using multi‐instruments over India during a moderate geomagnetic storm that occurred on 2 October 2013 where Dst minimum reached ~−80 nT. This storm produced unique signatures in the equatorial ionosphere such that equatorial electrojet strength showed signatures of an abrupt increase of its strength to 150 nT and occurrence of episodes of counter electrojet events. During the main phase of the storm, the interplanetary magnetic field Bz is well correlated with the variations in the equatorial electrojet/counter electrojet suggesting the role of undershielding/overshielding electric fields of magnetospheric origin. Further, observations showed the presence of strong F3 layers at multiple times at multiple stations due to undershielding electric field. Interestingly, we observed simultaneous presence of F3 layers and suppression of EPBs in the dusk sector during the recovery phase. While strong EPBs were observed before and after the day of the geomagnetic storm, suppression of the EPBs on the storm day during “spread F season” is intriguing. Our further analysis using low‐latitude station, Hyderabad, during the time of prereversal enhancement suggests that intense Esb layers were observed on the storm day but were absent/weak on quiet days. Based on these results, we suggest that the altitude/latitude variation of disturbance dynamo electric fields/disturbance winds may be responsible for simultaneous detection of F3 layers, occurrence of low‐latitude Es layers, and suppression of EPBs during the storm day along the sunset terminator.

PRELIMINARY RESULTS OF LOWER IONOSPHERE MONITORING BASED ON ANALYSIS OF TWEEK-ATMOSPHERICS

The altitude interval and the low concentration of charged particles in the lower ionosphere reduce the possibilities of its investigation by using radio sounding, balloons, rockets, or satellites. We use the tweek-atmospherics (tweeks) in this work for studying the lower ionosphere. Tweeks are the ELF–VLF radio pulses excited by lightning strokes in the Earth-ionosphere waveguide. A new method is suggested and used of automated identification and analysis of tweeks. On the basis of the tweek analysis recorded in August 2014, the relationship was investigated between the regular variations in the ionosphere height caused by changes in the solar zenith angle, which determines the main source of ionization in ambient night conditions - the radiation from geocorona. We demonstrate that elevation of the lower boundary of the ionosphere increases the flux rate of tweeks, and the effect is associated with the decrease of losses in the ionosphere. The elevation of ionosphere lower boundary was detected during the moderate geomagnetic storm. Thus, the diagnostic capabilities of the proposed method were demonstrated, which allow for locating the thunderstorm centers and to reveal the height variations of the Earth-ionosphere waveguide along the propagation paths of VLF radio waves excited by lightning discharges from different thunderstorms.

Interplanetary and Geomagnetic Consequences of Interacting CMEs of 13 – 14 June 2012

We report on the kinematics of two interacting CMEs observed on 13 and 14 June 2012. Both CMEs originated from the same active region NOAA 11504. After their launches which were separated by several hours, they were observed to interact at a distance of 100 Rs from the Sun. The interaction led to a moderate geomagnetic storm at the Earth with Dst index of approximately, -86 nT. The kinematics of the two CMEs is estimated using data from the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) onboard the Solar Terrestrial Relations Observatory (STEREO). Assuming a head-on collision scenario, we find that the collision is inelastic in nature. Further, the signatures of their interaction are examined using the in situ observations obtained by Wind and the Advance Composition Explorer (ACE) spacecraft. It is also found that this interaction event led to the strongest sudden storm commencement (SSC) (approximately 150 nT) of the present Solar Cycle 24. The SSC was of long duration, approximately 20 hours. The role of interacting CMEs in enhancing the geoeffectiveness is examined.