Superposed Epoch Analysis Method Research Articles

Energy Inflow into the Magnetosphere and Its Distribution During Magnetic Storm

The energy inflow from the solar wind into the magnetosphere and its dissipation in the circular current and auroral ionosphere during maximums of magnetic storm intensity are considered. All magnetic storms with Dst intensities from –18 nT to –422 nT for the period from 1996 to 2014 were divided into groups. For each group, the mean contribution of energy from the solar wind to the magnetosphere and subsequent characteristics of the energy dissipation in the auroral ionosphere and circular current were determined by the superposed epoch analysis method. The nonlinearity of the dependence of the intensity of magnetic storms on the energy coming from the solar wind into the magnetosphere was revealed. Anomalous behavior of magnetic storms with intensity |Dst| > 200 nT was detected. A discussion of the results is given.

Study of the evolution of the geomagnetic disturbances during the passage of high-speed streams from coronal holes in solar cycle 2009-2016

We study the geo-effectiveness of high-speed streams (HSS) detected during the consecutive eight years (2009 - 2016) period of solar cycle 24. These HSS are of different speeds and durations. During their passage, solar wind plasma and magnetic field parameters are also not similar. We utilize solar wind plasma and magnetic field parameters during the HSS passage in addition to two geomagnetic indices. Using the superposed-epoch analysis method, we study the time-evolution of geomagnetic disturbances concerning various solar wind parameters during the passage of HSS of different speeds and durations. We identify the solar wind parameter, which best follows the variations in geomagnetic parameters during the evolution of geomagnetic storms, from the start till recovery. Further, utilizing the level of geomagnetic activity due to individual HSS, we also search for solar wind parameter/function, which best correlates with the strength of geomagnetic storms due to these HSS.

Study of transient modulation of galactic cosmic rays due to interplanetary manifestations of coronal mass ejections: 2010 - 2017

We study the transient modulation of galactic cosmic rays (GCRs) due to coronal mass ejections (CMEs) and their associated structures detected during most part of solar cycle 24. The interplanetary counterpart of CMEs (ICMEs), while propagating in the interplanetary space, may be associated with the shock/sheath region. An ICME may (or may not) be the so-called magnetic cloud. Using the method of superposed-epoch analysis, we analyze cosmic-ray neutron monitor data together with various plasma and magnetic field data during the passage of interplanetary structures with distinct plasma and magnetic field properties. From a detailed analysis of plasma and magnetic field variations during the passage of the shock/sheath/magnetic cloud/non-magnetic cloud ejecta, together with the cosmic-ray neutron monitor data, we conclude that the high magnetic field regions are particularly effective in modulating the GCRs when the field inside them is high and turbulent also. Our results agree with the hypothesis that the transient decreases/Forbush decreases in GCRs are mainly caused by turbulent high field regions due to the scattering of the GCR particles.

A Statistical Study of the Subauroral Polarization Stream Over North American Sector Using the Millstone Hill Incoherent Scatter Radar 1979–2019 Measurements

AbstractThis work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019, which provides a comprehensive SAPS climatology using a significantly larger database of radar observations than was used in seminal earlier works. Key features of SAPS and associated electron density (Ne), ion temperature (Ti), and electron temperature (Te) are investigated using a superposed epoch analysis method. The characteristics of these parameters are investigated with respect to magnetic local time, season, geomagnetic activity, solar activity, and interplanetary magnetic field (IMF) orientation, respectively. The main results are as follows: (1) Conditions for SAPS are more favorable for dusk than near midnight, for winter compared to summer, for active geomagnetic periods compared to quiet time, for solar minimum compared to solar maximum, and for IMF conditions with negative By and negative Bz. (2) SAPS is usually associated with a midlatitude trough of 15–20% depletion in the background density. The SAPS‐related trough is more pronounced in the postmidnight sector and near the equinoxes. (3) Subauroral ion and electron temperatures exhibit a 3–8% (50–120 K) enhancement in SAPS regions, which tend to have higher percentage enhancement during geomagnetically active periods and at midnight. Ion temperature enhancements are more favored during low solar activity periods, while the electron temperature enhancement remains almost constant as a function of the solar cycle. (4) The electron thermal content, Te × Ne, in the SAPS associated region is strongly dependent on 1/Ne, with Te exhibiting a negative correlation with respect to Ne.

The Comparison of Responses to Geomagnetic Activity Changes of foF2 Predicted by IRI with Observations at Magnetic Conjugate Points for Middle and High Latitudes

In this study, the response to geomagnetic storms of the ionospheric F2 layer critical frequency (foF2) was investigated at the magnetic conjugate points. The hourly foF2 data observed at the magnetic conjugate points of middle and high latitudes for the geomagnetic stormy days around both of 1976 and foF2 data received from IRI-2016 Model for same points were used. The foF2 data observed in magnetic conjugate points and received from IRI Model were examined using the superposed epoch analysis method and the results obtained were compared. From the results of analysis, it was observed that the foF2 data observed at magnetic conjugate points and received from IRI Model simultaneously respond to geomagnetic activity changes for both the middle latitudes and the high latitudes.

Thermospheric Neutral Winds Above the Oukaimeden Observatory: Effects of Geomagnetic Activity

AbstractIn the context of space weather, we investigate the effect of geomagnetic activity on Earth's thermosphere above the Oukaimeden Observatory in Morocco (geographic coordinates: 31.206°N, 7.866°W; magnetic latitude: 22.77°N) over 3 years from 2014 to 2016. The observatory is equipped with a Fabry‐Perot interferometer (FPI) that provides measurements of thermospheric wind speed. In this study 41 disturbed nights (with SYM‐H ≤ −50 nT, Kp ≥ 5) were identified and analyzed. We have characterized the meridional and zonal winds variability and dependence on the solar cycle, during both quiet and disturbed conditions. We have classified the storm time meridional neutral winds into three types of variation. The first type is characterized by traveling atmospheric disturbance (TAD)‐induced circulation: the first TAD coming from the north and the second TAD being transequatorial, coming from the south. This type of storm with TAD‐induced circulation accounts for 59% of the cases. The second type exhibits only slight discrepancies between the disturbed and quiet night flows. These cases account for 33% of the cases. The third type is characterized by the transequatorial wind in whole the night. This last type accounts for 8% of the cases. Finally, we apply a superposed epoch analysis method on the FPI data, and the effect of each phase of the geomagnetic storm on the wind flow and vertical total electron content VTEC has been quantified.

The Responses of Ionospheric Conductivities on the Mid-Latitudes to Changes in the BZ Component of Interplanetary Magnetic Field

In this study, we investigate the response of the ionospheric conductivities on the middle latitudes to the southward and northward turnings of the BZ component of the interplanetary magnetic field (IMF). For this purpose, parallel conductivity (σ0), Pedersen conductivity (σ1), Hall conductivity (σ2) data calculated for Rome (41.8° N, 12.5° E); Juliusruh (54.6° N, 13.4° E); Uppsala (59.8° N, 17.6° E); Lycksele (64.7° N, 18.8° E) during the 22nd solar cycle are examined. The superposed epoch analysis method is used to investigate the response of the conductivities to the southward and northward turnings of the BZ component. From the analysis results it is observed that the turnings in the BZ component of the IMF have significant effect on all conductivities. At the event moment (zero time), the southward turnings of the BZ component increases all conductivities while the northward turnings of the BZ component decreases all conductivities. The effect on the conductivities of the changes in the BZ component occurs before about 18 h from the event moment, and this effect disappears after about 24 h from the event moment. Furthermore, all of ionospheric conductivities shows or does less reaction to changes in the BZ component as latitude increase.

The effect of different phases of severe geomagnetic storms on the low latitude ionospheric critical frequencies

In this paper, the effect of different phases of severe geomagnetic storms on low latitude ionospheric critical frequencies (foF2) is investigated. For this purpose, hourly ionospheric critical frequency (foF2) data measured at the low latitude ionosonde station Manila during 1981 and 1991 is examined. The investigation is carried out using superposed epoch analysis method considering the disturbance storm time index Dst ≤ −100 nT hours as event times. To examine depending on local time the effect of the phases of geomagnetic storms on foF2, this analysis was conducted for separately for the day hours, night hours, and all hours during both the main and the recovery phases of the severe geomagnetic storms and the results were compared with each other. It is observed that for both 1981 and 1991, the highest change (increase or decrease) in foF2 values occurs at the event times for all hours of day during both the main and the recovery phase of severe geomagnetic storms. Also, during the main phases of severe geomagnetic storms, the foF2 values increase at day hours, while the foF2 values decrease at night hours. However, during the recovery phases of severe geomagnetic storms, the foF2 values decrease at day hours, while the foF2 values increase at night hours. For both the day and night hours, the changes in the foF2 values during the recovery phase of severe geomagnetic storms are greater than the changes in the foF2 values during the main phase of severe geomagnetic storms.

The effect of geomagnetic activity changes on the ionospheric critical frequencies (foF2) at magnetic conjugate points

In this work, we investigate the possible effects of geomagnetic activity on the ionospheric critical frequencies (foF2) in geomagnetic conjugate points. For this purpose, hourly foF2 data measured for the year 1976 from the ionosonde stations Akita, St. John's and Resolute Bay in the Northern hemisphere and their corresponding magnetic conjugate ionosonde stations Brisbane, Halley Bay and Scott Base in the Southern hemisphere are examined. Planetary geomagnetic activity “3h-Kp” indices are used as a geomagnetic activity indicator. foF2 data in the magnetic conjugate points (MCP) are investigated by using a superposed epoch analysis method. This analysis is done depending on the response of foF2 to geomagnetic activity variations in MCP based on geomagnetic stormy days around equinoxes (March 21, September 23) and solstices (June 21, December 21), and the results obtained from these MCP are compared. From these results, it is found that foF2 values in magnetic conjugate pairs give similar reactions to geomagnetic activity variations simultaneously, although this relationship differs according to the seasons and magnetic latitudes of the stations.

Observation of a 27-day solar signature in noctilucent cloud altitude

Previous studies have identified solar 27-day signatures in several parameters in the Mesosphere/Lower thermosphere region, including temperature and Noctilucent cloud (NLC) occurrence frequency. In this study we report on a solar 27-day signature in NLC altitude with peak-to-peak variations of about 400 m. We use SCIAMACHY limb-scatter observations from 2002 to 2012 to detect NLCs. The superposed epoch analysis method is applied to extract solar 27-day signatures. A 27-day signature in NLC altitude can be identified in both hemispheres in the SCIAMACHY dataset, but the signature is more pronounced in the northern hemisphere. The solar signature in NLC altitude is found to be in phase with solar activity and temperature for latitudes ≳70°N. We provide a qualitative explanation for the positive correlation between solar activity and NLC altitude based on published model simulations.

The lunar semidiurnal tide at the polar summer mesopause observed by SOFIE

The polar summer mesopause, particularly the presence of noctilucent clouds (NLCs), exhibits pronounced temporal variability. Parts of this variability are thought to be caused by lunar tidal influences. We extract the semidiurnal lunar tide in various NLC related parameters by applying the superposed epoch analysis method to the dataset of the SOFIE satellite instrument. Analyzing the NLC seasons from 2007 to 2015 in the northern and southern hemisphere we, first, confirm the influence of the lunar tide on ice water content (IWC) and temperature. For both parameters the lunar influence had already recently been demonstrated in satellite measurements. Second, we apply the analysis to the variety of parameters observed by SOFIE including trace gases (H2O, O3, CH4, and NO), NLC properties (e.g., NLC altitudes and ice mass density), microphysical properties (e.g., particle concentration and mean radius), and mesopause properties. In all of these parameters we find signatures of the semidiurnal lunar tide, which is the first demonstration of this effect for all of these parameters. We quantify the lunar influence in terms of amplitudes and phases. Whereas the focus of the present study is providing observational evidence for the existence of lunar tidal signatures in various parameters, we do not aim at investigating the underlying mechanisms in detail, which is only possible with the utilization of comprehensive modeling approaches. Nevertheless, we briefly discuss the relations to known processes of the NLC evolution where appropriate, e.g., the relevance of the freeze-drying effect for the signature in H2O and the relation of IWC and NLC altitudes.

First identification of lunar tides in satellite observations of noctilucent clouds

Noctilucent clouds (NLCs) are optically thin ice clouds occurring near the polar summer mesopause. NLCs are a highly variable phenomenon subject to different sources of variability. Here we report on a poorly understood mechanism affecting NLCs, i.e., the lunar gravitational tide. We extract remarkably clear and statistically highly significant lunar semidiurnal tidal signatures in NLC occurrence frequency, NLC albedo and NLC ice water content from observations with the Solar Backscatter Ultraviolet (SBUV) satellite instruments using the superposed epoch analysis method applied to a data set covering more than 3 decades. The lunar semidiurnal tide is identified in NLC measurements in both hemispheres. In addition, lunar semidiurnal tidal signatures in polar summer mesopause temperature were extracted from space borne observations with the Microwave Limb Sounder (MLS) and the phases of the lunar tidal signatures in NLC parameters and temperature are demonstrated to be consistent. To our best knowledge these results constitute the first identification of the lunar tide in non-visual NLC observations.

Observed geomagnetic induction effect on Dst-related magnetic observations under different disturbance intensities of the magnetospheric ring current

Based on the spherical harmonic expansion of geomagnetic disturbance observed on the mid-latitude surface of the Earth, external and internal field separation is conducted in which the external component is magnetic disturbance caused by the magnetospheric ring current and the internal component is that raised by the correspondingly induced currents within the Earth. The objectives are to evaluate the influences of the induced internal field on the surface magnetic observations and to reveal the response performance of internal geomagnetic induction under different strengths of magnetospheric ring current fluctuations for better understanding of the disturbance storm time (Dst) index variations. The results show that the ratio of the internal component to surface observation does not remain constant in storm time. During the main phase of the storm, the ratio variation follows the pattern of logarithmic growth with storm evolution up to the top value at the Dst-minimum; then, the ratio slowly decreases in the long recovery phase. Multiple small logarithmic growths are superimposed on the traces of internal ratios, corresponding to temporary ring current intensification during the storm main phase and amplifying the effect of this intensification on surface magnetic observations. With the intensification of magnetospheric storms from the level of (−200 nT, − 100 nT) to (−300 nT, − 200 nT) and (−500 nT, − 300 nT) classified with the Dst-minimum, the top value of the ratio averaged for each storm group in the superposed epoch analysis method increases from the value of 0.295 ± 0.014 to 0.300 ± 0.016 and 0.308 ± 0.015, respectively. It is demonstrated that the geomagnetic induction exceeds the linear relation with the intensification of the external field, which is physically reasonable and coincident with the Faraday’s law of induction. Due to the effects of high induction of the oceans and lateral heterogeneity of electric conductivity distribution in the upper mantle of the Earth, the geomagnetic induction and its contribution to surface geomagnetic disturbance vary significantly among observatories. This factor should be considered in the research of magnetospheric current systems.

Statistical study of interplanetary condition effect on geomagnetic storms: 2. Variations of parameters

We investigate the behavior of mean values of the solar wind’s and interplanetary magnetic field’s (IMF) parameters and their absolute and relative variations during the magnetic storms generated by various types of the solar wind. In this paper, which is a continuation of paper [1], we, on the basis of the OMNI data archive for the period of 1976–2000, have analyzed 798 geomagnetic storms with Dst ≤ −50 nT and their interplanetary sources: corotating interaction regions CIR, compression regions Sheath before the interplanetary CMEs; magnetic clouds MC; “Pistons” Ejecta, and an uncertain type of a source. For the analysis the double superposed epoch analysis method was used, in which the instants of the magnetic storm onset and the minimum of the Dst index were taken as reference times. It is shown that the set of interplanetary sources of magnetic storms can be sub-divided into two basic groups according to their slowly and fast varying characteristics: (1) ICME (MC and Ejecta) and (2) CIR and Sheath. The mean values, the absolute and relative variations in MC and Ejecta for all parameters appeared to be either mean or lower than the mean value (the mean values of the electric field Ey and of the Bz component of IMF are higher in absolute value), while in CIR and Sheath they are higher than the mean value. High values of the relative density variation sN/〈N〉 are observed in MC. At the same time, the high values for relative variations of the velocity, Bz component, and IMF magnitude are observed in Sheath and CIR. No noticeable distinctions in the relationships between considered parameters for moderate and strong magnetic storms were observed.

HF radar observations of the F region ionospheric plasma response to Storm Sudden Commencements

Performance of the Super Dual Auroral Radar Network (SuperDARN) HF radars during geomagnetic storms is investigated by analyzing the data collected during storm events over a 5‐year period. Changes in the occurrence of F region HF backscatter observed by the 6 most equatorward radars are analyzed statistically using a superposed epoch analysis method with respect to a Storm Sudden Commencement (SSC). Regular diurnal variations of the echo occurrence during geomagnetically quiet days are produced and the amount of detected backscatter during storms is adjusted using quiet time curves. All radars considered in this study show a significant decrease in the number of detected echoes approximately 24 hours following SSC. Unexpected significant changes in occurrence levels are also present within a few hours of SSC, with most radars observing an increase in the amount of backscatter detected. The typical time evolution of F region echo occurrence is highly reminiscent of that of the electron density reported previously. Also considered is the ionospheric convection response to SSC observed by the zonally looking SuperDARN Unwin radar in New Zealand. It is shown that the initial response to SSC is instantaneous within uncertainty and appears to be independent of the magnetic latitude and local time. The observed convection response timing and morphology are discussed in the context of possible ionospheric propagation mechanisms.

Impact of aerosol on air temperature in Kuwait

This work uses MODIS aerosol optical thickness (AOT) data observed over Kuwait during the 7-year interval 2000–2007. The values of AOT and the Ångström wavelength exponent ( α) show a clear annual cycle. These data are categorized into two catalogues in terms of the values of the AOT of the 870 nm channel ( τ 870). One catalogue (71 days) includes days with high values of AOT ( τ 870 ≥ 0.75). The most probable “modal” value of α for these days is 0.52. The other catalogue (1162 days) consists of the background days with a modal value ~ 1.1 for the exponent α. This analysis is extended to include water vapor content (WVC), surface wind speed (V), visibility (Vis) and the diurnal temperature range (DTR). Chree's method of superposed-epoch analysis is applied to these parameters in order to compare the variation in the daily averages during days with high AOT values with respect to background days. The high values of AOT during the 71 days are positively correlated with aerosol size, near-surface winds and poor visibility. This concludes that the aerosol particles during these days were mostly dust. The mean daily value of the DTR (Δ T) and visibility reduced significantly during these days. This reduction on DTR is a direct result of increasing the atmospheric opacity due to the presence of dust.

Comparison of ionospheric ionization measurements over Athens using ground ionosonde and GPS-derived TEC values

[1] GPS-derived TEC measurements (GPSTEC) represent the electron content up to 20,000 km, while a new ionosonde technique estimates the ionospheric electron content (ITEC) from ground ionosondes from the bottom of the ionosphere through the peak up to 1000 km. The two independent techniques were applied to determine ITEC and GPSTEC for Athens (38°N, 23.5°E) using the vertical Digisonde observations at Athens and GPSTEC maps produced by DLR/IKN for Europe using GPS measurements. Comparison of the two quantities over a 12-month time period shows a general agreement at daytime and a systematic deviation of ITEC towards lower values during nighttime. The diurnal variation of their residual values (ΔTEC) exhibits a morning minimum and an evening maximum, which can be explained in terms of ionospheric-plasmaspheric ionization exchange. The prominent evening enhancement, observed in all seasons around 1800–2000 LT, is attributed to the plasmaspheric bulge. The application of the superposed epoch analysis method on the daily ΔTEC values for several geomagnetic storms showed a rapid decrease of ΔTEC just after the storm initiation and a consecutive increase over a time period of 9 days, a behavior that is consistent with the plasmaspheric depletion and successive replenishment following storm activity. The daily variation of the ionospheric slab thickness is compatible with the variation of the thermospheric temperature within a day. Concerning the total slab thickness, this behavior is altered by the nighttime increase, which is most prominent in winter, and it is due to the lowering of the O+/H+ transition height. In summary, this analysis presents additional proof that the residuals ΔTEC = GPSTEC − ITEC provide information about the qualitative characteristics of the plasmaspheric dynamics as deduced from their diurnal behavior and their variation during geomagnetic storms.

GEOTAIL observations of flow velocity and north‐south magnetic field variations in the near and mid‐distant tail associated with substorm onsets

Temporal changes in the plasma flow and magnetic fields of the Earth's magnetotail in the region of −7.5 > X(RE) > −52.5 and −15 < Y(RE) < 15 in the GSM coordinates were studied by applying a superposed epoch analysis method to 263 substorm events observed by GEOTAIL spacecraft. The onset times of the substorms were determined with Pi‐2 pulsations at Kakioka station. It was found that the tailward flow associated with plasmoids started about 2 or 3 min before the Pi‐2 onsets, and the earthward flow became somewhat coherent 1 min before the onsets. The variation of the northward magnetic field component was investigated by obtaining the deviation of this component from that in the interval from 10.5 to 7.5 min before the Pi‐2 onsets. We found that the northward magnetic field increased at X ∼ −10RE and decreased at X ∼ −28RE simultaneously in the premidnight tail immediately after the onsets. These variations definetely correspond to the dipolarization and plasmoid formation, respectively.

Geomagnetic, ionospheric and cosmic ray variations around the passages of different magnetic clouds

Thirty-four interplanetary magnetic clouds have been divided into two groups on the basis of Wilson's ( J. geophys. Res. 95, 215, 1990) classification: NS clouds (whose B z near cloud onset at Earth is directed northward, and soon after B z is turning southward) and SN clouds (those with an opposite behaviour with respect to B z ). Using the days of cloud onsets as key days, geomagnetic, ionospheric and cosmic ray data have been analysed by the superposed epoch analysis method for passages of both NS and SN clouds. On the basis of the daily ∑ K p values, geomagnetic activity is found to suddenly increase in the vicinity of both types of cloud passages. Afterwards, the variation shown by the geomagnetic indices is found to differ for NS clouds in comparison with SN clouds. Namely, on average the recovery to a normal activity level is much slower for NS clouds. Similarly, the enhancements in the ionospheric absorption of radio waves (the so-called “after-effects”) are found to show different signatures according to cloud type, an interpretation also valid for variations in cosmic ray intensity. The latter results are based on analyses of neutron monitor counts observed at two stations (Apatity: 67°N; and Moscow: 55°N).

A Critique of the Superposed Epoch Analysis Method: Its Application to Solar–Weather Relations

Abstract Superposed epoch analyses, based on solar sector boundary crossings as key times and the Vorticity Area Index as the response variable, are tested for significance using both parametric and randomization techniques. We conclude from a comparison of these techniques that the randomization procedure leads to markedly different results from those obtained from parametric tests. In particular, the results are strongly affected by the modest skewness of the Vorticity Area Index distribution.